POWER BALANCING IN A MULTI-PHASE SYSTEM
BACKGROUND
The power grid is a distribution network for delivering power from suppliers to consumers.
Traditionally, the electrical needs of consumers have been met by power companies distributing power
through the grid. A power grid provides the majority of the power that is made available to a region.
Power is generated at a location and distributed through the power grid to the surrounding areas. This
power is distributed in the form of alternating current (AC) power, which is transmitted at very high
voltages along power lines. The majority of the power transmitted through the grid is in three phases.
Power is transmitted in three phases to provide an even source of power which can be readily
distributed over long distances with efficient wiring costs. Three-phase power is AC power that is
transmitted along three separate power lines. Each line of power is in a different phase, with the phase of
each power line shifted relative to the other two power lines by 120 degrees. These phases are shifted
from each other to provide the consistency of a DC power source using AC power.
Three-phase power is a standard method for power distribution throughout the world. In the
United States, power is distributed in three phases, but the majority of all homes are run on a single phase.
The three-phase power is split into single phases at a transformer, where the voltages are stepped down
and the phases are split up. However, some homes and businesses are configured to receive three-phase
power.
Figure 1 illustrates energy usage in a home 105 wired for three-phase power from the power grid.
The home 105 may be any site where power is consumed or produced, such as an industrial building or an
isolated solar panel installation. Figure 1 shows three power lines 125, 130, and 135, the home 105, a
load 160, and a power grid 120. The three power lines 125, 130, and 135 provide power from the grid 120
to the load 160 in the home 105 in three different phases shown in graphs 145, 150, and 155 respectively.
The load 160 for the home 105 is powered by the three power lines 125, 130, and 135 supplied by
the power grid 120. The graphs 145, 150, and 155 of the three phases show that the phases of the power
lines 125, 130, and 135 are shifted in time with a phase difference of one-third of a cycle or 120 degrees.
The phase difference provides power such that one of the three phases will be reaching its peak at three
different points of a single cycle of a particular power line, which allows an AC power source to provide
the consistency of a DC power source. Some alternative power generators, such as solar panels, provide
DC power, but in order to be used with the grid, the DC power source must be converted to AC power.
Solar panels have become an increasingly common alternative source of energy. With
installations of varying sizes, consumers have also become producers, resulting in a multitude of power
producers, rather than a single producer. Consumers are able to produce their own power, reducing their
reliance on the power companies. Due to the nature of solar power energy being more readily available
during the day, residential customers may be producers during the day, but consumers during the night as
the needs of a particular site change throughout the day. As the amount of power that the solar installation
inject into the grid and the needs of the consumer change throughout the day, additional power may need
to either be drawn from the grid or returned to the grid. However, in order for the solar installation to
inject power into the grid, the DC power generated by the solar panels on the installation needs to be
converted to AC power in a form similar to the power provided by the grid.
The conversion of power from solar panels from DC power to AC power is often done using
inverters. Typically, the solar panels are wired in series and then connected with high-voltage cables to
connect the DC power to an inverter. These solar panel installations use a single, large inverter to convert
the generated power into power which can be used in the home or fed back into the grid.
Figure 2 illustrates energy usage in a home 205 wired for three-phase power where power
supplied for the home 205 is supplied from solar panels 215 which provide DC power to a single inverter
262. Similarly to Figure 1, the three power lines 225, 230, and 235 provide power to the load 260 of the
home 205. In addition, Figure 2 shows solar panels 215 and inverter 262. The solar panels 215 provide
DC power to the inverter 262 which converts the DC power and provides AC power to the three power
lines 225, 230, and 235. Single inverter installations require a large inverter as well as heavy, high-voltage
cabling to bring the DC power to the AC inverter.
In recent years there has been an emergence of interest in module-integrated electronics. The
solar micro-inverter in particular has been noted as a product that has a number of benefits over the
existing conventional solutions. Micro-inverters are smaller inverters which are installed on or near the
solar panels themselves. Rather than a single inverter for inverting all of the power provided by an
installation, micro-inverters invert the power of one or a few solar panels and provide AC power at the
source panels. Micro-inverters provide many benefits over traditional inverters. These benefits include:
improved energy harvest over the lifetime of the installation, particularly in scenarios of shading or other
causes of mismatch in solar photovoltaic (PV) installations and low voltage DC (less than 80V from a
single panel), which is safer and significantly reduces arcing faults. Additional benefits of an energy
harvesting system based on micro-inverters also include the ability to pin point failures or problems with
solar panels (or solar modules), and the ease of scalability when adding panels to an installation. The
installation process itself is also extremely easy and can be considered as a plug and play method.
BRIEF SUMMARY
Some embodiments of the invention provide a method for maintaining balanced energy
transmissions between the power lines of a multi-phase system. In order to maintain balanced energy
transmissions, the method of some embodiments identifies the phase of the power line to which each
micro-inverter is connected. The method of some embodiments calculates an aggregate power output for
each power line of the multi-phase system and controls the individual power output of each microinverter.
In some embodiments, rather than maintaining balanced energy transmissions, the invention may
be used to balance other aspects of the power lines of the multi-phase system. For example, some
embodiments of the invention provide a method for maintaining balanced voltage levels between the
power lines of a multi-phase system as discussed below. In some embodiments, the method is performed
by a gateway which communicates with each of the micro-inverters.
In some embodiments, the method receives identification messages from each of the microinverters
based on the phase of the power line to which the micro-inverter is connected and calculates an
offset based on the identification message to determine which power line the micro-inverter is connected
to. The method calculates an aggregate power output for each power line of the multi-phase system. In
some embodiments, the method receives power output messages from each of the individual microinverters
and uses the individual power output messages to determine an aggregate power output for each
power line of the multi-phase system. The method controls the power output of the individual microinverters
in order to adjust the aggregate power being output to each power line. In some embodiments,
the method sends control signals which cause a micro-inverter to shut down or to scale back its power
production in order to control the aggregate power output.
The preceding Summary is intended to serve as a brief introduction to some embodiments of the
invention. It is not meant to be an introduction or overview of all of the inventive subject matter disclosed
in this document. The Detailed Description that follows and the Drawings that are referred to in the
Detailed Description will further describe the embodiments described in the Summary as well as other
embodiments. Accordingly, to understand all the embodiments described by this document, a full review
of the Summary, Detailed Description and the Drawings is needed. Moreover, the claimed subject matters
are not to be limited by the illustrative details in the Summary, Detailed Description and the Drawing, but
rather are to be defined by the appended claims, because the claimed subject matters can be embodied in
other specific forms without departing from the spirit of the subject matters.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features of the invention are set forth in the appended claims. However, for purpose of
explanation, several embodiments of the invention are set forth in the following figures.
Figure 1 illustrates energy usage in a home wired for three-phase power from the grid.
Figure 2 illustrates energy usage in a home wired for three-phase power with a single inverter.
Figure 3 illustrates energy usage in a home wired for three-phase power with micro-inverters.
Figure 4 illustrates asymmetric power generation in a home wired for three-phase power.
Figure 5 illustrates the power of a three-phase system and the imbalance that may arise when the
power being returned to the grid is provided by independent power sources.
Figure 6 illustrates the output balancing system of some embodiments.
Figure 7 conceptually illustrates the process performed to provide balanced power to each power
line of a multi-phase system.
Figure 8 conceptually illustrates the process performed by a gateway to identify the power line to
which each micro-inverter is connected, in some embodiments.
Figures 9a-c illustrate the timing of messages that are sent from micro-inverters to the gateway at
specific points of the AC power cycle.
Figure 10 illustrates an example of the phase identification process of some embodiments.
Figure 11 illustrates a three-phase power system in which micro-inverters report instantaneous
voltage amplitudes to a gateway.
Figure 12 conceptually illustrates the process for analyzing the micro-inverter outputs and
adjusting the outputs to balance the power output to each power line of a three-phase system.
Figure 13 illustrates an example of the analysis performed by the gateway to determine the
aggregate output to each power line of a three-phase power system.
Figure 14 illustrates a method for controlling the aggregate output to different power lines by
shutting down individual micro-inverters.
Figure 15 illustrates a method for controlling the aggregate output to different power lines by
managing a threshold output level for a set of micro-inverters.
Figure 16 conceptually illustrates the process performed by each micro-inverter in a system to
identify itself and communicate with the gateway.
Figure 17 illustrates a simplified circuit layout of a micro-inverter of some embodiments.
Figure 18 illustrates an electronic system with which some embodiments of the invention are
implemented.
DETAILED DESCRIPTION
In the following detailed description of the invention, numerous details, examples, and
embodiments of the invention are set forth and described. However, it will be clear and apparent to one
skilled in the art that the invention is not limited to the embodiments set forth and that the invention may
be practiced without some of the specific details and examples discussed.
Some embodiments of the invention provide a system for providing a balanced output to a multi
phase power grid. The system comprises a gateway for monitoring and managing the output of a plurality
of micro-inverters, a plurality of micro-inverters for providing power output to each power line of the
multi-phase system, and a communication system for transmitting and receiving messages between the
gateway and the micro-inverters. The communication system, in some embodiments, is a wireless system
with a wireless transmitter and receiver located at the gateway and at each micro-inverter. In other
embodiments, the communication system may be a wired or some other alternative communication
system.
Figure 3 illustrates a three-phase power system 300 for supplying three-phase power to a home
305. The power of the three-phase power system 300 is supplied by the power company as well as by a
set of solar panels. Figure 3 illustrates a power grid 320 for supplying power to the home 305. Three
power lines 325, 330, and 335 provide power from the grid 320 to the load 360 in the home 305 in three
different phases. The voltage levels of the power lines 325, 330, and 335 are illustrated in graphs 345,
350, and 355 respectively. The micro-inverters 365, 370, and 375 convert DC power harvested by solar
panels 315 into AC power and inject the converted AC power into the three power lines 325, 330, and
335, respectively.
Although the micro-inverters 365, 370, and 375 are illustrated as being connected to a single line,
it should be apparent to one skilled in the art that the micro-inverters are additionally connected to either a
reference neutral (a fourth line), or to one of the other active lines. The reference neutral is used as a
return line for the power. In other cases, the micro-inverters may inject power into one line but also be
connected to a second line. For instance, micro-inverters 375 could have a first connection to line 335 as
drawn and have a second additional connection to line 330 as a return line. Similarly, micro-inverters 370
could have a first connection to line 330 as drawn and have a second additional connection to line 325 as
a return line. Similarly, micro-inverters 365 could have a first connection to line 325 as drawn and have a
second additional connection to line 335 as a return line.
The power lines 325, 330, and 335 power the grid 320 as well as the load 360. Since each of the
three power lines carries power in a different phase of the three-phase power system, a power line that is
used to carry power of a particular phase is sometimes identified by that particular phase in this invention.
For example, a micro-inverter that provides power to a power line of "phase l " of a three-phase system is
sometimes referred to as the micro-inverter of "phase 1", and the power line that carries the power for
"phase 1" of the three-phase system is sometimes referred to as the "phase 1" power line. A microinverter
that injects power into a power line of a particular phase is sometimes referred to as a microinverter
"of the particular phase" or a micro-inverter "associated with the particular phase".
Unlike the large inverter of Figure 2 that is used to convert the power of an entire installation of
solar panels, micro-inverters are smaller inverters that are used to convert the power produced by one or a
few solar panels. In the example of Figure 3, each micro-inverter in micro-inverter sets 365, 370, and 375
connects one or a few solar panels. Although the sets of micro-inverters are shown as separated into
groups according to the power lines to which they are connected, one skilled in the art will recognize that
the physical layout of the micro-inverters in an installation may vary. For instance, in some embodiments,
the micro-inverters attached to each power line of the power grid may be physically interspersed with the
micro-inverters attached to the other power lines.
In order for micro-inverters 365, 370, and 375 to inject generated power into the grid 320 (i.e.,
through power lines 325, 330, and 335), the voltage level being produced by the micro-inverters must
match the voltage level on the grid 320. Since the power on the grid 320 is AC power, the voltage level
on the grid is sinusoidal and cyclically goes through different phases (i.e., 0° to 360°). Hence in order to
match the voltage level of the micro-inverters with the voltage level of the grid, the phase of the power
being generated by the micro-inverters must match the phase of the grid 320. Since the grid 320 is a threephase
system with three different power lines 325, 330, and 335 that are on three different phases, a
micro-inverter injecting power into a particular power line must ensure that the phase of the power it
provides matches that phase of the power on the power line. In the example of Figure 3, the microinverter
375 aligns the phase of the power it injects into the power line 335 to match the phase of power
that is on the power line 335, the micro-inverter 370 aligns the phase of the power it injects into the power
line 330 to match the phase of power that is on the power line 330, and the micro-inverter 365 aligns the
phase of the power it injects into the power line 325 to match the phase of power that is on the power line
325.
In some embodiments, a micro-inverter aligns its power output with the power on the grid by
sensing the voltage on the power line. Each micro-inverter will produce power that is aligned with the
phase and amplitude of the voltage sensed on the power line of the power grid to which the micro-inverter
is connected. Descriptions of micro-inverters can be found in U.S. Patent Application Publication No.
2012/0057388 and U.S. Patent Application Publication No. 2012/0063177.
As long as the different sets of micro-inverters 365, 370, and 375 inject similar amount of power
into each of the three power lines, the three phases of the three-phase power system will remain balanced
or symmetrical. However, the power output injected by micro-inverters to different power lines of the
three-phase system is based on the production of individual micro-inverters. The power production of the
micro-inverters is in turn based on the power production of the solar panels. The production of the solar
panels can vary throughout the day based on weather, the angle of the sun, etc. The production of the
solar panels may also vary greatly from solar panel to solar panel depending on factors such as shading on
a solar panel, solar panel position, solar panel performance, solar panel failure, etc. Since each phase (i.e.,
power line) in the three-phase system 300 receives power from its own set of micro-inverters, and those
micro-inverters in turn receives power from its own set of solar panels, variations in power production
from the different solar panels may cause power being injected into the different power lines to differ
from each other significantly, causing an imbalanced or asymmetrical output to the multi-phase system. In
other words, when each power line in a three-phase power system is powered by an independent power
source (e.g., an independent set of micro-inverters and solar panels), imbalance or asymmetry may arise
in the three-phase system when some of the solar panels / micro-inverters fail to produce as much power
as other solar panels / micro-inverters.
Figure 4 illustrates asymmetric power generation in a three-phase power system 400. Figure 4
shows three power lines 425, 430, and 435 for delivering three-phase power to a home 405 from a grid
420. The micro-inverter set 475 injects power to the power line 435, the micro-inverter set 470 injects
power to the power line 430. The micro-inverter set 465 injects power to the power line 425. The amount
of power injected to the power lines 425, 430, and 435 are illustrated in graphs 445, 450, and 455
respectively. The graphs 445, 450, and 455 show that the power production from the three sets of microinverters
are different (the power production of the micro-inverter set 475 is less than the power
production from micro-inverter set 470, and the power production of the micro-inverter set 470 is less
than the power production from the micro-inverter set 465). Thus, the solar power being injected into
individual phases of the three-phase system is out of balance (as illustrated in the composite graph 490).
The imbalances in the power being fed to each power line can cause problems for power
distribution. When the total power output is relatively low, the level of imbalance in the power output to
each power line may be acceptable. In fact, many residential installations are low-power single-phase
installations which inject all of their generated power into a single power line of the power grid. However,
when the amount of power being generated by the solar panels at a particular site is significant, imbalance
between different power lines of a three-phase power system may cause voltage spikes, which may
disconnect a micro-inverter or cause appliances to shut down.
Figure 5 illustrates the power levels of a three-phase system and the imbalance that may arise
when the power being injected to the grid is provided by power sources that are independent of each
other. The imbalance may arise when the independent power sources provide different amounts of power
to the different power lines / phases. Figure 5 includes a first graph 505 for illustrating the voltage levels
of the three-phase system and a second graph 510 for illustrating the current levels of the three-phase
system.
The first graph 505 includes voltage level 520 for phase 1 power line of the three-phase system,
voltage level 525 for phase 2 power line of the three-phase system, and voltage level 530 for phase 3
power line of the three-phase system. When micro-inverters are used to produce power for each power
line of the three-phase system, each micro-inverter matches both the amplitude and phase of the voltage
levels at the power grid. Therefore, even when the power generated by a particular solar panel changes, it
will not affect the output voltage of the micro-inverter. However, because the power produced is a
product of the voltage and the current, when the voltage is held constant, changes in the power production
of the solar panels will be reflected in the current produced by the micro-inverter. The second graph 510
includes current level 560 for the phase 1 power line, current level 565 for the phase 2 power line, and
current level 570 for the phase 3 power line.
In this example, though the peak voltage levels of the three power lines are the same (as seen in
the voltage graph 505), the peak current levels generated for the three power line are different (as seen in
the current graph 510), which will result in differing power outputs for each power line. As illustrated in
the current graph 510, the peak output of the current level 565 of phase 2 and the current level 570 of
phase 3 differ by a peak current difference 550. This difference in peak current levels reflects the
differences in power output between the three phases. The differences in peak current levels may result
from decreased production by the solar panels attached to the micro-inverters connected to a particular
phase. Factors such as shading and solar panel performance may affect the current production of a
particular solar panel.
Some embodiments of the invention provide a communication system by which the imbalance
between the three power lines is detected and reported to a gateway. The gateway in turn controls the
micro-inverters to make adjustments in order to maintain balanced or symmetrical power output between
the power lines. For some embodiments, Figure 6 illustrates a system 600 that includes a gateway for
balancing the power outputs from micro-inverters to a three-phase system. The output balancing system
600 allows for the management of multiple power sources to create a balanced aggregate output to each
power line. Figure 6 illustrates three power lines 625, 630, and 635, a home 605, sets of micro-inverters
665, 670, and 675, and a grid 620. The sets of micro-inverters 665, 670, and 675 output power along the
power lines 625, 630, and 635 respectively. The three power lines 625, 630, and 635 provide power from
the grid 620 to the load 660 in the home 605 in three different phases. In addition, Figure 6 shows a
wireless communication system 684 by which the gateway 640 communicates with the micro-inverters
665, 670, and 675.
The communication system 684 communicatively couples the micro-inverters 665, 670, and 675
with the gateway 640 and allows information to be exchanged between devices in the communication
system. In some embodiments, the communication system is a wireless communication system. The
communication system can be implemented in any one of a number of wireless communication systems
such as ZigBee, Wifi, Bluetooth, Wireless MBus, etc. Though not illustrated, instead of or in addition to
wireless systems, some embodiments use power line communication, in which a data signal is modulated
over a lower frequency carrier signal that is typical of mains voltage.
The gateway 640, in some embodiments, is used to monitor and manage the micro-inverters of an
installation. In some embodiments, the gateway 640 communicates with the micro-inverters 665, 670, and
675 over the communication system 684. The gateway monitors and controls the output of each of the
individual micro-inverters that feed into each power line of the multi-phase system. In some
embodiments, the gateway may be located at a site remote from the micro-inverters. The operations of the
gateway for some embodiments will further described below in Sections I-III.
Figure 6 illustrates a power output balancing operation in two stages 601 and 602. The first stage
601 shows the power levels of the three power lines 625, 630, and 635 in graphs 645, 650, and 655
respectively and in a composite graph 690. The composite graph 690 superimposes the graphs 645, 650,
and 655 and shows that the phase of the power in each of the power lines is shifted from the others by 120
degrees (one third of a full cycle). As can be seen in the composite graph 690, the peak amplitudes on the
different power lines are different, and hence the power being injected into the three-phase system is
asymmetric and out of balance. Specifically, the output of the set of micro-inverters 675 to the power line
635 is less than the output of the set of micro-inverters 670 to the power line 630, and the output of the set
of micro-inverters 670 to the power line 630 is less than the output of the set of micro-inverters 665.
The first stage 60 1 also illustrates messages being communicated between the sets of microinverters
665, 670, and 675 and the gateway 640 over the communication system 684. The messages may
be used to identify the power line to which each micro-inverter is connected. The messages may also
provide updates to the gateway 640 regarding the power being output by each micro -inverter in the sets of
micro-inverters 665-675 (e.g., total power output, voltage levels, current levels, etc.).
In the second stage 602, the gateway 640 has received the messages and calculated the aggregate
power being output to each power line of the power grid 620 by the sets of micro-inverters 665, 670, and
675. Based on the calculations, the gateway determines whether the imbalance in the output of a set of
micro-inverters is acceptable with regard to a threshold amount (e.g., whether the difference in total
power injected by the different sets of micro-inverters to different power lines of the three-phase system is
greater than the threshold amount). The threshold amount may be based on government regulations.
When, as in this example, the gateway determines that the imbalance in the power output to the power
lines 625, 630, and 635 are outside of the threshold amount, the gateway 640 sends control signals 685 to
the individual micro-inverters in 665, 670, and 675 to control the power being produced by the individual
micro-inverters. By changing the power output of some of the micro-inverters, the power being injected
into the three different power lines will be balanced, as illustrated in the composite graph 695, which
super imposes the power levels of the three different power lines after the adjustments has been made to
the individual micro-inverters.
Figure 7 conceptually illustrates a process 700 for balancing power between the different power
lines of a multi-phase system. In some embodiments, the process 700 is performed by a gateway that is
configured to interact with sets of micro-inverters. The process 700 identifies (at 705) the power line to
which each micro-inverter is connected based on the phase of the power line. Since the phases of the
voltage and/or current of the different power lines are different, the gateway can determine which power
line a micro-inverter is connected to by looking at the phase of the voltage/current on the power line
connecting the micro-inverter.
Some embodiments use identification messages received from micro-inverters for determining
the phases of the voltage/current on the power lines. In some embodiments, the identification messages
from the micro-inverters are timed with respect to a particular point in a cycle of the AC power on the
power line. The gateway in some of these embodiments determines the phase of the power line
connecting a particular micro-inverter based solely on when the message is received from the particular
micro-inverter. Alternatively or conjunctively, the phase may be calculated based on the amplitude
measurement of the micro-inverter at the moment the message is sent. In some embodiments, the power
line of each micro-inverter is identified in other ways, such as through manual input at the gateway.
Identifying the power line/phase to which a micro-inverter injects power will be further described below
in Section I .
The process 700 receives (at 710) a power output measurement from each of the individual
micro-inverters. The power output measurement being reported a particular micro-inverter in some
embodiments includes voltage, current, and power being produced by the particular micro-inverter.
The process 700 then analyzes (at 715) the received measures and computes an aggregate power
being produced by the micro-inverters to each of the three power lines in the three-phase system. In some
embodiments, the aggregate power injected into a particular power line is the sum of the power produced
by individual micro-inverters connected to that particular power line.
Next, the process 700 determines (at 720) whether any adjustment is necessary. Adjustment is
necessary when the imbalance between the different power lines of the multi-phase system is beyond a
threshold value. Some embodiments set the threshold value based a state-regulated value, the expected
output of a single micro-inverter, a custom user-identified value, and/or other variables. In some
embodiments, a user of the gateway can modify the threshold amount of imbalance dynamically in order
to trigger an adjustment. If no adjustment is necessary (i.e., the imbalance is within threshold), the process
700 returns to 710. Otherwise, the process proceeds to 725.
At 725, the process 700 sends adjustment control signals to the micro-inverters to control the
output of the micro-inverters. These adjustment control signals, as described earlier by reference to
Figure 6, are transmitted from the gateway to the micro-inverters via a wireless communication system in
some embodiments. Some embodiments sends adjustment control signals that shut down individual
micro-inverters associated with a particular phase when the power output level to the power line of that
phase is too high (i.e., much higher than power output to other phases). Some embodiments send
adjustment control signals that reduce the output levels of individual micro-inverters in order to reduce
the aggregate output of all of the micro-inverters injecting power into a particular phase. Conversely, the
adjustment control signals may turn on individual micro-inverters (e.g., for micro-inverters that were
previously turned off) or increase output levels of individual micro-inverters in order to increase the
aggregate output of all of the micro-inverters injecting power into a particular phase.
After sending the adjustment control signals, the process 700 returns to 710 to continue
monitoring the output levels from the micro-inverters.
In some embodiments, the process 700 may be used to balance other properties of the power
being produced by micro-inverters in the different phases. For example, some embodiments use a process
similar to the process 700 to balance voltage levels between the three different phases of the three-phase
system. In some of these embodiments, rather than calculating an aggregate amount of power, the process
computes the average peak voltage of the micro-inverters for each of the three phases. In some
embodiments, process sends adjust and control signals to the micro-inverters such that the voltages of the
power being injected into the three different power lines are balanced. In some of these embodiments, the
voltage level of the power lines are being sensed at a point where the power being produced by the microinverters
is being injected into the three phase system.
Several more detailed embodiments of the invention are described below. Section I describes the
identification of the power line for each micro-inverter in a system. Section II describes the monitoring of
the aggregate outputs for each power line of a multi-phase system. Section III describes the control of the
power output of individual micro-inverters.
I. IDENTIFYING THE POWER LINE FOR EACH MICRO-INVERTER
In order to balance the power being injected to a three-phase power system from sets of microinverters,
some embodiments first identify which phase or power line of the three-phase system each
micro-inverter is connected to. In some embodiments of the invention, the power line / phase connected to
a micro-inverter is identified by the gateway. By identifying the power line associated with each microinverter,
the gateway is able to calculate the total amount of power being injected to each power line /
phase of the three-phase power system.
In some embodiments, the power line connecting a micro-inverter can be identified based on an
identification message sent by the micro-inverter. The identification message is used for identifying the
properties of the voltage on the power line (e.g., amplitude, phase, etc.). In some embodiments, the microinverter
sends an identification message only at a specified point in the cycle of the AC power on the
power line. The specified point in the cycle may be any point in the cycle, such as at a peak, a zero
crossing (positive to negative or negative to positive), 10% of peak level, etc. Each micro-inverter,
regardless of which power line it is connecting, sends the message at the same point in the cycle of the
voltage in the power line. Since the voltage/current on different power lines of a three-phase system are
always at different phases of the AC power cycle (120° apart), a micro-inverters connecting a particular
power line will reach the specified point in the cycle at a different time than another micro-inverter
connecting a different power line. The gateway can therefore determine which power line a microinverter
is connected to by analyzing the time when the message from the micro-inverter is received.
In some embodiments, the identification message provides information about the power on a
power line, such as an instant voltage amplitude measurement of the voltage on the power line when the
message is sent, which is used to calculate the phase of the voltage on the power line. By calculating the
phase of the voltage on the power line, the gateway can identify the power line to which the microinverter
is connected.
For some embodiments, Figure 8 conceptually illustrates a process 800 for identifying the power
line of each micro-inverter. The process 800 is performed by the gateway in some embodiments, which
identifies the connections between the micro -inverters and the power lines of the three-phase system. The
gateway identifies the power line connected to each micro-inverter in order to calculate the aggregate
power output to each power line. In some embodiments, the process 800 corresponds to operation 705 of
the process 700.
The process 800 receives (at 805) an identification message from one of the micro-inverters in the
system. In some embodiments, the identification message has a unique identifier which identifies the
micro-inverter to the gateway. In some embodiments, an identification message contains additional
information, such as the power output of the micro-inverter or the location of the micro-inverter within an
installation. In some embodiments, a micro-inverter sends its identification message as it is being added
to the system (such as during installation of the micro-inverters or when the micro-inverters are initially
powered up). In some embodiments, a micro-inverter sends its identification message to the gateway
when requested by the gateway. In some embodiments, micro-inverters send messages periodically (e.g.,
daily). In some embodiments, micro-inverters send messages upon the occurrence of certain events such
as when the gateway is reset, when the gateway comes online, or when the gateway sends a request for
identification messages from the micro-inverters.
In some embodiments, a micro-inverter sends timed identification messages that are sent at a
specified point in time of the AC power cycle on the power line. In some embodiments, the microinverter
may send messages that are not timed, but include other information about the power which the
gateway can use to calculate the phase of the power on the power line. The gateway of some of these
embodiments determines the phase of the power based on the other information in combination with the
timestamp of when the messages are received. Using messages from micro-inverters to identify the
connections between the micro-inverters and the power lines of the three-phase system will be described
below in Sub-Section I-A.
After receiving an identification message from a particular micro-inverter, the process 800
determines (at 810) whether the particular micro-inverter has been identified before. If the micro-inverter
has not been identified before, the process proceeds to 815. If the process 800 determines that this
message is from a micro-inverter which has been identified before, the process updates (at 820) the
timestamp associated with the micro-inverter based on the received message and proceeds to 825 .
At 815, the process generates (at 815) a timestamp for the newly identified micro-inverter based
on when the message is received. Generating the timestamp may include creating a new record for the
micro-inverter which records the unique identifier of the micro-inverter, the timestamp of the first
received message from the micro-inverter, as well as other information (e.g., voltage amplitude, location,
etc.).
Once the timestamp for the micro-inverter has been updated or created, the process 800 identifies
(at 825) the phase of the micro-inverter based on the timestamp. The gateway will analyze the timestamp
of the message to identify the phase (i.e., the power line) to which the micro-inverter is connected. Using
the timestamp of a micro-inverter to determine the phase (or the power line) associated with the microinverter
will be further described below by reference to Figures 9a-c.
Once the phase of the micro-inverter has been identified, the process 800 determines (at 830)
whether it has received any additional messages from any other micro-inverter in the system. If so, the
process returns to 805 to receive and process the message. Otherwise, the process 800 ends.
A. Generating Timestamps from Identification Messages
In some embodiments, the gateway uses a timed identification message of a micro-inverter to
determine the phase, and therefore the power line, of the micro-inverter. In some embodiments, this
determination is based on when the message is received within a cycle of the AC power. This
determination is possible because, in some embodiments, the micro-inverters send identification messages
at a specified point during the AC power cycle (i.e., timed with a particular phase of the AC power cycle).
In some embodiments, the specified point in the AC power cycle is when the voltage in the power line
reaches peak amplitude. In some embodiments, the specified point in the AC power cycle is when the
voltage in the power line crosses zero.
Figures 9a-c illustrate the timing of messages that are sent from micro-inverters to the gateway at
specific points of the AC power cycle. Figures 9a-c shows a three-phase power system 900 (e.g., a grid)
that receives power from micro-inverter sets 921-923. The three-phase power system has three power
lines 931-933 at three different phases 1-3. The micro-inverter set 921 injects power into phase 1 power
line 931, the micro-inverter set 922 injects power into phase 2 power line 932, and the micro-inverter set
923 injects power into phase 3 power line 933. Each of Figures 9a-c also includes a voltage level graph
950 showing the superposition of the AC voltage levels of phase 1-3 power lines. The solid dash box 955
in the graph 950 corresponds to one cycle of the AC power in the three-phase power system 900, during
which the voltages in each of the three power lines 93 1-933 completes one sinusoidal AC cycle.
Figure 9a illustrates micro-inverters that generate messages 941 at positive peaks of voltage
amplitudes. Specifically, during the AC cycle 955, a micro-inverter in the set 921 generates a message
941 when the voltage in the phase 1 power line 931 reaches a positive peak at the time tl, a microinverter
in the set 922 generates a message 942 when the voltage in the phase 2 power line 932 reaches a
positive peak at the time t2, and a micro-inverter in the set 92 1 generates a message 943 when the voltage
in the phase 3 power line 933 reaches a positive peak at the time t3.
Since the phase of each power line is shifted from the phases of the other power lines by 120
degrees, the three peaks of the three phases are evenly spaced within each AC power cycle. In the United
States, AC power is cycled at 60 Hz or 60 cycles per second. Each cycle at 60Hz is approximately 16.7ms
long. The evenly spaced positive peaks at tl, t2 and t3 would therefore be measured approximately 5.6 ms
(or one third of the full cycle 16.7 ms) from each other. Upon receiving the messages 941-943 from the
micro-inverters 921-923, the gateway 910 will therefore be able to determine that the micro-inverter 921,
which sends the identification message 941 at the time tl, is attached to a power line that has a different
phase than the micro-inverter 922, which sends the identification message 942 at the time t2, and that the
micro-inverter 923, which sends the identification message 943 at the time t3, is attached to a power line
that has a different phase than the micro-inverters 921 and 922.
One of ordinary skill would recognize that the micro-inverters not only send messages during the
positive voltage peaks within the AC cycle 955, but also in AC cycles before and after the AC cycle 955.
Furthermore, the gateway 910 may or may not receive all of the identification messages transmitted.
However, given the periodicity of the AC power cycle, the gateway is still able to separate the microinverters
into the three different phases based on the timing of the identification messages, because
messages from micro-inverters attached to a power line of a particular phase always transmit
identification messages at the same relative timing position of any AC cycle.
Micro-inverters in different embodiments send messages at different points (i.e., phase) of the AC
cycle. Figure 9b illustrates identification messages that are sent at both positive and negative peaks
during the AC power cycle. Like in Figure 9a, messages are sent when voltages in power lines 93 1-933
reach positive peaks at tl, t2, and t3, respectively. In addition, a micro-inverter in the set 921 generates a
message when the voltage in the phase 1 power line 931 reaches a negative peak at time t2', a microinverter
in the set 922 generates a message when the voltage in the phase 2 power line 932 reaches a
negative peak at time t3', and a micro-inverter in the set 923 generates a message when the voltage in the
phase 3 power line 933 reaches a negative peak at time tl'.
Since the micro-inverters transmit identification messages on both positive and negative peaks,
messages from micro-inverters of a particular phase / power line reach the gateway on two different time
slots of each AC cycle. For example, during the AC cycle 955, messages from micro-inverters in the set
922 are generated for the gateway at both time slot t2 and time slot t3'. Furthermore, in each half cycle,
the gateway may receive messages from micro-inverters of all three power lines / phases messages from
micro-inverters attached to a power line of a particular phase are always transmitted at the same relative
timing position of any half cycle (e.g., in the half cycle 956, the messages from phases 1-3 are
generated/transmitted at times tl, t2, and tl', respectively; in the half cycle 957, the messages from the
phases 1-3 are generated/transmitted at times t2', t3', and t3). Given the periodicity of the AC power
cycle, the gateway is able to separate the micro-inverters into the three different phases based on the
timing of the identification messages within any half cycle.
In some embodiments, micro-inverters transmit identification messages at times in an AC cycle
other than at the voltage amplitude peaks. For example, micro-inverters in some embodiments transmit
identification messages at zero crossings. Figure 9c illustrates identification messages that are sent at
zero crossings during the AC power cycle. Specifically, during the AC cycle 955, a micro-inverter in the
set 921 generates messages when the voltage in the phase 1 power line 931 crosses zero at the times t6
and t9, a micro-inverter in the set 922 generate messages when the voltage in the phase 2 power line 932
crosses zero at the times t5 and t8, and a micro-inverter in the set 923 generate messages when the voltage
in the phase 3 power line 933 crosses zero at the times t4 and t7.
Like a micro-inverter that transmits a message once at the positive peak and once at the negative
peak of an AC cycle, a micro-inverter that transmits at zero crossings also transmits messages twice every
AC cycle (once at the positive to negative zero crossing and once at the negative to positive zero
crossing) the gateway is able to separate the micro -inverters into the three different phases based on the
timing of the identification messages within any half cycle (e.g., in the half cycle 956, the messages from
phases 1-3 are generated/transmitted at time t6, t5, t4, respectively; in the half cycle 957, the messages
from the phases 1-3 are generated/transmitted at times t9, t8, and t7, respectively).
When the micro-inverters send identification messages to the gateway that are timed with the
phase of the power line to which the micro-inverter is connected, the gateway can use the delay of the
received timestamp relative to the beginning of a reference AC power cycle of the grid to determine the
phase of the corresponding power line. In some embodiments, as long as all of the micro-inverters send
identification messages at the same specified point of a cycle (e.g., the peaks, the zero crossings, etc.,) the
gateway will be able to determine the phase of the micro-inverter. Determining the power line based on
the timed identification messages is described below with reference to Figure 10.
In some embodiments, rather than the micro-inverter sending a timed message, the micro-inverter
will send identification messages that are not sent at a particular point in the cycle of the voltage sensed
on the power line of the micro-inverter. Instead, the micro-inverter message includes the instantaneous
amplitude of the voltage at the moment in time when the message was generated. Because the frequency
and the amplitude of the voltage on the power lines from the grid are well-regulated, the phase of a
particular micro-inverter can be calculated based on the voltage amplitude at different points during a
particular cycle of the voltage on the power line. Calculating the phase based on measurements of voltage
amplitudes will be further described below with reference to Figure 11.
B. Gateway (GW) Phase Identification
In some embodiments, the power line/phase identification process is performed by the gateway,
which allows the gateway to automatically determine the phase of the power line for each micro-inverter
with which the gateway communicates. For some embodiments, Figure 10 illustrates an example of the
power line/phase identification process of some embodiments. Figure 10 shows a three-phase power
output system 1001 and a data table 1002. The system 1001 includes a gateway 1050, micro-inverters
1020-1045, power lines 1005, 1010, and 1015 and power grid 1004. The data table 1002 is a table in a
database stored in a storage of the gateway 1050 for some embodiments.
The micro-inverters 1020-1045 send identification messages 1060-1085 to gateway 1050. The
micro-inverters 1020-1045 also provide power to the grid 1004. Micro-inverters 1020 and 1025 provide
power to the grid 1004 across a first power line 1005 of the first phase in the three-phase system. Microinverters
1030 and 1035 provide power to the grid 1004 across a second power line 1010 for the second
phase. Micro-inverters 1040 and 1045 provide power to the grid 1004 across a third power line 1015 for
the third phase.
Each micro-inverter is physically connected to a power line of a particular phase and produces
power aligned with that particular phase. The micro-inverters 1020-1045 also send identification
messages 1060-1085 to the gateway 1050. In some embodiments, a micro-inverter sends an identification
message when the micro-inverter powers up. In some embodiments, micro-inverters send messages on
regular intervals, such as once daily. In some embodiments, the sent identification messages contain
additional information related to the micro-inverter, such as the power output and/or the physical location
of the micro-inverter sending the message. In this example, the identification messages are timed with the
phase of the power on the particular power line to which each micro-inverter is connected, as described
above with reference to Figures 9a-c.
In this example, the micro-inverters 1020 and 1025 send messages timed with the phase of the
power on the power line 1005, the micro-inverters 1030 and 1035 send messages timed with the phase of
the power on the power line 1010, and the micro-inverters 1040 and 1045 send messages timed with the
phase of the power on the power line 1015. The gateway 1050 receives the identification messages 1060-
1085 and maintains a timestamp for each micro-inverter. The timestamp is a record of the time at which a
message is received. This time may be measured against a standard 12-hour clock or may be measured
against an internal clock of the gateway 1050. As seen in the data table 1002, the gateway 1050 records a
timestamp for each micro-inverter based on the received time of the message.
The gateway 1050 analyzes the timestamp of each micro-inverter to determine an offset for each
timestamp relative to the beginning of the AC power cycle of the grid. Since the length of a cycle and the
occurrence of the peaks (or zero crossings) for each power line within that cycle do not change, regardless
of what time is selected as the beginning of the AC power cycle, the offset of any messages aligned with a
particular phase will be constant relative to the beginning of the particular AC power cycle during which
the message is received. In some embodiments, the gateway sets the beginning of the first AC power
cycle upon receiving the first identification message. In some of these embodiments, the gateway sets the
beginning of the first AC power cycle by making the beginning of the first AC power cycle to coincide
with the timestamp of the first received identification message. In the example of Figure 10, the gateway
sets the beginning of the first AC power cycle to the start of the first received identification message 1060
(4:35:36:300000).
Once the gateway 1050 has set the beginning of the first AC power cycle, the gateway 1050 can
calculate the beginning of a current AC power cycle (can be the first AC power cycle or a subsequent AC
power cycle) that encompasses the timestamp of a currently received identification message. The
beginning of the current cycle is the beginning of the cycle containing the timestamp (i.e., the cycle which
begins immediately before the timestamp). Because a cycle takes a fixed period of time to complete, the
beginning of the current cycle will occur at some multiple of the length of a single cycle (e.g., 16.7 ms for
60Hz AC power cycle) after the beginning of the first AC power cycle. For example, if the length of a
single cycle was 10ms, the beginning of a current cycle could occur at any multiple of 10ms (e.g., 20ms,
30ms, 100ms, etc.) after the beginning of the first AC power cycle. The offset, then, is the difference
between the timestamp and the beginning of the current cycle. In some embodiments, the offset may be
calculated as follows:
Offset = Timestamp mod cycle length
In this example, timestamp is the amount of time that has passed since the beginning of the first cycle.
Cycle length is the length of a single cycle in milliseconds. By performing a modulus operation on the
timestamp with the cycle-length, the offset gives the length of time between the receipt of the currently
identification message and the beginning of the current cycle.
Once the offset has been calculated, the phase can be identified because all messages which are
aligned with a particular phase should generate similar offsets. In this example, the offset for the first
identification message 1060 is 0 because the gateway set the beginning of the cycle at the first
identification message's timestamp. Identification message 1065 receives a timestamp of 4:35:36:301667.
The difference from the beginning of the first cycle is 16.7 ms, 1 cycle after the first identification
message 1060. 16.7 mod 16.7 gives an offset of 0, which matches the offset of message 1060 because
messages 1065 and 1060 are aligned with the phase of the same power line 1005.
Identification message 1070 has a timestamp 5.6 ms after the first identification message 1060, so
the offset from the current cycle is 5.6 ms, which is one third of a single cycle. In a three-phase system
running at 60 Hz and using full cycles, 5.6ms is the expected offset for a level which is shifted from a
reference signal by 120 degrees. Identification message 1075 has a timestamp 3166.7 ms (or 190 cycles)
after the timestamp of message 1070, so identification message 1075 also has an offset of 5.6 ms. This
matches the offset of message 1070 which is aligned with the phase of the same power line 1010. As seen
in this example, the gateway 1050 may receive identification messages from micro-inverters connected to
any power line at any time and in any order.
Similarly for identification messages 1080 and 1085 from micro-inverters 1040 and 1045
connected to phase 1015, both messages are calculated to have an offset of 11. 1 ms, therefore identifying
them as being connected to phase 1015. As described above, some embodiments may use half-cycles and
positive and negative peaks, but the results will be the same.
There may be small differences in the offsets due to differences between the expected and actual
receive times. These differences may arise for various reasons, such as transmission times over the
communication system. In such cases, in some embodiments, the phase for a received message may be
identified as the phase which is nearest to the offset of the received message. In other embodiments, a
response may be sent back to the micro-inverter requesting that the identification message be resent.
Once the offset has been calculated, the gateway can identify that micro-inverters with a
particular offset are connected with a particular power line. Once the corresponding line for each microinverter
has been identified, the gateway can monitor the aggregate power outputs of the micro-inverters
to the lines as described below in Section II.
Some embodiments use identification messages that report instantaneous voltage amplitudes of
the power on the power line for identifying the phase, and therefore the power line, of a micro-inverter.
Assuming the AC power on the power line has well-regulated peak amplitude and frequency, a gateway
receiving two or more identification messages from a particular micro-inverter will be able to determine
which of the three phases / power lines of the three-phase power system the particular micro-inverter is
connected with. With two or more identification messages at different points in the AC power cycle,
along with the well-regulated peak amplitude and frequency, the gateway can calculate the phase of the
power on the power line of the micro-inverter because there is only one phase that will fit the set of
information.
Figure 11 illustrates a three-phase power system 1100 in which micro-inverters 1121-1 125 report
instantaneous voltage amplitudes to a gateway 1110. The three-phase power system 1100 (e.g., a grid)
receives power from the micro-inverters 1121-1 125. The three-phase power system has three power lines
1131-1133 at three different phases 1-3. The micro-inverter 1121 injects power into phase 1 power line
1131, the micro-inverters 1122 and 1123 inject power into phase 2 power line 1132, and the microinverters
1124 and 1125 inject power into phase 3 power line 1133. The figure also includes a voltage
level graph 1150 showing the superposition of the AC voltage levels of phase 1-3 power lines.
Specifically, the phase 1 power line 113 1 is at voltage v6 at time t6 and at voltage v8 at time t8; the phase
2 power line 1132 is at voltage v2 at time t2, at voltage v4 at time t4, at voltage v5 at time t5, and at
voltage v9 at time t9; and the phase 3 power line 1133 is at voltage v l at time tl, at voltage v3 at time t3,
at voltage v7 at time t7, and at voltage vlO at time tlO.
The micro-inverters 1121-1125 send identification messages to the gateway 1110. Specifically,
the micro-inverter 1121 sends identification messages at time t6 and t8 to report voltage amplitudes v6
and v6; the micro-inverter 1122 sends identification messages at time t2 and t9 to report voltage
amplitudes v2 and v9; the micro-inverter 1123 sends identification messages at time t4 and t5 to report
voltage amplitudes v4 and v5; the micro-inverter 1124 sends identification messages at time t l and t3 to
report voltage amplitudes v l and v3; and the micro-inverter 1125 sends identification message at time t7
and tlO to report voltage amplitudes v7 and vlO.
As mentioned above, the gateway uses two identification messages from a particular microinverter
to determine the phase / power line that the particular micro-inverter belongs to. Hence, for
example, the gateway is able to determine that the micro-inverter 1122 belongs to phase 2 based on the
messages received at times t2 (reporting instantaneous voltage v2) and at time t9 (reporting instantaneous
voltage v9). Likewise, the gateway is able to determine that the micro-inverter 1125 belongs to phase 3
based on the messages received at times t7 (reporting instantaneous voltage v7) and at time tlO (reporting
instantaneous voltage vlO). With two identification messages at different points in the AC power cycle,
the gateway can calculate the phase of the power on the power line of the micro-inverter because there is
only one phase that will fit the set of information.
Although two alternative methods for determining the phase of a power line based on an
identification message received from a micro-inverter are described, one skilled in the art will realize that
many other methods exist to identify the phase of the power line to which a micro-inverter is connected,
based on an identification message, where the identification message is based on the phase of the power
line to which the micro-inverter is connected. By determining the phase, and therefore the power line, for
each micro-inverter, the gateway can monitor the outputs of the micro-inverters by calculating an
aggregate output of all of the micro-inverters to each power line.
II. MONITORING MICRO-INVERTER OUTPUTS
Figure 12 conceptually illustrates a process 1200 for analyzing the micro-inverter outputs and
adjusting the outputs to balance the power output to each power line of a multi-phase system. The process
1200 is performed by a gateway in some embodiments to monitor a set of micro-inverters injecting power
into a three-phase power system. The process 1200 analyzes and controls the outputs of individual microinverters
to balance the aggregate output (or voltage level) to each power line of a multi-phase system.
The process 1200 receives (at 1205) data from each of the micro-inverters. Based on the received
information, the process determines which phase / power line each micro-inverter injects power into
according to the phase identification described above in Section I .
Next, the process 1200 aggregates (at 1210) the power output of the micro-inverters of each
power line. Once the output power of the micro-inverters of each phase has been aggregated, the process
1200 determines (at 1215) whether the maximum difference between the different power lines (in terms
of the total power outputs to each power line) is within a threshold value. In some embodiments, the
maximum difference or imbalance of the output power is the difference in the output power between the
highest-producing power line and the lowest-producing power line. Some amount of imbalance between
the different power lines is to be expected, but the threshold value determines when that imbalance is
beyond an acceptable level. The threshold value may be different in some embodiments (e.g., a stateregulated
value, the expected output of a single micro-inverter, a custom user-identified value, etc.). If the
imbalance is within the threshold value, the process 1200 returns to 1205 to continue monitoring by
receiving more micro-inverter data. If the imbalance is beyond the threshold value, the process proceeds
to 1230.
At 1230, the process determines the necessary amount of adjustment. In some embodiments, the
process powers on or shuts down individual micro-inverters to control the output to a particular power
line. In some of these embodiments, the granularity at which the output is controlled is in increments of a
single micro-inverter's output. Alternatively or conjunctively, all of the micro-inverters continue
production, but the gateway adjusts the output of individual micro-inverters by raising or lowering an
output threshold (i.e., a limit on the amount of power to be output by a particular micro-inverter).
The process 1200 then identifies (at 1235) the micro-inverter(s) which need to be adjusted or
shutdown. When the process 1200 identifies the micro-inverters to be adjusted, the process may select
micro-inverters in such way as to maximize the expected life of the entire set of micro-inverters by
distributing the usage of the different micro-inverters evenly so that the expected life of each microinverter
is similar to the others. The process 1200 may identify micro-inverters which have been running
for the longest amount of time to be shut down. This may mean that new micro-inverters are rarely shut
down. In other cases, it may be based on recent usage so that micro-inverters which have gone the longest
without being shut down are shut down first. The process 1200 may also select a micro-inverter based on
other properties of the micro-inverter, such as location within the array or recent power output
performance.
Once the process determines the necessary adjustment to send and identifies the micro-inverters
to be controlled, the process 1200 sends (at 1240) the control signal to the identified micro-inverters to
reduce the output by the determined adjustment amount. In some embodiments, the control signal
commands the identified micro-inverter to either turn on or off. In some embodiments, the control signal
changes the power output threshold of the micro-inverter so the micro-inverter increase or decrease power
output by a specified amount.
As described above, these messages may be sent wirelessly or through wired connections with the
micro-inverters. After sending the control signal, the process returns to 1215.
Figure 13 illustrates an example of the analysis performed by the gateway to monitor the
aggregate output to each power line of a three-phase power system. Figure 13 shows a three-phase power
output system 1301 and a data table 1302. The three-phase system 1301 includes a gateway 1350, microinverters
1320-1345, power lines 1305, 1310, and 1315, and grid 1304. The gateway 1350 communicates
with the micro-inverters 1320-1345, which provide power to the grid 1304 along the power lines 1305,
1310, and 1315. The micro-inverters 1320-1345 also send power output messages 1360-1385 to the
gateway 1350. Based on the received power output messages 1360-1385, the gateway 1350 maintains and
updates the data table 1302 in a storage.
The power output messages 1360-1385 are sent by the micro-inverters 1320-1345 to the gateway
1350. The power output messages 1360-1385 provide a measurement of the power output of the microinverters.
In some embodiments, this measurement is the average power output of the micro-inverter over
a specified period of time (e.g., 4 minutes). Each micro-inverter in the system 1301 may send the power
output messages at various times (e.g., at regular intervals, after a change in the power output, at the
request of the gateway, etc.). In this example, the power output messages 1360-1375 show that the microinverters
1320-1335 are each producing 100 Watts. However, due to shading or some other circumstance,
the micro-inverters 1340-1345 are only producing 40 Watts each.
When the gateway 1350 receives power output messages 1360-1385 from the micro-inverters
1320-1345, the gateway aggregates the power output of the micro-inverters of each power line according
to the phase identification described with reference to Figure 8 . The gateway calculates the total output
for all of the micro-inverters for a particular power line and monitors the total output to each power line.
The results of the calculations are shown in the data table 1302. The data table 1302 keeps track of the
power output of individual micro-inverters as well as the aggregate power output to each of the power
lines / phases of the three-phase power system. In this example, the micro-inverters 1320 and 1325
connected to the phase 1 power line 1305 have a total output of 200 Watts. Micro-inverters 1330 and
1335 connected to the phase 2 power line 1310 also have a total output of 200 Watts. However, microinverters
1340 and 1345 connected to the phase 3 power line 1315 only have a total output of 80 Watts.
The output to power line 1315 is 120 Watts less than the output to power lines 1305 and 1310.
In some embodiments, the micro-inverters do not send power output messages. The gateway in
some of these embodiments determines the aggregate output power to each power line in other ways. For
example, in some embodiments, sensors on each power line of the multi-phase system may detect the
aggregate output of the micro-inverters on the power line.
After determining the aggregate output, some embodiments control the output to each power line.
For the example system 1301, in order to bring the three-phase system back to being symmetrical, the
micro-inverter 1350 upon analyzing the table 1302 send control messages to adjust the output of
individual micro-inverters (e.g., by turning off some micro-inverters for phase 1 and phase 2, by turning
on some micro-inverters for phase 3, or by decrease power output of some micro-inverters for phase 1 and
phase 2, etc.).
Alternatively or conjunctively, some embodiments seek to balance other aspects of the power
output to each line. For example, rather than the actual power output to each power line, the gateway
1350 is used to balance the voltage levels of the three phases. In some embodiments, the power output
messages 1360-1385 may describe the voltage detected on each power line. The voltage on each power
line may vary when the loads or internal resistances of each power line are different. Rather than
balancing the aggregate output to each power line, the gateway 1350 may instead control the output of
each micro-inverter to balance the voltage levels on each power line. For the sake of simplicity, the
process will be described with reference to power outputs, but it should be clear to one skilled in the art
that other embodiments may be used to control other aspects of the power on the power line, such as the
voltage.
Figures 14 and 15 illustrate different methods for controlling the aggregated output to each
power line of a three-phase system for some embodiments. Figure 14 illustrates a method for controlling
the aggregate output to each power line by shutting down individual micro-inverters. Shutting down the
individual micro-inverters may allow for simpler circuit designs or for longer life expectancies for the
individual micro-inverters. In addition, micro-inverters run more efficiently when running at full power,
so it may be beneficial to shut down some micro-inverters and allow the remaining micro-inverters to
perform at maximum efficiency. Figure 14 illustrates a three-phase power system 1401 that is similar to
the three-phase power system 1301 of Figure 13. The system 1401 includes a gateway 1450, microinverters
1420-1445, power lines 1405, 1410, and 1415 and a grid 1404. The gateway 1450
communicates with the micro-inverters 1420-1445 with control messages 1460 and 1465. The microinverters
1420 and 1425 supplies power to the phase 1 power line 1405, the micro-inverters 1430 and
1435 supplies power to the phase 2 power line 1410, and the micro-inverters 1440 and 1445 supplies
power to the phase 3 power line 1415.
The gateway 1450 sends control messages 1460 and 1465 to individual micro-inverters which
have been selected to be temporarily shut down. In the example illustrated, the gateway 1450 is
transmitting control messages 1460 and 1465 to the micro-inverters 1420 and 1430, respectively. The
gateway 1450 may select micro-inverters in such way as to maximize the expected life of the entire set of
micro-inverters by distributing the usage of the different micro-inverters evenly so that the expected life
of each micro-inverter is similar to the others. Alternatively or conjunctively, the gateway 1450 may
identify micro-inverters which have been running for the longest amount of time to be shut down. This
may mean that new micro-inverters are rarely shut down. In other cases, it may be based on recent usage
so that micro-inverters which have gone the longest without being shut down are shut down first. The
gateway 1450 may also select a micro-inverter based on other properties of the micro-inverter, such as its
location within the array or recent power output performance. The control messages 1460 and 1465 then
shut down the micro-inverters 1420 and 1430.
Figure 14 also illustrates an updated data table 1402 (updated from the data table 1302 of Figure
13), which includes a micro-inverter data table and a phases data table. The data table 1402 shows the
result of the gateway's calculation of the aggregate outputs to each power line after the control messages
1460 and 1465 have been sent to shut-down micro-inverters 1420 and 1430. The power outputs to phase 1
and phase 2 are now reduced to 100 Watts each (from 200 Watts) and the power output to phase 3
remains at 80 Watts, leaving an imbalance of 20 Watts. Although the outputs to the different power lines
are still not perfectly balanced, the imbalance between the power lines has been reduced to no more than
the expected output of a single micro-inverter (i.e., 100 Watts).
Figure 15 illustrates a method for controlling the aggregate output to each power line by
managing a threshold output level for a set of micro-inverters for some embodiments. Managing the
threshold output level by setting a limit on the maximum output for a micro-inverter may allow the
gateway to be more precise in managing the output of a set of micro-inverters. Figure 15 illustrates a
three-phase power system 1501 that is similar to the power system 1401 of Figure 14. The three-phase
power output system 1501 includes a gateway 1550, micro-inverters 1520-1545, power lines 1505, 1510,
and 1515, and grid 1504. The gateway 1550 communicates with the micro-inverters 1520-1545 with
threshold control messages 1560-1575. The micro-inverters 1520 and 1525 supply power to the phase 1
power line 1505, the micro-inverters 1550 and 1535 supply power to the phase 2 power line 15 10, and the
micro-inverters 1540 and 1545 supply power to the phase 3 power line 1515.
The gateway 1550 sends control messages to micro-inverters. However, rather than the shutdown
control messages of Figure 14, the gateway 1550 in Figure 15 sends threshold control messages 1560-
1575. Threshold control messages 1560-1575 set threshold output levels for micro-inverters that receive
the messages. As illustrated, the threshold control message 1560 sets the threshold output level of microinverter
1520 to 40W, the threshold control message 1565 sets the threshold output level of micro-inverter
1525 to 40W, the threshold control message 1570 sets the threshold output level of micro-inverter 1530 to
40W, and the threshold control message 1575 sets the threshold output level of micro-inverter 1535 to
40W. The threshold output level, in some embodiments, causes the micro-inverter to cap its output at a
particular level. The micro-inverters scales back the amount of power that is converted and output.
Figure 15 also illustrates an updated data table 1502 (updated from the data table 1302 of Figure
13), which includes a micro-inverter data table and a phases data table. The data table 1502 shows the
result of the gateway's calculation of the aggregate outputs to each power line after the threshold control
messages 1560-1575 have been sent to adjust the power output of micro-inverters 1520-1535. The power
outputs to phase 1 and phase 2 are now reduced to 80 Watts each (from 200 Watts) and the power output
to phase 3 remains at 80 Watts. The three phases of the power system 1501 are therefore balanced /
symmetrical.
In some embodiments, all of the micro-inverters of a particular phase are sent the same threshold
control message, reducing the output of each micro-inverter by a similar amount. Alternatively or
conjunctively, the gateway 1550 may select micro-inverters to be scaled back based on criteria similar to
the shutdown control messages of Figure 14. For instance, the output thresholds of a subset of the set of
micro-inverters connected to a particular power line may be selected to be scaled back because they are
older or have been running at full power for a longer period of time. The method for scaling back a microinverter's
output is described further below with reference to Figure 17.
III. CONTROLLING INDIVIDUAL MICRO-INVERTER OUTPUTS
Figure 16 conceptually illustrates a process 1600 performed by each micro-inverter in a system
to identify itself and to communicate with a gateway. The process 1600 is performed continuously by
each of the micro-inverters as they interact with the gateway in a system similar to the system 600 of
Figure 6 in some embodiments.
The process 1600 determines (at 1605) whether it is necessary to refresh the phase information at
the gateway used for phase identification as described in process 800 of Figure 8 . In some embodiments,
such phase information is maintained in a data table that is similar to the data table 1002 of Figure 10.
Such a data table keeps track of the individual micro-inverters, ensuring that the phase for each microinverter
is properly identified. In some embodiments, the information at the gateway may need to be
refreshed after a particular period of time (e.g., once daily). In other embodiments, the phase
identification may only be performed at the initial installation of a micro-inverter or upon receiving a
request from the gateway.
If the phase information does not need to be updated, the process proceeds directly to 1620. If the
process 1600 needs to update the phase information, the process detects (at 1610) the phase of the voltage
sensed on the power line of the micro-inverter as described with reference to Figures 9a-c. The process
1600 then sends (at 1615) a message based on the detected phase (e.g., a timed message sent at a
particular point in the detected phase, a message with the instant sensed voltage amplitude at a particular
point in the phase, etc.) to the gateway which then updates the timestamp and phase information as
described at 820 of Figure 8 before continuing to 1620.
At 1620, the process 1600 determines whether it is necessary to refresh the output information for
the gateway. The output information is a description of the amount of power being generated by the
particular micro-inverter. In some embodiments, such output information is maintained in a data table that
is similar to the data table 1302 of Figure 13 (and the updated data tables 1402 and 1502). Such a data
table keeps track of the power output of individual micro -inverters as well as the aggregate power output
to each of the power lines /phases of the three-phase power system. The process 1600 in some
embodiments determines that it is necessary to refresh the output information at regular intervals (e.g.,
once every four minutes) or when a request for updated information is received from the gateway. The
output information may include the voltage, current, total power output, and other information about the
power produced by the micro-inverter. If the process determines that the output information needs to be
refreshed, it sends (at 1625) the output information to the gateway before proceeding to 1630. Otherwise,
the process directly proceeds to 1630.
The process 1600 determines (at 1630) whether a control signal has been received from the
gateway. If the process has not received a control signal from the gateway, it returns to 1605 to begin the
process again. If the process 1600 has received a control signal from the gateway, it adjusts (at 1635) the
power output of the micro-inverter accordingly. As described above, adjusting the power output of the
micro-inverter may include shutting down a micro-inverter or adjusting the power output threshold of the
micro-inverter. The process then returns to 1605 to begin the process 1600 again.
Figure 17 illustrates a simplified circuit layout of a micro-inverter 1710 of some embodiments.
The micro-inverter 1710 is one of several micro-inverters in communication with a gateway 1715 in order
to control the output of the several micro-inverters individually to balance the aggregate outputs to each
power line of a multi-phase system. The micro-inverter 1710 receives power from a DC source 1705 (e.g.,
solar panels) and converts the received DC power into AC power for an AC load 1760. In some
embodiments, the AC load 1760 is in a three-phase power system and the micro-inverter 1710 injects the
converted AC power into a power line of a particular phase of the three-phase power system. In some
embodiments, the three-phase power system is part of a power grid.
As illustrated, the micro-inverter circuit 1710 includes a DC-DC step-up power converter 1720, a
DC link 1725, a DC-AC power converter 1730, and a controller 1735. The DC-AC converter 1730
includes a driver circuit 1740, a current shifting stage 1745, and a current unfold stage 1750. The
controller 1735, in some embodiments, communicates with the gateway 1715, sending the power output
and identification messages to the gateway 1715 and receiving control messages back from the gateway
1715. The controller 1735 in turn controls the output of the micro-inverter 1710 to the AC power load
1760 based on the received control messages. The controller also controls the DC-AC power converter
1730.
The micro-inverter circuit 1710 converts the DC power into AC power to be output for the AC
load 1760. In some embodiments, the DC-DC step-up power converter 1720 uses a transformer (not
shown) to step up the voltage of the input to match the voltage of the power grid 1760. The DC power is
then fed into the DC link 1725, which in some embodiments includes a capacitor for storing the generated
power.
The DC-AC power converter 1730 converts the stored energy in the DC link 1725 into AC power
for the AC grid 1760. When the micro-inverter circuit 1710 receives threshold control messages from the
gateway 1715, the controller 1735 signals the driver 1740 of the current shifting stage 1745 to step down
and shift the current without changing the voltage, reducing the overall power output of the micro-inverter
circuit 1710. In some embodiments, the current shifting stage 1745 uses a buck converter to step down the
current by signaling the driver 1740 to reduce the current being output by the micro-inverter circuit 1710.
The current shifting stage shifts the current into rectified AC waveforms. Finally, The current unfold
stage 1750 takes the stepped down rectified AC power and unfolds it into AC power to be output to the
AC load 1760. Examples of a multi-stage micro-inverter can be found in U.S. Patent Application
Publication No. 2011/0205766.
Although a particular embodiment of a micro-inverter is described, one skilled in the art will
realize that the methods and systems described may apply to many different types of micro-inverters or
other power producing devices. Furthermore, although many of the examples described in this document
are three-phase power systems, one of ordinary skill would realize that the methods and systems
described in this document are applicable to multi-phase power systems that includes four or more phases.
IV. ELECTRONIC SYSTEM
Many of the above-described features and applications are implemented as software processes
that are specified as a set of instructions recorded on a computer readable storage medium (also referred
to as computer readable medium). When these instructions are executed by one or more processing unit(s)
(e.g., one or more processors, cores of processors, or other processing units), they cause the processing
unit(s) to perform the actions indicated in the instructions. Examples of computer readable media include,
but are not limited to, CD-ROMs, flash drives, RAM chips, hard drives, EPROMs, etc. The computer
readable media does not include carrier waves and electronic signals passing wirelessly or over wired
connections.
In this specification, the term "software" is meant to include firmware residing in read-only
memory or applications stored in magnetic storage, which can be read into memory for processing by a
processor. Also, in some embodiments, multiple software inventions can be implemented as sub-parts of a
larger program while remaining distinct software inventions. In some embodiments, multiple software
inventions can also be implemented as separate programs. Finally, any combination of separate programs
that together implement a software invention described here is within the scope of the invention. In some
embodiments, the software programs, when installed to operate on one or more electronic systems, define
one or more specific machine implementations that execute and perform the operations of the software
programs.
Figure 18 conceptually illustrates an electronic system 1800 with which some embodiments of
the invention are implemented. The electronic system 1800 can be used to execute any of the control,
virtualization, or operating system applications described above. The electronic system 1800 may be a
computer (e.g., a desktop computer, personal computer, tablet computer, server computer, mainframe, a
blade computer etc.), phone, PDA, or any other sort of electronic device. Such an electronic system
includes various types of computer readable media and interfaces for various other types of computer
readable media. Electronic system 1800 includes a bus 1805, processing unit(s) 1810, a system memory
1825, a read-only memory 1830, a permanent storage device 1835, input devices 1840, and output
devices 1845.
The bus 1805 collectively represents all system, peripheral, and chipset buses that
communicatively connect the numerous internal devices of the electronic system 1800. For instance, the
bus 1805 communicatively connects the processing unit(s) 1810 with the read-only memory 1830, the
system memory 1825, and the permanent storage device 1835.
From these various memory units, the processing unit(s) 1810 retrieve instructions to execute and
data to process in order to execute the processes of the invention. The processing unit(s) may be a single
processor or a multi-core processor in different embodiments.
The read-only -memory (ROM) 1830 stores static data and instructions that are needed by the
processing unit(s) 1810 and other modules of the electronic system. The permanent storage device 1835,
on the other hand, is a read-and-write memory device. This device is a non-volatile memory unit that
stores instructions and data even when the electronic system 1800 is off. Some embodiments of the
invention use a mass-storage device (such as a magnetic or optical disk and its corresponding disk drive)
as the permanent storage device 1835.
Other embodiments use a removable storage device (such as a floppy disk, flash drive, etc.) as the
permanent storage device. Like the permanent storage device 1835, the system memory 1825 is a readand-
write memory device. However, unlike storage device 1835, the system memory is a volatile readand-
write memory, such a random access memory. The system memory stores some of the instructions
and data that the processor needs at runtime. In some embodiments, the invention's processes are stored
in the system memory 1825, the permanent storage device 1835, and/or the read-only memory 1830.
From these various memory units, the processing unit(s) 1810 retrieve instructions to execute and data to
process in order to execute the processes of some embodiments.
The bus 1805 also connects to the input and output devices 1840 and 1845. The input devices
enable the user to communicate information and select commands to the electronic system. The input
devices 1840 include alphanumeric keyboards and pointing devices (also called "cursor control devices").
The output devices 1845 display images generated by the electronic system. The output devices include
printers and display devices, such as cathode ray tubes (CRT) or liquid crystal displays (LCD). Some
embodiments include devices such as a touchscreen that function as both input and output devices.
As shown in Figure 18, bus 1805 also couples electronic system 1800 to a network 1865 through
a network adapter (not shown). In this manner, the computer can be a part of a network of computers
(such as a local area network ("LAN"), a wide area network ("WAN"), or an Intranet, or a network of
networks, such as the Internet. Any or all components of electronic system 1800 may be used in
conjunction with the invention.
Some embodiments include electronic components, such as microprocessors, storage and memory
that store computer program instructions in a machine-readable or computer-readable medium
(alternatively referred to as computer-readable storage media, machine-readable media, or machinereadable
storage media). Some examples of such computer-readable media include RAM, ROM, read
only compact discs (CD-ROM), recordable compact discs (CD-R), rewritable compact discs (CD-RW),
read-only digital versatile discs (e.g., DVD-ROM, dual-layer DVD-ROM), a variety of
recordable/rewritable DVDs (e.g., DVD-RAM, DVD-RW, DVD+RW, etc.), flash memory (e.g., SD
cards, mini-SD cards, micro-SD cards, etc.), magnetic and/or solid state hard drives, read-only and
recordable Blu-Ray® discs, ultra density optical discs, any other optical or magnetic media, and floppy
disks. The computer-readable media may store a computer program that is executable by at least one
processing unit and includes sets of instructions for performing various operations. Examples of computer
programs or computer code include machine code, such as is produced by a compiler, and files including
higher-level code that are executed by a computer, an electronic component, or a microprocessor using an
interpreter.
While the above discussion primarily refers to microprocessor or multi-core processors that
execute software, some embodiments are performed by one or more integrated circuits, such as
application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some
embodiments, such integrated circuits execute instructions that are stored on the circuit itself.
As used in this specification, the terms "computer", "server", "processor", and "memory" all refer
to electronic or other technological devices. These terms exclude people or groups of people. For the
purposes of the specification, the terms display or displaying means displaying on an electronic device.
As used in this specification, the terms "computer readable medium," "computer readable media," and
"machine readable medium" are entirely restricted to tangible, physical objects that store information in a
form that is readable by a computer. These terms exclude any wireless signals, wired download signals,
and any other ephemeral signals.
While the invention has been described with reference to numerous specific details, one of
ordinary skill in the art will recognize that the invention can be embodied in other specific forms without
departing from the spirit of the invention. For example, while the examples shown illustrate splitting one
or more shapes of a design layout region into two exposures, one of ordinary skill in the art would
recognize that some embodiments would use similar processes to split shapes of a design layout region
into more than two (e.g., three, four, etc.) exposures. One of ordinary skill in the art will also recognize
that in some instances above, when referring to assigning shapes or portions of shapes to multiple
exposures, the shapes (or portions thereof) are actually assigned to multiple mask layouts that are used to
create multiple masks that enable a design layout layer to be printed in multiple exposures. Similarly, one
of ordinary skill would recognize that while many instances above refer to "drawing" a graph, some
embodiments do not actually draw the visible graph, but instead define the graph as a data structure.
In addition, a number of the figures (including Figures 7, 8, 12, and 16) conceptually illustrate
processes. The specific operations of these processes may not be performed in the exact order shown and
described. The specific operations may not be performed in one continuous series of operations, and
different specific operations may be performed in different embodiments. Furthermore, the process could
be implemented using several sub-processes, or as part of a larger macro process. Thus, one of ordinary
skill in the art would understand that the invention is not to be limited by the foregoing illustrative details.
CLAIMS
1. A method for maintaining balanced energy transmissions between different phases of a
multi-phase power system, the method comprising:
for each phase of the multi-phase system, identifying a set of micro-inverters associated
with the phase;
calculating an aggregate power output for each phase of the multi-phase system; and
controlling an individual power output of at least one micro-inverter of at least one set of
micro-inverters in order to reduce the imbalance between the power outputs to each phase of the multi
phase power system.
2 . The method of claim 1, wherein identifying a set of micro-inverters comprises:
receiving a message from each micro-inverter;
recording a received time of each message; and
using received time of each message to determine the phase for each micro-inverter.
3. The method of claim 1 or 2, wherein controlling the individual power output of the at
least one micro-inverter comprises sending a control signal to the micro-inverter over a communication
system.
4 . The method of claim 3, wherein the communication system is a wireless communication
system.
5. The method of claim 1, 2, 3 or 4 wherein controlling the individual power output
comprises setting a same threshold power output level for the at least one micro-inverter as for all of the
micro-inverters in the same set of micro-inverters.
6. The method of claim 1, 2, 3 or 4 wherein controlling the individual power output of the at
least one micro-inverter comprises setting a threshold output level for the micro-inverter that is different
from the threshold output level of a micro-inverter in a different set of micro-inverters of another phase.
7. The method of any preceding claim, wherein controlling the individual power output
comprises preventing the at least one micro-inverter from injecting power into the multi-phase power
system.
8. A multi-phase power system comprising:
a plurality of sets of micro-inverters, each set of micro-inverters for injecting power into
a power line of a particular phase of a multi-phase AC grid mains;
a gateway for (i) receiving a set of information comprising the individual power output
from each micro-inverter of the plurality of sets of of micro-inverters, (ii) calculating an aggregate power
output of each set of micro-inverters, and (iii) balancing the output to different phases of the multi-phase
AC grid mains, the balancing comprising controlling the output of at least one micro-inverter of at least
one set of micro-inverters.
9. The system of claim 8, further comprising at least one sensor for measuring the aggregate
power output to the power line of the particular phase of the multi-phase AC grid mains.
10. The system of claim 8 or 9, wherein controlling the output of at least one micro-inverter
comprises sending control signals to the at least one micro-inverter based on the set of information.
11. The system of claim 10, wherein the control signals comprise instructions to change the
output of the at least one micro-inverter.
12. The system of claim 10 or 11, wherein the control signals comprise instructions to
prevent a micro-inverter from injecting power into the multi-phase power system.
13. The system of claim 10, 11, or 12, wherein each micro-inverter comprises (i) a wireless
transmitter for transmitting the sets of information and (ii) a wireless receiver for receiving the control
signals.
14. The system of any one of claims 8 to 13, wherein balancing the output to the phases of
the multi-phase power further comprises ensuring that the output to each phase of the multi-phase power
grid is within a threshold value of the outputs to the other phases.
15. The system of any one of claims 8 to 14, wherein the set of information comprises a
value of at least one of voltage output, current output, and power output.
16. The system of claim 15, wherein the value is an average output over a period of time.
17. A non-transitory machine readable medium comprising a program to be executed on at
least one processing unit, the program comprising sets of instructions for:
identifying, for each power line of a multi-phase AC grid mains, a set of micro-inverters
that provides power to the power line;
calculating an aggregate power output for each set of micro-inverters; and
balancing the aggregate power output of each set of micro-inverters, the balancing
comprising controlling the output of at least one micro-inverter of at least one set of micro-inverters.
18. The non-transitory machine readable medium of claim 17, wherein the program further
comprises sets of instructions for receiving data from each set of micro-inverters.
19. The non-transitory machine readable medium of claim 17 or 18, wherein calculating an
aggregate power output comprises receiving data from sensors from each phase of the AC grid mains.
20. The non-transitory machine readable medium of claim 17, 18 or 19 wherein controlling
the output of at least one micro-inverter comprises updating the output threshold for all of the microinverters
providing power to a particular phase of the multi-phase system.
21. A method for controlling power being injected into different power lines of a multi-phase
power system, the method comprising:
receiving messages from a plurality of micro-inverters, wherein each message is
generated by a micro-inverter when a power line connecting the micro-inverter is at a particular phase of
an AC cycle;
identifying a first power line as a power line that is connected to a first micro-inverter
transmitting the first message;
calculating a timing differential between the first message received from the first microinverter
and a second message received from the second micro-inverter; and
identifying a second power line as a power line connected to a second micro-inverter
based on the calculated timing differential.
22. The method of claim 21, wherein each micro-inverter generates a message when a power
line connecting the micro-inverter is at a voltage peak of an AC cycle.
23. The method of claim 2 1 or 22, wherein each micro-inverter generates two messages in
each AC cycle.