POWER CONVERSION SYSTEM AND METHOD PROVIDING MAXIMUM
EFFICIENCY OF POWER CONVERSION FOR A PHOTOVOLTAIC
SYSTEM, AND PHOTOVOLTAIC SYSTEM EMPLOYING A
PHOTOVOLTAIC ARRAY AND AN ENERGY STORAGE DEVICE
BACKGROUND
Field
The disclosed concept pertains generally to converting energ from
photovoltaic (PV) arrays and, more particularly, to power conversion systems for a
PV system. The invention further pertains to methods of power conversion for a PV
system. The invention also pertains to PV systems.
Background Information
Photovoltaic (PV) arrays are typically configured in a series parallel
arrangement of a plurality of PV modules. The conventional practice is to ensure that
the generated direct current (DC) voltage of a string of PV modules, under worst case
conditions, does not exceed the insulation ratings of the PV modules. For example,
the National Electric Code (NEC) requires this voltage to be under 600 VDC.
An example PV array / inverter system 2 is shown in Figure 1. Each of
two example PV arrays 4,6 consists of a number of strings 8 of PV modules 10
electrically connected in parallel before the corresponding PV array is electrically
connected to an inverter 12.
The power output of a PV module for a level of solar radiation
depends, for example, on the temperature of the PV module, the conditio of the PV
module surface, the age of the PV module, and the technology of the PV module.
However, the general characteristics of the PV arras DC voltage (volts) PV away
DC current (amperes) 16 and PV array DC power output (watts) 18 with respect to
100% solar radiation (i.e., "insolation") 20 are shown in Figure 2. A plot 22 of the
array output current versus the array output voltage is also shown.
Referring again to Figure 1, the DC power from the PV arrays 4.6 is
converted into alternating current (AC) power by a power converter (e.g.. the inverter
12) before, for example, being injected into a utility grid 24. The power comverter 12
preferably ensures that the DC power from the PV arrays 4.6 is maximized The
maximization of energy from the PV arrays 4,6 is done by continuously changing the

operating point based on the Sun's radiation and the temperature of the PV modules
10. However, there are various efficiencies of conversion from DC power to AC
power. For example, the power conversion can be made with more than one power
converter (not shown). When more than one power converter is used, the efficiency is
dependent on the operating point of the various power converters. In addition, a sub-
array connected to each power converter could have a different operating sub-array
voltage for maximum power.
In this power conversion process, there is an additional inefficiency in
the transformer 26 between the inverter 12 and the utility grid 24. Example efficiency
curves 28,30 of the inverter 12 and the transformer 26 have convex characteristics as
shown in Figure 3. The inverter efficiency depends mainly on the input DC voltage.
inverter switching frequency and operating current. The efficiency of the transformer
26 is dependent on the design and operating point.
There is room for improvement in power conversion systems for a PV
system.
There is also room for improvement in methods of power conversion
for a PV system.
There is further room for improvement in PV systems.
SUMMARY
Since the cost of solar modules and their installation is relatively high.
extracting maximum energy from a photovoltaic (PV) array, an inverter and a
transformer is very important for the economic feasibility of solar PV power
conversion.
During the day. as the sunlight conditions and the tempera are of PV
modules change, operating a PV array, an inverter and a transformer at their
respective maximum efficiencies is important. The transformer efficiency is fixed for
a design and the efficiency characteristics of the transformer are chosen to have the
highest efficiency at near 100% when the inverter is not at its peak efficient .
Two or more inverters can be operated to maximize PV array outputs
individually. Based on the output AC power, the number of inverters and the numer
of independent PV arrays can be advantageously selected.

When energy storage is available, the system can be optimized for
economics in addition to energy conversion efficiency.
These needs and others are met by embodiments of the disclosed
concept, which maximize energy injected into a utility grid and/or a load by operating.
a PV system (e.g., without limitation, a PV array, a corresponding energ} converter
and a single transformer) at the maximum efficiency of conversion in addition to
maximizing the output from the PV array.
In accordance with one aspect of the disclosed concept, a method of
power tracking is for a photovoltaic system including a number of photovoltaic
arrays, a number of inverters, and a transformer. The method comprises: operating
the photovoltaic system including the number of photovoltaic arrays, the number of
inverters and the transformer to provide maximum efficiency of power conversion
therefrom; and maximizing power output from the number of photovoltaic arrays.
The method may further comprise employing as the number of
photovoltaic arrays a plurality of photovoltaic arrays, and paralleling the outputs of
the plurality of photovoltaic arrays.
The method may further comprise employing as the number ol
photovoltaic arrays a plurality of photovoltaic arrays; employing as the number ol'
inverters a plurality of inverters; for each of the plurality of photovoltaic arrays,
powering a corresponding one of the plurality of inverters from a corresponding one
of the plurality of photovoltaic arrays; and operating each of the plurality of
photovoltaic arrays at a corresponding independent maximum power point during a
time of about peak or peak energy production.
The method may further comprise determining energy and power at a
utility grid, determining the loading on the number of inverters and the transformer.
and operating the system at optimal stress levels.
The method may further comprise employing two photovoltaic arrays
as the number of photovoltaic arrays, employing two inverters as the number of
inverters, initially paralleling the outputs of the two photovoltaic arrays and powering
one of the two inverters from the paralleled outputs, determining when power output
from the one of the two inverters exceeds a predetermined power percentage and
responsively powering the one of the two inverters from one of the two photovoltaic

arrays and powering the other one of the two inverters from the other one of the two
photovoltaic arrays, operating the one of the two inverters at the maximum power
point of the one of the two photovoltaic arrays, and operating the other one of the two
inverters at the maximum power point of the other one of the two photovoltaic arrays.
The method may further comprise determining when power output
from both of the two inverters falls below a predetermined power percentage: and
determining when the one of the two inverters has operated for longer than a
predetermined time during a predetermined time interval, and responsively disabling
the one of the two inverters, paralleling the outputs of the two photovoltaic arrays and
powering the other one of the two inverters from the paralleled outputs.
As another aspect of the disclosed concept, a photovoltaic system
comprises: an energy storage device; a photovoltaic array; a converter including a first
input/output structured to input power from or output power to the energy storage
device, and a second input/output structured to input power or output power: an
inverter including an input structured to input power from the photovoltaic array and
an output structured to output power; and a transformer having a primary electrically
connected to the output of the inverter and to the second input/output of the converter.
and a secondary electrically connected to at least one of a local load and a .
The first input/output of the converter may input power from the
energy storage device, the second input/output of the converter may output power,
and the local load may be powered through the transformer by both of the converter
and the inverter.
The converter may be structured to operate as a voltage source and set
the frequency and the voltage of the local load using available power from the energy
storage device.
The first input/output of the converter may output power to the energy
storage device, the second input/output of the converter may input power, and the
local load may be powered through the transformer by the inverter.
The converter may be an active rectifier structured to charge the
energy storage device.
The second input/output of the active rectifier may be powered through
the transformer from the utility grid.

The first input/output of the converter may output power to the energy
storage device, and the second input/output of the converter may input power from the
output of the inverter.
Neither one of the local load or the utility grid may be powered, and
the second input/output of the converter may input power directly from the output of
the inverter.
The secondary of the transformer may only be electrically connected to
the local load, the first input/output of the converter may input power from the energy
storage device, the second input/output of the converter may output power, and the
local load may be powered through the transformer by both of the converter and the
inverter.
The converter may be structured to operate as a voltage source and set
the frequency and the voltage of the local load, and the inverter may be structured to
track the frequency and the voltage of the local load.
As another aspect of the disclosed concept, a power conversion system
comprises: a number of photovoltaic arrays; a number of inverters: a transformer: and
a processor structured to control the number of inverters and operate the power
conversion system to provide maximum efficiency of power conversion by the
number of photovoltaic arrays, the number of inverters and the transformer, and to
maximize power output from the number of photovoltaic arrays.
The number of photovoltaic arrays may be two photovoltaic arrays
each having an output, the number of inverters may be two inverters, and the
processor may be structured to selectively cause: (a) a corresponding one of the
inverters to be powered from a corresponding one of the two photovoltaic arrays of
(b) the outputs of the two photovoltaic arrays to be electrically connected in parallel
and one of the two inverters to be powered from the paralleled outputs.
BRIEF DESCRIPTION OF THE DRAWINGS
A full understanding of the disclosed concept can be gained from the
following description of the preferred embodiments when read in conjunction with
accompanying drawings in which:
Figure 1 is a block diagram of a photovoltaic (PV) array/ inverter
system.

Figure 2 is a plot of PV array voltage, current and power output with
respect to 100% solar radiation (i.e., "insolation").
Figure 3 is a plot of efficiency versus percent load for a transformer
and an inverter.
Figure 4 is a block diagram of a power conversion system in
accordance with embodiments of the disclosed concept.
Figure 5 is a plot of solar radiation, time of the day and inverter system
capacity showing a seasonal variation of solar energy capacity utilization for the
power conversion system of Figure 4.
Figure 6 is a block diagram in schematic form of the power conversion
system of Figure 4 including system controls.
Figure 7 is a plot of efficiency versus percent load for a transformer, an
inverter and the power conversion system of Figure 4.
Figure 8 is a flowchart of a routine executed by the system controller
of Figure 4.
Figure 9 is a plot of PV array output power and PV array output
voltage for two different levels of solar radiation for the power conversion system of
Figure 4.
Figure 10 is a plot of cycles to failure of inverters versus heatsink
temperature change for the power conversion system of Figure 4.
Figures 11A-1 IE are block diagrams of other power conversion
systems including an energy storage device showing different operations to improve
energy output of a PV array in accordance with other embodiments of the disclosed
concept.
Figure 12 is a flowchart of a routine executed by the system controller
of Figure 11 A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As employed herein, the term "number" shall mean one or an integer
greater than one (i.e., a plurality).
As employed herein, the term "processor" means a programmable
analog and/or digital device that can store, retrieve, and process data; a computer a
workstation; a personal computer; a microprocessor; a microcontroller a

microcomputer; a central processing unit; a mainframe computer; a mini-computer a
server; a networked processor; a controller; a system controller; a programmable logic
controller; or any suitable processing device or apparatus.
As employed herein, the term "inverter" means an apparatus or device
that converts electrical energy from a direct current form to an alternating current
form.
As employed herein, the term "converter" means an apparatus or
device that converts electrical energy in a first direction from a direct current form to
an alternating current form (e.g., without limitation, functioning as an inverter
powered from a direct current energy storage device), and/or that converts electrical
energy in an opposite second direction from an alternating current form to a direct
current form (e.g., without limitation, functioning as an active rectifier or other
rectifier to charge a direct current energy storage device).
Referring to Figure 4, different energy conversion efficiencies are
shown in triangles 40,42,44,46,48 in a multi-inverter power conversion system 50. At
each of these triangles, there is an operating point for maximum power. Based on the
power available, the power conversion system 50 can be reconfigured for the best
efficiency of conversion. An alternative configuration of this power conversion
system 50 could be with individual transformers (not shown) for the two example
inverters 52,54, although the one transformer 70 has a relatively higher efficiency
than two transformers.
The example power conversion system 50 includes three different
example modes of operation: (1) each of two example inverters 52,54 operates at the
maximum power point of the corresponding PV array 56.58. respectively electrically
connected thereto; (2) the first inverter 52 only operates with the two PV arrays 56.58
electrically connected to that inverter; and (3) the second inverter 54 only operates
with the two PV arrays 52,54 electrically connected to that inverter.
The example power conversion system 50 includes two inverters 52.54
and two PV arrays 56,58 (e.g., without limitation, of different sizes) connected to the
two inverters 52,54. respectively. When the a mbined PV array output is below the
capacity of one inverter, only one inverter 52 or 54 is operated. Then, this inverter
maximizes the power output from the combined PV arrays 56,58. When the

combined PV array output is greater than the capacity of one inverter 52 or 54. the PY
arrays 56,58 are split between the two respective inverters 52.54. The two inverters
52,54 then operate the respective PV arrays 56,58 at their respective peak power
output. The two inverters 52,54 preferably transition between the two PV arrays
56,58 smoothly by adjusting the PV array voltages each time before disconnecting or
connecting the PV arrays. It will be appreciated that although two example inverters
are shown, this and other power conversions systems can operate with more than two
inverters.
In the second mode of operation, a processor, such as a system
controller 60 (e.g., without limitation, a programmable logic controller (PLC)) (figure
6) communicates (e.g., without limitation, using a suitable communication channel or
network; Modbus; RS-485) with the two inverters 52.54 and with a power meter 62
(Figure 6). For example, every day, the first inverter 52 wakes up in the morning,
electrically connects to the utility grid 64 and exports power to the utility grid with
contactor K2 66 closed (i.e., the outputs of both PV arrays 56,58 being electrically
connected).
As the day progresses, the power output increases as well as the losses
from the first inverter 52. its filter 68 and the transformer 70. When the output of the
first inverter 52 reaches a suitable predetermined power percentage (e.g.. without
limitation, about 80%; any suitable percentage) as measured by the power meter 62.
the losses in the first inverter 52 will exceed the losses if both inverters 52.54 would
operate. The predetermined power percentage is used by the system controller 60 to
switch the second inverter 54 on and open contactor K2 66 (as is shown). This is the
first mode of operation in which the two inverters 52.54 operate and share the PV
array energy of their respective PV arrays 56,58. The first inverter 52 operates at the
maximum power point of the PV array 56 and the second inverter 54 operates at the
maximum power point of the PV array 58. Together, the two inverters 52.54 have a
relatively higher energy and power efficiency than one inverter 52 or 54 c perating
alone. On a cloudy day, only one inverter 52 or 54 could operate entirely, thereby
saving considerable losses if both inverters 52.54 were to operate togethei or if one
relatively large inverter (not shown) was employed, thus increasing the energy output
from the PV arrays 56.58.

The control employed by the inverters 52,54 to maximize energy from
their respective PV arrays 56,58 is discussed, below, in connection with Figure X.
When the total power percentage falls below the suitable predetermined power
percentage (e.g., without limitation, about 80%; any suitable percentage) and when
the first inverter 52. for example, has operated for more than a predetermined time for
the day (e.g., without limitation, 4 hours; any suitable time), the first inverter 52 is
turned off and the contactor K2 66 is closed with the second inverter 54 taking the lull
output of the two PV arrays 56,58. This third mode of operation ensures equal or
about equal number of hours of operation of the two inverters 52.54.
As will be discussed in connection with Figures 7 and 8. the power
conversion system 50 operates the example PV arrays 56.58. the example inverters
52,54 and the example transformer 70 to provide maximum efficiency of power
conversion therefrom. See, for example, the peak of the second system plot 92 of
Figure 7 for a desired percent power output to the utility grid 64 of Figure 4 or a local
load (not shown).
Figure 5 shows plots 72,74 of example typical daily variation of solar
radiation at a location employing "tracking" (not to be confused with power point
tracking) PV arrays, such as the example PV arrays 56.58 of Figure 4. For example.
such tracking PV arrays can employ two axes or three axes (not shown) that track the
sun and keep the PV modules (not shown, but see the PV modules 10 of Figure 1 )
perpendicular to the incident solar radiation. Fixed PV arrays (e.g., having a fixed
angle with respect to the sun) (not shown) do not track the sun's movement during the
day and have a slightly narrower curve than that of the PV arrays 56.58. There is also
a seasonal variation between the example summer plot 72 and the example winter plot
74. In all cases, the inverters 52,54 are not operating at full capacity during a good
portion of the day and, thus, there is the opportunity to select the best configuration
using a suitable routine as will be discussed, below, in connection with Figure 8. For
example and without limitation, the outputs of the PV arrays 56.58 can be paralleled
during a number of hours before noon and a number of hours after noon, in order to
provide a relatively higher energy output and balance the usage of the inverters 52.54.
A power conversion system 76 including a suitable system control
architecture is shown in Figure 6. The two example inverter controllers 78.80. which

are controlled by the system controller 60, operate the two example inverters 52.54 in
grid parallel mode (e.g., defined by IEEE 1547 and UL 1741). The output power into
the corresponding transformer 70 (only one transformer is shown in Figure 6) at the
output of the corresponding inverter 52 or 54 is controlled by the system controller
60. The power meter 62 electrically connected at the utility grid 64 suitably measures
the three-phase power into the utility grid. The system controller 60 also controls the
paralleling of the DC busses 82,84 into the two respective inverters 52.54. Based on
the power output and the efficiency curves of the inverters 52.54. the system
controller 60 decides whether the two inverters 52.54 operate separately or w hether
only one of the two inverters 52,54 operates. When the two inverters 52,54 are
delivering power from the two separate PV arrays 56,58, the system controller 60
commands the two inverter outputs. The individual inverter controllers 78.80 then
adjust the DC voltages across the corresponding PV arrays 56,58.
When necessary, the contactor K.2 66 between the two DC busses
82,84 is closed or opened; during this process, the system controller 60 coordinates
the transfer of power from one inverter 52 to the other inverter 54.
Figure 7 includes plots 86.88,90 of efficiency (%) versus percent
power output (load) for the respective transformer 70. one of the two inverters 52.54.
and the system 50 of Figure 4. The system efficiency with normal operation of both
inverters 52,54 operational all the time is shown by the system plot 90. Improved
system efficiency with one of the two inverters 52,54 operating for 50% of the power
is shown by the second system plot 92. As can readily be seen, the efficiency can be
higher at various percent power outputs, if the two inverters 52,54 are operated in a
selective manner as will be discussed in connection with Figure 8.
Figure 8 is a flowchart of a routine 100 executed by the system
controller 60 of Figure 4 including maximum power point tracking (MPPT). The
inverter controllers 78,80 of Figure 6 measure and report DC voltage from the
respective PV arrays 56,58 to the system controller 60. First, at 102. the routine 100
determines if the DC voltage of the DC busses 82.84 with contactor K2 66 being
closed is above a suitable wake up voltage (e.g.. without limitation. 400 VDC). If so.
then at 104, it is determined if the voltage and the frequency of the utility grid 64 are
within desired limits. If not at either 102 or 104. a sleep mode is entered at 106.

Then, after a suitable time, step 102 is repeated. Otherwise, if the utility grid voltage
and frequency are within the desired limits, then, at 108, the system controller 60
commands one of the inverter controllers 78,80 to cause one of the respective
inverters 52,54 to begin switching to excite the transformer 70.
Next, at 110, it is determined if the system output power of both
inverters 52,54 (e.g., without limitation, as aggregated by the system controller 60
from the inverter controllers 78,80) is greater than a predetermined power value (e.g..
without limitation, No_load_pwr = 600 W). If not, then the sleep mode is entered at
106. On the other hand, if the inverter power is greater than the predetermined power
value, then at 112, it is determined if the PV voltage (the DC voltage of the DC busses
82,84 with contactor K2 66 being closed) is above a predetermined value (e.g.,
without limitation, MinJVdc = 400 VDC). If not. then the sleep mode is entered at
106. Otherwise, if the PV DC voltage is greater than this predetermined value. then at
114, system controller 60 commands the inverter controllers 78,80 to cause the
respective inverters 52,54 to synchronize with the utility grid 64 and then causes the
contactor Kl 67 to close.
After 114, at 116, the system controller 60 commands one of the two
inverter controllers 78,80 to start DC MPPT on one of the respective inverters 52.54.
At this stage of the routine 100. only the inverter controller 78 starts DCMPPT.
Next, at 118, it is determined from the power meter 62 whether the power to the
utility grid 64 is greater than a predetermined value (e.g.. without limitat on. Min Pwr
= 200 W). If not, then the sleep mode is entered at 106. On the other hand, if the
power to the utility grid 64 is greater than this predetermined value, then at 120. it is
determined from the power meter 62 whether the power to the utility grid 64 is greater
than a predetermined value (e.g., without limitation. 80%: Max Pwr = 110 kw or
85% of the rating of the inverters 52.54; MaxPwr = 100 kW or 80% of the rating of
the inverters 52,54; any suitable value). If so. then at 122. the contactor K2 66 is
opened, the contactor K1-2 71 is closed, and the system controller 60 commands both
of the two inverter controllers 78,80 to start DC MPPT on both of the respective
inverters 52,54. This operates each of the PV arrays 56.58 at a corresponding
independent maximum power point during a time of about peak or peak energy
production, in order to provide better energy efficiency.

Next, at 124. it is determined from the power meter 62 whether the
power to the utility grid 64 is less than a predetermined value (e.g.. without limitation.
80%; Max_Pwr; 80% out of 200%) for one inverter 52,54; any suitable value). If not.
then at 126, the AC output power of both inverters 52,54 is monitored (e.g..
employing the power meter 62 or by aggregating the outputs in the system controller
60) after which, at 128, AC MPPT is started on both inverters 52,54. The system
controller 60 is pre-programmed with efficiency curves of the combined inverter and
transformer system for selecting the maximum efficiency point for operation. To
operate at the desired operating point, the inverters 52.54 setup power limits to meet
based on the combined system efficiency. The system controller 60 can integrate the
AC power output from the power meter 62 to determine the output energy, or the
power meter .62 can advantageously provide both power and energy values. In turn,
this information can be employed by the system controller to determine loading (e.g..
% power) of the inverters 52,54 and the transformer 70.
Next, at 130, it is determined (e.g.. if the inverter outputs fall below the
power limits) if the AC power is less than a predetermined value (e.g.. MaxPwr). If
not, then step 126 is repeated. Otherwise, at 132, the contactor K2 66 is closed and
one of the two inverters 52,54 is stopped depending upon the run time of each of the
inverters 52,54, in order to enter the third mode of operation, which ensures equal or
about equal number of hours of operation of the two inverters 52.54. for example
and without limitation, step 132 can determine when one of the two inverters 52.54
has operated for longer than a predetermined time (e.g., without limitation, four hours:
any suitable time) during a predetermined time interval (e.g.. without limitation. one
day; any suitable time). This operates the power conversion system 76 of figure 6 at
optimal stress levels, in order to increase the life of the inverters 52,54. For example,
by operating alternate inverters 52 or 54 daily, the inverter heat sink temperature
variation (see Figure 10) is reduced. After 132. step 116 is executed to start DC
MPPT on the one of the two inverters 52,54 that is now running.
Figure 9 shows plots 140,142 of PV array output power versus PV
array output voltage for two different levels of solar radiation (e.g.. 100% radiation
and 80% radiation, respectively) including an effective DC output voltage operating
range for the inverter systems 50.76 of Figures 4 and 6. Using this information, the

system controller 60 can determine the optimum PV array voltage for a given level of
solar radiation, in order to operate the PV arrays 56.58 at maximum power output forr
a particular solar radiation. As the PV arrays 56.58 age. the power output at the same
voltage will be lower. This method does not look for an absolute value of power, but
looks for the maximum power.
Another consideration is control of the PV arrays 56.58 during the day.
When the PV array output changes from point A 144 to point B 146. the inverter 52 or
54 sets the voltage at Vpv_B and stores the corresponding PV array voltage and
current. After a suitable time delay, the inverter 52 or 54 looks for a change in current
and determines if that current increased or decreased. Next, the positive sequence
voltage at the utility grid 64 is used to determine if the grid voltage increased or
decreased. Based on the sign of the change in current and the sign of the change in
grid voltage, the inverter voltage set point 148 (shown in dotted line in Figure 9) is
moved to the right or to the left with respect to Figure 9. This control ensures
tracking of the power available from the PV array 56 or 58 and provides a stable
operating voltage for the inverter 52 or 54 and the PV array 56 or 58. After each
interval, the output power from the two inverters 52,54 combined is saved. This
output power information is used to determine how many of the two example
inverters 52,54 are to be on. When the two array voltages connected to the two
inverters 52,54 are controlled to their respective Vmp (voltage at maximum power), the
efficiency can be higher due to variations in the PV arrays 56.58. The system
controller 60 can have a learning capability or a user selected power level when the
PV arrays 56,58 are separately controlled (this is achieved by opening contactor K2
66 of Figure 6). This is because there can be PV array to PV array variations (e.g..
without limitation, up to about 3%, which can be termed a mismatch factor) and
transformer to transformer variations.
This can be combined with other operating system components, like
transformers and inverters, at their maximum power point (see the triangles
40.42,44,46,48 of Figure 4). The opening of contactor K2 66 can be determined with
a learning algorithm in the system controller 60 as was described above.
Figure 10 shows plots 150,152.154 of the typical life of inverters (e.g..
cycles to failure) versus temperature change. Since the disclosed method of using two

inverters 52,54 rotates the one inverter (e.g., without limitation, inverter 52) that starts
in the morning, the temperature change seen by the two inverters 52.54 is reduced by
one-half. The second inverter (e.g., without limitation, inverter 54) that starts later in
the day does not see the same temperature change since the enclosure (not shown)
gets heated during the morning time.
Referring to Figures 11 A-l1E, two other example power conversion
systems 160 (Figures 11 A-l1D) and 162 (Figure 11 E) include an energy storage
device, such as an example storage battery 164. electrically connected to a fust
converter 166 and a PV array 176 electrically connected to a second inverter 1 70.
Figure 12 shows a routine 172 executed by the system controller 1 74
of Figure 11 A. By employing the energy storage device 164 with the converter 166
and the PV array 176 with the inverter 170, the PV energy management is different
than that of the power conversion system 50 of Figure 4. The inverter 170 operates to
maximize power from the PV array 176 and the converter 166 manages energy tor the
energy storage device 164. It is believed that such a system employing an energy
storage device, a PV array, and multiple inverters and/or converters is novel and non-
obvious.
Referring to Figure 11A, the example inverter/converter power
conversion system 160 including the PV array 176 and the energy storage device 164
is electrically connected to a local load 178 and to the example utility grid 180. In
this power conversion system 160, the converter 166 is electrically connected to the
energy storage device 164 (e.g., without limitation, a battery: an electric couble-layer
capacitor; a super-capacitor; an electrochemical double layer capacitor (EDLC); an
ultra-capacitor) and the inverter 170 is electrically connected to the PV array 1 76. In
this mode, the inverter 170 provides maximum power point tracking of the
corresponding PV array 176 by continuously adjusting the PV array output voltage in
order to receive the maximum power therefrom. The controllers 182.184. power
meter 186, filters 188,190 and transformer 192 function in a manner similar to the
respective controllers 78,80, power meter 62. filters 68.69 and transformer 70 of
Figure 6.
The converter 166 has a first input/output 165 structured to nput
power from or output power to the energy storage device 164. and a secor .1

input/output 167 structured to input power or output power. The inverter 1 7() has an
input 169 structured to input power from the PV array 176 and an output 171
structured to output power. The transformer 192 includes a primary 191 electrically
connected to the inverter output 171 by filter 190 and to the converter second
input/output 167 by filter 188, and a secondary 193 electrically connected to at least
one of the local load 178 and the utility grid 180.
In Figure 11B, while connected to the utility grid 180. the local load
178 is powered by both the PV array 176/inverter 170 and the energy storage device
164/converter 166. Hence, when the PV array output power and, thus, the output
power from inverter 170 are insufficient to meet the requirements of the local load
178, the converter 166 supplements the power to the local load 178 by discharging the
energy stored in the energy storage device 164 to the local load 1 78. In this mode, the
converter 166 continuously manages the local load 178 depending on the available
power from the inverter 170.
As shown in Figure 11C, when the inverter 170 can deliver all the
power to the local load 178/utility grid 180, the converter 166 changes control into,
for example, an active rectifier and charges the energy storage device 164. as needed.
The converter 166 can also charge the energy storage device 164 if the output power
from the inverter 170 is in excess of the load requirements.
In Figure 1 ID, no energy is delivered into the local load 1 78 utility
grid 180 and all energy from the PV array 176/inverter 170 is used to charge the
energy storage device 164 through the converter 166. The energy is not transferred
from the PV array 176 to the energy storage device 164 through the output
transformer 192 in this mode. It is believed that this is a novel and non-obvious
feature of the power conversion system 160. This approach also increases the
efficiency of the power conversion system 160 since the power is not transferred
through the transformer 192, and the IGBTs (not shown) of the converter 66 need
not be switching during most of the time. For example, the magnitude of the AC
output voltage from the inverter 170 connected to the PV array 176 is higher than the
DC voltage of the energy storage device 164. The converter 166 need not actively
rectify the AC. For example, anti-parallel diodes (not shown) across the IGBTs (not
shown) of the converter 166 can be employed for rectification.

As shown in Figure 11E. another inverter/converter power conversion
system 162 powers a local load 194 and is not electrically connected to a utility grid
(not shown). The converter 166 continuously operates as a voltage source setting the
frequency and voltage of the local load 194 since the utility grid is absent. The
inverter 170 operates in the grid-parallel mode, treating the output of the converter
166 in the same manner as the utility grid (not shown).
In Figures 11B and 11E, the load power demand is met by both of the
PV array 176 and the energy storage device 164. The output energy from the PV
array 176 can vary throughout the day since there is no control over when solar
energy is available, and since the local load 178 or 194 can change as well, the
converter 1.66 identifies the energy needs of the local load 178 or 194 and adjusts its
output power to power the local load 178 or 194 continuously. In some cases, the
energy from the PV array 176 can exceed the needs of the local load 178 or 194 (e.g..
this can occur over weekends or holidays). In those cases, the converter 166 charges
the energy storage device 164 and stores any excess energy from the PV array
176/inverter 170.
Referring to Figure 12, although not shown, the routine 172
advantageously includes maximum power point tracking (MPPT) for the PV array
176, inverter 170 and transformer 192, as was discussed above in connection with
Figures 4, 6 and 8. The converter 166 addresses the needs of the energy storage
device 164 (i.e., charge or discharge). The inverter 170 always tracks the maximum
power point of the PV array 176. However, in addition, the converter 166 adjusts its
output voltage to charge the energy storage device 164 when not connected to the
utility grid 180.
At 196 of the routine 172, it is determined if PV power is available
from the PV array 176. If not, then a sleep mode is entered at 198. The sleep mode
198 periodically wakes up to recheck the test at 196. On the other hand, if the test
passes, at 196, then at 200, the inverter 170 is turned on. Next, at 202. it is
determined if the power of the local load 178 or 194 is met. If not. then a' 204. is
determined if the cost of power from the utility grid 180 is suitably low. If not, then
the local load 178 (or a suitable portion thereof) is shed at 206. after which step 202 is
repeated. Otherwise, if the power of the local load 178 or 194 is met at 2 2. then at

208, it is determined if there is excess PV power available. If not. then at 210. it is
determined if the cost of power from the utility grid 180 is suitably low. If not. then
the local load 178 or 194 (or a suitable portion thereof) is shed at 206. On the other
hand, if the cost of power from the utility grid 180 is suitably low at either 204 or 210.
then at 212, the converter 166 operates as a charger. Next, at 214, it is determined if
sufficient battery energy, for example, from the energy storage device 164 is
available. If not, then step 212 is repeated. Otherwise, at 216. the converter 166 is
turned on as an inverter after which step 202 is executed. On the other hand, it excess
PV power is available at 208, then at 218, the output of the converter 166 is reduced
before step 216 is executed.
While specific embodiments of the disclosed concept have been
described in detail, it will be appreciated by those skilled in the art that various
modifications and alternatives to those details could be developed in light of the
overall teachings of the disclosure. Accordingly, the particular arrangements
disclosed are meant to be illustrative only and not limiting as to the scope of the
disclosed concept which is to be given the full breadth of the claims appended and
any and all equivalents thereof.

REFERENCE NUMERICAL LIST
2 PV array / inverter system
4 PV array
6 PV array
8 number of strings
10 PV modules
12 inverter
14 PV array DC voltage (volts)
16 PV array DC current (amperes)
18 PV array DC power output (watts)
20 100% solar radiation (i.e., "insolation")
22 plot
24 utility grid
26 transformer
28 efficiency curve
30 efficiency curve
40 energy conversion efficiency
42 energy conversion efficiency
44 energy conversion efficiency
46 energy conversion efficiency
48 energy conversion efficiency
50 multi-inverter power conversion system
52 inverter
54 inverter
56 PV array
58 PV array
60 a processor, such as a system controller
62 power meter
64 utility grid
66 contactor K2
67 contactor K.1
68 filter
69 filter
70 transformer
71 contactor K1-2
72 plot
74 plot
76 power conversion system
78 . inverter controller
.80 inverter controller
82 DC bus
84 DC bus
86 plot
88 plot
90 plot
92 plot
100 routine

102 step
104 step
106 step
108 step
110 step
112 step
114 step
116 step
118 step
120 step
122 step
124 step
126 step
128 step
130 step
132 step
140 plot
142 plot
144 point A
146 point B
148 inverter voltage set point
150 plot
152 plot
154 plot
160 power conversion system
162 power conversion system
164 energy storage device, such as an example storage batten
165 first input/output
166 first converter
167 second input/output

169 input
170 second inverter
171 output
172 routine
174 system controller
176 PV array
178 local load
180 utility grid
182 controller
184 controller
186 power meter
188 filter
190 filter
191 primary
192 transformer
193 secondary
194 local load

196 step
198 step
200 step
202 step
204 step
206 step
208 step
210 step
212 ' step
214 step
216 step
218 step

we claim
What is Claimed is:
1. A method of power tracking for a photovoltaic system
(50;76;160;162) including a number of photovoltaic arrays (56.58:1 76). a number of
inverters (52,54; 170), and a transformer (70; 192), the method comprising:
operating (122,128) the photovoltaic system including said
number of photovoltaic arrays, said number of inverters and said transformer to
provide maximum efficiency of power conversion therefrom: and
maximizing (116,122) power output from said number of
photovoltaic arrays.
2. The method of Claim 1 further comprising:
employing as said number of photovoltaic arrays a plurality of
photovoltaic arrays (56,58); and
paralleling (66) the outputs (82,84) of said plurality of
photovoltaic arrays.
3. The method of Claim 2 further comprising:
paralleling the outputs of said plurality of photovoltaic arrays
during a number of hours before noon and a number of hours after noon.
4. The method of Claim 1 further comprising:
employing as said number of photovoltaic arrays a plurality of
photovoltaic arrays (56,58);
employing as said number of inverters a plurality of'inverters
(52.54);
for each of said plurality of photovoltaic arrays, powering 022
a corresponding one of said plurality of inverters from a corresponding one of said
plurality of photovoltaic arrays; and
operating (122) each of Said plurality of photovoltaic arrays at a
corresponding independent maximum power point during a time of about peak or
peak energy production.
5. The method of Claim 1 urther comprising:
determining (60,62) energy and power at a utility grid 64):
determining (60,62) the loading on said number of inverters
and the transformer; and

operating (132) the system at optimal stress levels.
6. The method of Claim 1 further comprising:
employing as said number of photovoltaic arra\s a plurality of
photovoltaic arrays (56,58);
employing as said number of inverters a plurality of inverters
(52,54); and
for each of said plurality of photovoltaic arrays, powering (122)
a corresponding one of said plurality of inverters from a corresponding one of said
plurality of photovoltaic arrays.
7. The method of Claim 6 further comprising:
powering the transformer from each of said plurality of
inverters; and
powering a load (64; 178; 194) from the transformer.
8. The method of Claim 1 further comprising:
employing two photovoltaic arrays (56.58) as said number of
photovoltaic arrays;
employing two inverters (52.54) as said number of inverters
initially paralleling (66) the outputs (82,84) of the two
photovoltaic arrays and powering one (52) of the two inverters from the paralleled
outputs;
determining (120) when power output from said one of the two
inverters exceeds a predetermined power percentage and responsively powering 1122)
said one (52) of the two inverters from one (56) of the two photovoltaic arrays and
powering the other one (54) of the two inverters from the other one (58) of the two
photovoltaic arrays;
operating (122) said one of the two inverters at the maximum
power point of the one of the two photovoltaic arrays; and
operating (122) the other one of the two inverters a: the
maximum power point of the other one of the two photovoltaic arrays.
9. The method of Claim 8 further comprising:
determining (124) when power output from both or the two
inverters falls below a predetermined power percentage; and

determining (132) when said one of the two inverters has
operated for longer than a predetermined time during a predetermined time interval,
and responsively disabling said one of the two inverters, paralleling the outputs of the
two photovoltaic arrays and powering the other one of the two inverters from the
paralleled outputs.
10. The method of Claim 1 wherein the photovoltaic system
(160; 162) further includes an energy storage device (164), a photovoltaic array (176)
as said number of photovoltaic arrays, an inverter (170) as said number of inverters,
and a converter (166), said method further comprising:
including with the converter (166) a first input/output (165)
structured to input power from or output power to the energy storage device, and a
second input/output (167) structured to input power or output power:
including with the inverter (170) an input (169) structured to
input power from the photovoltaic array and an output (171) structured to output
power; and
including with the transformer (192) a primary (191)
electrically connected to the output of the inverter and to the second input/output of
the converter, and a secondary (193) electrically connected to at least one of a local
load (178) and a utility grid (180).
11. A photovoltaic system (160; 162) comprising:
an energy storage device (164);
a photovoltaic array (176);
a converter (166) including a first input/output (165 ) structured
to input power from or output power to the energy storage device, and a s;cond
input/output (167) structured to input power or output power;
an inverter (170) including an input (169) structured to input
power from the photovoltaic array and an output (171) structured to output power:
and
a transformer (192) having a primary (191) electrically
connected to the output of the inverter and to the second input/output of the converter.
and a secondary (193) electrically connected to at least one of a local load (178:194)
and a utility grid (180).

12. The photovoltaic system (160; 162) of Claim 1 1 wherein the
energy storage device is selected from the group consisting of a battery, an electric
double-layer capacitor, a super-capacitor, an electrochemical double layer capacitor
(EDLC), and an ultra-capacitor.
13. The photovoltaic system (160; 162) of Claim 11 wherein the
inverter is structured to provide maximum power tracking of the photovoltaic arra\ by
adjusting the output voltage of the photovoltaic array in order to receive maximum
power therefrom.
14. The photovoltaic system (160; 162) of Claim 11 wherein the
first input/output of the converter inputs power from the energy storage device:
wherein the second input/output of the converter outputs power; and wherein the local
load is powered through the transformer by both of the converter and the inverter.
15. The photovoltaic system (160:162) of Claim 14 wherein the
converter is structured to operate as a voltage source and set the frequency and the
voltage of the local load using available power from the energy storage device.
16. The photovoltaic system (160;162) of Claim 11 wherein the
first input/output of the converter outputs power to the energy storage device: wherein
the second input/output of the converter inputs power: and wherein the local load is
powered through the transformer by the inverter.
17. The photovoltaic system (160; 162) of Claim 16 wherein the
converter is an active rectifier structured to charge the energy storage device.
18. The photovoltaic system (160:162) of Claim 17 wherein the
second input/output of the active rectifier is powered through the transformer from the
utility grid.
19. The photovoltaic system (160; 162) of Claim 1 1 wherein the
first input/output of the converter outputs power to the energy storage device: and
wherein the second input/output of the converter inputs power from the output of the
inverter.
20. The photovoltaic system (160) of Claim 19 wherein neither one
of the local load or the utility grid is powered; and wherein the second input-output of
the converter inputs power directly from the output of the inverter.

21. The photovoltaic system (162) of Claim 11 wherein the
secondary of the transformer is only electrically connected to the local load (194):
wherein the first input/output of the converter inputs power from the energy storage
device; wherein the second input/output of the converter outputs power; and wherein
the local load is powered through the transformer by both of the converter and the
inverter.
22. The photovoltaic system (162) of Claim 21 wherein the
converter is structured to operate as a voltage source and set the frequency and the
voltage of the local load (194); and wherein the inverter is structured to track the
frequency and the voltage of the local load.
23. A power conversion system (50;76; 160:162) comprising:
a number of photovoltaic arrays (56,58:176);
a number of inverters (52,54; 170);
a transformer (70; 192); and
a processor (60) structured (100) to control said number of
inverters and operate (122.128) said power conversion system to provide maximum
efficiency of power conversion by said number of photovoltaic arrays, said number of
inverters and said transformer, and to maximize (116.122) power output from said
number of photovoltaic arrays.
24. The power conversion system (76) of Claim 23 wherein said
number of photovoltaic arrays is two photovoltaic arrays (56.58) each having an
output (82,84); wherein said number of inverters is two inverters (52.54): and wherein
said processor (60) is structured (100) to selectively cause: (a) a corresponding one of
the inverters to be powered (66,122) from a corresponding one of the two
photovoltaic arrays; or (b) the outputs of the two photovoltaic arrays to bt electrically
connected in parallel (66,132) and one of the two inverters to be powered .from the
paralleled outputs.

A power conversion system (50:76; 160:162) includes a number of
photovoltaic arrays (56,58:176), a number of inverters (52.54:170). a transformer
(70; 192), and processor (60). The processor (60) is structured (100) to control the
number of inverters and operate (122,128) the power conversion system to provide
maximum efficiency of power conversion by the number of photovoltaic arrays, the
number of inverters and the transformer, and to maximize (1 16.122) power output
from the number of photovoltaic arrays.