ADDING AND SHEDDING LOADS USING LOAD LEVELS TO DETERMINE TIMING
CLAIM OF PRIORITY
This application claims the benefit of priority of U.S. Patent Application
Serial No. 13/289,131, entitled "ADDING AND SHEDDING LOADS USING
LOAD LEVELS TO DETERMINE TIMING," filed on November 4, 2011,
which application is incorporated by reference herein in its entirety.
TECHNICAL FIELD
Embodiments pertain to a system and method for adding and shedding
loads.
BACKGROUND
The process of prioritizing loads that are connected to a power supply
that has limited capacity is typically referred as load shedding. As an example,
power may be supplied by a standby generator where load shedding is required
because the standby generator has a capacity that is less than the requirements of
the entire attached load.
Water heaters and air conditioners are among the commonly utilized
devices that are powered loads by a power source (e.g., a generator). These
loads may need to be shed when a residence is being supplied by a limited
capacity generator. Existing load shedding systems typically prioritize each load
and then determine if the limited capacity power source is able to supply the
loads before adding each load. If the limited capacity power source becomes
overloaded, then the load control system will remove one or more loads to allow
the power source to continue supplying power to the more important connected
loads.
Utilizing a load shedding system may allow a smaller standby generator
to be installed thereby decreasing the generator costs that are associated with
powering a facility. In addition, load shedding may decrease costs by limiting
the peak demand for power during certain times of the day because such systems
often allow a power generation utility to keep a less efficient generation plant
offline and then pass the savings on to the customer (i.e., the generator user).
One of the drawbacks with existing load shedding systems is that
although custom-designed and configured load shedding schemes work well
under some conditions; many load shedding systems do not work well when
operating a variety of loads under a variety of conditions.
One of the biggest challenges for a load shedding system is a highpriority
switching load. In one example scenario, a high-priority switching load
may be deactivated which allows less important loads to be added. Therefore,
once the high-priority switching load is eventually turned on, the power source
becomes overloaded. The load shedding system must then shed several loads
before the load that is actually causing the overload is removed. The additional
time that is required to shed multiple loads increases the likelihood of the power
source becoming overloaded for an undesirable period of time. Although many
existing load shedding systems are customized in an attempt to minimize
unintended power source dropouts, such systems are still often unable to
adequately handle high-priority switching loads.
Another drawback with conventional load shedding systems is that in
some scenarios, all of the loads may not be drawing power from the generator
during an overload condition. As an example, six loads may be activated by the
system even though only two of the loads are actually drawing power. As a
result, when an overload occurs after all these loads have been added, the system
may have to take unnecessary time to shed as many as five loads before actual
load on the power source decreases at all. This increase in time to shed the
appropriate load could result in the power source going offline.
Load shedding systems must also typically be carefully configured in
order to work in each application because standard load shedding logic does not
accurately match the load profile of a typical power source or a typical motor
load. As a result, these existing systems are typically unable to start large
motors that would otherwise typically lie within the starting capabilities of the
generator. Configuring a typical load shedding system to permit starting a large
motor will typically result in inadequate protection for the generator.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example load shedding system.
FIG. 2 illustrates an example engine driven generator that may be used
with the load shedding system shown in FIG. 1.
FIG. 3 illustrates an example of how time T varies when a given load is
added based on the generator load L and the available generator capacity at a
point in time as compared to a conventional method of adding loads.
FIGS. 4 and 5 illustrate an example of how time T varies for a given load
being shed based on the correspond overload of a generator as compared to a
conventional method of shedding loads.
FIG. 6A shows conventional under- frequency load shedding techniques
handling motor starting and overload conditions.
FIG. 6B shows under-frequency load shedding techniques handling
motor starting and overload conditions in accordance with some example
embodiments.
FIG. 7 illustrates decreasing the time to shed subsequent loads after a
previous load shedding operation in accordance with some example
embodiments.
FIG. 8 is a block diagram that illustrates a diagrammatic representation
of a machine in the example form of a computer system 400 within which a set
of instructions for causing the machine to perform any one or more of the
methodologies discussed herein may be executed.
DETAILED DESCRIPTION
The following description and the drawings sufficiently illustrate specific
embodiments to enable those skilled in the art to practice them. Other
embodiments may incorporate structural, logical, electrical, process, and other
changes. Portions and features of some embodiments may be included in, or
substituted for, those of other embodiments. Embodiments set forth in the
claims encompass all available equivalents of those claims.
A method of adding and shedding loads LI, L2, L3, L4 that are
connected to a generator 12 will now be described with reference to FIG. 1. The
method includes determining whether a plurality of loads LI, L2, L3, L4 is being
supplied with power by the generator 12 and then determining the total load that
the generator 12 is supplying to the plurality of loads LI, L2, L3, L4.
The method further includes determining whether to change a number of
the loads in the plurality of loads LI , L2, L3, L4 based on the amount of load L
that is being supplied by the generator 12. As shown in FIGS. 3-6, the method
further includes determining an amount of time T in which to change the number
of loads in the plurality of loads based on the amount of load L that is being
supplied by the generator 12.
In some embodiments, determining whether a plurality of loads LI , L2,
L3, L4 are being supplied with power by the generator 12 may include
monitoring the position of an automatic transfer switch 13. It should be noted
that the plurality of loads LI, L2, L3, L4 are being supplied with power by
generator 12 when the automatic transfer switch 13 is in an emergency position.
In alternative embodiments, determining whether a plurality of loads LI ,
L2, L3, L4 are being supplied with power by the generator 12 may include
measuring a position of a throttle 17 that is part of the generator 12 (see e.g.,
FIG. 2). It should be noted that the generator 12 may be established as
supplying power to the plurality of loads LI, L2, L3, L4 when the throttle 17
position is in a position other than a "no load" position.
In still other embodiments, determining whether a plurality of loads LI,
L2, L3, L4 are being supplied with power by the generator 12 may include
monitoring the generator load L. As examples, monitoring the generator load L
may be done by (i) measuring the generator 12 operating frequency; (ii)
measuring the generator 12 operating voltage; and/or (iii) measuring the
generator 12 current.
In addition, determining the total load L that the generator 12 is
supplying to the plurality of loads LI , L2, L3 , L4 may include (i) measuring the
generator operating frequency; (ii) measuring the generator operating voltage;
and/or (iii) measuring the generator current.
In some embodiments, determining the total load L that the generator 12
is supplying to the plurality of loads LI , L2, L3, L4 includes determining the
output torque of a prime mover (i.e., an engine) of the generator 12. The output
torque may be calculated by (i) measuring fuel injection time duration 18 within
the generator 12; (ii) measuring the intake manifold 16 pressure within the
generator 12; and/or (iii) measuring a position of a throttle 17 within the
generator 12. It should be noted the output torque may be calculated for sparkignited
and compression-ignited engines as well as other types of prime movers.
INCREASING THE NUMBER OF LOADS
In some embodiments, determining an amount of time T in which to
change the number of loads in the plurality of loads LI, L2, L3, L4 may be based
on the amount of load L that is being supplied by the generator 12 includes
increasing the number of loads based on an available load capacity of the
generator 12.
As used herein, the available load capacity of the generator 12 is the
difference between the maximum loading threshold of the generator 12 and a
load the generator 12 is supplying at a particular point in time. As examples, the
maximum loading threshold of the generator may be adjustable by a user via a
user interface 20 (see FIG. 1), and/or may be based on a rating determined by a
manufacturer of the generator 12. As examples, the user interface 20 may be
part of a load control module 14, automatic transfer switch 13, generator
controller 15 or a stand-alone device.
FIG. 3 illustrates an example of how time T varies when a given load is
added based on the generator load L and the available generator capacity at a
point in time as compared to a conventional method of adding loads. The
amount of time T to add a load is varied based on the available generator
capacity. As the available generator capacity increases, the time T to add a load
decreases.
Therefore, the method allows generator loads to be added more quickly
when there is substantial available generator capacity and more slowly when
there is limited available generator capacity. This time adjustment provides (i)
improved protection to the generator as the generator approaches maximum
capacity; and (ii) power load as quickly as possible when there is minimal
generator loading (as compared to conventional methods).
DECREASING THE NUMBER OF LOADS
In some embodiments, determining an amount of time T in which to
change the number of loads in the plurality of loads LI, L2, L3, L4 may be based
on the amount of load L that is being supplied by the generator 1 includes
decreasing the number of loads based on an overload of the generator 12.
As used herein, the overload of the generator 12 is a difference between a
generator load at a particular point in time and a maximum loading threshold of
the generator. As examples, the maximum loading threshold of the generator
may be adjustable by a user interface 20 (see FIG. 1), and/or may be based on a
rating determined by a manufacturer of the generator 12.
FIGS. 4 and 5 illustrate an example of how time T varies for a given load
being shed based on the corresponding overload of the generator 12 as compared
to a conventional method of shedding loads. The amount of time T to shed a
load is varied based on the overload of the generator 12. As the overload
increases, the time T to shed a load decreases.
Therefore, the method allows generator loads to be shed more quickly
when there is substantial generator overload and more slowly when generator 12
is not as heavily overloaded. This time adjustment (i) provides improved
protection to the generator 12 when there is substantial generator overload by
shedding loads more quickly (see e.g., FIG. 4); and (ii) permits motor starting
(see e.g., FIG. 5) (as compared to conventional methods).
As shown in FIG. 6B, determining an amount of time in which to change
the number of loads in the plurality of loads based on the amount of load that is
being supplied by the generator includes decreasing the number of loads based
on generator operating frequency. In some embodiments, the amount of time to
decrease the number of loads will decrease as the generator operating frequency
decreases.
As shown in FIG. 6A, conventional under-frequency load shedding
techniques shed load after the generator has remained below a fixed threshold
for a specified period of time. This type of operating parameter results in poor
power quality being supplied to loads and could also result in unintended
shedding during motor starting, especially when using heavily loaded large AC
motors.
Comparing FIGS. 6A and 6B demonstrates how the methods described
herein may improve on conventional under frequency load shedding techniques.
FIG. 6A illustrates conventional under frequency load shedding techniques for a
given motor starting load and a given overload while FIG. 6B illustrates the
under frequency load shedding techniques described herein for the same motor
starting load and the same overload.
It should be noted that while FIGS 3, 4, 5 and 6 illustrate linear time/load
curves, other embodiments are contemplated where these curves may be nonlinear.
The shape of these curves will depend on a variety of design
considerations.
FIGS. 1 and 7 illustrate a method of adding and shedding loads that are
connected to a generator in accordance with another example embodiment. The
method includes determining whether a plurality of loads LI, L2, L3, L4 is being
supplied with power by the generator 12 and determining the load L that the
generator is supplying to the plurality of loads LI , L2, L3, L4.
The method further includes determining whether to change a number of
the loads in the plurality of loads LI , L2, L3, L4 based on the amount of load
that is being supplied by the generator 12 and changing the number of loads in
the plurality of loads LI, L2, L3, L4. The method further includes determining
an amount of time in which to further change the number of loads where the
amount of time is determined by whether the number of loads increases or
decreases during the previous change of the number of loads.
In some embodiments, determining an amount of time in which to further
change the number of loads in the plurality of loads LI , L2, L3, L4 includes
increasing the amount of time to decrease the number of loads when the previous
change of the number of loads increased the number of loads.
Other embodiments are contemplated where determining an amount of
time in which to further change the number of loads in the plurality of loads
includes decreasing the amount of time to decrease the number of loads when the
previous change of the number of loads decreased the number of loads.
It should be noted that embodiments are also contemplated where
determining an amount of time in which to further change the number of loads in
the plurality of loads LI, L2, L3, L4 includes decreasing the amount of time to
decrease the number of loads when the previous change of the number of loads
decreased the number of loads.
In still other embodiments, determining an amount of time in which to
further change the number of loads in the plurality of loads LI, L2, L3, L4
includes increasing the amount of time to increase the number of loads when the
previous change of the number of loads decreased the number of loads.
FIG. 7 illustrates decreasing the time to shed subsequent loads after a
previous load shedding operation. In the example scenario that is illustrated in
FIG. 7, three of six loads are not demanding power from the generator which
results in no decrease to the generator load when these loads are shed. The
subsequent decreases in the time to shed each load allows these loads to be shed
before there is significant degradation to the quality of power being supplied to
these loads.
The methods described herein may permit load control operation that
work well when there a variety of loads that operate under a variety of
conditions. In addition, the methods may be able to more adequately handle
high-priority switching loads. The methods may also reduce the time to shed
multiple loads more quickly until the actual load on the power source decreases.
This decrease in time to shed the appropriate load may allow the power source to
remain online.
Example Machine Architecture
FIG. 8 is a block diagram that illustrates a diagrammatic representation
of a machine in the example form of a computer system 400 within which a set
of instructions for causing the machine to perform any one or more of the
methodologies discussed herein may be executed. In some embodiments, the
computer system 400 may operate in the capacity of a server or a client machine
in a server-client network environment, or as a peer machine in a peer-to-peer
(or distributed) network environment.
The computer system 400 may be a server computer, a client computer, a
personal computer (PC), a tablet PC, a set-top box (STB), a Personal Digital
Assistant (PDA), a cellular telephone, a Web appliance, a network router, switch
or bridge, or any machine capable of executing a set of instructions (sequential
or otherwise) that specify actions to be taken by that machine. Further, while
only a single machine is illustrated, the term "machine" shall also be taken to
include any collection of machines that individually or jointly execute a set (or
multiple sets) of instructions to perform any one or more of the methodologies
discussed herein.
The example computer system 400 may include a processor 460 (e.g., a
central processing unit (CPU), a graphics processing unit (GPU) or both), a main
memory 470 and a static memory 480, all of which communicate with each other
via a bus 408. The computer system 400 may further include a video display
unit 410 (e.g., liquid crystal displays (LCD) or cathode ray tube (CRT)). The
computer system 400 also may include an alphanumeric input device 420 (e.g., a
keyboard), a cursor control device 430 (e.g., a mouse), a disk drive unit 440, a
signal generation device 450 (e.g., a speaker), and a network interface device
490.
The disk drive unit 440 may include a machine-readable medium 422 on
which is stored one or more sets of instructions (e.g., software 424) embodying
any one or more of the methodologies or functions described herein. The
software 424 may also reside, completely or at least partially, within the main
memory 470 and/or within the processor 460 during execution thereof by the
computer system 400, the main memory 470 and the processor 460 also
constituting machine-readable media. It should be noted that the software 424
may further be transmitted or received over a network (e.g., network 380 in FIG.
3) via the network interface device 490.
While the machine-readable medium 422 is shown in an example
embodiment to be a single medium, the term "machine-readable medium" should
be taken to include a single medium or multiple media (e.g., a centralized or
distributed database, and/or associated caches and servers) that store the one or
more sets of instructions. The term "machine-readable medium" shall also be
taken to include any medium that is capable of storing, encoding or carrying a
set of instructions for execution by the machine and that cause the machine to
perform any one or more of example embodiments described herein. The term
"machine-readable medium" shall accordingly be taken to include, but not be
limited to, solid-state memories and optical and magnetic media.
Thus, a computerized method and system are described herein. Although
the present invention has been described with reference to specific example
embodiments, it will be evident that various modifications and changes may be
made to these embodiments without departing from the invention. Accordingly,
the specification and drawings are to be regarded in an illustrative rather than a
restrictive sense.
The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)
requiring an abstract that will allow the reader to ascertain the nature and gist of
the technical disclosure. It is submitted with the understanding that it will not be
used to limit or interpret the scope or meaning of the claims. The following
claims are hereby incorporated into the detailed description, with each claim
standing on its own as a separate embodiment.
CLAIMS
What is claimed is:
1. A method of adding and shedding loads that are connected to a generator,
the method comprising:
determining whether a plurality of loads is being supplied with power by
the generator;
determining the load that the generator is supplying to the plurality of
loads;
determining whether to change a number of the loads in the plurality of
loads based on the amount of load that is being supplied by the generator; and
determining an amount of time in which to change the number of loads in
the plurality of loads based on the amount of load that is being supplied by the
generator.
2. The method of claim 1, wherein determining whether a plurality of loads
are being supplied with power by the generator includes monitoring the position
of an automatic transfer switch.
3. The method of claim 1, wherein determining whether a plurality of loads
are being supplied with power by the generator includes monitoring the
generator load.
4. The method of claim 3, wherein monitoring the generator load includes
measuring the generator operating frequency.
5. The method of claim 3, wherein monitoring the generator load includes
monitoring the torque of a prime mover within the generator, wherein
monitoring the torque of a prime mover within the generator includes measuring
fuel injection time duration within the generator, wherein monitoring the torque
of a prime mover within the generator includes measuring the intake manifold
pressure within the generator,, wherein monitoring the torque of a prime mover
within the generator includes measuring a position of a throttle within the
generator.
6. The method of claim 1, wherein determining the load that the generator is
supplying to the plurality of loads includes measuring the current that the
generator is supplying to the plurality of loads, wherein determining the load that
the generator is supplying to the plurality of loads includes measuring the
generator operating frequency, wherein determining the load that the generator is
supplying to the plurality of loads includes measuring the generator operating
voltage.
7. The method of claim 1, wherein determining an amount of time in which
to change the number of loads in the plurality of loads based on the amount of
load that is being supplied by the generator includes increasing the number of
loads based on an available load capacity of the generator.
8. The method of claim 7, wherein the available load capacity of the
generator is a difference between a maximum loading threshold of the generator
and a generator load at a particular point in time.
9. The method of claim 8, wherein the maximum loading threshold of the
generator is adjustable by a user.
10. The method of claim 8, wherein the maximum loading threshold of the
generator is based on a rating provided by a manufacturer of the generator.
11. The method of claim 8, wherein the amount of time to increase the
number of loads will increase as the available load capacity decreases.
12. The method of claim 1, wherein determining an amount of time in which
to change the number of loads in the plurality of loads based on the amount of
load that is being supplied by the generator includes decreasing the number of
loads based on an overload of the generator.
13. The method of claim 12, wherein the overload of the generator is a
difference between a generator load at a particular point in time and a maximum
loading threshold of the generator.
14. The method of claim 13, wherein the maximum loading threshold of the
generator is adjustable by a user.
15. The method of claim 13, wherein the maximum loading threshold of the
generator is based on a rating provided by a manufacturer of the generator.
16. The method of claim 12, wherein the amount of time to decrease the
number of loads will decrease as the generator overload increases.
17. The method of claim 1, wherein determining an amount of time in which
to change the number of loads in the plurality of loads based on the amount of
load that is being supplied by the generator includes decreasing the number of
loads based on generator operating frequency.
18. The method of claim 17, wherein the amount of time to decrease the
number of loads will decrease as the generator operating frequency decreases.