U.S. patent application number 15/978585 was filed with the patent office on 2018-11-15 for energy storage system for photovoltaic energy and method of storing photovoltaic energy.
The applicant listed for this patent is DYNAPOWER COMPANY LLC. Invention is credited to John C. PALOMBINI, Apurva SOMANI.
Application Number | 20180331543 15/978585 |
Document ID | / |
Family ID | 62528837 |
Filed Date | 2018-11-15 |
United States Patent
Application |
20180331543 |
Kind Code |
A1 |
PALOMBINI; John C. ; et
al. |
November 15, 2018 |
ENERGY STORAGE SYSTEM FOR PHOTOVOLTAIC ENERGY AND METHOD OF STORING
PHOTOVOLTAIC ENERGY
Abstract
An energy system for renewable energy applications includes a
renewable energy source, a bidirectional inverter connected an AC
bus and a DC bus, an energy storage unit connected to the
bidirectional DC/DC converter, and a control system comprising one
or more controllers coupled to the bidirectional inverter and the
bidirectional DC/DC converter. The bidirectional inverter is
connected to the renewable energy source and a bidirectional DC/DC
converter through the DC bus. The system is configured to capture
low power of a photovoltaic (PV) array, energy typically lost to
inverter clipping, and through the utilization of ramp rate
control.
Inventors: |
PALOMBINI; John C.; (South
Burlington, VT) ; SOMANI; Apurva; (South Burlington,
VT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DYNAPOWER COMPANY LLC |
South Burlington |
VT |
US |
|
|
Family ID: |
62528837 |
Appl. No.: |
15/978585 |
Filed: |
May 14, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62506291 |
May 15, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J 7/35 20130101; H02M
3/33584 20130101; H02J 2300/26 20200101; H02J 3/381 20130101; H02J
7/0068 20130101; H02J 3/385 20130101; H02J 3/32 20130101 |
International
Class: |
H02J 3/38 20060101
H02J003/38; H02M 3/335 20060101 H02M003/335; H02J 7/35 20060101
H02J007/35; H02J 7/00 20060101 H02J007/00 |
Claims
1. A power system for capturing low voltage energy from a power
source, the power system comprising: the power source coupled to a
DC bus; a DC/DC power converter coupled to the DC bus and an energy
storage device; a power inverter coupled to the DC bus and an AC
bus; and a control system, the control system comprising one or
more controllers configured to: monitor a voltage produced by the
power source; determine whether the power source is producing a
voltage greater than a first predetermined threshold; determine
whether the power source is producing a voltage less than a second
threshold when it is determined that the power source is producing
a voltage greater than the first predetermined threshold; when it
is determined that the power source is producing a voltage between
the first threshold and the second threshold: control the DC/DC
power converter to operate in an MPPT mode and store energy
generated by the power source in the energy storage device; and
control the power inverter not to operate in an MPPT mode; and when
it is determined that the power source is producing a voltage
greater than or equal to the second threshold: control the power
inverter to operate in an MPPT mode and supply the energy generated
by the power source to a power grid through the AC bus; and control
the DC/DC power converter not to operate in an MPPT mode.
2. The power system of claim 1, wherein the first predetermined
threshold is equal to expected losses in the DC/DC power
converter.
3. The power system of claim 1, wherein the power inverter has a
wake up voltage being a voltage magnitude that a voltage at the DC
bus must reach for the power inverter to be operational, and the
second predetermined threshold is equal to the wake up voltage of
the power inverter.
4. The power system of claim 1, wherein the voltage produced by the
power source is monitored continuously, and the control system
continuously controls the DC/DC power converter and the power
inverter to transition between operating in an MPPT mode and not
operating in an MPPT mode.
5. The power system of claim 1, wherein in determining whether the
power source is producing a voltage less than the second
predetermined threshold, the control system is further configured
to monitor the voltage at the DC bus.
6. The power system of claim 5, further comprising a sensor sensing
the voltage at the DC bus and transmitting the sensed voltage to
the power system.
7. A power system for capturing clipped energy from a power source,
the power system comprising: the power source coupled to a DC bus;
a DC/DC power converter coupled to the DC bus and an energy storage
device; a power inverter coupled to the DC bus and an AC bus; and a
control system, the control system comprising one or more
controllers configured to: monitor an output power of the power
inverter; compare the output power of the power inverter to a
predetermined threshold; when the output power of the power
inverter is greater than the predetermined threshold, control the
DC/DC power converter to store output power of the power source
that exceeds the predetermined threshold in the energy storage.
8. The power system of claim 7, wherein the predetermined threshold
is a maximum power rating of the power inverter.
9. The power system of claim 7, wherein the output power of the
power inverter is monitored continuously, and the control system
continuously controls the DC/DC power converter and the power
inverter to transition between storing and not storing output power
of the power source in the energy storage.
10. A power system for selectively dispatching energy from a power
source, the power system comprising: the power source coupled to a
DC bus; a DC/DC power converter coupled to the DC bus and an energy
storage device; a power inverter coupled to the DC bus and an AC
bus; and a control system, the control system comprising one or
more controllers configured to: monitor parameters external to the
power system; and selectively control the DC/DC power converter to
store power generated by the power source in the energy storage in
accordance with the monitored parameters.
11. The power system of claim 10, wherein the parameters external
to the power system comprise: a PV energy pricing signal for energy
supplied to a power grid through the AC bus; and a curtailment
signal for ceasing or reducing an amount of energy supplied to the
power grid.
12. The power system of claim 11, wherein the DC/DC power converter
stores power generated by the power source in the energy storage
when a price in the PV energy pricing signal is below a
predetermined threshold.
13. The power system of claim 12, wherein the DC/DC power converter
supplies energy stored in the energy storage to the power grid
through the power inverter when a price in the PV energy pricing
signal is equal to or greater than the predetermined threshold.
14. The power system of claim 10, wherein the parameters external
to the power system are monitored continuously, and the control
system continuously controls the DC/DC power converter and the
power inverter to transition between storing and not storing output
power of the power source in the energy storage.
15. A power system for controlling a ramp rate, the power system
comprising: a power source coupled to a DC bus; a DC/DC power
converter coupled to the DC bus and an energy storage device; a
power inverter coupled to the DC bus and an AC bus; and a control
system, the control system comprising one or more controllers
configured to: monitor an output power of the power inverter and a
rate of change of the output power of the power inverter; compare
the rate of change of the output power of the power inverter with a
pre-defined ramp rate; and control the DC/DC converter to charge or
discharge the energy storage when the rate of change of the output
power of the power inverter differs from the pre-defined ramp rate
by more than a predetermined amount.
16. The power system of claim 15, wherein the output power of the
power inverter and a rate of change of the output power of the
power inverter is monitored continuously, and the control system
continuously controls the DC/DC power converter to charge or
discharge the energy storage until the rate of change of the output
power of the power inverter no longer differs from the pre-defined
ramp rate by more than the predetermined amount.
17. The power system of claim 15, wherein the DC/DC power converter
supplies power to the energy storage when the rate of change of the
output power of the power inverter is greater than the pre-defined
ramp rate by more than the predetermined amount.
18. The power system of claim 15, wherein the DC/DC power converter
discharges power from the energy storage to a power grid through
the power inverter when the rate of change of the output power of
the power inverter is less than the pre-defined ramp rate by more
than the predetermined amount.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to an energy storage system
and method for capturing photovoltaic (PV) energy in energy
storage.
[0002] Electric power conversion devices and associated control
systems may be used to interface various energy resources. For
example, a power system can include a variety of interconnected
distributed energy resources (e.g., power generators and energy
storage units) and loads. The power system may also connect to a
utility grid or a microgrid system. The power system employs the
electric power conversion to convert power between these energy
resources (e.g., AC/DC, DC/DC, AC/AC and DC/AC).
[0003] Power systems may be designed to supply power, regulate
power, and transfer power from one source to another with the goal
of providing continuous power to a load. It is desirable to provide
power in the most efficient manner possible, so that the maximum
possible amount of energy generation is used. However, topology
limitations and design requirements can be limitations the energy
generation that is ultimately used. Conventional PV installations
under-utilize power generated by a PV array by failing to capture
low voltage energy generated by a PV array when the PV array
voltage is lower than the wake up voltage of an inverter, failing
to capture "clipped" energy, and by failing to supply energy to the
grid in consideration of curtailment or energy price.
BRIEF SUMMARY OF THE INVENTION
[0004] Embodiments of the present invention include apparatus and
methods for utilizing power generated by a PV array.
[0005] In one aspect, a power system for capturing low voltage
energy from a power source includes: the power source coupled to a
DC bus; a DC/DC power converter coupled to the DC bus and an energy
storage device; a power inverter coupled to the DC bus and an AC
bus; and a control system. The control system may include one or
more controllers configured to: monitor a voltage produced by the
power source; determine whether the power source is producing a
voltage greater than a first predetermined threshold; determine
whether the power source is producing a voltage less than a second
threshold when it is determined that the power source is producing
a voltage greater than the first predetermined threshold; when it
is determined that the power source is producing a voltage between
the first threshold and the second threshold: control the DC/DC
power converter to operate in an MPPT mode and store energy
generated by the power source in the energy storage device; and
control the power inverter not to operate in an MPPT mode; and when
it is determined that the power source is producing a voltage
greater than or equal to the second threshold: control the power
inverter to operate in an MPPT mode and supply the energy generated
by the power source to a power grid through the AC bus; and control
the DC/DC power converter not to operate in an MPPT mode.
[0006] The first predetermined threshold may be equal to expected
losses in the DC/DC power converter.
[0007] The power inverter have a wake up voltage being a voltage
magnitude that a voltage at the DC bus must reach for the power
inverter to be operational, and the second predetermined threshold
is equal to the wake up voltage of the power inverter.
[0008] The voltage produced by the power source may be monitored
continuously, and the control system may continuously control the
DC/DC power converter and the power inverter to transition between
operating in an MPPT mode and not operating in an MPPT mode.
[0009] In determining whether the power source is producing a
voltage less than the second predetermined threshold, the control
system may be further configured to monitor the voltage at the DC
bus.
[0010] A sensor sensing the voltage at the DC bus and transmitting
the sensed voltage to the power system may also be included.
[0011] In another aspect, a power system for capturing clipped
energy from a power source may include the power source coupled to
a DC bus; a DC/DC power converter coupled to the DC bus and an
energy storage device; a power inverter coupled to the DC bus and
an AC bus; and a control system. The control system may include one
or more controllers configured to: monitor an output power of the
power inverter; compare the output power of the power inverter to a
predetermined threshold; when the output power of the power
inverter is greater than the predetermined threshold, control the
DC/DC power converter to store output power of the power source
that exceeds the predetermined threshold in the energy storage.
[0012] The predetermined threshold may be a maximum power rating of
the power inverter.
[0013] The output power of the power inverter may be monitored
continuously, and the control system may continuously control the
DC/DC power converter and the power inverter to transition between
storing and not storing output power of the power source in the
energy storage.
[0014] In an aspect, a power system for selectively dispatching
energy from a power source may include: the power source coupled to
a DC bus; a DC/DC power converter coupled to the DC bus and an
energy storage device; a power inverter coupled to the DC bus and
an AC bus; and a control system. The control system may include one
or more controllers configured to: monitor parameters external to
the power system; and selectively control the DC/DC power converter
to store power generated by the power source in the energy storage
in accordance with the monitored parameters.
[0015] The parameters external to the power system may include a PV
energy pricing signal for energy supplied to a power grid through
the AC bus; and a curtailment signal for ceasing or reducing an
amount of energy supplied to the power grid.
[0016] The DC/DC power converter may store power generated by the
power source in the energy storage when a price in the PV energy
pricing signal is below a predetermined threshold.
[0017] The DC/DC power converter may supply energy stored in the
energy storage to the power grid through the power inverter when a
price in the PV energy pricing signal is equal to or greater than
the predetermined threshold.
[0018] The parameters external to the power system may be monitored
continuously, and the control system may continuously control the
DC/DC power converter and the power inverter to transition between
storing and not storing output power of the power source in the
energy storage.
[0019] In an aspect, a power system for controlling a ramp rate may
include: a power source coupled to a DC bus; a DC/DC power
converter coupled to the DC bus and an energy storage device; a
power inverter coupled to the DC bus and an AC bus; and a control
system. The control system may include one or more controllers
configured to monitor an output power of the power inverter and a
rate of change of the output power of the power inverter; compare
the rate of change of the output power of the power inverter with a
pre-defined ramp rate; and control the DC/DC converter to charge or
discharge the energy storage when the rate of change of the output
power of the power inverter differs from the pre-defined ramp rate
by more than a predetermined amount.
[0020] The output power of the power inverter and a rate of change
of the output power of the power inverter may be monitored
continuously, and the control system may continuously control the
DC/DC power converter to charge or discharge the energy storage
until the rate of change of the output power of the power inverter
no longer differs from the pre-defined ramp rate by more than the
predetermined amount.
[0021] The DC/DC power converter may supply power to the energy
storage when the rate of change of the output power of the power
inverter is greater than the pre-defined ramp rate by more than the
predetermined amount.
[0022] The DC/DC power converter may discharge power from the
energy storage to a power grid through the power inverter when the
rate of change of the output power of the power inverter is less
than the pre-defined ramp rate by more than the predetermined
amount.
BRIEF DESCRIPTION OF THE FIGURES (NON-LIMITING EMBODIMENTS OF THE
DISCLOSURE)
[0023] FIG. 1 shows a power system employing an energy storage
system for photovoltaic energy according to an embodiment of the
present invention.
[0024] FIG. 2 illustrates solar array DC voltage and current from
the solar array over the course of a photovoltaic (PV) inverter
operation.
[0025] FIG. 3 illustrates capture of energy potentially lost during
inverter clipping.
[0026] FIG. 4 is a schematic diagram of an exemplary DC/DC
converter according to an embodiment of the present invention.
[0027] FIG. 5 is a control structure for a DC/DC converter
according to an embodiment of the present invention.
[0028] FIG. 6 is a flowchart illustrating a low voltage energy
capture method implemented by an energy storage system according to
an embodiment of the present invention.
[0029] FIG. 7 is a flowchart illustrating an inverter clipping
capture method implemented by an energy storage system according to
an embodiment of the present invention.
[0030] FIG. 8 is a flowchart illustrating a method for providing
dispatchable PV power implemented by an energy storage system
according to an embodiment of the present invention.
[0031] FIG. 9 is a flowchart illustrating a ramp rate control
method implemented by an energy storage system according to an
embodiment of the present invention.
DETAILED DESCRIPTION
[0032] Reference will now be made to the accompanying drawings,
which form a part hereof, and which show, by way of illustration,
specific exemplary embodiments. The principles described herein
may, however, be embodied in many different forms. The components
in the figures are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention. Moreover,
in the figures, like referenced numerals may be placed to designate
corresponding parts throughout the different views.
[0033] In the following description of the invention, certain
terminology is used for the purpose of reference only, and is not
intended to be limiting. For example, although the terms first,
second, etc. may be used herein to describe various elements, these
elements should not be limited by these terms. These terms are only
used to distinguish one element from another. As used in the
description of the invention and the appended claims, the singular
forms "a," "an," and "the" are intended to include the plural forms
as well, unless the context clearly indicates otherwise. It will
also be understood that the term "and/or" as used herein refers to
and encompasses any and all possible combinations of one or more of
the associated listed terms. It will be further understood that the
terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps operations, elements, components, and/or groups thereof.
[0034] Embodiments of the present invention include systems and
methods for capturing additional energy from solar PV installations
that typically goes to waste. Embodiments of the present invention
include interfacing storage with PV power generation for capturing
low voltage energy of a PV array. Other embodiments of the present
invention include interfacing storage with PV power generation for
capturing energy losses from inverter clipping. Other embodiments
of the present invention include interfacing storage with PV power
generation for providing dispatchable PV power. Other embodiments
of the present invention include interfacing storage with PV power
generation for providing ramp rate control.
[0035] Referring to FIG. 1, a PV plus storage generation system 100
includes a PV array 2, a PV inverter 31, energy storage 11, a DC/DC
converter 3, a controller 110, a DC bus 130 and an AC bus 120 that
may be connected to a utility grid, local loads, and/or a
microgrid.
[0036] In an embodiment, the control system 110 for the PV plus
storage generation system 100 may include a controller 110 that
coordinates the operation of the converter 3 and the inverter 31.
In another embodiment, the control system 110 for the PV plus
storage generation system 100 may include separate controllers for
each of the DC/DC converter 3 and the PV inverter 31. In the case
in which there are separate controllers for the DC/DC converter 3
and the PV inverter 31, the control system 110 may include a master
controller which coordinates with the controllers of the DC/DC
converter 3 and the PV inverter 31.
[0037] The PV inverter 31 is connected to an AC bus 120 on the AC
side of the inverter. The AC bus 120 is capable of being coupled to
a utility grid, microgrid, loads, and/or other AC connections.
Preferably, the DC side of the inverter 31 is connected to both the
DC/DC converter 3 and the PV array 2. For brevity, the array is
shown as a single connection, but it should be understood that in
embodiments of the present invention it is possible that panels are
connected in strings with the strings being connected in a
recombiner box prior to the inverter. Furthermore, in an
embodiment, the PV inverter 31 may be capable of more than one
Maximum Power Point Tracking (MPPT) inputs in which case multiple
converters 3 may be employed.
[0038] Preferably, the DC/DC converter 3 is connected to the DC
input of the PV inverter 31 and also to the energy storage 11.
Energy storage may include, for example, a battery, a battery bank,
etc.
[0039] In an embodiment, the PV inverter 31 may, e.g., be of a
central or string type.
[0040] Preferably, the battery 11, DC/DC converter 3, control
system 110, and PV inverter 31 are collocated within close
proximity of one another to minimize costs by reducing cable
lengths; and are located in a position to minimize any shading of
the solar panels such as the north side of the array. However, it
should be understood that the present invention is not limited as
such. Furthermore, embodiments of the present invention including
the storage 11, DC/DC converter 3, and controller 110 may be
installed with new construction or retrofitted to an existing solar
PV installation.
[0041] In embodiments of the present invention, the control system
110 can be connected to the DC/DC converter 3, energy storage 11,
and PV inverter 31 through a means of communication such as Modbus
TCP over copper or fiber, or wirelessly through short range
wireless communication, wireless local area networking, etc.
Additional communications connections may be made to any of the
assets of the power system by the owner, operator, or a third party
data collection service to monitor the operation and performance of
the system. These remote connections may be made, e.g., via
cellular, satellite, hardwired connection, etc.
[0042] FIGS. 4 and 5 show an exemplary bidirectional DC/DC
converter topology and control structure that could be used as the
bidirectional DC/DC converter 3 shown in FIG. 1. It should be
understood the DC/DC converter 3 is not limited to that shown in
FIGS. 4 and 5, and could be another DC/DC converter topology so
long as the converter is capable of bidirectional power flow. The
DC/DC converter of FIGS. 4 and 5 is described in detail in U.S.
application Ser. No. 15/895,565, which is incorporated by reference
in its entirety.
[0043] Referring to FIG. 4, a DC/DC converter 400 according to an
embodiment of the present invention may include a first conversion
stage 410 and a second conversion stage 420 connected to each
other. The first and second conversion stages 410, 420 form a
bi-directional DC/DC converter (i.e., the power flow is
bidirectional). The magnitude of the voltage on the first
converting stage 410 can be higher or lower than or roughly equal
to the magnitude of the voltage on the second converting stage.
Thus, either side of the DC/DC converter 400 can be used as a buck
or a boost converter.
[0044] In an embodiment, the first conversion stage 410 is
operative to convert the input/output voltage corresponding to the
battery to a desired magnitude at the input/output corresponding to
the PV array when the magnitude of the voltage of the input/output
corresponding to the battery is higher than the magnitude of the
voltage at the input/output corresponding to the voltage over the
PV array. The second conversion stage 420 is operative to convert
the input/output voltage corresponding to the PV array to a desired
magnitude at the input/output corresponding to the battery when the
magnitude of the voltage of the input/output corresponding to the
PV array is greater than the magnitude of the voltage at
input/output corresponding to the battery.
[0045] In an embodiment, the DC/DC converter 400 comprises a
cascaded connection of series H-bridges. The first conversion stage
410 comprises a first half bridge 412 and a second half bridge 414
connected in series. Each of the first half bridge 412 and the
second half bridge 414 may comprise a pair of switches Q1, Q2 and
Q3, Q4. The second converting stage 420 comprises a third half
bridge 422 and a fourth half bridge 424 that are connected in
series. Each of the third half bridge 422 and the fourth half
bridge 424 may comprise a pair of switches Q5, Q6 and Q7, Q8,
respectively.
[0046] In an embodiment, the first conversion stage 410 and the
second conversion stage 420 are interfaced using inductors L1 and
L2. In another embodiment, the first and second inductors L1 and L2
may be replaced by an isolation transformer T1 as shown in FIG.
5.
[0047] In the embodiment in which the first and second conversion
stages 410 and 420 are interfaced by the inductors L1 and L2, the
DC/DC converter 400 may further include an optional center point
connection. The center-point connection 450 may be advantageous,
for example, in a scenario in which the input/output is connected
to energy storage (e.g., battery/batteries) in that the noise on
the battery terminals is reduced by the neutral center-point
connection 450. However, there is a design trade-off to the
center-point connection 450 in that the ripple performance (i.e.
ripple current and voltage on the battery and PV ports) is
compromised to some extent.
[0048] In an embodiment, each of the half bridges 412, 414, 422,
424 may be close coupled to a DC bus capacitor C1-C4 for filtering
and semiconductor voltage overshoot reduction. For example,
capacitor C1 may be a filter capacitor for the half-bridge formed
by Q1 and Q2. Each of these capacitors C1-C4 may be an individual
capacitor or may be a series and parallel combination of several
discrete capacitors to reach the appropriate rating.
[0049] In an embodiment, switches Q1-Q8 are semiconductor switches
with back-body diodes. Examples of semiconductor switches that may
be used for Q1-Q8 include, but are not limited to, IGBT, MOSFETs,
etc.
[0050] FIG. 5 shows a control structure for a DC/DC converter
according to an embodiment of the present invention.
[0051] Referring to FIG. 5, the control structure 600 includes an
outer control loop 610 and an inner control loop 620. The outer
control loop 610 controls one of the interface inductor currents
(e.g. Im1), and the inner control loop 620 controls the magnitude
of the battery/PV current or the magnitude of the battery/PV
voltage.
[0052] In the embodiment shown in FIG. 5, the controller parameters
(e.g., the two PI parameters) may be tuned to adapt to hardware
parameters. The tuning depends on a few factors, for example: 1)
Speed of response required--the control bandwidth of the
system--e.g., whether it is desirable for the converter to reach
rated current in 1 ms or 100 ms; and 2) the hardware parameters of
the system--e.g., inductance, capacitance and switching frequency
values.
[0053] The outer control loop 610 receives as inputs a command of
battery current or PV voltage and feedback of battery current or PV
voltage. The command of battery current or PV voltage and feedback
of battery current or PV voltage may be the desired magnitude of
battery current or desired magnitude of PV voltage. The feedback of
battery current or PV voltage is the actual magnitude of the
battery current or actual magnitude of the PV voltage. The desired
magnitude is then compared to the actual magnitude by, for example,
taking the difference between the desired magnitude and actual
magnitude. This difference is inputted into a controller 612 for
controlling one of the interface inductor currents over one of the
inductors. The controller 612 then outputs the current command
Im_cmd for the interface inductor current to the inner control loop
620. Here, the current command Im_cmd may be a desired magnitude
for the interface inductor current that is compared to the actual
magnitude of the interface inductor current.
[0054] In the embodiment shown in FIG. 5, the controllers 612 and
622 are proportional-integral (PI) controllers. However, it should
be understood that these controllers are not limited to PI
controllers, and in fact, the controllers may be any closed loop
controller including, e.g., a proportional-integral-derivative
(PID) controller and a proportional (P) controller.
[0055] The inner control loop 620 receives as inputs the inductor
current command Im_cmd and the actual magnitude of the inductor
current Im1. The inductor current command Im_cmd is then compared
to the inductor current Im1 by, for example, taking the difference
between the inductor current command Im_cmd and the inductor
current Im1. This difference is then inputted into a controller 622
for calculating the duty value of the switching signals that are
input to switches Q1-Q8. Controller 322 outputs the duty value of
the switching signals to the DC/DC converter. The duty value
affects the duty cycle of the signals to the switches, which
affects the magnitude of the step up/step down of the DC/DC
converter 400. The duty ratio depends on the ratio of the voltages
on either side of the DC/DC converter 400.
[0056] The control structure 600 may be embodied on a controller
such as a digital signal processor (DSP), field programmable gate
array (FPGA), etc. However, is should be understood the controller
is not limited to these, and can be any type of processor. In
addition, the control structure 600 may be embodied on a single
controller or a plurality of controllers (e.g., a different
controller for the outer and inner loop).
[0057] As noted above, the DC/DC converter 3 is not limited to this
particular configuration, and may be any DC/DC converter capable of
bidirectional power flow.
[0058] Low Voltage Energy
[0059] FIG. 2 illustrates solar array DC voltage and current from
PV array 2 over the course of PV inverter 31 operation. FIG. 2 is
provided to aid in the explanation of an embodiment of the present
invention in which the PV plus storage generation system 100
implements a DC/DC converter 3 in order to store low voltage energy
that is below a certain threshold (i.e., a `wake up` voltage).
[0060] With a traditional PV inverter topology the PV inverter must
wait for a minimum DC voltage to be generated by the solar field
(e.g., solar array 2) in order to start producing power. This may
be referred to as the `wake up` voltage. In the embodiment shown
with reference to FIGS. 1 and 2, the addition of the DC/DC
converter allows the system to extract energy from the PV array
when the PV array voltage is lower than the inverter's wake up
voltage and the inverter is not operating (i.e., where the PV array
2, DC/DC converter 3, and the DC side of the inverter 31 are
connected).
[0061] FIG. 2 illustrates a typical PV inverter operation, with the
black trend (i.e., the top trend) being the solar array DC voltage
and the grey trend (i.e., the bottom trend) being the current from
the solar array. Topology limitations will limit a typical PV
inverter from trying to convert energy from the solar arrays to
grid energy until the PV array reaches the wake up voltage. When
referring to FIG. 2, it can be noted that the inverter is not able
to produce power from the array until the array voltage reaches the
wake up voltage, in this case roughly 700 VDC. Accordingly, from
the point at which sunlight is incident on the solar panels of the
solar array 2 to the point at which the array reaches the wake up
voltage, there is energy available from the panels. Traditional
implementations are unable to capture energy/power available below
the threshold of the wake-up voltage.
[0062] In an embodiment, for low voltage capture the DC/DC
converter 3 operates with a maximum power point tracking mode and
stores the PV generated energy into the energy storage 11. The
captured energy may then be used in a variety of ways. For example,
the low voltage captured energy may be discharged to the grid 120
via inverter 31 at a later time or may be used at a later time to
power local loads.
[0063] The control system 110 controls the operation of the DC/DC
converter 3 and PV inverter 31 so that the system 100 captures the
low voltage energy. For example, as shown in FIG. 6, in an
embodiment, at startup, the control system:
[0064] 210: Monitor the voltage produced by the PV array.
[0065] 220: Determines whether the PV array 2 is producing a
voltage greater than a first predetermined threshold. According to
an embodiment, the first predetermined threshold is set to be equal
to the expected losses in the DC/DC converter 3. The control system
110 determines whether the PV array 2 has enough available power by
using the voltage sensed on DC bus 130 and optional solar
irradiance sensors. This is done to ensure that the power available
in the PV array 2 is more than what would be lost in the DC-DC
converter 3 when it is operating. If the DC-DC converter 3 loses
more power than what it available in the PV array 2 during low
voltage operation, then the energy storage 11 may end up
discharging.
[0066] 230: When the control system 110 determines that the PV
array 2 is producing a voltage greater than the first predetermined
threshold, the control system 110 then determines whether the PV
array 2 is producing a voltage that is less than a second
predetermined threshold. In an embodiment, this second
predetermined threshold for voltage is set to be equal to the wake
up voltage of the PV inverter 31. The control system 110 determines
whether the PV array 2 is producing a voltage that is less than the
second predetermined threshold by monitoring the voltage on DC bus
130 to determine whether DC bus 130 voltage is less than the wake
up voltage. Such monitoring may take place through the use of
sensors that sense the magnitude of voltage on the DC bus 130.
[0067] 240: When the control system 110 determines that the PV
array 2 has available power that is greater than a first
predetermined threshold and is producing a voltage that is less
than a second predetermined threshold, the control system 110
controls the DC/DC converter 3 to operate with an MPPT mode and
stores the PV generated energy into the energy storage 11, and the
control system 110 controls the PV inverter 31 not to operate with
an MPPT mode.
[0068] While control system 110 controls DC/DC converter 3 to
operate with an MPPT mode, the control system 110 continues to
monitor the PV array voltage to determine whether the PV array
voltage has reached the second predetermined threshold (e.g., the
wake up voltage).
[0069] 250: When it is determined that the PV array voltage has
reached the second predetermined threshold, the control system 110
controls the PV inverter 31 to operate with an MPPT mode so that
energy produced by the PV array is provided to the grid 120. When
the controller 110 puts the inverter 31 into MPPT mode, the control
system 110 stops MPPT mode for the DC/DC converter 3.
[0070] Once the PV array voltage has reached or surpasses the
second predetermined threshold, the control system 110 continues to
monitor the PV array voltage to determine whether its magnitude
falls below the second predetermined threshold. This may occur when
clouds, dust, or other objects interfere with the sunlight incident
on the PV array 2, or when the sun begins to set. When the PV array
voltage falls below the second predetermined threshold, the control
system 110 again controls the DC/DC converter 3 to operate with an
MPPT mode so that energy is stored in energy storage 11, and stops
MPPT mode for the PV inverter 31.
[0071] Once the PV array voltage falls below the second
predetermined threshold, the control system 110 continues to
monitor the PV array voltage to determine whether its magnitude
again reaches the second predetermined threshold, at which point
the control system will again control the PV inverter 31 to operate
with an MPPT mode so that energy produced by the PV array 2 is
provided to the grid 120 and stops MPPT mode for the DC/DC
converter 3.
[0072] Although the above method is described for a case in which
the DC/DC converter 3 is connected to energy storage 11, it should
be understood that the present invention is not limited to this
specific case. For example, in another embodiment, a similar
control method is applied by the control system 110 to a DC/DC
converter 3 having one side coupled to a PV array 2 and the other
side coupled to the PV inverter 31. In this case, the DC/DC
converter 3 is not used to store energy, but rather, the DC/DC
converter 3 boosts the voltage to exceed the wake up voltage of the
PV inverter 31 in low voltage array PV output situations. Thus,
when the control system determines that the PV voltage is less than
the second threshold, the control system controls the DC/DC
converter 3 to boost the voltage above the wake up voltage of the
PV inverter 31.
[0073] Inverter Clipping Capture
[0074] FIG. 3 illustrates capture of energy potentially lost during
inverter clipping.
[0075] Inverter loading ratio (ILR) is defined as the ratio of
installed DC PV power to AC inverter (e.g., inverter 31) rating. An
ILR of 1 produces a continuous parabola when graphing the power
output of the solar system over the course of the day--assuming
ideal irradiance free of cloud cover and other variations. The
higher the ILR, the quicker the system will reach its output power
rating. For example, an ILR of 1 will have a slower ramp up to the
inverter maximum output power rating as compared to a larger ILR.
In contrast, a high ILR will produce a steeper ramp and quicker
time to reach the inverter maximum output power rating.
[0076] In order to maximize energy production from solar PV
installations, an ILR greater than 1 may be deployed, with ILR
values of 1.2 to 1.3 being common and ILR of greater than 2 not
uncommon. However, when employing the higher ILR values, while the
power output will reach inverter rating more quickly, inverter
clipping occurs. In the example shown in FIG. 3, there are
approximately 1.3 MW of PV panels and a 1 MW PV inverter (ILR=1.3).
This configuration limits the PV output power to 1 MW and will
harvest the energy of the dark grey shaded area. However, this
configuration is unable to capture all the energy available above 1
MW shown as the light grey area. In the embodiment shown in FIG. 1,
the converter and control system stores the `clipped` energy into
the energy storage 11, which can then be dispatched at a later
time.
[0077] The control system 110 controls the operation of the DC/DC
converter 3 and PV inverter 31 so that the system 100 captures the
clipped energy. For example, as shown in FIG. 7, in an embodiment,
the control system:
[0078] 310: Monitors the output power of the PV inverter 31. In an
embodiment, the control system 110 may monitor the voltage on the
AC bus 120. Such monitoring may take place through the use of
sensors that sense the magnitude of voltage and current output by
the PV inverter 31. Such a sensor may, for example, be placed at
the output of the PV inverter 31 or within a case of the PV
inverter. In an embodiment, the sensor may be incorporated into the
PV inverter.
[0079] 320: Determine whether the PV array power has reached a
predetermined threshold. In an embodiment, the control system 110
has stored therein the PV inverter 31 rating, and sets the PV
inverter 31 rating as the predetermined threshold. For example, if
there is a 1 MW solar inverter and 1.5 MW of solar panels, the
control system monitors the magnitude of the output power of the
solar inverter 31, and once the solar inverter becomes power
limited at 1 MW, the control system 110 controls the DC/DC
converter 3 to store any available excess power into the energy
storage 11.
[0080] 330: After the output power exceeds the predetermined
threshold, the control system 110 continues to monitor the output
power of PV inverter 31 to determine whether the output power falls
below the predetermined threshold, after which there is no longer
excess power to be stored.
[0081] Dispatchable PV
[0082] In an embodiment, the control system 110 stores energy
produced by the PV array 2 in the energy storage 11 so that it can
be dispatched a later time. The energy can then be used when the
solar installation is not curtailed or when the offtake (e.g.,
power company, large industrial facility, town, etc.) will pay a
premium for energy.
[0083] For example, in an embodiment, as shown in FIG. 8, the
control system 110:
[0084] 710: Monitors the grid parameters and energy pricing to
determine whether it is beneficial to charge the energy storage 11
using PV energy instead of sending PV energy to the grid. For
example, the control system 110 may receive pricing signal for
energy supplied to the grid, or the control system 110 may receive
a signal to reduce or stop supplying solar generation to the grid
from a utility or other entity.
[0085] 720: Determines that solar generated energy should not be
supplied to the grid, the control system 110 controls the DC/DC
converter 3 to store power from the PV array 2 in the energy
storage 11. The control system 110 may then determine that
curtailment ends by, for example a predetermined amount of time
passing or by receiving a signal from the entity (e.g., the
utility) or that energy price has increased that makes supplying
power to the grid more profitable.
[0086] 730: Once curtailment ends, or energy price increases, the
control system 110 may control the PV inverter 31 to provide power
to the grid 120, which may include local loads, the utility, large
industrial facility, town, etc.
[0087] This embodiment is advantageous in that if the solar array 2
at a solar installation is curtailed (even if curtailment were as
long as a day), instead of total loss, as much energy as possible
is stored in the energy storage. Then at a later point (e.g.,
nighttime) when the solar installation is offline because there is
no sunlight, the installation is able to discharge the energy
storage 11 to the grid.
[0088] Ramp Rate Control
[0089] PV power production is dependent upon sunshine, and thus, PV
power production can fluctuate with the passing of clouds or other
shading events. When these shading events occur down-ramping
happens. When the sunlight returns up-ramping happens. If there is
sharp up-ramping or down-ramping, damage may be done to the power
system or other systems that are connected to the power system
(e.g., a high ramp rate could cause over/under frequency events
which would cause system failures). For example, if substantial
cloud coverage comes while a solar farm is at full power, the
output power from the solar farm may go from at or near maximum
power to a very low value, and the grid and loads are not well
equipped to handle a very fast rate of change of power. In an
embodiment, the control system 110 and DC/DC converter 3 mitigate
both up-ramping and down-ramping events caused by shading by
partially charging during up-ramping events and partially
discharging during down-ramping events to maintain a pre-defined
ramp rate (rate of change of power with respect to time).
[0090] The control system 110 controls the operation of the DC/DC
converter 3 and the PV inverter 31 so that the system 100 operates
in ramp control to maintain a pre-defined ramp rate. For example,
in an embodiment, as shown in FIG. 9, the control system 110 is
configured to:
[0091] 510: When ramp control is initiated, the control system 110
monitors the output power of the PV inverter 21 to the grid. Such
monitoring may take place through the use of sensors that sense the
magnitude of voltage output by the PV inverter 31.
[0092] 520: The control system 110 determines whether the rate of
change of the power differs from a pre-defined ramp rate by a set
amount.
[0093] 530: when it is determined that the rate differs by the set
amount, the control system 110 controls the DC/DC converter 3 to
discharge or charge the energy storage 11 to slow the ramp-up or
ramp-down (e.g., supplement the lost solar production to slow down
the ramp rate of the output power).
[0094] In embodiments of the present invention, the DC/DC converter
3 facilitates capture of low voltage energy of a PV array 2,
capture of energy lost to inverter clipping, dispatchable PV and
ramp rate control. In embodiments, the DC/DC converter 3 will be
used between energy storage 11 and a PV array 2. The PV array 2 may
have an inverter connected with the utility AC grid. Therefore, the
power flow of the converter should be bidirectional (batteries
charging from PV, batteries discharging to grid via PV inverter).
The battery (energy storage) voltage could be either higher or
lower than or be roughly equal to the PV voltage with both
directions of power flow. So, either side of the converter could be
used as buck or boost.
[0095] In embodiments, the DC/DC converter 3 may also be used to
interface in parallel multiple batteries of different chemistries
to a single inverter, or to facilitate current sharing of batteries
when new batteries are added to upgrade the capacity of an existing
battery installation.
[0096] This system could also be used in microgrids where there is
no utility connection.
[0097] This system could also be used to service DC loads without
the need for an AC inverter.
[0098] Embodiments of the present invention make it possible to
capture additional energy from solar PV installations improving the
owner's return on investment (ROI). Additionally, embodiments of
the present invention make it possible to time shift the dispatch
of the solar PV energy production to address peaks and to dispatch
energy based on Time of Day (TOD) rates.
[0099] Embodiments of the present invention allow a user to
evaluate the production of a PV system based upon historic data or
some simulation software (e.g. PVSyst) to determine the energy lost
to inverter clipping or during low voltage array times and
calculate a revised ROI once the storage and converter are
added.
[0100] Although in certain exemplary embodiments discussed above,
the DC/DC converter 400 is described as being coupled between
energy storage and a PV array/inverter, it should be understood the
present invention is not limited to this application. It will be
readily understood to a person of ordinary skill in the art that
embodiments of the present invention are suitable for additional
applications, such as applications where DC/DC conversion is
required with overlapping voltages on the first and second
input/output sides. Additional examples include back up power in
variable frequency drive (VFD) applications. The DC/DC converter
may be interfaced with a VFD's DC bus. When the grid voltage is
present, the DC bus voltage is established by the grid and the VFD
is feeding the motor. When the grid goes away (e.g., a power
outage), the DC/DC converter can hold up the DC bus by discharging
the batteries into the VFD, allowing the VFD to run without
interruption.
[0101] It will be apparent to those skilled in the art that various
modifications and variations can be made to the disclosed power
system without departing from the scope of this disclosure. Other
embodiments of the present disclosure will be apparent to those
skilled in the art from consideration of the specification and
practice of the present disclosure. It is intended that the
specification and examples be considered as exemplary only, with a
true scope of the present disclosure being indicated by the
following claims and their equivalents.
* * * * *