Energy storage management system

Abstract

Disclosed herein is a method for managing an energy storage system (ESS) having one or more ultracapacitors, the method comprising: obtaining a desired power transfer between the ESS and a power grid; and determining a reference voltage for providing a desired power transfer between the ESS and the power grid based on a state of health SoH of the ESS. Further disclosed herein is a non-transitory computer-readable medium configured to perform such a method, an energy storage management system (ESMS) including the non-transitory computer-readable medium, and a power system including the ESMS.

Description

Technical Field

[0001] The present disclosure relates to a method for managing an energy storage system (ESS), an energy storage management system (ESMS) for implementing the method in the ESS, and a power system including the ESS having such an ESMS included therein. Background Art

[0002] Energy storage systems (ESS) are systems used to store large amounts of electrical energy. This energy may be generated inconsistently by power generation devices, and ESSs conveniently provide a way to store generated energy for later use on demand, even when the power generation devices themselves are off-grid. Consequently, ESSs can form a vital part of renewable energy distribution networks, including, for example, solar or wind power generation.

[0003] ESS can be integrated with converters to form an enhanced power grid system. An example of such a system is the enhanced static synchronous compensator (E-STATCOM), which integrates a converter system (e.g., a full-bridge-based modular multilevel converter (MMC)) with an energy storage system (ESS) based on supercapacitors.

[0004] An E-STATCOM is capable of supplying active and / or reactive power to the power grid. However, ESSs contain a finite amount of electrical energy, and therefore it is conventional practice for such systems, and in particular the ESMS therein, to track the ESS's SoC and operate accordingly (i.e., regulate the power transfer between the ESS and the power grid).

[0005] It would be desirable to provide improved methods for managing an ESS because such improvements could result in slower degradation of components of the ESS and thus increase the operational life of the ESS, and accordingly any E-STATCOM system in which the ESS is installed. Summary of the Invention

[0006] As part of this disclosure, it is recognized that the power and energy capabilities of an ESS can change over time due to component aging (e.g., reduced capacitance of a supercapacitor). Accordingly, aspects of this disclosure provide methods for managing an ESS that incorporate such varying capabilities into control operations, such as when the ESS is implemented in an E-STATCOM system. Aspects of this disclosure consider the ESS's System-of-System (SoC), System-of-Housing (SoH), current, power, and / or energy limitations, and provide an overall control strategy that advantageously improves the ESS's operational lifetime.

[0007] In particular, according to aspects of the present disclosure, a method for managing an ESS having one or more supercapacitors is provided. The method includes: obtaining a desired power transfer between the ESS and a power grid; and determining a reference voltage (i.e., a reference capacitor voltage) for providing the desired power transfer between the ESS and the power grid based on a state of health (SoH) of the ESS.

[0008] Then, the method may further include: initiating a desired power transfer between the ESS and the power grid using the determined reference voltage; and controlling the power transfer between the ESS and the power grid based on a desired state of charge SoC of the ESS using the controller.

[0009] According to another aspect of the present disclosure, the control method is implemented by an energy storage management system ESMS, which has a control unit implemented as or including a temporary or non-temporary computer-readable medium, etc., which computer-readable medium includes instructions that, when executed by a processor, cause the processor to implement the control method.

[0010] According to yet another aspect of the present disclosure, a power system is provided, the power system including: a converter coupled to a power grid; and an ESS operatively coupled to the converter and configured to transfer power to and / or from the power grid via the converter. The ESS includes the aforementioned ESMS configured to execute the control method described herein.

[0011] Those skilled in the art will appreciate that a STATCOM can inject power into and absorb power from a power grid. For example, if a frequency drop is detected in the power grid, an E-STATCOM can responsively supply power from an ESS to the power grid, thereby providing frequency support to the power grid.

[0012] Aspects of this disclosure primarily concern replenishing the energy stored within an ESS. That is, during normal operation of the E-STATCOM, the ESS is recharged using power from the power grid. However, as part of this disclosure, it is recognized that if the SoH of the ESS is not taken into account, excessive voltage levels during the discharge and recharge of the ESS (including one or more supercapacitors) can cause premature degradation.

[0013] Therefore, the life of the ESS can be extended by obtaining a desired power transfer between the ESS and the power grid based on the state of charge SoC of the ESS (i.e., indicating whether the SoC is below or above a set point) and determining a reference voltage for providing the desired power transfer between the ESS and the power grid based on the state of health SoH of the ESS.

[0014] In other words, the reference voltage across the ESS, used to power the ESS and thereby change its SoC, can be periodically adapted based on the SoH estimate, with the goal of minimizing the voltage across the ESS and thereby extending its lifetime. The reference voltage can be updated based on (i.e., in response to) SoH updates, and thus "automatically" updated with each SoH update.

[0015] The SoC of the ESS (such as the terminal voltage of the ESS) can be estimated using any suitable estimator, and the estimation can be performed online (i.e., during operation of the ESS) or offline (when the ESS is not operating). However, according to an advantageous refinement of the presently disclosed control method, the SoC is additionally or alternatively determined online based on an estimated (or measured) voltage across at least one of the one or more supercapacitors.

[0016] In some examples where the ESS includes multiple supercapacitors, a reasonable accuracy in estimating the SoC of the entire ESS may be achieved based on the voltage (and / or inferred SoC) of only one of the supercapacitors, since the SoCs of all supercapacitors can be expected to be balanced according to some balancing scheme.

[0017] The SoH of an ESS can also be estimated using any suitable estimator. For example, depending on the implementation, the SoH of the ESS can be determined based on the terminal voltage and / or current of the ESS. The SoH of the ESS may be affected by the temperature and / or voltage experienced by the capacitors of the ESS over their lifetime. Temperature fluctuations can be roughly estimated based on, for example, current values, or directly measured using a thermometer. However, it can be assumed that the temperature of the ultracapacitor will remain substantially constant over its lifetime, although it will be appreciated that non-constant temperatures will not substantially affect the resulting control.

[0018] Over its lifetime and as its SoH changes, the equivalent capacitance of an ESS also changes. Because the equivalent capacitance of an ESS changes over time due to aging, if the SoC regulator uses a fixed controller gain, the dynamic response of the ESS (i.e., during changes in the SoH, such as during recharging of the ESS) will change over time. It will be understood that a 'controller gain' is the controller gain used in a control function such as proportional-integral (PI) or proportional-integral-derivative (PID) control.

[0019] Therefore, according to an example improvement of the present disclosure, the control method further includes determining one or more gain factors based on the SoH of the ESS, the controller being configured to control the power transfer according to the one or more gain factors.

[0020] Since the control gain can be adapted during operation based on the SoH estimate (which takes into account changes in the ESS's equivalent capacitance), the dynamic response of the ESS can be maintained over its lifetime, even if the equivalent capacitance changes significantly due to SoH variations. This provides a more intuitive way to tune the parameters of the SoC control than directly selecting the controller parameters.

[0021] Optionally, the control method(s) according to aspects of the present disclosure may be suspended or disabled when the connected power grid leaves the defined frequency band. In other words, according to an optional refinement of the presently disclosed control method, if the power grid is determined to have a grid frequency outside the frequency band, power is not transferred between the ESS and the power grid. The determination that the power grid has a grid frequency outside the frequency band may be measured directly or indirectly, such as by using the output of a phase-locked loop controlling the converter reference signal, or in other ways.

[0022] Advantageously, this approach reduces the load on the grid when the frequency is not at its nominal value (e.g., 50 Hz). It can be assumed that the grid is susceptible when the frequency deviates significantly from the nominal frequency. Therefore, it is possible that recharging the ESS (i.e., transferring power between the ESS and the grid) will have a negative impact on the grid. Furthermore, it is possible that the grid will recover from low- or high-frequency episodes and return to nominal frequency, and the error between the reference and actual voltages may decrease again. If the ESMS is active during this period, there is a risk of overcharging (or discharging) the ESS, necessitating ESMS control again. Therefore, disabling the ESMS frequency band may result in reduced power exchange during frequency transients and ensure that power is exchanged for energy management purposes only during steady-state periods of the grid.

[0023] According to aspects of the present disclosure, a customizable reference voltage curve may be expressed as: .

[0024] Here, is the adaptable reference voltage (the minimum calculated reference voltage for SoH dependency), is the reference voltage at the end of the life of the ESS, and represents the voltage margin. In addition to the minimum voltage, a voltage margin can optionally be introduced as a buffer for reliable operation in power grid applications. Thus, adding the margin advantageously provides an additional degree of freedom, since the voltage can be selected depending on the specific implementation of the power system. In terms of available energy in the ESS, the margin represents a judicious trade-off between improved lifetime and increased safety, and can be chosen for different applications depending on, for example, the desired reliability level, the expected load range on the ESS, etc.

[0025] In other words, the control method according to the presently disclosed aspects may include the optional steps of determining a voltage margin based on an estimated reference voltage at the end of the life of the ESS and / or a current SoH of the ESS (i.e., a most recently determined SoH of the ESS), and further determining the reference voltage based on the determined voltage margin. The voltage margin may be determined at least in part based on a minimum required reference voltage of the ESS at the end of the life of the ESS.

[0026] In conventional power systems, most grid-forming control scheme designs assume the presence of large energy storage (e.g., batteries) with time constants much longer than the electromechanical dynamics associated with the inertial response required of the ESS. Consequently, the battery's SoC can be considered effectively constant when designing the control system.

[0027] However, in E-STATCOMs employing supercapacitors, for example, much smaller energy storage elements are typically used, and the SoC power is not well separated in time from the inertia simulation power required by the ESS—often on the order of seconds. Therefore, it is advantageous to consider the ESS's SoC when calculating its active and reactive power capabilities to reduce the risk of overcharging or undercharging the ESS. As used herein, the active and reactive power capabilities of an ESS are the amount of active or reactive power that the ESS is deemed capable of providing at any moment (e.g., from the perspective of the controller), i.e., the published availability of active or reactive power.

[0028] To this end, according to aspects of the present disclosure, dynamic active and reactive power capability strategies can be incorporated into the control method.Such strategies advantageously implement accurate and rapid enforcement of active and reactive power limitations provided by the underlying grid forming control scheme.

[0029] In particular, according to an optional refinement of the presently disclosed control technique, the method further includes dynamically changing a threshold power capability of the ESS based on the SoC of one or more ultracapacitors included in the ESS, the threshold power capability corresponding to a threshold amount of power that can be transferred between the ESS and the power grid. As mentioned above, the threshold power capability includes the minimum or maximum active and / or reactive power that can be transferred between the ESS and the power grid, or at least the minimum or maximum active and / or reactive power values ​​that can be provided to a controller when designing an overall control strategy for the ESS. Power transfer between the ESS and the power grid can then be limited based on the threshold power capability.

[0030] The threshold power capability may further vary based on the total energy storage capacity of the ESS and / or parameters indicative of converter capacity, such as one or more of SoC, DC voltage, AC side current, DC side current, modulation index, etc.

[0031] According to such methods, the SoC and / or energy storage current can be maintained between minimum and maximum values, and the converter AC-side output current amplitude can be maintained below the maximum value. In addition, the converter can be configured to operate in an active power priority mode so that the remaining current margin can be used for reactive power compensation.

[0032] It will be appreciated that the various features of the presently disclosed control methods may be implemented individually or in any combination, depending on the particular implementation, while still achieving the associated advantageous effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above and other aspects of the present disclosure will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a power system according to aspects of the present disclosure; and Figure 2 An example embodiment of an energy storage management system is schematically illustrated. DETAILED DESCRIPTION

[0034] The present disclosure is described below by means of several illustrative examples. It will be understood that these examples are provided for illustration and explanation only and are not intended to limit the scope of the present disclosure.

[0035] Figure 1 A power system 100 according to aspects of the present disclosure is schematically illustrated. The power system 100 includes a converter 120 coupled to an energy storage system (ESS) 130 to form an energy storage-enhanced converter system 140. The converter 130 is coupled to a power grid 110 and is configured to transfer power between the power grid 110 and the ESS 130, the details of which are beyond the scope of the present disclosure and are well understood by those skilled in the art.

[0036] According to the illustrated example, power grid 110 is an alternating current (AC) power grid. Therefore, converter 120 is configured to convert direct current (DC) from ESS 130 into AC for distribution on power grid 110, and to convert AC from power grid 110 into DC for providing power to ESS 130. In alternative examples, power grid 110 may be a high-voltage DC (HVDC) power grid, in which case converter 120 may be a DC-DC converter.

[0037] The converter 120 may be a static synchronous compensator (STATCOM), and thus the energy storage enhanced converter system 140 may be an energy storage enhanced STATCOM system (also referred to as an E-STATCOM system).

[0038] like Figure 1 The ESS 130 illustrated in FIG includes a plurality of supercapacitors 132 acting as energy storage units. In alternative examples, depending on the implementation, only one energy storage unit may be implemented in the ESS 130 and / or the energy storage unit(s) may be battery cells, fuel cells, or ordinary capacitors.

[0039] The operation and status of the ESS 130 are monitored and controlled by an energy storage management system (ESMS) 134 (such operation is generally referred to as the ESS being 'managed'). The ESMS may be implemented as hardware and / or software, and while the ESMS is shown as part of the ESS 130, it will be appreciated that in some examples, the ESMS may be implemented as a unit separate from the ESS 130 and remote from the ultracapacitor 132.

[0040] The ESMS may monitor, among other things, the state of charge (SoC) of the ultracapacitors 132, the terminal voltage of each ultracapacitor 132 and / or the ESS 130 as a whole, and / or the current flowing through the ESS 130 (e.g., including through each individual ultracapacitor 132). The ESS 130 may be configured with sensors to measure information relevant to such monitoring and control. Such sensors may communicate with a controller included in the ESMS 134 and configured to implement management processes. It will be appreciated that such a controller may be implemented as a transitory or non-transitory computer-readable medium containing instructions that, when executed by a processor, cause the processor to implement such management processes.

[0041] Figure 2 An example embodiment of an ESMS 134 is schematically shown, such as that described above with respect to Figure 1 Example Embodiments Discussed. As shown in this figure, the ESMS 134 can be considered to include multiple subsystems or modules.

[0042] In the illustrated example of FIG. 3 , the ESMS 134 includes a SoC estimation module 302 configured to estimate the SoC of the ESS, and a SoH estimation module 310 configured to estimate the SoH of the ESS.

[0043] The energy stored in a capacitor is therefore proportional to its capacitance and the square of its voltage. Therefore, the SoC estimation module receives as input the following: the terminal voltage of the ESS and / or the voltage across one or more capacitors 316, and the estimated equivalent capacitance 320 of the ESS's supercapacitor(s) from the SoH estimation module 310. The SoC estimation module 302 then provides the estimated SoC of the ESS as an output. This output of the estimated SoC 322 of the ESS is then provided as a usable output to, for example, the ESMS 134 to inform other processes in the power system. The output of the estimated SoC 322 of the ESS is further provided to other internal modules of the ESMS 134.

[0044] The ESMS 134 further includes a dynamic power capability calculation module 304 that receives as input the estimated SoC 322 of the ESS and provides as output the dynamic active and reactive power limits 328 of the ESS.

[0045] The ESMS 134 further includes an error reference calculation module 306 configured to determine the difference between a reference amount of energy stored in the ESS (i.e., a set point for the SoC) and the actual amount of energy stored in the ESS (or SoC). The error reference calculation module 306 receives as input an estimated SoC 322 of the ESS and outputs an error reference signal 326.

[0046] The error reference signal 326 is provided to a controller 308 , which is configured to control the transfer of power between the ESS and the power grid—ie, the charging and discharging of the ESS. Thus, the output of the controller 308 is a power reference signal 332 .

[0047] The gains used by the controller (e.g., as part of a PI or PID control scheme or the like) are adjusted based on the SoH of the ESS. That is, the estimated SoH, represented as an equivalent capacitance 320, is provided to a control parameter determination module 314, which determines control parameters (i.e., gains) for use by the controller 308 based on the estimated SoH 320 of the ESS and provides these control parameters to the controller 308.

[0048] The controller 308 is further provided with the dynamic power capability 328 of the ESS calculated by the dynamic power capability calculation module 304 so as to further base the control of power transfer between the ESS and the power grid on the power capability of the ESS.

[0049] The ESMS 134 further includes a voltage reference calculation module 312 configured to determine a reference voltage 324 for providing a desired power transfer between the ESS and the power grid based on the estimated SoH 320 of the ESS provided by the SoH estimation module 310. The reference voltage 324 is provided to the error reference calculation module 306 so that the controller 308 can control the desired power transfer according to the error reference 326.

[0050] It will be appreciated that the modules and their interconnections described above are purely schematic and are provided as examples to understand possible implementations of the control method as described herein. That is, the individual modules may be implemented in isolation or in any other combination, with or without additional possible modules, depending on the intended implementation.

[0051] Although the present disclosure is susceptible to various modifications and alternative forms, specific examples are shown and described with respect to the accompanying drawings in order to clearly explain the various advantageous aspects of the present disclosure. However, it should be understood that the detailed description herein and the accompanying drawings are not intended to limit the present disclosure to the specific forms disclosed. On the contrary, it is intended to cover all modifications, equivalents, and alternatives that fall within the scope of the appended claims, including possible combinations of the various elements of these specific examples.

Claims

1. A method for managing an energy storage system (ESS) having one or more supercapacitors, the method comprising: obtaining a desired power transfer between the ESS and a power grid; as well as A reference voltage for providing a desired power transfer between the ESS and the power grid is determined based on the state of health SoH of the ESS.

2. The method according to claim 1, further comprising: determining a desired power transfer between the ESS and the power grid based on a state of charge (SoC) of the ESS; as well as The determined reference voltage is used by a controller to control power transfer between the ESS and the power grid.

3. The method according to claim 2, further comprising: One or more gain factors are determined based on the SoH of the ESS, and the controller is configured to control the power transfer according to the one or more gain factors.

4. The method according to any one of claims 2 or 3, further comprising: During operation of the ESS, the SoC is determined online based on an estimated voltage across at least one ultracapacitor of the one or more ultracapacitors.

5. The method according to any preceding claim, further comprising: The SoH of the ESS is determined based on a terminal voltage and / or current of the ESS.

6. A method according to any preceding claim, wherein: If it is determined that the power grid has a grid frequency that is outside of a frequency band, power is not transferred between the ESS and the power grid.

7. The method according to any preceding claim, further comprising: determining a voltage margin based on a reference voltage estimated at an end of life of the ESS; as well as The reference voltage is determined further based on the determined voltage margin.

8. The method according to claim 7, wherein: The voltage margin is determined based at least in part on a minimum required reference voltage of the ESS at a current SoH and / or at end of life of the ESS.

9. The method according to any preceding claim, further comprising: A threshold power capability of the ESS is dynamically changed based on a SoC of one or more ultracapacitors included in the ESS, the threshold power capability corresponding to a threshold amount of power that can be transferred between the ESS and the power grid.

10. The method according to claim 9, wherein: The threshold power capability includes a minimum or maximum active and / or reactive power that can be transferred between the ESS and the power grid.

11. The method according to claim 9 or claim 10, wherein: The threshold power capability further varies based on the total energy storage capacity of the ESS.

12. The method according to any one of claims 9 to 11, further comprising: Power transfer between the ESS and the power grid is limited based on the threshold power capability.

13. A non-transitory computer-readable medium comprising instructions which, when executed by a processor, cause the processor to carry out the method according to any preceding claim. 14 . An energy storage management system (ESMS), comprising the non-transitory computer-readable medium according to claim 13 .

15. A power system comprising: a converter coupled to a power grid; as well as An energy storage system ESS is operatively coupled to the converter and is configured to transfer power to and / or from the power grid via the converter, wherein the ESS comprises the ESMS according to claim 14.

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