Control system for battery energy storage in micro-grid

Abstract

The invention provides a control system for battery energy storage in a micro-grid. The control system comprises a battery pack, a bidirectional DC / DC converter, a three-phase three-wire full-bridge voltage source converter (VSC), a BMS, an adaptive high-pass filter (AHPF), a dual-second-order generalized integral phase-locked loop (DSOG I-PLL), two Butterworth low-pass filters (BLPF) with different orders and a sine pulse width modulation module (SPWM). The bidirectional DC / DC converter is used for adjusting the voltage of the battery pack to the rated voltage of the DC bus and realizing adaptive frequency control; the VSC is responsible for realizing voltage regulation and unbalanced current compensation; the BMS is connected with the bidirectional DC / DC converter and is responsible for current and voltage measurement, battery maintenance and over-charge and over-discharge protection; the AHPF is integrated in an active current control loop, and the cut-off frequency of the AHPF is related to the state of charge of the battery and is used for frequency compensation; the DSOG I-PLL is used for acquiring a real-time system frequency; the two BLPFs are used for extracting positive sequence current and negative sequence current; the SPWM module is used for generating a switching signal for driving the three-phase full-bridge VSC.

Description

Technical Field

[0001] The present application relates to energy storage control technology, and in particular to a control system for battery energy storage in a microgrid. Background Art

[0002] With the continuous growth of the global economy and the continuous progress of society, energy demand has shown a rapid growth trend, and the tension in energy supply has become increasingly prominent. The development and utilization of renewable energy, such as wind power, solar energy and hydropower, has become a key path to solve the energy dilemma. Microgrids, as an emerging energy supply model, rely on their advantages such as being able to integrate a variety of distributed energy resources, improve energy utilization efficiency, and enhance power supply reliability. In microgrids, distributed power sources play a core role. These distributed power sources have many significant advantages. They can be distributed in different locations according to actual needs, effectively reducing the construction cost and energy loss of transmission lines; and can realize energy production and consumption locally. However, the output power of distributed power sources is intermittent and volatile, which makes microgrids face severe challenges during operation.

[0003] In order to solve the frequency and voltage instability problems caused by the intermittent and volatile output power of distributed power sources in microgrids, as well as the resulting degradation of power quality and increased system operation risks, effective frequency regulation, voltage support and imbalance compensation can be achieved by optimizing the control strategy of BESS, thereby improving the stability and reliability of microgrids. However, due to the complex and changeable operating conditions of microgrids, and the need to take into account the battery life and operation safety, traditional control methods are difficult to meet the requirements.

[0004] Traditional frequency control methods, such as simple droop control, have the problem of large steady-state frequency deviation; voltage regulation strategies often fail to fully consider the operating constraints of BESS; and methods for unbalanced current compensation also need to be improved in terms of compensation effect and system adaptability. In addition, some advanced control technologies are complex in calculation and have high requirements for system hardware, which increases the cost and difficulty of implementation. Summary of the invention

[0005] In view of the limitations of current technology, the present invention proposes a control system for battery energy storage in microgrids. This scheme can not only adaptively adjust the frequency compensation capability according to the SOC state of the battery to extend the battery life, but also stabilize the voltage under low voltage conditions through an effective voltage control loop, while using the remaining capacity of the BESS to compensate for unbalanced current. In addition, the control scheme of the present invention is flexible in structure and easy to implement, and has significant advantages and practical value in actual microgrid applications.

[0006] The system includes:

[0007] Battery pack, bidirectional DC / DC converter, three-phase three-wire full-bridge voltage source converter (VSC), battery management system (BMS), adaptive high-pass filter (AHPF), dual second-order generalized integral phase-locked loop (DSOGI-PLL), two different orders of Butterworth low-pass filters (BLPF) and sinusoidal pulse width modulation module (SPWM);

[0008] The battery pack provides electrical energy to the system;

[0009] The bidirectional DC / DC converter is used to adjust the battery pack voltage to the rated voltage of the DC bus and realize adaptive frequency control;

[0010] The three-phase three-wire full-bridge voltage source converter (VSC) is responsible for realizing voltage regulation and unbalanced current compensation;

[0011] The battery management system (BMS) is connected to the bidirectional DC / DC converter and is responsible for current and voltage measurement as well as battery maintenance and overcharge and over-discharge protection;

[0012] An adaptive high-pass filter (AHPF) is integrated in the active current control loop. The cutoff frequency of the AHPF is related to the battery state of charge and is used for frequency compensation.

[0013] A dual second-order generalized integral phase-locked loop (DSOGI-PLL) is used to obtain the real-time system frequency; two Butterworth low-pass filters (BLPF) are used to extract positive and negative sequence currents;

[0014] The Sinusoidal Pulse Width Modulation (SPWM) module is used to generate switching signals for driving the three-phase full-bridge VSC.

[0015] In some embodiments, the battery pack is connected to a bidirectional DC / DC converter, and the bidirectional DC / DC converter includes a switch tube S1, a switch tube S2 and a capacitor. The switch tube S1 and the switch tube S2 are respectively connected to two ends of the battery pack, one end of the capacitor is connected to the common end of the switch tube S1 and the switch tube S2, and the other end of the capacitor is grounded; the output end of the bidirectional DC / DC converter is connected to the DC bus of a three-phase three-wire full-bridge voltage source converter (VSC), and the three-phase three-wire full-bridge voltage source converter (VSC) is composed of six switch tubes to form a three-phase full-bridge structure; the battery management system (BMS) is connected to the battery pack and the bidirectional DC / DC converter to monitor the battery status in real time.

[0016] In some embodiments, in the adaptive frequency adjustment part, a dual second-order generalized integral phase-locked loop (DSOGI-PLL) is connected to a three-phase three-wire full-bridge voltage source converter (VSC) to obtain the real-time system frequency f act , frequency deviation f err After the PI controller and the output current limit value of the bidirectional DC / DC converter And the power setting value given by the secondary control Working together, we get the compensation power P comp , the cutoff frequency f of the adaptive high-pass filter (AHPF) c The output of the adaptive high-pass filter (AHPF) is determined by the average SOC of the battery pack and the P comp Together they act on the control of a bidirectional DC / DC converter.

[0017] In some embodiments, in the voltage regulation and unbalanced current compensation part, two Butterworth low-pass filters (BLPF) are respectively connected to the output current of the three-phase three-wire full-bridge voltage source converter (VSC) to extract the positive and negative sequence currents and the PCC voltage phase angle ω rt The positive sequence current is obtained by a dual second-order generalized integral phase-locked loop (DSOGI-PLL). By adjusting the DC capacitor voltage, From the voltage regulation loop, the negative sequence current and According to the unbalanced load current, a switching signal is finally generated through a sinusoidal pulse width modulation module (SPWM) to drive a three-phase three-wire full-bridge voltage source converter (VSC).

[0018] In some embodiments, when the control system is working, it tracks the energy storage / power supply power setting value (P D ), when SOC is lower than SOC min Or higher than SOC max hour, The control system obtains the real-time system frequency (f act ), calculate the frequency deviation (f err ) and the required active power (P comp ), P comp The current capacity of the bidirectional converter and limit:

[0019]

[0020] In some embodiments, the cutoff frequency (f c ) determines the active compensation power amplitude exchanged between the control system and the microgrid, f c Determined by the average SOC of the battery pack, the SOC is calculated by the battery voltage and current measured by the battery management system (BMS), where an algebraic polynomial function is used:

[0021]

[0022] Its coefficient k i Calculated based on the battery's long-term state of health (SOH):

[0023]

[0024] Among them, when SOH decreases, k i Decrease accordingly and lower the AHPF cutoff frequency.

[0025] In some embodiments, the AHPF used is a second-order Butterworth high-pass filter (BHPF), which provides a maximum flat response in the passband and is the best compromise between attenuation and phase response. Its transfer function is as follows:

[0026]

[0027] In some embodiments, two cutoff frequencies f are used to extract positive and negative sequence currents. c =20Hz Butterworth low-pass filter (BLPF), according to the different oscillation amplitudes of positive and negative sequences, the positive sequence uses the second-order BLPF1, and the negative sequence uses the fourth-order BLPF2:

[0028]

[0029] The voltage phase angle of PCC (ω rt ) is obtained by the DSOGI-PLL method, and the negative sequence phase angle can be obtained by inverting the positive sequence phase angle; By adjusting the DC capacitor voltage, From the voltage regulation loop, and Obtained based on the unbalanced load current; finally, using f sw =15kHz sinusoidal pulse width modulation (SPWM) method generates switching signals to drive the three-phase full-bridge VSC.

[0030] In some embodiments, the control system needs to obtain and The SRF method is adopted, obtained in the negative sequence by Park transform, and the average term is extracted as the reference value of the current control loop.

[0031] In some embodiments, when the frequency deviation of the control system exceeds a set threshold, the adaptive frequency control mode is preferentially entered; when the voltage fluctuates or is unbalanced, the voltage regulation or unbalance compensation mode is started accordingly.

[0032] The embodiments of the present application include at least the following beneficial effects:

[0033] (1) The converter topology of BESS consists of a battery pack, a bidirectional DC / DC converter and a three-phase three-wire full-bridge voltage source converter (VSC), with a clear structure and high functional integration. The bidirectional DC / DC converter can not only adapt the battery pack voltage to the rated voltage of the DC link, but also realize adaptive frequency control. The three-phase VSC integrates voltage regulation and unbalanced current compensation controller. The overall topology design is reasonable, without the need for complex additional equipment, reducing system cost and complexity, and having good economy and practicality. The battery management system (BMS) is connected to the bidirectional converter and is responsible for current and voltage measurement as well as battery maintenance and protection to prevent overcharging or over-discharging, further ensuring the safety of the system and the service life of the battery, and improving the overall reliability of the system.

[0034] (2) The proposed adaptive frequency control method uses AHPF to adjust the cutoff frequency according to the battery SOC, thereby controlling the degree of BESS participation in frequency regulation and effectively extending the battery life. At the same time, DSOGI-PLL is used to improve the frequency detection accuracy, which can respond quickly and accurately when the system is transient. The SOC estimation method combined with the coulomb counting method and the offline open circuit voltage method takes into account both the calculation burden and accuracy, so that the BESS can reasonably participate in frequency compensation under different SOC conditions, reduce the amplitude and duration of frequency deviation, and enhance the system frequency stability. The voltage regulation and unbalanced compensation functions are allocated to different control tasks according to system requirements by reasonably utilizing the current capacity of the VSC. In terms of voltage regulation, the voltage regulation method based on SRF has a fast response speed and can effectively compensate for voltage transient and steady-state deviations; in unbalanced current compensation, it can be achieved by using reactive current components without the need for additional active current sources, and through a reasonably designed control strategy and current saturation mechanism, it is ensured that the unbalanced current is effectively compensated without affecting the battery charging and discharging, thereby improving the system power quality.

[0035] (3) The control scheme realizes the coordinated work of multiple control functions and can automatically switch the control mode according to the actual operating status of the system, giving priority to ensuring the stability of the system frequency while taking into account voltage regulation and imbalance compensation. In the process of switching between different control modes, each control link cooperates with each other to avoid conflicts, ensuring the stable operation of the system under various working conditions and improving the system's adaptability to complex working conditions. The control scheme does not depend on the number, capacity and location of synchronous generators in the microgrid. It has strong structural adaptability and can be widely used in isolated synchronous generator microgrids with different structures. It enhances the versatility and scalability of the system and provides a comprehensive solution for the efficient application of BESS in isolated microgrids. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is the topological structure diagram of the battery energy storage system (BESS) converter;

[0037] Figure 2 It is an adaptive frequency control diagram proposed for DC / DC converters;

[0038] Figure 3 is a schematic diagram of a dual second-order generalized integral phase-locked loop (DSOG I-PLL) method for phase and frequency measurement;

[0039] Figure 4 It is a control scheme proposed for the voltage source converter (VSC) of the battery energy storage system (BESS). DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application is further described in detail below in conjunction with the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the embodiments of the present application, they are only examples of systems and methods consistent with some aspects of the embodiments of the present application.

[0041] It is understood that the terms "first", "second", etc. used in this application can be used to describe various concepts in this article, but unless otherwise specified, these concepts are not limited by these terms. These terms are only used to distinguish one concept from another concept. For example, without departing from the scope of the embodiment of the present application, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Depending on the context, the words "if" and "if" as used herein can be interpreted as "at the time of" or "when" or "in response to determination".

[0042] The terms "at least one", "multiple", "each", "any", etc. used in this application, at least one includes one, two or more, multiple includes two or more, each refers to each of the corresponding multiple, and any refers to any one of the multiple.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein are only for the purpose of describing the embodiments of this application and are not intended to limit this application.

[0044] The independent BESS multifunctional control scheme architecture proposed in this invention is as follows: Figure 1 , Figure 2 , Figure 3 , Figure 4As shown in the figure, it includes battery pack, bidirectional DC / DC converter, three-phase three-wire full-bridge voltage source converter (VSC), battery management system (BMS), adaptive high-pass filter (AHPF), dual second-order generalized integral phase-locked loop (DSOGI-PLL), two different-order Butterworth low-pass filters (BLPF), sinusoidal pulse width modulation (SPWM) module, etc. The battery pack provides power to the system; the bidirectional DC / DC converter is used to adjust the battery pack voltage to the rated voltage of the DC bus and realize adaptive frequency control; the three-phase three-wire full-bridge VSC is responsible for voltage regulation and unbalanced current compensation; the BMS is connected to the bidirectional converter and is responsible for current and voltage measurement as well as battery maintenance and overcharge and over-discharge protection; the AHPF is integrated in the active current control loop, and its cut-off frequency is related to the battery state of charge (SOC) and is used for frequency compensation; the DSOGI-PLL is used to obtain the real-time system frequency; the two BLPFs are used to extract positive and negative sequence currents; the SPWM module is used to generate the switching signal to drive the three-phase full-bridge VSC.

[0045] The specific connection method is described as follows:

[0046] (1) The battery pack is connected to a bidirectional DC / DC converter. The bidirectional DC / DC converter consists of switch tubes S1 and S2 and a capacitor. S1 and S2 are connected to the two ends of the battery pack respectively. One end of the capacitor is connected to the common end of S1 and S2, and the other end is grounded.

[0047] (2) The output end of the bidirectional DC / DC converter is connected to the DC bus of a three-phase three-wire full-bridge VSC. The VSC consists of six switches (S3-S8) forming a three-phase full-bridge structure.

[0048] (3) The BMS is connected to the battery pack and the bidirectional DC / DC converter to monitor the battery status in real time;

[0049] (4) In the adaptive frequency regulation part, the DSOGI-PLL is connected to the three-phase three-wire full-bridge VSC to obtain the real-time system frequency f act , frequency deviation f err After the PI controller and the output current limit value of the bidirectional DC / DC converter And the power setting value given by the secondary control Working together, we get the compensation power P comp , the cutoff frequency f of AHPF c The output of AHPF is determined by the average SOC of the battery pack. comp Work together on the control of bidirectional DC / DC converter;

[0050] (5) In the voltage regulation and unbalanced current compensation part, two BLPFs are connected to the output current of the three-phase three-wire full-bridge VSC to extract the positive and negative sequence currents and the PCC voltage phase angle ω rt The positive sequence current is obtained by DSOGI-PLL. By adjusting the DC capacitor voltage, From the voltage regulation loop, the negative sequence current and It is determined according to the unbalanced load current and finally generates a switching signal through the SPWM module to drive the three-phase full-bridge VSC.

[0051] Working principle:

[0052] This technology proposes a multifunctional control scheme suitable for independent battery energy storage system (BESS). The control part works in three operating modes: adaptive frequency control, voltage regulation, and unbalanced load compensation. The circuit working principles in different working modes are as follows: Figure 2 , Figure 4 As shown in the figure, the arrows in the figure represent the direction of energy transmission. The control mode is determined by the operating state of the BESS. When the BESS is running, it will cause changes in the system frequency, voltage or current, and the corresponding control loop will work to maintain system stability. Under different operating conditions, by detecting the change state of the system parameters, it is determined whether to adjust the control strategy to achieve the coordination of multiple control functions. The specific working principle of this control scheme is explained as follows:

[0053] (1) When BESS operates in an isolated microgrid, it needs to perform adaptive frequency control based on the battery state of charge (SOC) and system frequency. Usually, the synchronous generator (SG) is responsible for the frequency control of the power grid, but because its speed regulation system has a slow dynamic response, when there is an imbalance between power generation and power consumption, the system frequency will fluctuate significantly. BESS, based on its fast power exchange capability, can compensate for transient frequency changes in a short period of time. Considering the battery life and maintenance cost, it needs to be used within its operating constraints. Therefore, an adaptive high-pass filter (AHPF) is proposed to associate BESS participation in frequency regulation with the battery SOC.

[0054] (2) BESS first tracks the energy storage / power supply power setting value (P D ), when SOC is lower than SOC min Or higher than SOC max hour, The BESS obtains the real-time system frequency (f act ), calculate the frequency deviation (f err ) and the required active power (P comp). Since the primary task of BESS is to track P comp The current capacity of the bidirectional converter and limit:

[0055]

[0056] (3) To accurately detect the frequency, especially when the system is transient, a dual second-order generalized integral phase-locked loop (DSOGI-PLL) is used. It has a moderate computational burden and good accuracy. Compared with the traditional PI-PLL, it has better anti-interference, frequency tracking ability, faster transient response and low sensitivity to voltage imbalance / distortion. The SOC estimation uses the coulomb counting method combined with the offline open circuit voltage method, which takes into account both computational burden and accuracy. The initial SOC (SOC ini ) is based on the offline estimation of the battery pack open circuit voltage, and the SOC change is calculated according to the BESS current during operation.

[0057] (4) AHPF cutoff frequency (f c ) determines the active compensation power amplitude exchanged between BESS and microgrid, f c Determined by the average SOC of the battery pack, the SOC is calculated by the battery voltage and current measured by the BMS. Here, an algebraic polynomial function is used:

[0058]

[0059] Its coefficient k i Calculated based on the battery's long-term state of health (SOH):

[0060]

[0061] When SOH decreases, k i The AHPF used is a second-order Butterworth high-pass filter (BHPF), which provides the most flat response in the passband and is the best compromise between attenuation and phase response. Its transfer function is as follows:

[0062]

[0063] (5) When the BESS grid-connected converter (VSC) has excess current capacity, the orthogonal axis current (i q ) regulates the voltage amplitude of the connected feeder or busbar, compensating for voltage transients (such as sags, swells) and steady-state voltage drops, especially in radial topology microgrids. BESS can be strategically placed at or near the sensitive load busbar (SLB). In addition, the available current capacity of the BESS can also be used to mitigate the unbalanced load current in the SLB, because the compensating current required to balance the three-phase unbalanced load is only determined by the reactive term (iq ) composition, without the need for active current source, BESS can compensate for unbalanced current without charging or discharging.

[0064] (6) Performing the above auxiliary services will occupy the VSC current capacity. In steady state, the BESS contribution has no effect, but its contribution needs to be given priority in system transient state. For isolated islands or weak microgrids based on SG, maintaining the system frequency within the standard range is the primary task to maintain synchronization and prevent system collapse. Frequency regulation only requires the injection of positive sequence direct axis current (i d+ ), voltage regulation and unbalanced current compensation require positive sequence (i q+ ) and negative sequence (i q- ) of the orthogonal axis current.

[0065] (7) To extract the positive and negative sequence currents, two cutoff frequencies f are used. c =20Hz Butterworth low-pass filter (BLPF). According to the different oscillation amplitudes of positive and negative sequences, the positive sequence uses the second-order BLPF and the negative sequence uses the fourth-order BLPF:

[0066]

[0067] The voltage phase angle of PCC (ω rt ) is obtained by the DSOGI-PLL method, and the negative sequence phase angle can be obtained by inverting the positive sequence phase angle. By adjusting the DC capacitor voltage, From the voltage regulation loop, and Finally, f sw =15kHz sinusoidal pulse width modulation (SPWM) method generates switching signals to drive the three-phase full-bridge VSC.

[0068] (8) Unlike the traditional voltage control based on calculating the voltage root mean square (RMS) value of the full sinusoidal cycle, the voltage regulation method in this paper is based on the direct axis and orthogonal axis voltages in the synchronous rotating reference frame (SRF). Its main advantage is faster response, which enables the BESS to compensate for high-frequency interference. In unbalanced current compensation, it is necessary to obtain and Here, the SRF method is adopted, which is obtained in the negative sequence through Park transformation, and the average term is extracted as the reference value of the current control loop. It is assumed that the BESS is installed in the SLB or where most unbalanced loads are connected, and the priority of BESS performance according to the VSC current capacity is achieved through current saturation.

[0069] (9) This control scheme realizes seamless switching of BESS from grid following mode to grid supporting mode and vice versa. By detecting system parameters such as frequency, voltage and current, it automatically switches between different control modes according to preset control strategies and priorities. For example, when the system frequency deviation exceeds the set threshold, it will preferentially enter the adaptive frequency control mode; when the voltage fluctuates or is unbalanced, it will start the voltage regulation or unbalance compensation mode accordingly.

[0070] (10) In each control mode, the accuracy and effectiveness of the control are ensured through real-time monitoring and feedback control of relevant parameters. For example, in adaptive frequency control, the battery SOC and system frequency are continuously monitored, and the compensation power is dynamically adjusted; in voltage regulation, voltage changes are tracked in real time, and reactive current injection is accurately controlled. At the same time, the various control modes are coordinated with each other to avoid conflicts and jointly improve the stability and power quality of the isolated microgrid. For example, when performing unbalanced current compensation, it is necessary to ensure that there is no negative impact on frequency and voltage control, and the coordinated work of multiple control functions is achieved by reasonably allocating the current capacity of the VSC.

[0071] It can be understood that the advantages of this application include:

[0072] In view of the independent battery energy storage system (BESS) participating in the ancillary service market in the isolated synchronous generator microgrid, and the existing control schemes have problems such as ignoring the BESS operation constraints, complex control, and single function. In view of the multi-functional control of BESS and the improvement of system stability, this paper proposes a new BESS control scheme, whose advantages are specifically manifested in the following three aspects:

[0073] (1) The converter topology of BESS consists of a battery pack, a bidirectional DC / DC converter and a three-phase three-wire full-bridge voltage source converter (VSC), with a clear structure and high functional integration. The bidirectional DC / DC converter can not only adapt the battery pack voltage to the rated voltage of the DC link, but also realize adaptive frequency control; the three-phase VSC integrates voltage regulation and unbalanced current compensation controller. The overall topology design is reasonable, without the need for complex additional equipment, reducing system cost and complexity, and having good economy and practicality. The battery management system (BMS) is connected to the bidirectional converter and is responsible for current and voltage measurement as well as battery maintenance and protection to prevent overcharging or over-discharging, further ensuring the safety of the system and the service life of the battery, and improving the overall reliability of the system.

[0074] (2) The proposed adaptive frequency control method uses AHPF to adjust the cutoff frequency according to the battery SOC, thereby controlling the degree of BESS participation in frequency regulation and effectively extending the battery life. At the same time, DSOGI-PLL is used to improve the frequency detection accuracy, which can respond quickly and accurately when the system is transient. The SOC estimation method combined with the coulomb counting method and the offline open circuit voltage method takes into account both the calculation burden and accuracy, so that the BESS can reasonably participate in frequency compensation under different SOC conditions, reduce the amplitude and duration of frequency deviation, and enhance the system frequency stability. The voltage regulation and unbalanced compensation functions are allocated to different control tasks according to system requirements by reasonably utilizing the current capacity of the VSC. In terms of voltage regulation, the voltage regulation method based on SRF has a fast response speed and can effectively compensate for voltage transient and steady-state deviations; in unbalanced current compensation, it can be achieved by using reactive current components without the need for additional active current sources, and through a reasonably designed control strategy and current saturation mechanism, it is ensured that the unbalanced current is effectively compensated without affecting the battery charging and discharging, thereby improving the system power quality.

[0075] (3) The control scheme realizes the coordinated work of multiple control functions and can automatically switch the control mode according to the actual operating status of the system, giving priority to ensuring the stability of the system frequency while taking into account voltage regulation and imbalance compensation. In the process of switching between different control modes, each control link cooperates with each other to avoid conflicts, ensuring the stable operation of the system under various working conditions and improving the system's adaptability to complex working conditions. The control scheme does not depend on the number, capacity and location of synchronous generators in the microgrid. It has strong structural adaptability and can be widely used in isolated synchronous generator microgrids with different structures. It enhances the versatility and scalability of the system and provides a comprehensive solution for the efficient application of BESS in isolated microgrids.

[0076] The multifunctional control method of the battery energy storage system (BESS) proposed in this technical solution is particularly suitable for the situation where the BESS in an isolated / independently operated microgrid based on synchronous generators serves as an independent grid to support distributed energy resources. The overall control structure should be protected with emphasis, such as Figure 1 , Figure 2 , Figure 4As shown. Specifically, the BESS consists of a battery pack, a bidirectional DC / DC converter and a three-phase three-wire full-bridge voltage source converter (VSC). In the bidirectional DC / DC converter, the boost or buck mode is achieved by controlling the on and off of switches S1 and S2. At the same time, the converter also implements the proposed adaptive frequency control, and the battery management system (BMS) is connected to it, which is responsible for current and voltage measurement, battery maintenance and overcharge and over-discharge protection. The three-phase three-wire full-bridge VSC is used to achieve voltage regulation and unbalanced current compensation. Through a specific control strategy, its available current capacity is used to adjust the voltage amplitude of the connected feeder or bus, and compensate for the unbalanced load current in the sensitive load bus (SLB).

[0077] The working principle of the BESS control method of the present invention is that in terms of adaptive frequency control, the BESS first tracks the energy storage / power supply power set point provided by the secondary control, and at the same time obtains the real-time system frequency through the phase-locked loop (PLL), calculates the frequency deviation and obtains the required active power for compensation through the proportional integral (PI) controller. In order to extend the battery life, an adaptive high-pass filter (AHPF) is introduced, and its cut-off frequency is determined according to the average state of charge (SOC) of the battery, and the SOC is estimated by the coulomb counting method combined with the offline open-circuit voltage method. In terms of voltage regulation and unbalanced current compensation, when the VSC has excess current capacity, the quadrature axis current of the BESS is used to adjust the voltage amplitude and compensate for the transient and steady-state voltage drops; the positive and negative sequence currents are extracted through two Butterworth low-pass filters (BLPF) of different orders, and the positive and negative sequence current reference values ​​are obtained according to the PCC voltage phase angle and the corresponding control strategy, thereby controlling the switching action of the three-phase full-bridge VSC.

[0078] This scheme adopts a collaborative adaptive frequency control strategy. In the isolated microgrid, the synchronous generator (SG) is responsible for frequency control, but because its speed regulation system has a slow dynamic response, the system frequency will show obvious transient changes when power generation and power consumption are unbalanced. The converter-based energy storage system (ESS) has fast power exchange capability. The BESS participates in frequency regulation according to the battery SOC through the proposed AHPF, and works with the SG to compensate for transient frequency changes. In addition, the control scheme has strong structural adaptability and is not affected by the number, capacity and location of the SG in the microgrid.

[0079] The voltage regulation and unbalanced current compensation method of this technical solution is unique. In voltage regulation, the voltage amplitude is calculated based on the direct axis and quadrature axis voltages of the synchronous rotating coordinate system (SRF), which has a fast response speed and can compensate for high-frequency disturbances. For unbalanced current compensation, based on the characteristics that the compensation current required by the unbalanced three-phase load is only composed of reactive terms (quadrature axis current), the BESS uses the available current capacity of its VSC to calculate the positive and negative sequence quadrature axis current reference values ​​through a specific control strategy to achieve unbalanced current compensation. In the compensation process, the performance of the BESS is preferentially allocated according to the VSC current capacity through the current saturation strategy, and the contribution of the BESS is reasonably managed in steady state and transient state.

[0080] It can be understood that the contents of the above method embodiments are all applicable to the present system embodiments, the functions specifically implemented by the present system embodiments are the same as those of the above method embodiments, and the beneficial effects achieved are also the same as those achieved by the above method embodiments.

[0081] The embodiments described in the embodiments of the present application are intended to more clearly illustrate the technical solutions of the embodiments of the present application and do not constitute a limitation on the technical solutions provided in the embodiments of the present application. Those skilled in the art will appreciate that with the evolution of technology and the emergence of new application scenarios, the technical solutions provided in the embodiments of the present application are also applicable to similar technical problems.

[0082] Those skilled in the art will appreciate that the technical solutions shown in the figures do not constitute a limitation on the embodiments of the present application, and may include more or fewer steps than shown in the figures, or a combination of certain steps, or different steps.

[0083] The system embodiments described above are merely illustrative, and the units described as separate components may or may not be physically separated, that is, they may be located in one place or distributed on multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the present embodiment.

[0084] Those skilled in the art will appreciate that all or some of the steps in the methods disclosed above, and the functional modules / units in the systems and devices may be implemented as software, firmware, hardware, or a suitable combination thereof.

[0085] The terms "first", "second", "third", "fourth", etc. (if any) in the specification of the present application and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0086] It should be understood that in the present application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or multiple.

[0087] In the several embodiments provided in the present application, it should be understood that the disclosed systems and methods can be implemented in other ways. For example, the system embodiments described above are only schematic. For example, the division of the above units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of systems or units, which can be electrical, mechanical or other forms.

[0088] The units described above as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0089] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0090] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present application is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, and the computer software product is stored in a storage medium, including multiple instructions to enable a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods of various embodiments of the present application. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (Read-Only Memory, referred to as ROM), random access memory (Random Access Memory, referred to as RAM), disk or optical disk and other media that can store programs.

[0091] The preferred embodiments of the present invention are described above with reference to the accompanying drawings, but the scope of the rights of the present invention is not limited thereto. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present invention should be within the scope of the rights of the present invention.

Claims

1. A control system for battery energy storage in a microgrid, characterized in that: include: Battery pack, bidirectional DC / DC converter, three-phase three-wire full-bridge voltage source converter (VSC), battery management system (BMS), adaptive high-pass filter (AHPF), dual second-order generalized integral phase-locked loop (DSOGI-PLL), two different orders of Butterworth low-pass filters (BLPF) and sinusoidal pulse width modulation module (SPWM); The battery pack provides electrical energy to the system; The bidirectional DC / DC converter is used to adjust the battery pack voltage to the rated voltage of the DC bus and realize adaptive frequency control; The three-phase three-wire full-bridge voltage source converter (VSC) is responsible for realizing voltage regulation and unbalanced current compensation; The battery management system (BMS) is connected to the bidirectional DC / DC converter and is responsible for current and voltage measurement as well as battery maintenance and overcharge and overdischarge protection; An adaptive high-pass filter (AHPF) is integrated in the active current control loop. The cutoff frequency of the adaptive high-pass filter is related to the battery state of charge and is used for frequency compensation. A dual second-order generalized integral phase-locked loop (DSOGI-PLL) is used to obtain the real-time system frequency; two Butterworth low-pass filters (BLPF) are used to extract positive and negative sequence currents; The Sinusoidal Pulse Width Modulation (SPWM) module is used to generate switching signals for driving the three-phase full-bridge VSC.

2. The control system for battery energy storage in a microgrid according to claim 1, characterized in that: A battery pack and a bidirectional DC / DC converter, the bidirectional DC / DC converter includes a switch tube S1, a switch tube S2 and a capacitor, the switch tube S1 and the switch tube S2 are respectively connected to the two ends of the battery pack, one end of the capacitor is connected to the common end of the switch tube S1 and the switch tube S2, and the other end of the capacitor is grounded; the output end of the bidirectional DC / DC converter is connected to the DC bus of a three-phase three-wire full-bridge voltage source converter (VSC), and the three-phase three-wire full-bridge voltage source converter (VSC) is composed of six switch tubes to form a three-phase full-bridge structure; a battery management system (BMS) is connected to the battery pack and the bidirectional DC / DC converter to monitor the battery status in real time.

3. The control system for battery energy storage in a microgrid according to claim 2, characterized in that: In the adaptive frequency regulation part, a dual second-order generalized integral phase-locked loop (DSOGI-PLL) is connected to a three-phase three-wire full-bridge voltage source converter (VSC) to obtain the real-time system frequency f act , frequency deviation f err After the PI controller and the output current limit value of the bidirectional DC / DC converter And the power setting value given by the secondary control Working together, we get the compensation power P comp , the cutoff frequency f of the adaptive high-pass filter (AHPF) c The output of the adaptive high-pass filter (AHPF) is determined by the average SOC of the battery pack and the P comp Together they act on the control of a bidirectional DC / DC converter.

4. The control system for battery energy storage in a microgrid according to claim 3, characterized in that: In the voltage regulation and unbalanced current compensation part, two Butterworth low-pass filters (BLPF) are connected to the output current of the three-phase three-wire full-bridge voltage source converter (VSC) to extract the positive and negative sequence currents and the PCC voltage phase angle ω rt The positive sequence current is obtained by a dual second-order generalized integral phase-locked loop (DSOGI-PLL). By adjusting the DC capacitor voltage, From the voltage regulation loop, the negative sequence current and According to the unbalanced load current, a switching signal is finally generated through a sinusoidal pulse width modulation module (SPWM) to drive a three-phase three-wire full-bridge voltage source converter (VSC).

5. The control system for battery energy storage in a microgrid according to claim 1, characterized in that: When the control system is working, it tracks the energy storage / power supply power setting value P given by the secondary control D , when SOC is lower than SOC min Or higher than SOC max hour, The control system obtains the real-time system frequency f through a phase-locked loop (PLL). act , calculate the frequency deviation f err The active power P required for compensation is obtained through the PI controller comp , P comp The current capacity of the bidirectional converter and limit:

6. The control system for battery energy storage in a microgrid according to claim 5, characterized in that: The cutoff frequency f of the adaptive high-pass filter (AHPF) c Determines the active compensation power amplitude exchanged between the control system and the microgrid, f c Determined by the average SOC of the battery pack, the SOC is calculated by the battery voltage and current measured by the battery management system (BMS), where an algebraic polynomial function is used: Its coefficient k i Calculated based on the battery's long-term state of health SOH: Among them, when SOH decreases, k i Decrease accordingly and lower the AHPF cutoff frequency.

7. The control system for battery energy storage in a microgrid according to claim 6, characterized in that: The AHPF used is a second-order Butterworth high-pass filter (BHPF), which provides a maximum flat response in the passband and is the best compromise between attenuation and phase response. Its transfer function is as follows:

8. The control system for battery energy storage in a microgrid according to claim 7, characterized in that: To extract the positive and negative sequence currents, two cutoff frequencies f are used. c =20Hz Butterworth low-pass filter (BLPF), according to the different oscillation amplitudes of positive and negative sequences, the positive sequence uses the second-order BLPF1, and the negative sequence uses the fourth-order BLPF2: PCC voltage phase angle ω rt Acquired by DSOGI-PLL method, the negative sequence phase angle can be obtained by inverting the positive sequence phase angle; By adjusting the DC capacitor voltage, From the voltage regulation loop, and Obtained based on the unbalanced load current; finally, using f sw =15kHz sinusoidal pulse width modulation (SPWM) method generates switching signals to drive the three-phase full-bridge VSC.

9. The control system for battery energy storage in a microgrid according to claim 8, characterized in that: In the unbalanced current compensation, the control system needs to obtain and The SRF method is adopted, obtained in the negative sequence by Park transform, and the average term is extracted as the reference value of the current control loop.

10. The control system for battery energy storage in a microgrid according to claim 1, characterized in that: When the frequency deviation of the control system exceeds the set threshold, the adaptive frequency control mode is entered first; when the voltage fluctuates or is unbalanced, the voltage regulation or unbalance compensation mode is started accordingly.

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