Power equalization and circulating current suppression method for parallel operation of multiple grid-forming type energy storage converters

Abstract

The invention relates to the technical field of converter power averaging and circulating current suppression, in particular to a power averaging and circulating current suppression method for parallel operation of a plurality of grid-forming type energy storage converters, which averages power through communication interaction and generates frequency and voltage compensation quantity by adopting a first regulator and a second regulator to realize accurate power averaging; meanwhile, the total current is measured locally, and virtual impedance feedforward control is adopted, so that rapid suppression of the high-frequency circulating current is realized. According to the invention, high-precision active power and reactive power equalization is realized, the capacity utilization rate is improved, high-frequency circulating current is effectively suppressed, loss and thermal stress are reduced, dynamic performance and system reliability are considered, a cooperative control architecture with complementary advantages is formed, adaptability to complex working conditions and system stability are enhanced, and engineering applicability is improved.

Description

Technical Field

[0001] This invention relates to the field of power sharing and circulating current suppression technology for converters, specifically a method for power sharing and circulating current suppression when multiple grid-connected energy storage converters are operated in parallel. Background Technology

[0002] With the deepening of energy transition, energy storage systems have become a key infrastructure for smoothing renewable energy fluctuations, enhancing grid stability, and achieving peak shaving and valley filling. Grid-based energy storage converters, capable of autonomously constructing stable grid voltage and frequency, provide valuable inertia and damping support for the power system and are gradually becoming core equipment for building new power systems. In practical applications, to meet the power capacity requirements of large-scale energy storage, multiple grid-based energy storage converters are typically connected in parallel.

[0003] When multiple converters operate in parallel, two long-standing technical challenges severely restrict the system's performance and reliability: power sharing and circulating current. Each parallel unit should precisely share the total active and reactive load according to its rated capacity. However, in actual engineering projects, factors such as slight differences in the output impedance of each converter, line impedance mismatch, and errors in controller parameters and sampling circuits can lead to an imbalance in the active and reactive power output of each unit. This uneven power sharing causes some units to operate under overload for extended periods, accelerating their aging and even triggering protective shutdowns, while other units fail to fully utilize their capabilities, thus reducing the overall utilization and reliability of the energy storage system.

[0004] Circulating current refers to current that flows only between parallel units and does not flow into the power grid. It is mainly caused by the inconsistency in the amplitude and phase of the output voltage of each unit. The existence of circulating current not only significantly increases the conduction and switching losses of the converter and reduces system efficiency, but also causes current waveform distortion, generates heat, and may even induce system oscillations, seriously threatening the safe and stable operation of the system.

[0005] To address the aforementioned issues, traditional solutions fall into two main categories: The first is based on droop control or its advanced form, virtual synchronous generator control. This method simulates the power-frequency and voltage reactive power droop characteristics of a synchronous generator, achieving a certain degree of autonomous power distribution without relying on communication. However, to maintain system stability, a certain frequency and voltage deviation is necessary for power distribution, thus preventing precise, error-free power sharing, especially when line impedance mismatches occur, resulting in poor distribution performance. The second is current-sharing control based on low-speed communication, such as the average power current-sharing method. This method exchanges power information between modules via a communication network and uses this information to correct the unit's output voltage or frequency reference values, achieving higher-precision power sharing. However, this method relies on communication, its dynamic response speed is limited by the communication cycle, and it carries the risk of communication delays or interruptions. More importantly, it primarily addresses low-frequency power distribution problems, offering very limited suppression of high-frequency circulating currents caused by switching subharmonics or parameter asymmetry. No solutions have yet been proposed to address these technical issues. Summary of the Invention

[0006] To address the problems in related technologies, this invention proposes a power sharing and circulating current suppression method for parallel operation of multiple grid-connected energy storage converters. This method overcomes the aforementioned technical problems in existing technologies. The purpose of this invention is to achieve high-precision active and reactive power sharing, improve capacity utilization, effectively suppress high-frequency circulating currents, reduce losses and thermal stress, balance dynamic performance and system reliability, form a complementary collaborative control architecture, enhance adaptability to complex operating conditions and system stability, and improve engineering applicability.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for power sharing and circulating current suppression of multiple grid-connected energy storage converters operating in parallel, comprising the following steps:

[0008] Step S01: At each parallel grid-type energy storage converter, the three-phase voltage, three-phase current and total output current flowing into the common coupling point are collected in real time at the output terminal of the grid-type energy storage converter.

[0009] Step S02: Based on the three-phase voltage and three-phase current, calculate the active power and reactive power output by the converter;

[0010] Step S03: Through a low-speed communication network, exchange the active power and reactive power information of all converters between each converter controller, and calculate the average active power and average reactive power of all parallel converters.

[0011] Step S04: Based on the deviation between the active power and the average active power of the converter, the first regulator generates a compensation amount for adjusting the frequency of the converter.

[0012] Step S05: Based on the deviation between the reactive power of the converter and the average reactive power, a compensation amount for adjusting the output voltage amplitude of the converter is generated by the second regulator.

[0013] Step S06: The frequency compensation amount is superimposed on the rated angular frequency of the converter to generate a new frequency command, which is used for the frequency loop of the converter virtual synchronous machine or droop control.

[0014] Step S07: The voltage amplitude compensation amount is superimposed on the rated voltage amplitude of the converter to generate a new voltage amplitude command, which is used for the voltage loop of the converter virtual synchronous machine or droop control.

[0015] Step S08: Based on the total output current, calculate the circulating current suppression compensation voltage using a virtual impedance algorithm, and feed this compensation voltage forward to the output of the converter voltage loop to suppress the circulating current between parallel modules.

[0016] Preferably, in steps S04 and S05, both the first regulator and the second regulator are proportional-integral regulators, and the regulation rules of the first regulator and the second regulator are as follows:

[0017] Δω_i=-(K_p_p*(P_i-P_avg)+K_i_p*∫(P_i-P_avg)dt);

[0018] E_i=-(K_p_q*(Q_i-Q_avg)+K_i_q*∫(Q_i-Q_avg)dt)

[0019] In the formula, K_p_p and Ki_p are the proportional and integral coefficients of the active power equalization regulator, and K_p_q and Ki_q are the proportional and integral coefficients of the reactive power equalization regulator.

[0020] Preferably, in step S03, the low-speed communication network is one of CAN bus, RS-485 bus, or power line carrier communication, and the communication period is 10ms to 100ms.

[0021] Preferably, step S08 specifically includes the following steps:

[0022] Step S081: Bandpass filtering or high-pass filtering is performed on the collected total output current to extract the high-frequency components characterizing the circulating current.

[0023] Step S082: Multiply the extracted high-frequency component of the circulating current by a virtual impedance value to obtain the compensation voltage used to suppress the circulating current;

[0024] Step S083: The compensation voltage is superimposed on the reference voltage signal output by the local voltage loop at the feedforward point.

[0025] Preferably, the virtual impedance value is one of pure virtual resistance, pure virtual inductance, and resistive-inductive composite virtual impedance, and its expression is Z_v=R_v+jωL_v, where ω is the fundamental angular frequency.

[0026] Preferably, in steps S06 and S07, the new frequency command and the new voltage amplitude command are subject to rate limiting and amplitude limiting.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] (1) This invention is a power sharing and circulating current suppression method for multiple grid-type energy storage converters operating in parallel. The average power is obtained through a low-speed communication network, and the deviation between the local power and the average power is adjusted without error by a proportional-integral regulator. Combining the accuracy of communication control and the speed of local control, it can fundamentally eliminate the power distribution error caused by parameter differences, line impedance mismatch, etc., and ensure that each converter strictly shares the load according to its rated capacity ratio, avoiding the unreasonable situation of some units being overloaded while some units are lightly loaded, thereby improving capacity utilization and operational reliability.

[0029] (2) This invention is a power sharing and circulating current suppression method for multiple grid-type energy storage converters operating in parallel. It adopts virtual impedance feedforward based on local total current measurement to suppress circulating current. By extracting the high-frequency harmonic components in the total current and multiplying them by the virtual impedance, a compensation voltage with the opposite effect is generated and fed forward directly to the modulation stage to form a high-speed local negative feedback closed loop with extremely fast response speed. It can accurately and effectively suppress high-frequency circulating current caused by dead zone effect, switching frequency harmonics and parameter asymmetry, significantly reduce conduction loss and switching loss, reduce device heating, and improve operating efficiency and lifespan.

[0030] (3) This invention is a power sharing and circulating current suppression method for multiple grid-type energy storage converters operating in parallel. Low-speed communication is responsible for solving the low-frequency, global power sharing problem. It is not sensitive to communication delay and interruption, ensuring robustness. High-speed local virtual impedance loop is responsible for solving the high-frequency, local circulating current problem. It does not rely on communication at all, ensuring fast response. The combination of the two overcomes the disadvantage of low accuracy of droop control without communication, and makes up for the shortcomings of pure communication control that cannot suppress high-frequency circulating current and has slow dynamics, achieving the best balance between performance and reliability.

[0031] (4) This invention is a power sharing and circulating current suppression method for multiple grid-type energy storage converters operating in parallel. It limits the rate and amplitude of frequency and voltage commands, effectively preventing sudden changes in reference commands due to communication data jumps or drastic power fluctuations, avoiding excessive impact on the system, thereby enhancing the robustness and stability of the control system, enabling it to better adapt to complex operating conditions such as load changes and grid disturbances.

[0032] (5) This invention is a power sharing and circulating current suppression method for multiple grid-type energy storage converters operating in parallel. The power sharing loop can automatically compensate for the differences in line impedance and body parameters, reduce the stringent requirements on the length and specifications of connecting cables, simplify the complexity of on-site installation and maintenance, and improve the applicability and economy in actual engineering. Attached Figure Description

[0033] Figure 1 This is a flowchart illustrating the method of the present invention. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0035] Example

[0036] Please see Figure 1 This invention proposes a technical solution for power sharing and circulating current suppression of multiple grid-connected energy storage converters operating in parallel: The power sharing and circulating current suppression method for multiple grid-connected energy storage converters operating in parallel includes the following steps:

[0037] Step S01: At each parallel grid-connected energy storage converter, the three-phase voltage, three-phase current, and total output current flowing into the point of common coupling are collected in real time at the output terminal of the grid-connected energy storage converter. Specifically, at each grid-connected energy storage converter, the three-phase voltage, three-phase current, and total output current flowing into the point of common coupling (i.e., the current flowing through the output inductor of the converter, including the current flowing into the grid and the circulating current flowing to other parallel units) are collected in real time by a high-precision sensor. All analog signals are conditioned by the circuit and then synchronously sampled and digitized by the ADC module of the DSP at a sampling frequency of 20kHz.

[0038] Step S02: Based on the three-phase voltage and three-phase current, calculate the active power and reactive power output of the converter. Specifically, the DSP performs software phase-locked loop processing and Clark-Park coordinate transformation on the acquired three-phase voltage and three-phase current to obtain the components in the synchronous rotating coordinate system. The active power and reactive power of the grid-type energy storage converter are calculated using instantaneous power theory, and the formula is as follows:

[0039] P_i=1.5*(U_d_i*I_d_i+U_q_i*I_q_i)

[0040] Q_i=1.5*(U_q_i*I_d_i-U_d_i*I_q_i)

[0041] In the formula, P_i is the active power and Q_i is the reactive power;

[0042] The calculation results are passed through a first-order low-pass filter with a cutoff frequency of 50Hz to filter out switching frequency noise, and the power value used for control is obtained.

[0043] Step S03: Through a low-speed communication network, the active and reactive power information of all converters is exchanged among the converter controllers, and the average active and average reactive power of all parallel converters is calculated. Specifically, the controller broadcasts its filtered active and reactive power data to the network via the CAN bus (communication cycle set to 20ms). Each controller also receives power data from the other two controllers. Each controller independently calculates the global average active and average reactive power using the following formula:

[0044] P_avg=(P_1+P_2+P_3) / 3

[0045] Q_avg=(Q_1+Q_2+Q_3) / 3

[0046] In the formula, P_avg is the average active power and Q_avg is the average reactive power.

[0047] Step S04: Based on the deviation between the active power and the average active power of the converter, the first regulator generates a compensation amount for adjusting the converter frequency; specifically, the deviation formula is: ΔP_i=P_i-P_avg, ΔQ_i=Q_i-Q_avg.

[0048] Step S05: Based on the deviation between the reactive power of the converter and the average reactive power, a compensation amount for adjusting the output voltage amplitude of the converter is generated by the second regulator.

[0049] Step S06: The frequency compensation amount is superimposed on the rated angular frequency of the converter to generate a new frequency command, which is used for the frequency loop of the converter virtual synchronous machine or droop control; specifically, the frequency command is ω_i*=ω_0+Δω_i;

[0050] Step S07: The voltage amplitude compensation is superimposed on the rated voltage amplitude of the converter to generate a new voltage amplitude command, which is used in the voltage loop of the converter virtual synchronous machine or droop control; specifically, the voltage amplitude command is E_i*=E_0+ΔE_i, and rate limits are applied to ω_i* and E_i* (e.g., ±100πrad / s). 2 ) and amplitude limits (such as ω:49Hz-51Hz, E:300V-320V) to prevent command abrupt changes, and input the new command into the power-frequency loop and voltage-reactive loop in the VSG control algorithm of the converter as its new setpoint;

[0051] Step S08: Based on the total output current, calculate the circulating current suppression compensation voltage using a virtual impedance algorithm, and feed this compensation voltage forward to the output of the converter voltage loop to suppress the circulating current between parallel modules.

[0052] Furthermore, in steps S04 and S05, both the first regulator and the second regulator are proportional-integral regulators, and the regulation rules of the first regulator and the second regulator are as follows:

[0053] Δω_i=-(K_p_p*(P_i-P_avg)+K_i_p*∫(P_i-P_avg)dt);

[0054] E_i=-(K_p_q*(Q_i-Q_avg)+K_i_q*∫(Q_i-Q_avg)dt)

[0055] In the formula, K_p_p and Ki_p are the proportional and integral coefficients of the active power equalization regulator, and K_p_q and Ki_q are the proportional and integral coefficients of the reactive power equalization regulator.

[0056] Furthermore, in step S03, the low-speed communication network is one of CAN bus, RS-485 bus, or power line carrier communication, with a communication cycle of 10ms to 100ms.

[0057] Furthermore, step S08 specifically includes the following steps:

[0058] Step S081: Bandpass filtering or high-pass filtering is performed on the collected total output current to extract the high-frequency components characterizing the circulating current.

[0059] Step S082: Multiply the extracted high-frequency component of the circulating current by a virtual impedance value to obtain the compensation voltage used to suppress the circulating current;

[0060] Step S083: The compensation voltage is superimposed on the reference voltage signal output by the local voltage loop at the feedforward point.

[0061] Furthermore, the virtual impedance value is one of pure virtual resistance, pure virtual inductance, or a composite resistance-inductance virtual impedance, and its expression is Z_v=R_v+jωL_v, where ω is the fundamental angular frequency.

[0062] Furthermore, in steps S06 and S07, the new frequency command and the new voltage amplitude command are subject to rate limiting and amplitude limiting.

[0063] Under various load conditions, the active and reactive power deviations of the three converters are controlled within 2%, achieving precise equalization. At the same time, the high-frequency circulating current is suppressed to below 1A, the system loss is significantly reduced, and the current waveform quality is significantly improved, verifying the effectiveness and superiority of the method described in this embodiment.

[0064] This invention obtains average power through a low-speed communication network and uses a proportional-integral regulator to adjust the deviation between local power and average power without error. Combining the precision of communication control and the speed of local control, it can fundamentally eliminate power distribution errors caused by parameter differences, line impedance mismatch, etc., ensuring that each converter strictly shares the load according to its rated capacity ratio, avoiding the unreasonable situation of some units being overloaded while others are lightly loaded, and improving capacity utilization and operational reliability.

[0065] The virtual impedance feedforward suppression of circulating current is adopted based on local total current measurement. By extracting the high-frequency harmonic components in the total current and multiplying them by the virtual impedance, a compensation voltage with the opposite effect is generated and fed forward directly to the modulation stage to form a high-speed local negative feedback closed loop. The response speed is extremely fast, which can accurately and effectively suppress the high-frequency circulating current caused by dead zone effect, switching frequency harmonics and parameter asymmetry, significantly reduce conduction loss and switching loss, reduce device heat generation, and improve operating efficiency and lifespan.

[0066] Low-speed communication is responsible for solving low-frequency, global power distribution problems. It is insensitive to communication delays and interruptions, ensuring robustness. High-speed local virtual impedance loop is responsible for solving high-frequency, local circulating current problems. It does not rely on communication at all, ensuring fast response. The combination of the two overcomes the disadvantage of low accuracy of droop control without communication, and makes up for the shortcomings of pure communication control in suppressing high-frequency circulating current and slow dynamics, achieving the best balance between performance and reliability.

[0067] By limiting the rate and amplitude of frequency and voltage commands, the reference command can be prevented from changing abruptly due to communication data jumps or drastic power fluctuations, thus avoiding excessive impact on the system. This enhances the robustness and stability of the control system, enabling it to better adapt to complex operating conditions such as load changes and power grid disturbances.

[0068] By using a distributed loop, differences in line impedance and body parameters can be automatically compensated, reducing the stringent requirements for the length and specifications of connecting cables, simplifying the complexity of on-site installation and maintenance, and improving the applicability and economy in practical projects.

[0069] In the description of this invention, it should be understood that the terms "coaxial," "bottom," "one end," "top," "middle," "other end," "upper," "side," "top," "inner," "front," "center," "both ends," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0070] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "setting," "connection," "fixing," "screw connection," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two components or the interaction between two components. Unless otherwise explicitly limited, those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0071] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for power sharing and circulating current suppression of multiple grid-connected energy storage converters operating in parallel, characterized in that, The following steps are involved: Step S01: At each parallel grid-type energy storage converter, the three-phase voltage, three-phase current and total output current flowing into the common coupling point are collected in real time at the output terminal of the grid-type energy storage converter. Step S02: Based on the three-phase voltage and three-phase current, calculate the active power and reactive power output by the converter; Step S03: Through a low-speed communication network, exchange the active power and reactive power information of all converters between each converter controller, and calculate the average active power and average reactive power of all parallel converters. Step S04: Based on the deviation between the active power and the average active power of the converter, the first regulator generates a compensation amount for adjusting the frequency of the converter. Step S05: Based on the deviation between the reactive power of the converter and the average reactive power, a compensation amount for adjusting the output voltage amplitude of the converter is generated by the second regulator. Step S06: The frequency compensation amount is superimposed on the rated angular frequency of the converter to generate a new frequency command, which is used for the frequency loop of the converter virtual synchronous machine or droop control. Step S07: The voltage amplitude compensation amount is superimposed on the rated voltage amplitude of the converter to generate a new voltage amplitude command, which is used for the voltage loop of the converter virtual synchronous machine or droop control. Step S08: Based on the total output current, calculate the circulating current suppression compensation voltage using a virtual impedance algorithm, and feed this compensation voltage forward to the output of the converter voltage loop to suppress the circulating current between parallel modules.

2. The method for power sharing and circulating current suppression of multiple grid-type energy storage converters operating in parallel according to claim 1, characterized in that, In steps S04 and S05, both the first regulator and the second regulator are proportional-integral regulators, and the regulation rules of the first regulator and the second regulator are as follows: Δω_i=-(K_p_p*(P_i-P_avg)+K_i_p*∫(P_i-P_avg)dt); E_i=-(K_p_q*(Q_i-Q_avg)+K_i_q*∫(Q_i-Q_avg)dt) where K_p_p and K_i_p are the proportional and integral coefficients of the active power sharing regulator, and K_p_q and K_i_q are the proportional and integral coefficients of the reactive power sharing regulator.

3. The method for power sharing and circulating current suppression of multiple grid-type energy storage converters operating in parallel according to claim 1, characterized in that, In step S03, the low-speed communication network is one of CAN bus, RS-485 bus, or power line carrier communication, with a communication period of 10ms to 100ms.

4. The method for power sharing and circulating current suppression of multiple grid-type energy storage converters operating in parallel according to claim 1, characterized in that, Step S08 specifically includes the following steps: Step S081: Bandpass filtering or high-pass filtering is performed on the collected total output current to extract the high-frequency components characterizing the circulating current. Step S082: Multiply the extracted high-frequency component of the circulating current by a virtual impedance value to obtain the compensation voltage used to suppress the circulating current; Step S083: The compensation voltage is superimposed on the reference voltage signal output by the local voltage loop at the feedforward point.

5. The method for power sharing and circulating current suppression of multiple grid-type energy storage converters operating in parallel according to claim 4, characterized in that, The virtual impedance value is one of the following: pure virtual resistance, pure virtual inductance, and composite resistance-inductance virtual impedance. The expression is Z_v=R_v+jωL_v, where ω is the fundamental angular frequency.

6. The method for power sharing and circulating current suppression of multiple grid-type energy storage converters operating in parallel according to claim 1, characterized in that, In steps S06 and S07, the new frequency command and the new voltage amplitude command are subject to rate limiting and amplitude limiting, respectively.

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