Energy storage system and current sharing control method thereof

Abstract

The invention provides an energy storage system and a current sharing control method thereof, the energy storage system comprises a plurality of energy storage units and a plurality of power converters, and each power converter comprises a three-phase four-wire system power conversion circuit and a controller. The controller is used for controlling the three-phase four-wire system power conversion circuit to convert direct current output by the plurality of energy storage units into alternating current and output the alternating current to a load, the controller is also used for injecting an alternating current signal into the three-phase four-wire system power conversion circuit, and the frequency of the alternating current signal changes along with the output power of the three-phase four-wire system power conversion circuit; the target value is obtained by dividing the demanded power of the load by the number of the plurality of power converters, and the characteristic that the power is autonomously and evenly distributed when the power converters are connected to the load in parallel can be achieved through autonomous adjustment of each power converter.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy, and particularly relates to an energy storage system and a current sharing control method thereof. Background Art

[0002] The traditional power grid structure is centralized power supply. Although it is convenient for management, it also has disadvantages such as large transmission losses, high production costs, and low energy conversion efficiency. Facing the increasingly flexible electricity demand nowadays, it has gradually become unable to meet the requirements. Distributed generation has the characteristics of economy, flexibility, and environmental protection, and can effectively make up for the disadvantages of traditional centralized power supply. It is easier to meet the power supply needs of special areas such as mountainous areas and islands, and has become the focus of attention of countries around the world. However, there are also some defects in existing distributed generation technologies, mainly manifested as follows: 1) The distributed renewable energy generation capacity is small, the ability to independently support loads is limited, and off-grid operation is not supported; 2) Distributed generation is generally directly connected to the distribution network through a booster device, and its output power mostly has randomness and uncertainty. With the continuous increase in the penetration rate, it seriously affects the stability of the distribution network operation; 3) The traditional power operation and market mechanism cannot well adapt to the current situation of large-scale grid connection of renewable energy. In addition, there are a large number of unbalanced loads and nonlinear loads in the distributed system. Compared with the three-phase three-wire system, the three-phase four-wire system has a better power supply effect, and its related research has become one of the current hotspots.

[0003] In order to improve the reliability, expandability, and flexibility of the distributed generation system, the parallel power converter is an effective technical approach. However, due to the differences between devices, the incomplete identity of line parameters, and the difference in power-on timing, there will be a certain difference in the output voltage of the power converter, which will cause the output current of the power converter to flow from the high-level power converter to the low-level power converter, forming a circulating current. If the circulating current is too large, it will cause distortion of the voltage and current waveforms and a decline in power quality. In severe cases, it will damage the control equipment and lead to the collapse of the power system. Therefore, it is of great significance to study the parallel current sharing control strategy of power converters, especially the parallel current sharing control strategy of three-phase four-wire power converters. Summary of the Invention

[0004] An energy storage system and a current sharing control method thereof provided by the present application are used for autonomous current sharing control of power converters.

[0005] In a first aspect, the present application provides an energy storage system, which includes a plurality of energy storage units and a plurality of power converters. The DC terminals of the plurality of power converters are connected to the plurality of energy storage units, and the output terminals of the plurality of power converters are used to be connected to a load. Each of the plurality of power converters includes a three-phase four-wire power conversion circuit and a controller; the controller is used to control the three-phase four-wire power conversion circuit to convert the direct current output by the plurality of energy storage units into alternating current and output it to the load, and the controller is further used to inject an alternating current signal into the three-phase four-wire power conversion circuit, and the frequency of the alternating current signal changes with the magnitude of the output power of the three-phase four-wire power conversion circuit, so that the output power of the three-phase four-wire power conversion circuit reaches a target value, where the target value is the required power of the load divided by the number of the plurality of power converters.

[0006] The present application can achieve precise current sharing control of three-phase four-wire parallel power converters without relying on communication, and has significant advantages such as high reliability, low cost, plug-and-play, etc., bringing more convenience and safety to the application of distributed power generation systems.

[0007] In the present application, the controller is specifically used to generate an alternating current signal according to the output power of the three-phase four-wire power conversion circuit; inject the generated alternating current signal into the three-phase four-wire power conversion circuit; after injecting the alternating current signal into the three-phase four-wire power conversion circuit, generate positive-sequence, negative-sequence, and zero-sequence virtual impedances according to the alternating current signal components output by the power conversion circuit; adjust the total reference voltage value according to the positive-sequence, negative-sequence, and zero-sequence virtual impedances, and generate a drive signal for controlling the on / off of the switching tubes in the three-phase four-wire power conversion circuit based on the total reference voltage value, so that the output power of the three-phase four-wire power conversion circuit reaches the target value, that is, the characteristic of autonomously and evenly distributing power when each power converter is paralleled to the load can be achieved.

[0008] This application is based on the adaptive virtual impedance regulation technology, enabling the parallel three-phase four-wire power converters to actively regulate their own equivalent output impedance to ensure precise current distribution control. Moreover, considering that the equal-sharing conditions for positive-sequence, negative-sequence, and zero-sequence currents are not exactly the same, and the equivalent circuits of different phase sequences are independent of each other, this application independently regulates the virtual impedance of the positive sequence and the virtual impedance of the negative sequence and zero sequence respectively to ensure precise equal sharing of positive-sequence, negative-sequence, and zero-sequence currents. In addition, the principle for obtaining the virtual impedance in this application is as follows: Inject one or more AC signals into the three-phase four-wire power converter to establish a droop relationship between the AC signal frequency and the variables to be equal-shared (including the virtual impedance corresponding to the positive sequence, negative sequence, and zero sequence), and then establish a coupling relationship between the power of the AC signal and the virtual resistance, thereby realizing the adaptive regulation of the virtual impedance. Among them, the virtual impedance refers to adding an output virtual impedance regulation module outside the voltage closed-loop control of the power converter to regulate the equivalent output impedance of the power converter, thereby achieving certain control objectives, such as reducing the sensitivity of the power distribution of the droop control power converter to the differences in line impedance and design parameters, etc.

[0009] In this application, during the process of the controller adjusting the total reference voltage value, the frequency of the AC signal injected into the power conversion circuit fluctuates with the change of the output power of the power conversion circuit, and the generated virtual impedance of the positive sequence, negative sequence, and zero sequence also changes accordingly. Finally, after the output power of the power conversion circuit stabilizes to the target value, that is, after the power is autonomously and evenly distributed, the frequency of the AC signal and the generated virtual impedance of the positive sequence, negative sequence, and zero sequence also become fixed.

[0010] In some embodiments of this application, since the AC signal injected into the power conversion circuit will generate harmonics in the output of the power conversion circuit, therefore, after the output power of the power conversion circuit reaches the target value, the virtual impedance of the positive sequence, negative sequence, and zero sequence can be fixed, and the generated AC signal injected into the power conversion circuit can be stopped, that is, the injected AC signal is removed, and then the total reference voltage value is directly generated according to the fixed virtual impedance of the positive sequence, negative sequence, and zero sequence to eliminate the harmonic interference of the AC signal.

[0011] In some embodiments of the present application, after stopping injecting the generated AC signal into the power conversion circuit, that is, after completing the autonomous equalization power distribution, during the use of the power converter, due to reasons such as changes in load demand or changes in the number of parallel power converters in the energy storage system, new unequal current sharing problems may occur. Therefore, subsequently, a detection circuit can be used to detect the output of the power conversion circuit. When the controller determines that the fluctuation value of the output power of the power conversion circuit is greater than the set threshold, it indicates that a new unequal current sharing problem has occurred, and the above-mentioned self-help equalization distribution process is restarted, that is, an AC signal is injected into the power conversion circuit, and then positive-sequence, negative-sequence, and zero-sequence virtual impedances are generated according to the AC signal components, and the total reference voltage value is adjusted again according to the positive-sequence, negative-sequence, and zero-sequence virtual impedances, so that the output power of the power conversion circuit reaches the target value again, that is, the autonomous equalization power distribution is completed again.

[0012] In other embodiments of the present application, in order to avoid new unequal current sharing problems during the use after each completion of the autonomous equalization power distribution, after stopping injecting the generated AC signal into the power conversion circuit, an AC signal can be re-injected into the power conversion circuit every time a set time threshold is passed, so as to prevent unequal current sharing problems from occurring within the set time threshold.

[0013] In some embodiments of the present application, the controller is configured to: when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtain a fundamental angular frequency command value according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtain a fundamental angular frequency command value according to the positive-sequence active power of the three-phase four-wire power conversion circuit; generate a fundamental reference voltage command value according to the fundamental angular frequency command value. When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtain a frequency command value of the first AC signal according to the positive-sequence active power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtain a frequency command value of the first AC signal according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; generate a first AC signal reference voltage command value according to the frequency command value of the first AC signal. Obtain a frequency command value of the second AC signal according to the unbalanced power of the three-phase four-wire power conversion circuit, and generate a second AC signal reference voltage command value according to the frequency command value of the second AC signal. When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, calculate the corresponding reactive power according to the first AC signal component and the second AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the reactive power corresponding to the first AC signal component and the second AC signal component respectively; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, calculate the corresponding active power according to the first AC signal component and the second AC signal component output from the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the active power corresponding to the first AC signal component and the second AC signal component respectively; generate a virtual resistance voltage drop value according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit. Generate a total reference voltage value according to the fundamental reference voltage command value, the first AC signal reference voltage command value, the second AC signal reference voltage command value, and the virtual resistance voltage drop value.

[0014] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the fundamental angular frequency command value is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the fundamental angular frequency command value is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; the fundamental reference voltage command value is generated according to the fundamental angular frequency command value. When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the frequency command value of the first AC signal is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the frequency command value of the first AC signal is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; the first AC signal reference voltage command value is generated according to the frequency command value of the first AC signal. When the line of the three-phase four-wire power conversion circuit has a resistive characteristic and the line impedance angle of this device is equal to that of other power converters, the corresponding reactive power is calculated according to the first AC signal component output by the three-phase four-wire power conversion circuit, and the positive-sequence, negative-sequence, and zero-sequence virtual resistances are generated according to the reactive power corresponding to the first AC signal component; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic and the line impedance angle of this device is equal to that of other power converters, the corresponding active power is calculated according to the first AC signal component output by the three-phase four-wire power conversion circuit, and the positive-sequence, negative-sequence, and zero-sequence virtual resistances are generated according to the active power corresponding to the first AC signal component; the virtual resistance voltage drop value is generated according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit. The total reference voltage value is generated according to the fundamental reference voltage command value, the first AC signal reference voltage command value, and the virtual resistance voltage drop value.

[0015] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental angular frequency command value; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental angular frequency command value.

[0016] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal.

[0017] In some embodiments of the present application, the unbalanced power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the second AC signal.

[0018] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit exhibits a resistive characteristic, the reactive power corresponding to the AC signal component is negatively correlated with the virtual resistances of the positive sequence, negative sequence, and zero sequence; or, when the line of the three-phase four-wire power conversion circuit exhibits an inductive characteristic, the active power corresponding to the AC signal component is negatively correlated with the virtual resistances of the positive sequence, negative sequence, and zero sequence.

[0019] In a second aspect, the present application provides a current sharing control method for an energy storage system, and this method can be executed by a controller in a power converter. The method provided by the embodiments of the present application includes the following steps: injecting an AC signal into the three-phase four-wire power conversion circuits of multiple power converters, where the frequency of the AC signal varies with the magnitude of the output power of the three-phase four-wire power conversion circuit, so that the output power of the three-phase four-wire power conversion circuit reaches a target value, where the target value is the required power of the load connected to the output end of the three-phase four-wire power conversion circuit divided by the number of power converters included in the energy storage system.

[0020] The method provided by the embodiments of the present application specifically includes the following steps: generating an AC signal according to the output power of the three-phase four-wire power conversion circuit; injecting the generated AC signal into the three-phase four-wire power conversion circuit; after injecting the AC signal into the three-phase four-wire power conversion circuit, generating virtual impedances of the positive sequence, negative sequence, and zero sequence according to the AC signal components output by the power conversion circuit; adjusting the total reference voltage value according to the virtual impedances of the positive sequence, negative sequence, and zero sequence, and generating a driving signal for controlling the on / off of the switching tubes in the three-phase four-wire power conversion circuit based on the total reference voltage value, so that the output power of the three-phase four-wire power conversion circuit reaches the target value.

[0021] In some embodiments of the present application, it further includes: after the output power of the three-phase four-wire power conversion circuit reaches the target value, fixing the virtual impedances of the positive sequence, negative sequence, and zero sequence, and stopping injecting the generated AC signal into the three-phase four-wire power conversion circuit, and adjusting the total reference voltage value according to the fixed virtual impedances of the positive sequence, negative sequence, and zero sequence.

[0022] In some embodiments of the present application, after stopping injecting the generated AC signal into the three-phase four-wire power conversion circuit, when the fluctuation value of the output power of the three-phase four-wire power conversion circuit is greater than a set threshold, or, after a set time threshold, injecting an AC signal into the three-phase four-wire power conversion circuit.

[0023] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, a fundamental angular frequency command value is obtained according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, a fundamental angular frequency command value is obtained according to the positive-sequence active power of the three-phase four-wire power conversion circuit; a fundamental reference voltage command value is generated according to the fundamental angular frequency command value.

[0024] When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence active power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; a first AC signal reference voltage command value is generated according to the frequency command value of the first AC signal.

[0025] A frequency command value of the second AC signal is obtained according to the unbalanced power of the three-phase four-wire power conversion circuit, and a second AC signal reference voltage command value is generated according to the frequency command value of the second AC signal.

[0026] When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the corresponding reactive powers are respectively calculated according to the first AC signal component and the second AC signal component output by the three-phase four-wire power conversion circuit, and positive-sequence, negative-sequence, and zero-sequence virtual resistances are generated according to the reactive powers respectively corresponding to the first AC signal component and the second AC signal component; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the corresponding active powers are respectively calculated according to the first AC signal component and the second AC signal component output from the three-phase four-wire power conversion circuit, and positive-sequence, negative-sequence, and zero-sequence virtual resistances are generated according to the active powers respectively corresponding to the first AC signal component and the second AC signal component; a virtual resistance voltage drop value is generated according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit.

[0027] A total reference voltage value is generated according to the fundamental reference voltage command value, the first AC signal reference voltage command value, the second AC signal reference voltage command value, and the virtual resistance voltage drop value.

[0028] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, a fundamental angular frequency command value is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, a fundamental angular frequency command value is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; a fundamental reference voltage command value is generated according to the fundamental angular frequency command value.

[0029] When the line of the three-phase four-wire power conversion circuit exhibits a resistive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit exhibits an inductive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; a reference voltage command value of the first AC signal is generated according to the frequency command value of the first AC signal.

[0030] When the line of the three-phase four-wire power conversion circuit exhibits a resistive characteristic and the line impedance angle of this device is equal to that of other power converters, the reactive power corresponding to the first AC signal component output by the three-phase four-wire power conversion circuit is calculated, and virtual resistors of positive sequence, negative sequence, and zero sequence are generated according to the reactive power corresponding to the first AC signal component; or, when the line of the three-phase four-wire power conversion circuit exhibits an inductive characteristic and the line impedance angle of this device is equal to that of other power converters, the active power corresponding to the first AC signal component output by the three-phase four-wire power conversion circuit is calculated, and virtual resistors of positive sequence, negative sequence, and zero sequence are generated according to the active power corresponding to the first AC signal component; a virtual resistor voltage drop value is generated according to the virtual resistors of positive sequence, negative sequence, and zero sequence and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit.

[0031] A total reference voltage value is generated according to the fundamental wave reference voltage command value, the reference voltage command value of the first AC signal, and the virtual resistor voltage drop value.

[0032] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit exhibits a resistive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value; or, when the line of the three-phase four-wire power conversion circuit exhibits an inductive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value.

[0033] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit exhibits a resistive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal; or, when the line of the three-phase four-wire power conversion circuit exhibits an inductive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal.

[0034] In some embodiments of the present application, the unbalanced power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the second AC signal.

[0035] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit exhibits a resistive characteristic, the reactive power corresponding to the AC signal component is negatively correlated with the virtual resistances of the positive sequence, negative sequence, and zero sequence; or, when the line of the three-phase four-wire power conversion circuit exhibits an inductive characteristic, the active power corresponding to the AC signal component is negatively correlated with the virtual resistances of the positive sequence, negative sequence, and zero sequence.

[0036] For the technical effects achievable by any possible design in the second aspect, please refer to the technical effects achievable by any possible design in the above first aspect, which will not be repeated here. These aspects or other aspects of the present application will be more clearly understood in the following description of the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the framework structure of a distributed power generation system;

[0038] Figure 2 It is a schematic diagram of the structure of the energy storage system provided by the embodiments of the present application;

[0039] Figure 3a It is a schematic diagram of the operation of each module of the power converter provided by the embodiments of the present application under a resistive line;

[0040] Figure 3b It is a schematic diagram of the operation of each module of the power converter provided by the embodiments of the present application under an inductive line;

[0041] Figure 4 It is a schematic diagram of the current signal extraction provided by the embodiments of the present application;

[0042] Figure 5 It is a schematic diagram of the equivalent circuit after two power converters are connected in parallel based on a resistive-inductive line provided by the embodiments of the present application;

[0043] Figure 6a It is a schematic diagram of the positive sequence virtual resistance adjustment principle under a resistive or resistive-dominated line provided by the embodiments of the present application;

[0044] Figure 6b It is a schematic diagram of the negative sequence virtual resistance adjustment principle under a resistive or resistive-dominated line provided by the embodiments of the present application;

[0045] Figure 7 It is a schematic diagram of the voltage-current controller provided by the embodiments of the present application;

[0046] Figure 8a It is a schematic diagram of the operation of each module of the power converter under a resistive line or when the line impedance angles are equal provided by the embodiments of the present application;

[0047] Figure 8bThis is a schematic diagram of the operation of each module of the power converter provided by the embodiments of the present application under the condition that the inductive line or the line impedance angle is equal. Detailed implementation manners

[0048] In order to make the objectives, technical solutions, and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings. The specific operation methods in the method embodiments can also be applied to the device embodiments or system embodiments. It should be noted that in the description of the present application, "at least one" means one or more, and among them, "a plurality" means two or more. In view of this, in the embodiments of the present invention, "a plurality" can also be understood as "at least two". "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " generally represents an "or" relationship between the associated objects before and after, unless otherwise specified. In addition, it should be understood that in the description of the present application, words such as "first" and "second" are only used for the purpose of distinguishing descriptions, and cannot be understood as indicating or implying relative importance, nor can they be understood as indicating or implying an order.

[0049] It should be pointed out that "connection" in the embodiments of the present application can be understood as electrical connection, and the connection of two electrical components can be a direct or indirect connection between the two electrical components. For example, the connection between A and B can be either a direct connection between A and B or an indirect connection between A and B through one or more other electrical components. For example, for the connection between A and B, it can also be a direct connection between A and C and a direct connection between C and B, and the connection between A and B is realized through C. In some scenarios, "connection" can also be understood as coupling, such as the electromagnetic coupling between two inductors. In short, the connection between A and B enables the transmission of electrical energy between A and B.

[0050] In recent years, distributed generation has developed vigorously due to its economic, flexible, and environmentally friendly characteristics. Among them, the parallel connection of power converters can effectively improve the reliability and flexibility of distributed generation systems and has been widely used. However, due to factors such as inconsistent line and output impedance of power converters and control delay, the current and power sharing effects of the parallel system are poor, and a circulating current problem appears in the power converter parallel system. If effective suppression measures are not taken, it will lead to a decline in power quality, and in more serious cases, it will threaten the safe and stable operation of the distributed system.

[0051] At present, the existing current sharing control schemes are divided into communication-based current sharing control strategies and non-communication-based current sharing control strategies according to whether they rely on communication. Among them, the communication-based current sharing control scheme can achieve effective current sharing among parallel power converters. However, this technology is dependent on communication lines, and its reliability will be greatly affected during long-distance transmission. In addition, it will greatly increase the system cost and has a low cost performance. The non-communication-based current sharing control scheme has the advantages of high reliability, low cost, and support for plug-and-play, etc., and is more suitable for popularization and utilization. However, the existing schemes mostly focus on the equal sharing of positive-sequence current and negative-sequence current in a three-phase three-wire system. In a three-phase four-wire architecture, not only the positive-sequence and negative-sequence current sharing need to be considered, but also the zero-sequence current sharing needs to be considered, and the existing schemes rarely involve this. And in a three-phase four-wire system, if the zero-sequence current is not effectively shared, it will cause distortion of voltage and current waveforms and deterioration of power quality, seriously threatening the stable operation of the power system.

[0052] To solve the above problems, the present invention proposes an energy storage system and its current sharing control method. The core idea of the method is based on the adaptive virtual impedance regulation technology, enabling the parallel three-phase four-wire power converters to actively adjust their own equivalent output impedance to ensure precise current distribution control. And considering that the equal sharing conditions for positive-sequence, negative-sequence, and zero-sequence currents are not exactly the same, and the equivalent circuits of different phase sequences are independent of each other, therefore, this application uses the positive-sequence virtual impedance and the negative-sequence and zero-sequence virtual impedances to independently regulate respectively to ensure the precise equal sharing of positive-sequence, negative-sequence, and zero-sequence currents. In addition, the principle for obtaining the virtual impedance in this application is: injecting one or more AC signals into the three-phase four-wire power converter, constructing the droop relationship between the AC signal frequency and the variables to be equally shared (including the virtual impedances corresponding to positive-sequence, negative-sequence, and zero-sequence), and then constructing the coupling relationship between the power of the AC signal and the virtual resistance, so as to realize the adaptive regulation of the virtual impedance. Among them, the virtual impedance refers to adding an output virtual impedance regulation module outside the voltage closed-loop control of the power converter to adjust the equivalent output impedance of the power converter, so as to achieve certain control objectives, such as reducing the sensitivity of the power distribution of the droop control power converter to the differences in line impedance and design parameters, etc.

[0053] This application can achieve precise current sharing control of three-phase four-wire parallel power converters in the energy storage system without relying on communication, and has significant advantages such as high reliability, low cost, and plug-and-play, bringing more convenience and safety to the application of distributed generation systems.

[0054] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. First, the application scenarios of the power converter provided in the embodiments of the present application will be introduced. The power converter provided in the embodiments of the present application can be applied to scenarios such as photovoltaic power stations, energy storage power stations, or photovoltaic-energy storage power stations, etc.

[0055] Figure 1 Exemplarily, a schematic structural diagram of a framework of a distributed power generation system is shown.

[0056] Referring to Figure 1 , the distributed power generation system includes a plurality of photovoltaic modules or a plurality of energy storage units, and a plurality of power converters. Among them, the output terminal of one photovoltaic module or one energy storage unit is connected to the input terminal of one power converter, and the output terminals of the plurality of power converters are connected in parallel and then connected to the power grid. The power converter is used to convert the direct current stored in the energy storage unit or the direct current output by the photovoltaic module into alternating current and output it to the power grid. This application can be applied to a single-stage architecture in which the energy storage unit or the photovoltaic module is directly connected to the DC side of the power converter, and this application is also applicable to a two-stage architecture in which the energy storage unit or the photovoltaic module is connected to the DC side of the power converter through a DC / DC, that is, a DC / DC can be coupled between the energy storage unit or the photovoltaic module and the power converter.

[0057] Figure 2 Exemplarily, a schematic structural diagram of an energy storage system provided by an embodiment of this application is shown.

[0058] Based on the same inventive concept, the energy storage system includes a plurality of energy storage units and a plurality of the above-mentioned power converters of this application. Among them, the output terminal of one energy storage unit is connected to the input terminal of the power conversion circuit in one power converter, and the output terminals of the power conversion circuits in each power converter are connected in parallel and then connected to the power grid. Each power conversion circuit is used to convert the direct current stored in the energy storage unit into alternating current and output it to the power grid, and the controller in each power converter is used to balance the output power of the power conversion circuit.

[0059] Referring to Figure 2 , the energy storage system specifically includes a plurality of energy storage units and a plurality of power converters. Each power converter includes a three-phase four-wire power conversion circuit (hereinafter referred to as the power conversion circuit) and a controller. Each power converter may also include a detection circuit. Among them, the energy storage unit can be directly connected to the DC side of the power conversion circuit, or the energy storage unit can also be connected to the DC side of the power converter through a DC / DC. The output terminal of the power conversion circuit is used to connect to the load, and the controller is used to control the power conversion circuit to convert the direct current output by the energy storage unit into alternating current and output it to the load. The controller is also used to inject an alternating current signal into the power conversion circuit, and the frequency of the alternating current signal changes with the magnitude of the output power of the power conversion circuit, so that the output power of the power conversion circuit reaches the target value, where the target value is the required power of the load divided by the number of power converters. Since each power converter of the energy storage system can adjust the output power to reach the target value in the above manner, the characteristic of autonomous balanced power distribution can be achieved when each power converter is paralleled to the load.

[0060] In this application, the detection circuit is connected to the AC-side output terminal of the power conversion circuit. The detection circuit is used to detect the three-phase voltage values and three-phase current values (or four-phase current values) output by the power conversion circuit, and extract the positive-sequence component, negative-sequence component, and zero-sequence component of the output current of the power conversion circuit. When an AC signal (also called a small signal due to its small amplitude) is injected into the power conversion circuit, the AC signal component of the output current is also extracted. The controller is connected to the detection circuit. Specifically, the controller is used to generate an AC signal according to the output power of the power conversion circuit and inject the AC signal into the power conversion circuit. After injecting the AC signal into the power conversion circuit, the positive-sequence, negative-sequence, and zero-sequence virtual impedances are generated according to the AC signal component output by the power conversion circuit, the total reference voltage value is adjusted according to the positive-sequence, negative-sequence, and zero-sequence virtual impedances, and a drive signal for controlling the on / off of the switching tubes in the power conversion circuit is generated based on the total reference voltage value, so that the output power of the power conversion circuit reaches the target value. Since each power converter of the energy storage system can adjust the output power to reach the target value in the above manner, the characteristic of autonomous and balanced power distribution can be achieved when the power converters are paralleled to the load.

[0061] In this application, during the process of the controller adjusting the total reference voltage value, the frequency of the AC signal injected into the power conversion circuit fluctuates with the change of the output power of the power conversion circuit, and the generated positive-sequence, negative-sequence, and zero-sequence virtual impedances also change accordingly. Finally, after the output power of the power conversion circuit stabilizes to the target value, that is, after the autonomous and balanced power distribution is completed, the frequency of the AC signal and the generated positive-sequence, negative-sequence, and zero-sequence virtual impedances are also fixed.

[0062] In some embodiments of this application, since the AC signal injected into the power conversion circuit will generate harmonics in the output of the power conversion circuit, therefore, after the output power of the power conversion circuit reaches the target value, the positive-sequence, negative-sequence, and zero-sequence virtual impedances can be fixed, and the generated AC signal injection into the power conversion circuit can be stopped, that is, the injected AC signal is removed. Then, the total reference voltage value is directly generated according to the fixed positive-sequence, negative-sequence, and zero-sequence virtual impedances to eliminate the harmonic interference of the AC signal.

[0063] In some embodiments of the present application, after stopping injecting the generated AC signal into the power conversion circuit, that is, after completing the autonomous equalization and power distribution, during the use of the power converter, due to reasons such as changes in load demand or changes in the number of parallel power converters in the energy storage system, new current sharing problems may occur. Therefore, subsequently, a detection circuit can be used to detect the output of the power conversion circuit. When the controller determines that the fluctuation value of the output power of the power conversion circuit is greater than the set threshold, it indicates that a new current sharing problem has occurred, and the above-mentioned self-help equalization and distribution process is started again, that is, an AC signal is injected into the power conversion circuit, and then positive-sequence, negative-sequence, and zero-sequence virtual impedances are generated according to the AC signal components, and the total reference voltage value is adjusted again according to the positive-sequence, negative-sequence, and zero-sequence virtual impedances, so that the output power of the power conversion circuit reaches the target value again, that is, the autonomous equalization and power distribution is completed again.

[0064] In some other embodiments of the present application, in order to avoid new current sharing problems during the use after each completion of the autonomous equalization and power distribution, after stopping injecting the generated AC signal into the power conversion circuit, an AC signal can be injected into the power conversion circuit again every time the set time threshold is passed, so as to prevent the occurrence of current sharing problems within the set time threshold.

[0065] Figure 3a Exemplarily shows the working schematic diagram of each module of the power converter provided by the embodiments of the present application under the impedance line. Figure 3b Exemplarily shows the working schematic diagram of each module of the power converter provided by the embodiments of the present application under the inductive reactance line.

[0066] Refer to Figure 3a and Figure 3b The controller can be specifically divided into several links such as fundamental wave droop control, AC signal droop control, virtual impedance calculation, and voltage-current double-loop control. For the convenience of subsequent description, the controller can be divided into a fundamental wave droop control module, an AC signal droop control module, a virtual impedance calculation module, and a voltage-current double-loop control module according to the corresponding functions, which does not represent the actual module division in the controller. The functions implemented by each module will be introduced in detail below.

[0067] Figure 4 Exemplarily shows the schematic diagram of the current signal extraction provided by the embodiments of the present application.

[0068] The detection circuit is used to detect the three-phase voltage values and three-phase current values (or four-phase current values) output by the power conversion circuit, and extract the positive-sequence component, negative-sequence component and zero-sequence component of the output current of the power conversion circuit, as well as extract the AC signal current component additionally injected into the power conversion circuit from the output current of the power conversion circuit. The detection circuit can be specifically divided into a detection module and a current signal extraction module. The detection module is used to detect the three-phase voltage values and three-phase current values (or four-phase current values) output by the power conversion circuit, and the current signal extraction module is used to extract the positive-sequence component, negative-sequence component, zero-sequence component and AC signal current component in the output current. There can be multiple specific implementation methods for the current signal extraction module. For example, referring to Figure 4 , the current signal extraction module can be implemented based on a second-order generalized integral.

[0069] The fundamental wave droop control module can simulate the characteristics of a traditional synchronous generator, construct the droop relationship from positive-sequence reactive power (or positive-sequence active power) to frequency respectively according to whether the line is resistive (or inductive), and construct the droop relationship from positive-sequence active power (or positive-sequence reactive power) to voltage amplitude, so as to achieve the equal sharing of positive-sequence power among the power converters in the power converter parallel system. However, only one of the positive-sequence active power and positive-sequence reactive power can be accurately distributed and controlled through the droop relationship of the fundamental wave droop control module, which depends on whether the power converter is on a line dominated by inductance or resistance. For the other positive-sequence reactive power (or positive-sequence active power) that cannot be accurately distributed and controlled by the fundamental wave droop control, as well as the unbalanced power (including negative-sequence power and zero-sequence power) and neutral line current, their distribution is mainly affected by the line impedance. Taking a pure resistive or resistive-dominated resistive-inductive line as an example, the traditional fundamental wave droop control equation based on a resistive line is shown in formula (1):

[0070] w = w0 - m(Q0 - Q)

[0071] Formula (1)

[0072] E = E0 + n(P0 - P)

[0073] Among them, w and E are respectively the angular frequency and voltage amplitude output by the fundamental droop control; w0 and E0 are respectively the rated angular frequency and rated amplitude of the device; P and Q are respectively the positive-sequence active power and reactive power actually output by the power converter; P0 and Q0 are respectively the rated positive-sequence active power and rated reactive power of the power converter; m and n respectively represent the positive-sequence reactive power droop coefficient and positive-sequence active power droop coefficient. Under the fundamental droop control characteristic based on a resistive line, the distribution of positive-sequence active power is affected by the line impedance. Therefore, the droop relationship between the positive-sequence active power of the fundamental wave and the voltage amplitude can be ignored, and the output voltage amplitude of the power converter can be directly assigned the rated amplitude E0. Furthermore, the droop relationship constructed within the fundamental droop control module can be modified as shown in formula (2):

[0074] w = w0 - m(Q0 - Q)

[0075] Formula (2)

[0076] E = E0

[0077] In the fundamental droop control module of the controller, first, based on the three-phase voltage values v Cαβ detected by the detection circuit from the output of the power conversion circuit and the positive-sequence component i + 1αβ extracted from the output current by the current signal extraction module, fundamental power calculation is performed, and after low-pass filtering (LPF), the positive-sequence active power P and positive-sequence reactive power Q are obtained. Then, based on one of the positive-sequence active power P and positive-sequence reactive power Q output by the power conversion circuit, and the constructed fundamental droop relationship, a fundamental reference voltage command value is generated. Specifically, when the line of the power conversion circuit has a resistive characteristic, based on the positive-sequence reactive power output by the power conversion circuit and the constructed fundamental droop relationship from positive-sequence reactive power to frequency, a fundamental angular frequency command value is obtained, such that the positive-sequence reactive power output by the power conversion circuit is positively correlated with the fundamental angular frequency command value; when the line of the power conversion circuit has an inductive characteristic, based on the positive-sequence active power output by the power conversion circuit and the constructed fundamental droop relationship from positive-sequence active power to frequency, a fundamental angular frequency command value is obtained, such that the positive-sequence active power output by the power conversion circuit is positively correlated with the fundamental angular frequency command value; finally, based on the fundamental angular frequency command value and a fixed fundamental voltage command value, a fundamental reference voltage command value is generated. For example, in a resistive line, the fundamental angular frequency command value w* of the fundamental droop control output can be obtained according to the resistive droop relationship shown in formula (2), and then it is used to generate a fundamental reference voltage together with the fundamental voltage command value with a fixed value of √2E * to obtain the fundamental reference voltage command value v * 1αβγref .

[0078] Taking a resistive or resistive-dominated resistive-inductive line as an example, for the positive-sequence active power and unbalanced power (including negative-sequence power and zero-sequence power) whose distribution is mainly affected by the line impedance, in this application, the problem of its uneven distribution can be solved by the AC signal injection method. Among them, the injected AC signal can also be called a small signal due to its small voltage amplitude, and its signal frequency is different from the fundamental frequency of the power converter. By analyzing the output current of the power converter, the injected AC signal can be analyzed. Specifically, in the AC signal droop control module, a relationship similar to the above-mentioned resistive droop can be constructed. Specifically, the first AC signal can be used to construct the droop relationship between the frequency of the first AC signal and the positive-sequence active power, and the second AC signal can be used to construct the droop relationship between the frequency of the second AC signal and the unbalanced power. The AC signal droop control equation for the resistive line is shown in formula (3):

[0079]

[0080] where, w * ss1 is the frequency command value of the first AC signal corresponding to the positive-sequence active power; w ss10 is the rated frequency of the first AC signal corresponding to the positive-sequence active power; k ss1 is the droop coefficient of the positive-sequence active power AC signal, and its value is defined as a positive value; w * ss2 is the frequency command value of the second AC signal corresponding to the unbalanced power; w ss20 is the rated frequency of the second AC signal corresponding to the unbalanced power; k ss2 is the droop coefficient of the unbalanced power AC signal, and its value is defined as a positive value; Q u is the unbalanced power, which includes negative-sequence power and zero-sequence power.

[0081] In the AC signal droop control module of the controller, a first AC signal reference voltage command value can be generated according to the other one of the positive-sequence active power and the positive-sequence reactive power of the power conversion circuit (i.e., the positive-sequence power affected by the line impedance), and the constructed first AC signal droop relationship. Specifically, when the line of the power conversion circuit has a resistive characteristic, the frequency command value of the first AC signal can be obtained according to the positive-sequence active power output by the power conversion circuit and the constructed first AC signal droop relationship from the positive-sequence active power to the frequency of the first AC signal; wherein, in the first AC signal droop relationship from the positive-sequence active power to the frequency of the first AC signal, the positive-sequence active power is positively correlated with the frequency of the first AC signal, so that the positive-sequence active power output by the power conversion circuit is positively correlated with the frequency command value of the first AC signal. When the line of the power conversion circuit has an inductive characteristic, the frequency command value of the first AC signal is obtained according to the positive-sequence reactive power output by the power conversion circuit and the constructed first AC signal droop relationship from the positive-sequence reactive power to the frequency of the first AC signal; wherein, in the first AC signal droop relationship from the positive-sequence reactive power to the frequency of the first AC signal, the positive-sequence reactive power is positively correlated with the frequency of the first AC signal, so that the positive-sequence reactive power output by the power conversion circuit is positively correlated with the frequency command value of the first AC signal. Then, according to the frequency command value of the first AC signal and the set voltage command value of the first AC signal, for example, it can take values between 1V - 2V, a first AC signal reference voltage command value is generated. For example, in a resistive line, the frequency command value ω of the first AC signal can be obtained according to the droop relationship between the frequency of the first AC signal and the positive-sequence active power shown in formula (3). * ss1 , and the set voltage amplitude command value V of the first AC signal * ss1 After generating the AC signal reference voltage, the first AC signal reference voltage command value v generated based on the AC signal droop control is obtained * ss1αβγref .

[0082] In the AC signal droop control module of the controller, a second AC signal reference voltage command value can also be generated according to the unbalanced power of the power conversion circuit and the constructed second AC signal droop relationship. Specifically, the unbalanced power of the power conversion circuit can be calculated according to the negative-sequence current and zero-sequence current output by the power conversion circuit and the set voltage amplitude. For example, according to the negative-sequence component i - 1αβ and the zero-sequence component i 1γ extracted from the output current by the current signal extraction module, the unbalanced power calculation is performed, and the unbalanced power Q is obtained after low-pass filtering uAfter that, according to the unbalanced power of the power conversion circuit and the constructed second AC signal droop relationship from the unbalanced power to the frequency of the second AC signal, a frequency command value of the second AC signal is obtained. Among them, in the second AC signal droop relationship from the unbalanced power to the frequency of the second AC signal, the unbalanced power is positively correlated with the frequency of the second AC signal, so that the unbalanced power is positively correlated with the frequency command value of the second AC signal. For example, the frequency command value w of the second AC signal can be obtained according to the droop relationship between the frequency of the second AC signal and the unbalanced power shown in formula (3). * ss2 Finally, according to the frequency command value of the second AC signal and the set voltage command value of the second AC signal, which can take values between 1V and 2V for example, a reference voltage command value of the second AC signal is generated. For example, the frequency command value w of the second AC signal * ss2 and the set voltage amplitude command value V of the second AC signal * ss2 After generating the reference voltage of the AC signal, a reference voltage command value v of the second AC signal generated based on the AC signal droop control is obtained. * ss2αβγref 。

[0083] Figure 5 Exemplarily shown is a schematic diagram of the equivalent circuit after two power converters based on an inductive - resistive line are connected in parallel provided by an embodiment of the present application.

[0084] Referring to Figure 5 , taking an inductive - resistive line that is purely resistive or dominated by resistance as an example, when the line impedance satisfies that the line resistance and the line inductance are equal to each other or the value that satisfies a specific mathematical relationship such as formula (4) is equal to 0, the positive - sequence currents of each power converter in the parallel system can be evenly divided. And when the line impedance satisfies the condition that the voltage amplitudes are equal to each other, the negative - sequence current and the zero - sequence current can be evenly divided. Thus, it can be seen that when changing the equivalent impedance on the line of each power converter, precise power distribution control can be achieved.

[0085]

[0086]

[0087] B = R1R2+(R1 + R2)R L

[0088] where P1' and P2' are the active powers output by power converter 1 and power converter 2 respectively, R1 and R2 are the line resistances of the two power converters respectively, and R L is the common load, and respectively represent the phase differences between the output currents of two power converters and the common connection point, and X1 and X2 respectively represent the line reactances of the two power converters.

[0089] As can be seen from the above analysis, in order to achieve precise power distribution control, a virtual impedance that can be adaptively adjusted can be directly introduced into the system, thereby changing the equivalent impedance on the line, and realizing the adaptive adjustment of power under the condition that the line impedance information is unknown. In addition, it can be seen from the above analysis that in a resistive or resistive-dominated inductive line, only considering the introduction of a virtual resistor can still meet the condition of active power sharing. Therefore, in order to simplify the control, a control strategy of only introducing a virtual resistor without considering a virtual inductor can be adopted in this application.

[0090] In a resistive line, the reactive power of the injected AC signal is negatively correlated with the frequency of the AC signal. And if you want to adjust the distribution relationship between the positive-sequence active power and the unbalanced power, a virtual resistor under the corresponding phase sequence needs to be introduced to achieve it. However, there is no direct connection between the reactive power of the AC signal and the virtual resistor. Therefore, it is necessary to artificially construct the coupling relationship between the reactive power of the AC signal and the virtual resistors of the positive sequence, negative sequence, and zero sequence, so as to realize the adaptive adjustment of the virtual resistor, so as to achieve the precise equal sharing of the positive-sequence, negative-sequence, and zero-sequence currents in the voltage control link through the virtual impedance. The coupling relationship between the reactive power of the AC signal and the virtual resistor in a resistive line is shown in formula (5):

[0091]

[0092] where, R + v is the introduced positive-sequence virtual resistor; R + v0 is the positive-sequence virtual resistor bias, which is used to improve the stability of the system and at the same time increase the proportion of the resistive component of the line impedance in the positive-sequence equivalent circuit. It can be set to 0 for simplifying the analysis process; k R1 is the coupling coefficient between the reactive power of the first AC signal and the positive-sequence virtual resistor; Q ss1 is the reactive power of the first AC signal; R - v is the introduced negative-sequence virtual resistor; R - v0 is the virtual resistor bias of the sequence, similar to the positive-sequence virtual resistor bias. For simplifying the analysis process, it can be set to 0; k R2 is the coupling coefficient between the reactive power of the second AC signal and the positive-sequence virtual resistor; Q ss2 is the reactive power of the second AC signal.

[0093] The realization of the virtual resistor can be based on but not limited to the following formula:

[0094]

[0095]

[0096]

[0097] Among them, R + v is the positive-sequence virtual resistance; R - v is the negative-sequence virtual resistance; R 0 v is the zero-sequence virtual resistance.

[0098] Figure 6a Exemplarily shows a schematic diagram of the positive-sequence virtual resistance adjustment principle under a resistive or resistive-dominated line provided by an embodiment of the present application, Figure 6b Exemplarily shows a schematic diagram of the negative-sequence virtual resistance adjustment principle under a resistive or resistive-dominated line provided by an embodiment of the present application.

[0099] Referring to Figure 6a , in the positive-sequence equivalent circuit after the parallel connection of the power converters of two resistive lines, I in the figure L + is the positive-sequence equivalent current source on the load side, R1 and R2 are the line resistances of the two power converters respectively, L1 and L2 are the line inductances of the two power converters respectively, w * 1 and w * 2 are the fundamental voltage frequencies output by the two power converters respectively, w * ss11 and w * ss12 are the frequencies of the first AC signals injected into the two power converters respectively. The solid arrows in the figure represent the actual flow directions of the positive-sequence active powers P1 and P2 and the reactive powers Q ss11 and Q ss12 in the circuit. According to the relationships shown in formulas (3) and (5), these two formulas will be used to generate the frequencies w * ss11 and w * ss12 of the first AC signal and the positive-sequence virtual resistances R + v1 and R + v2 , as shown by the dashed arrows. At the same time, the thick arrows in the figure respectively represent the frequencies w * ss11 and w * ss12 of the first AC signal to the reactive power Q of the first AC signalss11 and Q ss12 The actual physical relationship between them, that is, they are negatively correlated; and the virtual resistance R in positive sequence + v1 and R + v2 to the actual physical relationship between the positive-sequence active powers P1 and P2, that is, they are negatively correlated.

[0100] Referring to Figure 6b , in the negative-sequence equivalent circuit after the parallel connection of the power converters in two resistive lines, I in the figure L - is the negative-sequence equivalent current source on the load side, w * ss21 and w * ss22 are respectively the frequencies of the second AC signals injected into the two power converters. The solid arrows in the figure represent the actual power flow directions in the circuit, including the unbalanced powers Q u1 and Q u2 and the reactive powers Q ss21 and Q ss22 of the second AC signal. The dashed arrows represent the relationships constructed in this application according to the formulas (3) and (5). These two formulas will be used to generate the frequencies w * ss21 and w * ss22 and the virtual resistance R in negative sequence - v1 and R - v2 . At the same time, the thick arrows in the figure respectively represent the frequencies w * ss21 and w * ss22 to the reactive powers Q ss21 and Q ss22 of the second AC signal, that is, they are negatively correlated; and the virtual resistance R in negative sequence - v1 and R - v2 to the unbalanced powers Q u1 and Q u2 of the second AC signal, that is, they are negatively correlated.

[0101] In the virtual impedance calculation module of the controller, a virtual resistor voltage drop value can be generated based on the AC signal components extracted from the power conversion circuit and the constructed virtual impedance coupling relationship. Specifically, when the line of the power conversion circuit exhibits a resistive characteristic, the reactive power of the AC signal can be calculated based on the AC signal components extracted from the power conversion circuit. For example, based on the reference voltage value v of the first AC signal ss1αβref and the first AC signal current component i in the output current extracted by the current signal extraction module + ss1αβ the reactive power Q of the first AC signal can be obtained through AC signal power calculation and then low-pass filtering ss1 . Based on the reference voltage value v of the second AC signal ss2αβref and the second AC signal current component i in the output current extracted by the current signal extraction module + ss2αβ the reactive power Q of the second AC signal can be obtained through AC signal power calculation and then low-pass filtering ss2 . Then, based on the reactive power of the AC signal and the constructed virtual impedance coupling relationship from the reactive power of the AC signal to the virtual resistors of the positive sequence, negative sequence, and zero sequence, virtual resistors of the positive sequence, negative sequence, and zero sequence are generated. Among them, in the virtual impedance coupling relationship from the reactive power of the AC signal to the virtual resistors of the positive sequence, negative sequence, and zero sequence, the reactive power of the AC signal is negatively correlated with the virtual resistors of the positive sequence, negative sequence, and zero sequence, such that the reactive power corresponding to the AC signal components is negatively correlated with the virtual resistors of the positive sequence, negative sequence, and zero sequence. For example, based on the coupling relationship formula between the reactive power of the AC signal and the virtual resistor shown in formula (5) and the relationship formula between the virtual resistors in formula (6), the virtual resistor R of the positive sequence + v , the virtual resistor R of the negative sequence - v and the virtual resistor R of the zero sequence 0 vSpecifically, when the line of the power conversion circuit is inductive, the active power of the AC signal can be calculated based on the AC signal component extracted from the power conversion circuit; then, based on the active power of the AC signal and the virtual impedance coupling relationship that constructs the virtual resistance of the active power of the AC signal to positive sequence, negative sequence, and zero sequence, the virtual resistances of positive sequence, negative sequence, and zero sequence are generated. Among them, in the virtual impedance coupling relationship of the active power of the AC signal to the virtual resistances of positive sequence, negative sequence, and zero sequence, the active power of the AC signal is negatively correlated with the virtual resistances of positive sequence, negative sequence, and zero sequence, such that the active power corresponding to the AC signal component is negatively correlated with the virtual resistances of positive sequence, negative sequence, and zero sequence. Finally, based on the virtual resistances of positive sequence, negative sequence, and zero sequence and the positive sequence, negative sequence, and zero sequence currents output by the power conversion circuit, after calculating the virtual resistance voltage drop, an adaptive virtual resistance voltage drop value v is generated. zαβγ 。

[0102] After calculating the virtual voltage drop generated on the virtual impedance using the output current according to the above method, the corresponding virtual voltage drop can be subtracted when calculating the voltage reference value in the subsequent calculation, so as to achieve the effect of simulating the line impedance. It should be noted additionally that although an AC signal is injected into the power converter, when calculating the virtual impedance voltage drop, the present application only considers the virtual impedance voltage drop at the fundamental frequency, and no virtual resistance is introduced at the respective AC signal frequencies.

[0103] The voltage-current double-loop control module first synthesizes the total reference voltage value v 1αβγ ref based on the fundamental wave droop control-generated fundamental wave reference voltage command value v* αβγ ref, the voltage drop value vz on the adaptive virtual resistance 1αβγ ref, and the reference voltage command values v*ss 2αβγ ref and v*ss αβγ ref of the two AC signals. The total reference voltage value v αβγ ref is used to generate the drive signal for controlling the on / off of the switching tubes in the power conversion circuit, and then a multi-resonant controller or other controller in the αβ coordinate system is used to achieve accurate tracking of the output voltage vc to the total reference voltage value v αβγ ref.

[0104] When synthesizing the total reference voltage value v αβγref ref, the relationship shown in formula (7) can be specifically adopted:

[0105]

[0106] Among them, v α ref, v β ref, v γ ref are the αβγ axis components of the reference voltage value of the voltage loop respectively.

[0107] Figure 7 The schematic diagram of the voltage-current controller provided by the embodiment of the present application is exemplarily shown.

[0108] Referring Figure 7 , in the present application, in order to ensure a better reference voltage tracking effect, the voltage control loop in the voltage-current dual-loop control can adopt proportional multi-resonant closed-loop control, while the current control loop can adopt proportional control.

[0109] In the present application, the voltage-current dual-loop control module not only considers the voltage control at the fundamental frequency, but also needs to consider the voltage control at two AC signal frequencies, which can ensure a better tracking effect on the reference voltage of the AC signal. At the same time, when the resonant controller works, it is necessary to correct the resonant frequency in the resonant controller in real time according to the actually generated frequency value, which can ensure a better control effect.

[0110] Figure 8a The schematic diagram of the operation of each module of the power converter provided by the embodiment of the present application under the condition of a resistive line or equal line impedance angles is exemplarily shown, Figure 8b The schematic diagram of the operation of each module of the power converter provided by the embodiment of the present application under the condition of an inductive line or equal line inductive reactance angles is exemplarily shown.

[0111] Referring Figure 8a , in some other embodiments of the present application, for the case of a resistive line or equal line impedance angles, equal line impedance angles mean that when designing the wiring of the power converter, the line impedance angle of one power converter device is equal to that of other power converters. For the positive-sequence active power and unbalanced power (including negative-sequence power and zero-sequence power) whose distribution is mainly affected by the line impedance, the condition for equal distribution of the positive-sequence active power and unbalanced power can be simplified to a certain extent. At this time, there is no need to consider the influence of the impedance angle on power distribution, and the distribution relationship of the positive-sequence active power and unbalanced power will change synchronously with the change of the line impedance amplitude. Therefore, as long as the line impedance amplitudes of each power converter are equal to each other, power equalization can be ensured, that is, satisfying the relationship formula (8) can ensure power equalization.

[0112]

[0113]

[0114]

[0115] In a power conversion circuit, when the line parameters of the three-phase ABC lines and the neutral line are the same, the virtual resistors for positive sequence, negative sequence, and zero sequence can be simplified by designing a unified virtual resistor. According to the relationship of the line impedances under different phase sequences, the corresponding virtual resistors for positive sequence, negative sequence, and zero sequence are generated to simulate the actual line and achieve power control. Specifically, formula (6) can be simplified to formula (9).

[0116]

[0117]

[0118]

[0119] Similarly, referring to Figure 8b , in the case of an inductive line or when the line impedance angles are equal to each other, for the positive-sequence active power and unbalanced power (including negative-sequence power and zero-sequence power) whose distribution is mainly affected by the line inductive reactance, the condition of equal sharing of positive-sequence reactive power and unbalanced power can be simplified to a certain extent. At this time, the influence of the inductive reactance angle on power distribution does not need to be considered, and the distribution relationship of positive-sequence reactive power and unbalanced power will change synchronously with the change of the line inductive reactance amplitude. Therefore, as long as the line inductive reactance amplitudes of each power converter are equal to each other, power sharing can be ensured.

[0120] When injecting an AC signal to achieve adaptive adjustment of the virtual resistor, only one AC signal can be injected to generate the total virtual resistor R v , and then the virtual resistors for positive sequence, negative sequence, and zero sequence are generated proportionally according to the fixed relationship of formula (9). The process of AC signal injection can refer to the above-mentioned first AC signal generation link and will not be elaborated here.

[0121] Based on the above embodiments of the power converter, the present application also provides a current sharing control method for an energy storage system, which can be executed by the controllers in each power converter. The method provided by the embodiments of the present application includes the following steps: injecting an AC signal into the three-phase four-wire power conversion circuit of multiple power converters, and the frequency of the AC signal changes with the magnitude of the output power of the three-phase four-wire power conversion circuit, so that the output power of the three-phase four-wire power conversion circuit reaches a target value, where the target value is the required power of the load connected to the output end of the three-phase four-wire power conversion circuit divided by the number of power converters included in the energy storage system.

[0122] The method provided by the embodiments of the present application specifically includes the following steps: generating an AC signal according to the output power of a three-phase four-wire power conversion circuit; injecting the generated AC signal into the three-phase four-wire power conversion circuit; after injecting the AC signal into the three-phase four-wire power conversion circuit, generating positive-sequence, negative-sequence, and zero-sequence virtual impedances according to the AC signal components output by the power conversion circuit; and adjusting the total reference voltage value according to the positive-sequence, negative-sequence, and zero-sequence virtual impedances, and generating a driving signal for controlling the on / off of the switching tubes in the three-phase four-wire power conversion circuit based on the total reference voltage value, so that the output power of the three-phase four-wire power conversion circuit reaches the target value.

[0123] In some embodiments of the present application, it further includes: after the output power of the three-phase four-wire power conversion circuit reaches the target value, fixing the positive-sequence, negative-sequence, and zero-sequence virtual impedances, and stopping injecting the generated AC signal into the three-phase four-wire power conversion circuit, and adjusting the total reference voltage value according to the fixed positive-sequence, negative-sequence, and zero-sequence virtual impedances.

[0124] In some embodiments of the present application, after stopping injecting the generated AC signal into the three-phase four-wire power conversion circuit, when the fluctuation value of the output power of the three-phase four-wire power conversion circuit is greater than the set threshold, or after the set time threshold, inject an AC signal into the three-phase four-wire power conversion circuit.

[0125] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtaining a fundamental angular frequency command value according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtaining a fundamental angular frequency command value according to the positive-sequence active power of the three-phase four-wire power conversion circuit; generating a fundamental reference voltage command value according to the fundamental angular frequency command value.

[0126] When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtaining a frequency command value of a first AC signal according to the positive-sequence active power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtaining a frequency command value of the first AC signal according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; generating a first AC signal reference voltage command value according to the frequency command value of the first AC signal.

[0127] Obtaining a frequency command value of a second AC signal according to the unbalanced power of the three-phase four-wire power conversion circuit, and generating a second AC signal reference voltage command value according to the frequency command value of the second AC signal.

[0128] When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the corresponding reactive power is calculated respectively according to the first AC signal component and the second AC signal component output by the three-phase four-wire power conversion circuit, and the positive-sequence, negative-sequence and zero-sequence virtual resistances are generated according to the reactive powers corresponding to the first AC signal component and the second AC signal component respectively; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the corresponding active power is calculated respectively according to the first AC signal component and the second AC signal component output by the three-phase four-wire power conversion circuit, and the positive-sequence, negative-sequence and zero-sequence virtual resistances are generated according to the active powers corresponding to the first AC signal component and the second AC signal component respectively; the virtual resistance voltage drop value is generated according to the positive-sequence, negative-sequence and zero-sequence virtual resistances and the positive-sequence, negative-sequence and zero-sequence currents output by the three-phase four-wire power conversion circuit.

[0129] A total reference voltage value is generated according to the fundamental wave reference voltage command value, the first AC signal reference voltage command value, the second AC signal reference voltage command value and the virtual impedance voltage drop value.

[0130] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the fundamental wave angular frequency command value is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the fundamental wave angular frequency command value is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; the fundamental wave reference voltage command value is generated according to the fundamental wave angular frequency command value.

[0131] When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the frequency command value of the first AC signal is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the frequency command value of the first AC signal is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; the first AC signal reference voltage command value is generated according to the frequency command value of the first AC signal.

[0132] When the line of the three-phase four-wire power conversion circuit has a resistive characteristic and the line impedance angle of this device is equal to that of other power converters, calculate the corresponding reactive power according to the first AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the reactive power corresponding to the first AC signal component; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic and the line impedance angle of this device is equal to that of other power converters, calculate the corresponding active power according to the first AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the active power corresponding to the first AC signal component; generate a virtual resistance voltage drop value according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit.

[0133] Generate a total reference voltage value according to the fundamental wave reference voltage command value, the first AC signal reference voltage command value, and the virtual resistance voltage drop value.

[0134] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value.

[0135] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal.

[0136] In some embodiments of the present application, the unbalanced power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the second AC signal.

[0137] In some embodiments of the present application, when the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the reactive power corresponding to the AC signal component is negatively correlated with the positive-sequence, negative-sequence, and zero-sequence virtual resistances; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the active power corresponding to the AC signal component is negatively correlated with the positive-sequence, negative-sequence, and zero-sequence virtual resistances.

[0138] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalent technologies, the present application also intends to include these changes and modifications.

Claims

1. An energy storage system, characterized in that, It includes multiple energy storage units and multiple power converters; The DC terminals of the multiple power converters are connected to the multiple energy storage units, and the output terminals of the multiple power converters are used to be connected to a load; Each of the multiple power converters includes a three-phase four-wire power conversion circuit and a controller; The controller is used to control the three-phase four-wire power conversion circuit to convert the direct current output by the multiple energy storage units into alternating current and output it to the load; The controller is further used to: inject an alternating current signal into the three-phase four-wire power conversion circuit, and the frequency of the alternating current signal changes with the magnitude of the output power of the three-phase four-wire power conversion circuit, so that the output power of the three-phase four-wire power conversion circuit reaches a target value, where the target value is the required power of the load divided by the number of the multiple power converters.

2. The energy storage system according to claim 1, wherein The controller is used to: generate an alternating current signal according to the output power of the three-phase four-wire power conversion circuit and inject the alternating current signal into the three-phase four-wire power conversion circuit; after injecting the alternating current signal into the three-phase four-wire power conversion circuit, generate positive-sequence, negative-sequence and zero-sequence virtual impedances according to the alternating current signal components output by the power conversion circuit, adjust the total reference voltage value according to the positive-sequence, negative-sequence and zero-sequence virtual impedances, and generate a drive signal for controlling the on / off of the switching tubes in the three-phase four-wire power conversion circuit based on the total reference voltage value, so that the output power of the three-phase four-wire power conversion circuit reaches the target value.

3. The energy storage system according to claim 2, characterized in that, The controller is used to: after the output power of the three-phase four-wire power conversion circuit reaches the target value, fix the positive-sequence, negative-sequence and zero-sequence virtual impedances, and stop injecting the generated alternating current signal into the three-phase four-wire power conversion circuit, and adjust the total reference voltage value according to the fixed positive-sequence, negative-sequence and zero-sequence virtual impedances.

4. The energy storage system according to claim 3, wherein, The controller is used to: after stopping injecting the generated alternating current signal into the three-phase four-wire power conversion circuit, inject the alternating current signal into the three-phase four-wire power conversion circuit when the fluctuation value of the output power of the three-phase four-wire power conversion circuit is greater than a set threshold, or after a set time threshold.

5. The energy storage system according to any one of claims 2-4, characterized in that, The controller is used to: When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtain a fundamental angular frequency command value according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtain a fundamental angular frequency command value according to the positive-sequence active power of the three-phase four-wire power conversion circuit; generate a fundamental reference voltage command value according to the fundamental angular frequency command value; When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence active power of the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; a reference voltage command value of the first AC signal is generated according to the frequency command value of the first AC signal; A frequency command value of the second AC signal is obtained according to the unbalanced power of the three-phase four-wire power conversion circuit, and a reference voltage command value of the second AC signal is generated according to the frequency command value of the second AC signal; When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the corresponding reactive powers are respectively calculated according to the first AC signal component and the second AC signal component output by the three-phase four-wire power conversion circuit, and positive-sequence, negative-sequence, and zero-sequence virtual resistances are generated according to the reactive powers respectively corresponding to the first AC signal component and the second AC signal component; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the corresponding active powers are respectively calculated according to the first AC signal component and the second AC signal component output from the three-phase four-wire power conversion circuit, and positive-sequence, negative-sequence, and zero-sequence virtual resistances are generated according to the active powers respectively corresponding to the first AC signal component and the second AC signal component; a virtual resistance voltage drop value is generated according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit; A total reference voltage value is generated according to the fundamental wave reference voltage command value, the reference voltage command value of the first AC signal, the reference voltage command value of the second AC signal, and the virtual resistance voltage drop value.

6. The energy storage system according to any one of claims 2-4, characterized in that, The controller is used for: When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, a fundamental wave angular frequency command value is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, a fundamental wave angular frequency command value is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; a fundamental wave reference voltage command value is generated according to the fundamental wave angular frequency command value; When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, a frequency command value of the first AC signal is obtained according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; a reference voltage command value of the first AC signal is generated according to the frequency command value of the first AC signal; When the line of the three-phase four-wire power conversion circuit has a resistive characteristic and the line impedance angle of this device is equal to that of other power converters, calculate the corresponding reactive power according to the first AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the reactive power corresponding to the first AC signal component; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic and the line impedance angle of this device is equal to that of other power converters, calculate the corresponding active power according to the first AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the active power corresponding to the first AC signal component; generate a virtual resistance voltage drop value according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit; Generate the total reference voltage value according to the fundamental wave reference voltage command value, the first AC signal reference voltage command value, and the virtual resistance voltage drop value.

7. The energy storage system according to claim 5 or 6, characterized in that, The controller is used for: When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence reactive power output by the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence active power output by the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value.

8. The energy storage system according to any one of claims 5 to 7, characterized in that, The controller is used for: When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence active power output by the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence reactive power output by the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal.

9. The energy storage system according to claim 5, wherein The controller is used for: The unbalanced power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the second AC signal.

10. The energy storage system according to any one of claims 5-9, characterized in that, The controller is used for: When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the reactive power corresponding to the AC signal component is negatively correlated with the positive-sequence, negative-sequence, and zero-sequence virtual resistances; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the active power corresponding to the AC signal component is negatively correlated with the positive-sequence, negative-sequence, and zero-sequence virtual resistances.

11. A current sharing control method for an energy storage system, characterized in that, The method includes: Inject an AC signal into the three-phase four-wire power conversion circuits of multiple power converters, and the frequency of the AC signal changes with the magnitude of the output power of the three-phase four-wire power conversion circuit, so that the output power of the three-phase four-wire power conversion circuit reaches a target value, where the target value is the required power of the load connected to the output end of the three-phase four-wire power conversion circuit divided by the number of power converters included in the energy storage system.

12. The current sharing control method according to claim 11, wherein, Specifically, it includes: Generate an AC signal according to the output power of the three-phase four-wire power conversion circuit; Inject the generated AC signal into the three-phase four-wire power conversion circuit; After injecting the AC signal into the three-phase four-wire power conversion circuit, generate positive-sequence, negative-sequence, and zero-sequence virtual impedances according to the AC signal components output by the power conversion circuit; Adjust the total reference voltage value according to the positive-sequence, negative-sequence, and zero-sequence virtual impedances, and generate drive signals for controlling the on / off of the switching tubes in the three-phase four-wire power conversion circuit based on the total reference voltage value, so that the output power of the three-phase four-wire power conversion circuit reaches the target value.

13. The current sharing control method according to claim 12, wherein It further includes: After the output power of the three-phase four-wire power conversion circuit reaches the target value, fix the positive-sequence, negative-sequence, and zero-sequence virtual impedances, stop injecting the generated AC signal into the three-phase four-wire power conversion circuit, and adjust the total reference voltage value according to the fixed positive-sequence, negative-sequence, and zero-sequence virtual impedances.

14. The current sharing control method according to claim 13, wherein After stopping injecting the generated AC signal into the three-phase four-wire power conversion circuit, when the fluctuation value of the output power of the three-phase four-wire power conversion circuit is greater than the set threshold, or after the set time threshold, inject the AC signal into the three-phase four-wire power conversion circuit.

15. The current sharing control method according to any one of claims 12-14, characterized in that, It includes: When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtain the fundamental angular frequency command value according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; or when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtain the fundamental angular frequency command value according to the positive-sequence active power of the three-phase four-wire power conversion circuit; generate the fundamental reference voltage command value according to the fundamental angular frequency command value; When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtain the frequency command value of the first AC signal according to the positive-sequence active power of the three-phase four-wire power conversion circuit; or when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtain the frequency command value of the first AC signal according to the positive-sequence reactive power of the three-phase four-wire power conversion circuit; generate the first AC signal reference voltage command value according to the frequency command value of the first AC signal; Obtain the frequency command value of the second AC signal according to the unbalanced power of the three-phase four-wire power conversion circuit, and generate the second AC signal reference voltage command value according to the frequency command value of the second AC signal; When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, calculate the corresponding reactive power according to the first AC signal component and the second AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the reactive powers corresponding to the first AC signal component and the second AC signal component respectively; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, calculate the corresponding active power according to the first AC signal component and the second AC signal component output from the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the active powers corresponding to the first AC signal component and the second AC signal component respectively; generate a virtual resistance voltage drop value according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit. Generate the total reference voltage value according to the fundamental wave reference voltage command value, the first AC signal reference voltage command value, the second AC signal reference voltage command value, and the virtual resistance voltage drop value.

16. The current sharing control method according to any one of claims 12-14, characterized in that Including: When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtain the fundamental wave angular frequency command value according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtain the fundamental wave angular frequency command value according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; generate the fundamental wave reference voltage command value according to the fundamental wave angular frequency command value. When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, obtain the frequency command value of the first AC signal according to the positive-sequence active power output by the three-phase four-wire power conversion circuit; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, obtain the frequency command value of the first AC signal according to the positive-sequence reactive power output by the three-phase four-wire power conversion circuit; generate the first AC signal reference voltage command value according to the frequency command value of the first AC signal. When the line of the three-phase four-wire power conversion circuit has a resistive characteristic and the line impedance angle of this device is equal to the line impedance angle of other power converters, calculate the corresponding reactive power according to the first AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the reactive power corresponding to the first AC signal component; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic and the line impedance angle of this device is equal to the line impedance angle of other power converters, calculate the corresponding active power according to the first AC signal component output by the three-phase four-wire power conversion circuit, and generate positive-sequence, negative-sequence, and zero-sequence virtual resistances according to the active power corresponding to the first AC signal component; generate a virtual resistance voltage drop value according to the positive-sequence, negative-sequence, and zero-sequence virtual resistances and the positive-sequence, negative-sequence, and zero-sequence currents output by the three-phase four-wire power conversion circuit. Generate the total reference voltage value according to the fundamental wave reference voltage command value, the first AC signal reference voltage command value, and the virtual resistance voltage drop value.

17. The current sharing control method according to claim 15 or 16, characterized in that When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the fundamental wave angular frequency command value.

18. The current sharing control method according to any one of claims 15 to 17, characterized in that, When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the positive-sequence active power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the positive-sequence reactive power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the first AC signal.

19. The current sharing control method according to claim 15, characterized in that The unbalanced power of the three-phase four-wire power conversion circuit is positively correlated with the frequency command value of the second AC signal.

20. The current sharing control method according to any one of claims 15-19, characterized in that, When the line of the three-phase four-wire power conversion circuit has a resistive characteristic, the reactive power corresponding to the AC signal component is negatively correlated with the positive-sequence, negative-sequence, and zero-sequence virtual resistances; or, when the line of the three-phase four-wire power conversion circuit has an inductive characteristic, the active power corresponding to the AC signal component is negatively correlated with the positive-sequence, negative-sequence, and zero-sequence virtual resistances.

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