A coordinated control method for energy storage and flexible multi-state switches in a distribution network system

Abstract

The present application discloses a coordinated control method for energy storage and flexible multi-state switches in a distribution network system, and relates to the field of power supply and distribution. The present application adopts a hierarchical control method to coordinate the energy storage system and the flexible multi-state switch; the coordinated control layer is composed of a centralized controller, which selects and switches the working mode in the networked operation mode according to the feeder load power, battery charge state, the operating status of each transformer, and the location and number of feeder faults; the control strategy of the flexible multi-state switch can change between a constant DC voltage control strategy, a constant power control strategy, and a droop control strategy; the control strategy of the battery can switch between constant current charging control, floating charge control, and constant voltage control; the feeder load in the equipment control layer can also implement a load switching control strategy. The coordinated control method of the present application can improve the system power supply reliability and power quality.

Description

Technical Field

[0001] The present application relates to the field of power supply and distribution technology, and in particular to a coordinated control method for energy storage and flexible multi-state switches in a distribution network system. Background Art

[0002] A flexible multi-state switch (FMSS) is a power electronic device installed in a distribution network, connected between two or more feeders, and regulating the flow of active power between them. Through power electronics technology and intelligent algorithms, it not only maintains the on / off states of a conventional switch but also enables continuous power control. It is a key technology and equipment support for future smart grids and new power systems. When distribution network feeders operate normally, FMSSs not only prevent power outages and loop surges caused by conventional switch switching operations, but also mitigate voltage sags and three-phase imbalances, promoting balanced feeder load distribution and improving power quality. However, in the event of a distribution network feeder fault, the lack of grid support and the switching of FMSS control strategies often lead to bus voltage fluctuations and reduced power quality. To address these issues, energy storage systems with energy reserve functions have been introduced into FMSSs. Therefore, achieving coordinated control of energy storage and FMSSs to improve system power supply reliability and power quality is a key direction for the future development of FMSSs. Summary of the Invention

[0003] The purpose of this application is to provide a coordinated control method for energy storage and flexible multi-state switches in a distribution network system, so as to improve the system power supply reliability and power quality through coordinated control of energy storage and FMSS.

[0004] To achieve the above objectives, this application provides the following solutions.

[0005] The present application provides a coordinated control method for energy storage and a flexible multi-state switch in a distribution network system. The distribution network system includes: a flexible multi-state switch, an energy storage system, and multiple AC power grids; the flexible multi-state switch is composed of multiple back-to-back voltage source converters; the AC port of each voltage source converter is connected to the feeder end of an AC power grid via a transformer; each feeder is connected to a corresponding feeder load; the DC ports of each voltage source converter are connected in parallel to form a DC bus, which is connected to an energy storage system composed of a battery and a bidirectional DC / DC converter;

[0006] The coordinated control method for energy storage and flexible multi-state switches in a distribution network system includes: coordinating the energy storage system and the flexible multi-state switches in a hierarchical control manner; wherein a coordinated control layer provides a centralized controller to achieve multi-directional operational control objectives of feeder load balancing, improving system power supply reliability, and extending the power supply time of faulty feeder loads, and sends the control strategy to the device control layer; the device control layer provides a separate local controller for each voltage source converter to complete local device control of each voltage source converter;

[0007] The coordinated control layer comprises a centralized controller that selects and switches the operating mode under the networked operation mode based on the feeder load power, battery state of charge, operating status of each transformer, and the location and number of feeder faults. The control strategy of the flexible multi-state switch in the device control layer can change between a constant DC voltage control strategy, a constant power control strategy, and a droop control strategy. When the feeders connected to the AC ports of all voltage source converters in the flexible multi-state switch are operating normally, the AC port of one of the voltage source converters adopts a constant DC voltage control strategy to stabilize the DC bus voltage, while the AC ports of the other voltage source converters adopt a constant power control strategy to control the power input from the AC grid to the system. When a feeder fault occurs at the AC port of one of the voltage source converters in the flexible multi-state switch, the voltage source converter corresponding to the port connected to the faulty feeder is switched to a droop control strategy. The control strategy of the battery in the device control layer can switch between constant current charging control, floating charge control, and constant voltage control. The feeder load in the device control layer can also implement a load switching control strategy.

[0008] Optionally, the centralized controller selects and switches the working mode in the networked operation mode according to the feeder load power, battery charge state, operating status of each transformer, and location and number of feeder faults, specifically including:

[0009] When all feeders connected to the AC ports of the voltage source converters in the flexible multi-state switch are operating normally, multiple AC power grids are interconnected through the flexible multi-state switch, which is called the flexible interconnected operation state;

[0010] When the centralized controller determines that the system is in a flexible interconnected operation state, it divides the system into a free-network operation mode and a current-limited network operation mode based on whether each transformer in the system has reached its own capacity limit. Based on the above two operation modes, the centralized controller determines the system operation mode and the control strategies of the coordination control layer and the equipment control layer according to the working status of the battery.

[0011] Optionally, when the centralized controller determines that the system is in the flexible interconnection operation state, if the difference between the total power of all feeder loads and the rated charging power of the battery is less than the sum of the maximum powers of all transformers, the system is determined to be in the interconnection free operation mode;

[0012] In the free-running mode, when the battery state of charge is less than the maximum state of charge, the flexible multi-state switch is selected as the voltage stabilizing device and the battery is used as the backup power supply to improve the system power supply reliability. At the same time, multiple AC grids are set to generate the same power in order to achieve feeder load balancing. , ;in Feeder load Power; is the number of feeder loads; is the rated charging power of the battery, and the battery output power is positive and the absorbed power is negative. The control strategy of the device control layer is: the battery adopts constant current charging control, and at the same time, a constant DC voltage control strategy is adopted for the AC port of one voltage source converter of the flexible multi-state switch, and a constant power control strategy is adopted for the AC ports of the other voltage source converters.

[0013] In the free-running mode, when the battery state of charge is greater than or equal to the maximum state of charge, the battery switches to floating charge control. At this time, the system power supply reliability has been maximized. The coordination control layer only aims to achieve feeder load balancing, and sets multiple AC grids to generate the same power. The control strategy of the equipment control layer is: the battery adopts floating charge control, and at the same time, the AC port of one voltage source converter of the flexible multi-state switch adopts a constant DC voltage control strategy, and the AC ports of other voltage source converters adopt a constant power control strategy.

[0014] Optionally, when the centralized controller determines that the system is in a flexible interconnection operation state, if the sum of the maximum powers of all transformers is greater than the sum of the total powers of all feeder loads but less than the difference between the total powers of all feeder loads and the rated charging power of the battery, it is determined that the system is in an interconnected current limiting operation mode;

[0015] In the networked current-limited operation mode, the coordination control layer aims to maximize the system power supply reliability, ensuring that each transformer outputs maximum power. The battery is charged with excess power within the system, and the battery is selected as the voltage-stabilizing port. The control strategy of the equipment control layer is: the battery adopts constant voltage control, and at the same time, the AC ports of all voltage source converters of the flexible multi-state switch adopt a constant power control strategy.

[0016] Optionally, the centralized controller selects and switches the working mode in the networked operation mode according to the feeder load power, battery charge state, operating status of each transformer, and location and number of feeder faults, specifically including:

[0017] If a fault occurs in part of the AC grid feeder connected to the flexible multi-state switch, the system is determined to be operating in the load transfer operation state; at this time, according to the different locations of the feeder faults, it is divided into feeder faults occurring at the voltage-stabilizing port and feeder faults occurring at the non-voltage-stabilizing port.

[0018] Optionally, when a feeder fault occurs at the voltage stabilizing port, if the difference between the total power of all feeder loads and the rated charging power of the battery is less than the sum of the maximum powers of all transformers connected to the normally operating feeders in the system, it is determined that the system is in the free-running mode;

[0019] In the free-running mode of the network, when the battery state of charge is less than the maximum state of charge, with the goal of improving system power supply reliability, a port in the flexible multi-state switch that has not experienced a feeder fault is selected as the DC voltage-stabilized port, and the battery is used as the backup power supply. With the goal of extending the power supply time of the faulty feeder load, the port that has not experienced a feeder fault is set to supply power to the faulty feeder load. At the same time, if a feeder fault occurs only on a single voltage-stabilized port, multiple AC power grids that have not experienced a feeder fault are set to output the same power to achieve feeder load balancing. The control strategy of the equipment control layer is as follows: the battery adopts constant current charging control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage-stabilized port adopts a constant DC voltage control strategy, and the other ports adopt a constant power control strategy.

[0020] In the free-running mode of the interconnected network, when the battery state of charge is greater than or equal to the maximum state of charge value, the system power supply reliability has been maximized and the battery stops charging. If a feeder fault occurs only at the voltage-stabilizing port, the coordinated control layer aims to extend the power supply time of the faulty feeder load and balance the feeder load. Multiple AC power grids that have not experienced feeder faults are set to output the same power. If a feeder fault occurs at the dual-port flexible multi-state switch, including the voltage-stabilizing port, the goal is only to extend the power supply time of the faulty feeder load. The control strategy of the equipment control layer is as follows: the battery adopts floating charge control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage-stabilizing port adopts a constant DC voltage control strategy, and other ports adopt a constant power control strategy.

[0021] Optionally, when the centralized controller determines that the system is in a load transfer operation state, if the sum of the maximum powers of all transformers connected to the normally operating feeders in the system is greater than the sum of the total powers of all feeder loads, but less than the difference between the total powers of all feeder loads and the rated charging power of the battery, the system is determined to be in a networked current limiting operation mode; and the control strategy of the coordination control layer is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the port where the feeder fault has not occurred is set to supply power to the faulty feeder load; with the goal of improving the power supply reliability of the system, the maximum output power of the transformer connected to the normally operating feeder is set, the battery is selected as the DC voltage stabilization port, and the excess power inside the system is used to charge the battery until it is fully charged; the control strategy of the equipment control layer is: the battery adopts constant voltage control, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy;

[0022] When the battery charging is completed, it switches to float charge control, and a port in the flexible multi-state switch without a feeder fault is selected as the DC voltage stabilization port. The DC voltage stabilization port adopts a constant DC voltage control strategy, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy.

[0023] Optionally, when the centralized controller determines that the system is in a load transfer operation state, if the sum of the maximum powers of all transformers connected to the normally operating feeders in the system is less than the sum of the total powers of all feeder loads, the system is determined to be in a networked current limiting operation mode; and the coordination control layer control strategy is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the battery is selected as the DC voltage stabilization port, and together with the port where no feeder fault has occurred, it supplies power to the faulty feeder load; the equipment control layer control strategy is: the battery adopts constant voltage control, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy.

[0024] Optionally, when a feeder fault occurs at the non-regulated port, if the difference between the total power of all feeder loads and the rated charging power of the battery is less than the sum of the maximum powers of all transformers connected to the normally operating feeders, it is determined that the system is in the networked free operation mode;

[0025] In the free-running mode, when the battery state of charge is less than the maximum state of charge, the battery is charged at rated power until it is fully charged, with the goal of improving system power supply reliability. To extend the power supply time of the faulty feeder load, ports that have not experienced feeder faults are set to supply power to the faulty feeder load. Furthermore, if only one non-regulated port in the system experiences a feeder fault, multiple AC grids that have not experienced feeder faults are set to output the same power, with the goal of achieving feeder load balancing. The control strategy at the equipment control layer is as follows: constant current charging control is used for the battery, droop control is used for the port connected to the faulty feeder, constant DC voltage control is used for the DC regulated port, and constant power control is used for all other ports.

[0026] In the free-running mode, when the battery state of charge (SOC) is greater than or equal to the maximum SOC value, the system power supply reliability is maximized and the battery stops charging. If a feeder fault occurs on only one unregulated port in the system, the coordinated control layer sets the ports connected to the normally operating feeders to jointly supply power to the faulty feeder load, with the goal of extending the power supply time of the faulty feeder load and balancing the feeder load. Multiple AC grids that have not experienced feeder faults are set to output the same power. If feeders connected to multiple unregulated ports in the system fail, the coordinated control layer sets the regulated ports to supply power to multiple faulty feeder loads, with the goal of extending the power supply time of the faulty feeder load. The control strategy of the equipment control layer is as follows: the battery adopts floating charge control, the port connected to the faulty feeder adopts droop control strategy, the DC regulated port adopts constant DC voltage control strategy, and other ports adopt constant power control strategy.

[0027] Optionally, when the centralized controller determines that the system is in a load transfer operation state, if the sum of the maximum powers of all transformers connected to the normally operating feeders in the system is greater than the sum of the total powers of all feeder loads but less than the difference between the total powers of all feeder loads and the rated charging power of the battery, the system is determined to be in a networked current limiting operation mode; and the control strategy of the coordination control layer is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the port where the feeder fault has not occurred is set to supply power to the faulty feeder load; with the goal of improving the power supply reliability of the system, the maximum output power of all transformers connected to the normally operating feeders is set, the battery is selected as the DC voltage stabilization port, and the excess power inside the system is used to charge the battery until it is fully charged; the control strategy of the equipment control layer is: the battery adopts constant voltage control, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy;

[0028] When the battery is fully charged, it switches to float charge control and selects a port in the flexible multi-state switch that has not experienced a feeder fault as the DC voltage-regulated port. The DC voltage-regulated port adopts a constant DC voltage control strategy, the port connected to the faulty feeder adopts a droop control strategy, and the other ports adopt a constant power control strategy.

[0029] If the sum of the maximum powers of all transformers connected to the normally operating feeder is less than the sum of the total powers of all feeder loads, the system is determined to be in the networked current-limited operation mode; and the coordination control layer control strategy is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the battery is selected as the DC voltage-stabilizing port, and together with the ports where no feeder fault has occurred, it supplies power to the faulty feeder load; the equipment control layer control strategy is: the battery adopts constant voltage control, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy.

[0030] According to the specific embodiments provided in this application, this application discloses the following technical effects.

[0031] The present application provides a method for coordinated control of energy storage and flexible multi-state switches in a distribution network system, in which a hierarchical control method is used to coordinate the energy storage system and the flexible multi-state switch. The control strategy of the flexible multi-state switch in the device control layer can change between a constant DC voltage control strategy, a constant power control strategy, and a droop control strategy. When all the feeders connected to the AC ports of the flexible multi-state switch are operating normally, one of the AC ports adopts a constant DC voltage control strategy to stabilize the DC bus voltage, while the other AC ports adopt a constant power control strategy to control the power input from the AC grid to the system. When a feeder fault occurs at one of the AC ports of the flexible multi-state switch, the port connected to the faulty feeder is switched to a droop control strategy, thereby relying on the droop control strategy to provide the corresponding phase angle. The control strategy of the battery in the device control layer can switch between constant current charging control, floating charge control, and constant voltage control to achieve coordination with the functions of the FMSS. In addition, the feeder load in the device control layer can also implement a load switching control strategy. The coordination of control strategies at each device level enables the centralized controller to achieve multi-directional operation control goals of feeder load balancing, improving system power supply reliability, and extending the power supply time of faulty feeder loads, significantly improving system power supply reliability and power quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0033] Figure 1 A topological diagram of the distribution network system constructed for this application;

[0034] Figure 2 This is the hierarchical control structure diagram of the energy storage system and FMSS;

[0035] Figure 3 Schematic diagram of power transmission between units of the distribution network system under mode 1;

[0036] Figure 4 Schematic diagram of power transmission between units of the distribution network system under mode 2;

[0037] Figure 5 Schematic diagram of power transmission between units of the distribution network system under mode 3;

[0038] Figure 6 Schematic diagram of power transmission between units of the distribution network system under mode 4;

[0039] Figure 7Schematic diagram of power transmission between units of the distribution network system under mode 5;

[0040] Figure 8 Schematic diagram of power transmission between units of the distribution network system under mode 6;

[0041] Figure 9 Schematic diagram of power transmission between units of the distribution network system under mode 7;

[0042] Figure 10 Schematic diagram of power transmission between units of the distribution network system under mode 8;

[0043] Figure 11 Schematic diagram of power transmission between units of the distribution network system under mode 10;

[0044] Figure 12 Schematic diagram of power transmission between units of the distribution network system under mode 11;

[0045] Figure 13 Schematic diagram of power transmission between units of the distribution network system under mode 12;

[0046] Figure 14 Schematic diagram of power transmission between units of the distribution network system under mode 13;

[0047] Figure 15 Schematic diagram of power transmission between units of the distribution network system under mode 14;

[0048] Figure 16 Schematic diagram of power transmission between units of the distribution network system under mode 15;

[0049] Figure 17 Schematic diagram of power transmission between units of the distribution network system under mode 16;

[0050] Figure 18 Schematic diagram of power transmission between units of the distribution network system under mode 18;

[0051] Figure 19 Schematic diagram of power transmission between units of the distribution network system under mode 19;

[0052] Figure 20 This is the planned feeder off-grid switching control timing diagram proposed in this application;

[0053] Figure 21 This is the power feedforward control diagram when the battery feeder is unplanned off-grid;

[0054] Figure 22 Flexible multi-state switch and Control strategy diagram;

[0055] Figure 23 Flexible multi-state switch with off-grid phase-locking function Control strategy diagram;

[0056] Figure 24 This is the block diagram of droop control with off-grid phase locking;

[0057] Figure 25 This is the block diagram of the dual-loop control: voltage outer loop and current inner loop;

[0058] Figure 26 This is the control strategy diagram of the energy storage system. DETAILED DESCRIPTION

[0059] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0060] This application proposes a coordinated control method for energy storage and flexible multi-state switches in a distribution network system, which improves the system power supply reliability and power quality through coordinated control of energy storage and flexible multi-state switches.

[0061] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0062] In an exemplary embodiment, a method for coordinated control of energy storage and flexible multi-state switches in a distribution network system is provided. The distribution network system (hereinafter referred to as the system) includes a flexible multi-state switch (FMSS), an energy storage system, and multiple AC power grids. The flexible multi-state switch is composed of multiple back-to-back voltage source converters (hereinafter referred to as converters). The AC port of each voltage source converter is connected to the feeder terminal of an AC power grid via a transformer. Each feeder is connected to a corresponding feeder load (hereinafter referred to as load). The DC ports of each voltage source converter are connected in parallel to form a common DC bus, to which an energy storage system consisting of a battery and a bidirectional DC / DC converter is connected. The following detailed description of the coordinated control method for energy storage and flexible multi-state switches in a distribution network system is given, using a three-port flexible multi-state switch as an example.

[0063] Figure 1 The topology of the distribution network system based on energy storage and flexible multi-state switch constructed in this application is shown. Among them, the three-port flexible multi-state switch uses three back-to-back voltage source converters. 、 、 Port 1, Port 2, and Port 3. 、 、 The respective AC ports are connected through transformers 、 and It is connected to the feeder ends of three independent 10kV AC grids (called AC grid 1, AC grid 2 and AC grid 3), realizing flexible interconnection of different feeders. It is the DC side capacitor, which can reduce the DC ripple. is the DC port voltage. 、 、 The DC ports of each device are connected in parallel to form a common DC bus. This DC bus is connected to an energy storage system consisting of batteries and bidirectional DC / DC converters. This improves system power supply reliability when the distribution network operates off-grid, and enhances the adaptability and control flexibility of the flexible multi-state switch to complex operating conditions. Considering the importance of loads in the power system, this embodiment sets three feeder loads with different priorities. Feeder Load 1 is the critical load with the highest priority, while Feeder Load 2 and Feeder Load 3 are both non-critical loads.

[0064] The coordinated control method of energy storage and flexible multi-state switch in the distribution network system includes: coordinating the energy storage system and FMSS in a hierarchical control manner, and the hierarchical control structure is as follows: Figure 2 As shown, it includes a coordination control layer and a device control layer. The coordination control layer sets a centralized controller to achieve multi-directional operation control goals of feeder load balancing, improving system power supply reliability, and extending the power supply time of fault feeder loads, and sends the control strategy to the device control layer. The device control layer is each voltage source converter 、 、 Separate local controllers are provided to complete local device control of each voltage source converter.

[0065] The coordination control layer is composed of a centralized controller to solve the problem of working mode selection and switching in the network operation mode according to the feeder load power, battery state of charge (SOC), the operating status of each transformer and the location and number of feeder faults. The control strategy of the flexible multi-state switch in the device control layer can be controlled at a constant DC voltage ( ) control strategy, constant power ( ) control strategy and droop ( When all the feeders connected to the AC ports of all converters in the FMSS are operating normally, the AC port of one of the converters is usually control strategy to stabilize the DC bus voltage while adopting Control strategy is used to control the power input from the AC grid to the system. When a feeder fault occurs at the AC port of one of the FMSS converters, the port connected to the faulty feeder needs to be switched to the Control strategy. The battery control strategy in the device control layer can switch between constant current charging control, floating charge control, and constant voltage control. The feeder load in the device control layer can also implement a load switching control strategy.

[0066] In actual operation, Figure 1 The distribution network system shown can operate both interconnected and islanded. When the system operates interconnected, flexible multi-state switches enable flexible power flow control and balanced feeder load distribution. When the system operates off-grid, energy storage supplies power to the loads on each feeder, thereby improving system power supply reliability. Based on this, the system's operating states can be further categorized into flexible interconnection and load transfer, taking into account whether the AC feeders connected to each port of the flexible multi-state switch are faulty and the number of ports experiencing feeder failures. This is explained as follows: 1) Flexible interconnection: If all three AC feeders connected to the flexible multi-state switch are operating normally, meaning that the three AC grids are interconnected via the flexible multi-state switch and the output power of the three AC grids can be adjusted via the flexible multi-state switch, this is considered the flexible interconnection state. 2) Load transfer: If one or two AC feeders connected to the flexible multi-state switch fail, the system operates in the load transfer state. Depending on the location of the feeder fault, this can be categorized as either a feeder fault on a regulated port or a feeder fault on a non-regulated port.

[0067] Furthermore, when the centralized controller determines that the system is in a flexible interconnected operation state, the centralized controller combines the transformers in the system to determine the system is in a flexible interconnected operation state. 、 and Whether the system has reached its capacity limit is further divided into free-connection operation mode and current-limited operation mode. Based on these two operating modes and the battery operating status, the system operating mode and control strategies for the coordination control layer and the device control layer are determined. A detailed analysis is as follows.

[0068] 1) Flexible interconnection operating status, including the following 1.1) to 1.2).

[0069] 1.1) Network free running mode.

[0070] If the total power of all feeder loads and battery rated charging power The difference is less than the sum of the maximum power of all transformers , confirm that the system is in free running mode. Feeder load power. is the number of feeder loads. In this embodiment =3. is the rated charging power of the battery, and the power emitted by the battery is positive and the power absorbed is negative. For transformer In free running mode, the battery state of charge Whether the maximum state of charge value has been reached , divided into the following mode 1 and mode 2.

[0071] Mode 1: In this mode 1, since there are three AC grids as strong power support and the power between the flexible multi-state switch and the AC grid is continuously adjustable, the control strategy of the coordinated control layer can be formulated as follows: ① Taking improving the system power supply reliability (or operation reliability) as the control target, the flexible multi-state switch is selected as the voltage stabilizing device and the battery as the backup power supply. ) takes the DC bus voltage stabilization port as an example, the power transmission between the units is as follows Figure 3 ② With the goal of balancing the feeder load distribution (i.e. achieving feeder load balance), set Figure 3 The power output of AC grid 2 and AC grid 3 is the same and both are , see formula (1).

[0072] (1);

[0073] In formula (1), 、 、 are the powers of feeder load 1, feeder load 2, and feeder load 3 respectively. is the battery state of charge obtained by the ampere-hour integration method. The specific calculation formula is shown in formula (2).

[0074] (2);

[0075] In formula (2), is the initial charge value of the battery state of charge; is the rated capacity of the battery; For the battery The charge and discharge current at each moment; is the charge and discharge time. is the maximum state of charge of the battery. The battery state of charge calculated at the current time, abbreviated as .

[0076] The control strategy of the equipment control layer is: battery (constant current charging control) + ( Control) + ( Control) + ( control). That is, the battery adopts constant current charging control strategy, and the AC port of one of the FMSS converters adopts Control strategy, the AC port of other converters adopts Control strategy.

[0077] Mode 2: When the battery in Mode 1 is fully charged, When the battery switches to floating charge control, the system power supply reliability is guaranteed to the maximum extent. The coordination control layer only aims to achieve feeder load balancing, and sets multiple AC power grids to generate the same power. , at this time the power The calculation of is shown in formula (3).

[0078] (3).

[0079] At this time, the power transmission between the units is as follows: Figure 4 As shown. The control strategy of the equipment control layer is: battery (floating charge control) + ( Control) + ( Control) + ( control).

[0080] 1.2) Network current limiting operation mode.

[0081] When the centralized controller determines that the system is in the flexible interconnection operation state, if , confirming that the system is in the grid-connected current-limited operation mode. In this operating mode, although all three AC grids are operating normally, the high load power cannot guarantee that the battery can be charged at the rated power. At this time, the system switches to Mode 3.

[0082] Mode 3: The coordinated control layer aims to maximize the reliability of the system power supply and make each transformer 、 、 Maximum output power 、 、 The battery is charged with the excess power inside the system. And since the output power of each port of FMSS is constant and at the maximum value, the battery is selected as the voltage stabilization port. The power transmission between each unit is as follows Figure 5 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control). Among them, the 3 ports of FMSS are all Control strategy, so that 、 、 Both output maximum power.

[0083] 2) Load transfer operation status, including the following 2.1) to 2.2).

[0084] If a fault occurs on a portion of the AC feeders connected to the flexible multi-state switch, the system is determined to be operating in a load-transfer mode. This fault can be classified as occurring on a regulated port or a non-regulated port, depending on the location of the feeder fault. When the system is operating in a load-transfer mode, an AC feeder fault reduces system power supply reliability. The coordinated control layer aims to extend the load supply time of the faulty feeder and achieve load balancing control. This analysis, combined with the location and number of feeder faults, provides a detailed analysis as follows.

[0085] 2.1) A feeder fault occurs at the voltage stabilization port, including the following 2.1.1) to 2.1.2).

[0086] 2.1.1) An AC feeder fault occurs on a single port and voltage-stabilizing port of a flexible multi-state switch, including the following 2.1.1.1) to 2.1.1.2).

[0087] Assume that port 1 of FMSS (i.e. ) is a voltage stabilizing port. When only one AC grid feeder fails and the port connected to the faulty feeder is a DC voltage stabilizing port, the AC grid connected to port 1 will no longer be able to provide energy to the system. At this time, the energy input to the system will decrease. However, since the feeder load power remains unchanged, it may occur and The output power reaches the upper limit. Based on this, the system operation can be further divided into free-running mode and current-limited mode.

[0088] 2.1.1.1) Networked free-running mode.

[0089] like , confirm that the system is in the free running mode of the network, then according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following modes 4 and 5.

[0090] Mode 4: In mode 4, the total power of the three feeder loads is small. and The power of the AC grid is continuously adjustable. The corresponding coordinated control layer control objectives can not only ensure the normal operation of the fault feeder load, but also ensure that the battery is charged at the rated power. Specifically, it can be expressed as follows: ① To improve the reliability of the system power supply, set port 2 as a DC voltage stabilization port, and charge the battery at a constant current until it is fully charged. The power transmission between the various units of the system is as follows Figure 6 As shown. ② With the goal of extending the power supply time of the fault feeder load, set ports 2 and 3 to supply power to feeder load 1. ③ With the goal of achieving feeder load balancing, set the output power of AC grids 2 and 3 as shown in formula (4):

[0091] (4).

[0092] The control strategy of the equipment control layer is: battery (constant current charging control) + ( Control) + ( Control) + ( control). Among them, due to The connected AC feeder is faulty, so Convert to Control strategy to ensure the normal operation of feeder load 1.

[0093] Mode 5: When the battery in Mode 4 is fully charged, the system power supply reliability has reached the maximum guarantee, the battery stops charging, and the coordination control layer aims to extend the power supply time of the fault feeder load and balance the feeder load. The output power of AC grid 2 and 3 is set as shown in formula (5). The corresponding power flow relationship is as follows: Figure 7 shown.

[0094] (5).

[0095] The control strategy of the equipment control layer is: battery (floating charge control) + ( Control) + ( Control) + ( control).

[0096] 2.1.1.2) Network current limiting operation mode.

[0097] Mode 6: If the total power of the three feeder loads is large, and Under normal load conditions, the maximum power of the battery cannot be guaranteed to be charged at the rated power. When the fault feeder load is in the fault state, the coordination control layer control strategy can be formulated as follows: ① To extend the power supply time of the fault feeder load, set the FMSS port 2 and port 3 to supply power to the feeder load 1. ② To improve the system power supply reliability, set and The output power is the maximum power. and The exchange power between the AC grid is not adjustable and reaches the maximum value 、 Therefore, the battery is selected as the DC voltage stabilization port and is charged with the excess power inside the system until it is fully charged. The power transmission between the various units in the system is as follows Figure 8 shown.

[0098] Equipment control layer control strategy: battery (constant voltage control) + ( Control) + ( Control) + ( control). Among them, due to The connected feeder fails, so it is control; and Both output the maximum power, so and for When the battery is fully charged, it switches to float charge control, and the voltage is stabilized by port 2 of the flexible multi-state switch. For the specific control strategy, see Mode 5.

[0099] Mode 7: If the total power of the three feeder loads is too large, greater than and The maximum power sum, the coordination control layer control strategy is: to extend the fault feeder load power supply time as the goal, set the battery as the voltage stabilization port, and discharge together with port 2 and port 3 to provide energy for the system. At this time, the power transmission between the various units of the system is as follows Figure 9 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control).

[0100] 2.1.2) An AC feeder fault occurs on a flexible multi-state switch with dual ports, including a voltage-stabilizing port, including the following 2.1.2.1) to 2.1.2.2).

[0101] When two AC grid AC feeders fail, and the ports connected to the faulty feeders include voltage stabilization ports, and Taking the fault of the connected AC feeder as an example, the following description is given in the free-networking and current-limiting operation modes.

[0102] 2.1.2.1) Networked free-running mode.

[0103] like , confirm that the system is in the free running mode of the network, then according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following modes 8 and 9.

[0104] Mode 8: If the load power of the three feeders is small, the coordinated control layer control strategy can be formulated as follows: ① To extend the power supply time of the fault feeder load, set the FMSS It is a DC voltage stabilization port, supplying power to feeder load 1 and feeder load 2. ② To improve the reliability of the system power supply, set the battery constant current charging until it is fully charged. The power transmission between the units in the system is as follows Figure 10 As shown. The control strategy of the equipment control layer is: battery (constant current charging) + ( Control) + ( Control) + ( control).

[0105] Mode 9: When the battery is fully charged, the system power supply reliability is maximized, so the system control strategy is set to extend the power supply time of the faulty feeder load.

[0106] 2.1.2.2) Network current limiting operation mode.

[0107] Mode 10: If the total power of the three feeder loads is large, but less than When the maximum power is reached and the battery cannot be charged at the rated power, When the fault feeder load is powered on, the control objectives of the coordination control layer are formulated as follows: ① To extend the power supply time of the fault feeder load, set the FMSS It is a voltage stabilizing port, supplying power to feeder load 2 and feeder load 3. ② To improve the reliability of system power supply, set Output maximum power, under the premise of ensuring the power supply of all AC loads in the system, make the battery a DC voltage stabilization port, and charge with the excess power in the system until it is fully charged. At this time, the power transmission between the units in the system is as follows Figure 11 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control).

[0108] Mode 11: If the total power of the three feeder loads is large and greater than The maximum power at this time In order to ensure the power supply to the fault feeder load, the battery is set as the DC voltage stabilization port and outputs power to make up for the power shortage in the system. Figure 12 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control).

[0109] 2.2) A feeder fault occurs at the non-regulated port, including the following 2.2.1) to 2.2.2).

[0110] 2.2.1) An AC feeder fault occurs on a single non-regulated port of the flexible multi-state switch, including the following 2.2.1.1) to 2.2.1.2).

[0111] When only one AC feeder fails and the port connected to the faulty feeder is a non-DC regulated port, such as the FMSS Voltage stabilization and When a feeder fault occurs, as the AC grid input system energy decreases, the voltage stabilization port will and The output power will change depending on whether it reaches the upper limit. The specific situation is discussed below.

[0112] 2.2.1.1) Networked free-running mode.

[0113] like , confirm that the system is in the free running mode of the network, then according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following modes 12 and 13.

[0114] Mode 12: In this mode 12, the total power of the three feeder loads is small and The connected feeder has not experienced any fault, so the voltage stabilization port remains unchanged. The control objectives of the coordination control layer can be formulated as follows: ① To extend the power supply time of the faulty feeder load, set the FMSS and Power is supplied to feeder load 3. ② With the goal of improving the reliability of the system power supply, the battery is charged at a constant current until it is fully charged. ③ With the goal of achieving feeder load balance, the output power of AC grids 1 and 2 is set to be the same as in formula (4). The power transmission between the units in the corresponding system is obtained as follows Figure 13 As shown. The control strategy of the equipment control layer is: battery (constant current charging control) + ( Control) + ( Control) + ( control).

[0115] Mode 13: When the battery in Mode 12 is fully charged, the system power supply reliability has reached the maximum guarantee, the battery stops charging, and the coordination control layer sets the output power of AC grid 1 and 2 in the same way as (5) with the goal of extending the power supply time of the fault feeder load and balancing the feeder load. The power transmission between the units in the corresponding system is as follows: Figure 14 When the battery is fully charged, the control strategy of the equipment control layer is: battery (floating charge control) + ( Control) + ( Control) + ( control).

[0116] 2.2.1.2) Network current limiting operation mode.

[0117] Mode 14: In this mode 14, the total power of the three feeder loads is large, but less than and The maximum power sum of .at this time, and The maximum power of the FMSS cannot guarantee constant current charging of the battery while maintaining normal load operation. The control strategy of the coordinated control layer can be formulated as follows: ① To extend the power supply time of the fault feeder load, set the and Supply power to feeder load 3. ② To improve the reliability of system power supply, set and The output power is the maximum power. and The exchange power between the AC grid is not adjustable, so the battery is selected as the DC voltage stabilization port and charged with the excess power in the system until it is fully charged. Figure 15 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control).

[0118] Mode 15: In this mode 15, the total power of the three feeder loads is greater than and The maximum power sum of the coordinated control layer control strategy is to achieve uninterrupted power supply to the fault feeder load, set the battery discharge, and the voltage stabilization port. At this time, the power transmission between the various units of the system is as follows Figure 16 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control).

[0119] 2.2.2) An AC feeder fault occurs on a dual-port flexible multi-state switch, excluding the voltage-stabilizing port, including the following 2.2.2.1) to 2.2.2.2).

[0120] When two AC grid AC feeders fail and the ports connected to the faulty feeders do not include voltage stabilization ports, and Taking the failure of the connected AC feeder as an example, the following description will be given in the following two operation modes: free networking and current limiting networking.

[0121] 2.2.2.1) Networked free-running mode.

[0122] like , confirm that the system is in the free running mode of the network, then according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following modes 16 and 17.

[0123] Mode 16: In this mode 16, the load power of the three feeders is small, and only It can supply power to all feeder loads and is sufficient to To this end, the coordinated control layer control strategy is formulated as follows: ① To extend the power supply time of the faulty feeder load, set the FMSS port 1 to supply power to feeder load 2 and feeder load 3. ② To improve the system power supply reliability, set the battery constant current charging until it is fully charged. The DC voltage regulated port of the FMSS port 1 remains unchanged. The power transmission between the units in the system is as follows Figure 17 As shown. The control strategy of the equipment control layer is: battery (constant current charging control) + ( Control) + ( Control) + ( control).

[0124] Mode 17: When the battery is fully charged, the system power supply reliability has reached the maximum guarantee. Therefore, in order to extend the power supply time of the fault feeder load, the control strategy of the equipment control layer is set as: battery (floating charge control) + ( Control) + ( Control) + ( control).

[0125] 2.2.2.2) Network current limiting operation mode.

[0126] Mode 18: When the total power of the three feeder loads is large but less than The maximum power, that is When the fault occurs, it is impossible to maintain the constant current charging of the battery while maintaining the feeder load. Therefore, the control objectives of the coordinated control layer are: ① To extend the power supply time of the fault feeder load, set port 1 of the FMSS to supply power to feeder load 2 and feeder load 3. ② To improve the power supply reliability of the system, set Output maximum power, the battery is charged with the excess power inside the system until it is fully charged. The output power of AC grid 1 is not adjustable, so the voltage is stabilized by the battery. The power transmission between the units in the system is as follows Figure 18 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control).

[0127] Mode 19: The total power of the three feeder loads is large and greater than The maximum power of the system is achieved. At this time, the control strategy of the coordinated control layer is to achieve uninterrupted power supply to the feeder load. The battery discharge and voltage stabilization ports are set to make up for the power shortage. At this time, the power transmission between the various units of the system is as follows Figure 19 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control) + ( Control) + ( control).

[0128] 3) Feeder off-grid switching control, including the following 3.1) to 3.2).

[0129] When the AC feeder connected to one or more ports of a flexible multi-state switch fails, or the AC grid needs to shut down for a specific reason (such as a power outage for maintenance), the corresponding ports on the flexible multi-state switch will experience a feeder disconnection. During the transition from grid-connected to off-grid, if the feeder disconnection is caused by an AC feeder fault, it is called an unplanned off-grid operation or an unplanned feeder disconnection. If the feeder disconnection occurs within a specific time period due to a specific reason, it is considered a planned or predictable feeder disconnection and is called a planned off-grid operation or a planned feeder disconnection.

[0130] Whether a feeder disconnection is planned or unplanned, the AC grid power output to the system from one port of the flexible multi-state switch will instantly drop to zero. This will cause changes in the control strategies and power of other ports of the flexible multi-state switch and the battery, resulting in changes in the bus voltage. Therefore, properly coordinating the control strategies between the flexible multi-state switch and the battery during the switching process is key to improving system efficiency.

[0131] Figure 20 This is the planned feeder off-grid switching control timing diagram proposed in this application, where There are four different control moments. When a planned feeder disconnection occurs in the system, as the power output from the AC grid to the system drops rapidly, the battery or flexible multi-state switch voltage regulator port needs to quickly replenish energy to stabilize the DC bus voltage, and even the battery or flexible multi-state switch control strategy is switched, resulting in large fluctuations in the bus voltage. In order to solve this problem, it is necessary to reduce the power exchange between the AC grid that is about to be disconnected and the system. Affected by the flexible multi-state switch load balancing control strategy, the output power of the AC grid depends on the feeder load power it carries and the power of the feeder loads at other ports. Therefore, in order to achieve the purpose of zero exchange power, it is necessary to At this moment, combined with the system operating conditions, the non-important feeder loads connected to the AC grid that is about to be disconnected from the grid are removed / shed in advance. The corresponding converter control strategy is changed at all times to make the power output from the AC grid to the system zero. At this moment, the exchange power between the AC grid and the large grid is reduced to near zero, and then the grid-connected to off-grid switching is carried out, and the voltage stabilization port is selected according to the load power to avoid the switching impact caused by the sudden drop in output power. At this moment, the load of the feeder that has been shed is restored.

[0132] 3.1) Unplanned feeder off-grid switching strategy.

[0133] Unplanned feeder disconnection is an emergency. Although the power of the flexible multi-state switch port and the feeder load power do not change suddenly, the change in the working mode after the switch may cause large DC bus voltage fluctuations. To reduce this fluctuation, power feedforward control is added to the battery control strategy when an unplanned feeder disconnection occurs to increase the battery output and quickly fill the unbalanced power in the system. The corresponding control strategy is as follows: Figure 21 shown. Figure 21 middle, is the battery inductance, is the battery inductor current. and For the switch tube. is the introduced feedforward power, is the battery power feedforward ratio value, is the battery terminal voltage. is the DC bus voltage reference power, is the DC bus voltage (that is, the DC port voltage of FMSS). The power feedforward control can be specifically expressed as:

[0134] (6);

[0135] Where, 、 、 They are the output powers of the three AC grids, that is, the output powers of the three transformers; It is the AC grid 1 on-grid and off-grid detection switch. When it is on-grid, 0; when running off-grid, is 1. 、 Definition .

[0136] Figure 21 The voltage and current monitoring part of the power grid mainly monitors the voltage and current of the AC power grid 1~3 outputs. Once it is found that the output current signals of the three AC power grids are all 0, the off-grid signal is activated, and the flexible multi-state switch operates in the island state, while outputting feedforward power. and compare this value with the battery terminal voltage Divide by, and you can get the added feedforward current The injection of feedforward current at the moment of off-grid switching reduces bus voltage fluctuations. After the bus voltage returns to the set value, the delayed shutdown in the battery control strategy takes effect, reducing the current input to the system to zero, thereby avoiding the effect of the battery constant voltage control in the later stage.

[0137] 3.2) Off-grid phase-locked control strategy.

[0138] When a fault occurs in the grid feeder connected to a port of the flexible multi-state switch, the corresponding port will be disconnected from the grid unplanned. Due to the interconnected nature of the flexible multi-state switch, the feeder load connected to the corresponding port can still continue to work, but since the grid at the port has been disconnected from the flexible multi-state switch, it can no longer obtain the phase angle from the grid and needs to rely on The control strategy provides the phase angle From this we can see that if the load is to work properly, it is necessary to Phase angle with other power grids Consistent, at this time In order to keep the phase angle after disconnection consistent with the grid, it is necessary to eliminate the periodic change of the phase difference sign. To this end, this application eliminates the change of the phase difference sign by taking a sine function of the phase difference on the basis of the traditional off-grid phase-locked control strategy, that is, using replace As the input signal of off-grid phase lock, Figure 24 middle The output of the controller maintains the same rate of change of the regulation signal, thereby shortening the phase angle synchronization process. From the trigonometric function formula, we can get:

[0139] (7);

[0140] Where, 、 are the positive and cosine values ​​of the voltage phase of the large power grid; 、 The sine and cosine values ​​of the voltage phase obtained for the load.

[0141] From formula (7), we can get: The difference between the angular frequency provided by the control strategy and the angular frequency of the normal power grid for:

[0142] (8);

[0143] Where, and are the proportional and integral adjustment coefficients of the off-grid phase-locked controller respectively.

[0144] On this basis, the improved off-grid phase-locked control is combined with Combined with the control strategy, the improved Control strategy, i.e. Figure 24 In order to speed up the tracking speed, the droop control strategy with off-grid phase locking function is Add the initial angular frequency of off-grid phase locking , which can be obtained by Droop control is achieved, and The off-grid phase-locked output angular frequency can be obtained by superposition and output angle .

[0145] 4) Control strategies for each converter in the device control layer, including the following 4.1) to 4.3).

[0146] As can be seen from the previous text, each port of the flexible multi-state switch can be 、 and When the feeder connected to a port in the three-port FMSS is working normally, the port is working under the control strategy. or Under the control strategy; when a feeder connected to a port in the three-port FMSS fails, the port works in Under control strategy. 、 and The control strategy is introduced as follows.

[0147] 4.1) Flexible Multi-state Switch 、 Control strategy.

[0148] If one of the three-port FMSS ports is a voltage-regulated port, then the port operates at Under the control strategy; if one of the ports of the three-port FMSS is a non-regulated port, the port will work in Under the control strategy. Among them, The control mainly adopts the grid voltage-oriented vector control technology based on the d-axis to stabilize the DC bus voltage; The control mainly adopts voltage and current dual loop and decoupling control to achieve the output of specific power, thereby achieving the purpose of feeder load balancing and maximum power output. 、 The impact on DC bus voltage during switching 、 The control adopts the common current inner loop structure. Take the example to illustrate the corresponding control strategy. Figure 22 shown.

[0149] Figure 22 middle, 、 for filter inductors and capacitors; DC bus input to of current. for Three-phase voltage on the AC side; for Three-phase current on the AC side. is the three-phase voltage of AC grid 1. SPLL is a phase-locked loop, is obtained after SPLL phase locking Phase angle on the AC side; is the angular frequency of the AC grid 1. and are the d-axis and q-axis currents after dq transformation. and are the d-axis and q-axis voltages after dq transformation. for Active power value under control. and are the d-axis and q-axis reference currents. and are the d-axis and q-axis voltage reference values ​​respectively. Figure 22 It can be seen that since the two control strategies are common current loop control in the d-axis, the current inner loop is decoupled to achieve independent current control of the d-axis and q-axis. Figure 2 When the switch S in the device control layer is set to 1, The control strategy of the corresponding voltage outer loop control strategy is shown in formula (9); when the switch S is set to 2, it is Control strategy, the corresponding current calculation formula is shown in formula (10).

[0150] (9);

[0151] (10);

[0152] in for Reactive power value under control; for Proportional and integral coefficients of the PI controller under the control strategy.

[0153] 4.2) Improvement with off-grid phase-locking function Control strategies include the following 4.2.1) to 4.2.3).

[0154] As we know from the above, when a port of FMSS is working Under the control strategy, stable voltage and frequency can be provided for the fault feeder load. In order to quickly synchronize the phase angle of the fault feeder load with the phase angle of the normal feeder load, an off-grid phase-locked link is added on the basis of traditional droop control. Take the example to illustrate the corresponding control strategy. Figure 23 shown.

[0155] Figure 23 and Figure 24 middle, is the filter inductor current; and for Output voltage and current. is the rated output power, is the reference reactive power. 、 are the active power and reactive power obtained from power calculation. for Control strategy output Reference phase angle. is the synthesized voltage control signal. and They are The d-axis and q-axis reference voltages in the control strategy. Figure 23 and Figure 24 It can be seen that the droop control based on The control strategy consists of a power calculation unit, a droop control unit with off-grid phase locking, and a voltage-current dual-loop controller, as shown below.

[0156] 4.2.1) Power calculation unit.

[0157] The power calculation unit calculates the voltage and current values ​​after dq transformation, and combines formula (11) to obtain the active power and reactive power :

[0158] (11);

[0159] Where, for The d-axis and q-axis currents obtained after dq transformation are: Figure 24 Abbreviated as ; for The d-axis and q-axis voltages obtained after dq transformation are: Figure 24 Abbreviated as .

[0160] 4.2.2) Droop control unit with off-grid phase locking.

[0161] See also Figure 24 , the droop control process is as follows: measure Output voltage and current values 、 , and calculate the active power and reactive power , after the droop characteristics expressed by equations (12) and (13), the inverter output voltage deviation signal can be obtained and frequency deviation signal . Then compare it with the rated voltage signal , rated frequency signal Superposition can generate voltage control signal and frequency control signal On this basis, the three-phase symmetrical electromotive force expression, as shown in formula (14), can be obtained as the three-phase voltage control signal , abbreviated as .

[0162] The corresponding droop characteristics can be expressed as:

[0163] (12);

[0164] (13);

[0165] in is the system rated frequency; is the system rated voltage. Frequency droop coefficient , voltage droop coefficient . is the maximum frequency allowed to be output by the system, for Maximum reactive power output; is the minimum voltage amplitude allowed to be output by the system, It is the maximum power allowed to be output when the system voltage drops.

[0166] (14);

[0167] in for Control the resulting angular frequency; Indicates time.

[0168] Further combining the three-phase stationary coordinate system-two-phase synchronous rotating coordinate system transformation formula, as shown in formula (15), the final The control signal and :

[0169] (15).

[0170] Combining the traditional droop control with the above equations (7) and (8), the block diagram of the droop control and voltage synthesis with off-grid phase locking is as follows: Figure 24 shown. Figure 24 in for The abbreviation of for The off-grid phase-locked control is the control strategy obtained by transforming Equation (8), where Substitute In order to speed up the off-grid phase-locking speed, the initial angle of the phase-locked loop is designed to be the frequency control signal obtained by droop control. The initial angular frequency obtained by transformation .

[0171] 4.2.3) Voltage and current dual-loop controller.

[0172] The voltage and current dual-loop controller adds voltage decoupling and current decoupling to the PI regulator respectively. The specific control strategy is as follows: Figure 25 shown. Figure 25 middle and for The d-axis and q-axis reference values ​​under control. and For the calculated Reference values ​​of the d-axis and q-axis inductor currents. and for The d-axis and q-axis inductor current values. and The d-axis and q-axis control voltage values ​​are output.

[0173] When controlling the current in the d and q axes, the current cross-coupling term is often affected. and To eliminate current coupling, current decoupling control is introduced into the current loop. Similarly, when controlling the voltage on the d and q axes, they are also affected by the voltage cross-coupling term, so voltage decoupling control is also introduced into the voltage loop.

[0174] 4.3) Energy storage system control strategy.

[0175] The energy storage system can switch between constant voltage control, constant current charging control, floating charge control and stop. Constant voltage control uses voltage and current dual loop control; constant current charging and floating charge control both use current loop control alone. In order to ensure that the energy storage system can quickly provide power when it is unplanned off-grid, a power feedforward control strategy is also added to the current inner loop. The overall control strategy of the energy storage converter is as follows: Figure 26 shown.

[0176] Figure 26 middle, The DC bus voltage reference value is set to 750V in this application. is the DC bus voltage. The constant current charging current of the battery is set to 36A in this application. It is the battery float charge voltage. is the battery inductor current, that is, the battery input current. Output current to the battery. is the power coefficient, which is the battery power feedforward ratio value, and is generally set to 0.8. is the power feedforward value.

[0177] Figure 26 When switch S is set to 1, the system is in floating charge control. achieve The battery is close to saturation and is charged with a small current. At this time, the voltage outer loop and current inner loop are controlled to make the battery terminal voltage Maintain at set value When the switch S is set to 2, the battery is charged under constant current control. At this time, the battery terminal voltage does not change much, and only the current loop control is needed to make the battery inductor current At the set value Constant power charging is achieved when switch S is set to 3. Constant voltage control is used. The battery stabilizes the DC bus voltage. When a sudden power surge occurs in the system, the delay switch in the constant voltage control closes, increasing the battery's current reference value. This allows the battery to provide more power at the moment of the sudden power surge, thereby reducing bus voltage fluctuations. When the bus voltage returns to near the set value, the battery's delay switch opens, disabling power feedforward and preventing the battery's constant voltage control effect.

[0178] In order to verify the effectiveness of the coordinated control method proposed in this application, Figure 1 A new distribution network system based on energy storage and flexible multi-state switches was built in Matlab / Simulink. 750V; 、 and The capacity is 100kW; the transformer ratio is 10kV / 0.4kV; 、 and The maximum power is 100kV·A; the battery capacity is 350A·h, is 0.9, terminal voltage It is 360 V. According to the simulation results under the corresponding flexible interconnection and load transfer operation modes, the coordinated control method of the present application greatly improves the reliability and stability of the system power supply.

[0179] This application designs the system and 、 and A set of batteries is added on the DC side of the FMSS three-port to improve the system power supply reliability. Combined with the coordinated control requirements of "source-grid-load-storage" in the new distribution network system, the Figure 2 The hierarchical control approach shown in the figure coordinates energy storage and FMSS. Each of the three-port FMSSs has a separate local controller to perform local device control at the device control layer, measure local parameters, and estimate the operating status of the FMSS and battery. A centralized controller is used to complete the coordinated control layer. By combining the operating status of the three ports of the flexible multi-state switch and feeder fault information, the different possible operating modes of the distribution network containing energy storage and FMSS, as well as the coordinated control strategy of energy storage and FMSS, are studied.

[0180] When a fault occurs in the feeder connected to the three-port FMSS or the fault is cleared, the control strategies of each part will inevitably switch, causing bus voltage fluctuations. In order to improve the stability and reliability of system operation, this application also studies the power feedforward and nonlinear anti-disturbance control technology of the battery to achieve rapid energy replenishment. During the switching process of fault occurrence and clearance, in addition to the DC bus voltage fluctuation, the phase angle will also change suddenly due to the different phase angle acquisition methods of the FMSS port in the two different states of grid connection and off-grid. Therefore, it is necessary to ensure a smooth transition of the phase when switching between grid connection and off-grid. Based on this, this application also studies the phase locking and grid connection pre-synchronization strategies to achieve a smooth transition of voltage and current on the AC side.

[0181] It should be noted that the terms "comprises," "includes," or any other variations thereof used in this application are intended to encompass non-exclusive inclusion, such that a product or system comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such a product or system. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the product or system comprising the element.

[0182] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0183] This application uses specific examples to illustrate the principles and implementation methods of this application. The description of the above examples is only intended to help understand the method and core concept of this application. At the same time, for those skilled in the art, based on the concept of this application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting this application.

Claims

1. A coordinated control method for energy storage and flexible multi-state switches in a distribution network system, characterized in that: The distribution network system includes: a flexible multi-state switch, an energy storage system, and multiple AC power grids; the flexible multi-state switch is composed of multiple back-to-back voltage source converters; the AC port of each voltage source converter is connected to the feeder end of an AC power grid via a transformer; each feeder is connected to a corresponding feeder load; the DC ports of each voltage source converter are connected in parallel to form a DC bus, which is connected to the energy storage system composed of a battery and a bidirectional DC / DC converter; The coordinated control method for energy storage and flexible multi-state switches in a distribution network system includes: coordinating the energy storage system and the flexible multi-state switches in a hierarchical control manner; wherein a coordinated control layer provides a centralized controller to achieve multi-directional operational control objectives of feeder load balancing, improving system power supply reliability, and extending the power supply time of faulty feeder loads, and sends the control strategy to the device control layer; the device control layer provides a separate local controller for each voltage source converter to complete local device control of each voltage source converter; The coordinated control layer is composed of a centralized controller that selects and switches the working mode under the networked operation mode according to the feeder load power, battery charge state, operating status of each transformer, and the location and number of feeder faults. The control strategy of the flexible multi-state switch in the device control layer can change between a constant DC voltage control strategy, a constant power control strategy, and a droop control strategy. When the feeders connected to the AC ports of all voltage source converters in the flexible multi-state switch are all operating normally, the AC port of one voltage source converter adopts a constant DC voltage control strategy to stabilize the DC bus voltage, while the AC ports of other voltage source converters adopt a constant power control strategy to control the power input from the AC grid to the system. When a feeder fault occurs at the AC port of one of the voltage source converters in the flexible multi-state switch, the voltage source converter corresponding to the port connected to the faulty feeder is switched to a droop control strategy. The control strategy of the battery in the device control layer can switch between constant current charging control, floating charge control, and constant voltage control. The feeder load in the device control layer can also implement a load switching control strategy. The centralized controller selects and switches the working mode in the networked operation mode according to the feeder load power, battery charge state, operating status of each transformer, and the location and number of feeder faults, specifically including: If a fault occurs in part of the AC grid feeder connected to the flexible multi-state switch, the system is determined to be operating in the load transfer state. At this time, according to the different locations of the feeder faults, it is divided into feeder faults at the voltage-stabilizing port and feeder faults at the non-voltage-stabilizing port. When a feeder fault occurs at the voltage stabilization port, if the difference between the total power of all feeder loads and the rated charging power of the battery is less than the sum of the maximum powers of all transformers connected to the normally operating feeders in the system, the system is determined to be in the free-running mode; In the free-running mode of the network, when the battery state of charge is less than the maximum state of charge, with the goal of improving system power supply reliability, a port in the flexible multi-state switch that has not experienced a feeder fault is selected as the DC voltage-stabilized port, and the battery is used as the backup power supply. With the goal of extending the power supply time of the faulty feeder load, the port that has not experienced a feeder fault is set to supply power to the faulty feeder load. At the same time, if a feeder fault occurs only on a single voltage-stabilized port, multiple AC power grids that have not experienced a feeder fault are set to output the same power to achieve feeder load balancing. The control strategy of the equipment control layer is as follows: the battery adopts constant current charging control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage-stabilized port adopts a constant DC voltage control strategy, and the other ports adopt a constant power control strategy. In the free-running mode of the interconnected network, when the battery state of charge is greater than or equal to the maximum state of charge value, the system power supply reliability has been maximized and the battery stops charging. If a feeder fault occurs only at the voltage-stabilizing port, the coordinated control layer aims to extend the power supply time of the faulty feeder load and balance the feeder load. Multiple AC power grids that have not experienced feeder faults are set to output the same power. If a feeder fault occurs at the dual-port flexible multi-state switch, including the voltage-stabilizing port, the goal is only to extend the power supply time of the faulty feeder load. The control strategy of the equipment control layer is as follows: the battery adopts floating charge control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage-stabilizing port adopts a constant DC voltage control strategy, and other ports adopt a constant power control strategy.

2. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 1, characterized in that: The centralized controller selects and switches the working mode in the networked operation mode according to the feeder load power, battery charge state, operating status of each transformer, and the location and number of feeder faults, specifically including: When all feeders connected to the AC ports of the voltage source converters in the flexible multi-state switch are operating normally, multiple AC power grids are interconnected through the flexible multi-state switch, which is called the flexible interconnected operation state; When the centralized controller determines that the system is in a flexible interconnected operation state, it divides the system into a free-network operation mode and a current-limited network operation mode based on whether each transformer in the system has reached its own capacity limit. Based on the above two operation modes, the centralized controller determines the system operation mode and the control strategies of the coordination control layer and the equipment control layer according to the working status of the battery.

3. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 2, characterized in that: When the centralized controller determines that the system is in the flexible interconnection operation state, if the difference between the total power of all feeder loads and the rated charging power of the battery is less than the sum of the maximum powers of all transformers, the system is determined to be in the free-connection operation mode; In the free-running mode, when the battery state of charge is less than the maximum state of charge, the flexible multi-state switch is selected as the voltage stabilizing device and the battery is used as the backup power supply to improve the system power supply reliability. At the same time, multiple AC grids are set to generate the same power in order to achieve feeder load balancing. , ;in Feeder load Power; is the number of feeder loads; is the rated charging power of the battery, and the battery output power is positive and the absorbed power is negative. The control strategy of the device control layer is: the battery adopts constant current charging control, and at the same time, a constant DC voltage control strategy is adopted for the AC port of one voltage source converter of the flexible multi-state switch, and a constant power control strategy is adopted for the AC ports of the other voltage source converters. In the free-running mode, when the battery state of charge is greater than or equal to the maximum state of charge, the battery switches to floating charge control. At this time, the system power supply reliability has been maximized. The coordination control layer only aims to achieve feeder load balancing, and sets multiple AC grids to generate the same power. The control strategy of the equipment control layer is: the battery adopts floating charge control, and at the same time, the AC port of one voltage source converter of the flexible multi-state switch adopts a constant DC voltage control strategy, and the AC ports of other voltage source converters adopt a constant power control strategy.

4. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 3, characterized in that: When the centralized controller determines that the system is in the flexible interconnection operation state, if the sum of the maximum power of all transformers is greater than the sum of the total power of all feeder loads but less than the difference between the total power of all feeder loads and the rated charging power of the battery, the system is determined to be in the interconnection current limiting operation mode; In the networked current-limited operation mode, the coordination control layer aims to maximize the system power supply reliability, ensuring that each transformer outputs maximum power. The battery is charged with excess power within the system, and the battery is selected as the voltage-stabilizing port. The control strategy of the equipment control layer is: the battery adopts constant voltage control, and at the same time, the AC ports of all voltage source converters of the flexible multi-state switch adopt a constant power control strategy.

5. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 1, characterized in that: When the centralized controller determines that the system is in load transfer operation, if the sum of the maximum power of all transformers connected to the normally operating feeders in the system is greater than the sum of the total power of all feeder loads, but less than the difference between the total power of all feeder loads and the rated charging power of the battery, the system is determined to be in network current limiting operation mode. The coordinated control layer control strategy is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the ports where the feeder fault has not occurred are set to supply power to the faulty feeder load. To improve system power supply reliability, the maximum output power of the transformer connected to the normally operating feeder is set. The battery is selected as the DC voltage stabilization port, and the battery is charged with excess power within the system until it is fully charged. The control strategy of the equipment control layer is: constant voltage control is used for the battery, droop control is used for the port connected to the faulty feeder, and constant power control is used for other ports. When the battery charging is completed, it switches to float charge control, and a port in the flexible multi-state switch without a feeder fault is selected as the DC voltage stabilization port. The DC voltage stabilization port adopts a constant DC voltage control strategy, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy.

6. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 5, characterized in that: When the centralized controller determines that the system is in the load transfer operation state, if the sum of the maximum powers of all transformers connected to the normally operating feeders in the system is less than the sum of the total powers of all feeder loads, the system is determined to be in the networked current limiting operation mode; and the coordination control layer control strategy is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the battery is selected as the DC voltage stabilization port, and together with the port where the feeder fault has not occurred, it supplies power to the faulty feeder load; the equipment control layer control strategy is: the battery adopts constant voltage control, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy.

7. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 1, characterized in that: When a feeder fault occurs on the non-regulated port, if the difference between the total power of all feeder loads and the rated charging power of the battery is less than the sum of the maximum powers of all transformers connected to the normally operating feeders, the system is determined to be in the free-running mode; In the free-running mode, when the battery state of charge is less than the maximum state of charge, the battery is charged at rated power until it is fully charged, with the goal of improving system power supply reliability. To extend the power supply time of the faulty feeder load, ports that have not experienced feeder faults are set to supply power to the faulty feeder load. Furthermore, if only one non-regulated port in the system experiences a feeder fault, multiple AC grids that have not experienced feeder faults are set to output the same power, with the goal of achieving feeder load balancing. The control strategy at the equipment control layer is as follows: constant current charging control is used for the battery, droop control is used for the port connected to the faulty feeder, constant DC voltage control is used for the DC regulated port, and constant power control is used for all other ports. In the free-running mode, when the battery state of charge (SOC) is greater than or equal to the maximum SOC value, the system power supply reliability is maximized and the battery stops charging. If a feeder fault occurs on only one unregulated port in the system, the coordinated control layer sets the ports connected to the normally operating feeders to jointly supply power to the faulty feeder load, with the goal of extending the power supply time of the faulty feeder load and balancing the feeder load. Multiple AC grids that have not experienced feeder faults are set to output the same power. If feeders connected to multiple unregulated ports in the system fail, the coordinated control layer sets the regulated ports to supply power to multiple faulty feeder loads, with the goal of extending the power supply time of the faulty feeder load. The control strategy of the equipment control layer is as follows: the battery adopts floating charge control, the port connected to the faulty feeder adopts droop control strategy, the DC regulated port adopts constant DC voltage control strategy, and other ports adopt constant power control strategy.

8. The coordinated control method for energy storage and flexible multi-state switches in a distribution network system according to claim 7, characterized in that: When the centralized controller determines that the system is in load transfer operation, if the sum of the maximum power of all transformers connected to the normally operating feeders in the system is greater than the sum of the total power of all feeder loads but less than the difference between the total power of all feeder loads and the rated charging power of the battery, the system is determined to be in network current limiting operation mode. The coordinated control layer control strategy is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the port where the feeder fault does not occur is set to supply power to the faulty feeder load. To improve system power supply reliability, the maximum output power of all transformers connected to normally operating feeders is set. Batteries are selected as DC voltage-stabilized ports, and the excess power within the system is used to charge the batteries until they are fully charged. The control strategy at the equipment control layer is: constant voltage control is used for batteries, droop control is used for ports connected to faulty feeders, and constant power control is used for other ports. When the battery is fully charged, it switches to float charge control and selects a port in the flexible multi-state switch that has not experienced a feeder fault as the DC voltage-regulated port. The DC voltage-regulated port adopts a constant DC voltage control strategy, the port connected to the faulty feeder adopts a droop control strategy, and the other ports adopt a constant power control strategy. If the sum of the maximum powers of all transformers connected to the normally operating feeder is less than the sum of the total powers of all feeder loads, the system is determined to be in the networked current-limited operation mode; and the coordination control layer control strategy is formulated as follows: with the goal of extending the power supply time of the faulty feeder load, the battery is selected as the DC voltage-stabilizing port, and together with the ports where no feeder fault has occurred, it supplies power to the faulty feeder load; the equipment control layer control strategy is: the battery adopts constant voltage control, the port connected to the faulty feeder adopts a droop control strategy, and other ports adopt a constant power control strategy.

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