Coordination control method for energy storage and flexible multi-state switch in power distribution network system

Abstract

The invention discloses a coordination control method for energy storage and a flexible multi-state switch in a power distribution network system, and relates to the field of power supply and distribution. The hierarchical control mode is adopted to coordinate the energy storage system and the flexible multi-state switch; the coordination control layer performs working mode selection and switching in a networking operation mode by a centralized controller according to the feeder load power, the charge state of a storage battery, the operation state of each transformer and the position and the number of feeder faults; the control strategy of the flexible multi-state switch can be changed among a constant DC voltage control strategy, a constant power control strategy and a droop control strategy; the control strategy of the storage battery can be switched among constant-current charging control, floating charging control and constant-voltage control; and the feeder load in the equipment control layer can also realize a load switching control strategy. By adopting the coordination control method, the power supply reliability and the power quality of the system can be improved.

Description

Technical Field

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

[0002] Flexible multi-state switch (FMSS) is a power electronic device installed in the distribution network, connected between two or more feeders, and adjusting the flow of active power between feeders. Through power electronics technology, intelligent algorithms and other controls, it not only has the on and off states of conventional switches, but also can achieve continuous control of power. It is the key technology and equipment support for future smart grids and new power systems. When the distribution network feeder operates normally, FMSS can not only avoid power outages and loop impact caused by conventional switch switching operations, but also alleviate voltage sags and three-phase imbalance, promote feeder load balancing and improve power quality. However, in the case of distribution network feeder failure, due to the lack of support from the power grid and the switching of FMSS control strategies, bus voltage fluctuations and reduced power quality are often caused. In order to solve the above problems, energy storage systems with energy storage functions are introduced into FMSS. Therefore, how to achieve coordinated control of energy storage and FMSS to improve system power supply reliability and power quality is an important direction for the future development of FMSS. 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 power supply reliability and power quality of the system 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 flexible multi-state switches in a distribution network system, wherein the distribution network system comprises: 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 port of each voltage source converter is connected in parallel to form a DC bus, and is connected to an energy storage system composed of a battery and a bidirectional DC / DC converter; The coordinated control method of energy storage and flexible multi-state switches in the distribution network system includes: coordinating the energy storage system and the flexible multi-state switch in a hierarchical control manner; wherein a centralized controller is provided in the coordinated control layer 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, and sending control strategies to the device control layer; the device control layer is provided with a separate local controller for each voltage source converter to complete the local device control of each voltage source converter; The coordination control layer is composed of a centralized controller, which selects and switches the working mode under the networking operation mode according to the feeder load power, the battery charge state, the operating state 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 all 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, and the AC ports of other voltage source converters adopt a constant power control strategy to control the power input from the AC power grid to the system; when a feeder fault occurs at the AC port of one of the voltage source converters of 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 charging control and constant voltage control; the feeder load in the device control layer can also implement a load switching control strategy.

[0006] Optionally, the centralized controller selects and switches the working mode in the networking operation mode according to the feeder load power, the battery charge state, the operating status of each transformer, and the location and number of feeder faults, specifically including: When all feeders connected to the AC ports of all voltage source converters in the flexible multi-state switch are operating normally, multiple AC power grids are interconnected and operated through the flexible multi-state switch, which is called the flexible interconnection operation state; When the centralized controller determines that the system is in a flexible interconnected operation state, it divides the system into a free-connection operation mode and a current-limited operation mode based on whether each transformer in the system has reached its own capacity upper 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.

[0007] Optionally, when the centralized controller determines that the system is in a 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, it is determined that the system is in a networked free 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 with the battery as the backup power supply to improve the system power supply reliability; at the same time, multiple AC power 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 power emitted by the battery is positive, and the power absorbed is negative; the control strategy of the equipment control layer is: the battery adopts constant current charging control, and at the same time, a fixed DC voltage control strategy is adopted for the AC port of one of the voltage source converters of the flexible multi-state switch, and a constant power control strategy is adopted for the AC ports of 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 power grids to emit 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 fixed DC voltage control strategy, and the AC ports of other voltage source converters adopt a constant power control strategy.

[0008] 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; Under the networked current-limiting operation mode, the coordination control layer aims to maximize the system power supply reliability, so that each transformer outputs the maximum power, the battery is charged with the excess power inside the system, and the battery is selected as the voltage stabilization 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.

[0009] Optionally, the centralized controller selects and switches the working mode in the networking operation mode according to the feeder load power, the battery charge state, the operating status of each transformer, and the location and number of feeder faults, specifically including: If a part of the AC grid feeder connected to the flexible multi-state switch fails, it is determined that the system is 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 at the voltage-stabilizing port and feeder faults at the non-voltage-stabilizing port.

[0010] 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 a networked free operation mode; In the free-running mode of networking, when the battery state of charge is less than the maximum state of charge value, with the goal of improving the system power supply reliability, a port in the flexible multi-state switch without feeder fault is selected as the DC voltage stabilization 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 without feeder fault is set to supply power to the faulty feeder load; at the same time, if only a single port of the voltage stabilization port has a feeder fault, with the goal of achieving feeder load balancing, multiple AC power grids without feeder fault are set to output the same power; the control strategy of the equipment control layer is: the battery adopts constant current charging control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage stabilization port adopts a fixed DC voltage control strategy, and other ports adopt a constant power control strategy; In the free-running mode of networking, when the battery state of charge is greater than or equal to the maximum state of charge value, the system power supply reliability has reached the maximum guarantee, and the battery stops charging. If a feeder fault occurs only at the voltage stabilizing port, the coordination control layer sets the same power output for multiple AC power grids that have not experienced feeder faults, with the goal of extending the power supply time of the faulty feeder load and balancing the feeder load. 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: 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.

[0011] 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 coordination 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 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 battery is charged with the excess power inside the system until it is fully charged; 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; When the battery charging is completed, it switches to floating charge control, and a port in the flexible multi-state switch where no feeder fault occurs 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.

[0012] 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.

[0013] 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 a networked free operation mode; In the free-running mode of networking, when the battery state of charge is less than the maximum state of charge value, the battery is charged at rated power until it is fully charged, with the goal of improving the system power supply reliability; 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; at the same time, if only one non-voltage-stabilized port in the system has a feeder fault, multiple AC power grids where the feeder fault does not occur are set to output the same power, with the goal of achieving feeder load balancing; the control strategy of the equipment control layer is: 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 fixed DC voltage control strategy, and other ports adopt a constant power control strategy; In the free-running mode of networking, when the battery state of charge is greater than or equal to the maximum state of charge value, the system power supply reliability has reached the maximum guarantee, and the battery stops charging. If only one non-voltage-stabilized port in the system has a feeder fault, the coordination control layer sets the ports connected to the normal 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, and sets multiple AC power grids that have not experienced feeder faults to output the same power; if multiple feeders connected to non-voltage-stabilized ports in the system fail, the coordination control layer only sets the voltage-stabilized 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: the battery adopts floating charge control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage-stabilized port adopts a fixed DC voltage control strategy, and other ports adopt a constant power control strategy.

[0014] 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 coordination 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 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 battery is charged with the excess power inside the system until it is fully charged; 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; When the battery is fully charged, it switches to floating charge control, and selects a port in the flexible multi-state switch that has no feeder fault 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. 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, it is determined that the system is 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 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.

[0015] According to the specific embodiments provided in this application, this application discloses the following technical effects.

[0016] In a coordinated control method of energy storage and flexible multi-state switches in a distribution network system provided by the present application, a hierarchical control method is adopted 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 fixed DC voltage control strategy, a constant power control strategy and a droop control strategy; when all feeders connected to all AC ports in the flexible multi-state switch are operating normally, one of the AC ports adopts a fixed DC voltage control strategy to stabilize the DC bus voltage, while other AC ports adopt a constant power control strategy to control the power input into the system by the AC power grid. When a feeder fault occurs in 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 providing a corresponding phase angle by relying on the 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 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 various device-level control strategies 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

[0017] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. 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 paying creative work.

[0018] Figure 1 A topological diagram of the distribution network system constructed for this application; Figure 2 It is the hierarchical control structure diagram of energy storage system and FMSS; Figure 3 It is a schematic diagram of power transmission between units of the distribution network system under mode 1; Figure 4 It is a schematic diagram of power transmission between units of the distribution network system under mode 2; Figure 5 It is a schematic diagram of power transmission between units of the distribution network system under mode 3; Figure 6 It is a schematic diagram of power transmission between units of the distribution network system under mode 4; Figure 7 It is a schematic diagram of power transmission between units of the distribution network system under mode 5; Figure 8 It is a schematic diagram of power transmission between units of the distribution network system under mode 6; Fig. 9 It is a schematic diagram of power transmission between units of the distribution network system under mode 7; Fig.10 It is a schematic diagram of power transmission between units of the distribution network system under mode 8; Fig.11 It is a schematic diagram of power transmission between units of the distribution network system under mode 10; Fig.12 It is a schematic diagram of power transmission between units of the distribution network system under mode 11; Fig.13 It is a schematic diagram of power transmission between units of the distribution network system under mode 12; Fig.14 It is a schematic diagram of power transmission between units of the distribution network system under mode 13; Fig.15 It is a schematic diagram of power transmission between units of the distribution network system under mode 14; Fig.16 It is a schematic diagram of power transmission between units of the distribution network system under mode 15; Fig.17 It is a schematic diagram of power transmission between units of the distribution network system under mode 16; Fig.18 It is a schematic diagram of power transmission between units of the distribution network system under mode 18; Fig.19 It is a schematic diagram of power transmission between units of the distribution network system under mode 19; Fig. 20 This is the planned feeder off-grid switching control timing diagram proposed in this application; Fig.21 This is the power feedforward control diagram when the battery feeder is off-grid unplanned; Fig. 22 For flexible multi-state switch and Control strategy diagram; Fig.23 It is a flexible multi-state switch with off-grid phase-locking function. Control strategy diagram; Fig.24 It is a block diagram of droop control with off-grid phase locking; Fig.25 This is the double loop control block diagram of voltage outer loop and current inner loop; Fig.26 This is the control strategy diagram of the energy storage system. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0020] The present application proposes a coordinated control method of energy storage and flexible multi-state switches in a distribution network system, which improves the power supply reliability and power quality of the system through coordinated control of energy storage and FMSS.

[0021] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0022] In an exemplary embodiment, a coordinated control method for energy storage and flexible multi-state switches in a distribution network system is provided. The distribution network system (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 (referred to as 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, referred to as load. The DC ports of each voltage source converter are connected in parallel to form a common DC bus, and an energy storage system composed of a battery and a bidirectional DC / DC converter is connected to the DC bus. Taking a three-port flexible multi-state switch as an example, the coordinated control method for energy storage and flexible multi-state switches in a distribution network system is described in detail below.

[0023] 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 adopts three back-to-back voltage source converters. , , Port 1, Port 2, and Port 3. , , The respective AC ports are connected through transformers , and The feeder ends connected to three independent 10kV AC grids (called AC grid 1, AC grid 2 and AC grid 3) realize flexible interconnection of different feeders. It is the DC side capacitor, which can reduce the DC measurement ripple. is the DC port voltage. , , The respective DC ports are connected in parallel to form a common DC bus, and an energy storage system consisting of a battery and a bidirectional DC / DC converter is connected to the DC bus to improve the system power supply reliability when the distribution network system is off-grid, and enhance the adaptability and control flexibility of the flexible multi-state switch to complex operating conditions. In view of the importance of the load in the power system, three feeder loads with different priorities are set in this embodiment, among which feeder load 1 is a critical load with the highest priority, and feeder load 2 and feeder load 3 are both non-critical loads.

[0024] 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 faulty feeder loads, and sends control strategies to the device control layer. The device control layer is each voltage source converter , , A separate local controller is provided to complete the local device control of each voltage source converter.

[0025] 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 the feeders connected to the AC ports of all converters in the FMSS are all 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 failure occurs at the AC port of one of the converters of the FMSS, the port connected to the faulty feeder needs to be switched to Control strategy. The control strategy of the battery in the device control layer can be switched between constant current charging control, floating charging control and constant voltage control. The feeder load in the device control layer can also implement load switching control strategy.

[0026] In actual operation, Figure 1The distribution network system shown can be connected to the network or operated in an island. When the system is connected to the network, the flexible multi-state switch can realize flexible power flow control and balanced feeder load distribution. When the system is off-grid, the energy storage is used to supply power to the loads of each feeder, thereby improving the reliability of the system power supply. On this basis, combined with whether the AC feeders connected to each port of the flexible multi-state switch are faulty and the number of ports with feeder faults, the system operation state can be subdivided into two operation states: flexible interconnection and load transfer. The specific description is as follows: 1) Flexible interconnection operation state: If the AC feeders connected to the three ports of the flexible multi-state switch are all working normally, that is, the three AC power grids are connected to the network through the flexible multi-state switch, and the output power of the three AC power grids can be adjusted through the flexible multi-state switch, it is called the flexible interconnection operation state. 2) Load transfer operation state: If one or two AC feeders connected to the flexible multi-state switch fail, the system operates in the load transfer operation state. According to the different feeder fault locations, it can be divided into feeder faults at the voltage-stabilizing port and feeder faults at the non-voltage-stabilizing port.

[0027] Furthermore, when the centralized controller determines that the system is in a flexible interconnection operation state, the centralized controller combines the transformers in the system to determine that the system is in a flexible interconnection operation state. , and Whether the upper limit of its own capacity is reached, it is further divided into the free operation mode and the current-limited operation mode. Based on the above two operation modes, the system operation mode and the control strategy of the coordination control layer and the equipment control layer are determined according to the working status of the battery. The specific analysis is as follows.

[0028] 1) Flexible interconnection operation status, including the following 1.1) to 1.2).

[0029] 1.1) Network free running mode.

[0030] If the total power of all feeder loads Rated charging power of battery 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 of power. is the number of feeder loads. =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.

[0031] Mode 1: In this mode 1, since there are three AC power grids as strong power support and the power between the flexible multi-state switch and the AC power 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. ) is taken as an example of the DC bus voltage regulator port. 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 , see formula (1).

[0032] (1); In formula (1), , , They 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).

[0033] (2); In formula (2), is the initial charge value of the battery’s 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 .

[0034] The control strategy of the equipment control layer is: battery (constant current charging control) + ( Control)+ ( Control)+ ( control). That is, the battery adopts a constant current charging control strategy, and at the same time, the AC port of one of the FMSS converters adopts Control strategy, adopt the AC port of other converters Control strategy.

[0035] Mode 2: When the battery in Mode 1 is fully charged, When the battery is switched to floating charge control, the system power supply reliability is guaranteed to the maximum. 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).

[0036] (3).

[0037] 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).

[0038] 1.2) Network current limiting operation mode.

[0039] When the centralized controller determines that the system is in the flexible interconnection operation state, if , confirming that the system is in the network current limiting operation mode. In this working mode, although the three AC power grids are working normally, the battery cannot be charged at the rated power due to the large load power, and it is converted to mode 3.

[0040] Mode 3: The coordinated control layer aims to maximize the reliability of system power supply and control each transformer , , Maximum output power , , The battery is charged with the excess power in the system. 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 three ports of FMSS are all Control strategy, so , , Both output maximum power.

[0041] 2) Load transfer operation status, including the following 2.1) to 2.2).

[0042] If some of the AC feeders connected to the flexible multi-state switch fail, 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 at the voltage-stabilizing port and feeder faults at the non-voltage-stabilizing port. When the system is operating in the load transfer operation state, the AC feeder fails and the system power supply reliability is reduced. At this time, the coordination control layer aims to extend the load power supply time of the faulty feeder and achieve load balancing control. Combined with the location and number of feeder faults, the specific analysis can be as follows.

[0043] 2.1) A feeder fault occurs at the voltage stabilization port, including the following 2.1.1) to 2.1.2).

[0044] 2.1.1) An AC feeder fault occurs on a single port and voltage-stabilizing port of the flexible multi-state switch, including the following 2.1.1.1) to 2.1.1.2).

[0045] 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 cannot continue 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, and The output power reaches the upper limit. Based on this, the system operation can be further divided into the networking free operation mode and the networking current limiting operation mode.

[0046] 2.1.1.1) Networked free-running mode.

[0047] like , confirm that the system is in the free running mode of networking, at this time, according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following mode 4 and mode 5.

[0048] 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 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 units of the system is as follows Figure 6 As shown. ② With the goal of extending the power supply time of the faulty 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): (4).

[0049] 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.

[0050] 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.

[0051] (5).

[0052] The control strategy of the equipment control layer is: battery (floating charge control) + ( Control)+ ( Control)+ ( control).

[0053] 2.1.1.2) Network current limiting operation mode.

[0054] Mode 6: If the total power of the three feeder loads is large, and The maximum power of the total under the normal operation of the load cannot guarantee that the battery is charged at the rated power, that is The coordination control layer control strategy can be formulated as follows: ① To extend the power supply time of the faulty feeder load, set the ports 2 and 3 of the FMSS to supply power to the feeder load 1. ② To improve the power supply reliability of the system, 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 charged with the excess power inside the system until it is fully charged. The power transmission between the units of the system is as follows Figure 8 shown.

[0055] 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 floating charge control, and the voltage is stabilized by port 2 of the flexible multi-state switch. For the specific control strategy, see mode 5.

[0056] 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 units of the system is as follows Fig. 9 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control)+ ( Control)+ ( control).

[0057] 2.1.2) The flexible multi-state switch has dual ports and includes a voltage stabilization port and an AC feeder fault occurs, including the following 2.1.2.1) to 2.1.2.2).

[0058] When two AC grid AC feeders fail, and the ports connected to the faulty feeders include voltage stabilization ports, and Taking the failure of the connected AC feeder as an example, the following is explained in the free-networking and current-limiting operation modes.

[0059] 2.1.2.1) Networked free-running mode.

[0060] like , confirm that the system is in the free running mode of networking, at this time, according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following mode 8 and mode 9.

[0061] 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 faulty 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 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 Fig.10 As shown. The control strategy of the equipment control layer is: battery (constant current charging) + ( Control)+ ( Control)+ ( control).

[0062] Mode 9: When the battery is fully charged, the system power supply reliability is guaranteed to the maximum extent, so the system control strategy is set with the goal of extending the power supply time of the faulty feeder load.

[0063] 2.1.2.2) Network current limiting operation mode.

[0064] 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 control objectives of the coordination control layer are as follows: ① To extend the power supply time of the faulty feeder load, set the FMSS It is a voltage stabilizing port, which supplies 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, the battery is used as a DC voltage stabilization port, and the excess power in the system is charged until it is fully charged. At this time, the power transmission between the units in the system is as follows Fig.11 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control)+ ( Control)+ ( control).

[0065] 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 faulty feeder load, the battery is set as a DC voltage stabilization port and outputs power to make up for the power shortage in the system. At this time, the power transmission between the units of the system is as follows: Fig.12 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control)+ ( Control)+ ( control).

[0066] 2.2) A feeder fault occurs at the non-regulated port, including the following 2.2.1) to 2.2.2).

[0067] 2.2.1) An AC feeder fault occurs on a single port and non-regulated port of the flexible multi-state switch, including the following 2.2.1.1) to 2.2.1.2).

[0068] 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 regulation 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.

[0069] 2.2.1.1) Networked free-running mode.

[0070] like , confirm that the system is in the free running mode of networking, at this time, according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following mode 12 and mode 13.

[0071] Mode 12: In this mode 12, the total power of the three feeder loads is small and The connected feeder has not failed, so the voltage stabilization port remains unchanged. The control objectives of the coordinated 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 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 formula (4). The power transmission between the units in the corresponding system is obtained as follows Fig.13 As shown. The control strategy of the equipment control layer is: battery (constant current charging control) + ( Control)+ ( Control)+ ( control).

[0072] 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 to extend the power supply time of the fault feeder load and balance the feeder load as the goal. The power transmission between the units in the corresponding system is as follows: Fig.14When the battery is fully charged, the control strategy of the equipment control layer is: battery (floating charge control) + ( Control)+ ( Control) + ( control).

[0073] 2.2.1.2) Network current limiting operation mode.

[0074] 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 operation of the load. 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 power 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. Fig.15 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control)+ ( Control)+ ( control).

[0075] 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, and the battery discharge is set as a voltage stabilization port. At this time, the power transmission between the units of the system is as follows Fig.16 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control)+ ( Control)+ ( control).

[0076] 2.2.2) The flexible multi-state switch has dual ports and does not include the voltage stabilization port. An AC feeder fault occurs, including the following 2.2.2.1) to 2.2.2.2).

[0077] 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, it is also explained in the free networking and current limiting operation modes.

[0078] 2.2.2.1) Networked free-running mode.

[0079] like , confirm that the system is in the free running mode of networking, at this time, according to the battery charge state Whether the maximum state of charge value has been reached , divided into the following mode 16 and mode 17.

[0080] Mode 16: In mode 16, the load power of the three feeders is small, and only All feeder loads can be powered, and the maximum To charge the battery. To this end, the control strategy of the coordinated control layer is formulated as follows: ① With the goal of extending the power supply time of the faulty feeder load, set port 1 of FMSS to supply power to feeder load 2 and feeder load 3. ② With the goal of improving the power supply reliability of the system, set the battery to charge at a constant current until it is fully charged. The DC voltage stabilization port of port 1 of FMSS remains unchanged. The power transmission between the units in the system is as follows Fig.17 As shown. The control strategy of the equipment control layer is: battery (constant current charging control) + ( Control)+ ( Control)+ ( control).

[0081] Mode 17: When the battery is fully charged, the system power supply reliability has reached the maximum guarantee, so the goal is to extend the power supply time of the fault feeder load, and set the control strategy of the equipment control layer to: battery (floating charge control) + ( Control)+ ( Control)+ ( control).

[0082] 2.2.2.2) Network current limiting operation mode.

[0083] 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 faulty feeder load, set port 1 of 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 the 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 Fig.18 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control)+ ( Control)+ ( control).

[0084] 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 coordination control layer control strategy is to achieve uninterrupted power supply to the feeder load, and the battery discharge and voltage stabilization port are set to make up for the power shortage. At this time, the power transmission between the units of the system is as follows Fig.19 As shown. The control strategy of the equipment control layer is: battery (constant voltage control) + ( Control)+ ( Control)+ ( control).

[0085] 3) Feeder off-grid switching control, including the following 3.1) to 3.2).

[0086] When the AC feeder connected to one or more ports of the flexible multi-state switch fails or the AC grid needs to stop supplying power due to some special reason (such as power outage for maintenance, etc.), the corresponding flexible multi-state switch port will be disconnected from the feeder. During the transition from grid-connected to off-grid, if the off-grid operation is caused by the failure of the AC feeder, it is called unplanned off-grid or unplanned feeder off-grid; if the feeder off-grid operation is caused by some special reason within a specific time period, it is a planned or predictable feeder off-grid, which is called planned off-grid or planned feeder off-grid.

[0087] Whether it is a planned or unplanned feeder off-grid, since the power output from the AC grid to a certain port of the flexible multi-state switch to the system will drop to zero, the control strategy and power of other ports of the flexible multi-state switch and the battery will change, thus causing the bus voltage to change. Therefore, how to reasonably coordinate the control strategy between the flexible multi-state switch and the battery during the switching process is the key to improving the system operation efficiency.

[0088] Fig. 20 This is the planned feeder off-grid switching control timing diagram proposed in this application, where 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 go offline and the system. Affected by the flexible multi-state switch load balancing control strategy, the output power of the AC grid depends on the power of its own feeder load and the power of the feeder loads at other ports. Therefore, in order to achieve the goal 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 / reduced in advance. The corresponding converter control strategy is changed at all times to make the power output from the AC power grid to the system zero. At this moment, the exchange power between the AC power grid and the large power 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 shock caused by the sudden drop in output power. At this moment, the load of the shed feeder is restored.

[0089] 3.1) Unplanned feeder off-grid switching strategy.

[0090] 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 of the working mode after switching may cause large DC bus voltage fluctuations. In order to reduce this fluctuation, when unplanned feeder disconnection occurs, power feedforward control is added to the battery control strategy to increase the output of the battery and quickly fill the unbalanced power in the system. The corresponding control strategy is as follows Fig.21 shown. Fig.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 feed-forward 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: (6); In the formula, , , They are the output powers of the three AC power 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 off-grid operation, is 1. , Definition .

[0091] Fig.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 . With the injection of feed-forward current at the moment of off-grid switching, the fluctuation of bus voltage is reduced. After the bus voltage returns to the set value, the delayed shutdown in the battery control strategy takes effect, reducing the current value input to the system to 0, thereby avoiding the effect of the battery constant voltage control in the later stage.

[0092] 3.2) Off-grid phase-locked control strategy.

[0093] When a grid feeder connected to a port of the flexible multi-state switch fails, the corresponding port will be unplanned off-grid. Due to the interconnection 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 normally, it is necessary to Phase angle with other power grids Consistent, at this time In order to keep the phase angle consistent with the grid after being off-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, Fig.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: (7); In the formula, , is the voltage phase sine and cosine value of the large power grid; , The sine and cosine values ​​of the voltage phase obtained for the load.

[0094] 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: (8); In the formula, and They are the proportional and integral adjustment coefficients of the off-grid phase-locked controller respectively.

[0095] On this basis, the improved off-grid phase-locked control is combined with Combined with the control strategy, the improved Control strategy, i.e. Fig.24 The droop control strategy has off-grid phase-locking function. In order to speed up the tracking speed, Add the initial angular frequency of off-grid phase locking , which can be obtained by Droop control is obtained, and The off-grid phase-locked output angular frequency can be obtained by superposition and output angle .

[0096] 4) The control strategies of each converter in the device control layer include the following 4.1) to 4.3).

[0097] 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 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.

[0098] 4.1) Flexible multi-state switch , Control strategy.

[0099] If one of the three-port FMSS ports is a voltage-regulated port, then the port operates at Under the control strategy; if one port of the three-port FMSS is a non-regulated port, the port works in Under the control strategy. Among them, The control mainly uses 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, so as to achieve the purpose of feeder load balancing and maximum power output. , The impact of switching on the DC bus voltage , The control adopts a common current inner loop structure. Take the example to illustrate the corresponding control strategy. Fig. 22 shown.

[0100] Fig. 22 middle, , for Filter inductors and capacitors; The DC bus input 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 power 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 reference currents for the d-axis and q-axis. and are the d-axis and q-axis voltage reference values ​​respectively. Fig. 22It can be seen that since the two control strategies share a common current loop control on the d-axis, the inner current 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).

[0101] (9); (10); in for Reactive power value under control; for Proportional and integral coefficients of the PI controller under the control strategy.

[0102] 4.2) Improvement with off-grid phase-locking function Control strategies include the following 4.2.1) to 4.2.3).

[0103] As can be seen from the previous text, when a port of FMSS works 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-locking link is added on the basis of traditional droop control. Take the example to illustrate the corresponding control strategy. Fig.23 shown.

[0104] Fig.23 and Fig.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. Fig.23 and Fig.24 It can be seen that based on droop control 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 follows.

[0105] 4.2.1) Power calculation unit.

[0106] The power calculation unit calculates the voltage and current values ​​after dq transformation, and the active power can be obtained by combining formula (11): and reactive power : (11); In the formula, for The d-axis and q-axis currents obtained after dq transformation are: Fig.24 Abbreviated as ; for The d-axis and q-axis voltages obtained after dq transformation are: Fig.24 Abbreviated as .

[0107] 4.2.2) Droop control unit with off-grid phase locking.

[0108] See also Fig.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 used to obtain the three-phase voltage control signal , which is abbreviated as .

[0109] The corresponding droop characteristic can be expressed as: (12); (13); in is the system rated frequency; is the system rated voltage. Frequency droop coefficient , voltage droop coefficient . is the maximum frequency allowed by the system to output, for Maximum reactive power output; is the minimum voltage amplitude allowed by the system to output, It is the maximum power allowed to be output when the system voltage drops.

[0110] (14); in for Control the resulting angular frequency; Indicates time.

[0111] Further combined with 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 : (15).

[0112] Combining the traditional droop control with the above equations (7) and (8), the block diagram of the droop control and voltage synthesis part with off-grid phase locking is as follows: Fig.24 shown. Fig.24 In for 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 .

[0113] 4.2.3) Voltage and current dual loop controller.

[0114] The voltage-current dual-loop controller adds voltage decoupling and current decoupling to the PI regulator. The specific control strategy is as follows: Fig.25 shown. Fig.25 middle and for The d-axis and q-axis reference values ​​under control. and For the calculated The d-axis and q-axis inductor current reference values. and for The d-axis and q-axis inductor current values. and The d-axis and q-axis control voltage values ​​for the output.

[0115] When controlling the current in the d and q axes, the current cross-coupling term is often affected. and In order to eliminate current coupling, current decoupling control is introduced in the current loop. Similarly, when controlling the voltage on the d and q axes, it will also be affected by the voltage cross-coupling term, so voltage decoupling control is also introduced in the voltage loop.

[0116] 4.3) Energy storage system control strategy.

[0117] 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 for the energy storage system to 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: Fig.26 shown.

[0118] Fig.26 middle, is the DC bus voltage reference value, which is set to 750V in this application. is the DC bus voltage. is the constant current charging current value of the battery, which 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 factor, that is, the battery power feedforward ratio, which is generally set to 0.8. is the power feed-forward value.

[0119] Fig.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 with constant current. 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 When the switch S is set to 3, it is a constant voltage control. At this time, the battery stabilizes the DC bus voltage. When the system experiences a power mutation, the delay switch in the constant voltage control will close, increasing the battery current reference value, and promoting the battery to provide more power at the moment of power mutation to reduce bus voltage fluctuations. When the bus voltage returns to near the set value, the battery delay switch is disconnected, and the power feedforward does not work, thereby avoiding the constant voltage control effect of the battery.

[0120] 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, the 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.

[0121] 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 reliability of the system power supply. Combined with the coordinated control requirements of "source-grid-load-storage" in the new distribution network system, the Figure 2 The hierarchical control method shown in the figure coordinates energy storage and FMSS. Among them, the three-port FMSS is equipped with a separate local controller to complete the local device control of the device control layer, measure local parameters, and estimate the working status of FMSS and batteries; a centralized controller is set to complete the work of the coordinated control layer. Combined with the operating status of the three ports of the flexible multi-state switch and the feeder fault information, the different possible operating modes of the distribution network containing energy storage and FMSS and the coordinated control strategy of energy storage and FMSS are studied.

[0122] 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, the present 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-connected and off-grid. Therefore, it is necessary to ensure a smooth transition of the phase when switching between on-grid and off-grid. Based on this, the present application also studies the phase locking and grid-connected pre-synchronization strategies to achieve a smooth transition of voltage and current on the AC side.

[0123] It should be noted that the terms "include", "comprising" or any other variations thereof used in this application are intended to cover non-exclusive inclusion, so that a product or system including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such a product or system. In the absence of further restrictions, an element defined by the sentence "comprising a ..." does not exclude the existence of other identical elements in the product or system including the element.

[0124] The technical features of the above embodiments may 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.

[0125] The present application uses specific examples to illustrate the principles and implementation methods of the present application. The above examples are only used to help understand the method and core ideas of the present application. At the same time, for those skilled in the art, according to the ideas of the present application, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present 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 comprises: a flexible multi-state switch, an energy storage system and a plurality of AC power grids; the flexible multi-state switch is composed of a plurality of back-to-back voltage source converters; the AC port of each voltage source converter is connected to a feeder end of an AC power grid via a transformer; each feeder is connected to a corresponding feeder load; the DC port of each voltage source converter is connected in parallel to form a DC bus, and is connected to an energy storage system composed of a battery and a bidirectional DC / DC converter; The coordinated control method of energy storage and flexible multi-state switches in the distribution network system includes: coordinating the energy storage system and the flexible multi-state switch in a hierarchical control manner; wherein a centralized controller is provided in the coordinated control layer 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, and sending control strategies to the device control layer; the device control layer is provided with a separate local controller for each voltage source converter to complete the local device control of each voltage source converter; The coordination control layer is composed of a centralized controller, which selects and switches the working mode under the networking operation mode according to the feeder load power, the battery charge state, the operating state 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 all 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, and the AC ports of other voltage source converters adopt a constant power control strategy to control the power input from the AC power grid to the system; when a feeder fault occurs at the AC port of one of the voltage source converters of 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 charging control and constant voltage control; the feeder load in the device control layer can also implement a load switching 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 networking operation mode according to the feeder load power, battery charge state, the operating state of each transformer, and the location and number of feeder faults, specifically including: When all feeders connected to the AC ports of all voltage source converters in the flexible multi-state switch are operating normally, multiple AC power grids are interconnected and operated through the flexible multi-state switch, which is called the flexible interconnection operation state; When the centralized controller determines that the system is in a flexible interconnected operation state, it divides the system into a free-connection operation mode and a current-limited operation mode based on whether each transformer in the system has reached its own capacity upper 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 a 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 a 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 with the battery as the backup power supply to improve the system power supply reliability; at the same time, multiple AC power 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 power emitted by the battery is positive, and the power absorbed is negative; the control strategy of the equipment control layer is: the battery adopts constant current charging control, and at the same time, a fixed DC voltage control strategy is adopted for the AC port of one of the voltage source converters of the flexible multi-state switch, and a constant power control strategy is adopted for the AC ports of 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 power grids to emit 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 fixed 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 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, the system is determined to be in an interconnection current limiting operation mode; Under the networked current-limiting operation mode, the coordination control layer aims to maximize the system power supply reliability, so that each transformer outputs the maximum power, the battery is charged with the excess power inside the system, and the battery is selected as the voltage stabilization 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: The centralized controller selects and switches the working mode in the networking operation mode according to the feeder load power, battery charge state, the operating state of each transformer, and the location and number of feeder faults, specifically including: If a part of the AC grid feeder connected to the flexible multi-state switch fails, it is determined that the system is 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 at the voltage-stabilizing port and feeder faults at the non-voltage-stabilizing port.

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 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 normal operating feeders in the system, it is determined that the system is in the free-running mode of networking; In the free-running mode of networking, when the battery state of charge is less than the maximum state of charge value, with the goal of improving the system power supply reliability, a port in the flexible multi-state switch without feeder fault is selected as the DC voltage stabilization 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 without feeder fault is set to supply power to the faulty feeder load; at the same time, if only a single port of the voltage stabilization port has a feeder fault, with the goal of achieving feeder load balancing, multiple AC power grids without feeder fault are set to output the same power; the control strategy of the equipment control layer is: the battery adopts constant current charging control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage stabilization port adopts a fixed DC voltage control strategy, and other ports adopt a constant power control strategy; In the free-running mode of networking, when the battery state of charge is greater than or equal to the maximum state of charge value, the system power supply reliability has reached the maximum guarantee, and the battery stops charging. If a feeder fault occurs only at the voltage stabilizing port, the coordination control layer sets the same power output for multiple AC power grids that have not experienced feeder faults, with the goal of extending the power supply time of the faulty feeder load and balancing the feeder load. 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: 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.

7. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 6, 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 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 the 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 does not occur is set to supply power to the faulty feeder load; To improve the reliability of system power supply, the maximum output power of the transformer connected to the normal operating feeder is set, the battery is selected as the DC voltage stabilization port, and the battery is charged with the excess power inside the system 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; When the battery charging is completed, it switches to floating charge control, and a port in the flexible multi-state switch where no feeder fault occurs 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.

8. The coordinated control method of 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 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 no feeder fault occurs, 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.

9. 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 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 free-running mode of networking; In the free-running mode of networking, when the battery state of charge is less than the maximum state of charge value, the battery is charged at rated power until it is fully charged, with the goal of improving the system power supply reliability; 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; at the same time, if only one non-voltage-stabilized port in the system has a feeder fault, multiple AC power grids where the feeder fault does not occur are set to output the same power, with the goal of achieving feeder load balancing; the control strategy of the equipment control layer is: 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 fixed DC voltage control strategy, and other ports adopt a constant power control strategy; In the free-running mode of networking, when the battery state of charge is greater than or equal to the maximum state of charge value, the system power supply reliability has reached the maximum guarantee, and the battery stops charging. If only one non-voltage-stabilized port in the system has a feeder fault, the coordination control layer sets the ports connected to the normal 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, and sets multiple AC power grids that have not experienced feeder faults to output the same power; if multiple feeders connected to non-voltage-stabilized ports in the system fail, the coordination control layer only sets the voltage-stabilized 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: the battery adopts floating charge control, the port connected to the faulty feeder adopts a droop control strategy, the DC voltage-stabilized port adopts a fixed DC voltage control strategy, and other ports adopt a constant power control strategy.

10. The coordinated control method of energy storage and flexible multi-state switches in a distribution network system according to claim 9, 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 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 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 does not occur is set to supply power to the faulty feeder load; To improve the reliability of system power supply, all transformers connected to the normal operating feeder are set to output the maximum power, and the battery is selected as the DC voltage stabilization port. The battery is charged with the excess power inside the system 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 the droop control strategy, and the other ports adopt the constant power control strategy. When the battery is fully charged, it switches to floating charge control, and selects a port in the flexible multi-state switch that has no feeder fault 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. 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, it is determined that the system is 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 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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