Optimal Coordinated Control Method for AC / DC Hybrid Power Grid Based on Flexible Multi-State Switches

Abstract

This invention provides an optimized coordinated control method for AC / DC hybrid power grids based on a flexible multi-state switch. The current operating mode is determined by the number of port faults of a three-port flexible multi-state switch. Based on the different operating modes, coordinated control of the ports of the three-port flexible multi-state switch and the lithium battery pack is achieved to realize the coordinated control of the bipolar DC microgrid and the AC distribution network. Voltage balance control of the bipolar DC microgrid is performed through the three-port flexible multi-state switch and a voltage balancer, depending on the operating mode. When the operating mode is load transfer mode or autonomous mode, SOC equalization control of the lithium battery pack is performed. From the user side, through FMSS and the bipolar DC microgrid, demand-side response coordination between sources and loads and effective power flow distribution are achieved, improving the system's power supply quality and reliability, and solving the voltage balance problem and the SOC equalization problem of the positive and negative batteries in the bipolar DC microgrid.

Description

Technical Field

[0001] This invention relates to the field of smart grid technology, and in particular to an optimized coordinated control method for AC / DC hybrid power grids based on flexible multi-state switches. Background Technology

[0002] To address the diverse electricity demands, large-scale distributed power source integration, and complex power flow coordination and control in new power systems, flexible multi-state switches (FMSS), as power devices capable of adjusting power flow between feeders, have been introduced into AC distribution networks. Depending on the number of FMSS ports, two to three or even more AC distribution networks can be organically connected together to form a unified system. Through power electronics technology and intelligent algorithms, FMSS not only possesses the on / off states of conventional switches but also achieves continuous and controllable power. Therefore, it is not only a key power distribution device for flexible switches in current distribution networks but also a key technology and equipment support for future smart grids and new power systems, making it of significant research importance.

[0003] Since each port of the FMSS is connected to a different AC distribution network, the operating status of the distribution network must be fully considered when controlling it. Different control strategies should be formulated for each port of the FMSS to achieve different control functions. For example, when the distribution network is operating normally, controlling the FMSS can avoid power outages and loop-closing impacts caused by conventional switching operations. It can also alleviate the three-phase imbalance caused by voltage drops, promote the equalization of feeder load distribution and improve power quality. However, in the event of a grid fault, the switching of control strategies between the ports of the FMSS often causes problems such as bus voltage fluctuations and reduced power quality. To solve these problems, microgrids, which can be used as "virtual power plants," can be introduced into AC distribution networks containing FMSS to play an energy support role. A microgrid is an independent micro power system that integrates generation, distribution, supply and storage within a certain area, mainly based on distributed energy. From the perspective of energy structure, microgrids are more adapted to the development trend of distributed energy and are more conducive to promoting the large-scale development and utilization of distributed energy. It is an important carrier for building a new type of power system.

[0004] With the vigorous development of microgrids, their ability to operate in parallel or in islanded mode makes them well-suited as an energy source for AC feeder loads at various FMSS ports during AC distribution network faults. Furthermore, with the increasing number of DC distributed power sources and loads, and the significant advancements in flexibility, security, reliability, and multi-voltage levels of bipolar DC microgrids, the advantages of AC / DC hybrid grids integrating bipolar DC microgrids and FMSS are becoming increasingly prominent. While the performance of AC / DC hybrid grids has been greatly improved, due to their hybrid interconnection characteristics, the coordinated control of bipolar DC microgrids and AC distribution networks containing FMSS is crucial for the stable and optimized operation of AC / DC hybrid grids under both normal and fault conditions of the AC distribution network. This is of great significance because, during normal grid operation, FMSS (Fast-Fast Grid System) alters the power flow direction within the system, providing energy reserves for bipolar DC microgrids and enabling them to assist in peak shaving and frequency regulation, thus improving the operational efficiency of AC / DC hybrid grids. During grid faults, the energy supply from the bipolar DC microgrid powers the faulty feeder loads, ensuring reliable power supply. Furthermore, to ensure reliable coordinated control, specific problems inherent in bipolar DC microgrids need to be addressed, such as bus voltage imbalance caused by asymmetric positive and negative load power, and state-of-charge (SOC) imbalance among multiple battery banks on the positive and negative sides due to line impedance and power distribution issues. These problems affect the power quality and battery life of bipolar DC microgrids to varying degrees. However, most research focuses on FMSS and bipolar DC microgrids individually, with few studies combining them for coordinated control to achieve better system characteristics. Summary of the Invention

[0005] In view of this, the present invention proposes an optimized coordinated control method for AC / DC hybrid power grids based on flexible multi-state switches. Based on different operating modes of AC / DC hybrid power grids, coordinated control, voltage balance control, and SOC equalization control can be achieved to ensure the stable operation of AC / DC hybrid power grids.

[0006] The technical solution of this invention is implemented as follows:

[0007] A method for optimized coordinated control of AC / DC hybrid power grids based on flexible multi-state switches is proposed. The AC / DC hybrid power grid includes a three-port flexible multi-state switch, an AC distribution network, a bipolar DC microgrid, a lithium battery pack, and a voltage balancer. The AC port of the three-port flexible multi-state switch is connected to the low-voltage side of the transformer in the AC distribution network. The three-port flexible multi-state switch is connected to the bipolar DC microgrid, which includes a positive, negative, and zero-pole bus. The lithium battery pack is connected to both the positive and negative bus. The voltage balancer is positioned between the positive and negative bus. The optimized coordinated control method for the AC / DC hybrid power grid includes the following steps:

[0008] Step S1: Determine the current operating mode based on the number of port faults of the three-port flexible multi-state switch. The operating modes include flexible interconnection mode, load transfer mode, and autonomous mode.

[0009] Step S2: Based on the different working modes, the ports of the three-port flexible multi-state switch and the lithium battery pack are controlled and adjusted to achieve coordinated control of the bipolar DC microgrid and the AC distribution network.

[0010] Step S3: Depending on the operating mode, voltage balance control of the bipolar DC microgrid is achieved through a three-port flexible multi-state switch and a voltage balancer.

[0011] Step S4: When the working mode is load transfer mode or autonomous mode, perform SOC equalization control of the lithium battery pack.

[0012] Preferably, the AC / DC hybrid power grid further includes a voltage source converter, and the AC port of the three-port flexible multi-state switch is connected to the bipolar DC microgrid through the voltage source converter.

[0013] Preferably, the following steps are also included:

[0014] Step S0: Design the capacity of the voltage source converter based on the AC feeder load power of each port of the three-port flexible multi-state switch.

[0015] Preferably, the specific steps of step S1 are as follows: when all three ports of the three-port flexible multi-state switch are fault-free, the working mode is flexible interconnection mode; when all three ports of the three-port flexible multi-state switch are faulty, the working mode is autonomous mode; and in other cases, the working mode is load transfer mode.

[0016] Preferably, when the working mode is flexible interconnection mode, the specific steps of step S2 include:

[0017] Step S21: Obtain the adjustable capacity of the three ports of the three-port flexible multi-state switch;

[0018] Step S22: Operate the port with the largest adjustable capacity in constant DC bus voltage control mode, and operate the other ports in constant power control mode.

[0019] Preferably, when the working mode is autonomous mode, the specific steps of step S2 include:

[0020] Step S23: Switch the three ports of the three-port flexible multi-state switch to constant voltage-frequency control mode and adjust the lithium battery pack to constant voltage control.

[0021] Preferably, when the operating mode is load transfer mode and the number of faulty ports of the three-port flexible multi-state switch is 1, the specific steps of step S2 include:

[0022] Step S24: If the operating mode before the fault occurred was constant DC bus voltage control mode, and the SOC of the lithium battery pack was within the allowable range, control the lithium battery pack to switch from constant current charging to constant voltage control, and switch the fault port to constant voltage-frequency control mode.

[0023] Step S25: When the SOC of the lithium battery pack reaches the lower limit, control the lithium battery pack to stop working, and switch the non-faulty port with larger adjustable capacity in the three-port flexible multi-state switch that is running in constant power control mode to constant DC bus voltage control mode.

[0024] Step S26: If the operating mode before the fault occurred was constant power control mode, switch the lithium battery pack to constant power control.

[0025] Preferably, when the operating mode is load transfer mode and the number of faulty ports of the three-port flexible multi-state switch is 2, the specific steps of step S2 include:

[0026] Step S27: If the non-faulty port of the three-port flexible multi-state switch is in constant DC bus voltage control mode, the faulty port is operated in constant voltage-frequency control mode, and the lithium battery pack is switched to constant power control.

[0027] Step S28: If the non-faulty port of the three-port flexible multi-state switch is in constant power control mode, switch the lithium battery pack to constant voltage control.

[0028] Preferably, step S3 includes the following specific steps:

[0029] Step S31: When the working mode is flexible interconnection mode, the positive bus voltage and negative bus voltage are stabilized by using the port of the three-port flexible multi-state switch that is in constant DC bus voltage control mode.

[0030] Step S32: Use a voltage balancer to stabilize the voltage of one of the busbars, and determine the direction of energy transfer in the voltage balancer by combining the magnitude relationship between the positive and negative busbars.

[0031] Step S33: When the working mode is load transfer mode or autonomous mode, the voltage balancer is used as the voltage regulator terminal. The bus voltage of the lithium battery pack where the SOC reaches the upper or lower limit is determined as the bus voltage that the voltage balancer needs to stabilize, and the energy flow direction in the voltage balancer is determined.

[0032] Preferably, step S4 includes the following specific steps:

[0033] Step S41: Construct a lithium battery pack SOC equalization power bridge with SOC deviation as the control quantity, and perform energy flow between the two poles of the voltage balancer according to the SOC deviation of the lithium battery pack.

[0034] Step S42: Design an adaptive virtual resistor for the lithium battery pack by adjusting the size of the virtual resistor in conjunction with the power flow control strategy of the voltage balancer.

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] This invention is applied to a hybrid AC / DC power grid containing a three-port flexible multi-state switch, an AC distribution network, and a bipolar DC microgrid. The current operating mode is determined by the port status of the three-port flexible multi-state switch. Based on different operating modes, coordinated control of the bipolar DC microgrid and the AC distribution network can be achieved, improving power quality. Simultaneously, a voltage balancer is introduced into the hybrid AC / DC power grid to achieve voltage balance control of the bipolar DC microgrid, realizing midpoint voltage balance under complex operating conditions and ensuring stable operation. When the operating mode is load transfer mode or autonomous mode, SOC equalization control of the lithium battery packs in the bipolar DC microgrid is performed, improving battery regulation capabilities and mitigating problems such as bus voltage instability, thus providing improved power quality and reliability. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only preferred embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 The flowchart shows the AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to the present invention.

[0039] Figure 2This is a topology diagram of the AC / DC hybrid power grid based on the AC / DC hybrid power grid optimization and coordination control method of the present invention.

[0040] Figure 3 The flowchart shows the specific steps of step S2 when the working mode is flexible interconnection mode.

[0041] Figure 4 The flowchart shows the specific steps of step S2 when the working mode is autonomous mode.

[0042] Figure 5 The flowchart of step S2 is as follows: when the working mode is load transfer mode and the number of fault ports of the three-port flexible multi-state switch is 1.

[0043] Figure 6 The flowchart of step S2 is as follows: when the working mode is load transfer mode and the number of fault ports of the three-port flexible multi-state switch is 2.

[0044] Figure 7 This is a flowchart illustrating the specific steps of step S3 in the AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches of the present invention.

[0045] Figure 8 This is a flowchart illustrating the specific steps of step S4 in the AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches of the present invention. Detailed Implementation

[0046] To better understand the technical content of this invention, a specific embodiment is provided below, and the invention will be further described in conjunction with the accompanying drawings.

[0047] See Figures 1 to 8 This invention provides an optimized coordinated control method for an AC / DC hybrid power grid based on a flexible multi-state switch. The AC / DC hybrid power grid includes a three-port flexible multi-state switch, an AC distribution network, a bipolar DC microgrid, a lithium battery pack, and a voltage balancer. The AC port of the three-port flexible multi-state switch is connected to the low-voltage side of the transformer in the AC distribution network. The three-port flexible multi-state switch is connected to the bipolar DC microgrid, which includes a positive, a negative, and a zero-pole bus. The lithium battery pack is connected to both the positive and negative bus. The voltage balancer is positioned between the positive and negative bus. The optimized coordinated control method for the AC / DC hybrid power grid includes the following steps:

[0048] Step S1: Determine the current operating mode based on the number of port faults of the three-port flexible multi-state switch. The operating modes include flexible interconnection mode, load transfer mode, and autonomous mode.

[0049] Step S2: Based on the different working modes, the ports of the three-port flexible multi-state switch and the lithium battery pack are controlled and adjusted to achieve coordinated control of the bipolar DC microgrid and the AC distribution network.

[0050] Step S3: Depending on the operating mode, voltage balance control of the bipolar DC microgrid is achieved through a three-port flexible multi-state switch and a voltage balancer.

[0051] Step S4: When the working mode is load transfer mode or autonomous mode, perform SOC equalization control of the lithium battery pack.

[0052] The optimized coordinated control method of this invention is applied to an AC / DC hybrid power grid, mainly composed of multiple AC distribution networks and bipolar DC microgrids. A three-port flexible multi-state switch is introduced into the hybrid power grid. The three AC ports of the three-port flexible multi-state switch are respectively located on the low-voltage side of the AC distribution network. Taking a single three-port flexible multi-state switch as an example, the number of AC distribution networks in this invention is also three, with a voltage level of 35kV. The corresponding transformers T1~T3 have a transformation ratio of 35kV / 400V and an operating voltage of 380V. A bipolar DC microgrid is introduced at the DC port of the three-port flexible multi-state switch. Compared to a unipolar DC microgrid, a bipolar DC microgrid has three buses (positive, negative, and zero), making it easier to form multiple voltage levels. This not only saves a significant number of AC / DC conversion stages but also... This invention provides power to high-voltage, low-voltage, or certain special loads. The positive and negative buses of the bipolar DC microgrid are relatively independent, preventing system paralysis due to a single bus failure. Based on current mainstream voltage levels in China, the positive and negative bus voltages of the bipolar DC microgrid are set to 750V, with +375V and -375V for the positive and negative buses respectively. To improve power supply reliability, a lithium battery pack is installed at each of the positive and negative buses, and a voltage balancer is added between the positive and negative buses to achieve voltage balance. Before implementing optimized coordinated control methods, the capacity configuration of the main components of the AC / DC hybrid grid and the parameters of the LC circuit in the main circuit are designed, completing the circuit structure construction and initialization of the AC / DC hybrid grid.

[0053] To achieve coordinated control of the bipolar DC microgrid and the AC distribution network at each port of the three-port flexible multi-state switch on the user side, it is necessary to formulate different operating modes and control methods based on various possible operating conditions during the operation of the three-port flexible multi-state switch and the normal operation of the AC distribution network. The operating mode is determined by whether there is a fault at any of the three ports of the three-port flexible multi-state switch. These operating modes include flexible interconnection mode, load transfer mode, and autonomous mode. Based on these three different operating modes, coordinated control of the bipolar DC microgrid and the AC distribution network is implemented. This coordinated control involves regulating each port of the three-port flexible multi-state switch and the lithium battery pack to achieve high-quality power supply. Specifically, when all ports of the three-port flexible multi-state switch are operating normally, the goal is to improve system reliability by implementing constant current discharge of the lithium battery pack. When one or two ports of the three-port flexible multi-state switch fail, the goal is to ensure uninterrupted power supply to the faulty feeder load. When all three ports of the three-port flexible multi-state switch fail, the goal is to extend the power supply time of the feeder load by planning load switching strategies.

[0054] In addition, depending on the operating mode, voltage balance control of the bipolar DC microgrid can be achieved using a three-port flexible multi-state switch and a voltage balancer. Based on operating conditions and the state of charge of the lithium battery packs, the required stable bus voltage can be determined autonomously to control the conduction and cutoff of the switching transistors in the voltage balancer. This achieves reasonable energy support for the positive and negative systems and midpoint voltage balance under complex operating conditions. Furthermore, to address the SOC balance issue of the positive and negative lithium battery packs under load transfer and autonomous operation modes, and to improve the regulation capability of the lithium battery packs, the voltage balancer is used as an energy transfer intermediary. A control strategy is employed to control the direction and magnitude of energy flow within the voltage balancer, enabling automatic and rapid SOC balance of multiple lithium battery packs. This prevents a single energy storage unit from shutting down due to overcharging or over-discharging caused by asymmetric positive and negative load power, ensuring the reliability of the bipolar DC microgrid operation.

[0055] Preferably, the AC / DC hybrid power grid further includes a voltage source converter, and the AC port of the three-port flexible multi-state switch is connected to the bipolar DC microgrid through the voltage source converter.

[0056] The voltage source converters VSC1~VSC3 are composed of IGBTs and are formed by a three-phase controllable rectifier bridge controlled by SPWM pulses. The AC port of the three-port flexible multi-state switch can be connected to the bipolar DC microgrid through the voltage source converter.

[0057] Preferably, the following steps are also included:

[0058] Step S0: Design the capacity of the voltage source converter based on the AC feeder load power of each port of the three-port flexible multi-state switch.

[0059] Before optimizing and coordinating control, the AC / DC hybrid power grid is initialized, including the design of the voltage source converter capacity, adjustment based on the load power of the AC feeder at each port of the three-port flexible multi-state switch, and, to improve the reliability of the system power supply, a wind-solar-storage bipolar DC microgrid with wind-solar complementary characteristics is adopted. Based on the actual load power in the AC distribution network and the bipolar DC microgrid, the ratio between wind turbines, photovoltaics, loads, and lithium batteries is determined, and the capacity configuration of each major component is set. At the same time, the parameters of the LC in the main circuit are designed to realize the initialization of the AC / DC hybrid power grid.

[0060] Preferably, the specific steps of step S1 are as follows: when all three ports of the three-port flexible multi-state switch are fault-free, the working mode is flexible interconnection mode; when all three ports of the three-port flexible multi-state switch are faulty, the working mode is autonomous mode; and in other cases, the working mode is load transfer mode.

[0061] By determining whether any of the three ports of a three-port flexible multi-state switch is faulty, the operating mode of the power grid can be identified. There are three operating modes: flexible interconnection mode, autonomous mode, and load transfer mode. When none of the three ports of the three-port flexible multi-state switch are faulty, the operating mode is flexible interconnection mode. When all three ports of the three-port flexible multi-state switch are faulty, the operating mode is autonomous mode. When one or two ports of the three-port flexible multi-state switch are faulty, the operating mode is load transfer mode.

[0062] Preferably, when the working mode is flexible interconnection mode, the specific steps of step S2 include:

[0063] Step S21: Obtain the adjustable capacity of the three ports of the three-port flexible multi-state switch;

[0064] Step S22: Operate the port with the largest adjustable capacity in constant DC bus voltage control mode, and operate the other ports in constant power control mode.

[0065] When the operating mode is flexible interconnection mode, based on the energy balance relationship, it is necessary to pay attention to the power flow and power support relationship between the AC distribution network containing the three-port flexible multi-state switch and the bipolar DC microgrid in this mode, in order to change the power flow distribution of the AC distribution network, achieve feeder load balancing, and improve the absorption of distributed power sources. First, based on the adjustable capacity (the difference between port capacity and apparent power) of the three ports of the three-port flexible multi-state switch, the port with the largest adjustable capacity is operated in constant DC bus voltage control mode (U). dc-Q mode) to stabilize the DC bus voltage, while the other ports operate in constant power control mode (PQ mode) to enhance system stability.

[0066] The bipolar DC microgrid operates in a flexible interconnection mode, with both wind power and photovoltaic power generation systems operating in maximum power point tracking mode to maximize the utilization of renewable energy. The positive and negative lithium battery packs are constantly charged with constant current until their state of charge reaches its upper limit, serving as backup energy in islanded mode to improve the reliability of system power supply during AC distribution network failures. When a power deficit occurs in the bipolar DC microgrid, the three-port flexible multi-state switch provides the power deficit; when the bipolar DC microgrid has a power surplus, the excess power is fed back to the AC distribution network, enabling the three-port flexible multi-state switch to participate in frequency and voltage regulation, feeder load balancing, and distributed power consumption in the AC distribution network.

[0067] Preferably, when the working mode is autonomous mode, the specific steps of step S2 include:

[0068] Step S23: Switch the three ports of the three-port flexible multi-state switch to constant voltage-frequency control mode and adjust the lithium battery pack to constant voltage control.

[0069] In autonomous mode, since all three ports of the three-port flexible multi-state switch are connected to AC distribution networks in a fault state, all three ports of the three-port flexible multi-state switch are switched to constant voltage-frequency control mode (U). ac In the -f mode, both the positive and negative lithium battery packs in the bipolar DC microgrid are adjusted to constant voltage control to provide energy and power the feeder loads. However, when their SOC approaches the lower limit, in order to extend the power supply time of important loads in the feeder loads as much as possible, a load switching control strategy is adopted to cut off non-important loads in the system.

[0070] Preferably, when the operating mode is load transfer mode and the number of faulty ports of the three-port flexible multi-state switch is 1, the specific steps of step S2 include:

[0071] Step S24: If the operating mode before the fault occurred was constant DC bus voltage control mode, and the SOC of the lithium battery pack was within the allowable range, control the lithium battery pack to switch from constant current charging to constant voltage control, and switch the fault port to constant voltage-frequency control mode.

[0072] Step S25: When the SOC of the lithium battery pack reaches the lower limit, control the lithium battery pack to stop working, and switch the non-faulty port with larger adjustable capacity in the three-port flexible multi-state switch that is running in constant power control mode to constant DC bus voltage control mode.

[0073] Step S26: If the operating mode before the fault occurred was constant power control mode, switch the lithium battery pack to constant power control.

[0074] When the operating mode is load transfer mode, two cases need to be considered: the number of faulty ports of the three-port flexible multi-state switch is 1 or 2. When the number of faulty ports of the three-port flexible multi-state switch is 1, it is necessary to determine the control mode before the port fault occurred. If the operating mode is constant DC bus voltage control mode and the SOC of the lithium battery pack is within the allowable range, in order to achieve rapid energy replenishment, the bipolar DC microgrid is regarded as a "virtual power plant", and the positive and negative lithium battery packs are switched from constant current charging to constant voltage control to regulate power and stabilize the DC bus voltage. At the same time, the faulty port is switched to constant voltage-frequency control mode to achieve uninterrupted power supply to the feeder load. However, when the SOC of the positive and negative lithium battery packs reaches the lower limit, in order to protect the lithium battery pack and improve the system economy, the lithium battery pack stops working and the non-faulty port of the three-port flexible multi-state switch with a larger adjustable capacity, which is operating in constant power control mode, is switched to constant DC bus voltage control mode to continue to balance internal power and stabilize the DC bus voltage.

[0075] If the port was operating in constant power control mode before the fault occurred, the lithium battery pack will be switched to constant power control to reduce the energy supply of the constant DC bus voltage control mode port in the three-port flexible multi-state switch.

[0076] Preferably, when the operating mode is load transfer mode and the number of faulty ports of the three-port flexible multi-state switch is 2, the specific steps of step S2 include:

[0077] Step S27: If the non-faulty port of the three-port flexible multi-state switch is in constant DC bus voltage control mode, the faulty port is operated in constant voltage-frequency control mode, and the lithium battery pack is switched to constant power control.

[0078] Step S28: If the non-faulty port of the three-port flexible multi-state switch is in constant power control mode, switch the lithium battery pack to constant voltage control.

[0079] When two ports are faulty, it is necessary to determine whether the non-faulty port of the three-port flexible multi-state switch is in constant DC bus voltage control mode. If the non-faulty port is operating in constant DC bus voltage control mode (i.e., a regulated port), then all faulty ports will be switched to constant voltage-frequency control mode. The positive and negative lithium battery packs in the bipolar DC microgrid will be switched to constant power control to discharge quickly, thereby continuing to supply power to the two faulty feeder loads. If the non-faulty port is operating in constant power control mode (i.e., a non-regulated port), then the positive and negative lithium battery packs will switch to constant voltage control to maintain bus voltage balance. Similar to the lithium battery pack control in a single AC distribution network fault, once the positive and negative lithium battery packs reach the lower limit of SOC, they will stop working to protect the lithium battery packs.

[0080] Preferably, step S3 includes the following specific steps:

[0081] Step S31: When the working mode is flexible interconnection mode, the positive bus voltage and negative bus voltage are stabilized by using the port of the three-port flexible multi-state switch that is in constant DC bus voltage control mode.

[0082] Step S32: Use a voltage balancer to stabilize the voltage of one of the busbars, and determine the direction of energy transfer in the voltage balancer by combining the magnitude relationship between the positive and negative busbars.

[0083] Step S33: When the working mode is load transfer mode or autonomous mode, the voltage balancer is used as the voltage regulator terminal. The bus voltage of the lithium battery pack where the SOC reaches the upper or lower limit is determined as the bus voltage that the voltage balancer needs to stabilize, and the energy flow direction in the voltage balancer is determined.

[0084] Depending on the operating mode, the midpoint voltage balance control strategy based on the voltage balancer also differs. For the voltage balance control of bipolar DC microgrids in flexible interconnection mode, the constant DC bus voltage control mode port in the three-port flexible multi-state switch can be used to stabilize the positive and negative bus voltages, and the voltage balancer can be used to stabilize the voltage of one of the buses. Combining the magnitude relationship between the positive and negative buses, the direction of energy transfer in the voltage balancer is determined to achieve an approximate balance between the positive and negative bus voltages. For the voltage balance control of bipolar DC microgrids in load transfer and autonomous mode, the voltage balancer can be used as a voltage regulator. The bus voltage of the lithium battery pack where the SOC reaches the upper or lower limit is set as the bus voltage that the voltage balancer needs to stabilize. This determines the direction of energy flow in the voltage balancer, and the power support of the other lithium battery pack is used to simultaneously stabilize the positive and negative bus voltages.

[0085] Preferably, step S4 includes the following specific steps:

[0086] Step S41: Construct a lithium battery pack SOC equalization power bridge with SOC deviation as the control quantity, and perform energy flow between the two poles of the voltage balancer according to the SOC deviation of the lithium battery pack.

[0087] Step S42: Design an adaptive virtual resistor for the lithium battery pack by adjusting the size of the virtual resistor in conjunction with the power flow control strategy of the voltage balancer.

[0088] To address the issue of automatic SOC balancing between positive and negative lithium battery packs in bipolar DC microgrids under load transfer and autonomous modes, when the SOC is within the allowable range, the constant voltage control strategy of the voltage balancer is changed to an SOC balancing control strategy. A SOC balancing power bridge is constructed between the positive and negative lithium battery packs, using the SOC deviation as the control variable. This bridge allows for energy flow between the two batteries based on their SOC deviations, ensuring the feasibility of SOC balancing. Secondly, the mechanism of SOC deviation generation among multiple lithium battery packs in a bipolar DC microgrid is analyzed in depth, summarizing the effects of source and load changes on SOC deviation. The influence of the control strategy was investigated, and theoretical analysis was completed to ensure its correctness and efficiency. Finally, an adaptive voltage complementary control strategy was added to the positive and negative lithium battery packs. The virtual resistance was designed by combining the difference in SOC between the positive and negative electrodes, the internal energy relationship of the system, and the influence of source and load changes on SOC deviation. This allows the virtual resistance of the positive and negative lithium battery packs to be adjusted during the SOC equalization process. Combined with the power flow control strategy of the voltage balancer (constant voltage control or constant power control), the maximum power flow in the voltage balancer is transferred under different operating conditions, thereby accelerating the SOC equalization speed.

[0089] The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches of the present invention uses RTDS for semi-physical simulation. All main circuits in the experiment (including three-port flexible multi-state switches, photovoltaic arrays, wind power generation, lithium battery packs, loads and various converters) are built in the RTDS simulation platform environment, and the converter control and energy coordination control are completed through the RCP controller.

[0090] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for optimized coordinated control of AC / DC hybrid power grids based on flexible multi-state switches, characterized in that, The AC / DC hybrid power grid includes a three-port flexible multi-state switch, an AC distribution network, a bipolar DC microgrid, a lithium battery pack, and a voltage balancer. The AC port of the three-port flexible multi-state switch is connected to the low-voltage side of the transformer in the AC distribution network. The three-port flexible multi-state switch is connected to the bipolar DC microgrid, which includes a positive, a negative, and a zero-pole bus. The lithium battery pack is connected to both the positive and negative bus. The voltage balancer is positioned between the positive and negative bus. The optimized coordinated control method for the AC / DC hybrid power grid includes the following steps: Step S1: Determine the current operating mode based on the number of port faults of the three-port flexible multi-state switch. The operating modes include flexible interconnection mode, load transfer mode, and autonomous mode. Step S2: Based on the different working modes, the ports of the three-port flexible multi-state switch and the lithium battery pack are controlled and adjusted to achieve coordinated control of the bipolar DC microgrid and the AC distribution network. Step S3: Depending on the operating mode, voltage balance control of the bipolar DC microgrid is achieved through a three-port flexible multi-state switch and a voltage balancer. Step S4: When the working mode is load transfer mode or autonomous mode, perform SOC equalization control of the lithium battery pack. The specific steps of step S3 include: Step S31: When the working mode is flexible interconnection mode, the positive bus voltage and negative bus voltage are stabilized by using the port of the three-port flexible multi-state switch that is in constant DC bus voltage control mode. Step S32: Use a voltage balancer to stabilize the voltage of one of the busbars, and determine the direction of energy transfer in the voltage balancer by combining the magnitude relationship between the positive and negative busbars. Step S33: When the working mode is load transfer mode or autonomous mode, the voltage balancer is used as the voltage regulator terminal. The bus voltage of the lithium battery pack where the SOC reaches the upper or lower limit is determined as the bus voltage that the voltage balancer needs to stabilize, and the energy flow direction in the voltage balancer is determined. The specific steps of step S1 are as follows: when all three ports of the three-port flexible multi-state switch are fault-free, the working mode is flexible interconnection mode; when all three ports of the three-port flexible multi-state switch are faulty, the working mode is autonomous mode; and in other cases, the working mode is load transfer mode.

2. The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to claim 1, characterized in that, The AC / DC hybrid power grid also includes a voltage source converter, and the AC port of the three-port flexible multi-state switch is connected to the bipolar DC microgrid through the voltage source converter.

3. The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to claim 2, characterized in that, It also includes the following steps: Step S0: Design the capacity of the voltage source converter based on the AC feeder load power of each port of the three-port flexible multi-state switch.

4. The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to claim 1, characterized in that, When the operating mode is flexible interconnection mode, the specific steps of step S2 include: Step S21: Obtain the adjustable capacity of the three ports of the three-port flexible multi-state switch; Step S22: Operate the port with the largest adjustable capacity in constant DC bus voltage control mode, and operate the other ports in constant power control mode.

5. The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to claim 1, characterized in that, When the working mode is autonomous mode, the specific steps of step S2 include: Step S23: Switch the three ports of the three-port flexible multi-state switch to constant voltage-frequency control mode and adjust the lithium battery pack to constant voltage control.

6. The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to claim 1, characterized in that, When the operating mode is load transfer mode and the number of faulty ports of the three-port flexible multi-state switch is 1, the specific steps of step S2 include: Step S24: If the operating mode before the fault occurred was constant DC bus voltage control mode, and the SOC of the lithium battery pack was within the allowable range, control the lithium battery pack to switch from constant current charging to constant voltage control, and switch the fault port to constant voltage-frequency control mode. Step S25: When the SOC of the lithium battery pack reaches the lower limit, control the lithium battery pack to stop working, and switch the non-faulty port with larger adjustable capacity in the three-port flexible multi-state switch that is running in constant power control mode to constant DC bus voltage control mode. Step S26: If the operating mode before the fault occurred was constant power control mode, switch the lithium battery pack to constant power control.

7. The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to claim 1, characterized in that, When the operating mode is load transfer mode and the number of faulty ports of the three-port flexible multi-state switch is 2, the specific steps of step S2 include: Step S27: If the non-faulty port of the three-port flexible multi-state switch is in constant DC bus voltage control mode, the faulty port is operated in constant voltage-frequency control mode, and the lithium battery pack is switched to constant power control. Step S28: If the non-faulty port of the three-port flexible multi-state switch is in constant power control mode, switch the lithium battery pack to constant voltage control.

8. The AC / DC hybrid power grid optimization and coordination control method based on flexible multi-state switches according to claim 1, characterized in that, The specific steps of step S4 include: Step S41: Construct a lithium battery pack SOC equalization power bridge with SOC deviation as the control quantity, and perform energy flow between the two poles of the voltage balancer according to the SOC deviation of the lithium battery pack. Step S42: Design an adaptive virtual resistor for the lithium battery pack by adjusting the size of the virtual resistor in conjunction with the power flow control strategy of the voltage balancer.

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