Power coordination control method for AC / DC hybrid power distribution system based on solid-state transformer

Abstract

The present invention is a power coordination control method for an AC / DC hybrid power distribution system based on a solid-state transformer. The system includes a main grid, a solid-state transformer and an AC / DC microgrid. The solid-state transformer is a three-level structure of a low-voltage level, an isolation level and a medium-voltage level. The control method is composed of a low-voltage level, an isolation level, a medium-voltage level control strategy and a pre-synchronization control strategy. The low-voltage level associates the low-voltage AC frequency and the low-voltage DC voltage representing the power state of the AC / DC microgrid, and realizes power coordination between AC / DC microgrids without communication. The isolation level and the medium-voltage level establish a connection between the medium-voltage AC frequency and the low-voltage DC voltage, and realize the power exchange between renewable energy and the medium-voltage AC bus according to its own state. The pre-synchronization control strategy enables seamless switching of various modes. Under the premise of no communication and no switching control strategy, the present invention realizes stable operation and seamless switching of various modes, improves system reliability, and enhances the level of renewable energy consumption.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a power coordination control method for an AC / DC hybrid power distribution system based on a solid-state transformer. Background Art

[0002] Large-scale use of renewable energy for power generation is one of the inevitable choices to achieve the goal of "carbon neutrality and carbon peak". With the large-scale access to the power grid of distributed renewable energy, distributed energy storage and other components, the future distribution system will develop into an AC / DC hybrid distribution system in which the distribution network and microgrid complement each other and interconnect multiple voltage levels. Solid-state transformers have AC / DC ports of multiple voltage levels, which can be used to build AC / DC hybrid distribution systems, reduce power conversion links, and support the flexible access of distributed renewable energy. Through the interconnection of multiple solid-state transformers, it is also possible to achieve mutual assistance of electric energy in a larger range and fully absorb renewable energy. However, renewable energy and loads are highly random. The network structure of the AC / DC hybrid distribution system based on solid-state transformers is complex and has multiple working modes. Therefore, how to economically and reliably realize the power coordination of the AC / DC hybrid distribution system based on solid-state transformers and the seamless switching between various modes is an urgent problem to be solved.

[0003] The existing publication number CN104852406B discloses a "hybrid microgrid system and power control method based on power electronic transformer". This method designs two operation modes for the hybrid microgrid system based on power electronic transformer, and designs control strategies respectively. However, this control method does not consider the switching method between the two modes, and cannot achieve seamless switching between modes, which reduces the power supply reliability. In addition, the publication number CN108258694B discloses a "coordinated control method of AC / DC microgrid based on power electronic transformer". This method divides the microgrid into load type and power supply type according to the relative size of renewable energy and load in the microgrid and the state of SOC. The solid-state transformer controls the power flow and size of the AC / DC microgrid according to the type of microgrid. However, this control method needs to obtain the power of renewable energy and load during operation, has high communication requirements, increases construction costs, and the system's dependence on communication is also not conducive to the reliable operation of the system.

[0004] In summary, the above schemes rely on communication during operation or use different control strategies in different operation modes, so the construction cost is high and the system reliability is low, which is not conducive to the large-scale consumption of renewable energy. Therefore, a power coordination control method is designed for the AC / DC hybrid distribution system based on solid-state transformers, which does not rely on communication during operation and does not require switching control strategies. Summary of the invention

[0005] The purpose of the present invention is to solve the problems existing in the above-mentioned prior art and provide a power coordination control method for an AC / DC hybrid power distribution system based on a solid-state transformer. In the method of the present invention, the system realizes stable operation of each mode and seamless switching between modes without relying on communication and changing the control strategy, thereby improving the power supply reliability of the system and thus improving the level of renewable energy consumption.

[0006] The present invention is achieved through the following technical solutions:

[0007] A power coordination control method for AC / DC hybrid power distribution system based on solid-state transformer:

[0008] The AC / DC hybrid power distribution system based on solid-state transformers includes a main grid, several solid-state transformers and multiple AC / DC microgrids, wherein the solid-state transformer has a three-level structure, namely, a low-voltage level, an isolation level and a medium-voltage level; the low-voltage level of the solid-state transformer constructs a low-voltage AC / DC bus and connects multiple AC / DC microgrids, the isolation level of the solid-state transformer interconnects the low-voltage level and the medium-voltage level, the medium-voltage level of each solid-state transformer is connected to the main grid, and a circuit breaker is provided at the connection between the main grid and the medium-voltage level of the solid-state transformer. According to the on-off state of each circuit breaker, the system is divided into three working modes: active interconnection mode, passive interconnection mode and solid-state transformer island mode; the system determines whether to perform mode switching according to the mode switching conditions, and if mode switching is required, the switching between modes is completed according to the preset mode switching steps;

[0009] The system power coordination control method is composed of control strategies of medium voltage level, isolation level and low voltage level. The control target of the low voltage level is to control its AC output voltage. The amplitude and frequency given values ​​of the low voltage AC output voltage are obtained by the low voltage reactive power droop control strategy and the low voltage DC voltage-AC frequency control strategy respectively; the control target of the isolation level is to control the medium voltage DC voltage. The medium voltage DC voltage given value is obtained by the low voltage DC-medium voltage DC control strategy; the control target of the medium voltage level is to control the medium voltage AC output voltage. The amplitude and frequency given values ​​of the medium voltage AC output voltage are obtained by the medium voltage reactive power droop control strategy and the medium voltage DC voltage-AC frequency control strategy respectively; by adopting the system power coordination control method, the system operation does not depend on communication, and there is no need to switch the control strategy when switching between the three working modes. The pre-synchronization control strategy is only used when switching to the active interconnection mode or the passive interconnection mode.

[0010] Furthermore, the AC / DC hybrid distribution system based on solid-state transformers includes a main grid, a No. 1 solid-state transformer, a No. 2 solid-state transformer, a No. 1 DC microgrid, a No. 2 DC microgrid, a No. 1 AC microgrid, and a No. 2 AC microgrid; in the No. 1 solid-state transformer, the No. 1 low-voltage stage is connected to the No. 1 DC microgrid and the No. 1 AC microgrid, the No. 1 isolation stage is connected to the No. 1 low-voltage stage and the No. 1 medium-voltage stage, and the No. 1 medium-voltage stage is connected to the main grid via the No. 1 circuit breaker and the main grid side circuit breaker; in the No. 2 solid-state transformer, the No. 2 low-voltage stage is connected to the No. 2 DC microgrid and the No. 2 AC microgrid, the No. 2 isolation stage is connected to the No. 2 low-voltage stage and the No. 2 medium-voltage stage, and the No. 2 medium-voltage stage is connected to the main grid via the No. 2 circuit breaker and the main grid side circuit breaker; the No. 1 solid-state transformer and the No. 2 solid-state transformer are connected in parallel.

[0011] Furthermore, the AC / DC hybrid distribution system based on the solid-state transformer has three working modes: when the main grid side circuit breaker, circuit breaker No. 1 and circuit breaker No. 2 are all closed, the system is in active interconnection mode; when the main grid side circuit breaker is disconnected and circuit breaker No. 1 and circuit breaker No. 2 are closed, the system is in passive interconnection mode; when the main grid side circuit breaker, circuit breaker No. 1 and circuit breaker No. 2 are all disconnected, the system is in solid-state transformer island mode.

[0012] Furthermore, when the system is in operation, if the mode switching conditions are met, the mode switching needs to be performed: when the solid-state transformer and AC / DC microgrid are to be started after fault maintenance or regular inspection, the system should switch from the shutdown mode to the solid-state transformer island mode; when it is to be re-integrated into the main grid, the system should switch from the solid-state transformer island mode to the active interconnection mode; when the system is in the active interconnection mode, if the main grid fails, the system switches from the active interconnection mode to the passive interconnection mode; when the system is in the passive interconnection mode, when the total power of renewable energy generation and the total power of energy storage devices in the system are insufficient to support the load power, the system Switch from passive interconnection mode to solid-state transformer island mode; when the system is in active interconnection mode, if the medium voltage level or isolation level of the solid-state transformer suddenly fails, the system switches from active interconnection mode to solid-state transformer island mode; when the solid-state transformer needs to be reconnected to the system in parallel due to its own fault maintenance, if the main grid fails, but the medium voltage AC bus voltage and frequency are supported by other solid-state transformers in the system, the system switches from solid-state transformer island mode to passive interconnection mode; when the system is in passive interconnection mode, if the voltage of the main grid returns to normal after the fault is eliminated, the system switches from passive interconnection mode to active interconnection mode.

[0013] Furthermore, when the system switches modes, it follows the preset mode switching steps: when the system switches from the shutdown mode to the solid-state transformer island mode, the energy storage unit in the DC microgrid is first started to establish the DC bus voltage, and then the photovoltaic unit in the DC microgrid is started to further increase the DC bus voltage; the low-voltage level of the solid-state transformer establishes a low-voltage AC bus according to the low-voltage level control strategy; then the energy storage unit and the photovoltaic unit in the AC microgrid are connected to the AC bus, and the system enters the solid-state transformer island mode; when the system switches from the solid-state transformer island mode to the active interconnection mode, the solid-state transformer isolation level is started according to the isolation level control strategy, and the medium-voltage level is started according to the medium-voltage level control strategy to obtain the main grid voltage information, and the medium-voltage level of the solid-state transformer performs pre-synchronization control to adjust the medium-voltage AC voltage to the same as the main grid voltage, close the circuit breaker, and the system enters the active interconnection mode; the system switches from the active interconnection mode to the solid-state transformer island mode, and the main grid side circuit breaker and the solid-state transformer medium-voltage level circuit breaker are disconnected. , the medium voltage level and isolation level of the solid-state transformer stop running; the system switches from the solid-state transformer island mode to the passive interconnection mode, the solid-state transformer isolation level starts according to the isolation level control strategy, the medium voltage level starts according to the medium voltage level control strategy, and obtains the medium voltage AC bus voltage information. The medium voltage level of the solid-state transformer performs pre-synchronization control, adjusts the medium voltage level AC voltage to the same as the medium voltage AC bus voltage, closes the circuit breaker, and the system enters the passive interconnection mode; the system switches from the passive interconnection mode to the active interconnection mode, and the medium voltage level of each solid-state transformer performs pre-synchronization control, adjusts the AC voltage of each medium voltage level to the same as the main grid voltage, closes the circuit breaker, and the system enters the active interconnection mode; the system switches from the passive interconnection mode to the active interconnection mode, obtains the main grid voltage information, and the medium voltage level of each solid-state transformer performs pre-synchronization control respectively, adjusts the AC voltage of each medium voltage level to the same as the main grid voltage, closes the circuit breaker, and the system enters the active interconnection mode; disconnect the main grid side circuit breaker, and the system switches from the active interconnection mode to the passive interconnection mode.

[0014] Furthermore, the control strategy of the low-voltage stage is as follows: the low-voltage stage of the solid-state transformer obtains the amplitude and frequency given value of the low-voltage stage AC voltage according to the following formula, inputs them into the voltage controller and the trigger pulse generator, inputs the trigger pulse signal into the low-voltage stage, and controls the low-voltage stage AC voltage to follow the voltage amplitude and frequency given value;

[0015]

[0016]

[0017] In the formula, f LA is the low voltage AC frequency; f LAn is the low voltage AC frequency rating; V LD is the low voltage DC voltage; V LDn is the low voltage DC voltage rating; K LV is the ratio of the low voltage DC voltage range to the low voltage AC frequency range; LA is the low voltage AC voltage; V LAn is the low voltage AC voltage rating; K LQ is the low voltage reactive power droop coefficient; Q LA It is the low voltage AC output reactive power; Q LAn It is the low voltage reactive power rating.

[0018] The frequency of the AC microgrid is associated with the DC voltage of the DC microgrid through the control of the low-voltage level, and the frequency of the AC microgrid and the voltage of the DC microgrid represent the power status of the AC and DC microgrids respectively. Therefore, the low-voltage level can sense the power status of the AC and DC microgrids at the same time without communication. When the DC microgrid has a power surplus, the low-voltage DC voltage increases, the frequency of the AC side of the low-voltage level increases, and the phase difference with the AC microgrid increases, and power is transmitted to the AC microgrid. Conversely, power is absorbed from the AC microgrid, realizing the mutual support of the AC and DC microgrids.

[0019] Furthermore, the control strategy of the isolation stage is as follows: the low-voltage DC voltage of the solid-state transformer isolation stage is determined by the state of the microgrid, and the isolation stage obtains the medium-voltage DC voltage set value of the isolation stage according to the following formula, which is input to the single-phase shift controller and the trigger pulse generator, and the trigger pulse signal is input to the isolation stage to control the medium-voltage DC voltage of the isolation stage to follow the medium-voltage DC voltage set value;

[0020] V MDref =n(V LD -V LDn )+V MDn

[0021] Where V MDref is the given value of medium voltage DC voltage; V MDn is the rated value of medium voltage DC voltage; n is the isolation level ratio.

[0022] The medium-voltage DC voltage of the isolation level is associated with the low-voltage DC voltage through the control strategy of the isolation level. The low-voltage DC voltage is related to the state of renewable energy, so the medium-voltage DC voltage also represents the state of renewable energy.

[0023] Furthermore, the control strategy of the medium voltage stage is as follows: the medium voltage stage of the solid-state transformer obtains the amplitude and frequency given value of the medium voltage stage AC voltage according to the following formula, inputs them into the voltage controller and the trigger pulse generator, inputs the trigger pulse signal into the medium voltage stage, and controls the medium voltage stage AC voltage to follow the voltage amplitude and frequency given value;

[0024]

[0025]

[0026] In the formula, fMA is the medium voltage AC frequency; f MAn is the medium voltage AC frequency rating; V MD is the medium voltage DC voltage; V MDn is the rated value of medium voltage DC voltage; K M V is the ratio of the medium voltage DC voltage range to the medium voltage AC frequency range; MA is the medium voltage AC voltage; V MAn is the medium voltage AC voltage rating; K MQ is the medium voltage reactive power droop coefficient; Q MA It is the medium voltage AC output reactive power; Q MAn is the medium voltage reactive power rating.

[0027] Through the control of the medium voltage stage, the medium voltage AC voltage frequency is associated with the medium voltage DC voltage. Therefore, the medium voltage AC frequency also represents the power state of renewable energy. When the power of renewable energy increases, the low voltage DC voltage increases, the medium voltage DC voltage increases, the medium voltage output frequency increases, and the phase angle between the medium voltage AC bus increases, transmitting power to the medium voltage AC bus. Conversely, power is absorbed from the medium voltage AC bus. Mutual assistance between the medium voltage AC side and the low voltage DC side is achieved. At the same time, the medium voltage stage has the ability to independently establish voltage and frequency, and there is no need to switch the control strategy when switching to the active interconnection mode.

[0028] Furthermore, when the system switches to the active interconnection mode or the passive interconnection mode, the medium voltage level of the solid-state transformer uses a pre-synchronization control strategy to keep the phase, amplitude, and frequency of the medium voltage output AC voltage consistent with the medium voltage AC bus voltage, thereby reducing the impact of the mode switching. The principle of the pre-synchronization control strategy is shown in the following formula:

[0029]

[0030]

[0031] Where V pre is the output of amplitude pre-synchronization; RMS is the effective value estimation function, V MAref is the medium voltage AC bus voltage; f pre Output for frequency pre-synchronization; and V MA and V MAref The phase angle, k p and k i are the proportional and integral gains of the PI controller in the phase pre-synchronization module, respectively.

[0032] In summary, the system of the present invention includes a main grid, several solid-state transformers and multiple AC / DC microgrids. The solid-state transformer is a three-level structure of low voltage level, isolation level and medium voltage level. The low voltage level is connected to the AC / DC microgrid, the isolation level interconnects the low voltage level with the medium voltage level, and the medium voltage level is connected to the main grid or other solid-state transformers. The method of the present invention is composed of a solid-state transformer low voltage level, an isolation level, a medium voltage level control strategy and a pre-synchronization control strategy; the low voltage level associates the low voltage AC frequency and the low voltage DC voltage representing the power state of the AC / DC microgrid, and realizes power coordination between AC / DC microgrids without communication; the isolation level and the medium voltage level establish a connection between the medium voltage level AC voltage frequency and the low voltage DC bus voltage, and realizes the power interaction between renewable energy and the medium voltage AC bus according to its own state; the pre-synchronization control strategy enables seamless switching of various modes.

[0033] Compared with the prior art, the system operation in the method of the present invention does not rely on communication, and there is no need to switch the control strategy when switching modes, which increases the system operation reliability, reduces the construction cost, and is conducive to the large-scale consumption of renewable energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a structural schematic diagram of an AC / DC hybrid power distribution system based on a solid-state transformer.

[0035] Figure 2 Schematic diagram of an AC / DC hybrid power distribution system based on solid-state transformers in active interconnection mode.

[0036] Figure 3 Schematic diagram of the AC / DC hybrid power distribution system based on solid-state transformers in passive interconnection mode.

[0037] Figure 4 This is a schematic diagram of an AC / DC hybrid power distribution system based on a solid-state transformer in solid-state transformer island mode.

[0038] Figure 5 Schematic diagram of the switching conditions of various modes in the AC / DC hybrid distribution system based on solid-state transformers.

[0039] Figure 6 The figure is a flow chart of a power coordination control method for an AC / DC hybrid power distribution system based on a solid-state transformer.

[0040] Figure 7 This is the experimental waveform diagram of the AC / DC hybrid distribution system based on solid-state transformer switching from shutdown mode to solid-state transformer island mode.

[0041] Figure 8 This is the waveform diagram of the isolation-level medium-voltage startup experiment.

[0042] Fig. 9This is the experimental waveform diagram of the AC / DC hybrid distribution system based on solid-state transformer switching to active interconnection mode.

[0043] Fig.10 Experimental waveform diagram of AC / DC hybrid power distribution system based on solid-state transformer switching to passive interconnection mode

[0044] In the figure: 1-main grid, 2-No. 1 solid-state transformer, 3-No. 2 solid-state transformer; 10-main grid side circuit breaker; 20-No. 1 circuit breaker, 21-No. 1 medium voltage level, 22-No. 1 isolation level, 23-No. 1 low voltage level, 24-No. 1 DC microgrid, 25-No. 1 AC microgrid; 30-No. 2 circuit breaker, 31-No. 2 medium voltage level, 32-No. 2 isolation level, 33-No. 2 low voltage level, 34-No. 2 DC microgrid, 35-No. 2 AC microgrid. DETAILED DESCRIPTION

[0045] The present invention is further described below in conjunction with the accompanying drawings. The following examples are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0046] The present embodiment provides a power coordination control method for an AC / DC hybrid power distribution system based on a solid-state transformer, wherein the AC / DC hybrid power distribution system based on a solid-state transformer includes a main grid, a plurality of solid-state transformers, and a plurality of AC / DC microgrids, wherein the solid-state transformer is a three-level structure, namely, a low-voltage level, an isolation level, and a medium-voltage level; the low-voltage level of the solid-state transformer constructs a low-voltage AC / DC bus and connects a plurality of AC / DC microgrids, the isolation level of the solid-state transformer interconnects the low-voltage level and the medium-voltage level, the medium-voltage level of each solid-state transformer is connected to the main grid, and a circuit breaker is provided at the connection between the main grid and the medium-voltage level of the solid-state transformer. According to the on / off status of each circuit breaker, the system is divided into three working modes: an active interconnection mode, a passive interconnection mode, and a solid-state transformer island mode; the system determines whether to perform mode switching according to a mode switching condition, and if mode switching is required, switching between modes is completed according to preset mode switching steps. The system power coordination control method is composed of control strategies for the medium voltage level, the isolation level, and the low voltage level. The control target of the low voltage level is to control its AC output voltage, and the amplitude and frequency set values ​​of the low voltage AC output voltage are obtained by the low voltage reactive power droop control strategy and the low voltage DC voltage-AC frequency control strategy respectively; the control target of the isolation level is to control the medium voltage DC voltage, and the medium voltage DC voltage set value is obtained by the low voltage DC-medium voltage DC control strategy; the control target of the medium voltage level is to control the medium voltage AC output voltage, and the amplitude and frequency set values ​​of the medium voltage AC output voltage are obtained by the medium voltage reactive power droop control strategy and the medium voltage DC voltage-AC frequency control strategy respectively; by adopting the system power coordination control method, the system operation does not depend on communication, and there is no need to switch the control strategy when switching between the three working modes. The pre-synchronization control strategy is only used when switching to the active interconnection mode or the passive interconnection mode.

[0047] In this embodiment, the AC / DC hybrid power distribution system based on solid-state transformers takes two solid-state transformers as an example. Figure 1 As shown, it includes a main grid 1, a No. 1 solid-state transformer 2, a No. 2 solid-state transformer 3, a No. 1 DC microgrid 24, a No. 2 DC microgrid 34, a No. 1 AC microgrid 25, and a No. 2 AC microgrid 35; in the No. 1 solid-state transformer 2, the No. 1 low-voltage stage 23 connects the No. 1 DC microgrid 24 and the No. 1 AC microgrid 25, the No. 1 isolation stage 22 connects the No. 1 low-voltage stage 23 and the No. 1 medium-voltage stage 21, and the No. 1 medium-voltage stage 21 is connected to the main grid 1 via the No. 1 circuit breaker 20 and the main grid side circuit breaker 10; in the No. 2 solid-state transformer 3, the No. 2 low-voltage stage 33 connects the No. 2 DC microgrid 34 and the No. 2 AC microgrid 35, the No. 2 isolation stage 32 connects the No. 2 low-voltage stage 33 and the No. 2 medium-voltage stage 31, and the No. 2 medium-voltage stage 31 is connected to the main grid 1 via the No. 2 circuit breaker 30 and the main grid side circuit breaker 10; the No. 1 solid-state transformer 2 and the No. 2 solid-state transformer 3 are in parallel relationship.

[0048] The process of the system power coordination control method is as follows: Figure 6 As shown, the specific steps include:

[0049] S1 collects the voltage on the main grid side, the voltage at each level of the solid-state transformer, and the frequency information, and uploads the information to each level of the solid-state transformer.

[0050] S2 central controller determines the system working mode according to the status of each circuit breaker;

[0051] S2-1 If the low voltage DC voltage of each solid-state transformer is zero, the system is in shutdown mode;

[0052] S2-2 If the main grid side circuit breaker 10, No. 1 circuit breaker 20 and No. 2 circuit breaker 30 are all closed, the system is in active interconnection mode. The structural diagram of the active interconnection mode is as follows: Figure 2 As shown;

[0053] S2-3 If the main grid side circuit breaker 10 is disconnected, the No. 1 circuit breaker 20 and the No. 2 circuit breaker 30 are closed, and the system is in a passive interconnection state. The passive interconnection mode structure diagram is as follows: Figure 3 As shown;

[0054] S2-4 If the main grid side circuit breaker 10, No. 1 circuit breaker 20 and No. 2 circuit breaker 30 are all disconnected, the system is in the solid-state transformer island state. The structural diagram of the solid-state transformer island mode is as follows: Figure 4 shown.

[0055] S3 determines whether the mode switching condition is met;

[0056] When the system is running, if the mode switching conditions are met, the mode switching needs to be performed: when the solid-state transformer and AC / DC microgrid are to be started after fault maintenance or regular overhaul, the system should switch from the shutdown mode to the solid-state transformer island mode; when it is to be re-integrated into the main grid, the system switches from the solid-state transformer island mode to the active interconnection mode; when the system is in the active interconnection mode, if the main grid fails, the system switches from the active interconnection mode to the passive interconnection mode; when the system is in the passive interconnection mode, when the total power of renewable energy generation and the total power of energy storage devices in the system are insufficient to support the load power, the system switches from the passive interconnection mode to solid-state transformer island mode; when the system is in active interconnection mode, if the medium voltage level or isolation level of the solid-state transformer fails suddenly, the system switches from active interconnection mode to solid-state transformer island mode; when the solid-state transformer needs to be reconnected to the system in parallel after its own fault maintenance is completed, if the main grid fails, but the medium voltage AC bus voltage and frequency are supported by other solid-state transformers in the system, the system should switch from solid-state transformer island mode to passive interconnection mode; when the system is in passive interconnection mode, if the voltage of the main grid returns to normal after the fault is eliminated, the system switches from passive interconnection mode to active interconnection mode; the schematic diagram of the switching conditions of each mode of the system is as follows Figure 5 shown.

[0057] If S4 needs to switch modes, follow the switching steps;

[0058] The S4-1 system switches from shutdown mode to solid-state transformer island mode: the energy storage unit in the DC microgrid starts first to establish the DC bus voltage, and then the photovoltaic unit in the DC microgrid starts to further increase the DC bus voltage; the low-voltage level establishes a low-voltage AC bus according to the low-voltage level control strategy; then the energy storage unit and photovoltaic unit in the AC microgrid are connected to the AC bus, and the system enters the solid-state transformer island mode;

[0059] The S4-2 system switches from the solid-state transformer island mode to the active interconnection mode: the solid-state transformer isolation level is started according to the isolation level control strategy; the medium voltage level is started according to the medium voltage level control strategy; the main grid voltage information is obtained, the solid-state transformer medium voltage level performs pre-synchronization control, adjusts the medium voltage level AC voltage to the same as the main grid voltage, closes the circuit breaker, and the system enters the active interconnection mode;

[0060] The S4-3 system switches from the active interconnection mode to the solid-state transformer island mode: the main grid side and the medium-voltage level circuit breaker of the solid-state transformer are disconnected, and the medium-voltage level and isolation level of the solid-state transformer stop operating;

[0061] The S4-4 system switches from the solid-state transformer island mode to the passive interconnection mode: the solid-state transformer isolation level is started according to the isolation level control strategy; the medium voltage level is started according to the medium voltage level control strategy; the medium voltage AC bus voltage information is obtained, the solid-state transformer medium voltage level performs pre-synchronization control, adjusts the medium voltage level AC voltage to the same as the medium voltage AC bus voltage, closes the circuit breaker, and the system enters the passive interconnection mode;

[0062] The S4-5 system switches from the passive interconnection mode to the active interconnection mode: the medium-voltage level of each solid-state transformer performs pre-synchronization control, adjusts the AC voltage of each medium-voltage level to the same as the main grid voltage, closes the circuit breaker, and the system enters the active interconnection mode;

[0063] S4-6 The system switches from passive interconnection mode to active interconnection mode: obtain the main grid voltage information, perform pre-synchronization control on the medium voltage level of each solid-state transformer, adjust the AC voltage of each medium voltage level to the same as the main grid voltage, close the circuit breaker, and the system enters active interconnection mode;

[0064] The S4-7 system switches from active interconnection mode to passive interconnection mode: the circuit breaker on the main grid side is disconnected.

[0065] The control strategies of low voltage level, isolation level, medium voltage level, pre-synchronization, etc. involved in the above mode switching steps are as follows:

[0066] Solid-state transformer low-voltage control strategy: The solid-state transformer low-voltage level obtains the amplitude and frequency set value of the low-voltage level AC voltage according to the following formula, inputs them to the voltage controller and the trigger pulse generator, inputs the trigger pulse signal to the low-voltage level, and controls the low-voltage level AC voltage to follow the low-voltage AC voltage amplitude and frequency set value;

[0067]

[0068]

[0069] In the formula, f LA is the low voltage AC frequency; f LAn is the low voltage AC frequency rating; V LD is the low voltage DC voltage; V LDn is the low voltage DC voltage rating; K L V is the ratio of the low voltage DC voltage range to the low voltage AC frequency range; LA is the low voltage AC voltage; V LAn is the low voltage AC voltage rating; K LQ is the low voltage reactive power droop coefficient; Q LA It is the low voltage AC output reactive power; Q LAn It is the low voltage reactive power rating.

[0070] Solid-state transformer isolation level control strategy: The low-voltage DC voltage of the solid-state transformer isolation level is determined by the state of the microgrid. The solid-state transformer isolation level obtains the medium-voltage DC voltage set value of the isolation level through the following formula, which is input to the single-phase shift controller and the trigger pulse generator. The trigger pulse signal is input to the isolation level to control the medium-voltage DC voltage of the isolation level to follow the medium-voltage DC voltage set value;

[0071] V MDref =n(V LD -V LDn )+V MDn

[0072] Where V MDref is the given value of medium voltage DC voltage; V MDn is the rated value of medium voltage DC voltage; n is the isolation level ratio.

[0073] Solid-state transformer medium voltage control strategy: The medium voltage level of the solid-state transformer obtains the amplitude and frequency set value of the medium voltage level AC voltage by the following formula, which are input to the voltage controller and the trigger pulse generator, and the trigger pulse signal is input to the medium voltage level to control the medium voltage level AC voltage to follow the voltage amplitude and frequency set value;

[0074]

[0075]

[0076] In the formula, f MA is the medium voltage AC frequency; f MAn is the medium voltage AC frequency rating; V MD is the medium voltage DC voltage; V MDn is the rated value of medium voltage DC voltage; K M V is the ratio of the medium voltage DC voltage range to the medium voltage AC frequency range; MA is the medium voltage AC voltage; V MAn is the medium voltage AC voltage rating; K MQ is the medium voltage reactive power droop coefficient; Q MA It is the medium voltage AC output reactive power; Q MAn is the medium voltage reactive power rating.

[0077] Pre-synchronization control strategy: When the system switches to active interconnection mode or passive interconnection mode, the medium voltage level of the solid-state transformer uses a pre-synchronization control strategy to keep the medium voltage output AC voltage consistent with the phase, amplitude, and frequency of the medium voltage AC bus voltage to reduce the impact of mode switching. The principle of the pre-synchronization control strategy is shown in the following formula:

[0078]

[0079]

[0080] Where V pre is the output of amplitude pre-synchronization; RMS is the effective value estimation function, V MAref is the medium voltage AC bus voltage; f pre Output for frequency pre-synchronization; and V MA and V MAref The phase angle, k p and k i are the proportional and integral gains of the PI controller in the phase pre-synchronization module, respectively.

[0081] Figure 7 This is the experimental waveform diagram of the system switching from shutdown mode to solid-state transformer island mode. ds is the power of the energy storage unit in the DC microgrid; P dp is the power of the photovoltaic unit in the DC microgrid; P as is the power of the energy storage unit in the AC microgrid; P ap is the power of the photovoltaic unit in the AC microgrid; P dt is the power of interaction between the AC and DC microgrids, with the power transferred from the DC microgrid to the AC microgrid being positive. As can be seen from the figure, at 0s, the system is in shutdown mode, and at t1 and t2, the photovoltaic unit and energy storage unit in the DC microgrid are started successively and operate in coordination. At t3, the low-voltage stage of the solid-state transformer is started, and at the same time, the photovoltaic unit and energy storage unit in the AC microgrid start working, and the system enters the solid-state transformer island mode. Since there is no load in the AC and DC microgrids, the power emitted by the photovoltaic unit is equal to the power absorbed by the energy storage unit. Moreover, since the configurations of the photovoltaic unit and the energy storage unit in the AC and DC microgrids are the same, there is no power interaction between the AC and DC microgrids, that is, P dt is zero.

[0082] Figure 8 This is the waveform diagram of the isolation level medium voltage level startup experiment. dse and P ase They are the sum of the photovoltaic and energy storage powers in the DC microgrid and AC microgrid, respectively. As can be seen from the figure, at 0s, only the low-voltage stage of the solid-state transformer is working. At t1, the isolation stage switch is started, and the AC / DC microgrid charges the medium-voltage DC capacitor through the isolation stage. Due to the large capacitance, the low-voltage DC bus voltage drops for a short time and then recovers, and the medium-voltage bus voltage rises to the set value. At t2, the medium-voltage stage of the solid-state transformer is started to establish the medium-voltage AC bus voltage.

[0083] Fig. 9 This is the experimental waveform diagram of the system switching to active interconnection mode. gis the active power flowing through the circuit breaker on the main grid side, with the direction flowing out of the main grid as positive; the phase difference in the figure is the phase difference between the medium voltage level voltage of the solid-state transformer and the main grid voltage. It can be seen from the figure that at 0s, the system works in the solid-state transformer island mode, at this time the distribution network is normal, at t1, the pre-synchronization instruction is executed to adjust the medium voltage output AC voltage, at t2, the phase difference with the distribution network is stable to zero, the circuit breaker S1 at the solid-state transformer is closed to complete the grid connection operation, and the system enters the active interconnection mode.

[0084] Fig.10 This is the experimental waveform diagram of the system switching to passive interconnection mode. LD1 is the low voltage DC voltage of the No. 1 solid-state transformer; f MA1 is the medium voltage AC frequency of the No. 1 solid-state transformer; V LD2 is the low voltage DC voltage of the No.2 solid-state transformer; f MA2 is the medium voltage AC frequency of the No. 2 solid-state transformer; the phase difference in the figure is the phase difference between the medium voltage AC voltage of the No. 1 solid-state transformer and the medium voltage AC voltage of the No. 2 solid-state transformer. As can be seen from the figure, the system needs to coordinate with other solid-state transformers to switch from the solid-state transformer island mode to the passive interconnection mode. In order to more clearly show the parallel process, a DC load is added to the No. 2 solid-state transformer to make its low voltage DC voltage and medium voltage AC frequency lower than the No. 1 solid-state transformer. Therefore, there is a phase difference between the medium voltage AC voltage of the No. 1 solid-state transformer and the No. 2 solid-state transformer before t1. The pre-synchronization link is started at t1, and the phase difference is stable to zero at t2. The switch is closed and the system enters the passive interconnection mode.

[0085] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can make many modifications without departing from the purpose of the present invention, which are all within the protection of the present invention. Any non-substantial changes to the present invention using this concept shall be deemed as an infringement of the protection scope of the present invention.

Claims

1. A power coordination control method for an AC / DC hybrid power distribution system based on a solid-state transformer, characterized in that: The AC / DC hybrid power distribution system based on solid-state transformers includes a main grid, several solid-state transformers and multiple AC / DC microgrids, wherein the solid-state transformer has a three-level structure, namely, a low-voltage level, an isolation level and a medium-voltage level; the low-voltage level of the solid-state transformer constructs a low-voltage AC / DC bus and connects multiple AC / DC microgrids, the isolation level of the solid-state transformer interconnects the low-voltage level and the medium-voltage level, the medium-voltage level of each solid-state transformer is connected to the main grid, and a circuit breaker is provided at the connection between the main grid and the medium-voltage level of the solid-state transformer. According to the on-off state of each circuit breaker, the system is divided into three working modes: active interconnection mode, passive interconnection mode and solid-state transformer island mode; the system determines whether to perform mode switching according to the mode switching conditions, and if mode switching is required, the switching between modes is completed according to the preset mode switching steps; The system power coordination control method is composed of control strategies of medium voltage level, isolation level and low voltage level. The control target of the low voltage level is to control its AC output voltage. The amplitude and frequency given values ​​of the low voltage AC output voltage are obtained by the low voltage reactive power droop control strategy and the low voltage DC voltage-AC frequency control strategy respectively; the control target of the isolation level is to control the medium voltage DC voltage. The medium voltage DC voltage given value is obtained by the low voltage DC-medium voltage DC control strategy; the control target of the medium voltage level is to control the medium voltage AC output voltage. The amplitude and frequency given values ​​of the medium voltage AC output voltage are obtained by the medium voltage reactive power droop control strategy and the medium voltage DC voltage-AC frequency control strategy respectively; by adopting the system power coordination control method, the system operation does not depend on communication, and there is no need to switch the control strategy when switching between the three working modes. The pre-synchronization control strategy is only used when switching to the active interconnection mode or the passive interconnection mode.

2. The method for power coordination control of an AC / DC hybrid power distribution system based on a solid-state transformer according to claim 1, characterized in that: The AC / DC hybrid distribution system based on solid-state transformer includes a main grid, a No. 1 solid-state transformer, a No. 2 solid-state transformer, a No. 1 DC microgrid, a No. 2 DC microgrid, a No. 1 AC microgrid, and a No. 2 AC microgrid; in the No. 1 solid-state transformer, the No. 1 low-voltage stage is connected to the No. 1 DC microgrid and the No. 1 AC microgrid, the No. 1 isolation stage is connected to the No. 1 low-voltage stage and the No. 1 medium-voltage stage, and the No. 1 medium-voltage stage is connected to the main grid via the No. 1 circuit breaker and the main grid side circuit breaker; in the No. 2 solid-state transformer, the No. 2 low-voltage stage is connected to the No. 2 DC microgrid and the No. 2 AC microgrid, the No. 2 isolation stage is connected to the No. 2 low-voltage stage and the No. 2 medium-voltage stage, and the No. 2 medium-voltage stage is connected to the main grid via the No. 2 circuit breaker and the main grid side circuit breaker; the No. 1 solid-state transformer and the No. 2 solid-state transformer are connected in parallel.

3. The power coordination control method of the AC / DC hybrid power distribution system based on solid-state transformer according to claim 2 is characterized in that: When the main grid side circuit breaker, No. 1 circuit breaker and No. 2 circuit breaker are all closed, the system is in active interconnection mode; when the main grid side circuit breaker is disconnected and No. 1 circuit breaker and No. 2 circuit breaker are closed, the system is in passive interconnection mode; when the main grid side circuit breaker, No. 1 circuit breaker and No. 2 circuit breaker are all disconnected, the system is in solid-state transformer island mode.

4. The power coordination control method of the AC / DC hybrid power distribution system based on solid-state transformer according to claim 1 is characterized in that: When the system is running, if the mode switching conditions are met, the mode switching needs to be performed: when the solid-state transformer and AC / DC microgrid are to be started after fault maintenance or regular inspection, the system should be switched from the shutdown mode to the solid-state transformer island mode; when it is to be re-integrated into the main grid, the system should be switched from the solid-state transformer island mode to the active interconnection mode; when the system is in the active interconnection mode, if the main grid fails, the system will switch from the active interconnection mode to the passive interconnection mode; when the system is in the passive interconnection mode, when the total power of renewable energy generation and the total power of energy storage devices in the system are not enough to support the load power, the system will switch from the passive interconnection mode to the passive interconnection mode. The system switches from the active interconnection mode to the solid-state transformer island mode; when the system is in the active interconnection mode, if the medium-voltage level or isolation level of the solid-state transformer fails suddenly, the system switches from the active interconnection mode to the solid-state transformer island mode; when the solid-state transformer needs to be reconnected to the system in parallel due to its own fault maintenance, if the main grid fails, but the medium-voltage AC bus voltage and frequency are supported by other solid-state transformers in the system, the system switches from the solid-state transformer island mode to the passive interconnection mode; when the system is in the passive interconnection mode, if the voltage of the main grid returns to normal after the fault is eliminated, the system switches from the passive interconnection mode to the active interconnection mode.

5. The power coordination control method of the AC / DC hybrid power distribution system based on solid-state transformer according to claim 4 is characterized in that: When the system is switching modes, it follows the preset mode switching steps: when the system switches from the shutdown mode to the solid-state transformer island mode, the energy storage unit in the DC microgrid is first started to establish the DC bus voltage, and then the photovoltaic unit in the DC microgrid is started to further increase the DC bus voltage; the low-voltage level of the solid-state transformer establishes a low-voltage AC bus according to the low-voltage level control strategy; then the energy storage unit and the photovoltaic unit in the AC microgrid are connected to the AC bus, and the system enters the solid-state transformer island mode; when the system switches from the solid-state transformer island mode to the active interconnection mode, the solid-state transformer isolation level is started according to the isolation level control strategy, and the medium-voltage level is started according to the medium-voltage level control strategy to obtain the main grid voltage information, and the medium-voltage level of the solid-state transformer performs pre-synchronization control to adjust the medium-voltage AC voltage to the same as the main grid voltage, close the circuit breaker, and the system enters the active interconnection mode; the system switches from the active interconnection mode to the solid-state transformer island mode, the main grid side circuit breaker and the solid-state transformer medium-voltage level circuit breaker are disconnected, and the solid The medium voltage level and isolation level of the transformer stop running; the system switches from the solid-state transformer island mode to the passive interconnection mode, the solid-state transformer isolation level starts according to the isolation level control strategy, the medium voltage level starts according to the medium voltage level control strategy, and obtains the medium voltage AC bus voltage information. The medium voltage level of the solid-state transformer performs pre-synchronization control to adjust the medium voltage level AC voltage to the same as the medium voltage AC bus voltage, closes the circuit breaker, and the system enters the passive interconnection mode; the system switches from the passive interconnection mode to the active interconnection mode, and the medium voltage levels of each solid-state transformer perform pre-synchronization control to adjust the AC voltage of each medium voltage level to the same as the main grid voltage, closes the circuit breaker, and the system enters the active interconnection mode; the system switches from the passive interconnection mode to the active interconnection mode, obtains the main grid voltage information, and the medium voltage levels of each solid-state transformer perform pre-synchronization control to adjust the AC voltage of each medium voltage level to the same as the main grid voltage, closes the circuit breaker, and the system enters the active interconnection mode; the system switches from the passive interconnection mode to the active interconnection mode, obtains the main grid voltage information, and the medium voltage levels of each solid-state transformer perform pre-synchronization control to adjust the AC voltage of each medium voltage level to the same as the main grid voltage, closes the circuit breaker, and the system enters the active interconnection mode; disconnect the circuit breaker on the main grid side, and the system switches from the active interconnection mode to the passive interconnection mode.

6. The power coordination control method of the AC / DC hybrid power distribution system based on solid-state transformer according to claim 5 is characterized in that: The low-voltage stage of the solid-state transformer obtains the amplitude and frequency set value of the low-voltage stage AC voltage according to the following formula, inputs them into the voltage controller and the trigger pulse generator, inputs the trigger pulse signal into the low-voltage stage, and controls the low-voltage stage AC voltage to follow the voltage amplitude and frequency set value; In the formula, f LA is the low voltage AC frequency; f LAn is the low voltage AC frequency rating; V LD is low voltage DC voltage; V LDn is the low voltage DC voltage rating; K L V is the ratio of the low voltage DC voltage range to the low voltage AC frequency range; LA is the low voltage AC voltage; V LAn is the low voltage AC voltage rating; K LQ is the low voltage reactive power droop coefficient; Q LA It is the low voltage AC output reactive power; Q LAn It is the low voltage reactive power rating.

7. The power coordination control method of the AC / DC hybrid power distribution system based on solid-state transformer according to claim 5 is characterized in that: The low-voltage DC voltage of the solid-state transformer isolation stage is determined by the state of the microgrid. The isolation stage obtains the medium-voltage DC voltage set value of the isolation stage according to the following formula, which is input to the single-phase shift controller and the trigger pulse generator. The trigger pulse signal is input to the isolation stage to control the medium-voltage DC voltage of the isolation stage to follow the medium-voltage DC voltage set value; V MDref =n(V LD -V LDn )+V MDn Where V MDref is the given value of medium voltage DC voltage; V MDn is the rated value of medium voltage DC voltage; n is the isolation level ratio.

8. The power coordination control method of the AC / DC hybrid power distribution system based on solid-state transformer according to claim 5 is characterized by: The medium voltage stage of the solid-state transformer obtains the amplitude and frequency set value of the medium voltage stage AC voltage according to the following formula, inputs them into the voltage controller and the trigger pulse generator, inputs the trigger pulse signal into the medium voltage stage, and controls the medium voltage stage AC voltage to follow the voltage amplitude and frequency set value; In the formula, f MA is the medium voltage AC frequency; f MAn is the medium voltage AC frequency rating; V MD is the medium voltage DC voltage; V MDn is the rated value of medium voltage DC voltage; K M V is the ratio of the medium voltage DC voltage range to the medium voltage AC frequency range; MA is the medium voltage AC voltage; V MAn is the medium voltage AC voltage rating; K MQ is the medium voltage reactive power droop coefficient; Q MA It is the medium voltage AC output reactive power; Q MAn is the medium voltage reactive power rating.

9. The power coordination control method of the AC / DC hybrid power distribution system based on solid-state transformer according to claim 5 is characterized in that: When the system switches to the active interconnection mode or the passive interconnection mode, the medium voltage level of the solid-state transformer uses a pre-synchronization control strategy to keep the medium voltage output AC voltage consistent with the phase, amplitude, and frequency of the medium voltage AC bus voltage to reduce the impact of the mode switching. The principle of the pre-synchronization control strategy is shown in the following formula: Where V pre is the output of amplitude pre-synchronization; RMS is the effective value estimation function, V MAref is the medium voltage AC bus voltage; f pre Output for frequency pre-synchronization; and V MA and V MAref The phase angle, k p and k i are the proportional and integral gains of the PI controller in the phase pre-synchronization module, respectively.

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