Flexible interconnection power distribution network load grading seamless transfer method and system

Abstract

The invention discloses a load grading seamless transfer method and system for a flexible interconnection power distribution network, relates to the technical field of power distribution networks, and solves the technical problem that voltage and frequency fluctuation and impact caused by transfer power imbalance during mode switching are difficult to consider by an existing method during FMS fault load transfer. According to the invention, a flexible interconnection distribution network architecture is constructed based on the FMS; establishing a flexible multi-state switch mode emergency switching seamless control strategy, and minimizing mode emergency switching fluctuation and impact; a flexible multi-state switch load hierarchical control strategy is established, and when a power distribution network breaks down, if the power provided by the FMS does not meet the current load requirement after the fault, selective power supply is carried out according to the importance degree of the load; according to the method, the time required for adjustment is shortened, the stability of the electrical parameters is effectively guaranteed during emergency conversion, and the problem that the electrical parameters exceed the limit due to disturbance or faults is avoided.

Description

Technical Field

[0001] The present invention belongs to the field of power distribution networks, and in particular to a method and system for seamlessly transferring loads in a flexible interconnected power distribution network. Background Art

[0002] It is urgent to build a new distribution network based on distributed renewable energy generation on the distribution network side. The flexible interconnected distribution network provides a technical basis for the large-scale access and efficient consumption of DG. On the one hand, the distribution network itself can maintain the optimal distribution and balance of energy to a certain extent, and realize the two-way flow of power between the external distribution network; on the other hand, the flexible multi-state switch (FMS), as a back-to-back power control device, can realize the flexible interconnection of the distribution network and the flexible adjustment of the flow, and realize the optimal configuration of adjustable resources in a larger range.

[0003] When the FMS transfers fault loads, it is necessary to ensure smooth switching of the distribution network mode in the emergency state and continuous stable operation. The existing technology adopts seamless mode switching. If the source and load power of the distribution network are unbalanced, it will cause large changes in frequency and voltage. Due to the sudden switching of the FMS control method, the output signal changes suddenly, causing a large voltage shock; when switching, if the load size to be transferred on the fault side exceeds the power that can be mutually assisted on the opposite side, it will cause overload or abnormality on the opposite side; switching fluctuations and shocks may cause short-term power outages and DG withdrawal in the distribution network, and even instability in severe cases. Therefore, the present invention provides a method and system for seamless load transfer in a flexible interconnected distribution network. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art; to this end, the present invention proposes a method and system for seamless load transfer in a flexible interconnected distribution network, which is used to solve the technical problem that when transferring FMS fault loads, the existing methods are difficult to take into account the voltage and frequency fluctuations and impacts caused by the imbalance of transferred power during mode switching.

[0005] To achieve the above object, a first aspect of the present invention provides a method for seamless load transfer in a flexible interconnected distribution network, comprising the following steps:

[0006] A flexible interconnected distribution network architecture is constructed based on FMS; the operation modes of the flexible interconnected distribution network include power regulation mode and emergency control mode;

[0007] Establish a flexible multi-state switch mode emergency switching seamless control strategy to minimize mode emergency switching fluctuations and impacts;

[0008] Establish a load classification control strategy for flexible multi-state switches;

[0009] When a failure occurs in the distribution network, it is evaluated whether the power provided by the FMS after the failure meets the current load demand; if yes, normal power supply is provided; if not, selective power supply is provided based on the importance of the load.

[0010] Preferably, the construction of a flexible interconnection distribution network architecture based on FMS includes:

[0011] The FMS is composed of a back-to-back converter, which includes a converter, a common DC capacitor, a power grid and a circuit breaker switch; the converter is arranged on both sides of the FMS, respectively marked as MMC1 and MMC2; the common DC capacitor is marked as C0; the power grid is arranged at both ends of the FMS, respectively marked as AC1 and AC2; the circuit breaker switches are respectively marked as K1 and K2, and the solid line represents the power flow, and the dotted line represents the information communication;

[0012] When K1 and K2 are closed, power exchange can be achieved between the distribution networks on both sides of the FMS. When the upper power grid on one side of the FMS loses power due to an abnormality or fault, the intelligent distribution system sends a switching command to the FMS, K1 or K2 is disconnected, and the FMS switches to the Uac-f control mode to provide voltage and frequency support to the distribution network of PCC1 or PCC2.

[0013] The present invention proposes a flexible interconnected distribution network structure based on FMS, which realizes flexible interconnection between distribution networks through back-to-back converters, improving the access and absorption capacity of distributed power generation resources. Compared with traditional rigid interconnection, this structure can achieve optimal allocation of resources in a wider range, improve the operating efficiency of the distribution network and the penetration rate of new energy.

[0014] Preferably, the power regulation mode of the flexible interconnected distribution network is applied to the distribution network Internet, using the PQ control method to control the output power of the FMS; the emergency control mode is applied to the abnormality or failure of the upper power grid, using the U ac -f control mode, keeping the distribution network voltage / frequency unchanged.

[0015] Preferably, MMC1 in FMS adopts PQ control mode, and MMC2 adopts U dc -Q control mode.

[0016] Preferably, the current reference value of the PQ control mode is:

[0017]

[0018] Among them, I dref ,I qref is the output current reference value of the flexible multi-state switch controller of the PQ control mode in the dq coordinate system; k pnis the proportional term coefficient in the flexible multi-state switch controller, kin is the integral term coefficient in the flexible multi-state switch controller, and n is 1, 2, 3, 4; P is the actual system active power, Q is the actual system reactive power, both of which are measured and collected quantities, P ref is the reference active power, Q ref is the reference reactive power;

[0019] U ac -f control mode current reference value:

[0020]

[0021] Among them, i dref 、i qref is U in the dq coordinate system ac -f current reference value of control mode; U pccd , U pccq It is the conversion value of the actual voltage at the bus connection point projected to the dq coordinate system after coordinate transformation. Both are measured and collected quantities; U dref , U pref is the port reference voltage of the flexible multi-state switch controller in the dq coordinate system.

[0022] Preferably, the establishment of a flexible multi-state switch mode emergency switching seamless control strategy includes:

[0023] When a fault occurs, the FMS receives an emergency mode switching command from the intelligent power distribution system and switches from the PQ control mode to the U ac -f control mode;

[0024] Establish a seamless switching control loop of the steady-state inverse model to predict the steady-state operating point after the fault:

[0025]

[0026] Among them, i d 、i q is the steady-state operating point after the fault; ΔP and ΔQ are the powers provided by the distribution network under normal operation.

[0027] Preferably, the evaluating whether the power provided by the FMS after the fault meets the current load demand includes:

[0028] The power provided by the FMS includes the rated power of the FMS and the mutual aid power of the opposite power grid; wherein the mutual aid power of the opposite power grid is the difference between the power when the opposite line is critically overloaded and the current power;

[0029] The current load demand is the sum of the loads on the faulty feeder, marked as the transfer load;

[0030] Determine whether the transferred load exceeds the rated power of the FMS and the mutual aid power of the opposite power grid; if so, the power provided by the FMS after the fault does not meet the current load demand; if not, the power provided by the FMS after the fault meets the current load demand.

[0031] Preferably, the selective power supply according to the importance of the load includes:

[0032] The load is divided into three levels, namely, primary load, secondary load and tertiary load; the importance of the load is primary load > secondary load > tertiary load;

[0033] Establish the FMS hierarchical power transfer control loop to transfer the loads in different levels according to the power requirements of each level of loads;

[0034]

[0035] Among them, P o is the hierarchical transfer control function; P all The total power provided by FMS; P load1 , P load2 , P load3 They are the power requirements of primary load, secondary load and tertiary load respectively.

[0036] It should be noted that primary loads are the most important, and they have extremely high requirements for the continuity and reliability of power supply, such as hospitals, data centers, critical infrastructure, etc.

[0037] The present invention can realize seamless switching of distribution networks in grid-connected and island modes, and can ensure safe and stable operation of the system regardless of small disturbances or major faults. In the event of disturbances or faults, the seamless switching of multiple working modes can be realized through the seamless switching control loop, reducing the fluctuations and impacts caused by the switching; and during the troubleshooting period, the hierarchical transfer control loop can be used to realize the reasonable use of limited power resources and ensure the normal operation of key facilities.

[0038] Preferably, the total power provided by the FMS is determined based on the mutually available power size when the opposite side is not overloaded and the rated power size of the FMS;

[0039] P all =min(P help +P FMS )

[0040] Among them, P help P is the power that can be mutually supplemented when the opposite side is not overloaded. FMS is the rated power of the FMS.

[0041] Preferably, the second aspect of the present invention is a flexible interconnected distribution network load classification seamless transfer system, comprising an emergency switching control module and a distribution network construction module and a load classification control module connected thereto;

[0042] The distribution network construction module builds a flexible interconnected distribution network architecture based on FMS;

[0043] Emergency switching control module: used to establish a seamless control strategy for emergency switching of flexible multi-state switch modes, minimizing fluctuations and impacts of emergency switching of modes;

[0044] The load classification control module is used to establish a flexible multi-state switch load classification control strategy.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] 1. The present invention comprehensively considers the coordinated control of microgrid and FMS, and solves the problem that the traditional control method cannot effectively deal with the switching delay and source-to-load ratio changes during the emergency switching process. The method optimizes the dynamic response characteristics of voltage and frequency, shortens the adjustment time, ensures the stability of electrical quantities during the emergency switching process, and avoids the problem of electrical quantity exceeding the limit caused by disturbance or fault;

[0047] 2. The present invention supports flexible access to distribution networks of different sizes and types. Through modular control logic, the system is easy to expand and upgrade to meet the needs of future power grid development. When a power grid abnormality occurs, the strategy can quickly identify and isolate the fault area while maintaining continuous power supply to the non-fault area, significantly improving the system's self-healing ability and power supply reliability. Through refined power regulation and optimized configuration, the strategy helps to improve the energy utilization efficiency of the entire distribution network, reduce energy loss, and embody the characteristics of sustainable development. During emergency switching, the strategy can give priority to ensuring the power supply of critical loads, ensuring the uninterrupted operation of key facilities such as hospitals and data centers, and has important social and economic value. It not only focuses on the stability and reliability of the system, but also considers economic and environmental impacts, supports the optimal balance between multiple goals of the smart grid, and provides technical support for the comprehensive development of the smart grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0049] Figure 1 It is a schematic diagram of the process of the present invention;

[0050] Figure 2 A typical topological diagram of the flexible interconnection distribution network of the present invention;

[0051] Figure 3 A control block diagram for mode switching of the present invention;

[0052] Figure 4 This is a simulation verification diagram of the example of the present invention. DETAILED DESCRIPTION

[0053] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0054] See also Figure 1 The first aspect of the present invention provides a method for seamless load transfer in a flexible interconnected distribution network, comprising the following steps:

[0055] A flexible interconnected distribution network architecture is constructed based on FMS; the operation modes of the flexible interconnected distribution network include power regulation mode and emergency control mode;

[0056] See also Figure 2 ,Typical topology of flexible interconnected distribution network.,Distribution network (a) and distribution network (b) are interconnected through flexible multi-state switch (FMS),,power can flow bidirectionally between sub-distribution networks.,The closing or shutting down of PCC switch determines the grid-connected or islanded operation mode of,the microgrid.

[0057] Specifically, the FMS is composed of a back-to-back converter, which includes a converter, a common DC capacitor, a power grid and a circuit breaker switch; wherein the converter is arranged at both sides of the FMS, respectively marked as MMC1 and MMC2; the common DC capacitor is marked as C0; the power grid is arranged at both ends of the FMS, respectively marked as AC1 and AC2; the circuit breaker switches are respectively marked as K1 and K2, and the solid line represents the power flow, and the dotted line represents the information communication;

[0058] When K1 and K2 are closed, power exchange can be achieved between the distribution networks on both sides of the FMS. When the upper power grid on one side of the FMS loses power due to an abnormality or fault, the intelligent distribution system sends a switching command to the FMS, K1 or K2 is disconnected, and the FMS switches to the Uac-f control mode to provide voltage and frequency support to the distribution network of PCC1 or PCC2, thereby achieving uninterrupted power supply to the distribution network and safe and stable operation of distributed power sources.

[0059] The power regulation mode of the flexible interconnected distribution network is applied to the distribution network Internet, using the PQ control method to control the output power of the FMS; the emergency control mode is applied to the abnormality or failure of the upper power grid, using the U ac -f control mode, keeping the distribution network voltage / frequency unchanged.

[0060] Establish a flexible multi-state switch mode emergency switching seamless control strategy to minimize mode emergency switching fluctuations and impacts;

[0061] Specifically, in order to minimize the fluctuation and impact of emergency mode switching, a seamless control strategy for mode switching is proposed. MMC1 in FMS adopts PQ control mode to achieve accurate power exchange between distribution networks, and MMC2 adopts U dc -Q control mode is used to maintain DC bus voltage stability and power balance of the entire system.

[0062] Current reference value of PQ control mode:

[0063]

[0064] Among them, I dref ,I qref is the output current reference value of the flexible multi-state switch controller of the PQ control mode in the dq coordinate system; k pn is the proportional term coefficient in the flexible multi-state switch controller, kin is the integral term coefficient in the flexible multi-state switch controller, and n is 1, 2, 3, 4; P is the actual system active power, Q is the actual system reactive power, both of which are measured and collected quantities, P ref is the reference active power, Q ref is the reference reactive power, and both the reference active power and the reference reactive power are set by those skilled in the art.

[0065] When a fault occurs, the FMS receives the emergency mode switching command from the intelligent power distribution system after a delay; the FMS mode switching control mode changes at the moment of switching, and the FMS switches from the PQ control mode to the U ac -f control mode.

[0066] U ac -f control mode current reference value:

[0067]

[0068] Among them, i dref 、i qref is U in the dq coordinate system ac -f current reference value of control mode; U pccd , U pccqIt is the conversion value of the actual voltage at the bus connection point projected to the dq coordinate system after coordinate transformation. Both are measured and collected quantities; U dref , U qref is the port reference voltage of the flexible multi-state switch controller in the dq coordinate system, and the reference voltage is set by those skilled in the art.

[0069] Establish a seamless switching control loop of the steady-state inverse model to predict the steady-state operating point after the fault:

[0070]

[0071] Among them, i d 、i q is the steady-state operating point after the fault; ΔP and ΔQ are the powers provided by the distribution network under normal operation.

[0072] It should be noted that the current loop PI regulator has reached a steady state before the control mode is switched, and the output signal is a steady-state value. After the switch, the current PI regulator needs to gradually adjust from the zero state to the steady state, which requires a certain adjustment time, thus generating a control switching shock. In order to speed up the FMS control switching adjustment speed, a seamless switching control loop based on a steady-state inverse model is proposed. The power ΔP and ΔQ required by the FMS are obtained according to the source-load imbalance of the power-lost feeder. The power value can be directly measured by the line measurement device, that is, the power size provided by the fault distribution network under normal circumstances, combined with the voltage reference value U dref , U qref Calculation; This formula is equivalent to predicting the steady-state operating point after the fault, compensating the difference between the zero state and the steady state, so it can directly provide the command value for the current inner loop and shorten the adjustment time. Among them, the control block diagram of the mode switching is as follows Figure 3 shown.

[0073] Establish a load classification control strategy for flexible multi-state switches;

[0074] When a failure occurs in the distribution network, it is evaluated whether the power provided by the FMS after the failure meets the current load demand. If yes, normal power supply is provided; if not, selective power supply is provided based on the importance of the load.

[0075] It should be noted that in the distribution network, once a fault occurs, the load of the faulty line must be transferred through the FMS fault. In this emergency situation, first evaluate whether the power that the FMS can provide after the fault is sufficient to support the current load demand;

[0076] And because the opposite power grid is still connected to the fault-side busbar through the FMS, the opposite distribution network can provide mutual aid power under certain circumstances, which is usually provided by the excess power of the opposite power grid when its own load is met.

[0077] Specifically, the scale of the transferred load is evaluated; the load size that needs to be transferred is equal to the total load on the faulty feeder, while taking into account the rated power PFMS of the FMS and the mutual aid power that the opposite power grid can provide; the mutual aid power size is the difference between the power at the critical overload of the opposite line and the current power.

[0078] Determine whether the transferred load exceeds the rated power of the FMS and the mutual aid power of the opposite power grid; if so, the power provided by the FMS after the fault does not meet the current load demand; if not, the power provided by the FMS after the fault meets the current load demand.

[0079] The load is divided into three levels, namely, primary load, secondary load and tertiary load; the importance of the load is primary load > secondary load > tertiary load;

[0080] For example, primary loads are the most important, and they have extremely high requirements for the continuity and reliability of power supply. These loads may include hospitals, data centers, critical infrastructure, etc., which require uninterrupted power supply under any circumstances. Next, these loads need to be graded and transferred according to the power that the FMS can provide. First, the power supply of primary loads must be ensured, and then the secondary and tertiary loads are gradually considered based on the remaining power supply capacity; secondary loads include commercial areas, industrial production, etc., while tertiary loads may be residential areas and other non-critical areas.

[0081] Establish the FMS hierarchical power transfer control loop to transfer the loads in different levels according to the power requirements of each level of loads;

[0082]

[0083] Among them, P o is the hierarchical transfer control function; P all The total power provided by FMS; P load1 , P load2 , P load3 They are the power requirements of primary load, secondary load and tertiary load respectively.

[0084] The total power provided by the FMS is determined based on the power that can be mutually supported without overloading the opposite side and the rated power of the FMS.

[0085] P all =min(P help +P FMS )

[0086] Among them, P help P is the power that can be mutually supplemented when the opposite side is not overloaded. FMS is the rated power of the FMS.

[0087] A second aspect of the present invention is a flexible interconnected distribution network load classification seamless transfer system, comprising an emergency switching control module and a distribution network construction module and a load classification control module connected thereto;

[0088] The distribution network construction module builds a flexible interconnected distribution network architecture based on FMS;

[0089] Emergency switching control module: used to establish a seamless control strategy for emergency switching of flexible multi-state switch modes, minimizing fluctuations and impacts of emergency switching of modes;

[0090] The load classification control module is used to establish a flexible multi-state switch load classification control strategy.

[0091] Compared with the hard switching process of the distribution network mode, the present invention can greatly reduce the frequency and voltage deviations in the switching process, shorten the recovery time, ensure the stability of electrical quantities in the emergency switching process, and avoid the problem of electrical quantity exceeding the limit due to disturbance or fault; the flexible interconnected distribution network structure and operation mode can realize the coordination and complementarity of FMS and distribution network control capabilities. The proposed strategy can realize seamless switching of distribution network modes, which is conducive to the safe and stable operation of DG with large-scale access and clustered consumption, and is of great significance to improving the continuous and stable operation capability of the new distribution network based on flexible interconnection technology.

[0092] On this basis, it is also necessary to implement sophisticated power control strategies. This may include adjusting voltage, frequency and phase, as well as using advanced load management techniques to optimize the allocation of power resources. Through these measures, the power supply to non-critical loads can be minimized without sacrificing the power supply to critical loads, thereby maintaining the stable operation of the power grid during faults.

[0093] Example simulation verification:

[0094] In order to verify the effectiveness of the proposed seamless transfer method, a Figure 2 The flexible interconnected distribution network operation scenario shown in the figure. The system is set to operate normally from 0 to 1.8 seconds, a 10kV feeder fails at 1.8 seconds, the PCC switch of the power grid is disconnected at 2 seconds, the FMS switches the control mode after the switching delay, and the grid is reconnected for 5 seconds. The source-load ratio is defined as the active power ratio of DG to load. Taking the energy network parameters of a university as an example, the parameters are shown in Table 1.

[0095] Table 1 Flexible interconnection distribution network operation parameters

[0096]

[0097]

[0098] Simulation scenario: system failure

[0099] like Figure 4 As shown in Figures (a) and (b), 1.8s after the fault occurs, the distribution network voltage drops rapidly, and the FMS transmission power becomes unstable. At 2s, the PCC switch is disconnected, and the distribution network enters a transition state. Within a 0.2s delay, since the source is smaller than the load, the voltage and frequency of the distribution network continue to drop, and the FMS transmission power remains at the initial value. At 2.2s, the distribution network enters the emergency power supply mode, and the voltage and frequency drop to the minimum. The FMS provides voltage and frequency support to the distribution network, and the voltage and frequency gradually recover. The FMS output power is the source-load difference of 0.3MW. Figure 4 As shown in the middle figure (c), after adopting the strategy proposed in this paper, the voltage drop recovers quickly and the frequency fluctuation is greatly reduced in the transition state of the distribution network. The voltage and frequency have a small impact at the switching moment, which is much smaller than the impact of hard switching of the distribution network mode.

[0100] Part of the data in the above formula is calculated by removing the dimension and taking its numerical value. The formula is a formula closest to the actual situation obtained by software simulation of a large amount of collected data; the preset parameters and preset thresholds in the formula are set by technical personnel in this field according to actual conditions or obtained through simulation of a large amount of data.

[0101] The above embodiments are only used to illustrate the technical method of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical method of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical method of the present invention.

Claims

1. A method for seamless load transfer in a flexible interconnected distribution network, characterized in that: The following steps are involved: A flexible interconnected distribution network architecture is constructed based on FMS; the operation modes of the flexible interconnected distribution network include power regulation mode and emergency control mode; Establish a flexible multi-state switch mode emergency switching seamless control strategy to minimize mode emergency switching fluctuations and impacts; Establish a load classification control strategy for flexible multi-state switches; When a failure occurs in the distribution network, it is evaluated whether the power provided by the FMS after the failure meets the current load demand; if yes, normal power supply is provided; if not, selective power supply is provided based on the importance of the load.

2. A method for seamless load transfer in a flexible interconnected distribution network according to claim 1, characterized in that: The flexible interconnection distribution network architecture based on FMS includes: The FMS is composed of a back-to-back converter, which includes a converter, a common DC capacitor, a power grid and a circuit breaker switch; the converter is arranged on both sides of the FMS, respectively marked as MMC1 and MMC2; the common DC capacitor is marked as C0; the power grid is arranged at both ends of the FMS, respectively marked as AC1 and AC2; the circuit breaker switches are respectively marked as K1 and K2, and the solid line represents the power flow, and the dotted line represents the information communication; When K1 and K2 are closed, power exchange can be achieved between the distribution networks on both sides of the FMS. When the upper power grid on one side of the FMS loses power due to an abnormality or fault, the intelligent distribution system sends a switching command to the FMS, K1 or K2 is disconnected, and the FMS switches to the Uac-f control mode to provide voltage and frequency support to the distribution network of PCC1 or PCC2.

3. A method for seamless load transfer in a flexible interconnected distribution network according to claim 2, characterized in that: The power regulation mode of the flexible interconnected distribution network is applied to the distribution network Internet, using the PQ control method to control the output power of the FMS; the emergency control mode is applied to the abnormality or failure of the upper power grid, using the U ac -f control mode, keeping the distribution network voltage / frequency unchanged.

4. A method for seamless load transfer in a flexible interconnected distribution network according to claim 2, characterized in that: MMC1 in FMS adopts PQ control mode, and MMC2 adopts U dc -Q control mode.

5. A method for seamless load transfer in a flexible interconnected distribution network according to claim 3 or 4, characterized in that: Current reference value of PQ control mode: Among them, I dref ,I qref is the output current reference value of the flexible multi-state switch controller of the PQ control mode in the dq coordinate system; k pn is the proportional term coefficient in the flexible multi-state switch controller, kin is the integral term coefficient in the flexible multi-state switch controller, and n is 1, 2, 3, 4; P is the actual system active power, Q is the actual system reactive power, both of which are measured and collected quantities, P ref is the reference active power, Q ref is the reference reactive power; U ac -f control mode current reference value: Among them, i dref 、i qref is U in the dq coordinate system ac -f current reference value of control mode; U pccd , U pccq It is the conversion value of the actual voltage at the bus connection point projected to the dq coordinate system after coordinate transformation. Both are measured and collected quantities; U dref , U qref is the port reference voltage of the flexible multi-state switch controller in the dq coordinate system.

6. A method for seamless load transfer in a flexible interconnected distribution network according to claim 1, characterized in that: The establishment of a flexible multi-state switch mode emergency switching seamless control strategy includes: When a fault occurs, the FMS receives an emergency mode switching command from the intelligent power distribution system and switches from the PQ control mode to the U ac -f control mode; Establish a seamless switching control loop of the steady-state inverse model to predict the steady-state operating point after the fault: Among them, i d 、i q is the steady-state operating point after the fault; ΔP and ΔQ are the powers provided by the distribution network under normal operation.

7. A method for seamless load transfer in a flexible interconnected distribution network according to claim 1, characterized in that: The evaluation of whether the power provided by the FMS after the fault meets the current load demand includes: The power provided by the FMS includes the rated power of the FMS and the mutual aid power of the opposite power grid; wherein the mutual aid power of the opposite power grid is the difference between the power when the opposite line is critically overloaded and the current power; The current load demand is the sum of the loads on the faulty feeder, marked as the transfer load; Determine whether the transferred load exceeds the rated power of the FMS and the mutual aid power of the opposite power grid; if so, the power provided by the FMS after the fault does not meet the current load demand; if not, the power provided by the FMS after the fault meets the current load demand.

8. A method for seamless load transfer in a flexible interconnected distribution network according to claim 7, characterized in that: The selective power supply according to the importance of the load includes: The load is divided into three levels, namely, primary load, secondary load and tertiary load; the importance of the load is primary load > secondary load > tertiary load; Establish the FMS hierarchical power transfer control loop to transfer the loads in different levels according to the power requirements of each level of loads; Among them, P o is the hierarchical transfer control function; P all The total power provided by FMS; P load1 , P load2 , P load3 They are the power requirements of primary load, secondary load and tertiary load respectively.

9. A method for seamless load transfer in a flexible interconnected distribution network according to claim 8, characterized in that: The total power provided by the FMS is determined based on the mutually available power without overloading the opposite side and the rated power of the FMS; P all =min(P help +P FMS ) Among them, P help P is the power that can be mutually supplemented when the opposite side is not overloaded. FMS is the rated power of the FMS.

10. A flexible interconnected distribution network load classification seamless power transfer system, based on a flexible interconnected distribution network load classification seamless power transfer method according to any one of claims 1 to 9, characterized in that: It includes an emergency switching control module and a distribution network construction module and a load classification control module connected thereto; The distribution network construction module builds a flexible interconnected distribution network architecture based on FMS; Emergency switching control module: used to establish a seamless control strategy for emergency switching of flexible multi-state switch modes, minimizing fluctuations and impacts of emergency switching of modes; The load classification control module is used to establish a flexible multi-state switch load classification control strategy.

Images