Control method for flexible interconnection device of medium and low voltage power distribution network

Abstract

The invention provides a control method of a flexible interconnection device of a medium and low voltage distribution network, the flexible interconnection device adopts a back-to-back voltage source type converter topological structure, and the flexible interconnection device comprises converters at two sides of a direct current bus, a direct current support capacitor, a filter circuit, a measuring unit and a control unit, in addition, an electrical quantity measurement and preprocessing module, a power instruction calculation module, a converter double-closed-loop control module, a fault detection module, a ride-through control module and a multi-terminal distributed coordination module are arranged. According to the invention, high-precision quick-response power control is realized, a distributed coordination control mechanism of the multi-terminal flexible interconnection device is established, and the fault identification and ride-through capabilities are enhanced.

Description

Technical Field

[0001] This invention belongs to the technical field of flexible interconnection devices for distribution networks, and specifically relates to a control method for a flexible interconnection device for medium and low voltage distribution networks. Background Technology

[0002] With the large-scale integration of distributed power sources into the distribution network, the traditional radial distribution network structure faces problems such as insufficient power supply reliability and limited load transfer capacity. Flexible interconnection devices, as a new type of power electronic equipment, can achieve flexible power transmission between different feeders, but existing technologies mainly have the following shortcomings:

[0003] 1) Low power control accuracy, making it difficult to adapt to dynamic load changes;

[0004] Traditional flexible interconnection devices mostly adopt power control strategies based on steady-state power flow, and control is carried out through preset power transmission plans. This method can work normally when the load is stable, but when the distribution network load fluctuates rapidly, the output of distributed power sources changes suddenly, or during fault transients, the control system response is lagging and cannot adjust the power transmission amount in time.

[0005] 2) Lack of multi-terminal coordination and control capabilities, and insufficient overall optimization of the interconnected system;

[0006] Existing research mainly focuses on control strategies for individual flexible interconnected devices. When there are multiple flexible interconnected devices in a distribution network, there is a lack of effective information exchange and coordination mechanisms between the devices. Each device makes independent decisions based on local information, which may lead to problems such as power circulating current and control conflicts.

[0007] 3) Insufficient fault-crossing capability; system safety and stability need to be improved.

[0008] During distribution network faults, flexible interconnection devices need to possess low-voltage ride-through capability to maintain stable system operation. Existing devices have shortcomings in fault detection, fault type identification, and fault response strategies. Fault detection mainly relies on threshold judgments of voltage or current amplitude, which is easily affected by normal operations such as load changes and capacitor switching, leading to misjudgments. Fault identification methods are simplistic, making it difficult to distinguish between symmetrical and asymmetrical faults, and unable to adopt differentiated control strategies for different fault types. During fault response, most devices adopt a conservative strategy of directly blocking and shutting down operation, failing to fully utilize their ability to support system voltage and isolate the impact of faults. Summary of the Invention

[0009] To overcome the shortcomings of the prior art, the present invention discloses a control method for a flexible interconnection device for medium and low voltage power distribution networks.

[0010] The technical solution provided by this invention is: a control method for a flexible interconnection device for medium and low voltage distribution networks. The flexible interconnection device adopts a back-to-back voltage source converter topology. The flexible interconnection device includes: converters on both sides of the DC bus, DC support capacitors, filter circuits, measurement units, and control units. In addition, it is equipped with an electrical quantity measurement and preprocessing module, a power command calculation module, a converter dual closed-loop control module, a fault detection module, a ride-through control module, and a multi-terminal distributed coordination module.

[0011] The control method is characterized by comprising the following steps:

[0012] 1) Configure flexible interconnect device parameters based on hardware topology;

[0013] Set the rated capacity of the flexible interconnect device;

[0014] 2) The electrical quantity measurement and preprocessing module collects and measures the electrical quantities of the feeders on both sides, and obtains the feeder operating status parameters through digital filtering;

[0015] The electrical quantity measurement and preprocessing module collects electrical quantities of both feeders in real time, including three-phase voltage, three-phase current, high-frequency noise and switching harmonics. The collected electrical quantities are sent to the preprocessing module for digital filtering to obtain the operating status parameters of both feeders.

[0016] 3) Calculate the power transmission command based on the operating status parameters of the negative two-sided feeders;

[0017] Input the operating status parameters of the two feeders obtained in step 2) into the power command calculation module to calculate the power command that the flexible interconnection device should transmit. The command control objectives include two aspects: load balancing and voltage regulation.

[0018] 4) The converter's dual closed-loop control module converts power commands into modulation signals;

[0019] The power command obtained in step 3) is sent to the converter dual closed-loop control module and converted into a current command. The converter dual closed-loop control module adopts a dual closed-loop mode of current inner loop and power outer loop. The modulation voltage is obtained by calculating the modulation voltage command based on the current error of the inner and outer loops. The modulation voltage generates a modulation signal through space vector PWM. The output voltage is precisely controlled by controlling the action time of each vector.

[0020] 5) The fault detection module performs rapid fault detection based on the modulation signal;

[0021] When a fault occurs in the distribution network, the flexible interconnection device can quickly identify the fault through the fault detection module, transmit the fault detection results to the fault ride-through control module, and take protective measures according to the severity of the fault.

[0022] 6) Cross-traffic control module control strategy;

[0023] The fault detection module adopts a control strategy based on the obtained fault detection results;

[0024] 7) Distributed coordinated control of multi-terminal flexible interconnected devices;

[0025] Multiple flexible interconnected devices exchange their respective power transmission status and optimization objectives through the communication network in the multi-terminal distributed coordination module, and use a consensus algorithm to iteratively calculate the globally optimal power allocation scheme to achieve system-level coordination optimization.

[0026] Preferably, in step 1), there are two converters, each consisting of three bridge arms. Each bridge arm includes two upper and lower insulated gate bipolar transistor power switch modules. The three-phase AC side of converter 1 is connected to feeder 1 through an LCL filter, and the three-phase AC side of converter 2 is connected to feeder 2 through an LCL filter. The DC side uses capacitor energy storage to maintain DC voltage stability.

[0027] Further preferred, the rated capacity calculation in step 1) includes: rated DC-side voltage, DC-side supporting capacitor capacity, converter-side inductance, and harmonic resonant frequency, wherein,

[0028] The rated DC side voltage is determined based on the AC side voltage, and the calculation formula is as follows:

[0029] (1)

[0030] In the formula, The rated DC voltage (V). This is the effective value (V) of the AC side line voltage. This represents the modulation coefficient margin.

[0031] The DC-side supporting capacitor capacity is determined based on power fluctuation and voltage fluctuation requirements, and the calculation formula is as follows:

[0032] (2)

[0033] In the formula, This represents the DC-side capacitance value (F). The rated power of the device is (W). To meet power response time requirements, The allowable DC voltage fluctuation;

[0034] The LCL filter parameters are designed based on the switching frequency and harmonic suppression requirements. The converter-side inductor... The main limitation is the current ripple at the switching frequency, calculated using the following formula:

[0035] (3)

[0036] In the formula, For switching frequency, Allowable current ripple;

[0037] Grid-side inductance With capacitor The formula for calculating the harmonic resonant frequency of a harmonic notch filter is as follows:

[0038] (4)

[0039] Further optimization, in step 2), the electrical quantity measurement and preprocessing module includes: a measurement unit and a control unit. The measurement unit includes a voltage transformer and a current transformer, which measure the three-phase voltage and three-phase current of the feeders on both sides, respectively. The measurement signals are digitized after passing through the signal conditioning circuit and sent to the controller. The control unit adopts a DSP+FPGA architecture. The DSP is responsible for complex power calculation and coordinated control algorithms, while the FPGA is responsible for high-speed PWM (pulse width modulation) generation and protection logic.

[0040] Further optimization, in step 2), the calculation method for the operating status parameters of the feeders on both sides is as follows:

[0041] The electrical quantity measurement and preprocessing module uses a second-order low-pass filter to eliminate high-frequency noise and acquires the three-phase voltage of the feeders on both sides in real time. and three-phase current ;

[0042] (5)

[0043] In the formula, This is the filtered signal. The original sampled signal, Forward coefficients, The feedback coefficients are determined using the Butterworth filter design method.

[0044] The filtered voltage and current signals are transformed into a synchronously rotating dq coordinate system using the Park transform:

[0045] (6)

[0046] In the formula, For the dq axis voltage components, To synchronize the rotation angle, a phase-locked loop (PLL) is used to extract the current signal from the grid voltage, and the current signal is also converted into a synchronous rotation angle. ;

[0047] The phase-locked loop adopts a second-order generalized integrator (SOGI) structure.

[0048] Calculate the active and reactive power of the feeders on both sides:

[0049] (7)

[0050] (8)

[0051] In the formula, Let be the active power (W) of feeder k. Reactive power (Var), subscript These represent feeder 1 and feeder 2, respectively.

[0052] Calculate the load factor of each feeder:

[0053] (9)

[0054] In the formula, Let k be the load factor of the feeder. Where k is the rated capacity of the feeder;

[0055] Voltage deviation is calculated as follows:

[0056] (10)

[0057] In the formula, For voltage deviation, Voltage amplitude, Rated voltage;

[0058] The calculation results of these electrical quantities ( This will be used as input for the next step of power command calculation.

[0059] Further optimization, in step 3), the power transfer command calculation method is as follows:

[0060] The load balancing objective function is defined as minimizing the difference in load rates between the two feeders:

[0061] (11)

[0062] In the formula, The objective function for load balancing;

[0063] The voltage regulation objective function is defined as minimizing the sum of squares of the voltage deviations between the two feeders:

[0064] (12)

[0065] In the formula, The objective function for voltage regulation is... The weighting coefficient is determined based on the importance of the feeder.

[0066] The overall objective function is:

[0067] (13)

[0068] In the formula, The balance coefficient is dynamically adjusted according to the current operating status. When the voltages on both sides are within the acceptable range, load balancing is the primary method; when the voltage on either side exceeds the limit, voltage regulation is the primary method.

[0069] Active power transmission commands are calculated using the gradient descent method:

[0070] (14)

[0071] In the formula, It is an active power transfer command (W). As the learning rate, the partial derivative of the objective function with respect to power is calculated using numerical differentiation methods:

[0072] (15)

[0073] In the formula, For small power disturbances;

[0074] To avoid power oscillations, the power command rate is limited:

[0075] (16)

[0076] In the formula, This represents the upper limit of the rate of change of power.

[0077] The reactive power transmission command is determined based on voltage regulation requirements. When the voltage of feeder 1 is low and the voltage of feeder 2 is high, the device sends reactive power to feeder 1.

[0078] (17)

[0079] In the formula, This is a reactive power transmission command. This is the voltage regulation coefficient;

[0080] Calculated and It will be sent to the converter's dual closed-loop control module for execution.

[0081] Further optimization, in step 4), the modulation signal is calculated as follows:

[0082] The power outer loop will control the power command. , Convert to dq axis current command. For converter 1 (connected to feeder 1), set the current reference value according to the power transmission direction:

[0083] (18)

[0084] (19)

[0085] In the formula, The dq axis current command (A) for converter 1;

[0086] For converter 2, to maintain DC voltage stability, its active current command is calculated by the DC voltage outer loop controller using the following formula:

[0087] (20)

[0088] In the formula, This is the proportional coefficient of the DC voltage loop. The integral coefficient is... This is the DC voltage reference value. To measure the DC voltage, the DC voltage is obtained through a DC-side voltage sensor.

[0089] The reactive current command for converter 2 is:

[0090] (twenty one)

[0091] In the formula, The q-axis current command (A) for converter 2;

[0092] The inner current loop uses a PI controller to calculate the modulation voltage command based on the current error.

[0093] (twenty two)

[0094] (twenty three)

[0095] In the formula, The dq axis modulation voltage (V) of converter k. This is the proportionality coefficient of the current loop. The integral coefficient is... For the power grid angular frequency, cross-coupling terms and Used to achieve decoupled control of the d-axis and q-axis;

[0096] The modulated voltage is transformed back to the abc coordinate system via inverse Park transform:

[0097] (twenty four)

[0098] In the formula, It is the three-phase modulated voltage (V).

[0099] Further optimization, in step 5), the protection measure is a current-limiting protection measure, and the calculation method for implementing the current-limiting protection measure is as follows:

[0100] The voltage jump is defined as the difference in voltage between adjacent sampling points:

[0101] (25)

[0102] In the formula, Let V be the voltage fluctuation at the nth sampling point. The current sampled voltage;

[0103] Define the current surge as:

[0104] (26)

[0105] In the formula, Let be the current surge (A) at the nth sampling point. The current sampling current amplitude ( );

[0106] The fault detection criteria are set as follows: both voltage surges and current surges exceed a threshold.

[0107] (27)

[0108] In the formula, This is the voltage change threshold. The threshold for sudden current change;

[0109] When the criteria are met, the fault identification process is initiated to calculate the positive-sequence voltage and negative-sequence voltage to determine the fault type.

[0110] (28)

[0111] (29)

[0112] In the formula, It is the positive sequence voltage (V). It is a negative sequence voltage (V). For rotation factor, The three-phase instantaneous voltages are obtained from the inverse Park transformation in step 2);

[0113] The unbalance is defined as the ratio of negative-sequence voltage to positive-sequence voltage:

[0114] (30)

[0115] In the formula, The degree of imbalance is used to determine the type of fault: when When, it is determined to be a symmetrical fault (three-phase short circuit); when When this occurs, it is determined to be an asymmetrical fault (two-phase short circuit or single-phase grounding).

[0116] Further optimization, in step 6), the implementation method of the control strategy is as follows:

[0117] For symmetrical faults, a current-limiting control strategy is adopted to limit the current command within the allowable range:

[0118] (31)

[0119] (32)

[0120] In the formula, The current command (A) during the fault period. This is the maximum allowable current of the device;

[0121] Simultaneously, reactive power is injected into the grid to support the voltage, and the reactive current command is determined based on the voltage sag depth:

[0122] (33)

[0123] In the formula, The reactive power support current is (A). Low voltage ride-through gain coefficient;

[0124] For asymmetrical faults, in addition to current limiting control, it is also necessary to suppress negative sequence current. Positive and negative sequence separation control is adopted to decompose the current into positive sequence and negative sequence components:

[0125] (34)

[0126] (35)

[0127] In the formula, These are the positive and negative sequence currents (A) along the d-axis, respectively.

[0128] The negative sequence current command is set to zero to reduce negative sequence contamination of the system.

[0129] (36)

[0130] The positive sequence current command is determined according to formulas (31)-(33);

[0131] When the fault duration exceeds the threshold Or the current exceeds the hardware protection limit. When the device is locked out of operation, the feeders on both sides are disconnected via the bypass switch:

[0132] (37)

[0133] In the formula, The fault duration is obtained by accumulating the control cycle using a counter.

[0134] Fault ride-through control, through current limiting and reactive power support, enables flexible interconnect devices to maintain operation as much as possible without damage, thereby improving system safety. Current limiting value... and reactive power support current The calculation result is sent back to the current command in step 4), replacing the power command in step 3) during normal operation.

[0135] Further optimization involves using a consensus algorithm for distributed coordination control in step 7), with the specific calculation method as follows:

[0136] Assume there are M flexible interconnected devices in the distribution network, and device i can communicate with its neighboring device j. The communication topology is represented by the adjacency matrix A:

[0137] (38)

[0138] In the formula, These are elements of the adjacency matrix;

[0139] Each device maintains a local state variable. This represents the power transmission amount of the device. The consensus algorithm iteratively updates the state variables of all devices to make them consistent.

[0140] (39)

[0141] In the formula, Let i be the state variable of device i during the k-th iteration. The coupling strength coefficient is represented by the summation term, which indicates the difference between the state of the device and that of the neighboring device.

[0142] The global optimization objective is to minimize the variance of the load factor of each feeder. The global objective function is defined as follows:

[0143] (40)

[0144] In the formula, Let N be the total number of distribution network feeders, and N be the global objective function. Let n be the load factor of feeder n. Average load factor;

[0145] The optimization variable for device i is its power transfer rate. The optimization problem is transformed into:

[0146] (41)

[0147] In the formula, The power transmission capacity limit for device i is 2MW.

[0148] The solution is obtained using distributed gradient descent, with each device calculating its local gradient.

[0149] (42)

[0150] Gradient information is exchanged using a consensus algorithm, and the global average gradient is calculated.

[0151] (43)

[0152] In the formula, For average gradient estimation;

[0153] Power update instruction based on average gradient:

[0154] (44)

[0155] In the formula, The step size.

[0156] The beneficial effects of this invention are:

[0157] 1) Achieved high-precision, fast-response power control:

[0158] A power transmission control strategy based on dual closed-loop vector control (Equation 18-24) is adopted. Through dq axis decoupling control and cross-coupling compensation (Equation 22-23), the mutual interference between active and reactive power control is eliminated, and independent adjustment is achieved. The control system can quickly adapt to dynamic disturbances such as load changes and distributed power output fluctuations, and maintain stable system operation.

[0159] 2) A comprehensive multi-terminal coordination and control mechanism has been established:

[0160] A distributed coordination control method based on consensus algorithm is proposed (Equations 39-44). Each device achieves information sharing and collaborative decision-making through proximity communication, eliminating the need for a central controller and avoiding the risk of single point of failure.

[0161] 3) Enhanced fault identification and traversal capabilities:

[0162] The fault detection method based on voltage and current mutations (Equations 25-27) has a fast detection speed and high accuracy, with an average detection time of 8ms. It accurately identifies the fault type by separating positive and negative sequences (Equations 28-30) and adopts differentiated control strategies for symmetrical and asymmetrical faults (Equations 31-36). During the fault, the device maintains operation and provides reactive power support, thereby improving the transient stability and power supply reliability of the distribution network. Attached Figure Description

[0163] Figure 1 A flowchart of a control method for a flexible interconnection device for medium and low voltage power distribution networks provided by the present invention. Detailed Implementation

[0164] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0165] Example

[0166] This invention provides a control method for a flexible interconnection device for medium and low voltage distribution networks. The flexible interconnection device adopts a back-to-back voltage source converter topology and includes: converters on both sides of the DC bus, DC support capacitors, filter circuits, measurement units, and control units. In addition, it is equipped with an electrical quantity measurement and preprocessing module, a power command calculation module, a converter dual closed-loop control module, a fault detection module, a ride-through control module, and a multi-terminal distributed coordination module.

[0167] The control method is characterized by comprising the following steps:

[0168] 1) Configure flexible interconnect device parameters based on hardware topology;

[0169] The flexible interconnection device adopts a back-to-back dual PWM voltage source converter structure. It achieves electrical isolation and power transmission between the AC systems on both sides through a shared DC bus. The rated capacity of the device is 2MVA, which is suitable for 10kV medium voltage distribution network. There are two converters, each consisting of three bridge arms. Each bridge arm contains two upper and lower insulated gate bipolar transistor power switch modules. The three-phase AC side of converter 1 is connected to feeder 1 through an LCL filter, and the three-phase AC side of converter 2 is connected to feeder 2 through an LCL filter. The DC side uses capacitor energy storage to maintain DC voltage stability.

[0170] The rated capacity of the flexible interconnect device is set, and the rated capacity calculation includes: rated DC-side voltage, DC-side supporting capacitor capacity, converter-side inductance, and harmonic resonant frequency. The rated DC-side voltage is determined based on the AC-side voltage.

[0171] (1)

[0172] In the formula, The rated DC voltage (V). This is the effective value (V) of the AC side line voltage. Assuming a modulation coefficient margin (taken as 1.2), for a 10kV system, the calculation is as follows: kV, but 17kV was actually selected;

[0173] The DC-side supporting capacitor capacity is determined based on power fluctuation and voltage fluctuation requirements:

[0174] (2)

[0175] In the formula, This represents the DC-side capacitance value (F). The rated power of the device is (W). The power response time requirement is 0.02s. The allowable DC voltage fluctuation (take) ), calculated mF, but a 30mF capacitor bank was actually selected;

[0176] The LCL filter parameters are designed based on the switching frequency and harmonic suppression requirements. The converter-side inductor... The main limiting factors are switching frequency and current ripple.

[0177] (3)

[0178] In the formula, The switching frequency is 10kHz. For the allowable current ripple (take 20% of the rated current), calculate as follows: mH, actually selected as 2mH;

[0179] Grid-side inductance With capacitor The harmonic resonant frequency of the harmonic notch filter is set as follows:

[0180] (4)

[0181] In the formula, The resonant frequency should be set between the switching frequency and the power frequency. kHz; Selected F, then the calculation is as follows mH;

[0182] The measurement unit includes a voltage transformer and a current transformer, which measure the three-phase voltage and three-phase current of the feeders on both sides, respectively. The voltage transformer has a transformation ratio of 10kV / 100V and an accuracy class of 0.2. The current transformer has a transformation ratio of 200A / 5A and an accuracy class of 0.5. After the measurement signal is processed by the signal conditioning circuit, it is digitized by a 16-bit A / D converter at a sampling frequency of 20kHz and then sent to the controller.

[0183] The control unit adopts a DSP+FPGA architecture. The DSP is responsible for complex power calculations and coordinated control algorithms, while the FPGA is responsible for high-speed PWM (Pulse Width Modulation) generation and protection logic. The control cycle is 100 seconds. The PWM switching cycle is 100. ;

[0184] 2) The electrical quantity measurement and preprocessing module collects and measures the electrical quantities of the feeders on both sides, and obtains the feeder operating status parameters through digital filtering;

[0185] The electrical quantity measurement and preprocessing module includes a measurement unit and a control unit. The measurement unit includes a voltage transformer and a current transformer, which measure the three-phase voltage and three-phase current of the feeders on both sides, respectively. The measurement signals are digitized after passing through the signal conditioning circuit and sent to the controller. The control unit adopts a DSP+FPGA architecture. The DSP is responsible for complex power calculation and coordinated control algorithms, while the FPGA is responsible for high-speed PWM (pulse width modulation) generation and protection logic.

[0186] The electrical quantity measurement and preprocessing module collects electrical quantities of both feeders in real time, including three-phase voltage, three-phase current, high-frequency noise and switching harmonics. The collected electrical quantities are sent to the preprocessing module for digital filtering to obtain the operating status parameters of both feeders.

[0187] The measurement unit acquires the three-phase voltage of the feeders on both sides in real time. and three-phase current The original sampled signal contains high-frequency noise and switching harmonics, which need to be digitally filtered.

[0188] A second-order low-pass filter is used to eliminate high-frequency noise.

[0189] (5)

[0190] In the formula, This is the filtered signal. The original sampled signal, These are the forward coefficients (taken as 0.0201, 0.0402, and 0.0201 respectively). The feedback coefficients (taken as -1.561 and 0.6414 respectively) are determined using the Butterworth filter design method, and the cutoff frequency is set to 1kHz.

[0191] The filtered voltage and current signals are transformed into a synchronously rotating dq coordinate system using the Park transform:

[0192] (6)

[0193] In the formula, For the dq axis voltage components, To synchronize the rotation angle, a phase-locked loop (PLL) is used to extract the current signal from the grid voltage, and the current signal is also converted into a synchronous rotation angle. ;

[0194] The phase-locked loop adopts a second-order generalized integrator (SOGI) structure, which can suppress the influence of grid voltage imbalance, with a phase-locking error of less than 0.5 degrees and a response time of less than 20ms;

[0195] Calculate the active and reactive power of the feeders on both sides:

[0196] (7)

[0197] (8)

[0198] In the formula, Let be the active power (W) of feeder k. Reactive power (Var), subscript These represent feeder 1 and feeder 2, respectively.

[0199] Calculate the load factor of each feeder:

[0200] (9)

[0201] In the formula, Let k be the load factor of the feeder. The rated capacity of feeder k (obtained from the distribution network topology database);

[0202] Voltage deviation is calculated as follows:

[0203] (10)

[0204] In the formula, For voltage deviation, Voltage amplitude, Rated voltage (10kV);

[0205] The calculation results of these electrical quantities ( This will be used as input for the next step of power command calculation;

[0206] 3) Calculate the power transmission command based on the operating status parameters of the negative two-sided feeders;

[0207] Input the operating status parameters of the two feeders obtained in step 2) into the power command calculation module to calculate the power command that the flexible interconnection device should transmit. The command control objectives include two aspects: load balancing and voltage regulation.

[0208] The load balancing objective function is defined as minimizing the difference in load rates between the two feeders:

[0209] (11)

[0210] In the formula, The objective function for load balancing;

[0211] The voltage regulation objective function is defined as minimizing the sum of squares of the voltage deviations between the two feeders:

[0212] (12)

[0213] In the formula, The objective function for voltage regulation is... The weighting coefficient is determined based on the importance of the feeder (feeders with important loads are assigned a higher weight, generally set to 1). );

[0214] The overall objective function is:

[0215] (13)

[0216] In the formula, This is a balance coefficient, dynamically adjusted based on the current operating status. When both voltages are within acceptable ranges ( When load balancing is the primary consideration, take... When the voltage on either side exceeds the limit, voltage regulation should be the primary method. ;

[0217] Active power transmission commands are calculated using the gradient descent method:

[0218] (14)

[0219] In the formula, It is an active power transfer command (W). The learning rate (set to 200) is used to calculate the partial derivative of the objective function with respect to power using numerical differentiation:

[0220] (15)

[0221] In the formula, For a small power disturbance (taken as 1kW);

[0222] To avoid power oscillations, the power command rate is limited:

[0223] (16)

[0224] In the formula, The upper limit of the power change rate (taken as 500kW / s);

[0225] The reactive power transmission command is determined based on voltage regulation requirements. When the voltage of feeder 1 is low and the voltage of feeder 2 is high, the device sends reactive power to feeder 1.

[0226] (17)

[0227] In the formula, It is a reactive power transfer command (Var). The voltage regulation coefficient (determined based on the system short-circuit capacity, taken as...) );

[0228] Calculated and The converter dual closed-loop control module sent to step 4) will be executed;

[0229] 4) The converter's dual closed-loop control module converts power commands into modulation signals;

[0230] The power command obtained in step 3) is sent to the converter dual closed-loop control module and converted into a current command. The converter dual closed-loop control module adopts a dual closed-loop mode of current inner loop and power outer loop. The modulation voltage is obtained by calculating the modulation voltage command based on the current error of the inner and outer loops. The modulation voltage generates a modulation signal through space vector PWM. The output voltage is precisely controlled by controlling the action time of each vector.

[0231] The modulated signal is calculated as follows:

[0232] The power outer loop will execute the power command in step 3). , Convert to dq axis current command. For converter 1 (connected to feeder 1), set the current reference value according to the power transmission direction:

[0233] (18)

[0234] (19)

[0235] In the formula, The dq axis current command (A) for converter 1;

[0236] For converter 2, in order to maintain DC voltage stability, its active current command is given by the DC voltage outer loop controller:

[0237] (20)

[0238] In the formula, This is the DC voltage loop proportionality factor (taken as 0.5). This is the integral coefficient (taken as 10). This is the reference value for DC voltage (17kV). The measured DC voltage (obtained via a DC-side voltage sensor).

[0239] The reactive current command for converter 2 is:

[0240] (twenty one)

[0241] In the formula, The q-axis current command (A) for converter 2;

[0242] The inner current loop uses a PI controller to calculate the modulation voltage command based on the current error.

[0243] (twenty two)

[0244] (twenty three)

[0245] In the formula, The dq axis modulation voltage (V) of converter k. This is the current loop proportionality factor (taken as 5). This is the integral coefficient (taken as 100). The angular frequency of the power grid ( rad / s), cross-coupling terms and Used to achieve decoupled control of the d-axis and q-axis;

[0246] The modulated voltage is transformed back to the abc coordinate system via inverse Park transform:

[0247] (twenty four)

[0248] In the formula, The three-phase modulation voltage (V);

[0249] The modulated voltage generates switching signals using a space vector PWM (SVPWM) algorithm. The SVPWM algorithm maps the three-phase modulated voltage to six effective voltage vectors and two zero vectors. Precise control of the output voltage is achieved by controlling the duration of each vector. The switching signals are generated by an FPGA, with a dead time set to 2 seconds. To prevent direct connection between switches on the same bridge arm;

[0250] The dual closed-loop control achieves precise control from the power command in step 3) to the actual power transmission through the fast current tracking of the inner loop and the power regulation of the outer loop. The current loop bandwidth is about 1kHz and the power loop bandwidth is about 50Hz, which meets the dynamic response requirements.

[0251] 5) The fault detection module performs rapid fault detection based on the modulation signal;

[0252] When a fault occurs in the distribution network, the flexible interconnection device can quickly identify the fault through the fault detection module, transmit the fault detection results to the fault ride-through control module, and take protective measures according to the severity of the fault.

[0253] Among them, the protection measure is a current limiting protection measure, and the calculation method for implementing the current limiting protection measure is as follows:

[0254] When a fault occurs in the distribution network, the voltage at the fault point drops sharply and the fault current increases dramatically. The device needs to quickly detect the fault and take protective measures.

[0255] The voltage jump is defined as the difference in voltage between adjacent sampling points:

[0256] (25)

[0257] In the formula, Let V be the voltage fluctuation at the nth sampling point. The current sampled voltage;

[0258] Define the current surge as:

[0259] (26)

[0260] In the formula, Let be the current surge (A) at the nth sampling point. The current sampling current amplitude ( );

[0261] The fault detection criteria are set as follows: both voltage surges and current surges exceed a threshold.

[0262] (27)

[0263] In the formula, Voltage change threshold (take) V), The threshold for sudden current change (take) A, where (A is the rated current)

[0264] When the criteria are met, the fault identification process is initiated to calculate the positive-sequence voltage and negative-sequence voltage to determine the fault type.

[0265] (28)

[0266] (29)

[0267] In the formula, It is the positive sequence voltage (V). It is a negative sequence voltage (V). For rotation factor, The three-phase instantaneous voltages are obtained from the inverse Park transformation in step 2.

[0268] The unbalance is defined as the ratio of negative-sequence voltage to positive-sequence voltage:

[0269] (30)

[0270] In the formula, The degree of imbalance is used to determine the type of fault: when When, it is determined to be a symmetrical fault (three-phase short circuit); when When this occurs, it is determined to be an asymmetrical fault (two-phase short circuit or single-phase ground fault).

[0271] 6) Cross-traffic control module control strategy;

[0272] The fault detection module adopts a control strategy based on the obtained fault detection results;

[0273] The implementation method of the control strategy is as follows:

[0274] For symmetrical faults, a current-limiting control strategy is adopted to limit the current command within the allowable range:

[0275] (31)

[0276] (32)

[0277] In the formula, The current command (A) during the fault period. The maximum allowable current of the device (take) A);

[0278] Simultaneously, reactive power is injected into the grid to support the voltage, and the reactive current command is determined based on the voltage sag depth:

[0279] (33)

[0280] In the formula, The reactive power support current is (A). The low voltage ride-through gain factor is taken as 0.02A / V.

[0281] For asymmetrical faults, in addition to current limiting control, it is also necessary to suppress negative sequence current. Positive and negative sequence separation control is adopted to decompose the current into positive sequence and negative sequence components:

[0282] (34)

[0283] (35)

[0284] In the formula, These are the positive and negative sequence currents (A) along the d-axis, respectively.

[0285] The negative sequence current command is set to zero to reduce negative sequence contamination of the system.

[0286] (36)

[0287] The positive sequence current command is determined according to formulas (31)-(33);

[0288] When the fault duration exceeds the threshold (Set to 150ms) or the current exceeds the hardware protection limit. (Pick When A) the device is locked out of operation, the feeders on both sides are disconnected via the bypass switch:

[0289] (37)

[0290] In the formula, The fault duration (obtained by accumulating the control cycle using a counter);

[0291] Fault ride-through control, through current limiting and reactive power support, enables the device to maintain operation as much as possible without damage, thereby improving system safety. Current limiting value... and reactive power support current The calculation result is sent back to the current command in step 4), replacing the power command in step 3) during normal operation;

[0292] 7) Distributed coordinated control of multi-terminal flexible interconnected devices;

[0293] Multiple flexible interconnected devices exchange their respective power transmission status and optimization objectives through the communication network in the multi-terminal distributed coordination module, and use a consensus algorithm to iteratively calculate the globally optimal power allocation scheme to achieve system-level coordination optimization;

[0294] When there are multiple flexible interconnected devices in the distribution network, each device exchanges information through a communication network and uses a consensus algorithm to achieve coordinated control.

[0295] Assume there are M flexible interconnected devices in the distribution network, and device i can communicate with its neighboring device j. The communication topology is represented by the adjacency matrix A:

[0296] (38)

[0297] In the formula, These are elements of the adjacency matrix;

[0298] Each device maintains a local state variable. This represents the power transmission amount of the device. The consensus algorithm iteratively updates the state variables of all devices to make them consistent.

[0299] (39)

[0300] In the formula, Let i be the state variable of device i during the k-th iteration. The coupling strength coefficient is 0.1, and the summation term represents the difference between the state of the device and that of the neighboring device.

[0301] The global optimization objective is to minimize the variance of the load factor of each feeder. The global objective function is defined as follows:

[0302] (40)

[0303] In the formula, Let N be the total number of distribution network feeders, and N be the global objective function. The load factor of feeder n is calculated from the sum of measurement data from each device in step 2. Average load factor;

[0304] The optimization variable for device i is its power transfer rate. The optimization problem is transformed into:

[0305] (41)

[0306] In the formula, The power transmission capacity limit for device i is 2MW.

[0307] The solution is obtained using distributed gradient descent, with each device calculating its local gradient.

[0308] (42)

[0309] Gradient information is exchanged using a consensus algorithm, and the global average gradient is calculated.

[0310] (43)

[0311] In the formula, For average gradient estimation;

[0312] Power update instruction based on average gradient:

[0313] (44)

[0314] In the formula, Step size (taken as 50);

[0315] Power command for coordinated control Alternative step 3) Single device optimization calculation The data is then sent to step 4) for execution. Through distributed coordination, multiple devices achieve global load balancing without the need for a central controller, avoiding power circulation and control conflicts.

[0316] Simulation verification was conducted based on the actual topology of a city's power distribution network. This network includes four 10kV feeders: Feeder 1 is 8 km long with a rated capacity of 5MVA; Feeder 2 is 6 km long with a rated capacity of 4MVA; Feeder 3 is 10 km long with a rated capacity of 6MVA; and Feeder 4 is 7 km long with a rated capacity of 4.5MVA. A 2MVA flexible interconnection device is installed between Feeder 1 and Feeder 2, and between Feeder 3 and Feeder 4. The load types include commercial loads, industrial loads, and residential loads. The commercial load reaches its peak during the daytime period from 10:00 to 18:00, the industrial load operates stably from 8:00 to 22:00, and the residential load reaches its peak during the daytime period from 18:00 to 22:00. The power distribution network is also connected to two 1.5MW distributed photovoltaic power stations, and the solar irradiance varies according to actual meteorological data.

[0317] A detailed model of the distribution network was built on the MATLAB / Simulink platform, including line impedance, transformers, loads, and flexible interconnection devices. The flexible interconnection devices were implemented using the control strategies described in steps one through seven. The simulation lasted for 24 hours with a simulation step size of 50 microseconds, recording the voltage, current, power, and device operation data for each feeder. Three scenarios were set up during the simulation: Scenario 1 was normal operation, verifying the load balancing and voltage regulation effects; Scenario 2 was a fault condition, where a three-phase short-circuit fault occurred on feeder 2 at the 10th hour, lasting for 0.15 seconds before being cleared, verifying fault ride-through capability; Scenario 3 was multi-device coordinated operation, comparing the effects of independent control of a single device and distributed coordinated control as follows.

[0318] Table 1 Comparison of Load Balancing Effects

[0319] Control method Feeder 1 load factor Feeder 2 load factor Feeder 3 load factor Feeder 4 load rate Load factor variance Network loss (kWh) No flexible interconnection 0.92 0.56 0.88 0.47 0.0421 1256 Single device independent control 0.74 0.74 0.68 0.67 0.0012 1089 Distributed coordination control 0.71 0.73 0.69 0.69 0.0003 1042

[0320] After adopting the distributed coordinated control strategy of this invention, the load factor balance of each feeder is significantly improved, and the load factor variance is reduced from 0.0421 without flexible interconnection to 0.0003, a reduction of 99.3%. Although independent control of a single device can improve the load balance of two directly connected feeders, the overall network coordination is insufficient. Distributed coordinated control achieves optimal load distribution across the entire network, reducing network losses from 1256 kWh to 1042 kWh, a reduction of 17.0%, resulting in significant economic benefits.

[0321] Table 2 Comparison of Voltage Regulation Effects

[0322] Control method Feeder 1 voltage (kV) Feeder 2 voltage (kV) Feeder 3 voltage (kV) Feeder 4 voltage (kV) Maximum voltage deviation Voltage qualification rate No flexible interconnection 9.52 10.21 9.48 10.18 7.2% 78.3% Only power regulation 9.78 9.89 9.73 9.91 2.7% 94.6% Active and reactive power coordination and regulation 9.92 9.96 9.91 9.94 0.9% 99.8%

[0323] The active and reactive power coordinated regulation strategy of this invention calculates the reactive power transmission command through formula (17), which effectively improves the voltage distribution. The maximum voltage deviation is reduced from 7.2% without flexible interconnection to 0.9%, and the voltage qualification rate is increased from 78.3% to 99.8%. When only active power regulation is used, although the voltage can be indirectly improved through load transfer, the effect is limited. After adding reactive power regulation, it can directly support the voltage of weak nodes, and the regulation effect is significantly improved.

[0324] Table 3 Fault Traceability Verification

[0325] Control strategy Fault detection time (ms) Fault identification accuracy Maximum current (A) during the fault Is the device locked? Voltage sag depth Voltage recovery time (ms) No crossing control (direct locking) 15 100% 0 yes 0.68pu 182 Traditional current limiting control 12 100% 168 no 0.65pu 156 This patented method 8 100% 171 no 0.58pu 89

[0326] The fault detection method based on voltage and current surges proposed in this invention (Equations 25-27) offers fast detection speed, with an average detection time of 8ms, 33% faster than traditional methods. The differentiated fault ride-through control strategy (Equations 31-36) injects reactive power support voltage while limiting current, reducing the voltage drop depth from 0.68pu to 0.58pu and shortening the voltage recovery time from 182ms to 89ms, thus improving the system's transient stability. The device maintained operation during the fault without blocking, verifying its fault ride-through capability.

[0327] Table 4 Control Response Speed ​​Test

[0328] Disturbance type Disturbance amplitude Traditional method response time (ms) The response time (ms) of this patented method Overshoot steady-state error Sudden increase in load +800kW 320 45 3.2% 0.5% sudden load drop -600kW 280 38 2.8% 0.3% Solar power output fluctuations ±500kW 410 52 4.1% 0.8% Topology switching Line switching 550 68 5.6% 1.2%

[0329] The dual closed-loop vector control (formulas 18-24) of this patent achieves rapid response, with an average response time of approximately 50ms, which is 7-8 times faster than traditional methods. The PI controller parameters are optimized using formulas (22)-(23), with overshoot controlled within 5% and steady-state error less than 1.5%, resulting in high control accuracy. The rapid response capability enables the device to adapt to the dynamic changes in the power distribution network and maintain stable system operation.

Claims

1. A control method for a flexible interconnection device in a medium- and low-voltage distribution network, wherein the flexible interconnection device adopts a back-to-back voltage source converter topology, and the flexible interconnection device comprises: The DC bus includes converters on both sides, DC support capacitors, filter circuits, measurement units, and control units. In addition, it is equipped with an electrical quantity measurement and preprocessing module, a power command calculation module, a converter dual closed-loop control module, a fault detection module, a ride-through control module, and a multi-terminal distributed coordination module. The control method is characterized by comprising the following steps: 1) Configure flexible interconnect device parameters based on hardware topology; Set the rated capacity of the flexible interconnect device; 2) The electrical quantity measurement and preprocessing module collects and measures the electrical quantities of the feeders on both sides, and obtains the feeder operating status parameters through digital filtering; The electrical quantity measurement and preprocessing module collects electrical quantities of both feeders in real time, including three-phase voltage, three-phase current, high-frequency noise and switching harmonics. The collected electrical quantities are sent to the preprocessing module for digital filtering to obtain the operating status parameters of both feeders. 3) Calculate the power transmission command based on the operating status parameters of the negative two-sided feeders; Input the operating status parameters of the two feeders obtained in step 2) into the power command calculation module to calculate the power command that the flexible interconnection device should transmit. The command control objectives include two aspects: load balancing and voltage regulation. 4) The converter's dual closed-loop control module converts power commands into modulation signals; The power command obtained in step 3) is sent to the converter dual closed-loop control module and converted into a current command. The converter dual closed-loop control module adopts a dual closed-loop mode of current inner loop and power outer loop. The modulation voltage is obtained by calculating the modulation voltage command based on the current error of the inner and outer loops. The modulation voltage generates a modulation signal through space vector PWM. The output voltage is precisely controlled by controlling the action time of each vector. 5) The fault detection module performs rapid fault detection based on the modulation signal; When a fault occurs in the distribution network, the flexible interconnection device can quickly identify the fault through the fault detection module, transmit the fault detection results to the fault ride-through control module, and take protective measures according to the severity of the fault. 6) Cross-travel control module control strategy; The fault detection module adopts a control strategy based on the obtained fault detection results; 7) Distributed coordinated control of multi-terminal flexible interconnected devices; Multiple flexible interconnected devices exchange their respective power transmission status and optimization objectives through the communication network in the multi-terminal distributed coordination module, and use a consensus algorithm to iteratively calculate the globally optimal power allocation scheme to achieve system-level coordination optimization.

2. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, In step 1), there are two converters, each consisting of three bridge arms. Each bridge arm contains two insulated gate bipolar transistor power switch modules, one above the other. The three-phase AC side of converter 1 is connected to feeder 1 through an LCL filter, and the three-phase AC side of converter 2 is connected to feeder 2 through an LCL filter. The DC side uses capacitor energy storage to maintain DC voltage stability.

3. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, Step 1) in the rated capacity calculation includes: rated DC-side voltage, DC-side supporting capacitor capacity, converter-side inductance, and harmonic resonant frequency. The rated DC side voltage is determined based on the AC side voltage, and the calculation formula is as follows: (1) In the formula, The rated DC voltage (V). This is the effective value (V) of the AC side line voltage. This represents the modulation coefficient margin. The DC-side supporting capacitor capacity is determined based on power fluctuation and voltage fluctuation requirements, and the calculation formula is as follows: (2) In the formula, This represents the DC-side capacitance value (F). The rated power of the device is (W). To meet power response time requirements, The allowable DC voltage fluctuation; The LCL filter parameters are designed based on the switching frequency and harmonic suppression requirements. The converter-side inductor... The main limitation is the current ripple at the switching frequency, calculated using the following formula: (3) In the formula, For switching frequency, For allowable current ripple; Grid-side inductance With capacitor The formula for calculating the harmonic resonant frequency of a harmonic notch filter is as follows: (4) In the formula, The resonant frequency should be set between the switching frequency and the power frequency.

4. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, In step 2), the electrical quantity measurement and preprocessing module includes a measurement unit and a control unit. The measurement unit includes a voltage transformer and a current transformer, which measure the three-phase voltage and three-phase current of the feeders on both sides, respectively. The measurement signals are digitized after passing through the signal conditioning circuit and sent to the controller. The control unit adopts a DSP+FPGA architecture. The DSP is responsible for complex power calculation and coordinated control algorithms, while the FPGA is responsible for high-speed PWM (pulse width modulation) generation and protection logic.

5. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, In step 2), the calculation method for the operating status parameters of the feeders on both sides is as follows: The electrical quantity measurement and preprocessing module uses a second-order low-pass filter to eliminate high-frequency noise and acquires the three-phase voltage of the feeders on both sides in real time. and three-phase current ; (5) In the formula, This is the filtered signal. The original sampled signal, Forward coefficients, The feedback coefficients are determined using the Butterworth filter design method. The filtered voltage and current signals are transformed into a synchronously rotating dq coordinate system using the Park transform: (6) In the formula, For the dq axis voltage components, To synchronize the rotation angle, a phase-locked loop (PLL) is used to extract the current signal from the grid voltage, and the current signal is also converted into a synchronous rotation angle. ; The phase-locked loop adopts a second-order generalized integrator (SOGI) structure. Calculate the active and reactive power of the feeders on both sides: (7) (8) In the formula, Let be the active power (W) of feeder k. Reactive power (Var), subscript These represent feeder 1 and feeder 2, respectively. Calculate the load factor of each feeder: (9) In the formula, Let k be the load factor of the feeder. The rated capacity of feeder k; Voltage deviation is calculated as follows: (10) In the formula, For voltage deviation, Voltage amplitude, Rated voltage; The calculation results of these electrical quantities ( This will be used as input for the next step of power command calculation.

6. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, In step 3), the power transfer command is calculated as follows: The load balancing objective function is defined as minimizing the difference in load rates between the two feeders: (11) In the formula, The objective function for load balancing; The voltage regulation objective function is defined as minimizing the sum of squares of the voltage deviations between the two feeders: (12) In the formula, Let the voltage regulation objective function be... The weighting coefficient is determined based on the importance of the feeder. The overall objective function is: (13) In the formula, The balance coefficient is dynamically adjusted according to the current operating status. When the voltages on both sides are within the acceptable range, load balancing is the primary method; when the voltage on either side exceeds the limit, voltage regulation is the primary method. Active power transmission commands are calculated using the gradient descent method: (14) In the formula, It is an active power transfer command (W). As the learning rate, the partial derivative of the objective function with respect to power is calculated using numerical differentiation methods: (15) In the formula, For small power disturbances; To avoid power oscillations, the power command rate is limited: (16) In the formula, This represents the upper limit of the rate of change of power. The reactive power transmission command is determined based on voltage regulation requirements. When the voltage of feeder 1 is low and the voltage of feeder 2 is high, the device sends reactive power to feeder 1. (17) In the formula, This is a reactive power transmission command. This is the voltage regulation coefficient; Calculated and It will be sent to the converter's dual closed-loop control module for execution.

7. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, In step 4), the modulation signal is calculated as follows: The power outer loop will control the power command. , Convert to dq axis current command. For converter 1 (connected to feeder 1), set the current reference value according to the power transmission direction: (18) (19) In the formula, The dq axis current command (A) for converter 1; For converter 2, to maintain DC voltage stability, its active current command is calculated by the DC voltage outer loop controller using the following formula: (20) In the formula, This is the proportional coefficient of the DC voltage loop. The integral coefficient is... This is the DC voltage reference value. To measure the DC voltage, the DC voltage is obtained through a DC-side voltage sensor. The reactive current command for converter 2 is: (21) In the formula, The q-axis current command (A) for converter 2; The inner current loop uses a PI controller to calculate the modulation voltage command based on the current error. (22) (23) In the formula, The dq axis modulation voltage (V) of converter k. This is the proportionality coefficient of the current loop. The integral coefficient is... For the power grid angular frequency, cross-coupling terms and Used to achieve decoupled control of the d-axis and q-axis; The modulated voltage is transformed back to the abc coordinate system via inverse Park transform: (24) In the formula, It is the three-phase modulated voltage (V).

8. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, In step 5), the protection measure is a current-limiting protection measure. The calculation method for implementing the current-limiting protection measure is as follows: The voltage jump is defined as the difference in voltage between adjacent sampling points: (25) In the formula, Let V be the voltage fluctuation at the nth sampling point. The current sampled voltage; Define the current surge as: (26) In the formula, Let be the current surge (A) at the nth sampling point. The current sampling current amplitude ( ); The fault detection criteria are set as follows: both voltage surges and current surges exceed a threshold. (27) In the formula, The voltage change threshold, The threshold for sudden current change; When the criteria are met, the fault identification process is initiated to calculate the positive-sequence voltage and negative-sequence voltage to determine the fault type. (28) (29) In the formula, It is the positive sequence voltage (V). It is a negative sequence voltage (V). For rotation factor, The three-phase instantaneous voltages are obtained from the inverse Park transformation in step 2); The unbalance is defined as the ratio of negative-sequence voltage to positive-sequence voltage: (30) In the formula, The degree of imbalance is used to determine the type of fault: when When, it is determined to be a symmetrical fault (three-phase short circuit); when When this occurs, it is determined to be an asymmetrical fault (two-phase short circuit or single-phase grounding).

9. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, In step 6), the implementation method of the control strategy is as follows: For symmetrical faults, a current-limiting control strategy is adopted to limit the current command within the allowable range: (31) (32) In the formula, The current command (A) during the fault period. This is the maximum allowable current of the device; Simultaneously, reactive power is injected into the grid to support the voltage, and the reactive current command is determined based on the voltage sag depth: (33) In the formula, The reactive power support current is (A). Low voltage ride-through gain coefficient; For asymmetrical faults, in addition to current limiting control, it is also necessary to suppress negative sequence current. Positive and negative sequence separation control is adopted to decompose the current into positive sequence and negative sequence components: (34) (35) In the formula, These are the positive and negative sequence currents (A) along the d-axis, respectively. The negative sequence current command is set to zero to reduce negative sequence contamination of the system. (36) The positive sequence current command is determined according to formulas (31)-(33); When the fault duration exceeds the threshold Or the current exceeds the hardware protection limit. When the device is locked out of operation, the feeders on both sides are disconnected via the bypass switch: (37) In the formula, The fault duration is obtained by accumulating the control cycle using a counter. Fault ride-through control, through current limiting and reactive power support, enables flexible interconnect devices to maintain operation as much as possible without damage, thereby improving system safety. Current limiting value... and reactive power support current The calculation result is sent back to the current command in step 4), replacing the power command in step 3) during normal operation.

10. The control method for a flexible interconnection device in a medium- and low-voltage distribution network according to claim 1, characterized in that, Step 7) uses a consensus algorithm for distributed coordination and control. The specific calculation method is as follows: Assume there are M flexible interconnected devices in the distribution network, and device i can communicate with its neighboring device j. The communication topology is represented by the adjacency matrix A: (38) In the formula, These are elements of the adjacency matrix; Each device maintains a local state variable. This represents the power transmission amount of the device. The consensus algorithm iteratively updates the state variables of all devices to make them consistent. (39) In the formula, Let i be the state variable of device i during the k-th iteration. The coupling strength coefficient is represented by the summation term, which indicates the difference between the state of the device and that of the neighboring device. The global optimization objective is to minimize the variance of the load factor of each feeder. The global objective function is defined as follows: (40) In the formula, Let N be the total number of distribution network feeders, and N be the global objective function. Let n be the load factor of feeder n. Average load factor; The optimization variable for device i is its power transfer rate. The optimization problem is transformed into: (41) In the formula, The power transmission capacity limit for device i is 2MW. The solution is obtained using distributed gradient descent, with each device calculating its local gradient. (42) Gradient information is exchanged using a consensus algorithm, and the global average gradient is calculated. (43) In the formula, For average gradient estimation; Power update instruction based on average gradient: (44) In the formula, The step size.

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