Load transfer method and system based on transformerless multi-port power router

Abstract

A load transfer method and system based on a transformerless multi-port power router includes the following steps: determining the fault location; if the fault location is in a higher-level line of a line connected to an MTUPFC, disconnecting the higher-level power grid and changing the control strategy of each MTUPFC port to constant AC voltage control; if the fault location is in a line connected to the MTUPFC, selecting one of the lines as a balancing line; determining whether the fault occurs on the balancing line; if not, changing the control strategy of the MTUPFC port connected to the faulty line to constant AC power control; if so, first disconnecting the faulty line; if the faulty line is a passive network, changing the control strategy of the MTUPFC port connected to the faulty line to constant AC voltage control; changing the control strategy of the port connected to the balancing line to optimal modulation index control; and if the faulty line is an active network, adjusting the control strategy of each MTUPFC port based on the power capacity of the active network. This invention can fully utilize the remaining capacity of other lines to maximize power restoration to the power-lost area.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid load transfer, and in particular relates to a load transfer method and system based on a transformerless multi-port power router. Background Art

[0002] With the large-scale integration of renewable energy, the distribution of power in the power grid has undergone significant changes in both time and space. Compared to traditional thermal power plants, the output power of renewable energy power plants fluctuates rapidly and is more difficult to control. This can lead to rapid power flow fluctuations, which in turn can trigger emergencies such as overloads. To address this challenge, power grid topology optimization is required to adapt to the high volatility of power output. Furthermore, with increasing loads, the structure of regional power grids is becoming increasingly complex, creating opportunities for transmission networks to implement load shifting and reduce load shedding. In the past, power grid structures were relatively simple. When a failure in a critical line or transformer caused an overload, even if the overall power generation and load balance was not affected, limited transmission capacity made it difficult to provide sufficient power to the load center. Consequently, partial load shedding was often used to resolve the emergency. However, with the increasing complexity of power grid structures, transmission lines are increasingly interconnected at multiple points. In this context, load shifting through interconnected transmission lines can effectively alleviate or even eliminate system emergencies, thereby reducing the need for load shedding, minimizing the scope of power outages, and minimizing economic losses.

[0003] The Multiport Transformer-less Unified Power Flow Controller (MTUPFC) is a new type of router. Compared with traditional UPFCs, MTUPFCs offer several significant advantages, including transformerlessness, lightweight, high efficiency, low cost, and easy port scalability. Previous research on MTUPFCs has primarily focused on power flow control and related control strategies, with no research specifically addressing load transfer within MTUPFCs. However, due to limitations in the UPFC's line structure, the active regulation capability of load transfer is insufficient. Furthermore, there is no control strategy that dynamically adjusts UPFC load transfer based on fault location and line characteristics, making it difficult to adapt to complex and changing grid operating conditions. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention proposes a load transfer method and system based on a transformerless multi-port power router.

[0005] A first aspect of the present invention provides a load transfer method based on a transformerless multi-port power router, characterized by comprising:

[0006] When a power grid fault occurs, the fault location is determined based on the protection devices in the grid and the current MTUPFC operating mode. If the fault is located in the upstream line of the line connected to the MTUPFC, the upstream grid is disconnected and the control strategy of each MTUPFC port is adjusted to constant AC voltage control to transfer power.

[0007] If the fault location is in the connected lines of the MTUPFC, one of the connected lines is selected as the balancing line; it is determined whether the fault occurs on the balanced line. If not, the control strategy of the MTUPFC port connected to the faulty line is adjusted to constant AC power control; if so, the faulty line is first disconnected. If the faulty line is a passive network, the control strategy of the MTUPFC port connected to the faulty line is adjusted to constant AC voltage control; and a balanced line is reselected from the remaining lines, and the control strategy of the port connected to the balanced line is adjusted to optimal modulation index control; if the faulty line is an active network, the control strategy of each MTUPFC port is adjusted according to the power capacity of the active network.

[0008] Preferably, the protection device includes a directional protection device and a distance protection device. The equivalent line voltage is obtained by subtracting the MTUPFC series side injection voltage from the voltage at the protection device measured by the directional protection device. The phase difference between the equivalent line voltage and the current measured at the protection device is calculated. If the phase difference is within a phase difference threshold range adaptively changed in combination with the current MTUPFC operation mode, it is a forward fault; otherwise, it is a reverse fault.

[0009] The distance from the fault point to the protection installation location is calculated based on the line unit length impedance and the impedance at the protection device measured by the distance protection device;

[0010] If the directional protection device determines that it is a forward fault and the distance from the fault point to the protection installation is within the set first range, the fault location is determined to be in the line connected to the MTUPFC; if the directional protection device determines that it is a reverse fault and the distance from the fault point to the protection installation is within the set second range, the fault location is in the upper line of the line connected to the MTUPFC; if it is other cases, the directional protection device and the distance protection device repeat the above process to make a judgment.

[0011] Preferably, in combination with the phase difference threshold range of the adaptive change of the MTUPFC operation mode at this time, the calculation formula is:

[0012]

[0013] Where, ; 、 are the upper and lower limits of the adaptive change range, respectively; 、 are the upper and lower limits of the initially set range, respectively; The mode coefficient is set. Different mode coefficients are set in different MTUPFC operation modes. for per unit value; for Phase; Injection voltage for the series side of MTUPFC; is the nominal voltage.

[0014] Preferably, the calculation formulas for the first range and the second range are set as follows:

[0015]

[0016] Where, is the upper limit of the first range; is the upper limit of the second range; 、 All are elongation coefficients; Between 0.8-0.85; Between 0.2-0.5; is the total length of the line where the corresponding protection device is located; is the total length of adjacent lines; from 0 to For the first range; from arrive For the second range.

[0017] Preferably, the selecting of a line from the lines connected to the MTUPFC as a balancing line is specifically:

[0018] Calculate the capacity margin and voltage sensitivity of each line connected to the MTUPFC and normalize them respectively; the capacity margin is the ratio of the rated active power capacity to the current actual active power transmitted; the voltage sensitivity is the partial derivative of the line voltage with respect to the active or reactive power injected into the line;

[0019] The normalized capacity margin and voltage sensitivity are weighted and summed according to the set weights, and the line with the largest weighted sum result is regarded as the balanced line.

[0020] Preferably, when performing constant AC power control and constant AC voltage control, the constraint condition of the remaining capacity of the remaining lines is satisfied, and the constraint condition formula is:

[0021]

[0022] Where, For the k The current active power of the remaining lines, is the current reactive power of the line, No. k Rated power of the remaining lines, No. k The actual transmission apparent power of the remaining lines, Apparent power required for failover; n is the total number of the remaining lines;

[0023] When the remaining capacity constraint of the remaining lines is not satisfied, the remaining lines are cut off according to the set priority until the remaining capacity constraint of the remaining lines is satisfied.

[0024] Preferably, the constant AC voltage control is to perform constant amplitude and constant frequency control on the voltage of the MTUPFC port; the voltage of the MTUPFC port is equal to the voltage at the connection point of the parallel CMI and the power flow regulation module PFCM of the MTUPFC minus the output voltage of the energy balance module PBCM, plus the output voltage of the power flow regulation module PFCM connected to the line in the MTUPFC; when performing constant amplitude and constant frequency control, a PI controller is used to perform voltage closed-loop regulation, and a feedforward term is added during the voltage closed-loop regulation.

[0025] Preferably, the optimal control of the modulation index is to adjust the modulation index within a set modulation index adjustment range until the objective function is minimized. The modulation index is the ratio of the modulation wave amplitude to the carrier amplitude when modulating the MTUPFC. The objective function is:

[0026]

[0027] Where t represents the time step; T represents the total time step; represents the sum of switching loss and line resistance loss at the tth time step; Indicates the rated power of MTUPFC; represents the DC voltage deviation at the tth time step; Indicates rated DC voltage; represents the line power deviation at the tth time step; Indicates the maximum transmission capacity of the line; Indicates the switching frequency; Indicates the maximum frequency allowed for switching; 、 、 、 is the set weight; is the penalty coefficient, is the constraint penalty term for the voltage over-limit and current over-limit at the t-th time step. The constraint penalty term is the sum of the squares of the voltage over-limit and current over-limit parts respectively.

[0028] Preferably, the control strategy for adjusting each port of the MTUPFC according to the active network power capacity is specifically:

[0029] Determine whether the power capacity of the active network meets the internal load of the active network. If so, the control strategy of each port of MTUPFC remains unchanged; if not, adjust the control strategy of the MTUPFC port connected to the fault line to fixed AC power control.

[0030] A second aspect of the present invention provides a load transfer system based on a transformerless multi-port power router, comprising a fault location module, an upper-level line fault adjustment module, a balancing line selection module, and a local-level line fault adjustment module, characterized in that:

[0031] Fault location module: When a power grid fault occurs, it determines the fault location based on the protection devices in the power grid and the MTUPFC operation mode at that time;

[0032] Upstream line fault adjustment module: If the fault location is in the upstream line of the line connected to the MTUPFC, it will disconnect the upstream power grid and change the control strategy of each port of the MTUPFC to constant AC voltage control to transfer power;

[0033] Balance line selection module: used to select one of the lines connected to the MTUPFC as a balance line if the fault location is in the lines connected to the MTUPFC;

[0034] This-level line fault adjustment module: Determine whether the fault occurs on the balanced line. If not, change the control strategy of the MTUPFC port connected to the faulty line to fixed AC power control. If so, first disconnect the faulty line. If the faulty line is a passive network, change the control strategy of the MTUPFC port connected to the faulty line to fixed AC voltage control. Reselect a balanced line from the remaining lines and change the control strategy of the port connected to the balanced line to optimal modulation index control. If the faulty line is an active network, adjust the control strategy of each MTUPFC port according to the power capacity of the active network.

[0035] The beneficial effects of the present invention are that, compared with the prior art, the present invention performs load transfer based on the MTUPFC port in the event of a fault. The present invention comprehensively determines the fault location based on various protection devices in the power grid and the operating mode of the MTUPFC at that time; the fault location is more accurate, different load transfers are performed for the upper line and the line connected to the MTUPFC, and one of the lines connected to the MTUPFC is selected as the balancing line; if the fault occurs in the balancing line, the faulty line is first disconnected to ensure the safety of the power grid; if the fault does not occur in the balancing line, the control strategy of the port connected to the balancing line is changed to optimal modulation control, which can quickly compensate for the power gap caused by the removal of the faulty line, maintain the voltage / frequency of the regional power grid within the allowable range, and avoid cascading failures; the present invention can actively distribute line power flow, so it is no longer restricted by the line structure and can fully utilize the remaining capacity of other lines to restore power to the power-lost area to the greatest extent, which not only improves the active regulation capability of the power grid but also significantly improves the power supply reliability of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a flow chart of the method of the present invention;

[0037] Figure 2 The topology of the transformerless multi-port power router of the present invention;

[0038] Figure 3 This is a schematic diagram of the connection between the transformerless multi-port power router and the circuit of the present invention;

[0039] Figure 4 This is a constant voltage control block diagram of the transformerless multi-port power router of the present invention;

[0040] Figure 5 is the active power of line 1 in this embodiment;

[0041] Figure 6 is the reactive power of line 1 in this embodiment;

[0042] Figure 7 is the active power of line 2 in this embodiment;

[0043] Figure 8 is the reactive power of line 2 in this embodiment;

[0044] Figure 9 is the active power of line 3 in this embodiment;

[0045] Figure 10 It is the reactive power of line 3 in this embodiment. DETAILED DESCRIPTION

[0046] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The embodiments described in this application are only part of the embodiments of the present invention, not all of them. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0047] like Figure 1 As shown, embodiment 1 of the present invention proposes a load transfer method based on a transformerless multi-port power router, which is characterized by comprising:

[0048] When a grid fault occurs, the fault location is determined based on the protection devices in the grid and the MTUPFC operating mode at the time. If the fault is located in the upstream line of the line connected to the MTUPFC, the upstream grid is disconnected, and the control strategy of each MTUPFC port is changed to constant AC voltage control to transfer power.

[0049] It should be noted that if Figure 2 As shown, the MTUPFC consists of multiple power flow control modules (PFCMs), a parallel converter (CMI), and an energy balancing module (PBCM). All PFCMs and PBCMs are connected via a common DC bus. The multiple power flow control modules (PFCMs) are multiple parallel modular multilevel converters (MMCs), responsible for power flow control on each transmission line. The parallel converter (CMI) is a STATCOM composed of full-bridge submodules (FBSMs), providing reactive power compensation and voltage support. The energy balancing module (PBCM) is a hub module connecting the PFCMs and CMI, achieving power balancing via the common DC bus.

[0050] If the fault is located in the lines connected to the MTUPFC, one of these lines is selected as the balancing line. It is determined whether the fault occurs on the balanced line. If not, the control strategy of the MTUPFC port connected to the faulty line is changed to constant AC power control. If so, the faulty line is first disconnected. If the faulty line is a passive network, the control strategy of the MTUPFC port connected to the faulty line is changed to constant AC voltage control. A balanced line is then reselected from the remaining lines, and the control strategy of the port connected to the balanced line is changed to modulation index optimal control. If the faulty line is an active network, the control strategy of each MTUPFC port is adjusted according to the power capacity of the active network.

[0051] Preferably, the protection device includes a directional protection device and a distance protection device. The equivalent line voltage is obtained by subtracting the MTUPFC series side injection voltage from the voltage at the protection device measured by the directional protection device. The phase difference between the equivalent line voltage and the current measured at the protection device is calculated. If the phase difference is within a range adaptively changed based on the current MTUPFC operating mode, it is a forward fault; otherwise, it is a reverse fault.

[0052] The distance from the protection device to the fault point is calculated based on the line unit length impedance and the impedance at the protection device measured by the distance protection device;

[0053] If the directional protection device determines that it is a forward fault and the distance from the distance protection device to the fault point is within the set first range, the fault location is in the line connected to the MTUPFC; if the directional protection device determines that it is a reverse fault and the distance from the distance protection device to the fault point is within the set second range, the fault location is in the upper line of the line connected to the MTUPFC; if it is other cases, the directional protection device and the distance protection device repeat the above process to make a judgment.

[0054] Preferably, considering the range of adaptive changes in the mode of operation of the MTUPFC at this time, the calculation formula is:

[0055]

[0056] Where, ; 、 are the upper and lower limits of the adaptive change range, respectively; 、 are the upper and lower limits of the initially set range, respectively; The mode coefficient is set. Different mode coefficients are set in different MTUPFC operation modes. for per unit value; for Phase; Injection voltage for the series side of MTUPFC; is the nominal voltage;

[0057] Preferably, the calculation formulas for the first range and the second range are set as follows:

[0058]

[0059] Where, is the upper limit of the first range; is the upper limit of the second range; 、 All are elongation coefficients; Between 0.8-0.85; Between 0.2-0.5; is the total length of the line where the corresponding protection device is located; is the total length of adjacent lines; from 0 to For the first range; from arrive For the second range.

[0060] Preferably, the selecting of a line from the lines connected to the MTUPFC as a balancing line is specifically:

[0061] Calculate the capacity margin and voltage sensitivity of each line connected to the MTUPFC and normalize them respectively; the capacity margin is the ratio of the rated active power capacity to the current actual active power transmitted; the voltage sensitivity is the partial derivative of the line voltage with respect to the active or reactive power injected into the line;

[0062] The normalized capacity margin and voltage sensitivity are weighted and summed according to the set weights, and the line with the largest weighted sum result is regarded as the balanced line.

[0063] Preferably, when performing constant AC power control and constant AC voltage control, the constraint condition of the remaining capacity of the remaining lines is satisfied, and the constraint condition formula is:

[0064]

[0065] Where, For the k The current active power of the remaining lines, is the current reactive power of the line, No. k Rated power of the remaining lines, No. k The actual transmission apparent power of the remaining lines, Apparent power required for failover; n is the total number of the remaining lines;

[0066] When the remaining capacity constraint of the remaining lines is not satisfied, the remaining lines are cut off according to the set priority until the remaining capacity constraint of the remaining lines is satisfied.

[0067] Preferably, the constant AC power supply control is to use the MTUPFC as an active power source, inject a controllable AC voltage into the line through the power flow regulation module PFCM, directly change the equivalent impedance and power angle of the line, and actively adjust the transmission direction and magnitude of active / reactive power; and the constant AC voltage control is to control the voltage at the MTUPFC port at a constant amplitude and frequency; the voltage at the MTUPFC port is equal to the voltage at the connection point between the parallel CMI of the MTUPFC and the power flow regulation module PFCM minus the output voltage of the energy balance module PBCM, plus the output voltage of the power flow regulation module PFCM connected to the line in the MTUPFC, and the formula is:

[0068]

[0069] in, is the port voltage, is the voltage at the connection between the parallel CMI and the power flow regulation module PFCM, is the output voltage of the energy balance module PBCM, is the output voltage of the power flow regulation module PFCM connected to the line.

[0070] like Figure 4 As shown, a PI controller is used to perform voltage closed-loop regulation when constant amplitude and constant frequency control is performed, and a feedforward term is added during the voltage closed-loop regulation.

[0071] Preferably, the optimal control of the modulation index is to adjust the modulation index within a set modulation index adjustment range until the objective function is minimized. The modulation index is the ratio of the modulation wave amplitude to the carrier amplitude when modulating the MTUPFC. The objective function is:

[0072]

[0073] Where t represents the time step; T represents the total time step; represents the sum of switching loss and line resistance loss at the tth time step; Indicates the rated power of MTUPFC; represents the DC voltage deviation at the tth time step; Indicates rated DC voltage; represents the line power deviation at the tth time step; Indicates the maximum transmission capacity of the line; Indicates the switching frequency; Indicates the maximum frequency allowed for switching; 、 、 、 is the set weight; is the penalty coefficient, is the constraint penalty term for the voltage over-limit and current over-limit at the t-th time step. The constraint penalty term is the sum of the squares of the voltage over-limit and current over-limit parts respectively.

[0074] Preferably, the control strategy for adjusting each port of the MTUPFC according to the active network power capacity is specifically:

[0075] Determine whether the power capacity of the active network meets the internal load of the active network. If so, the control strategy of each port of MTUPFC remains unchanged; if not, adjust the control strategy of the MTUPFC port connected to the fault line to fixed AC power control.

[0076] Embodiment 2 of the present invention proposes a load transfer system based on a transformerless multi-port power router, comprising a fault location module, an upper-level line fault adjustment module, a balancing line selection module, and a local-level line fault adjustment module, characterized in that:

[0077] Fault location module: When a power grid fault occurs, it determines the fault location based on the protection devices in the power grid and the MTUPFC operation mode at that time;

[0078] Upstream line fault adjustment module: If the fault location is in the upstream line of the line connected to the MTUPFC, it will disconnect the upstream power grid and change the control strategy of each port of the MTUPFC to constant AC voltage control to transfer power;

[0079] Balance line selection module: used to select one of the lines connected to the MTUPFC as a balance line if the fault location is in the lines connected to the MTUPFC;

[0080] This-level line fault adjustment module: Determine whether the fault occurs on the balanced line. If not, change the control strategy of the MTUPFC port connected to the faulty line to fixed AC power control. If so, first disconnect the faulty line. If the faulty line is a passive network, change the control strategy of the MTUPFC port connected to the faulty line to fixed AC voltage control. Reselect a balanced line from the remaining lines and change the control strategy of the port connected to the balanced line to optimal modulation index control. If the faulty line is an active network, adjust the control strategy of each MTUPFC port according to the power capacity of the active network.

[0081] like Figure 3As shown, in order to verify the effectiveness of the control strategy, a power flow control system based on MTUPFC was built in Matlab / Simulink. In this embodiment, MTUPFC connects three 220kV transmission lines, calculates and selects line 1 as the balanced line, and after a three-phase short circuit fault occurs, the section switch on line 1 and the circuit breaker at the port outlet of MTUPFC trip, and recalculate and select line 2 as the new balanced line. MTUPFC restores power to the 0.6pu active load originally supplied by the line. Before and after power supply restoration for the original load of line 1, the power flow conditions of each line in the simulation are shown in Table 1. The active load on line 1 is evenly distributed by line 2 and line 3;

[0082] The simulation timing is as follows: at t = 0.1s, the power restoration control strategy is started, and the active power reference value of line 3 decreases at a fixed slope. At t = 0.15s, it drops to 0.3 pu. All the above powers are per-unit values.

[0083] In this embodiment, the MTUPFC is connected to three 220kV transmission lines.

[0084] Table 1 Flow conditions of each line

[0085]

[0086] Figure 5 、 6 , 7, 8, 9, and 10 are the active power waveforms and reactive power waveforms of lines 1, 2, and 3 respectively. It can be seen from the waveforms that after line 1 loses power, the power supply of line 1 is restored through the constant voltage control of MTUPFC.

[0087] The present disclosure may be a system, method and / or computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0088] A computer-readable storage medium can be a tangible device that can hold and store instructions for use by an instruction execution device. A computer-readable storage medium can be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination thereof. More specific examples (a non-exhaustive list) of computer-readable storage media include: a portable computer disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punched card or raised structure in a groove on which instructions are stored, and any suitable combination thereof. As used herein, a computer-readable storage medium is not to be construed as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse passing through a fiber optic cable), or an electrical signal transmitted through an electrical wire.

[0089] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions to be stored in the computer-readable storage medium in each computing / processing device.

[0090] The computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages such as Smalltalk, C++, and conventional procedural programming languages such as "C" or similar programming languages. The computer-readable program instructions may be executed entirely on the user's computer, partially on the user's computer, as a stand-alone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, the state information of the computer-readable program instructions is used to personalize an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), so that the electronic circuit can execute the computer-readable program instructions, thereby implementing various aspects of the present disclosure.

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A load transfer method based on a transformerless multi-port power router, characterized in that: include: When a power grid fault occurs, the fault location is determined based on the protection devices in the power grid and the current MTUPFC operation mode; If the fault location is in the upstream line of the line connected to the MTUPFC, the upstream power grid will be disconnected, and the control strategy of each port of the MTUPFC will be adjusted to constant AC voltage control to transfer power; If the fault location is in the line connected to the MTUPFC, the capacity margin and voltage sensitivity of each line connected to the MTUPFC are calculated and normalized respectively; The capacity margin is the ratio of the rated active power capacity to the active power actually transmitted; the voltage sensitivity is the partial derivative of the line voltage with respect to the active or reactive power injected into the line; The normalized capacity margin and voltage sensitivity are weighted and summed according to the set weights. The line with the largest weighted sum is regarded as the balanced line. Determine whether the fault occurs on a balanced line. If not, adjust the control strategy of the MTUPFC port connected to the faulty line to constant AC power control. If so, disconnect the faulty line first. If the faulty line is a passive network, adjust the control strategy of the MTUPFC port connected to the faulty line to constant AC voltage control. Reselect a balanced line from the remaining lines and adjust the control strategy of the port connected to the balanced line to optimal modulation index control. If the faulty line is an active network, determine whether the power capacity of the active network meets the internal load of the active network. If so, the control strategy of each MTUPFC port remains unchanged. If it cannot be satisfied, the control strategy of the MTUPFC port connected to the faulty line is adjusted to fixed AC power supply control.

2. A load transfer method based on a transformerless multi-port power router according to claim 1, characterized in that: include: The protection device includes a directional protection device and a distance protection device. An equivalent line voltage is obtained by subtracting the MTUPFC series injection voltage from the voltage at the protection device measured by the directional protection device. A phase difference between the equivalent line voltage and the current measured at the protection device is calculated. If the phase difference is within a phase difference threshold range adaptively changed based on the current MTUPFC operating mode, a forward fault is detected. Otherwise it is a reverse fault; The distance from the fault point to the protection installation location is calculated based on the line unit length impedance and the impedance at the protection device measured by the distance protection device; If the directional protection device determines that the fault is in the forward direction and the distance from the fault point to the protection installation is within the set first range, the fault location is determined to be in the line connected to the MTUPFC; If the directional protection device determines that it is a reverse fault and the distance from the fault point to the protection installation is within the set second range, the fault location is on the upper line of the line connected to the MTUPFC; If it is other cases, the directional protection device and the distance protection device repeat the above process to make a judgment.

3. The load transfer method based on a transformerless multi-port power router according to claim 2, characterized in that: Combined with the phase difference threshold range of the current MTUPFC operation mode adaptive change, the calculation formula is: Where, ; 、 are the upper and lower limits of the adaptive change range, respectively; 、 are the upper and lower limits of the initially set range, respectively; The mode coefficient is set. Different mode coefficients are set in different MTUPFC operation modes. for per unit value; for Phase; Injection voltage for the series side of MTUPFC; is the nominal voltage.

4. The load transfer method based on a transformerless multi-port power router according to claim 3, characterized in that: The calculation formula for the first range and the second range is set as follows: Where, is the upper limit of the first range; is the upper limit of the second range; 、 All are elongation coefficients; Between 0.8-0.85; Between 0.2-0.5; is the total length of the line where the corresponding protection device is located; is the total length of adjacent lines; from 0 to For the first range; from arrive For the second range.

5. The load transfer method based on a transformerless multi-port power router according to claim 1, characterized in that: When performing constant AC power control and constant AC voltage control, the constraint condition of the remaining capacity of the remaining lines must be met. The constraint condition formula is: Where, For the k The current active power of the remaining lines, is the current reactive power of the line, No. k Rated power of the remaining lines, No. k The actual transmission apparent power of the remaining lines, Apparent power required for failover; n is the total number of the remaining lines; When the remaining capacity constraint of the remaining lines is not satisfied, the remaining lines are cut off according to the set priority until the remaining capacity constraint of the remaining lines is satisfied.

6. The load transfer method based on a transformerless multi-port power router according to claim 1, characterized in that: Constant AC voltage control is to control the voltage at the MTUPFC port with a constant amplitude and frequency. The voltage at the MTUPFC port is equal to the voltage at the connection point between the parallel CMI of the MTUPFC and the power flow regulation module PFCM minus the output voltage of the energy balance module PBCM, plus the output voltage of the power flow regulation module PFCM connected to the line in the MTUPFC. When performing constant amplitude and constant frequency control, a PI controller is used to perform voltage closed-loop regulation, and a feedforward term is added during the voltage closed-loop regulation.

7. The load transfer method based on a transformerless multi-port power router according to claim 1, characterized in that: The optimal control of modulation index is to adjust the modulation index within the set modulation index adjustment range until the objective function is minimized. The modulation index is the ratio of the modulation wave amplitude to the carrier amplitude when modulating the MTUPFC. The objective function is: Where t represents the time step; T represents the total time step; represents the sum of switching loss and line resistance loss at the tth time step; Indicates the rated power of MTUPFC; represents the DC voltage deviation at the tth time step; Indicates rated DC voltage; represents the line power deviation at the tth time step; Indicates the maximum transmission capacity of the line; Indicates the switching frequency; Indicates the maximum frequency allowed for switching; 、 、 、 is the set weight; is the penalty coefficient, is the constraint penalty term for the voltage over-limit and current over-limit at the t-th time step. The constraint penalty term is the sum of the squares of the voltage over-limit and current over-limit parts respectively.

8. A load transfer system based on a transformerless multi-port power router using the method according to any one of claims 1 to 7, comprising a fault location module, an upper-level line fault adjustment module, a balancing line selection module, and a current-level line fault adjustment module, characterized in that: Fault location module: When a power grid fault occurs, it determines the fault location based on the protection devices in the power grid and the MTUPFC operation mode at that time; Upstream line fault adjustment module: If the fault location is in the upstream line of the line connected to the MTUPFC, it will disconnect the upstream power grid and change the control strategy of each port of the MTUPFC to constant AC voltage control to transfer power; Balance line selection module: used to select one of the lines connected to the MTUPFC as a balance line if the fault location is in the lines connected to the MTUPFC; This level line fault adjustment module: determines whether the fault occurs on the balanced line. If not, changes the control strategy of the MTUPFC port connected to the faulty line to fixed AC power control; If so, the fault line is cut off first. If the fault line is a passive network, the control strategy of the MTUPFC port connected to the fault line is changed to constant AC voltage control; A balanced line is reselected from the remaining lines, and the control strategy of the ports connected to the balanced line is changed to modulation index optimal control; if the faulty line is an active network, the control strategy of each port of the MTUPFC is adjusted according to the power capacity of the active network.

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