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

Abstract

The invention discloses a load transfer method and system based on a transformer-free multi-port electric energy router. The method comprises the following steps: judging a fault position; if the fault position is a superior line of the line connected with the MTUPFC, disconnecting a superior power grid, and changing the control strategy of each port of the MTUPFC into constant alternating current voltage control; if the fault position is in the lines connected to the MTUPFC, selecting one line as a balanced line; judging whether a fault occurs on the balanced line or not, and if not, changing the control strategy of the MTUPFC port connected with the fault line into constant alternating current power supply control; if yes, the fault line is cut off firstly, and if the fault line is a passive network, a control strategy of an MTUPFC port connected with the fault line is changed into fixed alternating current voltage control; changing a control strategy of a port connected with the balance line into modulation degree optimal control; and if the fault line is an active network, the control strategy of each port of the MTUPFC is adjusted according to the power supply capacity of the active network. According to the invention, the residual capacity of other lines can be fully utilized to restore power supply for the power-losing area to the maximum extent.

Description

Technical Field

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

[0002] With the large-scale access of new energy, the distribution of power grid power has changed significantly in time and space. Compared with traditional thermal power plants, the output power of new energy power plants changes rapidly and is difficult to control, which may lead to rapid changes in power flow and trigger emergencies such as overload. To address this challenge, the power grid needs to be topologically optimized to adapt to the high volatility of power output. In addition, with the increase in load, the structure of regional power grids is becoming increasingly complex, which provides conditions for load transfer in the transmission grid and reduction of the load shedding amount. In the past, the power grid structure was relatively simple. When a key line or transformer failed and caused overload, even if the overall power generation and load balance was not affected, due to limited transmission capacity, it was difficult to provide sufficient power for the load center. Therefore, load shedding was often used to eliminate the emergency state. However, with the increasing complexity of the power grid structure, transmission lines are gradually developing towards a multi-terminal interconnected trend. In this context, if load transfer can be carried out through other interconnected transmission lines, it may effectively alleviate or even eliminate the emergency state of the system, thereby reducing load shedding measures and narrowing the power outage scope and economic losses.

[0003] The transformerless multi-port power router (Multiport Transformer-less Unified Power Flow Controller, MTUPFC) is a new type of router. Compared with traditional UPFC, MTUPFC has several significant advantages, including no need for a transformer, light weight, high efficiency, low cost, and easy expandability of ports. Previous research on MTUPFC mainly focused on power flow control and related control strategies, and there has been no research on load transfer for MTUPFC. For the load transfer of UPFC, due to the limitation of the UPFC line structure, the active regulation ability of load transfer is insufficient, and there is no control strategy for dynamically regulating UPFC load transfer according to the fault location and line characteristics, making it difficult to adapt to the complex and changing power grid operation state. 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] The first aspect of the present invention proposes a load transfer method based on a transformerless multi-port power router, which is characterized by including: When a power grid failure occurs, the fault location is judged according to the protection devices in the power grid and in combination with the current operation mode of the MTUPFC; if the fault location is on the upper-level line of the line connected to the MTUPFC, the upper-level power grid is disconnected, and the control strategies of each port of the MTUPFC are adjusted to constant AC voltage control for power transfer. If the fault location is in the connection line of the MTUPFC, one of the connected lines is selected as the balance line; it is judged whether the fault occurs on the balance line. If not, the control strategy of the MTUPFC port connected to the fault line is adjusted to constant 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 adjusted to constant AC voltage control; and a balance line is reselected from the remaining lines, and the control strategy of the port connected to the balance line is adjusted to modulation index optimal control; if the fault line is an active network, the control strategies of each port of the MTUPFC are adjusted according to the power capacity of the active network power source.

[0006] Preferably, the protection device includes a directional protection device and a distance protection device. The equivalent line voltage is obtained by subtracting the voltage injected by the series side of the MTUPFC from the voltage at the protection device measured by the directional protection device, and the phase difference between the equivalent line voltage and the current at the protection device measured is calculated. If the phase difference is within the phase difference threshold range adaptively changed in combination with the current operation mode of the MTUPFC, it is a forward fault; otherwise, it is a reverse fault. And according to the impedance per unit length of the line and the impedance at the protection device measured by the distance protection device, the distance from the fault point to the protection installation location is calculated. If the directional protection device judges a forward fault and the distance from the fault point to the protection installation location is within the set first range, it is judged that the fault location is in the line connected to the MTUPFC; if the directional protection device judges a reverse fault and the distance from the fault point to the protection installation location is within the set second range, the fault location is in the upper-level line of the line connected to the MTUPFC; if it is other situations, the directional protection device and the distance protection device repeat the above process for judgment.

[0007] Preferably, the phase difference threshold range adaptively changed in combination with the current operation mode of the MTUPFC has the following calculation formula:

[0008] In the formula, ; and are respectively the upper and lower limits of the adaptively changed range; and are respectively the upper and lower limits of the initially set range; is the set mode coefficient, and different mode coefficients are set for different operation modes of the MTUPFC; is per unit value; is the phase of is the injection voltage of the series side of the MTUPFC; is the nominal voltage.

[0009] Preferably, the calculation formulas for the set first range and second range are:

[0010] In the formula, is the upper limit of the first range; is the upper limit of the second range; , are both extension coefficients; is between 0.8 and 0.85; is between 0.2 and 0.5; is the total length of the line where the corresponding protection device is located; is the total length of the adjacent line; from 0 to is the first range; from to is the second range.

[0011] Preferably, when performing constant AC power source control and constant AC voltage control, the constraint conditions of the remaining capacity of the set remaining lines are satisfied, and the constraint condition formula is: 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 currently actually transmitted active power; the voltage sensitivity is the partial derivative of the line voltage with respect to the active or reactive power injected into the line; The line with the largest weighted sum result after weighted summation of the normalized capacity margin and voltage sensitivity according to the set weights is used as the balance line.

[0012] Preferably, when performing constant AC power source control and constant AC voltage control, the constraint conditions of the remaining capacity of the set remaining lines are satisfied, and the constraint condition formula is:

[0013] In the formula, is the k th current active power of the remaining line, is the current reactive power of the line, the k th rated power of the remaining line, the k th actual transmitted apparent power of the remaining line, Apparent power required for failover; n is the total number of the remaining lines; When the constraint condition of the remaining capacity of the remaining lines is not satisfied, the remaining lines are cut off according to the set priority until the constraint condition of the remaining capacity of the remaining lines is satisfied.

[0014] 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 of the shunt 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 for voltage closed-loop regulation, and a feedforward term is added during voltage closed-loop regulation.

[0015] Preferably, the modulation index optimal control 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 amplitude of the modulation wave to the amplitude of the carrier wave when modulating the MTUPFC. The objective function is:

[0016] In the formula, t represents the time step; T represents the total number of time steps; represents the sum of the switching loss and the line resistance loss at the t-th time step; represents the rated power of the MTUPFC; represents the DC voltage deviation at the t-th time step; represents the rated DC voltage; represents the line power deviation at the t-th time step; represents the maximum transmission capacity of the line; represents the switching frequency; represents the maximum allowable frequency of the switch; 、 、 、 are the set weights; is the penalty coefficient, is the constraint penalty term for voltage overlimit and current overlimit at the t-th time step. This constraint penalty term is the sum of the squares of the voltage overlimit and the current overlimit parts respectively.

[0017] Preferably, the control strategy for adjusting each port of the MTUPFC according to the active network power supply capacity is specifically: Judge whether the active network power supply capacity can meet the internal load of the active network. If it can be met, the control strategy of each port of the MTUPFC remains unchanged; if it cannot be met, the control strategy of the MTUPFC port connected to the faulty line is adjusted to constant AC power control.

[0018] The second aspect of the present invention proposes a load transfer system based on a transformerless multi-port power router, including a fault location module, an upper-level line fault adjustment module, a balance line selection module, and a local-level line fault adjustment module, characterized in that: Fault location module: When a power grid fault occurs, it is used to determine the fault location according to the protection devices in the power grid and in combination with the operation mode of the MTUPFC at this time; Upper-level line fault adjustment module: If the fault location is on the upper-level line of the line connected to the MTUPFC, the upper-level power grid is disconnected, and the control strategies of each port of the MTUPFC are changed to constant AC voltage control for load transfer; Balance line selection module: If the fault location is in the lines connected to the MTUPFC, one of these lines is selected as the balance line; Local-level line fault adjustment module: It judges whether the fault occurs on the balance line. If not, the control strategy of the MTUPFC port connected to the fault line is changed to constant AC power control; if so, the fault line is first cut off. 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; and a new balance line is reselected from the remaining lines, and the control strategy of the port connected to the balance line is changed to modulation index optimal control; if the fault line is an active network, the control strategies of each port of the MTUPFC are adjusted according to the power capacity of the active network power source.

[0019] The beneficial effects of the present invention are as follows. Compared with the prior art, the present invention performs load transfer based on the MTUPFC ports in case of faults. The present invention comprehensively determines the fault location according to various protection devices in the power grid and in combination with the operation mode of the MTUPFC at this time; the fault location is more accurate, and different load transfers are performed for the upper-level lines and the lines connected to the MTUPFC. One of the lines connected to the MTUPFC is selected as the balance line; if the fault occurs on the balance line, the fault line is first cut off to ensure the safety of the power grid. If it does not occur on the balance line, the control strategy of the port connected to the balance line is changed to modulation index optimal control, which can quickly compensate for the power gap caused by the removal of the fault line and maintain the voltage / frequency of the regional power grid within the allowable range to avoid cascading failures; the present invention can actively allocate the line power flow, so it is no longer limited by the line structure and can make full use of the remaining capacity of other lines to restore power supply to the power outage area to the greatest extent, which can not only improve the active regulation ability of the power grid but also significantly improve the power supply reliability of the power grid. Brief Description of the Drawings

[0020] Figure 1 It is a flowchart of the method of the present invention; Figure 2This is the topological structure of the transformerless multi-port power router of the present invention; Figure 3 This is the connection schematic diagram of the transformerless multi-port power router of the present invention and the line; Figure 4 This is the fixed voltage control block diagram of the transformerless multi-port power router of the present invention; Figure 5 This is the active power of Line 1 in this embodiment; Figure 6 This is the reactive power of Line 1 in this embodiment; Figure 7 This is the active power of Line 2 in this embodiment; Figure 8 This is the reactive power of Line 2 in this embodiment; Figure 9 This is the active power of Line 3 in this embodiment; Figure 10 This is the reactive power of Line 3 in this embodiment. Detailed implementation manners

[0021] To make the objectives, technical solutions and advantages of the present invention clearer, 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 a part of the embodiments of the present invention, rather than all the embodiments. Based on the spirit of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0022] As Figure 1 shown, Embodiment 1 of the present invention proposes a load transfer method based on a transformerless multi-port power router, which is characterized by including: When a power grid fault occurs, according to the protection device in the power grid, combined with the operation mode of the MTUPFC at this time, judge the fault location; if the fault location is on the upper-level line of the line connected to the MTUPFC, disconnect the upper-level power grid, change the control strategy of each port of the MTUPFC to fixed AC voltage control, and perform power transfer; It should be noted that as Figure 2As shown, the MTUPFC consists of multiple power flow regulation modules PFCM, a shunt converter CMI, and an energy balance module PBCM. All PFCM and PBCM are connected through a common DC bus. The multiple power flow regulation modules PFCM are multiple parallel modular multilevel converters MMC, which are responsible for the power flow control of each transmission line. The shunt converter CMI is a STATCOM composed of full-bridge sub-modules FBSM, which provides reactive power compensation and voltage support. The energy balance module PBCM is the hub module connecting PFCM and CMI, and realizes power balance through the common DC bus; If the fault location is in the line connected to the MTUPFC, then one of these lines is selected as the balance line; it is judged whether the fault occurs on the balance line. If not, the control strategy of the MTUPFC port connected to the fault line is changed to constant AC power control; if so, the fault line is first disconnected. 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; and a new balance line is selected from the remaining lines, and the control strategy of the port connected to the balance line is changed to modulation index optimal control; if the fault line is an active network, the control strategies of each port of the MTUPFC are adjusted according to the power capacity of the active network power supply.

[0023] Preferably, the protection device includes a directional protection device and a distance protection device. The equivalent line voltage is obtained by subtracting the voltage injected by the series side of the MTUPFC from the voltage measured by the directional protection device at this protection device. The phase difference between the equivalent line voltage and the current measured by this protection device is calculated. If the phase difference is within the range adaptively changed in combination with the operating mode of the MTUPFC at this time, it is a forward fault; otherwise, it is a reverse fault; And according to the impedance per unit length of the line and the impedance measured by the distance protection device at this protection device, the distance from this protection device to the fault point is calculated; If the directional protection device judges it to be a forward fault, and the distance from the distance protection device to the fault point is within the set first range, then the fault location is in the line connected to the MTUPFC; if the directional protection device judges it to be a reverse fault, and the distance from the distance protection device to the fault point is within the set second range, then the fault location is in the upper-level line of the line connected to the MTUPFC; if it is other situations, the directional protection device and the distance protection device repeat the above process for judgment.

[0024] Preferably, the calculation formula for the range adaptively changed in combination with the operating mode of the MTUPFC at this time is:

[0025] In the formula, ; 、 They are respectively the upper and lower limits of the range of adaptive change; and They are respectively the upper and lower limits of the initially set range; is the set mode coefficient, and different mode coefficients are set for different MTUPFC operating modes; is per unit value; is the phase of is the injection voltage on the series side of MTUPFC; is the nominal voltage; Preferably, the calculation formulas for the set first range and second range are:

[0026] In the formula, is the upper limit of the first range; is the upper limit of the second range; and are both extension coefficients; is between 0.8 - 0.85; is between 0.2 - 0.5; is the total length of the line where the corresponding protection device is located; is the total length of the adjacent line; from 0 to is the first range; from to is the second range.

[0027] Preferably, select one line from the lines connected to MTUPFC as the balance line, specifically: Calculate the capacity margin and voltage sensitivity of each line connected to MTUPFC; and normalize them respectively; the capacity margin is the ratio of the rated active power capacity to the currently actually transmitted active power; the voltage sensitivity is the partial derivative of the line voltage with respect to the active or reactive power injected into the line; Perform weighted summation on the normalized capacity margin and voltage sensitivity according to the set weights, and the line with the largest weighted summation result is used as the balance line.

[0028] Preferably, when performing constant AC power source control and constant AC voltage control, the constraint conditions of the remaining capacity of the set remaining lines are satisfied, and the constraint condition formula is:

[0029] In the formula, is the current active power of the k th remaining line, is the current reactive power of the line, the k th remaining line's rated power, Article k The apparent power actually transmitted by the remaining lines, The apparent power required for failover; n Is the total number of the remaining lines; When the constraint condition of the remaining capacity of the remaining lines is not satisfied, the remaining lines are cut off according to the set priority until the constraint condition of the remaining capacity of the remaining lines is satisfied.

[0030] Preferably, the fixed AC power supply is controlled by MTUPFC as the active power supply, and a controllable AC voltage is injected into the line through the power flow regulation module PFCM to directly change the equivalent impedance and power angle of the line, and actively adjust the transmission direction and magnitude of the active / reactive power; while the fixed 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 of the parallel CMI 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 MTUPFC. The formula is:

[0031] Wherein, Is the port voltage, Is the voltage at the connection of 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.

[0032] As Figure 4 Shown, a PI controller is used for voltage closed-loop regulation when performing constant amplitude and constant frequency control, and a feedforward term is added during the voltage closed-loop regulation.

[0033] Preferably, the modulation index optimal control is to adjust the modulation index until the objective function is minimized within the set modulation index adjustment range. The modulation index is the ratio of the amplitude of the modulation wave to the amplitude of the carrier wave when modulating MTUPFC. The objective function is:

[0034] In the formula, t represents the time step; T represents the total number of time steps; Represents the sum of the switching loss and the line resistance loss at the t-th time step; Represents the rated power of MTUPFC; Represents the DC voltage deviation at the t-th time step; Represents the rated DC voltage; Represents the line power deviation at the t-th time step; Represents the maximum transmission capacity of the line; represents the switching frequency; represents the maximum frequency allowed for switching; , , , are the set weights; is the penalty coefficient, is the constraint penalty term for voltage over - limit and current over - limit at the t - th time step, and this constraint penalty term is the sum of the squares of the voltage over - limit and current over - limit parts respectively.

[0035] Preferably, the control strategy for adjusting each port of the MTUPFC according to the active network power capacity is specifically as follows: Judge whether the active network power capacity can meet the internal load of the active network. If it can be met, the control strategy of each port of the MTUPFC remains unchanged; if it cannot be met, adjust the control strategy of the MTUPFC port connected to the faulty line to constant AC power control.

[0036] Embodiment 2 of the present invention proposes a load transfer system based on a transformerless multi - port power router, including a fault location module, an upper - level line fault adjustment module, a balance line selection module, and a local - level line fault adjustment module, characterized in that: Fault location module: When a power grid fault occurs, it is used to judge the fault location according to the protection devices in the power grid and in combination with the operation mode of the MTUPFC at this time; Upper - level line fault adjustment module: If the fault location is on the upper - level line of the line connected to the MTUPFC, disconnect the upper - level power grid, change the control strategy of each port of the MTUPFC to constant AC voltage control, and perform load transfer; Balance line selection module: If the fault location is in the lines connected to the MTUPFC, select one of these lines as the balance line; Local - level line fault adjustment module: Judge whether the fault occurs on the balance line. If not, change the control strategy of the MTUPFC port connected to the faulty line to constant AC power control; if so, first cut off 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 constant AC voltage control; and re - select the balance line among the remaining lines, and change the control strategy of the port connected to the balance line to optimal modulation control; if the faulty line is an active network, adjust the control strategy of each port of the MTUPFC according to the active network power capacity.

[0037] Such as Figure 3As shown, to verify the effectiveness of the control strategy, a power flow control system based on MTUPFC was built in Matlab / Simulink. In this embodiment, the MTUPFC is connected to three 220 kV transmission lines. The line 1 was calculated and selected as the balanced line. After a three-phase short circuit fault between phases occurred, the sectionalizing switch on line 1 and the circuit breaker at the port outlet of the MTUPFC tripped. Then, line 2 was recalculated and selected as the new balanced line. The MTUPFC restored power supply for the 0.6 pu active load that was originally supplied by the line. Before and after restoring power supply for the original load on line 1, the power flow conditions of each line in the simulation are shown in Table 1, and the active load on line 1 was evenly distributed between line 2 and line 3; The simulation time sequence is as follows. At t = 0.1 s, the power supply restoration control strategy was started, and the reference value of the active power of line 3 decreased at a fixed slope and dropped to 0.3 pu at t = 0.15 s. All the above powers are per-unit values.

[0038] In this embodiment, the MTUPFC is connected to three 220 kV transmission lines, Table 1 Power flow conditions of each line

[0039] Figure 5 and 6 Figures 7, 8, 9, and 10 are the waveforms of the active power and reactive power of lines 1, 2, and 3 respectively. It can be seen from the waveforms that after line 1 lost power, the power supply restoration of line 1 was achieved through the constant voltage control of the MTUPFC.

[0040] The present disclosure may be a system, a method, and / or a computer program product. The computer program product may include a computer-readable storage medium having computer-readable program instructions thereon for causing a processor to implement various aspects of the present disclosure.

[0041] A computer-readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. A computer-readable storage medium may 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 of the foregoing. More specific examples (a non-exhaustive list) of the computer-readable storage medium include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disc (DVD), a memory stick, a floppy disk, a mechanically encoded device such as a punched card or raised structures in grooves having instructions stored thereon, and any suitable combination of the foregoing. The computer-readable storage medium used herein is not construed as an instantaneous signal itself, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagated through a waveguide or other transmission medium (e.g., an optical pulse through an optical fiber cable), or an electrical signal transmitted through a wire.

[0042] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to various computing / processing devices, or downloaded to an external computer or external storage device through a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network may include a copper transmission cable, an optical fiber transmission, a wireless transmission, a router, a firewall, a switch, a gateway computer, and / or an edge server. A 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 for storage in a computer-readable storage medium in each computing / processing device.

[0043] Computer program instructions for performing the operations of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related 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++, etc., and conventional procedural programming languages such as the "C" language or similar programming languages. The computer-readable program instructions may be executed entirely on the user's computer, partially on the user's computer, executed 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 through 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., through the Internet using an Internet service provider). In some embodiments, by using the state information of the computer-readable program instructions to customize an electronic circuit, such as a programmable logic circuit, a field-programmable gate array (FPGA), or a programmable logic array (PLA), the electronic circuit can execute the computer-readable program instructions to implement various aspects of the present disclosure.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: it is still possible to modify the specific embodiments of the present invention or make equivalent replacements, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the protection scope 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 upper line of the line connected to the MTUPFC, the upper power grid is disconnected, and the control strategy of each port of the MTUPFC is adjusted to constant AC voltage control for power transfer; If the fault location is in the connected lines of the MTUPFC, one of the connected lines is selected as the balancing line; Determine whether the fault occurs on the balanced line. If not, adjust the control strategy of the MTUPFC port connected to the faulty line to fixed AC power control. If so, cut off 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 fixed AC voltage control. Reselect the 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, adjust the control strategy of each MTUPFC port according to the power capacity of the active network.

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. The equivalent line voltage is obtained by subtracting the injection voltage of the MTUPFC series side from the voltage at the protection device measured by the directional protection device. The phase difference between the equivalent line voltage and the current at the protection device measured is calculated. If the phase difference is within the 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; And according to the impedance per unit length of the line and the impedance at the protection device measured by the distance protection device, the distance from the fault point to the protection installation location is calculated; If the directional protection device determines that it is a forward fault and the distance from the fault point to the protection installation location is within a set first range, it is determined that 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 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 situation, the directional protection device and the distance protection device repeat the above process to make 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: In the formula, ; , are the upper and lower limits of the range of adaptive changes, 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 The phase of Inject voltage into 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 setting the first range and the second range is: In the formula, is the upper limit of the first range; is the upper limit of the second range; , All are elongation coefficients; Between 0.8 and 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 is the first range; 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: Select one line from the lines connected to MTUPFC as the balancing line, specifically: Calculate the capacity margin and voltage sensitivity of each line connected to MTUPFC and normalize them respectively; The capacity margin is the ratio of the rated active power capacity to the active power actually transmitted at present; 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, and the line with the largest weighted sum result is taken as the balanced line.

6. 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 is satisfied. The constraint condition formula is: In the formula, For the k The current active power of the remaining lines, is the current reactive power of the line, No. k The rated power of the remaining lines, No. k The actual transmitted apparent power of the remaining lines, The apparent power required for failover; n is the total number of remaining lines; When the constraint condition of the remaining capacity of the remaining lines is not met, the remaining lines are cut off according to the set priority until the constraint condition of the remaining capacity of the remaining lines is met.

7. The load transfer method based on a transformerless multi-port power router according to claim 1, characterized in that: The constant AC voltage control is to control the voltage at the MTUPFC port with a constant amplitude and a constant 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.

8. 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; It represents 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 by the switch; , , , 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, which is the sum of the squares of the voltage over-limit and current over-limit parts respectively.

9. The load transfer method based on a transformerless multi-port power router according to claim 1, characterized in that: The control strategy for adjusting each port of MTUPFC according to the active network power capacity is specifically as follows: It is determined whether the power capacity of the active network meets the internal load of the active network. If it does, the control strategy of each port of MTUPFC remains unchanged; if it does not, the control strategy of the MTUPFC port connected to the faulty line is adjusted to fixed AC power control.

10. A load transfer system based on a transformerless multi-port power router using the method according to any one of claims 1 to 9, 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, the fault location is determined based on the protection devices in the power grid and the operating mode of the MTUPFC at that time; The upper line fault adjustment module is used to disconnect the upper power grid if the fault location is in the upper line of the line connected to the MTUPFC, and change the control strategy of each port of the MTUPFC to fixed AC voltage control for power transfer; Balanced line selection module: used to select one of the lines connected to the MTUPFC as a balanced line if the fault location is in the lines connected to the MTUPFC; This level line fault regulation module: determines 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 fixed AC power supply control; If so, the faulty line is cut off first. 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; And reselect the balanced line from the remaining lines, and change the control strategy of the port connected to the balanced line to the optimal modulation control; if the faulty line is an active network, adjust the control strategy of each port of MTUPFC according to the power capacity of the active network.

Images