Method and system for quickly recovering transformer-free multi-terminal flexible interconnection device after failure

Abstract

The invention discloses a method and system for quickly recovering a transformer-free multi-terminal flexible interconnection device after a fault, and relates to the technical field of alternating current power grid power flow control, and the method comprises the steps: carrying out the fault isolation through a thyristor-based switch when a power transmission line connected with the transformer-free multi-terminal flexible interconnection device breaks down; cutting off a fault line, recovering power flow control of a series connection part, and recovering operation of a non-fault line; and judging whether a fault occurs in the balance line, and reselecting and adjusting a new balance line and a control strategy. According to the method for rapidly recovering the transformer-free type multi-terminal flexible interconnection device after the fault, the series converter of the transformer-free type multi-terminal flexible interconnection device is rapidly isolated from the power grid, the safety and reliability of the system are improved, the non-fault line is rapidly recovered to operate, the operation efficiency of the power grid is improved, and the service life of the power grid is prolonged. A new balance line is selected and adjusted, the flexibility of the system is improved, and better effects are achieved in the aspects of safety, flexibility and operation efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of AC power grid power flow control, and in particular to a method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault. Background Art

[0002] With the rapid development of economy and science and technology, the demand for electric energy in society continues to rise. The continuous development of new energy sources such as wind energy and photovoltaics has made the structure of modern power systems increasingly complex. The power supply and load are constantly increasing. The interconnection of regional power grids has formed a large network of power systems. This interconnected power grid has significantly improved the reliability of power supply and enhanced the ability of the power grid to optimize the allocation of power resources. However, this has also brought a series of problems, such as uneven distribution of natural currents in the system, insufficient reactive voltage support capacity, and electromechanical oscillations, which have brought new challenges to the safe and stable operation of the power grid. At present, power grid flow control mainly relies on electromagnetic and mechanical technologies, such as adjusting generator output, transformer tap switches, and line disconnection, but these means are no longer sufficient to meet the needs of new power grids.

[0003] A soft normally open point (SNOP) can fully utilize the capacity of existing AC power transmission lines until their thermal limit by controlling power flow. Compared with building new transmission lines, applying SNOP to power transmission is a more economical and promising solution. However, the commercial application of traditional SNOP is limited by the high equipment cost, which exceeds US$100 per kilovolt-ampere. In view of the large size, high installation, operation and maintenance costs of traditional SNOP devices, some researchers have proposed a transformerless multi-terminal flexible interconnection device (multiport transformer-less SNOP, MTSNOP). MTSNOP consists of two parts, namely a series converter and a parallel cascade multilevel inverter (cascade multilevel inverter, CMI). This architecture enables the transmission line to independently control active and reactive power. Compared with traditional SNOP, MTSNOP has many advantages, including no need for transformers, light weight, high efficiency, low cost, and easy port expansion. However, when a short circuit fault occurs in the AC transmission line, the series converter structure of MTSNOP will cause the fault current to propagate to other operating transmission lines. This phenomenon may cause potential damage to the device and the power grid. Summary of the invention

[0004] In view of the above-mentioned problems, the present invention is proposed.

[0005] Therefore, the technical problem solved by the present invention is: the existing fast recovery method has low fault isolation efficiency, slow recovery of non-fault lines, inflexible balance line selection and control strategy adjustment, and how to solve the optimization problem of incomplete fault isolation.

[0006] In order to solve the above technical problems, the present invention provides the following technical solutions: a method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault, comprising: when a transmission line connected to the transformerless multi-terminal flexible interconnection device fails, isolating the fault through a thyristor-based switch; cutting off the faulty line, restoring the flow control of the series part, and restoring the operation of the non-faulty line; judging whether the fault occurs in the balanced line, and reselecting and adjusting a new balanced line and control strategy.

[0007] As a preferred solution of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault according to the present invention, the fault isolation through a thyristor-based switch includes that when a fault occurs in the transformerless multi-terminal flexible interconnection device or the AC power grid, the thyristor switch is triggered to open, bypassing the series converter in the transformerless multi-terminal flexible interconnection device, and the control protection unit of the flexible interconnection device closes the bypass circuit breaker to provide a path for the fault current, replacing the thyristor switch, and based on the fast bypass characteristics of the thyristor switch, fault isolation is performed, and the bypass circuit breakers on both sides of the series converter are closed. When a transmission line fails, the equipment is isolated, and the transformerless multi-terminal flexible interconnection device immediately blocks the sub-unit, so that the output voltage of the sub-unit is clamped to 0V.

[0008] As a preferred solution of the method for rapid recovery after a fault of a transformerless multi-terminal flexible interconnection device described in the present invention, the fault isolation by a thyristor-based switch also includes combining the instantaneous change of the fault current, introducing an exponential decay function, combining the circuit time constant, describing the rapid change of the current, and based on the influence of the inductance on the current change, modeling the influence of the inductance by a differential operator, expressed as:

[0009]

[0010] Among them, I i (t) is the instantaneous current after fault isolation, R b is the resistance of the bypass circuit, L b is the inductance of the bypass circuit, V in Input voltage for the series converter of the transformerless multi-terminal flexible interconnection device.

[0011] As a preferred solution of the method for rapid recovery after a fault of a transformerless multi-terminal flexible interconnection device described in the present invention, the recovery operation of the non-fault line includes a power flow control equation, and the instantaneous active power of the non-fault line is calculated through the voltage and current of the non-fault line and the phase angle between the voltage and the current. The instantaneous active power reflects the operation state and load condition of the line. The current of the non-fault line is maintained by adjusting the output voltage of the series part. The output current of the fault isolation model is used as the input of the control model. When the operation is restored, the adjustment process of the output voltage is optimized through the coordinated work of the proportional integral controller in combination with the influence of the fault isolation on the system, which is expressed as:

[0012]

[0013]

[0014] Among them, V nf is the voltage of the non-fault line, I nf is the current of the non-fault line, P nf (t) is the instantaneous active power of the non-fault line, ω nf is the instantaneous angular frequency of the non-fault line, V s is the output voltage of the series part, K ps and K is They are the proportional and integral gains of the series part respectively.

[0015] As a preferred solution of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault as described in the present invention, the judgment of whether the fault occurs in a balanced line includes designating one line as a balanced line among all lines connected to the transformerless multi-terminal flexible interconnection device to balance the active power of all lines. When a line fault occurs, it is determined whether the fault occurs on the balanced line. When the fault occurs on an unbalanced line, the faulty line is isolated and the equipment is disconnected to restore the operation of the transformerless multi-terminal flexible interconnection device without changing the control strategy. If the fault occurs on a balanced line, a new balanced line is selected and the control strategy is modified.

[0016] As a preferred solution of the method for rapid recovery after a fault of the transformerless multi-terminal flexible interconnection device described in the present invention, wherein: the reselection and adjustment of the new balance line and control strategy include that when a fault occurs, the switch of the thyristor immediately activates the bypass device and the internal sub-unit to isolate the transformerless multi-terminal flexible interconnection device from the power grid and disconnect the fault line at the same time. When a fault is detected on the balance line, a new balance line is selected from the remaining operating lines, and the input of the phase-locked loop is switched from the three-phase voltage of the previous balance line to the voltage of the newly selected balance line, based on the output voltage of the parallel cascaded multi-level inverter and the system voltage level. Matching, give priority to restoring the operation of the parallel cascade multilevel inverter, the sub-units of the series part remain in the bypass state, after the parallel cascade multilevel inverter is restored, the series converter resumes operation, when switching the balance line, the line control is changed from line power control to series voltage modulation optimal control, when a fault occurs, the line flow loses control, and the PI controller enters the saturation state, first reset the PI controller to stabilize the internal energy of the series converter, after the line flow control is restored, restore the control of the overall capacitor voltage of the series converter, restore the internal energy balance control, balance the energy between phases, between arms and between sub-units.

[0017] As a preferred solution of the method for rapid recovery after a fault of a transformerless multi-terminal flexible interconnection device described in the present invention, the reselection and adjustment of the new balance line and control strategy also includes reselecting and adjusting the balance line after the fault is isolated and the non-fault line resumes operation. The balance line selection is based on the active power and reactive power of all lines. The new balance line is determined by minimizing the sum of the squares of the power. The power contribution of each line is combined to make the newly selected balance line balance the power distribution of the entire system. The new control strategy adjustment model uses the voltage and current of the new balance line, and is adjusted in combination with the output of the previous power flow control model. The phase of the phase-locked loop is recalculated based on the instantaneous angular frequency of the voltage and current of the new balance line, so that the new balance line is synchronized to the system, which is expressed as:

[0018]

[0019] Among them, P i and Q i are the active power and reactive power of the line respectively, θ p (t) is the phase of the phase-locked loop, Q b (τ) is the reactive power of the balanced line.

[0020] Another object of the present invention is to provide a transformerless multi-terminal flexible interconnection device post-fault rapid recovery system, which can perform fault isolation through a thyristor-based switch via a fault isolation module, thereby solving the current problem of serious damage to the internal components of the converter caused by fault current.

[0021] As a preferred solution of the post-fault rapid recovery system of the transformerless multi-terminal flexible interconnection device described in the present invention, it includes: a fault isolation module, a control recovery module, and a selection and adjustment module; when a fault occurs in the transmission line connected to the transformerless multi-terminal flexible interconnection device, the fault isolation module performs fault isolation through a thyristor-based switch; the control recovery module is used to cut off the faulty line, restore the power flow control of the series part, and restore the operation of the non-faulty line; the selection and adjustment module is used to determine whether the fault occurs in the balanced line, and reselect and adjust the new balanced line and control strategy.

[0022] A computer device includes a memory and a processor, wherein the memory stores a computer program, and is characterized in that the processor executes the computer program to implement a step of a method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault.

[0023] A computer-readable storage medium stores a computer program, wherein when the computer program is executed by a processor, the computer program implements the steps of a method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault.

[0024] Beneficial effects of the present invention: The method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault provided by the present invention quickly isolates the series converter of the transformerless multi-terminal flexible interconnection device from the power grid, thereby improving the safety and reliability of the system, reducing equipment damage and fault losses, and rapidly resuming operation of non-fault lines, thereby improving the operating efficiency of the power grid, rapidly selecting and adjusting new balancing lines, and improving the flexibility and response speed of the system. The present invention achieves better results in terms of safety, flexibility, and operating efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0026] Figure 1 This is an overall flow chart of a method for rapid recovery after a transformerless multi-terminal flexible interconnection device failure provided in the first embodiment of the present invention.

[0027] Figure 2 The topological structure and schematic diagram of interconnected multi-transmission lines of a method for rapid recovery after a fault of a transformerless multi-terminal flexible interconnection device provided in the second embodiment of the present invention.

[0028] Figure 3This is a current waveform diagram of line 2 during the fault recovery process of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault provided by the second embodiment of the present invention.

[0029] Figure 4 Three current waveforms of the line during the fault recovery process of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault provided by the second embodiment of the present invention.

[0030] Figure 5 Reactive waveform diagram of each line during the fault recovery process of the transformerless multi-terminal flexible interconnection device for rapid recovery after a fault provided by the second embodiment of the present invention.

[0031] Figure 6 Active waveform diagram of each line during the fault recovery process of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault provided by the second embodiment of the present invention.

[0032] Figure 7 This is a CMI capacitor voltage waveform diagram of the fault recovery process of the method for rapid recovery after a fault of a transformerless multi-terminal flexible interconnect device provided in the second embodiment of the present invention.

[0033] Figure 8 This is a waveform diagram of the voltage of the capacitor of the series converter during the fault recovery process of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault provided by the second embodiment of the present invention.

[0034] Fig. 9 This is an overall module diagram of a system for rapid recovery after a transformer-less multi-terminal flexible interconnection device failure provided in the third embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0036] Example 1

[0037] Reference Figure 1 , is an embodiment of the present invention, and provides a method for rapid recovery of a transformer-free multi-terminal flexible interconnection device after a fault, comprising:

[0038] S1: When a fault occurs in the transmission line connected to the transformerless multi-terminal flexible interconnection device, the fault is isolated by a thyristor-based switch.

[0039] Furthermore, fault isolation by thyristor-based switches includes when a fault occurs in a transformerless multi-terminal flexible interconnection device or an AC power grid.

[0040] It should be noted that the switch of the thyristor is triggered to open, bypassing the series converter in the transformerless multi-terminal flexible interconnection device, and the control protection unit of the flexible interconnection device closes the bypass circuit breaker to provide a path for the fault current, replacing the switch of the thyristor. Based on the fast bypass characteristics of the thyristor switch, fault isolation is performed and the bypass circuit breakers on both sides of the series converter are closed. When the transmission line fails, the equipment is isolated, and the transformerless multi-terminal flexible interconnection device immediately blocks the sub-unit, so that the output voltage of the sub-unit is clamped to 0V.

[0041] Furthermore, fault isolation by thyristor-based switching also includes incorporating transient changes in the fault current.

[0042] It should be noted that the exponential decay function is introduced and combined with the circuit time constant to describe the rapid change of current. Based on the influence of inductance on current change, the influence of inductance is modeled by differential operator, which is expressed as:

[0043]

[0044] Among them, I i (t) is the instantaneous current after fault isolation, R b is the resistance of the bypass circuit, L b is the inductance of the bypass circuit, V in Input voltage for the series converter of the transformerless multi-terminal flexible interconnection device.

[0045] S2: Cut off the faulty line, restore the flow control of the series part, and resume the operation of the non-faulty line.

[0046] Furthermore, restoring the non-faulty line to service includes the power flow control equations.

[0047] It should be noted that the instantaneous active power of the non-fault line is calculated by the voltage and current of the non-fault line and the phase angle between the voltage and current. The instantaneous active power reflects the operating status and load conditions of the line. The current of the non-fault line is maintained by adjusting the output voltage of the series part. The output current of the fault isolation model is used as the input of the control model. When the operation is restored, the adjustment process of the output voltage is optimized through the coordinated work of the proportional integral controller in combination with the impact of the fault isolation on the system. It is expressed as:

[0048]

[0049]

[0050] Among them, Vnf is the voltage of the non-fault line, I nf is the current of the non-fault line, P nf (t) is the instantaneous active power of the non-fault line, ω nf is the instantaneous angular frequency of the non-fault line, V s is the output voltage of the series part, K ps and K is They are the proportional and integral gains of the series part respectively.

[0051] S3: Determine whether the fault occurs in the balance line, reselect and adjust a new balance line and control strategy.

[0052] Furthermore, it is determined whether the fault occurs in the balanced line including all the lines connected to the transformerless multi-terminal flexible interconnection device.

[0053] It should be noted that one line is designated as the balanced line to balance the active power of all lines. When a line fault occurs, it is determined whether the fault occurs on the balanced line. When the fault occurs on the unbalanced line, the faulty line is isolated and the equipment is disconnected to restore the operation of the transformerless multi-terminal flexible interconnection device without changing the control strategy. If the fault occurs on the balanced line, a new balanced line is selected and the control strategy is modified.

[0054] Furthermore, new balancing lines and control strategies are reselected and adjusted including when a fault occurs.

[0055] It should be noted that the switch of the thyristor immediately activates the bypass device and the internal sub-unit, isolates the transformerless multi-terminal flexible interconnection device from the power grid, and disconnects the faulty line at the same time. When a fault is detected on the balancing line, a new balancing line is selected from the remaining operating lines, and the input of the phase-locked loop is switched from the three-phase voltage of the previous balancing line to the voltage of the newly selected balancing line. Based on the matching of the output voltage of the parallel cascade multilevel inverter with the system voltage level, the operation of the parallel cascade multilevel inverter is restored first, and the sub-units of the series part are kept in the bypass state. After the parallel cascade multilevel inverter is restored, the series converter resumes operation. When switching the balancing line, the control of the line is changed from line power control to optimal control of the series voltage modulation. When a fault occurs, the line flow loses control and the PI controller enters a saturated state. The PI controller is reset first to stabilize the internal energy of the series converter. After the line flow control is restored, the control of the overall capacitor voltage of the series converter is restored, and the internal energy balance control is restored to balance the energy between phases, between arms and between sub-units.

[0056] Furthermore, reselecting and adjusting a new balancing line and control strategy is also included after the fault is isolated and the non-fault line is restored to operation.

[0057] It should be noted that the balance line is reselected and adjusted. The balance line selection is based on the active power and reactive power of all lines. The new balance line is determined by minimizing the sum of the squares of the power. The power contribution of each line is combined to make the newly selected balance line balance the power distribution of the entire system. The new control strategy adjustment model uses the voltage and current of the new balance line and is adjusted in combination with the output of the previous power flow control model. The phase of the phase-locked loop is recalculated based on the instantaneous angular frequency of the voltage and current of the new balance line to synchronize the new balance line to the system, which is expressed as:

[0058]

[0059] Among them, P i and Q i are the active power and reactive power of the line respectively, θ p (t) is the phase of the phase-locked loop, Q b (τ) is the reactive power of the balanced line.

[0060] Example 2

[0061] Reference Figure 2-8 , which is an embodiment of the present invention, provides a method for rapid recovery of a transformer-free multi-terminal flexible interconnection device after a fault. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0062] Taking the transformerless multi-terminal flexible interconnection device connecting three transmission lines as an example, assuming that line 1 is the original balanced line, after a three-phase phase-to-phase short circuit fault occurs, the section switch on line 1 and the circuit breaker at the port outlet of the transformerless multi-terminal flexible interconnection device trip, and line 2 is reselected as the new balanced line. The phase-locked loop is oriented to line 2, and the parallel cascade multi-level inverter control is restored to maintain line voltage stability. Then the power flow control of the series converter part is restored. At this time, the control of line 2 is changed from line power flow control to modulation optimal control. While restoring the power flow control, the overall energy balance control of the series part is restored. Finally, the internal energy balance control is restored, and the power flow control system After the fault isolation and power flow control function restoration, the stable operation of the non-fault line is guaranteed. The system is simulated and verified by MATLAB / Simulink software. The power flow conditions of each line in the simulation are shown in Table 1, and the simulation parameters are shown in Table 2. The simulation timing is as follows: at t = 0.1s, a three-phase short circuit fault occurs in line 1; at t = 0.1015s, the MTSNOP device is locked and the TBS is triggered; at t = 0.14s, the fault line trips; at t = 0.15s, the reactive power compensation control of the cascaded multilevel inverter is restored; at t = 0.25s, the power flow control and the overall capacitor voltage control return to normal.

[0063] Table 1 Flow conditions of each line

[0064] Command name <![CDATA[P1]]> <![CDATA[Q1]]> <![CDATA[P2]]> <![CDATA[Q2]]> <![CDATA[P3]]> <![CDATA[Q3]]> Working conditions -0.8pu 0.2pu 0.4pu 0.2pu 0.4pu 0.2pu

[0065] Table 2 Simulation parameters

[0066] parameter Numeric Rated apparent power of transmission line <![CDATA[S N =2.5kV·A]]> Node 1 voltage <![CDATA[V 1,RMS =240V,θ1=0°<!-- 6 --> ]]> Node 2 voltage <![CDATA[V 2,RMS =240V,θ2=0°]]> Node 3 voltage <![CDATA[V 3,RMS =240V,θ3=0°]]> Transmission Line Inductance <![CDATA[L3=0.01p.u.]]> PFCM subunit capacitance <![CDATA[C PFCM =10.8mF,V PFCM =30V]]> Number of PFCM bridge arm subunits 2 Cascaded multilevel inverter subunit capacitors <![CDATA[C CHB =10.8mF,V CHB =30V]]> Number of cascaded multilevel inverter subunits 8 Switching frequency 3kHz

[0067] like Figure 3-Figure 8 As shown, the waveform diagram is a simulation result of the recovery of operation of the transformerless multi-terminal flexible interconnection device before the fault occurs and after the fault is restored. The post-fault rapid recovery strategy based on the transformerless multi-terminal flexible interconnection device of the present invention can accurately and quickly restore the stable operation of the non-fault line after the fault, and at the same time cooperate with the flow control of the non-fault line to realize flexible adjustment of the flow of the non-fault line.

[0068] Example 3

[0069] Reference Fig. 9 , as an embodiment of the present invention, provides a rapid recovery system for a transformer-free multi-terminal flexible interconnection device after a fault, comprising: a fault isolation module, a control recovery module, and a selection adjustment module.

[0070] Among them, the fault isolation module is used to isolate the fault through a thyristor-based switch when a transmission line connected to the transformerless multi-terminal flexible interconnection device fails; the control recovery module is used to cut off the fault line, restore the power flow control of the series part, and restore the operation of the non-fault line; the selection and adjustment module is used to determine whether the fault occurs in the balancing line, and reselect and adjust the new balancing line and control strategy.

[0071] If the function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium, including several instructions for a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the methods of each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, etc. Various media that can store program codes.

[0072] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For the purposes of this specification, "computer-readable medium" can be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0073] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering, or processing in another suitable manner as necessary, and then stored in a computer memory.

[0074] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc. It should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limited. Although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

[0075] 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 preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault, characterized in that: include: When a transmission line connected to the transformerless multi-terminal flexible interconnection device fails, the fault is isolated through a thyristor-based switch; Cut off the faulty line, restore the power flow control of the series part, and resume the operation of the non-faulty line; Determine whether the fault occurs in the balanced line, and reselect and adjust the new balanced line and control strategy.

2. The method for rapid recovery after a transformer-free multi-terminal flexible interconnection device failure as claimed in claim 1, characterized in that: The fault isolation by the thyristor-based switch includes that when a fault occurs in the transformerless multi-terminal flexible interconnection device or the AC power grid, the thyristor switch is triggered to open, bypassing the series converter in the transformerless multi-terminal flexible interconnection device, and the control protection unit of the flexible interconnection device closes the bypass circuit breaker to provide a path for the fault current, replacing the thyristor switch, and based on the fast bypass characteristics of the thyristor switch, fault isolation is performed, and the bypass circuit breakers on both sides of the series converter are closed. When a transmission line fault occurs, the equipment is isolated, and the transformerless multi-terminal flexible interconnection device immediately blocks the sub-unit, so that the output voltage of the sub-unit is clamped to 0V.

3. The method for rapid recovery after a transformer-free multi-terminal flexible interconnection device failure as claimed in claim 2, characterized in that: The fault isolation by the thyristor-based switch also includes combining the instantaneous change of the fault current, introducing an exponential decay function, combining the circuit time constant, describing the rapid change of the current, and based on the effect of the inductance on the current change, modeling the effect of the inductance by a differential operator, expressed as: Among them, I i (t) is the instantaneous current after fault isolation, R b is the resistance of the bypass circuit, L b is the inductance of the bypass circuit, V in Input voltage for the series converter of the transformerless multi-terminal flexible interconnection device.

4. The method for rapid recovery after a transformerless multi-terminal flexible interconnection device failure as claimed in claim 3, characterized in that: The restoration of the non-fault line includes a power flow control equation. The instantaneous active power of the non-fault line is calculated through the voltage and current of the non-fault line and the phase angle between the voltage and the current. The instantaneous active power reflects the operation status and load condition of the line. The current of the non-fault line is maintained by adjusting the output voltage of the series part. The output current of the fault isolation model is used as the input of the control model. When restoring operation, combined with the impact of fault isolation on the system, the adjustment process of the output voltage is optimized through the coordinated work of the proportional integral controller, which is expressed as: Among them, V nf is the voltage of the non-fault line, I nf is the current of the non-fault line, P nf (t) is the instantaneous active power of the non-fault line, ω nf is the instantaneous angular frequency of the non-fault line, V s is the output voltage of the series part, K ps and K is They are the proportional and integral gains of the series part respectively.

5. The method for rapid recovery after a transformer-free multi-terminal flexible interconnection device failure as claimed in claim 4, characterized in that: The determination of whether the fault occurs in a balanced line includes designating one of all the lines connected to the transformerless multi-terminal flexible interconnection device as a balanced line to balance the active power of all the lines. When a line fault occurs, determining whether the fault occurs on the balanced line, when the fault occurs on an unbalanced line, isolating the faulty line and disconnecting the equipment, restoring the operation of the transformerless multi-terminal flexible interconnection device without changing the control strategy, and if the fault occurs on a balanced line, selecting a new balanced line and modifying the control strategy.

6. The method for rapid recovery after a transformerless multi-terminal flexible interconnection device failure as claimed in claim 5, characterized in that: The reselection and adjustment of the new balancing line and control strategy include that when a fault occurs, the switch of the thyristor immediately activates the bypass device and the internal sub-unit, isolates the transformerless multi-terminal flexible interconnection device from the power grid, and disconnects the fault line at the same time; when a fault is detected on the balancing line, a new balancing line is selected from the remaining operating lines, and the input of the phase-locked loop is switched from the three-phase voltage of the previous balancing line to the voltage of the newly selected balancing line; based on the matching of the output voltage of the parallel cascade multilevel inverter with the system voltage level, the operation of the parallel cascade multilevel inverter is restored first, and the sub-units of the series part are kept in the bypass state; after the parallel cascade multilevel inverter is restored, the series converter is restored to operate; when switching the balancing line, the control of the line is changed from line power control to optimal control of the series voltage modulation; when a fault occurs, the line flow loses control and the PI controller enters a saturated state; the PI controller is reset first to stabilize the internal energy of the series converter; after the line flow control is restored, the control of the overall capacitor voltage of the series converter is restored, the internal energy balance control is restored, and the energy between each phase, between each arm and between each sub-unit is balanced.

7. The method for rapid recovery after a transformerless multi-terminal flexible interconnection device failure as claimed in claim 6, characterized in that: The reselection and adjustment of the new balancing line and control strategy also includes reselecting and adjusting the balancing line after the fault isolation and non-fault line restoration, wherein the balancing line selection is based on the active power and reactive power of all lines, and the new balancing line is determined by minimizing the sum of squares of the power, and the power contribution of each line is combined to make the newly selected balancing line balance the power distribution of the entire system, and the new control strategy adjustment model uses the voltage and current of the new balancing line, and is adjusted in combination with the output of the previous power flow control model, and the phase of the phase-locked loop is recalculated based on the instantaneous angular frequency of the voltage and current of the new balancing line, so that the new balancing line is synchronized to the system, which is expressed as: Among them, P i and Q i are the active power and reactive power of the line respectively, θ p (t) is the phase of the phase-locked loop, Q b (τ) is the reactive power of the balanced line.

8. A system using the method for rapid recovery after a transformerless multi-terminal flexible interconnection device failure as claimed in any one of claims 1 to 7, characterized in that: It includes fault isolation module, control recovery module and selection adjustment module; The fault isolation module performs fault isolation through a thyristor-based switch when a transmission line connected to the transformerless multi-terminal flexible interconnection device fails; The control recovery module is used to cut off the faulty line, restore the power flow control of the series part, and restore the operation of the non-faulty line; The selection and adjustment module is used to determine whether a fault occurs in a balanced line, and to reselect and adjust a new balanced line and control strategy.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault are implemented as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for rapid recovery of a transformerless multi-terminal flexible interconnection device after a fault are implemented as described in any one of claims 1 to 7.

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