Transformerless modular multi-terminal flexible interconnection controller series connection current limiting method and system

Abstract

The invention discloses a transformer-free modular multi-terminal flexible interconnection controller series connection current limiting method and system, and relates to the technical field of short circuit current limiting and power electronics, and the method comprises the steps: locking a power electronic device at the moment of a power transmission line fault, and stopping the current from passing through the power electronic device; a TBS bypass power electronic device is started, so that fault current passes through a bypass channel; a current-limiting inductor is put into the line, and short-circuit current flowing through the line is reduced through the current-limiting inductor. Through a transformer-free design, a current-limiting inductor, a TBS bypass, a power flow regulation module (PFCM), an SPWM modulation mode, voltage phase angle control and the like, the problems of large equipment size, high cost, short-circuit current diffusion, slow fault recovery, low control precision and the like in the prior art are effectively solved respectively. The beneficial effects are obviously superior to the existing technical scheme in the aspects of fault response, recovery speed, stability, cost control and the like of the system.

Description

Technical Field

[0001] The present invention relates to the field of short-circuit current limiting and power electronics technology, and in particular to a series current limiting method and system for transformerless modular multi-terminal flexible interconnected controllers. Background Art

[0002] In traditional interconnected power grids, the distribution of power flow is mainly determined by the grid structure of the system. It is usually very difficult to make large-scale adjustments using conventional methods, so the actual power flow distribution may not be ideal. In addition, large-scale interconnected power grids cover a wide area, and the regions affect each other. Once a fault occurs, it may cause a catastrophic large-scale power outage. In order to improve the stability of the system, it is necessary to adopt a control method that can respond quickly based on system parameters. FACTS technology is based on the traditional AC transmission system. It uses devices composed of high-power power electronic devices and advanced control technology to flexibly and quickly control the operating parameters and topological structure of the power system, realize the reasonable distribution of transmission power, reduce power loss and reduce power generation costs, thereby greatly improving the stability and reliability of the system. As the third-generation FACTS device, the unified power flow controller (Unified Power Flow Controller) has the ability to flexibly and reliably improve the power flow distribution of the AC power grid and enhance reactive support, significantly improve the stability and transmission capacity of the power system, and is of great significance to the construction of smart grids. However, due to the presence of power frequency transformers, the traditional UPFC has high equipment cost, large volume and difficulty in transportation, and low operating efficiency and slow response speed. These problems limit its low-cost and large-scale application in power grids.

[0003] In this regard, some scholars have proposed a transformerless modular multi-terminal flexible interconnection controller, which is mainly composed of two parts: a series converter and a parallel cascade multilevel converter (Cascade Multilevel Inverter, CMI). This series-parallel architecture provides sufficient freedom for independent control of active and reactive power. Compared with the traditional UPFC, the transformerless modular multi-terminal flexible interconnection controller has many significant advantages, including no need for transformers, light weight, high efficiency, low cost and easy scalability of ports. However, due to the mutual coupling of energy between the series parts, the fault characteristics of this structure are relatively complex, and the fault is easily transmitted to the non-fault port, so that the non-fault line also flows through the fault current, affecting the normal operation of the non-fault port. In this regard, it is urgent to propose a current limiting scheme to increase the voltage of the non-fault port and reduce the impact of the fault on the non-fault line. Therefore, the present invention proposes a series current limiting method for a transformerless modular multi-terminal flexible interconnection controller to limit the fault current and raise the voltage of the non-fault port. 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: how to quickly limit the current when a fault occurs, prevent the short-circuit current from spreading, and ensure the normal operation of other non-fault lines.

[0006] In order to solve the above technical problems, the present invention provides the following technical solution: a transformerless modular multi-terminal flexible interconnection controller series current limiting method, which comprises the following steps:

[0007] At the moment of transmission line fault, the power electronic devices are locked to stop the current from passing through the power electronic devices;

[0008] Start the TBS bypass power electronic device to make the fault current pass through the bypass channel;

[0009] Add a current-limiting inductor to the line to reduce the short-circuit current flowing through the line.

[0010] The series connection part is used for active current limiting to further reduce the short-circuit current.

[0011] As a preferred solution of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in the present invention, the current limiting inductor is connected to each port to limit the short-circuit current of each port and prevent the short-circuit current from being transmitted to other ports.

[0012] As a preferred scheme of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in the present invention, wherein: the starting of the TBS bypass power electronic device is to release the TBS bypass of the power electronic device, and enable the power flow regulation module PFCM in the modular multi-terminal flexible interconnection controller to be reconnected in series with the line and start working.

[0013] As a preferred scheme of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in the present invention, the PFCM operates in a static synchronous series compensator mode after reconnecting to the line, outputs a d-axis differential mode voltage matching the line impedance, increases the line impedance and suppresses the short-circuit current.

[0014] As a preferred solution of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in the present invention, the output voltage is controlled by SPWM modulation, and the maximum effective value of the output base frequency voltage is expressed as:

[0015]

[0016] in, is the rated voltage of the submodule capacitor, and N is the number of submodules in one bridge arm of the PFCM.

[0017] As a preferred solution of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in the present invention, among the multiple transmission lines connected to the multi-terminal flexible interconnection controller, the equivalent voltage phase angle of the non-fault line is uniformly calculated to make the PFCM output voltage of the fault line consistent with the equivalent voltage phase of the non-fault line.

[0018] As a preferred solution of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in the present invention, the equivalent voltage phase angle of the non-fault line is expressed as:

[0019]

[0020] Among them, δ i is the voltage phase angle of the ith line.

[0021] As a preferred solution of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in the present invention, wherein:

[0022] Another object of the present invention is to provide a transformerless modular multi-terminal flexible interconnection controller series current limiting system, which can effectively limit the fault current by using the current limiting inductor through series current limiting, and readjust the voltage and impedance of the line after the fault is removed through the power flow regulation module (PFCM), thereby restoring the normal operation of the system. The system not only solves the problems of large size, high cost and slow response of traditional power frequency transformers, but also overcomes the problem of short-circuit current easily spreading and affecting the operation of non-fault lines in the prior art.

[0023] In order to solve the above technical problems, the present invention provides the following technical solutions: a transformerless modular multi-terminal flexible interconnected controller series current limiting system, comprising: a TBS bypass module, a current limiting inductor module, and a power flow regulation module;

[0024] The TBS bypass module is activated when a power system fault occurs, bypasses the power electronic devices, and protects the power electronic components in the system. When the fault ends, the TBS bypass is released and the normal current path of the system is restored;

[0025] The current limiting inductor module limits the short-circuit current of the fault line by increasing the inductive reactance, reduces the current impact, and prevents the current from spreading to the non-fault line or port;

[0026] The power flow regulation module reconnects to the line after the fault is cleared, regulates the power flow by controlling the output voltage, operates in the SSSC mode, outputs the differential mode voltage to increase the line impedance, controls the voltage phase angle of the multi-terminal line, and keeps the voltage of the faulty line consistent with that of the non-faulty line.

[0027] A computer device includes a memory and a processor, wherein the memory stores a computer program, and when the processor executes the computer program, the steps of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller are implemented as described above.

[0028] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps of the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller as described above.

[0029] Beneficial effects of the present invention: The present invention eliminates the traditional power frequency transformer and adopts a modular multi-terminal flexible interconnection controller for power regulation. The traditional UPFC (unified power flow controller) requires a power frequency transformer, resulting in large equipment size, heavy weight, high installation and transportation costs, and low operating efficiency. Through the transformer-free design, the present invention significantly reduces the size and weight of the equipment, reduces the overall cost of the system, and improves the operating efficiency of the system. When a fault occurs, the present invention limits the increase and spread of the short-circuit current by adding a current-limiting inductor to the line. In a traditional system, the fault current is easy to spread rapidly through multiple lines, causing other non-fault lines to be affected. The present invention effectively limits the conduction of the fault current between the lines by connecting a current-limiting inductor at each port, ensuring that the non-fault line is not affected, thereby improving the stability of the system. At the moment of the fault, the power electronic device may not be able to respond in time, resulting in damage to the device. The present invention starts the TBS bypass module, and after the power electronic device is locked, a low-impedance channel is provided for the fault current, so that the fault current will not pass through sensitive power electronic devices, thereby protecting the safe operation of these devices and extending the life of the equipment. After the fault is removed, the system needs to resume normal operation, especially to restore the power flow and voltage regulation of the line. The present invention adopts a power flow regulation module (PFCM). After the fault is removed, the PFCM is reconnected in series with the line and operates in the static synchronous series compensator mode (SSSC), outputting a differential mode voltage that matches the line impedance, actively adjusting the line impedance, and further limiting the short-circuit current. At the same time, the PFCM can also adjust the voltage phase angle of the fault line and the non-fault line to ensure that the power flow regulation function is quickly restored after the fault and maintain the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] 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 work. Among them:

[0031] Figure 1A topological structure of a transformerless modular multi-terminal flexible interconnection controller series current limiting method and a system schematic diagram of interconnected multiple transmission lines provided in the first embodiment of the present invention;

[0032] Figure 2 A schematic diagram of a system in which a transformerless modular multi-terminal flexible interconnection controller is connected to three lines in a series current limiting method provided by a third embodiment of the present invention;

[0033] Figure 3 A schematic diagram of controller series current limiting equivalent to the transformerless modular multi-terminal flexible interconnection controller series current limiting method provided in the third embodiment of the present invention;

[0034] Figure 4 A controller series current limiting control strategy diagram of a transformerless modular multi-terminal flexible interconnected controller series current limiting method provided by a third embodiment of the present invention;

[0035] Figure 5 The current waveform of line 1 connected to the controller in the series current limiting method of the transformerless modular multi-terminal flexible interconnection controller provided in the third embodiment of the present invention;

[0036] Figure 6 A voltage waveform diagram of controller port 1 of a transformerless modular multi-terminal flexible interconnection controller series current limiting method provided by the third embodiment of the present invention;

[0037] Figure 7 A current waveform diagram of line 3 connected to the controller in the series current limiting method of a transformerless modular multi-terminal flexible interconnected controller provided in the third embodiment of the present invention;

[0038] Figure 8 This is a voltage waveform diagram of controller port 3 in the series current limiting method of the transformerless modular multi-terminal flexible interconnected controller provided in the third embodiment of the present invention. DETAILED DESCRIPTION

[0039] 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.

[0040] Example 1, reference Figure 1 to Figure 4 According to an embodiment of the present invention, a transformerless modular multi-terminal flexible interconnection controller series current limiting method is provided, comprising:

[0041] The present invention provides a series current limiting method for a transformerless modular multi-terminal flexible interconnection controller. By analyzing the current characteristics under fault conditions and combining the topological structure characteristics, an effective control strategy for series current limiting is proposed. The power flow control module (PFCM) of the series part is fully utilized to realize active current limiting under fault conditions, reduce the voltage drop of the fault line, and further realize the fault ride-through of the device and the recovery of the power flow regulation function after the fault.

[0042] See also Figure 1 The topological structure of the transformerless modular multi-terminal flexible interconnected controller series current limiting method and the system schematic diagram of its interconnected multiple lines; when a fault occurs in the external line of the system, since the power electronic devices are more sensitive to the fault current and have a lower tolerance to overvoltage and overcurrent, the traditional MMC-type UPFC usually adopts the TBS bypass method to avoid the fault current from damaging the switching device. However, in fact, when a short circuit fault occurs in the power grid, due to the inherent delay in the conduction of the TBS, the short-circuit current can still pass through the switching device during the period of TBS conduction, resulting in the risk of overcurrent lockout of the switching device. Moreover, since in this state, the device is completely out of operation, it is not conducive to the recovery of the device's power flow regulation function after the fault.

[0043] In this regard, it is possible to consider making full use of the fault current limiting capability of the device during the fault period, raising the non-fault line voltage and achieving fault ride-through. First, each port side of the device is equipped with an inductor. According to the topological structure of the transformerless modular multi-terminal flexible interconnection controller, when a fault occurs, under the action of the port inductor, the short-circuit current flowing through the port will be significantly reduced. If the fault current is still within the tolerance range of the device at this time, it can be ensured that the PFCM of the series part is still connected to the line, and it is controlled to operate as a static synchronous series compensator (SSSC). The differential mode voltage of the output d-axis component increases the line impedance and further limits the short-circuit current. The series equivalent schematic diagram is shown in the figure. Figure 3 As shown. Under the SPWM modulation mode, the maximum effective value of the output base frequency voltage of PFCM is

[0044]

[0045] in, is the rated voltage of the submodule capacitor, and N is the number of submodules in one arm of the PFCM. In order to ensure its maximum current limiting effect, the modulation index of the PFCM is fully utilized to make it operate at full modulation index. When the MTUPFC is connected to n+1 transmission lines, the equivalent voltage phase angle of the n non-fault lines is:

[0046]

[0047] Among them, δ i is the voltage phase angle of the i-th line. In order to maximize the use of the series voltage, the output voltage of the PFCM connected to the faulty feeder is consistent with the equivalent voltage phase angle of the non-faulty line. Based on the above analysis, the control strategy of PFCM when operating as SSSC can be obtained, as follows: Figure 4 shown.

[0048] Embodiment 2 is an embodiment of the present invention, which provides a system for a transformerless modular multi-terminal flexible interconnection controller series current limiting method, including: a TBS bypass module, a current limiting inductor module, and a power flow regulation module;

[0049] The TBS bypass module is activated when a power system fault occurs, bypassing the power electronic devices to protect the power electronic components in the system. When the fault ends, the TBS bypass is released and the normal current path of the system is restored;

[0050] The current limiting inductor module limits the short-circuit current of the fault line by increasing the inductive reactance, reducing the current impact, so that the current will not spread to the non-fault line or port;

[0051] The power flow regulation module reconnects the line after the fault is cleared and regulates the power flow by controlling the output voltage. When operating in the SSSC mode, the output differential mode voltage increases the line impedance and controls the voltage phase angle of the multi-terminal line to keep the voltage of the faulty line consistent with that of the non-faulty line.

[0052] If the functions are implemented in the form of software functional units and sold or used as independent products When Stored in a computer readable storage medium. Based on this understanding, the present invention The essence of the technical solution 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, which is stored in a storage medium and includes several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the method described in 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.

[0053] 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.

[0054] 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, if necessary, processing in another suitable manner, and then stored in a computer memory.

[0055] 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, a plurality of 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.

[0056] Embodiment 3: In this embodiment, in order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments. This embodiment experiments the existing traditional method and the method of this embodiment respectively.

[0057] See also Figure 2 to Figure 8 A transformerless modular multi-terminal flexible interconnection controller series current limiting method is provided in this embodiment. Figure 2 This is a schematic diagram of connecting three 10kV lines to the transformerless modular multi-terminal flexible interconnection controller in this embodiment. Figure 3This is an equivalent schematic diagram of the controller series current limiting method of the transformerless modular multi-terminal flexible interconnection controller. Figure 4 The controller series current limiting control strategy is a transformerless modular multi-terminal flexible interconnection controller series current limiting method. The details are as follows:

[0058] In order to verify the effectiveness of the control strategy, a power flow control system based on MTUPFC was built in Matlab / Simulink, and the simulation parameters are shown in Table 1. Assuming that line 1 is a balanced line and a three-phase short circuit occurs in line 2, the series current limiting scheme proposed in the present invention is used to actively limit the fault current. The simulation timing is set as follows: the fault occurs at t=0.1s, and first passes through the TBS bypass device. At this time, the short-circuit current is only limited by the port inductance of the MTUPFC. At t=1s, the PFCM control strategy is put into use to make it work in the SSSC operation state to further limit the short-circuit current. The simulation results are shown in Figure 1. Figure 5-Figure 8 shown.

[0059] Table 1 Simulation parameters

[0060]

[0061] Figure 5 and Figure 7 It is the non-fault line current of the transformerless modular multi-terminal flexible interconnection controller, that is, the current of line 1 and line 3. It can be seen that after the fault occurs, passive current limiting is first performed through the current limiting inductor, and then the active current limiting strategy is run at 1 second, and the fault current is further reduced. Figure 6 and Figure 8 is the voltage at the non-fault line port. It can be seen that the active current limiting strategy can increase the voltage at the non-fault port and assist the equipment in fault ride-through.

[0062] This scheme mainly proposes a short-circuit current limiting method for a new type of device, the transformerless power flow controller. Previously, the device used the passive current limiting method of the port inductor. On this basis, this scheme uses the device control to further actively limit the current. It can be seen from the simulation results that the fault current is further reduced after active current limiting.

[0063] For transformerless power flow controllers, the traditional fault current limiting scheme uses a current limiting inductor for passive current limiting, and the current limiting capability is limited. This scheme uses a device for active current limiting, and uses PFCM to operate in SSSC mode, and cooperates with the current limiting inductor to further limit the short-circuit current. From the simulation results, it can be seen that after the active current limiting control is put into use, the fault current is further reduced and the port voltage is raised. Therefore, compared with the traditional inductor current limiting, this scheme has a stronger current limiting capability when the same current limiting inductor is used.

[0064] The simulation results show that the series current limiting strategy based on the transformerless modular multi-terminal flexible interconnection controller proposed in this paper can effectively suppress the short-circuit current and voltage drop at the non-fault outlet, which is conducive to the realization of fault ride-through and recovery of the power flow regulation function after the fault of the transformerless modular multi-terminal flexible interconnection controller.

[0065] 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 transformerless modular multi-terminal flexible interconnection controller series current limiting method, characterized in that: include: At the moment of transmission line fault, the power electronic devices are locked to stop the current from passing through the power electronic devices; Start the TBS bypass power electronic device to make the fault current pass through the bypass channel; Add a current-limiting inductor to the line to reduce the short-circuit current flowing through the line. The series connection part is used for active current limiting to further reduce the short-circuit current.

2. The transformerless modular multi-terminal flexible interconnection controller series current limiting method according to claim 1, characterized in that: The current limiting inductor is connected to each port to limit the short-circuit current of each port and prevent the short-circuit current from being conducted to other ports.

3. The transformerless modular multi-terminal flexible interconnection controller series current limiting method according to claim 2, characterized in that: The starting of the TBS bypass power electronic device is to release the TBS bypass of the power electronic device, and enable the power flow regulation module PFCM in the modular multi-terminal flexible interconnection controller to be reconnected in series with the line and start working.

4. The transformerless modular multi-terminal flexible interconnection controller series current limiting method according to claim 3, characterized in that: After the line is reconnected, the PFCM operates in a static synchronous series compensator mode to output a d-axis differential mode voltage matching the line impedance, thereby increasing the line impedance and suppressing the short-circuit current.

5. The transformerless modular multi-terminal flexible interconnection controller series current limiting method according to claim 4, characterized in that: The output voltage is controlled by SPWM modulation, and the maximum effective value of the output base frequency voltage is expressed as: in, is the rated voltage of the submodule capacitor, and N is the number of submodules in one bridge arm of the PFCM.

6. The transformerless modular multi-terminal flexible interconnection controller series current limiting method according to claim 5, characterized in that: In multiple transmission lines connected by the multi-terminal flexible interconnection controller, the equivalent voltage phase angle of the non-fault line is uniformly calculated to make the PFCM output voltage of the fault line consistent with the equivalent voltage phase of the non-fault line.

7. The method for current limiting in series connection of transformerless modular multi-terminal flexible interconnection controller according to claim 6, characterized in that: The equivalent voltage phase angle of the non-fault line is expressed as: Among them, δ i is the voltage phase angle of the ith line.

8. A system using the transformerless modular multi-terminal flexible interconnection controller series current limiting method as claimed in any one of claims 1 to 7, characterized in that: Including TBS bypass module, current limiting inductor module, power flow regulation module; The TBS bypass module is activated when a power system fault occurs, bypasses the power electronic devices, and protects the power electronic components in the system. When the fault ends, the TBS bypass is released and the normal current path of the system is restored; The current limiting inductor module limits the short-circuit current of the fault line by increasing the inductive reactance, reduces the current impact, and prevents the current from spreading to the non-fault line or port; The power flow regulation module reconnects to the line after the fault is cleared, regulates the power flow by controlling the output voltage, operates in the SSSC mode, outputs the differential mode voltage to increase the line impedance, controls the voltage phase angle of the multi-terminal line, and keeps the voltage of the faulty line consistent with that of the non-faulty line.

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 series current limiting method of the transformerless modular multi-terminal flexible interconnection controller described in any one of claims 1 to 7 are implemented.

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 series current limiting method of the transformerless modular multi-terminal flexible interconnection controller as described in any one of claims 1 to 7 are implemented.

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