Flexible DC short-circuit fault protection method based on thyristor chain and superconducting current limiter

Abstract

The invention discloses a flexible DC short-circuit fault protection method based on a thyristor chain and a superconducting current limiter. The converter used in the flexible DC power transmission system is a modular multi-level converter based on cascaded half-bridge sub-modules: ABC three-phase structure, each phase includes an upper bridge arm and a lower bridge arm, each bridge arm is composed of several half-bridge sub-modules and a bridge arm inductor; the output port of the modular multi-level converter AC side is connected in parallel with a thyristor A superconducting current limiter is connected in series with each of the three-phase lines on the AC side of the modular multilevel converter; the upper ends of the three-phase upper arms of the modular multilevel converter are short-circuited and connected to the DC side through the first mechanical switch , the lower end of the three-phase lower bridge arm is short-circuited and then connected to the DC side through the second mechanical switch. The protection method realizes the current isolation of the AC and DC sides under fault conditions by opening the thyristor chain and reduces the fault current of the DC side to zero, and limits the fault current of the AC side through the superconducting current limiter, thereby realizing the DC short-circuit fault protection of the flexible DC transmission system.

Description

technical field [0001] The invention belongs to the technical field of multi-level power electronic converters, and in particular relates to a flexible DC short-circuit fault protection method based on a thyristor chain and a superconducting current limiter. Background technique [0002] Modular Multilevel Converter (MMC) is composed of multiple sub-modules with the same structure cascaded, and the voltage and power levels can be changed by adjusting the number of cascaded sub-modules, and any level of power can be realized. output, thereby effectively reducing the harmonic components of the output voltage and reducing the switching frequency and loss of the power switching device. With its high efficiency, easy expansion, easy redundancy design and many other advantages, MMC has shown its important engineering application prospects in high-voltage and high-power systems, and is gradually being applied to grid-connected renewable energy, motor drives, and rail transit. and ...

AI Technical Summary

Problems solved by technology

[0003] When a short-circuit fault occurs on the DC side of the MMC, due to the presence of freewheeling diodes in the half-bridge sub-modules, an uncontrolled rectification path will be formed. After each sub-module is blocked, the current on the AC side will feed current to the short-circuit point on the DC side through the freewheeling diodes. , will bring great harm to converters and transmission lines

In addition, in the traditional fault handling method, since the AC and DC sides are not isolated, the DC side current cannot be self-interrupted. Therefore, DC circuit breakers are often used to break the DC side fault current, which affects the breaking capacity of the DC circuit breaker. High requirements and high cost

In some fault handling methods that isolate the AC and DC sides, due to the direct short circuit on the AC side, the current on the AC side will increase sharply during faults, which is dozens of times that during normal operation, which will seriously endanger the safe operation of the power grid.

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