MMC converter modulation method and system capable of shutting direct current fault current

Abstract

The invention relates to an MMC converter modulation method and system capable of shutting direct current fault current. A switching state of a modular multilevel converter bridge arm submodule is determined according to an input modulation signal; steady-state voltage of a DC-DC converter is determined according to an equivalent circuit model of the DC-DC converter constructed by the bridge arm submodule capacitor voltage in the switching state; the number of the submodules input by a lower bridge arm under the steady-state voltage is calculated; according to the number of the input submodules, a DC-DC converter transfer function is defined, and the switching sequence is determined according to a preset manner to adjust the capacitor voltage of the modular multilevel converter bridge armsubmodule to be in a voltage stabilization state. The scheme can effectively provide safe, stable and reliable control protection for the DC-DC converter capable of shutting the direct current fault current.

Description

technical field [0001] The invention belongs to the field of power electronic device control, in particular to an MMC converter modulation method and system capable of blocking DC fault current. Background technique [0002] The development of DC grids is still limited by the lag of key equipment such as DC circuit breakers and DC converters, among which DC converters are of great significance for connecting different DC lines to form a DC grid. Compared with the traditional AC grid, it can be found that there are multiple different voltage levels in the AC grid, among which the higher voltage level is used for long-distance power transmission, while the lower voltage level is used for power distribution and power consumption. Similarly, in order to reduce the transmission loss in the DC power grid, a higher DC voltage level is required from the power generation center to the load center, while a lower voltage level should be used in the distribution network and the user sid...

AI Technical Summary

Problems solved by technology

[0003] The DC power grid is a "low inertia" system. If a DC fault occurs, a large short-circuit current may be generated, and it will instantly affect the entire DC power grid. It requires a high response time for the protection system. The traditional AC system protection method , such as overcurrent protection, distance protection and differential protection, etc., are not suitable for direct application to the DC power grid; and the DC power grid has a large scale, complex structure, diverse operation modes, and a large number of equipment, so it is not possible to simply use the two-terminal DC transmission system The protection method of cutting off the entire DC system from the AC side; at the same time, in the DC power transmission and distribution system, the development of large-capacity DC circuit breakers is relatively difficult, and the DC converter is usually required to have fault protection and isolation functions to prevent fault transmission from endangering the normal power supply system ; Therefore, the DC-DC converter is required to have the function of blocking the DC fault current

[0004] At present, the engineering application of DC circuit breakers in high-voltage and high-power occasions cannot be realized, and the MMC with a half-bridge sub-module structure used in actual engineering does not have the ability to block DC current, especially when the MMC has a bipolar short-circuit fault. Block the converter to cut off the fault short-circuit current, so the whole system can only cut off the fault current through the AC circuit breaker after the fault occurs, which has a serious impact on the reliability of the system

In order to avoid DC faults as much as possible, existing MMC projects use submarine cables or underground cables with low failure rate and high cost as transmission lines, which greatly restricts the application of MMC in long-distance power transmission and multi-terminal DC systems.

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