Starting method of modular multi-level flexible direct-current transmission converter

Abstract

The invention relates to a starting method of a modular multi-level flexible direct-current transmission converter. The method comprises the following steps: realizing switch between soft start and a normal operating circuit through on-off of a circuit breaker and a line contactor; in the soft start process, cutting off the highest direct current voltage of a sub module at a certain frequency to ensure even charge of a bridge arm sub module; and ensuring that the voltage of the bridge arm sub module reaches a system operational rated value by selecting the cutting number of the sub modules so as to create condition for the system for smooth switch from the soft start process to the control mode. A single bridge arm in the control stage is used as a unit control unit, and even charge control of the sub module is carried out for each bridge arm at the same time. Thus, charging is easier to realize without detecting the current of the bridge arm. The soft start process is remarkably shortened, and the even charging degree of the sub modules is very high.

Description

Technical field [0001] The invention belongs to the technical field of power system flexible transmission and distribution, power electronics and user power, and relates to a method for starting a modular multi-level flexible DC transmission converter. Background technique [0002] The flexible DC transmission technology is based on switchable power electronic devices and voltage source converter stations (VSC), mostly in BtB topology and uses underground cables and submarine cables as transmission media to realize power transmission. Among them, the voltage source converter (VSC) is the basic unit of the transmission system. Modular multilevel converter (MMC) is a multilevel converter topology that has attracted much attention in recent years. In the modular multilevel converter, each bridge arm is composed of several submodules (SM) connected in series. By controlling the input and removal status of each submodule, the output voltage of the AC side of the converter is controll...

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Problems solved by technology

[0010] Among the charging methods of the above two sub-modules, the first charging method is to charge each sub-module one by one, the charging time is long, and if the sub-module control circuit takes power from both ends of the DC capacitor of the sub-module, it will cause the sub-module that completes charging first The voltage will complete the discharge process within a certain period of time. For the actual system, this method is not very operable; one of the defects of the second charging method is that it does not consider the difference in circuit parameters of the sub-module itself and the difference in the DC power load of the sub-module. For the sub-module The influence of natural charging uneven voltage; the second defect is that the voltage equalization strategy adopted for sub-module charging after the soft start enters stability is selected with one phase unit (two bridge arms) as the control object. For a huge system, If the number of bridge arm sub-modules is large, the amount of calculation for sorting the voltages of the two bridge arm sub-modules will greatly increase, so the consumption of control system resources is relatively high. In addition, the modular multi-level valve control system is generally controlled by the bridge arm The controller is a unit, and the phase unit is used as a whole to carry out voltage equalization control, which will inevitably bring about huge communication requirements between the controllers of each phase bridge arm, which intensifies the internal dependence of the valve control system hardware.

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