A pre-charging method for a modular multilevel converter DC transmission system

Abstract

The invention relates to a charging method in the field of direct-current transmission, in particular to a pre-charging method for a modularized multi-level converter direct-current transmission system. The method includes two charging processes which are carried out in sequence, and the two charging processes are the latching charging process and the semi-latching charging process, wherein the latching charging process is an uncontrolled charging process mentioned in other references, the semi-latching charging process bypasses some sub-modules, and triggering pulse distribution and capacitor voltage balance control can be achieved by simplifying relevant control over the MMC in the steady state or latching some functions. Therefore, the charging method is simple and easy to operate, and charging can be performed to make the capacitor voltage reach a voltage value in steady operation.

Description

technical field [0001] The invention relates to a charging method in the field of direct current transmission, in particular to a precharging method for a modular multilevel converter direct current transmission system. Background technique [0002] The research on start-up control of DC transmission system is mainly aimed at two-level converters, but less involved in the start-up control of MMC-based DC transmission system. Since MMC contains a large number of capacitors, during the start-up process of MMC, it is necessary to Charge the capacitor, but the current research on the charging process of MMC is limited to uncontrolled charging on the DC side or the AC side, and the capacitor cannot be charged to the capacitor voltage during steady-state operation, resulting in the occurrence of current on the AC side during the unlocking process. overshoot or distortion. If the sub-modules are charged one by one, the capacitors of the sub-modules will be charged to the voltage o...

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Problems solved by technology

[0002] The research on start-up control of DC transmission system is mainly aimed at two-level converters, but less involved in the start-up control of MMC-based DC transmission system. Since MMC contains a large number of capacitors, during the start-up process of MMC, it is necessary to Charge the capacitor, but the current research on the charging process of MMC is limited to uncontrolled charging on the DC side or the AC side, and the capacitor cannot be charged to the capacitor voltage during steady-state operation, resulting in the occurrence of current on the AC side during the unlocking process. overshoot or distortion

If the sub-modules are charged one by one, the capacitors of the sub-modules will be charged to the voltage of the steady-state operation of the MMC, but this method is not practical, because the driving of the IGBTs in the sub-modules must take energy from the DC capacitors to drive the IGBTs. Live work, therefore, when the sub-module capacitor voltage is 0, it is impossible to drive the IGBT

An improved method is to use uncontrolled rectification to charge the sub-modules first, and then charge each sub-module one by one until the capacitance of each sub-module is charged to the voltage of the steady-state operation, but this method needs to be written dedicated control program

Since the sub-modules are not charged at the same time, and there is a loss in each sub-module, the capacitance voltage of each sub-module varies greatly after charging, which is not conducive to the stability of the MMC when unlocking

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