A start-up method for a modular multi-level flexible direct current transmission converter

Abstract

The invention relates to a method for starting a modularized multi-level flexible direct current transmission converter, which comprises the following steps: A. The DC side poles of each converter station are connected, the sub-modules of each station are locked, and at the same time, the active station closes the AC switch, and enters the Controlled charging stage; B. After the sub-module voltage of each converter station is higher than the corresponding charging threshold voltage, the controllable voltage equalization charging method is respectively used to enter the controllable charging stage, and the active station cuts off the soft start resistance of the AC side; C. Each converter station adopts the closed-loop voltage equalization charging method according to the AC and DC side voltage conditions. When the voltage of its sub-modules reaches the rated level and is stable, they are respectively unlocked and enter the unlocking operation stage. By adding a closed-loop charging method after the controllable charging, the cutting number can be adjusted in real time according to the current sub-module voltage, and the sub-module voltage can be stabilized at the rated value through closed-loop control, avoiding the electrical shock during unlocking operation.

Description

technical field [0001] The invention relates to a method for starting a modularized multilevel flexible direct current transmission converter. Background technique [0002] Multi-terminal flexible DC transmission can realize multi-source power supply and multi-drop power receiving, which is a flexible, reliable and fast power transmission method in the power system. The modular multilevel converter has gradually become the development trend of multi-terminal flexible DC transmission due to its advantages of high output voltage waveform quality, low switching loss, easy expansion and strong fault ride-through capability. [0003] The successful completion of the start-up process is the premise and basis for the normal operation of the multi-terminal system. A suitable start-up strategy should be able to coordinate and configure the start-up sequence of each converter in the multi-terminal system, suppress the electrical shock caused by their mutual influence, and avoid start-...

AI Technical Summary

Problems solved by technology

However, in actual engineering, due to the influence of line and sub-module loss, stray parameters and other factors, the sub-module voltage obtained by the theoretical cut-off number control will deviate from the rated value during charging steady state, and the equivalent sub-module voltage obtained by the current sub-module voltage is The AC voltage on the valve side is different from the actual AC voltage, and the lack of a sufficiently large damping element in the line will cause a large inrush current

[0009] Second, in the early stage of pre-charging, the voltage of the sub-module is low, and the control power supply of the sub-module that adopts the self-charging method cannot work, so the sub-module can only be locked for uncontrolled charging

According to the difference between the AC and DC side power supplies of the converter, the converter has different charging circuits, and the voltage of the power supply for charging the bridge arm or phase unit under each circuit is different, resulting in different steady-state voltages of the sub-modules, which also determines the charging threshold. The voltage is different. If the charging threshold voltage is set randomly, it will cause some stations to be too early, too slow or even unable to enter the controllable charging stage, which will cause a large electrical shock, increase the start-up time or even cause start-up failure.

At the same time, if the charging mode is not distinguished in the controllable stage, and the cut-off number is set randomly, it will cause different charging of the sub-modules, fail to meet the charging requirements, and even cause overvoltage damage to the sub-modules

[0010] Third, in a multi-terminal system, there is one and only one station for constant DC voltage control

The operation of the multi-terminal system requires a stable DC voltage. If a converter station is unlocked and operated before the fixed DC voltage control station, its injection or extraction of active power into the DC line will cause the DC voltage to increase or decrease, which is not conducive to operation control, and even cause system failure

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