Over modulation method of modular multi-level converter station

Abstract

The invention discloses an over modulation method of a modular multi-level converter station. The over modulation method of the modular multi-level converter station is used for direct-current transmission. The modular multi-level converter station comprises a plurality of valve units, a charging resistor and a knife switch used for a bypass charging resistor, and each valve unit comprises a capacitor. After the converter station is unlocked, a Udc, the capacitor voltage of a bridge arm, an average value Ucap and alternating current modulation voltages (Ua, Ub and Uc) generated by a current controller of the converter station are set according to the current direct current voltage, and the minimum common-mode voltages Un capable of being overlaid to the alternating current modulation voltages are worked out, so that after the minimum common-mode voltages Un are overlaid, the modulation voltages (Uan=Ua-Un, Ubn=Ub-Un, and Ucn=Uc-Un) of all phases are in a permissible range as much as possible; when the compensation effect is the same after overlaying, the common-mode voltage is small as soon as possible. Through the method, when uncontrolled charging of the converter station is finished, due to the fact that the capacitor voltage of the bridge arm is low, the alternating current modulation voltages capable of being output will be obviously improved, and starting impact current is further lowered; when the system operates stably, due to the facts that the capacitor voltage of the bridge arm has been raised and the overlaid modulation voltages are zero, the stable operation of the system is not affected.

Description

technical field [0001] The invention belongs to the technical field of flexible direct current transmission system control, and in particular relates to the start-up process of a converter station in a direct current transmission system. Background technique [0002] The voltage source DC transmission system, also known as the flexible DC transmission system, has the independent control of the active / reactive power of the converter station, does not require a huge AC filter, can support the black start of the power grid, and supports the island / wind farm and other weak It is an effective method to solve the problem of grid connection and consumption of new energy in China. At present, the modular multilevel converter is one of the main methods to realize the voltage source DC transmission system, but the control and protection technology of the modular multilevel converter station is complicated, especially during the start-up process, it is easy to cause a large inrush curr...

AI Technical Summary

Problems solved by technology

At present, the modular multilevel converter is one of the main methods to realize the voltage source DC transmission system, but the control and protection technology of the modular multilevel converter station is complicated, especially during the start-up process, it is easy to cause a large inrush current, affect the reliability of the system

[0003] At present, there are two methods to solve the start-up of the modular multi-level converter station: one method is to increase the leakage reactance of the AC side transformer and the bridge arm reactance of the converter, and suppress the start-up inrush current. cost, which increases the steady-state voltage drop on the transformer and bridge arm reactance, which is not conducive to the optimization of the converter station system design; another method [1] is to divide the bridge arm modules into two groups, and perform Charging can get a higher charging voltage. After the charging is completed, because the total bridge arm capacitor voltage is significantly higher than the grid voltage, too much inrush current is avoided; the method of [2] is similar, by bypassing a certain number of bridges Arm module, repeatedly charging the bridge arm bypassing different modules, can also get a higher bridge arm capacitor voltage, thereby reducing the start-up inrush current; both [1] and [2] methods significantly increase the charging resistance. loss, making the charging and starting process too complicated

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