A modular capacitor-commutated converter and method

Abstract

The invention belongs to the technical field of converters, and relates to a modular capacitor commutation converter and a method, including: a three-phase six-pulse converter valve, a converter transformer and a capacitor module; a three-phase six-pulse converter valve and a capacitor module connected in series, and the capacitor module is arranged between the converter transformer and the three-phase six-pulse converter valve; the capacitor module includes a capacitor and at least one set of converter bridges composed of sub-modules composed of reverse-parallel diodes that can turn off power electronic devices, The converter bridge is connected in parallel with the capacitor. It relaxes the angle limit of the thyristor commutation, improves the degree of freedom of the firing angle, makes the commutation possible before the natural voltage commutation point or after the natural cut-off point, reduces the reactive power consumption, and improves the active power of the LCC DC transmission system and reactive properties.

Description

technical field [0001] The invention relates to a modular capacitor commutation converter and a method, belonging to the technical field of converters. Background technique [0002] Grid-commutated DC transmission technology (LCC) is currently the most widely used DC transmission technology and plays an irreplaceable role in the field of long-distance and large-capacity power transmission. However, since LCC DC transmission technology uses thyristors as commutation devices, there are the following Essential defects: [0003] 1. During the commutation process, the commutation voltage must be provided by the connected AC system. The strength of the connected AC grid has a significant impact on the operation of the DC system. The LCC DC project cannot supply power to the passive system, and it is difficult to operate stably under a weak AC system. [0004] 2. In order to establish sufficient thyristor commutation forward voltage, the rated firing angle of the LCC converter mus...

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Problems solved by technology

[0003] 1. The phase commutation process must be provided by the connected AC system. The strength of the connected AC grid has a significant impact on the operation of the DC system. The LCC DC project cannot supply power to the passive system, and it is difficult to operate stably under a weak AC system.

[0004] 2. In order to establish sufficient thyristor commutation forward voltage, the rated firing angle of the LCC converter must generally be controlled at about 15°, and the commutation process requires a relatively large commutation angle, so the LCC converter consumes a large amount of power during operation. For reactive power, a large amount of reactive power compensation equipment must be installed and invested at the same time, generally a fixed-capacity AC filter and shunt capacitor directly connected to the high-voltage transmission network

These reactive power compensation devices are close to half of the area of ​​the converter station, and the investment is large, which increases the complexity of DC control; the reactive power devices with fixed capacity need to be switched continuously during the power change process, and the reactive power changes in steps during switching. , resulting in voltage fluctuations; reactive equipment relies on the action of the circuit breaker for switching, and the action of the circuit breaker takes a long time and cannot react sensitively. Power surplus and AC system overvoltage, which limit the speed of transmission power change of LCC DC project, greatly reduce transmission sensitivity; capacitors are directly connected to AC transmission network, which changes the harmonic impedance characteristics of the transmission network, which is easy to cause harmonic amplification

In addition, the reactive power generation mechanism of the LCC DC project makes the coupling of reactive power and active power too close, which cannot be decoupled, and cannot be adjusted as flexibly as another type of DC transmission-voltage source converter (VSC). Reactive power output greatly reduces the coordination with the AC system

[0005] 3. When the receiving end fails to connect to the power grid, causing the voltage amplitude of the busbar of the converter station to drop or the three-phase imbalance, the LCC converter is prone to commutation failure. At this time, the DC current increases greatly, and the active power of the sending end and the receiving end is interrupted. , excessive reactive power consumption at the sending end, overvoltage at the sending end and receiving end

However, the capacitor charging of the CCC converter is difficult to control, the capacitor voltage increases the insulation level of the converter valve and the converter transformer, and the overall harmonic characteristics of the converter are more complicated

Once the capacitor charging is out of control after commutation failure, the converter will easily lose the fault self-recovery ability, resulting in continuous commutation failure

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