电网电压跌落时CHB-STATCOM的限幅策略

Limitation Strategy of CHB-STATCOM Under Voltage Sags

静止同步补偿器(STATCOM)除补偿电网平衡无功功率外,在网侧电压跌落时,还需输出负序无功电流来实现低电压穿越(LVRT)以支撑电网。传统的LVRT电流基准应用于星形级联H桥STATCOM(CHB-STATCOM)拓扑,会因电流不平衡导致单相过流,此外,由于需要额外注入零序电压实现相间电压均衡,CHB-STATCOM的输出电压幅值将受到影响,从而引发过调制现象。在此根据当前电网电压的正负序分量、变换器的电流应力及单相链的直流侧电压基准,实时计算出基于电流限幅和电压限幅的最大功率,避免了变换器过流和过调制的现象,保证了变换器的安全工作。最后,在不同跌落情况下分别对该限幅策略的有效性进行了验证。

Besides compensating the balanced reactive power of the grid,the static synchronous compensator(STATCOM)is also used to support the grid by generating reactive current of negative sequence when voltage sag occurs,which is beneficial to realize low voltage ride through(LVRT).When applying the traditional LVRT current reference to star connected cascaded H-bridge STATCOM(CHB-STATCOM),overcurrent problem of single phase may occur as a result of unbalance current.In addition,the extra injection of zero sequence voltage to balance the direct current side voltage of three phases can lead to overmodulation because the output voltage is affected.The online calculation the maximum power reference is realized based on current limit and voltage limit according to the positive and negative sequence component of the grid voltage,the current stress of converter and the direct current side voltage of single phase,avoiding the problems of overcurrent and overmodulation and ensuring the safe of converter.Finally,the limitation strategy is verified under different voltage sag condition by experiment.