Research of the influence of rectifier alpha min limiter on the system stability

The ultra high voltage direct current, responsible for the transformation between DC and AC, can deliver long-range, high-voltage and large-capacity power. The voltage dependent current order limiter (VDCOL) and the current control amplifier (CCA) are key factors for the dynamic characteristic. The firing angle will be quickly reduced to the allowed minimum value when faults occur at the AC side of the rectifier station. With the AC faults removed and the AC voltage recovering, the direct current will increase rapidly if the firing angle is too small. The alpha min limiter (RAML) module of the rectifier is adopted to prevent that situation. However, RAML will affect the voltage stability of the sending end system. In this paper, the structure and function of RAML are described. The influence of RAML on DC and AC systems is analyzed using Thevenin equivalent system or single machine infinite bus power system connected with a generator set respectively. The influence is also explained by equal area rule. Finally, the influence of RAML on voltage stability is verified through the results of IEEE 10-machine 39-bus system.