Abstract
Harmonic instability is a phenomenon that occurs when the AC and DC side networks in an HVDC system have impedances which result in the amplification of voltages or currents of one particular frequency[5]. It is assumed that harmonic instabilities are caused by several reasons. A number of previous works have focus on an integer order harmonics including 2nd harmonics[6]. Some paper[8] [9] have reported concerning non-integer harmonics. But they have not fully explained mechanism and a quantitative analysis.
In this paper, a frequency scanning method is introduced to obtain frequency characteristic for HVDC system including the effect of controls. And instability is judged by Nyquist method using locus of open loop transfer function of the AC and DC system. Our analysis shows that countermeasures to reduce static condenser and to reduce lead time constant of automatic current regulator (ACR) are effective to prevent harmonic instability. Simulator test result and digital simulation result are also shown.
