A new practical approach to evaluate critical gain for an HVDC based damping controller using wide area measurement system

The paper proposes a comprehensive design approach for a robust wide area measurement system (WAMS) based inter-area swing mode (0.1Hz–1.0Hz) damping controller for networks with embedded multiple High-voltage Dc (HVDC) links. A simplified low-frequency model for the converter is first used to design the controller, which uses a specially designed feedback signal that greatly improves damping and is relatively robust against changes in operating point and loss of communication. However, the paper identifies a drawback, in that although using the simplified model does provide a good design, it can sometimes suggest unacceptable controller gains that cause instability. As predicted by the model, the damping of the targeted low frequency inter-area modes improves with increasing gain. However, if increased beyond a certain critical value, Electromagnetic Transient (EMT) simulation shows that the larger gain results in de-stabilization of the higher frequency modes of the converter. This is not predicted by the simplified representation in the design process which only considers the low-frequency converter behaviour. This critical gain can be determined using a generalized Nyquist criterion. This advocates a two-step process design. The simplified model can first be used for the basic design, and EMT simulation combined with Nyquist Analysis can then be used to determine the maximum limit for the controller gain.