Stability analysis of multiterminal HVDC links

The instability due to feed back control of the dc current in HVDC controls manifests itself through the form of limit cycle oscillations. In particular, such a limit cycle phenomena of 6 Hz frequency was observed on Nelson River multiterminal HVDC system.;Since limit cycle oscillations are distinctly a non-linear phenomena, analysis tools based on Z transform theory, which is linear, are not suitable for stability analysis. The HVDC converter is intrinsically a nonlinear device, and accordingly, the describing function technique can be used to evaluate the stability limit of closed loop control system. To evaluate the describing function, it is necessary to perform a harmonic analysis of the individual dc current of the HVDC converter. An off-line digital simulation program can be used for such harmonic analysis and the frequency response of the linear elements of the closed loop control elements can be used to predict the limit cycle oscillations.;In this thesis one such scheme of how to use an off-line digital simulation program to evaluate the describing function of HVDC converter has been explained in details. Moreover, stability analysis criteria of the multiterminal HVDC links using describing function has been derived. As an example, stability criteria of the Nelson River multiterminal HVDC links has been derived.;A digital simulation study of how to avoid such limit cycle oscillation, by reducing the time constant of the linear part of the control loop was confirmed by the field tests of the Nelson River HVDC system. And finally the inherent limitations of digital simulation are discussed to highlight the fact that results obtained with such stability studies should be taken with certain degree of uncertainty.