Improvement of Commutation Failure Prediction in HVDC Classic Links

In this thesis, an evaluation of the existing control system for ABB: s HVDC Classic Links is performed in order to investigate whether a possible improvement to commutation failure prediction is possible and to be recommended.

The thesis starts with a theoretical approach to the complexity of consequences of increasing the extinction angle (γ) in order to prevent commutation failure in inverter operation, which is later confirmed through using the simulation software PSCAD to evaluate coherence between simulation results and theory.

Dynamic power studies are performed through simulations in the electromagnetic time domain transient tool PSCAD in order to establish a possible improvement to the existing commutation failure prediction today used in ABB control systems for HVDC applications.

The thesis shows three possible detection signals to quickly detect increased risk for commutation failure within 1 ms after fault initiation. It addresses issues of network strength and robustness. The detection can be made by utilizing existing control system signals for recording zero-crossing times of the valve voltages, the sum of the AC voltage phases and to detect phase shifts using the error signal of the internal PLL-algorithm.

Several points of interest are presented in the thesis for a successful implementation of an improved predictor algorithm. Simulations show how there are limits to when an angle increase may be effective concerning point-of-wave of the AC voltages as well as point-of-wave in the extinction angle (γ) transient, where an effective use of an improved predictor is limited to areas between 0-54º and 144-216º on the AC voltage cycle.

The extent to which an extinction angle (γ) contribution is effective without giving consequences which leads to network collapse is highly related to the strength of the connected AC system and there might be applications in which an increase in (γ) is not to be advised.


My Bachelor’s thesis was approved by University West, 17th of March 2011. The electronic publication can be found in full text at the DiVA database for academic papers in the coming weeks.