Investigation of Non-Convergence in Waveform Relaxation Based Testing of Power System Controllers

A waveform relaxation (WR) based iterative real-time playback system for controller testing can be a low-cost and flexible alternative to conventional hardware-in-loop based testing, as it does not require a real-time simulator. Proof-of-concept simulations and laboratory experiments reported in the earlier literature show that the method has the potential to be a practical controller testing method. However, in some situations non-convergence was observed in the experiments, although it was not seen in the simulations of the method. In this paper, the cause of this behaviour is investigated using simple models. It is shown that high open-loop transient gain, coupled with the ingress of noise due to round-off can be a cause of non-convergence in practical applications. Measures are proposed to overcome this problem and their efficacy is demonstrated using illustrative examples.