Abstract
Commutation failures is the commonest perturbation can appear in the inverter valves during or after the faults. Therefore, understanding the system fault is very important for regulation and protection design. Simulation by means of digital real time simulator can provide quickly the simulation results, and also represents the most effective methods to find the system fault behavior. This paper proposes a simplified method to detect and avoid the commutations failures in an HVDC inverter. Besides, the inverter is connected to the AC system which is considered as a weak AC grid having a low short circuit ratio. The results are provided by means of the digital real time simulator using the RT-lab platform and achieved by the simulation in the loop approach. Although the real time simulation allows for testing the behavior of these kinds of systems under adverse conditions that rarely occur in the field but are important to evaluate. 12 pulses line-commutated converter HVDC system is used in this study (both stations the rectifier and the inverter are based on thyristor valves). The function of the protection function is validated against the real time simulation of the HVDC inverter following different kind of faults, both kind of fault are the single phase to ground AC fault and a short circuit at the inverter valve. The obtained results show clearly the effectiveness of the proposed method to avoid the commutation failure by ensuring and maintaining a safe commutation process on the inverter station in the worst case.