Commutation Failure Prevention for HVDC Improvement in algorithm for commutation failure prevention in LCC HVDC

Abstract

Line commutated HVDC systems are widely used due to their high power ratings. However, one of the disadvantages of such HVDC systems is the high risk of commutation failures when AC disturbances arise. These failures normally develop in the inverter station. When failed commutation occurs, the LCC HVDC system is greatly disturbed resulting in loss of power transmission. Moreover, the rapid increase in the direct current during unsuccessful commutation results in additional stresses on the thyristor valves. In an attempt to reduce the probability of unsuccessful commutation, a commutation failure prevention function is added to the HVDC system controls. When an AC system disturbance is detected, this function is activated with the aim of altering the firing order at the inverter station. Since the angle contribution from the function is independent of the minimum inverter extinction angle, this approach possesses limitations under certain AC faults. In this thesis, a commutation failure prevention function based on voltage-time area contribution was designed and implemented. Simulation results show that both the proposed and existing functions are equally ineffective in mitigating the first commutation failures when three phase faults are applied. However, the proposed function is more effective in mitigating the first commutation failure when single phase faults are applied compared to the existing function. In 17% of the investigated cases, improvements were registered when the proposed function was utilised. Moreover, in 25% of the cases when three phase faults were applied, the proposed function reduced the occurrence of multi-valve commutation failures.