Study Of The Impact Of AC Systems On HVDC Operation

With the development of the grid, a very complex AC-DC hybrid system has comeinto being in China, and the interaction between AC system and DC transmission hasbecome one of the primary focuses of the security and stability operation of power system.AC system failure may cause a decrease in commutating voltage and a phase shift in thezero crossover points of the commutation voltage. This would cause commutation marginangle decreasing and commutation failure. Continuous commutation failure may lead to dcblock, which imperils the safe and stable operation of the system greatly. Because of this,we need analyze a method to estimate whether commutation failure happens or not. Acontrol loop is used to synchronize the output of the firing control of the AC supplyfrequency. Any errors in the frequency will cause a change in the loop to correct theseerrors. During the course of the correction, whether the firing pulse could cause abnormalchanges in the DC system or not still needs further research.This paper focuses on the impact of the voltage and frequency on the commutation andfiring control. The main work is carried out as follows:(1) The basic principles of the commutation and the firing control are described in thispaper. The factors causing commutation failure and the sensitivity of the firing control tothe frequency change are elaborated. And then, model a bipolar DC transmission systembased on PSCAD/EMTDC.(2) Study on a method to calculate the minimum voltage reduction to causecommutation failure. By improving the previous method under symmetric voltage, a newmethod taking the change of the DC current and the AC failure into account is proposed.This method can also be used to calculate the commutation failure probability curve.(3) Based on this method, this paper proposes a way to calculate the reduction in caseof asymmetric. Simulations have done to verify the effectiveness and results show that theproposed method in this paper has higher precision.(4) The basic principle of the actual system MACH2is described, and the impact ofthe frequency on the zero-crossing is analyzed. Results show that because of themisjudgement of the PLO about the zero crossover point, the firing pulse would be abnormal for a moment. The changes of the DC system are mainly affected by the firingangle limiter, and the changes will be larger when there is a rise in the AC frequency on therectifier side or a drop in theAC frequency on the inverter side.