Analysis Of Commutation Failure Mechanism In Multi-infeed HVDC And Research On Its Defending Method

The commutation period of line-commutated-converter high voltage direct current (LCC-HVDC) project relies on the voltage of converter bus, thus commutation failures following AC system disturbances may frequently occur in LCC-HVDC systems. Moreover, the coupling effect of multi-infeed LCC-HVDC systems is more complicated, and concurrent commutation failures may lead to serious influence to AC system. Consequently, detailed study should be done to solve this problem.Firstly, the commutation characteristics of the inverter valves in LCC-HVDC, under different kinds of unsymmetrical AC faults, are analyzed. The actual commutation voltage of every valves of unsymmetrical faults, with different connection methods of transformer winding in 12-pulse converter, are obtained. Based on symmetrical components method in s-domain(complex frequency domain) circuit, transient response of converter bus voltage is deduced. Then the voltage-time commutation area of various valves are compared, and the valves which have higher possibility to suffer from commutation failure are acquired. Through simulation analysis, the valves which are vulnerable to commutation failure under unsymmetrical AC faults are obtained, and then the effectiveness of theoretical analysis is manifested.Secondly, in order to reveal the interaction between multi-infeed LCC-HVDC systems, symmetrical component method is used in s-domain circuit to calculate the transient expression of inverter bus voltage. In the analysis of AC fault’s effect on the DC systems with different placements, the multi-infeed interaction factor in s-domain form is used to represent the affected extent of the subsystems. In the research of the interaction caused by commutation failure, the current commutation failure component is defined to solve the voltage disturbance of the subsystems motivated by the commutation failure of a certain subsystem. Theoretical analysis shows that the influence of AC fault on DC systems is different, which is related to the parameter of AC network, the rated power of subsystems, etc. And the commutation failure interaction, superimposed on the AC fault, may lead to concurrent commutation failure. Simulation results verify the validity of the theoretical calculation.Finally, to reduce the probability of commutation failure in LCC-HVDC power transmission system, the mechanism of commutation failure is analyzed and the key factors to promote the commutation process are obtained. First of all, based on real-time monitoring device composcd of digital signal processor, and combined with the features of fault voltage and fault current, a power component detection method is proposed. Then, based on the fault detection results, a quick current order limiter based control method is put forward, by which DC current can be reduced quickly. Theoretical analysis verifies that the proposed power component detection method possesses better sensitivity and accuracy, and the quick current order limiter based control can reduce the commutation area while increasing the commutation voltage. A model of dual-infeed LCC-HVDC power transmission systems is constructed in PSCAD/EMTDC. Simulation results conform to the theoretical analysis, thus the superiorities of proposed power component detection method and quick current order limiter based control are demonstrated.