Resonance Problem Analysis And Active Suppression Strategy For VSC-HVDC Transmission System

Based on voltage source converter high voltage distribution current(VSC-HVDC) transmission system has a number of advantages, such as high-density and flexible control ability. Therefore, VSC-HVDC transmission system has a unique technical advantage in feeding of remote and isolate loads that are away from the land, as well as building the urban DC power distribution network and the connection of the new renewable energy power plants to a main grid. As an indispensable part of modern strong and smart grid, the most important thing is to be capable to maintain the system run steadily and securely for a long time. However, unbalanced faults in AC system of the VSC-HVDC happen from time to time, which will produce zero and negative sequence harmonic components of AC voltage, as well as deteriorate the DC bias problem of converter transformers and make transformers to produce a large number of low-order harmonics. And then increase the electrical stress of power electronic devices. Thereby the network resonance very easy occurs in the low-order harmonics due to capacitive AC impedance of VSC.In this paper, considering the problem that the network resonance under the asymmetric fault or unbalanced conditions in the AC side of the VSC-HVDC system, based on the mechanism of harmonic components transferring between the AC side and DC side in VSC-HVDC system, and then quantitative analysis of the impact of negative harmonic component in AC and DC side of the VSC. At the same time, building the positive-sequence and negative-sequence impedance calculation model, and further elaborating the cause of network resonance in the VSC-HVDC system. And then, the influence of the DC Capacitors and the SCR of the AC grid on network resonant frequency is studied. Furthermore, to overall suppress the over-voltage and over-current during network resonance, a novel method which is series a resistor to the AC impedance branch of the VSC is proposed, namely virtual harmonic resistor control.Finally, Simulation results in PSCAD/EMTDC show that the proposed control strategy can suppress the harmonic component delivered from AC side to DC side in the VSC-HVDC system. So the over-voltage and over-current caused by the network resonance between AC grid and the VSC-HVDC system can be fully suppressed, as well as the safety of the VSC-HVDC system under the condition of unbalanced faults is improved without affecting the system fundamental electrical power control. Moreover, it does not cause any power loss.