Converter Control in 800 kV DC Transmission System

Two 12-pulse converters in series will be applied in the recent ±800kV DC transmission projects in China.The basic control strategy for the double converters in series has been illustrated and analyzed in this paper.As many other HVDC systems,the basic operation control strategy of ±800kV DC transmission system is that DC current is controlled by two rectifiers simultaneously during normal operation while DC voltage is controlled by two inverters themselves or OLTC of inverters.The simulation results by EMTDC program demonstrate that independent control for each converter without inter-communication between them on this strategy can insure the performance of the system to operate symmetrically during steady operation and to response properly under fault conditions as varions AC and DC faults.It is discovered that the inherent unbalance between two controllers of converters in series will lead to control shift at steady statement.For example,one of controllers is working at min.control angle as 5° and the other one at some larger angle.The error elimination part should be included into control chain to avoid such control shift.As an essential method to improve the availability of HVDC system with two converters in series the by-pass switch is usually applied to the converter.The sequence control of fault converter isolating by by-pass switch and the de-blocking of isolated converter is also simulated in this paper without any reduction and interruption of DC power.