Study Of Control And Protection Strategies Of Capacitor Commuted Converter

Capacitor commutation technology is a new converter technology for High Voltage Direct Current transmission (HVDC). This new converter technology can increase the operation range of the converter valve extinction angle y and the firing angle a. It can also inhibit the occurrence of commutation failure and reduce the demand of reactive power from AC system, which makes HVDC can connect to weak AC power system. These properties are gained due to the series capacitor connecting between the converter transformer and the converter valves. Besides, series capacitor will charge or discharge unbalanced when any kind of fault occurs. So some new control and protection strategies are needed for Capacitor Commutated Converter (CCC). The main purpose of this paper is to solve the problem above, so that the advantages of CCC converter can be fully exploited.First of all, the steady state characteristics of 6 pulse CCC converter are discussed using Vector Analysis method. Its mathematical model is builted and the series capacitor’s charging or discharging characteristics are analized in this paper. Then the expression of peak voltage for series capacitor is derived, as well as the range of the compensation ratio K that is allowed.Secondly, taking Qinghai-Tibet ± 400kV HVDC project for example to analsis all kinds of faults of CCC converter using Matlab programming software after choosing a suitable compensation ratio for it. The main forcus lays on the differences between LCC converter (Line Commutated Converter) and CCC converter under the same fault, and to analsis what would happen if no measures are taken.What’s more, it is necessary to find a solution to the problems caused by the faults. In order to find the solution, the relationships among the series capacitors of three phases are studied under normal state. And the influence to the DC system is analysised when the voltage of series capacitors three phases are unbalanced. After that, a control and protection stratage is proposed which is called the unbalanced voltage of series capacitors protection, cooperating with LCC converter control and protection stratages.Finally, a simulation model of Qinghai-Tibet ± 400kV HVDC project is builted using PSCAD/EMTDC simulation software. The control and protection stratages are tested by the model under the self-recovery fault and the fault which can not recover itself.