A Practical Method to Calculate the AC-Side Short-Circuit Current of MMC-HVDC

With more and more MMC-HVDC connected to the AC network, the short-circuit current contributed by converter station should be taken into account when the short-circuit fault occurs at the AC-side. In this paper, the characteristics of short-circuit current contributed by MMC are analyzed and a simple and convenient method to calculate the fault current is proposed. Study shows that the characteristics of short-circuit current depend on the control method that MMC adopted. The development of short circuit current can be divided into transient-state and steady-state stage according to whether the controller responds completely or not. Considering the margin of IGBT, current limiters are set both in inner and outer controller. It can limit the bridge arm current and avoid overload. Therefore, AC-side current will change within the limit value. A practical method for the short-circuit current calculation is proposed based on the controllers and limiters. Through this method, both the symmetrical and asymmetrical fault short-circuit current can be calculated after the controllers enters steady state. The analysis in this paper shows that MMC can be equivalent to the current source at steady-state and participate in the calculation of AC system short-circuit. Finally, the accuracy of the proposed method is verified in the PSCAD simulation.