Series-Stacked Hybrid Modular Converter with DC Fault Blocking Capability for HVDC Application

Abstract

The three-phase series modular multilevel converter (SMMC) is an interesting option for high voltage direct current (HVDC) systems and also for tapping power from HVDC lines. The number of semiconductor switches required in SMMC for the same dc voltage is one third of that required in the conventional MMC. However, since SMMC also uses half-bridge submodules (SMs), it is unable to block the dc fault current and hence its further use will depend on the fast dc circuit breaker technology development. The bipolar SMs, like the full bridge SM, may also be used instead for dc fault current blocking in MMC. However, it results in increase in loss, cost and complexity of the converter. This paper proposes a series-stacked hybrid MMC, which uses both unipolar as well as bipolar SMs for attaining dc fault blocking capability. The bipolar SMs are used to limit or block the fault current by injecting negative capacitor voltages. The number of FBSMs required in the proposed topology is only one-third of the total number of SMs in the three-phase converter. On the other hand, in the conventional MMC, at-least half of the SMs should be FBSMs for dc fault blocking. Hence the proposed topology offers significant reduction in size, cost and loss. Three different circuit configurations are proposed on the basis of the proposed topology. This paper also presents capacitance size calculation of the SMs used in the proposed topology so that its voltage deviation remains within a predefined maximum limit. The effectiveness of the converter for HVDC system and its dc fault tolerant capability are verified using simulation studies in PSCAD/EMTDC.