Analysis of Transformerless MMC and Suppression of Third Order Harmonic Current

In the rapidly expanding electrical power network the modular multilevel converter (MMC) based high voltage DC (HVDC) presents the most effective mean for bulk power transmission. This recently developed MMC topology for the voltage source converter (VSC) has many advantages. The one unexplored advantage of MMC is transformerless operation. This paper analyses the transformerless operation of the MMC. The internal dynamic of the MMC generates third order voltage ripple, which has the potential to induce the third order harmonic current. With star-delta transformers as an interface between MMC and AC network, this harmonic current is blocked. However, with the transformerless MMC and physically grounded DC terminal, this third order harmonic current poses severe harm to the MMC as well as to the AC source. This paper also proposes a Proportional Resonant (PR) controller to suppress the third order harmonic current. To perform the analysis, electromagnetic transient (EMT) model of MMC under different connection has been developed on the real-time digital simulator (RTDS). The developed controller successfully suppresses the third order harmonic current.