Experimental study on the influence of MMC control design on the propagation of AC grid-unbalance to a DC network

The Modular Multilevel Converter (MMC) has become the leading technology for delivering High Voltage DC (HVDC) power transmission, due to its scalability, harmonic quality and ability to ride through AC network faults. However, unbalanced AC-Grid conditions create harmonics at twice the fundamental AC-Grid frequency that can propagate to the DC-side as a result of the unbalance in the arm energy. These harmonics may risk excitation of network resonances as well as the maloperation of connected assets, and thus control action is needed to suppress such harmonics. Several different control solutions have been proposed in the literature and the aim of this paper is to provide a comparison of the performance of a sample of these techniques on a 1.2 kV/12 kVA lab-scale MMC demonstrator and cable emulator under unbalanced AC-Grid conditions. The sample of control methods chosen showcases a range of design complexity from basic direct modulation techniques with no DC current control to advanced energy based controllers. Experimental testing of these controllers under the same test condition not only validates the operation of each individual controller but also allows like-for-like comparison of their relative performance. It was found that the controllers tested were capable of significantly suppressing double line-frequency components on the DC -bus compared to the base case, however the more complex controllers had the additional benefit of being able to tune the transient response.