MMC-based topology for grid connection of wind generators with phase and arm power balancing

Abstract

A new topology of a wind farm wherein the wind generators are integrated into the structure of a modular multilevel converter (MMC) is presented. It allows connecting a group of small/medium size wind turbines to the grid, replacing all the grid side converters of the wind generators by the MMC. The different power generated by each wind generator causes power imbalances that can lead to circulating currents; this paper addresses this problem by analyzing it in a simple way. The analysis shows that power differences between phases are naturally balanced by the DC component of circulating current, without the need to use a specific regulator. However, the power differences between the arms result in a high 50 Hz circulating current. This paper presents the design of a new regulator that corrects these power imbalances by generating a phase difference between the upper and lower arm voltages resulting in an optimal 50 Hz circulating current. The operation of the MMC-based wind farm topology has been tested when all the turbines generate the same power and when they generate significantly different powers. Numerical and real-time simulation results show that the proposed regulator keeps the powers of the MMC phases and arms balanced.