Optimal control and filter sizing of a multi-megawatt medium-voltage voltage-source converter for future grids

With increasing share of the intermittent renewable energy resources in the grid, battery energy storage systems (BESS) are becoming increasingly important for the primary reserve. One possible way to connect such large BESS to the existing ac grids is by using medium-voltage (MV) multi-megawatt voltage-source converters (VSC). In these grid-connected converters, an LCL filter is often used to comply with the grid code requirements of low harmonic content. The semiconductor devices used at these power and voltage levels can be switched only at fairly low switching frequencies because of the excessive switching loss. Operation at low switching frequency leads to control problems such as low phase margin of the current-control loop (CCL) because of the delay incurred by the digital implementation of the modulator which might lead to system instability. This issue is discussed in detail in this work using the double-sided Frequency Response Function (FRF) bode plots of the CCL. Active and passive damping of the LCL resonance is also discussed. An optimized variant of the discontinuous space-vector modulation based on a previous work is adapted to increase the switching frequency while maintaining the overall system efficiency. Consequently, the required LCL-filter size is reduced to one half, thus saving significant amounts of copper and steel.