Robust and Cost Effective Synchronization Scheme for a Multicell Grid Emulator

Steadily increasing requirements for grid connected converters, their growing unit power and output voltage call for flexible high-power medium-voltage grid emulators. Four-quadrant cascaded H-bridge topology features high output voltage resolution and high effective switching frequency which enable high-dynamic high fidelity grid emulation. This paper shows that despite nonidealities (turn ratios, phase-shift angles and different stray inductances) in a typical phase-shifting multiwinding transformer, active front-ends of the cells can be effectively synchronized to their respective supply voltages using transformer primary side voltage measurements and nameplate values. Consequently, cell input filters can be omitted, making the grid emulator more compact, cheaper, faster and exhibiting more robust grid synchronization. Theoretical developments and designs are verified by means of high fidelity time domain simulations.