Voltage Support and Electrical Stresses in MMC-HVDC Systems during AC Faults.

Modular multilevel converters must keep currents and voltages within limits as dictated by their component ratings. During AC faults, electrical stresses in the form of internal over-voltages and over-currents result from interactions between different parts of the control system. This work investigates how modern voltage support schemes affect these stresses and proposes a control strategy to reduce them. It was found that the arm capacitor voltage increases significantly when voltage support is done due to the rapid phase shift in the AC output current. By spreading this phase shift over multiple periods, it is possible to reduce these electrical stresses.