A Fast Acting Precharge Control Strategy for Modular Multilevel Converters

The modular multilevel converter (MMC) will play a vital role in medium and high voltage transmission especially when controllability is of utmost importance. A lot of research has been done to develop different pulse width modulation (PWM) techniques and controls to drive the MMC once it is online. An important and yet overlooked research topic is how to transition the MMC from black start to live operation in a fast and controllable manner. In this paper a novel closed loop constant current charging model using nearest level control (NLC) has been developed. To study the effectiveness of this method in the presence of capacitor imbalances, three simulation models were prepared. In the first simulation model, all capacitors were assumed to be identical, in the second, one was assumed to have 80% of the nominal value, and in the last, one was assumed to have 120 % of the nominal value. It was found that in all cases, the model was able to deliver a constant current and constrain the capacitor voltages in a well defined voltage tolerance band. One drawback was that the second and third simulation models needed on average 34 % and 30 % more switching instances to maintain a constant current. If the number of switching instances are constrained, the developed precharging scheme can serve as a powerful method to charge a MMC in a fast and controlled manner without any overshoots even in the presence of capacitor tolerances.