An enhanced modulator concept for the Modular Multilevel Converter

Abstract

This paper deals with an enhanced modulator concept for the Modular Multilevel Converter. Based on an overview on existing concepts including module sorting algorithms and measurement results from a hardware-setup in laboratory, a new concept is developed. Measurement results of the implemented new algorithm for module selection and capacitor energy balancing in various operating points of the converter are presented and analyzed in detail. This elaboration is completed with a classification of the suggested modulator concept compared to established concepts.

Performance comparison of phase shifted PWM and sorting method for modular multilevel converters

“…Generally, the method is based on an external control loop, such as the open loop Nearest Level Control (NLC), or closed loop: Tolerance Band (TB) modulation or Selective Harmonic Elimination (SHE) PWM. In each sampling period the external control algorithm provides a reference voltage which is approximated by the number of the inserted SMs [7][8][9][10]. Then, the sorting algorithms are based on a constant update frequency; in each update period it is calculated from the reference voltage the number of the modules which needs to be inserted as shown in Fig.…”

“…This creates extra switching losses and also voltage peaks and drops appears in the output voltage. A way to minimize the extra switching is to consider the previous insertion state as it is described in [8]. Another method to reduce the number of switching used to exchange a discharged cap with a charged one from the same arm, is to set a tolerance band.…”