A Modified Voltage Balancing Algorithm for the Modular Multilevel Converter: Evaluation for Staircase and Phase-Disposition PWM

Abstract

This paper introduces a low complexity implementation of the voltage balancing algorithm aiming to reduce the switching frequency of the power devices in modular multilevel converters (MMCs). The proposed algorithm features a relatively simple implementation without any conditional execution requirements and is easily expandable regardless of the number of submodules (SMs). Two modulation techniques are evaluated, namely the staircase modulation and the phase-disposition pulse width modulation (PD-PWM) under the conventional and the proposed algorithm. Using a circulating current controller in an MMC with 12 SMs per arm, PD-PWM yields better results compared to the staircase modulation technique. The test condition for this comparison is such that the power devices operate at a similar switching frequency and produce similar amplitudes to the capacitor voltage ripples in both modulation techniques. The results are verified through extensive simulations and experiments on a low power phase-leg MMC laboratory prototype.

Submodule power losses balancing algorithms for the modular multilevel converter

“…Capacitor voltage balancing is achieved by selecting the SM with lower voltage when the arm current is positive, and the SM with higher voltage when the arm current is negative. In order to reduce the switching losses, an algorithm that limits the number of SM transitions per switching period is implemented [13]. The voltage reference for the modulation algorithm is provided by a typical d-q current controller for grid-connected applications.…”

“…The voltage balancing algorithm used in this application is based on multiplying the capacitor voltages by the opposite sign of the arm current and sorting them in descending order [13]. That is, when the current goes in the charging direction (positive), the voltages are multiplied by −1, giving higher priority of activation to the ones that have the lowest values (highest values when inverted).…”

“…The first algorithm is based on balancing the number of switching transitions of each SM, and the second one is based on balancing the estimation of both switching and conduction losses. In this paper, a level-shifted pulsewidth modulation (LS-PWM) is used, and a voltage balancing algorithm with reduced switching frequency is used [13], [14].…”

Discontinuous modulation of modular multilevel converters without the need for extra submodules

“…In this paper, the voltage reference is a sinusoidal waveform with amplitude defined by the modulation index m a . It is modulated by a level-shifted PWM (LS-PWM) technique [12] and the capacitor voltages are balanced using the algorithm proposed in [13].…”

Comparison of bipolar sub-modules for the alternate arm converter

“…Various types of modulation techniques are applicable [3], [5], [6] in order to determine the number of SMs to be inserted, while the staircase type modulation methods are more efficient over the alternative techniques as the number of output voltage levels increases [7]. Different capacitor voltage balancing strategies [8], [9] are used and the SM sorting algorithms are the most common. Circulating current control is important for the MMC in order to regulate the capacitor voltages, maintain upper and lower arm energy balance, and efficiently utilize semiconductor ratings.…”