A new capacitor voltage balancing control for hybrid modular multilevel converter with cascaded full bridge

Abstract

Recently, a hybrid modular multilevel converter with cascaded full bridges (HMMC-CFB) is proposed. This converter has mainly two parts: a director part (DP) and a wave shaping part (WSP). The DP consists of a conventional modular multilevel converter (MMC) with half bridge submodules (HBS). The WSP is a series connection of full bridge submodules (FBS) and attenuates the voltage harmonics produced by DP. The energy exchanged by WSP changes with modulation index and causes improper capacitor voltage regulation. For satisfactory operation of HMMC-CFB, the total capacitor voltage of WSP should remain constant. This paper presents an analysis of energy exchanged by WSP and proposes a new control technique to regulate the total dc voltage of WSP. In the proposed control, the energy exchanged by WSP is controlled by changing the slope of the voltage reference signal of DP. In addition, analysis of circulating current of HMMC-CFB is presented. A circulating current suppressing controller (CCSC) is used to control the second harmonic component. The performance of the proposed control technique for the standalone model of HMMC-CFB is evaluated by using time domain simulation studies in the PSCAD software environment. To test fault blocking capability of converter, a grid connected model is built in PSCAD and analyzed for dc fault tolerant capability.

Capacitor‐voltage regulation and linear‐range extension of a hybrid cascaded modular multilevel converter

“…The main reason for the FB-submodule capacitor voltage depression is that the main power stage does not deliver enough average power to the ac network when the main power stage uses the same voltage reference as the phase voltage and operates with m > 1.0. A method for FB submodule capacitor voltage regulation using a trapezoidal PWM method is given in [24], and is briefly described in Section 3.2.1. However, this method suffers from the low-frequency harmonics embedded in its trapezoidal reference, which yield a large THD (see Section 3.3).…”

“…One method to compensate the energy deficiency in the FB submodules is to use a slope-adjustable trapezoidal PWM for the main power stage [24], as shown in Figs. 3a and b, where v cref-fb and v cavg-fb are the reference and average capacitor voltages of the FB submodules in the active-filter stage.…”

“…3a and b, where v cref-fb and v cavg-fb are the reference and average capacitor voltages of the FB submodules in the active-filter stage. The detailed derivations based on the zeronet energy absorbed by the stacked FB submodules are shown in [24]. However, [24] only analyses the relationship between the slope of the trapezoidal reference, k trap , and its equivalent…”

“…3a). The expressions for the trapezoidal reference for these two cases are as follows (see (8)) According to the detailed analysis of the trapezoidal PWM method in [24], the relationship between the converter modulation index, m, and the slope of the trapezoidal waveform, k trap , is expressed as…”

Hybrid modular multilevel converter with cascaded full bridges for medium-voltage motor drives

“…Further, the cascaded full-bridge SMs absorb the common mode voltage at the motor side, which reduces the harm to the motor. In addition, the published control strategies of HMMC-CFB (Li et al, 2015; Ghat et al, 2017; Ghat et al, 2016) are only suitable for the constant frequency applications, not suitable for motor drives applications, and the stability problem of the HMMC-CFB closed-loop system has not been discussed in the published literatures. Therefore, referring to the state-error feedback control of MMC proposed by Cisneros et al (2015) and Bergna-Diaz et al (2017), a state-error feedback control law of HMMC-CFB system is given by the established mathematical model in this paper.…”