An improved four‐layer capacitor voltage control strategy combined with arm current control for transformerless modular multilevel converters

Abstract

In order to improve the performance of the transformerless modular multilevel converters (MMC) under AC and DC grid fault conditions, this paper proposes an improved fourlayer capacitor voltage control strategy combined with arm current control. The proposed control strategy consists of overall submodule capacitor voltage averaging control, legbalancing control with flexible weight factor W, arm-balancing control with zero-sequence suppression, and inner-arm individual balancing control. The new concept of weight factor W for leg-balancing control is also originated, considering negative-sequence current and DC circulating current jointly from power point of view. The proposed approach effectively realizes the capacitor voltage balance, ac-side current tracking, and unwanted circulating current elimination. The practical implementation of the proposed control strategy and the experimental results from a downscale three-phase transformerless MMC prototype are presented to validate the performance of the proposed four-layer capacitor voltage control strategy under grid faults.

Circulating Current Suppression Strategy Based on Virtual Impedance and Repetitive Controller for Modular Multilevel Converter Upper and Lower Bridge Arm Capacitance Parameter Asymmetry Conditions

“…When the capacitance degradation causes asymmetry in the capacitance parameters of the upper and lower bridge arms, odd harmonics dominated by the fundamental frequency and triplet frequency components will be introduced in the circulating current of the upper and lower bridge arms, and the greater the degree of asymmetry, the greater the amplitude of the odd harmonics. In fact, the capacitor voltage and bridge arm current will generate more higher harmonic components through a series of coupling effects [23]. This harmonic will cause fluctuations in the DC side current and increase converter losses, so measures must be implemented to suppress it.…”

“…The strength of the differential link is determined by the time constant T. The regulation of the virtual reactance can be achieved by adjusting the corresponding T. However, in the actual control process, the pure differential link is not easy to implement, and it is easy to make the system over-tuned, which leads to system divergence. Therefore, a first-order inertial link is added to Equation (23) to correct it, which is…”

“…However, in the actual control process, the pure differential link is not easy to implement, and it is easy to make the system over-tuned, which leads to system divergence. Therefore, a first-order inertial link is added to Equation (23) to correct it, which is…”