Experimental Validation of a Nested Control System to Balance the Cell Capacitor Voltages in Hybrid MMCs

Mauricio Espinoza,Andres Mora,Alan Watson,Roberto Cardenas,Felipe Donoso,Jon Clare

doi:10.1109/access.2021.3054340

Mauricio Espinoza, Andres Mora + Show 4 more

Open Access

https://doi.org/10.1109/access.2021.3054340

Copy DOI

Abstract

In a hybrid modular multilevel converter (MMC), capacitor voltage balance between the Full-Bridge Sub-Modules (FBSMs) and Half-Bridge Sub-Modules (HBSMs) is only possible when the arm currents are bipolar. For a grid-connected MMC, operating at unity power factor, this is typically only achievable when the modulation index is less than 2. Previous control methodologies, based on open-loop feed-forward compensating currents, have been proposed to operate an MMC with a higher modulation index. However, these solutions do not minimize the compensating currents; they cannot compensate entirely for both the variations in the operating conditions and the parameters typically encountered in a real implementation; and they do not consider the actual capacitor voltage imbalance between the FBSM and HBSMs. In this paper, a new nested closed-loop control algorithm based on an outer voltage control loop with an inner current loop is proposed and experimentally validated. Feed-forward currents are still utilised in the inner loop, but they are calculated using a new optimising algorithm which minimises the required compensating currents. Moreover, to the best of our knowledge, this is the first work where explicit algebraic equations to calculate these compensating currents are provided. Experimental results to validate the approach, obtained with an 18-cell hybrid MMC, are presented and discussed in the paper.

Highlights

Nowadays, the modular multilevel converter (MMC) is a prominent solution for high voltage direct current (HVDC) transmission systems [1], [2] and for medium voltage drive applications [3], [4]
Considering the problems mentioned above, this paper proposes the use of a nested closed-loop control system to regulate the capacitor voltage imbalances between the Half-Bridge Sub-Modules (HBSMs) and Full-Bridge Sub-Modules (FBSMs)
BALANCE BETWEEN THE HBSMs AND FBSMs DURING LOW DC-PORT VOLTAGE The capacitor voltage imbalance problems produced in a hybrid MMC operating with a high modulation index m have already been reported in [9], [22]–[24]

Summary

INTRODUCTION

The modular multilevel converter (MMC) is a prominent solution for high voltage direct current (HVDC) transmission systems [1], [2] and for medium voltage drive applications [3], [4]. The main drawback of the methods discussed in [13], [21], [23], [24] is that the feed-forward circulating currents are calculated off-line and are imposed without considering the degree of imbalance between the capacitor voltages of the FBSMs and HBSMs, i.e., there is no closed-loop control of the FBSM-HBSM capacitor voltage imbalance, and the system lacks the capability to compensate for changes in the operating point and/or variations in the parameters of the hybrid MMC. The compensating currents are calculated off-line using a methodology based on an optimising algorithm and, the injected current is the minimum that ensures voltage balance between the FBSMs and HBSMs. The main contributions of this work are: 1) Unlike previous works [9], [23], [24] the proposed strategy implements a nested closed loop control system to regulate the FBSM-HBSM capacitor voltage imbalance. There is an Appendix where the algebraic equations for the CLC-I and CLC-II methods are given

MODELLING OF THE HYBRID MMC

OPTIMAL METHODOLOGY TO ENSURE THE LOCAL BALANCE OF THE HYBRID MMC

ADVANTAGES AND DISADVANTAGES OF CLC-I AND CLC-II

GLOBAL CAPACITOR VOLTAGE CONTROL

OPERATION OF AN UNCOMPENSATED HYBRID MMC IN THE OVER-MODULATION RANGE

VIII. CONCLUSION