Experimental validation of a parallel hybrid modular multilevel voltage source converter for HVDC transmission

Abstract

This paper discusses a small scale laboratory model constructed to validate the operation of a recently proposed parallel hybrid modular multilevel voltage source converter which is intended for HVDC applications. The converter is characterized by a 'chainlink' arrangement composed of a number of half bridge cells capable of operating with bidirectional current and an associated full bridge converter for each converter phase arrangement. Each half bridge cell is made up of two bidirectional switches and a local storage capacitor. The voltages on the local storage capacitors of the 'chainlink' cells need to be controlled within a tolerance band to ensure sustainable operation of the converter. The paper also presents a capacitor voltage balancing scheme for this converter and further demonstrates black start capability using a lower rated auxiliary DC source. Four quadrant operation of the converter is also demonstrated as would be expected of a grid connected voltage source converter. Simulation and experimental results are presented to support the converter operation.

Control of a hybrid modular multilevel converter during grid voltage unbalance

“…The model is implemented using PLECS. The power flow control between the power converter and the grid, the power balance between the power converter and the DC circuit and the cell voltage control are achieved with the control schemes discussed in [7,9,10,17]. …”

“…In [7,10,13], the use of triplen harmonics are explored for modulation ratio control in the parallel hybrid modular multilevel converter (PH-M2L-VSC) allowing PQ control and DC link voltage ripple reduction. In these papers, the studies considered a balanced symmetrical grid with equal amount of triplen harmonic injected in each chain-link to allow fundamental frequency ac converter voltage control.…”

“…Research into the performance of the converter in a balanced network is presented in [7,[9][10][11][12][13]. The converter uses a cascade of half bridge cells to produce a full wave rectified multilevel waveform for each phase according to (1) during balanced network operations according to (2).…”

Operation of a hybrid modular multilevel converter during grid voltage unbalance

“…Research into the performance of the converter in a balanced network is presented in [7,[26][27][28][29][30]. The authors of [31] investigated the operation of the converter with a depressed AC voltage by introducing voltage notches in the AC voltages which has an effect on the DC bus voltage similar to the staggered voltage operation of the converter chainlinks in [5].…”

The star-switched MMC (SSMMC) - a hybrid VSC for HVDC applications

“…Recently, hybrid voltage source converters such as the parallel hybrid VSC [5,17] and the Alternate Arm Converter (AAC) [6,18], combining features of two-level and modular multi-Level converters have been proposed to achieve high waveform fidelity, lower loss and/or DC fault blocking capability.…”