Direct Modular Multilevel Converter With Six Branches for Flexible Distribution Networks

Abstract

This paper presents a complete analysis of a direct ac-to-ac modular multilevel converter (direct MMC) applied in medium voltage distribution networks through the soft-openpoint concept. The direct MMC is capable of bidirectional power flow between two feeders at any power factor, even when the feeders have different nominal voltages and operate with a phase shift angle or unbalanced voltages. The converter has six branches, each one composed of full H-bridges cells connected in series to generate a multilevel voltage waveform, to share the blocking voltage of the power switches and to have fault tolerant operation.This paper presents a suitable control scheme and provides a discussion about the capabilities and limitations of the converter, the capacitor voltage balance control, the efficiency and the power loss mitigation at various operation points. Simulation results and power loss calculations are presented for a threephase 11 kV 16 MVA direct MMC with 10 H-bridge cells per branch. The direct MMC is simulated in a distribution network to demonstrate the features of the converter and control under various operation conditions, including grid faults. Index Terms-Direct power conversion, AC-AC power converters, matrix modular multilevel converters, soft open point (SOPs), power-flow controller, grid connected converters.

15 kVA Three-Phase Low-Voltage SOP Prototype Laboratory Tests Results

“…Finally, it should be noted that we do not consider in this paper SOP based on modular multilevel converter (MMC) topologies, which are recently proposed as an alternative to the B2B 2-or 3-level voltage source converter (VSC) configuration, especially for MV applications; see [19][20][21][22][23]. See [22] for a comparison between the two structures in terms of PQ operating area.…”

“…In [19], the author considers the use of medium-voltage power electronic compensation by considering modified versions of existing flexible AC transmission system (FACTS) or custom power devices. In [20], the authors propose the use of modular multilevel converters as power electronics topology for an MV SOP. A control strategy of a multi-port MMC-based SOP under steady-state operation of a distribution network is presented in [21].…”

Recent contributions and future prospects of the modular multilevel converters: A comprehensive review

“…They can be categorized as carrier‐based PWM (CB‐PWM), nearest level modulation (NLM), and selective harmonic elimination (SHE), which is rarely preferred . CB‐PWM strategies are generally chosen due to the suitability in MMC‐based works, which contains PSC‐PWM and level‐shifted PWM (LS‐PWM) . Besides, LS‐PWM includes phase disposition PWM (PD‐PWM) and phase opposition disposition PWM (POD‐PWM), as well as alternate phase opposition disposition PWM (APOD‐PWM) .…”

Dual-timescale control for power electronic zigzag transformer

“…In essence, PEZT is a power electronic voltage regulator. Its thyristor and VSC parts can be unitedly controlled to make the output voltage phasor track the expected amplitudes and phases formulated in (8).…”

“…Back-to-back voltage-source converter (BTB-VSC) has been approved worldwide in the operation of high-voltage direct-current (HVDC) transmission systems and low-voltage equipment. By the virtue of its full controllability and fast response, it was utilized in [6]- [8] as so-called soft normally -open points (SNOP) for mid-voltage distribution networks to take the place of mechanical interconnection switch. Although the potential benefits of SNOP was demonstrated, its cost, footprint and efficiency were always huge barriers in front of the real applications, because large amount of passive components were required in its modular multilevel converters (MMC) [9].…”