The augmented modular multilevel converter

Abstract

The Controlled Transition Bridge (CTB) is a class of converter topology that combines series connected semiconductor "director valves" with chains of switched capacitor modules, "chainlink circuits", in such a way that the director valves carry the main current for a significant portion of the period and the chainlink circuits provide a controlled traverse of voltage between different director valves conducting. This combination is applicable to HVDC where efficiency is paramount, since it allows thyristors or diodes to be used for the director valves to reduce the conduction losses, with the chainlink circuits providing commutation and direct control of the rate of change of transition voltage. Since the chainlink portion of the AMMC only has to manage the transition between the upper and lower director valves, the size of the capacitors in the individual sub-modules can be reduced, reducing the converter footprint. Also the trapezoidal waveform that results can be tailored to give reduced harmonic levels, so reducing filtering required for the AC waveform to meet regulations on distortion at the point of common coupling for the converter (PCC).An analysis is presented of an example of this type of converter where a modular multilevel converter (MMC) is combined with a conventional thyristor bridge, the Augmented MMC (AMMC). Various aspects of the operation of the bridge are discussed, including the management of the charge in the chainlink capacitors and the converter losses.

The transition arm multilevel converter — A concept for medium and high voltage DC-DC transformers

“…The TAMC structure is capable of administering controlled voltage transition for a trapezoidal ac output with lower silicon area compared to the CTB topologies described in [17] and [18]. The bi-state arm may be comprise of low-loss switching devices (e.g.…”

Controlled Transition Full-Bridge Hybrid Multilevel Converter With Chain-Links of Full-Bridge Cells

“…Such techniques are found in the literature such as trapezoidal modulation but with low order harmonic emergence [23]. To optimize the amount of low order harmonics due to the voltage clamp (saturation), multi-slope trapezoidal modulation is presented in [23,24]. Although the maximum AC voltage can be increased with these methods, their modulation control range is limited due to the narrowed wave-shaping ability.…”

“…Assuming the insulated gate bipolar transistor (IGBT) modules are used to build each converter, the total on-state losses are the summation of IGBT and diode losses. The forward voltage drop of IGBT and its anti-parallel diode are calculated from (24), where {V CE , V D } are the voltage drop in conduction mode; {I C , I D } are the instantaneous value of the device current; {V CEO , V DO } represent the device threshold voltages; {R V , R D } define the equivalent resistances for IGBT and diode respectively, and can be further obtained by (25) according to the nominal device operation points {V CEN , I CN } and {V DN , I DN } from the manufacturer datasheet [25,26].…”

Thyristor-Bypassed Submodule Power-Groups for Achieving High-Efficiency, DC Fault Tolerant Multilevel VSCs

“…The parallel operation of a traditional thyristor based LCC converter with VSC has also been considered, however this can be considered as two separate converter stations [11]. One of the most notable examples of a true hybrid converter integrating thyristors and SMs is the Controlled Transition Bridge (CTB) [12]- [14], which combines a sixpulse bridge of thyristors, providing the majority of bulk power processing, with additional of SM stacks that provide filtering and current control. In [15] the author presented an MMC topology that is capable of isolating a DC side fault by utilising a parallel thyristor bridge and low current, high voltage DC circuit breaker.…”