Active-Forced-Commutated Bridge Using Hybrid Devices for High Efficiency Voltage Source Converters

Abstract

In high-power converters, the on-state characteristics of semiconductor devices dictate its efficiency performance even if the optimized topologies are adopted. This letter proposes an active-forced-commutated (AFC) bridge that employs the hybrid power devices of thyristor and insulated-gate bipolar transistor (IGBT) to operate as a voltage source converter or the building blocks of complex multistage high-voltage high-power converters. In this scheme, the thyristors are placed in the main power path that conducts for most of the fundamental period to lower the on-state losses; while the IGBT-based full-bridge (FB) chain-link is used for controlled (soft) transition and forced commutation of the main thyristor bridge, operating in short period. This stepped transition voltage also leads to minimized dv/dt exerted on the interfacing transformer. To coordinate the operation of these two parts, the FB stack is designed to operate in a concave polygon stepped transition mode for the ordered turn-on and turn-off of thyristors according to different categories of commutation events. Detailed commutation analysis for the AFC-bridge is provided in this letter; also, high-level discussions and simulation results are presented to demonstrate its potential technical merits.Index Terms-Active forced commutation (AFC), controlled transition, high efficiency, hybrid device, insulated-gate bipolar transistor (IGBT), thyristor, voltage source converter (VSC).

Operation Analysis of Thyristor-Based Front-to-Front Active-Forced-Commutated Bridge DC Transformer in LCC and VSC Hybrid HVDC Networks

“…With phase-shift control, soft switching performances for the turn-on of self-commutated switches and turn-off of their antiparallel diodes can be achieved for much of the operation range in traditional DAB, which is inherited by the FB-CL parts in the F2F AFC-bridge [15]. Also, the main thyristors undergoes nearly zero voltage switching (ZVS) with stepped voltage transition manner controlled by the FB-CLs [20,28].…”

“…Alternatively, the reverse-bias of on-state thyristors can depend on the ac grid voltage, in a similar way of conventional LCC; while the FB-CL only protects itself with reduced dynamic rating burden on its cell capacitors. Importantly, this dc-fault blocking feature of AFC-bridge is achieved without sacrifice on the efficiency performance in contrast to a FB or mixed cell MMC; rather, it operates the low loss, low cost and robust thyristors as close as possible to a standard VSC, which further reduces the losses of HB-MMC and retains its key control flexibility [28]. Additionally, the semiconductor area and total cost of AFC-bridge are advantageous, considering that its total amount of cell capacitors and number of IGBTs (more expensive than thyristors) are significantly smaller than in a MMC, not to mention the protective thyristors installed in each cell of HB-MMC for over-current bypass [35].…”

“…2(b). Alternatively, when the power flow direction reverses, B1 will shift into an inverter mode with opposite current distribution among the symmetrical thyristors [28].…”

“…The active-forced-commutated (AFC) bridge has recently been proposed in [28] to establish a VSC scheme by using symmetrical thyristor-bridge as the main power paths and an insulated-gate bipolar transistor (IGBT) based FB cell chain-link (CL) for the controlled transition and forced commutation of main thyristors. With the assistance of FB-CL, AFC-bridge can fully control the turn-on and turn-off of its thyristors in a manner that allows elimination of operational dependency on the live ac network compared to conventional LCC.…”

“…The VSC nature of AFC-bridge is an essential difference compared to the combination of LCC with cascaded FB compensator that remains a current source nature in the main power circuit [12], making it suitable for developing the multi-terminal dc grid at reduced power losses [28]. This paper employs the AFCbridge in a dc transformer to link both LCC and VSC terminals in a hybrid HVDC network.…”