IGCT in HVDC Transmission: Hybrid Commutated and Modular Multilevel Converter

Despite advancements in VSC deployment, LCC technology remains valued for its high efficiency, proven reliability, and widespread use in large-scale projects. The main challenge is to preserve the efficiency and maturity of LCC systems while integrating the enhanced controllability of VSC technology. This need has led researchers to re-examine established power electronic devices to identify components capable of bridging these two technological approaches.

The Integrated gate-commutated thyristor (IGCT) is a well-established high-power device. During turn-off, it rapidly commutates the cathode current, achieving conduction losses comparable to those of thyristors and blocking capabilities similar to those of insulated-gate bipolar transistors (IGBTs), with the gate drive integrated into the device package [1]. Commercial IGCTs are available in asymmetric, reverse-conducting, and reverse-blocking variants for medium- and high-voltage conversion [2]. Asymmetric IGCTs (AS-IGCTs) provide voltage blocking in only one polarity and rely on an external antiparallel diode for reverse current conduction. They are typically used in voltage-source applications such as modular multilevel converter (MMC) submodules, where the separate diode allows optimised conduction and switching performance. Reverse-conducting IGCTs (RC-IGCTs) integrate the free-wheeling diode within the same package, enabling bidirectional current conduction while maintaining unidirectional voltage blocking. Reverse-blocking IGCTs (RB-IGCTs) incorporate additional structural insulation and a P-buffer emitter layer, providing full bidirectional voltage blocking, which are suited to current-source that demand both bidirectional blocking and active turn-off capability. Each IGCT variant is tailored to specific inverter topologies and operating conditions, balancing blocking strength, conduction losses, and packaging efficiency. Recent advancements have extended the voltage capability to 8.5 kV, enabling higher turn-off currents and reducing the number of series-connected elements required in converter valves [2], [3]. This higher-voltage capability has attracted HVDC researchers by offering the potential to simplify valve construction, enhance reliability, and reduce system costs.

The renewed interest in IGCT is particularly evident in China, where HVDC systems operate under complex multi-infeed conditions. No other region has as many long-distance ±500 kV and ±800 kV HVDC transmission lines feeding into adjacent consumption-heavy alternating current (a.c.) power networks. While these networks achieve an adequate power supply, they are also more vulnerable to disturbances. When multiple converters share a common a.c. system, a single commutation failure can trigger cascading disruptions due to a.c. voltage reduction. This phenomenon is well documented in CIGRE studies on commutation failures and multi-infeed interactions [4], [5]. Ensuring system stability in these circumstances has become a central challenge for power grids in China and other regions. As a result, engineers are exploring new converter topologies that combine current-source efficiency with enhanced controllability. The IGCT, with its gate-controlled turn-off capability and high current endurance, provides a logical foundation for these innovations.

 One line of research focuses on a line-commutated converter equipped with controlled turn-off capability, commonly referred to as a hybrid commutated converter (HCC)because its operating principle combines natural commutation as in an LCC, with active turn-off when required during the commutation process. In this configuration, the conventional thyristors in an LCC valve are replaced by RB-IGCTs, as shown in Figure 1 [6], [7]. During a.c.-side disturbances, RB-IGCTs can temporarily interrupt current to help commutation recovery without completely blocking the converter. Unlike a conventional LCC inverter whose reactive-power absorption rises during a.c. faults, the HCC can temporarily reduce its reactive power demand through a delayed-angle control mode enabled by active turn-off of RB-IGCTs. Simulation studies suggest that this control helps maintain direct-current (d.c.) voltage and can, depending on the fault location and system conditions, improve local voltage recovery [8]. The HCC retains current-source efficiency during normal operation while enabling a controlled response under fault conditions. A ±120 kV HCC converter has completed successful electrical and real-time digital simulator (RTDS) testing [9]. It has been installed as a retrofit to an existing LCC valve at the Lingbao back-to-back station in Henan Province, China, with commissioning scheduled for November 2025.

A second line of research focuses on advancing the modular multilevel converter (MMC) architecture by increasing submodule voltage and power density by replacing IGBT switches with press-pack IGCTs, as depicted in Figure 2 [10], [11], [12], [13]. This approach offers two main advantages. First, the lower on-state voltage of IGCTs reduces conduction losses at HVDC operating points, where switching frequency is moderate. Second, the higher device-voltage rating increases submodule voltage, reducing the number of cells required per arm and simplifying valve construction. The MMC platform also supports grid-forming operation. Progress in engineering verification has already been demonstrated in China, where a ±15 kV, 60 MW back-to-back installation completed a 180-day reliability trial at the Yunnan Mile wind hub [14]. An urban ±120 kV back-to-back HVDC project is under construction in Guangzhou [15], and a ±525 kV / 3 000 MW valve prototype has entered type testing [16].

Together, the hybrid-commutated converter and IGCT-MMC developments provide a comprehensive direction for HVDC technology by adapting established converter topologies to meet evolving operational requirements. The HCC technology addresses commutation failure risks in LCC-HVDC systems by enhancing system stability and supplying reactive power support during faults. Meanwhile, the use of IGCTs in MMCs supports the ongoing transition to MMC architectures by reducing conduction losses and improving efficiency. IGCT-based MMCs enable higher submodule voltages and provide robustness under fault conditions, complementing the capabilities of traditional IGBTs.

Several challenges must be addressed before IGCT-based converters achieve widespread adoption. The primary concern is the maturity of devices at 8.5 kV, with ongoing efforts focused on achieving industrial-scale production and consistent quality. Circuit designs require further optimisation, particularly to simplify IGCT clamping circuits. Standardisation must also progress to accommodate topologies with hybrid turn-off characteristics. Most pilot projects and standardization initiatives remain concentrated in China. Furthermore, the economic viability of these systems requires a comprehensive assessment over extended operational periods. Long-term field data, including device lifespan, will be essential for evaluating total system costs.

These initiatives are intended to expand the range of HVDC solutions rather than replace existing technologies. By re-evaluating established technologies, engineers can develop practical approaches that balance efficiency, controllability, and feasibility. Integrating IGCTs into HVDC systems builds on proven methods instead of introducing abrupt changes. As additional empirical evidence emerges, IGCTs may broaden the scope of high-power switching applications beyond the capabilities of current IGBT technology.

References

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