Hybrid-LCC Based on IGCTs in HVDC Transmission

Low-loss HVDC energy transmission systems are a critical part of the future decarbonized electricity generation and distribution strategy. Line-commutated converters (LCC) based on traditional thyristors have been widely used in HVDC systems; however, the use of these devices can make the system more susceptible to commutation failures. In this article, we summarize the concept of a Hybrid-LCC converter at the receiving end of the HVDC system that makes use of integrated gate-commutated thyristors (IGCTs) to enhance system robustness along with other performance and cost advantages.

HVDC power transmission

Let’s consider the power generation and transmission advances that China has made in recent years. They are one of the biggest producers and consumers of electricity in the world. China has made significant progress in renewable power generation. Large installations of solar and hydro power generation have been successfully deployed in the western parts of the country, where these resources are plentiful. 

The biggest load, however, is situated on the eastern coast of the country, where the population density in large cities is high. Hence, various sources of renewable energy need to be transmitted over thousands of kilometers. HVDC is the most efficient choice, since it minimizes energy loss and offers greater control in power flow. Over 30 HVDC projects have so far been deployed in China with a capacity of over 120 GW. Further, new HVDC transmission projects are accelerating as over 30 new ones are slated for construction by the year 2030.

Commutation failures (CFs) with LCC receivers

LCCs have been popular at the receiving end of HVDC lines due to their large capacity, low losses, and high reliability. In these converters, thyristors rely on the AC line voltage to provide the commutation voltage to transfer current from one valve to another, i.e., the turn-off of the device depends on the AC voltage. 

The firing sequence of the thyristors is based on a minimum extinction angle Ɣ, which ensures that the device going from the on to the off state has sufficient reverse recovery time and can block when the AC voltage changes polarity. A weak AC grid or AC faults, however, can disturb this extinction margin. This is depicted in Figure 1.

An insufficient reverse recovery period can prevent the thyristor from reaching blocking and turning on again. The result can be 2 valves in the same bridge arm conducting simultaneously, creating a short circuit on the AC side. 

This causes the DC current to rise sharply, the DC voltage to fall, and the active power transfer to be temporarily interrupted. Reactive power imbalance, harmonic distortion, and voltage instability can result in the receiving grids. Stress on the system components and reliability risks increase from repeated failures. 

Efforts to reduce CFs in LCC’s can involve adding auxiliary circuits and control optimization, though these can often increase cost and complexity. Voltage source converters (VSCs) that use IGBTs can be used, but at the expense of higher losses and overall system cost increases. The cost increase is particularly true if existing systems based on LCC have to be modified.   

IGCTs

IGCTs evolved from gate turn-off (GTO) thyristors in the mid-1990s. Its basic cross-section is shown in Figure 2. In the IGCT, the n-buffer layer placed between the n- base and the p-emitter results in a reduction in on-state voltage. The low inductive design in IGCTs that monolithically integrates the gate drive with the power device helps reduce turn-off times (to < 10 µs) and better dV/dt immunity (>4kV/µs for gate-powered devices) compared to GTOs. These current-controlled devices are the ideal choice in many high-power conversion applications such as MVD drives, renewable energy converters, and grid-use circuit breakers.  

Key advances over the last decade have led to several improvements such as reverse blocking capability (e.g. by making the device symmetric in layer construction: see Figure 2) of over 8 kV and current blocking capability over 5 kA, large diameters with single IGCTs on 150 mm Si wafers, reverse conducting capability with the addition of freewheeling diode alongside, and many improvements in the hermetic, press-pack packaging to allow higher power and temperature. 

Some key advantages over IGBTs include:

• IGCTs have significantly lower on-state voltage drops with a stronger modulation effect. As a result, conduction losses can be lower. A higher-density carrier plasma region allows higher surge current conduction.
• Bidirectional voltage withstand capability is key in a current source converter, and reverse blocking IGCTs with higher voltage and current ratings have been demonstrated compared to those with IGBTs (see Figure 3, lower right). 
• The reverse recovery characteristic of the IGCT is similar to that of a thyristor. Voltage equalization can be more easily achieved in series-connected devices.

Hybrid-LCC Systems with IGCTs

The team at Tsinghua University, China, has demonstrated a hybrid-LCC system [2] that replaces part of the thyristors in each bridge arm with IGCTs, allowing existing LCC systems to be made robust against CFs in a cost-effective way. 

A schematic of the H-LCC is shown in Figure 3. During a CF, the IGCTs are actively turned off upon detection that the current in the one that is supposed to turn off does not, and instead increases abnormally. Thus, the fault current can be actively broken and commutated to the other arm.

The goal is to create IGCTs that have reverse blocking capability over 8kV with low leakage and turn-off capability of 8kA for the HVDC project. Some of the device development highlights were:

• A thicker, lower-resistivity N base layer (> 1200 µm thick @ ~ < 1e13/cm³) achieved > 8 kV blocking with a leakage current at 8 kV of only 140 mA. A higher rating allows fewer devices to be used in series/parallel for the HVDC project.
• An isolated P emitter was used. Referring to Figure 1, this involves using a P- layer above the P+ emitter anode. A P- isolation region laterally adjacent to the non-isolated P+ emitter connects to this P- layer. This creates two parallel paths which serve to decouple the requirements during the high injection conduction state and the low injection blocking state [3]. An isolation ratio (i.e., relative area of the isolated to the total anode path) of 0.1 reduces forward off-state leakage by more than 50% without affecting on-state voltage.
• A shallow, highly doped P+ emitter (~ 15-20 µm @ 1×10¹⁹/cm³) creates a lower on-state voltage drop while 

This design enabled the development of an 8 kV bidirectional blocking, 4 kA conduction, and 8 kA turn-off-capable IGCT with a forward drop of 2.2 V at 5 kA. 

Topology enhancements included the use of separate metal-oxide varistor (MOV) clamping devices in parallel with each active device instead of sharing them across series-connected devices. This facilitates series voltage balancing that is forgiving to variations in individual device parameters. An ultra-fast active turn-off strategy was used based on a rising current and a timed turn-off mechanism. The method ensures a guaranteed turn-off under a CF condition.

Results and deployment

Tests conducted with this H-LCC showed a 20% reduction in steady-state reactive power demand, as well as a 60% reduction in surplus power (created from the current spike under a CF) during a fault. Harmonic reduction of 40% was achieved during CF. 

Fluctuations in power during CF events were reduced, enabling smoother ride-throughs. This demonstrated advantages compared to the standard LCC, both under normal operation and fault conditions. Compared to the LCC, the H-LCC valves showed a 15-20% decrease in valve loss ratio at full power, creating system cost savings. 

A 120 kV HCC converter was deployed at the Lingbao converter station in China in Dec 2025. It has successfully passed artificial short-circuit and CF tests and validated a reduction in the DC system recovery time from faults by 35 ms.   

Cover image: Adobe Stock 

References

1. Rong Zeng, “IGCT-based New Generation HVDC Technology: From Devices, Equipment to Applications,” IEEE PELS Webinar, 2026.
2. Chaoqun Xu, Zhanqing Yu, Biao Zhao, Zhengyu Chen, Zongze Wang, Chunpin Ren, and Rong Zeng, “A Novel Hybrid Line Commutated Converter Based on IGCT to Mitigate Commutation Failure for High-Power HVDC Application,” IEEE Transactions on Power Electronics, 37, 5, May 2022.
3. Chunpin Ren, Jiapeng Liu, Xiaozhao Li, Yue Song, Lihui Xu, Zongze Wang, Biao Zhao, Zhanqing Yu, Jinpeng Wu, and Rong Zeng, “Optimal Design of Reverse Blocking IGCT for Hybrid Line Commutated Converter,” IEEE Transactions on Power Electronics, 38, 11, Nov 2023

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