IGCT Research Articles

The First Optimisation of a 16 kV 4H-SiC N-Type IGCT

A challenge in the development of Silicon carbide (SiC) gate turn-off thyristors lie in an uneven transient behaviour, necessitating expensive snubbers. To address these limitations and simplify circuit topology we present an optimized 16 kV n-type SiC integrated gate commutated thyristor (IGCT) design, which utilises a novel highly doped base strip (HDBS). A particular focus is on optimizing the gate commutation of the GCT during switching, and the trade-offs in the HDBS base design were investigated. The findings reveal that compared with conventional GCT design, the HDBS design under high current conditions recorded a 11.8% reduction in turn-off power losses. When simulating the device in a high-voltage scenario, the HDBS IGCT demonstrated a 3.9% reduction in turn-off power losses and an improved turn-on power loss performance. This resulted in a reduction of power losses by 12.1% and 2.3% in high current and high voltage conditions, respectively. In summary, the novel SiC HDBS IGCT design paves the way towards a secure, high current density, and low loss switching SiC thyristor device.

On Fatigue Damage-Oriented Through-Life Optimization and Control of High-Power IGCT Converters in Wind Energy Systems

Integrated gate commutated thyristors (IGCTs) are critical components in high-voltage, high-current, and high-power conversion systems, particularly in offshore wind energy systems. However, the working environment of offshore wind energy conversion systems is extremely harsh. In this article, we propose an active damage control approach aiming at enhancing the reliability of the conversion system. By employing electro-thermal modeling for the equipment of the offshore wind energy conversion system, the junction temperature and fatigue damage of IGCT are simulated during the operation process. Using the improved model predictive current control (MPCC) method, active damage control effectively regulates the switching frequency of IGCT. IGCTs are symmetrically distributed on each leg of the converter, so the lifespan of the two IGCTs on each leg is also considered to be similar. This method balances the life of the IGCTs on the three legs of the converter and optimizes their utilization to the maximum extent. These measures effectively enhance the reliability of the conversion system and lower the operation and maintenance cost of high-power IGCT converters. The effectiveness of the proposed method is validated by co-simulation results by ANSYS and MATLAB/Simulink.

Improved assembly DC circuit breaker based on resonant current injection.

DC circuit breakers (DCCBs) with high breaking capacity and low cost are necessary for quick fault clearance in DC networks. The assembly DC circuit breakers (ADCCBs) have a main breaking section (MBS) and a sub-breaking sections (SBS) for each line, which greatly reduce the cost. But in conventional operation, it bears high voltage for a long time when there is a main switch grounding process in any line fault action. To address this problem, a multiport assembly circuit breaker based on current injection (CI-MPACB) is proposed, which is able to generate a resonant current with increasing amplitude by controlling the duty cycle of Integrated Gate-Commutated Thyristors (IGCTs). Then the resonant current is injected into the SBS to generate current zero crossing and arc extinction. A complex frequency domain circuit analysis is performed on the MBS to describe the action logic as well as the commutation characteristics. In addition, the parameters of each component of the MBS are subject to multiple constraints and reasonable design to ensure the fault current could be cut off quickly and reliably. The cost of existing design is greatly reduced due to the design idea of resonant current injection device parameter selection. Finally, a PSCAD/EMTDC simulation confirms the opening viability of CI-MPACB and the accuracy of the parameter design. The test results show that the designed CI-MPACB can cut off DC fault lines.

A novel controllable capacitor commutation based superconducting hybrid direct current breaker

Featuring low power loss and high reliability, voltage source converter medium voltage direct current (VSC-MVDC) systems have been widely employed for grid-tied renewable energy applications. To maintain high operational safety, circuit breakers are needed to isolate faulted powerlines by comprehensively considering response speed and installation cost. Research efforts have been put to realizing DC fault isolation by coordinating resistive type superconducting fault current limiter (R-SFCL) and integrated-gate-commutated-thyristor (IGCT) based hybrid DC circuit breaker. In this paper, a controllable current commutation based superconducting DC circuit breaker (CCCB-SDCCB) is proposed. By integrating R-SFCL with IGCT based hybrid DC circuit breakers, the current interrupting capacity can be greatly enlarged with the advantage of low cost and fast speed, and hence the overall cost for suppress large fault currents can be greatly reduced for MVDC systems. In addition, a new current injection circuit branch using H-bridge structure is designed to recycle the residual capacitor voltage from the previous fault stage to trigger the IGCTs without the capacitor pre-charging process. Simulation results show that the fault current can be successfully suppressed from 24.2 to 2.1 kA and fully interrupted within 4.11 ms by the proposed CCCB-SDCCB.

A Novel Mixture-Devices-Based Submodule for MMC by Using Low on-State Voltage IGCT and High di/dt Ability IGBT

At present, insulated-gate bipolar transistor (IGBT) is widely used as the converter component in the submodule of modular multilevel converter (MMC) for its excellent performance in driving circuit and switching speed, but the further application is restricted by its high conduction loss and cost rate. Integrated gate-commutated thyristor (IGCT) is another converter device used in MMC with low conduction loss and cost. However, the d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> /d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> capability of IGCT is limited so that a snubber circuit is required in the submodule for protection, which generates unwanted power loss for MMC. For better performance and reliability, this paper proposes a mixture-devices-based submodule (MS) for MMC based on the cooperation of parallel IGCT and IGBT devices. Double-pulse switching experiments for the MS are conducted to verify its effectiveness, and to get the data for the follow-up switching power loss calculation. The power loss of MMC based on IGBT, IGCT and mixture devices are calculated based on the model established in this paper. Furthermore, the comprehensive comparison for power loss among IGBT-based, IGCT-based and mixture-device-based MMCs are performed, proving the effectiveness of the mixture topology in terms of power loss reduction. Meanwhile, the proposed MS is cost-competitive.

Low-Frequency and Sub-Synchronous Power Oscillation Suppression and Analysis of Hybrid Line Commutated Converter for HVDC Grid

Building high-proportion renewable energy grid requires economical and reliable High Voltage Direct Current (HVDC) converters. Line Commutated Converter (LCC) is widely used because of its long transmission distance, low power loss, low manufacturing cost, and large transmission capacity, which modular multilevel converter (MMC) cannot achieve, but has commutation failure (CF) problems. Low-frequency and sub-synchronous power oscillation caused by CF will affect the safety of bulk power systems. This paper theoretically analyzes the power oscillation during CF and CF recovery, and then proposes a novel hybrid line commutated converter (H-LCC) based on integrated gate commutated thyristor (IGCT), which can effectively mitigate CF and suppress power oscillation. The principle of H-LCC suppressing power oscillation is analyzed. On this basis, the performance of LCC, MMC and H-LCC under the same fault conditions is compared by a simulation study, which proves that H-LCC can improve the security and stability of high-proportion renewable energy power systems.

Ultra-Low Reactive Power Operation and Control Strategy of Hybrid Line Commutated Converter Based on IGCT for HVdc Application

Existing line commutated converter (LCC) based high voltage DC (HVDC) systems require large reactive power during operation and reactive power compensation equipment under construction. However, the equipment is expensive and occupies a huge area. In this letter, a novel operation of hybrid line commutated converter based on integrated gate commutated thyristor (IGCT) is proposed to achieve ultra-low reactive power (LR-LCC), and the corresponding control strategy is developed by utilizing IGCT's active blocking and bidirectional voltage withstanding capability. It can almost eliminate the system's extinguishing angle (γ), and the power factor is close to 1. This novel LR-LCC can result in advantages including a much lower manufacturing cost and volume of reactive power compensation equipment for HVDC application.

Transient Current Balancing of Parallel IGCTs in 10-kV/10-kA Hybrid DC Circuit Breaker

Integrated gate-commutated thyristor (IGCT) is an emerging alternative for hybrid DC breaker (HDCB) to achieve fault current clearance in DC systems. However, the maximum controllable current of present IGCT products cannot cope with high-capacity interruption and IGCTs often need to be paralleled. Unlike insulated-gate bipolar transistor (IGBT) whose parallel methods have been widely studied, there lacks intensive study on the transient current balancing and optimization methods for parallel IGCTs, especially in critical current breaking applications. This paper proposes a practical solution to the transient current balancing of parallel IGCTs in HDCB. Theoretical and mathematical analysis shows that current spike occurs due to anode voltage discrepancy. Although self balancing effect and delayed gate signals can help relieve the current spike, IGCT is still possible to exceed the safe operation area (SOA). An improved topology of parallel IGCTs with bypass capacitors is used to deal with this problem, which is verified by 10 kA turn-off experiment. Successful 10 kA current breaking tests of 10 kV HDCB prototype demonstrate the feasibility and validity of the improved parallel IGCT switch.

Simulation study of integrated‐gate‐commutated‐thyristor based superconducting hybrid direct current circuit breaker

AbstractWith the development of a distributed generation, direct current (DC) load and energy‐storage equipment, voltage‐source‐converter‐based medium‐voltage DC systems (VSC‐MVDC) have attracted more attention due to its low power consumption, high reliability, independent power control and so on. However, VSC‐MVDC has the problem of DC fault isolation, which requires the fast‐acting DC circuit‐breakers to isolate faulty lines and ensure low cost. This problem can be solved by coordinating resistive type superconducting‐fault‐current‐limiter (R‐SFCL) and integrated‐gate‐commutated‐thyristor (IGCT) based hybrid circuit breaker. Based on this, IGCT based superconducting DC circuit breaker (SDCCB) is proposed and analysed. Combining R‐SFCL with IGCT could realise large current limiting and interruption and ensure low cost. In addition, the IGCT based hybrid DC circuit breaker (IGCT‐HDCCB) is compared with the traditional insulated gate bipolar transistor (IGBT) based hybrid DC circuit breaker (IGBT‐HDCCB) to evaluate which circuit breaker is more suitable for VSC‐MVDC. The results show that, coordination based on R‐SFCL and SDCCB, the fault current is successfully limited from 17.6 to 2.1 kA, and then interrupted within 3.8 ms. In addition, IGCT‐HDCCB overcomes the disadvantage that IGCT has less interrupting capacity than IGBT, retains the advantage of low cost of IGCT and is more suitable for MVDC system.

Comprehensive Analysis and Experiment of Extreme Faults in MMC Based on IGCT

High voltage direct current voltage source converter (HVDC-VSC) technology is one of most important solutions for transmission of large-scale renewable sources. Modular multilevel converter (MMC) is one of the most popular topologies for HVDC-VSC system. The extreme faults affect the safety and reliability of MMC greatly. This paper gives comprehensive analysis and experiment of extreme faults in MMC based on integrated gate commutated thyristor (IGCT). Causes and characterization of the extreme faults are analyzed in detail. A new protection solution based on IGCT with controlled punch-through design (CP-IGCT) and fast recovery diode with high surge current capability (HS-FRD) is proposed. The surge current capability of HS-FRD is validated via experiment successfully. Theoretical calculation shows that the protection solution based on CP-IGCT and HS-FRD can help reduce the maximum fault current below 50% when the anode inductor is 0.8 μH in the extreme shoot-through fault. Test results show that the protection solution can realize the discharge of the DC capacitor below 4.5 kV in the extreme shoot-through fault. The maximum fault current in MMC based on IGCT is only about 600 kA while the maximum fault current in MMC based on insulated gate bipolar transistor (IGBT) is over 1100 kA. Explosion proof of CP-IGCT is validated during the tests, which is similar to that of the protection thyristor. However, IGBT can't guarantee the stability of the package and may cause severe safety problem. The failed CP-IGCT has the short circuit failure mode (SCFM) and can be used for bypass of the faulty MMC sub-module (MMC-SM). Via comparison, the proposed protection solution has lowest cost and complexity, as well as the highest safety.

Hybrid Line Commutated Converter With Fast Commutation Characteristic Based on IGCT for HVDC Application: Topology, Design Methodology, and Experiments

In order to effectively mitigate commutation failure of line commutated converter (LCC), a concept of hybrid converter is proposed. The converter replaces a part of the thyristors by the integrated gate commutated thyristors (IGCT). In this article, the hybrid line commutated converter is further developed and reverse blocking IGCTs (RB-IGCT) are applied in each bridge arm to achieve fast commutation characteristic. RB-IGCT is capable of withstanding bidirectional voltages, and owing to the device control method of self-turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> , has almost zero commutation time and gives play to the advantage of fast commutation, thus earning recovery time for the thyristors. Therefore, the commutation performance of the hybrid converter is enhanced greatly. The topology, control strategy, and the hybrid device characterization analysis with fast commutation are introduced. On the basis, the design methodology of hybrid line commutated converter with fast commutation characteristic (FC-LCC) is presented. Finally, the simulation results based on CIGRE standard system and experimental results of a 30 kV/4.5 kA FC-LCC prototype verify the proposed converter's effectiveness and correctness and the fast commutation characteristic can provide sufficient safety time margin for system. The converter shows promising prospects for high voltage direct current system and has significant value for engineering application.

Shoot-Through Protection for an IGCT-Based ZVS Resonant DC Transformer

Integrated gate-commutated thyristors (IGCTs) have successfully demonstrated to be well-suited devices for an application in medium-voltage dc transformers based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter topology. This appli- cation enables unusually high switching frequencies in kilohertz range thanks to the zero-voltage switching while maintaining the high conversion efficiency. Moreover, the clamp circuit that is an inseparable part of every hard-switched IGCT-based converter can be completely omitted—further lowering the converter complexity and cost. Nevertheless, without the clamp circuit, the effect of shoot through becomes fatal, considering that the IGCTs have a defined mode of failure in short circuit and the desaturation effect is practically nonexistent. To prevent the shoot through, this letter proposes a simple, yet effective, protection method based on anode-voltage measurement that can be implemented locally on the IGCT gate unit. The validity of the solution is demonstrated using a custom IGCT gate unit in a medium-voltage resonant test setup.

A Novel Ultrafast Turn-Off Failure Detection Method of Integrated Gate Commutated Thyristor for VSC Application

Integrated gate commutated thyristor (IGCT) has prospects in voltage source converter application. However, it has no desaturation characteristic in conducting state like insulated gate bipolar transistor, for which fast turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure detection for blocking converter in deadtime is of great significance. In this letter, an ultrafast IGCT turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure detection method is proposed. First, turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> process and failure mechanism of IGCT is analyzed. It is clarified that turn-off capacitors of gate driver will continue to discharge in turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure, thus generating a voltage drop on turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off mosfet</small> , which can be the criterion of failure. Then prototype, threshold and timing sequence are designed. Analog circuit including amplifier, hysteretic comparator and reference circuit is used for fast and accurate measurement of voltage on <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> . The failure gate current and fall time of turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> trail current is measured in pulse test. Finally, experiments are carried out to verify the feasibility of the proposed scheme. The results show that the proposed scheme can achieve reliable turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> failure detection within 19 microseconds. Neither false detection nor missed detection occurred. Proposed method has great accuracy and rapidity, and is convenient in implementation. It will greatly decrease the extreme failure rate of IGCT-based converters.

Comprehensive Physical Commutation Characteristic Analysis and Test of Hybrid Line Commutated Converter Based on Physics Compact Model of IGCT

In order to mitigate commutation failure of high voltage direct current (HVdc) system, a novel hybrid line commutated converter (H-LCC) based on integrated gate commutated thyristor (IGCT) and thyristor has been proposed. To verify the proposed scheme's effectiveness and correctness, the physics-based compact model of reverse blocking IGCT (RB-IGCT) and physics-based model of thyristor are established in this article. By comparing the features of physics-based model with other simulation methods, it can be observed that the former has obvious advantages. On the basis, a simulation study of proposed H-LCC based on physics compact model of RB-IGCT and physics-based model of thyristor is conducted to analyze the physical commutation characteristics comprehensively, which is verified by equivalent commutation tests. The H-LCC can mitigate commutation failure of HVdc effectively and physics-based model of H-LCC is expected to be applied in the simulation of dc grid equipment and provide reference for commutation characteristics analysis and snubber parameter optimization that need rapid and multiple simulations.

A Novel Oscillating-Commutation Solid-State DC Breaker Based on Compound IGCTs

Medium voltage dc system has fast rise rate in the event of a short-circuit fault. To avoid high-peak fault current, it is necessary to configure dc circuit breaker with fast operation. Solid-state dc circuit breaker has short breaking time and high reliability, which is suitable for Dc system. In this letter, a novel oscillating-commutation solid-state dc circuit breaker based on compound integrated gate-commutated thyristors (IGCTs) is proposed. By combining IGCTs with different advantages, such as high current breaking and high withstand-voltage capability, the proposed dc breaker has the advantages at less breaking time, high performance, low cost, and small volume. The working principle of the proposed scheme is described in detail, and an 8 kV prototype with 7 kA current breaking ability is built to verify its feasibility.

IGCT Gate Unit for Zero-Voltage-Switching Resonant DC Transformer Applications

Recently, integrated gate-commutated thyristors (IGCTs) have shown a great potential for the soft-switching dc transformer applications based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant converter. The zero-voltage switching that is present in this converter topology enables an operation of IGCTs without clamping circuit and with significantly higher switching frequencies, while maintaining the exceptionally low conduction losses. The high voltage and current ratings of the IGCTs make them a preferred choice for an efficient bulk power transmission. This article shows how gate units for IGCTs can be optimized for this soft-switching application. It presents a simplification of the driving topology and the design process for high switching frequencies. This results in low power consumption and a very small size of the gate unit. The prototype of the gate unit is built and experimentally validated in a 2.5 kV 0.75 kA resonant operation of the IGCT.

A Novel Detachable Gate Driver Unit With Ultralow Inductance for Integrated Gate Commutated Thyristor

Under the background of clean energy connected to the grid, the high-capacity power electronics device integrated gate commutated thyristor (IGCT) has a bright future. In order to increase the convenience of replacement and longevity of service, a novel detachable gate driver unit (DGDU) concept for IGCT is proposed in this letter. To ensure the turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> capability, both connection scheme and board optimization on DGDU are applied to minimize the stray inductance. Besides, the effects because of the detachable connecter on the structure and thermal characteristics are analyzed. Afterward, an overlapping scheme of DGDU with ultralow inductance is proposed for IGCT. Finally, a 4-inch prototype and a single-phase full-bridge platform are developed for the test. The turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> capability of DGDU is verified by pulse experiment, which could turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> a 5 kA current successfully. The stray inductance of DGDU is calculated based on the experimental result and is only 4% larger than conventional GDU, which means the commutation capability has little decrease. Furthermore, the continuous running reliability is verified by 2200 V-dc-voltage, 1300 A-effective-current power cycling experiments.

Digital Control and Power Systems for the Pegasus-III Experiment

The PEGASUS-III experiment is a solenoid-free, low aspect ratio spherical tokamak that will serve as a dedicated U.S. platform for comparative nonsolenoidal tokamak plasma startup studies. Approximately 175 megavolt-ampere (MVA) of reconfigured and expanded programmable power systems, 7 MJ of new stored energy, and new digital control and protection systems for the facility are being commissioned to support PEGASUS-III upgrades. These include: increased toroidal field (0.15–0.6 T); new divertor and poloidal field coils; increased pulse length; local and coaxial helicity injectors for solenoid-free plasma initiation; radio frequency (RF) systems for heating and current drive; and a diagnostic neutral beam (DNB). A new real-time digital control system implements 16 proportional-integral differential (PID) feedback controllers with 25 kHz loop rates to control the electromagnets and helicity injectors. The poloidal field coils, helicity injector arc currents, and toroidal field are driven by 36 3.6 MVA (4 kA, 900 V) insulated-gate bipolar transistor (IGBT) buck converters. Helicity injector bias voltage and current will be provided by a set of four 10.8 MVA multi-level buck converters (MLBCs). Each is comprised of an 1800 V integrated gate-commutated thyristor (IGCT) stage and a ±900 V IGBT stage in series, providing controllable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{\mathrm {inj}}\le 4$ </tex-math></inline-formula> kA at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {inj}}\le 2.7$ </tex-math></inline-formula> kV. A field programmable gate array (FPGA)-based digital fault protection system multiplexes controller commands to individual power semiconductors in these supplies, monitors their operational status, and executes shutdown sequences within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\mu \text{s}$ </tex-math></inline-formula> of fault detection. An 80 kV, 4 A zero-voltage-switching (ZVS) resonant converter with < 1% output ripple is under development for the DNB and is being evaluated as a topology to drive RF sources.

An IGCT Unified Simulation Model Based on the Electrical Model and Thermistor

Integrated gate commutated thyristor (IGCT) is the core device of a new fusion power supply for Comprehensive Research Facility for Fusion Technology (CRAFT), which is very important to establish an accurate, suitable, and effective device model. To establish a unified simulation model (USM) suitable, this article analyzes the process of carrier motion in the process of IGCT turning on and off. The structure of the electrical model and the calculation method of related parameters are obtained by fitting the mathematical equations. Using standard passive components, such as resistance, capacitance, inductance, and an accurate bipolar junction transistor (BJT) model, converting the physical model into the electrical model can be realized. At the same time, because the physical model contains temperature parameters, compared with the traditional IGCT model, this model combines the thermal model with the electrical model, which makes the model unified. In this article, the 4000-A/4500-V IGCT power device in ABB is taken as an example to verify the rationality of the electric-thermal model under different currents and temperatures through MATLAB/SIMULINK simulation and experimental test.

Deciphering the Effect of Corrugated p-Base on Reverse Blocking IGCT

The corrugated p-base (CB) structure has been acknowledged as an effective method to improve the maximum controllable current (MCC) of asymmetric integrated gate-commutated thyristor (IGCT), but never clarified in symmetrical devices such as reverse blocking IGCT (RB-IGCT). This study focuses on the influence of the CB structure on the MCC and the failure mechanism of RB-IGCT. First of all, by employing the CB structure in the RB-IGCT samples, it is verified that the CB structure can improve MCC of the symmetrical devices by more than 50%, no matter under high or low dc-link voltages. Moreover, regarding the scenario under the high dc-link voltage, the effect of the CB structure on the dynamic avalanche is analyzed via simulation, and the improvement of MCC and the positive trend of MCC upon temperature are further clarified. As to the scenario under the low dc-link voltage, distinct failure phenomenon and negative temperature dependence of MCC are observed evidentially on the symmetric devices with CB structure. To explain these failure characteristics that are conflicting with the well-acknowledged dynamic avalanche failure, a novel failure mode named cathode retrigger failure is proposed in this study. It is believed that this study not only deciphers the fundamental mechanisms of the CB structure in RB-IGCT, but also provides practical strategy to optimize the symmetric IGCT devices.