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How have power semiconductors changed HVDC technology?

Bi-mode Insulated Gate Transistor Concept

High Voltage Direct Current (HVDC) technology offers greater stability, reliability and transmission capacity than HVAC technology. In bulk power transfer, DC transmission lines can carry power over longer distances with lower losses than AC lines, making them ideal for distributing power across the world. HVDC has many benefits over HVAC technology. One advantage is that it can transmit a large amount of power over a long distance, up to 3,000 MW over 2,000 km. For overhead lines, the break-even distance between DC lines and AC lines is around 600 km, while in underground lines, it is around 50 km. After these distances, it becomes cheaper to use DC transmission lines.

The optimisation of power semiconductors has led to improvements in their power handling capability and control, as well as enabling their use in solid state HVDC breakers for HVDC grid applications. The Bi-mode Insulated Gate Transistor (BIGT) is one of those developments.

The development and optimisation of power semiconductor devices

Over the past few years, efforts in developing power semiconductors has attempted to fully integrate high voltage and high current IGBT and diode structure on a single chip.

Bi-mode Insulated Gate Transistor ConceptFigure 1 Concept of the BIGT

The result of these developments was a Reverse Conducting Insulated Gate Bipolar Transistor (RC-IGBT) type device, which is referred to as the Bi-mode Insulated Gate Transistor (BIGT). The BIGT is able to operate in both the IGBT and diode modes under extreme conditions and exhibit low losses.

The BIGT

The goal of the BIGT is to obtain higher power, while also eliminating the need for an anti-parallel diode. The output current of the BIGT can be up to 15% higher than that of a separate IGBT/diode solution.

The BIGT can operate in both IGBT mode and freewheeling diode mode by making use of the same available silicon volume in each mode. This means that compared to standard modules, there is an increased silicon area available. The advantage of this is that the device can provide enhanced thermal utilisation as there is an absence of device inactive operational periods, resulting in improved reliability.

Unlike a conventional RC-IGBT, there is no typical snapback behaviour in the transistor in the on-state mode. The BIGT offers soft turn-off behaviour, which reduces the switching loss and noise.

The diode in the IGBT module presents a problem in modern power electronics applications as there is a major restriction regarding its losses performance and surge current capability. These limitations are a result of the package footprint design offering limited diode area. Although increasing the diode area is a potential solution, it is not a preferred one primarily because it is still constrained by the package footprint design.

BIGT StakPak modules

BIGT StakPak ModuleFigure 2 The BIGT StakPak module

The StakPak package utilises press-pack technology, allowing for simple mechanical and electrical series connections of semiconductor dies. By employing the BIGT devices in the StakPak package, the current conduction and switching capability is improved for a power semiconductor-based hybrid HVDC breaker.

HVDC breakers are switching devices that interrupt the high voltage direct current (HVDC) flowing in the network. When a fault develops in the circuit, the breaker should create current zero crossing in order to interrupt the current and will also need to withstand the voltage response of the network.

The primary advantage of the StakPak, in applications that require series connection, is that a failure in any of the chips will result in a short circuit, rather than an open circuit as is the case for normal insulated IGBTs. The BIGT StakPak will go into Short Circuit Failure Mode (SCFM), which is a stable low impedance state. With the modules being in series, the remaining modules are able to share the blocking voltage.

Conclusion

The development of power semiconductors, such as the BIGT, allows for higher power levels to be obtained with lower losses. By being able to operate in both the IGBT and freewheeling diode modes and use the same available silicon area when doing so, the BIGT offers greater reliability than standard modules.

The benefit to HVDC grid applications of using the BIGT is that the likelihood of a multi-terminal HVDC system collapsing is reduced.

Useful links

Power Semiconductors

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