Experience with IGCT based VFD
Introduction
This is a continuation of our Reliability and availability series. To access all posts in this series please click here.
In today’s post we will share an industrial experience with medium voltage IGCT based VFD. IGCT as heart of high-power frequency converter is used in applications such as motor drives, grid interties, static VAR compensation (STATCOM), circuit breakers and others. Two leading manufacturers of IGCTs are ABB [1, 2] and Mitsubishi.
IGCT power semiconductor
The Integrated Gate-commutated Thyristor, in short IGCT, is a high-end medium voltage power semiconductor. It is a fully controllable switch. In fact, it is an integration of a high-power semiconductor switch and a powerful gate unit. The device is known for its robustness and excellent reliability. The structure is thyristor based and inherently has very low conduction losses (–> high efficiency). The driver is integrated directly next to the power part. The tight integration minimizes the parasitic inductance of the firing circuit. The IGCT is turned on and off by a current injection through the gate. As normal thyristor, also IGCT can be heavily overloaded for a short time (overload integral). It makes the IGCT ideally suitable for a fuseless design.
History of IGCT
The IGCT semiconductor was developed in mid 1990s. The commercial use started in 1997. The device was first introduced by ABB. It emerged as a significant improvement of Gate Turned-Off (GTO) thyristors. IGCT is very robust due to its thyristor structure. It allows high power density. Therefore, this semiconductor is preferred in medium and high power range starting from several hundred kW up to multi-megawatt range. The device is available either with or without reverse blocking capability.
Compared to GTO the IGCT has lower switching losses and allows higher switching frequency. IGCT withstands higher voltage rise (dv/dt). The turn-off time is shorter than GTO thyristor. IGCT switch allows more compact and modular power converter design.
IGCT based VFDs
First VFDs with IGCT were marketed in 1997 by ABB [1]. Other VFD manufacturers adopted the IGCTtechnology in their products as well. The IGCT can be specifically optimized for target application. Depending on target switching frequency the ratio between switching losses and conduction losses can be tuned. Such tuning allows best possible efficiency and best thermal utilization.
Field experience with IGCT based VFD
First IGCT based VFD had been commissioned more than 20 years ago. The installed base is continuously growing. Nowadays IGCT is used as power switch in several VFD hardware platforms. More than 250,000 units have been installed worldwide. The cumulative field experience with IGCT power semiconductor as per December 2019 exceeds 1 million years! This number considers all delivered and operational medium voltage VFDs. Considering other power converters (wind converters, STATCOM, grid interties etc.) the number would be even higher.
>> Field experience with IGCT power semiconductors exceeds 1 million years. <<
Field case: Mine hoist used in Gotthard tunnel
A very interesting field case on IGCT based VFD has recently been deeply analyzed and published. The Gotthard Base Tunnel with its 57 km length is currently the world’s longest railway tunnel. It is called Switzerland’s construction of the century. This impressive masterpiece officially started in 1999 after few years of preparation work. The festive opening was celebrated in 2016. The Swiss Railways (SBB-CFF-FFS) coated one of their locomotives of Re 460 series (popularly known as Lok 2000) into “Gottardo 2016” advertisement.
A large amount of excavated material was transported to either entrance and to a central portal at Sedrun (see fig. 7). In Sedrun an elevator was installed that hoisted the material roughly 850 meters to the surface. The roll diameter of the elevator was 4.8 m. The mine hoist was driven by a variable speed motor supplied from medium voltage IGCT based VFD. During the 14 years of operation, the elevator carried out more than 84,000 lifts, removing 28,200,000 tons of gravel [5].
After completion of the construction the VFD was not needed anymore and was decommissioned. The IGCTs were returned to ABB semiconductor factory (Lenzburg). They underwent an in-depth analysis inxcluding many measurements. The data were bench-marked with he original data measured right after manufacturing.
The report [5] states:
“Their electrical parameters were remeasured and compared with the original production measurements of 15 years earlier. No significant shift or degradation of the electrical parameters were found. The hermetic ceramic housing of the power semiconductor was still perfectly sealed.
The device analysis focused on the turn-off circuit involving the gate unit and the parallel connection of electrolytic capacitors and MOSFETs (metal-oxide-semiconductor field-effect transistor). Original gate impedance measurements from the device qualification were compared against devices of different ages that were either in operation (including the ‘Gotthard devices’) or held in storage. Here, again, no significant degradation of the gate circuit impedance was found.“
>> The device from the Gotthard application does not show any degradation of the cathode metallization <<
Analysis such as reported in [4, 5] bring great value to compare mathematical models with measurements on real device and to assess the lifetime of IGCT under heavy duty load cycles.
Summary from Gotthard’s mine hoist
The analysis provided an extremely valuable insight into the IGCT switch and its lifetime. To summarize the findings:
– Electrical parameters are practically unchanged after 14 years of a heavy duty loading in mine hoist application
– No significant degradation of gate circuit incl. electrolytic capacitors and MOSFET transistors
Conclusion
IGCT is a well proven medium voltage power semiconductor. It has been successfully used since 1997 in power electronic converters such as medium voltage VFDs, grid interties, wind and hydro converters, STATCOMs etc. The structure of IGCT allows very high short time overload and is suitable for converter design without medium voltage fuses (fuseless design).
The experience shows that the device practically does not age. This fact was reconfirmed by analyzing the IGCTs of a mine hoist drive after 14 years of heavy duty operation. These findings support the suitability of IGCT for high performance applications with focus on highest possible reliability.
For more details on the reference field case please refer to complete reports [4, 5]. For more details on applications with IGCTs please refer to [6].
References
[1] ABB Semiconductors – Homepage: https://new.abb.com/semiconductors
[2] IGCT semiconductor from ABB, integrated-gate-commutated-thyristors-(igct)
[3] T. Wikström, T. Setz, K. Tugan, T. Stiasny and B. Backlund, “Introducing the 5.5kV, 5kA HPT IGCT”, PCIM Europe, Nuremberg 2012
[4] ABB Semiconductors: Reliability evaluation of IGCT based on demanding long-term application
[5] Case study: Gotthard tunnel’s power semiconductors give insights to IGCT reliability, Click here to download
[6] T. Setz, M. Lüscher, Applying IGCTs, ABB Switzerland Ltd, Semiconductors, October 2007
[7] Reliability and availability – complete series: https://mb-drive-services.com/category/reliability-and-availability/
