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Power electronic control in electrical systems 159

Fig. 5.5 Photograph of a 5-inch light-triggered thyristor with integrated breakover diode. (Courtesy of Siemens
and EUPEC.)

same valve from the blocking voltage point of view can be reduced. It is widely
accepted by manufacturers that the highest blocking voltage that results in optimized
device as far as cost, power losses, reliability and fabrication process are concerned is
about 8-kV (Ruff et al., 1999; Asano et al., 1998).
Today, manufacturers try to integrate the drive and protection circuitry within the
device. Specifically, the light and overvoltage triggering functions have been integrated
with the device. When compared with the ETT, the component count of the drive circuit is
reduced substantially (Lips et al., 1997). Furthermore, the overvoltage protection is also
integratedintothedevicewhichfurtherreducesthecomplexityofthecircuitryarrangement
required to ensure safe operating conditions and risk of failure reduction. An improved
8-kV LTT with the overvoltage protection developed by Siemens and EUPEC hasbecome
available (Schulze et al., 1996; Schulze et al., 1997; Ruff et al., 1999). An 8-kV LTT with
integrated diode (Niedernostheide et al., 2000) is shown in Figure 5.5.
Many LTT devices have been successfully used in Japan (Asano et al., 1998). The
recently developed 8-kV LTT by Siemens, AG was tested as commercial product at
Bonneville Power Administration's (BPAs), Celilo Converter Station at The Dalles,
Oregon, USA in 1997. Celilo is the northern end of BPAs 3.1-MW HVDC line from
the Columbia River system to Southern California.
Current R&D work aims at developing fully self-protected devices with break over
diode (BOD), forward recovery protection (FRP) and dv/dt protection as a high-
power LTT (Ruff et al., 1999).
5.2.4 Desired characteristics offully-controlled power
semiconductors

In switch-mode solid-state converters, the fully controlled power semiconductors
can be turned on and off with control signals applied to a third terminal and can