IEEE Power & Energy Magazine - March/April 2016 - 41

figure 10. Incipient capacitor failures are difficult to
detect. (Used with permission from EPRI).

Marketplace
Identical to the Adelanto SVC, the Marketplace SVC is
located near Las Vegas, Nevada. It was extensively damaged
in an event that took place on 30 June 1999. An explosion
and fire resulted from a 21-s ground fault following a scheduled maintenance period. Unfortunately, important protection circuits were blocked, and the fault was finally cleared
by manual tripping of a circuit breaker. One set of valves,
some bushings, arresters, buswork, and capacitors had to be
replaced. Changes were made to the control and protection.
Two of the three thyristor-switched capacitor branches were
returned to service in August 2000 and the most heavily
damaged branch in December 2000.

Thyristor-Controlled Series Capacitors
Series capacitors are often installed in long transmission lines
to reduce the end-to-end impedance. These series capacitors are
mounted on insulated platforms and operate at the same voltage as the transmission line. In the early 1990s, several large
electrical manufacturers realized that controllability of a series
capacitor could be used in a variety of ways to solve stability
or resonance problems often associated with series capacitors.
Thyristors along with a simple circuit including a reactor were
installed in parallel with the capacitors. By cleverly switching
the thyristors, the capacitors could be made to appear as having
higher capacitance than they actually have. A number of TCSCs
have been installed in South America on long ac lines. The stability augmentation provided by the TCSCs in these cases are
critical to operation of the lines.

Slatt
The Slatt TCSC was initially installed in 1993 as a research
project by the Electric Power Research Institute (EPRI).
Designed and constructed by GE, it was installed at a BPA
substation near the Columbia River and the town of Arlington, Oregon. The location of the Slatt TCSC was chosen for
its proximity to a large coal fired generator. One of the goals
in constructing the TCSC was to demonstrate that a series
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capacitor with sufficient smart control can eliminate the threat
of subsynchronous resonance in which there is an interaction
between the series capacitor and the generator. Past experience
has shown that serious damage to the generator shaft can occur
when there is a low impedance path between the capacitor and
the generator. Tests conducted by BPA, Portland General Electric, GE, and EPRI demonstrated the success of this concept.
Commissioned in 1993, the TCSC underwent a minor
renovation in 2006 when some control cards were replaced
on the platforms and fiber optic columns and junction boxes
were also replaced. Since GE has stopped supporting the
control technology used at Slatt, BPA was able to purchase
the remaining spare parts from them. Due to changes in the
BPA network and upgrades to other series capacitors, the
Slatt TCSC was taken out of service in 2015.

Kayenta
Also commissioned in 1993 this TCSC was provided by Siemens;
it is located in the Four Corners area near the point where Arizona,
Utah, Colorado, and New Mexico meet. The system is located
in a line owned by the Western Area Power Administration. Currently, it is in operation, and there are no plans for renovation.

Conclusions
Power electronics come in many forms. The workhorse is the
thyristor used in HVdc converters. From the above descriptions, there are a number of other applications using either thyristors or voltage source converter technology [gate turn-off
thyristor (GTO) or insulated-gate bipolar transistor (IGBT)] to
do a wide variety of tasks needed by HV power systems.
The first HVdc power system was contracted for use in Berlin, Germany, in 1941. The system was never commissioned
and subsequently moved to Russia where it went into service in
1951. It has since been removed from service. The first commercially successful HVdc link was built in 1954 by ASEA to provide power to the island of Gotland in Sweden. This mercury arc
converter design led ultimately to a number of other mercury arc
installations worldwide, all of which have now been removed
from service or replaced, except for the Vancouver Island HVdc
cable. Thyristors were next to dominate HVdc applications followed by IGBTs or GTOs. Oddly, there is no consensus as to
the usable life for thyristors. Some experienced HVdc engineers
will claim that 30 years is the useful life of thyristors, while at
least one manufacturer will claim 50 years. One of the most significant factors is the availability of replacement thyristors. If a
manufacturer continues support of an older project by supplying
parts, including thyristors, longer useful life can be achieved. If
the manufacturer has gone out of the business, other suppliers
may be able to support an older project. In some cases, even
though a manufacturer continues to support an older project,
the cost for spare parts may become prohibitive. Thyristor converters have been renovated or replaced, but it is difficult to use
the age of the thyristors as justification for replacement unless
there is evidence from physical inspections or higher incidences
of failures (see Figure 10). Cooling system components in the
march/april 2016

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