25-08-2017, 09:32 PM
Application of High Power Thyristors in HVDC and FACTS Systems
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Abstract
Both HVDC and FACTS systems use power electronic converters for the power conversion and power
quality control. High power thyristors have been serving as the key component in HVDC and FACTS converters for
several decades now and are still being further developed for higher power rating nowadays. This paper describes the
thyristor technology and its development in application in HVDC and FACTS. The fundamental features and
characteristics of high power thyristors is discussed with particular reference to its application in high voltage and
high current area. Many thyristors connected in series together with specially designed auxiliary mechanical and
electronic systems build so called thyristor valves, which form the HVDC and FACTS converters. An overview of
thyristor valve design is provided. Furthermore, the latest development in the thyristor and thyristor valve technology
and its application in the ultra high voltage DC application (800 kV) is introduced. A summary of technical key
parameters and design features of 6” thyristor valves are provided including the valve design date for the first
UHVDC application.
INTRODUCTION
There is an increasing demand for high efficiency and high
quality of power transmission world wide. In this context the
modern High Voltage DC Transmission (HVDC) and Flexible
AC Transmission Systems (FACTS) gains more importance
and utilization in today’s power transmission system. Both
HVDC and FACTS systems use power electronic converters
for the power conversion and power quality control. Therefore
the performance and quality of converter systems depend
much on the key component- high power thyristors. Since its
introduction in the HVDC application late sixties of last
century, thyristor technology has continuously further
developed to higher power rating over last decades (Fig.1).
The first thyristors used had a silicon wafer with a diameter of
33mm. They had a peak blocking voltage of 1600V and
supported a direct current of up to 1000 A. For higher current
ratings, thyristors were connected directly in parallel. Over the
last thirty years, the device ratings were permanently
increased. Today silicon wafers of 6 inch diameter can be
manufactured; the peak blocking voltage per device is 8000V
and a d.c. current of 4500A can be handled without parallel
connection.
STATE OF ART OF MODERN THYRISTOR TECHNOLOGY
In 1960 the development of thyristors (also called SCRs =
silicon controlled rectifier) was started; since that time many
development steps followed in order to increase the power
capability of the devices and to improve the reliability.
Power thyristors are manufactured from highly pure
monocrystaline silicon. They are so called NPNP
semiconductors. This means that they consist of four layers
which are doped alternately with P and N (Fig. 2). The outer,
highly doped zones are the emitting zones; the weakly doped,
inner layers are the base zones. The control connection G is
located on the P base; J1-J3 designate the junctions between
individual zones. The off-state voltage in the reverse direction
is blocked at junction J1 between P-emitter and N-base. The
off-state voltage in the forward direction is blocked at junction
J2 between P-base and N-base.
Electrical valve components
Due to the fact that a thyristor is not an ideal switch and to
properly perform their function in the series connection under
all steady state and transient conditions, the thyristors need to
be complemented by auxiliary components: snubber
capacitors, snubber resistors, non linear reactors, d.c. grading
resistors, and grading capacitors.
HIGH POWER THYRISTOR VALVES
Since the first commercial use of high voltage thyristor
valves in HVDC-transmission systems in the early seventies,
there has been a constant enhancement of performance
concerning the thyristors blocking as well as current carrying
capability.
That improvement of the thyristor characteristics results in
a drastic decrease of components in a thyristor valve: to
transmit the same amount of power as in the beginning of the
thyristor-era in HVDC-technique, only about 5% of the
thyristors (and snubber circuits) are necessary today.
Thus the reliability of the valves was considerably
increased and the way was pathed to the advantageous design
of modern thyristor valves resulting in a clear structured and
compact valve setup comprising easy assembly, easy
accessibility and easy maintainability
CONCLUSION
Modern power electronics gain increased importance for
the power transmission and distribution applications.
Particularly the power thyristors play a key role in the modern
HVDC and FACTS systems. During last decades the
technology of thyristors has been contentiously developing
both in performance and rating. The design of converter
valves shall fully utilize the capability of thyristors on one
side and meet various challenging requirements of
transmission systems on other side. Long time design and
manufacturing experience ensure the high quality of these
important products.