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Abstract:
Beginning with a brief historical perspective on the development of high voltage direct current (hvdc) transmission systems, this paper presents an overview of the status of HVDC systems in the world today. it then reviews the underlying technology of HVDC systems, and discusses the HVDC systems from a design, construction, operation and maintenance points of view. the paper then discusses the recent developments in HVDC technologies. the paper also presents an economic and financial comparison of HVDC system with those of an ac system; and provides a brief review of reference installations of HVDC systems. the paper concludes with a brief set of guidelines for choosing HVDC systems in today’s electricity system development.
INTRODUCTION:
A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required by the cable capacitance. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may still be warranted, due to other benefits of direct current links. HVDC allows power transmission between unsynchronized AC distribution systems, and can increase system stability by preventing cascading failures from propagating from one part of a wider power transmission grid to another.
THE HVDC TECHNOLOGY:
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion
NATURAL COMMUTATED CONVERTERS (NCC):
Natural commutated converters are most used in the HVDC systems as of today. The component that enables this conversion process is the thyristor, which is a controllable semiconductor that can carry very high currents (4000 A) and is able to block very high voltages (up to 10 kV). By means of connecting the thyristors in series it is
possible to build up a thyristor valve, which is able to operate at very high voltages (several hundred of kV).The thyristor valve is operated at net frequency (50 Hz or 60 Hz) and by means of a control angle it is possible to change the DC voltage level of the bridge. This ability is the way by which the transmitted power is controlled rapidly and efficiently.
CAPACITOR COMMUTATED CONVERTERS (CCC):
An improvement in the thyristor-based
Commutation, the CCC concept is characterised by the use of commutation capacitors inserted in series between the converter transformers and the thyristors valves. The commutation capacitors improve the commutation failure performance of the converters when connected to weak networks.
FORCED COMMUTATED CONVERTERS (FCC):
This type of converters introduces a spectrum of advantages, e.g. feed of passive networks (without generation), independent control of active and reactive power, power quality. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source Converters). Two types of semiconductors are normally used in the voltage source converters: the GTO (Gate Turn-Off Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Both of them have been in frequent use in industrial applications since early eighties. The VSC commutates with high frequency (not with the net frequency). The operation of the converter
is achieved by Pulse Width Modulation (PWM). With PWM it is possible to create any phase
Angle and/or amplitude (up to a certain limit) by changing the PWM pattern, which can be one almost instantaneously. Thus, PWM offers the possibility to control both active and reactive power independently. This makes the PWM Voltage Source Converter a close to ideal component in the transmission network. From a transmission network viewpoint, it acts as a motor or generator without mass that can control active and reactive power almost instantaneously.
THE COMPONENTS OF AN HVDC TRANSMISSION SYSTEM:
To assist the designers of transmission systems, the components that comprise the HVDC system, and the options available in these components, are presented and discussed. The three main elements of an HVDC system are:
A. THE CONVERTER STATION:
The converter stations at each end are replica’s of each other and therefore consists
of all the needed equipment for going from AC to DC or vice versa. The main component of a converter station are:
1. THYRISTOR VALVES:
The thyristor valves can be build-up in different ways depending on the application and manufacturer. However, the most common way of arranging the thyristor valves is in a twelve-pulse group with three quadruple valves. Each single thyristor valve consists of a certain amount of series connected thyristors with their auxiliary circuits. All communication between the control equipment at earth potential and each thyristor at high potential, is done with fibre optics.
2. VSC VALVES:
The VSC converter consists of two level or multilevel converter, phase-reactors and AC filters. Each single valve in the converter bridge is built up with a certain number of seriesconnected
IGBTs together with their auxiliary electronics. VSC valves, control equipment and cooling equipment would be in enclosures (such as standard shipping containers) which make
transport and installation very easy. All modern HVDC valves are water-cooled and air insulated.