30-07-2012, 01:13 PM
HVDC transmission using Voltage Source Converters
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ABSTRACT
Rapid developments in the field of power electronic devices with turn off capability like insulated gate bipolar transistors (IGBT) and gate turn off transistors (GTO), makes the voltage source converters (VSC) getting more and more attractive for High voltage direct current transmission (HVDC).This new innovative technology provides substantial technical and economical advantages for direct applications compared to conventional HVDC transmission systems based on thyristor technology. VSC Application for HVDC systems of high power rating (up to 200MW) which are currently in discussion for several projects are mentioned. The underlying technology of VSC based HVDC systems, its Characteristics and the working principle of VSC based HVDC system are also presented. This paper concludes with a brief set of guidelines for choosing VSC based HVDC systems in today’s electricity system development.
INTRODUCTION
The development of power semiconductors, especially IGBT's has led to the small power HVDC transmission based on Voltage Source Converters (VSCs). The VSC based HVDC installations has several advantages compared to conventional HVDC such as, independent control of active and reactive power, dynamic voltage support at the converter bus for enhancing stability possibility to feed to weak AC systems or even passive loads, reversal of power without changing the polarity of dc voltage (advantageous in multi terminal dc systems) and no requirement of fast communication between the two converter stations .Each converter station is composed of a VSC. The amplitude and phase angle of the converter AC output voltage can be controlled simultaneously to achieve rapid, independent control of active and reactive power in all four quadrants. The control of both active and reactive power is bi-directional and continuous across the operating range. For active power balance, one of the converters operates on dc voltage control and other converter on active power control. When dc line power is zero, the two converters can function as independent STATCOMs. Each VSC has a minimum of three controllers for regulating active and reactive power outputs of individual VSC.
HIGH VOLTAGE DIRECT CURRENT (HVDC)
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 distribution, HVDC systems are less expensive and suffer lower electrical losses. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful.
The major advantages are:
If the cost of converter station is excluded, the dc overhead lines and cables are less expensive than ac lines and cables. A dc link is asynchronous. The corona loss and radio interference are less. For dc line, reactive power compensation is not needed. However, reactive power support will be required at both ends as explained later. The line length is not restricted by stability. The interconnection of two separate ac systems via a dc link does not increase the short-circuit capacity, and thus the circuit breaker ratings of either system. The dc line loss is smaller than for the comparable ac line.
The major disadvantages are:
The converters generate harmonic voltages and currents on both dc and ac sides and therefore filters are needed. The converter consumes reactive power. The dc converter stations are expensive. The dc circuit breakers are difficult to design.
Bipolar:
Bipolar Under normal load, negligible earth-current flows, as in the case of mono polar transmission with a metallic earth-return. This reduces earth return loss and environmental effects. When a fault develops in a line, with earth return electrodes installed at each end of the line, approximately half the rated power can continue to flow using the earth as a return path, operating in monopolar mode. Since for a given total power rating each conductor of a bipolar line carries only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating. In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged.
Homopolar:
Homopolar In this type of link two conductors having the same polarity (usually negative) can be operated with ground or metallic return. Due to the undesirability of operating a dc link with ground return, bipolar links are mostly used. A homopolar link has the advantage of reduced insulation costs, but the disadvantages of earth return outweigh the advantages.
Back to Back:
Back to Back High Voltage DC technology enables the interconnection of two asynchronous AC networks. It also enables interconnection between two networks with completely different frequencies; e.g. 50Hz and 60Hz. An HVDC system takes electrical power in an alternating current (AC) system and converts it into high voltage direct current (DC) using a converter station. It then transmits the DC to a remote system, where it is converted back again to AC by another HVDC converter station.
VOLTAGE SOURCE CONVERTER FOR HVDC
The world of converters may be divided in to two groups that are to be distinguished by their operational principle. One group needs an AC system to operate and called as line commutate converters. Conventional HVDC systems employ line commutated converters. The second group of converters does not need an AC system to operate and is therefore called as self commutated converters. Depending on the design of the DC circuits this group can be further divided in to current source converters and voltage source converters. A current source converter operates with a smooth DC current provided by a reactor, while a VSC operates with a smooth DC voltage provided by storage capacitor. Among the self commutated converters it is especially the VSC that has big history in the lower power range for industrial drive applications.
BASIC WORK PRINCIPAL
The basic function of a VSC is to convert the DC voltage of the capacitor into AC voltage. The polarity of the DC Voltage of the converter is defined by the polarity of the diode rectifier. The IGBT can be switched on at any time by appropriate gate voltages. However if one IGBT of a branch is switched on, the other IGBT must have been switched off before to prevent a short circuit of storage capacitor. Reliable storage converter inter lock function will preclude unwanted switching IGBT. Alternating switching the IGBT’s of one phase module as shown successively connects the AC terminals of the VSC to the positive tapping and negative tapping of the DC capacitor. This results in a stair stepped AC voltage comprising two voltage levels +Vdc/2 and -Vdc/2. A VSC as shown is therefore called a 2 level converter.