HVDC TRANSMISSION USING VOLTAGE SOURCE CONVERTERS

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.

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CONCLUSIONS

In this paper, we have presented the analysis of High voltage DC transmission using VSC, the number of advantages associated with implementing VSC-based designs for HVDC applications that result in systems with high reliability and superior operating performance; these benefits including economic, environmental or technical aspects. Of particular note today is the ability to control power flow and prevent propagation of severe disturbances, thus limiting blackout extension. This ability to maintain in dependence of interconnected networks can be of prime importance when the two systems have different regulatory procedures, notably if two counties, and also technically if the load frequency control regimes are not compatible .These properties are further enhanced by using HVDC Light which gives independent control of reactive power at both stations, in addition to active power flow control.