A Paper Presentation on Hvdc Transmission Using Voltage Source

  • CONVERTERS VSC
  • Published 2011

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. VOLTAGE SOURCE CONVERTERS 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 commutated coverters.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. Diagrammatic Representation of VSC-HVDC BASIC WORKING PRINCIPLE The basic function of a VSC is to convert the DC voltage of the capacitor into AC voltages. Fig 2 illustrates the basic operating principle. 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 there fore called a 2 level converter. Due to switching frequency, that is considerably higher than the AC system power frequency the wave shape of the converter AC current will be controlled to vary sinusoidal. This is achieved by special Pulse Width Modulation. Besides the 2 level converters, so called 3 level converters have been used for high power applications. A three level VSC provides significant better performance regarding the total harmonic voltage distortion (THD).However, the more complex converter layout resulting in the larger footprint and higher investment costs makes 2 level technology the preferred solution for HVDC from today’s point of view. PULSE WIDTH MODULATION A converter for interconnecting two electric networks to transmit electric power from one network to the other, each network being coupled to a respective power generator station. The converter, having an AC side and a DC side, includes a bridge of semiconductor switches with gate turn-off capability coupled to a control system to produce a bridge voltage waveform having a fundamental Fourier component at the frequency of the electric network coupled to the AC side of the converter. The control system includes three inputs for receiving reference signals allowing to control the frequency, the amplitude and the phase angle of the fundamental Fourier component with respect to the alternating voltage of the network coupled to the AC side of the converter. Through appropriate feedback loops, the converter may be used to maintain at a predetermined level the power flowing therethrough or to keep at a preset value the voltage across the DC terminals of the converter and, in both cases, to maintain the frequency synchronism between the fundamental Fourier component and the alternating voltage of the network coupled to the DC side of

Cite this paper

@inproceedings{VSC2011APP, title={A Paper Presentation on Hvdc Transmission Using Voltage Source}, author={CONVERTERS VSC}, year={2011} }