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HVDC TRANSMISSION USING VOLTAGE SOURCE CONVERTERS (VSC) 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. The VSC based HVDC transmission system mainly consists of two converter stations connected by a dc cable. Usually the magnitude of AC output voltage of converter is controlled by Pulse width modulation (PWM) without changing the magnitude of DC voltage. 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 the converter. CHARACTERISTICS OF VSC-HVDC The principal characteristic of VSC-HVDC transmission is its ability to independently control the reactive and real power flow at each of the AC systems to which it is connected, at the Point of Common Coupling (PCC). In contrast to line- commutated HVDC transmission, the polarity of the DC link voltage remains the same with the DC current being reversed to change the direction of power flow. VSC-HVDC Transmission System Model The 230 kV, 2000 MVA AC systems (AC system1 and AC system2 subsystems) are modeled by damped L-R equivalents with an angle of 80 degrees at fundamental frequency (50 Hz) and at the third harmonic. The VSC converters are three-level bridge blocks using close to ideal switching device model of IGBT/diodes. The relative ease with which the IGBT can be controlled and its suitability for high-frequency switching has made this device the better choice over GTO and thyristors. Open the Station 1 and Station 2 subsystems to see how they are built. HARMONICS IN VOLTAGE SOURCE CONVERTERS (VSC) Like all power electronic converters, VSC’s generate harmonic voltages and currents in the AC and DC systems connected. In a simplified manner, from the AC system, a VSC can be considered a harmonic current source connected in parallel to the storage capacitor .This behavior is just opposite to those of conventional line commutated converters. Harmonics generated depends on the station topology (e.g. 6 pulse or 12 pulse) switching frequency of IGBT’S pulse pattern applied Using 12 pulse configuration instead of 6 pulse will improve harmonic conditions both on AC and DC side. Characteristic AC side harmonics will have the ordinal numbers Vac =12n+1; n=1, 2……… Characteristic DC harmonics will have the ordinal numbers Vdc=12n; n=1, 2……….. All harmonics will be cancelled out under ideal conditions. Due to its inherent harmonic elimination capability, the harmonic interface of VSC converter is rather small in comparison to the conventional line commutated converters.However, harmonic filters might be necessary on the AC and DC sides depending on the harmonic performance requirements both for AC and DC sides, AC system harmonic impedance, DC line/cable impedance and loss evaluation. VSC HVDC has the following advantages No need for short circuit power for commutation. Can even operate against black Networks. Can operate without communication between stations. Can operate to control the power continuously in one direction. No change of Voltage polarity when the power direction is changed. This makes easier to make multi-terminal schemes. Possibility to use robust and economically extruded cables for both land and sea. Small converters that reduce the requirement for space. VSC based HVDC does not add short circuit power, so there is a great freedom in choice of topology and interconnection points. A substantial reduction in system losses, mainly due to the elimination of the transformer and related equipment. Losses could be reduced by up to 25%. Other environmental benefit, e.g. the new motor is epoxy-free and therefore easy to recycle. APPLICATION’S OF HVDC TRANSMISSION USING VSC HVDC Light is a recent technology that utilizes Voltage Source Converters (VSC) rather than line commutated converters. HVDC Light offers advantages due to the possibility to independently control both active and reactive power HVDC Light employs Insulated Gate Bipolar transistors (IGBTs), plus other important technological developments: In the HVDC Light transmission schemes, the switching of the IGBT valves follows a pulse width modulation (PWM) pattern. This switching control allows simultaneous adjustment of the amplitude and phase angle of the converter AC output voltage with constant dc, PWM pattern and the fundamental frequency voltage in a Voltage Source Converter. With these two independent control variables, separate active and reactive power control loops can be used for regulation. With these two independent control variables, separate active and reactive power control loops can be used for regulation. MAIN DIFFERENCES BETWEEN HVDC LIGHT AND CONVENTIONAL MAIN DIFFERENCES BETWEEN HVDC LIGHT AND CONVENTIONAL -connected IGBTs -voltage dc capacitors 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.
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