HVDC TRANSMISSION USING VOLTAGE SOURCE CONVERTERS (VSC) GODAVARI INSTITUTE OF ENGINEERING AND TECHNOLOGY Presented by: ABSTRACT transmission based on Voltage Source Rapid developments in the field Converters (VSCs). The VSC based of power electronic devices with turn off HVDC installations has several capability like insulated gate bipolar advantages compared to conventional transistors (IGBT) and gate turn off HVDC such as, independent control of transistors (GTO), makes the voltage active and reactive power, dynamic source converters (VSC) getting more voltage support at the converter bus for and more attractive for High voltage enhancing stability possibility to feed to direct current transmission weak AC systems or even passive loads, (HVDC).This new innovative reversal of power without changing the technology provides substantial polarity of dc voltage (advantageous in technical and economical advantages for multi terminal dc systems) and no direct applications compared to requirement of fast communication conventional HVDC transmission between the two converter stations .Each systems based on thyristor technology. converter station is composed of a VSC. VSC Application for HVDC systems of The amplitude and phase angle of the high power rating (up to 200MW) which converter AC output voltage can be are currently in discussion for several controlled simultaneously to achieve projects are mentioned. The underlying rapid, independent control of active and technology of VSC based HVDC reactive power in all four quadrants. The systems, its Characteristics and the control of both active and reactive power working principle of VSC based HVDC is bi-directional and continuous across system are also presented. This paper the operating range. For active power concludes with a brief set of guidelines balance, one of the converters operates for choosing VSC based HVDC systems on dc voltage control and other converter in today’s electricity system on active power control. When dc line development. power is zero, the two converters can INTRODUCTION function as independent STATCOMs. The development of power Each VSC has a minimum of three semiconductors, especially IGBT's has controllers for regulating active and led to the small power HVDC 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. BASIC WORKING PRINCIPLE One group needs an AC system The basic function of a VSC is to operate and called as line commutated to convert the DC voltage of the coverters.Conventional HVDC systems capacitor into AC voltages. Fig 2 employ line commutated converters. illustrates the basic operating principle. The second group of converters The polarity of the DC does not need an AC system to operate voltage of the converter is defined by the and is therefore called as self polarity of the diode rectifier. The IGBT commutated converters. Depending on can be switched on at any time by the design of the DC circuits this group appropriate gate voltages. However if can be further divided in to current one IGBT of a branch is switched on, the source converters and voltage source other IGBT must have been switched off converters. A current source converter before to prevent a short circuit of operates with a smooth DC current storage capacitor. Reliable storage provided by a reactor, while a VSC converter inter lock function will operates with a smooth DC voltage preclude unwanted switching IGBT. provided by storage capacitor. Among Alternating switching the IGBT’s of one the self commutated converters it is phase module as shown successively especially the VSC that has big history connects the AC terminals of the VSC to in the lower power range for industrial the positive tapping and negative tapping drive applications. of the DC capacitor. This results in a Diagrammatic Representation of stair stepped AC voltage comprising two VSC-HVDC voltage levels +Vdc/2 and -Vdc/2. A VSC as shown is there fore called a 2 complex converter layout resulting in the level converter. larger footprint and higher investment The VSC based HVDC transmission system costs makes 2 level technology the mainly consists of two converter stations preferred solution for HVDC from connected by a dc cable. Usually the magnitude today’s point of view. of AC output voltage of converter is controlled by Pulse width modulation (PWM) without PULSE WIDTH MODULATION changing the magnitude of DC voltage. 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 Due to switching frequency, that is coupled to the AC side of the converter. considerably higher than the AC system The control system includes three inputs power frequency the wave shape of the for receiving reference signals allowing converter AC current will be controlled to control the frequency, the amplitude to vary sinusoidal. This is achieved by and the phase angle of the fundamental special Pulse Width Modulation. Fourier component with respect to the Besides the 2 level converters, so called alternating voltage of the network 3 level converters have been used for coupled to the AC side of the converter. high power applications. Through appropriate feedback loops, the A three level VSC converter may be used to maintain at a provides significant better performance predetermined level the power flowing regarding the total harmonic voltage therethrough or to keep at a preset value distortion (THD).However, the more the voltage across the DC terminals of contrast to line-commutated HVDC the converter and, in both cases, to transmission, the polarity of the DC link maintain the frequency synchronism voltage remains the same with the DC between the fundamental Fourier current being reversed to change the component and the alternating voltage of direction of power flow. the network coupled to the DC side of the converter. 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 CHARACTERISTICS OF VSC- converters are three-level bridge blocks HVDC using close to ideal switching device The principal characteristic of model of IGBT/diodes. The relative ease VSC-HVDC transmission is its ability with which the IGBT can be controlled to independently control the reactive and and its suitability for high-frequency real power flow at each of the AC switching has made this device the better systems to which it is connected, at the choice over GTO and thyristors. Open Point of Common Coupling (PCC). In the Station 1 and Station 2 subsystems to All harmonics will be cancelled out see how they are built. under ideal conditions. Due to its inherent harmonic HARMONICS IN VOLTAGE elimination capability, the harmonic SOURCE CONVERTERS (VSC) interface of VSC converter is rather Like all power electronic small in comparison to the converters, VSC’s generate harmonic conventional line commutated voltages and currents in the AC and DC converters.However, harmonic filters systems connected. In a simplified might be necessary on the AC and manner, from the AC system, a VSC can DC sides depending on the harmonic be considered a harmonic current source performance requirements both for connected in parallel to the storage AC and DC sides, AC system capacitor .This behavior is just opposite harmonic impedance, DC line/cable to those of conventional line impedance and loss evaluation. commutated converters. VSC HVDC has the following Harmonics generated depends on advantages the station topology (e.g. 6 No need for short circuit power pulse or 12 pulse) for commutation. Can even switching frequency of operate against black Networks. IGBT’S Can operate without pulse pattern applied communication between stations. Using 12 pulse configuration instead of Can operate to control the power 6 pulse will improve harmonic continuously in one direction. conditions both on AC and DC side. No change of Voltage polarity Characteristic AC side harmonics will when the power direction is have the ordinal numbers changed. This makes easier to Vac =12n+1; n=1, 2……… make multi-terminal schemes. Characteristic DC harmonics will Possibility to use robust and have the ordinal numbers economically extruded cables for Vdc=12n; n=1, 2……….. both land and sea. Small converters that reduce the -voltage dc requirement for space. capacitors VSC based HVDC does not add short circuit power, so there is a In the HVDC Light great freedom in choice of transmission schemes, the switching of topology and interconnection the IGBT valves follows a pulse width points. modulation (PWM) pattern. This A substantial reduction in system switching control allows simultaneous losses, mainly due to the adjustment of the amplitude and phase elimination of the transformer angle of the converter AC output voltage and related equipment. Losses with constant dc, PWM pattern and the could be reduced by up to 25%. fundamental frequency voltage in a Other environmental benefit, e.g. Voltage Source Converter. With these the new motor is epoxy-free and two independent control variables, therefore easy to recycle. separate active and reactive power control loops can be used for regulation. APPLICATION’S OF HVDC With these two independent control TRANSMISSION USING VSC variables, separate active and reactive HVDC Light is a recent power control loops can be used for technology that utilizes Voltage Source regulation. Converters (VSC) rather than line MAIN DIFFERENCES BETWEEN 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: - connected IGBTs HVDC LIGHT AND CONVENTIONAL MAIN DIFFERENCES BETWEEN HVDC LIGHT AND CONVENTIONAL 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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