Note for Advanced Power Electronics - APE by sunil manjani

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Chapter – 4 ANALYSIS OF HVDC CONVERTER TRANSFORMER Converter transformer is very important and costly equipment in the HVDC Transmission system. Thus great care has to be taken to check whether its insulation can withstand the impulse over voltages occurring in practice. Performance of the winding under impulse voltage has a significant importance in the design of high voltage transformer windings. Proper design of insulation not only provides trouble free operation of the transformer, but also reduces the overall cost of it. The purpose of impulse test is to ascertain the ability of the insulation of the transformer to withstand the application of the standard test voltage. Power frequency test by itself is not sufficient for the purpose. This may be due to fast rate of rise of the impulse voltage and wave shape which produces a non linear stress distribution through the transformer winding and is characteristically different from that of power frequency voltage. 4.1 ANALYTICAL MODELLING OF TRANSFORMER The equivalent circuit of a transformer winding can be imagined in the form of a string of inductors tapped by their parallel capacitance to 251

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ground and shunted by the parallel stray capacitance between the winding sections. The long duration power frequency voltage distribution in the windings is almost uniform. However, impulse voltage distribution is highly non uniform and is based on the inductance and capacitance of the winding. In the present context, the neutral current is visualized as a non stationary signal whose properties changes in time, when there is fault. Detection of major faults has never been an issue since the waveforms, which are being compared, show major difference for such fault. Hence, the focus is on being able to detect accurately the smallest or minor type faults. In order to understand the above phenomena prototype transformers were subjected to theoretical and experimental investigations. In this chapter, mathematical modeling of a converter transformer and calculation of neutral current has been discussed. 4.1.1 FUNCTION OF CONVERTOR TRANSFORMERS: Convertor transformers are specially designed oil insulated power transformers and are similar to AC power transformers in many respects of design and construction. They are oil insulated and OFAF cooled. They are fitted with OLIC, bushings other usual accessories. They are most vital and costly equipment in the HVDC convertor station. Convertor 252

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Transformers are subjected to very unusual times of stresses due to combination of AC, DC voltages and harmonics. Convertor transformers are used for transforming AC network voltage to suitable voltages for feeding the 12 pulse convertor bridges. The convertor transformer serves the following important functions which are not performed by usual power transformers in AC substations.  Convertor Transformer provides effective and natural barrier between AC network and DC line. AC voltages/currents are not mixed with DC voltages and currents, yet power is transferred from AC to DC (rectification) or DC to AC (inversion)  By tap changer fitted on the convertor transformer, the DC line voltage DC power flow is effectively controlled.  By means of tap changers reactive power requirement of converter is minimized.  The short circuit levels are precisely controlled by means of the closely matched impedance of converter transformer. Impedance values are selected for satisfactory operation of valves.  DC load is distributed symmetrically by converter transformers over 3 phase of AC network. 253

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 By suitable combination of star-star, star-delta transformer connections, the required phase shift and voltages are given at the leads on the valve side. Thus the 12 pulse converter is possible with the use of converter transformer and quadruple valves  The reactance of converter transformer help in minimizing non characteristic current harmonics. 4.1.2 SPECIAL FEATURES OF A CONVERTOR TRANSFORMER:  High speed, reliable on load taps changer with 24 or more steps and regulation of +17.5 and -12.5 % (or more). The tap changers operate frequently and should need only modest maintenance.  Specially built low loss, loss noise core. The magnetic circuit is suitable for higher V/F ratios and higher harmonic content.  Exceptionally long bushings to achieve desired creepage distances.  Very special design of insulation system and windings.  Higher percentage impedance to limit short circuit currents during convertor operation.  High reliability. 254