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- Advanced Power Electronics - APE
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**Rashtra Santh Tukduji Maharaj Nagpur University - RTMNU**- 3 Topics
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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

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

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

4.1.3 MODELING OF REPRESENTATION A OF TRANSFORMER EQUIVALENT AND ELECTRIC NETWORK. A converter transformer can be modeled by sub-dividing the windings into a large number of sections. The present transformer 315 MVA 400 206 3 3 KV is divided into 60 sections. Figure 4.1 shows the constructional geometry of one limb of the converter transformer comprising of tapping, HVAC and HVDC star windings. It has been evaluated that for reasonably accurate neutral current calculation, it is not required that the winding should be divided into such a large number eight sections. Even formation of network with relatively smaller number of sections can give accurate results. It also reduces the total commutation time. For the purpose of the neutral current calculation the tapping winding has been fully earthed as is done in a particular case of impulse tests. The HVAC winding is divided into 8 sections and HVDC winding is divided into 16 sections as shown in Figure 4.2. These sections can be represented by self inductance (L) and mutual inductance (M) with all other sections of the same winding as well as with the other windings. The series and shunt capacitances are calculated using standard formula [155] for parallel plate and cylindrical geometry respectively. Using these above values an equivalent electric network is derived. 255

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