I want to design a phase to phase single phase voltage transformer which is used for placing capacitor banks on the system. For this purpose, I need a voltage transformer with an air gap coil (in order to have constant L value). So, I need some advice about materials or document which i can use. thanks a lot.
Important is voltage rating, you can use same transformer used, for phase to neutral, to 230V, line to line.
You are right Mr. Desai , I didnt specify the voltage level. It is medium voltage level (12 kV ) and there is not any seconder for voltage transformer.
To discharge capacitor bank, you need a inductore with constant value, where the voltage level is 12KV. You can use any burned transformer with open secondary with having 12KV primary as inductore to discharge 12 KV capacitor bank.....
thank you Mr. Desai but there are some limitations such Tdischarge < 10 sec and Vcapacitor < 70 V. So, transformer inductor value is important for me. While capacitor is discharging, this transformer inductor value changes according to flux. Can i assume the transformer inductor value as constant ( i.e. L=500 H). Is it sufficient assumption?
According to the problem we have few things to be taken care of. 1) Voltage is 12 KV AC, Single phase(Between Phase to Phase). 2) Application is to discharge capacitor. 3) Inductance is to be kept around 500 Henry, Nothing can be suggested regarding value of Inductance unless you specify value of C and time constant you want to keep.
One of the Transformer windings can be used as inductor keeping the other winding open but for normal transformer with Ferromagnetic core, the inductance will obviously be nonlinear. Another point is the rate of discharge. For ferromagnetic core this will also have some effect. You can use normal transformer core, may be CRGO or CRNO and design a high value(500H) Inductor with air gap. As you can assume that up to a certain limit the L will be linear with more air gap but to make a linear L all through the range the cost will be very high so you can make some compromise to make the L approximately linear. Keep the area of cross section of core such that as a 50/60 Hz transformer the flux density is not higher than 0.9 Tesla. You may keep the air gap as per design formula.
Inductance(L), is variation of flux, with the variation of current(L=do/di), hence you can consider constant value by taking the discharge current constant, though will changed with time as the voltage reduces but not much. Inductance of transformer coil you used, may vary with capacity and core material used, hence first find resistance, applying D.C (R=V/I), then apply A.C to find impedance(Z=V/I), and then find inductance from found inductive reactance ( X= Squrt(Z sqr-Rsqr), and inductance (L=X/2pie.f).
Any voltage transformer will discharge the capacitor bank once it switched off. The technical issue to consider is that this will saturate the PT core and hence the discharge current and the time constant of discharge will be limited only by the resistance of the winding. This current may be so high that it will destroy it thermally. Be Careful! I high voltage resistor will limit the current and protect the PT if desired.
An alternate approach is to use a switching device that has a pre-insertion resistor. This limits inrush currents for both banks with a trapped charge as well as those which are already discharged. This is a commerically available solution. If the question is simply discharging the bank, the capacitor cans have built in resistors which will discharge them in 5 minutes or less.
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