Resonant Transformers

Resonant Transformers

Q. Describe the principle of operation of a resonant transformer ? How to advantageous over the cascade connected transformers ?

• Resonant transformer work on the principle that the load capacitance is variable and for certain loading when the capacitance is equal to inductance of the circuit resonance may occur.
• Under this condition then, the current will be very large and limited only by the resistance of the circuit. The waveform of the voltage across the test object will be purely sinusoidal.

The magnitude of the voltage across the capacitance of the test object will be

V_C = I –Jvx_C / R +j (X_L – X_C)
=V / R = X_C = V / ωCV

A simplified diagram for the series resonance ac test system is given in Figure B and that of a parallel resonant ac test system is given in Figure C.

In Figure B the inductance of the reactor L varied by varying its air gap and operating range is set in the ratio 10 : 1 capacitance C comprises of the capacitance of the test object, capacitance of the measuring voltage divider, capacitance of the HV bushing etc. The Q factors obtained in these circuits will be typically CR of the order of 50.

In the parallel reasonant mode (Figure C) the HV reactor is connected as an auto transform and circuit is connected as a parallel resonant circuit. The advantage of the parallel resonant circuit is that more stable output voltage can be obtained along with a high rate of rise of test voltage, independent of the degree of tuning and the Q – factor.

Single unit resonant test systems are built for output voltages upto 500 kV, while cascaded units for outputs upto 3000 kV, 50/60 Hz are available.

Figure A

Figure B

Rating :

Figure C