SERIES RESONANT CIRCUIT

The equivalent circuit of a single-stage-test transformer along with its capacitive load is shown in Fig.v2.15. Here  L 1  represents the inductance of the voltage regulator and the transformer primary,  L the exciting inductance of the transformer,  L 2  the inductance of the transformer secondary and C  the capacitance of the load. Normally inductance L is very large as compared to  L 1 and  L2  and hence its shunting effect can be neglected. Usually the load capacitance is variable and it is possible that for certain loading, resonance may occur in the circuit suddenly and the current will then only be limited by the resistance of the circuit and the voltage across the test specimen may go up as high as 20 to 40 times the desired value.

Similarly, presence of harmonics due to saturation of iron core of transformer may also result in resonance. Third harmonic frequencies have been found to be quite disastrous.

With series resonance, the resonance is controlled at fundamental frequency and hence no un-wanted resonance occurs.

The development of series resonance circuit for testing purpose has been very widely welcome by the cable industry as they faced resonance problem with test transformer while testing short lengths of cables.

With the static frequency convertor circuits the specific weight has come down to 0.5 kg/kVA.

It is to be noted that whereas the series resonant systems are quite popular for testing cables and highly loss free capacitive loads, cascaded transformers are more common in high voltage laboratories for testing equipment in  MV range and also for relatively high loads.