C.T. testing set is meant for determination of PHASE ANGLE ERROR AND RATIO ERROR OF Current transformer having secondary rated for 5 Amps. or 1 Amp. which are intended to be used on 50 Hz. The measurement is on principle of comparison and null indication on magnetic balance bridge.
The basic unit is sensitive magnetic comparator and also it incorporates electronic null detector to facilitate the balance. The reference Current 5A. or IA. obtained from Standard Current Transformer and in comparison to that the vector length and phase displacement is measured. The ratio error in percentage and phase displacement in minutes are directly compared and indicated on dials.
The two current vectors, one from Standard Current Transformer and the other from the Current Transformer under test energises the high permeability magnetic core and then the associates windings try to annual the residual magnetism which occurs due to error. The differential vectors will be proportional to phase error and ratio error magnitudes. The Standard Current Transformer injects proportional currents in phase and also of controllable value. The control is done by ratio error dial. The injection current tries to annual in phase component vectors.
The high resolution quadrature circuit fed from the same standard current transformer also injects proportional and controllable current in quadrature. Due to this injection the quadrature of differential vector is annualed. The phase angle dial controls this magnitude.
The amount of injection will be proportional to phase error and ratio error and thus two quantities can be measured directly if perfect balance is achieved. The electronic tuned amplifier detector circuit measures the flux value picking up the current from the balancing magnetic circuit. At perfect balance condition fundamental frequency flux in the comparator is negligible and the null is achieved.
The C.T. Testing Set provides the following ranges:-
PHASE ANGLE ERROR
0 to ± 20 min. with x 1 range.
0 to ± 0.5 % with x 1 range.
0 to ± 100 min. with x 5 range.
0 to ± 2.5 % with x 5 range.
0 to ± 200 min. with x 20 range.
0 to ± 10.0% with x 20 range.
The set provides the following components:-
1. Separate terminals to connect Standard C.T. and Test C.T. secondaries.
2. Ratio error and Phase angle error calibrated dials.
3. Range selecting switch.
4. Polarity check indicator.
5. Ammeter to measure current injected in percentage.
6. Null detector with sensitivity control.
7. Terminals to connect burden box.
8. Aux. power supply to energies electronic null detector.
9. High permeability magnetic core.
10. Secondary current selector 1A.or 5A.