CAPACITIVE VOLTAGE TRANSFORMER (CVT)
Capacitive Voltage Transformer Polarity Test Same as in CT or PT.
Ratio Test – Apply voltage to primary with secondary open. Measure secondary voltage and determine ratio.
In case of star/star configuration care has to be taken to ensure that primary & secondary neutral points are not connected together.
Measurement of Capacitance.
- Remove Earthing/PLCC connections from HF point.
- Connect LV terminal of the test kit at HF point.
- Remove neutral earthing of EMU unit.
- Take capacitance value & tans value at 2KV & 10KV.
- Discharge CVT and remove test kit connections.
- Connect HF point to Earth /PLCC.
- Restore neutral Earthing of EMU.
- Restore conductor jumpers of the CVT.
- Put the test kit.
- Remove conduct or jumpers from the CVT top terminals.
- Connect the HV terminal of the test kit.
Secondary Injection Test
Rated current 1A or 5A (as the case may be) shall be applied at the CT terminals and currents at the meter and really terminals should be measured. The voltage developed across the C.T. terminals should also be to measured to ensure that there is no loose connection and also to ensure that CTs are not connected with excess loads (burden). The secondary injections test ensures that connection have been made correctly.
The secondary injection tests should precede series and primary injections tests.
Primary Injection Tests
The test checks all parts of the protection system by injecting the test current through the primary circuit. This is required to prove that C.T.S. are correctly connected to relays.
these tests will usually include a check of the polarity of the 3 CTs in a group and often the comparison of the group with another. The sensitivity of the protection in terms of primary amps may also be dermined.
Following facts to be borne in mind: –
- Primary equipment must be dead.
- Safty earths have to be removed.
- Earth fault relay coils, stabilizing resistors and the like are often short time rated and so may be damaged unless suitable precautions are taken.
- Multipurpose instruments may be damaged unless they are on the correct range for the condition being applied.
The test kit gives variable output current. The output current can be varied by means of builtin Auto Transformers.
The output terminals of the test kit are connected to the primary of CT. the secondary current is measured at various points.
1 .Series injections test:
This test is carried out to confirm that the CT polarities have been correctly matched in a 3 phase system. All the 3 phases of the CT primary are connected in series & 1/3 of the rated current is made to flow through them.
2 .In case of star connected secondary
All three phase currents will be equal & through neutral 3 times phase current are flow.
In case of delta connected – All phase currents should be equal.
Also Read : SUB-STATIONS
Also Read : ELECTRICAL FAULT FINDING