Testing of CT

1. INSULATION RESISTANCE TESTS:

Insulation resistance between CT primary to secondary & primary to ground is usually checked by the use of conventional test instruments at 5 KV Megger for 1 min.  Insulation resistance between CT secondary and ground is usually checked by the use of conventional test instruments. Meggering core by core (to ground) keeping other cores temporarily grounded. It should be done by 500 V Megger for 1 min. The IR value should be compared with those of similar devices or circuits. Readings lower than those known to be good should be carefully investigated. The generally accepted min. IR value is 1 MΩ. One of the most common reasons for low readings is the presence of moisture. Drying out the equipment and retesting should be considered before it is dismantled.

For Example: If we are using 5 cores CT then there will be total 21 combinations between which we need to take Insulation resistance as follow:

Sr.No.	BETWEEN	AT VOLTAGE	REQUIRED VALUE
1	Primary to Earth	5 KV	> 10000 MΩ
2	Primary to Core-1	5 KV	> 10000 MΩ
3	Primary to Core-2	5 KV	> 10000 MΩ
4	Primary to Core-3	5 KV	> 10000 MΩ
5	Primary to Core-4	5 KV	> 10000 MΩ
6	Primary to Core-5	5 KV	> 10000 MΩ
7	Core-1 to Earth	500 V	> 1000 MΩ
8	Core-2 to Earth	500 V	> 1000 MΩ
9	Core-3 to Earth	500 V	> 1000 MΩ
10	Core-4 to Earth	500 V	> 1000 MΩ
11	Core-5 to Earth	500 V	> 1000 MΩ
12	Core-1 to Core-2	500 V	> 1000 MΩ
13	Core-1 to Core-3	500 V	> 1000 MΩ
14	Core-1 to Core-4	500 V	> 1000 MΩ
15	Core-1 to Core-5	500 V	> 1000 MΩ
16	Core-2 to Core-3	500 V	> 1000 MΩ
17	Core-2 to Core-4	500 V	> 1000 MΩ
18	Core-2 to Core-5	500 V	> 1000 MΩ
19	Core-3 to Core-4	500 V	> 1000 MΩ
20	Core-3 to Core-5	500 V	> 1000 MΩ
21	Core-4 to Core-5	500 V	> 1000 MΩ

Precaution:

If relays are left connected to the CT during test, the relay manufacturer should be consulted before test values above 500 V are used. Many solid-state relay designs have surge-suppression capacitors connected from input terminals to ground which may be damaged by use of a higher voltage.

2. INSULATION RESISTANCE TEST FOR PRIMARY TO TAN- δ POINT:

Insulation resistance between CT primary to Tan- δ point usually checked by the use of conventional test instruments at 5 KV Megger for checking the healthiness of insulation between inner conductor and dielectric medium of the CT.

Precaution:

Please reconnect the Tan-δ point to the earth as it was before testing.

3. POLARITY CHECK:

The lead polarity of any CT is a designation of the relative instantaneous direction of currents in its leads. Primary and secondary leads are said to have the same polarity when at a given instant the current enters the primary lead in question and leaves the secondary lead in question in the same direction as though two leads formed a continuous circuit.

The polarity check can be done by inductive kick with direct current method. In this test, a 6-10 V battery pack is connected momentarily to the primary P1-P2 of CT under test and the momentarily deflection of pointer of voltmeter (Zero centre) or analog multimeter in dc mAmp selection is connected to secondary is noted. If the connections are made as shown in the diagram then the deflection will be upscale when the battery is connected and downscale when the battery is disconnected, if the polarity is in accordance with the terminal marking. This test is also valid with the battery applied to secondary and the meter connected to primary. 

Precautions:

i. It is advisable to demagnetize any CT that is tested by impressing DC voltage across a winding.

ii. A dangerous voltage may be generated while disconnecting the battery from the transformer winding. Therefore, a knife switch is not used, a hot stick or rubber gloves must be used for connecting and disconnecting the battery.

4. CORE WINDING RESISTANCE TEST:

In this test the CT Secondary winding resistance is need to measure for checking whether it is proper or not, as per the design and specified by the manufacturer. It is measured by simple ohm meter keeping it in range of few ohms.

 5. RATIO TEST (Primary Current Injection Test):
In the ratio test of CT’s Primary winding of CT is provided with the current source having capacity greater than or equal to the 80% value of CT rated current. This test is done for checking the ratio of CT is proper or not as per the design and specified by the manufacturer. The secondary cores may have taps on it for selection of different current ratio hence during taking ratio test it is to be necessary to check ratio between those taps.

6. EXCITATION TESTS (Knee Point Voltage Test):

These tests are carried out during pre-commissioning stage to check if any turns of CT are short-circuited and to establish CT characteristics as well as capability of CT. An excitation test is made on CT’s for comparison with factory test results or previously measured data to determine if deviations are present.

To perform the test, an AC test voltage is applied to the secondary winding with the primary open circuited as shown in the diagram:

The test voltage applied to the secondary of the current transformer is varied up to 110% of rated KpV, and the current drawn by the winding at each selected value of voltage is recorded. Readings near the knee of the excitation curve are especially important in plotting a comparison curve. For current transformers with taps, the secondary tap should be selected to assure that the current transformer can be saturated with the test equipment available. The highest tap which can accommodate that requirement should be used.

The selection of instruments is especially important for this test. The ammeter should be an r.m.s instrument. The voltmeter should be an average reading voltmeter. It should be calibrated to give the same numerical indication as an r.m.s voltmeter on sine wave voltage. Any substantial deviation of the excitation curve for the current transformer under test from curves of similar transformers or manufacturer’s data should be investigated. Deviation from expected results may indicate a turn-to-turn short circuit, distortion of test supply voltage waveform, or the presence of a completed conducting path around the current transformer core.

7. CAPACITANCE AND TAN-δ MEASUREMENT:

These tests are carried out during pre-commissioning stage to check quality of dielectric medium by measuring the power loss in that. For the measurement of capacitance of dielectric medium almost Schering bridge of small capacitance measurement is used. Similarly Tan-δ is measured indirectly by manipulating power factor of dielectric medium.

            In dead tank CT the Tan-δ point is brought out for this measurement purpose at the bottom of CT which is always earthed. Only for measurement purpose this tap is made open otherwise in operative condition it is connected with ground.

            The value of Tan- δ should not be more than 0.7 %. The testing is to be done in UST (Ungrounded Specimen Test) mode.

Precaution:

It is very necessary to reconnect the Tan-δ point to earth for making the dielectric medium at zero potential after testing.

8.CONTACT RESISTANCE MEASUREMENT OF CONNECTORS/JOINTS:

Where ever clamps or joints are there we have to check its contact resistance after completion of all connections. The obtained value should be very less i.e. it must be less than 5 μΩ. This shows proper & solid connectivity between clamps and terminals/IPS.