TESTING OF CVT

1. INSULATION RESISTANCE TEST:

Insulation resistance between CVT 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 CVT 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. 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 3 cores CVT then there will be total 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	Core-1 to Earth	500 V	> 10000 MΩ
6	Core-2 to Earth	500 V	> 10000 MΩ
7	Core-3 to Earth	500 V	> 1000 MΩ
8	Core-1 to Core-2	500 V	> 1000 MΩ
9	Core-1 to Core-3	500 V	> 1000 MΩ
10	Core-2 to Core-3	500 V	> 1000 MΩ

Precaution:

Before doing megger of the CVT it is to be check that all fuse links are removed from the CVT terminal box.

If possible take the megger at terminal box and not at CVT marshalling box.

2. WINDING RESISTANCE TEST:

In this test the CVT Secondary winding resistance is need to measure for checking whether it is proper or not as per the design and specified by the manufacturer. If possible measure the resistance at CVT terminal box after removing fuses. 

3. RATIO TEST:

In the ratio test of CVT’s Primary of CT is provided with the voltage source up to capacity range of 0.0 Volts to 3000.0 Volts or whatever capacity of kit is available with testing team. This test is done for checking the ratio of CVT is proper or not as per the design and specified by the manufacturer. For example.: At 400 volts input we should get 0.110 Volts at secondary output.

4. FUSE CONTINUITY TEST:

All the fuses of the CVT should be checked for continuity.

5. CAPACITANCE AND TAN-δ MEASUREMENT:

These tests are carried out during pre-commissioning stage to check quality of dielectric medium (i.e. Capacitance) 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.

            Tan-δ of CVT is measured at HF point of CVT after removing its earth, at the same time the neutral point of EMU (Electro Magnetic Unit) is to be removed. HV lead of Tan- δ kit is connected with A terminal and LV lead is connected with B terminal as shown in the figure below for measurement of C1. This is to be done in UST mode. Similarly HV lead of Tan- δ kit is connected with B terminal and LV lead is connected with HF terminal as shown in the figure below for measurement of C2. This test is to be done in GST mode.

Precaution:

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