Current transformers play an important role in measuring electric current in our electric substations. To ensure their proper functioning, we need to perform certain testing on them such as insulation resistance tests. In this article, we are going to discuss an insulation resistance value test of a current transformer and how to perform this test.
Points to be checked while testing CTs
- Voltage class
- Indoor /Outdoor
- Oil filled?Resin cast? Ring type?
- Short Circuit rating
- Available ratios
- Secondary Current values
- Available cores
- Class of Accuracy
- Terminal Connections
- Overall dimensions etc.
IR Value Test in a Current Transformer
A current transformer develops an AC in its secondary output side by measuring the current in its primary side. Its output current is directly proportional to its input current. In the case of the insulation resistance test, we measure insulation in between both windings (primary and secondary), and in between the winding and ground.
The insulation resistance test is focused on determining the strength of a long-lasting transformer. In case of a bad value of resistance between the winding, there may be a short circuit and current can flow between the conductive parts of the transformer.
For its measuring, we use the IR tester Kit to apply voltage and measure the resistance. Usually, it should give a larger value in case of indication of good insulation value in windings. From its construction, we can see the following image.
Here P1 and P2 are primary terminals. In terminal boxes, we have 3 cores of CT from the secondary. We will test all that one by one in the next upcoming section.
The basic requirement for this test is insulation resistance with connecting codes. In the construction of CT, we have primary and secondary sides. CT can operate on 66kV or 220kV or more value. This voltage on P1, and P2 is called the live point and CT is grounded. So there is a resistance value in between the live point and ground.
Based on that value, we decide the working condition of CT in a power system. In the case of earth or low IR value by Megger Set, there will be more chances of flash in between the live and ground point.
Performing IR Test:
We need a Megger Set with connecting probes to measure the insulation resistance. It can measure insulation resistance on different voltage values. By pushing the test button on mete it can start working and providing values.
IR value test will be done by testing:
- Primary to earth
- Primary to Secondary
- Secondary to earth
- Core to Core (From Terminal Box) where the secondary terminal lies.
We will connect the negative (black) probe with the earth which is the CT body at its bottom side. Next, we will connect a red (positive) probe on the primary terminal that is present on the upper side. Normally, we take the primary test at 5kV for one minute. You can note the values in the following table.
Recording the Test Results:
- Place: Mention the Name of the Substation or Place
- Feeder/Transformer/spare: Write the Name of the Feeder or the Transformer
- Phase: R/Y/B
- CT Make, Serial Number & DOM: Say LAMCO/546544/01.05.2022
- Test Kit Make & Sr. No: Megger 12105
- Date of Calibration of the Test Kit:—/—/—–
- Atmospheric Condition: Sunny/rainy/ Cloudy Ambient Temperature:
- Taken by: Name of the Engineer
- Test Voltage: 1KV/5KV
- Test Time: 30 sec
|Sr. No.||Connection||Applied Voltage||Resistance value||Result|
|1||Primary to Earth||5kV||—— MΩ||Good|
|2||Secondary to Earth Core-1||500V||—— MΩ||Good|
|3||Secondary to Earth Core-2||500V||—– MΩ||Good|
|4||Secondary to Earth Core-3||500V||—- MΩ||Good|
In the second case we will connect the red probe to the secondary in the CT terminal box and our negative probe will connect to earth as in the previous state. We will use 500V for the secondary site due to its less resistance as compared to the primary.
Normally we should get values higher than 100MΩ. A value below this indicates that our CT insulation is in bad condition and we should repair or replace that specific CT to prevent any further disturbance.
We have taken different values, but for practical measurement, you should note resistance between all connections to ensure the integrity of a current transformer.
Basic Precautions of Testing:
Before testing the CT, you should verify the following conditions.
- Breaker is in an off-state
- Both isolators of CT are in open condition.
- Secondary wires should be removed from the CT terminal box (from the ground).
- Clean the surface of the insulating material to remove any dirt, moisture, or contaminants that could affect the accuracy of the test.
We have studied the insulation resistance test in the case of the current transformer. These current transformers are important in getting the lower measured values of input current by dividing the incoming value depending on their CT different ratio. So its protection and safety are compulsory.