Current Transformer CT Testing
Lets consider the CT installed in the panel of an 11kV feeder.
Following are the routine maintenance tests performed for a CT:
1. Continuity Test
2. Insulation Resistance Test
3. Ratio Verification Test
4. Saturation Test
5. Polarity Test
Continuity Test
Recommended testing equipment is a simple ohmmeter.
Continuity test for a CT is done as any other continuity test. The aim is to check for any broken circuit inside a CT. Flow of current from end to end is enough to verify this.
Insulation Resistance Test
The recommended testing equipment is Megger MIT 515.
In the first step test insulation resistance between phases - Red to Yellow, Red to Blue and Yellow to Blue.
In the second step test insulation resistance phase to ground for each of the phases - Red to Ground, Yellow to Ground and Blue to Ground.
Use input voltage of 5000V IR test.
Any measurement below 100 mega Ohms should be checked further.
Ratio Verification Test
The recommended testing equipment is ISA VIC 600.
Current in supplied at primary winding and output is supplied at secondary through a clamp-on meter. If the CT ratio is say 400/5 then 80 A at primary should give 1 A at secondary. Correct current transformation ratio means that turns ratio of CT is correct.
In the first step phase to ground current injection at primary should give transformed low current in the respective phase at secondary.
In the second step phase to phase current injection at primary should give transformed low current in respective phases at secondary but no current in ground at secondary.
Saturation Test
Recommended testing equipment is Megger MCT1600.
The saturation test is used to ensure that the CT is not saturated and will operate within specifications at the rated burden.
A saturation test is performed by supplying ac voltage to the CT secondary and increasing the volt¬age in steps until the CT is in satu¬ration. The testing voltage is slowly decreased to zero to demagnetize the CT. The test results are plot¬ted on a graph and evaluated based on the transition period between normal operation and saturation. This transition is called the knee of the curve and is directly related to the voltage burden rating of the CT.
Polarity Test
Recommended testing equipment is a simple DC analog meter. This test is also called DC kick/flick test.
Simply connect the positive and negative terminals of meter to CT terminals and note the direction of movement of needle.
CTs protect power equipment that is worth millions and this power equipment supply an industry that is worth billions. This shows the importance of instrument transformers and their testing.
Following are the routine maintenance tests performed for a CT:
1. Continuity Test
2. Insulation Resistance Test
3. Ratio Verification Test
4. Saturation Test
5. Polarity Test
Continuity Test
Recommended testing equipment is a simple ohmmeter.
Continuity test for a CT is done as any other continuity test. The aim is to check for any broken circuit inside a CT. Flow of current from end to end is enough to verify this.
Insulation Resistance Test
The recommended testing equipment is Megger MIT 515.
In the first step test insulation resistance between phases - Red to Yellow, Red to Blue and Yellow to Blue.
In the second step test insulation resistance phase to ground for each of the phases - Red to Ground, Yellow to Ground and Blue to Ground.
Use input voltage of 5000V IR test.
Any measurement below 100 mega Ohms should be checked further.
Ratio Verification Test
The recommended testing equipment is ISA VIC 600.
Current in supplied at primary winding and output is supplied at secondary through a clamp-on meter. If the CT ratio is say 400/5 then 80 A at primary should give 1 A at secondary. Correct current transformation ratio means that turns ratio of CT is correct.
In the first step phase to ground current injection at primary should give transformed low current in the respective phase at secondary.
In the second step phase to phase current injection at primary should give transformed low current in respective phases at secondary but no current in ground at secondary.
Saturation Test
Recommended testing equipment is Megger MCT1600.
The saturation test is used to ensure that the CT is not saturated and will operate within specifications at the rated burden.
A saturation test is performed by supplying ac voltage to the CT secondary and increasing the volt¬age in steps until the CT is in satu¬ration. The testing voltage is slowly decreased to zero to demagnetize the CT. The test results are plot¬ted on a graph and evaluated based on the transition period between normal operation and saturation. This transition is called the knee of the curve and is directly related to the voltage burden rating of the CT.
Polarity Test
Recommended testing equipment is a simple DC analog meter. This test is also called DC kick/flick test.
Simply connect the positive and negative terminals of meter to CT terminals and note the direction of movement of needle.
CTs protect power equipment that is worth millions and this power equipment supply an industry that is worth billions. This shows the importance of instrument transformers and their testing.
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