In this article, we are going to discuss the transformer polarity test in detail. This test is about the pre-commissioning test of the Current Transformer. We will study how it helps us to test the polarity of CT. Let’s start!
CT Polarity Test
We have a ring-shaped coil as secondary in CT and a straight coil passing through that ring-shaped coil as primary. The current transformer has a primary (P1 &P2) terminal and a secondary box with core terminals. Core terminals include tan delta point and CT earth point. Each core has three coils, named 1s1, 1S2, and 1S3. Same for the next core terminals. Our test discusses the polarity test with a dual ratio with a 600-300/1-1-1 ratio with three terminals in each core.
You can see the terminal box in the following image:
We have the following cores named:
- Metering core (1S1, 1S2, 1S3)
- Protection core (2S1, 2S2, 2S3)
- Special protection core (3S1, 3S2, 3S3)
The ratio for this CT is such a way that between 1S1 and 1S2 this ratio is 300/1 for all cores. Similarly in 1S1 and 1S3, this ratio is 600/1 for all three cores. It means 600A in the primary will induce 1A in the secondary for a 600/1 ratio and for 300/1, the 300A in the primary will induce 1A current in the secondary.
Requirement for Polarity Test
Here are following list that is needed while performing the test:
- One battery of 1.5V (cell).
- Extension wire.
- Galvanometer.
Next, we will discuss the test procedure.
Polarity in CT
The current in CT flows from primary P1 to primary P2 terminal and in secondary it flows from R1S2 to R1S1. Here R represents the “red phase” and similarly for other Y and B phases. We have to test this polarity to ensure the current is flowing in this direction.
Here are the connections for this test:
- We will connect P1 to a positive supply from the cell.
- We will connect P2 to the negative supply from the cell.
- From the secondary box, we will provide a positive supply from 1S1 from the galvanometer.
- From the secondary box, we will provide a negative supply from 1S2 from the galvanometer.
- If we get positive defection (in the right direction of a needle) from the galvanometer then it indicates that we have the right polarity for our CT.
Now if we change the ratio between the 1S1 and 1S3 for 600/1A, then we get positive deflection. You have to check the polarity between both ratios of your CT in the secondary terminal.
Here is the galvanometer deflection for positive deflection:
In case of negative polarity, the needle in the galvanometer will deflect in the left direction for both ratios in the secondary terminal.
Schedule for Polarity Test
Here is a table that represents the CT polarity schedule:
| Sr. No. | Primary Connection from Cell | Secondary Connection from Galvanometer | Result |
| 1 | +ve to P1 & – Ve to P2 | 1S1 to +Ve & 1S2 to -Ve | Positive Deflection |
| 2 | +ve to P1 & – Ve to P2 | 1S1 to +Ve & 1S3 to -Ve | Positive Deflection |
| 3 | +ve to P1 & – Ve to P2 | 2S1 to +Ve & 2S2 to -Ve | Positive Deflection |
| 4 | +ve to P1 & – Ve to P2 | 2S1 to +Ve & 2S3 to -Ve | Positive Deflection |
| 5 | +ve to P1 & – Ve to P2 | 3S1 to +Ve & 3S2 to -Ve | Positive Deflection |
| 6 | +ve to P1 & – Ve to P2 | 3S1 to +Ve & 3S3 to -Ve | Positive Deflection |
You have to decide according to your reasons. There are different metes available to check the polarity test for CT.
Conclusion
We have studied the polarity test for CT and we have studied the requirement for this test. If the galvanometer or any other meter for this test gives positive deflection then we have the correct polarity for passing current in the right direction.
