• Electrical India
  • Dec 5, 2016

Significance of Polarity in Current Transformers

For certain kinds of measurements, even today, we need  to know the correct polarity of the primary and secondary windings in current transformers...

- K Sivakumar



 Ensuring the correct polarity of the primary windings and secondary windings in Current Transformers (CT) is of paramount importance in various measurement and protection schemes in electrical power distribution networks. Even though this polarity may not matter much in modern day digital measuring instruments and protective relays, this is still of importance in certain measurement and protection requirements.

Polarity Test

  A typical polarity test kit schematic is given atop the next column.

Test Procedure

1. Identify & Mark permanently, the ‘P1’ & ‘P2’ Terminals in the CT to be tested for polarity.
2. Connect the ‘+’ve terminal of the 9V cell to the ‘P1’ terminal of the test CT, through a Test Push Button.
3. Connect the ‘-’ve terminal of the 9V cell to the ‘P2’ terminal of the test CT directly.
4. Assume the ‘S1’ & ‘S2’ Terminals in the test CT. (This has to be assumed as of now, as we are yet to know this polarity for sure)
5. Connect the ‘S1’ terminal of the test CT to the ‘+’ve terminal of a Centre Zero Analogue DC Micro Ammeter. (This ammeter can be of 48sq. mm size with a scale of -10 µA to 0 to +10µA).
6. Connect the ‘S2’ terminal of the test CT to the ‘-’ve terminal of the Centre Zero Analogue DC Micro Ammeter.
7. Now, the test set up is complete.
8. Now, when you press the Test Push Button, if the pointer of the Centre Zero Analogue DC Micro Ammeter deflects in the ‘+’ve side in the scale, then the assumed ‘S1’ & ‘S2’ are correct. Mark them permanently now.
9. If the pointer in the meter deflects in the ‘-ve’ side in the meter scale, then the assumed ‘S1’ is actually ‘S2’ and vice versa.
10. Reverse the meter connections & repeat the test to ensure positive deflection.
11. Caution: Do not extend DC voltage to the primary for more than a few seconds. Otherwise, the cell will drain soon.
12. Do not get confused between the ‘+’ve terminal of the meter and the ‘-‘ve marking in the meter dial on the same side. Terminal and dial are two different things.
13. Repeat this test for all cores in the CT and mark each CT secondary polarity permanently.
14. Do not use any digital ammeter or digital multimeter in place of the Centre Zero Analogue DC Micro Ammeter. They will not serve the purpose of this test.

CT Installation

1. The primary of the CT is always marked as ‘P1’ & ‘P2’.
2. The CT shall be mounted such that the primary current flow shall always be from ‘P1’ to ‘P2’. Take care while mounting CTs in Incomer Feeders and in Outgoing Feeders. The primary current flow direction and thus the P1/P2 orientation will change between incomers & outgoings.
3. For Bus Couplers, if CTs are installed in these feeders, consult engineering team for proper orientation of P1/P2 of the CTs.
4. Particular care must be taken in case of special schemes like Bus Differential

Polarity Test Kit Scheme...

Protection Schemes

5. Mark the CT secondary terminals as below:
Core-1: R Phase: 1s1r & 1s2r; Y Phase: 1s1y & 1s2y; B Phase: 1s1b & 1s2b
Core-2: R Phase: 2s1r & 2s2r; Y Phase: 2s1y & 2s2y; B Phase: 2s1b & 2s2b
Core-3: R Phase: 3s1r & 3s2r; Y Phase: 3s1y & 3s2y; B Phase: 3s1b & 3s2b
Core-4: R Phase: 4s1r & 4s2r; Y Phase: 4s1y & 4s2y; B Phase: 4s1b & 4s2b
(More than 4 Cores in a single CT is not anticipated)
For dual ratio CTs, an ‘-s3-’ will be added in all the above cases.
For triple ratio CTs, a further ‘-s4-‘will be added in all the above cases.
6. Connect the ‘S1’ terminal of the CT to the phase side terminal of the CT secondary burden (may be a meter or a relay or a Test Terminal Block or a meter selector switch)
7. Connect either ‘S2’ or ‘S3’ or ‘S4’ terminal of the CT – depending upon the ratio selected - to the neutral side terminal of the CT secondary burden (may be a meter or a relay or a Test Terminal Block or a meter selector switch)
8. Loop all ‘S2’, ‘S3’, ‘S4’ (as the case may be) of all the R, Y & B phase CTs together and earth ONLY at the CT end.
9. Please note that if you are using only S2 terminal, short only S2 terminals of R, Y & B Phase CTs. Leave S3 & S4 open in all the CTs. Follow similar for S3 & S4 too, if you select higher ratios.
10. It is always preferable to earth the CT Secondary (either S2 or S3 or S4) at the CT end.
11. If it is not possible to earth the CT Secondary at the CT end, then ONLY IF THERE IS NO OPTION, earth the CT Secondary at the CT Terminal Block (Disconnecting Type Terminal Block) in the LV Chamber.
12. It is not enough to only ‘see’ a green earth wire connected to the CT secondary terminal either at the CT or at the CT TB. Please ensure that this green wire is taken right upto the panel earth bus and is properly connected to the panel earth bus with proper bolt, nut, plain washers & spring washer.
13. The length of the wire connecting the CT Secondary and the Panel Earth Bus shall be as short as possible.
14. The size of the earth wire shall be a minimum of 2.5 sq.mm PVC Insulated Flexible Copper conductor, unless specified otherwise by the customer.
15. Do not earth the CT secondary by terminating the earth wire at the panel enclosure sheets. It MUST be directly connected to the panel earth bus. Ensure proper contact of this earth wire at the panel earth bus.
16. Ensure proper tightness of all terminations of CT Primary & Secondary connections.
17. Use proper tools for tightening screws/bolts/nuts. Do not use undersized or oversized screw drivers for tightening/loosening screw connections. And, do not use cutting pliers/nose pliers for tightening/loosening bolted joints/connections. Use proper type & size of spanners for the same.
18. Caution: DO NOT EARTH THE CT SECONDARY AT BOTH THE CT END AND AT THE LOAD END. This might lead to circulating currents and possible damage to CT insulation.
19. Design / Engineering departments may mark these polarities & CT Secondary earthing details appropriately & precisely in the drawings. This may be shown in GA/Cross section/SLD/Power Schematic, as applicable.
Note: It may please be noted that the earthing of the CT Secondary is done for reducing the stress on the secondary insulation and it, in no way, will affect the performance of the CT, as long as a closed path is established.
Do not get confused that if we do not earth the CT Secondary, the secondary current will not ‘return’ to the CT. It will definitely ‘return’ to the CT, through the CT Secondary Neutral Wire, even if you do not earth the CT Secondary.
Having said this, do not take the liberty of not earthing the CT Secondary on the presumption that ‘anyhow’ it is not going to affect the performance of the CT.
Safety is more important than performance.
20. Though incorrect polarity connections may not hamper performance in modern day digital instruments & in numerical relays, it is always a good engineering practice to observe & practise proper polarity in CTs. This will be of significance – just in case, someone still opts for olden days’ electro-mechanical type meters or relays. If proper polarity marking & wiring are practiced, it will help us in both the cases. We need not keep on changing the drawings/wiring based on the type of relay or meter.


K Sivakumar
Manager - Design & Training
Megawin Switchgear Pvt. Ltd.
Salem

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