Current Transformer (CT) polarity

You have to connect CT's with correct polarity because vector sum of three CT's should be same when connected together.

Yes you are correct, connection of each CT must have same polarity but over all polarity of CT do not matter.( whether all positive or all negative).

Generally in overcurrent applications polarity makes no difference, even if the relay is solid state.. You generally want to install everything correctly even though..

As long as you are not using any directional features such as kw metering, reverse power, differential, etc., the ct polarity does not normally matter..

What type of relay are you using? What voltage level? Is differential protection enabled?

Some relay mfgs do quirky things with the electronics in their relays, and maybe you have found a problem that needs a patch.. If you want to fix the polarity on the reverse ct with little effort, swap x1 & x2 on the ct secondary, but also perform continuity checks back to relay to confirm this has not already been done..

Test again..

more than likely you have a bad overload relay, but who knows..

Absolutely, The polarity is extremely important in this application. especially if it happens to be a 3 phase application.

The phase currents are summed algebraically with in the relay and 3 sequential component values are derived that have purpose in the relay logic.I am not familiar with that particular relay, but I am sure that the polarities are extremely important.

Also remember that most transformers have negative polarities, ie, current going in the polarity mark on the primary will give current out of the secondary polarity. There are some older transformers that have Positive polarity identification.

The connection schematic for the relay should show dots for the intended polarity marks.

If you reverse one or two phase of CT sensor then next logic assumes a phase failure and will trip due to phase failure detection. If two phase move in one direction and one in another direction then it is equal to not having one phase in active condition. Hence, you have no option but to place the CT polarity correctly.

If any CT is not functioning then also it is equal to phase failure and will result in phase failure tripping. If you insert the sense wire from wrong direction into CT then also it will result into polarity change.

You have to engineer it right way for a machine to function. Physics can explain but can not help you in your engineering. Do it properly or get it done right at the first place.

Vector sum of all 3 cts should be zero. polarities must be correct.

Dear Dr. Shyam.

Would you please clarify one detail for me. Would changing a polarity on secondary winding of the bushing CT (toroidal CT with one turn through the middle opening) reverse the direction of the field in the secondary, such that it would no longer act against the incident magnetic field and would add to it instead? So, by changing a polarity we actually amplify the e/m field so much that we overload and burn a CT?

Or this is not possible because no matter how we connect the polarity the secondary winding field will always oppose to the incident field created by primary turn?

Can you please confirm or comment?

Thank you.

To make the secondary ineffective against the primary coil field, wind it at 90 degree to primary coil or at zero degree to the magnetic core. Any wire, parallel to the field will not generate any current.

Winding in reverse / opposite direction means coupling two source of reverse polarity and then they will simply waste energy. You can easily play with transformer having two secondary coils. Wiring them in series correctly will add the voltages and in correctly wiring will subtract the voltages.

Dr, Shyam. Thank you for your response. My question was merely on the importance of maintaining the connection polarity for the bushing transformers. The primary winding is really the one turn insulated bus or bushing. Secondary winding is the CT itself. Question was: what happens if the dot marks are not followed. Will the CT burn? If not why not? What is the effect of incorrect polarity connection for a single phase application?

Thank you in advance for the answer.

Alex Lemberg

Some CT may have rectifier diode pre installed in the secondary winding and for these in pulse mode polarity of the input - primary may be important. Dot may signify the output voltage polarity direction.

Most of the CT measurement use Load resistor, rectifier diode, signal averaging capacitor and resistor in parallel. This gives DC output for measurement. If polarity is reversed and reverse voltage is fed into PLC ADC then it may not give any reading. Some ADC may read from -10V to +10V and such ADC may still read with polarity error.

Guest,

Did this motor starter ever work OK? if it did then the polarity of your CT's doesn't matter, frist try another OLR.

If you have 3 CT's lets say A B C with the connection X1 X2 depending on the ratio and make of the CT, but CT B is with the polarity changed the wiring in that CT |B should be X2 X1 to match the other 2 CT's

No, it never worked correctly, it tripped the first time we energized the motor.

We did correct the polarity orientation of the CT today, the problem went away. The SSOL did not have direct phase loss detection, but after consulting with the manufacturer it turns out my hunch was correct, its algorithm uses current imbalance to bias the thermal model to trip quicker because of the additional heating effect that imbalanced current will produce.

Thanks everyone for the opinions and support.

You are welcome !

I also have a 4160V 1.5 MW motor which uses CTs to an overload relay for protection.

Fortunately for me , my overloads are thermal.

In you case , the CT polarity may be an issue.

I have also had instances where the polarity mark on the CT was wrong.

I always check the polarity of the CT using a " flickometer': it is simply two or three dry cell batteries with a short jumper passing through the Ct window or in series with the load winding( if it is not a doughnut type CT) and using an analog voltmeter on the X1 X2 terminals.

(This is just a verification check and does not replace a complete mag curve check.)

Verify that the CT polarity is Ok before doing anything else.

Do you havbe another relay you can use?

How many of these relays do you have on your plant?

Do you have a current injection set you vcan use to test the overload?

Depending on the type of OLR installed it is possible that phase shift between the three phases is taken into account for accurate and fast measurement, you can check this in the manual of the OLR . It is possible that for imbalance ( difference between currents ) the protection device is taken the phase shift in to account, in that case an inverse polarity of a CT will give directly a wrong metering ( in stead of 120° phase shift it will be 120° + 180°) this will result in a very large imbalance so it will trip

Please check

1) Polarities of CT

2) Ration of CT

3) Setting of OLR.

I strongly see this to be related to CT connection problem . Also check the current in OLR if possible.IF the motor has not started from the beginning the chances are very high for improper connections of CT.