Different CT Ratio for Generator Differential Protection

[shkim2000]

Today I started up generator but ACB for generator has been tripped due to Differential Ocurrence. The reason is that CT's ratio are different from each other, which means that CT from Generator is 1250/5A and CT from Switchboard is 1250/1A. The protection relay is Alstom P343.
I am not sure if I am able to set two different CT setting and P343 is able to calculate equivalent current from two different CTs. At the moment, is P343 able to resolve problem by itself ?
If not, is there no other solution other than replacing same CT ratio ?
If any, can anybody advise any other protection to protect against differential ocurrence ?

 

[davidbeach]

I don't know that particular relay, but typically generator differential relays expect the same ratio at both ends. If the relay can also be used as a transformer differential relay, set it up so that the relay sees the generator as a 1:1 transformer and you might be able to get away with the CT mismatch. You could use 5:1 axillary CTs to match the two ends, but ultimately you will find the best results occur when you have matched CTs at both ends of the generator.

 

[dpc]

In theory this could be easily dealt with by a digital relay in the relay's program, but in practice relays expect all 5A secondaries or all 1A secondaries - not a mixture.

I'm not familiar with this relay, but as David Beach mentioned, some auxiliary CTs could be used to correct the ratio problem, at least temporarily.

So short answer: I don't know.

 

[stevenal]

Looks like this particular relay can accomodate either 1A or 5A secondaries. It should work if the proper relay taps are used.

http://www.areva-td.com/static/content/1132654775888-P34x_EN_TDS_A11.pdf

 

[stevenal]

Certainly connecting to the wrong input would cause a problem, and might even be the cause of tripping in this case. Good reason to double check all the secondary wiring. Increasing the complexity and chance for errors by adding another set of CTs makes little sense. The added burden of interposing CTs will likely increase any CT mismatch. The relay's disturbance recording function should be able to assist you in troubleshooting. Look around Areva's site for more information, manuals, etc.

 

[jtkirb]

This can be a big problem for through faults you don't want the 87 to pick up on. Putting a set of 5:1 CTs won't have the saturation capability as high as the larger units in the generator. A though fault will saturate the smaller CTs and make it look like a 87 trip took place.

Pick your solution wisely- either program it into the relay or put in new CTs

JTK

 

[stevenal]

Nothing to program regarding the 5 A versus 1 A. Slope is always a percentage of tap. Other settings in this relay are in terms of per unit of I nominal. I suspect a polarity problem.

 

[jtkirb]

generator differentials are always a problem on startup if the checkout crew has no idea how to make sure everything is correct. Polarity, saturation, current pushing, rotation, and actual knowlege of the system is required. I see way too many people assume everything is correct without doing time consuming checks (or know how).

I have not used this relay, so the solution you suggest may be the best course of action. Like I said- program the relay to handle the mis-match.

JTK

 

[Deansharafi]

It is wise to do a "Stability" test on the generator differential scheme first to establish the true nature of the problem. It can be something as simple as a reversed polarity.
Be careful with CTs of different type/class, as they can always mismatch and create problems in case of a through fault.

 

[Milko]

My friend I think you have a temporal solution for the problem.

Change the conection in the relay- the wiring from 1250/1 to the analog input for CT 1 A secondary. the same for the CT 1250/5 to input for CT 5 A secondary. Then check the differential current with secondary inyection.

Please respond if works.

Good luck

Milko

Melquiades
Protective Relay
Testing and Commissioning