Selection of Differential CT's

Obtain the correct transformers for the primary. The alternative could be for someone to have half-a-day-out with the Undertaker.

Oh.... The joys of reinventing the wheel...... There is going to be one happy 11 KV HT panel manufacturer as he will get a repeat order for panel after the exercise is completed. By the way why was correct HT CT not specified for HT panel if you had decided to go for differential protection for the transformer?

Please address the questions to Anonymous Poster #1.

If the CT is not a 11kV Ring type CT, What ever insulation you build up on 11kV Bus Bars, you should never dare to use LV Ring type CT on 11kV Bus Bar.

Reason being that

1. Flashover voltage (dry or wet) of CT is of Second highest of various components in the Power System (highest being Circuit Breaker) So that if any surge strikes, a post insulator can burst but not the CT as this has to measure fault current and protect the circuit (Known as Insulation Co-ordination in High Voltage Engg.).

2. There must be earth screen between 11kV system and the CT so that any leakage of 11kV goes to earth rather than in to secondary circuit.

You have 2 possible solutions:

1. Go for the 11kV Ring type CT (in case you do not have any place to fix Bar Primary or wound primary CT in the switchgear) or else 11kV Bar Primary CTs.

2. You can place LV ring type CT over each core of 11kV Cable at Cable Terminations in the terminal Box -PROVODED THE TERMINATIONS CONSIST OF HEAT SHRINK XLPE AND NOT COLD FIT/SLIPPED. But great care is required in this case, which is as follow:

1. The CT must be placed at location and approx. in the centre of Stress Relieving Tube (or semiconducting tube) portion of Termination kit. FYI in XLPE Heat Shrink Termination - to gradually terminate earth screen of each core of HV cable - a Stress relieving tube is first shrunk on the core. This is generally black in colour. Over this tube outer sheath is shrunk. Because up to semiconducting tubing there is earth screen between 11kv and the CT hence this is safe. ALSO MAKE SURE THAT IN DOING THIS YOU DO NOT MECANICALLY STRESS THE CORES AND CLEARANCE BETWEEN CORES AND CORES TO EARTH IS MAINTAINED AS GIVEN IN THE DIAGRAM OF XLPE TERMINATION KIT.

2. Tie the CT is correct position of 1 above with cable ties - so that under through fault current in the cable - these are not shifted from desired position.

Do not take any chance. In case you have a little doubt that criticality of above is not clearly understood - just go for 11kV CTs.

It is common for switchgear and generator manufacturers to use LV CT's on MV busbars and wiring. They feel that the air gap increases the insulation value. That being said they should be hard mounted with the conductor centered in the hole.

Class of CT not indicated.It should be PS and appropriately designed for a particular knee point voltage based on fault level.

We hope that he already has the correct type of CTs. Now a days with Numerical Relays have built in CT saturation detection and hence 5P20 class is adequate, Class PS (Class X) CTs are not required.

Sirs,

The CT class is CL-X, actually what happened, this diff. protn. is site modification, new requirement, already the CT chamber is ocupied, no space, inside the switchgear I checked, panel manufacturer are used LT CT's only. but they are using busbar bushings (resin cast) instead of bare busbars and fixed the CT's ove it. Now the available place in the panel is bare busbars only but since the size is bigger for HT CT's there is no space, but LT ct's we can accomadate. I think the suggestion of putting ct's in the cables with rachem sleeve is good. any way we made query to some other suppliers to check whether some body can supply smaller ct's suitable for our space limitation. please share one more thing.. Is it mandatory the diff. ct's for both primary and secondary should be from same manufacturer? I think no need ct is checking current only and knee point voltage should be as per calculation.

thanks to all

Are already bar CT's (3 Nos.) provided in HT panel?

If yes, it is just a matter of changing the CT's with additional core included in the new set of CT's suitable for differential protection. Please note that beyond HT CT's, you need to provide LT side interposing CT's to match the primary and secondary currents for proper operation of differential protection. The vector grouping of the main and interposing CT's also needs to be matched. This is a job that will require services of skilled technician well versed in the differential protection schemes.

Time to entrust the work to panel OEM or skilled technician.

presently there is ct's but its for some other purposes and it is ring type also. the new ct's are additional, we dont need interposing ct's because both are 1A secondary ct's. thanks.

Even if secondary of both sides are 1 amp - Most of the times it happens that Rated Primary currents of CT of the two sides do match with exact rated current ratio of transformer.

If this correction is not built in the differential protection relays, you will be required to use interposing CT to match CT ratios with that full load rating of transformer.

You can also take care of difference by adjusting %age bias but this is at the cost of sensitivity of the protection.

yes you are right, can u tell me one thing, is there any problem if we are using different ct manufacturer for primary and secondary? only values should be correct no? thanks.