Hi-Z REF Protection!

[YERUK123]

Hi,

This is wrt Hi-Z REF Protection.

The Phase CTs and Neutral CT parameters are same except the Rct Value. On Phase CTs the Rct(Ph) = 5 Ohm, On Neutral CT Rct(N) = 4 Ohm.

As the Rct Values are different there would be a Spill Current as the Dynamics are not Balanced.

1. Can I compensate for this Imbalance in the Stabilsing Resistor ?
2. If yes what is the factor , I should Compensate for?
3. Is it Advisable to go for Hi-Z REF for here ? As per the clients Specs Hi-Z REF is must.

Would appreciate All your valuable Inputs.

Thanks,

 

[YERUK123]

Hi,

This is wrt Hi-Z REF Protection.

The Phase CTs and Neutral CT parameters are same except the Rct Value. On Phase CTs the Rct(Ph) = 5 Ohm, On Neutral CT Rct(N) = 4 Ohm.

As the Rct Values are different there would be a Spill Current as the Dynamics are not Balanced.

1. Can I compensate for this Imbalance in the Stabilsing Resistor ?
2. If yes what is the factor , I should Compensate for?
3. Is it Advisable to go for Hi-Z REF for here ? As per the clients Specs Hi-Z REF is must.

Would appreciate All your valuable Inputs.

Thanks,

 

[inpran]

Yeruk123,

Are the magnetising characteristics of both the CTs same? If yes then you can adopt high impedence protection with the values of Rct. Ensure that you calculate the stabilising voltage with Rct=4 ohms.

It is difficult to generalise or give a standard compensating factor. It varies with voltage,current rating and total secondary lead resistance.

 

[slavag]

If I remember right, very important two points:
1. Same ratio of CT's
2. Vk > 2Us, Us is stabilization voltage.

Actually is not so important same Rct, important is fulfill Vk1 and Vk2 > Vk requirements.

 

[YERUK123]

Dear Inpan,

Vs' > If (Rct + Rp) and Rstab = Vs / Is. I think we should take the greater Value of Rct.

What is your opinion on this ?


Dear Slavag,

I dont agree with your comment! Can you please elobrate?


Thanks and Regards

 

[inpran]

Yeruk123,

The calculation what you have given holds good when the resistance of both the CTs are same. However due to different resistances there are possibility of circulating current in theory. In practice however , you will still observe zero current.

But to be on the safer side, especially at the time of energising, it is desired to set your stabilising voltage on the higher side. At the same time the value shall not be too high either.

Further, since all the CTs are paralleled in High Impedence, the effective resistance as seen by the relay will be the parallel combination of these two Resistances. Since the resistance values of these two CTs aren't that different. It is safe to consider the value of the lesser of the two resistances.

Also please confirm that the knee point voltage of both the CTs are same. In most of the cases I have come across, it was observed that the values knee point voltages are different necessiating a higher stabilising voltage value to be used.

 

[slavag]

Hi.
Vs= Isetting x Rstab operation level.
This is a point.
In your case, system have a some natural unbalnce. Rct1 not eq Rct2
But is not so important, if Iunbalnce< Isetting

for calculation, of corse, you must use a longer loop or bigest Rct+Rp.

 

[isquaredr]

Slavag is correct.
Use the higher ct resistance to calc you relay setting allowing 16*flc, your relay setting """MUST""" be less than half your ct knee point value. All ct's must have the same characteristics (class X).If resistance grounding is used calc the stablising resistor to allow between 10-25% primary current through you NER and of course prove by primary injection. If you follow these simple rules you will have excellent REF protection not suseptible to maloperation.