Primary CT Sizing for Transformer Earth Fault

[ukgraduate]

Hello,

I have an application where we have a 3MV Delta/Star transformer. The star system will be solidly earthed. We want to put a CT on the earth connection of the transformer for earth fault protection.

The transformer is for industrial uses and generally balanced as it is mainly running motors. Hence theoretically there will be 0 current through the CT.

The max earth fault calculated is approx 68kA.

We want to use a X:1 10P10 CT. I do not know how to calculate what the primary CT ratio would be?

We have used 100:1 previously however from reading various documents if I go too low the CT could saturate severely and causes issues with the protection relay. Is there a general rule of thumb to size these CT's?

Can anyone help here?

 

[dpc]

You need to look at the short-time thermal rating of the CT and what the fault duration could be. Saturation is an issue, but first priority is making sure the CT survives. It obviously does not need to be larger than the phase CTs so that would be the upper limit. I have specified one in the past that I think was probably too small. It hasn't failed yet, but I still worry about it a little.

 

[KuanYau]

ukgraduate,

1. I take it that you have a 3MVA (not 3MV) Dyn [HV delta and LV star with neutral terminal accessible] [TT system]. A five wire with L1, L2,L3, N and PE conductors. The Neutral is connected to the star point of the secondary wingding and is [isolated from earth externally]. An earthing PE wire is also connected to the star point of the transformer secondary winding. This PE conductor is connected to the [source earth electrode]. You wish to place a single CT on this PE wire for earth-fault protection.

2. Under healthy (no earth-fault) condition, the current in this PE wire is 0; irrespective of whether the loads [are balance motors or unbalance single-phase loads]. The current in this PE wire is also O under [an isolated from earth] three-phase or single-phase short-circuit fault. Current flows in this PE wire [only when there is an earth fault].

3. In a TT system, you plant a separate [installation earth electrode] in your plant, which is a separate electrode from the [source earth electrode]which is located close by to the transformer.

4. Assuming the [source earth electrode] earthing resistance of say 0.5 ohm and the [installation earth electrode] earthing resistance of say 0.5 ohm; the total earthing resistance would be 1ohm. The possible earth fault current would be the line to neutral voltage divided by the total earthing resistance. It is unlikely to be [approx. 65kA]; as you had calculated.
Attention: earth fault current is NOT the same as the [three-phase assumed zero-impedance short-circuit current].

5. We usually set the earth fault relay to operate at around 15% of the transformer full-load current [but limit down to maximum of 100A and with a definite time lag not exceeding 0.5s]
A 100:1 CT Class 10P10 is acceptable if the earth fault relay having [say 0-2A range input] or a 100:5 CT to suit the relay input current range. We had been using these settings for more the fifty years and have not encountered any problems.

 

[ukgraduate]

Thanks KuanYau,

In response to your comments please see below:

1)You are correct its a 3MVA Transformer. It is a TN-S system. The intention was to place the CT on the PE wire for earth fault protection.
2)I agree my first statement was misleading
3)There will be no earthroad near the equipment in the plant as such. All equipment is bonded together via earth cable/straps etc and all earth grids tied together.
4)Understood
5) Would not the CT saturate if there was a large fault current such that it may provide inaccurate readings? Or is this not an issue?


Thanks

Garrath

 

[KuanYau]

ukgraduate,

1. A TN-S system may be installed in two ways:
Type a) with six wires [L1, L2, L3, N, PE-1],and another [PE-2] conductor from the transformer star point to the source earthing electrode. The N shall be isolated from earth. The PE-1 conductor may be in contact with earth without any restriction.
In this case, both (two) PE-1 and PE-2 conductors shall go through a single CT. Attention: these two PE conductors shall be [insulated from the transformer up to the CT]. This is to ensure that any earth-fault current [either from PE-1 or PE-2] are sensed by the CT.
Type b) with five wires [L1, L2, L3, N, PE]. The PE wire is earthed by the [installation earth electrode] at the plant. The PE conductor may be in contact with earth without any restriction. A short length of [insulated conductor from the transformer star-point] goes through the CT and connected to the plant PE conductor. This is to ensure that [all earth-fault currents] are sensed by this single CT before reaching the transformer star-point. This CT is for [earth-fault protection] only.

3. "3)There will be no earthroad near the equipment in the plant as such. All equipment is bonded together via earth cable/straps etc and all earth grids tied together."
3A. There MUST be a insulated conductor* [from the transformer star-point] to the (plant) PE conductor where all equipments are bonded. There can be numerous earthing or bonding points in the plant. This [insulated conductor* from the transformer star-point goes through the single CT] which senses [all earth-fault currents]. Attention: Without this insulated conductor*, the plant PE conductor is ISOLATED (not) connected to the transformer start-point. It would be an IT system, which is [not permitted for use as public supply in UK].

5. See point 4 for earth-fault current calculation.
Even [say a total earthing resistance of 0.5 ohm] would limit the earth-fault current to a very low value.
By setting earth-fault [maximum of 100A and with a definite time lag not exceeding 0.5s] a 100:1 or 100:5 Class 10P10 CT [would not saturate and maintaining within reasonable (10%)accuracy] as the earth-fault current is unlikely to reach 10 times of 100A. Attention: see CT Standards for the characteristics of CT Class 10P10 .