CT Saturation 1

[mrbj]

Hi guys,
Two questions.
1. What happens to the CT, its' wiring, and connected protective device etc.. if a CT is driven into saturation from a high short circuit current ?.

2. Is it necessary for a set of CT's selected for REF protection ( low setting say 0.2In ) to have a knee point voltage that is high enough to cope with the Short Circuit Fault say 10In ?.

Regards
mrbj

 

[alehman]

1. Nothing "happens" except the CT output no longer accurately represents the primary current.

2. Only if you want the CT output to accurately represent the higher primary current. If you are using for any type of differential or impedance application, the CT must not saturate. The CT must be rated to withstand the available fault current for as long as it is likely to persist though.

 

[mrbj]

Thanks alehman,
thank you for your reply, to expand the questions further.

Q1. If the protective device failed to operate and allowed the SC current to persist for some time, would there be any damage to the CT's, its wring and its protective device.

Q2. Will the imbalance within the CT's , caused by the high SC current,(even if matched), cause the REF to operate
before the Overcurrent protection starts to operate.ie will the REF trip the circuit even though there is no fault, merely a high over current that is below the operating threhold of the Overcurrent / SC protective device(s).

You say that... ""the CT must be rated to withstand the available fault currnt for as long as it is likely to persist""...
Could you expand on this since it seems it may be relevant to my first question.

regards

 

[mrbj]

For Q2
Assume CT's in saturated area
regards

 

[RalphChristie]

Q1: It depends on the value of the current and the time it persists. Normally a CT has a STC-rating (short time current) of a certain value, like 50kA for 1sec. If the short-circuit current exceeds this value for the given time, you can and will certainly damage the CTs. For your protective devices and wiring: It depends on what their current ratings are, but they too can also be damaged.

Q2: a REF-scheme is a differential scheme and a type of zone protection. If there is a fault outside the zone, and the CTs match, it will not operate. The moment the CTs do not match, and the measured (differential) value exceeds the set value, it will trip. The setting of a REF-relay is such that is does not operate for through faults.
Now, lets say there is a mismatch between the CTs (such that it will operate the REF-relay) and there is a fault outside the zone. Normally a REF-scheme operates within a few cycles, and it will definitely trip before an O/C IDMT-curve. If you have an instantaneous O/C element, it just depends on which one of the two is the the fastest, because both will operate.

the CT must be rated to withstand the available fault current for as long as it is likely to persist
Your CTs must be rated to handle the short-circuit current at least for the time it takes the protection to clear the fault.

 

[RalphChristie]

Regarding your first post, question 2:

What kind of REF-scheme do you have? (High or low impedance) If you have a low-impedance scheme, the knee-point is not that important, because the burden on the CTs are usually low. However, for a high impedance REF-scheme, the knee-point value of the CTs must be at least twice the setting value.

Just something else:

For a high impedance (voltage operated) REF scheme, makes sure you have a metrosil connected across your relay. a Metrosil (non-linear resistor) protect the CTs, secondary wiring and relay from damage due to high voltages during a fault condition. This resistor is of a type which reduces in value as the voltage across it increases, and is selected with caracteristics which limit the voltage to between 1kV and 2kV.

 

[mrbj]

Many thanks RalphChristie,
You anticipated my next question.
The scheme employs
An electronic relay with input imp of 0.02ohm
The usuall 4 cts in parallel
Short lead resistance
A stabalizing resistor in series (circle) with device.

Does the knee point voltage have to be 2 x Iset x Rburden or must it be 2 x Ifault(sec) x Rburden ?.

Thanks for the advice on the Metrosil, I think the PD has one built in since it requires the fault voltage to be less than 3kV.

Regards

 

[mrbj]

Forgot to mention
scheme is fed from a 250kVa/400V alternator
CT's are 5P5 - 2.5VA - 400/5
REF set at 10%
regards

 

[RalphChristie]

I assume this scheme is on the generator? I am more familiar with REF-protection on transformers, but I think (correct me if I am wrong) it will be almost the same on generators. Normally a setting of approximately 1% of rated current can be obtained, but it is dependant upon the grounding method. Since the relay does not restrain on the load current, the same sensitivity is maintained for all load conditions.

Maximum voltage value is equal to the calculated maximum through current times the maximum CT loop impedance (Internal resistance of CT + wire resistance) in parallel with relay, plus a margin.

V = Imax x (Rct + Rwire)
V = maximum voltage developed during through fault.
Imax = max through current on secondary side.

The relay voltage setting is the maximum voltage value minus the voltage over the stabilizing resistor.

Vr = V - (i x r)
Vr = setting voltage of relay
i = operating current of relay
r = stabilizing resistor

The calculated setting shall not exceed 50% of the saturation voltage (kneepoint) of the CTs to wich the relay is connected.

Class X CTs are preferably required for this type of protection, however experience has shown that most protection type CTs are suitable for use with high impedance relays, providing the following basic requirements are met:
CTs should have identical turns ratio. Where turns error is unavoidable, it may be necessary to increase the fault setting to cater for this.
To ensure positive operation, the relay should receive a voltage of twice its setting. The knee-point voltage should be at least twice the relay setting voltage.
CTs should be of the low reactance type.

Hope it answers your question.

Regards
Ralph