Slope setting in differential prot. relays

[00123456]

Hi All,
I am just starting new thread. Today, most of the protection engineers know about numerical transformer differential prot. relays & most of the relays are having three SLOPE characteristics (i.e. 7UT612, P632, RET531,T60 etc.). What should be the criteria & considerations to set third SLOPE? Pl. explain in details. Is it necessary to set third slope or first & second is sufficient to keep the relay stable during heavy through faults?

Thanks

 

[DiscoP]

Of the relays you have listed, I am only familiar with P632, but I would imagine the others are the same.

On this relay and others, the 1st slope is fixed at 0%

The 2nd slope is a setting, used to bias the relay for tap-changer operations.

The 3rd slope is to bias the relay for large through faults. To accurately set this you need need to plot a graph of through fault vs differential current that would appear caused by CTs saturating.

Or you could be comfortable with what the relay manual suggests !!!

In older relays, there was no control over this slope. Most manufactures suggest a slope setting that will emulate the characteristic of it's electromechanical forefathers.

I have also noticed some modern numerical relays without this third slope - they have CT saturation detectors that are used to restrain the protection.

 

[davidbeach]

Three slopes? If DiscoP is correct in interpreting your question, with the first slope is fixed at 0%, then what you are describing is actually a dual slope (only two, not three) relay. The line that represents the minimum operate is not counted as a slope segment.

 

[521AB]

Keep default settings, if your CTs have passed the requirements indicated by the manufacturer, which means you have to calculate fault currents in the points indicated by the manufacturer and then veriyfy that the voltage across the CT is below the knee point voltage (or as today is called, Accuracy Limit Voltage, E_ALF_max).
If your CTs do not pass the requirement, ask the manufacturer (maybe you pay for that) to study your case and to propose new slope settings.

 

[subodhbhatnagar]

I have never come across adifferential relay with triple slope settings. There are three settings which need be done.
1 Pick up current of differential element which is normally default set.
2. First slope seeting is kept equal to {% mismatch of HV and LV side CT's(if any) plus percentage tap changer variation of the Xmer plus allowable relay inaccuracies and CT errors}.
3. The setting of second slope depends on saturation factors of both side CT's at maximum through fault currents anticipated on the Sub-Station with CT's inaccuracies considered.
In practice ,the name plate ratings of CT core in service, is considered enough for calculating dual bias/slope setting of the differential relay(This requires more knowledge about other details of secondary resistance , pilpt wire resistance etc).Whatever, generally an approximation and experienc can help u a lot in decision making .