differential relay setting 4

[SELEC]

I am going through IEEE buff book(2001 version.On page 431-432. It says" The second harmonic and some relays with higher harmonic(e.g., fifth,seventh, eleventh, thirteenth) are filtered to restrain them. The filtered harmonics are applied to the restrainig winding when the magnitude of the second harmonic exceeds 7.5% to 20% of the the fundaemntal current. the lower percentage is benificial during the normal no-faullt conditions because it provides rstaining action. but the lower percentage setting makes the relay less sensitive on an internal fault.' Can anybody explain this to me?

Thanks a lot in advance.

 

[unclebob]

The second harmonic has to be restrained because it is mainly present in the inrush curent and you don't want your relay to trip when energizing the transformer. During normal use, second harmonic is at a very low level if not absent.

 

[jghrist]

To add to what unclebob said:

The lower percentage setting makes the relay less sensitive on an internal fault because there may be 2nd harmonics in the current during an internal fault. Particularly if the CTs are close to saturation. Since the 2nd harmonic level is used to restrain the relay from tripping, if you set it below the value that might be in a real internal fault, then you might not trip for the internal fault.

This is usually not a problem, because there is normally an unrestrained differential element (pickup set higher than any inrush) that will trip for currents high enough to cause any CT saturation.

 

[stevenal]

2nd harmonic restrains for inrush. The 5th harmonic restrains for overexcitation. If you were to simply sum the currents, or the fundamentals, either condition may be a source of differential current and cause tripping during a non fault condition.

I am not familiar with the use of the higher orders.

A setting of 15% restraint on 2nd harmonic seems to do the job. After checking relay events after a recent energization, we saw 40 to 101% 2nd harmonic, well above the 15% setting.

 

[stevenal]

Update: According to IEEE C37.91, overexcitation is characterized by odd harmonics, so evidently those higher orders are also meant to restrain for that condition.

 

[amps21]

I have had an experience with a generator transformer having designed with lower flux density (1.45T) and the 2nd harm setting gave away (during inrush) being set at 15%. A lower setting of about 7.5% held in.

I guess what i want to say is, excitation curve from manufacturer can definitely help

Thanks

 

[mikey70]

2nd harmonic predominant on inrush, 5th on overexcitation just like stevenal says, but harmonics also appear at other times, i.e. sympathetic inrush when two transformers are in parallel and one is energized and the other is being energized. Harmonics appear in the energized bank as well as the one being energized.

Older relays had fixed harmonic restraint. Aproximately 35% on fifth and 15% on 2nd. Microprocessor relays can usually be adjusted to any percentage you want. Leave it at factory default unless you have a problem, like amps21. I know of no relay manufacturer using anything except 2nd and 5th to restrain tripping.

The idea of any protective relay is to optimize sensitivity and security. Since they work against one another it is always a juggling act.

 

[davidbeach]

Mikey mentions sympathetic inrush of transformers in parallel. That is the same terminology used in Blackburn and elsewhere, but actually the transformers only need to be fed from the same bus; the other side of transformers need not be connected to have sympathetic inrush. I've witnessed it in action on a radial system.