differential relay setting
differential relay setting
(OP)
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.
Thanks a lot in advance.






RE: differential relay setting
RE: differential relay setting
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.
RE: differential relay setting
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.
RE: differential relay setting
RE: differential relay setting
I guess what i want to say is, excitation curve from manufacturer can definitely help
Thanks
RE: differential relay setting
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.
RE: differential relay setting