Application Guide - NP900 Series
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A996A
incomer. This is typical behavior in old-fashioned relay protection which is not capable to
differentiate in between of normal consistent earth fault and intermittent earth fault. Since
the intermittent earth fault is transient type of fault where the actual fault is only few
hundred microseconds causes the traditional directional earth fault protection relays to
lose the directional sensitivity. When the directional decision algorithms of traditional
relays try to cope with intermittent signals the trip decision result will be totally up to luck.
Typical finding from this type of complete black out substation includes from all protection
relays logs found multiple directional earth fault starts and releases and finally the incomer
relay residual voltage trip. This is the worst case scenario. On the other typical scenario
few feeders have tripped in the same fault including, by luck the correct faulty feeder. In
this later case also in all of the relays logs can be seen the incorrectly started directional
earth fault events and releases.
Previously this kind of behavior was commonly ignored and put to mystery category since
it might have happened just once or twice per year and also disturbance recordings were
not commonly used in normal medium voltage substations for verification of the fault.
When disturbance recorders were introduced as common feature of the protection relay
this phenomena got name and characteristics. Its unique characteristics require
completely different tools for handling than the traditional directional earth fault protection
is capable of handling.
Typical characteristics of intermittent earth fault include high magnitude current spikes
which compared to residual voltage are in opposite direction of the current spike in the
faulted feeder and same direction in non faulted feeder. Following figures present few
intermittent earth fault situations seen by the relays in substation.
Содержание NP900 Series
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Страница 49: ...Application Guide NP900 Series 49 504 A996A Figure 3 2 1 3 25 Definite time operating characteristics...
Страница 50: ...Application Guide NP900 Series 50 504 A996A Figure 3 2 1 3 26 IEC predefined characteristics NI VI LTI and EI...
Страница 52: ...Application Guide NP900 Series 52 504 A996A Figure 3 2 1 3 28 IEEE predefined characteristics EI MI and VI...
Страница 243: ...Application Guide NP900 Series 243 504 A996A Figure 3 3 2 1 122 NPS biased thermal trip curves with kNPS values 7 and 10...
Страница 491: ...Application Guide NP900 Series 491 504 A996A 3 7 5 6 SETTING EXAMPLE...
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