Table. 5.3.4. - 59. Register content.
Date and
time
Event
code
Fault
type
Max I0
m
/I0
set
Fault
U
0
(%)
Operating
angle
Used SG
dd.mm.yyyy
hh:mm:ss.mss
8644-
8645
Descr.
L1-G…
L1-L2-
L3
The ratio between the highest measured
current and the pick-up value.
Residual
voltage
0...250°
Setting
group 1...8
active.
5.3.5. Intermittent earth fault (I0int>; 67NT)
The intermittent earth fault is a transient type of a single-phase-to-earth fault where the actual fault
phenomenon lasts for about a few hundred microseconds. The intermittent earth fault is commonly
seen in Petersen coil grounded (compensated) medium voltage networks. The intermittent earth fault is
commonly thought only as a cable network problem but it can also occur in overhead line networks.
The key point for this type of fault appearance is the compensation of earth fault currents with a
Petersen coil.
This phenomenon is becoming more frequent as more utilities networks are replacing overhead lines
with cables dug into the ground. This development in distribution networks is very understandable
as overhead lines are more vulnerable to possible seasonal storm damages. Also, the annual
maintenance costs as well as the annual power-down time are both signi cantly lower with
underground cable networks than with overhead line networks. However, the problem at hand is
caused by the increasing amount of cabling in the network which in turn causes dramatic increases
in the capacitive earth fault currents in the distribution networks. When the capacitive earth fault
current increases in the network, it becomes necessary to detect the earth fault current with a
Petersen coil.
Problems caused by intermittent earth fault are normally seen in compensated network substations: an
earth fault can trip multiple feeders simultaneously, or an entire substation can be tripped by residual
voltage back-up protection from the incomer. This is typical of old-fashioned relay protection as it is not
capable of differentiating between a normal consistent earth fault and an intermittent earth fault. As the
intermittent earth fault is a transient type of fault where the actual fault lasts only for a few hundred
microseconds, this causes traditional directional earth fault protection relays to lose their directional
sensitivity, and as a result their directional decision algorithms go haywire and the trip decisions will be
completely random. Typically, when a whole substation goes dark the logs of all protection relays show
how they have experienced multiple incorrect directional earth fault starts and releases, as well as an
incomer relay residual voltage trip. This is also the worst case scenario. In another typical scenario a
few feeders, including the correct faulty feeder, have tripped at the same time. In this case, as in the
previous, all the relays' logs show various incorrect directional earth fault starts and releases.
Previously, these scenarios were usually ignored and led under 'Mysteries of the universe'
because they only occured once or twice a year and because disturbance recordings were not
commonly used in normal medium voltage substations for fault veri cation. However, when disturbance
recorders were introduced as a common feature of protection relays this phenomenon received
a name and de ned characteristics. One such characteristic is the occurence of high magnitude
current spikes, which –compared to residual voltage– are in the opposite direction of the current spike
in faulty feeders and concurrent in healthy feeders. Handling these unique characteristics requires a
completely different set of tools than what traditional directional earth fault protection can offer. The
following gures present three intermittent earth fault situations experienced by relays in a substation.
AQ-E215
Instruction manual
Version: 2.01
79
© Arcteq Relays Ltd