Intermittent Ground Fault Protection; Description - Siemens siprotec SJ62 User Manual

2.13

Intermittent Ground Fault Protection

Applications

2.13.1 Description

Acquisition of Mea-
sured Quantities
Pickup/Tripping
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
A typical characteristic of intermittent ground faults is that they often disappear auto-
matically to strike again after some time. They can last between a few milliseconds
and several seconds. This is why such faults are not detected at all or not selectively
by the ordinary time overcurrent protection. If pulse durations are extremely short, not
all protection devices in a short-circuit path may pick up; selective tripping is thus not
ensured.
Due to the time delay of the overcurrent protection function such faults are too short
to initiate shutdown of the faulted cable. Only when they have become permanent
such ground faults can be removed selectively by the short-circuit protection.
But such intermittent ground faults already bear the risk of causing thermal damage to
equipment. This is why devices 7SJ62/63/64 feature a protective function that is able
to detect such intermittent ground faults and accumulates their duration. If within a
certain time their sum reaches a settable value, the thermal load limit has been
reached. If the ground faults are distributed over a long period of time or if the ground
fault goes off and does not re-ignite after some time, the equipment under load is ex-
pected to cool down. Tripping is not necessary in this case.
• Protection from intermittent ground faults which occur, e.g. in cables due to poor in-
sulation or water ingress in cable joints.
The intermittent ground fault can either be detected via the ordinary ground current
input (I ), the sensitive ground current input (I
N
the three phase currents (3 I0). Unlike the overcurrent protection which uses the fun-
damental wave, the intermittent ground fault protection creates the r.m.s. value of this
current and compares it to a settable threshold Iie>. This method accounts for higher
order harmonics contents (up to 400 Hz) and for the direct component since both
factors contribute to the thermal load.
When the pickup threshold Iie> is exceeded, a pickup message („IIE Fault
det", see Figure 2-79) is issued. The pickups are also counted; as soon as the
counter content has reached the value of parameter Nos.det., the message
„Intermitt.EF" is issued. A stabilized pickup is obtained by prolonging the pickup
message „IIE Fault det" by a settable time T-det.ext.. This stabilization is
especially important for the coordination with existing static or electromechanical over-
current relays.
The duration of the stabilized pickups „IIE stab.Flt" is summated with an inte-
grator T-sum det.. If the accumulated pickup time reaches a settable threshold
value, a corresponding message is generated („IEF Tsum exp."). Tripping takes
place, however, only while a ground fault is present (message „IEF Trip"). The trip
command is maintained during the entire minimum tripping time specified for the
device, even if the ground fault is of short duration. After completion of the tripping
command all memories are reset and the protection resumes normal condition.
The (much longer) resetting time T-sum det. (message T-reset) is launched si-
multaneously with „IEF Tres run." when a ground fault occurs. Unlike T-sum
det. each new ground fault resets this time to its initial value and it expires anew. If
T-reset expires and no new ground fault is recorded during that time, all memories
2.13 Intermittent Ground Fault Protection
), or it is calculated from the sum of
NS
220