Intermittent Ground Fault Protection; Functional Description - Siemens SIPROTEC 4 Manual

Functions

2.13 Intermittent Ground Fault Protection

2.13 Intermittent Ground Fault Protection
A typical characteristic of intermittent ground faults is that they often disappear automatically 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 7SJ80 feature a protective function that is able to detect such intermittent ground faults and accumu-
lates their duration. If the sum reaches a configurable value within a certain time, the limit of the thermal load
capacity 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 expected to cool down. Tripping
is not necessary in this case.
Applications
•
Protection from intermittent ground faults which occur, e.g. in cables due to poor insulation or water
ingress in cable joints.

Functional Description

2.13.1
Acquisition of Measured Quantities
The intermittent ground fault can either be detected via the ordinary ground current input (Ι
ground current input (Ι
current protection which uses the fundamental wave, the intermittent ground fault sprotection 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.
Pickup/Tripping
If the pickup value Iie> is exceeded, the pickup indication
The pickups are also counted; when the counter content reaches the value of parameter Nos.det., the indi-
cation Intermitt.EF is output. A stabilized pickup is obtained by prolonging the pickup indication IIE
Fault det by a settable time T-det.ext.. This stabilization is especially important for the coordination
with existing static or electromechanical overcurrent relays.
The duration of the stabilized pickups
accumulated pickup time reaches a settable threshold value, a corresponding message is generated (
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-reset (message
sum det. 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 are reset and the protection resumes normal position. T-reset thus determines the time during
which the next ground fault must occur to be processed yet as intermittent ground fault in connection with
the previous fault. A ground fault that occurs later will be considered a new fault event.
The message IIE Fault det will be entered in the fault log and reported to the system interface only until
the message Intermitt.EF is issued. This prevents a burst of messages. If the message is allocated to an
LED or a relay, this limitation does not apply. This is accomplished by doubling the message (message
numbers 6924, 6926).
198
), or it is calculated from the sum of the three phase currents (3Ι0). Unlike the over-
N
IIE stab.Flt is summated with an integrator T-sum det.. If the
IIE Fault det (see
IEF Tres run. ) is launched simultaneously with T-
E50417-G1140-C343-A8, Edition 12.2017
), the sensitive
N
Figure 2-86) is generated.
IEF
SIPROTEC 4, 7SJ80, Manual