Method Of Operation; Earth Fault Detection In Non-Earthed Systems - Siemens siprotec 7SA6 User Manual

6.11

Earth Fault Detection in Non-earthed Systems

6.11.1 Method of Operation

General
Pickup
Determination of
the Earth-faulted
Phase
7SA6 Manual
C53000-G1176-C133-1
In systems whose starpoint is either non-earthed or earthed through an arc
suppression coil (Petersen coil), single phase earth faults will not be detected by the
short circuit protection, since no significant earth fault current flows. Furthermore,
since network operation is not immediately affected by an earth fault (the voltage
triangle is maintained, Figure 6-88) rapid disconnection is not normally desired. It is
more important that the earth fault be recognized, indicated and, when possible,
localized also. After network reconfiguration it can then be cleared. But 7SA6 enables
the user to trip on directional earth fault in non-earthed systems.
U
L1
E
U
L3
a) Healthy network, without earth fault
Earth fault in non-earthed neutral network
Figure 6-88
Dependent upon the chosen model, a 7SA6 relay can be fitted with optional earth fault
detection module, which includes the following functions:
• Detection of an earth fault (pick-up) by monitoring the displacement voltage,
• Determination of the faulted phase by measuring the phase to earth voltages,
• Determination of the direction of the earth fault (residual) current by high accuracy
real and reactive component measurement.
The pickup is achieved when the settable threshold for the displacement voltage 3⋅U
is exceeded. To ensure measurement of stable values, all earth fault detection
functions are delayed until 1 second (settable) after inception of voltage displacement.
Further, each alteration of the earth fault conditions (e.g. altered direction) is
recognized only after this delay. Generally the pickup is only indicated if a fault was
detected for sure by the phase determination function (see next margin heading).
After recognition of displaced voltage conditions the first objective of the device is
selective detection of the earth-faulted phase. For this purpose the individual phase-
to-earth voltages are measured. The affected phase is the one in which the voltage is
below the settable threshold 8SKH PLQ when simultaneously the other two voltages
exceed an equally settable maximum threshold 8SKH PD[.
U
L2
U = 0
E
L1
U
U
L1–L2
L3–L1
U
L2–L3
b) Earth fault in phase L1
Functions
0
6-159