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2.12 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
200
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
2.12 Ground Fault Protection 64, 67N(s), 50N(s), 51N(s)
Depending on the variant, the fourth current input of the multi-functional protection
relays 7SJ62/63/64 is equipped either with a sensitive input transformer or a standard
transformer for 1/5 A.
In the first case, the active protective function is designed for ground fault detection in
isolated or compensated systems due to its high sensitivity. It is not very suited for
ground fault detection with large ground currents since the linear range is transcended
at about 1.5 A at the sensitive ground fault detection relay terminals.
If the relay is equipped with standard transformers for 1/5 A, also large currents can
be detected correctly.
Applications • Sensitive ground fault detection may be used in isolated or compensated systems
to detect ground faults, to determine phases affected by ground faults, and to
specify the direction of ground faults.
• In solidly or low-resistance grounded systems, sensitive ground fault detection is
used to detect high impedance ground faults.
• This function can also be used as supplementary ground fault protection.
2.12.1 Voltage Element 64
The voltage element relies on a pickup initiated by the displacement voltage V
0
or 3 ·
V
0
. Additionally, the faulty phase is determined. The displacement voltage V
0
can be
directly applied to the device, or the summary voltage 3 · V
0
can be calculated by the
device based on the three phase–to–ground voltages. In the latter case, the three
voltage inputs must be connected to voltage transformers in a grounded-wye configu-
ration (see also address
213 VT Connect. 3ph
in Section 2.1.3). If the device is
only provided with phase-to-phase voltages, it is not possible to calculate a displace-
ment voltage from them. In this case the direction cannot be determined.
If the displacement voltage is calculated, then:
3 · V
0
= V
A
+ V
B
+ V
C
If the displacement voltage is directly applied to the device, then V
0
is the voltage at
the device terminals. It is not affected by parameter
Vph / Vdelta
(address
206
).
The displacement voltage is used both to detect a ground fault and to determine direc-
tion. When the voltage element picks up, a preset time delay must elapse before de-
tection of the displacement voltage is reported to ensure measurement free quantities.
The time delay can be configured (
T-DELAY Pickup
) and its factory setting is 1 s.
Pickup initiated by the displacement voltage can be delayed (
64-1 DELAY
) for trip-
ping.
It is important to note that the total tripping time consists of the displacement voltage
measurement time (about 60 ms) plus the pickup time delay (address
3111 T-DELAY
Pickup
) plus the tripping time delay (address
3112 64-1 DELAY
).