Functions
2.9 Ground Fault Protection 50N(s), 51N(s)
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
147
2.9
Ground Fault Protection 50N(s), 51N(s)
Depending on the variant, the fourth current input of the multi-functional protection relays 7SJ61 is equipped
either with a sensitive input transformer or a standard transformer for 1/5 A.
In the first case, the protective function is designed for ground fault detection in isolated or compensated
systems due to its high sensitivity. It is not really 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.
• In solidly or low-resistance grounded systems, sensitive ground fault detection is used to detect high imped-
ance ground faults.
• This function can also be used as supplementary ground fault protection.
2.9.1
Current Elements 50Ns, 51Ns
The current elements for ground faults operate with the magnitudes of the ground current. They only make
sense where the magnitude of the ground current can be used to specify the ground fault. This may be the case
on grounded systems (solid or low-resistance) or on electrical machines which are directly connected to the
busbar of an isolated power system, when in case of a network ground fault the machine supplies only a neg-
ligible ground fault current across the measurement location, which must be situated between the machine ter-
minals and the network, whereas in case of a machine ground fault the higher ground fault current produced
by the total network is available. Ground current protection is mostly used as backup protection for high resis-
tance ground faults in solid or low resistance grounded systems when main fault protection does not pickup.
For ground fault detection, a two-step current/time characteristic can be set. Analogously to the time overcur-
rent protection, the high-set current element is designated as
50Ns-2 PICKUP
and
50Ns-2 DELAY
and is
provided with a definite time characteristic. The overcurrent element may be operated with either a definite time
delay (
50Ns-1 PICKUP
and
50Ns-1 DELAY
) or with a user-defined characteristic (
51Ns PICKUP
and
51NsTIME DIAL
). Additionally, a current element with logarithmic inverse characteristic or logarithmic inverse
characteristic with knee point is implemented. The characteristics of these current elements can be configured.
Settable Dropout Times
The pickup can be stabilized for ground fault protection with definite time curve by a settable dropout time. This
facility is used in power systems with intermittent faults. Used together with electromechanical relays, it allows
different dropout responses to be adjusted and time grading of digital and electromechanical relays to be im-
plemented.