•
Ground-fault and phase-phase short circuit protection of feeders in effectively
grounded distribution and subtransmission systems. Normally these feeders have
radial structure.
•
Back-up ground-fault and phase-phase short circuit protection of transmission
lines.
•
Sensitive ground-fault protection of transmission lines. NS4PTOC (4612) can
have better sensitivity to detect resistive phase-to-ground-faults compared to
distance protection.
•
Back-up ground-fault and phase-phase short circuit protection of power
transformers.
•
Ground-fault and phase-phase short circuit protection of different kinds of
equipment connected to the power system such as shunt capacitor banks, shunt
reactors and others.
In many applications several steps with different current pickup levels and time delays
are needed. NS4PTOC (4612) can have up to four, individual settable steps. The
flexibility of each step of NS4PTOC (4612) function is great. The following options
are possible:
Non-directional/Directional function: In some applications the non-directional
functionality is used. This is mostly the case when no fault current can be fed from the
protected object itself. In order to achieve both selectivity and fast fault clearance, the
directional function can be necessary. This can be the case for unsymmetrical fault
protection in meshed and effectively grounded transmission systems. The directional
negative sequence overcurrent protection is also well suited to operate in teleprotection
communication schemes, which enables fast clearance of unsymmetrical faults on
transmission lines. The directional function uses the voltage polarizing quantity.
Choice of time characteristics: There are several types of time characteristics available
such as definite time delay and different types of inverse time characteristics. The
selectivity between different overcurrent protections is normally enabled by co-
ordination between the operating time of the different protections. To enable optimal
co-ordination all overcurrent relays, to be co-ordinated against each other, should have
the same time characteristic. Therefore a wide range of standardized inverse time
characteristics are available: IEC and ANSI.
Table 30:
Inverse time characteristics
Curve name
ANSI Extremely Inverse
ANSI Very Inverse
ANSI Normal Inverse
ANSI Moderately Inverse
ANSI/IEEE Definite time
Table continues on next page
1MRK 505 370-UUS A
Section 7
Current protection
Busbar protection REB670 2.2 ANSI
205
Application manual
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