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 40: Inverse time characteristics
Curve name
ANSI Extremely Inverse
ANSI Very Inverse
ANSI Normal Inverse
ANSI Moderately Inverse
ANSI/IEEE Definite time
ANSI Long Time Extremely Inverse
ANSI Long Time Very Inverse
ANSI Long Time Inverse
IEC Normal Inverse
IEC Very Inverse
IEC Inverse
IEC Extremely Inverse
IEC Short Time Inverse
IEC Long Time Inverse
IEC Definite Time
Table continues on next page
Section 9 1MRK 504 163-UUS A
Current protection
550 Transformer protection RET670 2.2 ANSI
Application manual
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