ABB Relion 670 Series Applications Manual page 162

Section 8
Current protection
8.5.3 Application
Four step negative sequence overcurrent protection NS4PTOC is used in several applications
in the power system. Some applications are:
• Earth-fault and phase-phase short circuit protection of feeders in effectively earthed
distribution and subtransmission systems. Normally these feeders have a radial structure.
• Back-up earth-fault and phase-phase short circuit protection of transmission lines.
• Sensitive earth-fault protection of transmission lines. NS4PTOC can have better
sensitivity to detect resistive phase-to-earth-faults compared to distance protection.
• Back-up earth-fault and phase-phase short circuit protection of power transformers.
• Earth-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 operating levels and time delays are
needed. NS4PTOC can have up to four, individual settable steps. The flexibility of each step of
NS4PTOC 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
earthed 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 22:
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
156
Inverse time characteristics
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