9.4.2 Application
The directional residual overcurrent protection, four steps EF4PTOC (51N_67N) is used
in several applications in the power system. Some applications are:
• Ground-fault protection of feeders in effectively grounded distribution and
subtransmission systems. Normally these feeders have radial structure.
• Back-up ground-fault protection of transmission lines.
• Sensitive ground-fault protection of transmission lines. EF4PTOC (51N_67N) can
have better sensitivity to detect resistive phase-to-ground-faults compared to distance
protection.
• Back-up ground-fault protection of power transformers.
• Ground-fault 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. EF4PTOC (51N_67N) can have up to four, individual settable steps. The
flexibility of each step of EF4PTOC (51N_67N) 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 ground-fault protection in
meshed and effectively grounded transmission systems. The directional residual
overcurrent protection is also well suited to operate in teleprotection communication
schemes, which enables fast clearance of ground faults on transmission lines. The
directional function uses the polarizing quantity as decided by setting. Voltage polarizing
is the most commonly used, but alternatively current polarizing where currents in
transformer neutrals providing the neutral source (ZN) is used to polarize (IN · ZN) the
function. Dual polarizing, where the sum of both voltage and current components is
allowed to polarize can also be selected.
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 operate time of the different protections. To enable optimal co-
ordination all overcurrent protections, to be co-ordinated against each other, should have
the same time characteristic. Therefore a wide range of standardized inverse time
characteristics are available for IEC and ANSI.
Section 9 1MRK 506 369-UUS -
Current protection
508 Line distance protection REL670 2.2 ANSI
Application manual
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