ABB REL650 Applications Manual page 209

1MRK 506 334-UUS A
Application manual
• Ground-fault protection of different kinds of equipment connected to the power
system such as shunt capacitor banks, shunt reactors and others.
• Negative sequence direcitonal ground-fault protection of feeders with PTs connected
in Open Delta connection from which it is not possible to derive Zero sequence
voltage.
• Negative sequence directional ground-fault protection of double-circuit medium or
long transmission lines with significant mutual coupling.
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
(3V or V ) is most commonly used, but alternatively current polarizing (3I
0 2
currents in transformer neutrals providing the neutral (zero sequence) source (ZN) is used
to polarize (IPol · 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: IEC and ANSI. The time characteristic for step 1 and 4 can
be chosen as definite time delay or inverse time characteristic. Step 2 and 3 are always
definite time delayed and are used in system where IDMT is not needed.
Table 11: 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
Section 7
Current protection
or I ) where
0 2
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