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 36: 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
User Programmable
ASEA RI
RXIDG (logarithmic)
There is also a user programmable inverse time characteristic.
Normally it is required that the negative sequence overcurrent function shall reset as fast
as possible when the current level gets lower than the operation level. In some cases some
sort of delayed reset is required. Therefore different kinds of reset characteristics can be
used.
For some protection applications there can be a need to change the current pickup level for
some time. Therefore there is a possibility to give a setting of a multiplication factor
1MRK 506 369-UUS - Section 9
Current protection
Line distance protection REL670 2.2 ANSI 523
Application manual
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