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 17: 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
ASEA RI
RXIDG (logarithmic)
Power transformers can have a large inrush current, when being energized. This
inrush current can have residual current components. The phenomenon is due to
saturation of the transformer magnetic core during parts of the cycle. There is a risk
that inrush current will give a residual current that reaches level above the operating
current of the residual overcurrent protection. The inrush current has a large second
harmonic content. This can be used to avoid unwanted operation of the protection.
Therefore, EF4PTOC has a possibility of second harmonic restrain 2ndHarmStab if
the level of this harmonic current reaches a value above a set percentage of the
fundamental current.
1MRK 505 291-UEN A Section 6
Current protection
117
Application manual
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