ABB Relion 670 Series Applications Manual page 150

Section 8
Current protection
transmission systems. The directional residual overcurrent protection is also well suited to
operate in teleprotection communication schemes, which enables fast clearance of earth
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.
Table 21:
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 or RD (logarithmic)
It is also possible to tailor make the inverse time characteristic.
Normally it is required that EF4PTOC 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 operating level
for some time. Therefore, there is a possibility to give a setting of a multiplication factor
INxMult to the residual current pick-up level. This multiplication factor is activated from a
binary input signal ENMULTx to the function.
Power transformers can have a large inrush current, when being energized. This inrush current
can produce residual current component. 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
144
Time characteristics
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