ABB Relion 670 Series Applications Manual page 169

1MRK 505 370-UUS Rev. K
The maximum load current on the line has to be estimated. From operation statistics the load current up
to the present situation can be found. The current setting must be valid also for some years ahead. It is,
in most cases, realistic that the setting values are updated not more often than once every five years. In
many cases this time interval is still longer. Investigate the maximum load current that different
equipment on the line can withstand. Study components such as line conductors, current transformers,
circuit breakers, and disconnectors. The manufacturer of the equipment normally gives the maximum
thermal load current of the equipment.
There is also a demand that all faults, within the zone that the protection shall cover, must be detected by
the phase overcurrent protection. The minimum fault current I
be calculated. Taking this value as a base, the highest pick up current setting can be written according to
equation 35.
£
Ipu 0.7 Isc min
EQUATION1263 V2 EN-US
where:
0.7
I
scmin
As a summary the pick up current shall be chosen within the interval stated in equation 36.
Im ax
×
1.2
k
EQUATION1264 V2 EN-US
The high current function of the overcurrent protection, which only has a short delay of the operation,
must be given a current setting so that the protection is selective to other protection in the power system.
It is desirable to have a rapid tripping of faults within as large portion as possible of the part of the power
system to be protected by the protection (primary protected zone). A fault current calculation gives the
largest current of faults, I
have to be made to the risk of transient overreach, due to a possible DC component of the short circuit
current. The lowest current setting of the most rapid stage, of the phase overcurrent protection, can be
written according to
³
I 1.2
high
EQUATION1265 V1 EN-US
where:
1.2
k
t
I
scmax
The operate times of the phase overcurrent protection has to be chosen so that the fault time is so short
so that equipment will not be destroyed due to thermal overload, at the same time as selectivity is
assured. For overcurrent protection, in a radial fed network, the time setting can be chosen in a graphical
way. This is mostly used in the case of inverse time overcurrent protection. In the figure below is shown
how the time-versus-current curves are plotted in a diagram. The time setting is chosen to get the
Busbar protection REB670
Application manual
×
is a safety factor and
is the smallest fault current to be detected by the overcurrent protection.
£ £ ×
Ipu 0.7 Isc min
, at the most remote part of the primary protected zone. Considerations
scmax
× ×
k I
t sc max
is a safety factor,
is a factor that takes care of the transient overreach due to the DC component of the fault current and can be
considered to be less than 1.1
is the largest fault current at a fault at the most remote point of the primary protection zone.
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, to be detected by the protection, must
scmin
Section 7
Current protection
(Equation 35)
(Equation 36)
(Equation 37)
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