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1MRK 505 370-UEN D
6.1.3
Busbar protection applications
A busbar protection scheme design depends very much on the substation arrangement.
Complexity of the scheme can drastically vary from station to station. Typical applications
problems, for the most common busbar protection schemes, are described in this chapter.
6.1.3.1
General
A busbar protection is a device which protects busbars against short-circuits and earth-faults.
In the early development of electricity systems, no separate protection device was used for
busbar protection. Remote end line protections were used as main protection for busbar
faults. With the increased short-circuit power in the network separate differential IEDs for
busbar protection have to be installed in order to limit the damage caused by the primary fault
currents. At the same time, it is also a must to secure the network stability, as a delayed
tripping for busbar faults can also lead to network instability, pole slip of near-by generators
and even total system collapse.
For bus zone protection applications, it is extremely important to have good security since an
unwanted operation might have severe consequences. The unwanted operation of the bus
differential IED will have the similar effect from the operational point of view as simultaneous
faults on all power system elements connected to the bus. On the other hand, the IED has to
be dependable as well. Failure to operate or even slow operation of the differential IED, in case
of an actual internal fault, can have serious consequences. Human injuries, power system
blackout, transient instability or considerable damage to the surrounding substation
equipment and the close-by generators are some of the possible outcomes.
Therefore, Busbar differential protection must fulfill the following requirements:
1.
Must be absolutely stable during all external faults. External faults are much more
common than internal faults. The magnitude of external faults can be equal to the
stations maximum short circuit capacity. Heavy CT-saturation due to high DC components
and/or remanence at external faults must not lead to maloperation of the busbar
differential protection. The security against misoperation must be extremely high due to
the heavy impact on the overall network service.
2.
Must have as short tripping time as possible in order to minimize the damage, minimize
the danger and possible injury to the people who might be working in the station at the
moment of internal fault, and secure the network stability.
3.
Must be able to detect and securely operate for internal faults even with heavy CT
saturation. The protection must also be sensitive enough to operate for minimum fault
currents, which sometimes can be lower than the maximum load currents.
4.
Must be able to selectively detect faults and trip only the faulty part of the busbar system.
5.
Must be secure against maloperation due to auxiliary contact failure, possible human
mistakes and faults in the secondary circuits and so on.
6.1.3.2
Differential protection
The basic concept for any differential IED is that the sum of all currents, which flow to and
from the protection zone, must be equal to zero. If this is not the case, an internal fault has
occurred. This is practically a direct use of well known Kirchhoffss first law. However, busbar
differential IEDs do not measure directly the primary currents in the high voltage conductors,
but the secondary currents of magnetic core current transformers (CTs), which are installed in
all high-voltage bays connected to the busbar.
Therefore, the busbar differential IED is unique in this respect, that usually quite a few CTs,
often with very different ratios and classes, are connected to the same differential protection
zone. Because the magnetic core current transformers are non-linear measuring devices, under
high current conditions in the primary CT circuits the individual secondary CT currents can be
drastically different from the original primary currents. This is caused by CT saturation, a
Busbar protection REB670
Application manual
Section 6
Differential protection
SEMOD127456-1 v2
M12102-3 v4
M12103-3 v3
M12106-3 v4
93
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