Distinctive Features Of Busbar Protection Schemes; Differential Protection - ABB REB670 Applications Manual

Section 6

Differential protection

6.1.3.2
6.1.3.3
104
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.

Distinctive features of busbar protection schemes

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.
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,
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