ABB RELION 670 Series Applications Manual page 102

Section 6
Differential protection
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 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.
6.1.3.3 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 (that is, 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
phenomenon that is well known to protection engineers. During the time when any of the current
transformer connected to the differential IED is saturated, the sum of all CT secondary currents
will not be equal to zero and the IED will measure false differential current. This phenomenon is
especially predominant for busbar differential protection applications, because it has the strong
tendency to cause unwanted operation of the differential IED.
Remanence in the magnetic core of a current transformer is an additional factor, which can
influence the secondary CT current. It can improve or reduce the capability of the current
transformer to properly transfer the primary current to the secondary side. However, the CT
remanence is a random parameter and it is not possible in practice to precisely predict it.
Another, and maybe less known, transient phenomenon appears in the CT secondary circuit at the
instant when a high primary current is interrupted. It is particularly dominant if the HV circuit
breaker chops the primary current before its natural zero crossing. This phenomenon is
manifested as an exponentially decaying dc current component in the CT secondary circuit. This
secondary dc current has no corresponding primary current in the power system. The
phenomenon can be simply explained as a discharge of the magnetic energy stored in the
magnetic core of the current transformer during the high primary current condition. Depending on
the type and design of the current transformer this discharging current can have a time constant
in the order of a hundred milliseconds.
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1MRK 505 337-UUS A
M12105-3 v1
M12106-3 v3
Application manual