ABB Relion 670 Series Applications Manual page 100

Section 6
Differential protection
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 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, since 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 505 370-UUS Rev. K
M12106-3 v5
Busbar protection REB670
Application manual