ABB RELION REB670, Руководство по применению

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6.1.3.2 Differential protection
M12106-3 v4
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.
Consequently, all these phenomena have to be considered during the design stage of a
busbar differential IED in order to prevent the unwanted operation of the IED during
external fault conditions.
The analog generation of the busbar differential IEDs ( KA2, 87B, RADHA, RADSS,
REB 103) generally solves all these problems caused by the CT non-linear
characteristics by using the galvanic connection between the secondary circuits of all
1MRK 505 370-UUS A
Section 6
Differential protection
Busbar protection REB670 2.2 ANSI 111
Application manual