ABB REB670 Applications Manual page 111

1MRK 505 337-UUS -
Application manual
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.
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 (that is, 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 CTs
connected to the protected zone. These IEDs are designed in such a way that the current
distribution through the IED differential branch during all transient conditions caused by
non-linearity of the CTs will not cause the unwanted operation of the differential IED. In
order to obtain the required secondary CT current distribution, the resistive burden in the
individual CT secondary circuits must be kept below the pre-calculated value in order to
guaranty the stability of the IED.
Section 6
Differential protection
105