Figure 5–42: BIASED DIFFERENTIAL OPERATING CHARACTERISTIC
The protected zone input current with the highest magnitude is used as the restraining signal. Stability during heavy exter-
nal faults is achieved by dynamic CT saturation detection and current flow direction supervision without affecting sensitivity
and speed of operation for internal faults.
The differential operating characteristic is divided into two regions. In the region of low differential currents and lower slope,
the element operates on a 2-out-of-2 basis, applying both the differential and current directional tests. In the region of high
differential currents, the element operates on a dynamic 1-out-of-2 / 2-out-of-2 basis. If the differential current is in this
region and CT saturation is detected, both the differential and current directional tests are applied. If CT saturation is ruled
out by the saturation detector, the differential protection principle alone is capable of causing the element to operate.
The saturation detector is an integral part of the bus differential element. It has no settings but uses some of the differential
characteristic parameters. The flags indicating CT saturation are available on a per phase basis as FlexLogic operands.
The directional principle is an integral part of the biased bus differential element and has no associated settings. The direc-
tional element dynamically identifies what appears to be the faulted circuit and compares its current angle with that of the
sum of the remaining currents of the protected zone. The element declares a bus fault if the angle is less than 90°. Direc-
tional indicating flags signal operation on a per-phase basis and are available as FlexLogic operands.
The unbiased bus differential function checks the magnitude of the differential current against an adjustable threshold. Nei-
ther the bias nor the directional principles apply. The operation of the unbiased differential function is associated with sepa-
rate output operands. More information can be found in the Theory of Operation chapter.
The bus differential protection feature is available only when
Operation of this element is completely dependent on the dynamic bus replica which must be first defined under bus zone
1. The bus differential element 1 protects the differential zone defined as bus zone 1.
BUS ZONE 1 DIFF PICKUP: This setting defines the minimum differential current required for operation of the biased
bus differential protection element. This setting is chosen based on the maximum magnitude of the differential current
that might be seen under no-load conditions. This setting prevents relay maloperation in the situation when the bus
carries little power and the restraining signal is too low to provide enough bias in the first slope region of the differential
This setting may also be set above the maximum load level to ensure security during CT trouble conditions. However,
voltage supervision or a check-zone are better alternatives.
BUS ZONE 1 DIFF LOW SLOPE: This setting defines the percentage bias for the restraining currents, from zero to the
lower breakpoint (LOW BPNT). This setting determines the sensitivity of the relay for low current internal faults. The
value chosen should be high enough to accommodate the spurious differential current resulting from inaccuracy of the
CTs operating in their linear mode, i.e. in load conditions and during distant external faults. When adjusting this setting,
it must be kept in mind that the restraining signal used by the biased bus differential protection element is created as
the maximum of all the input currents.
B90 Low Impedance Bus Differential System
5.5 GROUPED ELEMENTS
is set to