GE B30 Instruction Manual page 237

5 SETTINGS
setting. This setting should be set above the maximum load current. The
LOW SLOPE
AC current under which all the CTs are guaranteed to transform without saturation. This includes the effect of residual
magnetism. When adjusting this setting, it must be kept in mind that the restraining signal is created as the maximum
of all the input currents.
• BUS ZONE 1 DIFF HIGH SLOPE: This setting defines the percentage bias for the restraining currents above the
higher breakpoint (HIGH BPNT). This setting affects stability of the relay for heavy external faults. Traditionally, the
value chosen for this setting should be high enough to accommodate the spurious differential current resulting from
saturation of the CTs during heavy external faults. This requirement may be considerably relaxed in favor of sensitivity
and speed of operation as the relay detects CT saturation and upon detection applies the directional principle to pre-
vent maloperation. When adjusting this setting, it must be kept in mind that the restraining signal is created as the max-
imum of all the input currents.
• BUS ZONE 1 DIFF HIGH BPNT: This setting defines the higher breakpoint of the dual-slope operating characteristic.
The percentage bias applied for the restraining current above the value specified as
setting. The
SLOPE HIGH BPNT
est CT feeding the relay. When adjusting this setting, it must be kept in that mind the restraining signal is created as the
maximum of all the input currents.
The dual-slope operating characteristic of the biased bus differential protection element is shaped to ensure true per-
centage bias for high restraining currents (see the following figure). This means that the straight line defining the upper
slope intersects the origin of the differential-restraining plane and a discontinuity appears between the low and high
slope regions (between the
operate/no-operate boundary of the characteristic using a certain "gluing" function. This ensures smooth transition of
the slope from
LOW SLOPE
The following parameters of the biased operating characteristic are used by the saturation detector:
, and . The saturation detector uses these settings to detect specific relations between the differential
SLOPE HIGH BPNT
and restraining currents. The values of these settings should be selected based on the aforementioned criteria related
to the art of bus differential protection.
• BUS ZONE 1 DIFF HIGH SET: This setting defines the minimum differential current required for operation of the unbi-
ased bus differential protection function. This setting is based on the maximum magnitude of the differential current
that might be seen during heavy external faults causing deep CT saturation. When selecting this setting, keep in mind
that the unbiased bus differential protection function uses the full-cycle Fourier measuring algorithm and applies it to
pre-filtered samples of the input currents. As a result, the transient measuring errors including the effect of the DC
component are below 2%. During heavy CT saturation when the currents are significantly distorted, the magnitude of
the differential current as measured by the relay and used by the unbiased bus differential function is significantly lower
than both the peak values of the waveform and the true RMS value. The measured magnitude practically reflects the
power system frequency component alone. This allows for lower values of the
The unbiased (high set) differential function can be virtually disabled by setting its operating threshold,
high.
• BUS ZONE 1 DIFF SEAL-IN: This setting defines the drop-out time of the seal-in timer applied to the
Logic™ operand.
More information on the bus zone differential settings can be found in the Application of settings chapter.
GE Multilin
setting should be set below the minimum AC current that is likely to saturate the weak-
and
LOW BPNT HIGH BPNT
(lower value) to (higher value).
HIGH SLOPE
B30 Bus Differential System
settings). This discontinuity is handled by approximating the
HIGH SET
5.6 GROUPED ELEMENTS
may be moved to the
LOW BPNT
is given by the
HIGH BPNT HIGH
,
LOW SLOPE HIGH
setting.
, very
HIGH SET
Flex-
BUS 1 OP
5-107
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