GE B90 Instruction Manual

Also see for B90: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

GE Multilin B90 Low Impedance Bus Differential System 5-875 SETTINGS 5.5 GROUPED ELEMENTS5it must be kept in mind that the restraining signal used by the biased bus differential protection element is created asthe maximum of all the input currents.• BUS ZONE 1 DIFF LOW BPNT: This setting defines the lower breakpoint of the dual-slope operating characteristic.The percentage bias applied for the restraining current from zero to the value specified as LOW BPNT is given by theLOW SLOPE setting. This setting should be set above the maximum load current. The LOW BPNT may be moved to theAC current under which all the CTs are guaranteed to transform without saturation. This includes the effect of residualmagnetism. When adjusting this setting, it must be kept in mind that the restraining signal is created as the maximumof all the input currents.• BUS ZONE 1 DIFF HIGH SLOPE: This setting defines the percentage bias for the restraining currents above thehigher breakpoint (HIGH BPNT). This setting affects stability of the relay for heavy external faults. Traditionally, thevalue chosen for this setting should be high enough to accommodate the spurious differential current resulting fromsaturation of the CTs during heavy external faults. This requirement may be considerably relaxed in favor of sensitivityand 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 HIGH BPNT is given by the HIGHSLOPE setting. The HIGH BPNT setting should be set below the minimum AC current that is likely to saturate the weak-est CT feeding the relay. When adjusting this setting, it must be kept in that mind the restraining signal is created as themaximum 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 upperslope intersects the origin of the differential-restraining plane and a discontinuity appears between the low and highslope regions (between the LOW BPNT and HIGH BPNT settings). This discontinuity is handled by approximating theoperate/no-operate boundary of the characteristic using a certain “gluing” function. This ensures smooth transition ofthe slope from LOW SLOPE (lower value) to HIGH SLOPE (higher value).The following parameters of the biased operating characteristic are used by the saturation detector: LOW SLOPE, HIGHSLOPE, and HIGH BPNT. The saturation detector uses these settings to detect specific relations between the differentialand restraining currents. The values of these settings should be selected based on the aforementioned criteria relatedto 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 currentthat might be seen during heavy external faults causing deep CT saturation. When selecting this setting, keep in mindthat the unbiased bus differential protection function uses the full-cycle Fourier measuring algorithm and applies it topre-filtered samples of the input currents. As a result, the transient measuring errors including the effect of the DCcomponent are below 2%. During heavy CT saturation when the currents are significantly distorted, the magnitude ofthe differential current as measured by the relay and used by the unbiased bus differential function is significantly lowerthan both the peak values of the waveform and the true RMS value. The measured magnitude practically reflects thepower system frequency component alone. This allows for lower values of the HIGH SET setting.The unbiased (high set) differential function can be virtually disabled by setting its operating threshold, HIGH SET, veryhigh.• BUS ZONE 1 DIFF SEAL-IN: This setting defines the drop-out time of the seal-in timer applied to the BUS 1 OP Flex-Logic™ operand.• BUS ZONE 1 DIFF SUPV: This setting specifies a FlexLogic™ operand that supervises operation of the element. Typ-ical applications include undervoltage condition and overall (check) zone. In a sense the setting is similar to the block-ing input, but does not stop the element from running. This is important for saturation detector, as the latter requiressome historical data. The Bus Differential element shall not be supervised from the blocking input. The blocking input(BUS ZONE 1 DIF BLK setting) is meant to block the element permanently, not dynamically from a fault depended condi-tion such as undervoltage.• BUS ZONE 1 DIFF TRIP: This setting specifies a FlexLogic™ operand that forces operation of the zone. Typical appli-cations include forced trip of the healthy phase from the faulted phase, or a trip command from the BF function to iso-late the entire zone of busbar protection.More information on the bus zone differential settings can be found in the Application of settings chapter.