9-8 B90 Low Impedance Bus Differential System GE Multilin9.6 OUTPUT LOGIC AND EXAMPLES 9 THEORY OF OPERATION99.6OUTPUT LOGIC AND EXAMPLES 9.6.1 OUTPUT LOGICThe biased differential characteristic uses the output logic shown below.For low differential signals, the biased differential element operates on the 2-out-of-2 basis utilizing both the differential anddirectional principles.For high differential signals, the directional principle is included only if demanded by the saturation detector (dynamic 1-out-of-2 / 2-out-of-2 mode). Typically, the directional principle is slower, and by avoiding using it when possible, the B90 gainsspeed.The dynamic inclusion/exclusion of the directional principle is not applied for the low differential currents but is included per-manently only because it is comparatively difficult to reliably detect CT saturation occurring when the currents are small, i.e.saturation due to extremely long time constant of the DC component or due to multiple autoreclosure actions.Figure 9–8: OUTPUT LOGIC OF BIASED DIFFERENTIAL PROTECTION9.6.2 INTERNAL AND EXTERNAL FAULT EXAMPLETwo examples of relay operation are presented: an external fault with heavy CT saturation and an internal fault.The protected bus includes six circuits connected to CT terminals F1, F5, M1, M5, U1 and U5, respectively. The circuits F1,F5, M1, M5 and U5 are capable of feeding some fault current; the U1 circuit supplies a load. The F1, F5 and U5 circuits aresignificantly stronger than the F5 and M1 connections.The M5 circuit contains the weakest (most prone to saturation) CT of the bus.Figure 8-10 presents the bus currents and the most important logic signals for the case of an external fault. Despite veryfast and severe CT saturation, the B90 remains stable.Figure 8-11 presents the same signals but for the case of an internal fault. The B90 trips in 10 ms (fast form-C output con-tact).836730A1.CDRDIFLDIRSATDIFHORANDANDORbiased busdifferentialDIFBIASED
