GE B90 Instruction Manual

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

Contents

8-6 B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALSLOPES AND HIGH SET THRESHOLD CHAPTER 8: APPLICATION OF SETTINGS8Table 8-3: Limits of linear operation of the CTSThe third and fourth columns of the table have the following significance.If an external fault occurs on circuit C-1, then CT-1 carries the fault current. As the fault current is higher than any of theother currents, the current supplied by CT-1 is used as the restraint signal. CT-1 is guaranteed to saturate if the currentexceeds 89.55 A secondary, or 17.9 times its rated current, or 8.96 pu of the bus differential zone. Consequently,considering CT-1, the value of 8.96 pu is used as the higher breakpoint of the characteristic.Considering CTs that can be connected (depending on the positions of the switches) to the North bus, the HIGH BPNT for theNorth bus zone is selected as the minimum of (8.96, 9.13, 31.17, 22.88, 24.57, 31.17), or 8.96 pu.Considering CTs that can be connected (depending on the positions of the switches) to the South bus, the HIGH BPNT for theSouth bus zone is selected as the minimum of (9.13, 31.17, 22.88, 24.57, 31.17), or 9.13 pu.8.3.3 Low breakpointThe DC component in the primary current can saturate a given CT even with the AC current below the suggested value ofthe higher breakpoint. The relay copes with this threat by using the Saturation Detector and applying a 2-out-of-2operating principle upon detecting saturation.The residual magnetism (remanence) left in the core of a CT can limit the linear operation of the CT significantly. It isjustified to assume that the residual flux can be as high as 80% of the saturation level leaving only 20% to accommodatethe flux component created by the primary current. This phenomenon can be reflected by reducing the saturation voltagein the calculations by the factor of 100% / 20%, or 5. This, in turn, is equivalent to reducing the limit of linear operation bythe factor of 5, hence the last column in the Limits of Linear Operations of the CTs table.For example, if the residual flux left in the core of the CT-1 is as high as 80% of its saturation level, the CT saturates at 17.92A secondary, or 3.58 times its rated current, or at 1.79 pu of the bus differential zone.The reduced limit of linear operation is used as the lower breakpoint of the biased differential characteristic (the LOW BPNTsetting). In this way the interval spanning from the lower to higher breakpoints covers the indistinct area of possiblesaturation due to the random factor of residual magnetism.The LOW BPNT is set at 1.79 pu for the North bus zone, and at 1.83 pu for the South bus zone.A combination of very high residual magnetism and a DC component with a long time constant can saturate a given CTeven with the AC current below the suggested value of the lower breakpoint. The relay copes with this threat by using a 2-out-of-2 operating mode for low differential currents.8.4 Slopes and high set threshold8.4.1 DescriptionTo set the higher slope and threshold of the high set (unbiased) differential operation, external faults must be analyzed.Consider an external fault for the North bus relay. It is justified to assume bus configurations that give maximum stress tothe maximum number of CTs. For this purpose we assume the tie breaker, B-7 closed; all the circuitry capable of supplyingthe fault current to be in service; moreover, they are connected to the South bus in order to analyze the CT-7 and CT-8carrying the fault current.CT Rs (Ω) I max (A sec) I max (pu)(no remanence)I max (pu)(80% remanence)CT-1 1.61 89.55 8.96 1.79CT-2 1.58 91.25 9.13 1.83CT-3 1.85 155.84 31.17 6.23CT-4 1.75 137.30 22.88 4.58CT-5, CT-6 1.63 147.42 24.57 4.91CT-7, CT-8 1.85 155.84 31.17 6.23