of busbar protection is increased, because there is now a dedicated, selective and fastbusbar protection functionality, which is based on the blockable overcurrentprotection principle. The additional time selective stages on the transformer HV- andLV-sides provide increased security degree of back-up protection for the transformer,busbar and also for the outgoing feeders.Depending on the overcurrent stage in question, the selectivity of the scheme in Figure63 is based on the operating current, operating time or blockings between successiveovercurrent stages. With blocking channels the operating time of the protection canbe drastically shortened, if compared to the simple time selective protection. Inaddition to the busbar protection, this blocking principle is applicable for theprotection of transformer LV terminals and short lines. The functionality andperformance of the proposed overcurrent protections can be summarized as seen inthe table.Table 59:Proposed functionality of numerical transformer and busbar over current protection.DT = definite time, IDMT = inverse definite minimum timeO/C-stage Operating char. Selectivity mode Operation speed SensitivityHV/3I> DT/IDMT time selective - + +HV/3I>> DT blockable/timeselective+/- +HV/3I>>> DT current selective + + -LV/3I> DT/IDMT time selective - + +LV/3I>> DT time selective - +LV/3I>>> DT blockable + +In case the bus-tie breaker is open, the operating time of the blockable overcurrentprotection is approximately 100 ms (relaying time). When the bus-tie breaker isclosed, that is, the fault current flows to the faulted section of the busbar from twodirections, the operation time becomes as follows: first the bus-tie relay unit trips thetie breaker in the above 100 ms, which reduces the fault current in to a half. After thisthe incoming feeder relay unit of the faulted bus section trips the breaker inapproximately 250 ms (relaying time), which becomes the total fault clearing time inthis case.Section 5 1MRS756378 DProtection functions136 REF615Application Manual
