As part of the analysis, you will have to consider faults at all points on the network, for both maximum and minimum fault conditions. The overcurrent relays must grade properly between themselves and all fuses under all fault conditions. Normally, you would use “standard inverse” on the overcurrent relays on the 33/6.6kV transformers and on the 6.6kV side of both distribution substation transformers, but you may wish to consider the ”extremely inverse” curve type for the low voltage sides of the distribution substations, to improve grading with the downstream fuses. When calculating faults, assume the transformers generator and motors are purely inductive. In particular, the following grading must occur- • Between the 6.6 kV overcurrent and 33kV overcurrent on the main supply substation transformers, for faults on the main 6.6 kV busbar, for both one and two transformers in service and with or without the local generator in service or the synchronous motor (i.e. between relays ‘R1’ and ‘R2’),
ELEC 5204 Mid-semester Assignment , 2011 P 4 of 6 • Between the 6.6 kV overcurrent and LV overcurrent on both the local distribution and VSD substation transformers (i.e. between relays ‘R4’ and ‘R7’ and relays R6 and R8 ), for faults anywhere on the low voltage systems (AC or DC sections), • Between the LV substation overcurrent and the outgoing low voltage lines, on both distribution substations (i.e. between relays ‘R7’ and fuses ‘F2,3 and4’; and relay ‘R8’ and fuses ‘F5’) for faults on the low voltage lines, • Between the AC fuse and DC fuse(s) on the VSDs system (i.e. fuse ‘F5’ and DC fuses ‘F6’ and ‘F7’) for faults on the DC systems. System Single-Line (Schematic) Diagram Refer to file “Model Industrial Network”.
ELEC 5204 Mid-semester Assignment , 2011 P 5 of 6 Fig B.1 - Standard Inverse, Very Inverse and Extremely Inverse Overcurrent Time-Current Characteristics for 1.0 Time Settings and 100 A current pick-ups. NB: At a given current multiple, operating time is proportional to the Time Lever setting
ELEC 5204 Mid-semester Assignment , 2011 P 6 of 6 0.01 0.1 1 10 100 1000 10000 100 1000 10000 Op Time, Sec Primary Current, A Time Grading Curves HV Fuse gG Fuse gR Fuse aM Fuse Fig B.2 - Standard Fuse Time-Current Characteristics for Common Fuse types and 100 A Fuses. NB: For fuses other than 100 A, operating time is proportional to the fuse rating/100. The above characteristics are for melting (pre-arc) times. For total clearance times, add +20% to currents. System single-line diagram and details of transformers, motors, etc are shown in Attachment “Model Industrial Network”.
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