LOAD, FAULT LEVEL ANALYSIS & PROTECTION DESIGN OF A REMOTE AREA MICROGRID
Assignment
Please undertake the following analyses:
1. Calculate the 3-phase fault levels on Bus 2, Bus 3 and Bus 4 with the main supply connected but DG's disconnected. Then calculate the fault levels on Bus 3 and Bus 4 with the microgrid islanded (synch switch open). To do this, assume that there is a total of (group A/B/C) 500/625/750 kW of DG output in the centre of each arm of the microgrid. Then calculate the fault levels on Bus 3 and Bus 4 with the microgrid DG's in service and the main grid also connected (parallel supply).
2. Calculate the voltage drop at Bus 3 with the main supply connected but DG's disconnected at peak loading. Assume even loading; the volt drop is then equal to (half total load at the end of the line) * (line impedance). Then calculate the voltage drop at Bus3 and Bus 4 with the microgrid islanded, again at peak loading times. Although microgrid loads and DG's are evenly spread around the whole microgrid ring, for the purposes of modelling assume that the total microgrid peak load of (group A/B/C) 1000/1250/1500 kW is met by the DG's modelled as before as two of (group A/B/C) 500/625/750 kW sources, one in the middle of each arm of the microgrid.
3. For protection purposes, overcurrent and earth-fault relays are installed at CB's CB3, CB4 and each CB1. The overcurrent relays at each CB has a pick-up current, as reckoned in primary amps, of 200% of the peak load flowing at that point (the extra 100% is to prevent spurious tripping under peak load conditions). The single-phase to earth pickup currents can be set at 20% of the overcurrent (three-phase) setting on the corresponding overcurrent relay.
Design the overcurrent and earth fault relay settings (pick-up currents and time dial settings) at CB's CB3, CB4 and each CB1 when the feeder is supplied by the main grid only. To do the design, start at the relays at CB1 and work your way up the feeder to the relays at CB3and CB4. Overcurrent relay pick-up currents will be set at 150% of load. The CB1relay time setting will be 0.1. Then, using time grading techniques (use 0.35 sec grading margin between relays) determine the settings for firstly overcurrent and then earth fault relays at CB2, CB3, and CB4. The earth fault relay pick-up current settings will be 20% of the corresponding overcurrent relay setting. Again, start off with a CB1 relay time setting of 0.1, and use the standard inverse (SI) characteristic. You can calculate directly from the SI relay formula, or use the overcurrent relay setting spreadsheet "Ocurrent.xls" which is provided for your use (or alternatively, make you own one!).
4. Then analyse the situation in the microgrid when it is islanded. Will the overcurrent relays at each CB1 (Bus #3) detect microgrid faults when fed by the DG's only? Suggest some solutions to the problem. As one possible solution, consider back-to-back directional overcurrent and earth fault relays at each CB1 and recommend settings for the backward direction.
Attachment:- Power Systems Assignment.rar