1. For the following distribution feeder systems shown in FIGURES 3(i), (ii) and (iii) determine the load voltage levels.
2. The following phase schematic diagram (FIGURE 4) shows an 11 kV, 50 Hz, 3-phase, short line feeding a load. By calculation or constructing the phasor diagram (use a scale of 1 mm = 2 A) for the load cuntnt with VR as reference, determine the capacitive current and
• calculate the capacitive reactance/ph such that the load power factor is increased to 0.98 lag
• calculate the percentage reduction in line current with this value of capacitive reactance in circuit.
3. (a) A short 3-phase II kV line delivers a load of 4.1 MW, 2.6 MVAr lag. If the series impedance of the line is (72 + j12.1) LI/ph, calculate the sending end voltage and load angle.
(b) Develop the receiving end performance chart for the above line and load to a 3-phase power scale. (Use a scale of 10 mm = 1000 V and 30 nun = 4.1 MW.)
4. Using the chart you produced in Question 3, determine:
(a) the real and reactive power when
(i) the transmission angle remains unaltered and the sending end 'oltage is increased by 28%
(ii) the original sending-end voltage is reduced by 15% and the transmission angle increased by 9°. (Use the same scale as that for Question 3 (b))
(b) the real power limit for the sending voltage in (ii) above (i.e. when reduced by 15%). FIGURE I shows four generators feeding into a sectionalised busbar with a fault limiting reactor.
5. Figure 1 shows four generators feeding into a sectionalised busbar with a fault limiting recactor.
Each generator is rated at II kV and 30 MVA and has a reactance of 02 p.u.
Calculate the value of the reactance to limit the fault level on a feeder to 500 MVA.
6. Compare FIGURES 2 and 4 with those of FIGURES I and 8 and state briefly why they are similar.
7. FIGURE 2 shows the sectionalised busbar of a two generator power station connected to the national grid.
Calculate, using a 40 MVA base, the fault level for a 3•phase symmetrical short circuit on the 11 kV feeder.
8. An 11 kV, single phase system is protected by a scheme using 1000/5 CTs and an IDMT relay having a plug setting of 150% and time multiplier setting of 0.68 (FIGURE 3).
Using the standard PSM/time characteristic, find the relay operating time (TMS = I) for a 236 MVA fault.
The circuit breakers CB,, CB2, CB3 and CB, are part of a scheme which uses 1000/5 as and IDMT relays.
If the maximum fault current capability is 6700 amps, clearance of which must be initiated in 225 secs, select, using the standard characteristic:
(i) plug setting multipliers for CB, at plug settings of 100% and 50%
(ii) the time multiplier setting of each IDMT relay to afford effective discrimination of 0.4 sec at a plug setting of 50%.