Part -1:
QUESTION 1.
Two loads are connected to a three phase system with Vi = 11kV .
Load 1 STAR 105 kW, PF 0.68 lagging
Load 2 DELTA 95 kVA, PF 0.73 lagging
For the above two (2) loads:
(i) draw a diagram of the system; Calculate the:
(ii) total power Pr dissipated by the loads in kW;
(iii) total reactive power Qr of the load in kVAr;
(iv) total apparent power Sr of the load in kVA;
(v) total magnitude and power factor of the line currents;
(vi) total capacitive kVAr (Qc) to correct the PF to 0.8 lagging.
QUESTION 2.
Tests on a single phase transformer were performed on the low voltage side due to safety issues with the high primary voltage:
RATING 45 kVA
PRIMARY VOLTAGE 132 kV
SECONDARY VOLTAGE 3.3 kV
FREQUENCY 50 Hz
The test data for the Open Circuit (0/C) and Short Circuit (S/C) Tests are below:
Low Voltage Side 0/C Test 3300 V 0.634 A 452 W
Low Voltage Side S/C Test 181 V 13.6 A 1070 W
For the above transformer:
(I) draw the approximate equivalent circuit referred to the primary, showing RE, XE, RC, XM, V1, E2, V2 & E2;
Calculate the:
(ii) approximate Equivalent circuit parameters referred to the High Voltage primary (show on above diagram);
(iii) input voltage at full load if the output voltage is 3.3kV;
(iv) voltage regulation at full load with a power factor (P.F.) of 0.89 lagging
(v) efficiency of the transformer at full load with a P.F. of 0.89 lagging;
(vi) secondary load impedance ZL for full load with a PF of 0.89 lagging;
Part -2:
The answer to each problem MUST include electrical diagrams where applicable. Marks have be allocated for diagrams.
In all calculations the minimum amount of work (presentation) for each stage of the solution is:
(i) draw circuit
(ii) state equation;
(iii) transpose if necessary;
(iv) substitute values into equation;
(v) answer.
Only Si. units are to be used for answers, eg. Newton-metres (Nm) for torque.