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FACULTY OF ENGINEERING LAB SHEET EEL2026 POWER TRANSMISSION AND DISTRIBUTION TRIMESTER 2 PTD1- Performance of Transmission Line under Different Loading Conditions PTD2- Parameters which affect Real and Reactive Power Flow *Note: On-the-spot evaluation may be carried out during or at the end of the experiment. Students are advised to read through this lab sheet before doing experiment. Your performance, teamwork effort, and learning attitude will count towards the marks. EEL 2026 Transmission and Distribution Instruction 1. Before coming to the laboratory read the lab sheet carefully and understands the procedure of performing the experiments. 2. Do not switch-on the power supply unless permitted by the lab supervisor. 3. Do not make or break any connection with the power supply on. 4. Handle the equipments with care. 5. Do the necessary calculation, draw the graphs and submit the report within the specified time of the lab session. Experiment # 1 Performance of Transmission Line under Different Loading Conditions Objectives To compare the voltage regulations of the given transmission line under resistive, inductive and capacitive loading conditions. To diagnose the reason for the voltage drop across the transmission line when the sending-end and receiving-end voltages have the same magnitude. To investigate the effectiveness of the shunt capacitors to improve the power transfer capability of the line. Introduction A short transmission line is modeled by a single reactance as shown in Fig. 1. A good understanding of the behaviour of most of the transmission lines can be obtained by the short line model. It is this model which will be used in this experiment. Depending upon the loading condition the phase angle difference between the sending-end and receiving-end voltages and the voltage drop along the line will vary. These effects can be easily understood from the phasor diagram shown in Fig. 1. It may also be observed that a significant voltage drop will exist across the line even when the sending -end voltage, E1 and the receiving-end voltage, E2 are equal in magnitude. (a) (b) I E1 XL E1 IX L E2 E2 I Fig. 1 (a) Transmission line (b) Phasor diagram We have studied that the voltage drop along the transmission line and the receiving-end voltage vary widely for inductive loads. In order to regulate the voltage at the receiving-end of the line in some way so as to keep it at as constant as possible we should adopt some type of compensation. One method commonly used is to connect shunt capacitors at the end of the line. These capacitors produce a significant voltage rise thus compensating for the voltage drop. Static capacitors are switched in and out in a practical system and their value is adjusted depending on the loads. For purely inductive loads, the capacitor should deliver reactive power equal to that consumed by the inductive load. For resistive loads, the reactive power, which the capacitor must supply to regulate the voltage, is not easy to calculate. In this experiment, we shall determine the reactive power (the value of capacitor) by trial and error, adjusting the capacitors until the receiving-end voltage is approximately equal to the sendingend voltage. For loads, which draw both real and reactive power, the same trial and error method is adopted. Note that for a short transmission line having a line reactance of X /phase and resistance neglected. The following formulas will be useful. Sending-end voltage (L-L) = E11; Receiving end voltage (L-L) = E22 Three-phase sending-end power = Three-phase receiving end Power E E Sin (1 2 ) = P1 = P2 = 1 2 X 2 E E E Cos(1 2 ) Three-phase sending-end reactive power = Q1 = 1 - 1 2 X X 2 E E Cos(1 2 ) E 2 Three-phase receiving-end reactive power = Q2 = 1 2 X X Apparent power at sending end = S1 Apparent power at receiving end = S 2 P Q ; P Q ; 2 1 2 1 2 2 2 2 Equipment required Three-phase transmission line (8329) Resistive load (8311) Inductive load (8321) Capacitive load (8331) AC voltmeter (8426) Phase meter (8451) Three-phase wattmeter/varmeter (8446) Power supply (8821) Connection leads (9128) Procedure 1. Set the impedance of the transmission line to 200 and connect the meters as shown in Fig. 2. The circuit should be connected to the three-phase variable supply. Note that watt/var meters and phase meter need 24V AC supply provided in the power supply unit. Connect all the loads in star. Verify your connections with the lab supervisor before switching on the power supply. 0-500V E2 0-500V E1 4 1 5 2 3 6 P1 1 5 2 3 6 8821 0-415V Q1 4 8446 8329 4 P2 Q2 5 6 8446 3-phase Yconnected LOAD 8311 8321 8331 Fig. 2 Connection diagram for steps 2, 3 and 4 2. Adjust the sending-end voltage E1 to 300 V and keep it constant for the reminder part of the experiment. Use a three-phase resistive load and increase the load in steps making sure that the loads are balanced. Take readings of sending end and receiving end voltages and powers, E1, Q1, P1, E2, Q2, and P2. Record your results in Table 1. 3. Switch off the power supply and connect a three-phase balanced inductive load in parallel with the balanced resistive load. Don’t remove any other connections shown in Fig.2. Increase the load in steps making sure that the loads are balanced. Take readings of sending end and receiving end voltages and powers, E1, Q1, P1, E2, Q2, and P2. Record your results in Table 2. 4. Switch off the power supply, remove the inductive load and connect a three-phase balanced capacitive load in parallel with the balanced resistive load. Take readings of sending end and receiving end voltages and powers,E1, Q1, P1, E2, Q2, and P2 for different loadings. Record your results in Table 3. 5. Draw three graphs of receiving end voltage, E2 (obtained from steps 2, 3, and 4) on the same graph paper as a function of the receiving-end power P2 and discuss your results. 6. Switch off the power supply and connect a phase meter to measure the phase angle difference between E1 and E2 and a voltmeter to measure the voltage across the transmission line as shown in Fig. 3. Note that the load consists of resistances in parallel with capacitances. Now for each resistive load, adjust the capacitive load so that the load voltage E2 is as close as possible to 300 V. Take readings of XC, E1, P1, Q1, E2, P2, Q2, and the phase angle for different loadings. Record your results in Table 4. 7. Draw the graphs of E2 and the phase angle difference between E1 and E2 as a function of P2 from the results in Table 4. Note that the addition of static capacitors has yielded a much more constant voltage, and further more, the power P2 which can be delivered has increased. On this curve, indicate the phase angle between E2 and E1 as well as the reactive power Q2 used for individual resistive load settings. 8. In this part of the experiment, we shall observe a significant voltage drop along the transmission line even when the voltages E1 and E2 are equal in magnitude. This voltage drop is due to the phase angle difference between the two voltages. Switch off the supply and insert an ammeter in series with the transmission line as shown in Fig. 3 to measure the line current without removing any other connection. Using the circuit shown in Fig. 3, set the load resistance per phase at 686 and E1 = 300 V, adjust the capacitive reactance until the load voltage is as close as possible to 300 V. Measure and record E1, Q1, P1, E2, Q2, P2, E3, the line current I and the phase angle. 8451 0-500V 1 2 3 4 E2 0-500V E1 686 8329 4 1 5 6 2 3 4 1 5 6 2 3 4 686 P1 Q1 P2 Q2 5 6 A 8821 686 8446 0-415V 8446 E3 3-phase 8311 8331 0-250V Fig.3 Connection diagram for Steps 6 – 8 9. Using the results of step 8, draw the phasor diagram of per phase values of E 1 and E2 to scale and draw E3. From the diagram compute E3 and compare it with the measured value. Also compute the real power, reactive power and apparent power consumed by the line. From the apparent power compute the line current and compare it with the measured value. Observations Table 1: Results of procedure step 2 R 4800 2400 1600 1200 960 800 686 E1 V P1 W Q1 var E2 V P2 W Q2 Var Table 2 Results of procedure step 3 R 4800 2400 1600 1200 960 800 686 Xl 4800 2400 1600 1200 960 800 686 E1 V P1 W Q1 var E2 V P2 W Q2 var P2 W Q2 var Table 3 Results of procedure step 4 R 4800 2400 1600 1200 960 800 686 Xc 4800 2400 1600 1200 960 800 686 E1 V P1 W Q1 var E2 V Table 4 Results of procedure step 6 R 4800 2400 1600 1200 960 800 686 Xc E1 V P1 W Q1 var E2 V P2 W Q2 var Angle degree Results of procedure step 8 E1= E2= P1= P2 = Q1= Q2= E3= Phase angle = Sample calculation Line current, I = This sample calculation is to help you to answer Exercise 3. Let E1 = 350 V E2 = 350 V E3 = 165 V P1 = 600 W P2 = 510 W Q1 = 170 var Q2 = -280 var Phase angle = 48o and Line current, I = 0.95 E1 per phase = 350/3 = 202 V E2 per phase = 350/3 = 202 V E3 = 165 V P1 per phase = 600/3 = 200 W P2 per phase = 510/3 = 170 W Q1 per phase = 170/3 = 56.7 var Q2 per phase = -280/3 = 93.3 var The phasor diagram of voltages to scale is shown in Fig.4. E1 -48o E3=165 E2 Fig. 4 Phasor diagram From the figure E3 = 165 V which is the same as the measured value. [The voltage E3 may also be calculated using the formula, E3 = 2*E1*sin(24o)] Real power consumed = 200 –170 = 30 W Reactive power consumed = 56.7 –(-93.3) = 150 var Apparent power in the line = 1502 302 153 VA Current through the line = 153/165 = 0.93 A The difference between the calculated value and the measured value is 0.02 A. Exercise 1. Analyze your results based on the graphs you have drawn in steps 5 and 7. From the graphs plotted predict the voltage regulations for load powers of 60W, 70W and 80W respectively under different loading conditions (resistive, resistive-inductive and resistive-capacitive) and compile the results. 2. Analyze your results obtained in step 9. 3. A three-phase transmission line has reactance of 100 per phase. The sending-end voltage is 100 kV and the receiving-end voltage is also regulated to be 100 kV by placing a bank of static capacitors in parallel with the receiving-end load of 50 MW. Predict (a) the reactive power supplied by the capacitor bank (b) the reactive power supplied by the sending-end side (c) the voltage drop in the line per phase (d) the phase angle between the sending-end and receiving-end voltages and (e) the apparent power supplied by the sending-end side. 4. If the 50 MW load in Exercise 3 is suddenly disconnected formulate the receiving-end voltage which would appear across the capacitor bank. What precaution, if any, must be taken? 5. Argue why it is not possible to raise the receiving-end voltage by static capacitors if the transmission line is purely resistive. 6. State briefly what you have learned from this experiment.