THREE-PHSE POER SYSTEMS ECE 5/55: Power Systems Laboratory Contributors: Dr... El-Keib Mr. Clifton Black Dr. Tim. Haskew Mr. Johnny Carlisle Mr. Neil Hutchins Objectives Learn how to perform measurements on three-phase systems. Understand phase sequence. Understand three-phase balanced operation. Understand voltages and currents in three-phase systems. Understand real and reactive power in three-phase systems. References Electromechanical Energy Devices and Power Systems, Zia. Yamayee and Juan L. Bala, Jr., John iley and Sons, Inc., New York, New York, 1989. - Section 3. Pre-Lab ssignment 1) For the system below, compute: (a) the three line-current phasors (b) the three-phase complex power delivered to the load 120 0 - + 2.5 j 120 120 - + I a 2.5 j 120 120 - + I b 2.5 j I c Load
2) balanced, 80, 60 Hz three-phase source is connected to a balanced three-phase load with a power factor of 0.85 lagging. The line current magnitude is 100. Compute: (a) the three-phase real power delivered to the load (b) the three-phase reactive power delivered to the load (c) the equivalent Y-connected load impedance (d) the equivalent -connected load impedance 3) For the provided system, compute: (a) the real power absorbed by the load (b) the reactive power supplied by the source 12 Balanced Three-Phase Source -j 12 12 208 60 Hz -j 12 12 -j 12 Load ) 60 Hz, balanced three-phase source is connected to a balanced load of 2815 k at 0.95 pf lagging through a short transmission line with negligible resistance and a series inductive reactance of 13. The load is operated at 25 k. Compute: (a) the line current magnitude (b) the line voltage magnitude at the source (c) the line-to-neutral source voltage (d) the power factor of the source ECE 5/55: Power Systems Laboratory 2
Equipment List Quantity Description Number 1 Three-Phase Power Supply EMS 8821 1 Phase ngle Meter EMS 851 2 Three-Phase Resistive Load EMS 8311 1 Three-Phase mmeter EMS 825 1 mmeter Fluke 5 1 Three-Phase att/r Meter EMS 86 1 Three-Phase Inductive Load EMS 8321 1 Three-Phase Capacitive Load EMS 8331 1 Three-Phase Squirrel-Cage Induction Motor EMS 8221 Procedure 1) Turn on the three-phase power supply (EMS 8821) and adjust the line voltage on terminal -5-6 to 208 using the onboard voltmeter. gain using the onboard voltmeter, measure the three line-to-neutral voltages. e define terminals, 5, and 6 as phases, B, and C, respectively. ith this definition and your measured data, record the three line-to-neutral voltage magnitudes and the three line-to-line voltage magnitudes. Does your data indicate the expected relationship between line-toneutral and line-to-line voltage magnitudes? N BN CN B BC C 2) Using the phase line-to-neutral voltage as your reference, determine the phase B and C line-to-neutral phase angles with the phase angle meter (EMS 851). The connection is illustrated below for measuring the B phase. Record your information. Is the source operating with positive or negative phase sequence? 8821 1 3 5 6 N 2 851 ECE 5/55: Power Systems Laboratory 3
N BN CN 3) gain using the phase angle meter (EMS 851), determine the phase angles of all of the line-to-line voltages. Record your data and construct a phasor diagram indicating all six source voltages. Your connection for measuring the phase angle of the B line-to-line voltage is illustrated below. Does your phasor diagram look like you would expect? State why or why not. Is the source balanced or unbalanced? Give reasons for your response. 8821 1 3 5 6 N 2 851 B BC C For the remaining steps in this laboratory exercise, connection figures will not be provided. You will be required to develop them on your own. Be sure to include the connection diagrams in your lab reports. ) Connect the three-phase power source (EMS 8821) to the three-phase resistive load (EMS 8311). The load should be Y-connected with a phase-to-neutral resistance of 300 and the neutral in place. Place the three-phase ammeter (EMS 825), using the 0.5 setting, in series with the load. Measure and record the three line current magnitudes and the three load current magnitudes (they are the same in the Y- connected case). Use an ammeter (Fluke 5) to measure and record the neutral current. Is the load balanced? State the reasons for your answer. I I N ECE 5/55: Power Systems Laboratory
5) Using the resistive load from step and a second resistive load bank (EMS 8311), create a -connected load that is equivalent to the Y-connected load in step. Record the value of the necessary phase resistance. Insert the ammeter in the circuit to measure the line currents. Record the line current magnitudes. re the loads equivalent? Insert the ammeter in the circuit to measure the load currents and record the magnitudes. Do the currents obey the expected relationships? R = I C 6) Connect the three-phase watt/r meter (EMS 86) and ammeter (EMS 825) between the source and a Y-connected load with each phase consisting of a 300 resistance in series with a 300 inductive reactance (EMS 8321). Measure and record the real and reactive power absorbed by the load and the three line current magnitudes. Compute and record the equivalent Y-connected impedance of the load. Using the known voltage and load impedance, compute the three-phase complex power absorbed by the load. Using the known value of current and load impedance, compute the three-phase complex power absorbed by the load. How do your computed results compare with your measured results? ZY = + j R I 7) Replace the inductive reactance with the three-phase capacitive load (EMS 8331) set at 300. Measure and record the real and reactive power absorbed by the load and the line current magnitudes. Compute and record the equivalent Y-connected impedance of the load. Using the known voltage and load impedance, compute the three-phase complex power absorbed by the load. How does your computed result compare with your measured result? Interpret the meaning of the sign on the reactive power absorbed by the load. ECE 5/55: Power Systems Laboratory 5
ZY = + j R I 8) Replace the load with a Y-connected load in which all three phases contain a series combination of a 600 resistance, a 600 inductive reactance, and a 600 capacitive reactance. Record the real and reactive powers absorbed by the load. Explain the results for the reactive power. Remove the capacitance and explain the results for the reactive power. ith Capacitive Load: R ithout Capacitive Load: R 9) Replace the load with the three phase squirrel-cage induction motor (EMS 8221) with no mechanical load. Record the real and reactive power delivered to the motor. hat does the real power delivered to the motor accomplish? hat does the reactive power delivered to the motor accomplish? Compute the power factor of the motor. R ECE 5/55: Power Systems Laboratory 6
Questions 1) Y-connected capacitor bank is to be inserted in parallel with a load to correct the power factor to unity. The load is 1 M at 0.75 power factor lagging and operates at 12.7 k. Compute the value of capacitance required in each phase. ssume 60 Hz operation. 2) -connected capacitor bank is to be inserted in parallel with a load to correct the power factor to unity. The load is 2 M at 0.75 power factor lagging and operates at 25 k. Compute the value of capacitance required in each phase. ssume 60 Hz operation. 3) 60 Hz, balance three-phase source supplies a balanced three-phase load of 175 k at 0.7 pf lagging and 80 through a line with a series resistance of 1.5. capacitor bank is used to correct the power factor. s the power factor is corrected from 0.75 leading to 0.75 lagging, plot: (a) the line current magnitude (b) the three-phase real power losses in the line (c) the source power factor (d) the line voltage magnitude at the source ECE 5/55: Power Systems Laboratory 7