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THREE PHASE CIRCUITSA.B.C.PREPARATION1.Three Phase Voltages and Systems2.The Determination of Phase Sequence3.Blondel's Theorem and Its Consequences4.ReferencesEXPERIMENT1.Equipment List2.Three Phase Power Supply3.Balanced Three Phase Resistive Loads4.Unbalanced Three Phase Resistive Loads5.The Three Phase TransformerREPORT1.Three Phase Power Supply2.Balanced Three Phase Loads3.Unbalanced Three Phase Loads4.The Three Phase TransformerThree Phase Circuits - 1
1.Three Phase Voltages and SystemsThree Phase Circuits - 2
Three Phase Circuits - 3
Three Phase Circuits - 4
Three Phase Circuits - 5
2.The Determination of Phase SequenceThree Phase Circuits - 6
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3Blondel’s Theorem and Its ConsequencesThree Phase Circuits - 8
Three Phase Circuits - 9
4Chapman, Stephen J., 2002. Electric Machinery and Power SystemFundamentals. McGraw HillThree Phase Circuits - 10
B.EXPERIMENT - THREE PHASE CIRCUITS1.Equipment Lista. The standard instrument rack found at each station.b. Various current shunts and probes.c. One single-phase wattmeter.d. One three-phase transformer.e. One Phase Sequence Indicator (PSI).f. One rheostat module with two 50-Ω rheostats rated at 4.5 Amp each.g. Various power cords.2.Three Phase Power Supplya. A schematic of the Phase Sequence Indicator (PSI) is provided on the next page.Plug this device into the Three Phase (3φ) wall power source using the 3-PhaseCord Set and determine the phase sequence (ABC or CBA) using the PSI. Besure the switch is in the NORMAL position. Also, observe the Phase A andPhase B waveforms on the oscilloscope using two 10X Probes or the A/100 andB/100 test points with coaxial cables. Determine their phase relationship and thephase angle between them using the scope. Copy this display. Now repeat theabove step for Phase A and Phase C and record the display. Next reverse thephase sequence by flipping the switch to REVERSE and observe the lamppattern. Be sure and return the switch to the NORMAL position for the rest of theexperiment. Finally, observe the voltage waveform for the Star Point (STAR) onthe Phase Sequence Indicator on the scope and copy this display.b. Being fully aware of the potential for an electric shock, and therefore careful toshut off the 3-phase breaker each time the DMM connections are moved,accurately measure the rms values for the three line-to-line voltages and thethree line-to-neutral voltages at your station using three DMMs.c. The "green" neutral wire on the 3-phase supply provides a return to ground. It iswell and thoroughly grounded and will remain at Mother Earth potential as longas there is no IR voltage drop, i.e, as long as the neutral return current is zero.This is the case for perfectly balanced three phase voltage sources and perfectlyThree Phase Circuits - 11
Input Phase CBACAThree Phase Circuits - 12Input Phase ABCReverseACNormalBB
balanced loads since no neutral current will exist to produce an IR voltage drop.Using a coaxial cable, determine the existence of any neutral point offsets byobserving on the scope the voltage waveform between the Neutral Point (N) onthe Phase Sequence Indicator and the "green" ground on its chassis and copyyour observation. Note that the scope is already referenced to ”green” ground.3.Balanced Three Phase Resistive Loadsa. Using a DMM to measure the left and right side resistances of the left rheostat,adjust it to make 2 equal resistances of approximately 25 Ω each. Record the 2resistance values. Now adjust the right rheostat until its left side resistance isequal to the two left rheostat resistances and record its value. Now connectthese three equal resistances to form a balanced Three Phase (3φ) Wye load byconnecting the yellow receptacles on the rheostats to form a load resistanceCommon Point (O). Connect the left side of the left rheostat to the Phase Aoutput of the Phase Sequence Indicator (PSI) and the right side to the Phase Boutput of the PSI. Connect the left side of the right rheostat to the Phase Coutput of the PSI. Connect the rheostat Common Point (O) to the Neutral Point(N) of the PSI to form a Neutral Return line. Show your set-up to the instructor.b. Now instrument the set-up, using 3 DMMs and 1 Wattmetter, to measure PhaseA line current (IA), Phase A load voltage (VAO), Phase A load power (PAO), andNeutral Return line current (ION). Use current shunts as required. Use a 10xprobe to observe Phase A rms voltage (VAG) and a coaxial cable to observe therheostat Common Point (O) rms voltage (VOG). Show your set-up to theinstructor before turning on 3φ power. Once your set-up is approved by theinstructor, turn on 3φ power and record all measurements and copy the scopedisplay. With power on, carefully move the 10X probe to also measure Phase B(VBG) and Phase C (VCG) rms voltages. Separately record these rmsmeasurements. Turn off 3φ power when finished recording all measurements.c. Now remove the “O” to “N” Neutral Return wire and change the DMM being usedto measure Neutral Return current to now measure rheostat Common Point (O)rms voltage (VON) relative to the Neutral Point (N) on the PSI. Turn on 3φ powerand record all measurements. Turn off 3φ power when finished.Three Phase Circuits - 13
4.Unbalanced Three Phase Resistive Loadsa. Now disconnect the Phase A and Phase B leads from the left rheostat. Connecta DMM to the left side of the left rheostat and adjust this rheostat until aresistance of 15 Ω is obtained. Measure and record both of the left rheostatresistances. Now reconnect the circuit as it was in Part B.3.c. above and show itto the instructor. When approved by the instructor, turn on 3φ power and recordall DMM and wattmeter measurements. Turn off 3φ power when finished.b. Obtain a second Wattmeter and reconfigure the instrumentation to measure loadpower using the 2 wattmeter method. Configure the DMMs to measure the threeload voltages relative to the rheostat Common Point (O). Show this set-up to theinstructor. When approved, turn on 3φ power and record the measured powersand load voltages. Be sure that both Wattmeters read upscale. Turn off 3φpower when finished.c. Remove both Wattmeters from the circuit and reconnect the Neutral Return wirebetween the rheostat Common Point and the Neutral Point of the PSI.Reconfigure the DMMs to measure Phase A line current, Phase A load voltage,and Neutral Return line current. Turn on 3φ power and record all DMMmeasurements. Turn off 3φ power when finished.5.The Three Phase TransformerBecause this portion involves exposed power points, it is electrically themost dangerous procedure of the course. You will therefore power uponly under the direct and continuous supervision of one of the faculty.NO EXCEPTIONS !!a. Using DMM voltage measurements, determine how to use the three individualtransformers in the step-down direction and calculate the three step-down ratios.Parts b and c below operate in the step-down direction.b. Connect the high voltage primaries of the transformers to form a wye. Thesecondaries of the transformers are to be connected as a delta, but beforeclosing the delta, you must determine that phasing is correct by measuringthe voltage across the open delta to see that it is approximately zero. WithThree Phase Circuits - 14
power off, close the delta through a rheostat set to 50 Ω. Turn the power backon and measure the voltage across the rheostat. If it still measuresapproximately zero, adjust the rheostat to minimum resistance and measure the3 secondary line-to-line delta voltages.c. With the power off and the primaries still wye connected, open the delta andreverse the connections for one of the secondaries, but leave the delta open.Now turn power back on and measure and record the voltage across the openterminals in the delta. Note that it is large. With the power off, close the deltathrough a rheostat set to 50 Ω. Now measure the ”ring” current in the 50 Ωrheostat by measuring the voltage across it.C.REPORT - THREE PHASE CIRCUITS1. Three Phase Power Supplya. What is the phase sequence provided by the 3-phase wall supply in thelaboratory. Assuming that the phase A voltage is the reference (0 phase shift),what are the measured phase angles for the phase B and C voltages? Did thePhase Sequence Indicator provide the same information that you deduced fromoscilloscope observation? Also, discuss and explain the STAR point waveformyou observed and recorded in part B.2.a.b. Make a table showing the three line-to-line voltages, their average, and theirdeviation from the average value. Make a second table showing the three lineto-neutral (phase) voltages, their average, and their deviation from averagevalue. What is the ratio these two averages? Is it in agreement with theory?Considering the voltage deviations from average shown in the tables, do youthink the laboratory supply is adequate for 3 phase circuit experiments?c. Comment on the Neutral Point (N) voltage you observed on scope. Does theNeutral Point appear to be well grounded to Mother Earth?2. Balanced Three Phase Resistive Loadsa. Based on data from part B.3.a, calculate the average value of load resistanceand make a table showing the load resistances and their deviation from theaverage value. Did you present a nearly balanced load to the 3φ power supply?Three Phase Circuits - 15
The measured values of the load resistors inherently include the resistances ofthe banana plug cords if the measurements were made using the DMM as anohmmeter. Comment on any effect these added resistances may have on theexperiments of parts B.3.b and B.3.c.b. Make a table comparing expected and measured values of IA, VAO, PAO, and IONfor the results of part B.3.b. In calculating the expected values of IA, PAO, and ION,use the 3 resistances measured in part B.3.a and the 3 phase voltagesmeasured in part B.2.b and assume that the phase shift between the voltages isexactly 120 in the proper sequence (CBA or ABC). Comment on the deviationsobserved. Why is the assumption that total power (PT) is equal to 3 PAO (PT 3 PAO) a reasonable assumption for a balance load with a neutral return line?c. Make a table comparing expected and measured values of IA, VAO, PAO, and VONfor the results of part B.3.c. In calculating the expected values of IA, PAO, andVON, use the 3 resistances measured in part B.3.a and 3 phase voltagesmeasured in part B.2.b and assume that the phase shift between the phasevoltages is exactly 120 in the proper sequence (CBA). Comment on thedeviations observed. Why is the assumption that PT 3 PAO a reasonableassumption for a balance load with no neutral return line?d. Make a table comparing measured values of IA, VAO, and PAO made in part B.3.band B.3.c. How should these values compare considering that the neutral returnline was not used in part B.3.c?3. Unbalanced Three Phase Resistive Loadsa. Based on data from part B.4.a, calculate the average value of load resistanceand make a table showing the load resistances and their deviation from theaverage value. Did you present an unbalanced load to the 3φ power supply?b. Make a table comparing expected and measured values of IA, VAO, PAO, and VONfor the results of part B.4.b. In calculating the expected values of IA, VAO, PAO,and VON, use the 3 resistances measured in part B.4.a and the 3 phase voltagesmeasured in part B.2.b and assume that the phase shift between the phasevoltages is exactly 120 in the proper sequence (CBA or ABC). Comment on theThree Phase Circuits - 16
deviations observed. Now calculate the expected values of VBO and VCO and thecorresponding expected values of PBO, and PCO. Obtain the total power PT asthe sum of these powers. Compare this total power to the sum of the powersmeasured using 2 Wattmeters. Why is the assumption that PT 3 PAO notreasonable for an unbalanced load with no neutral return line? For this circuit, itis clear that 3 Wattmeters could have been used to measure total power. Explainwhy total power can be measured using only 2 Wattmeters.c. Make a table comparing expected (calculated) and measured values of IA, VAO,PAO, and ION for the results of part B.4.b. In calculating the expected values of IA,VAO, PAO, and ION, use the resistances measured in part B.4.a and the phasevoltages measured in part B.2.b and assume that the phase shift between thephase voltages is exactly 120 in the proper sequence (CBA or ABC). Commenton the deviations between measured and calculated values shown in the table.Calculate PAO, PBO, and PCO and obtain the total power PT as the sum of thesepowers. Why is the assumption that PT 3 PAO not reasonable for anunbalanced load with a neutral return line? Obviously, 3 Wattmeters could beused to measure total power. Explain why total power cannot be measuredusing only 2 Wattmeters.d. Make a table comparing measured values of IA and VAO made in parts B.4.b andB.4.c. Also include the calculated values of PAO in this table. Include in the tablethe calculated values of PT for the unbalanced load with and without the NeutralReturn line. Comment on the results shown in the table.4. Three Phase Transformera. In part B.5.a, what is the step-down ratio for each transformer?b In part B.5.b, what are the secondary line-to-line voltages?c In part B.5.c, what are the open delta voltage and “ring” current with onesecondary phase wired backwards? Explain the last result with the aid of phasoralgebra and a phasor diagram.Three Phase Circuits - 17
Three Phase Power Supply . a. A schematic of the Phase Sequence Indicator (PSI) is provided on the next page. Plug this device into the Three Phase (3φ) wall power source using the 3-Phase Cord Set and determine the phase sequence (ABC or CBA) using the PSI. Be sure the switch is in the NORMAL position. Also, observe the Phase A andFile Size: 682KB
