THREE-PHASE POWER SYSTEMS ECE 454/554: Power Systems Laboratory

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THREE-PHASE POWER SYSTEMS

ECE 454/554: Power Systems Laboratory

Contributors: Dr. A.A. El-Keib Mr. Clifton Black Dr. Tim A. 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 A. Yamayee and Juan L. Bala, Jr., John Wiley and Sons, Inc., New York, New York, 1989.

- Section 3.4

Pre-Lab Assignment

1) For the system below, compute: (a) the three line-current phasors

(b) the three-phase complex power delivered to the load

-

+ 120 0 V I_a 2.5  j4 

-

+ 120 120V I_b 2.5  j4 

-

+ 120 120 V I_c 2.5  j4  Load

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2) A balanced, 480 V, 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 A. 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

4) A 60 Hz, balanced three-phase source is connected to a balanced load of 2815 kVA 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 kV. 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

12  -j 12  12  -j 12  12  -j 12  Balanced Three-Phase Source 208 V 60 Hz Load

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Equipment List

Quantity Description Number

1 Three-Phase Power Supply EMS 8821 1 Phase Angle Meter EMS 8451 2 Three-Phase Resistive Load EMS 8311 1 Three-Phase Ammeter EMS 8425 1 Ammeter Fluke 45 1 Three-Phase Watt/VAR Meter EMS 8446 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 4-5-6 to 208 V using the onboard voltmeter. Again using the onboard voltmeter, measure the three line-to-neutral voltages. We define terminals 4, 5, and 6 as phases A, B, and C, respectively. With 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-to-neutral and line-to-line voltage magnitudes?

Quantity Value Units

VAN V VBN V VCN V VAB V VBC V VCA V

2) Using the A 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 8451). The connection is illustrated below for measuring the B phase. Record your information. Is the source operating with positive or negative phase sequence?

4 5 6 N 8821 1 2 3 4 8451

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AN degrees

BN degrees

CN degrees

3) Again using the phase angle meter (EMS 8451), 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 AB 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.

AB degrees

BC degrees

CA degrees

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.

4) 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 8425), using the 0.5 A 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 45) to measure and record the neutral current. Is the load balanced? State the reasons for your answer.

IA A IB A IC A IN A 4 5 6 N 8821 1 2 3 4 8451

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5) Using the resistive load from step 4 and a second resistive load bank (EMS 8311), create a -connected load that is equivalent to the Y-connected load in step 4. Record the value of the necessary phase resistance. Insert the ammeter in the circuit to measure the line currents. Record the line current magnitudes. Are 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 = 

IA A IB A IC A IAB A IBC A ICA A

6) Connect the three-phase watt/VAR meter (EMS 8446) and ammeter (EMS 8425) 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 

P3 W

Q3 VAR

IA A

IB A

IC A

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.

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ZY = + j 

P3 W

Q3 VAR

IA A

IB A

IC A

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.

With Capacitive Load:

P3 W

Q3 VAR

Without Capacitive Load:

P3 W

Q3 VAR

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. What does the real power delivered to the motor accomplish? What does the reactive power delivered to the motor accomplish? Compute the power factor of the motor.

P3 W

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Questions

1) A Y-connected capacitor bank is to be inserted in parallel with a load to correct the power factor to unity. The load is 1 MVA at 0.75 power factor lagging and operates at 12.47 kV. Compute the value of capacitance required in each phase. Assume 60 Hz operation.

2) A -connected capacitor bank is to be inserted in parallel with a load to correct the power factor to unity. The load is 2 MVA at 0.75 power factor lagging and operates at 25 kV. Compute the value of capacitance required in each phase. Assume 60 Hz operation.

3) A 60 Hz, balance three-phase source supplies a balanced three-phase load of 175 kVA at 0.7 pf lagging and 480 V through a line with a series resistance of 1.5 . A capacitor bank is used to correct the power factor. As 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