SYNCHRONOUS GENERATOR OPERATION ECE 454/554: Power Systems Laboratory Contributors: Dr. Tim A. Haskew Objectives Be able to determine the equivalent circuit for a three-phase synchronous generator from test data. Understand the operational characteristics of three-phase synchronous generators. 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. - Sections 7.1-7.5 Pre-Lab Assignment 1) Open- and short-circuit test data for a three-phase, 2 pole, 60 Hz, Y-connected, 480 V, 300 kVA generator is provided below. The generator has negligible armature resistance. Field Amps (A) OC Voltage (V) SC Current (A) 0 0 0 1 125 71 2 250 143 3 350 214 4 425 286 5 480 357 6 520 429 7 540 500 8 560 571 9 580 643 Determine: (a) The unsaturated synchronous reactance. (b) The saturated synchronous reactance. 2) The generator in problem 1 has rotational losses of 24 kW. The generator supplies a 250 kVA load at a power factor of 0.8 lagging. Compute: (a) The line current. (b) The magnitude and phase angle of the back-emf. (c) The power required by the prime mover. (d) The torque required by the prime mover. (e) The voltage regulation of the generator. Equipment List Quantity 1 1 1 1 1 1 1 1 1 1 1 1 1 Description Benchtop Digital Multimeter Three-Phase Synchronous Generator/Motor DC Motor/Generator Three-Phase Power Supply Module Strobotach Three-Phase Voltmeter Oscilloscope Phase Shift Indicator Three-Phase Watt/VAR Meter Three-Phase Resistive Load Three-Phase Ammeter Three-Phase Inductive Load Three-Phase Capacitive Load Number Fluke 45 EMS 8241 EMS 8211 EMS 8821 ---EMS 8426 ---EMS 8909 EMS 8446 EMS 8311 EMS 8425 EMS 8321 EMS 8331 Procedure 1) Using a digital Ohmmeter (Fluke 45), measure the 3 line-to-line resistances of the three-phase synchronous generator (EMS 8241). Record your measurements below, and use them to compute the phase winding resistance. Quantity Rab Rbc Rca Ra = Value Units 2) Connect the dc motor (EMS 8211) in a shunt configuration with the field rheostat in the field circuit. Use the 8 A dc output on the power supply module (EMS 8821) to power the motor. The synchronous generator (EMS 8241) should be belt driven by the dc motor. Use the 2 A dc output of the power supply module for excitation of the generator field winding. Note that this adjustment can be accomplished with armature voltage and field current control on the dc motor. Using the strobotach, adjust the dc motor speed to 1800 RPM. You can sense the speed with the phase-shift indicator (EMS 8909), which should be installed on the shaft of the generator. Be sure that the generator field current is at a minimum value by setting the generator field rheostat to the maximum value. Connect the three-phase voltmeter (EMS 8426) across the armature of the generator to measure the line-to-line voltage magnitudes. With the system rotating at 1800 RPM, gradually increase the field current on the synchronous generator in discrete steps. Record each value of field current and the corresponding average line voltage magnitude. You should vary the field current up ECE 454/554: Power Systems Laboratory Synchronous Generator Operation 2 to the rated value in about 10 equal increments. The setup is illustrated below. Note that external field resistance may be required for the generator. a A2 dc motor synch. gen. A1 V(ac) b V(ac) c V(ac) F1 F2 A(dc) 0-120 V 120 V Field Current (A) Vab (V) Vbc (V) Vca (V) VL (V) 3) With the dc motor still serving as the prime mover, adjust the generator field rheostat to the minimum setting (minimum field current). Place the three-phase ammeter (EMS 8425) in series with the generator armature, and apply a three-phase short circuit after the ammeters, as illustrated below. From the nameplate data of the generator, compute the rated line current for the device. Record this value below. Irated = A Bring the system up to speed at 1800 RPM. Then, gradually ramp up the generator excitation in discrete steps while measuring the short-circuit current. Be careful not to allow the armature current to exceed the rated value. Record the field current and average short-circuit current in about 4 equal intervals. ECE 454/554: Power Systems Laboratory Synchronous Generator Operation 3 a A2 dc motor synch. gen. A1 F1 A(ac) b c A(ac) A(ac) F2 A(dc) 0-120 V 120 V Field Current (A) Ia (A) Ib (A) Ic (A) IL (A) With the results from steps 2 and 3, plot the occ and scc on the same graph. Using the graphs, compute the unsaturated and saturated synchronous reactances for the synchronous generator. 4) Adjust the field current on the generator (under no load) to provide rated output voltage. Connect the a-phase line-to-neutral output to the oscilloscope. Vary the speed of the generator by varying the armature voltage of the dc motor to achieve the speeds indicated below. At each speed record the armature voltage frequency and the peak line-to-neutral voltage magnitude. Plot the frequency vs. speed and the rms magnitude of the open-circuit line voltage vs. speed. Speed (RPM) 1800 1500 1200 900 f (Hz) Peak LN Voltage (V) RMS Line Voltage (V) 5) For this step, the generator is again to be operated at 1800 RPM. The phase shift indicator (EMS 8909) should still be placed on the shaft of the generator to adjust speed. Connect armature of the generator to a purely resistive load (EMS 8311) that is Y-connected. The three-phase watt/VAR meter (EMS 8446) and the three-phase voltmeter (EMS 8426) should be used to monitor the electrical output of the generator. A dc ammeter should be used to monitor the generator field current. The load resistance is to be varied as indicated in the table below. For each value of ECE 454/554: Power Systems Laboratory Synchronous Generator Operation 4 resistance, be sure to adjust the generator speed to 1800 RPM and the generator terminal voltage to the rated value. VL (V) Load Resistance () 1200 600 300 If (A) P3 (W) Using the measured load voltage and power with the equivalent circuit developed above, compute the required field current. How does the calculation compare with the measurement using both the unsaturated and saturated synchronous reactances of the machine for all three load resistance? 6) Now, use a resistive load bank (EMS 8311) and an inductive load bank (EMS 8321) to form a load of 300 + j300 . Operate the generator at 1800 RPM with rated terminal voltage. Monitor the line-to-neutral voltage of the generator with the threephase ac voltmeter (EMS 8426) and the three-phase complex power output of the generator with the three-phase watt/VAR meter (EMS 8446). Record your measurements. Switch the load off, and record the new output line-to-neutral voltage once the speed has been stabilized to 1800 RPM. VLN (V) 120 Full-Load P3 (W) Q3 (VAR) No-Load VLN (V) Use the appropriate equivalent circuit under full load to verify your measurements treating the load voltage and complex power as knowns. Use the full- and no-load voltage measurements to compute the voltage regulation. 7) Use a resistive load bank (EMS 8311) and a capacitive load bank (EMS 8331) to form a load of 300 - j300 . Operate the generator at 1800 RPM with rated terminal voltage. Monitor the line-to-neutral voltage of the generator with the three-phase ac voltmeter (EMS 8426) and the three-phase complex power output of the generator with the three-phase watt/VAR meter (EMS 8446). Record your measurements. Switch the load off, and record the new output line-to-neutral voltage. VLN (V) 120 Full-Load P3 (W) Q3 (VAR) No-Load VLN (V) Use the appropriate equivalent circuit under full load to verify your measurements treating the load voltage and complex power as knowns. Use the full- and no-load voltage measurements to compute the voltage regulation. ECE 454/554: Power Systems Laboratory Synchronous Generator Operation 5 8) Connect the fixed 208/120 V output of the three-phase power supply (EMS 8821) through the watt/VAR meter (EMS 8446) to the armature of the generator. Connect the generator field to the fixed 120 V dc supply. Set the field rheostat at the nominal mark, but leave the field switch open. Start the power supply. The synchronous generator has a squirrel-cage imbedded in the rotor. Hence, with the field winding open-circuited, the machine can be started as an induction motor. When the machine has reached steady-state speed, close the field switch. This will synchronize the synchronous motor. Now, record the real and reactive power absorbed by the synchronous motor at minimum and maximum field current. Minimum Field Current Maximum Field Current P3 (W) Q3 (VAR) Questions 1) Why does a synchronous machine equivalent circuit based on the saturated synchronous reactance not provide accurate results for light loading conditions? 2) Explain the frequency versus speed and voltage versus speed plots obtained in step 4. 3) Explain, in detail using the equivalent circuit, how the excitation of the synchronous generator alters the direction of reactive power flow in step 8. ECE 454/554: Power Systems Laboratory Synchronous Generator Operation 6