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AC Synchronous Machine Synchronization



AC synchronous generators are the backbone of electricity generation in power plants world-wide and are often used to stabilize the power grid. Matching the phase sequences, voltage magnitudes and frequencies of the synchronous generator to those of the power in network is essential. If the generator is out of phase with the grid, the generator can not deliver power. While automatic synchronizers are used in large power plants a simple method of manual synchronization is demonstrated here. This video will introduce three phase synchronous generators and demonstrate protocols for adjusting the voltage and frequency outputs for manual synchronization of the generator to the power grid.

AC synchronous machines consist of inner rotating core, the rotor, and the outer stationary ring, the stator. The rotor magnetic field is stationary induced by an applied DC voltage. The stator magnetic field is excited using three phase alternating current, each phase connected to its own separate set of stator coils. This induces a rotating magnetic field of constant magnitude and rotational frequency corresponding to oscillations in supply line current. The stator and rotor magnetic fields are coupled causing the rotor to spin at exactly the same speed as the stator's rotating magnetic field. For more information on characteristics of AC synchronous machines, please watch the JOVE's science education video, AC Synchronous Machine Characterization. When the synchronous machine is operated as a power generator, a prime mover applies torque to the rotor resulting in flex difference between the rotor and the stator magnetic fields. If the applied torque opposes the rotor motion, the machine absorbs reactive power from the system to bring the machine back into synchronization. If the applied torque instead boosts rotation, overexciting the machine, the generator delivers power to the system. A three lamp method can be used to provide visual confirmation that the generator is delivering power at the same voltage magnitude, frequency and phase sequence as the power grid. For synchronous generators, frequency is controlled through prime mover speed variation. If the generator and system power are out of phase, the lamps flicker. When voltage is match, a zero differential causes all three lamps to turn off and on at the same time. Now that the basic principles of synchronous generators have been explained, the manual synchronization of an AC synchronous generator to the power grid will be demonstrated.

Start by initializing a DC motor or dynamometer as the prime mover. Check that the three phase disconnect, synchronous motor and DC motor are all switched off. With the Variac set to 0%, wire it to the three phase outlet. Next, connect the set up as shown. Then, switch the three phase switch on the synchronous machine on. Finally, make sure S1 and the three lamps are connected in parallel. And note the polarities of the digital power meter probes. Then, check that the start run switches in the start position. With S1 switched off, set RF to its maximum resistance. Turn on the three phase disconnect switch and then turn on the high voltage DC power supply. Next, press the VI display button on the power supply to display the operating voltage on current and adjust the voltage to 15 volts. Then press START on the DC supply panel. The dynamometer should have a large transient current drawn from DC supply. However, if the over current limit or OCT light turns on, increase the over current limit. Now observe the synchronous machine spinning slowly. Finally, increase the DC supply output voltage to around 160 volts and measure the shaft rotational speed using the strobe light technique. Next, adjust the supply voltage in order to achieve 1,800 RPM rotational speed. Then record the DC current and voltage.

Now synchronize the generator using the three lamp method with the fully assembled apparatus as shown. Switch the start run switch on the synchronous machine side to run and check that the three lamps are on. Next adjust RF on the supply voltage iteratively to achieve a generator voltage of 120 volts. Adjust the frequency of the VG on the digital power meter to 60 Hz. Values within +/- 2% are acceptable. Then slightly increase the Variac output to 120 volts. At this stage, the grid and generator are both providing 120 volts at a frequency of 60 Hz. Record voltage, current and power readings on both power meters including + or - signs. Finally, use the lighting pattern of the lamps to confirm or adjust synchronization. In the three lamp method, once the desired AC voltage is achieved, the lamps cycle on and off at the same time. If a phase sequence of A, B, C from the grid is met with sequence A, C, B from the machine, the lamps' cycle as the voltages across the lamps never add up to zero on all three phases at the same time. If the three lamps instead cycle and flicker out of sync, then the generator and the grid have different phase sequences across the set of lamps. Identify the sequences. One as ABC, and the other as ACB. Then in order to adjust sequence, first turn the Variac back to 0% and press STOP on the power supply panel. After reducing the DC voltage back to 15 volts, finally switch phases B and C on the generator side. If the three lamps are all brighten and dim concurrently, then the generator and the grid have the same phase sequence and are synchronized correctly. Otherwise, repeat the phase sequence modification At the instant all the lights turn off, turn on switch S1. Now the lights should all remain off since S1 is now acting as a short circuit across their terminals. The generator is subsequently synchronized with the grid.

Synchronous machines are frequently used in industrial applications for stabilizing power. The machine's power factor demonstrates whether the machine can deliver reactive power under certain conditions. Storing and releasing energy to stabilize the grid. When functioning this way, the machine is termed Synchronous Condenser. In the use of wind as a renewable energy source, the wind power turbine is the prime mover of the synchronous generator. In order to prevent the generator from stalling out at high loads, turbine rotor blade angles are differentially controlled to optimize the rotation rate in variable wind speeds. To transmit generated wind power to the grid, wind turbines use an automatic synchronizer interface to transmit power safely to utility lines.

You've just watched JOVE's introduction to AC synchronous machine synchronization. You should now understand how to adjust the voltage and frequency outputs of three phase synchronous generators. Manually synchronize the generator to the power grid and measure effects of field current and speed variations on generator power output. Thanks for watching!

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