Without getting to technical, here's how it works. Most of you are some what aware of AC motors. Your, washing machine, the air compressor in you shop, your table saw etc. use what is known an an "Asynchronous" or "Induction" Motor". Their operating speed is based on line frequency and their individual "slip factor.
Without going into the theory of operation the above motor will operate at 1725 RPM due to slip. In this case the slip factor is negative 6%.
To further complicate things the motors are said to rotate at either 1800 RPM or 3600 RPM.. In actuality a 1800 RPM motor usually operates at either 1725 RPM or 1750 RPM, depending on the slip. Slip is the difference of 75 RPM of about 6% negative slip for the 1725 RPM motor. If the motor absorbed electrical power and did run at a synchronous speed of 1800 RPM it would produce no mechanical power at all. Any power produced would be eaten up by power to keep itself running to overcome windage & bearing friction. You see, the motor needs the slip to develop torque.
If we leave the motor connected to the electrical lines and cause the motor to turn faster (via say a hydraulic turbine) then it's synchronous speed by a factor of at least 2 X the slip factor then, the motor starts producing electrical power instead of absorbing it. That's were the positive slip comes into play. This means you have to design the turbine gearing & drive to turn the motor at 1875 RPM. Operating with positive slip the motor can now generate the same amount of power as it would have used as a motor (operating with negative slip)
The above two paragraphs are my definition of Asynchronous Generation
(Induction Generation). Below in the section Design and Operating
Behavior of an Asynchronous Plant you'll Ossberger's description
of asynchronous generation and plant operation.
Hydro power plants designed as asynchronous installations are usually more economical than synchronous generating sets. In the past, asynchronous plants were equipped with min-imal equipment. The turbine inlet gates were designed for the varying water flow and regulation was done manually. Unfortunately, with this type of hand-operated equipment, the turbine inlet gates had to be readjusted frequently to cope with varying water flows. To restart the units after shutdown and to synchronize them was a laborious task. Even maintenance-free turbines had to be adjusted continuously, if the yearly output was to be favorably exploited. Thus, the operator 5 costs reduced income to losses.
With a two-cell Ossberger set, start-up and paralleling the grid is generally accomplished by opening the smaller turbine cell first. This automatic governing sequence guarantees full utilization of all water flows to the highest efficiency.
Now, what's the point of all that? Well when you get right down to it, it's to keep you and others alive and to keep you out of the "poor house." The problem arises when for any given reason the utility grid loses power. You'd think that your motor acting as an asynchronous generator would cease to generate because it lost it's excitation. If it were just your generator and the power company's, the power would drop out like a heavy duty rock. However that's not the case. There's a factor knows as "resonance" to deal with that's inherit with power distribution systems. Some of it comes from power lines being parallel to each other, thus forming a "capacitor". However most of it comes from "power factor correction capacitors" the power company installs within it's system and inductance in transformers. Maybe some day I'll expand on those to things but for now lets stick with just the effects of resonance.
Normally if the power company loses a line nothing happens, except for the rumble of people griping about that stupid power company. The resonance causes no problem because when the line went dead it's "open" and there no where for power to go so issue is moot. However, you Mr. Junior Power Company Executive, are connected to the other side of that "dead" line with all those parallel lines, capacitors and transformers creating a resonant circuit. That resonate circuit can cause very erratic excitation of for your generator & there it's output will very very erratic. Some really high voltage can be feed right back into your power line. This voltage may even be aggravated to increase even more by resonance. Likely a power company repairman will be out there somewhere searching for the cause of his line being down. If he finds a break in the line, know one would know until it was to late. If you think you had trouble when you killed that little larva mentioned on the previous page, you aint seen nothing yet!
This is almost as bad as connecting a gas powered generator to your
house when there's a power outage and failing to "disconnect" the "main"
from the incoming line. If in that case the transformer feeding your house
was being feed by a 4,160 volt source, that's what you
would be putting on the power company side of the "dead"
line. He's aware that any line might be live and will be taking precautions
& wearing protection for 4160 volts. However if your using a
asynchronous generation and have not taken precautions and
are still connected to the power grid, you may be putting far in
excess of the expected 4160 volts on the power companies lines.
AC Synchronous: Over- and under-voltage relays and internal voltage
regula-tion protect equipment on line from voltage fluctuations. Speed
control must be used if controlled-frequency power is to be provided for
AC motors on line. The control may be hydraulic, mechanical, or electronic.
Electronic controls tend to be the least expensive and the most reliable
for installations under 20 kw, at this time.
Induction motor-generator: Over and under-voltage relays, as well as an off-frequency relay, protect the grid from power quality fluctuation. An electronic starter must be provided to bring the plant on line. In addition, a means must be provided to isolate the system from the grid in case of utility power outage. The recommended approach is to shut down the turbine / generator to avoid irregular voltage and frequency production in the system (voltage and frequency become erratic if excitation from the utility is cut off). One such system consists of a "fail-safe water bypass for the turbine which is held closed by utility power.
The above two diagrams were approved by and for
use by Haywood Electric in North Carolina in the 1970's. They are
to used as guides and all your interface controls must be approved by your
electric utility. The prices indicated are 1970's prices.