How does the auto-disconnect (Technical Detales) work for small grid connected generaters when the main power grid goes down. I beleave its called "ISLANDING"

re: Grid connect generaters	Warren Goodrich	6:54 pm monday september 17, 2001

I have not heard the term islanding that is a new one.

Auto transfer from a Utility Company factory power to a generator and back when the power is restored on the factory power grid is commonly done by emergency headquarter, hospital, goverment, etc.

An auto transfer switch is operated by magnetic coils.

The transfer activity in the switch operates same as a manual transfer switch only the switch activation is operated by magnetic coils

There are several styles and variations avaialable on the market. The following is one style.

There is a main set of contacts for the factory powered connection. Then there is a main set of contacts for the generator powered connection. The two groups of contacts disconnect all the hot wires and in some cases the neutral connection from the factory and the contacts hit the generator group of contacts redirecting the power supply source from the factory to the generator. The two sets of contacts [factory or generator] have a difinitive separation ensuring that it is an either or activity with no chance of both being connected. The factory grid supplied contacts are usually found at the top and the generator supplied contacts are usually found at the bottom of the transfer box. The center connections are usually found in the middle of the two power supplies and are used as the common of the A/B switching device. This layout design is usually the same whether auto or manual.

Then the differences appear in the auto side. There is a magnetic coil that holds the common group to the factory group as long as the factory grid is energized. When power fails that magnetic coil falls out and allows that contact to break. At the same time there is a second magnetic coil that has normally closed contacts that has also been held in the off position considering a set of contacts to start the generator motor. When the generator comes up to power and starts generating at full power this normally closed contact opens back up stopping the starter of the generator motor. Also at the same time a third magnetic coil that is normally open closes to keep the ignition on the motor of the generator. Then once the generator is up to power a fourth magnetic coil pulls the transfer switch common into contact with the B phase or generated phase of the transfer contacts. The power is retored to the building on generator power.

Then when the factory power comes back on and power reaches the factory side of the auto transfer box that third magnetic coil opens shutting of the generator, the second magnetic coil is held in by the factory power keeping the starter from trying to start the motor of the generator again and the magnetic coil holding the transfer contacts to the generator B side drops off breaking that contact. and the A sid magnetic coil pulls the common of the transfer to the factory power contacts almost instantaniously.

Little is noticed in the building when the power is switched from the generator power except maybe a blink of lighting.

When the factory power has failed and the generator is started and takes up the as the power source there is a dimming or black out experienced while the generator is started and brought up to speed.

Often times there is a clock type switch that activated the generator for exercising the motor and keep that generator in running condition about once or twice a month. The magnetic contact causing the transfer switching when the generator is running does not activate because the second magnetic coil is holding the contact in the open position so that this generator starting to be exercised only thus the factory power remains feeding the building and the generator is just running without load during the exercising activity that normally last about a half hour.

A few set back to this generator and the auto is maintanence or lack of. Often when the factory power fails the generator often fails to start also due to lack of maintenence of that generater to ensure the start up when needed.

A trick of the trade seems to be to buy an LP gas powered generator motor. The LP does not spoil and turn to a solid fuel in the carberator of that generator combustion motor. Regular gas turns stale, and spoils over time plugging the generator causing a failure when the generator is needed to start.

This is a generic approximation to the activity of an auto transfer box. Many variations in design of these auto transfer systems exist on the market.

Hope this helps

Wg