Induction Generators [RE-wrenches]

[Charlie Robb, Element Engineering]

Neil,

Induction generators are very commonly used on grid-connected R.E.
generating plant. They are cheaper and more robust than synchronous
generators and generally the control system required is simpler. Most large
wind turbines have induction generators; one of the advantages there is that
the "slip" of the generator, where its speed increases as the power
increases, helps to buffer large torque transients from the rotor. If a
synchronous generator is used, its speed is rigidly fixed by the grid
frequency, so the drivetrain has to absorb all of the torque peaks.

Usually an induction generator is just a standard induction motor, which
will generate instead of using power when driven above synchronous speed.
You don't have to say "induction" but just ask for a motor and that is what
you should get - you can buy them everywhere. My own 60kW wind turbine has a
standard (marine spec) 75kW induction motor. I work on other wind and hydro
turbines with standard induction motors and I recently did a retrofit of a
new induction motor in place of a synchronous generator on a 15kW hydro
turbine manufactured in 1946. It was a simple bolt-up job because
dimensions, including shaft size, were identical. With some mods to the
control system it was soon back in service having been off for decades.

Grid connection control is based on shaft speed rather than synchronisation
(of synchronous generators). In theory you could just close the breaker with
the machine stationary and it would motor up to speed, but the inrush
current (at least 7x rated) would likely trip you off the grid or at least
dim everyone's lights, so it is more normal to run the generator up to near
full speed, sensed by an inductive sensor (mounted near the generator shaft
key) and rpm sensing relay or circuit, then use a soft-start device to
gently energise the generator, then close the main contactor or circuit
breaker, bring on the (mechanical) power and start generating! You can often
get away with not using a soft start with motors below 10kW. In fact one
300kW hydro I work on has no soft start but uses the grid transformer to
limit the inrush. The same rpm sensor can be used to detect if the generator
starts motoring (if the wind drops or the spear valve blocks for example)
and disconnect the generator to avoid using electicity to make wind (or, er,
water).

Efficiency of induction motors bigger than 1kW is very good (>90%) at rated
power but poor at less than 50% rated - so size the generator just above the
max power output. Try to choose the "high efficiency" types if efficiency is
important to you. It is important to also consider power factor correction,
for efficiency reasons and particularly if you are going to be charged for
reactive power. Correction is often done using capacitors which are
connected when generating. An important safety issue, particularly if
capacitors are used for p.f. correction, is self-excitation. If the grid
goes down it is possible for the generator to self-excite and generate wild
voltages, so you need decent loss-of-mains protection relays.

Incidentally, most small (<3kW) portable petrol gennies are self-excited
induction motors. That is why the voltage is wild, and drops so badly when
you load them up heavily and the speed drops. List folks, if you have read
this far, on this issue of killing Trace FETs when running out of fuel: When
overspeeding in its dying coughs a capacitor excited generator will produce
very high voltages especially if unloaded. Proper AVR controlled synchronous
generators should not do this. Why not use a delay-on-energise (or a voltage
window) relay to control the small standby generator's connection?

I know at least one manufacturer in the UK who produces a grid connection
controller specifically for small induction hydro turbines which does all of
the water control, grid connection control and protection required. You just
press the start button!

Self-excitation can be useful if you wish to use use induction generators
for stand-alone mini-grids. If the correct size of capacitors are used the
generator can self excite and supply (slightly wild) power very cheaply -
this is popular in developing countries. It is important to have overvoltage
protection in case of overspeeds. Home Power has had a few articles on
people doing this which are excellent although the writers do suggest a
rather suck-and-see approach to system design and capacitor selection. If
you want to avoid reinvention of wheels and magic in general there is a
superb book (short with no unnecessary padding) which gets you there with
proper engineering formulae which I cannot recommend highly enough:

"Motors as Generators for Micro-Hydro Power" by Nigel Smith, Intermediate
Technology Publications Ltd. ISBN 1 85339 286 3     Price £7.50

This book covers everything from the priciples of induction motors, to grid
connection and capacitor selection for power factor control and stand-alone
self-excitation.

I'm no expert but I work with induction generators all the time and it is
not rocket science. Give it a go and read the book first. Hope this helps!

Regards
Charlie

Charlie Robb
Element Engineering
25 Sutherland Avenue
Riverside
Stirling
FK8  1LU      UK

Tel/Fax: 01786 472259
Mobile:   07887 991866

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