METHOD AND APPARATUS FOR COUPLING POWER GENERATOR TO UTILITY POWER GRID

METHODE ET CIRCUIT POUR COUPLER UNE GENERATRICE DE COURANT AU SECTEUR

English Abstract

A switching circuit to reduce flashing in a switching circuit, when coupling a
power generator to utility power grid. The reduction, in flash, is achieved by
monitoring AC sine waves of utility power grid and power generator for
compatibility in frequency, amplitude and phase. The switching occurs when
all monitored signals are, momentarily, equal in time.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE
DEFINED AS FOLLOWS:
1. A switching means for stabilized coupling of an induction generator to an
alternating current (AC) utility power grid, comprising:
monitoring frequency, amplitude and phase of utility power grid,
monitoring frequency, amplitude and phase of induction generator, and
hybrid switching means for coupling induction generator to utility
power grid when the monitored frequency, amplitude and phase are
equal in time between the induction generator and utility power grid.
2. A switching means as claimed in claim 1, wherein the hybrid switching
means comprises a solid state relay in series with a mechanical relay.
3. A switching means as claimed in claim 2, comprising delay means for
delaying the closing of solid state relay relative to closing of mechanical
relay.
4. A switching means as claimed in claim 2, comprising delay means for
delaying the opening of mechanical relay relative to opening of solid state
relay.

Description

CA 02214370 1997-10-28
Field of Invention
This invention relates to a switching circuit suitable for power generators and
particularly to an ~llxili~ry generating system for coupling an induction motor
type generator to an AC power line for ~d(lin3~ power to that line.
Back~round of the Invention
Various methods have been proposed, to couple a power generator and in
particular, windmill generated power using an induction type motor, to a
0 utility power grid.
Mechanical relays, used to couple high culrellls, have a limited operating life
due to excessive wear caused by fl~shing between the relay contacts when
they make or break.
Wind velocity is random, unpredictable and subject to rapid changes and
therefor complicates the controller in which the generator is connected to
utility power grid.
2 o The problem is further complicated, by the fact that utility power grid is not
stable either. In North America, the amplitude changes by as much as 7% and
the frequency by 0.5Hz, while in other parts of the world the changes are a lot
higher.
25 Naran Kanti (Patent number PCT WO 95/19659) teaches us a method of

CA 02214370 1997-10-28
using a solid state relay and a mechanical relay in parallel to reduce heat
within the solid state relay.
Frank J. Bourbeau ~'atent number US 4,656,413) teaches us a method of
5 using SCR, two per phase, to couple a power generator to a utility grid. The
algolilhlll used is complex and not suitable for unstable utility power grid.
Frank J. Nola ~atent number US 4,473,792) teaches us a method of
controlling a triac to connect the generator to utility power grid. The triac is10 turned on at a relatively late point in each half cycle to reduce power supplied
by the utility power grid to the induction motor type generator and the net
power delivered to the line is increased. This method does not monitor the
changes in the power grid before connecting the generator to the power grid.
Summary of the Invention
There remains a need to couple a power generator to utility power grid which
is not stable (frequency and amplitude), as seen in non-industrialized
countries to elimin~te in-rush cullell~ and fl~hing
It is the object of this invention to effectively reduce fl~.~hing when couplingto utility power grid by monitoring AC sine waves of utility power grid and
power generator, for compatibility in frequency, amplitude and phase.
2 5 Another object of this invention is to isolate the generator from the utility

CA 02214370 1997-10-28
power grid when generator is de-coupled.
A further object of this invention is to effectively reduce in-rush current whena generator, which has induction type motor, is coupled to utility grid.
Brief Description of the Dr~.will~s
In drawings which illustrate by way of example only a preferred embodiment
of the present invention,
Figure 1 is a block diagram of one embodiment of the present invention
showing connection between utility power grid and windmill power
generator;
5 Figure 2A is illustrating the frequency and ~mplitude of power utility grid;
and
Figure 2B is illustrating the frequency and amplitude of generator.
Detailed Descrintion of the Invention
As shown in Figure 1, in one embodiment the present invention comprises an
AC induction motor/generator 24 which is mechanically driven by propeller
26 of a win~lmill (not shown).

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Capacitors 42 and 44, are excitation or power factor correction capacitors,
which are brought on line by solid state relays 20 & 22 under the control of
controller 12.
Voltage sensors 30, 34 and 38 sense the voltage output of the generators
while sensors 32, 36 and 40 sense the cullelll ofthe generator.
Transformer 28 converts three phase output of generator 24 into a single
phase for connection to utility power grid 50 through solid state relay 14,
0 mechanical relay 16 and manual disconnect switch 18 which is required for
maintenance purposes.
Although, three phase output of the generator can be connected to the utility
power grid, for simplicity, only a single phase connection is shown in the
example.
In the initial state, all relays 14, 16, 20 and 22 are opened to isolate all
sections from each other and from power utility grid 50.
20 Wind velocity will start to turn propeller 26 which in turn, drives the
induction motor 24. The output of generator 24 is monitored by controller 12
via sensors 30 to 40 for frequency, amplitude and phase.
The presence of frequency and amplitude at sensors 30 to 40, indicates that
there is enough wind velocity to start the generator, in which case, capacitors

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42 and 44 are brought on-line to start the excitation process.
At a pre-programmed generator output value, mechanical relay 16 is brought
on-line while solid state relay 14 is still left open.
At this time, controller 12 starts to monitor utility power grid via sensors 46
and 48 and power generator via sensors 30 to 40 for compatibility in
frequency, amplitude and phase.
0 The acceleration of generator 24 is also monitored, via the frequency of the
output, to predict the time when the frequency output of the generator 24 will
equal the utility power grid 50.
Solid state relay 14 is enabled at zero-crossing, when all monitored signals
(frequency, amplitude and phase) between utility power grid 50 and power
generator 24 are momentarily equal in time.
As the connection between utility power grid 50 and generator 24 is made
only when both frequency and amplitude are equal, the variations in
20 frequency and amplitude within the utility power grid 50 and generator 24 are taken into account before connection is made.
As the speed of solid state relay is extremely fast, in the order of micro
seconds, the connection between utility power grid 50 and generator 24 is
25 made while frequency and amplitude are still equal and thus effectually

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reducing flash and in-rush cull~nl problems.
Current sensors 32, 36, 40 and 48 are mollilored by controller 12 during
operation, for flow of power. The flow of power lis from utility power grid 50
to generator 24 (generator acting as a motor) when the wind velocity is low
and from the generator 24 to utility power grid 50 when there is sufficient
wind velocity.
Generator 24 is disconnected from utility power grid 50, when the wind
10 velocity is low for a pre-programmed period of time or when the power
re~-lin~ from sensors 30 to 40 are out of sync with each other indicating a
problem or unbalancing of phases within generator 24.
Over heating of generator 24 (sensor not shown) will also disconnect the
generator 24 from utility power grid 50.
During disconnect phase, the solid state relay 14 is turned off first and then
mechanical relay 16 is disabled to isolate generator 24 from utility power grid
50.
Figure 2A shows the waveform of utility power grid 50 and Figure 2B shows
the output of generator 24 at start up. As shown in Figure 2A, the utility
power grid 50 is stable for the period of time, while the output of generator
24, as shown in Figure 2B, is ch~n~ing.

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As indicated in Figure 2A and 2B, solid state relay 14 is enabled at zero-
crossing 52, when all monitored signals (frequency, amplitude and phase)
between utility power grid 50 and power generator 24 are momentarily equal
m time.
A prerelled embodiment of the invention having thus been described by way
of example only, it will be apparent to those skilled in the art that
modifications and adaptations may be made without departing from the scope
of invention, as set out in the appended claims.

Representative Drawing