Note: Descriptions are shown in the official language in which they were submitted.
Case 2846
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APPARATUS FOR AUXILIARY POWER SUPPLY FROM
A GE~ERATOR FIELD
BACKGROU~D OF THE INVE~TION
This invention relates to apparatus for
providing auxiliary power from a generator field, and
in particular it relates to apparatus for mounting on
the rotor of a generator to provide a source of
auxiliary power on the rotor.
It is often desirable to have auxiliary
equipment mounted on the rotor of a generator, for
example to monitor the temperature of conductors in
the field winding or the insulation surrounding the
conductors, or to monitor electromagnetic radiation
~ that might occur in the stator windings because of
lS corona discharge, or to monitor other paramaters, and
to transmit the data representing the monitored
parameter from the rotor to an externally located
receiver. For example, Canadian Application Serial
No. 436,336, filed September 9, 1983 in the name of
James S~ Mark and assigned to Canadian General
Electric Company Limited describes a telemetry sy~tem
using infra red radiation for transmittin~ data
between the rotor of a generator and externally
located equipment. Auxiliary equipment of this nature
requires a supply of electrical power to operate. The
amount of power required by such equipment is
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relatively small, for example with a voltage up to
perhaps 24 volts d~ and a current of perhaps up to two
amperes and frequently less. This power can be
provided to the rotor through slip rings from an
external source, or it can be provided by a battery
mounted on the rotor.
It is often difficult to provide extra slip
rings on a generator for the purpose of providing
auxiliary power to the rotor because there is usually
little space available on the shaft of the rotor for
mounting auxiliary equipment and it is expensive to
provide space for this purpose. Additionally, extra
slip rings provide another complication to the
design. On the other hand, battery power is suitable,
but the battery must be replaced or it must be
recharged periodically. It is, of course, not
feasible to shut down a large generator in order to
replace a battery.
It will be apparent that there is, however,
a large source of power available on the rotor. There
must be direct current (dc) power to supply the
generator field windings. As one example only, a
typical 100 ~W generator might have a field supply
rated at + 3~0 volts dc at perhaps 1000 to 1300
amperes. With all this power available on the rotor
it should be possible to supply auxiliary equipment
re~uîring perhaps a voltage up to 24 volts at perhaps
one or two amperes. However it i5 not a simple matter
to derive suitable power with a supply of a
conveniently small size to mount on the rotor. It is
not efficient to use dropping resistors across the
field and then to tap off the required 9, 12 or 24
volts. Too much power would be wasted and there is
another problem related to control of the field
current.
A large power generator normally has a
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current regulated field supply to control the
generator output in accordance with a varying load.
The rate at which ~he generator output responds to
load changes mus-t be fa~t and it ls therefore quite
common to use an electronically regulated field
supply. Such field supplies are normally phase
controlled rectifiers which can quickly change the
time at which conduction starts in a thyristor
rectifier or silicon controlled rectifier (SCR). The
delay time, or angle of retard, after the vol~age
starts to go positive will vary the current conducted
by the rectifier and hence the average output voltaye.
Another complication is that forcing
voltages may be used to obtain rapid changes in field
current. The field current of a large generator may
be one or two thousand amperes and it will be fed in-to
the field winding which may have an inductance of
several henries. It is difficult to make rapid
changes of field current under these conditions and i-t
is common practice to "force" current changes up or
down by voltage changes which go rapidly up and down
in excess of normal changesO Forcing may be up to
eight or ten times normal which means for a 300 volt
dc supply the actual voltage could be up to 200Q volts
dc or even 300Q volts dc. Thus the field supply must
be able to pxovide a voltage of either polarity at any
given time. It is common practice to use a phase
controlled converter having two quadrant operation. A
description of such a phase controlled converter may
be found, for example, in the text book "Thyristor
Phase~Controlled Converters and Cycloconverters" by
B.R. Pelly, Wiley-Interscience, 1971, Chapter 3.
It is therefore not practical to place
dropping resistors across the field to try to obtain a
supply of 24 volts for auxiliary equipment on the
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rotor. It is, in fact, difficult to obtain a unipolar
supply of ~24 volts dc by any means from a field
supply which may be of either polarity with a value of
perhaps 3000 volts peak to peak negative to positive.
In a field current power supply using phase
controlled SCRs, each time the SCRs are turned on or
start to conduct at a time later than the time the
voltage is available to cause conduction, there is a
rapid change which generates high frequency components
which appear to extend into the 50 Khz range. These
high frequency components or disturbances in the
unfiltered field supply are undesirable ox at least
unnecessary insofar as the operation of the generator
i5 concerned, by they can be used by the apparatus of
the invention to provide a source of auxiliary power.
SUMMARY OF THE INVENTION
The present invention makes use of high
frequency ac components present in the field supply to
the rotor field windings. These high frequency
~O components are present when there i5 a current
regulated field supply and the regulation is
electronic regulation. The apparatus of this
invention would not function if all the high frequency
components were eliminated from the field supply, for
example by extensive high frequency filtering.
However, it is an unnecessary expense to provide high
frequency filtering for a generator field supply when
such filtering has no practical value.
The apparatus of this invention has a high
frequency transformer (sometimes ref~rred to as a
pulse transformer) with two windings. The first
winding is connected in series with a high voltage
capacitance across the field supply on the rotor of a
generator. The second winding is connected to a
rectifier. The rectifier output is preferably
filtered and has a current limiting device and a
Case 2846
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voltage control device. The limited and controlled
output may be used to keep a battery fully charged to
power auxiliary equipment.
It i5 another object of the invention to
provide a source o~ auxiliary power on the rotor of a
generator, where the source utili7es high ~requency ac
components in the electrical supply to the generator
field winding where the ac components are largely
created by the switching in an electronically
regulated field supply~
Accordingly there is provided a power supply
for the rotor of a generator having a field winding on
the rotor and a source of power for the field winding
having high frequency ac components comprising
a high frequency transformer having a first
and a second winding,
a capacitor for connection in series with
said first winding across said field supply of saicl
rotor,
a rectifier connected to said second winding
and rectifying ac voltages from said second winding,
and
~ilter means connected to said rectifier to
filter the rectified voltages from said rectifier and
provide a source of power for auxiliary equipment on
said rotor.
It is therefore an object of the invention
to provide an improved auxiliary power supply for
mounting on the rotor of a generator.
BRIEF DESCRIPTION OE' THE DRAWINGS
Figure 1 is a simplified schematic drawing
showing the invention, and
Figures 2 and 3 are graphs of voltaye
plotted against time showing an example of a partial
cycle of voltage in the converter providing the field
supply and the input of the supply of the invention.
Case 2~46
DESCRIPTION OF TH~ PREF~:RRED EM~3ODIM:~NT
Referxlny to ~igure 1 there is shown a
schematic drawing having conductors 10 and 11 on the
rotor of a generator which conduct current to the
rotor field windings (not shown). Across conductors
10 and 11 a capacitor 12 and a winding 14 of
transformer 15 are connected in series. The capacitor
12 must have a high voltage rating sufficient to
withstand the forcing voltages which may be applied to
conductors 10 and 11 to force a desired field current
through the field winding, and consequently it may in
practice be several capacitors in series.
Transformer 15 is a high frequency
transformer preferably capable of handling frequencies
which extend to at least several kilohertz. ~le
winding 16 of transformer 15 is connected to a bridge
rectifier 17. The individual rectifiers used in the
bridge rectifier 17 are high speed rectifiers, for
example type A115 (G~neral Electric Company). The
output of bridge rectifier 17 is on conductors 18 and
20 and capacitors 21 and 22 are connected across this
output. Capacitor 21 is to pass high frequency
transients and may be of the order of 0.33 ufd.
Capacitor 22 is a filter capacitor and may be of the
order of 5500 ufd at 75 volts dc. A zener diode 23
and a capacitor 24 are connected across conductors 18
and 20. The zener diode 23 is selected to break down
at a voltage which is very generally about twice the
desired ou~put voltage of the power supply. More
specifically, the zener diode 23 is selected to have a
breakdown voltage which will protect the three
terminal regulators used in the following circuitry
and described subsequently. If the desired power
supply voltage is 24 volts dc, the zener diode might
have a breakdown voltage of the order of 51 volts. A
resistor 25 connects the junction of zener diode 23
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and capacitor 24 to the base of $he transistor 26. A
resistor 27 connects the collector to conductor 18 and
the emitter of transistor 26 is connected to conductor
20. When zener diode 23 breaks down the transistor 26
is switched on causing current to flow through
resistor 27 and transistor 26 to drop the voltage
across conductors 18 and 20.
Conductor 18 is connected to a three
terminal integrated circuit connected as a current
limiter 28. The output terminal is connected through
resistor 30 to another three terminal integrated
circuit connected as a voltage limiter 31. The
control connection for current limiter 28 is connected
by conductor 32 to the input of voltage regulator 31.
A resistor network comprisiny resistors 33, 34 and 35
provides a desired control voltage on conductor 3~ for
the control input of voltage regulator 310 The output
from voltage regulator 31 is on conductor 37 which is
connected through diode 38 to output terminal 40. A
by-pass capacitor 41 is connected between conductor 37
and conductor 20. The power supply of Figure 1 is
adapted for mounting on the rotor of a generator.
While the power supply will directly power suitably
rated auxiliary equipment when the generator is
running and there is full voltage supplied by a phase
controlled converter, it is convenient to use the
supply to keep a battery in a charged condition. The
battery is indicated in broken lines as 42. The
battery and the supply of ~igure 1 provide power for
auxiliary equipment such as measuring and tele~etry
equipment. The diode 38 is to protect the apparatus
i~ the battery 42 should inadvertently be connected
with the incorrect polarity.
Referring for the moment to Figure 2, t~lere
is shown a graph of a partial cycle of ac voltage as
might appear in a field supply providing
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electronically regulated power for the field winding
oE a generator. The voltage increases to a point 50
at which time an SCR (not shown) in the field supply
is fired, i.e., begins to conduct. A description of
the operation of a phase controlled converter, which
is a suitable source for field power for a generator,
may be found in the aforementioned textbook "Thyristor
Phase-Controlled Conveters and Cycloconverters". When
conduction occurs the voltage drops very rapidly as
shown. It may, of course, go negative but is shown
for convenience at a zero reference. The voltage then
tends to oscillate at a high frequency as at 51. ~his
high frequency oscillation or high frequency ac is a
component of the field voltage across conductors 10
and 11. The high frequency ac component is coupled
through transformer 15 (Figure 1) and it appears
across winding 16 (Figure 1) as the ac component 52
shown in Figure 3.
It will be recalled that the field voltage
may take quite abrupt changes as required to force a
desired field current through the field windings, and
it may go negative. This does not matter insofar as
the apparatus of the present invention is concerned
because the apparatus uses only the high frequency ac
component on the field supply.
~ eferring again to Figure 1, it is believed
the operation of the power supply apparatus of the
invention is clear, however it will be discussed
briefly to ensure a complete understanding of the
invention. There is a fluctuating voltage across
conductors 10 and 11 which carry current to the field
winding. ~ high frequency ac volta~e component is
also present due to the rapid switchin-3 which occurs
in the phase controlled conver~er supplying power for
the field winding. ~apacitor 12 blocks the flow of
direct current, but the ac component is able to flow
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Case 2~46
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through capacitor 12 and winding 14. This produces a
voltage Oll winding 16 which may be similar to the
waveform of Figure 3. This is rectified by rectifier
17 and filtered. Any voltage fluctuations on
conductor 18 which exceed the level set by zener diode
23 will trigger transistor 26 to conduct and thereby
reduce the voltage. The supply includes a current
limiter and a voltage regulator. The output terminals
will therefore have a voltage regulated output whose
current is limited.
One example of a typical supply used the
following co~ponents:
capacitor 22 - 5500 ~fd at 75 volts
resistor 27 - 10 ohm, 20 watt
transistor 26 - 2N6249
resistor 25 - 100 ohm
diode 23 - lN5368
current limiter 28 - LM317HV
voltage regulator 31 - LM317HV
resistor 30 - one ohm, two watt
resistor 33 - 240 ohm
resisitor 34 - 5430 (adjust)
resistor 35 - 5.1 K (adjust)
diode 38 - lN5624
capacitor 41 - 1.0 ~fd
It will, of course, be apparent that the
values given above are by way of example and that
other values would be used to obtain different
characteristics. It is, of course, possible to use
different circuitry for limiting and controlling the
operation of the auxiliary power supply according to
the invention.
