Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Case 2543
1078017
This invention relates to a corona discharge
mon;tor system for dynamoelectric machines.
In dynamoelectric machines, and particularly in
larger salient pole dynamoelectric machines, the stator
winding normally comprises one or more conductors or bars in
slots, surrounded by insulation, with an armour jacket which
is often a partly conductive armour jacket overlying the
insulation on the side walls of the bar. The term "partly
conductive" or "partially conductive" is used only to indicate
a conductive material having a resistance which permits
current flow but is less conductive than the conductors used
in a dynamoelectric machine. The bars are fitted into the
respective slots in the stator core laminations and secured
by wedging or other means. Initially the partially
conductive armour covering on the bar or coil side makes
good electrical contact with the edges of the laminations
which define the slot in which it is located. Thus, the
partially conductive armour will be at the same potential as
the core. The bar vibration caused largely by the electro-
magnetic forces in an operating machine, coupled with thermal
expansion and contraction, will in time reduce the integrity
of the contact between the partially conductive armour jacket
and the laminations. The laminations are not all identical
and some project very slightly farther into the slot than
others. This non-uniformity in the slot wall may tend to
increase the tendency to lose contact resulting from
bar movement due to vibration and thermal expansion and
contraction. When the integrity of the contact is reduced
between the partially conductive armour and the laminations,
the capacitive current that is normally drained to ground
li ~>o~ h4 v~ ~
through the partially conductive armour will ~_wi~rit-low
resistance path through which to flow. As a result, voltage
Case 2543
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gradients sufficient to cause various types of corona and
slot qischarge may occur. The term "corona discharge" as
used herein is intended to cover a range of discharge
phenomena including a streamer type of irregular pulse
discharge, a steadier glow type discharge, and a breakdown
discharge or spark discharge which may be a complete slot
discharge. The invention will, depending on background noise,
detect lower levels of corona discharge, however the invention
is primarily aimed at the detection of the slot discharge or
spark which is of higher energy and causes the greatest
amount of damage to insulating materials. As higher opera-
ting voltages are used in dynamoelectric machines there is a
greater tendency for corona discharge to occur. It is well
known that in the presence of corona discharge particularly
a spark type of discharge an insulating material can be
eroded and eventually break down.
It is known in dynamoelectric machines that provision
of a resilient partly conductive material between the armour
and the core greatly reduces the tendency for corona discharge
to occur, but it does not eliminate it completely. There
are many dynamoelectric machines which use a hard conductive
paint. This appears to be inferior to the resilient conducting
material, i.e. resilient partly conductive material for
control of slot discharge.
It will be apparent that if the presence of corona
discharge could be determined in an operating dynamoelectric
machine, the machine could be shut down and repair made before
the problem developed into a serious fault. It will also be
apparent that if the extent and exact location of the corona
discharge could be determined, the repair could be effected
much more efficiently and quickly.
A corona discharge appears to provide electrical
excitation to the bar over a wide frequency spectrum. It
appears, for example, to provide energy from perhaps 0.5 MHz
1078017 Case 2543
to 100.0 MHz as well as .some energy at other fequencies.
In the past use has been made of the lower
frequencies, perhaps up to frequencies of the order of a
few hundred kHz, to detect corona discharge in dynamoelectric
machines. To determine corona discharge by this procedure,
the machine is disconnected from the line and run at no load,
full voltage configuration. Normally an oscilloscope is
connected to each phase through an isolating capacitor and
a filter and the voltage observed on an oscilloscope. With
the line frequency and harmonics filtered out, a corona
discharge will show up as small voltage pulses on the
oscilloscope trace indicating to an experienced eye that
there is corona activity on one or another phase of the
machine winding. At lower frequencies, the signals from one
phase can propagate through the entire winding via the neutral
connection making it difficult to determine the exact location
of the corona activity. When a high level of corona activity
is found, ~e machine is then shut down and the bars in the
phase which gives the largest corona signals are physically
examined to locate the region of corona discharge.
Not only is it impossible, without physical
inspection, to locate a particular slot or even a particular
area where a corona discharge has occurred using the detection
method just described, but there is considerable amount of
interference combined with the signal making it difficult to
separate the desired signal from the noise.
In the past attempts have been made to detect
corona discharge using acoustic energy. A detector adjacent
the dynamoelectric machine detects acoustic energy in the
sonic or ultrasonic region. This has not been satisfactory
and is impractical in an operating machine.
It is therefore a feature of this invention to
1078~17 Case 2543
provide a method of determining at least a particular area
involving only few slots where corona discharge events are
occurring.
It is also a feature of this invention to provide
apparatus capable of determining at least a particular area,
if not an exact slot, where corona discharge activity is
occurring.
In a system according to the present invention,
an antenna is mounted on a rotor pole to pick up radiated
energy in a radio frequency band, for example energy at a
particular radio frequency, and a coupling arrangement is used
to transfer the radio frequency signal energy from the rotor
to a signal conditioning unit. The signals are sampled
synchronously with the passage of the antenna over each bar
in a manner which allows location of a region of corona
discharge (if not the exact slot where corona discharge occurs).
Wedges have almost no influence on radiation at radio fre-
quencies. The radiated energy from a bar occurs at well
defined frequencies related to the length of the bar acting
as a half wave radiating antenna and are easily picked up.
These frequencies do not propagate into the rest of the
winding but radiate principally from the front of the slot
in which they occur. Consequently a good indication is
obtained of the precise region of corona discharge. This
procedure can be used while the machine is in service, and
this is an important feature.
Accordingly there is provided a corona discharge
monitor system for a dynamoelectric machine having a rotor and
a stator, said stator having slots therein with insulated
conductors in said slots, said system comprising antenna
means mounted on said rotor for rotation with said rotor for
receiving radio frequency signals resulting from corona
1078017 Case 2543
discharge in said slots in succession, means for transferring
said radio frequency signals received by said antenna means
to a place externally of said dynamoelectric machine, means
for detecting the rotational position of said rotor, and
thereby the rotational position of said antenna means and for
providing a positional signal representing the position, and
utilization means for receiving said radio frequency signal
from said means for transferring said radio frequency signal
and for receiving said positional signal from said means
for detecting the rotational position of said rotor and for
synchronizing said signals to provide an indication of the
occurrence of said radio frequency signals with reference to
the rotational position of said antenna means.
Also according to the invention there is provided
a method for locating corona discharge in a slot in the
stator of a dynamoelectric machine, comprising running said
dynamoelectric machine under load, receiving radio frequency
signals occurring in said slots with antemla means on the face
of the rotor of said dynamoelectric machine, transferring
the received radio frequency signals to a place external of
said dynamoelectric machine, detecting the rotational position
of said rotor and providing a positional signal representing
the position thereof, synchroni2ing the transferred radio
frequency signals and the positional signal to provide an
indication of the occurrence of radio frequency signals with
reference to the rotational position of the rotor.
The invention will be described with reference
to the accompanying drawings, in which:
Figure 1 is a schematic isometric view of a rotor
of a dynamoelectric machine with a simplified representation
of an embodiment of the apparatus of the present invention,
Figure 2 is an isometric view showing detail of a
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~ 1078017
particular antenna arrangement according to an embodiment
of the invention,
Figure 3 is an isometric view of the outer coupler
shown in broken lines in Figure 2,
Figure 4 is a simplified block diagram of suitable
signal conditioning circuitry for the present invention; and
Figure 5 is a schematic diagram showing in
diagrammatic form a multiplexer suitable for the circuitry
of Figure 4.
Referring now to Figure 1, a rotor 10 of a
dynamoelectric machine is shown in simplified form. Rotor 10
rotates in a stator core (not shown) as is well known in the
art. There are slots in the stator core which hold conductors
or bars and it is in these slots that corona discharge may
occur. In a stator winding each bar, depending on its place
in a particular coil, will vary in its R.M.S. voltage with
respect to the stator laminations. Consequently some bars
may only reach low levels of voltage while others may reach
voltages sufficient to result in all types of corona discharge.
It is importantto know the location of the slot in which any
type of discharge occurs, and it is also desirable to know the
severity of the discharge. To do this an antenna 11 is mounted
on the surface of a rotor pole. Thus~ the antenna 11 will
sweep across each slot in the stator core once in each
revolution. In a generator it is preferable to locate the
antenna behind the centre line of the pole by the average torque
angle for the particular machine. The voltage is maximum
when the rate of flux change is maximum, or as one pole leaves
a coil and the next pole approaches. This is true for no
load conditions. In a generator under load the armature
reaction causes the resultant flux to lag the pole flux by
the torque angle so under load the poles move ahead of the
Case 2543
1~78017
voltage wave formed ~y the torque angle. Thus the antenna
is preferably placed behind the centreline of the pole by
the average torque angle.
A transmission line 12 connects antenna 11 to a
rotating coupler 14 encircling the shaft 15 of rotor 10, or
other coupling means. Spaced radially outwardly from and
concentric with rotating coupler 14 is a fixed coupler 16.
The fixed coupler 16 is connected by transmission line 17
to a utilization means or apparatus 18. The utilization
apparatus 18 may include a radio frequency amplifier, a signal
conditioner such as a filter or filters, and some storage and
indicating or recording means. As will be explained in
more detail hereinafter it will also in a preferred form
include means for synchronously relating the radio frequency
signals with a positional signal from the rotor to relate the
indication to antenna position as a function of time.
Very briefly, as rotor 10 rotates on its shaft 15,
the antenna 11 sweeps across the slots in the stator core.
Any corona discharge taking place within a slot while the
antenna is adjacent to it will produce a burst of high
frequency energy and at least some of this energy will be
radiated at radio frequencies. Antenna 11 will pick up this
radio frequency energy and will provide a corresponding signal
at rotating coupler 14. The rotating coupler 14 and fixed
coupler 16 may be considered as a form of rotary transformer,
and the signal on the rotating coupler 14 is thus coupled
to the fixed coupler 16. Transmission line 17 conducts the
signal to the utilization apparatus 18 where it may ~e passed
through stages of filtering, amplification and detection.
The signal will thus be a representation of radio frequency
energy which is present as antenna 11 sweeps around the stator
core. By synchronously relating the signal with rotor rotation,
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Case 2543
1078017
any indication of corona discharge in the signal can be
readily associated with a particular slot or a sm~ll group
of slots in the stator core.
Referring now to Figure 2, there is shown a portion
of a rotor 10, enlarged with respect to Figure 1, with an
antenna 11 and rotating coupler 14 shown in more detail. The
antenna 11 in a preferred form is a balanced non-resonant
antenna comprising a 50 ohm stripline structure having a
base of dielectric material 21 with a ground plane in the form
of a metallic coating 22 on the side adjacent to the rotor 10
and conductive strips 23a and 23b on the opposite side. The
strip 23a is terminated at the outward end by a 50 ohm resis-
tance 24 connected from the strip 23a to the ground plane 22.
The strip 23b is terminated at its outward end by a 50 ohm
resistance 25 connected from strip 23b to the ground plane 22.
A coaxial cable 12a has its central conductor 26 connected
to the inward end of strip 23a and its outer conductor
connected to ground plane 22. Similarly a coaxial cable 12b
has its central conductor 27 connected to the inward end of
strip 23b and its outer conductor connected to the ground
plane 22.
The rotating coupler 14 comprises a dielectric
base 28 having a ground plane 30 on the side thereof adjacent
the surface of shaft 15, and a pair of parallel 50 ohm
striplines 31a and 31b which are on the radially outward side
of dielectric base 28 and which extend around a major portion
of shaft 15 as shown. The central conductor 26 of coaxial
line 12a is connected to one end of stripline 31a, and the
central conductGr 27 of coaxial line 12~ is connected to one
end of stripline 31b. The outer conductor of both coaxial
lines 12a and 12b is connected to ground plane 30. The end
of each stripline 31a and 31b, remote from the ends to which
Case 2543
1078017
the coaxial cables 12a and 12b are connected, are connected
to ground plane 30 by 50 ohm resistors 32 and 33 respectively.
Spaced radially outwards from rotating coupler 14 is fixed
coupler 16 which is shown in broken line in Figure 2 in
order to indicate its position. Fixed coupler 16 is shown in
more detail in Figure 3. The spacing between the rotating
coupler 14 and the fixed coupler 16 is not critical but should
be ~ept as small as possible to improve coupling in keeping
with the mechanical considerations.
Referring now to Figure 3, the fixed coupler 16
is shown having a flat, circular dielectric base 34 made
in two halves 34a and 34b for ease of assembly around the
shaft of the machine. There is a ground plane or ground
coating 35 on the outer surface of dielectric base 34. On the
inner surface of each half 34a and 34b are two 50 ohm
striplines. The striplines on the half 34a of base 34 are
designated 36a and 37a. The striplines on the half 34b of
base 34 are designated 36b and 37b. The ~trlpline~ 36a and
37a are terminated by 50 ohm resistors 40 and 41 respectively.
The striplines 36b and 37b are terminated by 50 ohm resistors
42 and 43 respectively. There is a 50 ohm coaxial cable
connected to the other end of each of the striplines. That
is, a coaxial cable 44a and 45a is connected respectively to
the other end of stripline 36a and 37a, and a coaxial cable
44b and 45b is connected respectively to the other end of
stripline 36b and 37b. In other words, the shielding or
outer conductor of each coaxial cable 44a, 44b, 45a and
45b is connected to ground plane 35, and the inner conductor
46a, 46b, 47a and 47b respectively is connected to the end
3Q of stripline 36a, 36b, 37a and 37b respectively.
The stripline coupler just described may be
referred to as a cylindrical balanced contra-directional
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. i~78017 Case 2543
stripline coupler.
The antenna of Figure 2 comprising ground plane 22
and striplines 23a and 23b, and the rotating and fixed coupler
of Figures 2 and 3 comprising respectively ground plane 30,
striplines 31a and 31b and ground plane 35 and striplines
36a, 36b, 37a and 37b, are all referred to as 50 ohm striplines
with appropriate termination. It should be emphasized that
striplines of other impedance values, with suita~le termination,
could be used, and also the antenna and rotating and fixed
couplers need not be in the form of stripline. While stripline
is suitable and is preferred because of its balanced
arrangement and ease of construction in this embodiment, it
is not essential to the operation of the invention.
Also, it is sometimes convenient for ~oth rotating
coupler 14 and fixed coupler 16 to use air as a dielectric,
that is to use solid dielectric supports at intervals around
the coupler to provide a constant air gap space from the
ground plane, thus providing air dielectric.
The length of the stripline portions in the coupler
will determine the frequency at which maximum coupling
occurs. It has been found in practice that the length of
the shorter portion of the coupler whether fixed or rotating
appears to have greater effect on determining the frequency
at which maximum coupling occurs. The length is preferably
made as one quarter the wavelength of the desired coupling
frequency, although other design factors as are known in
the art are used to keep the coupling as broad as possible.
The diameter of the coupler can be increased above the shaft
diameter if this is necessary to lower the coupling frequency.
Referring now to Figure 4, two balanced coaxial
cable pairs 44a, 45a and 44b 45b are connected to a balanced
radio frequency multiplexer 48. A rotation detector 50 provides
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iO78017 Case 2543
a signal representing each complete rotation of rotor 10,
or a multiple thereof, to a switch control 51. It may be
preferable to use two detectors 50, one on each side of the
rotor, in combination with a triggered multivibrator or
flip-flop which will give a convenient indication of which
half of the fixed coupler is in a desired position as will
be explained more fully hereinafter. When switch control 51
receives a signal representing rotor rotation it provides a
signal over conductor 52 to multiplexer 48 to control the
multiplexer to switch back and forth during each half cycle
of rotation of rotor 10. It will be recalled that on the
rotating coupler 14, the striplines 31a and 31b extend around
a major portion of the shaft 15, but that there is a gap
between the terminating ends of the stripline 31a and 31b and
the ends coupled to the coaxial cable 12a and 12b (see
Figure 2). If the fixed coupler were constructed in a similar
configuration the signal coupled from one to the other would
at certain frequencies cancel out due to a 180 phase shift
down the stripline. That is, if the stripline in the fixed
2n coupler is distributed over both ends of the rotating coupler
a loss of signal would occur at that location once each
revolution of the rotor because of the difference in phase of
the signal at each end of the rotating coupler. To avoid this
the fixed coupler 16 is made with two sets of stripline
antenna. The multiplexer 48, driven by the signal from switch
control 51, switches to receive the signals from cables 44a
and 45a when the gap in the stripline on the rotating coupler
is not opposite stripline 36a and 37a, and switches to receive
signals from cables 44b and 45b when the gap on the stripline
3Q on the rotating coupler is not opposite stripline 36b and 37b.
Thus the signal coupled from the rotating coupler 14 to the
fixed coupler 16 is always from the coupler half which is not
1~78017 Case 2543
over the gap in the rotating coupler. A simple arrangement
for carrying this out is shown schematically in Figure 5.
Referring for the moment to Figure 5, there is
shown in simplified schematic form, a circuit which could be
incorporated into block 48 (Figure 4) representing the
balanced radio frequency multiplexer to provide the necessary
switching from one set of stripline couplers to the other set.
The centre conductors 46a, 47a, 46b and 47b of coaxial cables
44a, 45a, 44b and 45b respectively, are connected to a pair
of double pole, double throw radio frequency switches. That is
conductor 46a and 47a are connected to switch members 53
and 54 respectively, and conductors 46b and 47b are connected
to switch members 55 and 56 respectively. Switch members
53, 54, 55 and 56 are actuated simultaneously by arm 57
operated by a drive 58. As shown in Figure 5, switch members
55 and 56 are in a closed position where conductor 46b is
connected to end 60 of a shielded balun transformer 61, and
conductor 47b is connected to end 62 of transformer 61. As
shown in Figure 5, switch members 53 and 54 are in the open
position. Thus, as shown in Figure 5, striplines 36a and 37a
are coupled to one side of transformer 61. When a signal from
switch control 51 is received over conductor 52 to cause
switching, then drive 58 operates arm 57 to move switch
members 53 and 54 to the closed position and switch members
55 and 56 to the open position. This will couple striplines
36b and 37b to the respective sides of balun transformer 61.
The winding 63 of transformer 61 will thus have signals
received from alternate sides of fixed coupler 16. The
switching action takes place twice each shaft revolution as
3Q was previously explained to obtain a signal unaffected by
the gap in striplines 31a and 31b (Figure 2) as the striplines
rotate on shaft li. While Figure 5 shows a mechanical switch
- 12 ~
. 1078~7 Case 2543
arrangement for simplicity of description, it will be apparent
that solid state radio frequency switches are preferred.
Referring again to Figure 4, the output from
balanced multiplexer 48 is applied over conductor 64 to a
broad band radio frequency (RF) amplifier 65 which provides
initial amplification. The output from amplifier 65 is
applied over conductor 66 to a tuned radio frequency (RF)
amplifier 67. Although the coupler tends to have a resonant
frequency, its response is relatively broad. There are
frequencies which are emphasized by resonance as a result of
the particular physical construction of the installation,
such as bar length. The tuned RF amplifier permits selection
of a specific frequency which is most significant. It has
been found that significant frequencies are generally in the
range of 25 MHz to 100 MHz, depending largely on bar length.
Frequencies in this range have not been used before to
detect and localize corona discharge in a dynamoelectric
machine. Frequencies in this range do not propagate to any
extent to the remainder of the winding and radiate principally
from the slot in which the discharge occurs. It is this
phenomena which allows location of the slot discharge.
There appears to be two factors of interest in
the signal on conductor 68 which represents the corona discharge
as the slots in the stator are scanned sequentially. These
factors are (1) the maximum amplitude of discharge for each
slot, and (2) the number of discharges which take place as the
antenna is scanning each slot. Each factor may have a
significance and it is preferable to determine a value for
each.
3Q The signal on conductor 68 is applied as an input
to envelope detector 70, which provides an output signal
on conductor 71 representing the envelope of the radio
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Case 2543
1~7B017
frequency burst for each slot discharge occurrence. The
conductor 71 provides one input to peak detect sample and
hold circuit 72. Conductor 73 provides a control input to
peak detect sample and hold circuit 72. A trigger pulse
generator 74 receives a signal on conductor 75 representing
the rotation of the rotor, and a reference timing signal on
conductor 76 from control 77. Control 77 provides a pulse
waveform comprising a series of pulses on conductor 73
representing the number of stator slots per second, scanned
by the antenna. Each pulse generated by trigger pulse
generator 74 on line 80 is synchronous with the pulses on
line 73 from control unit 77. However, the trigger pulse
generator 74 produces only one pulse per revolution either
coincident with a reference slot or delayed with respect to
the reference slot by a specific number of slots. The peak
detect sample and hold circuit 72 samples the signal on
conductor 71 as the antenna passes each slot and holds the
peak value until the antenna is adjacent the next slot. The
sampling process is repeated as the antenna passes each slot.
Thus, the output of circuit 72 on conductor 78 represents the
peak discharge amplitude for each slot during the time the
antenna is adjacent the slot. The output signal on conductor
78 goes to a storage apparatus or means 84. The storage
means 84 provides storage for a longer term than circuit
72 and may be a memory device. Preferably the storage means
84 stores signals from conductor 78 and averages them over a
number of revolutions of the rotor. The storage means 84
is synchronized with respect to rotor rotation by a signal
from conductor 80. The storage means 84 may include means
to digitize the signal from conductor 78 for a computer
printout to show discharge amplitude with reference to rotor
rotation. Storage means 84 may include some form of display.
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Case 2543
1~78017
For example, it may include a plotter which provides a plot
of corona discharge with reference to rotor rotation. In a
preferred form, storage means 84 may be a variable persistance
oscilloscope which provides a display of amplitude or degree
of corona discharge for each slot whereby an average amplitude
is indicated for a number of rotor revolutions.
A detector 81 also receives the signal on conductor
68. The detector has a short time constant and provides
a pulse for each burst of radio frequency energy on conductor
68, and provides an output pulse on conductor 82 for each
such burst. Conductor 82 is connected to a counter 83 which
counts the pulses for each slot. Counter 83 also receives a
signal via conductor 73 which latches the digital output
and resets the counter as the scanning antenna passes to the
next slot. When the counter 83 is latched it provides an
analog voltage proportional to the count for that slot on
conductor 85. Conductor 85 is connected to a storage
apparatus or means 84a which may be as before a plotter or
a variable persistance oscilloscope, properly synchronized
by the signal on conductor 80, to provide a plot or display
representing the number of discharges for each slot.
A suitable storage apparatus 84, 84a for this
particular implementation is a dual beam, variable persistance
oscilloscope with external triggering. The dual display will
show simultaneously the maximum amplitude of discharge for
each slot and the number of discharges for each slot. Thus
conductors 78 and 85 would provide the two display or vertical
inputs and conductor 80 would provide the necessary trigger
for the sweep.
It will be seen that the present invention provides
not only for detection of corona discharge under operating
conditions but provides for determining a particular slot (or
group of a few slots) as the location of the associated discharge.
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