Download Magnetic field measurement

Hydropower’92, Broch & Lysne (eds) 0 1992 Balkema, Rotterdam, lSBN 90 54 10 054 0
Magnetic field measurement
François Lalonde
IREQ, Institut de Recherche d’Hydro-Québec, Que., Canada
ABSTRACT: This paper describes a system to measure and analyse magnetic flux in the gap of an
electric machine. Our research group has contributed to new monitoring systems in the past. With
these new instruments, we have been able to determine that some machines still show strong
vibrations without any rotor/stator deformations. Magnetic unbalance is suspected, but can not be
measured while the generator is operating. The following paper describes a sure way to do so, the
preliminary results and the future goals.
1
INTRODUCTION
With time we have gained some experience
on large generator equipment. The research on
air-gap monitoring lead us to the development of
other instruments. One of latest instruments
tried at Hydro-Quebec is the magnetic field
monitoring system. We suspect that magnetic
unbalance may be the cause of many rotating
machine vibrations. When we suspect this kind
of defect there is no simple method available to
check the integrity of the rotor.
Most of Hydro-Quebec’s power come out of
hydraulic generators. For many years, HydroQuebec has acquired a wide experience in
hydropower generator operating, maintenance
and dynamic behaviour. Like many utility
companies, Hydro-Quebec is concerned with
equipment aging, In order to maintain high
plant availability, we need better monitoring.
Choosing the parameters that must be monitored
is not a simple task. We could tried to monitor
everything, taking the risk of being submerge
with data. Many new instruments are tested on
rotating machines for better monitoring, but not
all of those are beneficiary. On the other hand,
some instruments give us useful information
about the machine behaviour. The dynamic airgap monitoring system, which provides
information on rotor and stator shape under
various operating conditions, is a good example.
This system was developed by our research
group specially for large generators at HydroQuebec. We are not a rotating machinery
specialist team, our expertise is on electronic
instrumentation.
2
METHODS
Many different procedures are used to detect
winding shorts circuits. Our intention is to find
the simplest method specifically designed for
hydraulic generators!.
Pole Impedance Measurement (PIM)
During a normal maintenance outage, pole
impedance measurements are a complicated task
that require a lot of manpower and time. This
method gives indirect results which only allow
one to guess the operational uniformity of the
magnetic field. Nevertheless, shorted poles can
2.1
505
install this type of device in the air-gap because
it could hazard the machine or personnel
according to P.J. Tavner and al. (1986). Also,
many have found this sensor difficult to design
and install. Usually the search coil is installed
on the stator wedging system in turbinegenerators. The coils are made of many turns of
wire on a small form. One of the benefits of this
technique is that it is used on-line.
be detected, but is this enough? External
influences that modify the magnetic field are
temperature, dynamic air-gap changes and
mechanical stress. These influences are present
during the operation of a generator and must be
measured on-line, without stopping the
generator, especially during peak periods.
2.2
Time Domain Reflectometry (TDR)
According to many authors, the TDR
technique may be used to detect a short on a
winding. If a pulse is applied to the rotor
winding circuit, a short winding causes
reflections that can be analysed. This technique
is well described in J.W. Wood (1986) and is
called recurrent surge oscillograph (RSO). Up
to now, this testing technique may be used on
site. Future development may lead research to
on-line TDR testing.
Resume of the techniques
Some other techniques are described in
many papers but the above are the only one
selected to be useful in hydrogenerator
applications.
2.6
Table 1. Application of the selected techniques.
Method 1 Site testing [
On-line
2.3
Rotor Shaft Current (RSC)
According to Z. Posedel (1991), a short in a
pole winding will cause a current in the shaft.
Using suitable technique, one can analyse the
harmonic content of the shaft current to detect
the presence of a short. This technique may be
used on-line on some machines where only one
side of the shaft is grounded. In that case, this
may be very difficult to apply to
hydrogenerators.
3
TECHNIQUE SELECTION
Our goals are to get the simplest technique
unbalance on
to detect magnetic
hydrogenerators in normal operating condition.
So, the first two methods are rejected. We have
now to look at Rotor Shaft Current and Search
Coil.
The Rotor shaft current method could be
very attractive but we rejected it because it is not
always applicable on hydrogenerators and also it
can’t tell us where the defective pole is. It’s the
same with the split phase current method.
The coil in the air-gap may be difficult to
apply to every large generator since the air-gap
can be very small (<l0mm).
None of those techniques are exactly what
we are looking for. Consequently, we decided
to look what else could be done combining
2.4
Split Phase Current (SPC)
Unbalance split phase current may be
induced by magnetic unbalance. Also, other
problems on the generator may cause unbalance
split phase current. In this case, the method
gives us indications of the state of the machine,
but we can’t be sure what is the source of the
problem.
Search Coils (SC)
The magnetic flux variation in the air-gap
will induce a voltage in a coil placed in this airgap. It is obvious that this search coil will
detect a short winding. Many are reluctant to
2.5
506
techniques to get a simple method, compatible
with computer means for the development of a
positive analyses package.
4
THE NEW SYSTEM
In order to avoid hazard in the generator we
used our experience in air-gap measurement
with flat probes. We know how to build flat
sensors that can be installed on the stator
surface. For many years we had no problem
since the probes are made of special conductive
material over a tin material support.
With a different arrangement coupled to a
specially design electronic conditioning circuit
we have formed a new system to measure
magnetic flux in the gap of a rotating machine.
The probe is easily installed on the stator.
Usually it could be cemented on the stator
without removing the rotor. The probe is
installed on the stator wall by inserting it
between two poles. We have selected a fast
setting cement (1 min) for installation.
T h e p r o b e i s linked to electronic
conditioning circuits through 30 meter cables. It
is now possible to monitor, on-line, the
magnetic field of a large rotating machine. With
the addition of a key phasor probe, it is possible
to identify the magnetic field value of each
passing pole.
Modern computer technique will allow the
user to gain access to magnetic fields at
different time intervals, operating conditions
and to compare the field of each pole. Access to
the data is always available, therefore discarding
the need for outages during pole impedance
measurements. In the near future, we would
like to incorporate the newly developed
Magnetic-Field-Measurement-System to the Air
Gap Monitoring System (AGMS ®), which
would display, on a polar plot, the rotor/stator
roundness and centres and also the magnetic
field shape.
507
5
PRACTICAL RESULTS
The first 4 probes were fixed on a 60MW
hydrogenerator equipped with the Air Gap
Monitoring System. The method has proven
good results since April 1991. The system
showed that it may not give absolute
measurement value, yet it provides very good
relative value. In other words, it can’t tell the
exact magnetic flux value in Wb unit but it can
be used to compare all the poles in order to get
the difference between them. A well balanced
machine should have poles with the same
magnetic flux value. If an interturn short-circuit
occur on a pole, the magnetic flux of this pole
will be reduced and should be easily detected.
All the following data are from generator #l
in Manicouagan river. The magnetic values are
relative, and the air-gap values are in mm. All
the measurements are synchronised to pole 49 so
correlation are possible. This machine have 72
poles.
Hydro-Québec Manic1
,
1
1
0.004
/
0.008
Time (s)
Graphic 1. Raw signal from electronic
conditioning circuit. (10, 28 and 40MW from a
60MW machine)
It is difficult to analyse the magnetic
condition of the machine from the information
in graphic 3. The next step may be to zoom the
full size value and to see only the peaks from the
flux signal as shown in graphic 4.
From graphic 1, we can’t say very much
about the machine, since we see only two
consecutive poles. If we look at the zero
crossing of the signals, we can see that when the
power output increases, the phase angle
changes. Also, notice the slight slant on the top
of the pole at high power, which seemed to be
normal. Signals like this one give us confidence
in the method since it appeared to be close to
reality. If we zoom to the top of the poles, we
see some peaks that could be the damping bars
as we can see in graphic 2. This information
may be useful in the future to detect faulty
damping bars. More sophisticated data analyses
should be done for this kind of fault detection.
Manic1 #1 Peak value
1.07~~
__._________
i ______._..__.
i .._._._._. i .._.__.______
1.05
0
I
1
0.1
I
0.2
+ ____.______.
I
0.3
/
/ j
/
I
/
I
I
0.4
0.5
I
0.6
Graphic 4. Peak value of relative magnetic flux
for 1 turn.
0.002
0.004
0.003
O.OOS
0.001
Graphic 4 is a much better view of the
magnetic uniformity. Some may like it better in
a polar representation. With the use of proper
data acquisition and computer treatment, both
representations are easily done as we can see on
graphic 5.
0.007
Time (I)
Graphic 2. Top of one pole at 10MW
Manic1 #1 Polar view of peak values
-1.5 I -. -----+ -+----0
0.1
02
0.3
i
1
0.4
/
0.5
/
0.6
Time (s)
Graphic 3. Signal from electronic conditioning
circuit for one complete turn. (72 poles at
40MW)
Graphic 5. Peak value on a polar plot to observe
the roundness of the magnetic field.
508
The slight flux variations on this particular
machine visible on graphic 4 and 5 indicate that
there is no problem on the poles. Those waves
may be caused by air-gap variation. Air-gap is
easily measured with the AGMS. In graphic 6,
we can look at air-gap and relative magnetic
flux at the same time. This machine has no
problem. It is mechanically and magnetically
round. In the future, we would like to install a
temporary short in a pole to identify the
particular pattern for this kind of problem.
Note that full integration of air-gap and
magnetic field data analyses are not done yet.
So graphic 6 and 7 are real data plotted on the
same view but taken at different time on the
same machine. The information were manually
treated to fit on those graphics.
From all the information that we get from
our first set-up in Manicouagan 1, we think that
there is no single instrument able to get positive
diagnostic of generator vibration. Magnetic
field measurement may be very useful especially
when it is coupled with air-gap and power
measurement. In this case, only a single
magnetic sensor may be enough.
Magnetic flux measurements will detect
intertum short on most machines. According to
computer simulation, on a 72 poles machine
with 20 turns per pole, a single winding short on
one pole will reduce the magnetic flux by 5%
on this pole as shown on graphic 8.
Airgap & Relative flux
- Airgap (mm)
Simulation of interturn short
Graphic 6. Absolute air-gap and relative
magnetic flux.
Graphic 8. Simulation of a interturn short
causing a 5% magnetic flux diminution in
relative unit.
0
8
16
24
32
40
Pole No.
46
56
64
72
Graphic 7. Zoomed absolute air-gap and relative
magnetic flux for 1 turn.
6
FUTURE GOALS
In the future, we would like to understand
more about the measuring system and the
influence of magnetic field unbalance on the
509
machine. More sensors will be permanently
installed on machines and more specific tests
will be tried to study this system. The first tests
with the equipment was done on a machine in
good condition. In the future, the method will
be applied where we suspect rotor problems.
Also, we will focus on data acquisition, software
analyses and integration of other parameters like
air-gap, power etc... For this task, we will
cooperate w i t h o u r s u b s i d i a r y which
commercializes the Air Gap Monitoring System.
7
ACKNOWLEDGEMENT
The author wishes to thank Mr. J.M.
Bourgeois from Hydro-Quebec, the power plant
personnel in Manicouagan region and VibroSystM
staff for there cooperation in the whole project.
9
in generator rotor windings using airgap
search coils. IEE Conf. Publ. 254, 1985.
Posedel, Z. 1991. Arrangement for detecting
winding shorts in the rotor winding of
electrical machines. United States Patent,
5,006,769, Apr. 9, 1991
T a v n e r , P . J . & Gaydon, B . G . & W a r d ,
B.A. 1986. Monitoring generators and large
motors. IEE PROCEEDINGS, Vol. 133,Pt.
B, No. 3, May 1986.
CONCLUSION
There is a lack of magnetic field
measurement devices in the generator
instrument market. The development of the
described system will be very useful even if it
does not give calibrated data. Relative data is
enough to compare magnetic flux on each pole
and detect interturn shorts. More development
will lead us to more sophisticated analyses that
could help us. One day, maybe the method will
be applied on most of the generators as a
standard instrument.
8
Conolly H.M. & Lodge I. & Jackson R.J. &
Roberts I. 1971, Detection of interturn faults
REFERENCES
Albright D.R. 197 1. Intern short-circuit detector
for turbine-generator rotor windings. IEEE
Transaction on power apparatus and systems,
Vol. Pas-90, No. 2, March/April 1971.
510
Wood, J.W. 1986. Rotor winding short
detection. IEE PROCEEDINGS, Vol. 133,Pt.
B, No. 3, May 1986.