Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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.