Patent Application
20140070775
The present invention relates to a method to detect short-circuits in the electrical windings of rotor poles of rotating machines in the case where such poles are salient.
On electrical machines such as hydro-generators, the rotor poles are salient and the electrical windings are constituted by insulated strips of copper surrounding the pole so that each copper turn is isolated from the adjacent ones and from the rotor pole.
After years of operation, insulation aging will occur and may cause shorts in some turns, a condition which may degrade the performance of the machine.
To detect such shorted turns, two main methods are known:
The first method is known in the trade as the “Pole Drop” test and is practiced at a shutdown of the machine. The machine is opened and an AC voltage across each pole is measured. Shorted turns are detected by the smaller than average voltage drop across a pole. The biggest disadvantages of this method are that the machine has to be stopped, the tests may have to be conducted for a long period of time, and shorts that may be happening when the machine is rotating may not be detected when no rotation occurs because they are due to centrifugal force that does not act at machine standstill.
The second method is known as “Flux Monitoring”. A flat coil is glued to the stator and measures the radial magnetic flux emitted in the air-gap by each rotating pole. The voltage generated in the coil is proportional to the radial flux emitted by each pole, so that if there is a shorted turn in a pole, at a given load, an abnormal change in the flux profile of such pole in comparison to the other poles is believed to become apparent.
This method has the advantage of being an on-line tool that does not need the machine to be stopped and opened. However, although this method has been successfully used for many years for cylindrical rotors having non-salient poles, it has only been recently introduced for salient pole machines, and there have been instances where shorted turns that were not detected by such method have nevertheless been uncovered by Pole Drop tests.
The reasons why this Flux Monitoring method does not seem as satisfying with salient rotor poles as it is with non-salient rotor poles are not clear, the most common being that “interpretation of the flux patterns” of a salient pole “is not obvious” (S. R. Campbell et al.—Waterpower XIV—HCI Publications, 2005), probably due to the fact that salient pole machines have all their turns on one pole superimposed instead of being distributed along the pole as they are in the case of non-salient pole machines.) This creates a constant overlapping of the flux patterns created by the neighboring turns on the flux pattern of a defective pole so that the signature of that defective pole becomes undistinguishable from that of the neighboring ones.
Even more, the relationship between the Flux density and the Ampere-turns is not linear as shall be seen later. That non-linearity may dampen the effect of a small number of shorted turns so much so that it may become impossible to detect them when the machine operates on load in its standard use in the power grid.
Moreover, numerous observations and unsuccessful experiments have also led to believe that when a given hydro-generator is under load, and connected to a large grid that is fed by numerous other generators, there seems to be a “grid effect” that partly or totally dampens any short turn anomaly and seems to compensate for such deficiency.
One factor may be even more important than the ones cited above: the variation of the thickness of the air-gap between the poles and the stator in front of each pole of such salient pole machine may affect the resulting radial flux intensity from a pole more significantly than one or two shorted turns on such a pole.
Therefore, there is still a need for a method to detect shorted turns in salient rotor pole machines that is simple, not time-consuming and does not need a machine to be shut down and opened.
The present invention includes a two-step method to detect shorted turns in salient-pole rotors.
In a first step, it uses a coil to measure in real time the radial magnetic flux leaking in the air-gap of an operating electric rotating machine having salient rotor poles and also uses an air-gap sensor to measure the rotor-stator distance in real time as well, and calculates the influence of the air-gap thickness existing in front of each pole, so that major shorted turns are detected under normal working conditions of the machine.
In a second and optional step, in situations where shorted turns cannot be clearly diagnosed under the first step, the machine is taken off line, short-circuited and the magnetizing current applied to the rotor windings is totally cut off so that the magnetic flux is back to zero or almost zero; the magnetizing current is then gradually brought up to nominal capacity by gradual increases, while the radial flux is measured at each magnetizing current increasing fraction and the air-gap correction of the first step is applied to the measured radial flux.
With this second step, gradual magnetization of the rotor pole from a state where the magnetic remanence has a minimal or zero value, offers the maximum sensitivity in radial flux increases (ΔØ/ΔNI) in response to increases of the magnetizing current NI in the rotor windings, thus enabling to detect shorted turns at the maximum sensitivity level, for example with maximum accuracy.
Most shorted turns can be detected while the machine is operating, and for the remaining minor cases, it does not need any shut-down or any opening of the machine. The machine only needs to be taken off-line, to cut off the magnetizing current for a few seconds to let the magnetic flux return toward zero and to proceed with applying the first step of the method as the magnetizing current is gradually increased.
In accordance with a first broad aspect, there is provided a method for detecting shorted turns in the windings of salient rotor poles of an electric rotating machine, the method comprising: for each one of the salient rotor poles, measuring a radial magnetic flux between a stator of the electric rotating machine and the salient rotor pole, and measuring a thickness of an air-gap between the stator and the salient rotor pole; for each one of the salient rotor poles, determining an expected radial flux using the measured radial magnetic flux and the measured thickness of the air-gap; and identifying shorted turns by comparing, for each one of the salient rotor poles, the measured radial magnetic flux to the expected radial magnetic flux.
In one embodiment, the method further comprises, for each one of the salient rotor poles, determining a maximum radial magnetic flux from the measured radial flux, and a minimum air-gap thickness from the measured thickness of the air-gap.
In one embodiment, the step of determining the expected radial flux comprises: calculating an average value for the minimum air-gap thicknesses and an average value for the maximum radial magnetic fluxes; and for each one of the salient rotor poles, calculating the expected radial magnetic flux using the respective measured thickness of the air-gap, the average value for the minimum air-gap thicknesses and the average value for the maximum radial magnetic fluxes.
In one embodiment, the shorted turns are determined when a measured radial magnetic flux for a given salient rotor pole is less than the respective expected radial magnetic flux for the given salient rotor pole.
In one embodiment, the steps of measuring the radial magnetic flux and measuring the thickness of the air-gap are performed during at least one rotation of the electric rotating machine.
In one embodiment, the steps of measuring the radial magnetic flux and measuring the thickness of the air-gap are performed substantially continuously during the at least one rotation of the electric rotating machine.
In another embodiment, the steps of measuring the radial magnetic flux and measuring the thickness of the air-gap are performed in a stepwise manner during the at least one rotation of the electric rotating machine.
In one embodiment, the steps of measuring the radial magnetic flux and measuring the thickness of the air-gap are performed substantially concurrently using a magnetic radial flux sensor integrated with an air-gap sensor positioned in the air-gap.
In one embodiment, the method further comprises: bringing the electric rotating machine off-line; bringing a magnetic remanence of the salient rotor poles substantially near zero by shutting off a current flowing through the windings of the salient rotor poles; reinstating said current up to a nominal value; for each one of the salient rotor poles, measuring the radial magnetic flux and the thickness of the air-gap; for each one of the salient rotor poles, determining an expected radial flux using the measured radial magnetic flux and the measured thickness of the air-gap; and identifying shorted turns by identifying at least a given one of the salient rotor poles having a measured radial magnetic flux inferior to the corresponding expected radial magnetic flux.
In one embodiment, the method further comprises outputting an identification of a given one of the salient rotor poles for which shorted turns have been detected.
In accordance with another broad aspect, there is provided a system for detecting shorted turns in windings of salient rotor poles of an electric rotating machine comprising: a radial magnetic flux sensor for measuring a radial magnetic flux between the stator and the salient rotor pole, for each one of the salient rotor poles; an air-gap thickness sensor for measuring a thickness of an air-gap between the stator and the salient rotor pole, for each one of the salient rotor poles; and a calculation unit in communication with the magnetic radial flux sensor and the air-gap thickness sensor, the calculation unit for: for each one of the salient rotor poles, determining an expected radial flux using the measured radial magnetic flux and the measured thickness of the air-gap; and identifying shorted turns by comparing, for each one of the salient rotor poles, the measured radial magnetic flux to the expected radial magnetic flux.
In one embodiment, the radial magnetic flux sensor comprises a coil adapted to be positioned in the air-gap of the electric rotating machine.
In one embodiment, the radial magnetic flux sensor is integrated with the air-gap sensor.
In one embodiment, the calculation unit is further adapted to, for each one of the salient rotor poles, determine a maximum radial magnetic flux from the measured radial flux, and a minimum air-gap thickness from the measured thickness of the air-gap.
In one embodiment, the calculation unit is further adapted to: calculate an average value for the minimum air-gap thicknesses and an average value for the maximum radial magnetic fluxes; and for each one of the salient rotor poles, calculate the expected radial magnetic flux using the respective measured thickness of the air-gap, the average value for the minimum air-gap thicknesses and the average value for the maximum radial magnetic fluxes.
In one embodiment, the calculation unit is further adapted to identify the shorted turn when a measured radial magnetic flux for a given salient rotor pole is less than the respective expected radial magnetic flux for the given salient rotor pole.
In one embodiment, the radial magnetic flux sensor and the air-gap sensor are adapted to measure the radial magnetic flux and the thickness of the air-gap, respectively, during at least one rotation of the electric rotating machine.
In one embodiment, the system further comprises a control unit for: bringing the electric rotating machine off-line; bringing a magnetic remanence of the salient rotor poles substantially near zero by shutting off a current flowing through the windings of the salient rotor poles; and reinstating the current up to a nominal value, the measurement of the radial magnetic flux and the thickness of the air-gap, the determination of the expected radial flux, and the identification of the shorted turns being performed after the reinstatement of the current.
In one embodiment, the calculation unit is further adapted to output an identification of a given one of the salient rotor poles for which shorted turns have been detected.
In accordance with a further broad aspect, there is provided a computer program product comprising a computer readable memory for storing computer executable instructions thereon for detecting shorted turns in the windings of salient rotor poles of an electric rotating machine, the computer executable instructions for performing, upon execution by a computer, the steps of: receiving, for each one of the salient rotor poles, a measurement of a radial magnetic flux between a stator of the electric rotating machine and the salient rotor pole, and a measurement of a thickness of an air-gap between the stator and the salient rotor pole; for each one of the salient rotor poles, determining an expected radial flux using the measured radial magnetic flux and the measured thickness of the air-gap; and identifying shorted turns by comparing, for each one of the salient rotor poles, the measured radial magnetic flux to the expected radial magnetic flux.
In accordance with still a further broad aspect, there is provided a monitoring system for monitoring an air-gap thickness between a stator of a rotating machine and a rotor pole, and a magnetic flux between the stator and the rotor pole, the system comprising: an air-gap sensor for measuring the air-gap thickness and outputting a first electrical signal indicative of the air-gap thickness; a magnetic flux sensor for monitoring the magnetic flux and outputting a second electrical signal indicative of the magnetic flux, the second electrical signal having an electrical frequency lower than an electrical frequency of the first electrical signal; a triaxial cable having a first end connected to the air-gap sensor and to the magnetic flux sensor, and having a substantially low frequency bandwidth for transmitting the second electrical signal and a substantially high frequency bandwidth for transmitting the first electrical signal; a first capacitor operatively connected between the triaxial cable and the magnetic flux sensor; a first conditioner unit connected to a second end of the triaxial cable and adapted to receive the first electrical signal, extract the air-gap thickness form the first electrical signal, and output the air-gap thickness; and a second conditioner unit connected to a second end of the triaxial cable and adapted to receive the second electrical signal, extract a parameter indicative of the magnetic flux from the second electrical signal, and output the parameter; and a second capacitor operatively connected between the triaxial cable and the second conditioner unit.
In one embodiment, the monitoring system further comprises a coaxial cable operatively connected between the air-gap sensor and the triaxial cable.
In one embodiment, the magnetic flux sensor comprises a coil and is adapted to measure a voltage generated within the coil, the second electrical signal being indicative of the voltage and the second conditioner unit being adapted to extract the voltage from the second electrical signal.
In one embodiment, the second conditioner unit is further adapted to determine the magnetic flux between the stator and the rotor pole from the voltage, and output the magnetic flux.
In one embodiment, the method 10 is performed while the machine operates, i.e. while the rotor is rotating. Therefore, there is no need to stop the operation of the machine.
In one embodiment, the magnetic radial flux and the air-gap thickness are substantially continuously monitored during one or more rotation of the rotor. In another embodiment, the magnetic radial flux and the air-gap thickness are monitored in a stepwise manner during one or more turns
This demonstrates the need to correct any measured value of the magnetic flux created by a given pole that is above or under the average flux of all poles, by taking into account whether the air-gap thickness in front of the pole is under or above the average air-gap of all poles.
Here is how the separation between the air-gap effect and the shorted turn effect on the magnetic field of a pole may be approximated so that only real shorted turns may be identified.
For a given machine, the following calculations are made:
The air-gap thickness and the radial flux values are continuously measured. The minimum value of the air-gap thickness for each pole is calculated as well as the maximum (peak) value of radial flux for each pole over one complete rotation or turn of the rotor. Then, the average value of the minimum air-gap thickness and the average value of maximum radial flux are calculated. Once having these values, the expected maximum radial flux value for each pole is derived by the following Formula (1) which is illustrated in
For Pole n,Expected Flux=Average Flux over one complete turn+(Average Air-Gap over one complete turn−Measured Air-Gap for Pole n)×(Average Flux over one complete turn/Average Air-Gap over one complete turn).
As illustrated in
Table 1 is an illustration of the above mentioned calculation method in the case of a rotor with 8 salient poles.
Table 1 shows the measured values of air-gaps and radial fluxes in a given salient pole machine and their average values in rows 2 and 3. In the 4th row, fluxes are calculated according to Formula (1) and in the last row, the percentage of increase or decrease of the measured flux over the calculated flux for each pole is indicated.
Table 1 indicates that the only pole where the expected flux value is larger than the measured one is Pole 1, which shows the presence of at least one shorted turn on Pole 1. The more negative the percentage is, the more severe the shorted turn is.
In one embodiment, the method 10 is embodied as a system comprising a air-gap sensor for measuring the air-gap thickness, a magnetic flux sensor, and a calculation unit adapted to calculate the expected radial magnetic flux for each rotor pole and compare the measured radial magnetic flux to the corresponding expected radial magnetic flux, for each rotor pole, as described above.
In another embodiment, the method 10 is embodied as an apparatus comprising a processing unit, a memory or storing unit, and a communication unit for receiving the measurements of the air-gap sensor and the radial magnetic flux. The processing unit is configured for calculating the expected radial magnetic flux for each rotor pole and comparing the measured radial magnetic flux to the corresponding expected radial magnetic flux, for each rotor pole, as described above.
In a further embodiment, the method 10 is embodied as a computer program product comprising a computer readable memory for storing computer executable instructions thereon for detecting shorted turns in the windings of salient rotor poles of an electric rotating machine. Upon execution by a computer, the computer executable instructions perform the steps of: receiving, for each one of the salient rotor poles, a measurement of a radial magnetic flux between a stator of the electric rotating machine, and the salient rotor pole, and a measurement of a thickness of an air-gap between the stator and the salient rotor pole; for each one of the salient rotor poles, determining an expected radial flux using the measured radial magnetic flux and the measured thickness of the air-gap; and identifying shorted turns by comparing, for each one of the salient rotor poles, the measured radial magnetic flux to the expected radial magnetic flux.
In one embodiment, the triaxial cable is omitted so that the conditioner is directly connected to the coaxial cable for receiving the electrical signal outputted by the capacitive air-gap sensor. For example, such a configuration may be used when the conditioner is located at a substantially short distance from the capacitive air-gap sensor, and the capacitive air-gap sensor outputs a substantially low current electrical signal having a substantially high frequency.
In another embodiment, the coaxial cable is omitted so that the capacitive air-gap sensor is directly connected to the triaxial cable for transmitting the electrical current. For example, such a configuration may be used when the conditioner is located at a substantially great distance from the capacitive air-gap sensor, and the capacitive air-gap sensor outputs a substantially low current electrical signal having a substantially high frequency.
Another refinement and economic improvement of this method is illustrated in
In one embodiment, the flux sensor comprises a coil and is adapted to measure a voltage generated within the coil, and the conditioner is adapted to extract the voltage value from the second electrical signal, and optionally integrate the voltage value to obtain the corresponding magnetic flux.
In one embodiment, the coaxial cable is omitted so that the capacitive air-gap sensor is directly connected to the triaxial cable.
It should be understood that the conditioner may be seen as comprising two conditioner units, a first conditioner unit being adapted to extract the air-gap thickness and a second conditioner unit being adapted to extract the voltage or flux value.
It should be understood that the capacitive air-gap sensor and the coil are each adapted, i.e. sized and shaped, to be positioned in the air-gap between the stator and the rotor poles of the rotating machine.
For cases where expected fluxes are slightly above measured fluxes, and to reduce the occurrence of false alarms, the method characterized by Formula (1) may still be applicable in the following way:
As seen on
It should be noticed that, for a given variation of the current NI flowing through the pole windings, the variation of the magnetic radial flux Ø is more detectable when a first magnetization occurs (zone b), since for each magnetizing current increase, the slope of increase of the radial flux (ΔØ/ΔNI) is steeper.
It follows that if there exists a shorted turn on a given pole, maximum probability of its detection will happen when the machine operates along zone b, where sensitivity of detection is maximum, because ΔØ/ΔNI will be maximum and also because the machine being the only one to generate a voltage, the contribution of each pole can be observed without having the damping effect of any of the other machines on the grid.
To operate in zone b, the first condition is to reduce the magnetic remanence of the pole core as much as possible. This can be obtained by cutting off the magnetizing current and putting the electric rotating machine off-line. Given the ferro-magnetic nature of the material of the pole core, it should only take a few seconds to reduce the magnetic remanence of the pole core to zero or near zero.
Once this objective is attained, the magnetizing current may be increased step by step and the values of the radial flux and air-gap at each step should be measured in real time. Maximum probability of short turn detection should occur when the machine operates approximately between ¼ and ⅔ of the maximum magnetic field.
Then the above calculations of the expected fluxes according to Formula (1) may be applied and the shorted turns identified.
The embodiments described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the appended claims
| Number | Date | Country | |
|---|---|---|---|
| 61699327 | Sep 2012 | US |
