Abstract:
There is provided an apparatus and associated method for the testing of a laminated core ( 100 ) for an electrical machine. The apparatus supplies an excitation signal to the winding ( 102 ) of the electrical machine which is of a frequency higher than the rated operating frequency of the machine. There is also provided a phase sensitive detector ( 110 ) which receives, from the laminated core ( 100 ), a signal in response to the excitation signal. The phase sensitive detector ( 110 ) resolves the received signal into a number of components. These components are resolved relative to a reference signal. The size of the components determines the severity of any fault that exists within the laminated core ( 100.

Description:
BACKGROUND TO THE INVENTION 
   The present application claims priority of United Kingdom Patent Application Serial No. 0129020.4, filed 4 Dec. 2001, the contents which are hereby incorporated hereby in its entirety. 
   The present invention relates to the testing of a laminated core of an electrical machine. 
   The stator cores of generators and other electrical machines are built up of thin steel laminations which are each coated with a layer of electrical insulation. This insulation prevents the alternating magnetic flux, through the core, inducing unwanted eddy currents between the laminations. 
   The lamination insulation may, however, become damaged during assembly or maintenance of a stator, particularly during removal or replacement of the rotor. The insulation may also degrade during operation, due to, for example, wear of the insulation. 
   If the insulation becomes damaged, then conducting circuits may be formed between laminations, through which fault currents are induced by the alternating magnetic flux. These damaged regions may become hot in service and are referred to as hot spots. These hot spots may damage the insulation on adjacent sections of the stator and can cause electrical breakdown and failure of the machine. 
   There have been several techniques developed to identify damaged insulation and so allow repair to reduce the severity of the hot spots. One such test involves generating a large (around 80% of the normal operating value) magnetic ring flux around the stator core. This heats the hot spot allowing infra red equipment to locate the position of the hot spot. This type of test however requires a large amount of power and may not be able to detect deep seated core damage. Additionally, these tests are labour intensive as the excitation windings are large. This in turn has prevented the tests being carried out with the rotor in situ. Other tests have therefore been developed to mitigate these problems. Two examples using different techniques now follow. 
   A local excitation test is described in U.S. Pat No. 5,990,688. This uses a test head that is a C shaped section of laminated steel held against the stator teeth which locally excites the core. Using this test, only a low excitation flux is required to be generated in the stator. This means that a small amount of power is required to conduct this test. The test is also simple to set up as the test head need only be held against the stator teeth. 
   However, the test head is relatively large and heavy. This means that the test head cannot traverse the stator core very easily. Additionally, the test may not enable hot spots to be located precisely and so repairing the laminations may be difficult. 
   A second example is the Electromagnetic Core Imperfection Detector (ElCID) test. This test is described in GB-A-2 044 936 and requires that a low ring flux is generated around the stator core (typically only 4% of the normal operating value). This ring flux is produced by an excitation winding through the bore of the stator core. The ring flux induces fault currents in the stator core that flow through any potential hot spots, but these fault currents are too small to cause any detectable heating. Instead the current flowing through the fault is detected electromagnetically using a special pick-up coil such as, for example, a Chattock Potentiometer. Such coils also detect the magnetic fields produced by the excitation current that is typically much larger than those produced by the fault currents. The fault current tends to be in phase quadrature with the excitation current and so by identifying and measuring the component of the detected current that is in phase quadrature with the excitation current, a fault current can be quantified allowing the severity of the fault to be determined quickly, without the use of large number of personnel. Also it is possible, using ElCID, to locate the position of the fault, facilitating repair as well as providing a permanent record of the fault severity thereby allowing the effectiveness of the repair to be quickly assessed. ElCID testing is now common in the electrical power industries and is widely accepted as a reliable core testing method. 
   However there are several situations in which it is difficult to obtain reliable results using ElCID testing. These include testing of stators with rotors in situ and the testing of certain areas of hydrogenerator cores. 
   Rotor in situ testing is reasonably common in hydrogenerators as there is often sufficient access to test the stator bore, without major disassembly of the machine. There can however be difficulties because the current through the excitation winding induces eddy currents in solid steel components such as the rotor and rotor bearings. These eddy currents produce magnetic fields at the tips of the stator teeth that are not in phase with the excitation current and so can affect the ElCID test results such that faults are difficult to identify. 
   Many hydrogenerators are made up of split cores, where the core comes in two or more sectors which are assembled on site. Inevitably there are small gaps at the joins between the sectors of the laminated core. The ring flux around the core generates very large magnetic fields as it crosses the gaps and so increases the magnetic field at the stator teeth near the gap. The amplitude and phase of these fields vary along the length of the gap and the amplitude of these fields can be much greater than (typically 100 times greater than) those produced by a fault current. Since the phase and amplitude of these magnetic fields are not well defined it becomes very difficult using the standard ElCID test to determine whether there is a genuine fault in the core near the joins. This is a serious drawback as damage is likely to occur at the joins in the core. 
   It is an object of the present invention to address these problems. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention there is provided apparatus for testing a laminated core of an electrical machine, the apparatus comprising excitation signal generating means, arranged to generate an excitation signal at a first frequency which is higher than the rated operating frequency of said electrical machine, for supply to an excitation winding coupled to the laminated core of the electrical machine, and a phase sensitive detector arranged to receive a first, reference signal having a reference phase and to receive from the laminated core of the electrical machine a second, test signal having a test amplitude and phase, each in response to the excitation signal applied to the excitation winding, the phase sensitive detector being arranged to resolve the test signal into a plurality of components relative to said reference phase of said reference signal and to generate an output signal, the level of such signal being indicative of the severity of a fault between the laminations of said laminated core. 
   The present invention overcomes the problem experienced by traditional ElCID testing apparatus when testing split hydrogenerator cores by operating at higher frequencies because smaller magnetic fields are generated by the excitation current near the stator teeth. By increasing the frequency of the excitation signal, the current required to produce a given electric field along the stator core is reduced. This means that the large Magnetic Potential Difference (MPDs) across any gaps and between the stator teeth are also reduced. Therefore, as the current through a fault is primarily determined by the level of the electric field and by the resistance of the fault, the fault currents in the stator core near the gaps are detected more reliably because the level of the unwanted magnetic field is reduced. Also the present invention reduces eddy current flow within the solid metal components associated with many electrical machines, for example, the rotor and its bearings, and so reduces the quadrature magnetic fields generated at and near the stator teeth. This is because the amplitude of the eddy currents are proportional to the amplitude of the excitation current and both the amplitude and phase-shift of the external magnetic fields produced by the eddy currents tend to be independent of frequency. This increases the resolution and accuracy of the results. 
   It is preferable that the in phase and phase quadrature components of the test signal, relative to the reference signal, are used as an indication as to the severity of the fault. However, it is to be anticipated that either the substantially in phase or phase quadrature component can equally be used individually. 
   To optimise the effectiveness of the test it is preferable that the stator core is energised to between 0.01% and 5% of the rated operating electric field. 
   In a further embodiment of the present invention there is provided apparatus wherein said excitation signal generating means comprises a frequency generator, arranged to generate at least one further excitation signal having a respective frequency that is higher than the rated operating frequency of said electrical machine. 
   This is advantageous because one of the frequencies may typically be close to the optimum frequency. Another advantage is that it may be possible to analyse these signals to derive the effective resistance and inductance of the fault circuit at each frequency. These resistance and inductance values for the various frequencies may be then extrapolated back down to the rated operating frequency (usually 50 or 60 Hz), or even to DC, better to characterise the severity of the fault. 
   In a further embodiment of the present invention there is provided a system for testing laminated cores in electrical machines comprising-the testing apparatus of the present invention coupled to an excitation winding. 
   In a second aspect of the present invention there is provided a method for testing a laminated core of an electrical machine, comprising the steps of:
     generating an excitation signal at a first frequency, wherein the first frequency is higher than the rated operating frequency of said electrical machine;   receiving a first, reference signal having a reference amplitude and phase and a second, test signal having a test phase, each signal generated in response to the excitation signal applied to the excitation winding;   resolving the test signal into a plurality of components with respect to the reference phase of the reference signal; and generating an output signal, the level of said output signal being indicative of the severity of a fault between the laminations of said laminated core.   

   
     BRIEF DESCRIPTION OF DRAWINGS 
     One embodiment of the present invention will now be described by way of example only and with reference to the following drawings in which; 
       FIG. 1  shows a schematic diagram of a known configuration of equipment for an Electromagnetic Core Imperfection Detector (ElCID) test; 
       FIG. 2  shows a schematic diagram of a system for testing laminated cores which embodies the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a schematic of a typical known Electromagnetic Core Imperfection Detection (ElCID) test arrangement. This particular ElCID arrangement includes a laminated stator core  100 . The stator core  100  is made up of layers of thin steel and insulation as is known. In large turbogenerators and some hydrogenerators, the stator core  100  is built up from layers of laminations which overlap all around the core. Many large hydrogenerator cores, however, are formed from two or more sectors, each built up from overlapping laminations, and the sectors are assembled on site. Small gaps therefore occur between where the sectors are joined. In use, a rotor (not shown) passes through the centre of the core  100 . Typically during test, however, the rotor may be removed and an inserted excitation winding  102  passes through the centre of the core  100 . However, as noted previously the stator  100  may be tested whilst the rotor is in place. It is also to be noted that key bars (not shown) which traverse the length of the exterior wall of the stator core  100  may allow a conductive path to be formed between the laminations at the exterior wall. 
   The stator core  100  has stator teeth  104  around its inner core wall to house the stator windings. The stator teeth  104  are spaced evenly about the inner wall and are directed radially inwards towards the axis of the core. 
   As is indicated in  FIG. 1 , the excitation winding is connected to a power source (not shown). The power source operates at the rated operating frequency of the stator core  100  under test. Typically this frequency is 50 or 60 Hz depending on the stator core  100 . 
   Also the power supply (not shown) is arranged so that it provides to the excitation winding  102  sufficient current so as to induce in the stator core  100  a magnetic “ring” flux density of typically 4% of the rated operating magnetic flux density. This circumferential flux around the core  100  induces an electric field along the core  100 . This induced electric field is typically 4% of the rated electric field of the stator core  100 . For a large turbogenerator, the total Magneto Motive Force (MMF) required in the excitation winding  102  is typically 60 Ampere-turns and is about 250 Ampere-turns for a large hydrogenerator. 
   If the stator core  100  is fault free and there are no gaps around the core nor any solid metal in the vicinity, the MMF from the excitation winding  102  is fairly uniformly distributed around the core. The Magnetic Potential Difference (MPD) between each adjacent stator tooth  104  is then roughly equal to the total excitation MMF divided by the number of stator teeth  104 . Therefore, for a typical large turbogenerator having 60 teeth and requiring an MMF of 60 Ampere-turns, the MPD between adjacent teeth is about 1 Ampere. 
   If the stator core  100  is damaged, this may lead to catastrophic failure of the electrical machine in service as described in the introduction. In a damaged section of core, a fault current will flow between laminations, a typical closed circuit being formed by localised damage to the insulation on a group of laminations and by the electrical contact to the key bars at the back of the core. In an ELCID test, a fault current is also induced in such a circuit by the electric field generated by the excitation current. The amplitude of the current flowing through such a fault is usually much less than the MPD between adjacent stator teeth  104  produced by the excitation current. The fault current will however be predominantly in phase with the electric field along the core and therefore in phase quadrature with the excitation current. The total MPD between adjacent stator teeth  104  is the resultant of the component from the excitation current and the component from the fault current and the fault current, although small, is readily detected by measuring the component that is in phase quadrature with the excitation current. In the ElCID test, this phase quadrature component of the MPD is measured and used to detect faults and to quantify their severity. 
   Referring back to  FIG. 1 , as is known in the ElCID test, a Chattock Potentiometer  108  is placed over, in this case, two consecutive stator teeth  104 . The output voltage from the Chattock Potentiometer  108  is proportional to the line integral of the magnetic field along its length i.e. to the MPD between its ends. This characteristic of the Chattock Potentiometer  108  makes it suitable for ElCID tests although other types of pick up coil or magnetic field sensor, such as those using the Hall Effect or magneto-resistance devices may be employed instead. 
   It may be advantageous to place the Chattock Potentiometer  106  across more than two teeth  104 . More specifically, the Chattock Potentiometer  108  is typically arranged so that the ends of the Chattock Potentiometer  108  are placed over the farthest corners of two adjacent stator teeth  104 . The ends of the Chattock Potentiometer  108  are typically in contact with the stator teeth  104  although the ends may be placed just above the stator teeth  104 . 
   The output voltages of the reference coil  106  and the Chattock Potentiometer  108  are fed to a phase sensitive detector  110 . The reference coil  106  is placed in the stator bore so that the phase of its output voltage is determined by the phase of the magnetic flux in the stator bore that is generated by the excitation current. 
   The output voltage and the reference voltage are input to the phase sensitive detector  110 . The phase sensitive detector  110  is arranged to output two signals. One of the output signals is proportional to the component of the MPD, measured by the Chattock Potentiometer  108 , that is in phase with the excitation current and the second output signal is proportional to the MPD that is substantially in phase quadrature with the excitation current. As noted earlier, the phase quadrature component indicates a fault current. 
   The phase sensitive detector  110  is known and so will therefore not be explained in any further detail. 
   In test, the Chattock Potentiometer  108  is placed to span across two consecutive stator teeth  104 . The Chattock Potentiometer  108  then traverses the stator core  100 . As the Chattock Potentiometer  108  transverses the stator core  100 , both the in-phase and phase quadrature components are recorded or displayed as a function of distance along the core  100 . The in phase component and the phase quadrature component are therefore output to a recording device or may be displayed instead. This recording device may be a chart recorder, a computer or the like. 
   At the end of each scan, the Chattock Potentiometer  108  is repositioned so as to span across the next adjacent pair of stator teeth  104  which are then scanned and the two output signals recorded. This process is repeated until the whole stator core  100  has been tested. 
   If the phase quadrature component exceeds a certain threshold, typically 100 mA, then a significant fault current is deemed to be flowing at that point and more detailed analysis of the local area may take place. Such analysis may include using a smaller Chattock Potentiometer or the like to probe the locality. 
   Referring now to  FIG. 2 , a schematic drawing of a test apparatus is shown which embodies the present invention. The arrangement of  FIG. 2  also shows a known Chattock Potentiometer  108 , a stator core  100  under test, an excitation winding  102  and a phase sensitive detector  110 . 
   The excitation winding  102  passes through the bore of the stator core  100 ; as with the arrangement of  FIG. 1  above, the rotor has been removed prior to test. It should be noted however, that the rotor does not have to be removed before test. The excitation winding  102  is connected to a high frequency power amplifier  204 . Also connected to the high frequency power amplifier  204  is a signal generator  202 . 
   The signal generator  202  outputs a single, high frequency signal to the power amplifier  204  which amplifies this generated signal to provide the current to the excitation winding  102 . 
   The generated signal frequency exceeds the rated operating frequency of the stator core  100 . In this particular case, the generated signal will have a frequency of around 2 kHz. Other suitable frequencies that may be generated also include 500 Hz, 1 kHz, and 5 kHz although frequencies above and below these values may also be suitable. 
   The phase sensitive detector  110  is also connected to the Chattock potentiometer  108  and a reference phase sensor  200 . 
   In test, the high frequency signal generator  202  generates, in this case, a 2 kHz sinusoidal signal. This signal is amplified by the power amplifier  204  to supply current to the excitation winding  102 . The current output from the power amplifier is typically such that the stator core  100  is energised to only 0.01% to 5% of the rated operating electric field and may therefore be much less than in the known ElCID test where 4% of the rated operating electric field is used, as described above. 
   In this case, the reference phase sensor  200  is a wire loop around the stator core  100 . More specifically, the wire loop is a single loop that traverses the length of the stator core  100  along the inward face of one of the stator teeth  104 , the outside length of the stator core  100  and along both faces of the stator core  100 . The reference phase sensor  200 , in this case, measures the phase of the magnetic flux in the stator core  100  and so therefore a voltage signal representing the phase of the magnetic flux is input into the phase sensitive detector  110 . As the frequency of the excitation current is higher than the rated operating frequency, of the stator core  100 , the phase of the magnetic flux in the stator core  100  is substantially different from that of the magnetic flux in the stator bore. This is due to the skin effect as explained below. 
   As the frequency of the excitation current increases above the rated operating frequency of the stator core  100 , the magnetic flux is attenuated within the thickness of the laminations which make up the stator core  100 . This is because the laminations are designed to operate at a rated lower frequency, therefore the thickness of the laminations (determined by the manufacturer of the stator core  100 ), is dependent upon the operating frequency. As the frequency of the excitation signal increases beyond this rated operating frequency, the skin depth (the depth to which the excitation signal will penetrate) reduces. This means that it can no longer be assumed that the phase of magnetic flux within the stator bore produced by the excitation current is almost identical to the phase of the magnetic flux in the stator core  100 , as is the case in the prior art of FIG.  1 . The reference coil  106  as described in relation to  FIG. 1  can therefore not be used in the test of the present invention without modification to the system. 
   As an alternative, the reference phase signal for the phase sensitive detector  110  may be generated by other means. For example from a reference coil  106  in the stator bore or from a direct link to the high frequency signal generator  202 . In both these alternative cases however, the phase of the voltage input to the phase sensitive detector  110  is adjusted electronically within the phase sensitive detector  110  so that the resultant reference voltage is of a similar phase to that as would be produced by the wire loop arrangement described in relation to FIG.  2 . The phase is adjusted so that the phase quadrature output signal from the phase sensitive detector  110  is substantially zero when the Chattock Potentiometer is positioned over a section of the stator core  100  which is known to be undamaged, or a simulation of such a section. 
   To test a stator core  100  the Chattock Potentiometer  108  is arranged and scanned along the stator teeth  104  as previously described. Both the reference phase sensor  200  and the Chattock Potentiometer  108  are in communication with the phase sensitive detector  110 . The phase sensitive detector  110  is arranged as previously described with an in phase signal output and a phase quadrature output. Both the in-phase and the phase quadrature output signals from the phase sensitive detector  110  lead to an indicating device which may be an chart recorder, a computer or the like which records the output signals as a function of position along the stator teeth  104  as previously described. 
   Other modifications are contemplated to the described embodiment. For example, the signal generator  202  may be capable of generating a number of different frequency signals simultaneously. If several frequencies are generated simultaneously the phase sensitive detector  110  will output several pairs of in-phase and phase-quadrature signals, one pair for each different frequency. The results from the phase sensitive detector  110  may be used to calculate the effective resistance and inductance of the fault current circuit at each frequency. These results can be extrapolated back to determine the value of the resistance and inductance at the rated frequency, usually 50 Hz or lower, for example, D.C.