Abstract:
A method and a device for detecting a touching point (i.e. contact point) on a machine having a rotating shaft, in which the touching point (i.e. contact point) on the shaft is determined during operation. This is performed by measuring the grounding state of the shaft by measuring a characteristic (such as a current) of electricity flowing to ground via one or more ground connections provided on the shaft.

Description:
Priority is claimed to Swiss application CH 01332/04, filed Aug. 12, 2004 the entire disclosure of which is hereby incorporated by reference herein. 
   The present invention relates to a method and a device for monitoring rotating machines. In particular, it relates to a method for detecting touching points or contact points on machines having rotating shafts, such as generators, turbines, and motors, during operation. 
   In particular, the invention relates to the detection of shaft friction, i.e. the occurrence of touching points or contact points between the drive shaft and the housing. A particular intended use is monitoring the bearing insulation in generators. 
   BACKGROUND 
   Electrical machines, in particular large electrical machines such as those used in power stations, should be monitored continuously in order to ensure friction-free operation and/or analyzed from time to time in order to recognize faults in time and to avoid uncontrolled failures. 
   EP 0 271 678 A1 proposes, for example, a device with which statements can be made on the functional reliability and functional safety of the shaft grounding or the insulation of the shaft. The device proposed therein, which is actually envisaged for reducing shaft voltages, allows statements to be made on the entire grounding conditions of the electrical machine. 
   If the rotating metal shaft of a machine comes into contact with or touches the metal of the housing parts surrounding it, wear occurs. With generators, considerable electrical currents can also flow via the contact points and lead to instances of overheating, erosion or even melting processes. It is correspondingly desirable to detect such instances of the metal parts coming into contact with each other during operation and to localize the touching points. In this document, the terms “touching points” and “contact points” are used interchangeably. 
   It is known to provide double insulation for the bearings of machines, for example for the purpose of detecting touching points, and to arrange a conductive layer between the two insulating layers. Monitoring is then carried out to ascertain whether this conductive intermediate layer has a resistive conductive connection to the shaft or to ground. If this is the case, one of the insulating layers is defective. One disadvantage of this method is the fact that the insulating layers in the bearing are only tested selectively, but not the shaft insulation per se, i.e. the insulation state of the entire shaft cannot be determined. It is therefore, for example, not possible to establish whether the shaft is in direct contact with the housing. One disadvantage of this method is, inter alia, that only the insulating layers in the bearing are tested, but not the shaft insulation per se. It is not possible to establish whether the shaft is in contact with, for example, the housing. 
   Methods also exist in which currents through the shaft are measured by means of uniform-field coils. Here too, contact points between the shaft and the housing or ground are not detected directly, but only the indirect consequences of these contacts. 
   EP 0 503 846 A2 describes a method and a device for detecting faults in the insulation between a shaft of a generator and the bearings or seals which guide said bearings. For measurement purposes, a transformer is placed around the stationary shaft and is driven with alternating current. Then, a voltage measuring device, which is grounded on one side and is connected to the shaft on the other side at a point at which a fault is suspected to be, is used to measure the voltage. Such methods allow for effective measurement of faults in the rest state, but the touching points or contact points occurring exclusively during operation cannot be found in this way. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an improved method for detecting touching points or contact points on machines having rotating shafts, and a device for carrying out such a method. Of particular concern here is a method in which touching points or contact points are determined on the rotating shaft, i.e. during operation of the machine. 
   According to the present invention the grounding state of the shaft is measured, namely by determining an characteristic of an electrical flow to ground via at least one ground connection provided on the shaft, such as by determining all currents flowing to ground. The shaft of machines is normally grounded deliberately for safety reasons, for example a generator is often grounded at both ends by means of corresponding contact devices. In other words, the currents flowing out via these grounding devices may be measured. 
   Conclusions are drawn on the state of insulation of the shaft on the basis of electrical flow to ground which occur on the grounding devices. 
   In accordance with a first preferred embodiment of the present invention, the grounding state of the shaft is thus determined by means of at least one grounding module which is electrically connected to the shaft, by the voltage to ground being measured via a grounding resistor. In this case, in order to simplify the analysis, the procedure is preferably to evaluate currents which flow from the shaft to ground as being positive. 
   In a particularly advantageous manner, the method in accordance with a further preferred embodiment can be carried out if at least from time to time, i.e. for example intermittently, a quantitatively determined test current is injected into the shaft for the purpose of detecting the insulation behavior. The injection of a test current into the shaft makes it possible, in particular, to evaluate the insulation behavior quantitatively since it is essentially possible to test what component of the test current flows out via the grounding devices (this component is measured) and what component correspondingly flows out via unintentional contacts. The test current is preferably a current having a known direct-current component in the range from 10 to 100 mA, in particular having a direct-current component in the range from 20 to 40 mA. 
   In this case, the test current is preferably injected into the shaft via a ground connection, in particular via a grounding brush forming contact with the shaft or a corresponding sliding contact. It is possible for a separate contact device to be provided for injecting the test current, but the contact capability already provided by means of the grounding brush simplifies the method considerably. 
   The present method is used, for example, in generators. In the case of generators, grounding devices are normally already provided; these so-called grounding modules are in this case typically arranged both on the turbine side and on the other side of the generator. The method can be implemented particularly easily if, in accordance with a further preferred embodiment, the test current is applied to the shaft at least indirectly via one or else two of these grounding modules. In this case, the test current may either be injected into the grounding module or else it is possible for the test current to be injected via the grounding brush connected to the grounding module. The ground connections provided may be at least one grounding module which is in contact with the shaft and grounded via at least one grounding brush. 
   A quantitative evaluation, in particular in the case of an intermittent injection of test current, can be achieved in accordance with another embodiment by the detection of touching points or contact points taking place by means of a comparison of the value of the test current with the difference between the currents flowing to ground via ground connections provided on the shaft without the test current being applied (total ground current without test current) and the currents flowing to ground via ground connections provided on the shaft with the test current being applied (total ground current with test current). It is thus possible for the conductivity of the touching points or contact points to be determined directly from the comparison of the value of the test current with the difference. 
   In accordance with another preferred embodiment, of the currents which flow to ground via ground connections provided on the shaft, only the DC component is evaluated in order to average out AC components typically present on the shaft. A further improvement of the evaluation can be achieved by the currents which flow to ground via ground connections provided on the shaft being averaged over time and/or filtered. When the shaft is rotating in the range from 50 to 60 Hz, averaging over 5 s, preferably over 10 s, has proven to be sufficient. 
   The present invention furthermore relates to a device for carrying out a method, as is described above. The device is particularly characterized by the fact that at least one grounding module is provided by at least one grounding brush forming contact with the shaft (or another contact point, for example a sliding contact), the at least one grounding module having at least one grounding resistor. A unit is also arranged which has, at least indirectly via the grounding module, a contact for injecting a defined test current into the shaft and taps off or measures the voltage applied across the at least one grounding resistor or receives the corresponding measured values transmitted by the grounding module. 
   In accordance with one preferred embodiment, the unit has means for measuring and/or evaluating the touching points or contact points on the basis of the quantitative value of the test current and of the ground currents flowing out via the grounding resistors. This unit, which is in the form of a measurement and evaluation module, is optionally part of at least one of the grounding modules. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail below with reference to exemplary embodiments in conjunction with the drawings, in which: 
       FIG. 1  shows a schematic illustration of a rotating machine having grounding and measuring devices; and 
       FIG. 2  shows a schematic illustration of the electrical connectivity of a rotating machine with grounding and measuring devices as shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a schematic design, in which a turbine  3  and a generator  2  are arranged on a shaft  1 . The shaft is mounted via two bearings  4  and  5 . Further bearing points, for example behind the turbine, are not illustrated. The shaft of a generator  2  is normally grounded; this takes place via so-called grounding modules. In one specific case, two such grounding modules  8  and  9  are provided. 
   A first grounding module  8  is arranged on the turbine side, and contact is made between said first grounding module  8  and the shaft  1  via a grounding brush  6 . The grounding module  8  and the grounding brush  6  form a first ground connection. 
   A second grounding module  9  is arranged on the other side of the generator and for its part has a grounding brush  7 , by means of which the grounding module  9  is electrically connected to the shaft  1 . The grounding module  9  and the grounding brush  7  form a second ground connection. 
   The two grounding modules  8  and  9  are connected to ground  10 . Instead of the grounding brushes  6 ,  7 , sliding contacts or other means for making contact with the shaft may also be used. 
   Although this is not necessary for the present invention, the grounding modules  8  and  9  may in this case be much more complex components. For example, the grounding module  8  arranged on the drive side may be a so-called DE module, as is described, inter alia, in WO 2004/025316 A1. Such a DE module (cf. inlay in  FIG. 1 ) has a high-value resistor  22  (R DE,1 , 10Ω) for the contact device, in this case for the grounding brush  6 , to the shaft and a low-value resistor  23  (R DE,2 , 1Ω), which can be used, for example, as the grounding resistor  11 , to ground  10 , a fuse  25  being arranged in parallel with the high-value resistor  22 , and a measuring resistor  24  (0.1Ω) being arranged, for example, between ground and the low-value resistor  23 . For diagnosis purposes, the shaft voltage between the high-value resistor and the contact device and the ground connection and the shaft current can then be tapped off via the measuring resistor. 
   The second grounding module  9  may be a unit as is described in the abovementioned EP 0 271 678 A1. 
   However, one critical feature in connection with the two grounding modules  8  and  9  is that they each have a grounding resistor  11  and  12 , respectively, which makes it possible to determine the voltage measured across this resistor  11  or  12 , whose resistance value is known, and the current flowing out to ground  10 . 
   In the present case, an additional unit  13  in the form of a measurement and evaluation module is now provided. This measurement and evaluation module  13  on the one hand taps off the voltages occurring across the resistors  11  and  12  and measures them, as is indicated by the references  14  and  16 . 
   In addition, the measurement and evaluation module  13  has a current source  21  ( FIG. 2 ) which makes possible a well defined or known test current which is injected into the shaft  1  and allows the method to be carried out quantitatively. A typical DC test current has a value of 20 to 40 mA, for example of approximately 30 mA. The measurement and evaluation module  13  may also have hardware and software which makes possible deeper analysis. Furthermore, the measurement and evaluation module  13  may be part of one of the two grounding modules  8  or  9 . The measurement and evaluation module  13  also has a connection to ground  10 . 
   In order to be able to describe the method in more detail,  FIG. 2  shows the electrical connectivity of an arrangement as shown in  FIG. 1 . A friction-point contact is in this case illustrated schematically by a friction-point resistor  18  having the resistance value R 1 . A current i 1  flows to ground  10  via this fault in the insulation. Also shown is how a current i DE  flows out to ground  10  via the first grounding module  8 . This current i DE  can be determined via the voltage which can be measured across the first grounding resistor  11  having the value R DE  (for example a DC resistor having the value of 1 ohm). The first grounding module  8  has a brush contact resistance  19  to the shaft  1 . 
   The second grounding resistor  12  having the value R RC  (for example a DC resistor having the value of 470 ohms) is arranged in the second grounding module  9 , and a current i RC  flows to ground  10 . This current can be determined as a result of the known value of R RC  via the voltage u RC  applied across the resistor  12 . 
   It should be emphasized that the present method also functions with only one grounding point  8  or  9 . 
   The novel method is now distinguished, inter alia, by an electrical test current i in  having a known value being injected into the shaft  1  and, simultaneously therewith, all of the currents flowing out via the normal and intentional contacts (grounding brushes  6  and  7  or grounding modules  8  and  9 ), i.e. i DE  and i RC , being measured. 
   In the case of ideal shaft insulation, the total of all of the current values needs to result in the value 0, i.e. the total of the measured currents flowing in corresponds to the total of the measured currents flowing out. In the case of touching points, this balance is disturbed and the total of the currents flowing in is unequal to the total of the measured currents flowing out. 
   In order to carry out the method, the following measures are now taken: 
   Nonreactive resistors, i.e. the grounding resistors  11  and  12 , having a known value are inserted in all of the intentional grounding connections, i.e. the two grounding modules  8  and  9 , respectively, in order to create defined electrical conditions. This is used for estimating the “degree” of the touching point, i.e. of the average resistance R 1  over time of the touching point. 
   All of the currents flowing to ground via these intentional grounding contacts, i.e. i DE  and i RC , are measured. 
   In this case, advantageously only the DC components (DC) of the currents are measured. This is because the value of alternating currents additionally flowing in or flowing out (capacitive displacement currents) changes with time, is largely unknown and is normally difficult to measure. The DC component of currents additionally flowing in is unknown but is constant over the short term and medium term, and it primarily corresponds to the quantity of static electricity produced in the turbine. The DC components are determined by means of averaging over longer periods of time, for example over periods in the range from 1 to 10 s, or by filtering of the signals. This at the same time also eliminates very effectively the measurement inaccuracies owing to measurement noise. 
   A very advantageous measure is time differential measurement: in close time succession, the total S 0  of the currents i DE  and i RC  flowing out to ground without additional injected current i in  is measured and, and shortly thereafter or else shortly before, the total S 1  of the currents flowing out with additional injected current i in  The difference between the two current totals S 1 −S 0  is then compared with the value of the injected current i in  Given ideal insulation and measurement quality, both values are the same. This procedure serves the purpose of eliminating unknown, but temporally constant currents (such as the current caused by static charging of the turbine) from the balance. 
   The following statements can thus be made on the basis of the measured currents: 
   If i in =i DE +i RC , there is ideal shaft insulation, i.e. there are no friction points (R 1  is infinitely high). 
   If i in =i RC , the fuse in the second grounding module  9  must have blown since the total injected current flows only via the second grounding module  9  to ground  10 . 
   If u RC &gt;&gt;u DE  or, in other words, i RC &gt;&gt;i DE ·(R DE /R RC ), the brush contacts on the grounding brushes  6 ,  7  must be poor. 
   If u DE /i DE =R DE,1 , i.e. in a specific case where u DE /i DE =10 Ω, the fuse in the first grounding module must be defective. 
   In summary, it can be established that the above-described method or the device for carrying out this method is preferably distinguished by the following features: 
   The machine shaft  1  is electrically connected to ground  10  via-one or more grounding brushes  6 ,  7 , nonreactive resistors, i.e. grounding resistors  11 ,  12 , having a known value being inserted in all of these ground connections. 
   A measurement and evaluation module  13  measures all of the currents i DE  and i RC  which flow to ground  10  via the grounding modules  8 ,  9 . 
   The measurement and evaluation module  13  adds up these currents, with the correct mathematical sign, i.e. currents which flow to ground  10  from the shaft  1  are evaluated as being positive, for example, and thus the total ground current S 0  (i DE +i RC ) is determined which flows to ground  10  via these ground connections. 
   The measurement and evaluation module  13  which is also connected to ground  10  injects, at intermittent times, an additional test current i in  having a known value into the shaft  1 , the positive direction of the measurement and evaluation module  13  pointing towards the shaft  1 , and at the same time in turn the total of the currents S 1  flowing out to ground being determined. 
   The measurement and evaluation module  13  compares the value of the total ground current S 0,0  without injection of test current with the value of the total ground current S 0,1 , with injection of test current. The difference between the two values, i.e. the total ground current with injection of test current  17  minus the total ground current without injection of test current  17 , is then compared with the value of the injected current i in . If this difference in ground current differs from the value of the injected current, this is considered to be an indication of shaft touching and shaft friction. 
   The value of the difference, which must correspond to the current i 1  flowing out via the touching point, is then used to estimate the conductivity (or the resistance R 1 ) of the touching point, this taking place in accordance with the principles of current splitting via resistors.