ON-LINE MONITORING OF STATOR WINDING INSULATION THE MANUFACTURER AND SERVICE ENGINEERS VlEW

RM. Burbidge

GEC Alsthom helped to pioneer the installation of monitoring systems based on capacitive couplers in the UK and has now been engaged in this field of work for over ten years. During this period the number of companies involved in this field of work has increased. Many industries and utilities regard on-line monitoring of stator winding insulation as important. This is especially true of those operating hydroelectric generators and critical rotating machines on oil platforms. Electrical machine operators in other continuous process industries are also now using this form of monitoring.

On-line monitoring of stator insulation is an additional test method to add to the already large number of off-line tests. There is of course more than one method of on-line monitoring. The demand for this type of surveillance by machine operators has meant that the electrical manufacturer has another product to supply. Unfortunately there is no standard for this product, no B.S.4999 Part x as for a rotating machine or I.E.C.270 as for off-line partial discharge testing. Furthermore there is no Cigre Electra paper to give the equipment operator a basis for interpretation of the results, as there is for off-line partial discharge testing.

In practice the most successful on-line test commercially may eventually become the "de facto" standard. As long as any dominant producers of test equipment do not restrict the technology to themselves this should not be a problem. In the field of off-line partial discharge testing there are at least four manufacturers' products marketed in the U.K. which conform to 1.E.C.270. All of these products can be operated by the purchaser after a short period of training. Interpretation of the results is possible without referring to the equipment manufacturer. The results measured on one manufacturer's equipment at one site can be compared against those measured on another's at a different site, provided that the conditions of test and calibration recommended in I.E.C.270 are used for both tests. This is a very satisfactory situation for off-line p.d. testers.

On-line partial discharge test operators also may choose from at least four equipments in the U.K. Each of these equipments is built to the manufacturer's own standard. They appear to have different frequency bandwidths and give results in different units to each other. This makes comparison between test data recorded on different equipments difficult. It is unlikely that results from on-line systems can be compared with those from off-line systems even if the conditions of test are similar.

In the laboratory GEC Alsthom uses a form of on-line monitoring during accelerated life testing of a.c. stator coils and bars. During this work it has been found that a significant change in the parameter being measured, in this; case peak discharge, occurs after approximately 99% of the specimen's life has expired.

[RM. Burbidge is with GEC Alsthom Large Machines, Rugby.]

This pattern of main wall insulation failure has been noted on several examples of synthetic resin- bonded mica systems. It applies to both resin-rich and v.p.i. systems.

Breakdowns of the main wall in service are uncommon with modem insulation systems. This is fortunate since it appears that it is difficult to predict them until they are imminent. It may be that the prediction of 1% of remaining life is better than most operators are achieving with existing earth leakage and phase unbalance trips. It is possible that, using peak discharge as an indicator of a possible fault that the warning period will be long enough to enable appropriate action to be taken.

The preceding example of monitoring depended for its usefulness on the establishment of a consistent base level of peak discharge against which unusual readings could k compared. This base level was specific for each specimen and test and may be meaningless when applied to a complete winding. This brings us back to the problem of a lack of standards.

The insulation on the test samples quoted was micapaper tape bonded with epoxy resin . However there are many machines with natural resin based systems still in service which occasionally fail during operation. If it is considered that a particular winding of this type has useful remaining life then it could be retrofitted with detectors for monitoring. Since the winding would be well into its lifetime it might be difficult to establish a base level for it.

On-line monitoring of stator windings has been used by utilities to detect the onset of problems such as looseness of windings and erosion of semi-conducting coatings. The results of regular monitoring indicate when maintenance needs to be canid out. They are, therefore, useful in extending the life of windings. It may be that long term-prediction of failures due to internal erosion of the miin wall insulation are less important if other more common problems can be detected and rectified.

An obvious extension to monitoring machines in service is to use the same equipment for quality control in manufacture. This has already been tried by some equipment manufacturers. Here is another area where a standard is necessary.

If utilities press for the use of on-line monitoring equipment as a quality control on new windings or for coils in the factory or on site then all manufacturers will be forced to provide the technology and add the cost of equipment and testing to their products.

In the meantime, any manufacturer who wishes to be involved in providing confirmation of how his product performs in service is faced with developing his own standards. This will be an additional expense with no positive evidence that the technique will help in assessing the life of a modern winding in service or that this method will be accepted by any other company.

Until some general standards have been established, it will be difficult to assign pass and fail levels in absolute terms. In the interim period, on-line monitoring should be used as a tool for following changes of the parameters measured on operational machines. At this stage in the development of these techniques it may be sensible to monitor % changes from a base value specific to a particular machine. This would avoid the need to set absolute values of maximum discharge.

0 1995 The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place. London WCPR OBL. UK.