BS/EN 60034-27-3-2016 pdf download.Rotating electrical machines Part 27-3: Dielectric dissipation factor measurement on stator winding insulation of rotating electrical machines.
The guard ring electrode has to be fixed on top of the stress control layer near the overlapped part of the conductive slot coating and the stress control coating. Locating the physical end of the conductive slot coating can be difficult if it is overlapped with the stress control coating. Nevertheless, the beginning of the guard ring electrode should be positioned as near as possible to the end of the overlapped part of grading system, but without covering this overlapped part. This procedure is normally used on production bars and coils since it is known at that time where the exact overlap of these two coatings is located.
This configuration will only have a minor effect on the dielectric dissipation factor results if the guard ring electrode is implemented properly. The implementation depends on the electrical properties of the stress control coating, the possible presence of an insulating sleeve, covering the overlapping area and the contact resistance between guard ring electrode and the surface of the stress control layer.
This simplified guarding method may be useful in routine production testing when each test object needs to be measured individually.
5.2.3 Measuring procedure
To ensure a homogeneous production process quality, it is recommended to test such a sample size which is necessary for a statistically significant evaluation of the characteristic data of all bars or coils.
Dielectric dissipation factor measurement results may be affected during application of a test voltage. Therefore, some form of conditioning is recommended to stabilize partial discharge activity and dielectric losses, which influence the dielectric dissipation factor measurement. It is recommended to energize the bars or coils with high voltage for a consistent time (see IEC TS 60034-27). The high voltage test, which is a quality check of all new bars or coils. could be used for conditioning purpose.
Dielectric dissipation factor and capacitance values shall be recorded with stepwise increase of voltage according to Figure 7 and description in 5.1. The test results of each test object shall be recorded for reporting and possible statistical evaluation.
5.3 Complete windings
A dielectric dissipation factor measurement can be performed on the complete stator winding before as well as after assembly of the complete machine. Before assembly, the stator winding is specifically addressed, while e.g. bushings and other components may influence the measurement results after assembly. Dielectric dissipation factor measurements of complete windings In conjunction with other diagnostic methods are a useful tool for condition assessment of rotating electrical machines. Testing the new stator winding insulation provides the reference point for trending of ageing condition of the insulation system. A variation in dielectric dissipation factor values over a period of time may be an indication of a change in the condition of the main insulation, conductive slot coating, stress control coating, end windings, or slot support systems.
The test method and the test equipment differ from that when testing individual bars or coils. Contrary to the test method for individual bars or coils, guarding techniques are not practicable for complete windings. Therefore, the comparison of the results with those of tests on individual bar or coils should take account of the fact that results from the complete winding will be considerably influenced by the nonlinear resistance of the stress grading coatings relative to the length of the slot portion. In the case of measurement of an individual phase, the other phases should be connected to the stator frame.
Maximum information about end windings and slot portion condition may be obtained by measuring the dielectric dissipation factor on each individual phase winding and the complete winding (all phases together) up to the rated voltage UN for new machines.
In the case of on-site measurements on machines with an external neutral connection, only the individual phases may be measured to reduce the capacity requirement of the high voltage power supply.
Stator core and frame of complete machines are solidly grounded. When performing the dielectric dissipation factor measurement with an analogue Schering bridge an auxiliary pass adapter is needed to avoid operating the Schering bridge ungrounded, According to safety rules, handling an ungrounded instrument requires specially trained and experienced personnel. Digital equipment with long fibre optic connections between measuring device and control unit make it possible to connect the measuring box at the high voltage terminal of the tested phase to mitigate safety risks.
On grounded windings. different stray capacitances may influence the measurement. Nevertheless, because of the high phase winding capacitance, in most cases, these stray capacitances will not affect the readings significantly.
Dielectric dissipation factor measurement on large direct water cooled windings may be influenced by the resistance of water flow to ground potential. When the water conductivity and the dimensions of the insulation hoses are known, the water losses may be calculated and the measured dielectric dissipation factor corrected. Otherwise, the stator winding has to be drained and vacuum dried before starting the measurement.
In addition to the well-established type test of complete phase windings, a random sample test for global VPI-insulation systems may be agreed. A minimum of four coil-sides or two bars are manufactured as for the production machine, at the same time and under the same conditions as the production winding and accommodated in slot models.BS/EN 60034-27-3-2016 pdf download.