IEC 60034-27-3-2015 pdf free download.Rotating electrical machines – Part 27-3: Dielectric dissipation factor measurement on stator winding insulation of rotating electrical machines.
The dielectric dissipation factor versus voltage characteristic is affected to some extent by the temperature of the test object insulation. Stator winding temperature shall be uniform and close to ambient temperature (it may be checked by means of stator winding RTD5). The temperature of the main insulation, the ambient air temperature and the relative humidity shall be recorded.
NOTE The stray-capacitance of the test object to earth can influence the results of measurements. The operating Instruchons of the instrument usually describes how this stray capacitance to be handled in order to minimize the negative Impact.
5.2 Winding bars and coils
5.2.1 Test object preparation
The dielectric dissipation factor test is mainly used as a quality control test on newly manufactured bars and coils. It is an important method to prove the main insulation quality and to determine the consistency of the manufacturing process.
The test results are affected by the electrodes used to simulate the slot. To compare results between individual bars or coils, or groups of bars or coils, identical electrode systems shall be used.
In order to use the conductive slot coating as measuring electrode, it is not sufficient to contact it at a few single points because the contact resistance to the conductive slot coating would be much higher than In the slot. This high resistance would lead to unrealistic high dielectric dissipation factor measurements. Therefore, it is necessary to contact the conductive slot coating with metal plates, wrapped metal foil or spiralled wires along the entire length. Great care should be taken to minimize voids between the metal electrode and the conductive slot coating surface.
The use of guard electrodes Is necessary to prevent erroneous surface effects at the end of the conductive slot coating on the dielectric dissipation factor readings (see Figures 8 and 9). All strands shall be electrically connected to avoid breakdown of the insulation of individual strands. The bar or coil to be tested should be suitably insulated from earth.
5.2.2 Guarding techniques
5.2.2.1 General
Various techniques of using guard electrodes are available to minimize measuring errors caused by additional currents starting from the end of the conductive slot coating to the high voltage side. A grounded guard ring electrode placed near the ends of the conductive slot coating (varnish or tape) should be used in order to minimise the influence of these additional currents on the dielectric dissipation factor measurement. The type of guard electrode (metal foil or conductive paint) that would be the most suitable depends on the manufacturing process of the bars or coils.
Depending on bar design, the conductive slot coating area may cover only the straight part of the bar or may also include the bends and part of the involutes of the bar.IEC 60034-27-3 pdf download.