Analysing the dissipation factor (tanδ) is a traditional method to assess the condition of an insulation system. With the analysis of the dissipation factor, emphasis is more put on the overall health of the insulation system. In contrast, with partial discharge analysis, the focus is on individual defects producing discharge activity. Therefore, the application of tanδ measurements concentrates on insulation systems, which are relatively stable against partial discharge.
Most prominently, the health of epoxy-mica insulation of a rotating machine can be assessed using a tanδ analyser. Especially, it can detect humidity within the winding, the surface contamination of field grading elements, or the polarisation losses of improperly cured resin. Thus, the tanδ analysis is still a good complement to the partial discharge testing.
Principle of Operation
The TDAcompact simultaneously samples the AC drawn by the device under test and the current drawn by a reference capacitor. Subsequently, the two current traces are evaluated, and the capacitance, the tanδ, and the high voltage level are calculated. The unit continuously displays and refreshes these results.
Therefore, the instrument does not require any user interaction like the traditional Schering Bridge, nor does the display refresh take that long as with automatic adjusting bridge-type analysers. Furthermore, the essential resolution of the tanδ measurement is 10-4, which fulfils the requirements for rotating machine testing and testing on mass-impregnated cables.
The TDAcompact is designed as a stand-alone instrument. To measure the two currents, the basic configuration of the TDAcompact comes with a standard capacitor that has a built-in shunt capacitor, in combination with an external shunt. Optionally, the unit can be supplied with further shunt capacitors. Generally, the TDAcompact is with digital fibre-optic links to the precision shunts, which allow operation on any potential, including the high voltage connection of the device under test. The measuring frequency can vary between 20 and 500 Hz or is fixed to 0.1 Hz (VLF). Upon request, Power Diagnostix can supply complete portable tanδ testers, including the high voltage transformer.
Besides the stand-alone field test application of the TDAcompact, the instrument can also become a part of a larger and automated test system. Using the software HVpilot, the instrument will be read according to a pre-programmed test sequence. Besides tanδ measurements, such automated test systems further include partial discharge measurements and so-called step tests.