IEEE 43-2013 pdf free download – IEEE Recommended Practice for Testing Insulation Resistance of Electric Machinery.
Before any testing is conducted, the winding insulation must be discharged. It is not safe to begin testing before the discharge current is almost zero and there is no discernible return voltage (less than approximately 20 V) after the ground is removed (in general, the winding should not be left ungrounded). After completion of the test, the winding should be discharged through a suitable resistor, sized to limit the instantaneous current. A minimum discharge time, which is equal to four times the voltage application duration, is recommended. This time interval is based on the R (resistive), L (inductive), C (geometric capacitive), and absorptive characteristics of the circuit during charging (time of the application of the voltage) and discharging (elapsed time since the removal of the voltage source and subsequent grounding of the winding under test).
It is important to remember that the testing is not complete until the winding is discharged and there is no discernible voltage. It is recommended that subsequent testing not be conducted until the winding is fully discharged. During the test period, all appropriate safety measures for the voltages being used shall be taken. The lead between the test set and the winding must be appropriately insulated and spaced from ground; otherwise, surface leakage currents and corona loss may introduce errors in the test data. For safety considerations, and to avoid measuring stray currents, the leads may be shielded. Restriction of personnel access to the high voltages is mandatory. Use of personal protective equipment is recommended, as is the use of hot sticks, insulated ladders, etc.
If accessible, the phase neutral and line ends of each winding should be connected together during the test to minimize the effect of high-voltage surge reflections that may result from a winding failure. The safety measures described are by no means all encompassing. These are meant only to signify the nature of the hazards involved. It is the responsibility of the users of the test equipment to completely ascertain the possible hazards involved in the testing, to protect personnel from harm, and to eliminate the risk of damage to the equipment.
5. Insulation resistance—general theory The insulation resistance of a rotating machine winding is a function of the type and condition of the insulating materials used, as well techniques used to apply them. In general, the insulation resistance varies proportionately with the insulation thickness and inversely proportionately to the conductor surface area.
5.1 Components of the measured direct current The insulating system of a stator winding is comprised of numerous interfaces, which exist between different materials such as mica, glass and polymer matrix of either epoxy or polyester. As a consequence, the electrical conduction process is principally controlled by the interfacial polarization mechanism. When a direct-voltage field is suddenly applied across a stator winding insulating system, space charge build-up occurs at the interfaces due to the difference in the permittivities and conductivities of the contiguous dielectric materials forming the interfaces (see [B1] and [B2]). First, a rapid voltage division is established across the two abutting dielectrics at the interface because of their difference in permittivity. This is manifest by an almost instantaneous capacitive current, I c , whose duration is too short to influence the shape of the overall current and thus does not influence the one minute resistivity measurement. This capacitive current decreases exponentially with a time constant equal to the product of the winding’s capacitance and the instrumental resistance. The voltage drops across the two dissimilar dielectric layers, constituting the interface, that are characterized by two distinctly different conductivities, lead to the development of two currents of unequal magnitude. This causes charge accumulation or trapping at the interface until the counter field created by the trapped space charge equalizes the currents in the adjacent dielectric strata.IEEE 43 pdf download.