Concept of insulation resistance & Polarization Index
- 1. Concept of Insulation Resistance By: HARESH YADAV • BASICS OF INSULATION RESISTANCE • IMPORTANCE OF POLARIZATION INDEX • IEEE STD 43 -2000 & IS 732 • IR & P.I MEASUREMENT EQUATIONS
- 2. WHAT IS INSULATION RESISTANCE ??? • The capability of the electrical insulation of a winding to resist direct current (Flow of Electrons) 4 COMPONENTS OF CURRENT DROWN BY INSULATION WHEN DC APPLIED • Absorption (Polarization) current (Ia) • conduction (Leakage) current (Ig) • Capacitance Charging Current (Ic) • surface leakage current (IL):
- 3. Equivalent Circuit & Test connections
- 4. Absorption(Polarization): The absorption current, corresponding to the additional energy necessary for the molecules of the insulating material to reorient themselves under the effect of the electrical field applied. This current falls much more slowly than the capacitance charging current, sometimes requiring several minutes to reach a value close to zero. Leakage current: The leakage current or conduction current. This current characterizes the quality of the insulation. leakage current that flows through the insulation volume itself is of primary importance. This is the current that is used to evaluate the conditions of the insulation system under test. surface leakage current: The surface leakage current is due to the conduction on the surface of the insulation where the conductor emerges and points of ground potential. Clean the contaminated surface before testing. Capacitance Current: The capacitance charging current necessary to charge the capacitance of the insulation being tested. This is a transient current which starts relatively high and falls exponentially towards a value close to zero once the circuit being tested is charged electrically. After a few seconds or tenths of seconds, this current becomes negligible compared with the current to be measured.
- 6. Voltage Range for I.R Measurement (IEEE 43-2000)
- 7. Polarization Index: • The polarization index (P.I.) is traditionally defined as the ratio of the 10 min insulation resistance (IR10) to the 1 min insulation resistance (IR1), tested at a relatively constant temperature.. • In older insulation materials, such as asphaltic-mica, the absorption currents often take 10 min or more to decay to nearly zero • Modern types of film coated wire, as well as epoxy-mica or polyester-mica insulated stator windings, may approach a constant value of insulation resistance in 4 min or less. • If the winding is wet or dirty, a low steady value will usually be reached 1 min or 2 min after the test voltage is applied. • There is no standard for what time intervals the IR values are to be recorded. Different organizations use different ratios. • it usually takes several minutes after application of the electric field for the molecules to become reoriented, and, thus, for the current-supplied polarizing energy to be reduced to almost zero
- 8. Factors Affecting Insulation Resistance: • Surface Condition:Dust (or salts) on insulation surfaces, which are ordinarily nonconductive when dry, may become partially conductive when exposed to moisture or oil, and, thus, can lower the insulation resistance. Cleaning & Drying To be done to Restore the Value. • Effect of moisture: Older winding insulation systems are hygroscopic (easily absorb water) and moisture may be drawn into the body of the insulation from the humid ambient air. Absorbed moisture increases the conduction current (IG) and significantly lowers the insulation resistance. • Effect of temperature: in insulators, an increase in temperature supplies thermal energy, which frees additional charge carriers and reduces resistivity. The insulation resistance value of a winding depends upon the winding temperature and the time elapsed since the application of the voltage. • Effect of Voltage: The value of insulation resistance may decrease somewhat with an increase in applied voltage; however, for insulation in good condition and in a thoroughly dry state, substantially the same insulation resistance will be obtained for any test voltage up to the peak value of the rated voltage.
- 9. Recommended P.I Values: (IEEE 43-2000)
- 10. Minimum I.R Values: (IEEE 43-2000)
- 11. Minimum I.R Value Equation: (IS-732) • The insulation resistance of each phase winding against the frame and between the windings shall be measured. • Mega-ohmmeter of 500 V or 1000 V rating shall be used. Star points should be disconnected. • Minimum acceptable value of the insulation resistance varies with the rated power and the rated voltage of the motor. • If the resistance is measured at a temperature different from 25’C, the value shall be corrected to 25°C.
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