Diagnostics and testing of rotating electrical machines - DNV Kema

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16 ENERGY Non-destructive testing & plant diagnostics Electric testing of stators Reliable operation of a generator is highly dependent on the electrical and mechanical integrity of stator winding. Operating temperature, operating voltage stress and thermal-cycle stress cause damage to the stator insulation. Failure of a generator stator conductor due to electrical or mechanical problems will lead to long forced outage of the unit resulting into huge revenue loss. Therefore it is necessary for judgment of insulation deterioration to carry out a diagnostic test of the stator coil. To evaluate the residual life from experience and the accumulated data, DNV KEMA offers a full set of diagnostics to assess the generator stator condition, like: ■■ Insulation resistance ■■ Polarization index ■■ Capacitance and dielectric losses ■■ Partial discharge ■■ Voltage tests Measurement of the DC-resistance of the stator windings In order to detect presence of defects in the stator windings current path, the DC-resistance of the rotor windings is determined by means of a DC-current, which is conducted through the stator winding in series with a calibrated shunt. The test is normally performed between the slip rings. The DC-current must be large enough to enable accurate measurements but shall not result in significant thermal effects during the measurement. The voltage across the shunt and the stator winding is compared. During the measurements the temperature of the winding is recorded. For evaluation of the results a comparison is made with previous measuring results, such as obtained by the manufacturer in the factory. For this purpose the results are mostly corrected to a temperature of 75°C. If the results are deviating from the expected values the cause should be traced. Any bad connection or defect in the terminals or (partly) interruption in the windings or leads may cause large damage to the machine. Insulation resistance and polarization index of stator windings To determine the polarization index and insulation resistance of stator windings a dedicated test is performed. The insulation resistance of the stator windings is measured by means of a digital Megger. During the measurement a DC-voltage is applied ranging from 500 – 5000 V. The measurements are performed as a function of the time after application of the voltage. The measurements are performed on the stator winding with the
Non-destructive testing & plant diagnostics ENERGY 17 other machine winding earthed, together with the frame of the rotor. It is important to register the temperature of the winding for the evaluation of the result. A distinction can be made here between the time-dependent effects during the charging of the winding capacitance and the non-time-dependent effects which occur as a result of moisture and/or tracking inside or on the surface of the winding insulation. From the measured insulation resistance as a function of the time, the polarization index is determined. The ratio of the insulation resistance values measured 10 minutes and 1 minute after the application of the DC-voltage, is called the polarization index. The polarization index and the insulation time constant are measures for the evaluation of the insulation condition with respect to cleanliness and the presence of moisture. The temperature of the winding is of importance. For modern epoxy-resin insulated windings the insulation resistance should be more than 1000 MΩ at 20˚C and the polarization index PI should be in accordance with the IEEE 43 standard. Capacitance and dielectric losses (Tan-δ) Dielectric losses and capacitance of the winding will increase with the voltage because voids (gaps) inside the insulation will be short-circuited as a result of occurring partial discharges. The dielectric losses of the insulation represent the relation of the resistance to the capacitive impedance. The dielectric losses depend on temperature and frequency. The capacitance of the winding at 0.2U n is of vital importance to find out whether major changes have occurred in the insulation. The capacitance of the windings depends on the relative dielectric constant of the insulation. The capacitance is subject of voids and gaps in the insulation. For the purpose of these measurements a Schering bridge is used with a loss-free standard capacitor. The measurements are performed as a function of the voltage between for example 0.2 and 1.0U n in steps of 0.1U n. The measurements can be performed part-wise, mostly per phase, on the individual parts of winding with the other parts connected to earth and to the stator frame. The dielectric loss angle of the insulation and the capacitance of the stator windings are determined. The measurement can be done very accurately and reliable. In order to assess the condition of the generator stator, DNV KEMA offers a full set of diagnostics such as ■■ Insulation resistance ■■ Polarization index ■■ Capacitance and dielectric losses ■■ Partial discharge ■■ Voltage tests
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Diagnostics and testing of rotating electrical machines - DNV Kema

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