Diagnostics and testing of rotating electrical machines - DNV Kema
Diagnostics and testing of rotating electrical machines - DNV Kema Diagnostics and testing of rotating electrical machines - DNV Kema
14 ENERGY Non-destructive testing & plant diagnostics Electric testing of rotors DC-resistance of the rotor windings In order to detect the presence of defects in the rotor windings current path, the DC-resistance of the stator windings is determined by means of a DC-current, which is conducted through the rotor 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 rotor 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. Voltage withstand tests on the rotor windings Verification of the condition of the electrical coil insulation of rotor windings may be done by Megger testing followed by application of a prescribed power frequency voltage for some time. The coil insulation to earth or to neighbouring coils is tested with short-circuited rotor windings. Generally the voltage will be less than the level of the original factory acceptance test. For this purpose the results mostly are 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. Impedance of the rotor windings In order to detect presence of defects in the rotor windings current path, the 50/60 Hz power frequency impedance of the stator windings is determined by means of an AC-current, which is conducted through the rotor winding. Then voltage across the slip rings is measured. The test is normally performed between the slip rings. The AC-current must be large enough to enable accurate measurements but shall not result in significant thermal effects during the measurement. Subsequently the 50/60 Hz power frequency impedance of the rotor winding was determined. The accuracy of the measurement depends on several factors. For instance the wave shape of the harmonic current may be crucial. For evaluation of the results, a comparison is made with previous measuring results, such as obtained by the manufacturer in the factory.
Non-destructive testing & plant diagnostics ENERGY 15 RSO-measurement on the rotor windings In order to test the performance of the rotor windings RSO (Recurrent Search Oscillogram) measurements are conducted. For the test a transient signal with respect to the rotor shaft is injected in each of the slip rings. A voltage impulse generator is connected between one of the slip rings in turn and the rotor shaft. The impulse voltage and the resulting current are fed into a transient recorder. The admittance as a function of the frequency will be determined as the twenty-fold logarithmic ratio of current and voltage, for each frequency component resulting from the Fourier transform and is expressed in dB. From the resulting current the admittance can be derived. The admittance of the two branches of the rotor winding relative to the rotor shaft will be measured. The admittance characteristics should show no difference. The test is performed with the rotor in standstill mode, or if possible, in running mode, at various speed adjustments. Additional information about the condition of the rotor windings can be found by comparison of the transient behavior of symmetrical winding branches, each connected to a slip ring. Insulation resistance of the rotor windings and determination of the polarization index To determine the polarization index and insulation resistance of rotor windings a dedicated test can be performed. The insulation resistance of the rotor windings is measured by means of a digital Megger. During the measurement a DC-voltage of approximately 500 V is applied. The measurements are performed as a function of the time after application of the voltage. The measurements are performed on the rotor winding with the other machine winding earthed, together with the frame of the stator. It is important to register the temperature of the winding for the evaluation of the result. The leakage current that occurs when the winding insulation is subjected to a DC-voltage is a measure for the condition of its insulation. A distinction can be made here between the timedependent 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. The leakage current is measured at a specified DCvoltage level by means of an insulation resistance meter (Megger). 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. The admittance is presented in curves for the circuit inner slip ring to shaft (curves inner long and inner short) and for the circuit outer slip ring to shaft (curves outer long and outer short). The expression short and long relates to the length of the applied interconnecting cable wires between pulse generator and rotor. The length of the cable wires is of minor importance on the measurement. Furthermore, for frequencies up to 20 kHz, the admittance of both circuits is identical.
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14<br />
ENERGY<br />
Non-destructive <strong>testing</strong> & plant diagnostics<br />
Electric <strong>testing</strong> <strong>of</strong> rotors<br />
DC-resistance <strong>of</strong> the rotor windings<br />
In order to detect the presence <strong>of</strong> defects in the rotor windings<br />
current path, the DC-resistance <strong>of</strong> the stator windings is determined<br />
by means <strong>of</strong> a DC-current, which is conducted through the rotor<br />
winding in series with a calibrated shunt. The test is normally<br />
performed between the slip rings. The DC-current must be large<br />
enough to enable accurate measurements, but shall not result in<br />
significant thermal effects during the measurement. The voltage<br />
across the shunt <strong>and</strong> the rotor winding is compared. During the<br />
measurements the temperature <strong>of</strong> the winding is recorded.<br />
For evaluation <strong>of</strong> the results a comparison is made with previous<br />
measuring results, such as obtained by the manufacturer in the<br />
factory.<br />
Voltage withst<strong>and</strong> tests<br />
on the rotor windings<br />
Verification <strong>of</strong> the condition <strong>of</strong> the <strong>electrical</strong> coil insulation <strong>of</strong> rotor<br />
windings may be done by Megger <strong>testing</strong> followed by application <strong>of</strong> a<br />
prescribed power frequency voltage for some time. The coil insulation<br />
to earth or to neighbouring coils is tested with short-circuited rotor<br />
windings. Generally the voltage will be less than the level <strong>of</strong> the original<br />
factory acceptance test.<br />
For this purpose the results mostly are corrected to a temperature<br />
<strong>of</strong> 75°C. If the results are deviating from the expected values<br />
the cause should be traced. Any bad connection or defect in the<br />
terminals or (partly) interruption in the windings or leads may<br />
cause large damage to the machine.<br />
Impedance <strong>of</strong> the rotor windings<br />
In order to detect presence <strong>of</strong> defects in the rotor windings<br />
current path, the 50/60 Hz power frequency impedance <strong>of</strong> the<br />
stator windings is determined by means <strong>of</strong> an AC-current, which<br />
is conducted through the rotor winding. Then voltage across the<br />
slip rings is measured. The test is normally performed between<br />
the slip rings. The AC-current must be large enough to enable<br />
accurate measurements but shall not result in significant thermal<br />
effects during the measurement. Subsequently the 50/60 Hz<br />
power frequency impedance <strong>of</strong> the rotor winding was determined.<br />
The accuracy <strong>of</strong> the measurement depends on several factors.<br />
For instance the wave shape <strong>of</strong> the harmonic current may be<br />
crucial. For evaluation <strong>of</strong> the results, a comparison is made with<br />
previous measuring results, such as obtained by the manufacturer<br />
in the factory.
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