Online and offline Partial Discharge
Partial discharge (PD) occurs in the stator insulation system of rotating machines, where local electric field stress exceeds the local electrical strength. Compared with other dielectric tests, the differentiating character of partial discharge measurements allows localized weak points of the insulation to be identified.
PD measurements based on IEC 60034-27 standards can be performed on motors and generators either on-line during regular load service operation or off-line by energizing each winding successively during standstill of the machine.
Generally, several different partial discharge sources are present and active within the stator winding insulation at the same time. The key to successful PD measuring in stator windings is the separation of parallel active PD sources and the distinction between harmful.
PD, normal PD occurrences and outer noise, inevitably present in industrial surroundings. This is best achieved by synchronous, multi-channel partial discharge measurement. This separates internal PD sources from each other and from noise signals. PD sources are evaluated individually within the stator winding.
The insulation is the most sensitive part of rotating machines. The lifetime of a stator winding depends on how effectively the electrical insulation can prevent any occurrence of winding faults.
Weak spots can already occur in the insulation during manufacture or impregnation with cast resin, through the stresses of daily operation (for example in the form of soiling) or general aging.
With capacitance and dissipation factor measurements, also known as tan-delta measurements, changes in the insulation can be diagnosed. Aging processes, changes in the structure of the insulation or moisture ingress can, for example, increase the dissipation factor. The increase in the dissipation factor during the “tip-up test” reflects the condition of the insulation.
High Voltage Withstand Test
One of the more important tests to a motor or generator coil is the AC Withstand test. Only this test, using AC high voltage, can truly verify the integrity of insulation as it is stressed under operating conditions. This test applies an over voltage roughly twice the normal line to ground operating voltage, or 2 Un. The test voltage is held for 60 seconds. It is a simple pass or fail test, where the coil either holds the voltage or fails/arcs should there be a defect. It is a “destructive” test to defects that cannot hold the test voltage but has no harmful effects on the coil where it is not defective.
PI & IR measurements
Insulation starts to age as soon as it is made. As it ages, its insulating performance deteriorates. Any harsh installation environments, especially those with temperature extremes and/or chemical contamination, accelerates this process. This deterioration can result in dangerous conditions in power reliability and personnel safety. As such, it is important to identify this deterioration quickly so that corrective steps can be taken.
An insulation resistance (IR) test measures the total resistance between any two points separated by electrical insulation. The test, therefore, determines how effective the dielectric (insulation) is in resisting the flow of electrical current. Such tests are useful for checking the quality of insulation, not only when a product is first manufactured but also over time as the product is used.
A Polarization Index (PI) test is generally performed at the same voltage as the Insulation Resistance (IR) test. Where the IR test is performed for a period of one minute, the PI test is performed over a period of ten minutes. This gives the absorption (polarization) current ample time to decay and reveals a more detailed indication of the total leakage and conduction current. As such, PI is a good indication of winding contamination, moisture ingress (leakage currents), and/or bulk insulation damage (conduction currents).
Polarization Index testing is generally performed with an Insulation Resistance (IR) test set, commonly known as a Megger.
TVA Probe measurements
The Corona Probe (originally known as the TVA probe) is used to locate partial discharge in each slot with the winding energized phase by phase. It supplements online partial discharge monitoring.