Insulation Testing of HV Rotating Machines
HV rotating Machines have been for a long time a major component in many power systems such as generation of electricity and different types of motors. Due to its nature, any rotating machine needs to withstand electrical and mechanical stresses degrading the insulation materials causing irreversible and serious damage and even catastrophic and fatal to the entire system.
Preventing flash over on a rotating machine requires periodic inspection and testing of the insulation system. The point of the rotor's lowest insulation resistance will almost always be between the exposed metal surface of the slip ring and the rotor shaft. Lowered insulation resistance due to contamination built up around the rotor leads could easily lead to flash over (arcing between the rings or to ground) and damage the connections between the lead and the rings, the slip rings and possibly even the shaft. Hence, it is necessary to monitor and diagnose closely the insulation condition.
The accumulation of moisture, dirt and conductive contaminants on the stator insulation will also cause changes in winding insulation capacitance, resulting in corona formation.
CEL, Central Electrical Laboratory of Israel Electric Corporation shows 2 interesting examples from which one can learn on the importance of the role of a corona camera in a rotating machine predictive maintenance; For the diagnosis of the insulation condition and for the maintenance recommendations, CEL used the following tests:
- Insulation resistance and polarization index measurements
- Dielectric dissipation factor and capacitance measurements
- Partial distance measurements and corona detection with a corona camera
- HV testing
A 22,500 KVA, 11,000V generator during periodic testing showed high level of PD in one of the phases between the winding and stator core due to the degradation of the semiconductor layer protecting from corona as shown if fig. 1.
Fig. 1: Degradation of the insulation material protecting from corona discharge
Another generator of 133750 KVA and 11500 V showed corona discharge in generator winding at nominal voltage as shown in Fig. 2.
Fig 2: Corona detection with a Daycor® UV camera
With the help of the DayCor® corona camera that located the exact points of the failures the generator was repaired, isolation was strengthened with resin and the corona disappeared.
Regular testing of the rotor circuit is recommended to maintain a high level of dependability. In order to do so, use a DayCor® camera while ensuring the ground lead is connected to the shaft of the rotor. To get more accurate measurement ensure to inspect the slip rings and their insulation for any build up of contamination and clean if necessary.
Experience in Off-line insulation testing and diagnosis of high voltage rotating machines, A.Shkolnik, I.Litvin, Central Electrical Laboratory, Israel Electric Corporation