Statistics indicate that up to 37 percent of rotating equipment failures
occur due to stator insulation breakdown. For several decades, partial
discharge (PD) testing has been a very beneficial tool for assessing the
stator insulation condition of medium-voltage rotating apparatus. This article
examines how PD activity occurs and why PD testing is unique as applied to
generators and motors.
What is Partial Discharge?
Partial discharge is a localized partial breakdown within or on the surface
of insulation that causes a spark to occur. This partial failure of
the insulation is generally caused by poor design, flaws, voids,
or contamination that create a localized stress that exceeds the
dielectric breakdown strength of the insulation material. The
sparking activity then leads to further insulation decomposition
through a combination of thermal, chemical, and electrical phenomena
and persists until complete failure occurs. The detection
and monitoring of PD activity has been found to be very useful
in assessing the condition of electrical equipment. Although PD
can occur at lower voltages, equipment operating below 2,000
volts to ground usually does not discharge frequently and usually
fails by other mechanisms.
Rotating Apparatus Insulation
Switchgear, cable, and transformer insulation cannot tolerate
PD activity very well. Generally, PD activity will lead to further
destruction of the insulation and eventual failure. Therefore, these
types of equipment should always operate PD free.
The design of rotating apparatus creates inherent difficulties
that make producing PD-free equipment impossible. The complex
configuration of stator coil insulation (see Figure 1), difficulties
with the manufacturing impregnation process, and the problems
fitting the coils within the stator slots can all create voids which
lead to PD activity. Since PD in
rotating apparatus is inevitable, the
coil insulation employs mica tape
which is very resistant to partial discharge
damage. So, when assessing
the condition of rotating apparatus,
it becomes very important to monitor
and trend PD activity over the
course of many years as opposed
to assessing the condition of other
types of equipment insulation where
periodic PD testing may suffice.
PD Sensors
Effective on-line PD testing of switchgear, cable, and
transformer insulation generally involves taking regular (annual)
spot checks using temporary sensors that are safely attached
to grounded components of the apparatus. The main
goal of this procedure is to ensure a PD-free apparatus.
In order to best monitor rotating apparatus, permanent
coupling capacitors (see Figure 2) are connected to the machine
output bus during an outage. These sensors provide a
standardized method to decouple the PD signals. Additional
data can be obtained by using the RTD’s that are embedded
in the insulation as auxiliary PD measurement points. The
RTD’s act as antennas that can pick up the high frequency
pulses associated with PD.
Since the rotating equipment PD activity is trended
over many years, it is more economical to install permanent
monitors than conducting regular spot surveys. These monitors
(see Figure 3) accept other sensors for trending load,
temperature, and humidity data along with the PD data so
that correlations can be made that provide additional information
for more precise diagnostics of the cause, location,
and type of the PD activity occurring.
Valuable Data
As previously stated, the mica insulation is quite impervious
to most PD activity. However, large amplitude
discharges occurring from voids in the insulation or end
winding contamination can lead to direct insulation failure.
This is just one category of problems that can be detected.
Insulation can also fail from the thermal or mechanical
wear over the course of many years from overheating, thermal
cycling and loose wedging. In these cases, increases in
PD activity will show as a slow rise over time. This type of
PD activity relates directly to the machine’s overall thermal
or mechanical condition, as the PD activity is not the primary
problem but is actually an indicator of the presence
of other age-related problems.
Conclusion
Monitoring PD activity in rotating apparatus is very important
for increasing reliability and prolonging equipment
life. All generators and critical motors should be monitored
continuously and consideration should be given to performing
annual PD spot testing to the balance of plant medium
voltage rotating apparatus.
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