The winding does not behave as a simple test object. A discharge inside a slot couples to the terminals through the winding, and the path attenuates and reshapes the pulse. What the measurement sees depends on where the discharge is and on the winding geometry as much as on the discharge itself.

Why machines need their own PD approach

The winding has a resonant behaviour of its own. A pulse generated inside the winding propagates along the conductors and through the capacitance between coils, and at particular frequencies the winding presents a resonance that amplifies or attenuates the signal.

The coupling arrangement therefore matters more on a machine than on a cable. A capacitive coupler at the machine terminals sees the whole winding, and a discharge in a coil some distance from the terminals is attenuated before it arrives.

The interpretation differs for the same reason. On a cable, a discharge magnitude is compared against a limit derived from the acceptance criteria. On a machine, the magnitude at the terminals depends on the winding and the coupler, and the useful comparison is against the same machine measured under comparable conditions.

Sensor options for stator windings

The arrangement should be recorded in enough detail to be reproduced, because a different coupler position produces a different measurement on an unchanged machine.

Online measurement during operation

Online measurement is often the more practical option on a machine that is difficult to take out of service. It requires permanently installed sensors and a noise environment that permits detection, and it produces a record of the machine’s behaviour across its operating range.

The valuable information is the relationship between activity and operating conditions. A machine whose discharge activity rises with load is describing a condition that responds to thermal or mechanical changes, which is different from one whose activity is constant regardless of load.

The threshold should be set so that an alert leads to an outage test rather than an immediate intervention. Online measurement indicates that something has changed; the outage test establishes the magnitude and the location under controlled conditions. Method documentation for the measurement arrangement is published by suppliers such as OMICRON, and the general measurement method is defined in IEC 60270.

Interpreting activity in rotating machines

The most reliable interpretation is the machine’s own trend under comparable conditions. A measurement taken at the same load, the same temperature and the same coupling arrangement is comparable with the previous one, and a rise is a rise.

The relationship to load adds information. Activity that increases with load suggests a condition affected by thermal expansion or by electromagnetic force. Activity that is present at low load and changes little with load suggests a condition that is present whenever the machine is energised.

The phase-resolved pattern adds the defect family, as it does on other assets, but with the caveat that the winding alters the pulse shape before it reaches the coupler. The pattern is therefore indicative of the family rather than diagnostic of the site.

Correlating with insulation resistance and PI

Insulation resistance and the absorption ratio answer the moisture and contamination question, which the discharge measurement does not. A machine with a falling insulation resistance and rising discharge activity is describing two related conditions; one with rising activity and stable insulation resistance is describing a localised condition.

The correlation is also practical. Where the discharge activity has risen and the insulation resistance has fallen, the first response is usually to dry the machine out and repeat both measurements, because moisture can produce both effects.

Where the activity has risen and the insulation measurements are unchanged, the response is more likely to be an inspection of the winding. The machine requirements that the insulation measurements are judged against are defined in IEC 60034-1, and the framework for field testing high-voltage equipment is published as IEC 60060-3.

When to schedule an outage

An outage is justified when the activity has risen and cannot be explained by a change in conditions, when the rise is accompanied by a change in the insulation measurements, or when the machine is critical and the trend is moving in a direction that would eventually require intervention.

The outage test establishes the activity under controlled conditions, at a defined test voltage, with a known detection threshold. That measurement is what allows a judgement to be made about whether the winding can continue in service and on what monitoring interval.

Where the machine can be kept in service, the decision should be recorded with the interval at which the online measurement will be reviewed. A decision to continue without a stated review interval is a decision to stop managing the condition.

Recording for long-term trending

The record should state the coupling arrangement and its position, the test voltage or the operating load, the frequency band, the detection threshold achieved and the machine temperature.

It should also state the machine identification, its winding type and its operating history where a change in duty could explain a change in activity. A machine that has moved from base load to cycling duty is a machine whose thermal history has changed, and the insulation responds to that.

The value of the record accumulates. A machine with five years of comparable measurements is one whose condition can be judged; a machine with a single measurement is one whose condition is unknown whatever the magnitude. The machine test range is grouped on the generator detection testing hub, with the measurement instruments on the partial discharge testing hub.

Reporting to maintenance planning

The report should state the measurement, its conditions and its trend, and give a recommendation with a basis. A recommendation to continue in service with a defined review interval is as much a result as a recommendation to inspect.

Where the recommendation is an outage, the report should state what the outage would establish and what decision it would support. That framing allows the planning team to schedule the work against its consequence rather than against uncertainty.

Where the recommendation is to continue, the report should state the condition under which the recommendation would change. Those conditions are what make the interval meaningful, and they are what allow a future measurement to be judged without re-running the whole analysis. Research into machine insulation condition assessment is published by EPRI, and field experience is coordinated through CIGRE study committees.