Test · IEEE 1434 / IEC 60270

Partial Discharge Testing for High-Voltage Generators

Predictive diagnostic for the insulation of synchronous and turbogenerators in high-voltage service. Partial discharges (PD) are micro-electrical discharges inside the coil insulation that precede catastrophic failure by months or years — measuring them anticipates insulation breakdown and lets you schedule intervention before unplanned outage.

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Partial Discharge — offline + online

IEEE 1434IEC 60270IEC 60034-27CFE LAPEM W4200-12

Technical definition

What is partial discharge testing

Partial discharges (PD) are microscopic sparks inside voids, delamination or defects in the stator coil insulation of high-voltage generators. Each spark damages the insulation material slightly — accumulated over time, that degradation leads to insulation failure. Measuring PD in advance lets you anticipate the failure by months or years and plan intervention. Calibrated equipment such as the Megger ICMobserver is used for site measurements traceable to national metrology standards.

Reference standards: IEEE 1434, IEC 60270 and IEC 60034-27 (PD testing in rotating machines)
Offline mode: measurement with the machine stopped and energized by an external source
Online mode: measurement with the machine in normal operation — best for trending
Capacitive sensors coupled to the stator terminals
Analysis of magnitude (mV / pC), phase-resolved pattern and polarity
Time-domain trending to detect defect progression over months and years

Operational opportunity

When PD testing is recommended

PD is the test of choice for predictive maintenance on high-voltage generators (typically above 6 kV). It is applied:

As a baseline test post-rewind or post-commissioning
As a periodic measurement (annual or semiannual) in a predictive program
After a severe electrical event (overvoltage, overheating)
Before extending the service life of a generator with several years of operation
To validate class F/H insulation after modernization or rewind
When degradation is suspected with no obvious visible cause

Client value

What partial discharge testing detects

Voids in coil insulation (internal origin from poor impregnation)

Delamination between insulation layers (thermal or mechanical aging)

Surface discharges on coils (contamination or moisture)

Discharges at joints, end-winding bends and stator end connections

Time trends — growing defects vs stable ones

Approximate location of the highest PD activity zone

Deliverables

What we deliver at the end of the test

Each test is delivered with a signed documentation package under IEEE 1434, IEC 60270 and ISO 9001:2015. Full traceability by work-order number and signature of the responsible engineer.

1Signed protocol under IEEE 1434 and IEC 60034-27 with methodology
2Phase-resolved spectra (PRPD) in digital and PDF format
3Measured magnitudes (mV / pC) per stator phase
4Pattern analysis: type of defect detected and severity
5Comparative trending against previous measurements (when a baseline exists)
6Recommendation: acceptable / monitoring / preventive intervention / corrective intervention
7ISO 9001:2015 traceability with raw data for independent audit

Typical case

Annual PD program on a cogeneration plant turbogenerator

IEEE 1434

Applicable standard

4 years

Documented trend

Predictive

Before failure

Predictive maintenance program on a turbogenerator at a private cogeneration plant. Annual offline PD measurements with an established baseline. On the fourth measurement an increase in PD activity was detected on one phase — the recommendation was quarterly monitoring. A preventive intervention was scheduled for the next major maintenance window before the defect could progress to failure. The full PRPD spectra and trend chart were delivered with engineering interpretation.

FAQ

Frequently asked questions — partial discharge testing

Preguntas que recibimos con frecuencia. ¿No encuentras la tuya? Escríbenos a ventas@temisa.mx.

What is the difference between offline and online PD testing?

Offline PD measures with the machine stopped and energized by an external source — more control and higher sensitivity to small defects. Online PD measures with the machine in normal operation — more representative of real conditions, better for trending. The two are complementary and are used as operational opportunity allows.

What voltage range does PD testing apply to?

PD is most relevant for high-voltage generators (typically above 6 kV) where insulation is the dominant failure mode. On low-voltage generators (below 1 kV) failure modes are different (mechanical, thermal) and other tests are more relevant.

How many measurements are needed for a useful trend?

At minimum, three measurements spaced six months apart to identify a reliable trend. The first measurement is the baseline; the second and third begin to show progression or stability. Recommended predictive program: annual measurements with a post-rewind baseline.

Does partial discharge always indicate imminent failure?

No. All insulation systems have some PD from inevitable microscopic defects. What matters is the absolute magnitude relative to the standard's threshold and the time trend. A stable low-magnitude value can be acceptable; a growing value is the real warning signal.

Can you perform online PD without stopping the generator?

Yes. Online PD is executed with the machine in normal operation — no shutdown, no production loss. It requires installing capacitive couplers at the stator terminals (typically during a previous scheduled maintenance). After that, the measurement is non-intrusive.

How long does an offline PD test take?

Between 8 and 16 hours onsite for setup, energization with an external source, data capture per phase and disconnection. Analysis and protocol delivery is completed within 7 to 10 business days after the test.

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