In a rotating machine it is the insulation that fails, which is why most of this material is about partial discharge. The video summarises six technical webinars on motors and generators, the bus duct that connects them to the transformer, and what renewable duty cycles are doing to generator transformers.
Key points
- The RTD probes already in the stator winding can also detect partial discharge, and comparing their readings locates the defect.
- On the bus duct, a ground-path current sensor works at earth potential and tells which side of the joint the discharge came from.
- In one real case, discharge had been climbing for at least two months before the first temperature trip.
- In battery storage and wind, the transformer no longer sees a steady load but a duty cycle, and each part of it attacks something different.
- Moisture moves between paper and oil with temperature: the same transformer reads 10 ppm cold and over 100 hot.
Partial discharge: a tiny current in a narrow window
A discharge in a void causes a small dip in voltage across the sample, and the charge that replaces it is a high-frequency current pulse. That pulse is what the instrument actually measures. Published work puts the best detection between 30 kHz and 30 MHz; above 30 MHz only a small fraction of the signal is still available. The band matters more than the sensitivity: listening in the wrong window finds nothing, however good the instrument.
The sensors are already in the winding
The idea that runs through three of these sessions is that the stator RTDs, the temperature detectors buried in the winding, double as partial discharge sensors. They are more sensitive than a coupling capacitor, because the element covers a length of winding, whereas a coupling capacitor is a spot measurement. The method does not touch the RTD wiring, so the temperature measurement is unaffected, and there are typically two to six RTDs per phase, depending on the size of the machine. The pattern recognition rules developed for coupling capacitors still apply; only the sensitivity differs.
The highest reading is nearest the defect
This gives something conventional testing struggles with: location. Measure at every RTD around the stator, compare the magnitudes slot by slot, locate the defect near the one reading highest, trend that slot over time and act before the outage, not during it. The conventional alternative is a corona probe, which needs the machine opened and only reaches the zone you can physically get to.
The bus duct: nothing moves, so why watch it?
It is an inert system with no moving parts, which is the usual objection. But the bolted joints are thermally cycled every day the plant runs. Loose connections come from thermal cycling and incorrect torque; high-resistance joints from surface films, oxides or loss of plating; flex links are the usual location; cracks in the expansion joints let moisture in. And around three quarters of failures in this equipment are insulation related.
The sensor used there is elegant. A ground-path current sensor captures high-frequency pulses on the metal surface of the enclosure. It sits at earth potential, so there is no high-voltage risk in installing it, it is non-invasive (no modification to the bus itself), and the polarity of the pulse distinguishes the machine side from the bus side. It is combined with capsule temperature, humidity and pressure.
A case: two months of warning in the discharge
A disconnect switch was monitored by both partial discharge and temperature. Two trips were initiated when the temperature exceeded its set point, but the steep rise in discharge had begun at least two months earlier. The failure occurred at the corners, due to skin effect. Temperature confirmed it; discharge predicted it.
The step-up transformer: the one that cannot be swapped in a week
The generator step-up transformer is treated differently from a network transformer because it is not replaceable in a hurry and it never rests. Oil touches every component of the active part, so gas reports on all of it, and a sudden rise in dissolved gas is the best indicator of an incipient fault. About 90 % of the time the unit behaves entirely normally; the other 10 % is what the monitor exists for, and close surveillance can keep it running until a planned outage. For cost, the sessions recommend a single or two-gas monitor across a whole fleet rather than a full analyser on one unit.
Renewable duty cycles: each stress attacks something different
A transformer in a battery storage or wind installation no longer has a steady load profile. It has a duty cycle:
| Stress | What it attacks |
|---|---|
| Thermal cycling | insulation ageing |
| Harmonics | eddy-current heating, extra temperature rise |
| Moisture migration | reduced dielectric strength |
| Through faults | mechanical winding displacement |
| Fast ramping | thermal fatigue |
| High dV/dt | insulation stress |
Conventional transformers were designed for relatively stable loading; these duty cycles are fundamentally different.
Why the sample misleads: 10 ppm cold, 137 hot
Nearly all the water lives in the solid insulation, not the oil, and heating drives it out of the paper and into the oil. A 50 MVA unit can read 10 ppm at 20 °C and 137 ppm at 80 °C: the water did not change, only the temperature did. That is why operators see ppm spikes on load after a “safe” cold sample. Online relative saturation accounts for temperature automatically; a laboratory ppm figure does not.
What goes wrong in practice
- Treating a renewable-duty transformer like a conventional one.
- Relying on the oil and winding gauges alone, with poor hot-spot visibility.
- Ignoring harmonics: hidden overheating and accelerated ageing.
- Static alarm thresholds: nuisance alarms, or missed events.
- No baseline commissioning data, so no long-term trend is possible.
What the six sessions agree on
- In rotating machines the insulation fails, so discharge is the measurement.
- Listen in the right frequency window, or the instrument finds nothing.
- The sensors are often already installed: use the stator RTDs.
- Bus duct is not inert: bolted joints are cycled every day.
- Renewable duty cycles are a different problem, not a smaller one.
Related equipment
The Dynamic Ratings RMM rotating machine monitor and the PD Profiler monitor partial discharge in service; the rest of the range is under partial discharge detection. For step-up transformers, see the E3 transformer monitor and the GRIDSCAN5000 hydrogen sensor. To discuss your plant, contact our engineers.