Partial Discharge Measurement Techniques
- Visual – Blackout test:
This was used in the old days. A stator would be covered by a tarp to make it dark inside, and AC power was connected to the stator. With sufficient PD, the light emitted can be seen.
- Audio receiver:
Since discharges generate a crackling sound, audio receivers can be used to measure how loud the sound is, and that is related to the amount of Partial Discharge in a stator. AC power is connected to the stator to generate PD. The receiver is placed or held with a probe inside the stator.
- RF detector:Radio Frequency waves are generated with any discharge. The “driver” for PD is typically an AC voltage or an AC Hipot tester. In stators, the voltage of RF waves can be measured using an RF receiver and an oscilloscope. The receiver is placed on a probe and inserted into the stator. The highest concentration of PD can be located by moving the probe around inside the stator. Assembled motors cannot be tested since the RF signals are shielded from the receiver by the motor frame. RF noise from sources other than the motor can be an issue, particularly with the increasingly prevalent use of wireless devices.
- AC Hipot test with a capacitive coupling to an instrument:
Couplers can be used when AC power is connected to a stator or and assembled motor. The couplers are connected to the motor leads and act like a high pass filter, only passing high-frequency PD spikes to the instrument or oscilloscope the coupler is connected to.
- Surge test with a coupler:
Like an AC Hipot tester, a surge tester can be used as the “driver” to generate PD in a stator and in assembled motors. This partial discharge measurement technique is not sensitive to outside noise.
In the iTIG III Winding Analyzer with PD, the coupler/high-pass-filter is internal to the tester. It is connected to a second channel in the oscilloscope used by the surge test.
What is measured, and how large is a partial discharge?
The partial discharge measurement made by PD testers, using the surge test as the driver, is the voltage of the PD spikes. This voltage is proportional to the amount of discharge and measured in mV. The range measured is typically from a few mV to a few Volts.
A charge or discharge is measured in Coulombs. 1 Coulomb = 1A for 1 second. It is also the charge in a 1 Farad capacitor with a potential of 1 Volt.
Since most people do not relate to these numbers, here are some rough comparisons:
- Typical lightning strike: 15C to 350C
- Static discharge from a human hand to a doorknob: µC range
- Partial discharge: pC and nC range (represented by mV to Volts in the testers)
- 1 pC = 0.000 000 000 001C
It is important to note that all PD measurements are relative, and do not measure the absolute PD at the site of discharge. The PD voltage spikes attenuate between the discharge sites and the measurement location. The amount of attenuation depends on a list of factors and will vary from motor to motor. It will depend on the frequency range used by the instrument to detect PD, and so forth. Despite the relative nature of the measurement, it is an excellent way to track the condition of a motor and get early warnings of problems.