Sensor Technology

The major purpose of signal processing and diagnosis is to determine the fault type, fault extent, and aging status. Then the accurate estimation can be given to aid the decision on maintenance. The non-destructive measurement methods are often treated in the framework of the inverse problem theory.

Infrared thermal analysis: To accurately measure the surface temperature of an object, the most important thing is to identify emissivity of the object. As a measure index of an object’s ability to absorb and emit infrared energy, emissivity has a value from 0.0 to 1.0. The emissivity index of the cable is calibrated with a contact temperature device by adjusting the index until the infrared sensor has the same reading with the contact device. The real emissivity of the cable may change a lot due to aging of the cable and dusts on the surface of the cable, therefore the value of emissivity should be able to be adjusted according to the real case.

Acoustic partial discharge analysis: The amplitude and frequency components of acoustic waves generated from partial discharges are both factors in detection. They can be affected by geometrical spreading of the wave, interface between materials, absorption of the material (higher frequency components are removed), frequency-dependent propagation, and so on. Interpretation of the acoustic signal is a very complicated process since it involves many unknown parameters. Corresponding experiments will be performed to identify different types of partial discharges. The final aim is to identify the 3D location, void size, and void shape of partial discharges.

Time and frequency domain graphs of a partial ischarge

Acoustic Sensor

Fringing electric field sensing: Fringing electric field sensing is a tentative way to measure the dielectrometry properties of cables. The investigation on the relationship between dielectrometry properties and insulation status of insulation material plays an important role in the estimation of insulation status. If this relationship can be formulated or charted, the aging status, and the cable life remaining lifetime will be very easy to estimate. By changing the pattern of electrodes and driving-signal frequency, the best sensitivity can be attained.

Processing collected data is the most important step for identifying insulation status. Especially when several sensors are used for measurement of insulation properties at the same time, there are still no available signal-processing methods to integrate signals together. The signal-processing algorithm proposed should be able to find the incipient failures and estimate aging status of cables.

Water tree photos from a study by Elforskdagen in 2004


FEF sensor and FEF control board