Unit-7Lecture 47
Calibration of Discharge Detectors
Partial discharge detectors are connected across a measuring impedance Zm as shown in Fig. 9.21 and the signal measured across this impedance is read by the detector. The signal voltage developed across Zm depends on the circuit parameters Cx and Cc and also on the internal circuitry of the instrument (blocks 3,4,5 shown in Fig. 9.21).
Hence, the measuring instrument or detector is calibrated by injecting a pulse having a charge of known magnitude into the detecting system. For this purpose, a square wave generator and a calibrating capacitor (Ck) are usually used. The magnitude of the charge injected is q± = CkVk* where V^ is the magnitude of the voltage pulse. The rise time of the pulse is about 0.1 n sec, and the pulse width varies from 10 to 20 M. sec. With suitable attenuation, the output voltage of the pulse generator can be varied from a minimum output of about 10 JiV to a maximum value of 100 V in steps. The value of Cfr usually used, lies between 1000 and 2500 pF. If the calibrating pulse is directly injected at the H.V. terminal of the test object, the magnitude of the calibration
pulse will be C*. V*. If ^e pulse is injected across the measuring impedance (as shown in Fig. 9.21), then the calibrating pulse magnitude should be multiplied by (CX+CC)/CC.
Another method of calibration is to use a secondary standard, consisting of a point-hemisphere electrode system of specified dimensions (refer to LE. Publication 270,1968 (reference no. 8)).
This method is more accurate and is easily reproducible. With an over voltage of 10-20% applied above the discharge inception voltage, the arrangement gives discharges which are used for calibration purposes.
. In further investigations it was found that weak points in an insulation like voids, cracks, and other imperfections lead to internal or intermittent discharges in the insulation. These imperfections being small were not revealed in capacitance measurements but were revealed as power loss components in contributing for an increase in the dissipation factor. In modern terminology these are designated as "partial discharges" which in course of time reduce the strength of insulation leading to a total or partial failure or breakdown of the insulation.
If the sites of partial discharges can be located inside an equipment, like in a power cable or a transformer, it gives valuable information to the insulation engineer about the regions of greater stress and imperfections in the fabrication. Based on this information, the designs can be considerably improved.
Electrical insulation with imperfections or voids leading to partial discharges can be represented by an electrical equivalent circuit shown in Fig. 9.20. Consider a capacitor with a void inside the insulation (Ca). The capacitance of the void is represented by a capacitor in series with the rest of the insulation capacitance (Q,).
The remaining void-free material is represented by the capacitance Cc. When the voltage across the capacitor is raised, a critical value is reached across the capacitor Ca and a ischarge occurs through the capacitor, i.e. it becomes short circuited
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