The measurement of partial discharges is non-destructive Test on electrical apparatus or equipment. The measured quantities are valuable data to determine the quality of an insulation. In high-voltage Test technique, special importance is therefore attached to measurement of partial discharges.
The reading of partial discharge intensity is in Pico-coulombs (pC) or, alternatively, radio interference voltage (RIV) in microvolts (µV).
The partial discharge/RIV measuring system consists of,
Partial discharges in or on the Test object cause charge transfers in the high-voltage circuit, giving rise to voltage pulse variations on the measuring impedance (Quadripole). These are evaluated by the partial discharge meter. The filter with adjustable centre frequency allows to select parts of the high-voltage spectre for measurement purposes.
Partial discharge meter DTM
The measured partial discharge intensity is displayed either in pC or in µV in accordance with IEC 60 270 / IS 6209 or NEMA 107. According to IEC 60 270, the measured quantity shall be multiplied by a correction factor, which considers the circuit characteristics of the complete Test arrangement. With built-in correction circuit it is possible to incorporate the correction factor into the display. The actual partial discharge intensity can then be read directly without the usual calculations. At switch position "CORR", the correction factor is displayed directly. The PC pulses are tapped from the analogue output terminal and displayed on the built-in oscilloscope. The power frequency Test voltage can be displayed with the partial discharge pulses in an elliptical time-base and periodically recurrent interference pulse can be blanked in the oscilloscope display, automatically eliminating their evaluation in the partial discharge meter.
Built-in provision is available to conduct the bridge type PD detection, which is useful to suppress external noise having attenuation level of ³ 40 db.
A selective filter for variable center frequency and a broadband filter can be supplied.
Variable center frequencies from 600 kHz to 2400 kHz
Band width 9 kHz
Bandwidth 40 kHz to 220 kHz
The basic noise level of the instrument is less than 0.6 pC in the most sensitive measuring range.
The measuring accuracy and linearity correspond to IEC 60 270 and IS 6209
2 pC to 50,000 pC (IEC 60 270)
1 V to 25,000 µV (VDE, CISPR)
The measuring impedance (Quadripole) filters the HF voltage portions from the high-voltage Test circuit. It can be continuously loaded with 4 A. Built-in surge diverters protect the instrument against over voltages. Two Quadripoles are required for bridge measurements.
DIMENSIONS AND WEIGHT
19" rack (6 units of hight), 40 Kgs.
Co-axial cables to connect Quadripoles to meter.
Housing, in case the instruments is not intented for rack-mounting.
Fig. 1. Connection of partial discharge meter DTM between Test object and ground (Test circuit according to IEC 60270 and IS 6209).
Fig. 2. Connection of partial discharge meter DTM between coupling capacitor and ground (Test circuit according to NEMA Publ. 107)