PD detection methods on twisted-pair sample under repetitive bipolar impulse
K. Kimura, S. Ushirone, S. Ohtsuka, M. Hikita, W. Bitoh*
(Kyushu Institute of Technology, Japan *Mitsubishi Electric Corporation)
Stator winding of inverter-fed motors is exposed to repetitive bipolar impulses according to sharp square waveforms of PWM and reflection phenomena at cable/motor terminals. With a certain circuit condition, the overpotential impulse may exceed PD inception voltage (PDIV) of motor winding, which can lead to insulation failure. Although PDIV is very important parameter for safety design of inverter-fed motors, the measurement method is not yet established because of technical difficulties such as testing repetitive impulse generator and PD detection circuit which has to cope with large displacement current of impulse voltage.
For the suppression of the current, optical detection, differential amplifier and/or filtering techniques have been proposed. In our laboratory, we have investigated PD detection methods using a controllable high voltage impulse generator and several PD couplers. Each coupler has advantage and disadvantage. For a small sample such as twisted-pair, optical detection with photomultiplier tube (PMT) is the most sensitive, while measurements under highly repetitive impulses requires wide-band digital oscilloscope with large memory. When we observe long-time PD behavior under repetitive impulses, the combination of a capacitor in series to the sample and a data logger with large memory is recommended. The PD detection technique of the series capacitor is the same in principle as so-called Dakin’s “Capacitance bridge” published a couple of decades ago. With this technique it becomes possible to observe long-time PD behavior, which is not yet studied under repetitive bipolar impulse conditions in detail. When we use CT or antenna for PD detection, both a specific filter and a wide-band digital oscilloscope are necessary in order to discriminate weak PD signal to high displacement current of applied impulse voltage. In this paper we compare PD detection methods on twisted-pair samples. In addition the effect of prestress is also discussed for test procedure.
Author; Prof. Ken Kimura
Kyushu Institute of Technology, Electrical Engineering Deptartment
Tel/Fax: +81-93-861-3265
Email: kimura@ele.kyutech.ac.jp
URL: http://www.kifkit.com/