04_Montanari_relation_PD_space - IEEE Transactions on...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 11, No. 3; June 2004 393 Relation Between Space Charge Accumulation and Partial Discharge Activity in Enameled Wires Under PWM-like Voltage Waveforms D. Fabiani, G. C. Montanari, A. Cavallini and G. Mazzanti DIE-LIMAT University of Bologna Viale Risorgimento, 2 40136 Bologna, Italy ABSTRACT It has been observed that voltage waveforms generated by power electronic convert - ers may affect significantly the reliability of electric motor insulation. Since partial discharges are considered to be the main cause of the reliability loss, new enamel insulations for magnet wires are being developed in order to withstand better stress amplification. The electrical characterization of these insulating materials is often carried out through aging tests which may provide estimation of life under differ - ent stress levels and conditions. However, deeper investigation of aging phenomena due to supply voltage waveforms is needed, especially regarding the relation be - tween aging factors and stress conditions. This paper deals with this topic, show - ( ing experimental evidences of relation between partial discharge quantities e.g., ) inception voltage, repetition frequency, amplitude and electrical properties, asso - ciated with charge accumulation, which can be directly evaluated through space charge measurements. Characterization of insulating materials and comparison of materials candidate for application in power electronic waveform environment can be carried out resorting to the methodology proposed here. This approach can pro - vide, therefore, a useful feedback to wire manufacturers regarding, e.g., the choice of additive nature and enamel components for magnet wires in power-electronic controlled motors. Index Terms — Square waveforms, magnet wire enamel degradation, space charge trap depth, partial discharge inception voltage, enamel surface conductiv - ity. 1 INTRODUCTION Ž. OWADAYS adjustable speed drives ASD are N widely used in appliances where electrical motor speed has to be varied almost continuously. In particular, modern power converters implement the pulse width modulation PWM technique. This technique provides motor supply voltages characterized by square waveforms, width modulated according to a sinusoidal law, with rise front slew rate from 2 ] 3 to some tens of kV r m s and pulse repetition frequency up to some tens of kHz. Improvements in power converter technology, e.g., in- creasing rate of voltage rise front and pulse repetition fre- quency, can provide advantages in terms of switching loss reduction and improved stability of motor torque. On the contrary, it has been observed that reliability of electrical machines supplied by such adjustable speed drives can be Manuscript recei ® ed on 8 January 2003, in final form 28 July 2003.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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04_Montanari_relation_PD_space - IEEE Transactions on...

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