Partial Discharge measurements on ASD

 

The use of power electronics to supply induction motors is widely spreading out due to significant advantages in terms of speed and torque control of electrical machines. The most common are ASD (Adjustable Speed Drives) systems, lead by PWM (Pulse Width Modulation) techniques.

Unfortunately, it has been proved that this kind of power supply can cause overstress on electrical insulation in comparison to the sinusoidal 50/60 Hz voltage supply [1-13], thus decreasing drastically life and reliability of controlled motors.

The reliability of winding insulation, as it occurs for other insulation systems, can be investigated by Partial Discharge (PD) analysis.

Actually, this research is focused on:

  • on-line and off-line investigation of voltage distribution along windings;

  • development of UHF non-intrusive sensors, e.g. antennas, for partial discharge detection in motor  stator winding insulation;

  • reduction of radiated commutation noise signal of the inverter by high-pass filtering the signal picked-up by the sensor;

  •  characterization of defects through patterns relevant to multiple partial discharge acquisitions;

  • development of a non-intrusive diagnostic system for on-line motors.

Off-line testing

Figure 1 shows the off-line testing apparatus, TechSquare, developed by TechImp. It is composed of a HV squarewave generator controlled in amplitude, frequency, rise time and duty cycle. The voltage waveforms are acquired by the PDSolver system developed by TechImp. The motor under testing is usually supplied with open star, one phase connected to the inverter and the other two grounded. Increasing the supply voltage, it is possible to reach the partial discharge inception voltage (PDIV) of the insulation system.

Fig. 1: TechSquare with PD measurement system

Figure 2 shows an example of PD signal (2A) and pattern (2B) acquired with this configuration, respectively.

 

Fig. 2:  PD signal (2A) and PD pattern (2B) measured with PDSolver system

On-line testing

On-line measurements on motors supplied by  electronic power converters is another aspect of the research. An inverter bench is used for motor supply with the purpose of understanding the differences between on-line and off-line configurations, e.g. EM disturbs (sparks, mobile phones, etc.) and voltage distribution along motor winding.

 

References 

[1] W. Yin, “Failure Mechanism of Winding Insulation in Inverter-Fed Motors”, IEEE Electr. Insul. Mag., Vol. 13, n. 6, pp.18-23, November 1997.

[2]  G. C. Stone, R. G. van Heeswijk and R. Bartnikas, “Investigation of the Effect of Repetitive Voltage Surges on Epoxy Insulation”, IEEE Trans. on Energy Conv., Vol. 7, n. 4, pp. 754-759, December 1992.

[3] N. Foulon, J. P. Lucas, G. Barré, R. Mailfert and J. Enon, “Investigation of the Failure Mechanism of Insulation Subjected to Repetitive Fast Voltage Surges”, IEEE EIC and EMCW, Rosemont, IL (USA), September 1997, pp. 401-406.

[4] M. Kaufhold, G. Borner, M. Eberhardt and J. Speck, “Failure Mechanism of Low Voltage Electric Machines Fed by Pulse-Controlled Inverters”, IEEE Electr. Insul. Mag., Vol. 12, n. 5, pp.9-15, September 1996.

[5] D. Fabiani, G.C. Montanari and A. Contin, “Aging Investigation of Turn Insulation under Fast Repetitive Pulse Voltage with or without Partial Discharges”, invited paper, Int. Conf. Coil Winding, Insulation and Electrical Manufacturing (CWIEME’99), pp. 88-96, Berlin, Germany, June 1999.

[6] D. Fabiani and G.C. Montanari, “The Effect of Voltage Distortion on Aging Acceleration of Insulation Systems under Partial Discharge Activity”, IEEE El. Ins. Magazine, Vol. 17, No. 3, pp. 24-33, May 2001.

[7] D. Fabiani and G.C. Montanari, “On the Degradation of Winding Insulation of Ac-motors Supplied by Adjustable Speed Drives – an Overview”, Invited paper, 4th Int. Symp. on Advanced Electromechanical Motion Systems, pp. 321-326, Bologna, Italy, June 2001 and published on ELECTROMOTION, Vol. 8, No. 2, pp. 89-95, June 2001.

[8] D. Fabiani, G.C. Montanari and A. Contin, “Aging Acceleration of Insulating Materials for Electrical Machine Windings Supplied by PWM in the Presence and in the Absence of Partial Discharges” IEEE ICSD, pp. 283-286, Eindhoven, The Netherlands, June 2001.

[9] D. Fabiani, G.C. Montanari and A. Contin, “The Effect of Fast Repetitive Pulses on the Degradation of Turn Insulation of Induction Motors”, IEEE SDEMPED, pp. 413-418, Grado (GO), Italy, September 2001.

[10] A. Cavallini, M. Conti, D. Fabiani and G.C. Montanari, “Evaluation of Corona-resistant Magnet Wires through Partial Discharge and Space Charge Measurements”, IEEE EIC/EMCWA, pp. 11-16, Cincinnati, OH (USA), October 2001.

[11] D. Fabiani, Accelerated Degradation of AC Motor Insulation due to Voltage Waveforms Generated by Adjustable Speed Drives, Ph.D. Thesis, Bologna,  2002, published by Gedit Edizioni, Bologna, May 2003.

[12] D. Casadei, A. Cavallini, D. Fabiani, C. Rossi, G. Serra and G.C. Montanari, “The influence of power-electronic waveforms on partial discharge inception in low voltage rotating machines”, Invited paper, Int. Conf. Coil Winding, Insulation and Electrical Manufacturing (CWIEME’03), pp. 39-44, Berlin, Germany, June 2003.

[13] D. Fabiani, G.C. Montanari, A. Cavallini and G. Mazzanti, “Relation between Space Charge Accumulation and Partial Discharge Activity in Enameled Wires under PWM-like Voltage Waveforms”, to be published on IEEE Trans. on Dielectrics and Electrical Insulation.