It has been observed that voltage waveforms generated by power electronic converters 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 different stress levels and conditions. However, deeper investigation of aging phenomena due to supply voltage waveforms is needed, especially regarding the relation between aging factors and stress conditions. This paper deals with this topic, showing experimental evidences of relation between partial discharge quantities (e.g., inception voltage, repetition frequency, amplitude) and electrical properties, associated 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 provide, 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.

Relation between space charge accumulation and partial discharge activity in enameled wires under PWM-like voltage waveforms

This paper focuses on the features of harmonic distortion which may affect significantly the reliability of typical ac-power network equipment, such as low-voltage self-healing capacitors used for reactive power and harmonic compensation. Moreover, the effect of high-frequency pulse-like voltage generated by Adjustable Speed Drives (ASD) on electrical machine insulation is also investigated, resorting to life tests carried out on different insulating materials of the standard and "corona resistant" type, at electrical field levels able to incept partial discharges (PD).

The effect of voltage distortion on ageing acceleration of insulation systems under partial discharge activity

It is well known that the wide use of power electronic converters (PEC) is one of the main causes of current and voltage distortion in the electrical networks. These converters generate harmonic components propagating toward the supply network side and nonsinusoidal voltage at the load side. Distorted waveforms generated by power electronics can dramatically reduce the reliability of insulation systems. Such systems should be designed bearing this important factor in mind.

Power electronics and electrical insulation systems ߝ Part 1: Phenomenology overview

The problem of premature failure of low voltage rotating machines fed by inverters could have serious economic impact if it is not addressed. Indeed, even though much already has been written on the different topics related to the use of such power supplies, there is a great need for a better understanding of the physical mechanisms involved, particularly with regards to aging mechanisms of insulating materials. The aim of this paper is to highlight the effect of adjustable speed drives (ASD) on the stator finding insulating materials. Pulse-like electrical stresses have been applied to three types of samples (thin film, twisted pair, and layer to layer). Physical characterization and/or lifetime measurements have been performed. The results are presented and discussed according to the nature of the interaction existing between the electrical stress main characteristics (magnitude, frequency, dV/dt) and the material properties. According to these results, the development of a standardized insulation quality control test, based on a physical understanding of involved stresses and material behavior, appears now to be necessary.

Testing of low-voltage motor turn insulation intended for pulse-width modulated applications

This paper deals with partial discharge (PD) measurements on inverter-fed motors. They are performed by means of a broadband sensor-filter system that allows PD signal to be recorded, damping the noise generated by the commutation interference of electronic power supply. In this way, online or offline measurements can be carried out with the purpose of evaluating the presence of PDs in adjustable speed drives, which often constitute the main cause of winding insulation degradation and premature breakdown. An offline testing technique is also proposed to offer an insight into PD activity and a more precise location of PD sources. These techniques can allow improved quality control and diagnostics on power-electronic-controlled motors.

A UHF Technique for Advanced PD Measurements on Inverter-Fed Motors

Electrical rotating machines are subjected to nonsinusoidal wave shapes when supplied from high frequency inverters. PWM (pulse width modulated) voltage sources with steep wave fronts can be destructive to the insulation and criteria normally used under sinusoidal conditions cannot necessarily be transposed to this new case. In order to determine the influence of this electrical stress, laboratory experiments were carried out to determine whether such repetitive voltage surges can precipitate premature failure of high voltage motor windings. A fast voltage surge generator and laboratory cells have been built, to study partial discharge development and material degradation according to voltage magnitude, polarity, rise time and surge repetition rate. Partial discharges (PD) are strongly dependent on the voltage waveform, related to the statistical time lag which is necessary before PD inception. Physical and chemical analysis and surface potential decay measurements show that the ageing process is related to the voltage waveform (frequency, shape factor, etc.). It is suggested that under PWM operation, insulation ageing mechanisms are predominantly controlled through an electrostatic phenomenon, different from the 50/60 Hz sinusoidal case.

R. Mailfert

Papers

Investigation of the failure mechanism of insulation subjected to repetitive fast voltage surges