/pdf/electrical-insulation-for-rotating-machines-5cd6wk7493.pdf

Electrical Insulation for Rotating Machines

Considering the title of this paper, one might think that everything has been said, shown, proven and discussed already on ageing and failure of electrical insulating materials, based on at least half century of research work and field feedbacks. However, perhaps there is still room to collect ideas and speculate on apparently proven or brand new concepts, such as the interrelation between ageing mechanisms and physically measurable quantities such as space charge, partial discharges and conduction current. The latter is, indeed, the scope of this paper, which is taken from the Whitehead Memorial Lecture given at the 2010 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP). On one hand we will revisit the concepts of space charge accumulation in insulating (polymeric) materials, considering also the latest achievements which deal with ultra-fast charge pulses (solitons), on the other hand we will discuss how space charge can contribute to ageing rate under dc, ac or impulsive voltage, directly through electric field alteration, or indirectly modifying a fundamental ageing factor such as partial discharges.The goal is, at the very end, to show how much such basic investigations can have a fundamental impact on insulation design and electrical apparatus long-term performance/ reliability.

Bringing an insulation to failure: the role of space charge

This paper focuses on the influence of repetitive square wave voltage rise time on partial discharge pulse characteristics in the ultra-high frequency range. Paerial Discharge (PD) tests on single-point contact crossed pairs were carried out using repetitive square wave voltages with a wide range of rise times, from 200 ns to 400 μs, including also 50 Hz sinusoidal voltage waveforms. Experimental results show that rise time has a significant influence on magnitude and frequency spectrum of PD pulses. Voltages of shorter rise times generate larger PD pulses with a predominance of high frequency components in the energy spectrum. Interpretations are presented for this phenomenon.

Effect of rise time on PD pulse features under repetitive square wave voltages

This paper reviews the up-to-date research activities on electrical insulation techniques under repetitive surge voltages with steep wavefront for low voltage inverter-fed motors. Partial discharge (PD) inception, propagation, ageing and breakdown characteristics of magnet wires, including recently developed PD-resistant wires such as nanocomposite enameled wires, are described. PD measurement techniques under the steep-front voltages and IEC activity toward their standardization are also introduced.

Technical Development on Partial Discharge Measurement and Electrical Insulation Techniques for Low Voltage Motors Driven by Voltage Inverters

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

A new IEC test specification is under development for the insulation systems used in converter-driven machines with the aim of providing manufacturers with a reliable means to quality insulation systems for use in converter-fed machines and users with confidence that the machines they purchase contain insulation that is fit for their intended purposes. Much of the qualification testing for insulation systems subject to converter voltages may be undertaken with existing 50/60 Hz equipment. However, for testing of complete coils in low voltage machines, an impulse generator and an impulse PD test set are required. For large machines, which have stress-graded endwindings, it will be necessary for a source of converter voltage to be available.

J.C.G. Wheeler

Papers

Effects of converter pulses on the electrical insulation in low and medium voltage motors