ReviewEffect of humidity on partial discharge in a metal-dielectric air gap on machine insulation at trapezoidal testing voltages
Graphical abstract
The PD characteristics can be varied greatly with the increasing humidity. The transition from few big-amplitude discharges in dry air to plenty of small-amplitude discharges in humid air is mainly due to the increased surface conductivity in humid air.
Introduction
Stator insulation is a critical part of high-voltage rotating machines with respect to the efficiency and reliability of operation, manufacturing costs and maintenance. The stator insulation system is exposed to electrical, thermal, mechanical and ambient stresses simultaneously during its operation, resulting in the gradual deterioration of insulation properties which can reduce the machine's lifetime. Many failures of power generators failure are caused by insulation damage; the two main reasons leading to these damages are aging and partial discharge [1]. In general, several common PD sources exist in stator insulation, such as internal discharge, end-winding discharge, surface tracking, delamination and slot discharge [2], [3]. Even though modern machines based on epoxy-mica insulation system have been designed to be able to withstand an appreciable level of discharges, for instance, internal discharge in small volumes, some other PD activities are even more detrimental to the insulation system, such as slot discharge.
Slot discharge occurs in the air gap between the surface of the stator bar and the laminated magnetic core. It has been considered as the most severe damage to the stator winding groundwall insulation [4]. There are two main mechanisms that give rise to the slot discharge activity [5], [6]. One is the mechanical slot discharge, which is developed from loose bars in the slot due to, for instance, the shrinking of insulation; this allows vibration of the bar in the slot leading to abrasion of the conductive coating of the bar. The other one is the electrical slot discharge, which is caused by poorly manufactured semiconductor coating with inappropriate surface conductivity or uniformity of the coating. If the surface conductivity is too low, a significant voltage could build up between the surface of the bar and the core in parts where the bar surface is not in direct contact with the core. The PD activity that follows may lead to the breakdown of the insulation between the bar and the core. The calculation in Ref. [7] gave a maximum acceptable surface resistivity of 25 kΩ per square for the surface coating. Besides, poor electrical connection of the conductive coating to ground also can initiate the slot discharge. It is well known that the typical phase resolved partial discharge (PRPD) pattern of slot discharge is characterized by a strongly asymmetric pattern, which is triangular with a sharp slope at the onset of the positive discharge appearing during the negative polarity of the applied voltage [2]. However, this pattern varies with other conditions, such as temperature and humidity in the air.
One typical and systematic investigation of slot partial discharge has been presented in Refs. [8], [9], [10], including the influence of gap size, temperature and humidity, as well as the surface degradation caused by different stresses, such as electrical, thermal and mechanical stress, on the slot discharge activity. Moreover, it is apparent that the air humidity has a great impact on the PD activity in operating machines; one of the clear evidences from Ref. [11] is that by monthly on-line monitoring on the generator up to 9 years, the PD activities on generators in winter were higher than that in summer with a changing rate higher than 100 because of the seasonal changes in humidity. There exist a large number of studies about the effect of relative humidity on the PD activity [12], [13], [14], [15], [16], [17], [18], most of them focusing on the variation of PD inception and extinction voltage, PD intensity, breakdown strength, surface conductivity of the insulating material, space charge accumulation on the material and the statistical time lag of discharge, and so on. From those studies, it has been recognized that humidity has a negative correlation with PD activities: the discharge activity decreases as the humidity increases. For instance, it was observed in Ref. [15] that the external PD activities such as slot discharge are strongly reduced and even disappeared if the ambient air humidity goes above 50%. A similar behavior was reported in Ref. [16] for surface discharges on an epoxy bar disappeared at the eighth day of exposure to humidity of 80%. It has been understood this is due to the electronegative nature of water molecules, which can capture electrons and reduce the availability of free electrons to generate electron avalanches. However, lower PD activities at high-humidity do not directly mean that humidity reduces damage to the insulation surface; on the contrary, it was reported in Ref. [18] that degradation of epoxy resin exposed to PDs was more severe in moist air than in dry air.
In this paper, the effect of relative humidity on partial discharges which take place in the air gap between a spherical metal electrode and the surface of epoxy-mica machine insulation is investigated. The PD analysis is performed with trapezoidal voltage waveforms as stimuli. The reason for this novel method is to investigate the character of the PD behavior with voltage stimuli that have two features, first a constant changing voltage dU/dt during rising as well as falling voltage period and a second feature of including a short period of constant voltage between rising and falling voltage periods, rather than having continuously changing voltage that is the case with alternating sinusoidal voltages. The derivative of the applied voltage dU/dt were varied from the maximum possible (approximately square-wave) to the minimum possible (triangle-wave) for a given amplitude and period of the voltage. The influence of those voltage waveforms on the PD activity under different humidity levels was investigated, focusing on PD characteristics, such as PD repetition, average number of PD pulses per cycle, PD amplitude and delay time of PD appearance.
Section snippets
PD measurement system
The time-resolved PD measuring system consists of an Agilent 33120A function generator, a TREK 20/20 high-voltage amplifier, a detection resistance, an oscilloscope that was Yokogawa DL750 Scope Corder and a computer, as shown in Fig. 1.
The test voltage was generated by a function generator and then amplified by a high-voltage amplifier (with amplification factor 2000). The high voltage was applied to a steel spherical electrode with a diameter of 22 mm, in order to concentrate the discharges
PD at the relative humidity of 8%
In this study, the test sample was exposed to PD activities in dry air of RH = 8%. The applied voltage was Uapp = 7 kV (≈1.5–1.6 Uinc) with the varying values of dU/dt, that is, at the same voltage level but with different values of T1: 0 ms, 1 ms, 2 ms, 3 ms, 4 ms and 5 ms. Generally, the PD inception voltage Uinc was a little different for each waveform; it decreases slightly with the increasing dU/dt [20]. The time-sequential PD pulses were captured during consecutive 24 cycles of the
PD behavior for the dry case
It should be mentioned that the electric field from the applied voltage and from space charges will drive the initiation and evolution of the discharges in the air gap. Therefore, the benefit of the non-sinusoidal testing voltage waveforms is that the local conditions in the gap, such as electric field distribution, surface charge decay, vary with the applied non-sinusoidal voltage waveform. As a result, the PD process will also vary with the applied voltage waveforms. The application of
Conclusions
The relative humidity has a great effect on the PD activities, not only by reducing the effective free electrons in the air, but also by modifying the surface condition of the insulating material. This work presents the effect of relative humidity on the PD activities on machine insulation at trapezoidal testing voltage waveforms, with the derivative of the applied voltage dU/dt ranging from the maximum possible (approximately square-wave) to the minimum possible (triangle-wave). The PD
Acknowledgment
The project was funded by the Swedish Energy Agency, Elforsk AB, ABB AB and Swedish Railway Company via the ELEKTRA program (project No. 36161), which is gratefully acknowledged.
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