State of the Art and Objectives
Air as an insulator is probably the best conventional solution for many applications. The air gap thus is considered as one of the most important parameters for the design and dimensioning of insulating arrangements, in almost every electrotechnical application.
Through the previous decades in the majority of electrotechnical applications air gap models were used as compromised insulating solutions. In designing nearly every electrical arrangement air gaps are essential components that arise necessarily in constructions (gaps between power lines, or power lines and earth, gaps between electrical and electronic components in most devices, switches etc.)
In the last decades air gaps are necessary and operational components of modern constructions and devices, while their dielectric behaviour is the basis of their operation. Such devices and constructions are the electrostatic filters, the ionizing devices and air cleaners, the electrostatic painting systems, electrostatic loudspeakers, etc. Consequently the dielectric behaviour of different air gap arrangements is of great importance.
The dielectric behavior of real air gaps can be concluded by experimental investigation of symmetrical air gaps like sphere-sphere, plate-plate and rod-rod (or point-point), and especially of non symmetrical rod-plate (point-plate) air gaps.
The most determinant factor for the dielectric behaviour and especially for the dielectric strength of an inter-electrode air gap is the inhomogeniety of the electric field, and especially the maximum value of the field strength in the gap, which usually appears on the sharper edge of the electrodes, mostly on the tip of a rod, or a point [1-2]. Other factors are the waveform and the polarity of the applied voltage [3-4], as well as the corona effects , which take place when the field strength exceeds some specific value. In less inhomogeneous electric fields like small air gaps with relatively big diameters of the rod or the sphere electrodes, there are no corona effects before breakdown. In such gaps the values of the breakdown voltage depend on the grade of the field’s inhomogeneity, and definitely on the maximum value of the field strength in the gap [6-9]. It is well known that the grounding of one of the electrodes influences the values of the breakdown voltage of the spark gaps, especially in the vertical arrangements . This is stated due to the fact that the distribution of the electric field is influenced by the parasitic capacitances between the electrodes and the grounded surroundings . In a rod-plate gap the influence also depends on the electrode chosen to be grounded [5-6].
The basic effects which are referred as the dielectric behaviour of an air gap are:
1. The Corona Effects, that is the local partial discharges which appear in a gap at points or sharp edges where the field strength is higher than the air’s dielectric strength , and
2. The Breakdown voltage, which is the intense spark discharge which short-circuit a gap or an insulator.
The basic magnitudes which describe the dielectric behavior of an air gap are [1-3]:
1.The Corona or partial discharges onset voltage,
2.The Corona leakage current, and
3. The Breakdown Voltage.
The most known effects which influence the values of the above mentioned magnitudes are
The Polarity Effect, that is the influence of the polarity of the voltage applied on the arrangement on the value of the breakdown voltage, especially in the rod-plate arrangements with the plate grounded, , and
The Barrier Effect that is the influence of a dielectric insulating plate (barrier) placed in the gap  on the value of the breakdown voltage.
Other phenomena which have great influence on the dielectric behavior of the air gaps are:
The Ground Effect, that is the influence of the different ground positions of a gap (grounding rod, or grounding plate, or symmetrical charging of the electrodes) on the field distribution and hence the dielectric behavior of a gap [5-8]. In symmetrical arrangements (rod-rod or sphere-sphere gaps) with one electrode grounded, the Ground Effect refers to the differences of the values of the corona onset voltage, the corona current and the breakdown voltage recorded in comparison to the arrangements with symmetrically charged electrodes, while in a rod-plate air gap the big differences are recorded between the arrangements with the rod grounded in comparison to the commonly used arrangements with the plate grounded.
The Corona current effect that is the influence of the Corona current on the value of the breakdown voltage of a gap .
These two above phenomena have been formulated and first investigated lately in a study carried out in the High Voltage laboratory of the Electrical Department of the T.E.I. of Larissa, in cooperation with the N.T.U.A., and the proposed project aims to their further study and investigation of their influence towards the Optimization and the Prediction of the dielectric behavior of air gaps.
The Corona effect can be specified as local partial discharges which appear in a gap at points or sharp edges where the field strength is higher than the air’s dielectric strength (more than 30 KV/cm) .
The voltage value needed for the Corona effect to appear is called Corona Onset Voltage; it is different for every case and depends on the electrodes geometry, as well as the conditions of pressure and temperature, and generally on the electric field distribution in the gap. The Corona effects have been studied by many researchers mainly for the rod-plate (point – plate) gaps with the plate grounded, as well as rod-rod gaps with one rod grounded. In the last few years in the research project of T.E.I. of Larissa there was a first attempt to investigate the rod-plate air gaps with the rod grounded, and the rod-rod air gaps with symmetrically charged electrodes.
The Corona effect is one of the major factors that influence the operation of various installations and high voltage devices. Conductors and insulators, which are activated by high voltage, experience intense Corona effects and in general partial discharges. This phenomenon leads to energy and electric charge losses, disturbance in wireless communications and ozone production. Sometimes it even leads to the destruction of the insulating materials.
Thus it is very crucial to design common high voltage electric circuits and devices free from Corona Effects, or Partial Discharges, when installed and activated with operating voltage. The optimization towards that direction is a very important factor.
On the other hand many modern applications (electrostatic filters, electrostatic painting, electrostatic sound boxes, etc) rely on the existence and control of Corona effects in order to operate and function. Thus in these cases the optimization towards that direction of controlling the Corona effects is of major importance.
Many methods for controlling the corona effects have been proposed. The most effective method is to place a barrier, or a third electrode between the electrodes. In the proposed research project a new method will be investigated based on the results given from the influence of the grounding (Ground Effect) in connection to the Barrier and the polarity effect.
Breakdown is the phenomenon of intense spark discharge which can alter or destroy the insulating materials and usually appears after intense Corona or partial discharge effects. The voltage value needed to lead to breakdown is called breakdown voltage and is higher than the Corona onset voltage, if Corona appears before the breakdown. In gaps with less homogeneous electric field, breakdown can occur without intense or detectable Corona effects, and the value depends on the gap’s geometry and the conditions in the gap. In gaps with more inhomogeneous fields, Corona occurs before breakdown and the value of the breakdown voltage depends on the value of the Corona current, as well [5, 7]. It is well known that the Polarity of the applied dc or impulse voltage affects greatly the value of the breakdown voltage of the air gaps. In the rod-plate air gaps the breakdown voltage following Corona is higher when the applied voltage on the rod is negative (Polarity Effect), but so is the Corona current through the gap.
The Corona effects and the breakdown of air gaps have been experimentally investigated by many researchers for the most commonly used arrangements where the gap is formed between two electrodes one of which is grounded and the other is stressed by high voltage. Especially in rod – plate air gaps the majority and used so far technique of the experimental work concerns arrangements where the plate electrode is grounded and voltage is applied to the electrode of the rod [1-4]. Arrangements symmetrically charged have been just lately but not thoroughly investigated, as well as rod – plate air gaps with the rod grounded and the plate stressed by high voltage [5-9].
The influence of the polarity of the dc or Impulse voltage on the Corona and breakdown of the air gaps has been investigated by many researchers through the Polarity Effect, which mostly concerns rod – plate gaps with the plate grounded and rod – rod gaps with one rod grounded. It has been just lately investigated in the previous research program of the T.E.I. of Larissa that in air gaps the higher the Corona current is, the higher the Breakdown voltage will rise , and that the polarity effect is a manifestation of a more general phenomenon the Effect of the Corona Current. The optimization towards that direction is also a very important factor for applications.
The main scope of the last related research project carried out in the T.E.I. of Larissa (2004-2010) was the investigation of the dielectric properties of composite insulating materials and especially the investigation of the barrier effect in air gaps.
The research concerned:
1. The analysis of the field distribution using simulation methods,
2. The experimental investigation of the dielectric behavior of the air gaps (Corona and breakdown), and
3. The comparison and connection of the simulation and experimental results.
During the above research program two new phenomena emerged:
The influence of the grounding of one of the electrodes (especially of the rod electrode in a rod-plate air gap) on the field distribution and the dielectric behavior of air gaps, and
influence of the Corona current
There was a first investigation of these phenomena, mainly when stressed by dc Voltage and a suggestion to be referred to as “the Ground Effect” and “the Corona Current Effect” respectively [6-9]. These phenomena were combined with the Barrier Effect as well. The outcome of the above research was impressive. It was concluded that the Ground Effect and the Corona Current Effect influence significantly the dielectric behavior of air gaps. A new principle (rule) was expressed within the 3d law of Newton (Law of action – reaction), similar to Lenz’s Law for electromagnetic induction. These impressive results gave the motivation for a further and systematic investigation of the phenomena of Ground Effect and Corona Current Effect, as well as for a theoretical and mathematical investigation of the Corona current rule (within the 3rd law of Newton), which could be reformatted into a theorem after it is proven.
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