We hereby certify that the thesis titled "SIMULATION OF AIR GAP MECHANISM USING DIFFERENT ELECTRODES" submitted by A Srikant and Shekhar Chandra Pradhan has partially fulfilled the requirements for the degree of Bachelor of Technologist in Electrical Engineering at the National Institute of Technology, Rourkela ( Deemed University) is an authentic work done under my supervision and guidance. Research work related to this assignment was done at High Voltage Laboratory, Department of Electrical Engineering, National Institute of Technology (NIT), Rourkela. Karmakar, Professor, Department of Electrical Engineering, National Institute of Technology, Rourkela, for his consistent encouragement and tremendous support in our research work and sharing his technical knowledge which easily guided us through many difficulties.
D Subudhi, Professor and Head of the Department of Electrical Engineering, National Institute of Technology Rourkela, for allowing us to use the necessary facilities to carry out this thesis. It has been seen from several studies conducted by power engineers that one of the main problems in high voltage (HV) power equipment is insulation degradation, i.e., the quality of the insulation of the power equipment. In addition, the effect of breakdown voltage on different insulations such as lamiflex, leatheroid, plywood, craft paper and polyester fiber has been studied.
To observe the effect on insulation due to degradation mechanism, the insulation samples are collected both before and after the breakdown voltage test and analysis is done using a scanning electron microscope (SEM). To simulate the air breakdown voltage with and without the insulation barrier is experimentally investigated in the high voltage laboratory, a standard diameter of 25 cm spheres are used to measure air breakdown voltages and electric field in the high voltage equipment. The simulation of such an air breakdown voltage has been carried out in the COMSOL environment.
1 Breakdown voltage (Vbd) test using ball-ball electrode 17 2 Breakdown voltage (Vbd) test using different intermediate insulators.
Introduction
Introduction
The following sections include the experimental setups for the high voltage laboratory air breakdown test, the theoretical study, the computer simulation, and the analysis of the results.
Objective
Organization of the Thesis
3 Chapter 3: This chapter contains the experimental setup used and the measurement of the breakdown voltage of air and insulating paper using ball openings. The analysis of different electrode arrangements: sphere-sphere, rod-rod and rod-plate is discussed.
Air Breakdown Mechanism
- Basic Breakdown Process
- Primary Electrons
- Ionization
- Excitation
- Other Electron Processes
- Regeneration
- Townsend’s Mechanism
- Current Growth Equation
- Current growth in the presence of secondary process
- Townsend’s criterion for breakdown
- Streamer Theory
- Streamer Theory
- Numerical Methods for computation of Electric Field
- Finite Element Method
Let α be the average number of ionizing collisions caused by an electron per centimeter of travel in the direction of the field (α depends on the gas pressure p and E/p and is called Townsend's first ionization coefficient). 7 Therefore, the average current in a gap equal to the number of electrons traveling per second will be the same. The single avalanche process described in the previous section is completed when the initial set of electrons reaches the anode.
But because the electron gain [exp(αd)]. occurs in the field, additional new electrons are more likely to be released into the gap by other mechanisms, and these new electrons create further avalanches. the released positive ions may have enough energy to cause electrons to be released from the cathode when they strike it. excited atoms or molecules in avalanches can emit photons, which will result in the emission of electrons by photoemission. metastable particles can back-diffuse and cause electron emission. When the distance between the electrodes d increases, the denominator of the equation tends to zero and at some critical distance d = ds [4]. If d = ds, I and current will be limited by the resistance of the power supply and the external circuit.
For a given gap distance and at a given pressure, the value of voltage V that gives values of α and ϒ that satisfy the breakdown criterion is called the spark breakdown voltage Vs, and the corresponding distance ds is called the spark gap. Second, the mechanism assumes time delays of the order of 10-5 s, while in actual practice we observe that failure occurs in a very short time of the order of 10-10 s. For simplicity, the space charge at the top of the avalanche is assumed to have a spherical volume that contains the negative charge at the top due to higher electron mobility.
Under these conditions, the field at the top of the avalanche is reinforced with field lines from the anodes ending at the head. Furthermore, the field between electrons and ions at the bottom of the avalanche reduces the applied field (E). As the charge density in the avalanche approaches n = 108, the space charge filled field and the applied field will be the same size and this leads to the streamer.
Proper design of any high voltage apparatus requires a thorough knowledge of the electric field distribution. The potential, which is unknown throughout the problem domain, is approximated at each of these elements in terms of the potential at their vertices called nodes. Normally, a certain class of polynomial is used to interpolate the potential within each element in terms of their nodal values.
The coefficient of this interpolation function is then expressed in terms of the unknown node potentials. As a result of this, the interpolation can be performed directly relative to the nodal values.
Experiment Setup for Air Breakdown Voltage Using Standard Sphere-Sphere
- Apparatus required for measurement of air breakdown voltage
- Measurement of Voltage Breakdown of Air
- Measurement of Voltage Breakdown of Insulation Paper
- SEM analysis of the Insulation Paper
At the start of the interruption, the circuit is immediately disconnected from the supply and the breakdown voltage is recorded. A coupling device with connecting cable is associated with the measuring circuit for measuring the applied high voltage magnitude. The experimental setup for air breakdown study between the two sphere electrode is shown in Fig.
Again the experiment is carried out with insulator between the two spheres and the breakdown voltages for different insulating materials are observed. The breakdown strength of the polyester fiber is the highest, followed by lamiflex, leatheroid, craft paper, paper and plywood, which are depicted in Table 2. The electrons interact with the atoms that make up the sample and produce signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity.
Secondary electron detectors are common in all SEMs, but it is rare for a single machine to have detectors for all possible signals. The signals result from interactions of the electron beam with atoms at or near the surface of the sample. SEM test is conducted at SEM Laboratory, NIT Rourkela using SEM instrument JEOL-JSM-6480LV.
Figures 3.4, 3.5 and 3.6 show how the insulation paper deteriorates after the breakdown of the papers. From table 2 and figures it is seen that the Lamiflex paper has the highest breakdown voltage and from SEM photographs it is also shown to be the least degraded. So, if Lamiflex paper among the above tested insulation paper is used for insulation, the high voltage equipment protection will be effective.
Simulation of Electric Field for Different Electrode Configuration
Simulation of electric field of different electrode arrangements
- Sphere – Sphere Electrode
- Rod-Rod Electrode
- Rod-Plate Electrode Configuration
- Comparison of different electrode arrangements
The radius of the sphere is 12.5 cm and the gap distance is varied between the two spheres, and the maximum electric field (Emax) for the applied voltage is observed. Initially there is a sharp drop in the maximum electric field, but this gradually saturates as the gap distance increases. To observe the effect of the barrier, a barrier made of PVC (of ε = 2.9) with a thickness of 0.25 cm is introduced and the distribution of the electric field is observed.
This shows that the maximum electric field (Emax) for an arrangement with a barrier is greater than Emax for an electrode arrangement without a barrier. First, the electric field distribution for a rod-rod electrode arrangement without a barrier is observed, and then a PVC barrier (ε = 2.9) is introduced to observe the effect on the electric field. 4.5(b) shows that the electric field distribution is non-uniform for the rod-rod arrangement and that the electric field is symmetric about the y-axis.
The electric field is maximum at the tip of the rod and decreases along the axis connecting the rods and increases again. The electric field between the rods is much more inhomogeneous than in the sphere-sphere arrangement. 4.7 (b) shows that the electric field distribution between two rods is non-uniform and symmetric about the y-axis.
To observe the field distribution in rod plate arrangement with and without barrier, the arrangements are shown in fig. By varying the distance between the rod and the plate, the electric field distribution and the maximum electric field values were noted and plotted against the gap distance. 4.11 (b) shows that the electric field distribution in the bar plate gap is non-uniform.
It is seen from the graph that the plate-rod-electrode arrangement has the highest value of Emax followed by rod-rod and ball-ball. 4.5 (b) and 4.7 (b), but the rod-rod air gap is a symmetric arrangement, the electric field is much more inhomogeneous than the sphere arrangement. 4.9 (b) and 4.11 (b) it is shown that the rod-plate arrangement is of non-symmetrical arrangement.
Conclusion
The maximum value of electric field strength along the aperture axis at breakdown voltage tends to acquire a stable value in the case of symmetrical arrangements such as sphere-sphere, rod-rod etc. The test electrodes are kept in air medium without considering the temperature. and air pressure. The simulation can be performed for complex configurations such as transmission tower, circuit breaker, transformer, bus, high voltage reactors etc.
