Question

Q

a: What kinds stresses need to be considered in designing high voltage equipment?

b: Name the two theories or theorems that are used in order to estimate the electric field in high voltage equipment and elaborate on both of them.

c: Explain why it is important to estimate electric field and to assess suitability of dielectrics in high voltage systems.   

d: Discuss TWO (2) important characteristics of solid insulating material.

e: Describe analytical method for electric field estimation.

(please all answers are short )

Answer 1

a)What kinds stresses need to be considered in designing high voltage equipment?

Ans: Electrical, Temperature, Mechanical, Environmental stresses

b)Name the two theories or theorems that are used in order to estimate the electric field in high voltage equipment and elaborate on both of them.

Ans: In order to understand the breakdown phenomenon in gases, the electrical properties of gases should be studied. The processes by which high currents are produced in gases are essential. The electrical discharges in gases are of two types

I) non-sustaining discharges II) self-sustaining types.

The breakdown in a gas (spark breakdown) is the transition of non-sustaining discharges into a self-sustaining discharge. The build-up of high currents in a breakdown is due to the ionization in which electrons and ions are created from neutral atoms or molecules, and their migration to the anode and cathode respectively leads to high currents.

Townsend theory and Streamer theory are the present two types of theories which explain the mechanism of breakdown under different conditions as pressure, temperature, Electrode field configuration, nature of electrode surfaces and availability of initial conducting particles.

Townsend Theory

According to the Townsend theory; - firstly, current growth occurs as a result of ionization process only. But in practice, breakdown voltages were found to depend on the gas pressure and the geometry of the gap; secondly, the mechanism predicts time lags of order of 10-5 s, but practically it was observed to occur at a very short time of 10-8 s. Also the Townsend mechanism predicts a very diffused form of discharge, that actually discharges were found to be filamentary and irregular. Townsend mechanism failed to explain all these observed phenomena and as a result The Streamer theory was proposed.

Streamer theory

Raether, Loeb and Meek studied avalanche growth by means of Wilson cloud chambers during the breakdown of gaps for various pressures and gap lengths. They observed deviations from the Townsend theory (and Paschen’s Law) in the case of longer gaps.They also showed that, if the length of the avalanche becomes large, the negative and positive charges in the avalanche distort the field to such an extent that new avalanches form ahead of the original avalanche, eventually bridging the gap. The fields, caused by the space charge, strengthen the applied field in the regions ahead and towards the tail of the original streamer. The ionization coefficient α is a function of the field strength (E) and it is therefore increased in these regions - to such an extent that new avalanches are triggered by photons that emerge from the original avalanche.

The critical number of ions in an avalanche has been determined empirically to be of the order of 5. 108, i.e. αxc= 20,, with xc the critical avalanche length.

Eventually, the conductivity of the gap increases to such an extent that flashover follows.

For a non uniform field the following equation applies:

∫αdx=20

    X

c)Explain why it is important to estimate electric field and to assess suitability of dielectrics in high voltage systems.

Ans: Electric fields (e-fields) are an important tool in understanding how electricity begins and continues to flow. Electric fields describe the pulling or pushing force in a space between charges. Compared to Earth's gravitational field, electric fields have one major difference: while Earth's field generally only attracts other objects of mass (since everything is so significantly less massive), electric fields push charges away just as often as they attract them.

The direction of electric fields is always defined as the direction a positive test charge would move if it was dropped in the field. The test charge has to be infinitely small, to keep its charge from influencing the field.

We can begin by constructing electric fields for solitary positive and negative charges. If you dropped a positive test charge near a negative charge, the test charge would be attracted towards the negative charge. So, for a single, negative charge we draw our electric field arrows pointing inward at all directions. That same test charge dropped near another positive charge would result in an outward repulsion, which means we draw arrows going out of the positive charge.

The uniform e-field above points away from the positive charges, towards the negatives. Imagine a tiny positive test charge dropped in the e-field; it should follow the direction of the arrows. As we've seen, electricity usually involves the flow of electrons--negative charges--which flow against electric fields.

Electric fields provide us with the pushing force we need to induce current flow. An electric field in a circuit is like an electron pump: a large source of negative charges that can propel electrons, which will flow through the circuit towards the positive lump of charges.

d) Discuss TWO (2) important characteristics of solid insulating material.

Ans:Solid insulating materials are potentially better insulating materials than liquids and gases. Unlike gases, liquid and solid insulating materials are generally not self-restoring.

• Dielectric Constant is 3-6
• Dielectric Strenghth will be 200-400 KV/CM
• Can support conductors
• High dielectric strength
• Some types can be moulded (epoxies)
• Its Not a self restoring
• Can not fill the small spaces

e) Describe analytical method for electric field estimation.

Ans: Analytical method From Maxwell’s equations electric field strength at a specified point P(x,y) can be expressed as follows.

Where r is a conductor radius, h is a distance between the conductor and    the earth surface underneath & V1 is a conductor potential

In the finite difference method (FDM) and the finite element method (FEM), the relevant

field region is subdivided into a rectangular or triangular mesh, respectively. The

potentials at the node points are solved numerically, taking into account the boundary

conditions. In the case of the electrostatic field, this amounts to a numeric solution of La

Place’s equation for the region:

In the case of the charge simulation method (CSM), fictitious charges are placed inside

the electrodes (outside the field region). For the boundary element method (BEM),

surface charge elements are placed on the electrode surface. The values of these

fictitious charges are computed, taking into account the boundary conditions. Thereafter,

the fields can be computed for any point, such as P(x,y,z) within the field region.