Electric Stress and Electric Strength - Coursework Example

•  Currently, high-voltage power is widely used in industries, research laboratories, industrial X-ray equipment, etc (Kreuger. 1964).
• High voltages are indispensable in the transmission of huge amounts of power over long distances.
• Understanding electrical stress and electrical strength helps avoid discharges in electrical appliances that cause insulation failure. The discharges result from high working voltage and can be reduced by reducing the working voltages.  
• The electric stress of an insulating material is equal to the voltage gradient and to the electric field intensity.
• For the optimum design of electrical appliances, knowledge of the tendency to increase voltage stress is necessary. Proper selection of insulation material for reliability and economic purposes is done in relation to corona discharges and dielectric strength.
• It gives high-voltage engineers an understanding of the factors that affect the electrical strength of insulating materials, the nature of voltage being one of them.
• High voltage engineers must know the field intensities in different media that are under electric stress. This helps in the economical and efficient choice of insulators to ensure the reliable operation of the equipment and the correct electrode configurations.

Factors that affect the electric strength of insulating materials

• The thickness of the insulating material. A material’s electric strength increases at a rate less than that of a linear increase in its thickness. This means the electric strength for a thicker sample is lower than for a thin sample. Therefore increasing the thickness will greatly affect the electric strength of the insulating material (Malik et al 1998).
• Duration of voltage transients and operation frequency. When the operating frequency increases, the electric strength is reduced. The electric strength of a solid material increases fast and nearly reaches its intrinsic breakdown strength if voltages are applied for short periods, in the order of 10 nS.
• Humidity can reduce the electrical strength of an insulating material. The failures are accelerated if there is a difference in the humidity of the testing environment and that of the environment of operation.
• dielectric constant property of an insulating material can be greatly affected by temperature changes. This will in turn interfere with the signal integrity and result in unstable performance For insulators, for example, polyimide, their electrical breakdown behavior is constant in temperatures below room temperature. Beyond room temperature, there is an increase in the electric field stress required for breakdown. At 100o C, the dielectric strength of monolithic polyimide is about 80% of the room temperature value. At 200o C, its dielectric strength is reduced to about 60% of room temperature value. Due to the dependence on the quality of the polyimide interface and their complex structure, other insulators, for example, Polyimide laminates could be even more sensitive to temperature (Kreuger, 1964).
• Operation lifetime. Above a given voltage, the surrounding material is degraded by partial discharges. This makes the laminating material vulnerable to electric failures. With this degradation, electric failure may be reached over a few minutes to many years.
• When a solid insulating material is immersed in a liquid, a decrease in the liquid’s dielectric strength can affect the dielectric creepage strength of the solid Kreuger (1964).

How to determine the electric field strength in high-voltage structures

• Use of the field intensity/strength formula.

This is given as E=F/qtest

E represents electric strength, F represents force and quest represents the amount of test charge. From this formula, the ratio of force to the amount of test charge gives the field strength. It is given in Newtons per Coloumb (N/C). This gives the force available per coulomb of charge used as a test charge (Harris 1966).

Advantage

The formula is not complicated and gives the value of the electric field

Disadvantage

The direction of the field is not specified by the field intensity formula. This is because the direction of the force depends on the direction of force detected by a positive charge.

• Use of voltage. When Scalar's electric potential has been calculated, it can be used to calculate the electric field.

In any direction, the negative rate of change of potential is the component of the electric field of that direction. The differential voltage change is equal to the electric field component in that particular direction times the distance.

dV=-E.eds=-esds

Therefore the electric field is expressed as Es=dv/ds along ds or Es=-∂V/∂s

• Use of field lines

-Electric strength is related to the intensity of charges on the source but inversely related to the distance from the source.

-The direction in which a positive charge is pushed is the direction of the electric field. This is represented by a vector arrow.

-The number of arrows shows the amount of charge whereas the length of the arrows represents the magnitude of the electric field.  Arrow lengths are longer near the charge source than when away. 

Advantages

- Easy way of determining if a current is strong or weak by simply looking at the length of lines

-Gives a quick view of the amount of charge on a material

Disadvantages

-Simply tells if a current is weak or strong but not the exact magnitude

-The lines must be limited along an electric line pattern to ensure the readability of the patterns

-It is cumbersome to draw electric lines in highly charged materials

-Readability is reduced where more fields meet