21-09-2008, 10:21 AM
Introduction
With the development of modern power system to the direction of extra-high voltage, large capacity, far distance transmission and application of advanced technologies the demand on the safety, stability and economic operation of power system became higher. A good grounding system is the fundamental insurance to keep the safe operation of the power system. The good grounding system should ensure the following:
" To provide safety to personnel during normal and fault conditions by limiting step and touch potential.
" To assure correct operation of electrical devices.
" To prevent damage to electrical apparatus.
" To dissipate lightning strokes.
" To stabilize voltage during transient conditions and therefore to minimize the probability of flashover during the transients
As it is stated in the ANSI/IEEE Standard 80-1986 "IEEE Guide for Safety in AC substation grounding," a safe grounding design has two objectives:
" To provide means to carry electric currents into the earth under normal and fault condition without exceeding any operational and equipment limit or adversely affecting continuity of service.
" To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electrical shock.
A practical approach to safe grounding considers the interaction of two grounding systems: The intentional ground, consisting of ground electrodes buried at some depth below the earth surface, and the accidental ground, temporarily established by a person exposed to a potential gradient at a grounded facility.
An ideal ground should provide a near zero resistance to remote earth. In practice, the ground potential rise at the facility site increases proportionally to the fault current; the higher the current, the lower the value of total system resistance which must be obtained. For most large substations the ground resistance should be less than 1 Ohm. For smaller distribution substations the usually acceptable range is 1-5 Ohms, depending on the local conditions.
When a grounding system is designed, the fundamental method is to ensure the safety of human beings and power apparatus is to control the step and touch voltages in their respective safe region. step and touch voltage can be defined as follows.
Step Voltage
It is defined as the voltage between the feet of the person standing in near an energized object. It is equal to the difference in voltage given by the voltage distribution curve between two points at different distance from the electrode.
Touch Voltage
It is defined as the voltage between the energized object and the feet of the person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away from it.
In different season, the resistivity of the surface soil layer would be changed. This would affect the safety of grounding systems. The value of step and touch voltage will move towards safe region or to the hazard side is the main concerned question
