System earthing pdf

By selecting the proper horizontal conductor size, vertical electrode size and soil resistivity, the best choice of the project for safety can be performed. Substation earthing system is primarily not only to deliver the protection of people working in the surrounding area of earthed facilities and equipment against danger of electric shock, but to uphold proper function of electrical system.

Reliability and security are to be taken in contemplations as well as adherence to statutory obligations. Earthing Standards There are a variety of national and international standards available, which provide empirical formulae for the calculation of earthing design parameters and shock potential safety limits. Terms Associated with Earthing: 1.

This connection is not ideal due to the resistivity of the soil within which the earth grid is buried. During typical earth fault conditions, the flow of current via the grid to earth will therefore result in the grid rising in potential relative to remote earth to which other system neutrals are also connected.

This produces potential gradients within and around the substation ground area - this is defined as ground potential rise or GPR. The GPR of a substation under earth fault conditions must be limited so that step and touch potential limits are not exceeded, and is controlled by keeping the earthing grid resistance as low as possible.

These maximum permitted step and touch potentials are addressed within various national and international standards.

Also, the earthing conductors must have sufficient mechanical strength and corrosion resistance. It is normal practice to bury horizontal earthing conductors at a depth of between 0. The length of the earth rod is chosen so as to reach the more stable layers of ground below.

DOI: Fence earthing can be accomplished in two different ways: Electrically connecting the fence to the earth grid, locating it within the grid area or alternatively just outside Independently earthing the fence and locating it outside the earth grid area at a convenient place where the potential gradient from the grid edge is acceptably low.

This diverts part of the earth fault current to the tower footing earthing. Cable entering and leaving the site. The armouring of such cables is usually earthed to the substation earthing grid at both ends. Part of the earth fault current will thus be diverted to a remote earthing grid via the cable armouring. Description Notations Unit Value no.

The results for earthing system are obtained by computational method. For earthing conductor and vertical earth electrode, mild steel are used. They are inductive devices intended primarily to provide a neutral point for grounding purpose. The main purpose of this paper is to discuss about the difference between Earthing transformer and Power transformer and to discuss about the designing of earth transformers. An earthing transformer is a transformer primarily to provide a neutral point for grounding purpose.

The sole duty of the grounding transformer is to pass ground current during an earth fault. The desirable quantities of an earthing transformer are low zero impedance and low losses no load losses.

C with stand duration 10sec. Significance of zero sequence parameters In symmetrical components the positive sequence is one having normal phase sequence; the second is of negative sequence with reversed phase sequence; and the third that has no sequence and is called zero sequence — are important parameters. An earth fault is one that part of the current in the faulty phase returns to the supply through the earth.

This is due to a symmetrical component consisting of three single phase currents - one in each line, and all being in phase.

In general, the impedance of the transformer is the main factor which influences applications. The zero sequence impedance is used in short circuit calculations. When a zero sequence component of three line currents are compounded, they will add together to produce a resultant called a residue, whilst the positive and negative sequence components get cancelled in the transformer. The magnetic field produced by a zero-sequence set of currents is radically different from those produced by negative or positive sequence currents, and therefore zero sequence impedance is generally very different from positive and www.

A zero sequence reactance is equal to or less than a positive sequence and it depends on the form of core construction and disposition of the windings Why inter-connected star winding is preferred? The purpose to establish a suitable ground path can be achieved with the help of three pairs of concentric coils connected to oppose ampere turns.

As the fluxes oppose, the transformer takes a very small magnetizing current during normal condition. Lines of force enclosing both coils on one limb are therefore impossible. The earth fault current finds little impedance. This connection has unique characteristics, and is preferred to other types of neutral deriving transformers.

Rating and its inter-related parameters of an earthing transformer The earthing transformer is of short time rating 10 seconds to 1 minute. The rating of an earthing transformer is entirely different from that of a power transformer. Power transformers are designed to carry total load continuously, whilst an earthing transformer carries no load, and supplies current only if one of the lines becomes grounded.

It is usual to specify the single phase earth fault current, that the earthing transformer must carry for sufficient time. Since it is almost working on no-load, dictates to have low iron losses. Because of it being a short time device, its size and cost are less than that of a continuous duty transformer of equal KVA rating. Uploaded by Mark joedel Mendez. Did you find this document useful? Is this content inappropriate? Report this Document. Flag for inappropriate content.

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