Design of Earthing Grid
In designing the earthing grid for a sub-station, it is necessary to obtain the following information about the system:
(a)Maximum earth fault current.
(b)Resistivity of the soil at the sub-station site
(c) Fault clearing time.
(d) Area covered by the sub-station.
1. Earth Fault Current
The maximum earth fault current constitutes a major factor in the earthing system design this determines the cross section of the grounding conductor and total rise of the earthing system potential. The potential gradients are also a function of this current.
It is common to consider the symmetrical value of the maximum single line to ground fault.
When the fault duration is less than 0.5 sec, the rms value of the earth fault current is increased by a factor as given below to allow for D.C. offset and A.C. and D.C. decrement.
 |
| Fault Duration and it's Decrement Factor of Earth Fault Test |
To allow for the future growth of the system and the consequent increase in fault current a multiplication factor of 1.2 to 1.5 depending on the state of growth of the system is also adopted.
The current diverted away from the earthing system by the earthing wires connected to the station earthing is neglected.
2. Soil Resistivity
The resistivity of earth varies within extremely wide limits; between 1 and 10,000 ohm-meters.
To design the most economical earthing system for a sub-station it is necessary to obtain accurate data on the soil resistivity and its variation at the station site. Even at the same site the resistivity of the soil may vary considerably at different times of the year.
It is, therefore, necessary that the measurements are made a number of times during different parts of the year. Where time does not permit, measurements should be made in dry season to set the maximum value of the soil resistivity.
A number of test locations should be chosen at each site to cover a wide area of the soil. When 4 to 5 test sites are chosen and measurements spread over a period of full one year, it may be quite realistic to adopt for earth resistivity - the arithmetic average of various reading.
3. Fault Clearing Time
There are many considerations which influence the choice of fault clearing time such as the system stability, the type of switchgear and relaying used. The Russian practice is to adopt 0.2 sec. as the time of adopting 4 sec. i.e. the same duration as is used for short time rating of switchgear.
The short time rating of switchgear as per Indian practice is based on 3 seconds. Therefore, 3 sec. time may be adopted as the fault clearing time for Earthing Calculations.
However the system design engineer has to use his judgement to adopt any lower value consistent with the operating times of circuit breakers are relays used on the system (0.5 to 1.0 sec.)
4. Area Covered by Sub-Station
The area covered by the sub-station is governed by the number and type of equipment. After the layout has been decided and sub-station plan finalized it is a simple matter tocalculate the area covered by the sub-station.
The next steps in the design procedure is the :
(a) Selection of earthing material.
(b) Determination of size of earthing conductor.
(c) Preliminary arrangement of earthing conductors.
(d) Determination of conductor length required for gradient control.
(e) Calculation of resistance of earthing system.
(f) Checking of design for various voltages.
Comments
Post a Comment