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Senin, 18 April 2011

Protective earthing


7.1 Introduction
Earthing is a general term broadly representing grounding of power systems and bonding of equipment bodies to grounded electrodes. Earthing associated with current carrying power conductors, usually neutral conductor, is normally essential for the stability of the system and is generally known as system earthing. Earthing of non-current carrying metal works of equipment bodies is essential for the safety of life and property and is generally known as safety equipment earthing. The basic requirements of any earthing system are
(i) It should consist of equipotential bonding conductors capable of carrying the prospective earth fault current and a group of pipe/rod/plate earth electrodes for dissipating the current to the general mass of the earth without exceeding the allowable temperature limits in order to maintain all non-current carrying metal works reasonably at earth potential and to avoid dangerous contact potentials being developed on such metal works.
(ii) It should limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting.
(iii) It should be mechanically strong, withstand corrosion and retain electrical continuity during
the life of the installation. Earth electrodes, which form part of the earthing system, are provided to dissipate fault current during earth fault and to maintain the earth resistance to a reasonable value so as to avoid rise of potential of the earthing grid. The resistance to earth of an electrode of given dimensions is dependent on the electrical resistivity of the soil in which it is installed. In addition to the measurement of soil resistivity at the design stage, it is essential to repeat the measurement at the pre-commission stage
also, as the effectiveness of the earthing system depends on the value of soil resistivity . Hence before energising electric supply lines and apparatus it is necessary that all components of the earthing system including the soil are inspected and tested to ensure efficient functioning of the system.
7.2 Pre – commission inspection and checks
(i) General Layout
· Check whether the layout of earthing is as per the scheme approved by the department of Electrical Inspectorate.
· Check whether the number of plate electrodes and pipe electrodes are as per the approved scheme.
· Check whether the spacing between the electrodes are as per the approved scheme- 5 metres for pipe electrodes and 8 metres for plate electrodes.
(ii) Earth electrodes
· Check whether all the earth electrode terminals are visible and numbered. The numbering shall be done both on the top of trough cover and inside the trough.
· Check the size of earth electrodes used – 1200 x 1200 x 12.6 mm for cast iron plate and 600 x 600 x 6.3 mm for copper plate – One cast iron plate is equivalent to 4 copper plates of standard size.
· Check the dimension of earth electrode trough – 1000 x 500 x 600 mm for plate electrodes and 500 x 400 x 350 mm for pipe electrodes – This is for easiness of connections and convenience of testing .
· Check the class of pipe used for pipe electrodes – at least class B pipes shall be used.
· Check whether permanent watering arrangement is provided at sub-stations and where earth resistivity is relatively high.
· Check whether funnels are provided for watering the electrodes.
· Check the size of the earth mat of EHT stations with the designed values (spread of earth mat, mesh size, conductor size, size of risers, depth of laying etc.)
· In the case of earth mats, check the size of blue granite jelly, its depth and area of spread. The area of spread shall extend beyond fencing atleast by 1.5 metres.
· Check whether all the earth electrodes are interconnected to form a closed mesh.
(iii) Earth continuity strips and earthing conductors.
· Check whether two continuity strips have been taken from the plate electrodes to the top connector link.
· Where GI is used for earthing, check whether hot dip galvanized GI strips and conductors are used. GI is allowed where corrosion factor is within permissible limits and earth resistivity is more than 100 ohm-metre.
· Check the size of main earth bus for conformity with the approved size.
· Check the size of the sub earth buses and their interconnections.
· Check the size and effectiveness of connections of horizontal and vertical earth buses of cubicle type switch board sections.
· Check the interconnection of earth bus sections in switch boards.
· Check whether duplicate earthing of adequate size is provided for switches, isolators and control gears.
· Check whether duplicate earthing is provided for body of transformers, motors and other equipments. The two connections shall be taken from opposite sides.
· Check whether duplicate earthing is provided for the neutrals of transformers and generators . There shall be one direct connection from each neutral to a separate earth electrode but interconnected with the earthing system.
· Check whether continuous earth strip is run from the top of lattice type towers and structures of EHT stations and lines.
· Check whether the bottom of each High voltage bushing is earthed using earthing strip.
· Check whether outdoor CT, PT, breaker units, isolators, lightning arrestors etc. are directly earthed to the risers of the earth mat / earthing grid.
(iv) Connections and joints
· Check whether connections in the earthing system are made properly . The contact surfaces shall be properly tinned and contacts perfectly bonded and seated – Riveting, bracing or bolting shall be done effectively.
· Inspect the welded joints of GI earth strips and conductors – The welded surfaces shall be covered with zinc dichromate painting / bituminous coating.
· Check the quality of galvanisation of bolts and nuts used for earth lead connections – Hot dip galvanised rust free bolts and nuts shall be used.
· In the case of earth mats, check the perfection of welding of mesh joints.
7. 3 Precommission tests
7. 3.1 Earth Testers and principle of measurement
The most commonly used earth tester is the four terminal tester. The tester comprises of a current source and a meter in a single instrument. The resistance is directly read in the tester from which the earth resistivity is computed. Wenner’s four electrode method is followed for earth resistivity measurement. When four electrodes are driven along a straight line at equal intervals and a current is passed through the two outer electrodes, the current flowing into the earth produces an electric field proportional to the current density and the resistivity of the soil. The voltage measured between the two inner electrodes is therefore proportional to the field. Consequently the resistivity will be proportional to the ratio of the
voltage to current. The earth resistivity of the soil is given by
7 .3.2 Earth Resistivity – Test Procedure
The resistivity of soil varies over a wide range depending on the composition and moisture content of the soil. It is therefore advisable to conduct earth resistivity tests during dry season in order to get conservative results. In the case of sub – stations and generating stations, at least eight test directions shall be chosen from the centre of the station to cover the entire site. For very large station sites this number may be increased. In the case of transmission lines, the measurements shall be taken along the direction of the line throughout the length, at least once in every 4 kms. The connections for the test are given in fig 7.1
The four electrodes are driven into the earth along a straight line at equal intervals . The depth of driving the electrodes in the ground shall be of the order 10 to 15 cms. The earth megger is placed on a steady and approximately level base. The links between the terminals are opened and the four electrodes connected to the instrument terminals. Appropriate range in the instrument is selected to obtain accurate readings. The readings are taken while turning the crank at around 135 revolutions per minute. The resistivity is calculated by substituting the value of R obtained from the test in the equation in para 7.3.1.
If the resistance of the electrodes (two inner potential electrodes) is comparatively high, a correction of the test result is necessary depending on its value. For this purpose, the resistance of the voltage circuit of the instrument Rp is measured by connecting the instrument as shown in fig. 7.2.
Average earth resistivity at the site
The resistivity of the soil at many sites have been found varying with the depth of the soil and also with horizontal distances. Variation of the resistivity with depth is mainly due to stratification of earth layers and is found predominant when compared to the variation with horizontal distances. For the correct computation of earth resistivity, it is desirable to get information about the horizontal and vertical variations of earth resistivity at the site under consideration. The vertical variations may be detected by repeating the measurements at a given location in a chosen direction with different electrode spacing. The spacings may be increased in steps of 2, 5, 10, 15, 25 and 50 metres or more. The horizontal variations are studied by taking measurements in various directions from the centre of the station. If the variation in the earth resistivity readings for different electrode spacings in a direction is within 20 to 30 percent, the soil is considered to be uniform. When the spacing is increased gradually from low values, a stage will be reached at which the resistivity readings become more or less constant irrespective of the increase in the electrode spacing. This value of the resistivity is noted as the resistivity in that direction. Similarly, the resistivity for at least eight equally spaced directions from the centre of the site are measured. These resistivities are plotted on a graph sheet. A closed curve is plotted on the graph sheet joining the resistivity points to get a polar resistivity curve (see fig. 7.3). The area inside the polar curve is measured and the circle of the equivalent area is found out. The radius of the equivalent circle is the average earth resistivity of the site under consideration. The value will be reasonably accurate when the soil is homogeneous. If the soil is not
homogeneous, a curve of resistivity versus electrode spacing shall be plotted and this curve further analysed to decide stratification of the soil into two or more layers of appropriate thickness or a soil of gradual resistivity variation. Computation of earth resistivity of heterogeneous soil is highly involved and reference to text books may be made.
7.3.3 Measurement of earth electrode resistance
The same four terminal earth tester described under para 7.3.1 can be used for measurement of earth electrode resistance. One of the current and potential terminals are shorted to form a common terminal which is connected to the test electrode and the other current and potential terminals connected to two auxilliary electrodes. Alternately, 3 terminal earth testers with common terminal to be connected to the test electrode and independent current and potential terminals for connections to auxiliary electrodes are available for measurement of earth electrode resistance. Two standard auxiliary electrodes supplied with the instrument are used for the measurement. The depth of driving of auxiliary electrodes shall be low compared to the spacing between the electrodes. Generally, the auxilliary electrodes are driven at 15 metres and 30 metres from the test electrode. The connections may be checked before taking the measurement .
Resistance of individual electrodes
Resistance of individual electrodes is measured after disconnecting all interconnections to the electrode. Earth leads from the earth bus, neutral of transformers/generators and interconnections from other earth electrodes are disconnected before taking the measurement. Connections to the earth tester are made as described above. The cranking lever of the earth tester is rotated at the specified speed. The reading of the earth tester gives the earth resistance of the particular electrode. Resistance of all earth electrodes shall be measured in the above manner and the values recorded in a register for future reference.
Effective earth resistance of the station
After measuring the resistance of individual electrodes, reconnect all earth leads including interconnection of earth electrodes. Now measure the earth resistance at the outer most electrode, driving the auxiliary electrodes in the outward direction. The value so measured gives the effective earth resistance of the station.
Points to note
· The test electrode and the auxiliary electrodes shall be in a straight line.
· The spacing between the electrodes shall be approximately equal.
· The auxiliary electrodes shall be driven to approximately the same depth and the depth shall be very low compared to the spacing between electrodes.
· The tester shall be cranked at the specified and uniform speed.
Acceptable limits of earth resistance
The acceptable limits of earth resistance values for various systems are given below:
7.3.4 Earth Continuity Test
Non-current carrying metal parts of equipments, control gears and devices are provided with duplicate earth connections to ensure effective equipotential bonding with the earth bus and thereby to the earth electrodes. Duplicate earth leads are provided to ensure that failure of one lead does not result in the disconnection of the equipment from the earthing system. In order to ensure the effectiveness of the protective earthing system, it is necessary to test the continuity of various earthing conductors in the system. The test procedure for earth continuity test is the same as that for the measurement of earth electrode resistance. The earth tester is set ready with the auxilliary electrodes driven at 15 m. and 30 m. from the outer most earth electrode of the earthing grid. The common terminal of the earth tester is connected to a long flexible copper cable. The other end of the cable is connected or held tight to the body of the equipment under test. The tester is now cranked to the specified speed and the reading noted. The reading shall be very low and near to the combined earth resistance of the system. Repeat the test for all equipments and devices connected to the system.
A high value of earth resistance in the earth continuity test is an indication of loose contact in the terminations/joints or a break in the earth leads/conductors. A thorough check shall be carried out to locate the fault and corrective action taken.
7.3.5 Measurement of Earth Loop Impedance
When a line to earth fault occurs, the fault current shall have a value sufficient to discriminately operate the protective devices. The value of the fault current is determined by the impedance of the closed loop available for the fault current to circulate. The earth loop impedance includes the impedance of the line conductors, fault, earth continuity conductors, earth leads, earth electrodes etc.
Measurement of earth loop impedance is of greater importance in the case of HT and EHT installations where system earthing and equipment earthing are connected to the same grid or bus. Connections for measurement the earth loop impedance is shown in fig. 7.5. When HRC fuses are used to protect the circuit, approximately five times the rated current of the fuse is taken as the minimum required current for fast clearing of the fault. When protective relays are used, a fault current of around two times the setting of the relay is considered the minimum required current. The measured value of earth loop impedance shall be low enough to produce the above fault currents. The earth loop impedance may be measured at different levels of distribution i.e. at the fag end, DB level,
SSB, MCC, MSB etc.
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