TESTRING, INSTALLATION AND COMMISSIONING OF LT METERS

Er. K.V.Surya Prakasa Rao

The meters are required to. be tested to ensure proper working so that the accuracy is maintained within permissible limits under I.E. Rules. The meters are to be checked and calibrated with various adjustments. As per Indian standard specification-722 (ISS-722) a meter should pass the following two types of tests

• Type test.

• Routine test

Type Test

These tests are carried .out by the manufacturers at the National Testing Laboratory or at any approved testing Laboratory. Test certificates are to be submitted to the purchaser. Utilities also depute a representative from time to time for random selection of three meters from a lot for testing in the laboratory in the presence of the representative.

1. Insulation Resistance TestThe insulation resistance with a 500 V megger, between meter current circuit, voltage circuit, both coupled together and meter body shall have a value not less than 5 Mega ohms.

2. High Voltage TestThe insulation between current circuit and potential circuit and with respect to body shall withstand application of 2000 volts (R.M.S.) for one minute.

3. Creeping Test In creeping test with no current in the current coil and with rated frequency, the rotor of the meter shall not make a complete revolution at any voltage between 80 percent of the lowest and 110 percent of the highest rated voltage.

4.. Starting Current TestThe rotor of a meter shall start and continue to run when meter carries the following currents at maximum value of voltage range and U.P.F.•Percept _of basic • Current•(A) Meter with dial and pointer type to register 0.5 percent•(B) Meter with cyclometer type to register 0.75 percent•(C) Meter with M.D. Mechanism and 1.0•Cyclometer type to register

5 Limits of Error TestPermissible error of the meter at rated voltage, frequency for single phase and

three phase meters are given below Single Phase Meters

(2.5/5 maximum & 10/20 maximum Amps.)Load current PF Errors2.5 percent of full load 1.0 ± 2.5 percent 5 percent to 100 percent of full load 1.0 ± 2.0 percent 5 percent of full load 0.5 lag± 2.5 percent 10 percent to 160 percent of full load 0.5 lag± 2.0 percent

Three Phase Meters Load current PF Errors5 percent of full load 1.0 ± 2.5 percent to

-3.0%10 percent to 12.5 percent of full ,load 20 1.0 ±2.0 percentpercent to 125 percent of full load 0.5 lag ± 2.0 percent

6. Power Loss TestThe loss in the meter voltage circuit and current circuit shall not exceed the following values of the rated values :

Rated value Power loss Max.Voltage circuit loss Up to 240 V 1.5 wattsCurrent circuit loss Up to 10 A 1.5 watts

10A 50A 2.0 wattsabove 50 A 4.0 watts

7 Temperature Rise Test

With each current circuit carrying the maximum current for which the meter is specified to carry continuously and with each voltage circuit carrying 1.2 times

the rated voltage, the temp. rise of winding of current circuit of the meter shall not exceed 50°C over the steady value at 40°C ambient temp. In case of

windings of bare copper or enamelled copper, the temp rise shall not exceed 60°C. The temperature rise is to be measured by change of resistance of one of the current coils at the meter terminals.

8. Energy Register TestThe energy registering mechanism register shall be capable of integrating

continuously, i.e., before passing again through its starting position, the number of kWh corresponding to 2500 hours of service for 1 phase meters and 1000

hours for 3-phase meters at rated values. Reverse running stopper shall not effect the speed of rotor at 5 percent load current at rated voltage and U.P.F. by more than 1.5 percent.

9.Change of error due to Variation of Voltage A variation of 10 percent above or below the rated voltage at any current from

100 to 20 not cause a change of more than 1 percent in the error of meters.

percent of the rated current at U.P ,F. shall

10. Change of error due to Variation in Frequency

A variation of 5 percent above or below the rated frequency shall not cause the charges of error more than (a) 1.0 percent at rated voltage, U.P.F. and basic current (b) 1.5 percent at Rated voltage, U.P.F. and 20 percent basic current, (c) 2.0 percent at Rated voltage, 0.5 P.F (Lag) and basic current.

11. Effect of Heating by Main CurrentThe change in registration of meter shall not exceed the limits as follows and total error in rate of registration should not exceed permissible limits of errors.

Percentage basic of PF Percentage change in rateRegistration

100 1.0 1.0100 0.5 lag 1.5200 1.0 2.0

12. Temperature Coefficient TestThe meter shall not have a temperature co-efficient greater than 0.1 percent per

degree centigrade at the Rated voltage and at the following points calibrated at 272°C and valid between limits of 0 to 40°C ambient temperature.Percent Basic Current PF100 1.010 1.0100 0.5 lag

13. Effect of Oblique Suspension TestA change in 3 degree in level, in any direction shall not change the error of the meter by more than 1 percent at the basic current IJ.P.F or by more than 3 percent at 5 percent basic current and U.P.F.

14. Effect of External Magnetic Field : ,

The change in the percent error of the meter caused by an external field of 0.5 MT (5 gauss) produced by a current of the same frequency as that of the voltage applied to the meter shall not exceed 3 percent at basic current rated voltage and U.P.F

15. Range of Adjustment Test Facilities for adjustment for the purpose of calibration shall be capable of further movement or adjustment.

(a) Brake magnet adjustment should provide at least 4 percent increase and 6 percent decrease in speed at rated current, voltage and U.P.F.

(b) Low load adjustment should provide at least plus and minus 3 percent change in speed at 5 percent rated current voltage and U.P.F.

(c) Inductive load adjustment should provide on each energy element plus and minus 1 percent change hi speed when me meter current circuit of the element along is carrying rated current at 0.5 p. F. Lag.

(d) Torque balance adjustment should be provided so that speed of elements may be controlled separately and the meter so adjusted that each element contributes its true position of the total registration when the system load is unbalanced, the device for making adjustments shall be such that adjustment may be effected gradually and the sense of adjustment being suitably indicated and device shall be capable of being . locked in position after adjustment.

16. Independence of Adjustment Test(a) In low load adjustment 2 percent adjustment shall not effect the speed at 0,5 p.F at 50% rated

current by more than 8%(b) In inductive load adjustment - 1 % adjustment on energy element shall not effect the error by more

than 0.5% at 5% of rated current

17 Continuous Current TestThe meter shall not be injured by current of 200 percent of the basic current applied continuously and its errors shall remain within permissible limits.18. Effects of Short Circuit Test A meter shall be able to carry for 0.5 sec. a current equal to 30 times the basic current when this does not exceed 10 A and 20 times the basic current when this exceeds 10A. The variation of error shall not exceed 1.5 percent.

19. Sustained Accuracy Test Initial test for accuracy shall be carried at the rated voltage, frequency and following loads and percent error shall be noted.200 percent basic current U.P.F100 percent basic current U.P.F10 percent basic current U.P.F

After initial test the meter shall be run continuously at 200 percent basic current at U.P.F. for 1000 hours. On completion of this run, the meter shall be retested and percent error shall not differ by more than 1 percent from the error of meter under initial test

II. Routine TestThe routine-tests are carried out as a routine by the manufacturers and in the laboratory of the Board. The routine tests carried out in the board's laboratory are:

1. Long period dial test.2. Non-registration with voltage along-creep test.3. Starting current test.

2 and 3 have already been described.The long period dial test is done with the help of a R.S.S. (Rotating sub-standard) meter.The current coils of the meter under test and R.S.S. meter arc connected in series, while potential coils are connected in parallel. The meters are started and stopped simultaneously after connecting to the same load for a fixed period of say half an hour then

Percent error = (R - A) / R x 100

R = Energy recorded by kWh. meter under test

A = R.S.S meter

The revolutions of both the meters could also be counted. The procedure is to reset the R.S.S. to zero and start the meter (under test). As soon as the disc-spot of this meter reaches the zero position, the R.S.S. is started with a snap switch. The number of revolutions made by R.S.S. for a chosen number revolutions of the test meter are noted thenPercent error = ( Calculated Rev. of R.S.S.- observed Rev. of R.S.S)/Observed Rev. of R.S.S

If meter constants are

Test meter 300 revs. / Kwh. Ratio 1:2

RSS 600 revs./ Kwh

When Test meter makes 10 revs. R.S.S. meter makes 20. If R.S.S. revs, are observed as 19.5 then

Percent Error = 20-19.5/19.5 x 100The error is positive and the meter is recording fast ± 3 percent is the limit of variation. It is preferable to keep the error positive. The comparison could be made for load and P. F. conditions.

Annual loading and phantom loading methods are adopted to test the meters under different load conditions.

Actual Loading MethodThis method is employed generally for testing of single phase meters in laboratory. On a meter testing bench which is equipped with various tappings to facilitate meter testing at different load conditions. The loan is provided by standard resistances or lamps obviously 'powers factor is unity. 1/10th F.L. and F.L. adjustments are done at U.P.F.It is important to note in this connection that consumers P.F. varies from 0.5 to unity (or 85). Therefore meter must be calibrated at 0.5 P.P. 50 percent load to achieve this condition the current coil is energized by R phase and voltage coil by Y phase and the polarity of pressure coil is reversed. Under these conditions p.f. becomes 0.5 because of 1200 phase shift between voltage and current vectors.

Phantom or Fictitious Loading MethodWhen the capacity of meters .under test is very high testing with actual loading method would involve a considerable Joss of power and not economical too, therefore, to avoid wastage of power during testing, phantom loading method is adopted, on this method pressure circuit is supplied from a circuit of normal voltage source and the current circuit from a separate low voltage source. Thus total power consumed during testing becomes very small due to small pressure coil current at normal voltage plus load current at very less voltage for current circuit meter under test and R.8.S. act as a load on phantom loading kit. This point is illustrated by following two circuits: (a) Normal LoadingPower consumed in pressure and current circuit = (220 2/ 8800) + ( 220 X 5) = 5.5+1100 = 1105.5 W. (b) Phantom Loading Power consumed in pressure and current circuit = (2202/8800)+ (6X5) = 5.5 + 30 = 35.5 WFrom Equations (1) and (2) it is obvious that with phantom loading power consumed is very small as compared with normal loading.

Installation and Commissioning of Meters I. Apart from testing and calibrating the meters they must be properly installed, as per the connection diagram. After installation of meter its performance should be checked at site also. If we take 100 watt lamp (resistive load and time constant of energy meter is 2400 rev/ kWh. Assuming normal supply voltage the consumption in one hour will be 100 watt hours. The matterdisc should therefore rotate. (2400 x 100) / 1000 = 240 rev. / hour Therefore tune taken for one revolution is 60x60/240 =15 seconds If the time taken is more, the meter is running slow and vice-versa if the rime is less. Suppose 16 seconds instead of 15 seconds are taken then the meter is slow by(160-15) x 100/15 = 66.6% It should, therefore, be adjusted.For synchronous motors power factor can be taken unity. For Induction motors it is 0.7/0.8 and for welding transformer it is about 0.6. The motors contribute inductive load in the system.

II.C.T, Operated MetersFor C.T. "operated meters:1. Ratio test by primary injection kit and

2. Polarity test should be carried out.It should be borne in mind that C.T. Secondary terminals should not be

kept open..Following points must be kept in view while installation of meters:

(a) Position of meter should be such that reading is easily visible.(b) Mounting of meter should be on solid wall or D.P. structure are on

panel board.(c) For H.T. metering equipment, CTs should be right on the incoming

side before the print of isolation.(d) Meter should be fitted in protected meter box, nowadays provided

by the Board, It should be installed in a place where rain-water should not enter.

(e) The meter box has facility of sealing from outside. It should therefore be sealed.

(f) The meter-recording should be checked at site as explained.

Meter Reading I. Registering or counting mechanism is to record continuously a number. This

recording is proportional to the revolutions made by the moving system. The rotor shaft drives a series of five or six pointers. These rotate on round-dial which are marked with ten equal divisions. There are two types of registration system: (a) Pointer type ;,(b) Cyclometer type. Reading could be made from both the types of meters.

II. C.T. Type MetersIn case of C.T, operated meters the multiplying factor should be calculated. It depends upon ratio, meter rating and dial factor. An example is given below(a) (i) Meter rating 100/5A 400V

(ii) C.T.ratio200/5A400V(iii) Dialfactor48Overall M.F = [(C.T Ratio) / meter C.T rating] x Dial factor= [(200 / 5 x 400 ) / 100 / 5 x 400 ] x 48 = 96

The performance of the meter should be checked on load after installation and commissioning. It should be ensured for correct recording applying multiplying power-factor, which Could be noted on paper. This could be pasted in the meter box and one copy be kept in file/ledger. Many disputes have arisen due to this omission. If there is delay by the consumer in connecting the load, the commissioning of meter could be deferred till-the consumer starts drawing power.

Routine Tests of L.T. Meters and Various Checks/tests while in Service

All single and three-phase meters are tested in the Testing Laboratory after their receipt in the area stores from the manufacturers. Similarly disputed meters where consumers suspect that their meters are recording excess consumption are also sent to the laboratory for testing after the consumers deposit the testing charges for such contested meters.

In addition, routine checking and testing of old meters is to be carried out periodically to ensure their proper working and accuracy. This is (he requirement of LE; Rules.

Type tests are carried out on prototype specimens at National Testing Laboratories. No meters are marketed till Type tests are got done. Routine tests are carried out in the Board's laboratory and by the manufacturers.Adjustments in the Laboratory

Adjustments are carried out in energy meters at various load and power factor conditions in the laboratory I so that they read correctly and their errors within allowable limits. The sequence of these adjustments is as follows:

(1) Preliminary Light Load AdjustmentsThe disc is so positioned that the holes, are not underneath electromagnets and rated voltage is applied to the potential coil. The light load adjustment device is adjusted until the disc just falls to start When the meter carries minimum values of current as shown in starting current test.

(2) Full Load AdjustmentIn energy meters braking torque is developed by means of eddy currents induced in aluminium disc which moves in the air gap of a permanent (brake magnet) magnet Due to continuous motion of the disc there is a continuous braking action. The current in the disc interacts with the flux Of permanent magnet and produces a braking torque which is directly proportional to product of current, flux and effective radius R.With rated supply across pressure coil, full load current at UPF is passed through the current coil braking torque is adjusted hi two ways to ensure that the meter revolves at the speed within the permissible limits of errors. . .(a) By Rotating the Brake Magnet about its Axis of Fixation; By angular movement of the brake magnet about its axis of fixation, the effective radius from axis if disc is varied. Therefore with decrease of the radius R braking torque will decrease and meter becomes fast(b) By Linear Movement of Magnetic Shunt: Normally brake magnets are provided in pairs with unlike poles adjacent to each other. This arrangement provides definite advantage because eddy currents produced by this arrangement take shorter path through the disc. This results in lower resistance and hence larger braking torque is produced.The distance of iron magnetic shunt (in the form of screw) from (he magnets is varied by taking it away from the magnets or near to the magnets and to by pass lesser or greater portion of the flux.If the magnetic shunt is brought near the poles it will bypass larger amount of flux, so the disc will cut through a smaller amount flux resulting in reduced braking torque. So the meter becomes fast. On the other hand if it is moved away from the magnets, it will bypass lesser amount of flux giving a larger braking torque. The energy meter under this condition becomes slow.

(3) Inductive Load Adjustment or L% Adjustment or Power Factor AdjustmentThe deflecting torque of an energy meter is created by die interaction of two magnetic fields produced by the current and potential which causes the disc to move thereby. This torque is proportional to the product of fluxes and to the sin of phase shift angle between them.The arrangements for adjusting the mmf of the Lag Coil are:

(1) Adjustable Resistance . A few turns of fairly thick wire are placed around the centre limb of shunt magnet and the circuit is closed through a low adjustable resistance. The resistance of this circuit is altered to adjust the lag angle of flux.

(2) Shading Bands. In this arrangement copper shading bands LI are placed around central limb of shunt magnet instead of a lag coil with adjustable resistance. In this case the adjustment can be done by moving the shading band along the axis of limb. As, the shading bands are moved, up to limb they

embrace more flux.'(3) Lag Plate ;

In some meters a single turn lag coil consisting of a punched lag plate is used. The material and cross, section of the lag plate are such that appropriate values of impedance and mmf are obtained. The lag plate is situated in the air gap directly beneath the central limb of shunt magnet and the Jag angle is adjusted by moving the plate either readily with respect to the axis of the disc or parallel to the axis of the disc so that it links more or less shunt magnet flux.

' ; Thus its induced voltage and consequently current and mmf are altered.

(4) Low-Load Adjustment Despite every care is to be taken in the design, friction errors are liable to be serious particularly at light loads. Therefore, in order to ensure accurate registration at low loads it is necessary to arrange for a small torque which is particularly independent of load o& fee-meter and act in the direction of rotation. This torque is approximately equal to the friction torque acting in opposite direction of rotation.It is obtained by means of a small shading loop (or vane) situated between centre pole of shunt magnet and the 'disc and slightly to one side of the centre line of the pole. The interactions between the portions of the flux which are shaded and unshaded by this loop and the currents induced by these fluxes in the disc results in a small deriving torque whose value van be adjusted by the lateral movement of the loop. This lest for correct adjustment is free from serious errors at light load and starting currents other methods of non-symmetrical shading are also in use. 'In meters where lag plate is used for power factor adjustment the additional torque to compensate for the friction can be produced by displacing the plate in a direction parallel to the direction of motion of disc.

During this adjustment 5 percent or 10 percent F.L. current at UPF is passed through the meter current coil and the rated supply voltage is applied across pressure coil adjustment is done till meter runs at correct speed.

(5) Anti creep AdjustmentAs a final check on light load adjustment the pressure coil is energised by 110 percent of rated voltage with zero load current. If light load adjustment is done correctly the meter should not creep under these conditions.Creeping is continuous but slow rotation of meter at no load when only the pressure coil is energizedMain cause for creeping is over compensation for friction of the compensating device (low load adjustment is adjusted to give a driving torque to compensate for starting friction which is sufficiently higher than running friction meter disc will tend to run in the same direction (creep) at no load because the friction compensating torque is independent of load current. The other causes for creeping are, vibrations, stray magnetic fields and excessive voltage across the potential coils.Creeping is undesirable so it is prevented by two ways.

1. By two Diametrically Opposite Holes. Two diametrically opposite poles are drilled in the disc. The disc comes to rest with one of the hole under the pole of voltage

magnet thus the rotation being limited to a maximum of 1800. When hole comes under the edge of poles circular eddy current paths in the disc are distorted, the effective, centre of eddy current paths is shifted from A to 'A", which is the central point of the equivalent magnetic pole produced by the currents. The polarities are such that there is a resulting force on the disc tending to move A' away from the pole axis A. Thus as soon as hole reaches a position near the edge of a pole further movement of the disc is opposed by the torque described above. The magnitude of this torque is insufficient to affect the action of meter on load-

2. By two Magnetic Vanes. One small iron piece is attached to the spindle and 2nd below voltage coil so when pressure coil is energized it is attracted by vane provided

on voltage coil or when magnetic vane-comes near to brake magnet it is attracted by the iron piece attached on the spindle and creeping is stopped.

In some meters back stoppers are also provided either in the rotor shaft or counting train is blocked in the -ear operated mechanism

(6) Torque Balance AdjustmentThis is provided on poly phase meters only to confirm that each element may be controlled separately because in some cases one common disc is used in place of three separate discs. It is therefore, necessary to check the operation of each element when it is carrying load current to ensure that none of the element is having negative torque, which cans .happen even when the overall torque is positive. The meter is adjusted in such a way that contribution of each element is true to the total record when the system load is unbalanced.

Regulating screws are provided on each voltage core for torque balance which controls the proportion of useful voltage coil flux cutting the rotor.

In case of two element energy meter (3 ph.3 wire type) torque developed by each element is equal only at unit p.f. at. other p.f. conditions torque produced by the two elements are not equal and are of trying proportion. The torque produced by R element being more at lagging p.f. and that by 'R' element at leading p.f. conditions.In addition to normal compensating device attached to each element an adjustable magnetic shunt is provided on one or both elements to balance the torques of the two. The necessary adjustment is made with the coils energized from a single phase supply. The pressure coils are connected in parallel and the current coil series in such a manner that the torque produced by the two elements opposes each other. The magnetic shunt is adjusted to a position, where the two torques are exactly equal and opposite so that the meter does not RotateWith three-phase supply set on the test bench kit p.f. is kept unity with the help of phase shifter and the connection of one of the current coils of Rand B phase element is reversed, necessary adjustment is made until the meter fails to rotate.

In case of three-phase four wire meters contribution of each element is equal to the total torque developed by the meter. Therefore during torque balance adjustment, one element is disconnected and connections of 2nd element reversed. The meter should not rotate under these conditions because torque developed by one element is balanced by the other. This is true at all p.f. conditions.

Repairs of Minor Defects in the LaboratoryFollowing main defects are generally found in meters : . , ,

1. Rotor shaft found bent. 2. Counting train found defective.3. Rotor disc found bent.4. Pressure disc has discontinuity.5. Meter excessive slow fast, even after an adjustment.'6. 6. Upper guide pin broken. 7. Current coil short with body.8. Wrong name plate meter constant as the revolutions of

meter does not tally with constant 9. Demagnetized brake magnets.10.Broken glass plates.

• Out of new meters received from area Stores, defective ones are replaced free of cc by supplier and for carrying out repairs of old meters collected from field by area store repairing is being carried out at meter, repairing section by canalisation or by providing spa parts required. After due repairs meters are sent to L T meter testing for calibration, it is very necessary when electromagnet gaps have been distributed or when any component is replaced

•• First apply the rated voltage to each element in him for

3ph. meters without any current in current circuit and adjust the low load adjuster to a point where the rotor disc becomes stationary. If this adjuster is moved in either direction the disc should start to creep

The routine testing of meters could be once in 3/5 years. In addition, the consumer installation and meters should be checked from time to time surprisingly. The important points to be paid attention while checking are

(i) Connections of meter are proper.(ii) Seals are not tampered.(iii) No loose or direct connection exists.(iv) Meter is not damaged to record less.(v) CTs are properly connected. No reversal of any phase connection exists. Multiplying factor is properly calculated and applied, (vi) Meter is well-protected from rain, vermin and dust etc.Links are connected.viiiTerminal block is not damaged. ix It may also be mentioned that with the help of load current of known magnitude (preferably of resistive type) and a wrist watch having a second hand, one can off-hand check at site the performance of the meter as also its accuracy to some extent, as indicated below:

Let the resistive load be a bulb of 40 Wand the time constant of the energy meter 120 Rev /kWh. Assuming normal supply voltage, the consumption in one hour is 40 watt hour. The meter disc could therefore rotate

@ 1200 x 40 71000 = 48 Rev./hour. Say for 1 Rev. the disc would take (60x60)/48=75 seconds, , ,If the time taken is more, it means the meter is running slow and vice-versa. In -fact, the percent error can also be calculated from the time it actually takes. In this particular case, if the time taken is 78 sec, the meter is slow by

(78-75) / X 100 /75=3%If the load current is not resistive but inductive or capacitive, the power factor of the current is to be assessed first depending upon the nature of the load, and the energy consumption worked out. For example, a 10 HP flour mill drawing 10 Amps, current at 400 volts can be presumed to be running at 0.8 power factor and having √3 x l 0 x 400 x 0.8 = 5.54 kW load, for the purpose of such checking.

Installation and Testing of Meters with C.T. & P.T.

In order to increase the range of an ammeter shunts are generally used. A low resistance shunt is placed in parallel with an ammeter to measure current than that which the instrument itself can carry. Similarly in order to increase the range of a voltmeter, a resistance (with negligible inductance) is used in series with the instrument. This type of arrangement is used in multi-range testing instruments,A very general method of increasing range of A-C. instruments is to use instrument transformers with ammeters, voltmeters or watt meters etc. The transformers are either current transformers or potential transformers, which are respectively used to increase current or voltage range of instruments. The advantages of use of instrument transformers are as follows :

(i) Single range instruments can be used to cover large current or voltage range. The usual ranges of ammeters and voltmeters are 5 amperes and 110 volts. .

(ii) The indicating instruments can be located away from the H.T. circuit and do not require insulation for higher voltage.(iii) A split core C.T. with primary winding on which secondary winding is also wound can be used to measure the current in heavy current bus-bar without breaking the current circuit.

Installation of MetersFor L.T. Consumers, where the demand of load is 50 amperes or more, it is normal practice to use energy meters with current-transformers. When the demand is more than 100/150 HP, L.T. supply is not given but the consumer has to avail H.T. supply. H.T. supply should be given at 33 kV and higher voltage by extending line from the sub-station so that independent feeder is available to the industrial consumer. The feeder may not be governed by the rural discipline. Hence interruptions would be rare. Extension of supply at 11 kV should be avoided.

Most of the H.T. Consumers prefer two part tariff. They are required to be billed for maximum demand also and units consumed. Hence poly phase meters are provided with maximum demand indicator to record the demand in kVA / kV, over a fixed period (half an hour) each time, when moves with the help of a time switch. The pointer for maximum demand is left behind recording the consumption and is reset every month after taking the reading.

Proper Selection of Meters .For accurate metering it is necessary that meter normally operates at the higher ranges where its accuracy is better. To achieve this CT ratio is properly selected. It is recommended that the CTs used are of single ratio, as far as possible, and the ratio selected are in accordance with the following table to have better working torque for the meter

S. No CT Ratio Suitability for max. loading in kV A (With 20% overloading of L T) (A) For supply at 11 ::1. 5/5 Amps UptolI5kVA 2. 7.5/5 Amps 115 to 150 kV A 3. 10/5 Amps 150 to 225 kVA4. 15/5 Amps 225 to 340 kV A 5. 20/5 Amps 340 to 450 kV A6. 25/5 Amps 450 to 570 kV A

(B) For supply at 33 kV 1. 2.5/5 Amps Up to 225kVA2. 5/5 Amps 225 to 345kVA3. 10/5 Amps 345 to 675 kV A4. 20/5 Amps 675 to l350 kVA5. 30/5 Amps 350 to 2100

kVA6. 50/5 Amps 2100 to 3450

kVA7. 75/5 Amps 3450 to 5200 kV

A8. 100/5 Amps 5200 to 6900 kV

A9. 150/5 Amps 6900 to 11300

kVA10. 250/5 Amps 11300 to 14000

kVA

• It may be said as a guideline that while' selecting CT ratio the same should be related to 80% of the ultimate contract demand that may be expected in about a year's time of the commissioning of the consumer. As the current transformers are designed for 20% overloading, the CT would be adequate where the load exceeds 80% of the contract demand. It may be prescribed that the CT ratio should be checked in relation to the maximum demand once every year, or whenever there is increase in the contract demand.

• It may be added that whenever metering on L,T. side is provided for H,T. Consumer, 3 percent of the energy recorded is added for the transformer losses. It is further to be ensured that a 3 phased-4 wire meter is only installed.

Testing of MetersBefore installation of meters with CTs and P.T.s, testing has to be carried out. The metering equipment has to be tested for1 . Correct polarity of C.T.S.2. Continuity test of primary and secondary windings.3. Ratio test by primary injection kit.4. Insulation resistance.5. Terminal markings as per name plate details.After the above checks, special checks to be applied for, are as follows :

I. Multiplying FactorSince C.T. and P .T. ratios have a direct relation with the consumption to be recorded by a meter, proper working-out of the multiplying factor has to be carried out, unless the dial of a meter is originally calibrated for the C.T.s and P.T.s to be used therewith. The multiplying factor is given by (C x P) / C1xP1) where C and P are ratios of the C.T.s and P.T.s actually used, while CI and PI are ratios of C.T.s and P.T.s for which the meter is calibrated. Then to the energy consumption = ( W1 x C x P) / C1xP1where W1 is energy consumption as recorded by the meter.

Suppose W1 = 1000 unitsC = 200/5 ampsP = 33000/110 voltsC1 = 10015 ampsP1 = 11000/110 volts

Then Wl = ( 1000 x 33000 / 110 x 200 / 5 ) /11000 / l10 x 100 / 5 The actual consumption = 6000 units.Similarly for MD., the multiplying factor would have to be applied.

(II) Power Factor Check .,(A) Speed of kW or. disc

(a) Both PT fuses of Rand B Phases cut in-Seconds/Revolution = T1 .(b) R phase PT fuse cut-out—Seconds/Revolution = t 1(c) B phase PT fuse cut-out—Seconds/Revolution = t 2

(B) Speed of kVA hr. disc for both Rand B phase fuses in circuit—Seconds/Revolution =

Then(i) P.F. = cos [ tan -1 T 1 / T2 ] ……......(1) (ii) Also with (b) and (c) conditions under (A) P.F. = cos [tan-1 ( t 2 – t 1) / ( t 2 + t 1) ] ……… (2)

The result from both the equations should be the same and tally.

(IV) Power Check(i) Energy can be calculated by line current, line voltage and P.P. worked-out

under II x time.(ii) Energy calculated under (i) should be equal to the recording by kWh disc

when P.T. fuses of both Rand B phases are in circuit.The above tests are to ensure that H.T. meters are properly installed and tested. It is to ensure that the potential and current coil connections are on the same phase: phase sequence of die meter is correct so as to give cumulative effect of loads on all three phases.

For C.T. and P -T. operated meters, it is preferred to use a test terminal block (TIB) for C.T., P.T. and meter wiring. This TIB provides the facilities of shorting CT secondaries for testing purpose without any interruption to the consumer. The C.T. leads should be directly connected to the meter while PT supply to the meter should be connected through the fuses of proper rating. To eliminate the possibility of tempering with the fuses. The fuses should be provided with protective cover and properly sealed.

Now a days pilfer box is provided for this type of metering equipment.. It should be used so that no outside agency can temper with the equipment, (details in Annexure). Since the meter records correctly in vertical position, the box should be welded to the structure to avoid changing of position.

(V) Testing and Checking of HT Meters at SiteAfter installation and commissioning of meters or during routine testing of H.T.

Meters, it must be ensured mat same phase C.T. and P.T. are entering in one element of meter and phase sequence at the meter terminals is correct. Following checks are made after charging on supply:

1 . Voltage at meter terminals2. Phase sequence at meter terminals3. Current on each phase should be approximately equal for three phase-load4. Element wise rotation of disc5. Load test, it should tally with the connected load of consumers,

If results are satisfactory M.F. meter details reading etc, should be noted and consumers signature should be taken on result 'sheet. ;The meters of H.T. consumers may be tested at the following periodicity: i1 . H,T. Consumers having - After every 3 months demand of more than 25 MW2. H.T. Consumers having - After every 6 months demand between 5 and 25 MW 3. H.T. Consumers below 5 MW - After every one year

Electronic Tri-Vector Meters and L.T. MetersDigital electronic meters are now used for installation at S/s and consumer's premises. Due to display system, tampering with meter is difficult. Being electrostatic, the life and accuracy is expected to be better. The meter is described with various new features in the earlier chapter. In addition to the above tests the visual features are to be checked so that the meter cannot be tampered by outside agency.

Effect of Wrong Connections Effect of wrong connections on energy registration is different under different omissions and errors. These are shown in a tabular form for information and adoption so there is no loss of revenue and proper demand could be raised to cover the loss due to the error/omission.

• (iii) Checking of Connection• SI Element kWh meter

RkvAh meter• No• (i) If 'R' element is connected to the Disc should rotate in

Disc should rotate in• supply along forward direction

forward direction• (ii) If 'B' element is connected to the Disc should rotate in

Disc should stop• supply alone forward direction• (iii) If both 'R', 'B' elements connected to Disc should rotate in

Disc should rotate in• supply forward direction

forward direction• The rotation of disc on element-wise, depends on the power factor of the

installation. The above analysis is for the P.F. = 0.866. This has been explained with the help of phase or diagram as shown in Annexure.