Chapter-15
TESTING AND COMMISSIONING OF
ELECTRICITY ENERGY METERS
15.1 ENERGY METERS COMMISSIONING.
A short note on Commissioning as applied here serves as a key to open this chapter,
in order to signify its importance and simultaneously to literate the reader being “Testing and
Commissioning” person, to feel his responsibilities. Elaborate discussion on testing of energy
meters will then be followed after this brief glimpse.
Commissioning is carried out to testify the incorporated metering scheme along with
proper working of the associated wiring and ancillary equipment and to validate it.
Commissioning is considered to be essential when set-up was laid down initially or any
change or alteration has been incorporated subsequently at later stage, in existing metering
scheme or wiring re-routing. Testing and Commissioning Engineer certifies that the scheme
is appropriate to the objective and test it thoroughly, in order to certify the correctness of
associated wiring and intervening protection, healthiness and also declare that the accuracy
class of metering confirms to the manufacturer’s specifications. He also certify that the
complete lay out also confirms to “as built drawing’ which will be the replica of schematic
diagram. After certification, the case has been put up for approval from the competents
concerned for energization. Alternatively, commissioning is concerned about, complete
functionality of the metering scheme along with all the necessary peripherals, fully prepared
and kept as ready to energize.
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15.2.1 ENERGY METER TESTING.
The testing of electricity energy meters for commercial work has been standardized
initially by the provisions of the Electricity Supply (Meters) Act, 1936 in the United
Kingdom, encompasses different clauses covered under B.S.S (British Standards
Specifications) commonly known as B.S.S.
.
Similarly Working on the parallel platform, later on, International Electro-Technical
Commission (I.E.C) regulate and standardize, among other, The “Testing of the Electricity
Energy Meters” Act 1956 introduced in France, subsequently revised and reached in present
shape in 1986 under S.I units (stands for Standards de International or International
Standards) and now accepted in all the Europe, commonly known as S.I Units. For example,
Accuracy Class: Active Energy 0.2S or 0.5S (IEC 62053-22), Reactive Energy: Class 2, 3
(IEC 62053-23), calibrated up to 0.5 %, Apparent Energy: Calibrated up to 0.5 % etc. and lay
down different provisions regarding testing of the electricity energy meters.
The Electricity Commissioners (in the United Kingdom) issued a series of papers*,
which contain details of the methods of testing and should be employed and the approved
apparatus which should be installed in meter-testing stations / labs for the purpose of such
tests. These papers should be studied by the reader who is interested in the subject of energy
meters testing. Due to scarcity of space, only few of the more important provisions can be
given here,
* E.g. Electricity Supply (Meters) Act, 1936 : approved apparatus for testing stations, etc.; Explanatory Memorandum concerning the Testing of Electricity Meters; The apparatus approved for use in Meter Testing Stations, etc.; Further Explanatory Notes concerning the Testing and Certification of Electricity Meters, etc.: Supplementary Notes in Amplification of the Explanatory Memorandum, etc, published by H.M. Stationery Office, United Kingdom.
15.2.2 APPROVED APPRATUS
Dealing with the approved apparatus first, this is listed as follows:-
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Standard Apparatus
D.C. Potentiometer.
Standard Testing Set of desired Accuracy Class. i.e. 0.5, 0.1 or 0.02 etc.
A.C. Rheostat for having Proper input voltage to the testing set.
Substandard Apparatus
Indicating Wattmeter.
Rotating Meter.
Phase Sequence detector.
Ammeter.
Voltmeter.
Clamp-On Multimeter
Electrolytic Ampere-Hour Meter.
Time Standard
Ship’s Chronometer or Pendulum-Type Clock.
Time Substandard
Stop watch or some other timing device.
Tools Required
Screw driver set.
Precession Screw drivers set.
Pair of pliers and nose pliers.
Wire Stripper.
Screw driver type tester.
Banana plugs, crocodile clips, thimbles (sufficient stock).
Connecting wires, (ample stock).
Three Phase / Single Phase extension boards with earth wires.
Soldering iron with stand holder, and soldering wire / Rasin.
Hand Drill Machine with steel/ concrete / wooden bits.
Air Blower, Cleaning brushes, Cotton swabs.
Grease, WD-40 for loosening of rusted parts / nut & bolts.
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Contact cleaner.
Emery / Sand paper.
It is laid down that the rheostat and potentiometer shall be tested at intervals at the
National Physics Laboratory, and that the other apparatus shall be tested at regular intervals
of time (i.e. Annually, or bi-annually) against some standard apparatus or against substandard
indicating instruments at stated intervals.
Specifications are given for the approved apparatus, and these include constructional
requirements and allowable accuracy limits.
15.2.3 PERSONNEL PROTECTIVE EQUIPMENT (P.P.E)
As per international standards, “P.P.E” : Personnel Protective Equipment is as
essential as breathing is to life. No job is permitted without the wear of the properly
maintained P.P.E. This is for the individual betterment and concern about overall safety and
simultaneously lay downs favorable working environment for all, and can be attributed as
hereunder.
The priority level and work methodology should adhere to;
1. Personnel safety.
2. Equipment safety.
3. Then Comes the Work.
The objective of safety is to provide such a means as to protect the individuals from
any possible hazard and provide a healthy working environment to get maximum output from
the limited and highly expert human resource.
Then comes the equipment safety : Provide such a safe working practices as to protect
the costly equipment against any possible damage. By protecting the equipment, two ultimate
objectives are attainable:
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1. Suppress the wide spread catastrophe, so that major collapse of the system could be
averted, giving rise to stable continuity of service and hence better consumer
confidence.
2. Could save heavy financial and time constrained loss, which improves upon the cost
(minimized) and minimizing the planned outages.
Having all of the foregoing practices, there comes the execution of work. As every
individual have strived to the noble cause of providing a shield against hazards, he actually
endeavor to better service quality and to improve upon building consumer confidence.
Additionally every one feels the responsibility, protecting the environment, by saving against
the hazards, and reduction in material processing (as mass of the universe is constant, so
appearing new things merely is the fast material processing i.e. simply decomposing and
recomposing).
Now Came to the essentials to P.P.E (Personnel Protective Equipment):
Helmet: To provide coverage to head against falling / hitting objects.
Pair of Gloves. : Protects hands against mess and stresses.
Pair of Goggles. : Protects eyes from foreign bodies.
Chest Shield. : Protects chest against impacts.
Jute Matting. : To provide electrical insulation while on work.
Safety Shoes : Steel toed, safety shoes, protects feet from fallen heavy Loads.
Harness Belt : To provide safe overhead working.
15.3 SAFETY SLOGANS :
Here are few Safety Slogans; they are worth it, on the subject of Safety. These
Quotes, emphasis the importance of Safety, at our work place, daily lives and all
around us.
It is really a life.
Simply: Walk like you are on a slippery surface.
Additionally: Safety never takes a holiday.
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Safety Slogans in English.
Chance takers are accident makers.
Courtesy and Common Sense Promote SAFETY.
Do your work with pride, put safety in every stride.
Don't be a fool, cause safety is cool, so make that your rule.
Don't be safety blinded, be safety minded.
To avoid a scene keep your work place clean.
Working without safety is a dead end job.
Ignoring a warning can cause much mourning.
The greatest gift you can give your loved ones is to come Home Safely.
My job provides my paycheck, but SAFETY takes me Home!
Reduce Incidents With a Steady Diet of Safety!
Winter, Spring, Summer, Fall: Play it safe throughout them all!
The freedom to choose....choose to work safely.
If someone trip, he will be not in safe grip.
Wear Safety Gears, Protection Everywhere.
People don't Plan to Fail.........They Fail to Plan!
Safety doesn’t slow the job down but mishaps do.
Safety is our mission, Not an intermission.
Walk like you are on a slippery surface.
The right shoes will save your toes, may be your Life!
Hands are my friends along with Policies and Procedures.
Remember! Don’t Dare Take Care, Observe Safety Everywhere.
There is no face like Your Own, wear face Protection.
Safety is a cheap and effective insurance.
Safety never takes a holiday.
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Safety Slogans in Urdu.
!سیفٹی کو اپناؤ، خود کو حادثات سے بچاؤسیفٹی میں غیر محتاط، زندگی میں حادثات
ااسان سیفٹی سے ہر کام، ترقی کی منزلیں ہو جائیں ممشکل کام کو مات ایک ہی سوچ ایک ہی بات، سیفٹی سے ہر
سیفٹی میں سرمایہ کاری، زندگی میں لائے خوشحالی
Courtesy: Mr.Sohail, Manager Electrical,
Engro-Gen, Ghotki, Sindh, Pakistan.
5.4 METHODS OF TESTS ;
15.4.1 ELECTRO-MECHANICAL ENERGY METERS.
For motor meters, three methods of test are dealt with, as described hereunder;
Method A;
Long-period dial tests, using substandard rotating meters.
Method B;
(a) Tests (other than long-period dial tests) using substandard rotating meters.
(b) One long-period dial test.
Method C;
(a) Tests by substandard indicating instruments and stop watch.
(b) One long-period dial test.
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It is laid down that “Method C alone is to be used for testing direct current motor
meters.”
All meters are to be tested (a) at the lowest percentage of their marked current
specified in the limits of error for meters of their class under the electricity (Supply) Acts; (b)
at one intermediate load; and (c) at the highest percentage of marked current specified in the
limits of meter.
In the case of a.c meters, they are also to be tested at marked current and marked
voltage at 0.5 power factor lagging.
Watt-hour must also be tested for “creep” by applying 10 percent overvoltage to the
voltage coil, the main circuit (current-coil circuit) being open. The meter must not run under
these conditions.
It will be observed that the testing engineer has the liberty of choice between the
above mentioned three methods of test (in a.c. meters), but all three methods involve at least
one dial test.
Numerous sorts of the tests of electricity energy meters are applicable in market.
From manufacturer to end users, these tests are to affirm assurance that the behavior of these
meters remain satisfactory under prevailing market conditions and to ensure their endurance
under toughest of operating conditions. Apart from all these assurances, majority from the
lots get defective and provides way out to manufacturer and applicator (end users) to sit
together in order to fix the reasons behind these failures and improve upon the quality, to
have a customer satisfaction. Despite these challenges, the Testing Engineer has given the
responsibility to sort out these type of problems in the field and should have a competency
level of such an extent, that he should be able to convince and tackle the stakeholders, in
order to provide viable solution to these problems.
However, these tests are mainly classified under two sub-sectors.
1) In-House or Laboratory Testing:
a) Also covers In-Door Testing, Production Testing, Quality Control Testing
commonly known as FAT (i.e. Factory Acceptance Tests).
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b) With-Stand Tests: Electrical, Mechanical / Thermal and Environmental Tests,
such as High Frequency interference with-stand test, Dielectric strength
with-stand tests, Shock / Impact with-stand tests, Thermal stress with-
stand tests, Temperature, Humidity and Rain with-stand tests.
c) Preventive Maintenance Tests / Routine Tests, etc.
Such as Dial Tests, Creepage Tests and Accuracy Tests, either off-load or
on-load tests, etc.
2) Out-Door Tests / Field Tests
a) Routine / Preventive Maintenance Tests.
Bi-Annually or Annual Tests.
Such as Dial tests, Creepage tests and Accuracy tests either off-load or on-
load tests, etc.
b) Sector Specific Tests.
To cover up subsequent difference of recording (after commissioning),
between Primary and Back-up Metering due to errors introduced by
instrument transformers i.e. C.Ts and P.Ts, and to fix the problem. i.e. to
adopt a compensation factor or to add the compensation resistors in C.Ts
and P.Ts circuits.
Or
Drastic change in recording by the energy meter has been observed.
Or
Improper recordings of some measurands which demands re-testing and
for fixing the problem (i.e. improper recording of the reactive energy
when import of active energy is too low on a high tension, less loaded,
longer line or too low a power factor results in high penalties to
consumers) i.e. re-programming the energy meter, for recording of
reactive energy quadrant wise for proper reactive energy recording.
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Or
Summation Metering or data-logger requirements may arise, when multi-
feed in and out arrangement is there for the dispersal of power at the
interconnection point i.e. grid station of the power house.
* 2b above applies generally to digital energy meters.
Though there is no sharp line of demarcation between these tests, their suitability
and provision is solely detrimental to Pre-facture / utility policy and how much utility
wants to invest-in, on testing and commissioning phases of the metering. As a guide line,
where un-visible financial fragmentation has been carried out in electrically integrated
infrastructure, the pricing of energy quantum transaction is important rather than the
cost of metering facilities. So emphasis should be on the recording of energy
transactions, but not on the cost involvement on lay downing the metering structure ,
in this way, financial disputes ( worth billion of rupee) can be reduced among different
utilities and smooth operation of the system is possible.
Alternate Classification ;
Another classification for testing of the energy meters is as under:
D.C Energy Meter Testing.
A.C Energy Meter Testing.
Active Energy Meter Testing.
Reactive Energy Meter Testing.
Special Purpose Meter Testing.
15.4.2 SOLID STATE / DIGITAL ENERGY METERS ;
OPENING A DIALOGUE TO DECIDE WHICH WAY TO ADOPT; .
With static / digital energy meters, the manufacture of components, previously
under the control of the manufacturer in respect of electromechanical designs, is largely in
the hands of specialist electronic components manufacturers.
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This means that to ensure the requisite, high reliability, energy meter manufacturer
must under take extensive quality control tests on electronic components before they are
used. With the rapid advancement of semiconductor technology and the widespread
acceptance of the static energy meters, the problems of the design, manufacture and testing
of these energy meters necessitated increased attention.
The use of integrated circuits, firstly linear operational amplifiers, then digital
gates and logic, followed by large scale digital integrated circuits, memories and
microprocessors, accentuated the problems of quality control of components and
production tests methods of complex electronic assemblies.
As the volume and complexities of static energy meters outgrew, the early
production quality control and testing methods were become obsolete and automatic
programmable testing equipment was introduced to ensure effective production
methods.
Production testing and quality control techniques have two main objectives;
i) To maximize in-service reliability.
ii) To minimize production costs.
To minimize production costs: it might be supposed that the best procedure would
be to test only the complete equipment. This would be so if every equipment passed its
test, but failing this, there would be the problems of diagnosing and rectifying some
inevitable failures in extremely complex equipment. Further, failure of one component or
one assembly error may cause consequential failure of other components or assemblies,
so magnifying the cost of failures and complicating the task of diagnosing the failure. It is
generally held that the cost of rectifying a failure increases by a factor of ten for every
stage of assembly.
A philosophy of testing of each production stage is therefore followed.
Although repeated testing will increase the reliability of the product, it is necessary to
build in additional procedures to eliminate early life failures.
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These are usually in the form of “burn-in” or “Heat Soak” tests. Parameters of the
components are examined for drift, which is taken as an indication of possible early life
failure, even though the devices may not have actually failed at the conclusion of the test.
Automatic testing procedures have become essential with increasing production
volume, variety and the technical complexity of equipment. Equipment is designed so that
printed circuit boards are complete functional entities in order that they can be tested and
fault diagnosed at each stage. This is done by means of software tests and diagnostic
routines. Calibration takes place at the module test stage, followed by a heat soak at 70o C.
15.4.3 WHAT THE MICROPROCESSORS HAVE DONE:
GAVE BIRTH TO SECOND GENERATION OF ENERGY METERS:
SOLID STATE / DIGITAL ENERGY METERS;
Microprocessors are extremely attractive to use in Energy Metering Schemes to
replace hard wired contact logic used in remote metering. The hardware of digital energy
meters comprising input and output interfaces, central processing unit (C.P.U), memory,
clock and power supply can be standard design, type tested to meet all environmental
requirements. A new scheme can then be programmed into this standard hardware using a
program held on a separate EPROM (Erase-able Programmed Read Only Memory) chip used
as memory unit. Often several schemes can be held in one energy meter as to record / register
the energy dealing of another meters as summation metering in place of data-loggers, where
the in-out arrangements of Transmission lines at one Grid Station / Power House necessitates
the Net Energy Measurements (Import and Export) by means of a modem (modulator and de-
modulator) and a solid state switch (Thyrister, Triac, Diac or Power Transistors) for
incorporating different schemes. As the program runs continuously, scheme of this type can
be largely self-monitoring and arranged to give an alarm, should the program cease to run
correctly.
Although analogue to digital converters can be used with microprocessors to perform-
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-analogue measurements, the speed of processing can introduce problems with high speed of
measurements in digital energy meters. Simple microcomputers were developed with simple
analogue to digital converters on the chip and a small on-chip memory which is mask
programmed to a consumer’s unique requirements by the processor manufacturer. This
requires large scale production of Energy Meters.
Peripherals to the Microprocessors
The input circuit comprises of an interposing transformers in the meter, the out-put of
which is full wave rectified. The out-put is fed into a group of current setting resistors,
selected by five setting switches. The voltage output from the current setting resistors is used
as a signal for the microcomputer and at the setting current, the voltage is the same for all
current settings.
`The signal processing circuit comprises an eight bit microcomputer which (1)
converts the incoming signal into a digital count, (2) accepts inputs from the instantaneous
current setting switches, (3) process the input signals and initiates measurements by
microprocessor and storage of appropriate data in appropriate registers i.e. Energy, Mw,
Event logs called as registers.
The output is provided through attracted armature elements of solid state relay. Each
relay is fitted with one change-over contact (Normally close contact) and one normally open
contact. These relays are controlled by the microcomputer out-puts buffered transistors.
Measurement operations are indicated by blinking LED,s (Light Emitting Diodes) for active
and reactive energy respectively which are similar to rotational disc in electromechanical
energy meters. Availability of appropriate three phase voltages and currents to the energy
meters are indicated by L1, L2, L3 indications on the LCD (Liquid Crystal Display) display of
the energy meter. The digital energy meter holds data (energy and load with respect to, preset
time intervals i.e. half hourly) in different registers. One can have the requisite data either
manually or through auto-scrolling feature of the display or either can down-load the data
through a computer and a print-out can be had from. The down-loaded data can be stored as
soft-form on C.D (Compact Disc) or hard-form as a print-out chronologically via printer.
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Digital meters have three sort of displays, namely;
1. Manual read-out of requisite data from the L.C.D display.
2. Auto-Scroll feature of the L.C.D Display.
3. Computerized down-loading of the requisite data.
A Soft Copy or A Hard Copy versions.
Data types for digital energy meter are also of three types, namely:
1. Basic Measurands like individual phase voltage, current, frequency, Power Factor etc.
Commonly termed as Grid Status.
2. Derived Measurands like energy in Mwh, Power in Mw from energy or Mw logs.
3. Additional logs, if necessary or where required, such as event-logs, which are like
chronological histogram of the events, like meter cover opening, abrupt change in
grid condition: C.Ts currents, P.Ts voltages changes appreciably. System frequency
variations, power factor variations are also recorded in event-logs by the latest type of
energy meters.
This data provides any significant change in inputs to the energy meter which
drastically affect the recorded energy by the energy meter. For example any input phase
voltage missing due to operation of intervening protection, meter will record energy almost
33 % less than it would be with all the phase voltages available. Similarly any possible
change in the C.T circuit, either at terminal points or at the Martialling-Kiosk affects the
proper energy recording by the meter.
One important precaution while programming the energy meter, encompass the minor
change in C.T / P.T ratio at software level, which will be amplified by instrument
transformers. Example being a P.T having a actual ratio of 132 KV / 110 Volts give rise to a
value of 1200, but if software has been accidentally / mallifiedely fed as 132 KV / 100 Volts,
a value of 1320, energy meter would record 1320-1200 /1200 X 100 = 10 % more energy i.e.
110 % energy instead of 100 % energy. This is very crucial and such sort of incident should
not occur. Event log detects the happening of such an event (if mallafied) and provides
sufficient clue to approach the default. Consideration for C.T ratio miss-feed can similarly
affect the proper recording of the actual energy, which should also be avoided. To counter
such a mallafied intentions, access to energy meter should not be given to third party alone or
to any single party alone or double party. Such sort of access should only be permitted in the
presence of at least three members including the stack holders.
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The energy meter programming is at different levels, and instrumental transformer
data is being fed at initial stage with date and time setting format. Usually ordinary data cable
can be employed for such sort of programming and the availability of such sort of data cables
can not be restricted / guaranteed to authorize personnel only.
15.5 BASIC TESTS OF ENERGY METERS ;
15.5.1 Initial / Speed Test of the Energy Meter
First consider the electromechanical energy meter. You simply need a Chronograph
i.e. a stop watch and a calculator and paper, pencil. For all the electricity energy meters, their
name plates carries, its dial n = revolutions of the disc for 1 Kwh of the energy is being
provided like 70 rev / Kwh, 400 rev / Kwh etc. For the time being we will consider 400 rev /
Kwh. It means that rotating disc of the electromechanical energy meter completes 400
revolutions in an Hour ( 60 min X 60 seconds in each minute of the time = 3600 seconds), if
its connected load is 1 Kw or meter disc will complete 400 rev in half a hour, if its
connected load is 2 Kw or conversely we can say that meter disc will complete 200 rev in
half an hour, if its connected load remains the same i.e. 1 Kw. Similarly it will complete 800
revs in an hour time, if its connected load will be 2 Kw.
It will be time cumbersome to measure the meter speed for a complete hour. Instead,
calculate for smaller interval of time, the number of disc revolutions for some specified
connected load as follows:
400 rev / Kwh = 400 rev / Kw during one hour
Or
400 rev / Kwh = 400 rev / hour, for a connected load of 1 Kw.
Switch on 10 X 100 Watts lamps only or connect up 1000 watt heater element as a
load for, say 5 minutes of time.
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Than,
400 rev / Kwh = 400 rev / hour, for a connected load of 1000 watt (i.e.1 Kw)
400 rev / h, for 1 Kw connected load = 400 revs / 3600 seconds
Now 400 revs / 3600 seconds of time, for 1 Kw connected load,
400 revs in 3600 seconds for same load,
Hence, 1 revolution will take = 3600 sec / 400 revs = 9 seconds
For 1 minute of time, disc will complete 60 / 9 = 6.66 rev.
Alternatively, for 50 revolutions of disc to complete, you may need this much of
time: 50 x 9 = 450 seconds = 450 / 60 = 7.5 minutes.
So observe the time taken to complete 50 revolutions of the disc, with connected
load of 1 Kw. If meter disc takes lesser time as compared to calculated one, than
meter will be fast and vice versa i.e. If disc takes longer time than calculated time,
meter will be slow. Percentage fastness or slowness can easily be worked out as
follows;
% Slowness 0r Fastness = Time calculated - Time observed X 100
Time calculated
Percentage slowness if answer is negative, and percentage fastness if answer is
positive. If answer is zero, it means energy meter is measuring perfectly.
Also, work-out the energy consumption during above test / experiment by noting
initial and final energy readings. Then comparison between calculated and recorded
energy will give the rough idea about meter accuracy.
Similarly digital energy meters do have impulse rate i.e. Imp / Kwh instead of rev /
Kwh, as no moving parts are their in these type of energy meters. If Imp / Mwh are given on
dial (usual values ranges from 0.01 to 20 Imp / Mwh) than convert it to Imp / Kwh by
dividing Imp /Mwh by 1000. On the energy meter dial, two blinking LED represent Imp /
Kwh or KVArh for active and reactive energy respectively.
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Likely value ranges from 500 to 100,000 Imp / Kwh and Imp / Kvarh each. As more
energy will pass on through a meter, blinking will go fast and will be slow with less energy
passage through the energy meter accordingly. A visual inspection of the blinking L.E.Ds.
(Light Emitting Diodes), which provide a rough idea about how much energy passage will be
there, through the meter.
15.5.2 ACCURACY TEST OF THE ENERGY METER;
Accuracy Test on energy meters, usually employ a comparison with some Standard
Energy Meter of higher accuracy class, of the meter (of lower accuracy class) under
consideration. The accuracy test can be performed in two ways i.e.
- On Load Accuracy Test.
- Off Load / Phantom Load Accuracy Test.
15.5.2-1. On-Load Accuracy Test.
This test is easier to perform and takes less time to complete it. The only limitation is
that, load and power factor can not be varied as in phantom load test method, unless load
current remains constant.
Meter under consideration remains connected in the original circuit and working as it
is. Only the input quantities are taken for standard meter, from the meter under test, in such a
way that its performance can not be hampered. For taking current: clamp-on C.T are
employed, the out-put of these C.Ts can than be fed to standard meter. As for as the P.T
voltages are concerned, there is no need of disturbing the voltage circuit of the meter under
test, simply these voltages (three-phase, four-wire) are tapped from voltage terminals of the
meter under test and can be fed to standard energy meter. The data from the energy meter
under test is provided by a scanner head, to the standard energy meter. A meter constant (C.T
ratio X P.T ratio X Impulses / Kwh) relating to meter under test need to be fed to the
microprocessor. Sufficient time is being allowed to make the energy measurements by both
of the meters.
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The standard energy meter than compares the recordings over a specified
period of time (say 05 minute interval) by both the meters and its built-in
microprocessor calculates and displays the percentage error of the meter under test.
15.6 D.C ENERGY METERS TESTING
15.6.1 PERCENTAGE ERROR EVALUTION TEST
A- REVOLUTIONS EVALUATION METHOD
All supply meters of the motor type have a meter constant marked on its dial. This
constant is expressed as ampere-seconds per revolution or revolution per kilowatt-hour. The
full load current and line voltage for which meter is intended are also stated. From these data
the number of revolutions per minute which the meter should make when tested with a
certain fraction of its full load can be calculated. The number of revolution per minute which
it actually does make when tested at this load is then observed and the error (if any) deduced
therefrom.
Thus, if K is the constant of the meter, the number of revolutions per minute which it
should make when tested at some fraction a, of its full load is given by:
K. a W
60
Where W is its full load in Kilowatts.
Following example will illustrates such a test :
Example:
For a 20 amp, 230 volt meter, the number of revolutions per Kwh is 480. If, upon test
at the full load ( 4,600 watts), the disc makes 40 revolutions in 66 seconds, calculate the
error.
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Solution:
In 1 hour, at full load, the meter should make 4.6 X 480 revolutions.
This corresponds to a speed of 4.6 X 480 = 36.8 r.p.m
60
Thus, the correct time for 40 revolutions to complete = 40 X 60 = 65.2 Seconds
36.8
Time taken by disc to complete 40 rev – correct time for making 40 rev
i.e. 66 - 65.2 = 0.8 Seconds.
Hence:
Time taken is 0.8 Sec too long,
And the meter under test is 0.8 X 100 = 1.2 per cent slow.
65.2
As an alternative to the above method of testing, a standard watt-hour meter is often used
instead of timing a definite number of revolutions of the meter under test with a stop watch.
B- STANDARD WATT-HOUR METRE METHOD TEST
In such cases, the current coil of the standard instrument is connected in series with
that of the meter under test with and their voltage coils are connected in parallel. This ensures
that power operating the two meters is exactly the same. The standard meter records single
revolutions and is readable to a small fraction of a revolution. An arrangement is also
incorporated so that the register of the standard instrument can be started and stopped
instantaneously.
Specifications for such rotating standards are given in the Electricity Commission’s
paper on “Approved Apparatus”.
The procedure, after adjusting the load to the required value, is to allow the meter
under test to make a certain number of revolutions, and to observe the number of revolution-
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- made by the standard instrument in the same time.
If the constants of both meters are the same, the error in the meter under test can then
be obtained directly.
If the constants are unequal,
Let Ku = No. of revolutions per Kilowatt-hour for the meter under test (nominal value)
Ks = No. of revolutions per Kilowatt-hour for the standard meter.
And
Nu = No. of revolutions made in a certain time during the test by the meter under test.
Ns = No. of revolutions made by the standard meter in the same time.
Then, the energy indicated by the test meter in the time of the test is:
Nu (Kwh)
Ku
And that indicated by the standard meter is:
Ns (Kwh)
Ks
These should obviously be equal if the meter under test is correct,
if not, the error is given by:
Nu - Ns
K u Ks X 100 per cent.
Ns
Ks
or
{Nu X Ks - 1 } X 100 per cent.
{Ku Ns }
In order to ensure that the meters have attained a steady temperature by the time the
test is made, it is necessary that they shall be connected in circuit, with the load on, for some
15 to 30 minutes before making the test.
15-20
The advantage of this method of test, as compared with the method using a stop-
watch, are that errors introduced by the latter are avoided and also that variations in the load
during the test time affect both meters equally and are therefore unimportant.
15.6.2 DIAL TESTS
In addition to the short-time tests described above, it is usual ( and is required by the
Minister of Power U.K in commercial testing) to carry out a “dial” test extending over an
hour or more with the load maintained constant at its full value, in order to check the gearing
and that the readings given by the recording train are correct.
This test gives an opportunity of discovering errors due to self-heating, and it is usual,
at the end of such a test, to repeat the short time test at full load to investigate the effect of
this heating upon the rate of revolution of the disc.
15.6.3 OTHER TESTS
The British Standard Specifications states that a meter should start at one-hundredth
full load and also that it should not run when a voltage of 10 percent over the normal is
applied, the load current being zero. Tests must be made, extending for at least for ten
minutes, to ensure that meter complies with these requirements in addition to those regarding
errors for which the foregoing tests are carried out.
15.6.4 “PHANTOM OR FICTITIOUS LOADS” TESTS
When the capacity of a meter under test is high, a test with the ordinary loading
arrangements would involve a considerable waste of power. To avoid this, ”Phantom” or
“Fictitious” loading is employed which consists in supplying the voltage circuit with the
required ( normal ) voltage, and the current circuit from separate low voltage supply. This
means that the total power supplied for the test is that due to small voltage-coil current at-
15-21
- normal voltage plus that due to the load current at a low voltage ; and the total power
supplied is therefore small.
If a high capacity meter is to be tested whilst in service, the voltage coil is supplied
from the line in the normal way ; but the current coil is removed from the consumer’s load
circuit and replaced by a short-circuiting connection. The current coil is then supplied from a
battery or other low voltage source for purposes of testing.
15.6.5 STABLE D.C SUPPLY
In meter testing stations, a fairly high-voltage d.c supply, of good stability, is
essential. It replaces high-voltage accumulators (battery banks). The voltage supplied can be
of any value between 25 and 600 volts at upto 120 mA, with a short term stability of 1 in
50,000 and a long term one of 1 in 10,000. A variety of d.c stabilizers have been developed
from battery bank versions popular during early stage of electricity meter testing in 1940’s.
During world war-II (1942-1945), significant milestones have been achieved by electronics
industry because of valves and tubes, so is the stabilizers. Just after that, semiconductor
technology have taken place during 1950’s, against thermionic valves, but the rapid
advancement in semiconductor technology gave birth to solid state voltage stabilizers.
Development of “Microprocessors” turns the way again, its use encompasses almost every
field and become heart of the automated and digital stabilizers from 1980’s onward. The chip
technology evolution gives another turn to electronics and presented what is called as
modular structure. Modular design facilitates testing individually i.e. at each module stage
and saving in space and replacement time, should any fault arose at any stage. Unfortunately
we will see shortly “Micro-Circuitry Disposable Technology” where one can not see the
individual components, instead one will get million even billion of components-features
fabricated on a single chip and the circuitry be, of the shape of “Piece of a textile”. Simply
plug-in, play, plug-out, discard and replace with a new one technology is on its pace and we
will find it sooner.
Having drastic change in design and fabrication of D.C Supply over the past few
decades, better quality power supplies are in the laboratories worldwide. A range of a.c
mains operated power-supply units having output voltage variable upto 500 volts are-
15-22
-available in the market. The voltage stability is 1 part in 50,000 with a ripple of less than 1
mV r.m.s. Output currents upto 500 mA (0.5 Amps) can be provided for field use and upto 1
Amp for the laboratory purposes. Improved Harmonic and ripple control circuit designs have
been incorporated with the passage of time, experience as a result of technological
advancement. However need verses cost should have justified.
15.7 A.C ENERGY METERS TESTING
In testing a.c meters, it is not necessary the voltage and load current, but in addition,
the relative phases of the two quantities must be altered; in other words the power factor of
the load must be variable also. Variation of power factor may be brought about by making up
the load circuit with variable resistors, inductors, and capacitors, and adjusting these as
required. This method is, however, inconvenient in most cases, since it involves the waste of
the power in the load and necessitates several fairly expensive pieces of apparatus, these
objections / reservations being particularly apparent if the capacity / rating of the meter is
high.
For this reason the load employed for testing purposes is usually a fictitious one, as in
the case of d.c meters testing. Provisions must, of course, be made for varying the phase of
the current in the current circuit relative to that of the voltage circuit, in addition to the
variations of the magnitude of the current.
The magnitude of the load-current can be altered by changing the out-put current of
the phantom-load set, by varying the knob position calibrated in ampere with fractional
marks (The Fine-Setting). Usually the phantom-load set has two levels of output current-
ranges i.e 10 amps and 100 amps (The Course-Setting). The meter under test must be
connected to the appropriate current-level range of the phantom-load set. As far as fine-
settings are concerned, load current can be adjusted in steps from 100 percent to 50 percent,
25 percent and 10 percent of the full-load rating of the meter under-test while testing is being
carried out.
15-23
i- Either by using, as supplies, two similar alternators coupled together and driven by a
motor, with the stator of one of them capable of rotation through any desired angle relative to
the position of the stator of the other. The arrangement is such that one alternator supplies the
voltage-coil circuits of the meter under test, and the other supplies the current-coil circuits, a
step down transformer being used to obtain the (possibly) heavy currents required for the
current coils. By rotating the alternator stator supplying the current, using a worm-wheel and
a graduated circular scale, the phase can be altered relative to that of the voltage. The angle
through which the alternator stator is moved, gives a measure of the phase angle between
current and voltage. The frequencies of both the current and voltage are, of course, identical,
since the two alternators are direct-coupled.
ii - By using a phase-shifting transformer. When this method is being employed, instead of
motor-alternator set, the stator must be supplied with two or three phase current, in order to
produce rotating magnetic field, or, if the supply is single phase, a phase shifting device( may
be phase-shifting transformer) must be used. The phase of the current induced in the rotor of
the phase-shifter can then be altered as required by rotating through the appropriate angle.
The magnitude and power factor of the load having been adjusted according to the
requirements of the test, the errors of the meters under test can be obtained either by timing a
certain number of revolutions-the load being measured by an indicating watt-meter or
comparing their rates of revolutions with that of standard watt-hour meter connected so as to
measure the same load.
If the power factor adjustment of a meter has been made at some high value of the
power factor, so that the error of the meter is zero, no error, due to power factor variations,
will be observed at the lower power factor. If, however, the adjustment has been made so that
the error is (say) 0.5 per cent at 0.8 power factor, it will be found that the error due to this
cause at 0.1 power factor is more than 6 percent. This will be explained by a reference to the
correction curves given in Fig-1 below :
15-24
Fig-1 : Correction Factors.
15.7.1 SINGLE PHASE ENERGY METERS
Following figure Fig-2, shows the connections for the testing of the single phase
energy meter X (Meter under test) , using two coupled alternators A1 and A2 for the supply
to the voltage- and current- coil circuits respectively. The stator of A1 is capable of rotation
for phase variation. A2 is connected first of all, to step-down transformer T whose secondary
then supplies the current-coils of the two meters X and S, the latter being either a substandard
indicating watt-meter or a standard watt-hour meter, according to the method of test
employed. The variable resistor R is for the adjustment of the current in the current-coil
circuit, V and A are a voltmeter and ammeter for the measurement of the requisite
measurands i.e. load voltage and current for evaluation of power. A phase shifting
transformer may, of course, be used instead of the two alternators. The “equalizing lead” as
shown, is necessary when a fictitious load is used, in order to ensure that the potential
difference between the voltage and the current-coils of the meter is zero, as it is when they
are in service. As implied in Fig-2, the link between voltage and current-coils in each meter
must be disconnected for this test.
15-25
Fig-2 : Connection for Testing 0f Single Phase Energy Meter.
15.7.2 POLYPHASE ENERGY METERS
Fig-3: below, provides a diagram, illustrating connections for the testing of the
three-phase three-wire energy meters.
Fig-3: Connections for Testing of the Three-Phase, Three-Wire Energy Meter.
15-26
In this case there is a three-phase supply. This is used for two purposes: (a) to
supply the stator of the phase shifting transformer, (b) to supply three step-down
transformers, two of whose secondaries supply the current coils of the meter. A
regulating choke coil is connected in the primary circuits of these three transformers. The
rotor of the phase-shifting transformer supplies two auto-transformers having variable
secondary tappings. These supply the voltage coil of the meter, the latter consisting of a
double standard wattmeter and one or more meter to be tested. These are connected as
shown. A1, A2, and A3 are ammeters and V1 and V2 voltmeters, for adjustment of the
load.
15.7.3 METERS USED FOR SPECIAL PURPOSES
a- Pre-Payment Meters :
In addition to the form of supply meters described above, there are several special
form of meters, the use of which has been made necessary by the various existing
methods of charging for electrical energy i.e. “Pre-payment Metering”. Thus, there are
“pre-payment meters”, which are fitted with a coin box and a prepayment mechanism,
and are arranged so that only a limited amount of energy can be obtained after the
insertion of the coin. Such maters are used in premises, where owner ship of small
portion within a compound or complex, may be rented for short times i.e. where the
consumer changes frequently ; guest house and a hotel bed-room.
Essentially, these are not a special type of meter from the point of view of
principle of operation, but consists of mater of one of the types described above, in which
is incorporated a device which causes the load and meter circuit to remain open until a
coin is inserted in the mechanism, thus closing a switch in the meter, which automatically
opens the circuit again after a certain definite amount of energy has been supplied
subsequent to this insertion.
Many different type of mechanisms are employed by the various manufacturers,
in order to produce the operation required. These do not, however merit individual
description here. The principal requirements of these meters are :-
a- The switch regulating the opening and closing of the circuit must
make good contact and must operate correctly after an amount of
energy corresponding to that prepaid for has been supplied.
15-27
b- The meter must be proof against interference by any but authorized
persons.
c- The coins inserted must not jam.
d- The meter must be capable of any easy adjustments in the case of
alteration of the price per unit i.e. Rs. / Kwh.
e- The manipulation by the consumer must be of simple nature.
f- Allowance must be made in the operating mechanism for the fact
that all the coins inserted will not be of same weight and thickness
g- The number of coins which have been inserted should be indicated.
Figure-4: .Illustrates the internal connections of one type of pre-payment
energy meter.
Fig-4: Internal Connections of Pre-Payment Energy Meter.
15-28
b- Maximum Demand Indicators :
The chief requirement of these indicators is that they shall record the maximum
power, or rate of using energy, taken by a consumer during any particular quarter , the charge
for the quarter then being based upon this maximum demand, as well as the total number of
units consumed. In order that momentary heavy demand shall not be taken into account, and
the consumer penalized, it is also necessary that the indicator shall only record the average
power over successive predetermined periods, such as half an hour. Such indicators may
either take the form of a separate instrument, or may be in the form of an attachment to a
motor- or clock-type meter of ordinary kind.
c- Write Maximum Demand Indicator :
This was one of the earliest versions of demand indicators to be introduced. It
indicates the maximum current taken by a consumer load, the voltage (and power factor, if
used on a.c.) being neglected. The principle of this instrument is illustrated by the line
drawing of Fig-5 below.
Fig-5: Line Drawing of Write Maximum Demand Indicator.
15-29
The instrument is essentially a differential air thermometer. It consists of a glass U-
tube whose upper ends are closed by two bulbs of similar capacity, as shown. One of these
bulbs is surrounded by a metal heater strip through which the current in the supply circuit
flows heating the air inside this bulb as it does so. Near the upper end of the other limb of the
tube, and just below the bulb closing it, is a side tube leading to a narrow-bore index tube
alongside which is a scale. The U-tube contains a liquid with a small coefficient of expansion
–which acts as an indicator. When the indicator is set, initially, the right-hand limb of the
tube is filled with sulphuric acid upto the level of the branch tube. Acid is than added until its
level in the index tube is at zero mark on the scale.
Now, when current flows through the heater, the air in the bulb which the heater
surrounds expands and causes the sulphuric acid in the right hand limb of the U-tube to
overflow into the index tube. The greater the current, the greater is the expansion of the
liquid, and, therefore, the greater the overflow to the index tube. After the heater has cooled,
the current in it must subsequently reach a higher value than any previous one if any further
overflow into the index tube is to be obtained. The level of the liquid in the index tube gives,
therefore, a true record of the maximum current which has passed through the instrument
during any given period. The constrictions and traps in the limbs of the tube are for the
purpose of preventing the passage of air from one bulb to the other when the indicator is
reset, the latter operation being carried out by tilting the instrument and allowing the
sulphuric acid from the index tube to drain back into the main tube.
Momentary heavy currents do not cause the indicator to register, owing to the length
of the time taken for the temperature of the heater to rise and also on account of the low
thermal conductivity of the tube which it surrounds.
d- Price and Belsham Maximum Demand Indicator :
This instrument is shown in Fig-6: below, which shows the constructional details. The
instrument is thermally operated, its movement consisting of two matched, flat coils of bi-
metal strip - the actuating coil and a compensating coil - the former being surrounded by a
heater element carrying the load current (or a definite fraction of it,
15-30
Fig: 6 Price and Belsham Maximum Demand Indicator
obtained by the use of the shunt or current transformer). The two coils are coupled together at
their outer ends, the inner end of the actuating coil is being fixed and the inner end of the
compensating coil carrying the needle. The movement of the needle depends upon the heat
produced in the heater, and hence upon the load current, and the needle carries forward with
it a maximum demand pointer which remains fixed, by a special friction arrangement, at the
maximum reading.
Through the use of the two matched coils the readings are unaffected by changes of
ambient temperature.
e- Merz Price Demand Indicator :
This indicator is not in the form of separate instrument, but is a fitting which can be
used with any type of motor or clock meter to indicate the maximum consumption during a
half hour or other period throughout a quarter of a year (or other periods between consecutive
re-settings of the instrument).
15-31
Unlike the two preceding instruments, this indicator can be used to record either
maximum current or maximum power.
Fig:7 MERZ PRICE Maximum demand indicator
The principle of operation is as follows: A separate dial is fitted, inside the
instrument, the pointer of which is driven by the spindle of a moving system of the meter in
the ordinary way. After this dial has been in gear for the certain period-usually half an hour-a
device comes into operation which brings the mechanism of the disc back to zero position.
The pointer, however, does not return to zero but is lightly held by a special friction device
and continues to indicate the number of units consumed in previous half hour period. The
position of this pointer will not be moved forward unless the number of units consumed in
some subsequent period exceeds the number recorded by the pointer. In this way the
maximum demand, in units per half-hour, for any given period of time, is obtained.
The device for returning the driving mechanism of the demand dial to zero at the end
of each half-hour period is operated by clockwork inside the meter or by a separate “time-
switch” external to the meter. These “time-switches” consist of a clock, either electrically or
hand wound, which makes and breaks certain contacts, at predetermined intervals of time.
These intervals depend upon the setting of the switch, which can be altered as required.
15-32
An advantage of this demand indicator is that a heavy demand must be existent for an
appreciable time for it seriously to influence the number of units consumed in so long a
period as half an hour, and, at the same time, short-period heavy demands are not neglected
by the device.
The construction of a Merz pattern maximum-demand indicator, as manufactured by
Messrs. Landies & Gyr, is shown in Fig-7 above. Normally the pin (7) drives forward the
pointer (8) for (say) a half-hour period, and the energy consumed by the connected load,
during this time is indicated on the dial. At the end of the period the cam (14) momentarily
disengages the pinion (12) by means of the bell-crank (13). This allows the pin (7), with its
driving mechanism, to return to its zero position under the action of the spring (5). The
pointer is how-ever, left stationary and continues to record the energy consumed during the
previous half-hour period.
During the next half-hour period the pin (7) is again driven forward, but the pointer is
only moved forward if the energy consumed in a subsequent period exceeds that consumed in
all preceding periods.
Fig:8 Poly Phase energy meter with Max.demand indicator
15-33
Fig:9 Connections of A.E.I Poly Phase Meter with Max.Demand indicator.
Fig,s: -8 and - 9 shows an Associated Electrical Industries poly-phase meter with
maximum demand indicator, and the diagram of connections of such a meter, respectively.
Hill Shotter Maximum Demand KVA Indicator :
Another instrument which is now quite commonly used in connection with power-
factor tariffs, is the Hill Shotter maximum demand KVA indicator, which is manufactured
by Messrs. Aron Electricity Meters Ltd. One type of this instrument is really an a.c. ampere-
hour meter, and is of the induction type, having a shaded pole and a continuously driven disc.
A permanent magnet is used for breaking. It is used with a timing device of the Merz pattern,
and records the maximum ampere-hours consumed during successive time intervals of 15
minutes. The dial is marked in KVA corresponding to the voltage of the circuit on which
15-34
indicator is to be used. The instrument readings are independent of the power factor,
and the frequency, within such variations of the latter as might be expected in the normal
supply system the theory of this indicator is fully discussed by G.W.Stubbings {Reff : Paper.
(4)}.
f- Summation Metering :
When a number of circuits, on an electrical supply system, work in parallel, the total
energy supplied to them is often required. This total can be obtained by addition of the
individual totals for the circuits so that special metering for the purpose is not essential. The
measurement of the total maximum demand is, however a different matter ; it can not, in
general, be obtained by arithmetical addition of the individual maxima because these are very
unlikely to occur at the same time. Arithmetic addition will give too high a value. Summation
metering is commonly used, therefore, to determine the simultaneous maximum demand for
a number of circuits.
Fig-10 : Switchboard pattern Summator for 20 Circuits.
15-35
There is no space here for full discussion of this subject (Summation Metering),
which is a large one, and the references at the end of the chapter should be consulted for a
detailed study.
The most important of the methods used are ;
(i) A meter with several current coils: this method can be used satisfactorily for
summation in the case of two or three parallel circuits.
(ii) Paralleled current transformers: This method is similar to (i) except, that
only one current coil is needed per meter element.
(iii) Meter with a summation current transformer: If the number of circuits to be
summated is greater than can be handled by methods (i) and (ii), a transformer
having a number of primary windings-one for each summated circuit-can be
used. The transformer has a single secondary circuit which supplies the
(single) current coil of the meter element. The primary windings are supplied
from current transformers in the various load circuits.
(iv) Summation meter with multiple elements: This has a single rotor operated by
a number of driving elements-one for each circuit. There is obvious difficulty
in constructing such a rotor for more than a small number of circuits.
(v) Summation by impulsing meter: This method can be used for a large number
of circuits. The energy meter in each circuit is fitted with a contact-making
device which, for a pre-determined amount of energy, sends an impulse to a
mechanism in a summator which records the total number of such impulses
received. This system was introduced by Chamberlain and Hookham, Ltd. in
1932 and now they make a summator for 22 circuits (maximum). This
instrument shown in Fig-8 below, for 20 circuits, registers the consumption
for each circuit, gives the total consumption and also the simultaneous
maximum demand.
g- Tariff Metering :Multiple Rate Metering;
As to conserve energy, a measure to load management, day-time and night-time-
15-36
-tariffs, off-peak, medium peak and peak time tariffs, requires multi-tariff meters, which find
their way in the marketplace as to cope with the energy crises issue by reduction of pressure
on the supply undertaken in such a way to educate the ultimate user of the energy, to manage
their loads and to modify their living habits.
The individual customers need of maximum demands do not be persistent at the same
time. There total demand, nation wide, on the connected and running system constitutes
system demand, which keeps on varying, instant to instant, minute to minute. Though “Load-
Frequency Control” feature takes care of these small time changes of maximum system
demand, but the half-hourly or an hourly demands of maxima on the power system, can be
reduced by managing load over the twenty-four hour basis, during the season pattern,
through-out the year
Fig-11 : Metering Methods (Diagrammatic)
To educate the ultimate consumer, , in such a way, that there maximum load demands
can be reduced during any time interval over the day ( 24 hour-basis) by compelling him to
manage his load, in order to lessen the over-all burden on the electrical power system. The
strategy constitutes, offering different tariff in different zones or areas during different time
zones. This is, what technique, the supply under-taken adopted in bifurcating tariff-wise,
wide-spread areas, time-zone wise, in order to lessen the pressure on its electrical power
system.
15-37
By working on these lines, multi-part tariff (two or three-part tariff) metering
was introduced earlier to get the adequate benefits. To encourage the valued
consumers, they have offered , low rates of electrical energy during off-peak timings,
and subsequently intermediate rate during medium-demand timings and a bit higher
rate tariff during the peak-load timings. The energy supplied over these intervals is
being recorded, separately, by a switching mechanism, in case of electro-mechanical
designs, and, by way of employing different registers appropriate to each tariff in case
of the digital (solid-state) energy meters.
These meters are required in connection with a two-rate or double tariff
method of charging which differs from the maximum-demand system in that two
different rates of charge per unit are made, according to the time of the day at which
energy is consumed. By charging at a lower rate for energy consumed during periods
of light load on the generating station, the supply undertaking endeavours to
distribute the demand more uniformly over the day and so improve its “Load Factor”
this being the ratio Average load / Maximum load . Assume the average load remain
constant. The load factor is obviously increased by reducing the maximum load ; and
this can be achieved by causing some of the load to be taken at times of light load
rather than when the load on the station is heavy.
A two-rate meter of the ordinary type, but has two registering trains of wheels
and dials, both of which are operated- but not at the same time- by the moving system
of the meter. These registers are put into gear with the driving spindle alternately and
are used at such times, and for such periods, as are desired by the supply undertaking
for the purpose of double tariff method of charging. The time of operation of high-
rate and low-rate trains are usually governed by a time switch such as was mentioned
with the maximum demand system.
The time switches are set so that, in general, the high-rate register is in
operation during the evening when the load is heavy, the low-rate register operating
during the remaining portion of 24 hours, when the load is lighter.
15-38
15.8.1.1 Measurement of Kilovolt-ampere.
As has been mentioned previously, the system of charging for energy which penalize
low-power factor requires the use of another meter in addition to that measuring the total real
energy supplied (i.e. a watt-hour meter). Dorey (Ref.(19) :. illustrate the two alternate
methods of metering for such purposes by vector diagram as shown in Fig-8. In these
diagrams OI represents the active power, while OV represents the KVA. In diagram (a),
meter A is watt-hour or “cosine” meter, and measures the real energy supplied, while meter B
records Kilovolt-ampere-hours. In other words, A records ∫ KVA . cos Ө dt , while B records
∫ KVA . dt.
In diagram (b), meter A records the true energy as before, and meter C records the
reactive energy, or ∫ KVA . sin Ө dt. The angle Ө is the phase angle between voltage and
current in the circuit. Obviously,
(KAV sin Ө )2 + (KVA cos Ө )2 = (KVA)2
In the first diagram, meter B may be fitted with a maximum demand indicator which
gives the KVA demand. The average power factor of the load or average “energy factor”
may be obtained in case of (a) by the ratio ;
Reading of mater A
Reading of mater B
The reading being for corresponding period of time. In case of (b) the average energy
factor is given approximately by ;
cos Ө = Reading of meter A
(Reading of meter A)2 + (Reading of meter C)2
This expression is not, however, exact if the power factor is subject to considerable
variations during the period of time to which the reading refer (see E.W Hill’s remarks in
the discussion on the paper mentioned in Ref.(16).
15-39
In one method of charging the total charge is made up of two parts : one at so much
per unit of energy, and the other based on the quantity ;
Kilawatt-hours X P , where Cos Ө is the average energy factor
Cos Ө
and P is the “lowest permissible power factor”, and is laid down by the supply
undertaking.
Obviously, if cos Ө is less than P, the consumer is charged, in the second part of the
tariff, for greater number of units than those actually consumed, whereas if his power factor
is higher than P, the charge is for less units than the number actually consumed.
Fig-12 : The gearing mechanism of Trivector Meter for Apparent energy measurement.
An induction watt-hour meter can be compensated to read KVAh with in a limit of 2
or 3 percent, by adjusting the flux of the voltage coil circuit so that it lags by 90o + Өa
behind the applied voltage instead of by merely 90o , Өa being the average phase angle of the
load circuit. Thus if the phase angle of the load circuit is , at any time, exactly Өa , the meter-
15-40
-behaves as though the total applied voltage were exactly in phase with the current ; i.e. it
indicates volt-ampere-hours. For other power factors, the speed of the meter will not be
exactly proportional to the volt-ampere, but to volt-ampere X cos (Ө - Өa ), where Ө is the
actual phase angle.
The power in an a.c. circuit : Volt X amperes X Cos Ө - is very nearly the same as
the volt-ampere if the angle Ө is small, when Cos Ө is nearly unit. As an example, if Ө is
20o, Cos Ө is 0.9397 ; the power measured, in this case, is thus less – by 6.30 per cent – than
the volt-ampere in the circuit. This will apply weather the power factor is lagging or leading.
If than, the meter is adjusted so that it runs 3 per cent fast at unity power factor, it will
measure the volt-ampere in the circuit correct to with in + 3 per cent , provided the phase
angle “Ө” of the load circuit does not exceed 20o (lagging or leading) .
The required compensation of a watt-hour meter in order to cause it to indicate as
above may be obtained by using quadrature loops of abnormal thickness on the shunt
magnet.
15.8.2 Landis and Gyr Trivector Meter :
This meter measures KVAh and also KVA of maximum demand. Referring to Fig-
13. , it can be seen that, if the power factor of the circuit changed during a period of time
from Cos Ө to Cos Ө/ , Cos Ө// , etc., the true total of KVAh is given by the sum of
Ad, dd/ , d/d/ / ,etc. If an attempt is made to measure this total, simply by means of KWh
meter together with a reactive KVAh meter, the value obtained will be
(AB)2 + ( BD)2 = AD . This value is obviously, in general, less
than the true value.
15-41
---- KVAh calculated from KWh and KVAh SinΦ meters.
…. KVAh obtained from the KVAh meter.
Fig-13: Landis and Gyr Trivector Meter.
The Trivector Meter registers the true value of KVAH correct to within + 1 per cent.
The meter consists of KWh meter and a reactive KVAh meter in the same case with a special
summator mounted between them. Both meters drives the summator through somewhat
complicated system of gearing which arranges for the summator to register KVAh correctly
at all power factors.
The general principle on which this gearing is arranged can be understood from Fig-
13. As pointed out above, for phase angles of the system between 0o and 10o, a KWh
meter will register KVAh correctly within narrow limits, just as reactive KVAh meter
will register total KVAh very closely for phase angles of 80o to 90o. The gearing
arranges, therefore, that the summator shall be driven principally by the KWh meter
when the phase angle is small (curve 5, Fig.12) above, and principally by the reactive
KVAh meter when the phase angle is nearly 90o (curve 9, Fig-12.). At intermediate phase
angles, both meters are responsible for the drive of the summator through different
combinations of gears, the result is that the KVAh, measured at various power factors, is
almost constant, as shown by the full line curve at the top of the graph.
15-42
For the fuller discussion of the question of KVA measurement, the reader is referred
to Refs. (4), (7), (8), (11), (19), (24).
15.9 BIBLIOGRAPHY AND REFERENCES:
(1) Electricity Meters, C.W.Gerhardi.
(2) Electrical measurements, F.A Laws.
(3) Electrical measuring instruments, C.V Dersdale and A.C.Jolley.
(4) Commercial A.C. Measurements, G.W.Stubings.
(5) Electrical measuring instruments, D.J.Bolton, Jour. I.E.E., Vol. LXIV, p.
633.
(6) Dictionary of Applied Physics, Vol. II, Section on “Watt-hour meters”.
(7) “Electricity meters with notes on Meter Testing”, H.A.Retcliff and
A.E.Moore, Jour .I.E.E., Vol-XLVII, p. 3.
(8) “Recent Developments in Electricity Meters, with particular references to
those for special purposes,” J.L.Carr, Jour. I.E.E., Vol. XLVII, p. 859.
(9) “An investigation of the Frequency Variations in Induction Watt-hour
Meters,” A.E.Moore and W.T.Slater, Jour. I.E.E., Vol. XLVII, p. 1023.
(10) “Current Transformer Summators,” E.W.Hill and G.F.Shooter,
Jour .I.E.E., Vol. LXIX, p. 1251.
(11) “The metering of Three Phase Supplies,” O.Howarth, Jour .I.E.E., Vol.
LXIX, p. 381.
(12) “Apparatus and Methods for accurate maintenance of large A.C Energy
Meters,” E.Fawssett and G.E.Moore, Jour. I.E.E., Vol. LXIX, p. 647.
(13) “The Rotor Bearing of Electricity Meters,” W. Lawson, Jour. I.E.E., Vol.
LXVII, p. 1147.
(14) “A new Prepayment Meter,” Metropolitan-Vickers Gazette (July, 1927), P.
159.
(15) “Load Levelling Relays and their Application in Connection with Future
Metering Problems,” W. Holmes, Jour .I.E.E., Vol. LXVII, p. 296.
15-43
BIBLIOGRAPHY AND REFERENCES(Continued)
(16) “The Improvement of Power Factor,” G.Kapp, Jour. I.E.E., Vol. LXI, p.
89.
(17) “Some Technical Considerations concerning Power Factor in Relation to
Tariffs,” E.W.Hill, Jour. I.E.E., Vol. LXVII, p. 1228.
(18) “Power Factor: Its Technical and Commercial aspects,” H.E.Yerbury,
Jour. I.E.E., Vol. LXI, p. 675.
(19) ”The Improvement of Power Factor,” E.W.Dorey, Jour. I.E.E., Vol.
LXIV, p. 633.
(20) “The Metering Arrangements for the “Grid” Transmission System in
Great Britain,” C.W.Marshall, Jour. I.E.E., Vol. LXVIII, p. 1497.
(21) Handbook of Electric Metermen, published by the Meter Committee,
Technical National Section, National Electric Light Association, 29 West,
39th , Street, New York.
(22) “The Mean Error of an Electricity Meter,” G.W.Stubings, Jour. I.E.E.,
Vol. LIX, p. 335.
(23) “Integrating Electricity Meters,” E.Fawssett, Jour. I.E.E., Vol. LXIX, p.
545.
(24) Meter Engineering, J.L.Ferns.
(25) “The use of Grassot Flux Meter as a quantity Meter,” E.W.Golding, Jour.
I.E.E., Vol. LXXVI, p. 113.
(26) “Grid Metering” J.Henderson, Jour. I.E.E., Vol. LXXV, p. 185.
(27) “The Thermal Maximum Demand Indicator,” G.W.Stubbings,
Distribution of Electricity, April 1934.
(28) “An Improved Three-Phase, Three-wire, Reactive volt-ampere-hour
Meter,” G.A.Cheetham, Jour. I.E.E., Vol. LXXIII, p. 233.
(29) “The Metering of E.H.T. Supplies on the Secondary side of the Step-
Down-Transformer,” S.H.C. Morton, Jour. I.E.E., Vol. LXXI, p. 507.
15-44
BIBLIOGRAPHY AND REFERENCES(Continued)
(30) “Some Suggestions on the Equipment and Routine of the Meter
Department of Supply Undertaking,” J.L.Ferns, Jour. I.E.E., Vol. LXXVI, p.
369.
(31) “A Study of the Induction Watt-hour Meter with special reference to the
Cause of Errors on Very Low Loads,” T.Havekin, Jour.I.E.E.,Vol. LXXVII,
p. 335.
(32) “The History and Development of the Integrating Electricity Meter,”
A.E.Moore, Jour. I.E.E., Vol. LXXVII, p. 851.
(33) “The Measurement of Large Supplies of Electrical Energy for Costing
Purposes,” W. Casson and A.H.Gray, Jour. I.E.E., Vol. LXXVIII, p. 681.
(34) “The Metering of Mercury-Arc Rectifier Supplies and Outputs,”
Dannat, Jour. I.E.E., Vol. LXXXI, p. 256.
(35) “Torque in a Bi-Polar Induction Meter,” R.A.Morton, Trans. A.I.E.E.,
Vol. LV, p. 354.
(36) “Development of a Modern Watt-hour Meter,” I.F.Kinard and H.E.Trakell,
Trans. A.I.E.E., Vol. LVI, p. 172.
(37) “The Remote Indication of Meter readings,” C.Myers, G.E.C. Journal,
Vol. VIII, p. 201, August 1937.
(38) “Organization of a Meter Test Department of a Large Supply Under
Taking with special reference to the Electricity Supply (Meters) Act, 1936,
C.W.Hughees, Jour.I.E.E.,Vol. LXXXII, p. 410.
(39) “The Use of Auxiliary Current Transformers for Extending the Range of
Metering Equipment,” G.F.Shooter, Jour. I.E.E., Vol. LXXXIV, p. 128.
(40) “Reactive Metering,” V.B.Shah, Elect. Rev., June 13th, 1947.
(41) “Utilization of Watt-hour Meter,” A.Salzmann, Distribution of Electricity,
July, 1951, p. 9.
(42) “The Electrical Transmission of Flow and Level Records,” A.Linford,
B.E.A.M.A. Journal, October, November and December, 1942, pp. 28, 335
and 367.
15-45
BIBLIOGRAPHY AND REFERENCES(Continued)
(43) Electricity Supply Meters, A.E.B.Perrig.
(44) Electricity Meters and Meter testing, G.W.Stubbing.
(45) Electricity Meters and Instrument Transformers, S.James.
(46) Voltage-Stabilized Supplies, F.A.Benson (London, Mac Donald, 1957).
(47) Automatic Voltage regulators and Stablizers, G.N.Patchett (London,
Pitman, 1954
(48) Mr. Sohail, Plant Manager (Electrical), EngroGen, Daharki, Sindh,
Pakistan.
15-46
Recommended
