Download Chapter-16 Computerized Testing of Energy Meters

Chapter-16
Computerized Testing Of Electricity Energy Meters.
With the rapid advance of semiconductor and widespread acceptance of static /solid
state/ digital energy meters, the problem of the design, manufacture and testing of these
digital energy meters necessitated increased attention.
The use of integrated circuits, firstly linear operational amplifier, then digital gates
and logic, followed by large scale digital integrated circuits, memories, and microprocessors
accentuated the test methods.
As the volume and complexity of digital energy meters outgrew, the early testing
methods were left behind, as they cover few of the tests pertaining to latest digital energy
meters with advanced capabilities, automatic programmable test equipment was introduced
to ensure timely testing of the volumetric energy meters.
Testing and quality control techniques have two main objectives:
i.
To maximize in-service reliability.
ii.
To minimize testing cost and time.
To maximize in-service reliability of digital energy meters, repeated scheduled
maintenance testing has to be carried out. This will increase the testing cost, hence the
automatic programmable testing will help, save time and cost.
There are two approaches to automatic testing of the energy meters.
a.
Use of computer (laptop) in conjunction with microprocessor base energy
meter test set.
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b.
Use of microprocessor based automatic / manual , energy meter test set like
Cal port + Cal source, Calmate and MTE Type PTS 3.3 C ( Portable Test
System, Microprocessor Based)
a. Use of computer (laptop) in conjunction with microprocessor base energy
meter test set.
The target energy meter has to be connected to Reference energy meter of the
microprocessor based energy meter test set. A computer (usually laptop) is then attached to
the microprocessor based energy meter test set via serial port.
The much computer software is available for automated testing of the energy meter.
The off-load / on-load testing is usually performed through this method. The computer
controls the microprocessor based test set through these soft ware. Even automatic switching
of the microprocessor based energy meter test set can be controlled through computer for
load-point testing.
Following is the short list off the tests that can be performed automatically.

Dial test of electromechanical energy meter.

Creepage test of both electromechanical / digital energy meter.

Percentage error measurement of both electromechanical / digital
energy meter by energy measurement method.

Percentage error measurement of both electromechanical / digital
energy meter by power measurement method.
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1.
Dial test of electromechanical energy meter.
Speed or Dial Test of the Energy Meter
First consider the electromechanical energy meter. You simply need a computer
software setting instead of 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.
Rough assessment of speed and speed test of the digital energy meter.
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 so there is no dial test. 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 and KVArh for representation of active and reactive energy
passage respectively.
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These pulses can be read through a digital scanner and can be fed to standard meter
for comparison purpose. The computer also requires ( feed in ) the meter constant of the
target meter, and can be worked out as follows..
Meter Constant of the Target Meter.
M.C = PT ratio X CT ratio X Revolution or impulses / KWh
The standard energy meter of the microprocessor based test set than compares both
the energy meters ( target meter and the standard meter) by way of impulses read through
scanning head and meter constant fed to computer and provide percentage slowness or
fastness.. This type of test can be performed automatically with the help of computer
software.
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.
2.
Creepage test of both electromechanical / digital energy meter.
Watt-hour must also be tested for “creep” by applying 10 percent over-voltage to the
voltage coil, the main circuit (current-coil circuit) being open. The meter must not run under
these conditions.
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3
Percentage error measurement of both electromechanical / digital energy meter
by energy measurement method.
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 upto 0.5 power factor lagging i.e Cos 60o.
For percentage error measurements, computer software is configured in steps (a), (b),
and (c) mentioned above along with the time step for each feed. Power factor up to 0.5 must
also be fed in case of a.c energy meter testing.
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-
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-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.
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
“PHANTOM OR FICTITIOUS LOADS” TEST / OFF LOAD TEST.
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
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 (CT shorting links at the test plug). The
current coil is then supplied from a battery (in case of D.C energy meter) or other low voltage
source (in case of A.C energy meters) for purposes of testing.
For percentage error measurements, computer software is configured in steps (a), (b),
and (c) mentioned above along with the time step for each feed. Power factor up to 0.5 ( Cos
60o) must also be fed in case of a.c energy meter testing. Testing in Pakistan is carried out at
0o, 30o,60o lagging phase angles for each load step i.e. 100 %, 50 %, 20 %, and 10 % of rated
load current on the C.T secondary.
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4
Percentage error measurement of both electromechanical / digital energy meter by
power measurement method.
By automatic computer testing for percentage error measurement by power
measurement method, power levels are described to computer software by feeding the
corresponding spaces provided for this purpose.
For percentage error measurement by power measurement method, computer
software is configured in steps (a), (b), and (c) mentioned above along with the time step for
each feed. Power factor upto 0.5 ( Cos 60o) must also be fed in case of a.c energy meter
testing.
b.
Use of microprocessor based automatic / manual , energy meter test
set like
Cal port + Cal source, Calmate and MTE Type PTS 3.3 C
( Portable Test System, Microprocessor Based).
The computer need to be connected with Cal port + Cal source, Calmate test sets and
can perform automatic testing.( i.e semi-automatic testing)
The microprocessor contained in the case of MTE Type PTS 3.3 C test set performs
many of the functions to be taken by a computer. Semi-automatic testing features are
percentage error measurement or load point testing.
The load points along with time steps to feed in the meter under test have to be selected from
the default options already configured in the microprocessor. Target Meter type (single phase
or three-phase) with type of connection 3-wire or 4-wire must be defined. Meter constant of
the target meter must be fed to standard test set along with rated voltage and current of the
P.T and C.T secondary respectively.
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