Sci.Int.(Lahore),26(5),2063-2069,2014
ISSN 1013-5316; CODEN: SINTE 8
2063
PERFORMANCE OF ENERGY METERS UNDER HARMONIC
GENERATING ENVIRONMENT
Syed Safdar Raza, Masood Ahmad*, M. Shoaib Perveiz
Department of Electrical Engineering, COMSATS Institute of Information Technology (CIIT), 1.5 km Defense Road,
off Raiwind Road, Lahore - Pakistan
ABSTRACT: One of the major problems in utility power supply is the voltage and current harmonic distortion. Non linear loads produce harmonics which increases power losses and causes overheating of power system equipments. This paper discusses the effect of harmonic distortion on energy measurement of meter and behavior of current transducers under overloaded condition. It also presents a comparative study
between solid state electronic meters and electromechanical watt hour meters. The total voltage and current harmonic distortion, displacement power factor is simulated by HIOKI 3197 power quality analyzer.
The Microvip3 ELCONTROL energy analyzer is used to record the actual power consumption. It has been
found that the energy measurement error in solid state energy meters is much less than the electromechanical meters if they are tested in same harmonic generating environment. The whole current operated meter
slows down when large amount of current flows through the current coil and ultimately burnt. The CT operated meters have separate current transformer for each phase and if large amount of current flows
through it, it saturates and metering stops.
Key Words: Harmonics, non linear loads, solid state energy meters, electromechanical watt hour meters.
*Corresponding Author: masoodjaffar@ciitlahore.edu.pk
1. INTRODUCTION
The electricity tariff has been increasing day by day. At the
same time, all types of consumers demand better services
from the distribution companies in the form of power quality, electricity tariff and measurement. Energy meters are
basically used to measure electric energy consumption at
domestic, commercial and industrial level. In our country
most of the energy meters are electromechanical (rotating
disc) meters. In late 1970’s, electronic energy meters were
introduced. Now distribution companies are rapidly replacing the old conventional electromechanical meters with solid
state electronic meters because of higher accuracy and greater stability. The greatest advantage of these solid state electronic/digital meters is that they work well in all environmental conditions [1]. Manufacturers of both electromechanical and electronic meter claim that their meter fulfills all the
requirements of international standard (e.g. accuracy etc) and
consumer demand (e.g. price etc). The manufacturers
check/test their energy meters under purely sinusoidal conditions, but in practical scenario the case is quite different. The
extensive use of power electronics in many household and
commercial appliances like adjustable speed drives, static
power converters, switch mode power supplies, computers,
television set, battery charger, compact fluorescent lamps is
causing new unknown problem of harmonics. The consumption of energy/power in these power electronic equipments is
very low. Such type of equipments are more reliable and of
good quality than the previous one. Although these power
electronic equipments have many benefits but on the other
side they generate harmonics which increases iron and copper losses, insulation stress, overheating and measurement
error. These harmonics badly disturb the utility power factor,
current and voltage waveforms. Under this scenario the accurate measurement of power consumption is very important
for both consumer and the power distributer. Consumer is
interested in accurate measurement in order to ensure that
they are getting what they pay for and power distributer is
interested in order to ensure the integrity of the network.
Electromechanical watt hour meter and solid state energy
meters are frequently used for the measurement of electrical
energy at domestic, commercial and industrial level. The
behavior of these energy meters under non-sinusoidal voltage and current waveform is not known.
Initially theoretical models are developed to observe the effect of harmonics on energy measurement. Authors in [2, 3,
4] developed a theoretical model but the results obtained are
not significant because these models does not consider the
resistance of voltage coil and inductance of the disk in driving torque. Others [5], continued their work by developing a
model in which he considered the disk inductance and voltage coil resistance in order to calculate the torque, but the
results obtained are very misleading because he concludes
positive measurement error. Authors in [6] Present a quasilinear model in which the behavior of single phase induction
watt hour meter in the presence of voltage and current harmonics are studied and concludes that induction watt hour
meter cannot register the harmonic power and has negative
measurement error.While in [7] workers discuss experimentally the behavior of both single phase electromechanical and
solid state electronic energy meters under non sinusoidal
conditions. The laboratory setup consists of a device which
produces utility power disturbances (harmonics unbalance
voltage, frequency) and a resistive load. The results obtained
are compared with the reference meter whose accuracy is
less than 1% from 0 – 2.5 KHz. Paper concludes those solid
state electronic meters are more stable than the electromechanical meters. Performs experiment on 14 telephone exchanges and grouped them according to the harmonic concentration[8]. The paper concludes that measurement error is
greater if capacitor bank is installed at unbalanced load condition than without the capacitor bank. [9] Performs an experiment on electronic energy meter at high and medium
voltage customers in the locality of West Java and Benten
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Sci.Int.(Lahore),26(5),2063-2069,2014
and concludes that measurement error is within permissible
limits.
2. ENERGY METERS
Energy/power consumed by domestic, commercial and industrial consumers is measured by two types of energy meters. Theoretical description of these energy meters is illustrated below.
2.1 ELECTROMECHANICAL WATT HOUR METER
These are the rotating disc meters in which rotation of the
disk is proportional to the power consumed by consumer.
Meters which were manufactured before 1970’s had a disk
shaft between the bearings and disk rotates on a lubricated
ball. The metallic parts of the integrative devices were also
lubricated for proper movement of metallic disk numerator.
But this lubricating oil becomes hard with the passage of
time and causes undesirable error in measurement due to
friction.
In order to overcome this problem, magnetic suspension type
electromechanical energy meters were introduced. In electromechanical energy meters, the shunt magnet is wound
with a fine wire of many turns and is connected across the
main supply so that the current flow through it is proportional to the supply voltage. Since the coil of the shunt magnet
has large number of turns and the reluctance of its magnetic
circuit is very small due to the presence of small air gap,
which makes the coil highly inductive. Thus the current (and
hence the flux) lags the supply voltage by 90 . In comparison to shunt magnet, the series magnet is wound with a
heavy wire of few turns and is connected in series with the
load so that it carries the load current. The coil of series
magnet is highly non inductive so that the angle of lead or
lag is determined by the load.
A thin aluminum disk is mounted on the spindle placed between the shunt and series magnets so that it cuts the fluxes
of both the magnets. The permanent magnet near the rotating
disc produces braking torque so that the disk rotates in the
permanent magnet field. This permanent magnet induce eddy
current in the disk which produce braking or retarding torque
that is proportional to the disk speed. The power factor compensator (short circuited copper loop) is placed on the central
limb of the shunt magnet. The flux produce by shunt magnet
can be made to lag behind the supply voltage exactly by 90 ,
if the position of this loop is adjusted. Gearing mechanism is
used to record the energy consumption in KWh.
Such meters are lower in price and have high reliability. It
also measures both active and reactive power consumption at
sinusoidal supply. Its life is approximately 20 years if manufactured according to IEC standards. Error occurs in measurement if they are not mounted vertically and accuracy is
greatly affected by dirt and humidity. Fraud can be done
easily due to simple mechanism [10].
2.2 SOLID STATE ENERGY METERS
Solid state electronic energy meter has no mobile part as in
electromechanical energy meter and has been introduced in
market since last 20 years. Figure 1 shows the block diagram
of the digital solid state energy meter [11].
Figure.1. Block diagram of solid state meter [11]
Voltage and current transducers converts the signal into
smaller equivalent voltage signals which is then presented to
ADC for conversion. The digital signal is processed by digital circuit in order to calculate active, reactive and apparent
power, power factor, MDI etc. EEPROM is used to store
program for calibration purpose. Power supply monitoring
and watch dog is used to ensure the correct operation during
power up and power down. The block diagram of DSP based
solid state energy meter is shown in Figure.2 [11].
Figure. 2 DSP based solid state meter [11]
In solid state electronic energy meter, user programmable
DSP is used as a main computational engine of the meter.
The SAR-ADC is integrated on the main chip. The designer
of the DSP based meter has to write software for the required
parameters. MCU is used to drive the display and other duties in the meter. Supervisory circuit is used to monitor the
supply voltage and to ensure correct operation during power
up and power down. Price of such energy meter is reasonably low and greatest advantage of these meters is that accuracy is not affected by position, humidity etc. Life expectation is approximately 20-25 years [11].
2.3 ENERGY METERS IN PAKISTAN
Distribution companies in Pakistan take energy meters from
both national and international companies. Major energy
meter supplier are Pak Elektron Limited, MicroTech Industries (Pvt.) Ltd, Syed Bhai (Pvt.) Ltd, Creative Engineering
Group Lahore, S.B. Electronics and Control Engineering,
ESCORT Pakistan Ltd. Lahore. Single phase electromechanical watt hour meters are available in 10(30), 5(20), 10(40),
10(60), 20(80), 15(100) A range and poly phase electromechanical watt hour meter are available in two categories.
Sci.Int.(Lahore),26(5),2063-2069,2014
ISSN 1013-5316; CODEN: SINTE 8
Whole current operated meter are available in 15(60),
15(90), 15(120), 30(120) A and CT Operated meter 2.5(10),
5(10) A range. Single phase electronic meters are available
in 10(40) and 10(60) A range. Similar to poly phase electromechanical watt hour meter, poly phase electronic meters are
also available in two categories. Whole current operated meter is available in 10(100) A and LT CT/ HT CT PT operated
meter is available in 5(10) A range.
3.
POWER CONSUMPTION OF ENERGY METERS
The self-consumption and starting current of both whole
current and CT operated meters are summarized in Table 1.
Table 1: Power consumption of energy meters
Starting
Operational
Energy meters
current
Losses
0.5% of
1-φ ElectromechaniBasic cur< 0.8 W
cal.
rent.
Whole cur0.5% of
3-φ Electromechanirent
Basic cur< 1.5 W
cal.
Operated.
rent.
1-φ Electronic.
≤ 40 mA
< 0.75 W
3-φ Electronic.
≤ 40 mA
<2W
CT operat3-φ Electronic.
≤ 10 mA
<2W
ed.
4.
CURRENT TRANSDUCERS
Current transducers are used to sense current in the energy
meters. These current transducers are designed especially to
fulfill the requirements of international standards such as
accuracy, dc tolerance etc and of purchaser such as price,
availability etc [12]. For measuring purpose, following three
ways are used to sense the current.
4.1
SHUNT RESISTANCE METHOD
This method is rarely used now a day and is the low cost
current measurement method but offers high accuracy. In
current measurement, the parasitic inductance affects the
measurement at high frequencies. This parasitic inductance
produces the phase mismatch at low power factor. So in their
design it is quite necessary to take care and ensure that current and voltage are completely matched. As it is a resistive
element, there exists the self-heating problem which is proportional to the square of current [13].
4.2
CURRENT TRANSFORMER
The accuracy of the current transformer depends on the
properties of the magnetic material from which the core is
made. The core used in current transformers does not fulfill
the requirements of the designer because of high core losses
at high frequencies. The extensive use of power electronics
in our daily life adds harmonics in power system. So it is a
great challenge for the designer to increase the accuracy of
the current transformer at high frequencies and reduce the
cost of production [14]. For metering purpose two types of
current transformer are used.
4.2.1 LINEAR CURRENT TRANSFORMER
Earlier the core of current transformer is made of silicon iron
alloy but core of such magnetic material increases the power
losses. So it is replaced by other magnetic material such as
amorphous metal and nano crystalline magnetic material
which are annealed in axial magnetic field. These magnetic
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materials have very high permeability and low core losses
but are very expensive. The core of power transformer is
made of amorphous metal but this is not used in the construction of core of current transformer because of higher
price [12].
4.2.2 COMPOSITE CORE CURRENT TRANSFORMER
The core of such types of current transformers is made of
different materials having different permeability placed side
by side and winding is wound around it. The first core is
made of that ferromagnetic material whose permeability is
very high and provides very low phase displacement. The
second core is made of that magnetic material whose permeability is low and provides good immunity against magnetic
flux. When high permeability core become saturated because
of high current or dc component in the current, the current
transformer will not stop measuring the power consumption.
Current transformer will continue measuring the power consumption by passing the magnetic flux through low permeability core. As both core of current transformer are made of
low quality material. So they are cheaper than the linear current transformer [12].
HALL EFFECT METHOD
Hall Effect transducers are of two types: closed loop and
open loop. In energy meters, open loop type Hall Effect
transducers are used because of low cost. Hall Effect transducers measures large amount of current and have excellent
frequency response. The major drawback of Hall Effect
transducers is that it require stable external current source
because of large temperature drift [13].
5.
IMPACT OF HARMONICS
Voltage and current harmonics in real power system badly
affects the true power factor which is given as follows.
(1)
Pavg
PFTrue 
 THDI 
VRMS I RMS 1  

 100 
PFTrue 
2
 THDV 
1 

 100 
2
(2)
PFDisplacement
 THDI 
1 

 100 
2
 THDV 
1 

 100 
2
(3)
From (3), it is clear that distortion factor has a negative impact on the true power factor and in the presence of voltage
and current harmonics; the unity value of true power factor
cannot be achieved. Voltage THD is below 5% if current
THD lies between 80-100%. In real power system, current
THD has the dominant effect on the voltage THD. Voltage
THD is expressed as follows.
hmax
THDV 
V
h 1
2
h
VF
For distorted waveform, the RMS value is
(4)
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VRMS 
Since V1
hmax
 Vh2
(5)
h 1
 VF , eq. (5) becomes
hmax
VRMS  VF2  Vh2
(6)
h 1
From (4) and (6) we get
VRMS  VF2  THDV VF 
2
VRMS  VF 1  THDV2
(7)
(8)
From (4) we can write (7) as
VRMS  VF2  Vh2
(9)
Similarly
I RMS  I F2  I h2
(10)
I RMS  I F 1  THDI2
(11)
Since THD changes the shape of voltage and current waveforms which will affect the performance of the energy meters
because measuring instruments are calibrated on purely sinusoidal supply but they are operated in distorted power
supply conditions which produce significant error. The magnitude and direction of harmonic power is very important for
metering because the directions of flow of harmonic power
decide the meter error. Due to harmonics both positive and
negative metering error occurs.
The electromechanical energy meters has series and shunt
electromagnets which produces driving torque and the permanent magnet produce braking torque on the aluminum
disk. Both the driving and braking torques are primary torques. Secondary flux producing elements are used for compensation purpose in order to improve the accuracy of measuring instrument and to compensate error due to registering.
The response of energy meters to frequencies outside the
design parameter is abnormal and ultimately recording error
occurs. The total power (PT) seen by a meter is given by:
PT  PDC  PF  PH
PT  VDC I DC  VF I F cos F  VH I H cos H
Where,
PT = Total Power, PDC = DC Power, PF = Power at fundamental frequency (50Hz), PH = Power at harmonic frequen-
Sci.Int.(Lahore),26(5),2063-2069,2014
cy, VDC = DC Voltage, IDC = DC Current, VF = Voltage at
fundamental frequency, IF = Current at fundamental frequency, VH = Voltage at harmonic frequency, IH = Current at
harmonic frequency, cos F = Phase angle between VF and IF
at fundamental frequency, cos H = Phase angle between VH
and IH at harmonic frequency.
The energy meter will not measure DC power (PDC). It
measures PF accurately and PH inaccurately. The harmonic
power (PH) is obtained by adding all components of power at
frequencies above and below the fundamental frequency.
Any DC power supplied or generated by the customer will
cause an error which is proportional to
and the error sign
depends on the direction of flow of power. Similarly, in
measuring harmonic power an error will occur which is represented by 'X' and is given by . The factor 'X' is dependent
on the frequency response characteristics of the meter and
error sign again related to the direction of flow of power.
Direct current (IDC) distorts the working flux and changes the
permeability of the core. Fluxes produced by harmonic currents (IH) combine with spurious fluxes of the same frequency and produces secondary torques. Thus, DC power and
harmonic power affects the capability of the energy meter to
measure total power [15].
6. EXPERIMENTAL MEASUREMENT SETUP
To study the effect of harmonics on the single phase electromechanical and electronic energy meters, the experimental setup was made in the electrical distribution room of
electrical engineering department at COMSATS institute of
information technology Islamabad on the circuit of Computer Lab-2. And to study the effect of harmonics on three
phases electronic energy meter, the experimental setup was
made on the induction arc furnace having load 2250KW at
Karachi Steel Industry, plot #191, sector I-10/3 Industrial
area, Islamabad. The saturation tests of the energy meters
were performed at Rawat laboratories, Rawat.
The PT’s of both energy analyzer and power quality analyzer
are connected to the phase and neutral line and CT’s are attached around phase line with the arrow pointing towards the
flow of current. The harmonics are recorded by HIOKI 3197
power quality analyzer and readings on the meters are compared by energy analyzer in order to get the measurement
error. The specifications of meters and current transformers
which were used for experiment are given below.
M1: Manufacturer A, Electronic single phase energy meter,
2008, Class 1.00, 3200 imp/KWh, 240V, 10(40)A, 50Hz.
M2: Manufacturer B, Electromechanical single phase 2 wire
energy meter, 1995, Class 2.00, 400 Rev/KWh, 240V,
10(40)A, 50Hz.
M3: Manufacturer C, Three phase, 3 elements, 4 wire Multirate solid state wire energy meter. HT Type CT & PT operated, 2009, Class 1.00, 10000 imp/KWh, 10000
imp/KVARh, 3x63.5/110V, 5(10) A, 50Hz.
M4: Manufacturer D, Creative Electronics (Pvt.) Ltd. Three
phase, 4 wire Multi-rate whole current operated static energy
meter, 2011, Class 1.00, 1000 imp/KWh, 1000 imp/KVARh,
3x230/400V, 10(100), 50Hz.
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M5: Manufacturer E, Three phase, 4 wire whole current operated energy meter, 2002, Class 2.00, 50 rev/KWh,
3x230/400V, 15(90)A, 50Hz.
M6: Manufacturer A, Three phase, 3 elements, and 4 wire
Multi-rate solid state energy meter. LT Type CT operated,
Class 1.00, 10000 imp/KWh, 10000 imp/KVARh,
3x230/400V, 5(10) A, 50Hz.
CT1-3: Manufacturer C, Class 0.5, 50Hz, Sec factor < 5, Ext
range 200%, Specification DDS-80-07, 100/5, VA. 5(LT)
10(HT), Purpose: Metering
7. TEST RESULTS AND DISCUSSIONS
The results and discussions are categorized according to
impact of harmonics and saturation which is illustrated below.
Figure. 3(a): Single phase electronic energy meter
Figure. 3(b). Single phase electromechanical energy meter.
Figure. 3(c). Three phase electronic energy meter
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Table2: Technical details at harmonic generating source
Energy
Meters
1-φ
Electronic
1-φ
Electromech
anical
3-φ
Electronic
Harmonic
Source
THDV
(%)
THDI
(%)
Computer
5.2 –
6.1
Computer
Induction
Arc
Furnace
P.F
Disp.
P.F
Distortion
Factor
75 –
78
0.770
0.999
0.788 –
0.80
3.2 –
4.7
86.3 –
87.9
0.725
0.991
0.751 –
0.757
3.4 –
4.7
21.6 –
22.5
0.721
0.836
0.975 –
0.977
From Table 2, it is clear that when electronic energy meter is
going to be test under harmonic generating environment, the
THDV vary between 5.2-6.1% and THDI vary between 7578%. The 3rd & 5th harmonic current have values 66.19 %
and 32.17 % respectively of the fundamental frequency value
of the current.
7.1 IMPACT OF HARMONICS
The electronic and electromechanical energy meters (M1,
M2 and M3) are tested at the harmonic generating load
(Computers, induction arc furnace). Current, voltage, power
factor, displacement power factor, bar chart and list of harmonics, THDV and THDI snapshots are taken by HIOKI
3197 power quality analyzer which is shown in Figure 3(a),
3(b), 3(c).harmonic voltage distortion for bus voltage of less
than 69KV should not exceed 5% and total harmonic current
distribution for large concentration of low power loads in
building and offices should not exceed 30%. It is quite clear
that both THDV and THDI exceed the IEEE limits in this
case.
Since THDV and THDI have higher values than the prescribed IEEE limits. These higher values of THDV and THDI
increase the distortion factor (0.788-0.80). As we know from
equation (3) that true power factor is equal to the product of
displacement power factor and distortion factor. So, as the
distortion factor increase the true P.F decreases and low P.F
badly affects the overall efficiency of the power system. As
the electronic energy meters are calibrated on fundamental
frequency, these will measure the power at harmonic frequencies and the actual consumption of consumer is not determined by the energy meter. In this experiment also the
electronic energy meter records only 0.782 KWh of unit’s
instead actual power consumed is 0.82KWh. Thus because
of harmonics the electronic meter measures about 3.61% less
than the actual consumption. Similar is the case with single
phase electromechanical and three phase electronic energy
meters. Under the above specified conditions the measurement of the energy meters at harmonic generating load is
illustrated in the Table 3.
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Sci.Int.(Lahore),26(5),2063-2069,2014
IMPACT OF SATURATION
In Pakistan two types of energy meters are used to measure
the consumption of energy. One of them is the whole current
operated energy meter and other one is the CT operated or
MDI energy meter. To study the behavior of energy meters
under overloaded condition, the experimental setup has been
established on energy meter test bench at Rawat Laboratories, Rawat.
When experiments are performed on whole current operated
energy meters (M1, M2, M4 and M5), the reading on the
energy meters and that obtained from the ELCONTROL
energy analyzer are approximately same up to the maximum
rated current. But the meters show abnormal behavior when
the current exceeds the rated capacity. The test is also performed on the LT CT operated three phase electronic energy
meter (M6) having rated capacity of 5(10) A and current
transformers (CT1-3) having rating 100/5 are used for each
phase. Up to 100A of current on the primary side, the current
measured on the secondary side was 5 A. But when the current exceeds beyond 100 A, the current on the secondary side
also exceed in proportion to the primary current. When the
primary current reaches at 200 A, the secondary current observed was about 9.8A and when the current exceed beyond
the 200A, the current on the secondary side instead of increasing start decreasing and meter stops recording reading.
It means that current transformer has been saturated and
there is no output from it. The reading observed under over
loaded conditions is summarized in Table 4.
current path. This increase in reactance lowers the driving
torque and ultimately measurement error occurs which depends on the current distortion factor, true power factor and
displacement power factor. In comparison to it, solid state
electronic energy meter gives better results.
Table 3: Behavior of energy meters at harmonic generating
load
KWh ReadDifferings
Rated
Energy Meters
ence
Capacity MeAna(%)
ter
lyzer
Whole
1-φ Elec10(40)
12.4
12.9
3.87
Current
tronic.
Operated. 1-φ Electromechan
10(40)
14.4
15.8
8.86
ical.
3-φ Elec10(100)
38.5
37.9
1.5
tronic.
3-φ Electromechan
15(90)
18.4
19.7
6.59
ical.
Up to 100
14.2
14.32
0.84
A.
LT CT
5(10)
Operated. 101 to 200
22.2
23.7
6.32
A.
9. REFERENCE
[1] Ortiz A, Lehtonen Matti, Mana M, Renedo C, Muranen
S, Equiluz L.I. Evaluation of Energy Meters accuracy based
on a Power Quality Test Platform. Electric Power components and systems 35: 221-237. 2007
[2]. Downing W.C. Watt hour meter accuracy on SCR controlled resistance loads. IEEE Trans. Power Apparatus and
System. PAS-93: 1083 – 1089.1974
[3]. Emanuel A.E, Levitsky F.J, Gulachenski E. M. Induction watt hour meter performance on rectifier/inverter circuit. IEEE Trans. Power Apparatus and System, PAS-100:
4422-4427.1981
[4]. Emanuel A.E, Hynds B.M, Levitsky F.J. Watt hour
meter accuracy on integral cycle controlled resistance loads.
IEEE Trans. Power Apparatus and System, PAS-98: 15831590.1979
[5]. Fuchs E.F, Roesler D.S, Kovacs K.P. Sensitivity of
electrical appliances to harmonics of the power system’s
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April, Barcelona, Spain. 2004
8. CONCLUSION
The distortion in current and voltage waveform due to extensive use of nonlinear loads produces measurement error. The
deviation in the measurement of energy consumption is due
to voltage and current total harmonic distortion. From the
results obtained during the experiment, it is clear that when
the energy meters of both types are tested in asame harmonic
generating environment, the measurement error is found
greater in electromechanical energy meters than the electronic energy meters.
In electromechanical energy meters, meter disk slows down
due to harmonics because reactance increases in the eddy
Table 4: Reading observed under overloaded condition.
Energy meters.
1-φ Electronic meter.
1-φ Electromechanical meter
3-φ Electronic meter
KWh Readings.
Meter Analyzer
0.782
0.82
Difference
(%)
3.61
0.651
0.721
9.63
17.20
17.29
0.52
When energy meters are operated under overloaded condition, the whole current operated meters slow down because
there is a current coil instead of the current transformer. The
current coil is made up of copper. When large amount of
current flows through current coil, it heats up. The effect of
high temperature on the current coil is that it increases the
resistance. The high resistance hinders the flow of current
and sparking occurs when current exceeds the permissible
limits. In CT operated meter/MDI meter separate current
transformer is used for each phase. The tolerance value of
such current transformer is double than the rated capacity.
When large amount of current flow through current transformer, it saturates and stops metering.
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