1

“DETERMINATION OF ERRORS IN INSTRUMENT

TRANSFORMERS USING LAB-VIEW”

Project report submitted in partial fulfillment of the requirements

For the award of the degree of

BACHELOR OF TECHNOLOGY

IN

ELECTRICAL AND ELECTRONICS ENGINEERING

By

M.GOUTHAM REDDY (08241A0266)

A.HEMANTH (08241A0269)

M.PRADEEP (08241A0282)

P.VINAY CHANDER (08241A02B5)

Under the guidance of

Ms. D.RAMYA

Assistant Professor

Department of Electrical and Electronics Engineering

GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING &

TECHNOLOGY, BACHUPALLY, HYDERABAD-72

2

2012

GOKARAJU RANGARAJU INSTITUTE OF ENGINEERING AND

TECHNOLOGY

Hyderabad, Andhra Pradesh.

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

C E R T I F I C A T E

This is to certify that the project report entitled “DETERMINATION OF ERRORS IN

INSTRUMENT TRANSFORMERS USING LAB-VIEW” that is being submitted

by Mr. M.GOUTHAM REDDY, Mr. A.HEMANTH, Mr. M.PRADEEP, Mr. P.VINAY

CHANDER in partial fulfillment for the award of the Degree of Bachelor of Technology

in Electrical and Electronics Engineering to the Jawaharlal Nehru Technological

University is a record of bonafide work carried out by him under my guidance and supervision.

The results embodied in this project report have not been submitted to any other University or

Institute for the award of any graduation degree.

Mr. P.M.Sarma Ms. D.RAMYA

HOD, EEE Assistant Professor.

GRIET, Hyderabad GRIET, Hyderabad

(Internal Guide)

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Acknowledgement

This is to place on record my appreciation and deep gratitude to the persons without

whose support this project would never seen the light of day.

I have immense pleasure in expressing my thanks and deep sense of gratitude to my

guide MS. D.RAMYA, Assistant Professor Department of Electrical Engineering, and G.R.I.E.T for his guidance throughout this project.

I also express my sincere thanks to Mr.P.M.Sarma, Head of the Department, and

Mr. M. Chakravarthy, Associate Proffessor G.R.I.E.T for extending his help.

I express my gratitude to The Dr.S.N.Saxena, Project Supervisor G.R.I.E.T for his

valuable recommendations and for accepting this project report.

Finally I express my sincere gratitude to all the members of faculty and my friends who

contributed their valuable advice and helped to complete the project successfully.

M.GOUTHAM REDDY

A.HEMANTH

M.PRADEEP

P.VINAY CHANDER

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ABSTRACT In electrical engineering, current transformer (CT) is used for measurement of electric currents.

Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are

known as instrument transformers. When current in a circuit is instruments. A current

transformer also isolates the measuring instruments from what may be very high voltage in the

monitored circuit. Current transformers are commonly used in metering too high to directly

apply to measuring instruments, a current transformer produces a reduced current accurately

proportional to the current in the circuit, which can be conveniently connected to measuring and

recording and protective relays in electrical power industry.

Measurement of errors of current transformers is very

essential for measurement of current accurately. There are several methods for the measurement

of errors of instrument of current transformers. The comparison method is a convenient one for

the measurement of the errors of current transformers. Conventionally, the comparison method is

implemented using an AC bridge of an AC potentiometer. We now present a LAB-VIEW based

technique for the determination of the errors of instrument transformers, dispensing the AC

bridge/potentiometer. Dispensing the bridge involves explicit measurements and trigonometric

computations which can be done much efficiently using LAB-VIEW.

LAB-VIEW is system design software that provides

engineers and scientists with the tools needed to create and deploy measurement and control

systems through unprecedented hardware integration. You can get more done in less time with

LAB-VIEW through its unique graphical programming environment; built-in engineering-

specific libraries of software functions and hardware interfaces; and data analysis, visualization,

and sharing features.

5

CONTENTS:

S.NO TITLE Page No.

1. Introduction 8 2. CURRENT TRANSFORMER

2.1 Principle of operation

2.2 Design

2.3 Types of current transformers

2.4 Accuracy

2.4.1 Burden

2.4.2 Knee –point Voltage

2.4.3 Rating factor

2.5 Safety and Precaution

2.6 Tests of CT

2.7 Applications

9 9

11 12 13 13 18 19 19 19 21 22

3. DATA ACQUISTION 3.1 Sensor

3.2 DAQ Device

3.2.1 Signal Conditioning

3.2.2 Analog-to-Digital Converter

3.3 Computer Bus

3.4 Advantages of NI DAQ

3.4.1 High-Performance I/O

3.4.2 Easy Sensor Connectivity with

22 22

23 24 26

33

36

6

3..4.3 Integrated Signal Conditioning

3..4.4. Improved Productivity through Software 37 37 38 38 38

4. LAB-VIEW 4.1 Introduction

4.2 Application Areas

4.2.1 For Acquiring Data and Processing Signals

4.2.2 For Instrument Control

4.2.3 For Monitoring and Controlling Embedded Systems

39 39 40 40 41 42 42 43

5. HARDWARE DESIGN 5.1 Priciple Of Measurement

5.2 Working

5.3 Waveforms And Results

45 45 46 47 48

6 CONCLUSION AND DISCUSSIONS 6.1. Results Acheived 6.2.Difficulties Encountered 6.3. Future Scope

49 49 50 51

7 REFERENCES 52

8 LIST OF FIGURES 54

8 APPENDIX 8.1 Software used 8.2 NI daq 6009 Specifications

55 55 56

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CHAPTER 1

INTRODUCTION

Instrument transformers, namely current and voltage (potential) transformers (CT and VT or PT),

are employed in power systems for measurement and protection purposes. These instrument

transformers are characterized by their ratio and phase errors. Direct and comparison methods

have long been in use for measurement of the errors of instrument transformers. The principal

drawback of conventional test equipment has been the tedious process of manual balancing. Zinn

was first to introduce a semi-automated scheme using analogue electronics for the measurement

of current transformer errors. The circuit by Iwansiw has AC potentiometer under the control of

microprocessor. Also a unit is developed to determine errors of instrument transformer without

need for an AC potentiometer. In our project we present the development of a unit employing of

a data acquisition unit to take the input currents of both standard and test specimen transformer

currents and using LAB-VIEW we calculate ratio and phase errors of test specimen CT.

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CHAPTER 2

CURRENT TRANSFORMER

A current transformer is defined as “as an

instrument transformer in which the secondary current is substantially proportional to the

primary current (under normal conditions of operation) and differs in phase from it by an angle

which is approximately zero for an appropriate direction of the connections.” This highlights the

accuracy requirement of the current transformer but also important is the isolating function,

which means no matter what the system voltage the secondary circuit need be insulated only for

a low voltage.

A current transformer also isolates the measuring instruments from what

may be very high voltage in the monitored circuit. Current transformers are commonly used in

metering too high to directly apply to measuring instruments, a current transformer produces a

reduced current accurately proportional to the current in the circuit, which can be conveniently

connected to measuring and recording and protective relays in electrical power industry.

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FIG 2.1 Basic current transformers

2.1. PRINCIPLE OF OPERATION:

The current transformer works on the principle of variable flux. In the

“ideal” current transformer, secondary current would be exactly equal (when multiplied by

the turn’s ratio) and opposite of the primary current. But, as in the voltage transformer, some

of the primary current or the primary ampere-turns are utilized for magnetizing the core, thus

leaving less than the actual primary ampere turns to be “transformed” into the secondary

ampere-turns. This naturally introduces an error in the transformation. The error is classified

into two-the current or ratio error and the phase error.

CT is designed to minimize the errors using the best quality electrical steels for

the core of the transformer. Both toroidal (round) and rectangular CT s are manufactured.

A current transformer is a transformer, which produces in its secondary winding a current,

which is proportional to the current flowing in its primary winding. The secondary current is

usually smaller in magnitude than the primary current.

The principal function of a CT is to produce a proportional current at a level of

magnitude, which is suitable for the operation of measuring or protective devices such as

indicating or recording instruments and relays. The rated secondary current is commonly 5A

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or 1A, though lower currents such as 0.5A are not uncommon. It flows in the rated secondary

load, usually called the burden, when the rated primary current flows in the primary winding.

The primary winding can consist merely of the primary current conductor passing once

through an aperture in the current transformer core or it may consist of two or more turns

wound on the core together with the secondary winding. The primary and secondary currents

are expressed as a ratio such as 100/5. With a 100/5 ratio CT, 100A flowing in the primary

winding will result in 5A flowing in the secondary winding, provided the correct rated

burden is connected to the secondary winding. Similarly, for lesser primary currents, the

secondary currents are proportionately lower.

It should be noted that a 100/5 CT would not fulfill the function of a 20/1

or a 10/0.5 CT as the ratio expresses the current rating of the CT, not merely the ratio of the

primary to the secondary currents. The extent to which the secondary current magnitude

differs from the calculated value expected by virtue of the CT ratio is defined by the

[accuracy] “Class” of the CT. The greater the number used to define the class, the greater the

permissible “current error” [the deviation in the secondary current from the calculated

value].

Except for the least accurate classes, the accuracy class also defines the

permissible phase angle displacement between primary and secondary currents. This latter

point is important with measuring instruments influenced both by magnitude of current and

by the phase angle difference between the supply voltage and the load current, such as kWh

meters, wattmeter’s, vary meters and power factor meters.

2.2. Design:

Like any other transformer, a current transformer has a primary winding, a magnetic current

and a secondary winding. The alternating current flowing in the primary produces a magnetic

field in the core, which then induces a current in the secondary winding circuit. A primary

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objective of current transformer design is to ensure that the primary and secondary circuits are

efficiently coupled, so that the secondary current bears an accurate relationship to the primary

current.

The most common design of CT consists of a length of wire wrapped many times around a

silicon steel ring passed over the circuit being measured. The CT's primary circuit therefore

consists of a single 'turn' of conductor, with a secondary of many tens or hundreds of turns. The

primary winding may be a permanent part of the current transformer, with a heavy copper bar to

carry current through the magnetic core. Window-type current transformers are also common,

which can have circuit cables run through the middle of an opening in the core to provide a

single-turn primary winding. When conductors passing through a CT are not centered in the

circular (or oval) opening, slight inaccuracies may occur.

Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical

examples of low voltage single ratio metering current transformers are either ring type or plastic

moulded case. High-voltage current transformers are mounted on porcelain bushings to insulate

them from ground. Some CT configurations slip around the bushing of a high-voltage

transformer or circuit breaker, which automatically centers the conductor inside the CT window.

The primary circuit is largely unaffected by the insertion of the CT. The rated secondary current

is commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would provide an output

current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can

be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or

burden, of the CT should be of low resistance. If the voltage time integral area is higher than the

core's design rating, the core goes into saturation. towards the end of each cycle, distorting the

waveform and affecting accuracy.

2.3 Types of Current transformers (CT’s)

They are available in 3 basic configurations:

1. Ring Core CT’s

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There are available for measuring currents from 50 to 5000 amps, with windows (power

conductor opening size) from 1″ to 8″ diameter.

2. Split Core CT’s

There are available for measuring currents from 100 to 5000 amps, with windows in

varying sizes from 1″ by 2″ to 13″ by 30″. Split core CT’s have one end removable so that

the load conductor or bus bar does not have to be disconnected to install the CT.

3. Wound Primary CT’s

There are designed to measure currents from 1 amp to 100 amps. Since the load

current passes through primary windings in the CT, screw terminals are provided for the load

and secondary conductors. Wound primary CT’s are available in ratios from 2.5:5 to 100:5

2.4. Accuracy

The accuracy of a CT is directly related to a number of factors including:

Burden

Burden class/saturation class

Rating factor

Load

Externa and external magnetic field

Temperature

Physical configuration.

The selected tap, for multi-ratio CTs

For the IEC standard, accuracy classes for various types of measurement are set out in IEC

60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate

measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is

1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also

important especially in power measuring circuits and each class has an allowable maximum

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phase error for specified load impedance. Current transformers used for protective relaying also

have accuracy requirements at overload currents in excess of the normal rating to ensure accurate

performance of relays during system faults

2.4.1 Burden

The secondary load of a current transformer is usually called the "burden" to distinguish it from

the load of the circuit whose current is being measured. The CT burden is the maximum load (in

VA) that can be applied to the CT secondary.

The CT secondary load = Sum of the VA’s of all the loads (ammeter, wattmeter, transducer etc.)

connected in series to the CT secondary circuit + the CT secondary circuit cable burden (cable

burden = I2 x R x L, where I = CT secondary current, R = cable resistance per length, L = total

length of the secondary circuit cable. If the proper size and short length of wire is used, cable

burden can be ignored).

The CT secondary circuit load shall not be more than the CT VA rating. If the load is less

than the CT burden, all meters connected to the measuring CT should provide correct

reading. So, in your example, there should not be any effect on Ammeter reading if you use a

CT of either 5 VA or 15 VA (provided the proper size and short length of wire is used for the

CT secondary side).

Accuracy of a CT is another parameter which is also specified with CT class. For example, if

a measuring CT class is 0.5M (or 0.5B10), the accuracy is 99.5% for the CT, and the

maximum permissible CT error is only 0.5%.

CT burden is the load imposed on CT secondary during operation.

The burden is mentioned as x-VA for the CT.

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In the case of Measuring Current transformer, the burden depends on the connected meters

and quantity of meters on the secondary i.e. no of Ammeters, KWh meters, Kvar meters,

Kwh meters, transducers and also the connection cable burden (I x I x R of cable x 2L) to

metering shall be taken into account.

where 2L is the to &fro distance of cable length-L from CT to metering circuits

R=is the resistance of unit length of connecting cable

I=secondary current of CT

Total burden of Measuring CT=Connecting cable Burden in VA + sum of Meters Burden in

VA

Note Meters burden can be obtained from manufacturer catalogue

Selected CT burden shall be more than the calculated burden.

In the case of Protection CTs the burden is calculated in the same way as above except the

burden of individual protective relays burden shall be considered instead of meters. The

connecting cable burden is calculated in the same way as metering CT

Total burden of Protection CT=Connecting cable Burden in VA + sum of Protective relays

Burden in VA.

Selected CT burden shall be more than the calculated burden.

The burden can be expressed in two ways. The burden can be expressed as the total

impedance in ohms of the circuit or the total volt-amperes and power factor at a specified

value of current or voltage and frequency.

The engineer can convert a volt-amperes value to total impedance in ohms by dividing the

volt-amperes by the square of the secondary amperes.

15

A typical calculation would be to convert 50 volt-amperes to total impedance in ohms.

Dividing 50 volt amperes by 52 /25 would be an impedance of 2 ohms.

To determine the total impedance, both active and reactive, one must sum the burden of the

individual devices connected to the current transformer.

The individual devices may only be the current transformer, a short run of wire and a meter.

In contrast, the circuit may have the current transformer, a lone run of wiring, a relay, a

meter, an auxiliary current transformer and a transducer. While the latter configuration would

not be used today, one may be required to make this calculation on an existing system.

All manufacturers can supply the burden of their individual devices. Although not used very

often these days, induction disk over-current devices always gave the burden for the

minimum tap setting. To determine the impedance of the actual tap setting being used, first

square the ratio of minimum divide by the actual tap setting used and, second multiply this

value by the minimum impedance.

Suppose an impedance of 1.47 + 5.34j at the 1-amp tap. To apply the relay at the 4-amp tap

the engineer would multiply the impedance at the 1-amp tap setting by (1/4)2. The

impedance at the 4-amp tap would be 0.0919 + 0.3338j or 0.3462 Z at 96.4 power factor.

The external load applied to the secondary of a current transformer is called the “burden”

The burden is expressed preferably in terms of the impedance of the load and its resistance

and reactance components.

Formerly, the practice was to express the burden in terms of volt-amperes and power factor,

the volt-amperes being what would be consumed in the burden impedance at rated secondary

current (in other words, rated secondary current squared times the burden impedance). Thus,

a burden of 0.5-ohm impedance may be expressed also as “12.5 volt-amperes at 5 amperes,”

if we assume the usual 5-ampere secondary rating. The volt ampere terminology is no longer

standard, but it needs defining because it will be found in the literature and in old data.

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The term burden is applied not only to the total external load connected to the terminals of a

current transformer but also to elements of that load. Manufacturer’s publications give the

burdens of individual relays, meters, etc., from which, together with the resistance of

interconnecting leads, the total CT burden can be calculated.

The CT burden impedance decreases as the secondary current increases, because of

saturation in the magnetic circuits of relays and other devices. Hence, a given burden may

apply only for a particular value of secondary current. The old terminology of volt-amperes

at 5 amperes is most confusing in this respect since it is not necessarily the actual volt

amperes with 5 amperes flowing, but is what the volt-amperes would be at 5 amperes

If there were no saturation. Manufacturer’s publications give impedance data for several

values of over current for some relays for which such data are sometimes required.

Otherwise, data are provided only for one value of CT secondary current.

If a publication does not clearly state for what value of current the burden applies, this

information should be requested. Lacking such saturation data, one can obtain it easily by

test. At high saturation, the impedance approaches the d-c resistance. Neglecting the

reduction in impedance with saturation makes it appear that a CT will have more inaccuracy

than it actually will have. Of course, if such apparently greater inaccuracy can be tolerated,

further refinements in calculation are unnecessary. However, in some applications neglecting

the effect of saturation will provide overly optimistic results; consequently, it is safer always

to take this effect into account.

It is usually sufficiently accurate to add series burden impedances arithmetically. The results

will be slightly pessimistic, indicating slightly greater than actual CT ratio inaccuracy. But, if

a given application is so borderline that vector addition of impedances is necessary to prove

that the CTÕs will be suitable, such an application should be avoided.

If the impedance at pickup of a tapped over current-relay coil is known for a given pickup

tap, it can be estimated for pickup current for any other tap. The reactance of a tapped coil

varies as the square of the coil turns, and the resistance varies approximately as the turns. At

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pickup, there is negligible saturation, and the resistance is small compared with the reactance.

Therefore, it is usually sufficiently accurate to assume that the impedance varies as the

square of the turns. The number of coil turns is inversely proportional to the pickup current,

and therefore the impedance varies inversely approximately as the square of the pickup

current.

Whether CT is connected in wye or in delta, the burden impedances are always connected in

wye. With wye-connected CTÕs the neutrals of the CTÕs and of the burdens are connected

together, either directly or through a relay coil, except when a so-called zero phase-sequence-

current shunt (to be described later) is used.

It is seldom correct simply to add the impedances of series burdens to get the total, whenever

two or more CTÕs are connected in such a way that their currents may add or subtract in

some common portion of the secondary circuit. Instead, one must calculate the sum of the

voltage drops and rises in the external circuit from one CT secondary terminal to the other for

assumed values of secondary currents flowing in the various branches of the external circuit.

The effective CT burden impedance for each combination of assumed currents is the

calculated CT terminal voltage divided by the assumed CT secondary current. This effective

impedance is the one to use, and it may be larger or smaller than the actual impedance which

would apply if no other CTÕs were supplying current to the circuit.

If the primary of an auxiliary CT is to be connected into the secondary of a CT whose

accuracy is being studied, one must know the impedance of the auxiliary CT viewed from its

primary with its secondary short-circuited. To this value of impedance must be added the

impedance of the auxiliary CT burden as viewed from the primary side of the auxiliary CT;

to obtain this impedance, multiply the actual burden impedance by the square of the ratio of

primary to secondary turns of the auxiliary CT. It will become evident that, with an auxiliary

CT that steps up the magnitude of its current from primary to secondary, very high burden

impedances, when viewed from the primary, may result.

Burden is depending on pilot lead length

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VA Applications

1 To 2 VA Moving iron ammeter

1 To 2.5VA Moving coil rectifier ammeter

2.5 To 5VA Electrodynamics instrument

3 To 5VA Maximum demand ammeter

1 To 2.5VA Recording ammeter or transducer

2.4.2 Knee-point voltage

The knee-point voltage of a current transformer is the magnitude of the secondary voltage after

which the output current ceases to follow linearly the input current. This means that the one-to-

one or proportional relationship between the input and output is no longer within declared

accuracy. In testing, if a voltage is applied across the secondary terminals the magnetizing

current will increase in proportion to the applied voltage, up until the knee point. The knee point

is defined as the point at which an increase of applied voltage of 10% results in an increase in

magnetizing current of 50%. From the knee point upwards, the magnetizing current increases

abruptly even with small increments in voltage across the secondary terminals. The knee-point

voltage is less applicable for metering current transformers as their accuracy is generally much

tighter but constrained within a very small bandwidth of the current transformer rating, typically

1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to

protection current transformers, since they are necessarily exposed to currents of 20 or 30 times

rated current during faults.

2.4.3 Rating factor

Rating factor viqar is a factor by which the nominal full load current of a CT can be multiplied to

determine its absolute maximum measurable primary current. Conversely, the minimum primary

current a CT can accurately measure is "light load," or 10% of the nominal current (there are,

however, special CTs designed to measure accurately currents as small as 2% of the nominal

current). The rating factor of a CT is largely dependent upon ambient temperature. Most CTs

19

have rating factors for 35 degrees Celsius and 55 degrees Celsius. It is important to be mindful of

ambient temperatures and resultant rating factors when CTs are installed inside pad-mounted

transformers or poorly ventilated mechanical rooms. Recently, manufacturers have been moving

towards lower nominal primary currents with greater rating factors. This is made possible by the

development of more efficient ferrites and their corresponding hysteresis curves.

2.5. Safety and Precautions:

For personnel and equipment safety and measurement accuracy, current measurements on

conductors at high voltage should be made only with a conducting shield cylinder placed

inside the CT aperture. There should be a low electrical impedance connection from one end

only to a reliable local ground. An inner insulating cylinder of adequate voltage isolation

should be between the shield cylinder and the conductor at high voltage. Any leakage,

induced or breakdown current between the high voltage conductor and the ground shield will

substantially pass to local ground rather than through the signal cable to signal ground.Do not

create a “current loop” by connecting the shield cylinder to ground from both ends. Current

flowing in this loop will also be measured by the CT.

Unless a burden (i.e. meters, relays, etc.) is connected to the CT, current

transformers should always be shorted across the secondary terminals. The reason is very

high voltages will be induced at the terminals. Think of the CT as a transformer, with a 1 turn

primary and many turns on the secondary. When current is flowing through the primary, the

resulting voltage induced in the secondary can be quite high, on the order of kilovolts. When

a CT fails under open circuit conditions, the cause of failure is insulation breakdown, either

at the shorting terminal strip, or at the feedthrough (in the case of oil filled apparatus),

because the distances between terminals are not sufficient for the voltages present. CT is

connect in series with the load, when CT is shorted or connected to certain load, the

impedance become negligible compared to circuit its connected, so voltage drop across CT

will be negligible. When CT gets open circuited, the impedance of CT become infinity,

hence the voltage drop across CT primary will try to rise up to to the rated supply voltage,

but in this process insulation fails.

20

The reason is that the fidelity of the current transformer is highest when the

secondary is short circuited.The purpose of a current transformer is to provide a scaled-down

version of the primary current with the highest possible fidelity, that is, the secondary current

should be a faithful replica of the primary current. In many applications it is not just the

magnitude of the measured current that is important, but also faithful reproduction of the

phase and high-order harmonics.

Thus, the question becomes, why is the fidelity highest when the

secondary is shorted? At first this seems counter-intuitive, for the fidelity of the more

familiar voltage transformer is most definitely NOT optimised by shorting the secondary!

Indeed, as a voltage transformer is loaded more heavily by reducing the secondary load

resistance, the secondary voltage 'sags', phase error is increased, and frequency response

decreased, all leading to a loss of fidelity of the secondary voltage waveform. This occurs

mainly due to winding resistance and leakage inductance, which would both be zero in an

'ideal' transformer. For a voltage transformer, the fidelity of the secondary voltage is highest

with a high secondary load resistance, which draws very little current.

However, with current transformers, just about everything turns out to be

reversed compared to voltage transformers, which can be confusing. As with all

transformers, the secondary load resistance is reflected back to the primary side, scaled by

the square of the turns ratio. Therefore, if we short the secondary, then we also short the

primary. Note that the primary current is fixed and constant, being set by the external circuit.

Therefore, from V=IR, the voltage developed across the primary winding is approximately

zero when the secondary is shorted. Of course, the secondary voltage is also zero when

shorted. But why does this improve the current fidelity? Without going into too much detail,

the 'magnetizing current' of an transformer depends on the winding inductance and the

winding voltage. The inductance is set by the number of turns and the core material and

geometry, which are fixed for any given current transformer. However, by reducing the

winding voltages, by shorting the secondary, the magnetizing current is also reduced to near

zero. That is a GOOD THING, because the magnetizing current is an error, representing a

21

proportion of the primary current that DOES NOT end up being reflected in the secondary

current. The magnetizing current results in an error in both the magnitude and phase of the

secondary current, definitely not a good thing where high fidelity is required. To say the

same thing in a different way, shorting the secondary permits larger currents to be measured

with acceptable fidelity, and without core saturation.

2.6. Tests of CT

A number of routine and type tests have to be conducted on CT s before they can meet the

standards specified above. The tests can be classified as :

1. Accuracy tests to determine whether the errors of the CT are within specified limits.

2. Dielectric insulation tests such as power frequency withstand voltage test on primary

and secondary windings for one minute, inter-turn insulation test at power frequency

voltage, impulse tests with 1.2u/50 wave, and partial discharge tests (for voltage >=6.6kv)

to determine whether the discharge is below the specified limits.

3. Temperature rise tests.

4. Short time current tests.

5. Verification of terminal markings and polarity.

2.7. APPLICATIONS

6. The variety of applications of current transformers seems to be limited only by ones

imagination. As new electronic equipment evolves and plays a greater role in the

generation, control and application of electrical energy, new demands will be placed upon

current transformer manufacturers and designers to provide new products to meet these

needs.

7. Current transformers are used extensively for measuring current and monitoring the

operation of the Power grid. Along with voltage leads, revenue-grade CTs drive the

electrical utility's watt-hour meter on virtually every building with three-phase service

and single-phase services greater than 200.

8. The CT is typically described by its current ratio from primary to secondary. Often,

22

multiple CTs are installed as a "stack" for various uses. For example, protection devices

and revenue metering may use separate CTs to provide isolation between metering and

protection circuits, and allows current transformers with different characteristics

(accuracy, overload performance) to be used for the devices.

CHAPTER 3

DATA ACQUISTION (DAQ) Data acquisition (DAQ) is the process of measuring an electrical or physical

phenomenon such as voltage, current, temperature, pressure, or sound with a computer. A DAQ

system consists of sensors, DAQ measurement hardware, and a computer with programmable

software. Compared to traditional measurement systems, PC-based DAQ systems exploit the

23

processing power, productivity, display, and connectivity capabilities of industry-standard

computers providing a more powerful, flexible, and cost-effective measurement solution.

PARTS OF DAQ SYSTEM

FIG 3.1: Basic working of DAQ

3.1. SENSOR The measurement of a physical phenomenon, such as the temperature of a room, the

intensity of a light source, or the force applied to an object, begins with a sensor. A sensor, also

called a transducer, converts a physical phenomenon into a measurable electrical signal.

Depending on the type of sensor, its electrical output can be a voltage, current, resistance, or

another electrical attribute that varies over time. Some sensors may require additional

components and circuitry to properly produce a signal that can accurately and safely be read by a

DAQ device.

Essential to any data acquisition is a transducer sensor that converts real-world

phenomena, such as temperature and pressure, into measurable currents and voltages. This set of

tutorials is designed to provide you with essential information about using and understanding

various types of sensors to acquire data.

Common Sensors:

Sensor Phenomenon

24

Thermocouple, RTD, Thermistor Temperature

Photo Sensor Light

Microphone Sound

Strain Gage, Piezoelectric Transducer Force and Pressure

Potentiometer, LVDT, Optical Encoder Position and Displacement

Accelerometer Acceleration

pH Electrode pH

3.2.DAQ DEVICE DAQ hardware acts as the interface between a computer and signals from the outside

world. It primarily functions as a device that digitizes incoming analog signals so that a computer

can interpret them. The three key components of a DAQ device used for measuring a signal are

the signal conditioning circuitry, analog-to-digital converter (ADC), and computer bus. Many

DAQ devices include other functions for automating measurement systems and processes. For

example, digital-to-analog converters (DACs) output analog signals, digital I/O lines input and

output digital signals, and counter/timers count and generate digital pulses.

Key Measurement Components of a DAQ Device:

3.2.1 Signal Conditioning

Signals from sensors or the outside world can be noisy or too dangerous to measure directly.

Signal conditioning circuitry manipulates a signal into a form that is suitable for input into an

ADC. This circuitry can include amplification, attenuation, filtering, and isolation. Some DAQ

devices include built-in signal conditioning designed for measuring specific types of sensors.

Many applications require environment or structural measurements, such as

temperature and vibration, from sensors. These sensors, in turn, require signal conditioning

before a data acquisition device can effectively and accurately measure the signal. Key signal

conditioning technologies provide distinct enhancements to both the performance and

accuracy of data acquisition systems.

25

FIG 3.2: Signal conditioning provides more accurate sensor measurements.

Amplification

Amplifiers increase voltage level to better match the analog-to-digital converter (ADC) range,

thus increasing the measurement resolution and sensitivity. In addition, using external signal

conditioners located closer to the signal source, or transducer, improves the measurement signal-

to-noise ratio by magnifying the voltage level before it is affected by environmental noise.

Attenuation

Attenuation, the opposite of amplification, is necessary when voltages to be digitized are beyond

the ADC range. This form of signal conditioning decreases the input signal amplitude so that the

conditioned signal is within ADC range. Attenuation is typically necessary when measuring

voltages that are more than 10 V.

Isolation

Isolated signal conditioning devices pass the signal from its source to the measurement device

without a physical connection by using transformer, optical, or capacitive coupling techniques.

In addition to breaking ground loops, isolation blocks high-voltage surges and rejects high

common-mode voltage and thus protects both the operators and expensive measurement

equipment.

Filtering

Filters reject unwanted noise within a certain frequency range. Oftentimes, lowpass filters are

26

used to block out high-frequency noise in electrical measurements, such as 60 Hz power.

Another common use for filtering is to prevent aliasing from high-frequency signals. This can be

done by using an antialiasing filter to attenuate signals above the Nyquist frequency.

Excitation

Excitation is required for many types of transducers. For example, strain gages, accelerometers

thermistors, and resistance temperature detectors (RTDs) require external voltage or current

excitation. RTD and thermistor measurements are usually made with a current source that

converts the variation in resistance to a measurable voltage. Accelerometers often have an

integrated amplifier, which requires a current excitation provided by the measurement device.

Strain gages, which are very-low-resistance devices, typically are used in a Wheatstone bridge

configuration with a voltage excitation source.

Linearization

Linearization is necessary when sensors produce voltage signals that are not linearly related to

the physical measurement. Linearization is the process of interpreting the signal from the sensor

and can be done either with signal conditioning or through software. Thermocouples are the

classic example of a sensor that requires linearization.

Cold-Junction Compensation

Cold-junction compensation (CJC) is a technology required for accurate thermocouple

measurements. Thermocouples measure temperature as the difference in voltage between two

dissimilar metals. Based on this concept, another voltage is generated at the connection between

the thermocouple and terminal of your data acquisition device. CJC improves your measurement

accuracy by providing the temperature at this junction and applying the appropriate correction.

Bridge Completion

27

Bridge completion is required for quarter- and half-bridge sensors to comprise a four resistor

Wheatstone bridge. Strain gage signal conditioners typically provide half-bridge completion

networks consisting of high-precision reference resistors. The completion resistors provide a

fixed reference for detecting small voltage changes across the active resistor(s).

3.2.2 Analog-to-Digital Converter (ADC)

Analog signals from sensors must be converted into digital before they are manipulated by digital

equipment such as a computer. An ADC is a chip that provides a digital representation of an

analog signal at an instant in time. In practice, analog signals continuously vary over time and an

ADC takes periodic “samples” of the signal at a predefined rate. These samples are transferred to

a computer over a computer bus where the original signal is reconstructed from the samples in

software.

a) Bandwidth is defined as the measure of a circuit or transmission channel to pass

a signal without significant attenuation over a range of frequencies. Bandwidth is

measured between the lower and upper frequency points where the signal amplitude falls

to -3 dB below the pass-band frequency. The -3 dB points are referred to as the half-power

points.

Units

Hertz (Hz)

Example

-3 dB = 20 LOG (Vppout / Vppin)

Where

Vppout = Peak to peak Voltage of the output waveform

Vppin = Peak to peak Voltage of the input waveform = 1 V (in the above example)

If you input a 1 V, 100 MHz sine wave into high-speed digitizer with a bandwidth of 100

MHz, the signal will be attenuated by the digitizer’s analog input path and the sampled

waveform will have amplitude of approximately 0.7 V. The value of ~0.7 V can be

calculated by using the following equation:

28

-3 = 20 LOG (Vppout / 1)

Vppout = 0.7079 V = 0.7 V approximately

FIG 3.3: Attenuation of a 100 MHz sine wave when passed through a 100 MHz Digitizer

FIG 3.4: Typical 100 MHz Digitizer Input Response

Theoretical amplitude error of a measured signal

It is recommended that the bandwidth of your digitizer be 3 to 5 times the highest

frequency component of interest in the measured signal to capture the signal with minimal

amplitude error (bandwidth required = (3 to 5)*frequency of interest). The theoretical

amplitude error of a measured signal can be calculated from the ratio (R) of the digitizer's

bandwidth (B) in relation to the input signal frequency (fin)

29

Where

R = B / fin

Rise Time

Another important topic related to the bandwidth is rise time. The rise time of an input signal is

the time for a signal to transition from 10% to 90% of the maximum signal amplitude and is

inversely related to bandwidth.

FIG 3.5: Rise time for a signal is the time span from 10% to 90% of its maximum amplitude

It is recommended that the rise time of the digitizer input path be 1/3 to 1/5 the rise time of the

measured signal to capture the signal with minimal rise time error. The theoretical rise time

measured (Trm) can be calculated from the rise time of the digitizer (Trd) and the actual rise

time of the input signal (Trs).

b) Sampling rate is the rate at which data is sampled. Sampling rate is not directly related to the

bandwidth specifications of a high-speed digitizer. Sampling rate is the speed at which the

digitizer’s ADC converts the input signal, after the signal has passed through the analog input

30

path, to digital values. Hence, the digitizer samples the signal after any attenuation, gain, and/or

filtering has been applied by the analog input path, and converts the resulting waveform to digital

representation. The sampling rate of a high-speed digitizer is based on the sample clock that

controls when the ADC converts the instantaneous analog voltage to digital values.

There are several products available in the market like National Instruments M-series Data

Acquisition, Digital Signal Acquisition, Digital Multimeters and several others that have

different specifications for the maximum sampling rate. The choice of the most appropriate

device for your application will depend on the signal you are measuring.

c) Nyquist Theorem: Sampling rate (f s) > 2 * highest frequency component (of

interest) in the measured signal The Nyquist theorem states that a signal must be sampled at a

rate greater than twice the highest frequency component of interest in the signal to capture the

highest frequency component of interest; otherwise, the high-frequency content will alias at a

frequency inside the spectrum of interest (pass-band).

31

FIG 3.6: Effects of various sampling rates while sampling a signal

d) If a signal is sampled at a sampling rate smaller than twice the Nyquist frequency,

false lower frequency component(s) appears in the sampled data. This phenomenon is called

Aliasing.

The following figure shows a 5 MHz sine wave digitized by a 6 MS/s ADC. The dotted line

indicates the aliased signal recorded by the ADC. The 5 MHz frequency aliases back in the pass-

band, falsely appearing as a 1 MHz sine wave

FIG 3.7:. Sine wave demonstrating Aliasing

32

Alias frequency

The alias frequency is the absolute value of the difference between the frequency of the input

signal and the closest integer multiple of the sampling rate.

Alias Freq. = ABS (Closest Integer Multiple of Sampling Freq. – Input Freq.)

where

ABS means the absolute value.

e) Quantization is defined as the process of converting an analog signal to a digital

representation. Quantization is performed by an analog-to-digital converter (A/D converter or

ADC).

If we can convert our analog signals to a stream of digital data, we can take advantage of the

power of the personal computer and software to do any manipulation or calculation on the

signals. To do this, we must sample our analog waveform at well-defined discrete (but limited)

times so we can maintain a close relationship between time in the analog domain and time in the

digital domain. If we do this, we can reconstruct the signal in the digital domain, do our

processing on it, and later, reconstruct it into the analog domain if we need to.

FIG 3.8: When converting an analog signal to digital domain, signal values are taken at discrete

time instants.

The time resolution we have is limited by the maximum sampling rate of the ADC. Even if we

were able to increase our sampling rate forever, it would still never be purely “continuous time”

as is our input signal, as shown in figure 9. For most real world applications, this is still very

useful despite its limited nature. But obviously the usefulness of our digital representation

increases as our time and amplitude resolution increases. The amplitude resolution is limited by

the number of discrete output levels an ADC has.

33

FIG 3.9: Quantization error when using a 3 bit ADC

f) Dithering: During Quantization, in the time domain, we could almost completely

preserve the waveform information by sampling fast enough. In the amplitude domain we

can preserve most of the waveform information by dithering.

Dithering involves the deliberate addition of noise to our input signal. It helps by

smearing out the little differences in amplitude resolution. The key is to add random

noise in a way that makes the signal bounce back and forth between successive levels. Of

course, this in itself just makes the signal noisier. But, the signal smoothes out by

averaging this noise digitally once the signal is acquired.

34

FIG 3.10: Effects of dithering and averaging on a sine wave input

3.3.Computer Bus

DAQ devices connect to a computer through a slot or port. The computer bus serves as the

communication interface between the DAQ device and computer for passing instructions and

measured data. DAQ devices are offered on the most common computer buses including USB,

PCI, PCI Express, and Ethernet. More recently, DAQ devices have become available for 802.11

Wi-Fi for wireless communication. There are many types of buses, and each offers different

advantages for different types of applications.

All PC buses have a limit to the amount of data that can be transferred in a

certain period of time. This is known as the bus bandwidth and is often specified in megabytes

per second (MB/s). If dynamic waveform measurements are important in your application, be

sure to consider a bus with enough bandwidth.

Depending on the bus that you choose, the total bandwidth can be shared among several

devices or dedicated to certain devices. The PCI bus, for example, has a theoretical bandwidth of

132 MB/s that is shared among all PCI boards in the computer. Gigabit Ethernet offers 125 MB/s

35

shared across devices on a subnet or network. Buses that offer dedicated bandwidth, such as PCI

Express and PXI Express, provide the maximum data throughput per device.

When taking waveform measurements, you have a certain sampling rate and resolution that need

to be achieved based on how fast your signal is changing. You can calculate the minimum

required bandwidth by taking the number of bytes per sample (rounded up to the next byte),

multiplied by the sampling speed, and then multiplied by the number of channels.

For example, a 16-bit device (2 bytes) sampling at 4 MS/s on four channels would be

our bus bandwidth needs to be able to support the speed at which data is being acquired, and it is

important to note that the actual system bandwidth will be lower than the theoretical bus limits.

Actual observed bandwidth depends on the number of devices in a system and any additional bus

traffic from overhead. If you need to stream a lot of data on a large number of channels,

bandwidth may be the most important consideration when choosing your DAQ bus.

While there are many different buses and form factors to choose from, this section focuses on the

seven most common buses, including:

b) PCI

c) PCI Express

d) USB

e) PXI

f) PXI Express

g) Ethernet

h) Wireless

Figure 1 shows these buses organized into a PC-bus hierarchy of NI data acquisition products,

from internal plug-in options to hot-swappable external buses.

36

USB

FIG 3.11: DAQ which connected to computer through USB.

The Universal Serial Bus (USB) was originally designed to connect peripheral devices, such as

keyboards and mice, with PCs. However, it has proven useful for many other applications,

including measurement and automation. USB delivers an inexpensive and easy-to-use

connection between data acquisition devices and PCs. USB 2.0 has a maximum theoretical

bandwidth of 60 MB/s, which is shared among all devices connected to a single USB controller.

USB devices are inherently latent and nondeterministic. This means that single-point data

transfers may not happen exactly when expected, and therefore USB is not recommend for

closed-loop control applications, such as PID.

On the other hand, the USB bus has several characteristics that make it easier to use than some

traditional internal PC buses. Devices that connect using USB are hot-pluggable, so they

eliminate the need to shut down the PC to add or remove a device. The bus also has automatic

device detection, meaning that users do not have to manually configure their devices after

plugging them in. Once the software drivers have been installed, the operating system should

detect and install the device on its own.

37

FIG 3.12: The required signals are fed to computer for analysis.

3.4. Advantages of NI DAQ

Designed for performance, NI data acquisition devices provide high-performance I/O, industry-

leading technologies, and software-driven productivity gains for your application. With patented

hardware and software technologies, National Instruments offers a wide-spectrum of PC-based

measurement and control solutions that deliver the flexibility and performance that your

application demands. For more than 25 years, National Instruments has served as more than just

an instrument vendor, but as a trusted advisor to engineers and scientists around the world.

3.4.1. High-Performance I/O

Measurement accuracy is arguably one of the most important considerations in designing

any data acquisition application. Yet equally important is the overall performance of the

system, including I/O sampling rates, throughput, and latency. For most engineers and

scientists, sacrificing accuracy for throughput performance or sampling rate for resolution is

not an option. National Instruments wide selection of PC-based data acquisition devices

have set the standard for accuracy, performance, and ease-of-use from PCI to PXI and USB

to wireless.

38

3.4.2. High-Accuracy Designs

Many scientists and engineers mistakenly evaluate DAQ device error by just

considering the bit resolution of the DAQ device. However, the error dictated by the device

resolution, or quantization error, might account for only a very small amount of the total

error in your measurement result. Other types of errors, such as temperature drift, offset,

gain, and non-linearity can vary drastically by hardware design. Through years of

experience, NI has developed several key technologies to minimize these errors and

maximize the absolute accuracy of your measurements.

3.4.3. Easy Sensor Connectivity with Integrated Signal Conditioning

Traditionally, measuring sensors required separate front-end signal conditioning systems

cabled to a data acquisition system. New technologies and miniaturization have enabled the

integration of sensor-specific signal conditioning and analog to digital conversion on the

same device. NI DAQ devices with integrated signal conditioning deliver higher-accuracy

measurements by eliminating error-prone cabling and connectors and reduce the number of

components in a measurement system. NI has also partnered leading sensor vendors to

provide easy, tool-free sensor connectivity and automatic sensor configuration with TEDS

technology.

3.4.4. Improved Productivity through Software

One of the biggest benefits of using a PC-based data acquisition device is that you

can use software to customize the functionality and visualization of your measurement

system to meet your application needs. When examining the cost of building a data

acquisition system, software development often accounts for 25 percent of total system cost.

Obtaining easy-to-use driver software with an intuitive application programming interface

makes a big impact on completing a project on time and under budget. National Instruments

provides a wide array of software tools that make you more productive at accomplishing

your measurement or automation tasks.

39

FIG 3.13: DAQ(NI usb 6009)

40

CHAPTER-4

LABVIEW 4.1 INTRODUCTION:

LabVIEW is system design software that provides engineers and scientists with the tools

needed to create and deploy measurement and control systems through unprecedented hardware

integration. LabVIEW inspires you to solve problems, accelerates your productivity, and gives

you the confidence to continually innovate.

FIG 4.1: LABVIEW Logo

LabVIEW is an ideal platform for prototyping, designing, and deploying high-quality

products to market fast. You can use one development environment to quickly iterate on your

embedded hardware and software designs and then reuse the best parts in a final product. The

complexity of products that engineers need to test is increasing rapidly. Markets are demanding

improved quality with additional features. LabVIEW reduces the time to test these products by

helping you develop a flexible and efficient system that synchronizes multiple measurements and

analysis within your software. This results in faster inspection times across I/O.

APPLICATION AREAS:

1. For Acquiring Data and Processing Signals:

LabVIEW is system design software that accelerates your productivity by automating

several measurements from a wide variety of sensors. With tight hardware integration,

you can connect to more than 200 NI data acquisition and third-party devices, and with

41

the unparalleled data analysis, visualization, and sharing features, you can save time as

you translate results into decisions. Experience the flexibility of LabVIEW for

measurements.

• Add Power and Flexibility Through Software

One of the biggest benefits of using a PC-based DAQ device is that you can use

software to customize your measurement system functionality and visualization to meet

your application needs. Taking measurements by hand is costly, slow, and error-prone

whereas software defined systems perform quickly and consistently. NI LabVIEW software

provides a single programming interface to DAQ devices, resulting in seamless hardware

and software integration. You can automate measurements from several devices, analyze

data in parallel with acquisition, and create custom reports all in a matter of minutes with

LabVIEW.

• Accelerate Your Productivity

LabVIEW makes you more productive by focusing on data and the operations

performed on that data – as well as abstracting much of the administrative complexity of

computer programming such as memory allocation and language syntax. Built-in

engineering-specific software libraries allow for easy acquisition, analysis, control, and

data-sharing, so that you can focus on your data, not on programming.

• Build on Industry-Leading Innovation

National Instruments has been a premier virtual instrumentation hardware and

software developer for more than 25 years. Feel safe knowing that you are building on a

platform supported by an extensive R&D team, alliances with leading technology partners,

and experience making innovative software and dependable hardware that work hand in

hand.

• Distribute Stand-Alone Applications

You can create stand-alone applications for programs that need to be deployed to

42

other systems with the LabVIEW Application Builder. Distribute royalty-free copies of

software as end-use applications, or provide developers with shared libraries for use in

other development environments. LabVIEW makes it easy for you to create installers that

bundle all of the drivers and required run-time engines with your executables.

• Save Time by Using Pre-built Components

You do not need to create your entire DAQ system from scratch. Save up to 80

percent of your development time using the built-in LabVIEW Express functions and

example programs. Express functions help you program using interactive windows and

simple pull-down menus, and take you step-by-step through the configuration, so you can

apply custom scaling and engineering units. LabVIEW also includes several open-and-run

example programs for most common measurement tasks, ranging from simple single-

channel measurements to advanced timing, triggering, and synchronization across multiple

devices. Configuring voltage, current, temperature, strain, sound, and other sensor

measurements has never been easier.

• Turn Raw Data Into Results

LabVIEW includes thousands of advanced analysis functions created specifically

for engineers and scientists, all with detailed help files and documentation. With these

powerful tools, you can perform advanced signal processing; frequency analysis;

probability and statistics; curve fitting; interpolation; digital signal processing; and more.

You also can extend LabVIEW with toolkits for sound and vibration; machine vision; RF

communications; and transient and short-time duration signal analysis. For additional

analysis, you can integrate LabVIEW with algorithms developed in third-party software.

• Drag and Drop Your Way to a Custom User Interface

You can quickly create a GUI using hundreds of drag-and-drop controls,

graphs, and 3D visualization tools. Built for engineers and scientists, UI elements work

seamlessly with acquired and analyzed data without complicated reformatting and custom

development. You also can incorporate custom imagery and logos, or modify the default

controls to provide a custom appearance.

43

• Log Data and Generate Reports

Writing your data to disk or creating a custom report is as simple as calling

one function within LabVIEW. Native file formats are optimized for high-speed streaming,

and LabVIEW easily integrates with NI DIAdem software so you can quickly locate,

inspect, analyze, and report on measurement data. LabVIEW works with spreadsheet

applications such as Microsoft Excel. You also can use it to attach descriptive information

to your measurements, making them easier to reference offline.

• Stay Informed and Protect Your Investment

Whether you are a single-seat user or have a business with multiple licenses, the

NI software maintenance and support program can help you maximize your software

investment. Stay up to date on the latest technology improvements by automatically

receiving software updates and maintenance releases. Reduce your application development

time with direct access to technical support from NI applications engineers. Be informed

through special online software training modules that highlight features, application uses,

and development best practices.

• Increase Your Industry Value

NI training courses help you quickly pick up new skills, and NI certification helps

you prove your understanding of proper development and documentation practices that

make your applications easier to develop, support, and maintain. It also gives your

customers, peers, and employers confidence in your abilities, which can potentially lead to

new business, promotions, career opportunities, and an increased salary.

2. For Instrument Control:

NI LabVIEW is a graphical programming environment that makes it easy to

control and acquire data from any instrument over any bus. You can automate

measurements from several devices, analyze data as you acquire it, and create custom

reports all in a matter of minutes. Avoid spending hours learning how to take

measurements from a particular device. With LabVIEW, you can focus on the results

rather than the process of obtaining them.

44

• Work Faster With a Graphical Approach

Develop and debug applications using drag-and-drop graphical icons and

flowchart representations instead of writing lines of text.

• Quickly Automate Any Instrument Using Free Instrument Drivers

Download time-saving LabVIEW drivers for virtually any instrument free of

charge. The Instrument Driver Finder helps you install drivers in seconds directly from

LabVIEW. Consistent driver APIs eliminate the need for you to learn low-level instrument

commands specific to each instrument.

• Get Started Immediately With Open-and-Run Examples

Never start developing code from scratch. Every certified instrument driver

includes ready-to-run examples.

3. for Monitoring and Controlling Embedded Systems:

LabVIEW is system design software that is used by engineers and

scientists to efficiently design, prototype, and deploy embedded monitoring and control

applications.It combines hundreds of prewritten libraries, tight integration with off-the-shelf

hardware, and a variety of programming approaches including graphical development, .m file

scripts, and connectivity to existing ANSI C and HDL code. Whether designing medical

devices or complex robots, you can reduce time to market and the overall cost of embedded

monitoring and control with LabVIEW.

Reasons to Use NI LabVIEW for Designing Embedded Systems:

Prototype Faster with a Graphical Design

EnvironmentEasily Reuse Embedded Code and Existing IP

45

CHAPTER 5

HARDWARE DESIGN

PRICIPLE OF MEASUREMENT:

The figure below depicts the principle of error measurement for a CT using the

comparison method. The errors of the test CTs, CTx are determined by comparing it with a

standard CT, CTs having same ratio as the test CT but possessing very low, or known errors.

The errors can be expressed as,

FIG 5.1: Basic circuit for comparison test.

To find ratio and phase errors we need to take both current signals of standard

transformer and test specimen transformer. Considering Is1 and Is2 as currents of both the

transformers. Now to determine the ratio error we need to know differential current flowing in

between the two current signals. The ratio error is defined as ratio of cosine component of

differential current to standard current. Phase error is defined as the differential angle with

46

respect to the reference value. The differential current and standard current values are explicitly

measured using data acquisition system and errors are compiled.

WORKING:

In order to find ratio and phase errors we need the current signals of both the

transformers standard and test specimen. These signals are fed to DAQ as voltage signals by

taking signals from resistors connected parallel to them. Now from here the software part comes

into picture. These two voltage signals are sent to the software LAB-VIEW using DAQ and

waveform is generated in the computer, now the rms values of both the signals are calculated. To

find differential current we need substract those two signals and we get rms value of differential

current and the ratio error and phase error can be calculated using above formulae.

The DAQ receives the two voltage signals which are analog and these signals are

continuously sampled using DAQ assistant part of LAB-VIEW. After connecting the CT’s to

DAQ, the DAQ should be connected to computer then in LAB-VIEW we need to configure the

signals which are taken for calculations. The figure shows the block diagram of the project.

FIG 5.2: Test and standard transformers are connected to DAQ

47

FIG 5.3: Block diagram of our project.

Firstly, the signals are converted into respective rms values, then these rms values are subtracted

to obtain the rms value of differential current now the phase angle can be obtained using special

functions in LAB-VIEW now the error is the ratio of cosine component of differential current

with standard current, the cosine of phase angle is obtained by functions then multiplied with rms

value of differential current and ratio of this value is taken with rms value of standard current. To

calculate the phase error sine component is used and inverse tangent is used and phase is also

calculated.

In the figure shown below the two current transformers, standard and the test specimen

transformers primary sides are connected in series with a load. The load used is 100w bulb. Then

supply is connected to primary. A resistor is connected to both the secondaries and a resistor is

connected to both secondaries and standard current flows through the resistors. Standard voltage

waveform and differential voltage waveform are taken from the terminals of standard resistor

and differential resistor.These two terminals from standard and differential resistors are

connected to DAQ. From DAQ it is connected to computer through LAB-VIEW.

48

FIG: schematic diagram of current transformers

RATIO ERROR:

The secondary current is less than the expected value. The secondary is less in

magnitude. This diffence is known as ratio error.

PHASE ANGLE ERROR:

The angle between the expected and actual secondary current is known as phase error.

49

WAVEFORMS AND RESULT: The voltage waveforms of both current transformers are shown below,

FIG 5.4: Waveforms of both currents.

FIG: These are the voltage signals of test and standard transformers.

The obtained values of ratio error and phase error are given below,

FIG 5.5: Results obtained in LABVIEW

FIG: The ratio error and Phase error are calculated in LAB-VIEW.

50

The signals are scaled because the signal obtained is very low and finally the ratio error and

phase error values are obtained. Multiple measurements can be easily implemented and average

value is obtained. Comparing other methods DAQ based unit for , measurement is accurate easy

method to find ratio and phase errors.

CHAPTER-6

CONCLUSION AND DISCUSSIONS 6.1. RESULTS ACHIEVED IN THE PROJECT

In this project, we have calculated ratio error and phase angle error of current

transformer with reference to standard current transformer using DAQ. The results obtained from

lab-view are very accurate so we can get correct values when compared to other methods such as

comparison method etc.

FIG 6.1. RATIO AND PHASE ERROR CALCULATED IN LAB-VIEW

The above figure shows the values of ratio and phase error of current transformer. We can

see the rms values of both current transformers, phase difference between the two signals. The

advantage of the proposed method lies in the fact that the measuring time is independent of the

51

magnitude of error.

6.2.DIFFICULTIES ENCOUNTERED : To calculate the exact resistance to be connected across secondary of the cuurent

transoformer so that current flowing through it will be in permissible limits.

During simulation of the circuit, we have faced the problem with signals as they are not

perfectly sinusoidal.

FIG 6.2: These are the voltage signals of test and standard transformers

While calculating ratio error and phase error in lab-view ,as the values are very small we

have calibrated the values so that we can easily use it for further calculations.

52

FIG 6.3: IMPLEMENTATION OF LOGIC IN LAB-VIEW

The figure shows how the mathematical calculations are done in lab-view to

calculate ratio error and phase error.

6.3. FUTURE SCOPE The ratio error and phase angle error of current transformer are calculated using

lab-view. Calculation of errors in current transformers is very important because while we use

current transformers for measurement purposes, errors plays very important role in calculation

of the accurate result.

The future scope is to calculate the ratio error and phase error using

microprocessor so that we can get more accurate result. The advantage of using microprocessor

lies in the fact that the measuring time is independent of the magnitude of error. Multiple

measurements can be made and averaging can be easily implemented in the software to take into

account noise in the ZCD, if felt necessary.

53

REFERENCES

DAQ ASSISTANT BOOKS REFERED

[1] LabVIEW for Data Acquisition- Bruce Mihura

[2] Introduction to Data Acquisition with LabVIEW- Robert H. King

INSTRUMENT TRANSFORMERS BOOKS REFERED

54

[1] Instrument transformers: their theory, characteristics and testing; a

theoretical and practical handbook for test-rooms and research

laboratories-Bernard Hague

[2] Current transformers: their transient and steady state performance -

Arthur Wright

LIST OF FIGURES

FIG 2.1 Basic current transformer

8

FIG 3.1: Basic working of DAQ 22 FIG 3.2: Signal conditioning provides more accurate sensor measurements. 24 FIG 3.3: Attenuation of a 100 MHz sine wave when passed through a 100 MHz

Digitizer

27

FIG 3.4: Typical 100 MHz Digitizer Input Response 27 FIG 3.5: Rise time for a signal is the time span from 10% to 90% of its

maximum amplitude

28

FIG 3.6: Effects of various sampling rates while sampling a signal 30

55

FIG 3.7:. Sine wave demonstrating Aliasing 30 FIG 3.8: When converting an analog signal to digital domain, signal values are

taken at discrete time instants

31

FIG 3.9: Quantization error when using a 3 bit ADC 32 FIG 3.10: Effects of dithering and averaging on a sine wave input 33 FIG 3.11: DAQ which connected to computer through USB. 35 FIG 3.12: The required signals are fed to computer for analysis. 36 FIG 3.13: DAQ(NI usb 6009) 38 FIG 4.1: LABVIEW Logo 39 FIG 5.1: Basic circuit for comparison test. 45 FIG 5.2: Test and standard transformers are connected to DAQ 46 FIG 5.3: Block diagram of our project. 47 FIG 5.4: Waveforms of both currents. 48 FIG 5.5: Results obtained in LABVIEW 48 FIG 6.1. RATIO AND PHASE ERROR CALCULATED IN LAB-VIEW 49 FIG 6.2: These are the voltage signals of test and standard transformers 50 FIG 6.3: IMPLEMENTATION OF LOGIC IN LAB-VIEW 50

APPENDIX

SOFTWARE USED – LABVIEW

LabVIEW is a highly productive graphical programming environment that combines easy-touse

graphical development with the flexibility of a powerful programming language. It offers an

intuitive environment, tightly integrated with measurement hardware, for engineers and

scientists to quickly produce solutions for data acquisition, data analysis, and data presentation.

Integrated Hardware

LabVIEW has built-in compatibility with hardware libraries for: GPIB/VXI/PXI/Computer-

based instruments RS-232/485 protocol Plug-in data acquisition Analog/digital/counter timer I/O

Signal conditioning Distributed data acquisition Image acquisition and machine vision Motion

control PLCs/data loggers.

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Powerful Analysis

LabVIEW features comprehensive analysis libraries that rival those of dedicated analysis

packages. These libraries are complete with statistics, evaluations, regressions, linear algebra,

signal generation algorithms, time and frequency-domain algorithms, windowing routines, and

Digital filters.

Open Development Environment

With the open development environment of LabVIEW, you can connect or communicate to any

other application through ActiveX, the Internet, DLLs (dynamic linked libraries), shared

libraries, SQL (for databases), DataSocket, TCP/IP, DDE, and numerous other protocols.

Optimal Performance

All LabVIEW applications execute at compiled speed for optimal performance. With the

abVIEW Professional Development System or Application Builder, you can also build stand-

alone executables for secure distribution to operator station.

NI 6009 DAQ SPECIFICATIONS

Application and Technology

The USB-6008 and USB-6009 are ideal for applications where a low-cost, small form factor and

simplicity are essential. Examples include the following:

• Data logging – quick and easy environmental or voltage data logging

• Academic lab use – student ownership of data acquisition hardware for completely

interactive lab-based courses (Academic pricing available. Visit ni.com/academic for

details.)

• OEM applications as I/O for embedded systems

57

Recommended Software

National Instruments measurement services software, built around NI-DAQmx driver software,

includes intuitive application programming interfaces, configuration tools, I/O assistants, and

other tools designed to reduce system setup, configuration, and development time. National

Instruments recommends using the latest version of NI-DAQmx driver software for application

development in NI LabVIEW, LabVIEW SignalExpress, LabWindows™/CVI, and

Measurement Studio software. To obtain the latest version of NI-DAQmx,

visit ni.com/support/daq/versions.

NI measurement services software speeds up your development with features including the

following:

• A guide to create fast and accurate measurements with no programming using the DAQ

Assistant.

• Automatic code generation to create your application in LabVIEW.

• LabWindows/CVI; LabVIEW SignalExpress; and C#, Visual Studio .NET, ANSI C/C++,

or Visual Basic using Measurement Studio.

• Multithreaded streaming technology for 1,000 times performance improvements.

• Automatic timing, triggering, and synchronization routing to make advanced applications

easy.

• More than 3,000 free software downloads available at ni.com/zone to jump-start your

project.

• Software configuration of all digital I/O features without hardware switches/jumpers.

• Single programming interface for analog input, analog output, digital I/O, and counters

on hundreds of multifunction DAQ hardware devices. M Series devices are compatible

with the following versions (or later) of NI application software – LabVIEW,

LabWindows/CVI, or Measurement Studio versions 7.x; and LabVIEW SignalExpress

2.x.

Every National Instruments DAQ device includes a copy of LabVIEW SignalExpress LE data-

logging software, so you can quickly acquire, analyze, and present data without programming.

58

The NI-DAQmx Base driver software is provided for use with Linux, Mac OS X, Windows

Mobile, and Windows CE operating systems.