What is the working principle of a transformer Questions and Answers

20 Questions :

1. If a transformer is not marked, how could you test it for polarity?
2. A 10-kVA rating, that are connected in a closed delta arrangement, you would have a capacity of 30 
K VA. If one transformer is taken out of the bank, what would be the output capacity of the remaining 10
K VA transformers?
3. When connecting transformers in parallel, what factors must be taken into consideration?
4. What is a split-coil transformer?
5. Where may autotransformers be used?
6. What is the purpose of the markings on transformer leads?
7. In a bank of three single-phase transformers that are connected in a delta, each transformer delivers 240 volts at 10 amperes. What are the line voltages and line currents?
8. If you have a bank of three single-phase transformers that are connected in a closed delta arrangement, and one transformer burns up, how would you continue operation on the remaining two transformers?
9. What special precaution must be taken when using a booster transformer?
10. What is a three-phase transformer?
11. If transformers with different electrical characteristics are connected in parallel, what will happen?
12. Describe a current transformer.
13. What precautions must be taken when working with current transformers? Why?
14. What is an induction regulator?
15.Is it possible to connect two single-phase transformers to secure a three-phase output from a three-phase input?
16. When connecting an ordinary transformer as a booster transformer, what important factors must be considered?
17. When you use a bank of two single-phase transformers in an open delta arrangement, do they supply their full output rating?
18. How are current transformers rated?
19. Describe a potential transformer.
20. What is the phase relation between the three phases of a three-phase circuit?

Answers Above Questions:

1.Connect the transformer as shown in Figure 6-7. If it has subtractive polarity, V will be less than the voltage of the power source; if it has additive polarity, V will be greater than the voltage of the power source.
2.Each transformer would deliver 8.66 kVA, and you would have a bank capacity of 17.32 k VA.
3.Their electrical characteristics, such as voltage ratio, impedance percentage, and voltage regulation.
4.A transformer that has the coils on the low or high side in separate winding's so that they can be connected in series or parallel for higher or lower voltages, as desired.
5.(a) Where the system being supplied contains an identified grounded conductor that is solidly connected to a similar identified
6.They are there for standardization, so that transformer polarities are recognizable for any type of use.
7.The line voltages are each equal to 240 volts; however, the line current in each phase would be the current of each transformer multiplied by 1.732 (the square root of 3), or 17.32 amperes.
8.By merely disconnecting the leads to the disabled transformer.
9.There must be no fusing in the high side, or primary. Because the booster transformer is similar to a current transformer, an extremely high voltage could be built up on the secondary side if the fuse should blow.
10.A transformer that is the equivalent of three single-phase transformers, which are all wound on one core and enclosed within one common case.
11.They won’t distribute the load equally; one transformer will tend to assume more of the load than the other. This leads to overheating and, in severe cases, the destruction of the transformers.
12.A current transformer has a primary of a few turns of heavy conductor capable of carrying the total current, and the secondary consists of a number of turns of smaller wire. The primary winding is connected in series with the circuit carrying the current that is to be measured.
13.The secondary must never be opened when the primary circuit is energized. If it is necessary to disconnect an instrument while the circuit is energized, the secondary must be short-circuited. If the secondary is opened while the circuit is energized, the potential on the secondary might reach dangerously high values. By short-circuiting the secondary, damage is avoided and the voltage on the secondary is kept within safe limits.
14.This device is similar to a booster transformer. It has a primary and a secondary winding, which are wound on separate cores. The primary can be moved in either direction; this is usually done by an electric motor. In turning, the primary bucks or boosts the line voltage, as required. The amount of bucking or boosting is anticipated by the current being drawn by the line.
15.Yes, they would have to be connected in an open delta.
16.The high side of the transformer must be able to handle the approximate voltage of the line; the low side must have a voltage of approximately the value by which you wish to boost the line voltage and must also have a current capacity that is sufficient to carry the line current.
17.No. Each transformer is only capable of supplying 86.6 percent of its output rating.
18.They are rated at 50 to 5, 100 to 5, etc. The first number is the total current that the transformer is supposed to handle, and the second figure is the current on the secondary when the fullloadcurrent is flowing through the primary. For example, a 50- to-5 rating would have a multiplier of 10 Kilo.
19.A potential transformer is built like the ordinary isolation transformer, except that extra precautions are taken to ensure that the winding ratios are exact. Also, the primary winding is connected in parallel with the circuit to be measured.
20.They are 120 electrical degrees apart.

Transformer Principles Questions and Answers For Engineering Students and Workers

QUESTIONS:

1. What is the difference between the primary and the secondary of a transformer?
2. What is an oil-immersed transformer?
3. What is a transformer?
4. What factors affect the amount of induced electromotive force emf in a transformer?
5. Why is oil used in a transformer?
6. Is it possible to connect two single-phase transformers to secure a three-phase output from a three-phase input?
7. What is an air-core transformer?
8. What are eddy currents?
9. What means can be taken to keep eddy currents at a minimum?
10. Is hysteresis objectionable?
11. Are transformers normally considered to be efficient devices?
12. What factors constitute the major losses produced in transformers?
13. There are two basic types of transformers. What are they?
14. Is there a definite relationship between the number of turns and voltages in transformers?
15. What are instrument transformers?
16. Ordinarily, what is the phase relationship between the primary and secondary voltages of a transformer?
17. Is it possible to have the primary and secondary of a transformer in phase?
18. How are the leads of a transformer marked, according to ANSI (American National Standards Institute)?
19. What is mutual inductance?
20. What is a booster transformer?

ANSWERS:

1. The primary of the transformer is the input side of the transformer and the secondary is the output side of the transformer. On a step-down transformer, the high-voltage side is the primary and the low-voltage side is the secondary; on a step-up transformer, the opposite is true
2. The core and coils are immersed in a high-grade mineral oil, which has high dielectric qualities.
3. A device that transforms electrical energy from one or more circuits to one or more other circuits at the same frequency but usually at a different voltage and current. It consists of a core of soft-iron laminations surrounded by coils of copper-insulated wire.
4. The strength of the magnetic field, the speed at which the conductors are cut by the magnetic field, and the number of turns of wire being cut by the magnetic field.
5. To increase the dielectric strength of the insulation, to keep down the possibility of arcing between coils, and to dissipate heat to the outer case so that the transformer can carry heavier loads without excessive overheating.
6. Yes, they would have to be connected in an open delta.
7. A transformer that does not contain oil or other dielectric
compositions but is insulated entirely by the winding insulations and air.
8. Circulating currents induced in conductive materials (usually
the iron cores of transformers or coils) by varying magnetic
fields.
9. The iron used in the core of an alternation-current transformer
is laminated, or made up of thin sheets or strips of iron,
so that eddy currents will circulate only in limited areas.
10. Yes, it is a loss and affects the efficiency of transformers.
11. Yes, they have one of the highest efficiencies of any electrical device.
12. Power loss of the copper I2R losses, eddy currents, and hysteresis losses.
13. The isolation type, in which the two windings are physically isolated and electrically insulated from each other, and the autotransformer type, in which there is only one coil with a tap or taps taken off it to secure other voltages the primary is part of the secondary and the secondary is part of the primary.
14. Yes, the voltage varies in exact proportion to the number of turns connected in series in each winding.
15. In the measurement of current, voltage, or kilowatt-hours on systems with high voltage or high current, it is necessary to use a device known as an instrument transformer, which reproduces in its secondary circuit the primary current or voltage while preserving the phase relationship to measure or record at lower voltages
or lower amperages, and then to use a constant to multiply the readings to obtain the actual values of voltage or current. Current transformers CTs are used to measure the current, and potential transformers PTs are used to register the potential.
16. They are 180º out of phase.
17. Yes, by changing the connections on one side of the transformer.
18. The high side of the transformer is marked H1, H2, etc. The low side of the transformer is marked X1, X2, etc.
19. The linkage of flux between two coils or conductors, caused by the current flowing within one or both of the coils or conductors.
20. A transformer arrangement that is often used toward the end of a power line in order to raise the voltage to its desired value. These are often called “Buck-boost” transformers.

Electrical Transformer tutorials Current Potential and Auto tutorials

Current Transformers The primary of this transformer consists of a few turns or even a single turn to carry the current to be measured and is connected in series with the main circuit. The secondary winding with large number of turns supplies a reduced current to the ammeter. The meter scale is calibrated directly in terms of the primary circuit current.The core is worked at low flux density so that, at all loads, secondary current is a constant ratio of the main circuit current. When the cuurent is flowing in the primary circuit, the secondary circuit should not be opened. In that case high voltage may be induced in the secondary and the core may become over saturated, heated up and thereby damage the magnetic properties permanently.

Potential Transformer
This is used to reduce the primary voltage to a safe value for operation of voltmeter and other instrument. Primary is connected to the H.T. to be measured and the secondary to a voltmeter. It is so designed that the ratio of primary to secondary is constant throughout. To limit the short circuit in case of failure of the transformer, limiting resistances are placed in series with the H.T. winding.

Auto-transformer works on the priciple of self Induction
It has only one winding which performs the function of both primary and secondary winding. As in ordinary transformer, the transformation ratio in autotransformer, is also equal to the turns ratio.
In case of step down transformer, the complete winding acts as primary winding while the tapped section of this winding works as secondary winding. In the step up transformer, the whole winding works as secondary winding and its there is much saving of copper. These transformers are used as regulating transformers where only a small variation of voltage is required. It is mainly used for starting and speed control of induction motors. It suffers from a disadvantage that the two windings are not electrically separate and in case of failure of insulation between the two, either a sever shock may be felt on the low voltage side.

Various parts of a Transformer
Primary winding
Secondary winding
Oil level
Conservator
Breather
Drain cock
Tube for cooling
Transformer Oil
Earth Point
Explosion vent
Buchhol's Relay
H.T. Terminals
L.T Terminals


Free Pdf file about the kinds of Transformer

Current Transformers
Current transformers are used in electrical grids for measurement and protective applications to provide signals to equipment such as meters and protective relays by stepping down the current of that system to measurable values.

Instrument Transformers
Technical Information and Application Guide

Selecting Current Transformers
As engineers, we are aware that electrical power systems have grown. How much have they grown?

Current Transformer Principles and Operation
Used with watt transducers enable the owner to control demand as well as monitor building and/or tenant power consumption. When CT's are used with Current Transducers, the result is an excellent method of diagnosing the performance of fans,pumps, chillers.

Potential Transformer
Voltage transformers connected line-to-ground cannot be considered to be grounding transformers and must not be operated with the secondaries in closed delta because excessive currents may flow in the delta.
VOLTAGE TRANSFORMERS
Potential transformer are so small that they may be neglected for protective-relaying purposes if the burden is within the "thermal" volt-ampere rating of the transformer. This thermal volt-ampere rating corresponds to the full-load rating of a power transformer.

Auto Transformer
The currents drawn by these two windings are out of phase by 180◦. This prompted the use of a part of the primary as secondary. This is equivalent to fusing the secondary turns into primary turns.

Transformers vs. Autotransformers
A transformer converts audio from one voltage and impedance to a different voltage and impedance. Transformers are passive which means they do not require a  power supply to operate.

All About Electrical Distribution and Power Transformer Tutorials

This is All About Transformer its principles, Definition, Testing Facts and Tutorials

Transformer Classification

Transformer Definition
It is a static device for transforming electrical energy from one alternating current circuit to another without any change in frequency. It changes voltage from high to low and low to high with a corresponding increase or in decrease current. If the voltage is increased it is said to be stepped up. If it is decreased, then it is said as stepped down.

Electrical Transformer Principle
When one coil like primary is connected to the alternating current supply current flows and an alternating flux is set up in the core. Most of this flux links with the second coil secondary. Law of electromagnetic induction.If the circuit is completed, current will flow. The secondary voltage depends upon the ratio of secondary turns to primary turns.

The Testing of the Transformer

Two test are performed on the transformer which are open circuit test and short circuit test. These tests are performed to determine the parameter or constants of transformer, efficiency and regulation.

1. Open Circuit Test
This is also called no load test. It determined the iron losses and the no load current. One winding of the transformer, usually the low voltage side is connected to its normal supply with an ammeter to measure the voltage applied to the winding and a wattmeter to measure taken by transformer at no load. the high voltage winding is kept open. Under these conditions normal flux will be set up in the core, therefore, normal iron losses will occur. The current taken will be wattmeter will indicate the iron losses.

2. Short Circuit Test
This test is used to determine the full load copper losses and the equivalent resitances and reactances referred to the metering side. In this test on the high voltage winding a reduced value of the voltage is increased until full load current is flowing in this winding. The applied voltage is a small fraction of the normal working voltage, the mutual flux produced is very small and hence the core losses at this voltage can be neglected. The wattmeter during this test gives the total coppery losses.

What is Distribution Transformer?
Transformer up to a size of 200 KVA, used to step down the distribution voltage to a standard service voltage, are known as distribution transformers. They are kept in operation all the 24 hours a day whether they are carrying load or not. Energy is lost in iron losses throughout the day while the copper losses account for loss in energy when the transformer is loaded. Therefore, the distribution transformer should have their iron losses small as compared to full load copper losses, in other words, they should be designed to have maximum efficiency at a load much lower than full load about 50 percent. Owing to low iron loss, the distribution transformer have good all day efficiency. These transformer have a good voltage regulation.

What is Power Transformer?
These transformers have rating about 20 KVA and are in generating stations and substations at each of a power transmission line for stepping up or stepping down the voltage. They may be either single or three phase units. They are put in operation during load periods and are disconnected during light load periods. Therefore the power transformer should be designed to have maximum efficiency at or near full load. Power transformer are designed to have considerable greater leakage reactance than that permissible in distribution transformers because in the case of power transformer, voltage regulation is less important than current limiting effect of higher leakage reactance.

Important Facts About Transformer
1. In and ideal transformer on no load, the primary applied voltage is balanced by the secondary.
2. The concentric windings are used in core type transformer with LT winding placed next to core.
3. Cross over windings are used for high voltage winding of small rating transformers.
4. The magnitude of mutual flux in a transformer is same at all levels.
5. The induced emf in the transformer secondary will depend upon frequency, flux and number of turns in the secondary.

Current Electricity Flow of Electrons Engineering Tutorials

The Nature of Electricity
The modern theory defines matter as electrical in nature. All matter is made up of extremely small particles called molecules, each of which has the properties of matter of which it is a constituent. The molecules, in turn are made up of atoms, which are the smallest units of several elements. An atom consists of central nucleus of positive charge around which small negatively charged particles, called electrons, revolve in different paths or orbits.

Free Electrons
The electrons which can be easily removed or detached from an atom are called free electrons.
In metals, the atom lie very close together in regular formed crystal lattice. The nearness of one atom to another causes the electron in the outer orbit or valence electrons of each atom to be attracted by the nucleus is relatively weak. As a result, a large number of electrons are free to drift about interatomic space from atom to atom. These are called free electrons. The free electrons can be easily removed by applying a small amount of external energy.

Charge
A body is said to be charged if it has either excess or deficit of electrons from normal due share. If an electron is freed from a neutral atom, the proton predominate and the atom is said to posses a positive electric charge.

Unit
The unit of charge is coulomb and 1 coulomb is equal to charge on 628 by 10 to the power of 16. We say that a body has a positive discharge of ano coulomb, it means that it has a deficit of 628 by to the power of 16 electrons from normal due share.

Electric Potential
It is a measure of the density and sign of the electric charge at a point relative to that at some time. Thus electric potential, like electric charges and electric current is a phenomenon of displaced charges that exist in matter. For example, consider a copper rod of six meters length having the same number of electrons and protons distributed uniformly throughout.
If by some means, such as intercepting the flux in a magnetic field, one billion electrons can be shifted towards one end of the rod, that end will posses positive charge. The two ends of rod will have a different of potential. Work has been done to create this difference of potential energy.

Nucleus
It is the central part of an atom and contains protons and neutrons. A proton is positively charged particle while the neutron has nearly the same mass as proton, but has no charge. Therefore, the nucleus of an atom is positively charged. The sum of protons and neutrons constitute the entire weight of an atom and is called its atomic weight, it is because the particles in the extra nucleus has negligible weight as compared to protons or neutrons.

Extra Nucleus
It is the outer part of an atom and contains electrons only. An electron is a negatively charged particle having negligible mass. The charge on an electron is equal but opposite to that on a proton. Also, the number of electrons is equal to the number of protons in an atom under ordinary conditions. Therefore, an atom is neutral as a whole. The number of electrons or protons in an atom is called its atomic number.

Free Relay Substation Guide

Electric Power Systems Cyber Security
The aim of this case study was security analysis of a software interlocking system consisting of mutual interlocking disconnectors, circuit breakers and earthing switches to assure safety of switching operations carried out in a substation during operation and maintenance of the substation.


Relay Protection and Substation Automation
Of Modern Power Systems, the coordination function for the load resistance vectors with the help of load encroachment in the distance protection characteristic’s resistance area. However existed methodical directions for the load encroachment setting calculation do not take into account all the possible worst conditions for the 330 kV transmission network.


Reliability of Substation Protection System
Although the wide use of microcomputer relaying and integrated substation automation system in China has improved operation reliability, there are still many problems in substation protection system, for example, the complex insulation of conventional transducer


Protection and Power System Utilization
In the areas of power system automation and substation automation, there are two parallel trends in different directions: centralization and decentralization. More and more functions are moved from
local and regional control centers toward the central or national control center. At the same time we also observe more and more “intelligence” and “decision-power” moving closer toward the
actual power system process.


Control, Protection and Access to Information
Describes a system concept for integrated control, protection, condition monitoring, condition supervision, asset management and outage management system, which assures higher availability of plants and enhancement of the overall utility performance.


Using Microprocessor Relays
A subsstation is a complete, self contained substation designed to supply a single distribution circuit. Each substation consists of a power transformer
ranging in size from 1000 to 2500 kva and a dedicated low side circuit breaker to provide feeder protection and contrl.


How to Draw a Sequence Diagram
A sequence diagram has two dimensions: the vertical dimension represents time and the horizontal dimension represents the objects participating in the interaction. Time flows from top to bottom. Objects (or classifier roles, more generally) are shown as vertical lines (called lifelines) and messages as horizontal arrows extending from a sender object to a receiver object.

Computational Electronics, CMOS Active Inductors and Transformers, Electric Motor Drives

Computational Electronics
which deals with compact behavioral models for devices and sub-circuits relevant for circuit simulation in commercial packages.The goal of this book is to provide simulation tools that capture the essential physics while at the same time minimizing the computational burden so that results are obtained within a reasonable time frame.
http://depositfiles.com/files/6130333
http://w16.easy-share.com/1700700243.html
http://www.filefactory.com/file/5c5354/
http://rapidshare.com/files/124133414/ComputElectr.rar


CMOS Active Inductors and Transformers
The content of the book is drawn from recently published research papers and are not available in a single, cohesive book. Equal emphasis is given to the theory of CMOS active inductors and transformers, and their emerging applications. Major subjects to be covered in the book include: inductive characteristics in high-speed analog signal processing and data communications, spiral inductors and transformers.
http://depositfiles.com/files/6088467
http://w13.easy-share.com/1700683570.html
http://www.filefactory.com/file/7634f9/


The Essence of Dielectric Waveguides
the primary goal of this book is to provide an insight into this essence.
Review of the wave guiding structures over the whole electromagnetic spectrum shows that, for frequencies below 30 GHz, mostly metal-based structures are used, and for frequencies above 30 GHz, increasing skin-depth losses in metal requires that these structures be made without the use of metallic material.
http://depositfiles.com/files/6087982
http://w13.easy-share.com/1700683407.html
http://www.filefactory.com/file/902338/


Introductory Circuit Analysis 10th Edition
The book where they are discussed. Current and Voltage. Resistance. Ohm's Law, Power, and Energy. Series Circuits. Parallel Circuits. Series-Parallel Networks. Methods of Analysis & Selected Topics. Network Theorems. Capacitors. Magnetic Circuits. Inductors. Sinusodial Alternating Waveforms.
http://depositfiles.com/files/6051658
http://w15.easy-share.com/1700668165.html
http://www.filefactory.com/file/9baad1/
http://rapidshare.com/files/145118325/IntCircAn.rar


Theatre Engineering and Stage Machinery
http://depositfiles.com/files/5995825


Electric Motor Drives
Modeling, Analysis, and Control,
http://depositfiles.com/files/5993722
http://www.megaupload.com/?d=TLJSPQCB


CCTV Surveillance, Second Edition
is completely updated with the recent advancements in digital cameras and digital recorders, remote monitoring via the Internet, and CCTV integration with other security systems. Continuing in the celebrated tradition of the first edition, the second edition is written to serve as a useful resource for the end-user as well as the technical practitioner.
http://www.filefactory.com/file/635174/
http://www.mediafire.com/?3cosm1hdzue
http://rapidshare.com/files/121835130/bh-cctvsvpat2.rar.html
http://www.uploading.com/files/1LGE8NPJ/bh-cctvsvpat2.rar.html

Free eBooks Adaptive WCDMA, Circuits and System, Electric Machinary and Fundamentals, Power Transformer, Power Cable, Instrumentation

Adaptive WCDMA
Cover the latest theoretical principles of WCDMA and explain why these principles are used in the standards. Starting with a general overview, the more advanced material is then gradually introduced providing an excellent roadmap for the reader.
http://rapidshare.de/files/8618598/0470848251_-_Adaptive_WCDMA.rar.html




Circuits and Systems Based on Delta Modulation
Linear, Nonlinear and Mixed Mode Processing (Signals and Communication Technology, for students and professionals who are interested in the field of digital signal processing of delta-sigma modulated sequences. The overall focus is on the development of algorithms and circuits for linear, non-linear, and mixed mode processing of delta-sigma modulated pulse streams.
http://mihd.net/hwe81i
http://rapidshare.com/files/41347916/Circuits_and_Systems_Based_on_Delta_Modulation.pdf




Dynamic Simulations of Electric Machinery
This book and its accompanying website offer a complete treatment from background theory and models to implementation and verification techniques for simulations and linear analysis of frequently studies machine systems. They include: Three- and single- phase transformers, modeling of core saturation.
http://mihd.net/mgeb18
http://mihd.net/qiul0o
http://www.mathworks.com/matlabcentral/files/9941/Dynamic%20simulation%20of%20machine.zip





Electric Machinery Fundamentals
Has the rare ability to strike a golden balance between theory and practice, and between rigour and informality, such that the theoretical actually illuminates the practical.
http://rapidshare.com/files/24775788/Electric_Machinery_Fundamentals_4th_Edition.rar





Electric Circuits Nilsson
http://rapidshare.com/files/4070169/ecnsm.rar





Electronic Circuit Analysis
Provides a foundation for analyzing and designing analog and digital electronic circuits. is divided into three parts. Part 1 covers semiconductor devices and basic circuit applications. Part 2 covers more advanced topics in analog electronics, and Part 3 considers digital electronic circuits.
http://mihd.net/9m8j7a
http://rapidshare.com/files/24126142/electronic_circuit_analysis_and_design.part01.rar
http://rapidshare.com/files/24126920/electronic_circuit_analysis_and_design.part02.rar
http://rapidshare.com/files/24126946/electronic_circuit_analysis_and_design.part03.rar
http://rapidshare.com/files/24127267/electronic_circuit_analysis_and_design.part04.rar
http://rapidshare.com/files/24127293/electronic_circuit_analysis_and_design.part05.rar
http://rapidshare.com/files/24127802/electronic_circuit_analysis_and_design.part06.rar
http://rapidshare.com/files/24127750/electronic_circuit_analysis_and_design.part07.rar
http://rapidshare.com/files/24128443/electronic_circuit_analysis_and_design.part08.rar
http://rapidshare.com/files/24128240/electronic_circuit_analysis_and_design.part09.rar
http://rapidshare.com/files/24128666/electronic_circuit_analysis_and_design.part10.rar
http://rapidshare.com/files/24133838/electronic_circuit_analysis_and_design.part11.rar
http://rapidshare.com/files/24134917/electronic_circuit_analysis_and_design.part11.rar





Electric Power Transformer Engineering
This book provides the background required to understand the basic operation of electromagnetic induction as applied to transformers. The book is divided into three fundamental groupings: one stand-alone chapter is devoted to Theory and Principles, nine chapters individually treat major transformer types, and fourteen chapters cover many ancillary topics associated with power transformers.
http://mihd.net/dkw827
Password (if any): www.AvaxHome.ru
http://rapidshare.de/files/8801376/Electric_Power_Transformer_Engineering_MAZ.rar





Electrical Power Cable Engineering
Discusses basic dielectric theory; conductors; cable characteristics; electrical properties of insulating materials; shielding power cables; sheaths, jackets, and armors; standards and specifications; splicing and terminating; ampacity; treeing; lightning protection; corrosion; and more.
http://rapidshare.com/files/18808544/0824799763.Electrical.Power.Cable.Engineering.rar
http://gigapedia.org/redirect.id:a35e207f095dfde991e530507f79ff00.url





Electric Circuits
Emphasize the relationship between conceptual understanding and problem solving approaches; 3) To provide readers with a strong foundation of engineering practices. Chapter topics include Circuit Variables; Circuit Elements; Simple Resistive Circuits; Techniques of Circuit Analysis;
http://rapidshare.com/files/4064763/ec5251.rar
http://rapidshare.com/files/4075619/ec5252.rar
http://rapidshare.com/files/4068362/ec5253.rar





Instrumentation Reference Book
Is a comprehensive and authoritative collection of technical information, which is of direct practical value to instrumentation and control engineers as well as all instrument technicians and users. It is also an indispensable addition to any academic library for the benefit of engineering and science students.
http://rapidshare.com/files/34906779/Instrumentation.Reference.Book.3rd-0750671238.zip
http://gigapedia.org/redirect.id:d947fbe073dedb49a57e08d38671b366.url

INSTRUMENT TRANSFORMERS

OBJECTIVES
DEFINITION OF TERMS
CURRENT TRANSFORMER SPECIFICATIONS
VOLTAGE TRANSFORMER SPECIFICATIONS

OBJECTIVES
To discuss the different kinds of Instruments Transformers
To know the difference between a Current Transformer VS Voltage Transformer
How to specify the different kinds of Instrument Transformers.
To know how to interpret the different instruments at suppliers catalog.

DEFINITION OF TERMS
instrument transformer: One that is intended to reproduce in its secondary circuit, in a definite and known proportion, the current or voltage of its primary circuit with the phase relation substantially preserved.

current transformer (CT): An instrument transformer intended to have its primary winding connected in series with the conductor carrying the current to be measured or controlled.

voltage transformer (VT): An instrument transformer intended to have its primary winding connected in shunt with the voltage to be measured or controlled.

burden of an instrument transformer: That property of the circuit connected to the secondary winding that determines the active and reactive power at the secondary terminals.
NOTE
—
The burden is expressed either as total ohms impedance with the effective resistance and reactance components, or as the total voltamperes and power factor at the specified value of current or voltage, and frequency.

continuous thermal current rating factor (RF): The number by which the rated primary current of a current transformer is multiplied to obtain the maximum primary current that can be carried continuously without exceeding the limiting temperature rise from 30 °C average ambient air temperature. The RF of tapped-secondary or multi-ratio transformers applies to the highest ratio, unless otherwise stated. (When current transformers are incorporated internally as parts of larger transformers or power circuit breakers, they shall meet allowable average winding and hot spot temperature limits under the specific conditions and requirements of the larger apparatus.)
Excitation losses for an instrument transformer: The power (usually expressed in watts) required to excite the transformer at its primary terminals.
NOTE
—
Excitation losses include core, dielectric, and winding losses due to the excitation current.

multiple-secondary current transformer: One that has three or more secondary windings, each on a separate magnetic circuit, with all magnetic circuits excited by the same primary winding.

multi-ratio current transformer: One with three or more ratios obtained by the use of taps on the secondary winding.

marked ratio or nominal ratio: The ratio of the rated primary value to the rated secondary value as stated on the nameplate.

ratio correction factor (RCF): The ratio of the true ratio to the marked ratio. The primary current or voltage is equal to the secondary current or voltage multiplied by the marked ratio times the ratio correction factor.

percent ratio correction: The difference between the ratio correction factor and unity, expressed in percent [(RCF -1) x 100]

phase angle of an instrument transformer (PA): The phase displacement, in minutes or radians, between the primary and secondary values. The phase angle of a current transformer is designated by the Greek letter beta (b) and is positive when the current leaving the identified secondary terminal leads the current entering the identified primary terminal. The phase angle of a voltage transformer is designated by the Greek letter gamma (g) and is positive when the secondary voltage from the identified to the unidentified terminal leads the corresponding primary voltage.

phase angle correction factor (PACF): The ratio of the true power factor to the measured power factor. It is a function of both the phase angles of the instrument transformers and the power factor of the primary circuit being measured.
NOTE
—
The phase angle correction factor corrects for the phase displacement of the secondary current or voltage, or both, due to the instrument transformer phase angle(s).

For a current transformer, PACF:
PACF = cos (q2 + b)/ cos (q2)

For a voltage transformer, PACF:
PACF = cos (q2 -g)/ cos (q2)

When both voltage and current transformers are used, the combined phase angle correction:
PACF = cos (q2 + b- g)/ cos (q2)
q2 - is the apparent power factor angle of the circuit being measured.


rated current: The primary current upon which the performance specifications are based.

rated voltage: The primary voltage upon which the performance specifications of a voltage transformer are based.

rated secondary current: The rated current divided by the marked ratio.

rated secondary voltage: The rated voltage divided by the marked ratio.

thermal burden rating of a voltage transformer: The volt-ampere output that the voltage transformer will provide continuously at rated secondary voltage without exceeding the specified temperature limits.

transformer correction factor (TCF): The ratio of the true watts or watthours to the measured secondary watts or watthours, divided by the marked ratio.
NOTE
( = RCF x PACF )
The transformer correction factor for a current or voltage transformer is the ratio correction factor multiplied by the phase angle correction factor for a specified primary circuit power factor.

The true primary watts or watthours are equal to the watts or watthours measured, multiplied by the transformer correction factor and the marked ratio.

The true primary watts or watthours, when measured using both current and voltage transformers, are equal to the current transformer ratio correction factor multiplied by the voltage transformer ratio correction factor multiplied by
the marked ratios of the current and voltage transformers multiplied by the observed watts or watthours. It is usually sufficiently accurate to calculate true watts or watthours as equal to the product of the two transformer correction factors multiplied by the marked ratios multiplied by the observed watts or watthours.

Current Transformers
Terms in which ratings shall be expressed The ratings of a current transformer shall include:

a) Basic impulse insulation level in terms of full-wave test voltage (see tables 2 and 3)
b) Nominal system voltage, or maximum system voltage (see tables 2 and 3)
c) Frequency (in Hertz)
d) Rated primary and secondary currents (see tables 7 and 8)
e) Accuracy classes at standard burdens (see 6.3, 6.4, and tables 6 and 9)
f) Continuous thermal current rating factor based on 30 °C average ambient air temperature (see 6.5)
g) Short-time mechanical current rating and short-time thermal current rating (see 6.6)

* - ******
ACCURACY CLASS FOR RELAYING C.T.
“the accuracy ratings assigned to a metering current transformer might be C400, K200, and T100.”
C, K, or T, classification.
C or K classification covers current transformers in which the leakage flux in the core of the transformer does not have an appreciable effect on the ratio or ratios within the limits of current and burden outlined in this subclause, so that the ratio can be calculated in accordance with 8.1.10.
T classification covers current transformers in which the leakage flux in the core of the transformer has an appreciable effect on the ratio within the limits specified
An appreciable effect is defined as a 1% difference between the values of actual ratio correction and the ratio correction calculated in accordance with 8.1.10.

Secondary terminal voltage rating.
This is the voltage the transformer will deliver to a standard burden at 20 times rated secondary current without exceeding 10% ratio correction. Furthermore, the ratio correction shall
be limited to 10% at any current from 1 to 20 times rated secondary current at the standard burden or any lower standard burden used for secondary terminal voltage ratings

******
C100 RELAYING C.T.
For example, on a current transformer with 5 A rated secondary current, relay accuracy rating C100 means that the ratio can be calculated and that the ratio correction will not exceed 10% at any current from 1 to 20 times rated secondary current with a standard 1.0 ohm burden (1.0 ohms x 5 A x 20 x rated secondary current = 100 V).

Voltage Transformers
Terms in which ratings shall be expressed The ratings of a voltage transformer shall include:

a) Basic impulse insulation level in terms of full-wave test voltage (see tables 10–14 and figures 6a–6h)
b) Rated primary voltage and ratio (see tables 10–14 and figures 6a–6h)
c) Frequency (in Hertz)
d) Accuracy ratings (see 5.3)
e) Thermal burden rating (see 7.4)


Group 1 voltage transformers are for application with 100% of rated primary voltage across the primary winding when connected line-to-line or line-to-ground. (For typical connections, see figures 6a and 6b.) Group 1 voltage transformers shall be capable of operations at 125% of rated voltage on an emergency basis (this capability does not preclude the possibility of ferroresonance), provided the burden, in voltamperes at rated voltage, docs not exceed 64% of the thermal burden rating, without exceeding the following average winding temperatures: 105 °C for 55 °C rise types, 115 °C for 65 °C rise types, and 130 °C for 80 °C rise types. This will result in reduction of life expectancy.

*******
********
Group 2 voltage transformers are primarily for line-to-line services, and may be applied line-to-ground or line-to-neutral at a winding voltage equal to the primary voltage rating divided by the square root of 3. (For typical connections see figures 6c and 6d.) Note that the thermal burden capability will be reduced at this voltage. .
*********
***********
Group 3 voltage transformers are for line-to-ground connection only and have two secondary windings. They may be insulated-neutral or grounded,neutral terminal type. Ratings through 161 000 Grd Y/92 000 shall be capable of the square root of 3 times rated voltage (this capability does not preclude the possibility of ferroresonance) for 1 min without exceeding 175 °C temperature rise for copperconductor or 125 °C rise for EC aluminum. Ratings 230 000 Grd Y/138 000 and above shall be capable of operation at 140% of rated voltage with the same limitation of time and temperature. (For typical connections, see figure .) Group 3 transformers shall be capable of continuous operation at 110% of rated voltages, prodded the burden in voltamperes at this voltage does not exceed the thermal burden rating.

Group 4 voltage transformers are for line-to-ground connection only. They may be insulated-neutral or grounded-neutral terminal type. (For typical connections of Group 4A, see figure 6f. For typical connections of Group 4B, see figure 6g.) Group 4 transformers shall be capable of continuous operation at 110% of rated voltages, provided the burden in voltamperes at this voltage does not exceed the thermal burden rating. Group 4A voltage transformers shall
be capable of operation at 1257percnt; of rated voltage on an emergency basis (this capability does not preclude the possibility of ferroresonance), provided the burden, in voltamperes at rated voltage, does not exceed 64% of the thermal burden rating, without exceeding the following average winding temperatures: 105 °C for 55 °C rise types, 115 °C for 65 °C rise types and 130 °C for 80 °C rise types. (This will result in a reduction of normal life expectancy.)

Group 5 voltage transformers are for line-to-ground connection only, and are for use outdoors on grounded systems. They may be insulated-neutral or grounded-neutral terminal type. They shall be capable of operation at 140% of rated voltage for 1 min without exceeding 175 °C temperature rise for copper conductor or 125 °C rise for EC aluminum conductor. (This will result in a reduction of normal life expectancy.) Group 5 voltage transformers shall be capable of continuous operation at 110% of rated voltage, provided the burden, in voltamperes at this voltage, does not exceed the thermal burden rating. This capability does not preclude the possibility of ferroresonance.
ACCURACY CLASS FOR V.T.
“the accuracy ratings assigned to a metering current transformer might be 0.3 W and 0.6X”