potential and current rup

7/28/2019 Potential and Current Rup

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AReport

onPotential and Current

TransformersBy

Rupak bhattacharjeeA report submitted in partial fulfillment of the

requirement of ee-592: technical report writing

Supreme knowledge foundation group of institutions

(approved byaicte and affiliated to wbut)1, khan road, p. o.- mankundu (chandannagar),

Hooghly-712139, w.b.

Year- 2012-2013


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Transformer

A transformer is a device that transfers electrical energy from one circuit to another

through inductively coupled conductorsthe transformer's coils. A varying current in the

first or primary winding creates a varying magnetic flux in the transformer's core, and thus avarying magnetic field through the secondary winding. This varying magnetic field induces a

varying electromotive force (EMF) or " voltage" in the secondary winding. This effect is

called mutual induction.

Figure 1: Transformer-Basic Construction

If a load is connected to the secondary, an electric current will flow in the secondary

winding and electrical energy will be transferred from the primary circuit through thetransformer to the load. In an ideal transformer, the induced voltage in the secondary

winding (Vs) is in proportion to the primary voltage (Vp), and is given by the ratio of the

number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:


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By appropriate selection of the ratio of turns, a transformer thus allows an alternating

current (AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down"

by making Ns less than Np. In the vast majority of transformers, the coils are wound around

a ferromagnetic core, air-core transformers being a notable exception.

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden

inside a stage microphone to huge units weighing hundreds of tons used to interconnect

portions of national power grids. All operate with the same basic principles, although the

range of designs is wide. While new technologies have eliminated the need for transformers

in some electronic circuits, transformers are still found in nearly all electronic devices

designed for household ("mains") voltage. Transformers are essential for high voltage power

transmission, which makes long distance transmission economically practical.


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Instrument transformers

Instrument transformers are used for measuring voltage and current in electrical power

systems, and for power system protection and control. where a voltage or current is too

large to be conveniently used by an instrument, it can be scaled down to a standardized, low

value. Instrument transformers isolate measurement, protection and control circuitry from

the high currents or voltages present on the circuits being measured or controlled.

Figure 2: Current transformers, designed for placing around conductors

A current transformer is a transformer designed to provide a current in its secondary coil

proportional to the current flowing in its primary coil.

Voltage transformers(VTs), also referred to as "potential transformers"(PTs), are designed

to have an accurately-known transformation ratio in both magnitude and phase, over a

range of measuring circuit impedances. A voltage transformer is intended to present a

negligible load to the supply being measured. The low secondary voltage allows protective

relay equipment and measuring instruments to be operated at lower voltages.

Both current and voltage instrument transformers are designed to have predictable

characteristics on overloads. Proper operation of over-current protection relays requires

that current transformers provide a predictable transformation ratio even during a short

circuit.


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Potential Transformer (PT)

The instrument potential transformer (PT) steps down voltage of a circuit to a low value

that can be effectively and safely used for operation of instruments such as ammeters,

voltmeters, watt meters, and relays used for various protective purposes.

Figure3: A Potential Transformer (PT)

Potential transformers can be used with voltmeters for voltage measurements or they can

be used in combination with current transformers for watt-meter or watt-hour meter

measurements. They are used also to operate protective relays and devices, and for many

other applications, since they are used in a monitoring capacity, they generally require much

greater accuracy in design.

Potential Transformer is designed for monitoring single-phase and three-phase power line

voltages in power metering applications. The primary terminals can be connected either inline-to-line or in line-to-neutral configuration.

A Potential Transformer is a special type of transformer that allows meters to take

readings from electrical service connections with higher voltage(potential) than the meter is

normally capable of handling without at potential transformer.


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Potential transformers are designed to provide as accurate a voltage step-down ratio as

possible. To aid in precise voltage regulation, loading is kept to a minimum: the voltmeter is

made to have high input impedance so as to draw as little current from the PT as possible.

As you can see, a fuse has been connected in series with the PTs primary winding, for safety

and ease of disconnecting the PT from the circuit.

A standard secondary voltage for a PT is 120 volts AC, for full-rated power line voltage. The

standard voltmeter range to accompany a PT is 150 volts, full-scale. PTs with custom

winding ratios can be manufactured to suit any application. This lends itself well to industry

standardization of the actual voltmeter instruments themselves, since the PT will be sized to

step the system voltage down to this standard instrument level.

The potential transformer is designed for measuring the voltages ranging in kV or even

higher. The simple voltmeters may easily burn upon measuring these voltages. So these

transformers are designed to eliminate this harm and successfully recording the powersystem voltages.


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Principle of operation

The standards define a voltage transformer as one in which "the secondary voltage is

substantially proportional to the primary voltage and differs in phase from it by an angle

which is approximately zero for an appropriate direction of the connections."

This, in essence, means that the voltage transformer has to be as close as possible to the

"ideal" transformer. In an "ideal" transformer, the secondary voltage vector is exactly

opposite and equal to the primary voltage vector, when multiplied by the turns-ratio.

In a "practical" transformer, errors are introduced because some current is drawn for themagnetization of the core and because of drops in the primary and secondary windings due

to leakage reactance and winding resistance. One can thus talk of a voltage error, which is

the amount by which the voltage is less than the applied primary voltage, and the phase

error, which is the phase angle by which the reversed secondary voltage vector is displaced

from the primary voltage vector.


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The Task of Measuring Voltage

In the world of electricity, the typical measurement is that of voltage. A volt meter will

measure the voltage, usually within a range of up to hundreds of volts. In the power

industry, the transmission of electricity occurs at thousands and tens of thousands of volts.

This would destroy nearly all typical voltmeters. How then, do technicians measure the high

voltages they encounter every day? The trick is to use a potential transformer.

The Potential Transformer

The potential transformer works along the same principle of other transformers. It

converts voltages from high to low. It will take the thousands of volts behind power

transmission systems and step the voltage down to something that meters can handle.

These transformers work for single and three phase systems, and are attached at a point

where it is convenient to measure the voltage.

What Gives a Transformer Potential?

The biggest feature that a potential transformer has over regular transformers is the

voltage conversion is constant and linear. That is to say, if the first day of operation 50,000

volts is stepped to 50 volts, then on the last day of operation 50,000 steps to 50 volts.

Linearity states that when the voltage drops in a linear fashion, then the stepped down

voltage drops accordingly. This feature ensures that the meter will scale accordingly. The

potential transformer makes the measure of very high voltages much easier.


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Applications

The potential transformers that we manufacture are widely used to scale down the line to

neutral voltage of Wye (Y) system or the line-to-line voltage of a Delta system to the rated

input scale of the meter (typically 120 V).

Transformers can also be used in electrical instrumentation systems. Due to transformers'

ability to step up or step down voltage and current, and the electrical isolation they provide,

they can serve as a way of connecting electrical instrumentation to high-voltage, high

current power systems.

They are used in the transmission lines for the purpose of voltage measurement, power

metering, and the protection of the lines.

The potential transformers (like the current transformers) are used for the substation

service.


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Current Transformer (CT)

The instrument current transformer (CT) steps down the current of a circuit to a lower

value and is used in the same types of equipment as a potential transformer. This is done by

constructing the secondary coil consisting of many turns of wire, around the primary coil,

which contains only a few turns of wire. In this manner, measurements of high values of

current can be obtained. A current transformer should always be short-circuited when not

connected to an external load. Because the magnetic circuit of a current transformer is

designed for low magnetizing current when under load, this large increase in magnetizing

current will build up a large flux in the magnetic circuit and cause the transformer to act as a

step-up transformer, inducing an excessively high voltage in the secondary when under no

load.

Figure 4: A simple Current Transformer (CT)

Because CTs are designed to be powering ammeters, which are low-impedance loads, and

they are wound as voltage step-up transformers, they should never, ever be operated with

an open-circuited secondary winding. Failure to heed this warning will result in the CTproducing extremely high secondary voltages, dangerous to equipment and personnel alike.

To facilitate maintenance of ammeter instrumentation, short-circuiting switches are often

installed in parallel with the CT's secondary winding, to be closed whenever the ammeter is

removed for service: (Figure 5)


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Figure 5: Short-circuit switch allows ammeter to be removed from an active current

transformer circuit.

Though it may seem strange to intentionally short-circuit a power system component, it is

perfectly proper and quite necessary when working with current transformers.

The reason why a current transformer is used in place of the ordinary ammeter (current

measuring device), is that the ammeter cannot measure the very high amounts of current in

the high voltage (HV) applications. Therefore there is a need for an instrument that can

measure the high values of currents without any harm or burning of the device itself.


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Principle of operation

A current transformer is defined as "as an instrument transformer in which the secondarycurrent 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.

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 turns-ratio)

and opposite to the primary current. But, as in the voltage transformer, some of the primary

current or the primary ampere-turns is 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.


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Applications

The current transformers are used for control circuits as well as for instrumentation in

high current power applications.

These transformers can be used to detect single-phasing on a three phase power system.

If one phase of the power line to a three phase motor becomes defective, the motor would

run under increased current and stress. The current transformer on each phase could detect

the loss of current in the line and open a control relay to take the motor off the circuit.

The current transformers perform the roles for safety protection and current limiting.

They can be used for the current monitoring of the power systems, when the current

reaches a specified level. Current monitoring is necessary at frequencies from the 50 Hz/60

Hz power line to the higher frequencies of transformers that range into the hundreds of kHz.


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Basic Difference between the PT & CT

The potential transformers are used as the high voltage voltmeters, as the ordinary

voltmeters. The current transformers are used in place of the ordinary ammeters, to

measure the high values of the currents in the high voltage power applications.

There are several different windings of the primary in the potential transformer. But in

the current transformer, there is only the single winding i.e. the main live line.

PT is used to step-down the higher magnitudes of the power transmission voltages,

stepping-up the current. On the other hand, the CT steps-down the high values of currents

and steps-up the voltage at the same time.

There is one potential transformer required for the three phase transmission line. But for

the same three phase line, three current transformers are needed, one on each phase.


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Summary

Transformers can be used to transform impedance as well as voltage and current. When

this is done to improve power transfer to a load, it is called impedance matching.

A Potential Transformer (PT) is a special instrument transformer designed to provide a

precise voltage step-down ratio for voltmeters measuring high power system voltages.

A Current Transformer (CT) is another special instrument transformer designed to step

down the current through a power line to a safe level for an ammeter to measure.

Transformers can be used in electrical instrumentation systems and for the protection

purposes.

The current transformers are used for control circuits as well as for instrumentation in

high current power applications. They can also be used to detect single-phasing on a three

phase power system. The current transformers perform the roles for safety protection and

current limiting.


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ACKNOWLEDGEMENT

I have no appreciating words to express my sincere thanks to my parents and my family

members who have made unbelievable sacrifice while I snatched away valuable time and

money from them throughout this period of time.

I deem it is a pleasure to express my deep gratitude and grateful thanks to Mr. Shiladitya

Sarkar, Lecturer, Department of Electrical Engineering, Sir J. C. Bose School of Engineering,

Supreme Knowledge Foundation Group of Institutions, Mankundu, Hooghly, who directed

me through her timely advice which eased my task of completing this Technical Report.

I also express my thanks to all my classmates for their continuous inspiration and help.

Finally, I am indebted to Professor (Dr.) B. N. Biswas, Chairman (Educational Division) of

Supreme Knowledge Foundation Group of Institutions, Mankundu, for the inspiration I

received from him.


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STATEMENTS BY THE CANDIDATE

RUPAK BHATTACHARJEE, B. Tech, 3rd

year

Department of Electrical Engineering

Sir J. C. Bose School of Engineering

Mankundu, Hooghly.

I hereby state that the Technical Report on POTENTIAL AND CURRENT TRANSFORMER

is a report done by me as a part of the Technical Report Writing: EE- 592 during the period

July12- December12.

Signature

(RUPAK BHATTACHARJEE)


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STATEMENTS BY THE SUPERVISER

Mr. SHILADITYA SARKAR,M. Tech (Electrical Engineering)

Lecturer

Department of Electrical Engineering

Sir J. C. Bose School of Engineering

Mankundu, Hooghly.

This is to certify that the Technical Report writing on POTENTIAL AND CURRENT

TRANSFORMER is arecord of work done by the candidate during the period July12 to

December12 to affiliated to West Bengal University of Technology.

Signature

(Mr. SHILADITYA SARKAR)

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