IEEE UMT, Presented By Muhammad Saqib

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Instrument transformersPresented to:

Sir, ARIF SAEEDPresented by:

Majid AliHafiz Zahid Rasool

DEPARTMENT OF ELECTRICAL ENGINEERING SCHOOL OF SCIENCE AND TECHNOLOGY

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Instrument transformers

A device that serves as an input source of currents and voltages from an electric power system to instruments, relays, meters, and control devices.

Two special-purpose transformers are uses to

take measurements: potential and current transformers.

Examples: Voltmeters Ammeters Watt-hour Meters Relays

Definitions:

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Why use Instrument Transformers?Circuit Isolation

Reduce voltage and currents to reasonable working levels.

Phasor combinations for summing and measuring power

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Importance of Instrument transformers : 

Importance of Instrument transformers In dc circuits for current and voltage measurement we use ammeters and voltmeters. For measurement of high current ,it is usual to use low range ammeter with suitable shunt. For measurement of high voltage, low range voltmeter are used with high resistance connected in series. But for measurement of high A.C. current and voltage we cannot use these methods. We use specially constructed instrument transformers.

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Common uses of instrument transformers

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Current Transformer (CT)

An instrument transformer used to reflect a primary current into a secondary current through a magnetic medium. Always connected in series with the primary conductor. The nominal secondary current is often a 5 amp basis for ease of measurement.

Definitions

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Current Transformer (CT)Secondary Winding Primary

Conductor

Iron Core

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Construction of C.TC.T. has a primary coil of one or more turns of thick wire connected in series with the line whose current is to be measured. The secondary consist of large number of turns of fine wire, is connected across the ammeter terminals.  

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• Transformer ratio (TR)The ratio of a primary current or voltage to

a secondary current or voltage.

• Examples: A 100:5 window CT will deliver 5 amperes of secondary current when 100 amperes of primary current is passed through a center window. The current can be calculated.

Current = ratio × ammeter reading

Transformer ratio (TR)

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Ammeter resistance is very low ,the current transformer normally works short circuited. If for any reason the ammeter is taken out of secondary winding then the secondary winding must be short circuit with the help of short circuit switch s. If this is not done, then due to high m.m.f. will set up high flux in the core and it will produces excessive core loss which produce heat and high voltage across the secondary terminals Hence the secondary of current transformer is never left open.

Importance of short circuit

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15kV Indoor CT 15kV Outdoor CT

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POTENTIAL TRANSFORMERS

VP

Vs

Relay

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Definitions

A Potential Transformers is a step down transformer having many primary turns but few secondary turns. In step down the voltage decreases and current increases, thus voltage can be easily measured by using low range voltmeter. The voltage is stepped down in known ratio called voltage ratio.

Potential Transformers

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A potential transformer has many primary windings but few number of secondary windings that makes it step down transformer. Voltmeter is connected to secondary winding usually voltmeter of 150 v is suitable. Working Primary terminals are connected across the line to which the voltage is to be measured. The voltmeter gives the transformed value of voltage at secondary. The deflection of voltmeter when divided by transformed ratio gives the actual voltage at primary.

Working of Potential Transformers

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• Transformer ratio (TR)

The ratio of a primary current or voltage to a secondary current or voltage.

Transformer ratio (TR)

Where transformation ratio K = V2/V1

The deflection of voltmeter when divided by transformed ratio gives the actual voltage at primary.

Line voltage = deflection / K

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Since the secondary of Potential is connected to relays, their ratings are usually 40 to 100 Watts. For safety purpose the secondary should be completely insulated from the high voltage primary and should be in addition grounded.

Precaution for Potential Transformers

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Potential Transformers

Electromagnetic Voltage Transformers Capacitive Voltage Transformers

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SOME TYPES OF POTENTIAL TRANSFORMERS:

SHELL TYPE

DRY TYPE

OIL TYPE

Types of Potential Transformers

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May ALLAH Show HIS Countless Blessings upon us & our Homeland

IEEE UMT, Presented By Muhammad Saqib

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To be Conti………………………

IEEE UMT, Presented By Muhammad Saqib

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