April 8, 2023 1

Instrument Transformers

April 8, 2023 2

Good For Everything

Group

April 8, 2023

Group Members

3

Sl No.

NAME ID TOPIC

01 Mehedi Hasan

11105140

Introduction of Instrument X former

02 Abu Jauad Khan Aliv (Leader)

11105137 Design & Function of CT

03 Farzana Farzin Rimi 11105192 Types & Accuracy Limit of CT

04 Ab-E-Zamzam

11105199

Equivalent Circuit, Safety & Usage of CT

05 Prodip Kumar Das 11105136 PT Design & Function

06 Humayun Kabir

11105167

Error & cause of Error Of PT & conclusion

April 8, 2023

Overview1. Introduction to Instrument Transformer 2. Its Classification3. Design & Function of CT4. Construction, Accuracy Limit & Equivalent CKT5. Safety Precaution & Usage of CT6. Design & Function of PT7. Construction & Error of PT8. Cause of Error in PT9. Conclusion

4

Introduction

Isolating the protection, control & measurement equipment from the high voltages of a power systemSupplying the equipment with the appropriate values of current and voltage

Instrument transformers are necessary for

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Types of Instrument Transformers

Two Types of Instrument X-formers

Current Transformer (CT) Potential Transformer (PT)

Current transformer & Potential transformer

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Current Transformer (CT)

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Like any other transformer, a current transformer has a primary winding, a magnetic core and a secondary winding

The primary winding of a current transformer is connected in series with the power circuit

The impedance is negligible compared with that of the power circuit

Design

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The alternating current flowing in the primary produces a magnetic field in the core & induces a current in the secondary winding

The CT's primary circuit 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

Design

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Function Reduce power system current

to lower value for measurement

Insulate secondary circuits from the primary

Permit the use of standard current ratings for secondary equipment

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Current Transformer Construction

BAR PRIMARY WOUND PRIMARY

Primary

Secondary

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Bar-Primary Type CT

RELAY

1A 1000A

1000 turns sec

Insulation covered wire, giving inter-turn insulation & secondary to core insulation

Generator, or system voltage source

‘Feeder’ or ‘Bus-bar’ forming 1 turn of primary circuit

Insulation to stop flash-over from HV primary to core & secondary circuit

Laminated ‘strip’ wound steel toroidal core

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Accuracy

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

Burden Rating factor Load External electromagnetic fields Temperature Physical configuration. The selected tap, for multi-ratio CTs

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Accuracy

• 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

• Rating factor Rating factor is a factor by which the nominal

full load current of a CT can be multiplied to determine its absolute maximum measurable primary current 14

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C.T. Equivalent Circuit

ZbN

P1

ZCT

S1

VtZe

IsIp

Ip/NIe

Es

Ip = Primary rating of C.T. Ie = Secondary excitation currentN = C.T. ratio Is = Secondary current

Zb = Burden of relays in ohms Es = Secondary excitation voltage

(r+jx) Vt = Secondary terminal voltage

ZCT = C.T. secondary winding across the C.T. terminals

impedance in ohms (r+jx)Ze = Secondary excitation

impedance in ohms (r+jx)

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Safety Precaution

The secondary of a current transformer is not disconnected from its load while current is flowing in the primary

The transformer secondary will attempt to continue driving current across the effectively infinite impedance.

This will produce a high voltage across the open secondary

This high peaks of voltage may not be measured by conventional voltmeter. But these high peaks of induced voltage may breakdown the CT insulation, and may case accident to personnel

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Usage

Current transformers

are used:

Measuring current

A CT for operation on a 110 kV grid

Monitoring the operation of the power grid

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Potential Transformer (PT)

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Design A Potential Transformer theory is just like

theory of general purpose step down transformer

Primary of this transformer is connected across the phases or and ground depending upon the requirement

PT has lowers turns winding at its secondary

In an ideal Potential Transformer when rated burden connected across the secondary the ratio of primary and secondary voltages of transformer is equal to the turns ratio

But in actual transformer there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages

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Types of Potential Transformer

Two main basic types are available:

Electromechanical VT`s Similar to a power transformer May not be economical above 132kV

Capacitor VT`s (CVT) Used at high voltages Main difference is that CVT has a capacitor

divider on the front end20

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Construction of PT

a. Output – Seldom more than 200-300VA. Cooling is rarely a problem

b. Insulation – Designed for the system impulse voltage level. Insulation volume is often larger than the winding volume

c. Mechanical Design – Not usually necessary to withstand short-circuit currents. Must be small to fit the space available within switchgear

The construction of a voltage transformer takes into account the following factors

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Provides isolation from high voltages

Must operate in the linear region to prevent accuracy problems - Do not over specify VT

Must be capable of driving the burden, specified by relay manufacturer

Protection class VT will suffice

Function

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Errors in PT

Is - Secondary CurrentEs - Secondary induced emfVs - Secondary terminal voltageIp - Primary currentEp - Primary induced emfVp - Primary terminal voltageKT - numbers of primary turns/number of secondary turnsIo - Excitation Currentβ - phase angle error

The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio error in a potential transformer

% voltage error = Vp − KT.Vs /Vp X 100 %

The angle β between the ′ ′primary system voltage Vp and the reversed secondary voltage vectors KT.Vs is the phase error

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The voltage applied to the primary of the potential transformer first drops due to internal impedance of primary

Transformed voltage across secondary winding will again drops due to internal impedance of secondary before appearing across burden terminals

Cause of Error in PT

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ConclusionCurrent Transformer

Measuring current

Monitoring the operation of the power grid

• CT secondary should not be kept open

Potential Transformer

Measuring Voltage Provides isolation from high voltages

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THANK YOU