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Instrument transforme rs - ABB

Edvard

Introduction to instrument transformers

Inst rument transf ormers (ITs) are designed to transform

voltage of current from the high values in the

transmission and distribution systems to the low values

that can be utilized by low voltage metering devices.

There are three primary applications f or which ITs are

used: metering (f or energy billing and transact ion

purposes); protection contro l (fo r system protection and

protective relaying purposes); and load survey (f or

economic management of industrial loads).

Depending on the requireinents f or those applications,

the IT design and construction can be quite dif f erent.

Generally the metering ITs require high accuracy in the

range of normal operat ing voltage and current. Protect ion

ITs require linearity as a wide range of voltages and

currents . During a disturbance, such as system fault or

overvoltage transients, the output o f the IT is used by a

protective relay to initiate an appropriate action (open or

close a breaker, reconf igure the system, etc) to mitigate

the disturbance and protect the rest of the power system.

Inst rument t ransf ormers are the mos t common and economic way to detect a disturbance. Typical output leve

of instrument t ransf ormers are 1-5 amperes and 115-120 volts f or CTs and VTs, respectively. There are

several classes of accuracy for instrument transf ormers def ined by the IEEE, CSA, IEC, and ANSI standards.

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Figure 1: Current and Voltage Transforme r Symbo ls and Simplified Concep ts

Figure 2: Instrument Transformer Connections

Figure 1 presents a

conceptual design of

CTs and VTs.

Figure 2shows how the

polarity markers are used

to keep the direction of

current f low as the

meters exactly the same,

as if the primary circuit

was carried through the

meters. Grounding of the

secondary circuit is mos t

important, but in

complicated three-phase

connections, the best

point to ground is not

always easily determined.

A The currenttransf ormer is designed

to connect in series with

the line to t ransf orm the

line current to

the s tandard 5 amperes

suitable

f or the

meter or

relay.

The

voltage

transf ormer is designed to connect in parallel with the line to transf orm the line voltage to 115 or 120 volts

suitable for the meter or relay.

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To keep the voltage at the meters and relays at a saf e value, the secondary circuit must be grounded.

B The po larity markers indicate t he relative instantaneous directions of current in the windings. The polarity,

or instantaneous direction of current, is o f no signif icant dif f erence fo r current-operated or voltage-

operated devices.

Correct operation of current- current, voltage-voltage, or current- voltage devices usually depends on the

relative instantaneous directions.

Instrument Transformers

The main tasks of instrument transformers are:

To transf orm currents or voltages f rom a usually high value to a value easy to handle f or relays and

Instruments.

To insulate the metering circuit f rom the primary high voltage system.

To provide poss ibilities of standardizing the instruments and relays to a f ew rated currents and

voltages.

Inst rument transf ormers are special types of transf ormers intended to measure currents and voltages. The

common laws f or t ransf ormers are valid.

Current transformers

For a short- circuited transf ormer the f ollowing valid:

This equation gives current transf ormation in proport ion to the primary and secondary turns. A

current t ransf ormer is ideally a short- circuited t ransf ormer where the secondary terminal

voltage is zero and the magnetizing current is negligible.

Voltage transformers

For a t ransf ormer in no bad the f ollowing is valid:

This equation gives voltage transf ormation in propo rtion to t he primary and secondary turns. A

voltage transf ormer is deally a transf ormer under no- load conditions where the load current is

zero and the voltage drop is only caused by the magnetizing current and is thus negligible.

Types of Instrument Transformer Construction

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Type s of Instrument Transforme r Co nstruction

Resources:

ABB Instrument Trans f ormers - Technical Inf ormation and Application Guide

INSTRUMENT TRANSFORMERS PART-1 CURRENT & VOLTAGE TRANSFORMERS (FOR ELECTRIC

T&D, POWER PLANTS &, INDUSTRIAL APPLICATIONS)