Paralelogramos De Exactitud. Transformadores De Medición.

2004ABB_Cover_1.qxp

12/17/2004

11:52 AM

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Instrument Transformers

1VAP420003-TG December 2004 Rev. A (replaces TB3.2.8-1A)
© Copyright 2004 ABB. All rights reserved.

Technical Information and Application Guide

ABB Inc.
3022 NC 43 North
Pinetops, NC 27864
Tel: +1-252-827-3212
www.abb.com/mediumvoltage

Table of Contents

Technology Review
An introduction toinstrument transformer fundamentals,
a discussion on power quality, and a detailed explanation of design
considerations to assist in selecting an instrument transformer.

3

Product Description
A general overview of ABB’s current and voltage transformers.
This section is complemented with photographs and describes the
specific classes and uses for instrument transformers.

37Technology Review

Introduction

4

Types of Construction

6

Performance Characteristics

8

Accuracy Classifications for Metering

12

Burdens

13

Current Transformers: Accuracy Classes for Relaying

15

Insulation Systems

16

Voltage Ratings

18

Power Quality

22

Service Conditions

24

Auxiliary Current Transformers

31

Fusing Voltage Transformers32

New Design Technologies

33

3

Technology Review
Introduction

Instrument transformers (ITs) are designed to transform voltage
or 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 for which
ITs are used: metering (for energy billing andtransaction purposes); protection control (for system protection and protective
relaying purposes); and load survey (for economic management
of industrial loads).
Depending on the requirements for those applications, the IT
design and construction can be quite different. Generally, the
metering ITs require high accuracy in the range of normal operating voltage and current. Protection ITs requirelinearity in a
wide range of voltages and currents. During a disturbance, such
as system fault or overvoltage transients, the output of the IT is
used by a protective relay to initiate an appropriate action (open
or close a breaker, reconfigure the system, etc.) to mitigate the
disturbance and protect the rest of the power system. Instrument
transformers are the most common and economic way todetect
a disturbance. Typical output levels of instrument transformers
are 1-5 amperes and 115-120 volts for CTs and VTs, respectively.
There are several classes of accuracy for instrument transformers
defined by the IEEE, CSA, IEC, and ANSI standards. Figure 1
presents a conceptual design of CTs and VTs.
Figure 2 shows how the polarity markers are used to keep the
direction of current flowin the meters exactly the same, as if the
primary circuit was carried through the meters. Grounding of the
secondary circuit is most important, but in complicated threephase connections, the best point to ground is not always
easily determined.

4

Technology Review
Figure 1: Current and Voltage Transformer Symbols and Simplified Concepts
Symbol of a
Current Transformer

Conceptualpicture of
a Current Transformer

I1
I1
I2

I2
Symbol of a
Voltage Transformer

Conceptual picture of
a Voltage Transformer

V1

V1
V2
V2

Figure 2: Instrument Transformer Connections

Current Transformer
H1
H2

Lines in which either current or voltage is too high to
permit running the lines directly to the meter or relay
H1

H2

H1 X1
X2

X1

Voltage
TransformerH2 X2

Current
Operated
Relay

Transformer
or Power
Sub-Station

A

R

+
-

+
-

+
-

A

V

X2

Volt Meter

R

Ammeter

X1

Voltage Operated Relay

W+
Watt Meter
R+
Current-Voltage Relay
R+
Current-Current Relay

B

A. The current transformer is designed to connect in series with the line to transform the line current to the

standard 5 amperes...