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Current Transformers (CT)

Introduction

These are special types of transformers used for the measurement of currents. As the name suggests, these transformers are used in conjunction with some relevant instruments such as ammeters and control relays.

Current transformers are usually used when the AC currents exceeds the safe value for the measuring instruments.

Also, a current transformer is an instrument transformer which is used to measure alternating current of large magnitudes through the stepping-down by transformer action. The primary winding of CT is connected in series with the line in which current is to be measured and the secondary winding is connected to the proper instruments or control devices.

Introduction

Uses of Current Transformers CTs are used for:

1. Insulating the high voltage circuits from the measuring circuits in order to protect the measuring instruments from burning

2. To make it possible to measure the high currents with lower range ammeters.

3. These transformers are also used in controlling and protecting electrical circuits, to operate relays, to operate circuit breakers, etc. The working of these transformers is similar as that of ordinary transformers.

Measuring current with CT

The primary winding is so connected that the high current to be measured passes through it and a low induced current is passed through the secondary winding connected to an Ammeter.

Advantages of CTs

The power loss in CT instrument transformers is very small as compared to power loss due to the resistance of aiding devices such as shunts or multipliers.

By using current transformers with tong testers, the currents in the heavy current circuits can easily be measured.

Disadvantages of CTs

The only main disadvantage of CTs is that these instruments can not be used in DC circuits.

Current Transformer Designs

The secondary winding of a CT has very small load impedance (burden) which represents the current coil of an ammeter. The primary side has a few number of thick turns and the secondary side has a large number of fine turns. The primary winding carries a full load current and this current is stepped down to a suitable value which is within the range of ammeters and control devices ratings.

Current Transformer Designs In order to minimize the exciting ampere turns

(IT) required (for economic reasons), the core of the CTs must have lower reluctance (Resistance of the magnetic core to the flow of flux) in order to minimize the iron losses.

The following three types of core constructions are generally employed to build CTs:

1. Core type

2. Shell type

3. Ring type

1. Core type CTs

It is a rectangular form iron core type. The laminations are of L-shaped that can be assembled together.

The winding are placed on one of the shorter limbs, with the primary usually wound over the secondary to maximize the induced EMF. The main advantage of this type of core is that sufficient space is available for insulation/taping and is suitable for high voltage application designs.

Core type

2. Shell type

In shell type current transformers, the primary and secondary windings are placed over each other at the central limb, thus it gives a better protection for both the insulated windings and tap connections.

Shell type

3. Ring type

• Ring type core is commonly used when primary current is large. The secondary winding is distributed around the iron ring and the primary winding is a single bar that carries high current.

• It has a joint less core and therefore a very small reluctance and leakage reactance.

Ring type

Burden of Current Transformer • The operation of current transformers differs

slightly from the operation of power transformers. In case of current transformers, the secondary winding has a very small impedance or “Burden” , so the current transformer practically operates on short circuit conditions.

• The rated burden of this Current Transformer is the volt-ampere loading which is permissible without errors exceeding the limits.

Burden of Current Transformer

• Burden across the secondary of a current transformer is also defined as the ratio of the rated secondary voltage to the rated of secondary current.

That is:

ZL = secondary voltage/ secondary current

= V / I

The units of burden are ohms.

18/03/2013

18/03/2013

Demagnetization of CT’s

18/03/2013

Phasor Diagram of CT

Taking flux φm as the reference vector, the induced e.m.f. in the primary and secondary sides are E1 and E2 leading ahead and lagging behind the flux by 90o respectively as drawn. The magnitudes of e.m.f. are proportional to their respective number of turns.

The no load current Io drawn by the primary has two components, the magnetizing component Im and the active working component Iw .

The secondary current I2 lags behind the voltage by an angle of у .

The angle α between V2 and I2 is the angle produced by the burden connected on the secondary side (load power factor angle).

The secondary current I2 is now transferred to the primary side by reversing I2 and multiplying it by K, where K is the current turns ratio of that CT.

Phasor Diagram of CT

Phasor Diagram of CTs

Errors in Current Transformers

There are two types of errors in these transformers :

1. Ratio error, depends on Iw

2. Phase angle error, depends on Im

Ratio Error

For normal operation of these instrument transformers, the current transformation ratio should be constant and within the limits. It has been seen that this ratio are not constant but do vary with the power factor. So this error is known as Ratio Error.

Ratio Error

The ratio of the working component Iw of the magnetization no-load exciting current Io to the secondary current I2 of the instrument transformer is called its ratio-error.

Also, the ratio between the actual current ratio of the CT and its normal-ratio (i.e., the manufacturer’s name-plate current-ratio) is known as the Ratio Correction Factor, R.C.F.

R.C.F. = Actual Ratio/ Normal Ratio

= K/ KN

Phase Angle Error

The phase angle error is due to the magnetizing component Im of exciting current Io. This is the angle by which the secondary current I2, when reversed to the primary side (becomes I’2) differs in phase from the primary current I1.

In case of CT, the current ratio error is more important, while phase angle error is of a little importance so long as it is connected with an ammeter as a burden.

Methods to minimize CT Errors

As we know, the ratio error mainly depends upon the working component Iw of the no load primary current Io and the phase angle error depends upon the magnetizing component Im of the same current.

To minimize these errors, the following methods should be employed :

Methods to minimize CT Errors 1. In order to minimize these errors, the

working and magnetizing components (Iw and Im ) must be kept at low value. This is possible only by using core materials of high permeability, short magnetic paths and large cross-sectional area for the core.

The material may be of the following types :

-Hot rolled silicon

-Cold rolled grain oriented silicon steel

-Nickel iron alloys


High permeability nickel iron cores are used for precision current transformers. Among them: (75 % Nickel, 17 % Fe) and (50 % Fe, 50 % Nickel) alloys are used. These materials have high permeability at low flux densities, therefore they are commonly used.

The construction of these cores should have a minimum number of joints. Therefore to avoid the joints in building of cores, the cores are made of two types, Ring-type and Spiral-type cores


2. By providing a suitable turn ratio i.e. number of turns of the secondary can be reduced by one or two turns.

3. Leakage reactance also increases the ratio error. Therefore the two windings should be close to each other to reduce the secondary winding leakage reactance.

4. If the current on the secondary is too large, it should be reduced by using a shunt resistor. The shunt also reduces the phase angle error.

Types of Current Transformers

As far as the construction of CTs is concerned, these are the types of CTs:

1. Bar type CT

This type of CT is placed on the panel board to measure the current of bus bars. The bus bar whose current is to be measured is made to pass through the CT. It is of circular or ring type, on which secondary winding is placed. The ammeter is connected in the secondary windings.

Types of Current Transformers

2. Clamp on / Tong tester

This type of CT can be used with a single conductor. The core of the CT can be split with the help of a trigger switch and therefore, the core can be clamped around a live conductor to measure the current. The single conductor acts as a primary and the secondary is wound on the core of CT. The ammeter is connected in the secondary. This is a portable instrument and generally used in laboratories.

Clamp on / Tong tester

Application of Current Transformers

The following are the applications of CTs:

1. Current transformers are used in panel board of sub station or grid station to measure the bus bar current which is very high.

2. Current transformers are widely used in power measuring circuits. For example, the current coil of the wattmeter is connected with CT.

3. Current transformers are also used in power houses, sub stations etc. in conjunction with the relays.

Difference between CT and PT The few important points regarding the

difference in the working of CTs and potential transformer PTs are given below :

1. The current transformer is also known as series transformer. The secondary of CT is virtually under short circuit conditions when the primary is energized.

2. The potential transformer is also known as parallel transformer. The secondary of PT can be left open circuited without any damage being caused either to the transformer or to the operator.

Difference between CT and PT

3. Under normal conditions, the line voltage of the PT is nearly constant. The flux density and the exciting current of a PT varies between small range whereas the primary current and excitation of a CT varies over a wide range under normal working conditions.

4. The current in the primary of CT is independent of secondary winding conditions whereas current in the primary of PT depends upon the secondary circuit burden.

Difference between CT and PT

5. The primary winding of the PT is connected across full line voltage, whereas the CT is connected in series with one of the lines and therefore a small longitudinal voltage exists across its terminals. However the current transformer carries full line current.

Setup to generate high current by CTs

current transformers (ct) - engineering home pageengineering.richmondcc.edu/courses/eus...

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