Instrument Transformers...Current Transformer Ratio (CTR) The Current Transformer Ratio, CTR,...

$: Instrument Transformers...Current Transformer Ratio (CTR) The Current Transformer Ratio, CTR, expressed as a fraction, is the ratio between the magnitudes of the primary and the secondary$
Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

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Instrument

Transformers


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Power System protection Dr. Mohamad Tawfeeq

Instrument Transformers


� CT – Current Transformer � Current Scaling

� Isolation

� VT – Voltage Transformer � Voltage Scaling

� Isolation

1.Current Transformers

Current and voltage transformers are responsible for

scaling primary system signals.

Typical nominal secondary values are 5 A and 1 A for

current transformers and 110 V for voltage transformers.

Conventional magnetic current and voltage transformers

are still in wide use in power systems. For voltage levels

greater than 230 kV, we substitute capacitive-coupled

voltage transformers (CCVT) for VTs.CT saturation and

bad transient response of CCVTs create protection

problems.

New trends in current and voltage transducers are low-

power-output current and voltage transformers (recently

included in an IEC Standard) and magneto-optic current

and voltage transducers (MOCT and MOVT). MOCTs use

the Faraday Effect, and MOVTs can use either the Faraday

or the Pockels effect.


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Principles of Current Transformers

The initial problem was how to connect a low-voltage device to the high-voltage

system and have the ability to handle large fault currents (kilo-Amps). How can we

make the relay measure the currents flowing in the high-voltage system in order to

detect these faults?

The solution consists of using a special type of transformer, called a current

transformer.

The main parts of a current transformer are:

• Iron core

• Secondary winding

• Primary conductor

• External insulation

Some current transformers do not have a primary conductor. In those cases the

primary is the line or bus itself. Sometimes the core and its secondary winding are

directly installed in the bushing of the circuit breakers or transformers. These CTs are

called “bushing CTs”.

Some current transformers may have a primary that consists of several turns.

Typically there are no more turns than the natural, which is equivalent to say that the

primary number of turns is 1.

The total load connected to the CT terminal (g and h in this case) is called “burden”.

Ideally, the secondary current of a CT is perfectly proportional to the primary current.

It will be shown later that in reality this is sometimes not true.


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Core and Secondary Winding Example

High-Voltage CT Example


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Note that in all cases there are polarity marks. The

following conventions are used to mark the

reference for AC currents:

ANSI: Polarity marks

IEC: P1, P2, S1, S2

VDE: K, L, k, l


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CT Common Connections

These are the two of the most common connections of current

transformers in three phase systems.

At the left, the “Y” connection provides the line currents at the

secondary.

At the right, the “∆” connection provides the difference currents

(delta currents) to the secondary loads.


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Current Transformer Ratio (CTR)

The Current Transformer Ratio, CTR, expressed as a fraction, is

the ratio between the magnitudes of the primary and the

secondary current for ideal working conditions of the current

transformer.

� Denominator is the Secondary Rated Current

� Typically: 1 A or 5 A

� Numerator Is Not Always the Primary Rated Current


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Current Transformer Equivalent Circuit


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CT Performance Calculations

The performance of a C.T may be found from two approaches :

1. The formula method.

2. The saturation curve method.

2- The saturation curve and error method.

The relation between Es and Ie is not linear like the relation shown in the Fig.1 due to the saturation of C.T core. Therefore the C.T. has ratio error.

Fig.1 Magnetizing curve


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The deviation of Is’ from Is is called the C.T error, and can be expressed as a percentage.

%100%100.'

xI

Ix

I

IITC

s

e

s

sserror

Where:

Is –Is’ = Ie

Is: Total current in the CT secondary Is’

: Current due to the CT load

Example:

Assume that a C.T has rated current ratio of 500/5 A. The impedance of

the secondary winding Z2 = 0.242 Ω, and the burden impedance ZB =

0.351 Ω .The core area A = 0.00193 m2. The C.T must operate at

máximum primary current of 10 kA.If the frequency is 60 Hz and the

core is built from silicon steel:

(a) Determine whether or not the C.T will saturate.

(b) Determine the C.T error.

Solution

Is= 10,000 x (5/500)= 100 A

If we neglect the excitation current Ie.

Is’ ≈ Is= 100 A.

Es = Is (ZB+Z2)

= 100 x (0.351 +0.242) = 59.29 V

Es = 4.44 * f * N2 * A * Bm

So:

T

xxx

15.1

001935.05

5006044.4

29.59Bm

As we know that the lower limit for the silcion steel saturation is

Bm = 1.2T Hence the C.T will not saturate.

From the saturation curve the core corrsponding Ie for Es = 59.29 V

is Ie = 0.1 A.


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%1.0%100100

1.0%100= % C.T

'

error xxI

I

s

e

Saturation Curve for Silicon Steel Used in Current

Transformer Cores for different CTRs.


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CT Classes and accuracies

Accuracy Class

Class

% error Applications

0.1 ± 0.1

Metering 0.2 ± 0.2

0.5 ± 0.5

1.0 ± 1.0

5P ± 1 Protection

10P ± 3

C.T specifications:

A rated burden at rated current or (VA).

An accuracy clases

(ALF) accuracy limit factor an upper limit beyond which the accuracy is not guaranteed.

Example: 1. 15 VA clases 0.5→mettering C.T 2. 15 5p 20 →protection C.T

Rated burden 15VA Accuracy class 5p ALF= 20 (20 times the rated current)


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Power System protection Dr. Mohamad Tawfeeq


2.Voltage Transformers

Voltage transformers are connected across the points at which the voltage

to be measured.

Types of voltage Transformers:

There are three main types of voltage transformers:

Magnetic voltage transformers (ordinary two winding type – used

for L.V. and M.V).

Capacitive Voltage Transformers (CVT), used for high and extra

high voltages.

Magneto—optic voltage transformers (new ).

2.1 Magnetic Voltage Transformer (VT)

Line

Vs = 110 or 120


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VT Equivalent Circuit Referred to the Secondary Side

2.2 Capacitive Voltage Transformers (CVT) There are two types of CVTs:

Coupling capacitor voltage transformer

Capacitor – bushing voltage transformer

These types are shown in figure below:

Figure : Capacitor voltage transformers: (a) coupling-capacitor voltage divider

(b) capacitance-bushing voltage divider .


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CVT equivalent circuit


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VT error : Errors in magnitude can be calculated

from :

Error VT= {(n Vs - Vp) / Vp} x 100%.

Table : Voltage transformers error limits Class Primary

voltage Voltage

error

(±%)

Phase error

(±min)

0.1 0.8 Vn , 1.0

Vn and 1.2

Vn

0.1 0.5

0.2 0.2 10.0

0.5 0.5 20.0

1.0 1.0 40.0

0.1

0.5 Vn

1.0 40.0

0.2 1.0 40.0

0.5 1.0 40.0

1.0 2.0 80.0

0.1

Vn

0.2 80.0

0.2 2.0 80.0

0.5 2.0 80.0

1.0 3.0 120.0

Vn =Nominal voltage


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VT connections

(b) delta – wye connection (a) Open delta connection

wye—wye connection (c)

Instrument Transformers...Current Transformer Ratio (CTR) The Current Transformer Ratio, CTR,...

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