AY: 2021 – 22 �BRANCH : ELECTRICAL ENGINEERING�SEMESTER : 4TH �SUBJECT : ENERGY CONVERSION – I �

Chapter : 05�Topic : Instrument Transformer�PREPARED BY : ER Subhalaxmi Rout

contents

• Current Transformer
• Ratio Error
• Phase angle Error
• Burden

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• Potential Transformer
• Ratio Error
• Phase angle Error
• Burden

Current Transformer

• Primary winding has single turn winding and carries full load current.
• Secondary winding has large number of turns.
• Ratio of the primary current and secondary current is known as current transformer (CT) ratio.

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Current Transformer

Phasor diagram:

• The main flux is taken as reference.
• The primary and secondary voltages are lagging behind the main flux by 90 degrees.
• The magnitudes of the primary and secondary voltage s depend on the number of turns in the windings.

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Current Transformer

Ratio Error:

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Current Transformer

Phase angle Error:

• The phase angle between primary and secondary of the current transformer must be 180 degree.
• The primary and secondary current must be out of phase. The deviation in the phase angle of the primary and secondary current is called the phase angle error.
• The phase angle between primary and secondary current of the CT is denoted by angle θ.

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Current Transformer

• As high current is stepped down to lower value, open CT secondary will cause 0A current in secondary, which will ultimately lead to a high voltage which may burn the CT secondary winding and can damage the system.
• So it is necessary to load the CT secondary continuously and ultimately it acts a burden on CT, and that’s why the load on CT is named as a burden.

potential Transformer

• It is an instrument transformer used for the transformation of voltage from a higher value to the lower value. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watt-hour meters, etc.

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potential Transformer

• The main flux is taken as a reference. The primary current is the vector sum of the excitation current I_o and the current equal to the reversal secondary current I_s multiplied by the ratio of 1/k_t. The V_p is the voltage applied to the primary terminal of the potential transformer.
• The voltage drops due to resistance and reactance of primary winding due to primary current is given by I_pX_p and I_pR_p.
• This primary emf of the transformer will transform into secondary winding by mutual induction and converted into secondary induced emf E_s. This emf will drop by the secondary winding resistance and reactance, and the resultant voltage will appear across the secondary terminal voltage V_s..

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potential Transformer

Ratio Error –

• The ratio error is the change in the voltage ratio due to the variation in load. Varying load changes the magnetizing current & the core losses that affect the secondary voltage of the PT.
• In simple words, its nominal ratio differs from its actual ratio. Ratio error is given by

Ratio Error = (Nominal Ratio – Actual Ratio) / Actual Ratio

Ratio Error = (K_n – R)/R

% Ratio Error = {(K_n – R)/R} x 100

Where

K_n = Nominal Ratio (Rated Ratio)

R = Actual primary to secondary voltage Ratio

The nominal ratio is the ratio of rated primary voltage to rated secondary voltage.

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potential Transformer

Phase angle error:

• The phase angle error is the difference between the phase of primary voltage & the reversed secondary voltage.
• Ideally, the primary voltage is in phase with the secondary voltage in reverse.
• But practically, there is the reactance of the windings that shifts the phase of the secondary voltage creating phase angle error.
• The increases in the number of instruments in the relay connected to the secondary of the potential transformer will increase the errors in the potential transformers.

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potential Transformer

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Burden:

• The burden is the total external volt-amp load on the secondary at rated secondary voltage.
• The rated burden of a PT is a VA burden which must not be exceeded if the transformer is to operate with its rated accuracy.
• The limiting or maximum burden is the greatest VA load at which the potential transformer will operate continuously without overheating its windings beyond the permissible limits.
• This burden is several times greater than the rated burden.�

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