1. STANI MEMORIAL COLLEGE OF ENGINEERING & TECHNOLOGY
POWER SYSTEM DESIGN LAB
EXPERIMENT NO. 6
Aim: - Instrument Transformers: Design considerations of CTs & PTs for measurement and
protection.
Apparatus Requirements: - Current Transformer, Potential Transformer, AC supply,
Watt Meters, Patch Cords.
Theory:Instrument transformers are high accuracy class electrical devices used to isolate or transform
voltage or current levels. The most common usage of instrument transformers is to operate
instruments or metering from high voltage or high current circuits, safely isolating secondary
control circuitry from the high voltages or currents. The primary winding of the transformer
is connected to the high voltage or high current circuit, and the meter or relay is connected to
the secondary circuit.
Current Transformer:A current transformer (CT) is used for measurement of electric currents. Current
transformers, together with voltage transformers (VT) (potential transformers (PT)), are
known as instrument transformers. When current in a circuit is too high to directly apply to
measuring instruments, a current transformer produces a reduced current accurately
proportional to the current in the circuit, which can be conveniently connected to measuring
and recording instruments. A current transformer also isolates the measuring instruments
from what may be very high voltage in the monitored circuit. Current transformers are
commonly used in metering and protective relays in the electrical power industry.
Current transformers (CT) are a series connected type of instrument transformer. They are
designed to present negligible load to the supply being measured and have an accurate current
ratio and phase relationship to enable accurate secondary connected metering.

2. Potential Transformer:Potential transformers (PT) (also called voltage transformers (VT)) are a parallel connected
type of instrument transformer. They are designed to present negligible load to the supply
being measured and have an accurate voltage ratio and phase relationship to enable accurate
secondary connected metering.
The Current Error or Ratio Error in Current Transformer or CT:From passer diagram of C.T. it is clear that primary current Ip is not exactly equal to the
secondary current multiplied by turns ratio, i.e. KTIs. This difference is due to the primary
current is contributed by the core excitation current. The error in current transformer
introduced due to this difference is called current error of CT or sometimes Ratio Error in
Current Transformer.
Voltage Error or Ratio Error in Potential Transformer (PT):The difference between the ideal value Vp/KT and actual value Vs is the voltage error or
ratio error in a potential transformer, it can be expressed as:
Circuit Diagram:-
Fig. 6.1 Equivalent Circuit Diagram of C.T.

3. Fig 6.2 Vector Diagram of C.T.
Fig. 6.3 Vector Diagram of P.T.
Precautions:1. Do not touch any live conductor.

4. 2. Please keep distance from electrical equipments because voltage level very high i.e. 132
kV, 220 kV, and 400 kV.
Procedure:-
Error Measure in Current Transformer:I.
II.
III.
Using a core of high permeability and low hysteresis loss magnetic materials.
Keeping the rated burden to the nearer value of the actual burden.
Ensuring minimum length of flux path and increasing cross – sectional area of the
core, minimizing joint of the core.
IV.
Lowering the secondary internal impedance.
Is – Secondary Current
Es – Secondary induced emf
Ip – primary Current
Ep – primary induced emf
KT – turns ratio = numbers of secondary turns/number of primary turns
Io – Excitation Current
Im – magnetizing component of Io
Iw – core loss component of Io
Φm – main flux.
Let us take flux as reference. EMF Es and Ep lags behind the flux by 90o. The magnitude of
the passers Es and Ep are proportional to secondary and primary turns. The excitation current
Io which is made up of two components Im and Iw.
The secondary current Io lags behind the secondary induced emf Es by an angle Φ s. The
secondary current is now transferred to the primary side by reversing Is and multiplied by the
turns ratio KT. The total current flows through the primary Ip is then vector sum of KT Is and
Io.

5. Error Measure in Potential Transformer:As in the case of Current Transformer and other purpose Electrical Power Transformer, total
primary current Ip is the vector sum of excitation current and the electric current equal to
reversal of secondary current multiplied by the ratio 1/KT
Hence, Ip = Io + Is/KT
If Vp is the system voltage applied to the primary of the PT then voltage drops due to
resistance and reactance of primary winding due to primary current Ip will comes into picture.
After subtracting this voltage drop from Vp, Ep will appear across the primary terminals. This
Ep is equal to primary induced emf. This primary emf will transform to the secondary
winding by mutual induction and transformed emf is Es. Again this Es will be dropped by
secondary winding resistance and reactance, and resultant will actually appear across the
burden terminals and it is denoted as Vs
So if system voltage is Vp, ideally Vp/KT should be the secondary voltage of PT, but in reality
actual secondary voltage of PT is Vs.
Is – Secondary Current
Es – Secondary induced emf
Vs – Secondary terminal voltage
Rs – Secondary winding resistance
Xs – Secondary winding reactance
Ip – Primary current
Ep – Primary induced emf
Vp – Primary terminal voltage
Rp – Primary winding resistance
Xp – Primary winding reactance
KT – turns ratio = numbers of primary turns/number of secondary turns
Io – Excitation Current
Im – magnetizing component of Io
Iw – core loss component of Io
Φm – main flux
β – Phase angle error

6. Observation Table:-
Calculation:-
Result: - We have successfully studied about Instrument Transformers, Design
considerations of CTs & PTs for measurement.

7. Viva- Voice Questions:-
Ques. 1 What is Instrument Transformer?
Ques. 2 Write the Expression of Error in C.T.
Ques. 3 For which Instrument transformer Secondary must not be open?
Ques. 4 Difference between C.T. and P.T.

8. Viva- Voice Answers:Ans. 1 Instrument transformers are high accuracy class electrical devices used to isolate or
transform voltage or current levels. The most common usage of instrument transformers is to
operate instruments or metering from high voltage or high current circuits, safely isolating
secondary control circuitry from the high voltages or currents.
Ans. 2
Ans. 3 for Current Transformer.
Ans. 4
1) Ans. 4 CT is used for Current Measurement while PT is used for Voltage
Measurement.
2) CT always connected with in the series or main circuit while PT is always connected
with parallel with the main circuit.