What is an Instrument Transformer?

By Aina Parasher|Updated : June 6th, 2022

Instrument Transformer: When one has to deal with the measurement of operating voltages and currents in the power system, the conventional methods of measuring voltage and current with electromechanical instruments are far from reality. Generally, the instruments used for the measurement of such high voltages and currents are the instrument transformers. The instrument transformer is a combination of conventional measuring instruments and step-down transformers. 

In this article, we will discuss the importance of an instrument transformer, the types of instrument transformers that are the current transformer, and potential transformers, and the operating characteristics of an instrument transformer.

Table of Content

What is the Need for an Instrument Transformer? 

In commercial metering, high values of currents and voltages are involved, so we cannot measure them with the voltmeters and ammeters of such high ratings. One may think that we can extend the range of the voltmeter and ammeter with the help of multipliers and shunt connections, but practically the size of the instrument will be large, and cost-wise it is not even so wise to think about it. Apart from size and cost, we have the key operational difficulties with such extensions, that is the inaccuracy in A.C. circuits, the power consumed by the shunt is high for such large ratings, and the key issue is it is difficult to employ proper insulation for the instrument at the high voltages. 

So, to measure these high voltages and currents we must step down these quantities so that we can measure them with the instruments of moderate size, this is what exactly is done by the instrument transformers. Apart from stepping down the quantities to be measured, the instrument transformers are also providing isolation from the high voltage power circuits, so that we can ensure the safety of the operator. Hence, the development of instrument transformers enables the measurement of high voltage such as 132 kV by using an instrument of a nominal 110 V rating, we can also measure a 1000 A of current with the help of a nominal 5 A ammeter. 

Types of an Instrument Transformer

There are two types of instrument transformers, they are namely  

  1. Current transformer (C.T) 
  2. Potential transformer (P.T), also known as the voltage transformer. 

Current Transformer

The instrument transformer that is connected in series with the high-power circuit to measure the current is known as the current transformer. Apart from the measuring applications, we can also use the current transformers to feed the protective relays in the power system protection. The below figure shows the basic circuit arrangement of the current transformer. 

Unlike the power transformer, the primary current in the current transformer is not determined by the load connected in the secondary, since the primary winding of the current transformer is connected in series with the current-carrying line, whereas in the power transformer, the primary winding has a shunt connection with its source. Generally, the current ratings of secondary of current transformers are 5 A or 1 A. 

  • The primary winding of the current transformer has a smaller number of turns with thick conductors to withstand the high line currents, whereas the secondary side has a large number of turns. 
  • In order to protect the equipment in the event of insulation failure, one terminal of the C.T. secondary winding is grounded. 

For the current transformers employed for the measuring applications the saturation current will be 1.25 times the rated secondary current, but for the C. T.s employed in protection applications they must withstand high fault currents, so their core is designed with cold rolled grain oriented (CRGO) steel.  So that the C.T. can withstand these surges for a few more seconds and allow the relay to disconnect the faulty section. 

Potential Transformer

The primary winding of the potential transformers is connected in a shunt with high power circuit. The basic connection arrangement of the potential transformer is shown in the figure. The primary winding of the potential transformer has a large number of turns, whereas the secondary winding has a smaller number of turns. 

The secondary winding of the potential transformer is connected to the voltmeter, as shown above, as the voltmeter has a very high internal resistance, the operating condition of the potential transformer is almost similar to the open circuit transformer. To ensure the safety of the operator one terminal of P.T secondary is connected to the earth. Generally, the secondary voltage rating of the potential transformer is 110 V or 120 V line to line. 

Basic Terminology Associated with an Instrument Transformer

There is a standard terminology associated with the instrument transformers, such as the burden of the instrument transformer, transformation ratio (R), nominal ratio (Kn), turns ratio (n) and ratio correction factor (R.C.F), here we will discuss the definitions of these terms. 

Transformation Ratio (R)

The transformation ratio is the ratio of the magnitude of the primary phasor to the secondary phasor of the instrument transformer. 

R=Magnitude of primary phasor/Magnitude of the secondary phasor

For the current transformer, 

R=magnitude of primary current (Ip)/Magnitude of the secondary current (Is)

For the potential transformer, 

R=magnitude of primary voltage (Vp)/Magnitude of the secondary voltage (Vs)

Nominal Ratio (Kn)

The nominal ratio is the ratio of the rated value primary phasor to the rated secondary phasor of the instrument transformer. 

For the current transformer, 

Kn=Rated primary current (Ip rated)/Rated secondary current (Is rated) 

For the potential transformer, 

Kn=Rated primary voltage (Vp rated)/Rated secondary voltage (Vs rated)

Turns Ratio(n)

If Nis the number of turns in the primary winding and,

Nis the number of turns in the secondary winding of the instrument transformer then, 

Turns ratio of the current transformer (C.T) is, 

n=Ns/Np

Turns ratio of the potential transformer (P.T) is, 

n=Np/Ns

Burden

The load connected on the secondary side, including the impedance of the measuring instrument, is known as the burden of the instrument transformer. The rated burden of the instrument transformer is the volt-ampere loading of the transformer, at which errors do not exceed a specified limit.  

If Ze is the equivalent impedance on the secondary side of the instrument transformer that includes the secondary impedance winding, then  

Total secondary burden=Es2/Ze=Is2/Z

Here Eis the secondary induced voltage of the instrument transformer. 

If Vis the secondary terminal voltage of the instrument transformer and Zis the impedance of the load connected to the secondary then, the secondary burden due to load=Vs2/ZL=Is2/Z

Ratio Correction Factor (R.C.F)

The ratio of transformation ratio to the nominal ratio of the instrument transformer is known as the ratio correction factor. 

R.C.F=RKn

Errors in Current Transformer

There are two errors that occur with the current transformer, they are namely 

  1. Ratio error 
  2. Phase angle error 

Ratio Error

Ratio error=[(Kn−R)/R]×100

⇒Ratio error=(Kn/R−1)×100

⇒Ratio error=(1/R.C.F−1)×100

Phase Angle Error

For an ideal instrument transformer, the angle between the primary and secondary current will be 180°, but in practice, this angle is less than 180° which is 180°−θ 

Causes of Errors in Current Transformer

The causes of errors in the current transformer are,  

  • Losses in the core of the transformer. 
  • Losses are associated with the winding resistance. 
  • Flux leakage in the transformer. 
  • Core saturation. 

Reduction of errors in current transformer

The errors in the current transformer can be reduced by using the high permeable materials for the core construction, that is CRGO steel, nickel-ferrous alloy, and silicon steel stampings to minimize core losses. In addition, reduce the air gap between the primary and secondary windings to avoid leakage flux. 

Why the secondary winding of the C.T should not be open-circuited? 

In a power transformer, the primary current is dependent on its secondary current. If secondary is open-circuited, the primary current also reduces to the value of no-load current. Whereas in the current transformer (C.T), the primary winding is connected in series with the power circuit, so the primary current is constant irrespective of the condition of the secondary winding, hence the primary M.M.F is constant. 

During normal operation, the secondary M.M.F will oppose the primary M.M.F as per Lenz’s law. If we suddenly disconnect the C.T secondary, then there is no counter force for the primary M.M.F which is constant. This primary M.M.F will increase the core flux and causes it to saturate or if the core is still capable, it will produce a huge flux linkage at the C.T secondary which has a large number of turns. This flux linkage will produce a high voltage across the secondary which can damage the insulation and can also be capable of harming the operator responsible for the secondary winding opening. To avoid this, prior to removing the burden from C.T, it will be short-circuited using an earth switch.  

This short circuit will not affect the C.T, as the C.T is designed to operate with an ammeter across it, we know that the internal resistance of the ammeter is almost negligible, so the C.T is generally designed to operate with a negligible resistance almost equivalent to the short circuit across its secondary winding.    

Comments

write a comment

FAQs on Instrument Transformer

  • The instrument transformers are more advantageous than the shunts or multipliers used for the instrument extension. These are a few such advantages 

    • We can measure high quantities of voltages and currents with the instruments of moderate ratings.  
    • The measuring circuit is isolated from the high-power circuit. 
    • The readings of instrument transformers don’t have a dependency on their circuit parameters like resistance, inductance, and capacitance, whereas if we use shunts and multipliers for the instrument extension, the reading will be affected by these parameters. 
  • Yes, we can measure the power flow in the high-power circuit by the proper connection of the potential transformer, current transformer, and the watt meter. The current transformer secondary will be connected across the current coil and the secondary winding of the potential transformer is connected to the pressure coil of the watt meter. 

  • A current transformer is an instrument transformer that is used to measure the high currents in the power system and industrial applications. The instrument is a combination of a stepdown transformer and an ammeter. The current transformer primary is connected in series with the power circuit and steps down the current to the level measurable with the moderate rating ammeter. Generally, the rating of the secondary is 5 A or 1 A. 

  • For the current transformer used in the measurement, the core will saturate for the current slightly greater than the rated current. The current transformers used in the protection have to deal with high fault currents, so this current transformer is designed with a superior core material that is CRGO steel so that the C.T. can withstand these surges for a few more seconds and allow the relay to disconnect the faulty section. 

  • The potential transformer is an instrument transformer, that step down the high voltages in the power circuit to facilitate their measurement with a voltmeter of the moderate range. The primary winding of the potential transformer is connected in a shunt with the power circuit just like the power transformer, the secondary winding is connected across the voltmeter. 

Follow us for latest updates