What is an Instrument Transformer : Types and Their Differences

We know that the voltages and currents within a power system are very large. Thus, direct measurement of voltage & magnitude with high magnitude is not possible. So we need measuring instruments which have a high range of measurements or there is another technique like using the property of conversion within AC currents as well as voltages A transformer is used to transform the current or voltage down when turns ration is known after that determining the stepped down magnitude using a usual range of the device. The unique magnitude is decided by simply multiplying the outcome with the conversion ratio. So such kind of transformer with a precise turn ratio is known as Instrument transformer. This article discusses an overview of the instrument transformer and it’s working.

What is an Instrument Transformer?

Definition: A transformer that is used to measure electrical quantities like current, voltage, power, frequency and power factor is known as an instrument transformer. These transformers are mainly used with relays to protect the power system.


Instrument-Transformer
instrument-transformer

The Purpose of the instrument transformer is to step down the voltage & current of the AC system because the level of voltage & current in a power system is extremely high. So designing the measuring instruments with high voltage & current is difficult as well as expensive. In general, these instruments are mainly designed for 5 A & 110 V.

The measurement of high-level electrical quantities can be done using a device namely instrument transformer. These transformers play an essential role in current power systems.

Types of Instrument Transformers

Instrument transformers are classified into two types such as

  • Current Transformer
  • Potential Transformer

Current Transformer

This type of transformer can be used in power systems to step down the voltage from a high level to a low level with the help of a 5A ammeter. This transformer includes two windings like primary and secondary. The current in the secondary winding is proportional to the current in the primary winding as it generates current in the secondary winding. The circuit diagram of a typical current transformer is demonstrated in the following figure.

PCBWay

Current-Transformer
current-transformer

In this transformer, the primary winding consists of few turns and it is connected with the power circuit in series. So it is called a series transformer. Likewise, the secondary winding includes a number of turns and it is connected to an ammeter directly because the ammeter includes small resistance.

Thus, the secondary winding of this transformer works almost in the condition of a short circuit. This winding includes two terminals where one of its terminals is connected to ground to evade the huge current. So insulation breakdown chances will be reduced to guard the operator from huge voltage.

The secondary winding of this transformer in the above circuit is short-circuited before disconnecting the ammeter with the help of a switch to avoid the high voltage across the winding.

Potential Transformer

This type of transformer can be used in power systems to step down the voltage from a high level to a lower level with the help of a small rating voltmeter which ranges from 110 Volts to 120 Volts. A potential transformer typical circuit diagram is illustrated below.

This transformer includes two windings like a normal transformer like primary & secondary. The primary winding of the transformer includes a number of turns and it is connected in parallel with the circuit. So it is called a parallel transformer.

Potential-Transformer
potential-transformer

Similar to the primary winding, the secondary winding includes fewer turns and that is connected to a voltmeter directly because it includes huge resistance. Therefore the secondary winding works approximately in open circuit condition. One terminal of this winding is connected to the earth to maintain the voltage with respect to the earth to protect the operator from a huge voltage.

Difference between Current Transformer and Potential Transformer

The difference between the current transformer & potential transformer is discussed below.

Current Transformer (CT)

Potential Transformer (PT)

The connection of this transformer can be done in series with the power circuitThe connection of this transformer can be done in parallel with the power circuit
The secondary winding is connected to an ammeterThe secondary winding is connected to a voltmeter
The design of this can be done by using the lamination of silicon steel.

 

The designing of this can be done by using high-quality steel which operates at low-flux densities
The primary winding of this transformer carries the current.The primary winding of this transformer carries the voltage
 

It includes less number of turns

It includes a number of turns
The secondary winding of this transformer works

in the condition of a short circuit.

The secondary winding of this transformer works in the condition of an open circuit.
The primary current mainly depends on the flow of current within the power circuit

 

The primary current mainly depends on the secondary load.

 

The insulation breakdown can be avoided by connecting the secondary winding of this transformer to the earth.The secondary winding can be connected to the earth to protect the operator from a huge voltage
The range  of this transformer is 1A or 5AThe range of this transformer is 110v
This transformer ratio is highThis transformer ratio is low
The input of this transformer is the constant currentThe input of this transformer is a constant voltage
This type of transformers is classified into

two types like wound type & closed core.

This type of transformer is classified into two types like electromagnetic & capacitor voltage
The impedance of this transformer is lowThe impedance of this transformer is high
These transformers are used to measure current, power, monitoring the operation of power grid & protective relay.These transformers are used to measure, operating protective relay & power source.

Advantages & Disadvantages of Instrument Transformer

The advantages of instrument transformers are

  • These transformers use ammeter & voltmeter to measure high currents & voltages.
  • By using these transformers, several protecting devices can be operated like relays otherwise pilot lights.
  • Instrument transformer based transformers are less cost.
  • Damaged parts can be easily replaced.
  • These transformers offer electrical isolation among measuring instruments & high voltage power circuits. So that electrical insulation requirements can be reduced in protective circuits & measuring instruments.
  • By using this transformer, various measuring instruments can be connected to a power system.
  • Low power consumption will be there in protective & measuring circuits because of the low level of voltage & current.

The only disadvantage of instrument transformer is, these can be used simply for AC circuits but not for DC circuits

Testing of Instrument Transformer

Instrument transformers like CTs or current transformers play an essential role while monitoring and protecting electrical power systems. These types of instrument transformers mainly used for changing the current form to a diminished secondary current by using relays, meters, control devices & other instruments.

Testing of an instrument transformer is essential when metering, mixing up connections and protection fault occurs otherwise high degree of exactness can be reduced drastically. Simultaneously, electrical changes within a current transformer will occur.

Due to these reasons, it is necessary to verify & adjust current transformers along with their connected devices at normal intervals. There are some electrical tests are employed for these transformers to ensure exactness & optimal service reliability like ratio, polarity, excitation, insulation, winding & burden test.

FAQs

1). What is CT & PT in the instrument transformer?

The current transformer (CT) & potential transformer (PT) are measuring devices used in AC systems

2). What is the function of an instrument transformer?

These transformers are used to measure & protect the equipment

3). What is kVA in transformers?

The kVA stands for Kilovolt-amp and it is an apparent power unit, 1 kVA = 1000VA

4). Why the current transformer is used?

This type of transformer is used to multiply or reduce an alternating current

5). What is the advantage of an instrument transformer?

This transformer gives electrical isolation among the circuit like high voltage power & measuring devices to reduce the necessity of electrical insulation.

Thus, this is all about an overview of instrument transformer. These are high accuracy electrical devices, mainly used for isolating, transforming current or voltage levels. The transformer’s primary winding can be connected to the high voltage or high current circuit & the relay or the meter is connected to the secondary circuit. These transformers are also used as an isolation transformer by using secondary quantities in phase-shift keying without have an effect on other devices. Here is a question for you, what is the main purpose of the instrument transformer?

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