Instrument transformer (current transformer & Voltage transformer) -Types based on application, Installation.

Instrument Transformer.

An instrument transformer is a static device used in a substation.

Purpose using this transformer

Instrument transformers used for measurement and protection purposes in substations.

Parts and working principle of instrument transformer.

The instrument transformer has three main parts 

1. Primary winding
2. secondary winding
3. Core

The alternating current or voltage generated at the secondary winding is directly proportional to the alternating current or voltage at its primary winding. In simple words, Instrument transformers are the type transformer which reduces the voltage and current from a higher value to a lower value for measurement, protection, and controls purpose.

 A Primary winding receives a high amount of current or voltage from the main bus bar and converts it into a suitable value for measurement, Protection, and control purposes.

Instrument transformers have a very low burden value (VA ratings) so this cannot be used for power transmission. This is a major difference between the instrument transformer and the power transformer.

Classification of instrument transformer based on its installation

• If the transformer is installed at the switchyard then it is called 'Outdoor instrument transformer'.
• If the transformer is installed at the switchgear inside the panels then it is called 'Indoor Instrument transformer'. 

Can we use an instrument transformer for measuring current and voltage?

Yes, it can be used to measure current and voltage.

Classification of instrument transformer based on its application

Instrument transformers used for measurement and protection purposes in substations are the Current transformer and Voltage transformer.

Measurement purpose.

If the secondary winding of the instrument transformer is connected to the measuring devices such as Analog Ammeter, Analog Voltmeter, watt-meter, or Digital meters then this type of transformer is for measurement purposes.

Protection purpose.

If the secondary winding of the Instrument transformer is connected to the IED devices (Intelligent Electronic Device) such as Numerical relay, Differential relay, etc then this type of instrument transformer is used for protection purposes.

From the above mentioned, we came to know that Current transformers(CT) and Potential Transformers(PT) are called instrument transformers.

• To reduce "current" from a higher rating to a lower rating, we have to use the current transformer (CT). Secondary current is 5 Amp or 1 Amp (r.m.s).
• To reduce "voltage" from a higher rating to a lower rating, we have to use the potential transformer(PT). This is also called 'Voltage transformer'.Secondary voltage is 110 volts (r.m.s).

Current Transformer.

The current transformer reduces current for metering purposes in its secondary winding which is proportional to the current in its primary winding. The current transformer is a "step-up transformer" because the number of turns in the secondary winding is more when compared with the primary winding. The current transformer is a series-connected type instrument transformer.

Ns > Np

The current transformer is also called CT. The secondary side of the current transformer(CT) must be shorted while operating because the high voltage is generated at the secondary terminal, high voltage may cause arching or blast in CT. 

According to the connection of primary winding, the current transformer is basically classified into.

1. Wound-type current transformer.
2. Toroidal-type current transformer.
3. Bar-type current transformer.

1.Wound-type current transformer.

The primary side of the wound-type current transformer is directly connected with the live conductor.

2.Toroidal-type current transformer.

The toroidal transformer doesn't have a primary side. It has a window-type hole through which the conductor will pass.

3.Bar-type current transformer.

Bar type current transformers consist of a bar on the current transformer. The Primary winding is surrounded on the bar and ends of the bar connected to the conductor.

Based on its application, the Current transformer can be categorized into. 

1. Measuring core current transformer.
2. Protective core current transformer.

How to identify "Measuring core current transformer"?

To identify the (measuring) application of a current transformer, we have to check the "class" of the current transformer on the nameplate.

If the accuracy class is represented with 0.1 to 1.0 then the current transformer is for metering purpose. This transformer can reduce the current for measurement purposes.

Nameplate of the metering current transformers

Because of its metering application, this type of current transformers requires more accuracy and precision. So the value of class does not exist beyond 1%. The current transformer is connected with metering devices.

Single-line diagram for CT measuring the conductor of R-phase.Each phase(Y, B) is connected with a separate CT to measure the current flow in the conductor.

If the current transformer's class is equal to 0.5 means +\- 0.5% of ratio errors will be shown by the meter due to copper-current loss in the winding of the CT.

For example: From the above-mentioned nameplate.

• Ratio = 1250/1 A. 
• Class-0.5.

Formula to calculate %ratio error.

For a full-load current of 1250 Amps, the ammeter should show 1 Amp, but due to class-0.5% ratio error, the ammeter will show only 0.995 at 100% load and 1.005 at 120% load.

How to identify "Protective core current transformer"?

The transformer which reduces current for protection purposes is called a protective current transformer. 

To identify the application (protective transformer) of the transformer we have to check the class, if the class is denoted as 5 P-10 or 5 P -20 then this current transformer is a protective current transformer.

P represents protection. 

Sometimes 'class' is represented as a PS class (special protection).

Single-line diagram CT is connected to the IED device to monitor the faulty current.  

The accuracy class 5 P 10 means accuracy will change when 10 times of current flow through the current transformer. 

So in this type of transformer's accuracy and precision is very low. If the class is 5 P-10 then it means 5 % of error will occur when measuring 10 times of current flows on the conductor connected with CT.

Nameplate of the protective current transformer.

Double-core current transformer.

Double-core current transformer has two secondary winding one winding can use for measurement and the other one can be used for protection.

Nameplate detail for double-core current transformer

By seeing this single line diagram you can understand more about a double-core current transformer.

Potential transformer.

The potential transformer reduces the voltage on its secondary winding which is proportional to the voltage on its primary winding. A Potential transformer or voltage transformer is the 'step down transformer' its secondary winding is less when compared with the primary winding.

Ns<Np

A primary side of the potential transformer is connected from phase to phase or phase to earth i.e., we can use it for three-phase and for a single phase. Voltage transformers may be either single-phase or three-phase.

A potential transformer or voltage transformer connection should be in parallel. This is a parallel-connected type of instrument transformer. 

There are two types of constructions:

• Electromagnetic Potential transformer in which the primary side and secondary side are wound as a normal transformer.
• Capacitor potential transformer in which the primary side is taken through a series capacitor group i.e., Primary voltage flow through a series capacitor. The voltage across one of the series capacitor is taken to the auxiliary voltage transformer. The secondary side of the voltage transformer is taken for measurement or protection purposes.

How to find measuring voltage transformer?

As same as the current transformer, the voltage transformer also has 'Class'. If the class is 0.1 to 1.0 then it is for measurement purposes. The ratio error is due to the impedance of the voltage transformer winding.

How to find the protection voltage transformer?  

If the class is 1P to 10P then this type of transformer is used for a voltage transformer.

 Nameplate for measuring and protection voltage transformer.

Double core Potential transformer

From the above-mentioned name-plate potential transformer have two cores that are represented as 'Term' (a1-n1 and a2-n2). One core 'Class' represented as 0.5 and another one is represented as 3P.

Conclusion

All sensing devices' protective devices and measuring devices are connected to the secondary winding of the instrument transformer. The primary winding of the instrument transformer is connected to the main bus bars or lines.