What is an On-Load Tap Changing Transformer & Its Working

An electrical device which works on the principle of faraday’s law of induction is a transformer, where faraday’s law states that the magnitude of the emf produced inside a conductor is due to electromagnetic induction. A transformer consists of two types of windings like primary & secondary. The main function of this is to transfer electrical energy from one circuit to another circuit. When a voltage is supplied to a transformer, it should be controlled properly. Hence, in order to maintain the stability of the voltage supply based on the capacity of the transformer, we use the tapping concept. Where the number of turns in a transformer can be variably selected by a tap changing mechanism by connecting taps at various points in a transformer to either primary or secondary windings. This mechanism can be done automatically in two ways, one way is (NLTC) No-LoadTap Changing Transformer and another way is (OLTC) On-Load Tap Changing Transformer. This article briefs about OLTC.

What is On-Load Tap Changing Transformer (OLTC)?

Definition: An On-Load Tap Changing Transformer (OLTC) consists of an open load tap changer, it is also known as an on-circuit tap changer (OCTC). They are used in areas where there is an interruption in the power supply due to an unacceptable tap change. The ratio of the number of turns can be changed without breaking the circuit. It consists of 33 taps where out of which 1 tap = center rated tab and 16 taps = increases the ratio of the windings and the remaining 16 taps = decrease the ratio of the windings.


Location of Tapping

The location of the tapping is made at the ending of phase, or at the winding center or at a point of neutrality. By placing them at various points it has following advantages like

  • If the tap is connected at the ending of phase the insulators of bushing can be reduced
  • If the tap is connected at the winding center, there will be a decrease in insulation between various parts.

Theses kind of arrangement is necessary for larger transformers.

Construction

It consists of a center tap reactor or a resistor, with a voltage V1 employees HV – high voltage winding and LV – low voltage winding, a switch S which is present is a diverter switch, 4 selector switches S1, S2, S3, S4, 4 & Taps T1, T2, T3, T4. Taps are placed in a separate oil-filled compartment where the OLTC switch is present.

This tap changer operates remotely and also manually for safety purposes. There is a provision of a sperate handle for manual control. If the selector switch breaks down, it leads to a short circuit and damages the transformer. Hence in order to overcome this, we use resistor/reactor in the circuit which provides impedance, thereby reducing short circuit effect.

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On-Load Tap Changing Transformer using a Reactor

The transformer enters into the operating stage when the diverter switch is closed and the selector switch1 is closed. Now if we want to change the selector switch from 1 to 2 then this can be done by adjusting the tap, by following the below steps.

On Load Tap Changing Using a Reactor
On Load Tap Changing Using a Reactor

Step1: Firstly open the diverter switch, which indicates no current flows through selector switches

Step2: Connect tap changer to selector switch 2

Step3: Open the selector switch 1

Step4: Close the diverter switch, at this state current flows in the transformer.

Only half a portion of reactance is connected for limiting the current while adjusting the tap. The secondary output voltage can be increased or decreased by changing the number of turns ratio using the selector switch and the diverter switch. Due to the larger power system application, it is necessary to change the transformer taps several times to maintain the required voltage on the system as per load demand. Basically the demand for the continuity of supply does not permit the transformer from disconnecting the supply. Hence an on-load tap changer is employed with a continuous supply.

On-Load Tap Changing Transformer (OLTC) using a Resistor

The on-load tap changing transformer using a resistor can be explained as follows

It consists of resistors r1 and r2 and 4 taps t1, t2, t3, t4. Based on tap position the switches get connected and current flows which are shown in the below case figures.

Case(I): If diverter switch is connected at tap1 and tap2, the load current flows from top to tap1 as shown below

On-Load Tap Changing Transformer Connected Between Tap1 and Tap2
On-Load Tap Changing Transformer Connected Between Tap1 and Tap2

Case(ii): If the diverter switch is connected at tap2, the load current flows from r1 to tap

On-Load Tap Changing Transformer Connected at Tap2
On-Load Tap Changing Transformer Connected at Tap2

Case(iii): If the diverter switch is connected between tap 2 and tap3, current flows in the opposite direction which is represented as (I/2 – i) from r1 and (I/2 +i) from r2 as shown below

Connected between Tap2 and Tap3
Connected between Tap2 and Tap3

Case (iv): If the diverter switch is connected between tap3 and r2, then the current flows from r2 to tap

Connected between Tap3 and r2
Connected between Tap3 and r2

Case(v): If the diverter switch is connected at tap3 the current I is shorted as shown below

Connected at Tap3
Connected at Tap3

The main objective of using a resistor in the OLTC transformer is to maintain the voltage by controlling the flow of current using switches.

Advantages

The following are the advantages

  • The ratio of voltage can be varied without de-energizing the transformer
  • Provides voltage control in the transformer
  • OLTC increases the efficiency
  • It provides adjustment of voltage magnitude and flow of reactive.

Disadvantages

The following are the disadvantages

  • The transformer used is costlier
  • Huge maintain ace
  • Less reliability.

Applications

The following are the applications

FAQs

1). What is on load and offload tap changer?

In the No-load tap-changing transformer (NLTC), the main supply connection is disconnected while changing the tap. Whereas on-load tap changing transformer (OLTC) there will be continuous power supply even when tap positions change.

2). What is the tapping of the transformer?

Whenever a voltage is supplied to a transformer it should be controlled properly, hence in order to maintain the stability of the voltage supply based on the capacity of the transformer we use the tapping concept.

3). On which side the tap changer is usually located and why?

Tap changers can be connected at various points in a transformer to either primary or secondary windings. It becomes easy to access HV windings when a tap is placed on the HV side because HV is wounded with LV and also it reduces lightening risk when breaking down.

4). How do taps work on a transformer?

Taps control secondary voltage in a transformer.

5). What is the principle of the transformer?

The transformer works on faraday’s law of induction, where faraday’s law states that the magnitude of the emf produced inside a conductor is due to electromagnetic induction.

A transformer is an electrical device that works on the principle of faradays law of induction. A transformer consists of two types of windings primary windings and secondary windings. To maintain the stability of the voltage supply based on the capacity of the transformer we use tapping concept. Where the number of turns in a transformer can be variably selected by a tap changing mechanism, by connecting taps at various points in a transformer to either primary or secondary windings. This mechanism can be done automatically in two ways, one way is no load tap-changing transformer (NLTC), and another way is (OLTC) On-LoadTap Changing Transformer.

This article briefs about OLTC. In off load tap changer transformer, the main supply connection is disconnected while changing the tap. Whereas on-load tap changer transformer there will be continuous power supply even when tap positions change. The main advantage of OLTC is that is can operate without disconnecting. They are mainly used in power transformer.

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