Tap Changing Transformer is an electro-mechanical device. All the electrical equipment we use today is designed to function properly at a specific voltage level. However, the power supplied to electrical equipment can sometimes exceed or exceed its rated voltage. This can be due to a variety of factors, such as voltage drops in distribution lines or over-current power consumption by consumers.
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That is why tap changing transformers are installed to control the voltage so that the consumer can get stable and safe voltage.
What is a Tap Changing Transformer?
The tap-changer is a mechanical and electrical combination connected to the winding of the transformer, with the help of which different turn points of the winding are selected.
The electrical device with the help of which the turn ratio of the winding in the transformer is changed and the output voltage is changed is called tap changing Transformer.
By changing the transformer’s turn ratio, the output voltage is changed, but the supply voltage remains the same as required by the consumer. Tap chambers can be installed on either the primary winding or the secondary winding of the transformer.
It depends on the consumer providing the required voltage.
Why is Required Tap Changing Transformer?
When the load in a transformer changes, the output voltage changes. If the voltage per turn decreases, the core flux decreases, resulting in the core not being fully utilized. However, this situation reduces core loss.
In the second situation, when the voltage per turn of the capacitor is increased, the core flux increases, leading to magmatic saturation. Consequently, the core loss increases, and the magnetizing current increases, thus activating the third harmonic.
To avoid these problems and to provide a fixed voltage to the consumer, a heap chamber is required in the transformer.
Tap Changing Transformer Working Principle
The tap changer transformer acts on the principle of changing the number of turns in the winding.
Assuming
- V1 and N1 are the primary voltage and primary turns,
- V2 and N2 are the secondary voltage and secondary turns.
If the number of primary turns N1 of the transformer is reduced, the EMF V1/N1 generated per turn increases. Due to which the secondary output voltage (V1/N1)*N2 also increases.
if the primary side n1 of the transformer is kept constant and the turn n2 of the secondary side is increased, the output voltage increases
It is clear that the effect of reducing the primary turn of the transformer is the same effect as the effect of increasing the secondary turn n2, and it is on this principle that the tap changer works.
E2= E1N2/N1
Thus,
- E2 – decreases if
- N1 – increases or
- E2 – decreases if
- N2 – decreases
Necessity of Voltage Regulation by Tap Changer
The need for voltage regulation by tap-changer in a transformer is due to the following reasons:
- To change the secondary voltage to the desired value
- To match the output voltage of the transformer with the consumer terminal voltage
- To keep the output voltage constant even when the input voltage changes
- To control the flow of real and reactive power in the power network
- To provide neutral points in the system
Types of Tap Changing Transformers
Tap-changers are mainly classified into two types based on the way they function and use:
- No-Load (or Off-Load) Tap Changer
- On-Load Tap Changer
Off-Load Tap-Changing Transformers
The No Load Tap Changer is mainly used in adjusting the possible voltage variation. In the No Load Tap Changer, the tap change is done in the No Load or Off Load position of the transformer.
When the No Load Tap Changer is mounted on the winding of the transformer, its leads are connected to the numbered status (e.g. one to six). These positions are stable and they are arranged in a circular form.
The face plate holding the studs can also be mounted on the yoke of the transformer or any other convenient location, such as a separate box.
A rotatable arm is brought in contact with these studs. This arm is attached to a hand-wheel, which is placed on the outside of the transformer tank and can be rotated manually. In large transformers, this arm can also be rotated with the help of a motor and gear drive, while the hand-wheel is also available for manual operation.
Effect of Rotatable Arm Position
The determination of how many terms will be included in the winding of a transformer with a no load tap changer depends entirely on the position of the rotating arm. If the winding of the transformer is tapped at a voltage interval of 2%, then the windings involved in the circuit in different positions will be as follows: Operation of the tap changer. The step-by-step below determines the output voltage by changing the turn ratio of the winding of the transformer.
- Studs 1–2: Stays in the entire winding circuit
- Studs 2–3: 98% of the winding remains in the circuit
- Studs 3–4: 96% of the winding remains in the circuit
- Studs at 4–5: 94% of the winding remains in the circuit
- Studs at 5–6: 92% of the winding remains in the circuit
Thus, the voltage is changed step-wise by the no-load tap-changer.
On-Load Tap-Changing Transformers (OLTC)
The on-load tap-changer is used for daily or short-period voltage regulation. The most important feature of this type of tap-changer is that it can control the output voltage without stopping the supply.
Important Operating Conditions
During the operation of the On-load tap-changer, the following points are taken into account:
- The main circuit should never be opened at the time of tap-change, otherwise dangerous sparking may occur.
- No part of the tapped winding should be short-circuited.
Centre-Tapped Reactor-Based On-Load Tap Changer
A common form of on-load tap-changer uses a centre-tapped reactor. The main function of this reactor is to prevent short-circuiting of the winding during tap-change.
Under normal operating conditions:
- The main switch remains off
- The corresponding tap switches remain connected in the set position
- The entire winding remains in the circuit
Half of the total current flows in opposite directions in the two halves of the reactor. Since the entire reactor is winded in the same direction, the MMF generated by the two halves is opposite each other.
Due to the value of these MMFs being equal, their net effect becomes almost zero. As a result, the reactor behaves almost non-inductive and has very low impedance. For this reason, the voltage drop in a center-tapped reactor is negligible.
Diagram of Tap Changing Transformer
In the diagram of the tap changer, different tap points given on the transformer winding are shown, with the help of which the effective turns of winding are changed.
In the diagram of the No-load tap changer, the tapped winding is connected to the stationary studs and the required tap is selected with the help of a rotatable arm. Changing the position of the rotatable arm changes the number of turns that occur in the circuit.
The diagram of the On-load tap changer shows the centre-tapped reactor, which prevents the winding from being short-circuited during tap-changing. Due to this system, voltage regulation is possible without interrupting the supply.
Construction of Tap Changing Transformer
Construction of Tap Changer: A tap changer is an internal and electrical combination that is installed in the winding of a transformer. With the help of which the output voltage of the transformer is kept constant. Most of the tap changers are mounted on high voltage windings, because the high voltage winding has low current and the tap changing gear is easy to handle. The parts in the tap changer are mentioned below, which are as follows:
Main Constructional Parts
1. Tapped Winding
Connections are made at different points (taps) on the primary or secondary winding of the transformer. Each tap offers a different turn ratio.
2. Tap Selector
The task of the tap-selector is to select the required tap. It does not carry the load current, but only selects the next tap.
3. Diverter Switch
The function of the diverter switch is to safely transfer the load current during tap-change. It controls the arcing and continues the supply unhindered.
4. Insulating Medium
The tap-changer is typically placed in transformer oil or special insulating oil, to quench the arch and maintain insulation.
5. Operating Mechanism
The tap-changer is operated by a manual or motor-operated mechanism, making the tap-change smooth and controlled.
Operation of Tap Changing Transformer
Operation of the tap changer. The step-by-step below determines the output voltage by changing the turn ratio of the winding of the transformer.
Step-by-Step Tap Changer Operation:
- Step First:- When the output voltage rises or falls due to a load in the transformer.
- Step Two: – The mechanism of the tap changer instantly selects the appropriate tap and changes the number of turns of the winding.
- Step Three: – If the output voltage is to be increased, reduce the number of turns of the primary winding, causing the EMF per turn to be increased. Increases and the output voltage also increases.
- Step Four: – If the output voltage has to be reduced, the primary increases the number of turns, causing the EMF per turn to decrease. Decreases and the output voltage also decreases.
- Step Five: – The diverter switch provides a safe passage to the load current during tap changing in the On Load Tap Changer, causing no interpretation in the power supply.
Important Operational Points
- The tap-change is usually done near the neutral side, where the voltage is low
- The physical position of the tap-changer is placed in the middle of the winding, so that the axial force is reduced
- Mechanical strength is very important in the event of a short-circuit
Location and Selection of Tap Changing Transformer
The tap changer transformer can be mounted on either the primary side or the secondary side of the winding, but the decision on which side is appropriate is based on the output voltage of the transformer.
Tap Changing Transformer on Primary Side
If the power to the transformer is coming from a variable source, such as the receiving end of the transmission line, then the tap changer is placed on the primary side.
Tap Changing Transformer on Secondary Side
The primary voltage in the transformers installed at the generating stations remains almost constant; Therefore, tap changers are always installed on the secondary side there.
Factors Affecting Selection of Tapping Side
Power transformers with high turn ratios have a tap changer HV (High Voltage) side installed, as it can easily control the output voltage. If you put the tap changer LV (low voltage) side in a transformer, the output voltage changes in big steps, which is not easy to control
The tap changing gear will have to handle less current when the tap changer is installed on the HV side, but this will give the tap changer more insulation. The LV side winding is difficult to install a tap changer, as it is placed near the core for better insulation and the LV winding is inside the HV winding. Due to which it is easy to install a tap changer in HV windings.
Mechanical Forces and Mid-Point Tap Changing Transformer
Radial Forces in Windings
The radial force presses the inner coil towards the core and throws the outer coil away from the core. This repelling force acts along the line connecting the center of gravity of the primary and secondary coils.
Axial Forces During End Tapping
If the winding is tapped at one end and a few turns are cut, the radial force as well as the axial force is generated.
In the event of a short-circuit, these axial forces become too high and begin to press the coil axially, which can cause mechanical damage.
Need for Mid-Point Physical Tapping
To avoid these axial forces, the physical position of the tapped coil is placed in the middle of the winding. This does not generate axial force even after a few turns have been cut and the mechanical stability of the winding is maintained.
Electrical and Physical Position of Tap Changer
Tap Changer in Star-Connected Transformer
In a star-connected transformer, the tap-changer is electrically connected to the point where the voltage relative to neutral is minimal.
However, the physically tapped coil is placed in the middle of the winding.
Tap Changer in Delta-Connected Transformer
This arrangement is not possible in a delta-connected transformer. Here, the electrically tapped coil is also required to be placed in the middle of the winding, so that the tap-changing gear is at a sufficient distance from the line and the chances of flashover are low.
Advantages of Tap Changing Transformer
Stable Output Voltage
The biggest feature of the Tap Changing Transformer is that it keeps the output voltage under control. The consumer gets an almost constant voltage despite changing the load or input voltage.
Better Power Quality
When the voltage is in the right range, the motor, transformer, and other equipment work smoothly. This reduces overheating, noise, and losses.
Protection of Electrical Equipment
Both over-voltage and under-voltage are harmful to electrical equipment. Tap changer protects devices from both of these situations.
Efficient Power System Operation
With the help of a tap changer, the real and reactive power flow can be better controlled, which increases the efficiency of the entire power system.
Flexibility in Operation
In a transformer with an on-load tap changer, the voltage can be changed without shutting off the supply, which is very useful for systems with a continuous supply.
Disadvantages of Tap Changing Transformer
Complex Construction
The construction of a tap changing transformer is more complex than a normal transformer, especially in the case of an on-load tap changer.
Higher Cost
Due to the tap changer mechanism, control system and insulation, its initial cost is high.
Maintenance Requirement
Mechanical parts and contacts require regular inspection and maintenance. Faults can occur if proper maintenance is not done.
Mechanical Wear and Tear
Frequent tap changes can cause wear in contacts and moving parts, which affects reliability.
Oil Deterioration (OLTC case)
Oil can degrade quickly due to arcing in the on-load tap changer, so oil testing and replacement becomes necessary.
Applications of Tap Changing Transformer
Power Generation Stations
Tap changing transformers in generating stations are used to control the grid voltage, so that a stable power supply can be maintained.
Transmission Substations
Voltage regulation is very important in a transmission network. With the help of a tap changer, the voltage drop in long transmission lines is compensated.
Distribution Systems
Tap changers in distribution transformers are used to provide the correct voltage at the consumer end.
Industrial Plants
Industries have heavy motors and sensitive equipment, which require a stable voltage. Tap changing transformers are very useful here.
Power Network Control
Tap changing transformers play an important role for load flow and reactive power control in modern power systems.