Off-Load Tap Changer – Construction, Logic & Operation
Tap changer is used in a transformer is to keep the output voltage within an acceptable range when system conditions vary. Voltage of the system varies due to many reasons such as it changes with load, feeder length, system impedance, and upstream voltage fluctuations.
Tap changing gives flexibility to the transformer to adjust its effective turns ratio. When you increase the number of turns on the HV side using tap changer, the secondary voltage decreases. When you reduce the HV turns through tap changer, the secondary voltage increases. This simple adjustment helps maintain a stable output voltage across a wide range of load variations.
What is an Off-Load Tap Changer?
An Off-Load Tap Changer (OLTC-offload type) is a tap-changing mechanism that allows you to adjust the transformer’s voltage ratio when the transformer is in de-energized condition.
The transformer must be switched off, isolated, and grounded before changing the tap position.
Difference from OLTC
OLTC can change taps when transformer is in operating condition. Off-load tap changers cannot do so. OLTC uses diverter switches, arc-quenching systems (oil/Vacuum), and complex mechanisms. Off-load tap changer has simple and mechanical as current during tap changing is zero. Off-load tap changers are cheaper, easier to maintain, and are more reliable because there don’t use any arcing contact which are main sources of wear and tear in OLTC. Off-load tap changers are used where voltage is mostly stable and changes are rare.
Operating Condition
While operating taps of off load tap changer transformers, the load must be disconnected. The transformer must be completely de-energized, meaning no voltage should be applied, no current should be flowing and it should be properly grounded for safe working. The zero current prevent any arcing while changing of taps while grounding is done to discharge stray charges and adds safety for operator.
Construction & Working
Off-load tap changers uses a manual or mechanical tap selector. The operator changes the tap position using a handle or rotary switch when the transformer is de-energized.
Pair-wise Tap Changing
Taps are changed in pairs. This keeps the winding symmetrical on both sides. Pair-wise addition or removal of turns at the center prevents uneven magnetic forces.It reduces axial stresses inside the transformer.Generally odd-numbered taps are arranged on one side. Even-numbered taps are kept on the opposite side. This layout maintains electrical balance when selecting different tap positions.
Mechanical stoppers are provided between first and last taps. These stoppers prevent crossing into positions that may cause asymmetry. They also protect the winding from excessive axial forces that occur if taps are changed too far in one direction. This ensures safety and prevents mechanical or electrical stress.
Tap Change Example (Step-by-Step)
Taps are provided at 2.5% steps. See Fig-1, Tap position is kept between 2 and 3. This makes 2.5% less turns to be connected between phase and neutral. In Fig-2, Taps are connected between 3 and 4, this makes 2.5% less turns from both above and below coils i.e. total 5% less than original turns. In this position symmetry is restored.
The taps are changed alternately from above and below coils. This makes the winding either completely symmetrical or slightly skewed. This type of tap changing arrangement is recommended to prevent generation of axial forces in transformer winding.
Advantages and limitations of No-Load Tap Changing Transformer
No Diverter Switches Needed
The tap changing is done only when the transformer is de-energised. Since no current flows during tap operations, there is no need for diverter switches or arc-quenching systems. This reduces complexity and improves life.
No Arcing During Tap Change
Tap change occurs under zero current condition. No arcing happens during tap changing operation and hence no carbonization, no contact wear, and no need for arc-interruption oil/vacuum compartments. Lifespan of contacts increases significantly.
Simpler Insulation Requirements
Without any high voltage moving contacts, the insulation design is simpler. Manufacturing becomes easier and more cost-effective.
Lower Axial Forces
Axial forces develop mainly during faults and inrush currentsA symmetric tap arrangement helps distribute forces evenly. By changing taps in symmetric pairs, the winding maintains balance and mechanical stress remains low.
Very Low Cost, Low Maintenance & High Reliability
When compared to OLTC, it has fewer parts (no diverter/bypass/vacuum switches) and less complex operating mechanism. Hence it costs less compared to OLTC.
With no diverter contacts, now vacuum switch, parts which are prone to wear and tear are less. Thus maintenance required is also minimal.
Its simple operational mechanism makes it suitable for distribution system. Ideal for power system where voltage adjustment is rare.
Limitations of No-Load Tap Changing Transformer
1. Cannot Regulate Voltage Under Load. It requires transformer to be de-energised.
2. Requires Transformer Outage and hence system downtime increases.
3. Not Suitable for Fine Voltage Control. Usually offers limited tap steps (±5% or ±2.5%). Cannot provide continuous voltage regulation.
Applications
Used in Distribution Transformers
- Most 11 kV / 415v distribution transformers use off-load tap changers.
- Voltage rarely needs frequent adjustment, so shutdown-based tap change is acceptable.
Used for Seasonal or Occasional Voltage Adjustment
- Utilities adjust taps based on:
- Seasonal voltage variation
- Long-term load growth
- Feeder voltage profile improvements
- Changes may be done once or twice a year.
Used in Industrial Transformers With Stable Load
- Plants where the incoming utility voltage is stable.
- Minor voltage tuning may be required during periodic maintenance.
Used in Small Power Transformers
- Economical choice for 1 MVA – 10 MVA class transformers.
- OLTC is unnecessary and too costly for these ratings.
Used to Match Transformer Ratio With System Voltage
- Taps at primary side are adjusted to match the incoming voltage supply. Tap with closest voltage is selected. This helps in maintaining the secondary voltage close to rated voltage.
Used in Rural and Semi-Urban Networks
- Voltage drops are predictable and slow-changing.
- Utilities use off-load tap changers to set a “best average” voltage.
Used in Transformers of Renewable and Auxiliary Systems
- Wind/Solar farm auxiliary transformers.
- Battery energy storage LV/MV interface transformers.
FAQ
What is an Off-Load Tap Changer?
An off-load tap changer is a device used in transformers to change the turns ratio. Tap changing is done only when the transformer is switched off. No load current flows during tap change. That is why it is also called a no-load tap changer.
Why is tap changing allowed only in de-energized condition?
There is no diverter switch in an off-load tap changer. It cannot handle current during switching. If tap change is done under load, arcing will occur. This can damage contacts and winding insulation.
What is the purpose of an Off-Load Tap Changer?
The main purpose is voltage adjustment. It helps match the transformer to available supply voltage. It is used for long-term or seasonal voltage variation. It is not meant for frequent voltage control.
Where are off-load tap changers usually provided?
They are mostly provided on the high-voltage winding. HV winding has more turns for fine voltage steps. Current on HV side is low. This reduces sparking risk during tap change.
When should tap changing be carried out?
The load must be disconnected first. The transformer must be completely de-energized. Only then the tap position should be changed. After that, the transformer can be energized again.
Why off-load tap changers cannot regulate voltage continuously?
They work only in no-load condition. They cannot operate while the transformer is supplying power. Each tap change needs shutdown. So continuous voltage regulation is not possible.
What type of switching mechanism is used?
A simple mechanical tap selector is used. It may be rotary or linear in design. Contacts are fixed and robust.No arc-quenching system is required.
Is arcing possible during tap change?
No arcing occurs during proper operation. There is no current flow during tap change. Hence, contacts remain safe.This increases reliability.
What is meant by pair-wise tapping?
Turns are added or removed in symmetrical pairs. This keeps magnetic forces balanced. Axial force on the winding remains low. Mechanical stability improves.
Why symmetry in tapping is important?
Asymmetrical tapping causes unbalanced forces. This can stress the winding mechanically. During faults or inrush, damage may occur. Symmetric tapping avoids these problems.
What is end-tapped winding in off-load tap changers?
Taps are taken from the end of the winding. This construction is simple. But axial forces are higher. Good mechanical support is required.
Are off-load tap changers automatic?
No, they are not automatic. Tap changing is done manually. Operator intervention is required. Shutdown is compulsory.
Where are off-load tap changers commonly used?
They are used in distribution transformers. They are used in industrial installations. They suit stable voltage networks. They are ideal where tap change is infrequent.
Conclusion
A no-load tap changer is a simple and reliable method of voltage regulation in transformers. It allows the transformer ratio to be adjusted only when the unit is de-energized and the load is disconnected. Because no current flows during tap changing, there is no arcing, no diverter switch, and minimal insulation stress. This makes the design mechanically stable, low in cost, and easy to maintain. However, its main limitation is that voltage cannot be regulated during operation, so shutdown is required for any tap change. Due to these characteristics, no-load tap changers are best suited for applications where supply conditions are stable and frequent voltage adjustment is not required.
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