Discover what a tap changer in a transformer is, its types (On Load and Off Load), working mechanism, advantages, applications, and its role in maintaining voltage stability and efficiency in power systems.
What is a Transformer?
A transformer is an electrical device that uses the principle of electromagnetic induction to transfer energy between two electric circuits. It is designed to either increase (step up) or decrease (step down) the voltage of an alternating current (AC) while keeping the frequency unchanged. This energy transfer occurs without any direct electrical connection between the circuits. Instead, it relies on Faraday’s Law of Induction, which explains how a changing magnetic field can induce an electromotive force (EMF) in a nearby circuit.
What is a Tap Changer?
A tap changer is a device used in transformers to regulate their output voltage. It works by altering the number of turns in one of the windings, thereby changing the transformer’s turns ratio and adjusting the voltage accordingly.
Tap changers are mainly classified into two types:
- OLTC – On-Load Tap Changer: Allows voltage adjustment while the transformer is energized and under load.
- DETC – De-Energized Tap Changer (Off-Load Tap Changer): Requires the transformer to be powered down before any adjustment can be made.
It’s important to note that not all transformers are equipped with tap changers—they are typically found in power transformers where voltage regulation is necessary.
System Voltage Management Using Tap Changer
Tap changers play a crucial role in managing and regulating system voltage in electrical power systems. Effective voltage control is necessary for the following reasons:
- Maintaining Voltage Within Permissible Limits:
Ensures that the voltage at the consumer’s terminals remains within the specified range to prevent damage to equipment and maintain power quality. - Responding to Load Variations:
Allows voltage adjustment in response to fluctuations in load demand, helping to stabilize the power system. - Regulating Real and Reactive Power:
By adjusting the voltage, tap changers indirectly help control both active (real) and reactive power flow within the network. - Adapting Secondary Voltage:
Enables adjustment of the secondary (output) voltage of a transformer based on system operating conditions, ensuring optimal performance.
Different Types of Taps in Transformers
Principal taps in a transformer can be either positive, negative, or both. A tap is considered principal when it allows the transformer to deliver the rated secondary voltage when the rated primary voltage is applied.
- Positive taps: Provide a secondary voltage higher than the principal tap voltage. These taps increase the number of turns in the winding, resulting in a higher secondary voltage than the voltage at the principal tap.
- Negative taps: Provide a secondary voltage lower than the principal tap voltage. These taps reduce the number of turns in the winding, leading to a lower secondary voltage compared to the principal tap voltage.
Why Are Taps Usually Placed on the High Voltage (HV) Winding?
Transformer tap points are typically located on the high voltage winding, and here’s why:
- Greater Number of Turns:
The HV winding has more turns, which allows for finer and more efficient voltage variation by changing just a few turns. - Lower Current Levels:
Since HV windings carry lower current compared to low voltage (LV) windings, switching taps on the HV side is safer and easier due to reduced thermal and mechanical stress. - Physical Construction Advantage:
The low voltage winding is usually wound closer to the transformer core, while the high voltage winding is wound on the outside. This makes it more accessible and practical to install tap connections on the HV winding.
OLTC – On Load Tap Changer
An On-Load Tap Changer (OLTC) is an essential component in large power transformers that allows the transformer to adjust its turns ratio without interrupting the power supply. This means voltage regulation can be achieved even while the transformer remains connected to the load, making OLTCs highly effective for maintaining system voltage stability under varying load conditions. These tap changers are standard in most large power transformers due to their ability to improve overall system efficiency and reliability.
One of the main advantages of OLTCs is that they allow tap changing without affecting the main power circuit, ensuring continuous operation. Additionally, the switching mechanism is designed to prevent dangerous arcing during tap transitions, which helps protect the internal components of the transformer and ensures a longer service life.
The OLTC mechanism typically involves a mechanical selector switch, housed in a separate oil-filled chamber for insulation and arc suppression. This selector switch is motor-operated and can be controlled either locally or remotely. For emergencies, a manual handle is provided to operate the tap changer manually.
The selector switch operates on the “make-before-break” principle. During tap transition, a temporary connection is made between adjacent taps, creating a brief short circuit. To safely manage this, an impedance element—either a resistor or a reactor—is included in the circuit to limit the short-circuit current. Most modern OLTC systems use a pair of resistors to perform this current-limiting function, ensuring smooth and safe tap-changing operations.
Location of Tapping in OLTC
In an On-Load Tap Changer (OLTC), tap connections can be placed at various positions within the transformer winding—typically at the phase end, the center of the winding, or at a neutral point. The selection of the tap location depends on the design and voltage level of the transformer, and each position offers specific advantages:
- Tapping at the End of the Phase:
Placing the tap at the phase end helps to reduce the number and size of bushing insulators, simplifying the transformer’s external insulation requirements. - Tapping at the Winding Center:
This configuration reduces the insulation stress between different winding sections, which is especially beneficial in high-voltage transformers. - Tapping at the Neutral Point:
Common in many designs, this location provides better symmetry and insulation grading, particularly in larger transformers where voltage control and insulation management are critical.
The choice of tap location is a key factor in optimizing transformer design for performance, reliability, and cost-effectiveness.
OLTC( On-Load Tap Changer) Working Function
An On-Load Tap Changer (OLTC) operates by shifting the tap positions on the transformer winding while the transformer remains energized and under load. This process involves a carefully sequenced operation of selector switches and a diverter switch to ensure that voltage adjustment occurs without interrupting the power supply or causing harmful arcing.
When a voltage change is needed, the selector fingers (A and B) are moved from one tap segment to another—for example, from segment 1 to segment 2—following this sequence:
- Open Switch ‘Y’:
When switch Y is opened, the entire transformer load current flows through the lower half of the reactor, which is highly inductive. This causes a significant voltage drop. Therefore, the reactor must be designed to temporarily carry the full load current during this phase. - Move Finger B to Segment 2:
Since switch Y is open, finger B carries no current. This allows it to be safely moved to segment 2 without generating sparks or arcing. - Close Switch ‘Y’:
Once switch Y is closed, the winding between taps 1 and 2 is now connected across the reactor, creating a path for current through both tap positions. - Turn On Switch ‘X’:
The entire load current now begins flowing through the upper half of the reactor. - Move Finger A to Segment 2 and Close Switch ‘X’:
After finger A is safely moved to segment 2, switch X is closed. At this point, the winding section between taps 1 and 2 is completely disconnected from the main circuit, and the transformer is now operating at the new tap position (segment 2).
This controlled switching process ensures a smooth and spark-free transition between tap positions, maintaining continuous operation and adjusting the voltage as needed. The same procedure is repeated for additional tap changes depending on system voltage requirements.
Advantages of OLTC (On-Load Tap Changer)
- Voltage Adjustment Without Shutdown
OLTC allows the voltage ratio of the transformer to be changed without switching off the transformer, ensuring uninterrupted power supply during tap changes. - Precise Voltage Control
It enables real-time voltage regulation, helping maintain the desired voltage level at the secondary side under varying load conditions. - Improved Efficiency
By keeping the voltage within optimal limits, OLTC enhances the overall efficiency of the transformer and the power system. - Control of Voltage and Reactive Power Flow
OLTC facilitates adjustment of both voltage magnitude and reactive power flow, contributing to better system stability and power quality.
Disadvantages of OLTC (On-Load Tap Changer)
- Higher Cost
Transformers equipped with OLTC are more expensive due to their complex design, additional components, and control mechanisms. - High Maintenance Requirements
OLTC systems involve moving parts, contact switches, and arcing during operation, which require frequent inspection and maintenance to ensure reliable performance. - Lower Reliability
Compared to simpler tap changers (like DETC), OLTC units tend to have reduced dependability due to their mechanical complexity and the possibility of component wear or failure over time.
Applications of OLTC (On-Load Tap Changer)
- High Voltage Power Transformers
OLTCs are widely used in high voltage power transformers where continuous voltage regulation is essential without interrupting the load. - Power Generation Stations
They are employed in power generation plants to stabilize and regulate voltage before power is transmitted through the grid. - Substations (Above 110 kVA)
OLTCs are commonly used in substations rated above 110 kVA, where voltage adjustments are frequently required to meet varying load demands. - Industrial Power Systems
In large industrial facilities, OLTC-equipped transformers help maintain stable voltage levels, ensuring smooth operation of sensitive machinery.
Diagnosis of OLTC
1).Electrical field test of OLTC
Exciting current
Exciting current tests are highly effective in detecting various issues related to transformer tap changers—both De-Energized Tap Changers (DETC) and On-Load Tap Changers (OLTC). These tests can help identify:
- Tap misalignment
- Coking and wear of contacts
- Loose or poorly seated movable contacts
- Improper wiring between the tap winding and OLTC mechanism
- Reversed connections in the OLTC’s preventative autotransformer (PA)
- Open or shorted turns in the winding
- High resistance connections in the tap circuit
By analyzing abnormalities in the exciting current waveform, technicians can pinpoint mechanical or electrical problems early, thereby enhancing transformer reliability and reducing maintenance costs.
DC Winding Resistance Test
The DC winding resistance test is a diagnostic method used to evaluate the condition of a transformer’s winding and tap changer circuit. It helps identify faults that affect the integrity of the winding’s current-carrying path between terminals.
This test is particularly effective for detecting:
- Partial open-circuit conditions
- Loose or high-resistance connections
- Poor contacts in tap changers
- Broken strands within the winding
By applying a low DC current and measuring the resulting voltage, the resistance is calculated and compared with standard or previous readings. Significant deviations may indicate potential faults or deterioration that could affect transformer performance and reliability.
Dynamic Winding Resistance Test
The dynamic winding resistance test is a specialized diagnostic method used to measure DC current and resistance continuously while the On-Load Tap Changer (OLTC) changes tap positions. This test is particularly valuable for assessing the mechanical and electrical integrity of components involved in OLTC operation.
It is especially effective in identifying issues such as:
- Faulty diverter switches
- Worn or damaged diverter switch contacts
- Defective or degraded transition resistors
The test helps detect any irregularities during the tap transition, such as abnormal resistance spikes, drops, or unstable current flow. In general, it provides insight into the condition of any component responsible for making, carrying, or breaking current during OLTC switching.
This method enhances preventive maintenance by uncovering problems before they lead to operational failures.
SFRA (Sweep Frequency Response Analysis)
Sweep Frequency Response Analysis (SFRA) is a powerful diagnostic technique used to assess the mechanical integrity of a transformer’s internal components, especially the tap windings and their leads.
During an SFRA test, a range of low to high-frequency signals is injected into the transformer windings, and the output response is measured. The mid- to upper-frequency ranges are particularly sensitive to changes in the geometry and positioning of winding structures and connections.
This test is highly effective in detecting:
- Mechanical displacements of windings
- Loosened or deformed tap leads
- Core or clamping structure movement
- Shorted turns or open circuits
By comparing current test results to baseline data or factory fingerprints, even subtle mechanical faults—often caused by short-circuit forces or transportation stress—can be identified.
2).OLTC Oil Test
Dissolved Gas Analysis (DGA) for OLTC
Each On-Load Tap Changer (OLTC) design exhibits a unique normal gassing pattern, which results from the natural deterioration of its insulating materials during operation. These patterns typically remain consistent under normal conditions.
Dissolved Gas Analysis (DGA) of OLTC oil samples is a valuable diagnostic method used to detect abnormal operating conditions. Changes in the type and ratio of dissolved gases can indicate issues such as:
- Localized overheating
- Excessive arcing
- Insulation degradation
- Contact wear
A shift from the OLTC’s typical gas profile—especially in the hydrocarbon gas ratios—often signals the onset of faults. Early detection through DGA allows for timely maintenance and reduced risk of failure, ensuring reliable transformer performance.
Dielectric Strength of OLTC Oil
The dielectric strength of oil in an On-Load Tap Changer (OLTC) is a critical parameter that ensures the oil can withstand high voltages without electrical breakdown. It must remain above a defined minimum threshold to maintain reliable insulation and prevent arcing during operation.
Several factors influence the dielectric strength of OLTC oil:
- Water content: Higher relative saturation of water reduces the insulating capability of the oil.
- Presence of conductive particles: Both the number and size of metallic or carbon-based particles suspended in the oil can significantly lower its breakdown voltage.
- Aging and contamination: Degraded oil with oxidation by-products or contaminants has a reduced dielectric performance.
Regular testing and conditioning of the OLTC oil are essential to ensure safe operation and longevity of the tap changer.
Moisture in OLTC Oil
Moisture is a critical contaminant in the oil of an On-Load Tap Changer (OLTC). Excessive water content significantly reduces the dielectric breakdown strength of the oil, increasing the risk of electrical discharge and insulation failure.
In addition to compromising dielectric performance, high moisture levels can:
- Accelerate contact aging and corrosion
- Promote the formation of acids and sludge
- Lead to localized overheating and arcing
Routine moisture testing helps in early detection and allows for corrective actions such as oil filtration or dehydration, ensuring reliable tap changer performance.
3).Other Test
Infrared Test
Infrared (IR) Test for OLTC
The Infrared Test is a non-invasive thermal imaging technique used to detect temperature anomalies in a transformer, particularly between the main tank and the tap changer compartment.
Under normal operating conditions, the tap changer compartment should be cooler than the main tank. If the infrared scan reveals the tap compartment to be as hot as or hotter than the main tank, it may indicate:
- Internal arcing or contact wear
- Increased electrical resistance
- Poor ventilation or oil circulation
- Mechanical or insulation issues
Early detection of such abnormalities through IR testing helps prevent potential failures and allows for timely maintenance of the OLTC system.
DETC – De-Energised Tap Changer (Off Load Tap Changer)
A De-Energised Tap Changer (DETC), also known as an Off-Load Tap Changer, is typically found on the high-voltage side of a transformer and is used to adjust the transformer’s turns ratio. However, any adjustment can only be made when the transformer is completely de-energised, hence the name. DETCs generally have five fixed tap positions, commonly labeled as 1 to 5 or A to E, with the operating handle usually mounted on the transformer’s cover.
In terms of function, tap position 1 (+) provides the highest turns ratio, resulting in the lowest voltage on the low-voltage side, whereas tap position 5 (-) gives the lowest ratio, delivering the highest voltage. In transformers designed with dual primary or secondary voltage options, a selector switch located on the cover is used to toggle between settings. These tap changers are not meant for real-time voltage regulation but are ideal for seasonal adjustments to maintain voltage stability under varying load conditions.
The internal design often features winding taps at 2.5% intervals, connected through six studs arranged in a circle. A rotatable arm (R), manipulated via a handwheel outside the tank, is used to change the tap. The different stud positions correspond to different amounts of the winding being active in the circuit:
- stud 1–2 connects 100% of the winding,
- 2–3 connects 97.5%,
- 3–4 connects 95%,
- 4–5 connects 92.5%,
- 5–6 connects 90%.
The exact position of this arm determines the output voltage of the transformer. To ensure safe operation, a stud stop is built in to prevent over-rotation, which could otherwise short-circuit the winding and cause damage.
Since DETCs are not make-before-break in design, the transformer must be switched off before any tap change is performed. Attempting to operate the tap changer while the transformer is live can lead to severe arcing, potentially damaging the insulation and internal components. Regular operation is also recommended to prevent mechanical stiffness or failure due to infrequent use.
Application of DETC
The De-Energised Tap Changer (DETC) is commonly used in distribution transformers, particularly those in the range of 250 KVA to 500 KVA. These transformers are typically installed in power distribution networks to step down high transmission voltages to usable levels for residential, commercial, and small industrial consumers.
Since the voltage in distribution networks can vary due to seasonal changes in load demand, DETCs provide a practical way to adjust the transformer’s voltage ratio during planned outages. This ensures the voltage delivered to consumers remains within acceptable limits. DETCs are especially suitable for these applications because they offer a cost-effective and reliable solution where real-time voltage regulation is not required.
Why tap changer is required in a transformer?
A tap changer is essential in a transformer to maintain a constant or nearly constant voltage at the load side, despite variations in load current or fluctuations in the supply voltage. The voltage on the secondary (load) side can change due to these variations, affecting the performance of connected equipment.
The transformer’s basic voltage relationship is given by:
V₁ / V₂ = N₁ / N₂
or,
V₂ = V₁ / K,
where K = N₁ / N₂
Here, V₁ and V₂ are the primary and secondary voltages, and N₁ and N₂ are the number of turns in the primary and secondary windings, respectively.
To regulate and maintain the secondary voltage V₂ close to its desired value, the turns ratio (K) must be adjusted. Since V₂ is inversely proportional to K, changing the number of turns in one of the windings—typically through a tap changer—allows fine control over the output voltage. A tap changer accomplishes this by adding or removing turns from the winding, thereby adjusting the transformer’s voltage ratio and stabilizing the voltage supplied to the load.
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