On-Load Tap Changer (OLTC) transformer – Working of OLTC and Uses of OLTC transformer
An OLTC (On-Load Tap Changer) is a mechanism in a transformer that allows the voltage ratio to be adjusted while the transformer is energized and carrying load, without disconnecting with the power supply. It works by changing the number of effective turns in the transformer’s winding through a carefully controlled switching process that avoids arcing, usually using diverter switches, selector switches, and transition resistors or reactors. OLTCs are essential in transmission and distribution networks because they help in maintaining constant output voltage despite fluctuations in input voltage or load demand.
Table of Contents
Why an on-load tap changer is needed
Tap Changers are of two types. One is an onload tap changer and the other one is no load tap changer. The onload Tap changer is provided with transformer which operates continuously without any break. Transformer connected to power transmission grid are used continuously. If any voltage variations occurs in their primary or any load fluctuations occurs which causes voltage disturbances on the secondary side, then the end user will not get the rated voltage. This voltage variation will damage the equipment. High voltage damages insulation while low voltages damage windings of motors. In such transformers tap changer are used to adjust the voltage levels such that the end user can get rated voltage. These voltage adjustments are done while the transformer is in charged state i.e it is connected to the grid and is supplying power to the load.
Principle of Operation
The on-load tap changer (OLTC) works on make-before-break mechanism. In this mechanism the new tap is connected before the old tap is disconnected during tap changing process. The load current is never interrupted during tap changing. Diverter switches, resistor/reactors are used to make smooth transition without any spark. The diverter switches carry the load current for brief period of time while the transition resistors or reactors control the circulating current and reduce arcing during the tap changing. As a result, the tap change happens smoothly and the transformer continues supplying power without any break.
Detailed Step-by-Step OLTC Operation
Before diving into step by step OLTC operation, lets get familiar with the components of LOTC changer.
Selector Switch
Selector switch is used to choose the next tap position. Its job is to select and prepare the tap that will be used next. The selector switch does not break or make heavy current and it has less wear and tear. This makes OLTC operation safer, smoother, and more reliable.
Reactors
During tap changing, the load current should not break. Hence there will be some instance when two taps are selected at once using selector switch. At this instance there will be circulating current. The reactor will be limit this circulating current.
Vacuum switch
A vacuum switch is used to safely interrupt current during tap changing. In a vacuum, there is no air, so arcing is very small. This helps the switch break the current without arcing. The vacuum switch protects the diverter contacts from wear and tear. It also increases the life of the OLTC.
Bypass switch
A bypass switch in an OLTC is used to take over the current path during tap changing. It never makes or breaks the load current. It provides an alternate route for the load current while the diverter switch moves between taps. It makes the tap-changing process smoother and protects the OLTC components from damage.
Now lets understand the working principle of OLTC step by step.
Here’s a short, clear step–by–step explanation that follows the ten diagrams (1)–(10). Each step uses plain sentences.
- (1) The transformer is on tap 1 and carrying the full load current through the diverter/selector path. Half of the current flows through both the reactors towards neutral.
- (2) The bypass switch moves to position A. Sector switch still connects tap 1. Now full load current flows through both diverters towards neutral via point A.
- (3) In this step vacuum switch is operated. Now full load current flows through only one of the reactors. Tap is still at position 1 and full load current flows through point A.
- (4) The selector switch (q) reaches and makes contact with the new tap (tap 2). Now both taps are connected but load current still flows through selector p and bypass switch A only.
- (5) At this moment, vacuum switch is closed. Now Tap2 and Tap1 both are connected to neutral through bypass switch A. Since both taps are at different voltage levels, there will be a circulating current. This circulating current will be limited by both the reactors.
- (6) At this moment neutral connection is shifted to bypass switch B. both bypass switch A and B were in contact for a brief moment before shifting completely to B. This is make before break phenomena. Both reactors are still sharing load current and the circulating current is limited by these reactors.
- (7) Now Vacuum switch is opened to stop the circulating current. Atthis moment Full load current is flowing through only one of the reactors.
- (8) At this stage selector switch (p) has changed its position to Tap 2.
- (9) Now vacuum switch is turned On again. With this Load current is being shared by both the reactors.
- (10) Final state: both bypass switches A and B are connected to neutral point. Thus both reactors, both bypass switches and both selector switches are carrying equal load current through Tap 2.
8. Advantages & Limitations
Advantages of OLTC
- It adjusts transformer voltage without switching off the load.
- It maintains stable output voltage during varying system loads.
- It improves power quality by controlling voltage dips and rises.
- It allows continuous regulation, providing finer voltage control.
- It reduces the risk of equipment damage due to voltage fluctuation.
- It supports automatic remote voltage regulation through AVR.
- It increases system reliability for critical feeders and substations.
- It avoids supply interruption, improving consumer satisfaction.
Limitations of OLTC
- It is mechanically complex and needs regular maintenance.
- It has moving contacts, which wear out over time.
- OLTC oil gets carbonized faster due to arcing.
- It makes the transformer more expensive than off-load tap changers.
- Failure of OLTC can cause major transformer downtime.
- It needs high-quality insulating oil for proper arc quenching.
- OLTC operation is slower compared to electronic tap controls.
- It requires skilled technicians for troubleshooting and overhaul.
FAQ:
How can OLTCs, automatic voltage regulators, and power electronics interfaces help maintain steady output without overloading the transformer?
They adjust voltage automatically during load changes. OLTC changes turns ratio, AVR corrects output voltage, and power electronics smooth fast variations. Together they keep voltage stable and reduce stress on the transformer.
When do you use a tap changer vs. a capacitor bank to control voltage?
Use a tap changer for long-term, slow voltage regulation. Use a capacitor bank for reactive power support and improving power factor and voltage at load end.
How do you account for tap changer effects when setting differential protection?
You include the tap range in relay settings. The relay uses percentage bias and vector matching so that tap changes do not create false differential currents.
Can a tap changer voltage step increment be negative?
Yes. Negative steps reduce the output voltage by decreasing the number of turns.
Secondary voltage after tap change from 33 kV to 35.475 kV?
Voltage ratio = 33,000 / 433.
New secondary = 35,475 ÷ (33,000 / 433) ≈ 465 V.
Why is a tap changer installed on the high-voltage side?
Because HV winding carries lower current, making tap changing easier, safer, and mechanically simpler.
Why does the tap changer flash during on-load?
It happens due to arcing during contact switching. Poor oil quality, worn contacts, or high load can increase the flash.
Conclusion
Electrical Power System is getting complex day by day. Vide variety of loads with varying sizes are being added day by day. Maintaining voltage level of grid has become challenging these days. An On-Load Tap Changer is an essential system for maintaining steady voltage in modern power networks. It allows transformers to adjust taps without interrupting the load, ensuring reliable and high-quality power supply. Although OLTCs are more complex and require regular maintenance, their ability to provide continuous voltage regulation makes them indispensable in substations, industrial plants, and long-distance transmission systems. Overall, OLTCs improve system stability, protect equipment, and enhance consumer voltage reliability.
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