Tap Changing Transformer: Working principle, Nameplate readings, Advantages & Applications
The purpose of tap changer in a transformer is to keep the output voltage within an acceptable range when system conditions vary. In practice, the voltage on the power network keeps changing. It changes with load, feeder length, system impedance and upstream voltage fluctuations. During light load conditions, the secondary voltage of transformer will be higher, similarly during high load (power) demand , the secondary voltage of the transformer will be lower.
Tap changer gives flexibility to the transformer to adjust its effective turns ratio. For any given input voltage, when you increase the number of turns on the primary side, the secondary voltage decreases. When you reduce the number of turns turns on primary, the secondary voltage increases. This simple adjustment helps maintain a stable output voltage across a wide range of load variations.
In power systems, particularly in long transmission or sub-transmission lines, variable loading causes noticeable voltage drops. The tap changer compensates for these drops so that downstream users receive voltage within statutory limits.
On-Load Tap Changer and No Load Tap Changer
On-load tap changer corrects the voltage while the transformer is energized, which is essential for large substations and grid transformers. Off-load tap changers are used where occasional manual adjustment is required, such as distribution transformers supplying households.
Where are the Taps usually provided
Tap changing arrangement are usually provided on the high-voltage (HV) winding of a transformer. Placement of tap changer on HV side has a clear advantage.
- The HV side contains a larger number of turns, which allows finer and more accurate voltage adjustment with each tap step.
- Since the HV winding is wound on the outer side while LV winding are wound on the inner side of the core, HV windings are more easily accessible for installation and maintenance of Taps.
- Additionally, the HV side operates at higher voltage and it carries lower current, hence it reduces the chances of arcing or sparking during tap switching.
Basics of Nameplate reading provided on Tap Changing Transformer
There are some standard information provided on the nameplate of any transformer such as KVA ratings, Rated primary voltage, Rated secondary voltage, connection configurations such as start/delta connections etc. Our main objective is to understand the Tap related information on this nameplate.
In the nameplate provided above, the KVA rating of the transformer is 3333 KVA, HV rated voltage is 13800V, LV rated voltage is 480V. In this tap changing transformer taps are provided at step of 2.5% above or below. Usually taps are provided to both increase and decrease the voltage levels.
2.5% above rated voltage 13800V is 14145V, 5% above rated voltage (13800v) is 14490v. Similarly 13455v is 2.5% below the rated 13800v and 13110v is 5% below the rated 13800v. One can clearly see these voltage levels on the nameplate of tap changer transformer.
Types of Tapping Arrangements
Tapping arrangements are provided usually on High voltage side. The rated voltage tap is provided in the center. Taps can be changed in both directions to increase and decrease the voltage levels.
Positive tapping:
When Tap position is changed in such a manner that effective number of turns used after tap changing is more than the default turns it is called positive tapping. Voltage increases in positive tapping due to increase in numbers of turns.
Negative tapping:
When Tap position is changed in such a manner that effective number of turns used after tap changing is less than the default turns it is called negative tapping. Voltage decreases in negative tapping due to decrease in numbers of effective turns.
Principal tapping:
When tap is adjusted to its default position i.e. to rated supply voltage, it is called to be in principle tapping. The transformer when used in principle tapping position, it generates rated voltage.
Center tapping vs end tapping
Since in tap changing transformer we have to increase and decrease the number of turns. It is only possible to do so by adding or removing the turns at the end of the coil. But the physical location of the end can be done to make it physically present either at the center or at the end.
As the name suggests, in center tapping, the physical location of taps are at the center of the windings and in end tapping the physical location of taps are at the end of the winding.
Mechanical Forces in Tapped Winding Transformer
If two conductors carry current in opposite directions they repel each other and when they carry current in the same direction they attract each other. Due to this attraction and repulsion there will be certain Mechanical forces developed in the transformer windings.
Radial and axial forces:
In the transformer, Radial and axial forces arises when two current carrying conductor interact with each other as current in primary and secondary winding of the transformer. When LV and HV windings of the transformer are carrying current in opposite direction, they will repel each other. Since windings are symmetrically located they produce an axial force to repel each other. In such case we can safely say that forces are acting in radial directions at each point.
Now suppose after tap changing in end tapped transformer, few of the end turns are not used. In such case the center of the repulsion forces will shift on one direction as a result axial forces will come into picture.
These axial forces can cause the turns to dislocate from its position. During nominal/low load conditions these forces are low and cause less stress to the winding. The risk increases during inrush current and fault condition. These forces are so strong during fault conditions that it may damage the transformer.
The axial forces are predominant in end tapped transformer. In end tapped transformer upon changing taps, winding does not remains symmetrical with each other. To overcome this issue a small modification in the windings are done to physically bring the end winding to the middle.
In center tapped transformer, HV windings are divided into three parts, These parts are are placed in such a manner that the end part comes in the middle. The electrical connection in these parts are in such a manner that MMF gets added jus as normal coil. So when taps are changed they are changed from a symmetrical location and hence minimal/no axial forces are developed.
Conclusion
Tap-changing transformers are part of our power system for delivering a stable and regulated voltage to electrical transmission and distribution networks. Now Tap changing schematic can be used in two ways, one is on-load tap changers that allow seamless voltage correction without interruptions and other is off-load tap changers used in applications where occasional manual adjustment is sufficient. In both these systems the fundamental purpose remains the same—maintaining system voltage within acceptable limits while protecting equipment. Tap-changing transformers enhance reliability, improve power quality, and support the overall flexibility of modern power systems.
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