A transformer consists of low voltage (LV) & high voltage (HV) windings that are carefully arranged for performance and safety.
- What is LV Winding?
- What is HV Winding?
- Construction of LV (Low Voltage) Windings
- Location of LV Winding
- Conductor Size
- Winding Arrangement
- Applications
- Difference between HV and LV Windings in a Transformer
- HV Windings vs LV Windings
- Leakage Reactance in Transformer Windings
- Why LV Winding is placed near the Core?
- Reduced Insulation Requirement
- Lower Leakage Reactance
- Insulation Needs for HV and LV Windings
- Core-Type vs. Shell-Type Transformer Winding Position
- Summary
One popular design is to locate the LV winding nearer to the core.
The LV winding is placed near to the core because this design layout provides
- Lower insulation,
- Low leakage reactance and
- Convenient tap changer positioning at the exterior HV winding.
This post discusses the fundamental significance of positioning the LV winding near to the core.
What is LV Winding?
The LV winding of a transformer is the coil attached to the low-voltage side, which is built with a smaller number of thick conductors to transmit high current at a low voltage.
It is typically located near the core for improved insulation and provides the necessary utilization voltage to the load.
What is HV Winding?
In a transformer, the HV winding is the coil that is attached to the high voltage side.
It has a large number of turns of smaller conductors for managing high voltage and low current.
For insulating considerations, the HV winding is often installed above the LV winding & supplies (or) receives power at transmission (or) distribution voltage levels.
Construction of LV (Low Voltage) Windings
The LV winding of a transformer is intended to carry a large current at a low voltage.
The transformer’s construction is determined by its voltage rating, current capacity & use.
Location of LV Winding
The LV winding is always located nearer to the transformer core.
This decreases insulation requirements because the voltage differential between the core (at ground potential) & the LV winding is small when compared to the HV side.
Conductor Size
Because it carries more current the LV winding has a thicker conductor and a bigger cross-sectional area. This helps to reduce resistive losses & overheating.
Winding Arrangement
In concentric designs, the LV winding is attached directly on the core while the HV winding is put above it.
In disc or sandwich designs, LV windings are alternately interleaved with HV windings to enhance magnetic coupling & voltage control.
Applications
The LV winding provides the usage voltage required by loads (e.g., 415, 230 V in distribution transformers). Its design enables consistent current monitoring while remaining efficient and safe.
Difference between HV and LV Windings in a Transformer
HV Windings vs LV Windings
To figure out why the LV winding is located closer to the core, let us first differentiate the fundamentals of transformer windings.
A three-phase transformer has primary as well as secondary windings looped around the core.
The conductor utilized for the high-voltage (HV) winding is smaller in cross-sectional area than the low-voltage (LV) winding.
This is because, while transferring the same power from primary to secondary, the secondary voltage is stepped down, causing an increase in current on the LV side.
Hence, the LV winding requires an increased size conductor to carry the higher current, but the HV winding carries less current and hence employs a smaller conductor.
| Aspect | HV (High Voltage) Winding | LV (Low Voltage) Winding |
| Conductor Size | Smaller cross-sectional area (thin conductor) | Larger cross-sectional area (thick conductor) |
| Current Level | Carries lower current | Carries higher current |
| Voltage Level | Connected to high voltage side | Connected to low voltage side |
| Placement | Usually wound over the LV winding (outer side) | Placed near the core (inner side) |
| Reason | Needs better insulation from the surroundings. | Placed near the core to minimize insulation requirements. |
Leakage Reactance in Transformer Windings
When the primary winding of a transformer is turned on, the current creates a magnetic field which induces flux in the core.
Ideally, this flux should be entirely integrated with the secondary winding.
However, in practical terms, some of the flux does not connect with the secondary which is known as leakage flux.
The presence of leakage flux causes leakage reactance (or leakage inductance), that appears in series with both the primary & secondary winding.
Higher leakage reactance causes weaker voltage regulation in transformer.
Why LV Winding is placed near the Core?
There are 2 main reasons for placing the low-voltage (LV) winding near the transformer core:
Reduced Insulation Requirement
Because the core is at ground potential, positioning the LV winding near it reduces the amount of insulation required.
Lower Leakage Reactance
Closer positioning lowers the distance between windings, resulting in improved coupling & lower leakage flux.
Insulation Needs for HV and LV Windings
In a concentric winding design, the LV winding is placed near to the core, while the HV winding wraps around it. The design:
The LV side requires less insulation (since it is so near to the core).
Makes sure that the HV winding that needs additional insulation is located externally wherein insulation can be maintained more effectively.
Frequently contains a tap changer on HV winding to regulate output voltage.
If the HV winding were installed immediately adjacent to the core, greater insulation would be required between the winding & the grounded core. This would increase the transformer’s size and cost.
Core-Type vs. Shell-Type Transformer Winding Position
Core-Type Transformer: The LV winding is always located closest to the core, while the HV winding surrounds it.
Shell-Type Transformer: The HV & LV windings are interleaved alternately to reduce leakage reactance & improve coupling.
Eg: Insulation Requirement Calculation
Consider a transformer rated at 110/33 kV.
In this case:
- The LV winding (33 kV) is located near to the core.
- The HV winding (110 kV) is placed above the LV winding.
This arrangement of position results in less insulation required, increased efficiency & an optimized transformer design.
Case-1: If LV Winding is placed near the Core
If the LV winding (33 kV) is located near the core & the HV winding (110 kV) is wound over (above) it:
Insulation required for LV = Insulation between (LV & Core) + insulation between HV & LV winding.
Therefore, Insulation Required = 33 kV + (110 – 33) kV = 110 kV
This configuration used to reduces insulation requirements as compared to the positioning of the HV winding nearer the core that resulting in a smaller & less expensive transformer at overall comparsion.
Case-2: If HV Winding is placed near the Core
If the HV winding (110 kV) is located near to the core & the LV winding (33 kV) is located outside (outer):
Insulation needed for HV = insulation between HV & Core + insulation between LV & HV winding.
Therefore, Insulation Required = 110 kV + (33 kV) = 143 kV
This design in primary concern have significantly more insulation, increasing the size, cost & complexity of the transformer.
As a result, in practice, the LV winding is always located near the core to maximize efficiency and economy.
Summary
In a transformers, the LV winding is always placed nearest to the core since it decreases the insulating requirements, leakage reactance, costs and increases efficiency.
This design provides consistent performance while also reducing the transformer’s cost.