Why is the Low Voltage (LV) winding placed near to the core of a transformer ?

Usually in the transformer the primary winding and the secondary winding are interleaved in the core. We know the windings are generally placed in different limbs or they not coincide (by observing circuit diagram) that means two windings are separated. But in practical it is not like that, the two windings are wounded over each other i.e. interleaved.

Why is interleave winding preferred ?

The two windings are concentrated but they have different radius or different cross section. If the two windings are wounded in the same core then the flux linkage between the windings is very well that means more amount of flux is linked from one winding to other winding. So we have high coefficient of coupling. That means wasted flux or leakage flux is lesser but the linking flux in the secondary winding is high. That's why we prefer interleave winding for shell type transformer.

Interleaved winding design of transformer

Basic Concept

In a transformer the primary and secondary winding is placed on the core. The cross section area of Low Voltage winding is more compare to High Voltage winding. In case of step down transformer the secondary winding voltage is low but current is high as power is constant at the both side. 
For example -  A transformer in delta connection having rating of 6.6/0.440 KV, 50Hz, 1500KVA
∴Turns Ratio = Ratio of primary to secondary voltage (line - line)
 = 6600/440 =15
Primary line current Ip = 150000/(√3 * 6600) =131A
In secondary side the line current will be 15times of the primary side because the voltage of at secondary side is reduced by 1/15 times of the primary voltage. 
Line current in the secondary side Is = 131*15 = 1965 A
So from the above example it is very clear that the current rating of LV side is more than the current rating of HV side.

Placement of Winding

 First placed low voltage winding near to core and then placed high voltage winding. There are some specific reasons behind it, why low voltage winding is placed inside.

Reasons 

Insulation Requirement :

Insulation of a winding determine the voltage rating and diameter of the conductor determine the current rating. For higher current application we use thick wire and temperature rise determine the KVA rating.

If the high voltage(HV) winding  is placed close to core, the more insulation is required to insulate the HV winding of the transformer and the core. So thicker insulation is required. So for same rating of the transformer, if the HV winding is placed near to the core, more insulation is required and more insulation lead to increase in size and also increase the cost of the transformer. 

As Low voltage (LV) winding is placed inside that mean it has low voltage rating and the requirement of  insulation is also be lesser. So, the insulation which lies between the core and low voltage winding is reduced. So the cost is also be reduced. The high voltage winding (HV) is placed around the low voltage winding(LV).

Let's understand the importance of insulation in the core style transformer after placing the winding in different ways - 
A transformer having the voltage rating of 132/6.6 KV  and the LV winding is placed near to the core and the HV winding is placed after the LV winding.

Placement of Winding of the transformer

Lets check how the insulation requirement is gets increased due to changing the placement of HV and LV winding.

Case i : If the Low Voltage(LV) winding is placed near to the core.
Required insulation for LV  = Insulation required for insulation the LV and core + Insulation required for insulating the LV and HV.
The insulation requirement for LV winding = 6.6 +(132-6.6) = 132 KV

Case ii : If the High Voltage(HV) winding is placed near to the core.
Required insulation for HV  = Insulation required for insulation the HV and core + Insulation required for insulating the HV and LV.
The insulation requirement for LV winding = 132 +(132-6.6) = 257.4 KV
As a result, the increase in insulation results in increased transformer cost and size.

%increase in insulation = (257-132)/132 ⨯ 100 = 94.69%

Reduce Leakage Reactance :

As the leakage reactance depends on the distance between HV winding and the core. The leakage reactance of the primary winding is low when the HV winding is positioned farthest to the core. The low leakage reactance means that most of the flux produced in the primary winding gets linked to the secondary winding. The inrush current of the transformer is reduced and the voltage regulation of the transformer is also improved.

Placement of Tap changer on High voltage side :

Tap changing transformer means in which we can alternate the number of turn of the winding. In tap changing transformer we can change the output voltage as per our required. The current in the HV winding is low so there is less wear in the contact of tap changer. The taps are usually provided in the HV winding because we can have more variation in the voltage. When it comes to the high voltage winding, it has low current rating and less cross sectional area that's why HV winding is kept outside, so that we can easily tapped the winding. If the tap changer is connected with LV winding, the large cross section area conductor is needed and because of more current, there will be more contact wear of the tap changer surface. Thus, positioning the low-voltage winding close to the core and high-voltage winding after low-voltage winding facilitates ease to tap changer connections at high-voltage winding and thus more life of tap changer transformer can be obtained .