The LV winding is placed next to the core because with this arrangement, we get advantage of reduced insulation, low leakage reactance and easily placement of the tap changer at outer HV winding.
First, let us understand the basic concept in order to understand the reason for the placement of the LV winding nearer to the core.
In a three-phase transformer, the primary and secondary winding are placed around the core. The cross-section area of the high voltage winding conductor is less as compared to the low voltage winding, and for transferring the same power from primary to secondary, the voltage is stepped down at the secondary end. As a result, the current in secondary will increase in the same proportion to the reduction of the secondary voltage.
For example – A transformer with a delta star configuration of rating 6.6/0.440 KV, 50 Hz, 1500 KVA,
The ratio of primary to secondary voltage (line-line)=6600/440= 15——— (i)
Primary line current Ip = 1500*1000/ √3 *6600 = 131 Amp.
The line current on the secondary side will be 15 times that of the line current in the primary because the voltage at the secondary is reduced by 1/15 times of the primary voltage, therefore the current at the secondary side of the transformer will increase by 15 times of the primary line current.
Line current at secondary side = 131 *15= 1965 Amp.
From the above example, it is very clear that the current rating of the low voltage side of the transformer is more than the current at the high voltage side.
When the primary winding of the transformer is connected to the supply, the current flowing in the primary winding produces a magnetic field and induces flux in the core. The flux travels through the magnetic core and links to the secondary winding. Practically, all of the flux produced in the primary does not link to the secondary, and the flux that does not link to the secondary is considered as leakage flux.
The primary flux that does not link to the secondary is known as the leakage flux. The reason of the leakage inductance is due to the fact that the flux between primary and secondary winding is not perfectly coupled. The flux produced in the primary does not link to every turn of the secondary and some parts of the flux remain unlinked to the secondary and, it introduces leakage inductance in series with the primary and secondary winding of the transformer. The higher the leakage inductance is, the lower the regulation of the transformer.
Why LV winding is placed next to the core?
There are two reasons of placing the low voltage winding nearer to the core.
In the concentric arrangement of placing of HV and LV winding, less insulation is required when LV winding is placed near the core which is at ground potential. The HV winding is placed around the LV. The HV winding is provided with the tap changer to maintain the desired voltage. If the high voltage(HV) winding is placed near the core, more insulation is needed to insulate the high voltage winding of the transformer and the core. The thicker insulation is required if the HV is placed near the core. For the same rating of transformer, if high voltage winding is placed near the core, more insulation will be required and more insulation lead to an increase in the size and cost of the transformer.
If low voltage winding is placed near the core, the size and cost of the transformer reduce. In a core type transformer, the LV winding is always placed near the core. In a shell-type transformer, HV and LV winding is interleaved alternately to reduce the leakage flux.
Let us understand the insulation requirement in the core type transformer after the placement of winding in different ways.
Let the transformer voltage rating is 132/6.6 KV. The low voltage winding is placed near the core and the high voltage winding is placed after LV placement.
How the insulation requirement gets increased with change in position of HV and LV winding.
Case 1: If LV winding placed near the core
The insulation required for LV =
Insulation required to insulate LV and Core + Insulation required to insulate HV and LV winding
The insulation required for LV = 6.6 + (132-6.6) =132 KV
Case 2: If HV winding placed near the core
The insulation required for HV =
Insulation required to insulate HV and Core + Insulation required to insulate HV and LV winding
The insulation required for HV = 132 + (132-6.6) =257 KV
Percentage increase in insulation = (257-132)/132 x 100 = 94.69 %
Thus, the increase in insulation result in increase of cost and size of the transformer.
Low Leakage Reactance:
The leakage reactance depends on the distance between the core and HV winding. When the HV winding is placed farthest to the core the leakage reactance of primary is low. The low leakage reactance means most of the flux produced in the primary winding gets linked to the secondary. The inrush current of the transformer gets reduced. Furthermore, the voltage regulation of the transformer is improved.
Placement of Tap Changer on HV side
The tap changer is placed on the high voltage side of the transformer. The current flowing in the HV winding is low hence, there is less wear on the tap changer contacts. Also, it is easy to bring the connections of the HV winding at the tap changer because the cross-section area of connecting lead is less. If the tap changer is connected at the LV side of the transformer, the large cross-section area conductor is needed and there would be more contact wear of tap changer contact because of more current. Thus, placing the low voltage winding near the core and high voltage winding after low voltage winding facilitates ease of tap changer connections at high voltage winding and thus the more life of transformer tap changer can be achieved.
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