ALL
DAY EFFICIENCY
 The performance of the Distribution Transformer cannot be judged by ordinary efficiency.
 As the primary winding of the transformer is energized throughout the day ( 24 hours ) , iron loss occurs continuously whether the transformer is loaded or unloaded.
 The copper loss occurs only when the transformer is loaded therefore the distribution transformer is designed such that the core losses are very small.
 The performance of such transformers can accurately judge by following equation.
All
day efficiency = Output in ( kWh ) /
Input in ( kWh )
 Although the all day efficiency depends on kWh not in kW, its value depends upon load cycle ( from hour to hour ) during the day.
 The all day efficiency is always is less the ordinary efficiency because the distribution transformer does not supply rated load throughout the day.
Example
Find
out allday efficiency of 600 kVA transformer having core loss and full load
copper losses are 3 kW and 4 kW. The transformer is loaded as shown below during
the day.
Load 
Power factor 
Hours during day 
450 kW 
0.8 
5 
400 kW 
Unity 
6 
300 kW 
0.5 
6 
No load 

Rest of the day 
Solution
Data given
600 kVA
transformer
W_{i} =
3 kW
W_{cu}
= 4 kW
To find out
All day
efficiency = ?
Output
Energy = kW_{1} h_{1} + kW_{2} h_{2} + kW_{3}
h_{3}
= (450 × 5) + (400 × 6)
+ (300 × 6)
= 6450 kWh
Iron
losses throughout the day = 3 × 24 = 72 kWh
The
copper loss varies as the square of the load kVA therefore
kVA_{1}
= kW_{1} / Cos Φ_{1}
= 450 / 0.8
= 562.5
kVA_{2}
= kW_{2} / Cos Φ_{2}
= 400 / 1
= 400
kVA_{3
}= kW_{3} / Cos Φ_{3}
= 300 / 0.5
= 600
Copper
losses throughout the day
=
{[ kVA_{1} / kVA ]^{2} × W_{c} × h_{1}}
+{ [
kVA_{2} / kVA ]^{2} × W_{c} × h_{2} }
+{ [
kVA_{3} / kVA ]^{2} × W_{c} × h_{3} }
=
{ [ 562.5 / 600 ]^{2} × 4 × 5 } + { [ 400 / 600 ]^{2} × 4 × 6 }
+ { [ 600 / 600 ]^{2} × 4 × 6 }
=52.25
kWh
All
day efficiency = { Output energy ( 24 hours ) / Input energy ( 24 hours ) } ×
100%
=
{ Output energy / [ Output energy + Iron
losses + Copper losses ] }× 100%
=
6450 / [ 6450 + 52.25 + 72 }× 100%
=
98.10%
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