The voltage regulation of transformer is defined as the change in secondary terminal voltage (V₂) from no-load to full load at constant primary voltage and temperature. It is expressed as a percentage of the secondary no-load voltage.
 
Mathematically, % Regulation of transformer = (E₂ – V₂) x 100 / E₂
 
No-load voltage: The secondary terminal voltage of transformer when no load is connected to the transformer is known as the no-load voltage of the transformer. At no load, the secondary terminal voltage will be equal to induced EMF in the secondary winding.
 
So, no-load voltage = E₂ volts
 
Full load voltage: It is the secondary terminal voltage of transformer when a rated load is connected to the transformer. We will denote it by V₂.
 
When a transformer is loaded a voltage drop in primary and secondary impedances of transformer takes place. As the load current increases, this voltage drop will increase. This will reduce the secondary terminal voltage V₂. The ideal value of voltage regulation of transformer is 0%.

Voltage Regulation of Transformer | Formula

The approximate expression for the total voltage drop (E₂ – V₂) in a transformer as referred to secondary is given by
 

 
Where,
R₀₂ = equivalent resistance of transformer referred to secondary
X₀₂ = equivalent reactance of transformer referred to secondary
R₀₁ = equivalent resistance of transformer referred to primary
X₀₁ = equivalent resistance of transformer referred to primary
 
Here it has been assumed that φ₁ = φ₂ = φ.
 
The positive sign is used for a lagging power factor and the negative sign for a leading power factor.
 
It is clear from the above expressions, the voltage regulation of transformer does not depend only on the magnitude of load current. But it also depends on the type of load. The transformer regulation is positive for the resistive and inductive loads but it can be negative for the capacitive loads.
 
We can determine the values of R₀₁, X₀₁, R₀₂, X₀₂ of a transformer from short circuit test and calculate percentage regulation of transformer.

Calculate Voltage Regulation of Transformer

Example: A 100 kVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 Ω and 0.01 Ω respectively and the corresponding leakage reactances are 1.1 and 0.035 Ω respectively. The supply voltage is 2200 V. Calculate:

• equivalent impedance referred to primary and,
• the voltage regulation and the secondary terminal voltage for full load having a power factor of 0.8 leading.

 
Solution: K = 80/400 = 1/5,
 
R₁ = 0.3 Ω,
R₀₁ = R₁ + R₂/K² = 0.3 + 0.01/(1/5)² = 0.55 Ω
X₀₁ = X₁ + X₂/K² = 1.1 + 0.035/(1/5)² = 1.975 Ω
Z₀₁ = 0.55 + j 1.975 = 2.05 ∠74.44^o
 
Z₀₂ = K²Z₀₁ = (1/5)² (0.55 + j 1.975) = (0.022 + j 0.079)
 
No-load secondary voltage = KV₁ = (1/5) × 2200 = 440 V,
I₂ = 10 × 103/440 = 227.3 A
 
Full-load voltage drop as referred to secondary
= I₂ (R₀₂ cos φ − X₀₂ sin φ)
 
= 227.3 (0.022 × 0.8 − 0.079 × 0.6 ) = − 6.77 V
 
% regn. = − 6.77 × 100/440 = − 1.54
 
Secondary terminal voltage on load = 440 − (− 6.77) = 446.77 V
 
Thanks for reading about ‘voltage regulation of transformer’.
 

 

• Single Phase Transformer Working Principle
• Ideal Transformer
• Construction of Three Phase Transformer
• Types of Transformers
• Equivalent Resistance and Reactance of Transformer
• Equivalent Circuit of Single Phase Transformer
• Power Loss in a Transformer
• Open Circuit Test of Single Phase Transformer
• Short Circuit Test on Single Phase Transformer
• Transformer Efficiency
• Regulation of Transformer
• Autotransformer
• Instrument Transformers
• Polarity of Transformer Windings
• Significance of Vector Group of Transformer
• Buchholz Relay Construction | Working
• Why current transformer secondary should not be opened
• Dielectric Strength Test of Transformer Oil
• Transformer Moisture Removal Process

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