Basics of Transformer Ratings and KVA rating

Basics of Transformer Ratings and KVA rating: KVA Rating and Impedance Rating are two common transformer ratings.

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Basics of Transformer Ratings

There are two common transformer ratings:

1. KVA Rating
2. Impedance Rating

Read: Best Books for Transformer Design and Electrical Machine Design

KVA Rating

A transformer is generally rated in VA, kVA, or MVA rather than watts.

$$\boxed{{\displaystyle VA=V\times I}}$$

where:

• $V$ = voltage in volts
• $I$ = current in amperes

Common units:

• 1 kVA = 1,000 VA
• 1 MVA = 1,000,000 VA

As the physical size of the transformer increases, its rating is usually expressed in kVA or MVA.

Power Transformer vs Distribution Transformer

According to the notes in the image:

• Power transformer: rated 500 kVA and above
• Distribution transformer: generally below 500 kVA

Transformer rating is expressed in VA/kVA/MVA because the heating of the transformer depends mainly on voltage and current, rather than directly on the power factor.

Impedance Rating of a Transformer — Simple Explanation Transformer impedance is the opposition offered by the transformer to the flow of alternating current (AC).For a transformer, we usually refer to its equivalent impedance.1. Impedance VoltageTo determine the equivalent impedance:

• One winding is short-circuited.A voltage is applied to the other winding.The applied voltage is gradually increased until full-load current flows in the short-circuited winding.This applied voltage is called the impedance voltage.

Percentage ImpedanceTransformer impedance is commonly specified as a percentage (%) of rated voltage.For example, if:

Transformer = 2400/240 V

Impedance = 3%Then:$$V_Z=\frac{3}{100}\times 2400$$$$\boxed{{\displaystyle V_Z=72V}}$$So, 72 V is required on the high-voltage side to produce full-load current when the low-voltage winding is short-circuited.

Meaning of 3% ImpedanceA 3% impedance means that the voltage drop caused by the transformer’s impedance at full-load current is approximately 3% of its rated voltage.For the 2400-V winding:

$$\boxed{{\displaystyle \text{Voltage drop}=72V}}$$ Important Exam Formula

$$\boxed{{\displaystyle V_Z=\frac{Z\mathrm{\%}}{100}\times V_{\text{rated}}}}$$And if actual impedance in ohms is required:$$\boxed{{\displaystyle Z=\frac{V_Z}{I_{\text{rated}}}}}$$For the example, if the full-load current is 90 A:$$Z=\frac{72}{90}=0.8\mathrm{\Omega }$$$$\boxed{{\displaystyle Z=0.8\mathrm{\Omega }}}$$

⭐ Remember KVA rating → tells us the transformer’s capacity.Impedance rating (%) → tells us the transformer’s voltage drop/impedance at full load.3% impedance → 3% voltage drop at full-load current.

$$\boxed{{\displaystyle Z=\frac{V}{I}}}$$Where:

• $Z$ = transformer impedance in ohms (Ω)$V$ = impedance voltage drop in volts$I$ = rated full-load current in amperes

• Transformer = 2400/240 VImpedance rating = 3%Full-load current = 90 A

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