Kapp regulation diagram (or in short Kapp diagram) - a graphical method of determining the voltage regulation in a transformer caused by changes in load and power factor.¹⁾

Kapp regulation diagram²⁾

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

The output voltage of a mains power transformer when loaded reduces for inductive load (power factor lagging) and increases for capacitive load (power factor is leading).

The Kapp diagram is helpful in finding the voltage reduction or increase (voltage regulation). The main disadvantage is that the voltage regulation phasors are much smaller than the radii of the main circles, so the diagram has to be drawn on a very large scale to get sufficiently accurate results.³⁾

In order to create the diagram it is necessary to know the equivalent reactance X₀₂ and resistance R₀₂ of the transformer as referred to the secondary side.

The following algorithm should be used:

1. Draw phasor OL representing secondary terminal voltage V₂ on load
2. Draw OX representing the phase of the secondary current at an angle Φ₂ to OL such that cos(Φ₂) is the power factor of the load
3. Draw phasor LM (where I₂·R₀₂ is the voltage drop on resistance referred to the secondary side) parallel to OX, and then MN (where I₂·X₀₂ is the voltage drop on reactance referred to the secondary side). The resulting NL is the total voltage drop.
4. Transfer the impedance triangle NLM to OO'P which gives O'L = ON = ₀V₂. Therefore, for a given secondary current the locus of N is the circle with centre O and radius ₀V₂, while the locus of L has the same radius but with the centre O'
5. To find the voltage drop on full load at any power factor the radius OS (joining the three points OQS) should be drawn at at angle Φ to OX. If the impedance triangle is drawn at the position UQT then OU = OS. The length of QS represents the sought-after voltage drop.

• Voltage regulation