Transformer Tap Changer Formulas
Transformer tap changer formulas: secondary voltage at a primary tap, the tap for a target voltage, the nearest tap step and the voltage error.
The formulas
| To find | Formula |
|---|---|
| Secondary voltage at a primary tap t | Vs = Vs rated × Vp actual / (Vp rated × (1 + t / 100)) |
| Tap for a target voltage | 1 + t / 100 = (Vp actual / Vp rated) × (Vs rated / Vs target) |
| Nearest tap | round(t / step) × step |
| Voltage error | (Vs − Vs target) / Vs target × 100 |
The tap is on the primary winding. A positive tap adds turns to the primary, which lowers the secondary voltage, and a negative tap raises it. Vp actual is the supply voltage on the primary, Vs rated the secondary voltage at the rated tap and rated primary voltage, and t the tap in percent. The voltage also falls with the load current by the voltage regulation of the transformer, which the formulas above do not include. Check the tap convention on the nameplate because some makers number the taps the other way.
Worked example: 11 kV / 415 V transformer, the supply is 10.6 kV
The primary taps are ±5% in steps of 2.5%, and the target secondary voltage is 415 V.
- At the rated tap, Vs = 415 × 10.6 / 11 = 399.9 V
- The tap needed is 10.6 / 11 − 1 = -3.636%
- The nearest tap is -2.5%, which gives 410.2 V, an error of -1.166%
Secondary voltage on a 10.6 kV supply, by tap
| Tap | Secondary voltage | Error from 415 V |
|---|---|---|
| +5% | 380.9 V | -8.225% |
| +2.5% | 390.2 V | -5.987% |
| 0% | 399.9 V | -3.636% |
| -2.5% | 410.2 V | -1.166% |
| -5% | 421 V | 1.435% |
Frequently asked questions
Which direction does a tap move the voltage?
A tap that adds turns to the primary winding lowers the secondary voltage. A tap that removes turns raises it.
What is the difference between an off-circuit and an on-load tap changer?
An off-circuit tap changer is changed with the transformer de-energised. An on-load tap changer changes the tap under load, and is used for voltage control in a network.
Why is the voltage not exactly the target?
The taps move in steps, so there is a leftover error. The load current also lowers the voltage below the no-load value.
How do I choose the tap when the load varies a lot?
Check the voltage at the minimum and maximum load. Choose the tap that keeps both within the allowed band, or use an on-load tap changer.
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