For grid connection engineers, transformer buyers, and BESS developers
OLTC Calculator
An on-load tap changer is specified by the voltage range it has to cover, not by the number of taps. The taps follow from the range and the step size. Enter the transformer ratio and the regulation range the connection requires, choose a step, and the calculator builds the full tap table — every position, and the exact voltage it produces on each winding.
Transformer & OLTC settings
Nominal ratio, the winding that carries the tap changer, the regulation range, and how you define the steps.
Voltage at each tap
Every position on the tap changer, drawn as a ladder around the nominal tap. Drag the slider, or click a rung or a table row, to select a position.
| Tap | Deviation | Primary voltageat nominal secondary (kV) | Secondary voltageat nominal primary (kV) |
|---|
How to use this calculator
Enter the transformer's nominal ratio — for example 110 kV to 30 kV, or 60 kV to 30 kV — and the regulation range the connection has to cover, as a ± percentage of nominal. Most main power transformers are specified against a fixed band such as ±10%, so the range is the requirement, not something you choose freely.
Then define the taps. You can enter the step size (the percentage change per tap) and the calculator derives the number of positions, or switch to defining the number of steps directly and read back the resulting step size. The tap table updates live and shows two voltages for every position: the primary voltage that tap is matched to while the secondary sits at nominal, and the secondary voltage that results if the primary is held at nominal instead.
How the tap voltages are calculated
Each tap changes the effective turns ratio by a fixed percentage. For a tap deviation of p percent from nominal:
- Primary voltage at nominal secondary = nominal primary × (1 + p). This is the primary voltage at which that tap holds the secondary exactly at its nominal value — in other words, the grid voltage the tap is matched to.
- Secondary voltage at nominal primary = nominal secondary ÷ (1 + p). This is what the secondary becomes if the primary stays at nominal and you move to that tap.
The number of positions follows directly from the range and the step: positions per side = range ÷ step, and the total number of tap positions = 2 × (range ÷ step) + 1, including the center (nominal) tap. If the step size does not divide evenly into the range, the calculator rounds to the nearest whole number of steps and reports the range that actually results.
Which winding carries the tap changer
On most power transformers the OLTC sits on the HV winding. The current on the HV side is lower, so the tap changer switches less current and can be built smaller, cheaper and more reliably. The tap voltages are then expressed on the HV winding. Placing the OLTC on the LV winding is possible — the calculator supports both — and simply changes which winding the tap voltages are read against.
Choosing the range and the step size
The range and the step size are two separate decisions, and both are engineering choices in their own right.
The range has to cover the voltage band the connection requires. If the connection can swing ±10% and the OLTC only reaches ±8%, the tap changer cannot hold the plant-side voltage on its own, and the PCS rarely has the steady-state headroom to close the gap. That makes the range a necessary condition to check before the transformer is ordered.
The step size is chosen on its own merits. Each tap movement produces a discrete voltage step on the network, so the step has to be small enough that a single operation does not cause an objectionable voltage change. It also sets how tightly the voltage can be held: the automatic voltage regulator resolves the target to about one step, so a coarser step leaves a wider band around the setpoint. Pulling the other way, a smaller step over the same range means more tap positions, a more complex tap changer, and more operations across the transformer's life. The step is where regulation quality is traded against mechanical complexity and cost.
Who this calculator is for
- Grid connection engineers translating a voltage band requirement into an OLTC range and step size.
- Transformer buyers checking that a quoted tap range and step actually cover the connection requirement.
- BESS developers confirming the main power transformer carries the full voltage regulation the connection requires.
- Engineers and students learning how tap positions, step size, and turns ratio relate to the voltage at each tap.
Frequently asked questions
What is an on-load tap changer (OLTC)?
An OLTC is a mechanism that changes a transformer's turns ratio while the transformer is energized and carrying load, without interrupting supply. It moves between fixed tap positions using a diverter switch and a transition impedance, so voltage can be regulated continuously as grid or load conditions change. It contrasts with a de-energized (off-circuit) tap changer, which can only be moved with the transformer switched out.
What is the difference between the regulation range and the step size?
The regulation range is the total voltage span the OLTC can cover, expressed as ± a percentage of nominal. The step size is the voltage change produced by a single tap movement. The range is usually fixed by the grid requirement (for example ±10%); the step size then determines how finely the voltage can be trimmed and how many discrete positions the mechanism needs. A smaller step gives finer control but more positions and more mechanical operations over the transformer's life.
Why does the ±10% voltage requirement usually fall on the main power transformer?
Grid connection conditions commonly require the plant to stay within a voltage band at the Point of Interconnection (POI) as the transmission voltage varies. The main power transformer's OLTC is the primary means of holding the plant-side voltage within band, because the PCS has limited steady-state voltage headroom. In practice the tap changer carries most of the steady-state voltage regulation, which is why a ±10% requirement usually has to be handled by the main power transformer OLTC.
Does a "+" tap raise or lower the voltage?
The raise/lower sense depends on how the transformer nameplate defines its tap positions, and conventions differ between manufacturers. In this calculator a "+" tap adds turns to the tapped winding, so it is matched to a higher primary voltage — equivalently, if the primary is held at nominal, that tap drives the secondary slightly lower. Always confirm the convention against the actual nameplate before applying it to a real transformer.
What is the difference between an on-load and an off-circuit tap changer?
An on-load tap changer (OLTC) changes taps while the transformer is energized and under load. An off-circuit or de-energized tap changer (DETC) requires the transformer to be de-energized before the tap can be moved. OLTCs are used where voltage must be regulated frequently and automatically; DETCs are used for seasonal or one-time adjustment where the ratio rarely needs to change.