In short. Buck-boost regulation places a small series transformer in the supply path to add or subtract a fraction of the voltage. Because it only handles the correction, its kVA can be much smaller than the load, which makes it economical for modest corrections.
Principle
Supply voltage → series transformer adds or subtracts a correction → load voltage within target. The transformer carries the full load current but only a fraction of the voltage.
Transformer kVA ≈ load kVA × (correction voltage ÷ load voltage)
Worked example (illustrative)
A 100 kVA load at 415 V needs correction of up to 40 V. The series transformer handles about 40 ÷ 415 of the load kVA, so roughly 10 kVA of transformer rating, plus margin for tap steps and losses. A full 100 kVA isolating transformer would be larger and costlier.
| Aspect | Buck-boost |
|---|---|
| Isolation | None; the output is connected to the input |
| Size and cost | Small for modest correction |
| Correction range | Limited by transformer ratio |
| Fault levels | Short-circuit passes through; protection needed |
Where it appears
- Inside servo and static stabilisers as the correction stage. See servo and static.
- Standalone correction for fixed voltage offsets.
- Utility voltage regulators on feeders.
Limits
- No isolation: noise and earth issues pass through. See isolation transformers.
- Large corrections need larger ratios and cost more.
- Series impedance affects fault current and voltage drop.
- Protection must consider the series winding.
Illustrative example
A fixed 20 V low voltage on a lighting panel is corrected by a small boost transformer sized at under a tenth of the panel kVA. If the supply later swings widely, the engineer replaces it with an adjustable stabiliser.
A second worked example: fixed boost for a remote panel
A lighting and small-power panel at the end of a long cable consistently receives 380 V on a 415 V system. The load is 30 kVA. The correction needed is about 35 V.
Series transformer kVA ≈ 30 × (35 ÷ 415) ≈ 2.5 kVA, then add margin
A small series transformer of several kVA with a suitable tap could correct the offset. But if the supply swings from 360 to 440 V during the day, a fixed boost would over-voltage the load at high supply. In that case a variable stabiliser is the correct choice.
Selecting taps and ratios
- Record the supply range over time, not one reading.
- Decide whether the offset is fixed or varies.
- If fixed, select a ratio that centres the output without exceeding limits at the highest supply.
- If varying, use an adjustable or controlled correction.
- Check that the series transformer carries the full load current.
Protection concerns
| Concern | Why |
|---|---|
| Full fault current flows through the series winding | Winding must withstand it or be protected upstream |
| Open circuit of the secondary side of a series transformer can be hazardous | Follow the maker’s rules on wiring and protection |
| Inrush | Magnetising surge when energised |
Comparison with other methods
- Transformer tap change: simple, offline, for fixed offsets.
- Stabiliser: automatic and adjustable for varying supply.
- Utility-side regulation: best when the supply fault is on the network.
Verification
After installing, measure voltage at the panel at light and heavy load and at high and low supply. Confirm the output stays within the equipment’s tolerance in all cases.
The master engineering question
Is the correction fixed or variable, and how much of the load voltage must be added or removed?
What to send us
Send voltage and load data. Send your application details.
Questions engineers ask
Is it a stabiliser?
It is the correcting element; a stabiliser adds sensing and control.
Does it isolate?
No. Use an isolation transformer for that.
Why smaller than load kVA?
It only carries the correction voltage.
What about short circuits?
Fault current flows through the series winding; protect accordingly.
Values are illustrative. Design needs a calculation for the actual site.