The off-circuit tap changer: ±3×2.5 % explained, and when you need an on-load tap changer
Every distribution transformer has a tap changer on the HV winding to correct the network voltage. How the taps work, how to choose the position, and the cases where only an on-load tap changer will do.
The medium-voltage network is rarely at exactly its nominal value. Near the primary substation it may be 5 % high; at the end of a long rural feeder 5 % low. The tap changer on the HV winding lets the transformer compensate, so that the LV side delivers 400 V ± a few percent regardless of where in the network it stands.
How the off-circuit tap changer works
The HV winding has extra turns with taps brought out to a rotary switch on the lid (oil-immersed) or to bolted links on the coils (cast resin). Selecting a tap changes the number of active HV turns and therefore the ratio. PNACHO transformers are supplied with ±3 × 2.5 % (7 positions) as standard; ±2 × 2.5 % (5 positions) or ±5 % are available on request.
| Position (7-step changer) | HV voltage for 21 kV winding | Use when the network voltage is… |
|---|---|---|
| 1 (+7.5 %) | 22.58 kV | consistently high |
| 2 (+5 %) | 22.05 kV | high |
| 3 (+2.5 %) | 21.53 kV | slightly high |
| 4 (0 %) | 21.00 kV | nominal – factory setting |
| 5 (−2.5 %) | 20.48 kV | slightly low |
| 6 (−5 %) | 19.95 kV | low |
| 7 (−7.5 %) | 19.43 kV | consistently low |
The logic feels backwards to many technicians: to raise the LV voltage you select a lower HV tap, because fewer HV turns give a higher LV/HV ratio.
Three rules
- Only operate de-energised. "Off-circuit" means exactly that: the switch has no arc-quenching capability. Isolate and earth the transformer, move the switch through its detent, lock it, and record the position.
- Same position on paralleled units – otherwise a circulating current flows.
- Measure before you change. A voltage logger on the LV side for a week tells you the real range; set the tap for the average, not the extreme.
When an off-circuit tap changer is not enough
If the network voltage swings by more than ±5 % during the day, no fixed tap will do. Typical cases:
- PV plants on weak networks: the voltage rises when the plant feeds in at noon and falls at night. The grid operator may require an on-load tap changer (OLTC) or a reactive-power control that the inverters provide.
- Long rural feeders with large daily load variation.
- Industrial plants with voltage-sensitive processes.
An OLTC switches under load – typically 9 to 17 positions, ±10 % – and is controlled by a voltage regulator relay. It adds cost and a mechanical component that needs periodic inspection, so it is specified only where needed. PNACHO oil-immersed units can be built with an OLTC on request.
Grid-operator connection conditions in many EU countries define the permitted tapping range and the factory position. Send us the connection agreement and we set the tap changer accordingly before dispatch – the position is recorded on the test report.
Tapping range and position options are listed in the "Options" section of every PNACHO-O and PNACHO-D product page.
Further reading: What does medium voltage transformer ratio mean? (energeks.com, EN) · Co oznacza przekładnia transformatora średniego napięcia? (energeks.pl, PL)
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