The distribution transformer (ONT, short for the German “Ortsnetztransformator”) is the transformer inside a local distribution substation and thus the heart of the final transformation level before the end consumer. It steps down medium voltage (usually 10 to 20 kV) to a low voltage of 400 V, supplying residential areas, commercial parks, and rural regions directly with electricity. As decentralized generation and new loads such as heat pumps or charging infrastructure continue to expand, a regulated variant – the regulated distribution transformer (rONT) – is rapidly gaining importance, as classic ONTs reach their limits.
How is a distribution transformer designed and where is it used?
A typical ONT falls within a power range of 250 to 1,000 kVA and is built with either oil or cast-resin insulation. It is usually permanently installed in a local distribution substation – free-standing in concrete or sheet steel, as a compact pole- or tower-mounted station, or underground in densely built-up areas – together with the associated medium-voltage switchgear and the low-voltage distribution to the surrounding buildings. Classic ONTs only have an off-circuit tap changer (DETC) for coarse adjustment to the respective grid level.
What is a regulated distribution transformer (rONT)?
A regulated distribution transformer (rONT) is an ONT equipped with an on-load tap changer (OLTC) that automatically adjusts the secondary voltage during operation to match the current grid situation. This is particularly valuable in grid areas with high photovoltaic feed-in: around midday, feed-in from many rooftop systems can cause a sharp rise in low-voltage grid voltage, while in the evening high loads – for example from EV charging points or heat pumps – can lower the voltage again. An rONT actively compensates for these fluctuations, keeps the voltage within permissible tolerances, and thereby makes it possible to integrate significantly more decentralized generation and new loads into an existing grid – often without the need for expensive and time-consuming line reinforcement.
Key Characteristics at a Glance
- Power range typically 250 to 1,000 kVA
- Oil or cast-resin design depending on location and requirements
- Classic ONT: tap changer switchable only in a de-energized state (DETC)
- rONT: on-load tap changer (OLTC) for automatic voltage regulation during operation
- rONT increases the grid’s capacity to accommodate PV feed-in and e-mobility
- Increasingly equipped with sensors and remote control technology for smart grid applications
- Service life typically 30 to 40 years
Frequently Asked Questions
How does an rONT differ from a classic ONT?
The decisive difference lies in the tap changer: a classic ONT can only be set to a fixed tap in a de-energized state. An rONT, by contrast, adjusts the voltage continuously and automatically using an on-load tap changer – comparable to the OLTC systems of large power transformers, just designed for the low-voltage level.
When is using an rONT worthwhile compared with grid reinforcement?
Whenever voltage problems caused by high PV feed-in or new loads such as wallboxes are the limiting factor, but not the thermal capacity of the lines. An rONT can usually be implemented significantly faster and more cost-effectively than classic line or substation reinforcement, and can often delay or completely avoid the need for it.
Is an rONT significantly more expensive to purchase than a classic ONT?
Yes, the additional control technology and sensors noticeably increase the investment cost compared with a classic ONT. Compared with grid reinforcement involving new lines or additional substations, however, the investment is in many cases considerably lower – especially if the measure can be avoided altogether as a result.
What special requirements does winter operation place on a distribution transformer?
At low temperatures, the oil viscosity of oil-cooled ONTs increases, which can slow down cooling. Heated control cabinets, frost-resistant materials, and careful ventilation are standard in wintry regions. Modern ONTs are designed for temperatures down to -40 °C and operate reliably even under extreme conditions.
Are you modernizing distribution substations or planning new builds with intelligent components? We supply suitable distribution transformers – including regulated rONTs – and provide comprehensive advice. Contact our team.