MV/LV transformers with on-load tap changer (OLTC) – a solution to voltage problems in modern LV networks

The rapid development of modern power engineering, the emergence of new energy sources, the dynamic growth of the number of prosumers and the transition to a distributed energy model constitute significant challenges for the power and distribution grid. Key problems include, among others, significant voltage fluctuations occurring in medium and low voltage networks in the daily cycle. In order to reduce them, network operators are increasingly reaching for modern possibilities – such as MV/LV distribution transformers with solutions that allow for compensating voltage changes in the network and maintaining its value within the ranges permitted by applicable standards.
In the face of the ever-increasing demand for energy and the need to ensure stable energy supplies and maintain their appropriate quality, as well as changes in the production and distribution model, energy systems must evolve to meet the growing requirements. An example of such a challenge is the growing number of photovoltaic installations and other renewable energy sources, the operation of which can contribute to network disruptions. A low-voltage network saturated with RES installations is exposed to voltage fluctuations above and below the values allowed by standards. Significant voltage differences in different points of a single network require the use of solutions that will allow maintaining the voltage value within the desired range. One such solution is MV/LV distribution transformers with an on-load tap changer (OLTC).
MV/LV transformer – role and application
The construction of LV networks requires the use of transformers, i.e. devices that will effectively reduce voltage. Transformers are the central element of transformer stations, they can also be used in distribution stations, as well as for larger recipients – e.g. in enterprises. In the past, solutions that did not have the function of changing voltage under load were commonly used. Voltage fluctuations that appear more and more often in modern networks require the use of more advanced solutions, such as a transformer with an on-load tap changer OLTC.
Construction and operation of OLTC transformers
The basis for developing transformers equipped with a OLTC were classic distribution transformers. However, the construction of a power transformer equipped with an on-load tap changer required design changes that take into account the presence of the tap changer under the transformer cover. Therefore, compared to traditional solutions, modern OLTC transformers are usually slightly higher. However, a significant advantage of the solution used today is that it has been possible to maintain its relatively compact dimensions.
The method of voltage regulation in a transformer equipped with an on-load tap changer is similar to that in a transformer with a voltage-free tap changer. The main difference is that the voltage regulation process does not require interrupting the current flow, i.e. disconnecting the transformer from the network, thanks to the use of voltage switches on the load. Thus, regulation can take place at any time and in a short time, and the range of voltage regulation is greater than in the case of an off-load tap changer, which makes maintaining permissible voltage values in the power network less of a challenge. Thanks to the possibility of using voltage measuring devices in the substation, as well as deep in the network and using an appropriate algorithm, automatic voltage regulation is possible in the event of detecting irregularities.

Application of OLTC transformers
When describing the construction of OLTC transformers, their compactness was emphasized. The size of these devices is important because they are used in transformer stations with compact dimensions, where there is no space to place large devices. The use of OLTC transformers is becoming more and more common and occurs, among others:
- in network operators with a large number of RES installations, i.e. distributed generation sources
- in wide area networks, where there are significant voltage drops between the power source and the end user
- in networks, where there are significant differences in power consumption and the range between the minimum and maximum power drawn over a long period of time
OLTC transformers – summary
The use of transformers equipped with on-load tap changers is a solution that can bring a number of benefits and solve voltage problems in modern LV networks. Considering the dynamics of the development of distributed energy and the changes taking place in the energy system, the wider use of OLTC transformers can be a relatively simple and economical part of the network modernization process. In Poland, the first solutions based on on-load tap changers have been operating for several years and the experience gathered on their basis can be used to develop and disseminate this solution in the national energy system.