On May 1 and 2 this year, for the first time ever, five Slovenian power distribution companies fed more electricity into the transmission network than they drew from it. This shift is creating challenges for distribution grid operators while also highlighting the core reality of the energy transition.

Indeed, the circumstances were extraordinary during the first two days of May, writes Slovenia’s Naš stik. The weather was clear, but not warm enough for people to use air conditioners, while solar power plants connected to the distribution grid were generating electricity at nearly full capacity.

Industrial plants were not working due to the holiday, and many people were away from home, so electricity consumption from the transmission network fell sharply, reaching only 150 MW between 12 and 1 p.m. In winter, peak hourly consumption reaches around 2,200 MW.

Pumped storage hydropower plant Avče was operating at full capacity

Pumped storage hydropower plant Avče was operating at full capacity, receiving 157 MW from the transmission network. All Slovenian power distribution companies supplied more electricity to the transmission network than they received from it.

Elektro Ljubljana and Elektro Primorska drew energy from the transmission grid, but Elektro Maribor, Elektro Celje, and Elektro Gorenjska delivered significant amounts, resulting in a negative overall balance.

For four hours, the distribution network was a net electricity producer

The distribution network as a whole was a net producer of electricity for four hours on May 1, between 11 a.m. and 3 p.m. On the same day last year, the minimum hourly load of the transmission network was 770 MW, and power distribution companies drew 450-500 MW from the grid. The same thing happened on May 2.

It is worth noting that the total capacity of solar power plants in Slovenia is 1.4 GW, half of which was installed in 2023 and 2024, with the largest part connected to the distribution grid. The overall electricity generation capacity in Slovenia is 7.47 GW.

Managing voltage profiles on the grid poses the greatest challenge

The most pronounced change in energy flows was recorded at Elektro Celje. Boštjan Turinek, director of operations and development, said that a reverse flow of energy from the distribution network to the transmission network was first recorded in July 2022. At that time, the amount was minimal, around 4 MW.

However, in 2023, the reverse flow reached 40 MW, and this year it has already hit 100 MW. The biggest challenge, he explains, is managing voltage profiles on the grid.

The distribution network was built for one-way “traffic” – toward the end user – with the highest voltage at substations and the lowest at end consumers. The mass integration of distributed energy sources has disrupted these voltage profiles, Turinek stressed.

Distributed power plants like emergency vehicles

Ordinary consumers usually don’t notice this – perhaps only a slightly shortened lifespan of their LED bulbs, he said. However, according to him, solar power plant owners often experience automatic inverter shutdowns caused by overvoltage.

Besides holidays, changes in energy flows also occur during collective shutdowns of Slovenian industrial plants during the summer and spring breaks. If the weather is sunny at that time, the output from distributed energy sources is very high.

Turinek recalls that distributed power plants have been granted the same rights as emergency vehicles, so their output always takes priority, regardless of the state of the grid.

If the power system is to be properly managed, he adds, these plants should be placed on an equal footing with all others.

The net metering system for prosumers has had many positive effects, but it has also created numerous problems, Turinek concluded.