Bulgarian’s electricity transmission system operator (ESO EAD) has published its ten-year network development plan (TYNDP), subject to final approval by the Bulgarian Energy and Water Regulatory Commission (EWRC), which has made the 400kV network the backbone of the electricity transmission network in Bulgaria, whose geographical location suggests there will be great commercial interest in using its transmission network for electricity transit.
Since July 2023, the new interconnection power line Maritsa East in Bulgaria to Nea Santa in Greece has been in operation.
Investment interest in connecting new renewables to the network in southeastern and northeastern Bulgaria, however, will require significant development of the electricity transmission grid. These planned developments include a new 400kV ring: Maritsa East to Uzundzhovo (the existing substation to be upgraded to 400/110kV) to Lyubimets 2 (new) to Asenovo (new). In northeastern Bulgaria, a new 400kV ring will be constructed: Varna to Dobrich 2 (which will replace the old Belgun/Senokos double-circuit substation) to the General Toshevo 2 substation (new) to the Svoboda substation (which will replace the Druzhba and Saedinenie inter-system substations).
According to the TYNDP, the 220kV transmission network should be reduced in favour of 400kV and 110kV networks. Part of the planned changes in the transmission network are related to the possible cessation of electricity production from coal-fired power plants connected to the 220kV network. According to the plan, part of the routes for the existing 220kV power lines will be used to construct the new 400kV power lines.
The importance of the 110kV network is predominantly local and its development will be determined by the following:
- improving the security of power supply to consumers;
- improving the exchange of electricity with the distribution networks;
- connecting direct consumers with the necessary security category; and
- connecting generating facilities – direct and in the distribution networks.
To control voltages in the maximum mode, capacitor banks were used in the Burgas and Varna North substations. In the minimum mode, shunt reactors were used in the Varna, Sofia West, Chervena Mogila, Blagoevgrad and Tsarevets substations. The calculations show that in the absence of wind in northeastern Bulgaria, which is the furthest from any generating sources, critically low voltages occur. As a result, several options have been considered, with the conclusion being that at least two substations should be equipped with compensation devices with ± 200MVAr capabilities. One compensation device should be located in one of the new 400/110kV substations in the Dobrich region, which will be used to connect to new wind power plants. The other device must be installed in the Obraztsov Chiflik substation.
According to the ESO EAD, the declared investment intentions for the construction of new renewables is three times greater than Bulgaria’s existing conventional generating capacities.
In response to the threat of a transformer power shortage in a 110kV/MV substation due to the connection of a new renewable generation, the ESO EAD prescribes replacing existing transformers with new ones with higher rated power and primary and secondary switching facilities, according to the rated power of the new transformers. There are substations where the investment planned for connecting new renewables exceed transformer capacity, including the transformers slated for replacement. Specific technical solutions are determined for these sites, such as reconstruction, expansion, or construction of a new substation in the relevant area.
The ESO has technical solutions for the development of the electricity transmission network, after the connection of the 2400 MW of nuclear capacity at Kozloduy NPP and the 2000 MW of nuclear capacity at the site of Belene NPP.
There is a possibility of obtaining "synthetic inertia" from battery energy storage systems (BESS), but its contribution to the inertia of the system is limited. BESS are limited facilities for accumulating electricity. Maintaining the operability of the units in large thermal power plants and nuclear power plants is essential for ensuring the country’s inertia reserve.
By Kostadin Sirleshtov, Managing Partner, and Borislava Piperkova, Partner, CMS Bulgaria
