Analysis of the dynamics of the development of alternative energy plants in terms of their connection to the power transmission networks

In this article, the author presents an original approach to analyzing the dynamics of power plants’ development with renewable energy sources. This approach is based on the characteristics of their connection to (high-voltage or distribution) power transmission networks during both the pre-war and post-war periods. The article describes the impact of military aggression on the current status of power generation from alternative energy sources. The article not only suggests the main directions of post-war reconstruction but also defines approaches to developing regulatory mechanisms to promote the growth of renewable energy at the regional power system level of the United Energy System of Ukraine.


Introduction
Economy, energy, and ecology are areas that play a significant role in society.Energy is of great importance because it influences both the development of the economy and ecology.It determines, to a great extent, the economic potential of the state, the well-being of its population, and also has the most significant impact on the environment (all environmental problems directly or indirectly related to the production or use of energy).
In recent years, both in Ukraine and worldwide, there has been steady development of small or low-capacity power plants using renewable energy sources (RES), which gradually replace traditional methods of generation.According to data from the State Agency for Energy Efficiency and Energy Saving of Ukraine [1], the installed capacity of power generators from alternative energy sources increased by 9.5 from 2014 to 2021, as shown in figure 1.
However, after Russia's full-scale invasion, Ukraine's energy industry has faced difficult times.Some of the generation facilities and energy infrastructures have been damaged, destroyed, or are located in the occupied territories, including renewable energy generation facilities.After the war, the energy industry needs to be restored and significantly expanded.Therefore, it is crucial to find approaches and methods to support and develop RES producers in the war and post-war period.
There has been a hectic discussion in various professional circles about decentralizing the power system [2] in the context of post-war reconstruction reflections.The concept of decentralization includes measures to ensure reliable power supply to critical infrastructure objects, houses, etc. in case of damage to important objects of the power system.Distributed power generation is defined as an energy production and transmission system that involves a large number of consumers who, at the same time, are producers of electricity and heat for their own needs.They have the ability to transfer excess energy produced to the general power grid.According to the Law of Ukraine "On Electricity Market"distributed generation is defined as "a power plant with an installed capacity of 20 MW or less, connected to the electrical energy distribution system" [3].
To conduct research on the development of distributed generation in Ukraine and introduce new approaches and methods of supporting renewable energy in the war and post-war periods, it is essential to have data on the current state of renewable generation, as well as information on the capacities of RES facilities that are connected to the distribution network.
There are a number of scientific publications and information resources that analysed the dynamics of the development of alternative energy in Ukraine and the world.For example, the energy development strategy of Ukraine is reviewed in [4], the analysis of prospects for the development of renewable energy in Ukraine, the influence of military actions on the state of the industry, ways of development of RES in the post-war period, etc. are reflected in studies [5][6][7][8][9][10][11].Also the analysis of using of renewable energy sources on the development of energy systems, using of "smart technologies"for renewable energy are shown in research [12][13][14][15][16][17][18].However, none of them studies the state of alternative energy in terms of data on the connection of RES facilities to power transmission networks.
At the same time, in the research of Lukashevych and Evdokimov [19] describes the information system for monitoring the state and dynamics of development of RES power plants, its characteristics and structure.Besides, the described information system contains already formed and implemented in Excel data fields about RES objects of Ukraine, which should be used for the analysis of previous experience in development of power generation, its current state and formation of proposals for directions of post-war reconstruction.
The aim of the research is to analyze the dynamics of the development of alternative energy facilities in terms of their connection to power transmission networks.
The tasks of the study are: • to determine the distribution structure of RES in terms of connection to the power grid in the pre-war period; • to describe the impact of military aggression on the state of alternative power generation; • to identify the approaches and methods of support and development of RES producers in the wartime and postwar periods.

Analysis of alternative generation in terms of connection to power transmission networks by 2022
The research [19] contains information about main RES facilities (name, address, type of source, amount of installed capacity of the facility), including data on the type of power transmission network to which the stations are connected -high-voltage or distribution.The data on the connection of individual renewable energy generation plants to the distribution or highvoltage comes from the analysis of all PPA contracts (Power Purchasing Agreements -long-term contracts for the purchase of electricity from renewable energy sources) signed by alternative energy producers with the state-owned company Guaranteed Buyer (SE "Guaranteed Buyer") from 2019 to 2022.However, the system does not yet include information on alternative generation facilities of households that have signed power purchase and sale agreements with universal service providers (USPs).
Based on the available information, we can perform an analysis of the dynamics of development of alternative energy facilities in terms of their connection to the high-voltage or distribution power transmission networks.The calculations were performed for operating plants (for power generation) that have a "green" tariff, without taking into account households that sell electricity to the USP.
Figure 2 shows the dynamics of the development of renewable energy producers for the period from 2019 to July 2022.to 6168 MW in 2022.Thus, there is a trend towards the predominant connection of alternative generation plants to distribution grids, likely due to the low installed capacity of RES producers (with 98% of the total number of RES plants having an installed capacity of less than 50 MW) and the relative simplicity and cost-effectiveness of connecting to the distribution grid compared to the high-voltage grid.
Additionally, figure 2 shows that alternative generation peaked in 2021.
The distribution of installed capacities of operating alternative generation stations in 2021 is presented in figure 3.In 2021, 77% of total power generation came from solar power plants (SPP), 19% from wind power plants (WPP), and 4% from other power plants (small or micro hydropower plants; power plants using energy from biomass, gas from organic waste, sewage gas: biogas, etc.).1068 MW or 13% of all alternative generation capacities were connected to the high-voltage grids, including 408 MW of wind power plants and 660 MW solar power plants.RES plants connected to the high-voltage networks were located in the Dnipro, Lviv, Mykolaiv, Kherson, and Khmelnytskyi regions, as shown in figure 4.
5532 MW of solar power plants, 1123 MW of wind power plants and 350 MW of plants generating electricity from other sources were connected to the distribution grid.These plants are distributed throughout the territory of Ukraine, taking into account the geographical and climatic conditions of the regions, as shown in 5. Thus, the development of solar energy can be observed practically on the entire territory of Ukraine, with the predominance of central and southern regions, wind power generation mainly in the south, small or low-power hydroelectric plants along riverbeds in the northwestern, central and eastern regions, and power plants using energy from biomass or biogas throughout the country.Separately, it is worth noting the relatively low development of RES in the northern and eastern regions of Ukraine.
In general, in 2021, the share of electricity generated from RES in Ukraine's energy balance was 12.8 TWh or 8%, of which 56% was generated by solar radiation, 33% by wind energy, almost 8% by biomass and biogas combustion, and 3% by small hydropower [20].

Analysis of the impact of military operations on alternative sources producers
In 2022, as a result of the military invasion, some of the alternative generation facilities were destroyed, damaged, or landed in occupied territories.Thus, the vast majority of renewable energy facilities currently installed in the country are located in the southern and southeastern regions of Ukraine, where active combat operations have being taken place in the last six months.According to various expert estimates, as of August 2022, 30-40% of RES in these regions has already been affected in one way or another [20].
For example, according to the Ukrainian Wind Energy Association, more than three-quarters of Ukraine's wind energy capacity has been shut down since the war began, and five wind turbines in the Kherson region installed at the wind farms in Myrne, Siva, and Novotroitsky have been destroyed now.Due to the damage of 330 kW of power transmission lines in Melitopol, almost 600 MW of wind power capacities in Zaporizhzhya region are also out of service.More than 1 MW of bioenergy capacity in Chernihiv was damaged, while bioenergy plants with total capacity of 2 MW are located in occupied areas of Donetsk region, namely in Volnovakha and Mariupol.Fortunately, the small hydroelectric power plants, located mainly in the western regions of Ukraine, were not affected by the hostilities and continue to supply electricity [20].
As shown in figure 6, as of July 2022, 6828 MW of operating stations were connected to the United Energy System of Ukraine, of which 88% were SPPs, 7% were WPPs, and 5% were energy stations generating electricity from other sources.660 MW of solar power plants in Dnipro, Lviv, Mykolaiv, and Khmelnytsky regions remained connected to the high-voltage network, which makes up 10% of all capacities of RES producers.The structure of stations connected to distribution networks is as follows: 5358 MW -SPPs, 476 MW -WPPs, and 334 MW -others.
Overall, wind and solar power generation has declined by more than half from pre-war levels.The following factors may serve as explanations: first, the destruction of power generation as a result of direct hostilities; second, the increasing problems related to the destruction of demand and the high inflexibility of power generation, especially solar power generation, considering that the cumulative capacity of solar power generation has currently exceeded 6 GW; and third, problems related to damage to energy infrastructure -networks, substations, etc. -will increase.
At the same time, the financial crisis in the RES market is deepening and the percentage of payment under the "green"tariff is decreasing.According to SE "Guaranteed Buyer" -as of December 26th, 2022 -the overall level of settlements with RES producers for the supplied electricity in 2021 was 99%, while in 2022 this indicator is 53.5% [21].

The directions of the post-war reconstruction of the RES producers
Renewable energy is one of the components of Ukraine's energy independence.Their potential can be used to create new jobs, boost the economic activity of regions, and renew outdated main production facilities in the power industry.Currently, the only mechanism to promote the development of RES in Ukraine is the "green"tariff -a special tariff for the purchase of electricity from RES.The implementation of this mechanism is carried out by SE "Guaranteed Buyer", which is obliged to purchase all electric energy from RES generators at a special tariff on the basis of the Law on the Electric Energy Market [3] adopted in 2017.Since March 2022, a number of legislative changes were introduced, which significantly changed the "green"tariff (in particular, the peg to the euro was removed), the conditions and the mechanism of payments were changed as well.As mentioned above, the state's debt to producers under the "green"tariff increased significantly due to the military actions.Therefore, the question arises whether the state will be able to support such a mechanism in the future.Therefore, it is extremely important and urgent to find new mechanisms and ways to develop RES producers.
According to the Ten-Year Plan for Reconstruction of Ukraine [22], published by the National Council for Reconstruction of Ukraine after the consequences of war and presented for the first time at the URC 2022 (Ukraine Recovery Conference) on July 4-5, 2022 in Lugano, Switzerland, the main directions of development of the Ukrainian energy industry are defined as follows: • Integration with the energy systems of EU countries; • Decarbonization; • Optimization of the energy mix and balancing of the energy system; • Increase of energy efficiency.
In particular, the Plan presents the project of the National Program "Energy Independence and Green Course" [23], which includes points such as the construction of an additional 30 GW of RES and batteries, the construction of smart grids.
As the research shows, 90% of the capacities of producers from alternative sources are connected to distribution networks.Therefore, the development of renewable energy is one of the important points on the way to a decentralized energy system.Also, the government proposes to provide "mini power plants and small generation facilities that will be integrated into the existing power grid" [24].In particular, the development of distributed generation will provide energy independence and an increase in the security of energy supply to the communities of Ukraine.
Another step towards the development of distributed generation and the development of RES is the introduction of virtual power plants.A virtual power plant is a high-tech system that combines electricity from several producers and consumers at once.Producers in this case can be distributed generation facilities.Consumers are electricity consumers who have refrigeration units, air conditioners, drainage pumps, crushing units, etc.It should be noted that such objects of consumers and producers must be flexible in the production or consumption of electricity.The virtual power plant does not intervene directly in the production process.It regulates facilities that provide support to the power system when the balance between production and consumption is disturbed.
In addition, the concept of creating a cellular energy network (CEN) is being considered for territorial associations [25].CEN is a set of energy generation and storage centers connected to each other and to consumption centers in a given area.
Considering the experience of the development of RES, i.e., the spread of such facilities throughout the territory of Ukraine, it is reasonable to take into account the available data when studying the above approaches to the development of the national power industry.At the same time, the implementation of these ideas can be one of the methods of support and development of RES producers.Therefore, the approaches of decentralization of the power supply system or creation of a power supply system are promising for solving some consequences of military aggression on the territory of Ukraine and require further research.

Conclusions
The analysis of the dynamics of renewable energy development in Ukraine in recent years was carried out, taking into account the specifics of connecting plants to high-voltage or distribution networks.Also, the research shows the distribution of the capacities of RES facilities in the regions of Ukraine.
As a result of Russia's armed aggression, the power industry of Ukraine in general, and more than 1000 MW of RES capacity in particular, need restoration.At the same time, the existing methods of supporting RES producers are insufficient.Renewable energy needs new mechanisms and methods of development of the industry.
One direction for the evolution of the domestic power industry is the development of a decentralized energy system to provide energy independence and security for the citizens of Ukraine.The results of research show that about 90% of generation capacities are connected to distribution networks and only 10% to high-voltage networks.Accordingly, renewable energy is an important part of the distribution network.
Therefore, the issues of supporting alternative generation and the development of distributed generation are interrelated, mutually complementary, relevant, and require further research.

Figure 2 .
Figure 2. Distribution of installed capacities of operating alternative generation stations in terms of connection to power transmission networks for 2019-2022, MW.

Figure 3 .
Figure 3. Distribution of installed capacities of operating alternative generation stations by sources of alternative energy and connection to power transmission networks in 2021, MW.

Figure 4 .
Figure 4. Distribution of installed capacities of operating stations of alternative generation connected to high-voltage networks in 2021.

Figure 5 .
Figure 5. Distribution of installed capacities of operating alternative generation stations connected to distribution networks in 2021.

Figure 6 .
Figure 6.Distribution of installed capacities of operating alternative generation stations by sources of alternative energy and connection to power transmission networks in July 2022, MW.