The rapid growth of renewable energy capacity, in particular photovoltaic systems, is creating challenges associated with changing the rate of transient processes in the power system. This is due to the approach PV systems are connected to the grid using power converters and the absence of a rotating mass in the PV power plant. One of the most pressing challenge is the participation of PV stations in the process of frequency control in power systems, including in emergency modes. Simultaneously with PV power plants, it is efficient to use energy storage systems, including hydrogen ones. This is due to the fact that it is possible to obtain hydrogen for such energy storage systems using excess energy from PV power plants. The article proposes to solve the problem of frequency regulation in the power system by using an algorithm that allows to control the frequency in the power system using a synthetic inertia block of PV station, including at different levels of insolation and temperature of PV panels. The robustness of the proposed algorithm allows it to be used at different levels of power generated by the PV station, as well as in emergency modes.

Energy resources based on green hydrogen are nowadays beginning to enjoy wide adoption and draw considerable interest. There are different types of hydrogen production, but it is green hydrogen that is becoming an attractive energy development capable of replacing blue and grey hydrogen as well as fossil fuels, Thereby the carbon footprint will be reduced for a number of industrial processes. Proton exchange membrane electrolyzers used for the production of environmentally friendly hydrogen have one key shortcoming, which is the short service life of the equipment. This paper presents a comparative study of the commonly applied PLL control of AC-DC converters. To improve the control of the proton exchange membrane of the electrolyzer, a vector control arrangement is adopted, which allows, by relying on cascade control loops, maintaining different DC link voltage levels and thereby extending the DC link input voltage variation range.

Fossil fuel completely dominates the world energy supply, providing about 80% of the total world energy consumption. The most common types of fossil fuels burned at the thermal power plants are natural gas and coal. In the production of all thermal and electrical energy in the world, coal energy accounts for 36%. In China and India, coal-based generation is over 70%. In the Russian Federation, the share of "coal" generation is about 13%, while more than 40% of electricity in Russia is generated by energy sources without greenhouse gas emissions into the atmosphere (NPPs, HPPs, RES). The dominant source of energy in the Russian Federation is gas making about 46%.
Compared to natural gas, coal is the least environmentally friendly fuel, because when it is burned, not only nitrogen oxides and greenhouse gases are emitted into the atmosphere with flue gases, as when natural gas is burned, in a much larger amount, but also sulfur oxides and ash particles are emitted. In addition, the coal ash, formed at the coal-fired thermal power plants and collected in ash collectors, is mainly stored at the ash and slag disposal sites, polluting the air due to dusting of particles, getting into the aquifer due to filtration of toxic and radioactive solutions, and also leading to soil degradation in the area of ash and slag disposal sites location. Improving the environmental performance of the coal-fired thermal power plants is possible through the use of the best available environmental technologies to reduce emissions of harmful substances, including both regime measures carried out during fuel combustion and the use of modern flue gas cleaning installations, as well as through the introduction of the best available technologies in the field of the coal ash handling in order to maximize the level of the coal ash use in various sectors of the economy.
In 2019, about 1.1 billion tons of the coal combustion by-products (CCPs) were formed in the energy sector of the countries of the world, the main of which are the ash and slag named the coal ash. The coal ash utilization in many developed and developing countries exceeds 50%, and in the EU countries, more CCPs are utilized than generated at the solid fuel thermal power plants. These results are facilitated by the introduction of the best available environmental technologies, stringent requirements of national environmental legislation and stimulation of the CCP use in the circular economy, in which waste from one production becomes a raw material for another. Each country implements its own environmental policy based on the national environmental legislation, develops and amends national standards for the coal ash use in various applications, depending on their properties and characteristics. In total, there are more than 300 areas of the coal ash use in the world.
The main large-scale coal ash applications are:
• manufacture of the construction materials and the coal ash application in construction projects;
• road construction;
• backfilling, including filling the mines and restoration of quarries, etc.
The article considers several examples describing implementation of various projects using the coal ash in different countries around the globe construction of buildings and erection of structures, artificial porous wood and ceramic tiles, road construction; agriculture; filling mines and reclamation of quarries; production of sorbents, etc. Proposals are given to increase the level of the coal ash utilization in Russia in order to reduce the disposed of coal ash, and to introduce low waste and waste-free technologies in the field of coal ash handling.

Integration of renewable energy sources (RES) together with energy storage systems (ESS) changes processes in electric power systems (EPS) significantly. Specifically, rate of change and the lowest values of operating conditions during the emergencies are got influenced. Such changes can cause incorrect actions of relay protection (RP) as it was designed and adjusted with no regard for influence of RES and ESS. Detailed research on processes during the different normal and abnormal modes in both EPS and primary transducers and also in RP devices should be done to take preventive actions. To do this research mathematical modelling based on detailed and authentic models of all elements including RP should be used. HRTSim (which was developed by authors) software for simulating EPS provides the opportunity to create such models of EPS of any size without simplifications and limits. Using of this instrument together with detailed mathematical models of RP which were developed before provided the opportunity to investigate them rigorously in RES-integrated EPS. Settings providing adequate action of RP in certain conditions were performed as a result of this investigation. Fragments of these investigations are performed in this paper. Results of these investigations would be useful for designing new methods and tools of RP adjustment.