Studying possible technical solutions to prevent the long-term existence of ferroresonance phenomena in 10 kV power grids
Students Name: Nadtochyi Oleksandr
Qualification Level: magister
Speciality: Electrical Energetics, Electrical Engineering and Electromechanics
Institute: Institute of Power Engineering and Control Systems
Mode of Study: part
Academic Year: 2025-2026 н.р.
Language of Defence: ukrainian
Abstract: The master’s thesis is devoted to a comprehensive study of the conditions for the emergence, development, and prevention of the long-term existence of ferroresonance processes (FRP) in 10 kV distribution networks with an isolated neutral. Ferroresonance is a complex nonlinear electromagnetic phenomenon caused by the interaction between the magnetic inductance of a voltage transformer and the capacitances of electrical networks. It is characterized by the appearance of subharmonic and quasi-periodic oscillations, overvoltages, increased magnetizing current, and thermal overloads [1]. In real 6–10 kV networks, such processes have a probabilistic nature and depend on the network configuration, the capacitive earth-fault current, and nonlinear properties of transformer magnetic cores, which makes them particularly dangerous and difficult to predict [2], [3]. The practical importance of studying FRP is determined by the fact that the long-term existence of subharmonic oscillations may lead to voltage transformer damage, malfunction of relay protection, false signaling, neutral shift, unintended circuit-breaker operations, insulation breakdown, and ultimately to the interruption of power supply to critical consumers [1], [4]. For 10 kV networks, the most hazardous conditions occur when the capacitive earth-fault current Is lies within 0.25…4 A, for which both literature and experimental data confirm a high probability of ferroresonance initiation [5]. Object of research - is ferroresonance processes in 10 kV electrical networks with an isolated neutral. Subject of research - the conditions for the development of FRP and technical solutions for their reliable suppression in networks equipped with voltage transformers of the NTMI-10 and NAMI-10 types. Goal of research - increase the electromagnetic stability and reliability of 10 kV networks by developing a universal method for suppressing FRP that is effective across the entire range of possible operating conditions. To achieve this aim, a detailed analytical review of the physical mechanisms of ferroresonance development was performed, the characteristics of different voltage transformer types and their role in forming a ferroresonant circuit were summarized [1], [6]. Structural and mathematical models of network elements were developed, including equivalent circuits of voltage transformers, magnetic cores, transmission lines, and busbars. Modeling was carried out in the “RE” software environment, which made it possible to determine the FRP existence zones for typical substation configurations. A numerical analysis of the influence of network parameters, capacitive current magnitude, transformer design features, and load type on the oscillation behavior was performed. The study examined five technical approaches to suppressing FRP: connection of single-phase and three-phase capacitive elements, connection of inductive elements, combined schemes, and a method based on connecting a damping element to the neutral of the voltage transformer. It was found that methods based on capacitive or inductive components are effective only within limited ranges of Is and do not provide universal applicability, which is consistent with modern research findings in this field [3]. The scientific novelty of the work consists in proposing a new universal method for damping ferroresonant oscillations by connecting an active resistor to the neutral of the high-voltage winding of the voltage transformer. This approach provides stable suppression of FRP in all verified modes, eliminates subharmonic and quasi-periodic oscillations, and ensures normal VT operation in the absence of network disturbances. Numerical simulation confirmed that the optimal resistance value is approximately R ? 5500 ?, which ensures complete suppression of ferroresonance for the entire range of Ic = 0.25…2.5 A. The practical value of the results lies in the possibility of applying the proposed resistive damper in existing substation schemes without significant reconstruction of primary or secondary circuits. The method can be implemented in distribution networks to improve power-supply reliability, extend the operational lifetime of voltage transformers, and prevent emergency shutdowns [1], [5]. The obtained results may also be used in the modernization of relay protection schemes, the design of new voltage transformers with built-in damping elements, and further research on nonlinear electromagnetic dynamics.