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The purpose of this work is to develop a diagram version of the compensating device, and its control strategies using a phase-shifting transformer device. The purpose has been achieved performing a complex of research based on structural-simulation models both for the compensating device as a whole, and for the phase-shifting transformer. Based on the phase-shifting transformer model, calculation experiments in idle, short-circuit and under load conditions have been carried out. The obtained data allowed to determine the energy characteristics of the object of research and to perform analysis of its characteristics during the process of adjusting the angle of phase shift. The paper also provides the results of research in the regime's parameters issue of the device as a whole. The obtained data confirmed the functionality of the proposed reactive power source. The novelty of the work consists in the development of innovative circuit variant of controlled compensating device. Its component parts are susceptances, allowing carrying out a symmetrical adjustment of reactive power both consumed and offered to the network in a predetermined range. A control diagram for adjusting windings of a phase-shifting transformer has been proposed. The control's principle of power electronic keys provides control of the phase shift of the voltage at the output terminals of the device in the range from 0° to 60° at 15 switching positions. The proposed technical solution provides symmetrical smooth control of reactive power in a wide range at the point of connection of the device to the network.

The object of the study is a frequency converter based on IPC technology. The purpose of the study is to determine the device's performance characteristics by means of a mathematical apparatus describing the area of existence of operating modes for active and reactive power. On the basis of combinatorial laws, variants of non-repeating states of thyristor keys were obtained. The initial phases of active and reactive power components are determined by varying in accordance with the cosine law with two non-repeating combinations of key switching on each pair of conjugate reactive elements. The input of two constants made it possible to simplify the arguments influencing the magnitude and direction of the transmitted power by the device. A comparative analysis of the regime parameters and the characteristics obtained with the help of the simulation model and calculated by the proposed method confirms the identity of the results. The error of the method fluctuates between 0.5% and 1.5%. The developed method made it possible to determine a number of unique variants of the transmitted power level when adjusting the angle \(\delta_{sr}\) in the range from 0º \(\div\)360º . This allows to adjust the magnitude and direction of the transmitted power when "sliding" or slow and arbitrary frequency changes. Thus, the proposed method made it possible to determine with a high degree of accuracy the domain of existence of the regime parameters with the construction of switching diagrams of keys to maintain an ordered unidirectional power flow with frequency matching by the device under study.

Purpose of this work is in the development and modification of control scheme and of basic algorithms of discontinuous space-vector modulation for synchronous and balanced regulation of multi-inverter topology of six-phase open-end winding installation with four insulated dc sources. This purpose has been achieved due to including in the control and modulation scheme of specialized control correlations connecting coefficients of modulation of inverters, required power ratio between dclinks, and voltages of the corresponding dc sources. So, basic novelty of this research is in the fact, that the elaborated and investigated control and modulation algorithms for installation on the base of inverters with discontinuous space-vector modulation (with both 300 -non-switching intervals and 600 - non-switching intervals) insure both required power sharing capability between dc links and advanced harmonic composition (without even harmonics and subharmonics) of spectra of the phase and line voltages of six-phase installation. Mutual comparison of behavior of system with two basic schemes of modified discontinuous pulsewidth modulation (PWM) has been executed. Investigation of harmonic composition of the phase voltage of multi-inverter system illustrates an important fact, that for the all analyzed control modes, connected with balanced adjustment of drive under different conditions, phase voltage of installation has symmetry for any control regimes, including control modes with fractional frequency ratios between switching frequency of converters and fundamental frequency of installation, with absence in its spectra of undesirable subharmonics. Factor of minimization of subharmonics in spectra of voltage and current of ac drives of different topologies is especially important for the medium-voltage medium-power installations on the basis of PWM converters.

Sustainable economic development is also determined by the availability of primary energy supply. Geographical incoherence of energy production and consumption areas leads to new challenges in providing consumers with energy. The purpose of this work is to analyze and assess trends and problems related to the provision of energy resources, mainly of the natural gas for Republic of Moldova in the context of new challenges in the natural gas market and rhe changes in their transport infrastructure. The investigative methodology is based on the analysis of time series of production, consumption and import of energy resources at global and regional level in 1990-2017. It is proposed to using the linear approximation functions. The coefficients and the its signs of these functions ("plus" or "minus") and the value of and the determination coefficient are used for the quantitative analysis of the production, consumption and import trends of primary energy and natural gas. Europe and Asia ensure their energy balance by massive energy imports. The Southeast European region, including the Republic of Moldova, imports large sums of primary energy resources. The need to import natural gas in the Romania-Moldova-Ukraine sub-zone is currently estimated at about 12 million tons of oil equivalent. The new energy corridors for the transportation of natural gas will lead to new challenges regarding the natural gas supply of the Republic of Moldova. Information on possible scenarios of natural gas supply to the Republic of Molodova is presented and the estimated costs for natural gas transport for these scenarios are presented.

The paper presents deal with the heat pump designed mainly for heat supply systems using the qualitative law of regulation of the heat supply mode (constant flow rate of the heat carrier). For this purpose, an air-to-water heat pump with carbon dioxide as a refrigerant and compressor driven be gas piston unit is used. The aim of the work is to develop a scheme in which the position of the operating point of the heat pump compressor does not depend on fluctuations in the refrigerant flow rate, as well as using the heat produced by the gas engine - driven by the heat pump compressor. This goal achieves by elaboration of internal heat exchangers with an adjustable heat exchange surface area. The heat exchange surface area regulates by installing of an intermediate heat-conducting cylindrical element between the refrigerant and thermal agent circuits. The intermediate cylindrical element moved by using, for example, an electric stepper drive. The conditions, under which the sleeve can be considered as a thermally thin body. To increase the COP of the heat pump (HP), the additional heat exchanger, installed at the outlet of the ejector used in the pump has been used. It is. In the heat exchanger, the working fluid has been heated by using the waste heat of the gas piston unit (GPU. It had been shown that in the temperature control loop the PI controller may be used, and to compensate for the flow rate pulsations, it is necessary to use a combined control system.

The work deals with the development of a control system for heat exchangers with variable surface area of heat transfer, used, in particular, for heat pumps intended primarily for heat supply systems using the high-quality law of regulation of the heat supply mode (with a constant flow rate of the heat carrier). The aim of the work is to develop a scheme for the control of a heat exchanger with a variable surface area of heat exchange in which main disturbances will be compensated: i.e. the temperature of the heat carrier, heat carrier’s flowrates, and changes in the thermophysical parameters of the heat carriers. This goal has been achieved using a custom PID controller in the contour of control of the drive of the moving heat-insulating insert. To solve this problem, we obtained the equations of the statics of the heat exchanger with regard to the adjustable insert, the dynamic model of the heat exchanger taking into account this insert. The novelty of the work is control laws of an adjustable heat-conducting insert between the primary and secondary coolants of the heat exchanger, which allow compensating the main disturbances, which influence the heat exchanger. A method has been developed for solving differential-integral equations of heat exchanger statics with an insert (HEI), which allowed analytically expressing the ratio of average integral heat carrier temperatures in a heat exchanger depending on the heat exchanger length, which allows building a control system for a heat exchanger with a variable heat exchange surface area.

The scope of the work is to study the steady-state symmetrical load modes of operation of a new promising phase-regulating transformer circuit. This allows obtaining the value of the designed capacity of device. The device is based on a three-phase transformer windings connection into hexagon circuit, equipped with additional three phase regulating autotransformer, connected to hexagon circuit in a special manner. Besides, it should be equipped with semi-conductor switching keys, which give the possibility to control fast transition processes. The proposed circuit gives the possibility to round regulation of phase angle in the range of 0-360°, so the device could be used as frequency converter for flexible tie between energy systems with different frequencies and belongs to the group of FACTs controllers. The autotransformer provides the possibility of fine adjustment of the phase angle of the output three-phase voltage, so maintaining the flow of active power in the interconnection line. One of the most important features of such devices is the value of calculated designed capacity, which determines demand of the active materials for such devices. The scope of the work was achieved by conducted analytical study, based on electrical and transformer circuit’s theory. The currents in the windings of the device in symmetrical load modes were calculated for several variants of the phase-regulating unit circuit’s diagrams. On their basis, the maximum values of the devices currents and designed capacity for a number of the considered variants of circuits were determined, allowing a comparative analysis of different circuit modifications.

The paper aims to provide argumentation of centralized district heating system use from the consideration of harmlessness for both public health and environment, in comparison with the use of individual heating units, installed in residential areas of the Chisinau municipality. Proposed goal have been reached performing the comparison on quantitative harmful emissions resulting from the operation of these two types of heating, using emissions factors, deducted from the specific consumption of thermal energy. The issue's relevance is proved by the existence of considerable number of disconnected users from the centralized heating, though last years, this trend is decreasing. As regard to new buildings, they continue to give priority to the individual heating. Statistical data provided by specialized institutions of the municipality underlines the increased share of incidents of intoxication with combustion products, namely carbon oxide, followed by poisoning from gas leaks, shingles caused by explosions in violation of the operating rules of such equipment, also registering lethal cases. New results are the determination of the difference in emission values for both cases. The importance of the obtained results is that for the generation of thermal energy in a centralized heating system using cogeneration technology, harmful emissions into the environment are 28% less than when using individual heat supply systems. This received ascertainment could serve as an important argumentation for the obvious priority of the centralized heating system in the municipality versus the use of individual units.