Sergei Gulakov’s research while affiliated with Chernihiv Polytechnic National University and other places

This article describes the existing methods of heat energy stabilizing, which are realized in thyristor power supplies for resistance welding machines. The advantages and features of thyristor power supplies have been described. A control system of power supply for resistance welding machine with stabilization of heat energy in a welding spot has been developed. Measurements are performed in primary winding of a welding transformer. Weld spot heating energy is calculated as the difference between the energy, consumed from the mains, and the energy losses in the primary and secondary circuits of the welding transformer as well as the energy losses in the transformer core. Algorithms of digital signal processing of the developed control system are described in the article. All measurements and calculations are preformed automatically in real-time. Input signals to the control system are: transformer primary voltage and current, temperature of the welding circuit. The designed control system ensures control of the welding heat energy and is not influenced by the supply voltage and impedance changes caused by insertion of the ferromagnetic mass in the welding circuit, the temperature change during the welding process. The developed control system for resistance welding machine makes it possible to improve

To solve the problem of nonsinusoidality, unbalance and reactive power compensation in electric grid, active filters and devices for balancing are widely distributed. These devices are static compensators, active power filters, unified power flow controllers. Method of symmetrical components is widely used to calculate passive active filters and devic-es for balancing. It allows to obtain the desired active filters and devices for balancing conductivity values. It is shown that the unbalance compensation can be achieved using only the capacitive elements. But for active filters and devices for balancing containing only capacitive elements, simultaneous compensation of unbalance and reactive power is im-possible. In this case, the concept of the objective function is introduced. It is a function which determines the possibil-ity of "undercompensation": acceptable levels of unbalance and consumption (generation) of reactive power. The paper deals with a type of active active filters and devices for balancing, so-called "Dynamic capacitors" or inverter-less ac-tive filters, which consists of a capacitor bank connected to the grid through a direct AC/C converter. Load balancing using Dynamic capacitors is discussed. Dynamic capacitors allow for smooth control of reactive power, moreover, they can perform limited active harmonic filtering, similar to a shunt active power filter.

In modern electrical networks thyristor-switched capacitors (TSC) are most used devices for VAR compensation. These devices don’t contain rotating parts and mechanical contacts, provide a stepwise control of reactive power and no generation of harmonics to the network. However, with the help of TSC it’s not possible to ensure smooth control of reactive power and capacitor banks (CB) are exposed to the negative impact of higher harmonic components of the network voltage. Hybrid VAR compensator don’t have such drawbacks. It consists of active filter (AF) and capacitor bank with discrete regulation. The main drawback of such systems is the necessity of accessing all six terminals of CB, while most of them are manufactured with three terminals, internally delta-connected. In the article, the topology and control system of hybrid VAR compensator free from beforementioned drawback, is proposed. The control system provides operating modes of overcompensation or undercompensation reactive power. VAR distribution regulator performs redistribution of reactive power between active filter and capacitor banks with the condition to minimize active filter’s power. Scheme of the hybrid VAR compensator, which includes a three-phase three-terminal delta-connected capacitor banks, is shown. Proposed approach allows to provide smooth control of reactive power, isolate the capacitor bank from harmonic currents and use a more effective low-voltage power components.