This paper presents a control philosophy for multiterminal DC grids, which are embedded in the main AC grid. DC transmission lines maintain higher power flow at longer distances compared with AC lines. The voltage losses are also much lower. DC power transmission is good option for Russian north. Arctic seashore regions of Russia don't have well developed electrical infrastructure therefore power line lengths are significant there. Considering above it is possible to use DC grids for supply mining enterprises in Arctic regions (offshore drilling platforms for example). Three different control layers are presented in an hierarchical way: local, primary and secondary. This whole control strategy is verified in a scaled three-nodes DC grid. In one of these nodes, a modular multilevel converter (MMC) is implemented (five sub-modules per arm). A novel model-based optimization method to control AC and circulating currents is discussed. In the remaining nodes, three-level voltage source converters (VSC) are installed. For their local controllers, a new variant for classical PI controllers are used, which allow to adapt the values of the PI parameters with respect to the measured variables. Concerning the primary control, droop control technique has been chosen. Regarding secondary level, a new power flow technique is suggested. Unbalance conditions are also verified in order to show the robustness of the whole control strategy.

At enterprises, electricity metering devices are used everywhere, which incorporate various formulas for calculating power. The division of power into active, reactive and full is valid only in the case of sinusoidal currents and voltages. In the case of non-sinusoidal waveforms of current and voltage, which are most often found in industrial networks of enterprises, it is necessary to use additional concepts and correct formulas for calculating powers in order to avoid errors. The article compares the methods of calculating the power in the non-sinusoidal mode, which can be used to more correctly record electricity in industrial networks of enterprises.

The existing approach to electricity metering, which underlies the formation of payment for consumed electricity, has a significant drawback. It can be correctly used in systems with a sinusoidal symmetrical mode, but needs to be improved in other network modes. The article defines indicators that numerically reflect the effect of distortions on the modes of distribution electric networks, as well as indicators that take into account consumed inactive power (by analogy with reactive power in a sinusoidal mode) when paying electricity bills to an electricity supplier. The proposed principles of electricity consumption metering includes the influence of such indicators as the spectral composition of voltage and current, the ratio of linear and nonlinear loads of consumers, as well as the influence of the topology of the electric network.

The variable frequency drive (VFD) is sensitive to distortions at the point of common coupling (PCC). Decreasing of the voltage at the PCC for more than 50 ms leads to the shutdowns of VFD and breaks the process. As a result, it leads to emergency situations, defects and under-release of products. The VFD of rolling mills, gas pumping compressors, stations for maintaining reservoir pressure and other powerful electrical equipment are sensitive to voltage dips. The phenomena of a voltage dip often caused by short circuits or starts of a powerful load. Often short circuits accurse in one or two lines and lead to asymmetric voltage dips. The article describes the D-STATCOM control method to compensate for asymmetric voltage dips, although usually D-STATCOM is used to reduce symmetrical voltage dips. The obtained analytical expressions for solving the inverse problem of determining the voltage of a series-connected compensator show the possibility of using it to compensate for voltage dips.

The problem of the stability of a frequency-controlled drive in the conditions of voltage failures is topical, since modern production is sensitive to process disturbances. Of particular interest are the issues of preserving the parameters of the drive and the drive mechanism without the use of additional drives and converters. However, these methods of passing through voltage sags require accurate and rapid determination of the DC link voltage parameters for power grid voltage deviations. The article presents a method that allows, in less than half a period of the supply voltage, to determine both the shape of the DC bus voltage of a frequency-controlled drive and the value of its individual parameters (maximum instantaneous value and constant component). The accuracy of the proposed method for voltage failures of various kinds is determined in comparison with simulation results in the Simulink MATLAB.

This article contains analysis of the instantaneous reactive power in case of three phase balanced grid with high harmonics. Methodology and theories are particularly descripted. The type of electric load can be determined based on the proposed reactive power vector analysis.