The connection of renewable energy sources with significant nominal power (in the order of MW) to the medium-voltage distribution grid affects the operating conditions of that grid. Due to the increasing number of installed renewable energy sources and the limited transmission capacity of medium-voltage networks, the cooperation of these energy sources is becoming increasingly important. This article presents the results of a six-year study on a 2 MW wind power plant and a 1 MW photovoltaic power plant in the province of Warmia and Mazury, which are located a few kilometers away from each other. In this study, active energy, currents, voltages as well as active, reactive, and apparent power and higher harmonics of currents and voltages were measured. The obtained results show the parameters determining the power quality at different load levels. Long-term analysis of the operation of these power plants in terms of the generated electricity and active power transmitted to the power grid facilitated estimating the repeatability of active energy production and the active power generated in individual months of the year and times of day by a wind power plant and a photovoltaic power plant. It also allowed us to assess the options of cooperation between these energy sources. It is important, not only from a technical but also from an economic point of view, to determine the nominal power of individual power plants connected to the same connection point. Therefore, the cooperation of two such power plants with the same nominal power of 2 MW was analyzed and the economic losses caused by a reduction in electricity production resulting from connection capacity were estimated.

To achieve acceptable dynamical behavior for large rotating machines operating at subcritical speeds, the balancing quality check at the planned service speed in the installation location is often demanded for machines such as turbo-generators or high-speed machines. While most studies investigate the balancing quality at critical speeds, only a few studies have investigated this aspect using numerical methods at operational speed. This study proposes a novel, model-based method for inversely estimating initial residual unbalance in one and two planes after initial grade balancing for large flexible rotors operating at the service speeds. The method utilizes vibration measurements from two planes in any single direction, combined with a finite element model of the rotor to inversely determine the residual unbalance in one and two planes. This method can be practically used to determine the initial and residual unbalance after the balancing process, and further it can be used for condition-based monitoring of the unbalance state of the rotor.