Mineral fillers are mainly utilized in the production of printing and writing papers (P&W) to improve their optical features and their vulnerability to printing. With the high cost of pulp, the aim of their introduction has been to increase mineral loading in paper and reduce the overall cost of production. For many years the only method of paper formation was acid technology, while the only raw material of choice for filling and coating paper and cardboard was kaolin (in the beginning of the 1970s it was 80% of fillers and 94% of coating grades used in Western Europe, while in the USA – 92% and 96%, respectively). The onset on new methods of acid-free (alkaline) paper forming caused a drastic reduction in the kaolin demand for cost-competitive calcium carbonate: GCC – Ground Calcium Carbonate) and PCC – Precipitated Calcium Carbonate. This also resulted from the progressive self-destruction of machine-made acid papers. In 2013, the share of calcium carbonate in the total production of fillers was 83%, while kaolin accounted for 10%, and talc – 7%. The article presents the parameters of principal mineral fillers for the paper industry and the main reasons why they are suitable for particular kinds of paper. Kaolin, due to the platy nature of its main mineral constituent – kaolinite, is preferred in multiple coating papers. The choice of GCC is beneficial because of its low price and properties (especially whiteness). PCC , due to the possibility of shape and particle size modification serves as filler in uncoated woodfree papers, the key features of which are expected to be lightness and opacity. Size distribution is the main difference between PCC and GCC . The article also presents tendencies observed over the last several years in the paper market in Poland, i.e. in a significant growth in coated paper and board for packaging, as well as the decreasing demand for newspaper, which is a consequence of progressive digitalization.

The disadvantages of the conventional model predictive current control method for the grid-connected converter (GCC) with an inductance-capacitance-inductance (LCL) filter are a large amount of calculation and poor parameter robustness. Once parameters of the model are mismatched, the control accuracy of model predictive control (MPC) will be reduced, which will seriously affect the power quality of the GCC. The article intuitively analyzes the sensitivity of parameter mismatch on the current predictive control of the conventional LCL-filtered GCC. In order to solve these issues, a model-free predictive current control (MFPCC) method for the LCL-filtered GCC is proposed in this paper. The contribution of this work is that a novel current predictive robust controller for the LCL-filtered GCC is designed based on the principle of the ultra-local model of a single input single output system. The proposed control method does not require using any model parameters in the controller, which can effectively suppress the disturbances of the uncertain parameter variations. Compared with conventional MPC, the proposed MFPCC has smaller current total harmonic distortion (THD). When the filter parameters are mismatched, the control error of the proposed method is smaller. Finally, a comparative experimental study is carried out on the platform of Typhoon and PE-Expert4 to verify the superiority and effectiveness of the proposed MFPCC method for the LCL-filtered GCC.

Most of the basic control methods of the grid-connected converter (GCC) are defined to work with a sine wave grid voltage. In that case if the grid voltage is distorted by higher harmonics, the grid current may be distorted too, which, in consequence, may increase the value of the THD of the grid voltage. The paper deals with an improved finite control set model predictive control (FCS-MPC) method of an LCL-filtered GCC operating under distorted grid conditions. The proposed method utilizes supplementary grid current feedback to calculate the reference converter current. The introduced signal allows to effectively improve the operation when the grid is subject to harmonic distortion. The paper shows a simulation analysis of the proposed control scheme operating with and without additional feedback under grid distortions. To validate the practical feasibility of the proposed method an algorithm was implemented on a 32-bit microcontroller STM32F7 with a floating point unit to control a 10 kW GCC. The laboratory test setup provided experimental results showing properties of the introduced control scheme.