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Harmonics current emission from personal computers (PCs) is of special interest to power quality researchers, due to their wide spread application and the fact that they are usually concentrated in large groups (clusters). The input current total harmonic distortion (THDI) dependence on number of connected PCs in a cluster (NPC) is affected by network parameters and load side parameters of each cluster's PC simultaneously. As the impact of grid parameters on the THDI=f(NPC) dependence is analyzed in the author's previous papers, special emphasis is given on the impact of the DC side smoothing capacitor (C) in PCs power supply unit. For engineering application it is convenient to present the THDI=f(NPC) function with simple mathematical expressions which include these effects. In the paper authors are proposing an improved mathematical expression to presents impact of load side (DC capacitance) parameters. To achieve this goal, results of the measurements of harmonic emission from a large computer center with 167 clustered PCs and the computer simulation for even larger number of PCs are used

Besides many benefits deriving from the energy transition process, it is not uncommon for modern power systems to be faced with difficulties in their operation. The issues are dominantly related to the non-dispatchable nature of renewable energy sources (RES) and the mismatching between electricity generation and load demand. As a consequence of a constant peak load growth, this problem is particularly pronounced during the daily peak hours. Therefore, it is of great importance to conduct all necessary activities within the system in order to preserve the system stability and continuity of operation. Energy storage systems have been recognized as a major facilitator of renewable energy, by providing additional operational flexibility. Since pumped hydroelectric energy storage (PHES) accounts for almost 97% of the world’s storage capacity, in this paper, we have investigated the benefits of using pumped-storage hydropower in modern power systems characterized by high penetration of RES and the liberalized electricity market. A novel operation algorithm has been developed which finds the balance between providing additional flexibility by alleviating the peak load and obtaining financial revenue to justify the high investment costs associated with PHES. The algorithm has been tested for the daily and monthly operation of the Tonstad PHES in the dynamic environment of the Norwegian power system.

Purpose – Low power devices with switched-mode power supply represent harmonic generating apparatus in widespread use nowadays. The influence of personal computers (PCs), which affect the supply voltage, is considered. Harmonic level due to simultaneous PC operation is quantified by the total harmonic distortion of input current (THD I ). The purpose of this paper is to propose a multi-parameter mathematical model for the THD I calculation. The model is convenient for practical engineering application. Design/methodology/approach – The model is derived using the measured and simulated data. The model coefficients are obtained in the least squares sense. Findings – Mathematical modeling of THD I is the least expensive and the most convenient solution for engineering application. The models proposed in the literature have many drawbacks, which motivated the authors to develop a more comprehensive solution. Grid stiffness, capacitance of PC power supply unit and PC cluster size represent the major parameters that affect THD I , and as such they are taken into account in the proposed model. The influence of other existing parameters from both line and load side is also discussed and the reasons for their omitting from the model are explained. The model considers various PC configurations within the cluster and it enables the THD I calculation for an arbitrary PC cluster size. Practical implications – Due to its comprehensiveness and mathematical simplicity, the model is suitable for practical use, and its accuracy is verified through conducted measurements presented in the paper. Originality/value – The proposed model is more comprehensive than the existing ones, and it overcomes their shortcomings. The THD I calculation is simplified to the level of applying basic arithmetic operations only, without jeopardizing the accuracy. The validity of the model is supported by additional measurements carried out in sites characterized by grid conditions quite different from that used for model developing.

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