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Contingency analysis (CA) is a well-known function in power system planning and operation. In accordance with CA results, the system operator dispenses information regarding static security of the power system (overloads and/or voltage outside tolerable limits). However, classic CA with remedial action schemes cannot distinguish safe operating regimes from potentially dangerous ones in terms of voltage (in)stability. In fact, voltage instability is considered as one of the major threats leading to power system insecurity. Therefore, in this study an enhanced contingency analysis (ECA) is presented where the classical CA is extended with static voltage analysis based on the modal analysis. The article presents a dedicated methodology for the proposed ECA tool, with special emphasis on the analysis of corrective measures provided by the system operator, intended for enhancing power system security (regulation transformer action, distributed generation and energy storage). Also the influence of the load model was analyzed by simulation and the main conclusions are presented. The study demonstrated the advantages that distributed generation resources and energy storage can provide in the context of voltage stability. Also, the simulations acknowledged the importance of correct load modeling, since over or under estimation of a certain load-type component can result in too optimistic or too pessimistic power system operation limits.

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Photovoltaic (PV) system inverters usually operate at unitary power factor, injecting only active power into the system. Recently, many studies have been done analyzing potential benefits of reactive power provisioning, such as voltage regulation, congestion mitigation and loss reduction. This article analyzes possibilities for loss reduction in a typical medium voltage distribution system. Losses in the system are compared to the losses in the PV inverters. Different load conditions and PV penetration levels are considered and for each scenario various active power generation by PV inverters are taken into account, together with allowable levels of reactive power provisioning. As far as loss reduction is considered, there is very small number of PV inverters operating conditions for which positive energy balance exists. For low and medium load levels, there is no practical possibility for loss reduction. For high loading levels and higher PV penetration specific reactive savings, due to reactive power provisioning, increase and become bigger than additional losses in PV inverters, but for a very limited range of power factors.

The optimization of overcurrent relays’ operation is a topic associated with protection coordination of distribution networks. Usually, this refers to medium-voltage networks, since they are protected by numerical relay devices, as opposed to low-voltage networks, where utility operators allocate fuses. Correct setting of relays and optimal coordination is becoming a serious challenge to Distribution Network Operators around the world, since their networks’ passive operation has been greatly altered in the past two decades. Distributed generation units, a growing liberalized electricity market and more stringent legislation for distribution network planning and operation by state regulatory bodies have all indirectly affected the evolving of protection philosophy for distribution networks. In this paper the traditional optimization problem of overcurrent relay operation will be addressed and critically examined from both a theoretical and practical point of view. Optimization function, constraints and relay parameters will all be observed and compared with solutions used in distribution networks, and their modifications and improvements will be proposed and elaborated in detail.

No abstract

Photovoltaic (PV) system inverters usually operate at unitary power factor, injecting only active power into the system. Recently, many studies have been done analyzing potential benefits of reactive power provisioning, such as voltage regulation, congestion mitigation and loss reduction. This article analyzes possibilities for loss reduction in a typical medium voltage distribution system. Losses in the system are compared to the losses in the PV inverters. Different load conditions and PV penetration levels are considered and for each scenario various active power generation by PV inverters are taken into account, together with allowable levels of reactive power provisioning. As far as loss reduction is considered, there is very small number of PV inverters operating conditions for which positive energy balance exists. For low and medium load levels, there is no practical possibility for loss reduction. For high loading levels and higher PV penetration specific reactive savings, due to reactive power provisioning, increase and become bigger than additional losses in PV inverters, but for a very limited range of power factors.

Renewable energy sources have become a considerable part of electric transmission networks as well as medium and low voltage distribution networks. Understanding the overall process from design stage up to the installation stage, followed by the commissioning and startup of renewable energy sources plants is essential knowledge that electric engineers nowadays should posses. Therefore, in the first part of the article activities, conducted at the Faculty of Engineering, University of Rijeka, Croatia, necessary for the installation of a fully operational, grid connected photovoltaic power plant with dual-axis tracking system have been described. Consequently, upon photovoltaic plant's installation and commissioning, students are able to have 'hands-on' on a fully functional photovoltaic power plant and perform supervised, 'live' measurements and compare it with previously calculated values. Therefore, new-dedicated laboratory sessions have been introduced in an existing subject to make the most of the photovoltaic installation in the teaching process. In the second part, the article is mainly focused on the newly introduced laboratory sessions as well as on the educational framework and methodology. Some of the experiments that our students are able to perform include alternating current and direct current operating values measurements (photovoltaic string and inverter voltages, currents, power, efficiency, etc.), environmental parameters measurements (irradiance, air temperature, wind direction, velocity, etc.) and grounding parameters measurements (soil resistivity, photovoltaic plant's grounding resistance). The acquired knowledge gained from the activities performed during our educational photovoltaic plant project realization give us the ability to propose a methodology that can be used as the key model for other universities and faculties.