The world’s demand for electrical energy is increasing rapidly while the use of fossil fuels is getting limited more and more by energy policies and the need for reducing the impact of climate change. New sources of energy are required to fulfill the world’s demand for electricity and they are currently found in renewable sources of energy, especially in solar and wind power. Choosing the optimal PV nominal power minimizes the unnecessary surplus of electrical energy that is exported to the grid and thus is not making any impact on the grid more than necessary. Oversizing the PV system according to the Croatian net-metering model results in switching the calculation of the costs to the prosumer model which results in a decrease of the project’s net present value (NPV) and an increase in the payback period (PP). This paper focuses on formulating and solving the optimization problem for determining the optimal nominal power of a grid-connected PV system with a case study for Croatia using multiple scenarios in the variability of electricity production and consumption. In this paper, PV systems are simulated in the power range that corresponds to a typical annual high-tariff consumption in Croatian households. Choosing the optimal power of the PV system maximizes the investor’s NPV of the project as well as savings on the electricity costs. The PP is also minimized and is determined by the PV production, household consumption, discount rate, and geographic location. The optimization problem is classified as a quadratically constrained discrete optimization problem, where the value of the optimal PV power is not a continuous variable because the PV power changes with a step of one PV panel power. Modeling and simulations are implemented in Python using the Gurobi optimization solver.
This paper investigates the applicability of shortcircuit ratio (SCR) as a system strength indicator in power systems with a high penetration of voltage source converters (VSCs). In power systems dominated by synchronous generators, the SCR has been widely used to estimate the system strength by using short-circuit level information obtained at the relevant bus. However, the emerging converter technology such as VSCs has different short-circuit characteristics from synchronous generators, in the sense that the short-circuit currents are typically lower in magnitude and more controllable. Moreover, SCR calculations are based on the static analysis of the power system, neglecting the dynamic and control aspects of VSCs. These aspects raise the question whether the SCR can still be used to evaluate system strength of converter-dominated systems. To this end, in this paper, the fundamental definition of the SCR is revisited, and factors influencing its applicability in converter-dominated power systems are listed. A case study is provided to demonstrate the limitation of the SCR in converter-dominated systems, and the impact of the control strategy on the dynamic performance of the system is also discussed.
Voltage Source Converter based High Voltage Direct Current (VSC HVDC) technology is used more and more in modern power systems. Consequently, in the ac network, an increasing number of converters are appearing in mutual electrical proximity, thus forming Multi-Infeed (MI)-HVDC systems. For VSC technology, several aspects of MI-HVDC system operation are less well understood compared to Line Commutated Converter (LCC) HVDC systems. This paper studies an MI system which consists of two VSC HVDC schemes based on Modular Multilevel Converters (MMC) interconnected with an ac line and connected to an external grid. The aim of the study is to identify the small-signal stability properties of such a system, thereby focusing on the impact of different outer control loops for reactive power and ac voltage regulation. INDEX TERMS Modular multilevel converters, multi-infeed HVDC, reactive power and voltage regulation, small-signal stability.
Multi-terminal Direct Current Transmission (MTDC) is an emerging and promising technology for the transmission of electricity and the main initiator of the development of MTDC grids is offshore wind generation. However, prior to their construction, a thorough investigation of different aspects of their implementation and operation is required. In this research, an MTDC grid with voltage margin control consisting of voltage source converters (VSCs) and a high frequency cable model was implemented in Matlab/SIMULINK (R2015b, The MathWorks, Inc., Natick, MA, USA). Small-signal stability analysis was carried out to investigate the sensitivity of the grid's interaction modes to the operating point, the structure of the grid, and the selection of the voltage controlling converter. Based on the findings of these analyses, a strategy for droop control method is proposed and demonstrated.
Urban heat islands (UHI) represent increase of temperature inside of cities in regard to their rural environment. Causes of their formation are diverse and multiple: reduced amount of vegetation and waterproofing of surfaces, changed radiation and thermal characteristics of materials, urban geometry. Consequences are also diverse and significant: increasing consumption of energy for cooling, negative impact on human health, deterioration of air quality. Conventional methods for mitigating their impact include reflective and green roofs and introduction of green and water surfaces in cities. Different renewable energy technologies can also have positive impact on urban heat islands, but at the same time they contribute to greater energy independency of cities what is goal of future urban development. Proposed and described technologies are solar cooling, groundair heat exchanger, passive cooling, solar pond.
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