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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.

Time-averaged electromagnetic (EMT) power electronics models can comprise tens of state variables, and in upcoming converter-dominated power systems, this can cause a substantial computational burden. The level of detail in their modeling, however, is typically much greater than what is typically necessary for transient stability (RMS) tools. Hence, there is a reasonable need for using reduced-order models of power electronics devices in accordance with the study needs and the related frequency range of interest. This paper focuses on modeling the time-averaged voltage source converter (VSC) for the application in ac system stability studies where the small-signal assumption holds for converter variables. The impact of simplifications and converter parameters in VSC's model accuracy is evaluated first as observed from the ac-side admittance. Subsequently, based on the previous assessment, a limited set of VSC reduced-order models is established and analyzedwhich is the main paper contribution. The parametric sensitivity analysis is conducted to establish the validity boundaries of the proposed models in the frequency domain. Finally, the recommendations for the use in different power system studies are provided.

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.