**Preface to "Distribution Power Systems and Power Quality"**

Today, a lot of renewable power generation units, such as wind power systems, photo-voltaic and small biomass fired combined heat and power plants, are integrated into the power system. The first two generate power depending on weather conditions, and therefore have fluctuating power production. Often, the power plants produce power in an on–off controlled way, dependent on heat demand, which also leads to power fluctuation. Furthermore, a lot of the new power generation units are equipped with electronic power converters, which may inject harmonics into the power system. This can also affect power quality. Moreover, at distribution level, the hosting capacity of the lines is not only affected by new, small power generation units, which may lead to the voltage rising above the limit, but also new, large loads which are seen in the grid, such as electrical vehicles and heat pumps, which might lead to voltages below the lower limit. These load units might also cause harmonic injections, together with other converter- and rectifier-based loads in the grid. Another concern is the reliability of such systems; some claim that, in the future, there will be less interruptions, due to higher possibilities for ancillary services from all small units, but others claim that the integration of new units will lead to more interruptions, since they will replace some of the central power plants. Furthermore, the protection system might be affected by reverse power flow and shifting short circuit level.

Therefore, this Special Issue focuses on the hosting capacity of distribution grids, how to counteract voltage fluctuations and harmonics, and how to ensure the reliability and stability of the future power system, with a special focus on distribution systems with high dispersed power generation. In the papers, you will find different applied methods to analyze the grid behavior using, for instance, Monte Carlo simulations, piecewise bound constrained optimization, S-transform and probabilitic neural networks and wavelet methods.

> **Birgitte Bak-Jensen** *Special Issue Editor*

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