1949-3053 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TSG.2017.2712633, IEEE Transactions on Smart Grid TSG-00872-2016 1 Abstract —In recent years, the number of active grid components for voltage regulation in distribution grids has increased significantly. Besides voltage regulators (VRs), such as transformers with On Load Tap Changers (OLTCs), distributed generators can provide a certain voltage support by means of reactive power control (RPC). The different control entities, OLTC and RPC by photovoltaic (PV) systems, usually operate based on local measurements and control characteristics. Hence, unintended interactions between the control entities cannot be excluded in general. This study analyses the parallel operation of OLTC transformers with a voltage based control and PV systems with different RPC strategies (e.g. watt/power factor control PF(P), volt/var control (Q(V)) in a distribution system environment. The focus is on unintended interactions, such as an increase of OLTC switching operations by PV RPC. The contribution and novelty of this paper is to raise awareness for the likelihood of these unintended interactions and to provide a first methodology to assess the parallel operation of OLTC control and PV RPC in detail. The results show that the impact of PV RPC on the number of OLTC switching operations and the effectiveness in parallel operation can differ considerably between the applied PV RPC strategies. Index Terms—voltage regulation, reactive power control, on load tap changer, voltage support, volt/ var control I. I NTRODUCTION he electric power supply systems in various countries are undergoing a change towards a high share of renewable energy sources (RES). The worldwide RES capacity increased from 950 GW in the year 2004 to 1,990 GW in 2015 [1]. A high share of the RES, such as photovoltaic (PV) systems, is connected to the distribution grid and hence can be characterized as distributed generators (DGs). One major challenge for distribution system operators (DSOs) is the voltage regulation in distribution grids with a high DG penetration. Advanced DG functions (e.g. [2], [3]) like reactive power control (RPC) for voltage support can help to reduce the impact of DG feed in on the local voltage magnitude. Studies (e.g. [4]–[7]) show that the application of DG RPC can increase a grid’s hosting capacity for DG feed in, for instance, and hence avoid or at least delay grid M. Kraiczy is with the Fraunhofer Institute for Wind Energy and Energy System Technology IWES (e mail: markus.kraiczy@iwes.fraunhofer.de), 34119 Kassel, Germany. T. Stetz is Professor at the Technische Hochschule Mittelhessen, 35390 Gießen, Germany. M. Braun is Professor at the University of Kassel, 34125 Kassel, Germany, and also with Fraunhofer IWES, 34119 Kassel, Germany. reinforcement measures. In the studies [6] and [7], the impact of DG RPC on the grid hosting capacity is also analyzed for different OLTC control strategies. These studies highlight that advanced OLTC configurations and DG RPC have high potential to improve the voltage regulation in distribution grids with a high DG penetration. Nowadays, DSOs in several countries are taking advantage of the grid support functionalities of state of the art DG by requiring RPC in their interconnection guidelines (e.g. [8] to [11]). However, the IEEE working group on distributed generation integration expressed its concerns that DG RPC could conflict with other voltage regulation schemes applied by the DSO [13]. The study [14] showed, that a volt/var control (Q(V) control) can operate stably under all grid condition, if the Q(V) control settings are appropriately. However, the focus in [14] was not set on the parallel operation with other voltage regulators (VRs). In the literature, especially the parallel operation of DGs and VRs operating with a line drop compensation (LDC) algorithm were analyzed in detail (e.g., [15] to [17]). In Germany and other European countries, a voltage-based control method is usually applied for VRs, like OLTC transformers. Also the German Grid Technology/Grid Operation Forum (FNN) identified a research gap in parallel operation of DG RPC with OLTC transformers [12]. So far, a few studies have addressed the parallel operation of OLTC control and DG RPC in detail. The analysis in [18] showed that PV RPC can lead to a significant increase of reactive power fluctuations over the OLTC transformer and therefore to increased voltage fluctuations at the secondary transformer busbar. These voltage fluctuations can cause a relevant increase of OLTC transformer switching operations. The study in [19] evaluates different time delay algorithms for OLTC control and their effectiveness in grids with high DG penetration regarding the objective of minimizing the number of OLTC switching operations and keeping the voltage within the permissible bandwidth. The contribution and novelty of the paper at hand is to raise awareness for the likelihood of unintended interferences between autonomously controlled entities in the context of smart grids (such as volt/var control and OLTC operation, for example). The focus is set on the following interferences: • unintended OLTC switching operations caused by PV RPC, • increase of the number of OLTC switching operations caused by PV RPC. Parallel Operation of Transformers with On Load Tap Changer and Photovoltaic Systems with Reactive Power Control M. Kraiczy, T. Stetz, M. Braun (Senior Member) T