# Solutions to Increase PV Hosting Capacity and Provision of Services from Flexible Energy Resources
## 1 Introduction  
The potential of active (\(P\)) and reactive power (\(Q\)) control of distributed energy resources (DER) connected at distribution networks (MV and LV) is expected to be utilized increasingly in the future power systems to manage different renewable energy sources (RES) variabilities related challenges. These challenges related to larger share of RES based generation [1], concern (a) power generation fluctuations and (b) higher variability in power system inertia and fault levels. Moreover, network topologies need to be adapted by active network management (ANM) functionalities including occasionally intended island operation. DER consisting of distributed generation (DG), energy storages (ESs), demand response, or electric vehicles (EVs) can provide flexibility and resiliency for local (distribution system operator, DSO) and system-wide (transmission system operator, TSO) needs. Flexibility can also enable larger scale integration of RES and EVs and minimize the system and societal costs. The effective utilization of flexibilities requires combination and coordination of different types and sizes of resources from all voltage levels (LV, MV, and HV). Traditionally used fixed on-load-tap-changer's (OLTC's) setting value is rather high because it considers only the existence of customer loads. Additionally, the same OLTC setting value is typically used during the whole year. These traditional OLTC's settings, together with fixed and uncoordinatedDER units' droop control settings limit the PV hosting capacity of distribution networks and DER ability to provide power system frequency control support during very low load situations. In order to overcome these limitations, more active and adaptive distribution network operation and control principles are needed. Flexibility services provision must be enabled by DER management schemes, regulation, market structures, and business models. DER participation requires new distribution and transmission network operation and planning principles based on active utilization of flexibilities [2]. \(P\) and \(Q\) related flexibility services by DER can be realized by various inverter local control modes or functions. Inverter control functions such as constant power factor (cos\(\varphi\)), fixed \(Q\), _Q_(_P_), cos\(\varphi\)(_P_), _QU_, _PU_, and _Pf_, where \(f\) is frequency and \(U\) is voltage, are already required by different network connection and operation codes. _QU_- droop control seems to be the preferred and most effective inverter-based DG units' voltage control method in distribution networks [3; 4]. In order to increase DG hosting capacity and avoid curtailment of PV active power \(P\), _QU_- droop is used as a primary voltage control method and \(P\) curtailment by _PU_- droop control as a secondary method. In order to achieve the best possible voltage level control in both MV and LV networks simultaneously, the control settings of distribution grid connected DER need to be coordinated with the HV/MV substation's OLTC settings. This improves further DER hosting capacity in distribution networks and reduces network losses [5; 6; 7]. Different DER unit control modes, settings, and coordination with OLTC settings should be increasingly considered in the planning stage to obtain the most benefits from the coordinated operation. Amongst the various DER, battery energy storage systems (BESSs) are expected to play an important role in future smart grids due to their fast and controllable dynamics and potential to provide multiple different flexibility services in stationary applications. In many cases the use of BESS for only one purpose, for example, for (i) improving electricity supply reliability (in case of intended islanding or microgrid operation) or for (ii) increasing distribution network/PV hosting capacity [8; 9] may not be an economically viable solution. Therefore, multi-purpose use cases are normally needed for the distribution network connected BESSs.