## 1 Introduction  
In the recent years, the share of the distributed energy resources (DER) in the distribution grids has been growing continuously, due to environmental and economical concerns. The decreasing cost of photovoltaic (PV) systems accelerates the penetration of DER even more compared to previous years [1]. The electric power system has been developed as a one-way system, meaning that power flows in one direction, i.e., from the source to the load. By integrating DERs closer to the customers, reverse power flows may occur when there is lack of demand. Therefore, the distribution system operators (DSO) may face power quality challenges such as overvoltages. Due to the single-phase DER and asymmetrical loading voltage unbalance appears, which additionally decreases the hosting capacity of the low-voltage (LV) feeders [2]. There are several solutions for the DSOs to prevent power quality issues. A radical measure to mitigate the overvoltages and voltage unbalance is grid reinforcement for feeders. However, it is not preferred due to it its high cost and it is not practical for large-scale upgrades. Special equipment like dynamic voltage restoreers (DVR) and distribution static compensators (DSTATCOM) can be installed to improve the power quality, but these devices are installed at medium voltage (MV) grid near the LV grids with high penetration level of DERs [3; 4]. This implies that such a solution will not effectively solve nor mitigate problems with power quality at the end of the low-voltage buses. Furthermore, if similar problems occur in another LV grid connected to the same MV feeder but further geographically located from the DSTATCOM, the impact of the latter one will be small. Therefore, another optimal placement of these devices must be found and reinstalled [5]. On-load tap changers (OLTC) are very efficient devices for overvoltage mitigation, but since phase decoupled OLTC are not commercially popular, the voltage unbalance still remains unsolved. In addition, OLTCs might be able to solve overvoltages in saturated feeders, but they might cause undervoltages in other feeders. As mentioned earlier, in order for the DSO to mitigate power quality problems, sometimes dedicated equipment like DSTATCOMs, dynamic voltage restorers (DVR), active power filters (APF), unified power factor correctors (UPFC) and OLTC may need to be installed. Nevertheless, if this device stops providing its service due to faults, the power quality problems will appear again. Another option is provision of ancillary services by the DERs themselves [6; 7]. Unlike the dedicated equipment, the DERs are distributed across the LV grid and they can provide these ancillary services only when power quality improvement is needed [8]. Furthermore, the large number of DERs and the possible provision of ancillary services will increase the redundancy and thus reliability of the distribution grid and help the DSO to better maintain the power quality in their distribution grid. In recent years, many research is carried out, which examines the ancillary services for voltage control provided by inverter-connected DERs and reactive power. Fixed power factor (FPF), variable power factor (PF), and volt/var control are control strategies that have been developed and examined in [9; 10; 11]. Nevertheless, due to the high ratios of \(R/X\) in LV grids, the reactive power is very ineffective. This is very well demonstrated in [12]. Usually, DERs are designed to inject all energy into the distribution grid to maximise the profit. Nevertheless, during peak production and low loading periods, some active power drooping can be used to achieve a more effective voltage control, compared to the reactive power exchange. This eventually leads to some loss for the prosumer, which will extend the payback period of the investment. To reduce the curtailed active power, a battery energy storage system (BESS) can be incorporated in the DER.