# Research on Optimal Voltage Control of Distribution Network with the Participation of EVs and PVs
## 1 Introduction
[28] proposes a distributed control strategy for solar PV-based EVCS to achieve optimal power allocation within the EVCS. In [29], the impact of the grid integration locations of PV-rich EVCSs on the performance of distribution voltage support is investigated. Ref. [30] proposes a coordination scheme between PV power generation and EV charging to alleviate voltage rise in feeders with PVs, using coordinated EV fleets as a virtual storage system. To summarize, the voltage regulation methods applied to distribution networks with EVs and PVs connected can be generally categorized into four types, including local voltage control, centralized voltage control, distributed voltage control, and multi-time scale voltage control. Specifically, the local voltage control is generally in the form of the voltage droop control of smart PV inverters and EV chargers, which dynamically adjust their active or reactive power outputs based on the monitored terminal voltages [31; 32]. This type of control is easy to implement, and no communication systems are required, although it cannot achieve the globally optimized voltage distribution patterns. The centralized voltage control relies on a dedicated communication system to collect information on the electrical operating conditions of all buses [33]. Afterward, the collected data will be sent to a centralized voltage controller, which determines the operation settings of all voltage support resources via optimal power flow analysis. Because the communication systems of low-voltage and medium-voltage distribution systems are still imperfect, it requires extensive investment in communication infrastructure to implement centralized voltage control for a medium-size or large-size distribution network with high penetrations of EVs and PVs. To overcome the drawbacks of centralized voltage control, distributed voltage control has attracted widespread attention from scholars. In distributed voltage control, the distribution network with EVs and PVs connected is first partitioned into several regions based on the similarity of bus voltage sensitivity. Then, regional voltage control is applied to each regional distribution network via the active and reactive power control of their member EVs and PVs [34; 35]. As the integration of EVs and PV have a direct influence on the bus voltage sensitivity, the dynamic partition of distribution networks is required. As a modern distribution network is featured by the co-existence of diversified voltage support resources which respond at different time scales, multi-time-scale voltage regulation is required to achieve collaborative operation of those resources [36]. To make the most of various voltage support resources, dedicated coordinated control schemes are required. Finally, the advantages and disadvantages of all the above-mentioned distribution voltage regulation methods are summarized in Table 2.  
The limitations of the existing literature on the selected topic are summarized in the following. First, the current literature on EVs' participation in distribution voltage regulation only considers snapshot operations of EV chargers. However, multiple time-step operations of EV chargers should be considered in the coordinated control strategy to ensure that the desired battery SoC of all EV owners is met. Second, most literature applied centralized control of EVs for their participation in distribution voltage regulation by assuming that the charging behaviors of all EV owners are known in advance within a predicted period. In practice, EV drivers' willingness to charge can be affected by a variety of factors, including the charging price, the location of the EV charging station, and the planned mobility of EV drivers, etc., resulting in inherent randomness of EV charging behaviors. Third, the impact of EV transport type on the voltage support capability of aggregate EVs is rarely considered in the existing literature.