# Research on Optimal Voltage Control of Distribution Network with the Participation of EVs and PVs
## 1 Introduction  
With the progress of global decarbonization, distributed photovoltaics (PVs) and electric vehicles (EVs) are increasing their penetration into the power grid. According to the data disclosed by International Energy Agency (as in Figure 1), an exponential increase in the EV stock was seen in the past decade, and the global EV stock reached 11.27 million by 2020, with the dominant EV application belonging to light-duty passenger vehicles [1]. For the worldwide expansion of solar PVs, a steady increase was seen in the installed capacity of solar PVs, which was close to 900 GW by 2020 [2]. Although the high surge in the grid penetration of EVs and PVs is beneficial to the decarbonization of power supply and transportation sectors, the uncertainties of PV power generation and the stochastic EV charging behaviors further deteriorate the supply voltage quality, resulting in issues like voltage violations, voltage fluctuation, etc. The traditional voltage regulation of distribution networks is achieved with OLTC transformers and switchable capacitor banks (CBs). As OLTC transformers and CBs are characterized by inherent drawbacks in discrete voltage control and slow response, they are not capable of mitigating the voltage deviation and voltage fluctuation issues of distribution networks with high penetrations of EVs and PVs. By contrast, PVs and EVs are connected to the distribution networks via fully controllable power electronic converters, making them supplementary voltage support resources. The application of EVs and PVs for grid voltage support has been widely investigated in the existing literature. Conventionally, the distribution network voltage regulation is in the charge of the local distribution network operator (DNO) and is conducted in a centralized way with the operational settings of OLTC transformers and SCBs globally optimized. To make EVs and PVs get involved in the voltage regulation of distribution networks, both system-level optimal scheduling of EVs and PVs and device-level control of EV chargers and PV inverters are required, as illustrated in Figure 2. For system-level control, the optimal coordination schemes of EVs, PVs, and conventional voltage control devices have to be defined under the operational constraints of all available voltage support resources and the power grid. For device-level control, the optimal operation modes and operation setting parameters have to be defined to meet the system-level voltage support requirements. Acting as the grid-interaction interfaces, EV chargers, and PV inverters receive the commands from the device-level controller and provide real-time voltage support for distribution networks. The voltage regulation mechanism and the main features of typical voltage support resources available in distribution networks with EVs and PVs are summarized in Table 1.  
Figure 1: The global growth of EVs and PVs in the last decade: (**a**) The EV stocks by transport types; (**b**) The installed capacity of solar PVs with different application purposes. Figure 2: The general control framework for participating EVs and PVs in the voltage regulation of distribution networks. Many studies have been carried out to investigate the distribution voltage regulation with the participation of EVs and PVs. Regarding the research on the participation of EVs in voltage regulation of the distribution network, ref. [3] carried out simulation modeling of a bidirectional onboard EV battery charge and investigated its capability of providing distribution voltage support. In [4], the impact of uncontrolled EV charging behaviors on the bus voltage profiles of distribution networks is evaluated via the Monte Carlo (MC) simulations, and a coordinated EV charging scheme based on bi-level dynamic game theory is proposed for alleviating the bus voltage deviation. Ref. [5], researchers designs a scheduling strategy for EV participation in the voltage regulation of a residential distribution network.