Electric Vehicle Charging Station: Cause and Solution to Grid System
By Shamsul Aizam Zulkifli
Since the last decade, the number of electric vehicles (EVs) has significantly increased. This is due to the new advanced technology especially in electric motor design and also the government policies to reduce the amount of combustion engine produced carbon footprint into the atmosphere. As a result, more and more countries around the world such as United Kingdom, has taken the initiative to reduce the dependency on the usage of internal combustion engine vehicles to t combat this problem. Due to the reasons mentioned earlier, EV’s will soon be the main focus in transportation changes. Moreover, most of the EV aspects such as the charging/discharging of the battery, battery energy management for driving the electric motor or vice versa, the concepts of Vehicle to Grid (V2G) or Grid to Vehicle (G2V) are well studied by the researchers around the world. However, less focus has been given to the issues related to charging stations.
As known, the EV charging station consists only a power converter topology. Then, it is connected to the existing electrical grid using a rectifier and a DC converter to transfer the energy between the EV to the existing electrical grid or vice versa, but it can always remain the stability of the processes. As an example, when the EV is connected to the charging station, the behaviour of the charging station will change accordingly to the converter operation either rectification or inverter mode to the EV. Therefore, the EV sees the power converter as a nonlinear input source during rectification mode. In the meantime, the EV battery will actively contribute to the frequency variation/disturbance of the power grid system if not be controlled. In this case, if the grid is continuously disturbed, the power flows between the grid and the battery cannot be consistent for providing a fast charging condition for the EV. This problem does not only affect the power flows but also it reduces the power factor and grid frequency at the grid supply. Therefore, it is important for the EV to behave as a continuous primary frequency control to the grid side that will not interrupt the stability of the grid. Meanwhile, as seen from inside the EV perspective, the ability to give a high response to the state of charge (SOC) of the battery is also essential. However now a consider to any harmonic injection due to line impedance to the EV battery rectification mode is also needs to be considered.
For example, one EV charging station does not create a significant problem, but when hundreds of charging stations are connected at the same Point of Common Coupling (PCC), a major issue is created. Here, it will impose a tremendous problem on the grid stability due to frequency fluctuation and also reducing the power factor at the PCC. Therefore, it is necessary to make sure that the EV charging station can minimize these impacts to the electric grid by providing a high quality power factor, flexible frequency restoration and while the EV battery can be charged at the very short of time. Several researchers have suggested using two-stage power converters or two-stage controllers. The two stages power converters are made by combining the converter (rectifier) with a DC converter before it connects to the EV in order to maintain the SOC of the battery during charging, while maintaining the voltage for electrical grid quality. It can also improve the life cycle of the battery before it’s been saturated. Moreover, this configuration needs dual loop controllers for the converters for rectifier: first it needs to maintain the DC link voltage at the DC converter input using outer voltage loop control and then it needs to have a current flow control as an inner loop control to maintain the power factor at the AC grid level. Due to the complexity of the controllers which has to maintain the stability gain and two converters circuit at the same time. The best solution is to use only one power converter and one control loop where it should be able to keep the voltage charging at the EV at a constant rate and at the same time control the quality of the grid current. The idea is to implement the synchronous motor converter control formulation suggested in or any voltage control with has faster power quality solution and frequency restoration. By applying this idea, it is predicted that EV source and electrical grid will benefit from a good quality power while at the same time the EV will be the primary driver of next power generation to the electrical system for the future.
Shamsul Aizam Zulkifli was born in Selangor, Malaysia, in 1980. He obtained his B.Eng. (Hons) and M.S. from University Putra Malaysia, Malaysia, and PhD in Control System Engineering from Loughborough University, UK, in 2003, 2006 and 2012, respectively. He is currently a Associate Professor with the Department of Electrical Power Engineering, University Tun Hussein Onn Malaysia, Malaysia. His current research interests include robust control system on power electronics application, parallel inverter applications and smart grid topology for inverter-grid operation.