EV charging power levels
Research paper into EV charging power levels. Image: 123rf

Electric vehicles including plug-in hybrid, hybrid, and pure battery electric vehicles are expected to lead the clean vehicle industry in the near future. In support of this trend, electrification is considered as the most feasible way to achieve efficient and clean transportation.

The article first appeared in ESI Africa Issue 4-2020.
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Transportation electrification gained a lot of attention in the past few years, due to its potentially positive impact on the environment and on the decrease of our dependence on non-renewable and polluting resources. Electric vehicles (EVs) are one of the most important manifestations of this electrification, and most car manufacturers are contributing to the progress of this domain by developing and marketing several EVs and their corresponding infrastructure, such as charging stations.

The general structure of EV chargers is presented in Figure 1, where the charger is composed of an AC/DC converter connected to a DC/DC converter so that the interface between the EV battery and the electrical grid is completed.

Figure 1. General structure of an EV battery charger

With the progress made in the power electronics department, bidirectional converters are now efficient enough to be used in transportation applications, due to the improvement made on their efficiency and power density. This progress offered many possibilities for EVs and made them more attractive to the public and utilities worldwide, by allowing the reverse power flow between the battery and the grid, such as vehicle-to-grid (V2G), vehicle-to-building (V2B) and vehicle-to-home (V2H). This has many advantages, including:

  • peak shaving and valley filling, made possible by the EVs injecting power back to the grid to balance supply and demand;
  • voltage and frequency control, allowed by an appropriate charger structure;
  • power quality improvement, which is achieved by the charger acting as an active power filter and/or injecting or absorbing reactive power; and
  • energy storage systems for the renewable energy systems connected to the grid.

Several research papers have studied structures for EV bidirectional charging and presented results proving their effectiveness. This article is based on the paper Comparison between Isolated and Non-Isolated DC/DC Converters for Bidirectional EV Chargers and compares two structures that have a big difference, which is electrical isolation

EV charging power levels: The station structure

The Society of Automotive Engineers’ standard defines the EV charging power levels, as shown in Table 1. However, these levels are quickly changing as several car manufacturers tend to have their own standards (such as the Tesla EV charging power levels standard). Also, charging power is quickly increasing to make EVs more competitive with ICE vehicles and to meet public expectation of reduced charging time.

Table 1. EV charging power levels

Taking the EV charging power levels in Table 1 into consideration, the charging power used in this paper is in Level 3. This links to the trend that several standards set, like the CHAdeMO standard that recently released a 150kW standard and the Tesla protocol that uses a 170kW charging process. As seen in Figure 1, the charger contains two converters that can be on- or off-board. But, with the increase of the power flow, on-board chargers seem to have many drawbacks, such as the size needed and the heat management. That is why an offboard charger seems like a better idea and will be adopted. The charging station will therefore provide a charging power that is able to conduct a bidirectional power flow between the battery and the grid.

Going into details of the structure, the authors chose the three-phase full bridge as an AC/DC converter as a unified converter for the comparison. This converter, shown in Figure 2, is bidirectional and composed of six switches and can easily be controlled in order to provide its basic functions, such as regulate the DC Bus voltage and make sure that the grid current is in phase with the grid voltage and has a THD (Total Harmonic Distortion) that respects the IEEE norm during the battery charging, and act as an inverter while making sure that grid current is completely out of phase from the grid voltage during the reverse power flow.

Figure 2. The full bridge AC/DC converter

Rs and Ls are the grid’s resistance and inductance, while RF and LF constitute the grid-side filter. The DC/DC converters chosen have to fulfil several basic functions, such as providing a charging current to the battery that respects the manufacturer’s datasheet regarding the ripple. For example, communicating with the Battery Management System (BMS) to receive error messages and whether the power flow is to be started or stopped. The EV battery will be represented by its electrical model that takes into consideration its voltage and internal resistance that can change depending on the battery State of Charge (SOC).

DC/DC converter structures modelling and control

Interleaved Buck/Boost converter
The non-isolated DC/ DC converter chosen for this study is the four-phase interleaved bidirectional Buck/Boost converter. The main advantage of interleaving basic structures is the increase of the maximum output power level and the improvement of the power factor, while reducing the size of the passive components, especially the inductance L. In fact, the topologies placed in parallel allow the inductance to have the value shown in this equation:

Where n is the number of phases connected in parallel, i is the phase number (1≤ ≤i n). This topology also allows the reduction of the input and output current ripple. However, the increased number of phases results in an increase in the switches, which can negatively affect the efficiency in case the switching losses become high. Hence, a compromise between the number of phases used and the efficiency should be made. The graph in Figure 3 shows the evolution of the efficiency with the increase of the number of phases. This is obtained through a Matlab simulation of the multi-phase bidirectional Buck/Boost converter. The reason behind the decrease of efficiency when the phases exceed 4 is that the switching losses become high with the high number of switches.

Figure 3. Evolution of the efficiency with the number of
phases

According to Figure 3, the efficiency reaches its highest values for phases between 2 and 4, so the chosen structure will be a four-phase converter to have the same number of switches as the nonisolated dual active bridge, which is eight. The non-isolated structure is represented via the small-signal model. The transfer function between the duty cycle and the output current is represented below:

The output current, which is the battery current, will be controlled through a PID controller, with parameters chosen so that the closed-loop converter represented by the transfer function in (2) and the chosen PID provide a satisfying performance. The control of the interleaving phases consists of delaying the switches commands by Ts/4, where Ts is the switching frequency.

Dual active bridge converter
The isolated dual active bridge (DAB) converter is composed of two full bridges connected by a high-frequency transformer. One of the advantages of the DAB is its simple control that consists of controlling the two bridges by simple square wave signals with a delay between the control of the first bridge and the second bridge. This delay determines the amount of power that passes through the converter. The control of the DAB converter requires the transfer function between the phase delay and the output current.

In summary, the first configuration is a nonisolated four-phase interleaved Buck/Boost converter and the second is an isolated dual active bridge (DAB). Simulation of bidirectional power flow between the battery and the grid is done using Matlab/ Simulink/SimPowerSystems. Comparing the necessity of the isolation, the Buck/ Boost needs an additional isolation circuit to ensure that it respects the charging and discharging norms and standards, while the DAB already contains a high-frequency transformer that allows it to realise a galvanic isolation between the battery and the AC/DC converter, and ensures the safety of users. ESI

Acknowledgement
This article is an adaption of a paper titled Comparison between Isolated and NonIsolated DC/DC Converters for Bidirectional EV Chargers, written by Innocent Kamwa, Kamal Al-Haddad and Rawad Zgheib – of the Laval University. View a full list of references and diagrams. www.academia.edu/35889919