The ac bus-based architecture for EV charging is a crucial component of the EV infrastructure, enabling the efficient and reliable charging of electric vehicles. This artile will discuss the pros and cons of the architecture and then will dive into a case study to explore numbers on Total Harmonics Distortion (THD) on the electric grid side of the architecture.
The ac bus-based architecture primarily revolves around a three-phase ac supply (250 – 480 V, line-to-line) to which the electric grid is connected through a transformer. From the ac bus, an ac-dc converter and then a dc-dc converter are connected to feed the EV. The complete illustration is shown in Figure 1.
Figure 1. A generalized ac bus-based architecture for EV charging. (Image: Rakesh Kumar, Ph.D.)
Such a power ecosystem integrates ac power supply systems, charging stations, and the management systems that oversee the operation and distribution of electrical energy for charging EVs. The role of the power electronics converters is for power factor correction, rectification, isolation, and voltage control.
Figure 2 shows the advantages and disadvantages of using the ac bus-based architecture. More explanation on the same follows up immediately.
Figure 2. Assessment of ac bus-based architecture for EV charging. (Image: Rakesh Kumar, Ph.D.)
What are the advantages of using ac bus-based architecture?
- Control active and reactive power: By controlling reactive power, ac charging stations can minimize losses, reduce voltage fluctuations, and contribute to electrical grid stability, particularly in areas with a high density of EVs.
- Low protection device complexity: Ac systems benefit from relatively straightforward protection schemes compared to their dc counterparts. Devices like circuit breakers, fuses, and ground fault circuit interrupters are well-understood, widely available, and cost-effective.
- Established control techniques: There are various established control techniques for ac-dc rectifiers, such as feedback PWM control, dual control loop, cascaded PI control, and outer voltage control loops, to name a few. These time-tested techniques make ac bus-based architecture reliable.
- Direct to local loads: Ac power is the standard for most residential, commercial, and industrial loads. The ac bus architecture allows electric vehicles to integrate seamlessly into existing electrical infrastructures without the need for complex conversion systems.
- Mature Technology: With a broad base of available technology and a comprehensive set of standards, ac power systems are well established. This maturity ensures that components and systems for ac charging are readily available, reducing lead times for setting up new charging stations.
- Greater scalability: The modular nature of ac charging infrastructure, combined with established standards, makes it highly scalable. Operators can start with a few charging points and expand as demand grows, adding more stations or upgrading existing ones to higher power levels.
Does the architecture have disadvantages?
- Integration issues with renewable energy sources: Most renewable energy sources generate dc power, and therefore, integrating them directly into an ac bus architecture often necessitates an additional dc-dc conversion stage.
- Fast chargers add conversion stages: For fast charging purposes, additional conversion stages can incorporate complex filtering and power conditioning systems to prevent conversion harmonics from degrading power quality or interfering with grid devices.
- High-power quality challenges: Maintaining high power quality in an ac bus-based architecture can be challenging due to the fluctuations in load as vehicles connect and disconnect, the variability in charging rates, and the potential for harmonics.
- Higher converter count: Ac bus-based systems require converters at various stages of the charging process. Each EV needs an onboard converter to transform ac from the charging station to dc for battery storage.
- Increased cost: The requirement for an onboard ac-dc converter in each vehicle contributes to the overall cost of the EV. Additionally, ac charging stations may require converters to manage power quality and compatibility with the grid.
Case study
A collaborative research study was conducted by Ontario Tech University, Canada, Aligarh Muslim University, India, and Taibah University, Saudi Arabia, to understand the impact of ac-bus-connected architecture on EV charging station’s THD generation. The findings of the research study are shown as a chart in Figure 3 and Figure 4.
The charts represent the THD in voltage and current measurements during the charging of EV batteries under two different loading conditions, represented as “Fully Loaded Bus” and “Lightly Loaded Bus.” THD is a measure of distortion in the ac waveform and is a significant factor in power quality.
Figure 3. Voltage THD (%) at ac bus compared for four different transformer configurations under fully and lightly loaded bus conditions. (Image: Rakesh Kumar, Ph.D.)
Figure 4. Current THD (%) at the ac bus compared for four different transformer configurations under fully loaded and lightly loaded bus conditions. (Image: Rakesh Kumar, Ph.D.)
Here are some of the observations from the above two charts:
- For all transformer configurations, the voltage THD percentage is higher when the bus is fully loaded than when it is lightly loaded. However, with the current THD, it is precisely the opposite.
- The voltage THD percentage is lowest for delta-delta and star-star configurations, but the current THD percentage for the same configurations is the highest among all four.
- The delta-delta and star-star configurations have zero differences in the voltage and current THD percentages, indicating that both configurations have the same effect on THD.
Summary
People prefer an ac bus-based EV infrastructure because it works with ac-powered power lines, making integration easy and cheap. It is also a viable option for charging at home or the office. It can be installed with few changes to current electrical systems, which makes it easier for many people to use.
The main problem is that ac charging is slower than dc fast charging. This means ac stations are not as good for quick top-ups or long trips needing fast charge. Using the car’s built-in charger also limits the charging speed to the charger’s highest capacity, which is different for each EV model.
References
- Review of Electric Vehicle Charging Technologies, Standards, Architectures, and Converter Configurations, IEEE Access
- Comparison of common DC and AC bus architectures for EV fast charging stations and impact on power quality, Elsevier
- A comprehensive review on system architecture and international standards for electric vehicle charging stations, Elsevier
- Electric Vehicles Charging Stations’ Architectures, Criteria, Power Converters, and Control Strategies in Microgrids, MDPI
Images
- Figure 1, Rakesh Kumar, Ph.D.
- Figure 2, Rakesh Kumar, Ph.D.
- Figure 3, Rakesh Kumar, Ph.D.
- Figure 4, Rakesh Kumar, Ph.D.
Filed Under: Charging, FAQs