Flexible Grids Boost EV Capacity
The global automotive industry is currently navigating a profound transformation driven by the urgent necessity to decarbonize transportation. As electric vehicle adoption accelerates worldwide, the infrastructure supporting these zero-emission machines faces unprecedented stress. The surge in electric mobility is not merely a challenge for automakers but represents a critical test for power distribution networks. A recent groundbreaking study from China highlights a technological pathway that could resolve the bottleneck between grid capacity and the growing demand for electric charging. Researchers have developed a new model for assessing the maximum supply capability of distribution networks that incorporate low-voltage flexible interconnection technology. This advancement suggests that the grid of the future can be far more adaptable than previously thought, potentially unlocking significant capacity for electric vehicle charging without requiring massive infrastructure overhauls.
The context for this research is the ambitious dual carbon goals set by major economies, aiming for carbon peaking and carbon neutrality. These environmental targets necessitate the large-scale application of low-carbon technologies, including distributed generation, energy storage, and notably, electric heating and electric vehicles. However, the existing distribution network was designed for a different era of energy consumption. It faces severe challenges such as insufficient carrying capacity, load imbalance, and power quality issues when subjected to the stochastic and heavy loads introduced by modern electrification. Traditional grid upgrades are often costly and time-consuming. Consequently, engineers and planners are seeking smarter solutions that maximize the utility of existing assets. Low-voltage flexible interconnection technology has emerged as a potent candidate to address these challenges, offering a way to upgrade or construct distribution network contact nodes using power electronic flexible interconnected devices.
The core of this new research lies in the concept of Total Supply Capability. In the realm of power distribution, this metric is a classic indicator used for planning, assessment, and safety analysis. It defines the maximum load a network can supply while maintaining safety standards. Historically, models for calculating this capability were established for traditional medium-voltage networks. However, the emergence of low-voltage flexible distribution networks introduces new complexities. The presence of flexible interconnected devices allows for dynamic power flow control and fault isolation, enabling flexible closed-loop operation. This capability is distinct from traditional rigid networks where power flow is determined by network parameters and mechanical switch operations. The study posits that by leveraging these flexible devices, the grid can achieve a higher level of efficiency and reliability, which directly correlates to the ability to support more electric vehicle charging stations and residential charging points.
The research team, comprising experts from State Grid Tianjin Electric Power Company and Tianjin University, approached the problem by first defining the typical structure and operation modes of these flexible networks. They identified two basic structures for low-voltage station area flexible interconnection: centralized and distributed. In a centralized setup, various station areas connect to a public direct current bus through converters. This method is manageable but limited in the number of direct current ports available. Conversely, the distributed approach connects station areas locally via direct current cables and switches. This method is more conducive to large-scale access of distributed generation and direct current loads, such as electric vehicle chargers, without requiring additional site selection for converter stations. The choice between these structures depends on the specific spatial and temporal characteristics of the load, a factor crucial for planning EV charging infrastructure in dense urban environments.
A significant portion of the study focuses on the operational dynamics during both normal conditions and fault scenarios. Under normal operation, the flexible interconnected devices facilitate load balancing among connected station areas. This is particularly relevant for electric vehicle charging, where load spikes can occur unpredictably. The devices can mitigate the risk of transformer overload by shifting power from heavily loaded areas to those with spare capacity. Furthermore, the independent reactive power output function of these devices provides voltage support, addressing power quality issues caused by intermittent distributed generation. This stability is vital for the sensitive electronics within electric vehicles and charging equipment. When a fault occurs, such as a transformer failure, the system can rapidly transfer load to interconnected transformers via the flexible devices. This rapid transfer minimizes time, ensuring that charging services remain available even during grid disturbances.
The researchers established a comprehensive model for calculating the Total Supply Capability that accounts for these flexible interactions. Unlike traditional models where only user load is a variable, this new model includes the port power of the flexible interconnected devices as variables. This adds a layer of complexity, transforming the problem into a nonlinear non-convex planning model. To solve this, the team proposed a method based on the branch and bound algorithm. This computational approach allows for the rapid traversal of sub-problems and convergence to a global optimal solution. The robustness of this algorithm is essential for practical application, as grid planners need reliable data to make investment decisions. The model also incorporates N-1 security constraints, ensuring that the network remains safe even if a single component fails. This is a critical requirement for maintaining the reliability expected by electric vehicle users who depend on consistent charging availability.
To validate their theoretical model, the team conducted case studies using an actual distribution network configuration. The test system included multiple main transformers, medium-voltage feeders, and flexible interconnected devices with varying port capacities. The results were illuminating. The calculation showed that the Total Supply Capability could be significantly enhanced through flexible interconnection. In the specific case study, the network achieved a supply capability of over 53 megavolt-amperes under optimal conditions. More importantly, the system passed the N-1 security verification, confirming that even under fault conditions, the network could reconfigure itself to maintain supply without violating safety constraints. This finding suggests that utilities can defer costly upgrades to main transformers and feeders by strategically deploying flexible interconnection devices at the low-voltage level.
One of the most valuable insights from the research is the analysis of how the capacity of the flexible interconnected devices influences the total supply capability. The study revealed a distinct relationship characterized by three stages. Initially, as the device capacity increases, the supply capability grows linearly and rapidly. This is because the bottleneck is the capacity of the flexible device itself. As the capacity continues to increase, the growth rate slows down. Eventually, the supply capability plateaus. This saturation point occurs when the bottleneck shifts from the flexible device to the capacity of the distribution transformers themselves. This phenomenon provides a clear guideline for infrastructure planning. It indicates that there is an optimal range for installing these devices. Installing devices with excessive capacity beyond this point yields diminishing returns. For planners looking to support electric vehicle growth, this means investments can be optimized to achieve maximum grid flexibility without overspending on unnecessary power electronic capacity.
The implications of this research for the automotive and energy sectors are profound. As electric vehicle penetration rates climb, the load on low-voltage distribution networks will intensify. Residential charging, in particular, occurs at the low-voltage level where these flexible interconnection technologies operate. By enabling load transfer between station areas, the technology allows neighborhoods to share capacity. If one neighborhood has a high concentration of electric vehicles charging simultaneously, power can be drawn from a neighboring area with lower demand. This peer-to-peer energy sharing at the grid level reduces the need for every transformer to be sized for peak theoretical load. It effectively creates a virtual increase in grid capacity. For automakers and charging network operators, this translates to greater confidence in the grid’s ability to support high-power charging stations in urban areas where space for new substations is limited.
Furthermore, the study highlights the importance of secondary load transfer. In traditional rigid distribution networks, load transfer is typically a one-time operation involving medium-voltage switches. However, in a flexible low-voltage network, load can be transferred again after the initial reconfiguration by adjusting the power distribution of the flexible devices. This secondary transfer capability enhances the flexibility and reliability of the network. For electric vehicle infrastructure, this means that even if a primary power source is compromised, the system has multiple layers of redundancy to keep chargers operational. This level of reliability is becoming a key differentiator in the competitive charging market. Users are increasingly demanding not just availability, but reliability. A grid that can self-heal and reconfigure dynamically offers a superior user experience compared to traditional infrastructure.
The research also touches upon the economic and maintenance aspects of deploying this technology. While flexible interconnected devices involve higher initial costs and more complex maintenance compared to traditional switches, the long-term benefits in asset utilization efficiency are significant. By releasing potential grid capacity, utilities can serve more customers without building new lines. As the cost of power electronics continues to decline and operation maintenance technologies improve, the economic case for low-voltage flexible interconnection becomes stronger. Additionally, the technology is highly adaptable to the new power system characteristics, facilitating the integration of direct current sources and loads. Since many electric vehicle chargers and renewable energy sources operate on direct current, reducing the number of conversion stages can improve overall efficiency. This alignment with the technical nature of electric mobility makes the technology a natural fit for future grid evolution.
Looking ahead, the authors suggest several avenues for future research that will further refine the application of this technology. One critical area is the consideration of uncertainty introduced by photovoltaic generation and electric vehicle charging behavior. The power injected or consumed at network nodes is not always predictable. Future models will need to account for this stochastic nature to ensure robust planning. Another area of focus is the impact of flexible interconnection on power quality and the hosting capacity for distributed generation. As more homes install solar panels and electric vehicles, the bidirectional flow of power becomes more common. Understanding how flexible devices manage these flows will be key to maintaining grid stability. Finally, the development of methods for locating and sizing these flexible devices based on the Total Supply Capability model will help utilities deploy resources where they are needed most.
The collaboration between State Grid Tianjin Electric Power Company and Tianjin University underscores the practical orientation of this research. It is not merely a theoretical exercise but a study grounded in the realities of urban power distribution. The involvement of a major utility company ensures that the findings are relevant to actual grid operations. The use of real-world case data adds credibility to the results. For the international automotive community, this research serves as a signal that the energy infrastructure is evolving to meet the demands of electrification. It demonstrates that engineering innovations are actively working to remove the barriers that often slow down the transition to sustainable transport. The grid is becoming smarter, more flexible, and more capable.
In conclusion, the development of a Total Supply Capability model for low-voltage flexible distribution networks represents a significant step forward in power system engineering. By quantifying the benefits of flexible interconnection and providing a robust method for calculation and verification, the study offers a blueprint for modernizing distribution networks. The findings indicate that with the right technology, the grid can support a much higher density of electric vehicles than previously assumed. The key lies in optimizing the capacity of flexible devices to match transformer limits, ensuring cost-effective upgrades. As the automotive industry continues its push towards electrification, such advancements in grid technology will be indispensable. They provide the foundational support required to sustain the growth of electric mobility, ensuring that the promise of a low-carbon future can be realized without compromising on reliability or performance. The synergy between advanced grid management and electric vehicle deployment is becoming increasingly clear, paving the way for a more integrated and sustainable energy ecosystem.
Authors: Zu Guoqiang, Hao Ziyuan, Huang Xu, Zhang Chi Affiliations: Electric Power Research Institute of State Grid Tianjin Electric Power Company; Key Laboratory of Ministry of Education on Smart Power Grids (Tianjin University); Chengdong Power Supply Branch of State Grid Tianjin Electric Power Company Journal: Automation of Electric Power Systems DOI: 10.7500/AEPS20211116005