With the rapid development of renewable energy technologies, DC bus on-grid systems have taken a prominent role in the power sector, offering efficient and flexible energy management capabilities. This article delves into the traditional DC bus parallel structure, exploring its technical features, advantages, and challenges, serving as a reference for industry insiders and other interested readers.

Structure of the DC Bus On-Grid System

The DC bus on-grid system, also known as a centralized energy storage system, connects multiple battery clusters with identical technical parameters via a DC bus. The collective DC power is then converted to AC power by a centralized inverter and fed into the grid. In detail, each battery cluster is connected in series to boost the DC voltage to around 840V, after which it joins the DC bus, where the inverter converts it to AC power, which is then fed into the grid through an isolation or booster transformer. This structure simplifies system design and reduces costs, making it one of the most widely used solutions in early energy storage applications.

Advantages of the DC Bus On-Grid System Structure

1. Simplified System Design and Lower Costs

The DC bus on-grid system employs a two-stage topology, where a centralized inverter connects directly to the battery clusters. This minimizes the need for intermediate conversion equipment, significantly reducing design complexity and overall system construction costs. Specifically, centralized energy storage inverters have a lower cost per watt-hour (Wh) compared to other inverter types, resulting in a more economical energy storage system.

2. Precise Initial Capacity Configuration

In the initial design phase, rigorous cell screening and grouping ensure that each battery cluster within the same storage unit has consistent nominal capacity. Based on this, the DC bus structure can flexibly configure battery clusters to meet project-specific capacity requirements, preventing over-provisioning and optimizing the cost per watt-hour.

3. Rapid System Dispatch and Control

As the energy storage industry has evolved, large-scale projects exceeding 100 MW have become increasingly common. The DC bus structure supports this trend by upgrading from a 1000V to a 1500V DC, significantly increasing single-cluster capacity and inverter power. In large storage projects, this centralized design greatly reduces the complexity of data communication storage and control dispatch, achieving response speeds within 200 ms, which is superior to distributed energy storage systems.

Challenges in the DC Bus On-Grid Structure

Despite the numerous advantages of DC bus on-grid systems, several challenges emerge in practical applications, often becoming apparent after 1000-2000 operating cycles. These issues can have a profound impact on system performance and economics.

1. Reduced Usable Capacity

Battery degradation over extended operation can lead to capacity decline, especially in systems with large numbers of parallel-connected cells where inconsistencies in the State of Health (SOH) can cause capacity mismatch. After 1000-2000 cycles, some battery clusters may experience significant capacity drop, reducing the system’s usable capacity to below 70% of its expected level, severely affecting availability and economic returns.

2. Shortened System Lifespan

Battery lifespan is heavily impacted by temperature. In a DC bus structure, capacity imbalance between clusters can cause circulating currents, leading to increased battery temperatures, accelerated aging, and a shorter system lifespan. Additionally, extreme temperatures can accelerate cell performance degradation, further affecting system longevity.

3. Increased Operational and Maintenance Complexity

The DC bus on-grid system entails a higher maintenance complexity. With long-term Battery Management System (BMS) operation, State of Charge (SOC) estimation drift may occur, requiring periodic recalibration by professional engineers, which directly impacts the system’s availability. Additionally, when circuit breaker protection trips or circulating currents arise, engineers must respond urgently on-site, driving up labor costs and resulting lost revenue opportunities for the energy storage station.

Summary of the DC Bus On-Grid System Characteristics

Conclusion

In summary, DC bus on-grid systems have several significant advantages in energy storage applications but also face several pressing challenges. With ongoing advancements and innovation, these issues are expected to improve, driving energy storage systems toward greater efficiency and reliability in the future.

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