Designing Hybrid PV & BESS Plants for Weak Grid Conditions

by Egemen Ustundag, design engineer at Detra Solar.

This article draws on insights from Shahnazian et al. (2025), “Hybrid power plant control to enhance voltage stability in weak grid,” and reflects Detra Solar’s practical perspective on hybrid PV & BESS design.

Understanding Weak Grid Conditions

Weak grid conditions are becoming increasingly common in large-scale renewable projects. A grid is typically considered weak when the short circuit power at the point of connection is low. In such environments, voltage becomes highly sensitive to changes in active and reactive power.

For utility-scale PV plants, this often means that installed capacity does not directly translate into usable export capacity. Even when equipment is properly sized, voltage stability can limit how much power the grid can safely absorb.

As more renewable power plants are connected to the grid, these constraints are no longer exceptional – they are becoming part of everyday project development.

Short Circuit Power as a Stability Indicator

In practice, the grid’s physical strength at the connection point is often evaluated using the Short Circuit Ratio (SCR). SCR is calculated as the ratio of the grid’s short-circuit apparent power to the plant’s rated power.

More than just an abstract measure of “strength,” SCR indicates voltage stiffness. A low SCR (typically below 3) indicates that the grid voltage is highly sensitive to the active and reactive power your plant injects.

While SCR is a commonly used reference metric, the absolute short circuit power often determines the real stability boundary. Under typical weak-grid conditions, stable transferable power can be limited to approximately half of the available short-circuit power when renewable assets operate independently. This limitation is not commercial – it is a system-level stability constraint.

Understanding this relationship early in the design phase is critical for realistic export capacity planning.

Role of Hybrid Architecture

Hybrid renewable plants – combining PV, wind, and battery storage behind a shared transformer that acts as a single Point of Connection (PoC) to the grid – allow coordinated control at the plant level rather than the asset level.

From a system perspective, this single-PoC topology creates a significant advantage.

When active and reactive power are managed centrally:

Voltage support becomes more effective
Grid interaction improves
Effective impedance seen by the grid is reduced
Stability margins increase

This coordination can allow more stable power transfer compared to individually controlled assets.

Hybridization, therefore, is not only a commercial or market optimization strategy – it is a technical integration strategy.

Reactive Power Strategy and High-Voltage Control

Reactive power management plays a central role in voltage stability.

In weak grids, controlling reactive power locally at inverter level may not provide sufficient voltage support at the point of common coupling. Coordinating reactive power at the high-voltage side of the plant enables more effective voltage regulation.

This design decision directly influences:

Export capability
Compliance with grid codes
Stability margins

Reactive power strategy should therefore be defined during conceptual design and grid studies, not treated as a secondary adjustment.

Expanding Role of BESS in Weak Grids

Battery Energy Storage Systems are often introduced for energy shifting, peak shaving, or revenue optimization. However, in weak grids, BESS also contributes structurally to grid stability.

With fast dynamic response capability, storage systems can:

Provide rapid voltage support
Stabilize power fluctuations
Improve overall system robustness

In many large-scale projects, BESS is no longer only a financial asset – it becomes part of the plant’s stability architecture.

Practical Design Considerations

In weak-grid projects, early design decisions directly influence export capability and technical feasibility.

At Detra Solar, grid constraints are considered from the earliest design stages. Through system layouts, single line diagrams, and energy yield assessments, we help define realistic export assumptions for PV and hybrid PV & BESS projects.

As the design progresses, we ensure that electrical configuration, transformer selection, and plant integration remain aligned with grid connection requirements and study outcomes.

Where advanced grid stability simulations are required, these are typically performed by specialized consultants. Our role is to ensure that their findings are consistently reflected in the final plant design.

In weak-grid environments, alignment between grid studies and practical design implementation is essential.

Final Perspective

Weak grids do not prevent renewable development – but they require more deliberate design decisions.

Installed megawatts alone do not determine project success. Export capability increasingly depends on how intelligently the plant interacts with the grid.

Hybrid PV and BESS architecture, coordinated control, and early-stage electrical analysis are becoming central to unlocking stable and efficient renewable integration.

In large-scale renewable projects, grid-aware design is no longer optional – it is fundamental.

Let’s Talk About Your Next Project

If you’re planning a PV or hybrid PV & BESS project in a weak-grid area, our engineering team can help assess grid constraints, optimize control strategy, and support your project from concept to detailed design.

References:

Shahnazian, F., Das, K., Yan, R., & Sørensen, P. E. (2025). Hybrid power plant control to enhance voltage stability in weak grid. https://doi.org/10.1016/j.ijepes.2025.111070