High Power Energy Storage Projects

When designing battery energy storage systems for backup and microgrid use, transfer switches are often the unsung hero of system reliability. Choosing the wrong switching hardware can lead to unexpected blackouts, equipment damage, or failure to meet site backup requirements. Three main switch types dominate ESS installations: manual transfer switches, automatic transfer switches (ATS), and static transfer switches (STS). Each differs drastically in switching speed, cost and suitable load types. 1. Manual Transfer Switch A manual transfer switch relies entirely for human operation. When the utility grid fails, technicians must physically operate the switch to isolate the ESS from mains power, before the PCS can enter off-grid island mode to supply critical loads. Once grid power is restored, site personnel need to manually reconnect the grid side to return to grid tied operation. Pros & Cons Switching time: Several minutes, depends on on-site staff response Low hardware cost, simple wiring  ❌ Cannot realize automatic backup; requires people on-site during power outages Best fit: Small, non-critical sites with limited project budget, where short-duration outages are acceptable. 2. ATS (Automatic Transfer Switch) ATS is an electrical mechanical automatic change-over switch. Without human intervention, it can switch load supply between utility grid, diesel generators or battery ESS. When mains power drops, ATS detects grid fault and switches loads over to backup power source. Typical switching time ranges from several to tens of seconds, creating a short power interruption for downstream loads. After grid recovery, ATS automatically switches loads back to utility supply. Pros & Cons Fully automatic operation, no on-site operator required Cost sits between manual switch and STS ❌ Second-level blackout occurs during switching; mechanical contacts wear with frequent operations Best fit: Hybrid backup projects combining ESS and diesel generators, sites that can tolerate brief power loss. 3. STS (Static Transfer Switch) STS uses solid state semiconductor components with zero moving mechanical parts. It can complete grid isolation and power path switching within milliseconds, achieving near seamless power transfer for end loads. During normal operation, loads are supplied jointly by grid and energy storage. Once grid voltage or frequency exceeds protection thresholds, STS instantly cuts off the faulty utility connection. The ESS and PV continue powering local loads without perceptible power drop. When grid quality recovers, STS re-synchronizes and reconnects to mains. Pros & Cons Millisecond-level near zero-break switching No mechanical abrasion, suitable for frequent mode switching ❌ Higher procurement cost; cabinet thermal dissipation must be properly designed Best fit: Mission-critical loads that cannot tolerate any power cut.   4. Comparison Device Switching Principle Transfer Time Load Power Interruption Typical Case Manual Transfer Switch Human mechanical operation Minutes Long Outage Low-budget small sites ATS Electro mechanical motor-driven Several ~ tens seconds Short blackout allowed ESS + diesel hybrid backup STS Solid-state static semiconductor Millisecond Near-zero cut Critical load, seamless islanding   There is no one‑size‑fits‑all transfer switch for energy storage. Your decision should be guided by how much power interruption your loads can accept, system power rating and total project budget. Most general‑purpose backup sites select ATS, while STS is reserved for high‑value critical‑load applications.   5. Sail Solar Solution Sail Solar has different solutions about such hybrid solar energy storage projects: with integrated STS module and external STS module, Interlocking and coordinated operation with the PCS are achieved via hardwired dry contact signals and CAN/RS485 communication. Two configuration options are available:     Small-to-Medium Power Energy Storage Systems (30–500 kW)   These systems incorporate hybrid inverters equipped with STS functionality; A hard-wired dry contact interlock ensures a grid-tied to off-grid switching time of ≤10 ms, enabling seamless power supply; the STS can be independently maintained without shutting down the entire PV system. High-Power Energy Storage Projects (500 kW and above) It utilizes a complete, standalone high-power STS cabinet. It supports simultaneous connection to the utility grid, energy storage PCS, and diesel generators; features high power capacity and excellent thermal performance; and is suitable for complex microgrid applications involving diesel generators. Conclusion In practical ESS applications, ATS and STS can be flexibly combined based on specific project needs, offering enhanced power reliability and system flexibility. Clients need to reasonably select transfer equipment according to actual load sensitivity, power continuity requirements, system power rating and project budget.   For small and medium-sized energy storage systems pursuing cost-effectiveness and automatic backup performance, external ATS configuration is sufficient to meet daily switching demands.   For high-power industrial energy storage and key power guarantee scenarios, independent STS cabinet configuration can achieve stable, fast and frequent grid-off-grid switching.   With flexible collocation of different transfer switches and coordinated control with PCS and energy storage systems, it is able to greatly improve the power supply stability, safety and intelligence of solar energy storage microgrids, realizing reliable power conversion and seamless energy scheduling under both grid-tied and off-grid operating conditions.