String and centralised storage: architectures, enclosures and system boundaries

Storage integration follows two principal routes, string-based and centralised. Within the string route, two equipment arrangements have also developed.

The first distinction is architectural: traditional centralised and string systems differ fundamentally in whether battery clusters are connected in parallel or decoupled on the DC side. The second distinction concerns equipment arrangement: a separate battery enclosure plus string-converter/transformer unit differs from an integrated string AC/DC enclosure in physical integration and DC-side topology.

The differences have practical consequences. In some projects and operating conditions, string systems achieve better overall efficiency, discharged energy and availability. Centralised systems can offer advantages in unit capital cost, supply-chain maturity, concentrated management of large capacity and common spare parts. This comparison examines where those differences arise and whether the advantages can be reproduced.

Part one: centralised and string architectures

1. Parallel connection or decoupling

In a traditional centralised arrangement, battery clusters connect in parallel within a DC enclosure. Their combined output feeds a large central PCS for AC/DC conversion, followed by a transformer for grid connection. Large unit capacity and relatively few power devices make this a mature approach used in power electronics for decades and widely represented in the installed fleet.

A string architecture divides the system into independently operating units. Each battery cluster has its own string PCS and is managed independently. Clusters are not paralleled on the DC side; outputs combine on the AC side before voltage is stepped up for grid connection.

The fundamental distinction is therefore whether the DC side is paralleled or decoupled.

Parallel clusters must share the same terminal voltage, even though cell resistance, temperature and SOC differ. The article explains that this can make the weakest cluster constrain the enclosure and create circulating currents between clusters. Decoupling removes that inter-cluster DC path and the associated weakest-cluster constraint.

2. Efficiency and discharged energy

The article attributes a string system's efficiency advantage to its architecture. Without parallel DC connections, the physical path for inter-cluster circulating currents is removed, reducing the persistent losses associated with those currents.

It identifies several contributors: removal of circulating-current losses; intelligent thermal control that adjusts liquid-cooling power and auxiliary consumption; lower-switching-loss control in string PCS; and shorter internal cables in integrated AC/DC equipment. Together, these can improve efficiency relative to the compared centralised configurations.

Thermal control also has a long-term effect. Fully liquid-cooled battery and PCS arrangements can keep cell-temperature differences within a narrow range and adapt cooling to ambient conditions. Managing temperature differences can reduce uneven ageing and help sustain efficiency later in the equipment's life.

3. Availability and fault boundaries

With one PCS per cluster, a PCS failure removes only its associated cluster while other units continue operating. The article describes millisecond-level isolation and maintenance of a faulty unit while most remaining capacity continues participating in dispatch and market operation, supporting high annual online availability.

A large central-converter fault can remove all battery clusters connected beneath it, with an outage at the converter's complete capacity level. The economic effect depends on contractual availability measures and penalties: stricter and more granular requirements increase the value of a smaller fault boundary.

4. Illustrated architecture comparison

Qualification: efficiency and other figures come from manufacturer technical material and public reports under particular testing and statistical boundaries. Actual project performance depends on configuration, operating strategy and site conditions. Project measurements and contractual agreements take precedence.

Part two: separate and integrated string equipment

5. Two arrangements within the string architecture

In a separate string arrangement, batteries remain in a dedicated DC enclosure and string PCS units are placed in a separate converter/transformer enclosure. AC-side control is more granular, but long DC cables still connect the battery enclosure to the PCS, so the DC path has not been shortened.

An integrated string AC/DC enclosure places string PCS and battery clusters together in a standard 20-foot enclosure. Each cluster connects to its own PCS locally, with DC wiring remaining inside. AC outputs combine before the grid-connection transformer.

6. Keeping DC connections within the enclosure

Keeping the DC path inside the enclosure is an immediately visible but sometimes underestimated difference between integrated and separate string equipment.

Separate equipment requires long DC cable runs between batteries and PCS. A 12-cluster system can involve 24 high-voltage DC cables, with many joints and fixing points and corresponding insulation and arc-risk locations. Different cable lengths also create different impedances, affecting consistency between clusters.

An integrated arrangement connects clusters directly to PCS within the same 20-foot enclosure. DC runs can shrink from tens of metres between enclosures to tens of centimetres internally. The article identifies three potential benefits:

Safety: less exposed external high-voltage DC wiring and fewer field-crimped terminals reduce opportunities for arcs and short circuits. Efficiency: shorter paths reduce cable losses and help equalise cluster-to-PCS impedance. Delivery: factory-prefabricated and tested DC connections reduce extensive field crimping and insulation testing, helping shorten installation and commissioning.

7. Footprint and construction

Integrated enclosures remove external PCS enclosures and much of the DC cabling. The compared configurations typically reduce footprint by about 30%. Factory preassembly and commissioning can also reduce site PCS installation, DC wiring, communication tests and charge/discharge testing. The article describes delivery and whole-station commissioning moving from months toward weeks for some large projects, reflecting a more standardised, factory-based delivery approach.

The value of a smaller footprint depends on location. Land constraints in eastern load centres can make it particularly valuable, while large renewable bases with abundant land may place less weight on it. Shorter construction periods have broader value because earlier grid connection can bring earlier revenue.

8. Illustrated equipment-arrangement comparison

Qualification: delivery periods describe general industry examples. Actual performance depends on project scale, site conditions and construction organisation.

Part three: issues shared by both routes

9. Safety: reduce exposure and retain layered protection

Safety needs to be considered at two levels.

At architectural level, the fault-energy boundary differs. A centralised DC branch combines currents from multiple clusters, while a string branch corresponds to one cluster. At equipment level, an integrated enclosure uses short, standardised internal DC connections to reduce exposed external paths that could experience DC arcs or short circuits.

DC decoupling does not remove the possibility of cell thermal runaway. PACK-level fire protection, pressure relief and water-based fire protection remain important final barriers in both routes and equipment arrangements. Integrated systems can add module- and enclosure-level suppression, enclosure water systems, inter-cell insulation, multiple fuse stages and coordinated control/protection for battery and AC-side assessment and fault response.

10. Grid-forming functions and intelligent O&M

Grid-forming capability belongs principally to PCS control algorithms and is not exclusive to string systems; centralised systems can provide it too. The article identifies coordinated development of BMS, EMS and PCS as a further differentiator. Integrated control and protection can coordinate battery data, power instructions and dispatch within one system, simplifying fault diagnosis and responsibility boundaries.

High-speed communications in integrated string systems can synchronise large numbers of PCS units, support AGC/AVC dispatch and inertia controls, and serve demanding weak-grid or grid-forming black-start applications. Time-series EMS data can record and display cell voltages and temperatures at second-level intervals, supporting historical tracing and fault-waveform capture and moving O&M toward earlier warning.

Part four: cost and selection

11. Initial capital and lifetime economics

Performance comparisons must include cost. Centralised systems have structural advantages in large unit capacity, fewer devices, simpler combining arrangements, mature supply chains and common spares. These can lower cost per unit of power and remain attractive where initial capital is constrained, tariff structures are simple, availability penalties are limited and land is plentiful.

The article places the integrated string arrangement's disadvantage in initial investment and its potential benefits later in operation. It identifies three components: more discharged energy, using an assumed additional 10% DOD in its comparison; lower outage losses because faults affect a cluster rather than a complete converter unit; and the financial value of earlier grid connection where commissioning is shortened from months to weeks.

Over a 10–20-year lifecycle, additional discharged energy, reduced downtime and slower ageing can accumulate into substantial differences in returns. The article describes potential differences measured in tens of millions of renminbi, supporting its argument that lifetime benefits can exceed a modest initial premium.

12. Choosing a technical route

The article identifies integrated string equipment as particularly relevant where land is constrained, compact layouts are needed, connection deadlines are fixed, construction time is short, peak/off-peak price differences and cycling are high, or availability requirements carry strict penalties.

Centralised or separate string systems may suit large renewable bases with plentiful land, projects prioritising low initial cost, or extensions of existing stations that need a common spare-parts system.

For weak grids requiring grid-forming support, either route can be feasible. The decisive evidence is measured PCS control performance; topology alone does not establish that capability.

Conclusion

The comparison asks whether an additional initial investment in DC decoupling and closer AC/DC integration is justified by lifetime discharged energy, availability and construction time. Integrated string equipment removes inter-cluster DC parallel paths, external PCS enclosures, long DC runs and some field-wiring tasks. Centralised systems use scale and supply-chain maturity to control cost.

Selection should focus on lifecycle energy cost and availability rather than declaring one route universally more advanced. Site land conditions, connection deadlines, electricity-market arrangements and availability requirements provide the key inputs.