Power Conversion Systems
The converter sets the power rating, the fault behaviour and the grid services a storage plant can actually be paid for.
The PCS is where a battery becomes a grid asset
Cells store energy. The power conversion system is what turns that into something a grid operator will accept — it sets the power rating, the response time, the fault behaviour and, increasingly, the grid services the plant can be paid for. Two projects with identical batteries and different PCS platforms are not the same asset.
The significant change is that the PCS is no longer a commodity converter. Grid codes now require behaviours — ride-through, frequency response, reactive power at zero real power, and in some markets grid-forming operation — that live in the converter and its controls, not in the battery. Buying on cost per kilowatt alone selects against exactly the capabilities that determine whether the plant can be interconnected and what it can earn.
| Family | Typical configuration | Indicative lead time | Standards |
|---|---|---|---|
| Central PCS SkidsUtility-scale conversion, usually skid mounted with the MV transformer as one deliverable. | 1–5 MW per unit 630–800 V AC output Outdoor rated |
~38 weeks | UL 1741 SB IEEE 1547 IEEE 2800 |
| String / Modular PCSMultiple smaller converters in parallel. Partial failure degrades output rather than losing a block. | 100–400 kW per unit Paralleled to block rating Hot-swappable |
~26 weeks | UL 1741 SB IEEE 1547 |
| Grid-Forming PCSEstablishes voltage and frequency rather than following them. Required for black start and for weak-grid interconnection. | 1–5 MW per unit Virtual synchronous control Black start capable |
~44 weeks | IEEE 2800 UL 1741 SB ISO-specific |
| C&I Bidirectional InvertersBehind-the-meter conversion, frequently combined with solar input on a shared DC bus. | 30–250 kW 480 V AC Hybrid PV + storage |
~16 weeks | UL 1741 SA/SB IEEE 1547 Rule 21 |
Specifications that decide the selection
Continuous rating at site conditions
Nameplate power is quoted at a reference ambient. Real derating at design-day temperature and site elevation can remove a meaningful fraction of it, and that is the number the plant actually dispatches.
Reactive capability at zero real power
Many interconnection agreements require reactive support when the battery is neither charging nor discharging. Not every converter can do it, and the ones that can may derate elsewhere to achieve it.
Grid-following or grid-forming
Grid-following converters need a stiff grid to synchronise to. On weak interconnections, or where black start is required, grid-forming control is not an upgrade — it is the entry requirement.
DC window versus battery voltage
The converter’s usable DC voltage range must cover the battery’s full state-of-charge range at the design C-rate. A mismatch quietly strands capacity at the ends of the range.
Selection criteria in practice
- Start from the interconnection agreement, not the datasheet. The required grid support functions, fault ride-through profile and reactive range are specified by the utility or ISO. Those requirements select the converter; the battery selection follows.
- Match the PCS platform to the study assumptions. Interconnection studies are run on a specific converter model. Substituting a different platform after the study can require rework, and rework means re-queuing in some markets.
- Weigh central versus modular against your service model. Central skids are cheaper per kilowatt and simpler to interface. Modular converters degrade gracefully and can be repaired without a crane. The right answer depends on site access and who does the maintenance.
- Confirm the harmonic and short-circuit contribution. Converter-based sources behave nothing like synchronous machines under fault. Protection coordination on the collector system depends on the converter’s actual current contribution and duration.
- Establish the firmware governance early. If certification is tied to firmware, then patching, cybersecurity requirements and certification maintenance are the same conversation. Agree who authorises updates before commissioning.
Typical applications
Utility-scale storage blocks pair a central PCS skid with an MV transformer, repeated across the site. Weak-grid and islanded projects, and any plant expected to black start, need grid-forming converters and the study work that goes with them. String PCS suits sites where availability matters more than capital cost, or where crane access is poor. Behind-the-meter commercial installations use bidirectional inverters, very often sharing a DC bus with rooftop or carport PV so a single conversion stage serves both.
Work the numbers before you specify
Related reading
Common questions
What is the practical difference between grid-following and grid-forming?
A grid-following converter measures the grid’s voltage and frequency and injects current in step with it — it cannot operate without a reference. A grid-forming converter creates that reference itself, behaving more like a synchronous generator. On a strong grid the difference is largely invisible; on a weak one, or during a black start, it is the difference between a plant that works and one that cannot energise.
Should the PCS and the battery come from the same vendor?
Integrated supply removes an interface and usually shortens commissioning, which is a real benefit. The cost is a longer commercial dependency: augmentation, spares and warranty all route through one party. Split supply preserves options but puts the controls integration risk on you, and that risk is not small.
How much oversizing is sensible?
Very little on the converter itself, but the derating at site conditions must be understood first — what looks like oversizing is often just the honest rating. Where genuine headroom is worth buying is in reactive capability, because interconnection requirements have tightened repeatedly and retrofitting that capability is expensive.
Does a PCS need its own protection, or does the MV switchgear cover it?
Both, and they must be coordinated. Converters current-limit rather than delivering a large fault contribution, so protection schemes designed around synchronous sources may not see a fault at all. This is a study question, and it needs to be settled before the switchgear relay settings are finalised.
Family reference pages
Each family below has its own page: how it is specified, the standards it is built to, its indicative lead time and market price band.
Working out a conversion rating?
Run the load and current numbers, then build the circuit from source to point of interconnection.