Battery Storage · Equipment Category

Balance of Plant & MV Equipment

Transformers, collector switchgear, protection and auxiliary power — the items that usually set the energisation date.

Balance of plant is where storage schedules actually slip

The battery and the converter get the attention, but they are rarely what delays energisation. The medium-voltage transformer, the collector switchgear, the auxiliary supply and the protection relays sit between a finished block and a plant that can export — and several of them carry the longest lead times on the project.

Balance of plant also carries most of the interfaces. It is where the storage vendor’s scope ends and the EPC’s begins, where the utility’s requirements land, and where the protection scheme has to reconcile converter-based sources with conventional overcurrent practice. Scope gaps here are the single most common cause of avoidable rework on a storage project.

Family	Typical configuration	Indicative lead time	Standards
MV Step-Up TransformersTakes converter output to collector voltage. Often supplied on the PCS skid, but not always — confirm the scope split.	1–5 MVA 630 V to 13.8 / 34.5 kV Pad-mount or substation type	~60 weeks	IEEE C57.12 DOE 2016 efficiency
Collector SwitchgearCombines block feeders onto the collector bus and provides the protection interface to the utility.	15–38 kV Vacuum interrupters Metal-clad or metal-enclosed	~50 weeks	IEEE C37.20.2 IEEE C37.04
Protection Relays & Plant ControllerExecutes the interconnection requirements — trip settings, ride-through, and the dispatch interface to the ISO.	Multifunction digital relays DNP3 / IEC 61850 Redundant controllers	~24 weeks	IEEE 1547 IEEE C37.2 NERC CIP
Auxiliary Power & Station ServiceFeeds thermal management, controls and lighting. Must remain available when the battery is not.	50–500 kVA Utility or on-site backup UPS-backed controls	~22 weeks	NEC Article 706 IEEE 446

Auxiliary power is the quiet single point of failure. A storage plant whose thermal management, controls and communications all depend on the plant being able to export has an availability problem that will not appear in any datasheet. Station service needs to survive the conditions the battery exists to ride through, which usually means a genuinely independent supply and UPS-backed controls. Lead times shown are indicative modelled positions for North America, reviewed monthly, published with their basis on the Equipment Lead-Time Index.

Specifications that decide the selection

Transformer duty is not standard

A storage step-up transformer is bidirectional, loaded near rating for hours at a time, and fed by a converter with real harmonic content. Specifying it like a distribution transformer understates the thermal duty.

Fault contribution from converters

Converters current-limit at roughly their rating rather than delivering several times it. Protection schemes assuming a large fault contribution may simply not pick up. This has to be settled by study.

Collector voltage choice

34.5 kV reduces collector losses and conductor cost on large sites; 13.8 kV suits smaller ones and often has better equipment availability. The decision propagates to every transformer and cable on the site.

Communications architecture

The plant controller has to reach every PCS and BMS, and the ISO has to reach the plant controller. Latency, redundancy and cybersecurity scope belong in the specification, not in commissioning.

Selection criteria in practice

Order the transformer first. On most storage projects the MV step-up transformer is the critical path item by a wide margin. It should be released against the interconnection study, ahead of items with far more design freedom.
Write the scope split down explicitly. Whether the step-up transformer, the MV breaker and the auxiliary supply are inside the storage vendor’s scope varies by supplier. Every one of these has been missed on real projects because both parties assumed the other had it.
Coordinate protection against converter behaviour, not textbook sources. Settings derived from a conventional generator model can leave a converter-fed fault undetected. The study must use the actual converter characteristic.
Treat the plant controller as long-lead engineering, not a commodity. Its configuration encodes the interconnection agreement. Utility witness testing depends on it being right, and that testing is often the last gate before commercial operation.
Confirm spare parts for the relays and controllers. A digital relay with a six-month replacement lead time is an availability risk on a plant expected to run for two decades.

Typical applications

A utility-scale storage plant repeats a standard block — enclosures, PCS, MV step-up transformer — and collects those blocks onto a 34.5 kV bus through collector switchgear, with protection relays and a plant controller providing the interface the ISO contracts with. Smaller commercial installations compress this considerably, often to a single pad-mount transformer and a service-entrance-rated disconnect, but the auxiliary power and controls questions do not get smaller with the plant.

Work the numbers before you specify

CalculatorsLoad, current and sizing calculators PracticeBuild a circuit from source to load ToolBuild a bill of materials Templatespecification checklist

Common questions

Why is the transformer usually the critical path?

Medium-voltage transformer capacity has been constrained for several years, and storage projects compete for it against data centres, renewables and utility replacement programmes simultaneously. The design information needed to release the order is available early, so there is rarely a good reason to wait — and waiting is what causes the slip.

Is 13.8 kV or 34.5 kV the right collector voltage?

It follows from plant size and distance. Higher collector voltage reduces current, and therefore conductor cost and losses, which matters as the site grows. Below roughly 20 MW that advantage often does not repay the higher equipment cost, and 13.8 kV equipment is generally easier to source.

Who owns the plant controller configuration?

This should be settled in writing before award. The configuration implements the interconnection agreement, so it touches the utility’s requirements, the converter vendor’s capabilities and the owner’s dispatch strategy. Leaving ownership ambiguous reliably produces a dispute during witness testing, at the worst possible point in the schedule.

Does a storage plant need station service if it has a battery?

Yes. The battery cannot power its own thermal management and controls under all the conditions in which those systems must run — including a fault that has isolated the plant, or a state of charge at the bottom of the range. Independent station service with UPS-backed controls is standard practice, and skipping it converts a minor event into an extended outage.

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.

Mapping the scope split?

Work through what belongs in each package at specification time, using the completeness checklist.

Open the specification checklist