AC Collection & Substation

Everything the plant produces has to leave through this equipment

The AC collection system takes inverter output, steps it to medium voltage, gathers it across the site and delivers it to the point of interconnection. It is the part of a renewable plant that most resembles conventional utility work — and it carries the longest lead times, the heaviest regulatory scrutiny and the least design freedom once the interconnection study is complete.

It is also the part least amenable to schedule recovery. A module delivery can be accelerated; a 34.5 kV collector breaker or a step-up transformer generally cannot. On most projects the AC collection equipment defines the energisation date, which is why it should be released against the study rather than held for later design refinement.

Specifications that decide the selection

Selection criteria in practice

• Sequence procurement by lead time, not by design order. The main power transformer and collector switchgear should be released first even though they are conceptually downstream. Everything else has slack; these do not.
• Verify cable ampacity against a site-specific thermal model. Soil thermal resistivity varies widely and dries out under load, which reduces capacity further. Published ampacity tables are a starting point, not a design.
• Coordinate protection for converter-based sources explicitly. Inverters current-limit rather than delivering several times rated current into a fault. Collector protection designed on conventional assumptions may under-reach, and this must be resolved by study before relay settings are issued.
• Confirm the utility’s metering and telemetry requirements in writing. Revenue metering, ISO telemetry and the communications path are utility-specified and frequently discovered late. They are also a common cause of delay between mechanical completion and commercial operation.
• Check domestic content requirements against the transformer and switchgear. Where incentives depend on domestic content, these large items carry disproportionate weight, and qualifying supply has its own constrained lead time.

Typical applications

A utility-scale PV or storage plant repeats an inverter block with its pad-mount step-up transformer, ties blocks together with collector feeders at 34.5 kV, and brings those feeders into a collector substation through collector switchgear and a main power transformer to the transmission system. Distributed generation connecting at distribution voltage compresses this to a single step-up transformer and a utility-specified interconnection cabinet — but the metering, protection and telemetry requirements remain, and they remain utility-specified.

Work the numbers before you specify

Related reading

Common questions

Why are main power transformer lead times so long?

Large power transformers are engineered to order, built in a small number of factories, and depend on constrained inputs — grain-oriented electrical steel, bushings, and skilled winding labour. Demand from grid replacement, data centres and renewables has risen simultaneously against capacity that expands slowly, because a new transformer factory takes years to build and staff.

Is 34.5 kV always the right collector voltage?

It is the common choice for utility-scale plants because it keeps collector currents and losses manageable across a large site. Smaller plants, and those interconnecting at distribution voltage, are often better served at 13.8 kV, where equipment is cheaper and more readily available. Plant size and cable run length decide it.

When should the transformer order be placed?

As soon as the system impact study fixes the rating, voltages and impedance — which is typically well before the rest of the plant is designed. Holding the order until design completion adds the full lead time to the schedule for no engineering benefit, and this is one of the most common avoidable delays on renewable projects.

Does the collector system affect the power factor obligation?

Yes, and it is regularly overlooked. The obligation is measured at the point of interconnection, but MV cable contributes charging current and the transformers consume reactive power. The inverters have to make up the difference, which consumes some of their reactive capability before any grid support is delivered.

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.