Auxiliary transformer

Where the supply is derived

The commonest arrangement takes the primary off the block's own low-voltage AC bus, between the power conversion system and its step-up transformer. SMA's summary of auxiliary supply arrangements for its Sunny Central Storage and MV power station products describes the 8.4 kVA low-voltage auxiliary transformer as connecting from the AC busbar of the inverter, between the inverter and the medium-voltage transformer.

Huawei's JUPITER smart transformer station wraps the same idea into the station: its manual lists a dry-type auxiliary transformer of 5 kVA, vector group Ii0, wound 800 V/230 V/127 V, or an optional 50 kVA three-phase Dyn11 at 800 V/400 V or 800 V/220 V — both wound for the inverter bus, not for the collection system.

Power Electronics feeds the auxiliary services of its GEN3 inverters from an auxiliary services transformer inside the machine with multiple taps, one of them dedicated to ventilation. The attraction is that nothing outside the block has to exist for the block's own fans and controls to run; the consequence is that the supply has the same parent as the power train and shares its outages.

The second derivation hangs the primary on the medium-voltage system instead, so the supply survives the step-up transformer being switched out. Power Electronics' PCSM Econ Mode application note specifies exactly such a unit: single-phase, primary connected between one medium-voltage phase and ground at 34500/√3 ±10 %, secondary 120 V, thermal power 900 VA, 60 Hz, insulation class/withstand/BIL quoted as 38/70/150 kV.

Its purpose is to keep the inverter powered while the motorised medium-voltage switch is open and the main transformer is de-energised, avoiding that unit's no-load losses during long idle periods.

Read the rating before assuming what it covers: Power Electronics states the Econ Mode transformer is sized for inverter controls and communications, that power for the forced-air cooling system still comes from the main transformer, and that up to 190 VA of the 900 VA is reserved for customer equipment such as SCADA or communications gear, behind a 2 A single-phase breaker in the user cabinet specified per the US National Electrical Code.

The third derivation buys the supply from outside — a separate utility service, or terminals where the owner brings their own low-voltage source. Power Electronics offers external-supply terminals in the user cabinet at 230, 120 or 277 Vac, included by default at 230 Vac on IEC-market HEMK and PCSK units and at 120 Vac on UL-market ones, and not included by default on the larger HEM and PCSM. SMA's option list covers an external single-phase 230 V feed and an external three-phase-plus-neutral 230/400 V ±10 % feed, in which case the customer supplies the low-voltage transformer themselves.

A variant of that second derivation is worth knowing because it couples two scopes: some grounding transformers are built with a secondary winding that also serves station service, so an auxiliary-supply outage becomes a grounding outage — the grounding-system entry carries that trap. Three parents, three different sets of things that can take the supply away, and only the one-line diagram records which was actually ordered.

The rating comes from the cooling plant

Controls are cheap and cooling is not, which is why an auxiliary transformer sized on the electronics is wrong by more than an order of magnitude. Power Electronics' published GEN3 consumption figures for a HEMK/PCSK move from 0.4 kVA with ventilation off and electronics only, through 3.1 kVA with ventilation running at zero load, to 9.6 kVA with four modules at full load — a factor of about twenty-four on the same machine, driven by fans and load rather than by logic.

The larger HEM/PCSM frame runs 10.6 kW to 16.5 kW across the same span. Huawei's JUPITER order page makes the same point as a choice: 5 kVA single-phase or an optional 50 kVA three-phase unit for one station, a factor of ten apart. Direction of energy flow does not enter into it. Auxiliary draw follows module count, load and ambient, so a plant importing energy pays for the same fans it pays for while exporting, and in cold climates the pre-charge heating load is the other candidate for the design point.

The number to demand alongside the kVA is the duty. Power Electronics publishes its Advanced customer panel for solar inverters at 45 kVA and then qualifies it — 25 kVA in continuous operation, 40 kVA on a 40 % ON / 60 % OFF duty cycle — while the Basic panel gives the client 5 kVA and the Advanced storage panel is quoted as 20+5 kVA. Two auxiliary ratings are not comparable until both carry a duty statement and an ambient.

SMA splits the job across two transformers on the same skid: 8.4 kVA for the inverter's own auxiliaries and a 2.5 kVA unit for the medium-voltage station and external loads, itemised against the loads it feeds — an MV power station ventilation fan at 230 W, cabinet lighting at 50 W, communications at 100 W, plus general-purpose outlets on a pair of 16 A miniature circuit breakers.

Whole-site station service is a different animal again from a per-block supply. SMA's own undated project-reference deck covering storage projects from 2017 to 2021 lists a single 850 kVA auxiliary transformer at one site — nearly a thousand times the 900 VA of the Econ Mode control supply above, and a reminder that the words on the drawing are identical in both cases. Two design points then decide the rating.

The first is coincidence: the worst case is the hottest ambient occurring at the same time as maximum charge or discharge, not an annual average of either. The second is starting behaviour, because a board with chillers and pumps on it draws motor starting current, and the binding constraint on a small transformer is often the voltage dip at start rather than steady-state kVA.

Earthing the auxiliary system

The auxiliary secondary is normally a four-wire wye, solidly grounded, with a single neutral-to-earth bond — the arrangement IEC 60364 classifies as TN and the US National Electrical Code handles under its separately-derived-system rules. Three reasons converge on that answer, and all of them are local to the auxiliary board. The loads are single-phase and need a neutral to connect to. The available fault current at a few tens of kVA is small enough for ordinary moulded-case breakers to interrupt, so there is nothing to limit.

And residual-current and earth-fault protection only works when the return path is defined. This is the one bus in the plant where solid grounding is the routine answer: medium-voltage collection is usually impedance-grounded and the 1500 V class DC bus typically floats with insulation monitoring, and the grounding-system entry owns why the plant carries different answers at different levels.

The vector group is what makes the secondary separately derived in the first place. Huawei's optional three-phase auxiliary unit is a Dyn11 — delta on the primary side, wye secondary with the neutral brought out — and a delta primary severs the zero-sequence path from the bus feeding it, which is precisely why the secondary needs an earth reference manufactured on its own terms rather than inherited. The single-phase 5 kVA option is an Ii0 instead.

Do not generalise from one product: which winding carries the delta varies by market and design, and the transformer-vector-group entry has the letters and the clock numbers. What does generalise is the bond count. The neutral is bonded to earth exactly once, at the transformer or at the first distribution board, and a second bond puts normal load current into the earthing conductor — the neutral-current entry covers what that does, along with why the neutral on a board full of single-phase switch-mode supplies deserves its own sizing check for triplen harmonics.

The last question is whether the plant's protection can see a fault in the auxiliary transformer itself. Power Electronics is explicit about it for the medium-voltage-tapped case: because the Econ Mode transformer's primary sits between one phase and ground, leakage-current protection at the medium-voltage substation at the point of interconnection has to be configured to detect a fault in that transformer, since Econ Mode operation opens the switchgear and leaves it as the only current path into the inverter.

That is the general principle in miniature. A small transformer hung off the medium-voltage system is a fault path on a bus whose ground protection was set for feeders, so the earthing and protection of the auxiliary system come out of the grounding-system and protection studies rather than off a datasheet.

What is alive when the plant is not

An auxiliary transformer fed from the block's own AC bus is dead whenever that bus is dead, which makes availability a wiring question rather than an equipment question.

Power Electronics' Econ Mode is the deliberate version of that outage: to shed the no-load losses of the main medium-voltage transformer during long periods of inactivity, the control opens the motorised medium-voltage switch after a ventilation cool-down delay the note gives as typically 30 to 60 minutes, and the internal auxiliary transformer, connected to the low-voltage side of that MV step-up transformer, is de-energised with it.

Power Electronics states plainly what remains in that condition — control and communication functions only, with no reactive power at night, no internal heating and nothing from the internal auxiliary supply.

The answer is layering, not a bigger transformer. Power Electronics' GEN3 auxiliary application note describes three possible supplies for communications, control and switching duty, rectified onto a common capacitor board and discriminated by bus voltage in a fixed priority: the auxiliary services transformer first, then a client-supplied external supply, then the DC side — PV or BESS.

The Econ Mode note puts figures on the discrimination, roughly 300 Vdc from the battery against 230 × √2 ≈ 326 Vdc from the auxiliary transformer, so the transformer wins while it is energised and the battery takes over when it is not.

The capacitor board then delivers 120 Vdc for operating the equipment — DC switch-disconnectors, AC circuit breaker, medium-voltage switchgear, filter contactors — and 24 Vdc for the control boards. Crucially, the DC feed is taken between the busbar and the DC switch-disconnectors, so it survives their opening and, in Power Electronics' words, holds as long as there is energy in the batteries. Note what that path carries: controls, communications and switching, not the thermal plant.

Some of this is a grid-code obligation and it is jurisdiction-specific. Power Electronics' German compliance note cites VDE-AR-N 4110 clause 11.4.21 as requiring generating units to have an independent power source able to supply the equipment's auxiliary loads for five seconds on loss of mains — a German medium-voltage connection rule, not a number to quote in any other market.

For a longer ride, Power Electronics offers a 1 kVA UPS whose datasheet quotes 12 minutes autonomy at 75 % of rated load and power factor 0.7 on sealed lead-acid cells with a stated expected life of three to five years; in application Power Electronics quotes 10 minutes on a Twin Skid and 20 minutes on an MV Skid, sized against a load list of communications switch, inverter control and switchgear operation.

Black start raises the requirement a level: SMA makes a continuous or UPS supply for the inverter's critical loads a required option for a grid-forming black-start application, and names a 2.5 to 4 kVA transformer for it. A plant that intends to restart itself has to have decided, at order time, what feeds the machine that does the restarting.

How it shows up in specs, studies and contracts

Large plants draw the auxiliary system on its own sheet, and four questions settle the design on that sheet before anything is ordered. Which derivation was bought — block AC bus, medium-voltage tap or outside service? What are the secondary voltage and the vector group, and is the neutral brought out? Is the rating continuous or duty-cycled, and at what ambient? And where is the single neutral-to-earth bond? All four are cheap to change on a drawing and expensive to change in steel, and the last two are the ones most often left to whoever wires the board.

If station service comes from outside, somebody signs for it. An outside utility feed means a service agreement with its own metering point and standing charge, an owner obligation that has to appear in the operating budget rather than as an assumption.

It also has to be live earlier than most people expect, because an internally derived auxiliary supply cannot energise itself: first energisation means backfeeding through the medium-voltage system, so the sequence belongs in the commissioning plan and in the switching request to the utility. The related question of which meter ends up seeing the auxiliary consumption is the measurement-boundary and auxiliary-load territory, but the wiring decision that determines the answer is made here.

At commissioning, two records are worth insisting on. The first is per-phase current at the auxiliary transformer under representative load — thermal management running, not the quiet site of an acceptance day — because rebalancing single-phase circuits across the phases costs minutes before handover and an outage request afterwards; the phase-unbalance entry explains why.

The second is ride-through and autonomy stated in minutes together with the load list they were measured against. A UPS autonomy figure sized for a communications switch and a few contactors says nothing about how long the cells stay in their temperature window, and those are two different questions asked of the same supply.