Airport Transformer: How to Size the MV Distribution for a Terminal and an Airfield, Why 400 Hz Ground Power and Series Runway Lighting Change the Spec, and Where Dry-Type Beats Oil

The answer first: an airport transformer is sized from an area load list with airport-specific diversity, not from a watts-per-square-metre benchmark, and two of its loads are not ordinary distribution loads at all. The terminal behaves like a large commercial building with a very high base load and a short summer peak, so its substation is sized on demand factor by area and then fixed by N+1 firm capacity. The airfield behaves like a critical infrastructure network: runway lighting runs on 6.6 A series circuits where every lamp has its own isolation transformer, and aircraft ground power runs at 115/200 V, 400 Hz, which makes it a frequency-converter product rather than a 50 Hz transformer with a different nameplate. Get those three load families separated in the load list — terminal, airfield, and 400 Hz — and the kVA, the impedance, the K-factor and the dry-type-versus-oil decision all follow from the calculation instead of from habit.

Key takeaways

• Size the terminal substation from a load list by area with demand factors of roughly 0.5–0.9, then add 10–15% design margin and apply N+1 firm capacity. A 120,000 m² terminal typically lands near 11–12 MVA of demand at 0.93 power factor, which is two 12.5 MVA units rather than one large unit.
• 400 Hz ground power is a separate electrical system. Core loss rises with frequency and eddy loss with the square of frequency, so a 400 Hz transformer uses thin laminations, Class H insulation and forced cooling, and it arrives as part of a 90–180 kVA converter, not as a utility transformer.
• Runway and taxiway lighting is a constant-current series circuit at 6.6 A, fed by thyristor constant current regulators and protected per fixture by series isolation transformers. A failed lamp must not open the loop, and the open-circuit voltage at a broken circuit is what the isolation transformer limits.
• Harmonics at an airport come from three different places — 6-pulse drives in baggage handling and HVAC, phase-controlled regulators at low brightness steps, and IT switch-mode power supplies — and they add at different times of day. Specify a K-factor or a harmonic study, not a generic 5% impedance.
• Dry-type cast-resin units dominate inside terminals and concourses because there is no oil to burn and no bund to build; oil-immersed units still win at the 132/33/11 kV primary substation and at remote airfield substations where the ambient is high and the enclosure is outdoor.
• The standby generator is usually sized by transformer inrush, not by load. Energizing a 1,250 kVA transformer from a 2,000 kVA generator can dip the bus by more than 50%, which is why airport black-start schemes sequence transformer energization instead of closing everything at once.

What makes an airport electrical load different from an industrial plant?

An industrial plant has one dominant process load and a load profile that follows production. An airport has four load families running on four different clocks, and only one of them looks like a building. The terminal follows passenger flow and weather, the baggage system follows the flight schedule with a peak that starts three hours before departure, the airfield is almost constant and almost entirely critical, and the 400 Hz system is idle most of the day and then draws its full rating for twenty minutes at a time.

That mixture produces a load duration curve with a high base and a narrow peak. A terminal that draws 6 MW at 03:00 and 11 MW at 14:00 on the hottest day has a load factor near 0.55, but the 03:00 load is not process load that can be shed — it is security screening, IT, apron lighting, airfield lighting and the minimum ventilation needed for the building. Load shedding at an airport is therefore limited to concessions, some HVAC and non-critical signage, which means the transformer rating cannot be reduced on the assumption that peak load can be managed.

The table also explains why a single unit is a poor choice at an airport even when the arithmetic allows it. The terminal, the airfield and the 400 Hz system have different outage tolerances, different harmonic spectra and different maintenance windows. Splitting them across separate transformers costs one more circuit breaker and a little more floor space, and it buys the ability to work on one system without darkening another.

How much power does a terminal draw per square metre?

Published benchmarks for terminal load density range from about 40 to 100 W/m² of demand, and the spread is not a measurement error — it is climate, glazing ratio, chilled-water versus DX cooling, and whether the terminal includes a train station. The numbers below are the connected-load densities QDTB uses as a starting point for a temperate-climate international terminal with central chilled water; they are industry experience values, not a standard, and they should be replaced by the actual load list as soon as the MEP design exists.

Note on the difference between connected and demand load: the connected total at a terminal is often 110–130 W/m² while the actual demand is 55–75 W/m². Quoting a transformer on connected load doubles the price; quoting it on a benchmark without checking the local climate under-sizes it. The only defensible method is a load list with a demand factor per group, reconciled against the utility's measured maximum demand for a comparable building.

How do you size a terminal substation? A worked 120,000 m² example

Take a 120,000 m² international terminal with central chilled water, twelve wide-body-capable stands and a basement baggage system. The load list below is deliberately modest: it assumes LED lighting, VFD-driven chillers and fans, and no electric heating. The demand column is what the transformers have to carry on a 38°C design day.

At a corrected power factor of 0.93 the demand is 10,884 kVA. Add 10% for load growth and for the difference between a design-day calculation and a real summer, and the requirement is 11,972 kVA. That number, not the connected load, is what the transformer has to deliver continuously at an ambient of 40°C. Two 12.5 MVA units give 12.5 MVA firm with one unit out, which covers the requirement with 4% headroom; a single 16 MVA unit would cover it with less headroom and no redundancy.

The loading of 44% at normal demand looks wasteful until the peak and the contingency are counted. With one unit out, the remaining unit carries 87% of design demand on the hottest day, which is inside the continuous rating but leaves nothing for a second contingency. That is why airports with a 2N philosophy for IT and airfield loads often go to three units and a bus tie rather than two: the third unit is not for growth, it is for the hour when a chiller plant trips during a heat wave and the remaining transformer has to carry the whole terminal.

Why is 400 Hz ground power a different transformer, not a different nameplate?

Aircraft on the stand do not run their engines and do not run their APU if the airport can avoid it. They take 115/200 V, 400 Hz from a ground power unit through a cable to the aircraft receptacle, and that 400 Hz is generated locally by a static converter or a motor-generator set. The transformer inside that unit is a 400 Hz transformer, and the physics is not a rescaling of a 50 Hz design.

The transformer EMF equation is E = 4.44 × f × N × Φ_m. For the same voltage and the same peak flux density, increasing frequency from 50 Hz to 400 Hz reduces the required core area by a factor of eight, which is why a 400 Hz transformer of a given kVA is dramatically smaller and lighter. The savings are limited by two loss mechanisms. Hysteresis loss per unit volume rises with frequency, and eddy-current loss rises with the square of frequency and with the square of lamination thickness, so the laminations have to become much thinner — typically 0.1–0.15 mm grain-oriented steel instead of the 0.23–0.35 mm used at 50/60 Hz — and the core is often wound from strip rather than stacked.

Note on the 400 Hz supply: a 90 kVA ground power unit at 0.8 power factor delivers 72 kW to the aircraft, and a wide-body on a long turnaround can draw most of that for air conditioning and galley loads while the aircraft is being serviced. Twelve stands with a 0.35 diversity factor give about 378 kVA of demand, which is small against the terminal but is concentrated on one or two 1 MVA transformers at the pier. If those transformers also feed the jet bridges and the pier lighting, the 400 Hz converters become the dominant harmonic source on that bus, not the largest load.

The practical consequence for a buyer is that the 400 Hz equipment is normally procured with the ground support equipment package, not with the MV transformers. What belongs in the transformer specification is the interface: a dedicated 0.4 kV board per pier, a defined fault level, and a K-factor or harmonic study that accounts for the converter input current. Buying a 400 Hz transformer from a distribution transformer factory and expecting a standard 50 Hz unit with a 400 Hz nameplate is a common and expensive misunderstanding — the unit will run hot, noisy and short-lived.

What is a series runway lighting circuit, and why does every lamp need its own transformer?

Airfield ground lighting is not wired like a building. Runway edge lights, taxiway centreline lights, approach lights and threshold lights are connected in series on a constant-current loop fed by a constant current regulator, usually at 6.6 A. The reason is dimensional consistency: a lamp at the far end of a 4 km approach must have the same brightness as the first lamp, and in a series loop every lamp carries identical current regardless of cable length or lamp age.

The problem with a pure series loop is that one failed lamp opens the circuit and the whole runway goes dark. The solution is a series isolation transformer at every fixture: the primary is connected in series with the loop, the secondary feeds one lamp. The transformer passes the loop current to the lamp, and if the lamp fails or is removed, the transformer presents a high impedance that keeps the loop closed while limiting the open-circuit voltage at the empty socket to a safe level. That is why a runway with 200 lights has 200 small single-phase transformers buried in the base cans.

LED retrofit changes the numbers but not the architecture. A 45 W LED edge light draws less than a 200 W halogen lamp, so the loop voltage and the regulator kVA fall sharply. The trap is that many constant current regulators are designed for a minimum load, and a circuit that has been reduced to 30% of its original load may fall below the regulator's stable operating range. When an airport replaces lamps with LED, the regulator settings, the series transformer ratio and the open-circuit protection all have to be checked together rather than swapped fixture by fixture.

Note on standards for airfield lighting: series transformers and regulators are usually specified under IEC 61823 for AGL series transformers, the IEC 61820 series for aeronautical ground lighting installations, and in the United States under FAA advisory circulars, with the constant current regulator and the isolation transformer carrying FAA equipment designations L-828 and L-830 respectively. Contract documents normally require the manufacturer's type test evidence against the applicable circular, not just a CE declaration.

How much of the airfield load is actually critical?

Almost all of it, which is the point. The airfield draws one or two percent of the airport's power and causes most of the outage consequences. A terminal that loses a baggage carousel for ten minutes delays bags; a runway that loses edge lights closes the runway. The redundancy and the transfer scheme therefore have to be designed per system, not per megawatt.

This table is the reason an airport often has more transformer capacity in small units than the load list suggests. The airfield substation may carry only 500 kVA, but it needs two transformers, two feeders and a transfer scheme, because the cost of a runway closure is measured in diversions, not in kWh. The same logic applies to the 400 Hz system: a failed converter does not delay a flight by much, but a converter that trips the pier board because nobody studied its harmonics can delay several.

Where does dry-type win at an airport, and where does oil still win?

Fire is the deciding factor indoors, and ambient temperature is the deciding factor outdoors. A cast-resin dry-type transformer contains no oil, produces no bund, and can be installed in a terminal basement, a concourse plant room or a pier without a fire-rated cell. An oil-immersed transformer at the same rating needs a fire compartment, an oil containment bund, and often a deluge system, which costs more than the transformer itself when it has to be built inside an operating terminal.

There is a middle case that is worth knowing: a dry-type transformer with an IP54 enclosure installed outdoors at a pier. It works, but it derates faster than an oil unit in the same ambient, and the enclosure traps heat. If the pier substation is outdoors and the ambient reaches 45°C, a 2,500 kVA cast-resin unit may only be good for about 2,100 kVA after temperature and altitude correction, while the oil unit keeps its nameplate with ONAF. The comparison in the oil-immersed versus dry-type selection guide covers the full decision matrix; at an airport the insurance and fire strategy usually settles it before the thermal calculation does.

How do airport harmonics differ from a factory?

A factory usually has one dominant harmonic source — a bank of drives, an arc furnace, a rectifier — and the spectrum is predictable. An airport has three sources with different signatures and different schedules, and the worst case is not the sum of their peaks. Baggage handling drives peak during the departure bank, chillers peak in the afternoon, and the airfield regulators run at low brightness steps in the evening, which is when their distortion is worst.

The constant current regulator deserves its own line because its distortion is inversely related to brightness. A phase-controlled regulator reduces lamp current by delaying the thyristor firing angle, which chops the waveform and produces a poor displacement factor at low steps. At 10% brightness a 25 kVA regulator can draw 30–50% current distortion with a displacement factor near 0.4. The airfield is the one place where the worst harmonic case happens at night, when the terminal load is lowest and the airfield transformer looks lightly loaded — and a lightly loaded transformer with a distorted current is exactly the case that IEEE C57.110 harmonic derating is written for.

Two practical consequences follow. First, do not specify a generic 5% impedance and assume the harmonics are somebody else's problem: ask for the harmonic spectrum at the point of common coupling, and if the study has not been done, specify a K-factor of at least K-13 for terminal boards with 6-pulse drives and K-20 for airfield boards dominated by regulators. Second, check the neutral. Third-harmonic current from LED drivers and IT supplies adds in the neutral of a four-wire system and can exceed the phase current, which is why the neutral bar and the transformer's neutral terminal have to be rated for 100% or 200% of phase current. The harmonics guide covers the overheating mechanism and the mitigation options; the K-factor selection guide covers the rating method.

What impedance should you specify on an airport 11 kV network?

Impedance is chosen to keep the low-voltage switchgear inside its fault rating without making the voltage dip at motor start unacceptable. At an airport the two constraints pull in opposite directions more than usual, because the terminal has large chiller motors and the airfield has long cables with low fault levels at the far end.

The calculation is worth doing explicitly because the answer is not intuitive: the LV fault current from a transformer is I_sc = S / (√3 × U × u_k), so a 1,250 kVA unit at 6% impedance delivers about 30 kA at 400 V, and doubling the rating to 2,500 kVA doubles the fault current to 60 kA unless the impedance is raised. That is why large terminal substations sometimes specify 8–10% impedance: not to save money on the transformer, but to keep the downstream board at 50 kA instead of 65 kA. The trade-off is a larger voltage dip at chiller start and a slightly higher load loss. The short-circuit current calculation guide walks through the same formula with the source impedance included.

How does N+1 redundancy change the transformer rating?

N+1 does not mean buying one extra transformer of any size. It means that with the largest single unit out of service, the remaining units can still carry the design demand. That definition forces the rating up: two units at 50% each give no firm capacity at all if either fails, so the same total installed capacity has to be split into units that are each large enough to carry the load alone, or into three units where two can carry it.

There is a second-order effect that surprises designers: N+1 with two units means each transformer normally runs at 44% load, and a transformer at 44% load has a lower efficiency and a worse power factor than the same unit at 70%. The no-load loss is constant, so the loss per kVA delivered rises. At 8,000 operating hours a year the extra energy can be material, which is why loss capitalisation is worth doing even when redundancy drives the rating up. The total cost of ownership guide shows the arithmetic for a unit that spends its life at part load.

Why does the standby generator have to be sized for transformer inrush?

Airport standby generators are usually specified for life safety, security and airfield lighting, not for the whole terminal. The mistake is to size the generator to the connected load and then discover that it cannot energize the transformer that feeds that load. A transformer draws a magnetizing inrush current of roughly 5–12 times its rated current for the first few cycles when it is energized, and the generator sees that inrush as a short circuit through its subtransient reactance.

Take a 1,250 kVA transformer at 400 V with 6% impedance. Its full-load current is 1,804 A, and an inrush of 8 times rated is 14,400 A. A 2,000 kVA generator with a subtransient reactance of 0.15 per unit has a fault current of about 19,200 A at 400 V. The inrush is 75% of the generator's fault current, so the voltage dip on the generator bus is of the same order — far beyond the 15–20% that airport loads tolerate. The generator does not have to be 8 times the transformer rating, but it does have to be large enough that the dip stays inside the allowable window, or the scheme has to energize the transformer in a controlled sequence.

Note on black-start sequencing: the practical solutions are (1) size the generator at three to four times the largest transformer it must energize, (2) energize the largest transformer first with all load disconnected and accept a controlled dip, (3) use a soft-start or controlled-switching device on the transformer breaker, or (4) keep the transformer energized from the utility and transfer only the load. Option 4 is the cheapest and is why many airports have a generator bus that is separate from the transformer bus. Whichever route is chosen, the inrush case belongs in the generator sizing study, not in the transformer datasheet. The inrush current guide covers the physics and the mitigation methods.

How do coastal, hot and high-altitude airports change the specification?

Airports are frequently built on flat coastal land, which is exactly where salt-laden air and high humidity meet a transformer that was painted for an inland industrial estate. The corrosion category and the ambient temperature together change the rating, the enclosure and the maintenance interval.

Two of these deserve a specification line rather than a note. The first is the ambient temperature: a transformer rated 40°C maximum ambient and installed in a plant room that reaches 48°C in summer is overloaded at nameplate, and the manufacturer will point to the rating plate when it fails. State the design ambient and the enclosure type, and require the derating calculation in the offer. The second is noise: a 2,500 kVA cast-resin unit at 62 dB(A) at 1 m is quiet in a factory but audible through a concrete slab in a terminal at 03:00, and the sound level has to be a guaranteed value with a test method, not an approximate figure. The noise level guide gives the typical dB(A) by rating and the specifying language.

Does an airport need 690 V, or is 400 V enough?

Most airports stay at 400 V for the low-voltage distribution and take the large chillers at 11 kV directly. The 690 V level that dominates data centres and paper machines appears at airports only where a very large continuous drive load is concentrated in one plant room — a district cooling plant serving several terminals, for example. At 690 V a 4 MVA load draws 3,347 A instead of 5,774 A at 400 V, which halves the cable and switchgear count. For a terminal with 5 MW of chillers spread across three plant rooms, the extra voltage level is not worth the spares and the training; for a single 20 MW cooling plant, it usually is.

The choice interacts with the transformer because a 690 V secondary changes the fault level and the impedance. A 4 MVA transformer at 690 V with 6% impedance delivers about 55.8 kA, which needs 63 kA switchgear; the same transformer at 400 V delivers 96.2 kA, which is beyond standard LV gear. That is the real reason 690 V exists, and it is worth checking before a design mixes 400 V terminal boards with a 690 V plant room. The distribution system sizing guide covers the voltage-level decision with the same fault-current arithmetic.

What should an airport transformer datasheet contain?

An airport enquiry that arrives with a one-line diagram and a kVA number will produce a quotation that is comparable on price and not comparable on scope. The datasheet below is the minimum that lets two suppliers quote the same transformer, and each line has a reason that comes from the airport duty rather than from a generic template.

The vector group line is not boilerplate. An airport often has an existing 11 kV ring with Yyn0 units and a new substation with Dyn11 units, and paralleling them without a vector group check produces a circulating current that trips protection within seconds. The vector group guide explains the symbol and the paralleling conditions, and the parallel operation guide covers the five conditions in detail.

What are the six most common specification mistakes at airports?

• Sizing from a benchmark instead of a load list. A watts-per-square-metre figure produces a kVA that is either 40% too high or 20% too low, and neither survives the first summer. The load list by area, with demand factors and a design ambient, is the only defensible basis.
• Treating 400 Hz as a 50 Hz transformer with a different frequency. The core, the laminations, the insulation class and the cooling are all different, and the unit is part of a converter package. Specify the interface, procure the converter as a system.
• Putting an oil-immersed transformer inside the terminal to save money. The fire compartment, the bund and the deluge system cost more than the price difference, and the insurance approval can delay the project by months.
• Ignoring the constant current regulator in the harmonic study. At low brightness steps the regulator has a displacement factor near 0.4 and a current distortion above 30%, and the airfield transformer sees its worst case at night when it is lightly loaded.
• Sizing the standby generator to the load rather than to the inrush. A generator that can carry the load may still not be able to energize the transformer that feeds it, and the failure appears only during a real black start.
• Forgetting the neutral and the corrosion class. Third-harmonic current adds in the neutral, and a C5-M coastal site destroys a standard paint system in three to five years. Both are cheap to specify at the enquiry stage and expensive to retrofit.

The common thread is that all six are interface decisions rather than transformer decisions. The transformer factory can build almost anything; what it cannot do is guess the load list, the harmonic spectrum, the ambient or the fire strategy. An enquiry that states those four things gets a quotation that can be compared; an enquiry that states only a kVA gets a price and a hope.

How is an airport transformer ordered, tested and delivered?

An airport project usually runs on a fixed opening date, and the transformer is on the critical path because it has the longest manufacturing lead time of any electrical item. A 12.5 MVA 11/0.4 kV cast-resin unit is typically 10–14 weeks from drawing approval, and an oil-immersed 40 MVA 132/11 kV unit with OLTC is 20–28 weeks. The dates that matter are drawing approval, type test evidence and factory acceptance test, not the day the order is placed.

Two documents decide most disputes later. The first is the loss guarantee: a 12.5 MVA unit with 1.5 kW more no-load loss than quoted costs about 12,000 kWh a year at 8,000 hours, which is small against the contract price but compounds over twenty years, and the guarantee should be a tested value with a penalty clause rather than a calculated one. The second is the sound level guarantee, because a transformer that measures 3 dB above the quoted figure cannot be fixed on site without an acoustic enclosure.

For a coastal airport, add the corrosion inspection to the FAT: coating thickness readings, salt-spray test certificates for the paint system, and stainless-steel hardware verification. A C5-M coating system that is applied over a poorly prepared surface fails at the first monsoon, and the repainting has to be done in a plant room that was not designed for it. The pre-shipment inspection checklist covers the packing and documentation items that airports most often discover missing when the crate is opened on site.

How does the transformer choice affect the airport's energy bill?

An airport runs 8,000 hours a year or more, so no-load loss is not a rounding error. A 2,500 kVA dry-type unit with 3.2 kW of no-load loss and 18 kW of load loss at full load, operated at 45% average load, dissipates about 3.2 + 18 × 0.45² = 6.85 kW continuously. Over 8,000 hours that is 54,800 kWh per transformer per year. At USD 0.10/kWh it is USD 5,480, and over twenty years with a 6% discount rate the present value is roughly USD 63,000 — which is often more than the price difference between an ordinary and a high-efficiency unit.

The calculation changes the specification in two ways. First, it justifies a low-loss core even when the utility does not require it, because the airport pays for the loss, not the manufacturer. Second, it argues against oversizing for a vague future load: a transformer at 25% load has a poor loss per kVA delivered, and N+1 schemes that leave every unit at 44% are already paying that penalty. The TCO guide and the efficiency standards comparison give the method and the regulatory minimums; the airport-specific part is the 8,000-hour operating year and the fact that most of the load is present around the clock.

If you are preparing an enquiry for a terminal or airfield substation, the fastest route to a comparable offer is to send the area load list, the single-line diagram, the harmonic study or the drive schedule, the design ambient and the corrosion category together. Our engineering team can turn that into a sized transformer specification, a harmonic derating calculation and an indicative budget in one pass. You can also start from the power system calculators to check the fault level, the voltage dip and the loss capitalisation before you approach any supplier — and if the question is how the airport's 50 Hz or 60 Hz supply changes the transformer, the 50 Hz versus 60 Hz guide covers the V/f rule and the derating.

Sources / 资料来源

Source: IEC 60076-1 — Power transformers, general requirements.
Source: IEC 60076-2 — Temperature rise for liquid-immersed transformers.
Source: IEC 60076-3 — Insulation levels, dielectric tests and external clearances in air.
Source: IEC 60076-5 — Ability to withstand short circuit.
Source: IEC 60076-7 — Loading guide for oil-immersed power transformers.
Source: IEC 60076-10 — Determination of sound levels.
Source: IEC 60076-11 — Dry-type power transformers, including climatic, environmental and fire behaviour classes.
Source: IEC 60137 — Insulated bushings for alternating voltages above 1,000 V.
Source: IEC 60214 — Tap changers.
Source: IEC 61820 series — Electrical installations for aeronautical ground lighting at aerodromes.
Source: IEC 61823 — AGL series transformers for aeronautical ground lighting.
Source: FAA AC 150/5345-47 — Specification for series to series isolation transformers for airport lighting systems.
Source: FAA equipment designations L-828 (constant current regulator) and L-830 (series isolation transformer).
Source: IEEE C57.110-2018 — Establishing transformer capability when supplying non-sinusoidal load currents.
Source: IEEE 519-2014 — Harmonic control in electric power systems.
Source: IEEE C57.12.00 — General requirements for liquid-immersed distribution, power and regulating transformers.
Source: GB/T 6451-2015 — Technical parameters and requirements for three-phase oil-immersed power transformers.
Source: GB 20052-2020 — Minimum allowable values of energy efficiency and energy efficiency grades for power transformers.
Source: Commission Regulation (EU) No 548/2014 — Ecodesign requirements for small, medium and large power transformers.
Source: U.S. Department of Energy, 10 CFR Part 431 Subpart K — Distribution transformer energy conservation standards.
Source: ISO 12944-2 — Corrosivity categories for steel structures, category C5-M for coastal atmospheres.
Source: NFPA 70 (National Electrical Code) Article 450 — Transformers and transformer vaults.
Source: QDTB engineering experience — airport terminal area load lists, airfield series circuit sizing, 400 Hz ground power interface practice, and harmonic derating for constant current regulators. Values marked as industry experience are typical ranges from airport and infrastructure projects, not published standards.