One Central Transformer or Distributed Transformers? A Factory Hall Decision Guide

Executive answer. One central transformer can reduce the number of medium-voltage nodes, but it can create long, high-current low-voltage feeders to separated production halls. Distributed transformers move medium voltage closer to load centres and can reduce LV voltage-drop and loss exposure, but they add MV switchgear, protection zones, rooms, civil interfaces and operating duties. Select the architecture that passes a documented comparison of coincident P/Q demand, route geometry, voltage drop, losses, fault levels, continuity, expansion, fire/civil access and maintainability—not the option with the fewest symbols on the single-line diagram.

This guide is for an FDI factory owner preparing a concept design or RFQ in Vietnam. Every number below is hypothetical and bounded. It is not a QuangAnhcons project record, transformer selection, utility approval, price claim or statement of legal compliance. Final design must use the actual site, manufacturer data, applicable Vietnamese requirements and the electricity supplier’s approved connection conditions.

Define “central” and “distributed” on the real site

A central arrangement usually places one transformer or transformer group beside a main electrical room, then distributes at LV through busduct or parallel cables. A distributed arrangement extends MV around the campus and places a transformer and local LV board near each major hall. A hybrid can serve compact loads centrally while giving a remote high-demand hall its own transformer.

Compare the same service requirement. If the central option has two transformers and a normally open tie while the distributed option has three single-ended units, location, source redundancy and maintenance states have all changed. Define acceptable interruption time, essential loads, permitted switching states and the expansion horizon before comparing cost.

The QuangAnhcons guide on two transformers with a normally open bus tie addresses surviving load after one transformer is lost. This article owns a different decision: where transformation should occur across geographically separated halls. If resilience matters, run both studies.

Build a time-aligned electrical and geographic model

Use coincident real power P, reactive power Q and apparent power S for each hall, not connected motor kilowatts alone. For a balanced three-phase screen, S = P/PF, Q = P × tan(acos(PF)) and I = S/(√3 × VLL). The PF must describe the same interval as P. Harmonics, cycling, motor starting and converter behaviour need additional checks.

Assume three fictional halls at a common peak: A uses 760 kW at PF 0.92; B, 560 kW at 0.90; and C, 410 kW at 0.88. Their apparent demands are about 826.1, 622.2 and 465.9 kVA. Reactive powers are 323.8, 271.2 and 221.3 kVAr. Vector addition gives P = 1,730 kW, Q = 816.3 kVAr and S = √(1,730² + 816.3²) ≈ 1,912.9 kVA, aggregate PF about 0.904.

The total current is only an equivalent at 400 V. A central design may divide it among sections; a distributed design may transform each hall locally. Do not select transformer ratings from this table alone. Apply measured or justified diversity, growth, ambient and altitude conditions, harmonic heating, cooling mode, overload policy and actual OEM guarantees.

Add coordinates and constraints: horizontal length, vertical rises, permissible corridors, fire compartments, road crossings, flood level, duct/soil condition, room space and equipment replacement access. A load schedule without route geometry makes a distant hall look electrically adjacent.

Screen current-distance, voltage drop and losses

For a fictional remote hall using 600 kW at PF 0.90, balanced current at 400 V is 600,000/(√3 × 400 × 0.90) ≈ 962 A. At 22 kV, the current scale is about 17.5 A, a ratio near 55:1. This does not select a cable: MV and LV insulation, conductor size, installation and fault duty differ. It explains why carrying MV closer to a remote load can be worth studying.

Assume a complete LV route with per-phase operating-temperature resistance R = 0.012 Ω and reactance X = 0.006 Ω. These are fictional aggregate impedances. With sinφ ≈ 0.436, line-to-line drop is √3 × I × (R cosφ + X sinφ) ≈ 22.36 V, or 5.59% of 400 V. Resistive loss is 3I²R ≈ 33.3 kW.

If a local-transformer option shortens the comparable LV segment so R and X are one-tenth of those illustrative values, the LV screen becomes about 0.56% and 3.33 kW. A 30 kW difference sustained for 4,000 hours is 120 MWh. That is energy, not cost: MV cable, transformers, switchgear and their losses are excluded, and no tariff or operating profile is assumed.

Detailed design uses the actual cable or busduct configuration, current sharing, temperature, harmonics, transformer taps and source voltage. IEC 60364-5-52:2009 with Amendment 1:2024 covers wiring systems within its scope; its catalogue identity does not create a project-specific acceptance threshold for this example.

Compare transformer losses using guaranteed data and a common duty profile. For each energized unit, model no-load loss P0 × hours plus load loss at actual load fraction and temperature. Do not assume three lightly loaded units are automatically better or worse than one larger unit. Identify whether an idle unit can be de-energized without compromising continuity.

Compare four viable architectures

Central transformer group with LV trunks

This can suit compact campuses with short routes and a strong main room. It concentrates MV interfaces and may simplify spares. Risks include high-current trunks, common-room events, voltage-drop sensitivity and difficult later tie-ins. One transformer remains one transformation point unless a surviving source is genuinely provided and studied.

Local transformer for each major hall

Local transformation shortens LV routes and can make expansion modular. It adds MV switchgear, cables, earthing interfaces, fire/civil provisions, alarms and maintenance locations. Units sharing one upstream MV source are not automatically redundant.

MV ring or loop with a normally open point

A ring may isolate a faulty section and resupply healthy sections, subject to ratings, protection, interlocking and operating rules. It requires trained operators. Do not call it self-healing unless the actual automation has been designed and tested.

Hybrid arrangement

A central substation can serve nearby utilities and small halls while a remote large hall gets a dedicated transformer. This often reflects geography best, but it creates interfaces between protection, metering, earthing, SCADA and load shedding.

Protection, earthing and continuity can reverse the preference

Calculate maximum and minimum fault currents for every permitted normal, contingency and maintenance state using actual utility source data, transformer impedance and taps, conductor impedance, motor contribution and converter behaviour. A central unit close to a main LV board may raise LV fault duty; distributed units create more MV protection zones. Continuous-current rating alone does not prove breaker adequacy.

Define selectivity and clearing boundaries. Coordinate upstream MV devices with transformer protection and LV incomers; confirm CT ratios/classes, relay functions, breaker times and communications. A central bus with ties needs each source/tie state checked and unsafe paralleling prevented. Use the factory electrical interface register to assign signals and settings across packages.

Earthing is a site system. Record utility conditions, transformer neutral treatment, bonding, building grids, screens and transferred-potential risks. Distributed MV routes add screen-termination and bonding decisions. Qualified designers must reconcile touch/step safety and protection using actual soil and fault data.

Continuity must be stated. Geographic separation can reduce common exposure but does not give N−1. If a process must survive transformer loss, define the surviving source, transfer path, loading, interruption time and tested sequence. The substation interconnection documentation guide helps identify upstream ownership before the separate continuity study.

Civil, fire, HSE and maintainability are design inputs

Check transport from gate to final position, floor loading, lifting and replacement routes, weather, drainage/flood level, clearances, ventilation, noise, fire compartments, escape paths and access control. Distributed rooms can improve routing but place hazards nearer production; outdoor kiosks add heat, corrosion, drainage and security duties.

No generic article can set safe clearances, fire ratings, lifting capacity or ventilation openings. Use actual equipment drawings, loss data and applicable project requirements. IEC 61936-1:2021 provides common rules for installations above 1 kV AC within its scope; it does not replace Vietnamese legal review, fire requirements or OEM instructions.

Plan work under approved isolation and lockout/tagout. Separate excavation, MV pulling, placement, termination, testing and energization. Identify buried services, lifting exclusions, approach controls, temporary supply, backfeeds and switching authority. The factory retrofit isolation and tie-in guide expands this outage-planning discipline.

Make the BOQ, RFQ and test plan auditable

Ask bidders to use the same load basis, route revision, operating states and battery limits. Require deviations instead of hiding assumptions in a lump sum. Keep supply, civil work, installation, studies, testing, utility interface and owner items visible.

A practical ITP includes design-basis approval; source-data receipt; load, drop, loss and fault-study review; approved SLD/layout; OEM document release; civil readiness; FAT; receipt inspection; installation checks; cable/earthing tests; protection secondary injection and trip-path checks; interlock/SCADA tests; controlled energization; load observation; suitable-condition thermography; training; and as-built handover.

FAT verifies the ordered configuration and package logic. SAT verifies installation wiring, settings, interlocks, alarms and communications. Neither proves complete factory performance unless integrated criteria say so. Do not create live faults; use approved test methods. Energize under an authorized switching programme with abort criteria and record voltage, current, PF, alarms and actual load distribution.

For a concept comparison, provide the latest SLD, hall load model, route drawings, future phases, outage constraints and responsibility matrix. The QuangAnhcons electrical construction overview can frame a discussion, but any proposal must use the real site and agreed scope. No price, percentage saving or preferred transformer count is claimed.

Frequently asked questions

Is distributed transformation always more efficient?

No. It can reduce LV route losses but adds transformer no-load loss, MV cable loss and switchgear. Compare guaranteed data under one duty profile. The 120 MWh example covers only two fictional LV segments.

Can one large transformer be cheaper than three smaller units?

Only current, normalized quotations can answer. Include MV/LV gear, routes, civil rooms, protection, losses, spares, maintenance, outages and future phases.

Does one transformer per hall provide N−1?

No. A hall with one transformer can still lose supply. N−1 requires a defined surviving source, transfer path, loading and tested sequence.

What should the owner send first?

Send a redacted layout, SLD, interval P/Q data by hall, major motor/converter schedule, future phases, utility interface, route constraints, outage policy and essential-load list. Do not send credentials or confidential pricing for publication.

Sources, applicability and editorial limits

• IEC 60076-1:2011: official scope for general power-transformer requirements.
• IEC 60076-7:2018: official scope for loading mineral-oil transformers, operating temperatures and thermal ageing.
• IEC 60076-8:1997: application-guide scope including connections, fault current, parallel operation and voltage change.
• IEC 60364-5-52:2009+AMD1:2024 CSV: wiring-system scope.
• IEC 61936-1:2021: common rules for power installations above 1 kV AC within scope.
• IEC 61439-1:2020: general rules for LV assemblies, used with the relevant product part and OEM verification.
• Schneider Electric Electrical Installation Guide: public distribution-design guidance, not project approval or endorsement.

Official catalogue pages and manufacturer guidance were checked on 23 October 2026. Licensed IEC clauses were not accessed or copied. Arithmetic was independently calculated from stated fictional inputs. Applicable TCVN/QCVN, Vietnamese law, fire requirements and electricity-supplier rules must be identified for the actual location; no clause or compliance result is invented.

Engineering enquiry. Send a redacted layout, load schedule and SLD to projects@quanganhcons.com for a project-specific scope discussion. No rating, price, completion date, approval or performance outcome is promised by this educational example.