33kV Transformer Tender Checklist: What Commonwealth-Country Engineers Must Verify Before Awarding a Contract

Introduction

The 33 kV voltage level is the backbone of sub-transmission and primary distribution in virtually every Commonwealth country — the United Kingdom, India, Pakistan, Bangladesh, Nigeria, Ghana, Kenya, Malaysia, Sri Lanka, and many island nations across the Caribbean and Pacific. A 33 kV transformer is not a trivial purchase: even the smallest 5 MVA unit represents a six-figure USD investment with a 35–40 year service life, and a specification error discovered at the Factory Acceptance Test costs weeks of delay and reputational damage.

I have reviewed tender specifications from a dozen Commonwealth jurisdictions, and the patterns are consistent: engineers copy-paste an old specification from a 1980s UK Electricity Board document, adjust the kVA, and issue it. The gaps are predictable — BIL not stated, OLTC vs. OCTC not decided, N+1 architecture not reflected in the tender, and the distinction between a main transformer and a distribution transformer lost in the write-up.

This article provides a structured tender evaluation checklist for 33 kV power and distribution transformers, organized so that both the specification writer and the bid evaluator can work from the same reference.

The 33 kV Landscape: Not All "33 kV" Is the Same

A 33 kV transformer in the UK (Distribution Network Operator, typically 33/11 kV or 33/6.6 kV, up to 60 MVA) is a very different machine from a 33 kV transformer in rural Nigeria (DISCO, typically 33/0.415 kV, perhaps 500 kVA to 5 MVA). Yet both carry the "33 kV" label. The key differentiator is the primary-side system voltage class.

IEC 60076-3 places 33 kV equipment in the Um = 36 kV class:

IEC Designation	Um (kV)	BIL (LI, kV peak)	Power-frequency withstand (kV rms)
36 kV class	36	170	70
36 kV class (reduced)	36	145	70

The standard BIL is 170 kV peak. The reduced BIL of 145 kV is available but, like the reduced BIL of 95 kV for 24 kV class, I advise against it in lightning-prone regions (most of the Commonwealth tropics).

The most common specification error I see: engineers write "33 kV, BIL 125 kV" — which is the insulation level for 24 kV class (22 kV equipment) — because they have a 22 kV transformer specification open in another tab. BIL 125 kV on a 33 kV transformer will fail dielectric type testing. A 33 kV (Um=36 kV) transformer needs at least BIL 145/170 kV.

Tender Specification Table – Fill and Issue

Parameter	Specification	Notes
Project name	_____	Include substation name
Rated power	_____ / _____ MVA	ONAN / ONAF if dual-rated
Number of units	_____	Include spare if N+1 architecture
HV rated voltage	33,000 V ± _____	Nominal 33 kV
LV rated voltage	_____ V	11,000 V / 6,600 V / 415 V common
Highest voltage (Um)	36 kV	HV side
Frequency	50 Hz	Commonwealth standard
Vector group	_____	Dyn11 for dist.; YNd11/YNyn0 for GSU
BIL (LI) – HV	170 kV peak	IEC 60076-3, Um=36
BIL (LI) – HV neutral	_____ kV peak	Depends on earthing
BIL (LI) – LV	_____ kV peak	Depends on LV Um
Power-frequency withstand – HV	70 kV rms	1 minute
Impedance voltage	_____ % at 75°C	6–12% depending on MVA and application
Cooling	_____	ONAN / ONAF / OFAF
Tapping type	_____	OFF-CIRCUIT (OCTC) / ON-LOAD (OLTC)
Tapping range	_____% in _____ steps	±5% to ±15% common
Insulating liquid	_____	Mineral oil / Natural ester / Synthetic ester
Winding material	HV _____, LV _____	Cu specified for ≥ 5 MVA
Corrosion class	_____	C3 / C4 / C5-M per ISO 12944
Sound level	≤ _____ dB(A)	IEC 60076-10

OLTC vs. OCTC: When Each Is Appropriate

The on-load vs. off-circuit tap changer decision for a 33 kV transformer is binary and expensive:

	Off-Circuit (OCTC)	On-Load (OLTC)
Cost adder	Baseline	+25–40% of transformer cost
Tap range	±2×2.5% or ±5%	±10% to ±20% in 1.25% steps
Maintenance	None beyond handle lubrication	Diverter switch oil change every 50k–100k operations; motor drive inspection annually
Control	Manual, de-energized	Automatic voltage regulator (AVR) or SCADA
Failure mode	None (passive device)	Mechanical wear, contact erosion, oil carbonisation

When OCTC is sufficient:

• The 33 kV source voltage is stable (grid-connected, short feeder from a bulk supply point).
• Voltage regulation for the secondary bus is managed by the upstream transformer's OLTC.
• The transformer is a backup or standby unit in an N+1 configuration.
• Load variation is predictable and seasonal, not rapid.

When OLTC is necessary:

• The transformer is the terminal unit at the end of a long 33 kV radial feeder with no upstream regulation.
• The secondary load is highly variable (industrial arc furnace, mine hoist, large motor starting).
• The transformer supplies a dedicated industrial customer with a voltage-quality contract.
• The substation is unmanned and SCADA-controlled voltage regulation is required.

> Tender blank: On-load tap changer: REQUIRED / NOT REQUIRED > > If required: tapping range ± _____%, step size _____%, control: LOCAL / REMOTE / SCADA

N+1 Substation Architecture and the Tender

Many Commonwealth utilities specify N+1 transformer configurations for primary substations — e.g., two 20 MVA, 33/11 kV transformers where the firm capacity is 20 MVA and one unit can cover the load if the other is out of service.

What the tender must address for N+1:

• Identical rating: Both transformers must have the same MVA and impedance. If Unit A is 10% impedance and Unit B is 8%, the paralleled load sharing will be unequal — Unit B carries 55% of load, potentially overloading while Unit A is at 90%. Specify: "MVA rating, impedance, and X/R ratio shall be identical within ±2% tolerance across all units."

• Vector group: For paralleled operation, both units must have the same vector group. Two Dyn1 units can parallel; a Dyn1 and a YNd1 cannot without a phase-angle mismatch that circulates current.

• Busbar configuration: The tender should reference the single-line diagram and state whether the transformers operate radially (one open bus coupler) or with a closed bus coupler (paralleling). Paralleling increases fault level and may require higher-rated switchgear — flag this.

• Cooling staging: If ONAN/ONAF dual-rated, confirm whether the firm capacity is based on ONAN or ONAF rating. Most utilities treat ONAN as firm and ONAF as emergency/cyclical overload. Write this explicitly.

• Cold standby vs. hot standby: A spare transformer stored on-site (cold standby) needs an oil preservation system, space heaters for moisture protection, and periodic insulation testing specified in the scope of supply.

> Tender blank: Parallel operation with existing/other contract transformer: YES / NO > > If YES: existing transformer MVA _____ , impedance _____ %, vector group _____

Main Transformer vs. Distribution Transformer – The Combination Trap

A common tender structure for a 33 kV substation bundles a primary (main) transformer (33/11 kV, e.g., 20 MVA) and one or more distribution transformers (11/0.415 kV, e.g., 1000 kVA) for station auxiliary supply into a single contract. This is efficient for procurement but creates specification risks:

Main transformer (33/11 kV):

• Power transformer class (IEC 60076, power transformer)
• BIL 170 kV, impedance 10–12%, OLTC typical
• Winding temperature indicator, Buchholz relay, pressure relief device mandatory
• DGA monitoring often specified
• Transport weight may require special logistics (20 MVA unit ≈ 35–45 tonnes oil-filled)

Distribution transformer (11/0.415 kV, station auxiliary):

• Distribution transformer class
• BIL 75 kV, impedance 4–5%, OCTC standard
• Basic accessories only (oil level, thermometer, breather)
• Transport weight: 2–5 tonnes

Do not apply the main transformer's BIL, impedance, or accessory schedule to the distribution transformer in the same tender package. Clearly separate the two specification tables and mark them "SCHEDULE A – MAIN TRANSFORMER" and "SCHEDULE B – STATION AUXILIARY TRANSFORMER."

Bid Evaluation: Red Flags in Transformer Tenders

When evaluating bids, look for these patterns that indicate a bidder is not fully compliant or is cutting corners:

• BIL = 145 kV offered when 170 kV specified: The bidder's standard design is for the reduced insulation level. Unless the bidder provides a technical justification accepted by the employer's engineer, reject on technical grounds. A reduced-BIL unit has smaller clearances and will fail a 170 kV impulse test if specified.

• Impedance offered at ±15% of specified value, no comment: A transformer's impedance is set by its core and winding geometry during design. A 15% deviation implies a completely different core cross-section or winding arrangement. This is not an "equivalent" transformer — it will behave differently under fault conditions.

• OLTC manufacturer not named or a low-tier brand substituted: The OLTC is the most failure-prone component of the transformer. Name the acceptable OLTC manufacturers in your tender (e.g., MR Maschinenfabrik Reinhausen, Huaming, ABB/ Hitachi Energy). If the bidder substitutes an unbranded or unknown OLTC, reject unless they provide a reference list of 50+ units in service for 5+ years.

• Bushing creepage distance below specification: For coastal or polluted sites, specify creepage distance ≥ 31 mm/kV for heavy pollution (Class IV per IEC 60815). If the bidder offers 25 mm/kV (standard for light pollution), this is a non-compliance that leads to flashover in service.

• Auxiliary voltage mismatch: The tap changer motor, cooling fans, and control cabinet all need an auxiliary power supply. If your station has 110 V DC battery supply and the bidder offers 230 V AC auxiliaries, you have a site wiring problem. Always match auxiliary voltage to the station design.

FAQ

Q: Our existing 33/11 kV transformer is BIL 170 kV. The new unit tender asks for BIL 170 kV but one bidder is offering BIL 200 kV at a premium. Should I accept?

A: BIL 200 kV is the standard insulation level for Um = 52 kV (the 45 kV class) — it is what you would specify for a 66 kV transformer. A 33 kV transformer built to BIL 200 kV has unnecessarily large clearances, a larger core, more steel, more oil, and higher cost (typically 12–18% premium) for no incremental protection benefit. The 170 kV BIL already provides sufficient insulation coordination for a 36 kV system with standard surge arresters. Accepting BIL 200 kV is technically harmless — the transformer will run cooler and last longer — but you are paying for insulation you do not need. I recommend rejecting it as non-compliant (not technically unsafe, but deviating from the specified level without justification) unless the bidder can demonstrate that their BIL 200 kV price is equal to or lower than the BIL 170 kV compliant bid — which is almost never the case.

Q: The tender says "rated at 50 Hz, site altitude 1,500 m." Do I need a de-rated transformer?

A: Altitude affects air as an insulating and cooling medium. At 1,500 m above sea level, air density is approximately 85% of sea-level density. This reduces the dielectric strength of external air clearances and reduces the cooling capacity of natural-draft radiators. Per IEC 60076-3, the correction for external clearances is a 7% increase per 1,000 m above 1,000 m — so at 1,500 m, add approximately 3.5% to all air clearances. For cooling, the temperature derating is approximately 1°C per 500 m above 1,000 m — at 1,500 m, the ambient temperature assumed for the temperature-rise limit should be reduced by 1°C, or alternatively the transformer should be specified with a lower temperature-rise class (e.g., 55°C rise instead of 65°C). State the altitude in the tender and require the bidder to confirm that the offered transformer is suitable without derating, or to state the derated MVA. This is especially critical for naturally cooled (ONAN) units where radiator performance depends on chimney-effect air movement.

Q: Can I parallel a new 33/11 kV transformer with an existing unit that is 20 years old?

A: Yes, but with three mandatory checks. (1) The vector group must be identical — you cannot parallel a Dyn1 with a YNd1 even if both are 33/11 kV. (2) The impedance voltages must match within ±10% per IEC 60076-5 (clause 4.6). If the existing unit is 10% impedance and the new one is 12%, the new unit will carry only 45% of the load while the old unit carries 55% — potentially overloading the older, less reliable unit. (3) The short-circuit withstand of the existing transformer must be re-evaluated: two paralleled transformers double the fault level on the 11 kV bus, and if the 11 kV switchgear was rated for a single transformer's fault contribution, it may now be under-rated. Request a system study as part of the bidder's scope or perform it independently before closing the tender.

Q: What accessories should I specify as "mandatory, with spares" vs. "mandatory, no spares"?

A: Accessories with moving parts or consumable media should have spares specified in the tender. Minimum spare list for a 33 kV transformer:

Item	Quantity	Reason
Silica gel breather charge	2 sets	Breather charge is consumed by moisture absorption
Buchholz relay spare float assembly	1	Float failure from vibration, rare but high-impact
Winding temperature indicator (WTI) capillary	1	Mechanical damage during transport/installation
Gasket set (complete)	1 set	For any future gasket replacement
Oil level indicator (prismatic glass)	1	Breakage during transport
Pressure relief device rupture disc	2 sets	Operates once and must be replaced
Fan motor (if ONAF)	1 per 4 fans	Bearing failure in tropical conditions

Q: The bidder's delivery is 52 weeks but the competitor offers 36 weeks. Is the faster bid cutting corners?

A: Not necessarily, but investigate the difference. A 33 kV transformer from an established manufacturer with an existing design library for your exact voltage/ MVA/ vector group combination can be produced in 30–36 weeks because the design engineering is complete and only manufacturing/shop drawing time is needed. A 52-week lead time may indicate: (a) a new design (no existing reference), (b) long-lead items such as imported CRGO steel or a specific OLTC model, or (c) a full production queue. Ask each bidder to break down the schedule into design engineering (weeks 1–6), procurement of materials (weeks 3–14), manufacturing (weeks 8–24), testing (weeks 24–28), and shipping (weeks 28–36). The breakdown will reveal which activities drive the difference. A faster bid that compresses design review and testing is more risky than one that compresses shipping by using air freight for small units.

References and Standards

Standard	Title
IEC 60076-1:2011	Power transformers – Part 1: General
IEC 60076-2:2011	Power transformers – Part 2: Temperature rise
IEC 60076-3:2018	Power transformers – Part 3: Insulation levels, dielectric tests
IEC 60076-5:2006	Power transformers – Part 5: Ability to withstand short circuit
IEC 60076-7:2018	Power transformers – Part 7: Loading guide for mineral-oil-immersed transformers
IEC 60076-10:2016	Power transformers – Part 10: Determination of sound levels
IEC 60076-22-7:2020	Power transformer and reactor fittings – Accessories and fittings
IEC 60815	Selection and dimensioning of high-voltage insulators for polluted conditions
ISO 12944	Corrosion protection of steel structures by protective paint systems

About the Author

Du Fu is a Production Engineer at ZY POWER. He has prepared and evaluated tender technical specifications for 33 kV transformers destined for projects in Nigeria (TCN), Kenya (KPLC), Pakistan (DISCOs), and Bangladesh (BREB/PGCB), with voltages up to 33 kV and ratings up to 31.5 MVA.