Doc #69 — Transformer Rewinding and Fabrication

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RECOMMENDED ACTIONS (BY URGENCY)

First month (Phase 1)

1. National transformer fleet assessment: Request all lines companies and Transpower to provide fleet data — number of transformers by type, age, and condition. Compile a national register. Identify the oldest and highest-risk units.
2. Inventory transformer oil stocks nationally: Including oil at Transpower, lines companies, petroleum distributors, and transformer repair shops.
3. Inventory transformer oil testing and reconditioning equipment nationally: Identify all DGA-capable laboratories, mobile oil treatment plants, vacuum dehydration units.
4. Identify all transformer repair and manufacturing capability in NZ: Through the skills census (Doc #8), identify businesses and individuals with transformer winding, testing, and repair skills.
5. Inventory spare transformers and bushings: Transpower and lines companies hold spares — compile a national register of all spare units by type and rating.
6. Secure all transformer-related materials: Copper conductor, insulating paper, pressboard, transformer oil, bushings, and tap changer parts at all locations in NZ. These are strategic materials.

First year (Phase 1–2)

7. Establish transformer oil reconditioning program: Prioritise testing and reconditioning of oil in the oldest and most heavily loaded transformers. Deploy mobile treatment plants on a systematic national schedule.
8. Begin knowledge capture from experienced transformer technicians: Structured interviews with senior winding technicians, oil chemists, and transformer design engineers.
9. Establish rewinding capability at Etel or equivalent facility: Confirm capability, identify gaps, begin training additional technicians.
10. Develop standardised rewinding procedures for the most common distribution transformer types in the NZ fleet.
11. Begin DGA monitoring program for all sub-transmission and transmission transformers nationwide.
12. Assess copper conductor supply for rewinding — inventory existing stocks and establish recycled copper recovery pipeline (Doc #70).

Years 1–3 (Phase 2)

13. Achieve routine distribution transformer rewinding capability: Target 50+ units per year.
14. Test NZ-produced paper for dielectric suitability as transformer insulation.
15. Develop porcelain bushing manufacturing capability for 11 kV distribution class.
16. Begin designing standardised NZ distribution transformer (Section 7.3).
17. Cross-train motor rewinders for transformer winding work.

Years 3–7 (Phase 3)

18. Begin new distribution transformer manufacture from salvaged core steel and NZ-produced materials.
19. Scale up copper wire drawing for transformer conductor (Doc #70).
20. Develop sub-transmission transformer rewinding capability (33 kV class).
21. Assess NOES production feasibility at Glenbrook (Doc #89).

Years 7–15 (Phase 4)

22. Scale distribution transformer production to meet replacement demand.
23. If NOES production is achieved, begin building new transformer cores from domestically produced steel.
24. If trade with Australia develops, prioritise import of GOES, high-voltage bushings, and transformer oil.
25. Develop 33 kV transformer manufacturing capability for sub-transmission network support.

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ECONOMIC JUSTIFICATION

The cost of transformer failure

A single distribution transformer failure affects 10–50 households. A sub-transmission transformer failure affects 2,000–50,000 consumers depending on the zone substation and network configuration. A transmission transformer failure affects tens of thousands to hundreds of thousands. In each case, the affected area loses electricity until the transformer is repaired, bypassed, or replaced.

Under isolation, “replaced” means either installing a spare from existing stock (finite), or repairing/rewinding the failed unit (requires the capability described in this document). If neither option is available, the outage is permanent.

Investment required

Rewinding program (Stage 1):

• Facility: Existing transformer repair workshop (Etel, or equivalent) — no new construction required, though capacity expansion may be needed.
• Equipment: Existing winding machines, test equipment, oil processing equipment — supplemented as needed.
• Training: 10–20 technicians trained in transformer rewinding, drawing from the existing pool of motor rewinders and electricians. Training period: 6–12 months under experienced supervision.
• Materials: From existing stocks initially (copper, paper, oil), with domestic substitution developing in parallel.
• Estimated labour: 5–10 person-years to establish capability, plus 10–20 person-years per year of ongoing production at 50–200 units per year.

Distribution transformer manufacture (Stage 2):

• Additional facility development: Core disassembly and restacking workshop, expanded winding capacity.
• Design engineering: Standardised transformer designs (see Section 7.3).
• Estimated labour: 10–20 person-years to develop capability, plus 15–30 person-years per year of production at 20–100 units per year.

Comparison with the alternative

The alternative to developing transformer manufacturing capability is progressive loss of electrical distribution as transformers fail and cannot be replaced. Over 10–20 years, a significant fraction of NZ’s distribution transformer fleet will fail. Each failure that is not repaired means permanent loss of electricity supply to the affected area.

The value of electricity to NZ’s recovery is difficult to overstate. It powers milking sheds, water pumping, grain milling, machine shops, hospitals, communications, and lighting. Loss of electricity in an area means loss of these specific functions — milking reverts to hand methods (reducing dairy output by an order of magnitude), water pumping stops (requiring manual or gravity-fed alternatives), and machine shops lose power tools. Maintaining electricity distribution through transformer maintenance and replacement is one of the highest-return investments in the entire recovery program.

The rewinding program (Stage 1) breaks even — in terms of preventing permanent outages — with its first successfully rewound transformer, since the alternative (permanent outage) has enormous cost. The economic case is overwhelmingly positive.

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1. TRANSFORMERS IN THE NZ GRID

1.1 What a transformer is

A transformer transfers electrical energy between two circuits through electromagnetic induction. AC current flowing through a primary winding creates a changing magnetic field in the steel core; this changing field induces a voltage in the secondary winding. The ratio of turns in the two windings determines the voltage ratio. No energy is created or destroyed — if voltage goes up, current goes down proportionally (minus losses).

The physical components:⁴

• Core: Stacked laminations of grain-oriented electrical steel (GOES), typically 0.23–0.35 mm thick, coated with an insulating layer to reduce eddy current losses. The core provides a low-reluctance path for the magnetic flux. Core quality directly determines transformer efficiency — poor core steel means higher losses, more heat, and shorter life.
• Windings: Coils of copper (occasionally aluminium) conductor, insulated with paper, pressboard, or enamel. The primary winding connects to the higher-voltage side; the secondary to the lower-voltage side (for step-down transformers, which are the majority in distribution). Winding geometry varies: concentric (cylindrical layers), disc (flat pancake coils stacked axially), or interleaved designs.
• Insulation system: Paper and pressboard insulation between windings and between windings and core, impregnated with transformer oil. The insulation system determines transformer life — when insulation degrades, the transformer fails.
• Oil: Mineral insulating oil fills the tank, providing both electrical insulation (it has higher dielectric strength than air) and cooling (it circulates by convection or forced pumping, carrying heat from the windings to external radiators). Oil quality is the single most important maintainable parameter.
• Tank: Steel enclosure containing the core, windings, and oil. Must be sealed to prevent moisture ingress. Includes radiators or cooling fins for heat dissipation.
• Bushings: Insulated feedthroughs that bring the electrical connections through the tank wall. High-voltage bushings on large transformers are complex porcelain-and-oil or resin-impregnated-paper assemblies. Bushing failure is a common cause of transformer failure.⁵
• Tap changer: A switching mechanism that changes the transformer turns ratio to regulate output voltage. On-load tap changers (OLTCs) operate under load and contain moving contacts in an oil-filled compartment — they are the only part of a transformer with moving parts and the component most prone to mechanical failure.⁶
• Accessories: Buchholz relay (detects gas from internal faults), pressure relief device, oil level gauge, temperature indicators, breather (controls moisture exchange between oil and atmosphere).

1.2 Transformer types in NZ

NZ’s grid uses transformers at several voltage levels:⁷

Transmission transformers (Transpower):

• Grid transformers: 220/110 kV or 220/33 kV, typically 50–200 MVA. Found at major substations connecting the 220 kV transmission backbone to the 110 kV or 33 kV sub-transmission network. These are the largest, most expensive, and hardest to replace. NZ has approximately 100–200 of these units nationally.⁸ Each weighs 50–200+ tonnes and contains thousands of litres of oil.
• HVDC converter transformers: Specialised transformers at Benmore and Haywards for the Cook Strait HVDC link. Highly specialised, very large, and essentially irreplaceable under isolation conditions.⁹

Sub-transmission transformers (Transpower and lines companies):

• 110/33 kV, 33/11 kV, or similar voltage combinations. Typically 5–60 MVA. Found at zone substations throughout the country. NZ has several hundred of these. Smaller than grid transformers but still large, heavy, and complex.

Distribution transformers (lines companies):

• Ground-mounted (pad-mounted): 11 kV/400V, typically 200–1,000 kVA. Found in urban and suburban substations, often in green metal enclosures. Modern units are compact and sealed.
• Pole-mounted: 11 kV/400V, typically 15–200 kVA. The most numerous type in NZ — estimates range from 20,000 to 40,000+ units nationwide.¹⁰ Found throughout rural NZ, mounted on power poles. These are the smallest grid transformers and the most feasible domestic manufacturing target.
• Indoor/commercial: Various sizes for commercial and industrial buildings.

The replacement priority ladder: If a 220 kV grid transformer fails, it affects hundreds of thousands of people and cannot be replaced domestically. If a pole-mounted 50 kVA distribution transformer fails, it affects perhaps 10–50 households and is — in principle — within the range of domestic manufacture. The strategy described in this document works from the bottom of this ladder upward: build capability to make and repair small transformers first, and work up to larger units as skills and materials allow.

Community function as a priority factor: Within each voltage class, transformer priority should account for the community function served, not only metered load. Transformers supplying marae and other community hubs warrant higher priority than their residential load alone would suggest, because marae serve as community coordination centres during emergencies — for food preparation, shelter, and social organisation (Doc #122, Doc #144). The same principle applies to transformers serving hospitals, water pumping stations, and communications facilities.

1.3 NZ’s transformer fleet condition

The condition of NZ’s transformer fleet is not publicly documented in aggregate. What is known:

• Transpower publishes asset management plans that discuss transformer replacement programs and condition assessments, but specific unit-by-unit condition data is not public.¹¹
• Some lines companies (Vector, Orion, PowerCo, Unison, etc.) have published asset management plans noting aging transformer fleets. Several companies have reported that significant proportions of their distribution transformer fleet are at or near the end of their expected service life.¹²
• NZ has been investing in transformer replacement under normal conditions — meaning that some of the fleet is relatively new and in good condition. But under isolation, the replacement pipeline stops.

Estimate: A significant minority of NZ’s distribution transformers — perhaps 15–30% — are likely over 40 years old and approaching the end of their expected service life. The exact figure requires a national fleet assessment, which should be a Phase 1 priority. Some of these aging units will fail in the first decade of isolation, and the failure rate will accelerate over time as the fleet ages without replacement.

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2. OIL-IMMERSED TRANSFORMER MAINTENANCE

Oil management is the single highest-value maintenance activity for extending transformer life. Most transformer failures are ultimately insulation failures, and oil condition directly determines insulation degradation rate.¹³

2.1 Transformer oil functions

Transformer oil (mineral insulating oil) serves two functions:

1. Insulation: Oil has a dielectric breakdown strength of approximately 30–70 kV/cm when clean and dry — far higher than air.¹⁴ It fills all gaps between windings, between windings and core, and between windings and tank, preventing electrical breakdown.
2. Cooling: Oil circulates through the windings and core (by natural convection in smaller units, by pumped circulation in larger ones), absorbing heat and transferring it to external radiators or cooling fins where it dissipates to the atmosphere.

Both functions degrade as oil deteriorates. The primary degradation mechanisms:

• Moisture contamination: Water enters through breathing (expansion and contraction of oil with temperature changes draws moist air into the tank through the breather), through seal failures, and as a byproduct of insulation degradation (cellulose paper insulation releases water as it ages). Moisture reduces dielectric strength — even a few tens of parts per million (ppm) of dissolved water significantly reduces breakdown voltage.¹⁵
• Oxidation: Oil reacts slowly with oxygen (from breathing and dissolved air) to form acids, sludge, and other degradation products. These deposit on windings and in cooling passages, reducing cooling efficiency and accelerating insulation degradation. Temperature accelerates oxidation — every 6–8°C increase in operating temperature roughly doubles the oxidation rate.¹⁶
• Dissolved gases: Internal faults (arcing, overheating) decompose oil and cellulose into characteristic gases — hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, carbon dioxide. Dissolved gas analysis (DGA) of oil samples can identify the type and severity of internal faults before they cause catastrophic failure.¹⁷
• Particulate contamination: Carbon particles from arcing, fibres from degrading insulation, and other particulates reduce dielectric strength and can cause tracking (partial discharge paths along contaminated surfaces).

2.2 Oil testing

Regular oil testing is the foundation of transformer condition monitoring. The key tests:¹⁸

Dielectric breakdown voltage (BDV): Measures the voltage at which a standardised oil sample breaks down electrically. Test procedure: apply increasing voltage across a standardised electrode gap in an oil sample. Minimum acceptable BDV for in-service oil is approximately 30 kV (for transformers up to 72.5 kV) to 50 kV (for transformers above 170 kV).¹⁹ Low BDV indicates moisture and/or particulate contamination.

Moisture content: Measured by Karl Fischer titration — a laboratory technique that requires specialised glassware and reagents (methanol, Karl Fischer reagent). Target: below 20 ppm for large transformers, below 30–40 ppm for distribution transformers.²⁰ NZ has laboratory capability for this test at transformer oil testing laboratories and some lines company labs — maintaining this capability is essential.

Acidity (neutralisation number): Measures acid content from oxidation products. Measured by titration with potassium hydroxide. Increasing acidity indicates oil aging and the need for reconditioning or replacement.

Dissolved gas analysis (DGA): The most powerful diagnostic tool. Oil samples are analysed for dissolved gases using gas chromatography.²¹ This requires a gas chromatograph — a significant piece of laboratory equipment. NZ has several transformer oil testing laboratories with DGA capability, including at Transpower and some lines companies, and at commercial testing laboratories such as those operated by Intertek or similar firms.²² Maintaining at least one DGA-capable laboratory nationally is a high priority.

Gas interpretation follows well-established diagnostic methods (Duval triangle, Rogers ratios, IEC 60599):²³

• High hydrogen alone → partial discharge (corona)
• High hydrogen + methane + ethane → thermal fault (low temperature)
• High ethylene → thermal fault (high temperature, >700°C)
• High acetylene → arcing (very high temperature, >1,000°C)
• High CO and CO₂ → cellulose degradation (paper insulation overheating)

Interfacial tension (IFT): Measures the oil’s surface tension against water — decreases as oil degrades. A screening test for overall oil condition.

Colour and visual inspection: Degraded oil darkens. Fresh transformer oil is pale yellow; badly aged oil is dark brown or black.

2.3 Oil filtration and reconditioning

When oil testing indicates degradation, reconditioning can restore oil quality and extend transformer life significantly:

Filtration: Circulates oil through fine filters (typically 1–5 micron) to remove particulates. Can be done on-line (while the transformer remains energised) or off-line. Mobile filtration units exist in NZ — Transpower and major lines companies own or contract these. They must be identified and maintained as strategic assets.

Vacuum dehydration (degassing): Passes oil through a vacuum chamber where dissolved moisture and gases are extracted under reduced pressure. This is the most important reconditioning process — it restores dielectric strength by removing moisture and removes dissolved gases. Vacuum dehydration equipment is available in NZ and should be maintained.²⁴

Fuller’s earth (clay) treatment: Passes oil through columns of activated clay (fuller’s earth, attapulgite, or bentonite) which adsorbs acids, sludge precursors, and polar degradation products. Restores oil colour and reduces acidity. Fuller’s earth is consumed in the process and must be replaced. NZ has some bentonite deposits, though the suitability for oil treatment would need to be verified.²⁵

Oil replacement: In severe cases, the oil is drained and replaced with new oil. Transformer oil stocks in NZ exist at Transpower and lines company stores, and at petroleum distributors who supply transformer oil (typically Shell Diala or equivalent mineral insulating oils). These stocks are finite — no new transformer oil will be produced in NZ without petroleum refining capability (Marsden Point ceased refining in 2022).²⁶ Oil replacement should be reserved for cases where reconditioning cannot restore acceptable quality.

NZ oil reconditioning capability: The critical question is whether NZ has enough mobile oil treatment equipment and the skilled operators to manage the national transformer fleet. Under normal conditions, this work is done by specialist contractors and by lines company in-house teams. An inventory of all oil treatment equipment in NZ — including equipment owned by Transpower, lines companies, and private contractors — is a Phase 1 priority.

2.4 Maintenance of dry-type transformers

Not all transformers use oil. Dry-type transformers (cooled by air rather than oil, with solid insulation) are used in some indoor, commercial, and industrial applications. They are generally simpler to maintain (no oil to manage) but more vulnerable to environmental contamination (dust, moisture) and overheating if ventilation is restricted. Dry-type units are typically smaller and lower-voltage than oil-immersed units. The principles of insulation monitoring apply, though the testing methods differ — insulation resistance measurement and partial discharge testing rather than oil analysis.²⁷

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3. TRANSFORMER FAILURE MODES AND DIAGNOSIS

3.1 Why transformers fail

Transformer failures fall into several categories:²⁸

Insulation failure (most common): Gradual degradation of paper and oil insulation leads to reduced dielectric strength. Eventually, the voltage stress exceeds the insulation’s withstand capability, causing an internal arc (flashover). This is catastrophic — the arc generates enormous energy in a confined space, decomposing oil into gas (potential explosion), carbonising insulation, and potentially melting copper conductors. The transformer is destroyed unless the arc is very localized and the protection system disconnects the unit before the damage spreads.

Winding failure: Open circuit (a winding conductor breaks) or short circuit (insulation failure between turns, between windings, or between winding and core). Short circuits cause very high currents that can melt conductors and destroy insulation if protection does not disconnect quickly.

Bushing failure: Moisture ingress into bushings, cracking of porcelain, or internal tracking leads to flashover — either through the bushing itself or over its external surface. Bushing failures are a significant proportion of all transformer failures and are particularly dangerous because they can cause external fire.²⁹

Tap changer failure: Mechanical wear, contact erosion, and oil contamination in on-load tap changers. OLTC failures can cause arcing within the tap changer compartment, which may propagate to the main tank.

Core failure (rare): Insulation breakdown between core laminations or between core and frame, leading to circulating currents, localised heating, and gas generation. Core faults are difficult to repair without removing the core from the tank.

External causes: Lightning strikes (bushings are vulnerable), switching surges, overloading, through-faults (short circuits on the connected network that impose mechanical stress on windings through electromagnetic forces).

3.2 Identifying rewindable vs. unrepairable units

When a transformer fails, the first assessment is whether it can be repaired:

Rewindable (windings damaged, core intact):

• Insulation failure localised to windings
• Short-circuit damage to windings (deformed, displaced, or melted conductors)
• Moisture-damaged insulation throughout windings (but core steel undamaged)

If the core is intact and the tank is undamaged, the transformer can potentially be rewound — the old windings are removed, the core is cleaned and tested, and new windings are installed. This is a significant but feasible repair.

Repairable without rewinding:

• Bushing replacement
• Tap changer overhaul
• Oil reconditioning
• Tank leak repair
• External component replacement (radiators, fans, pumps)

Unrepairable or not worth repairing:

• Core damage (shorted laminations, physical distortion)
• Tank rupture or severe deformation
• Fire damage throughout
• Small distribution units where the labour cost of rewinding exceeds the value of the unit (under normal economics — under isolation, this calculation changes dramatically)

Important: Under isolation conditions, the threshold for “worth repairing” changes fundamentally. A 50 kVA pole-mounted distribution transformer that would normally be scrapped and replaced with a new imported unit becomes worth spending significant labour to rewire, because no replacement exists. The economic calculus is entirely different when imports are unavailable.

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4. TRANSFORMER REWINDING

4.1 What rewinding involves

Rewinding a failed transformer means removing the damaged windings from the core and installing new ones. The process varies significantly with transformer size, but the basic sequence for a distribution transformer is:³⁰

1. Drain oil and remove from service. For pole-mounted units, this means lowering the transformer from the pole — typically requiring a crane truck or suitable lifting equipment.
2. Remove the transformer from its tank. Unbolt the cover (many distribution transformers are “tank-down” design where the core-and-coil assembly lifts out of the tank, or “tank-up” where the tank lifts off the base).
3. Record all winding data. Before removing old windings, document: number of turns on each winding, conductor size and type, insulation type and thickness, winding geometry (number of layers, disc arrangement), tap positions, connection arrangement (delta, star, zigzag), and physical dimensions. This data is essential for fabricating replacement windings. Without it, the transformer cannot be correctly rewound.
4. Remove old windings. Cut or unwind the damaged copper from the core. Recover the copper for recycling (Doc #70 — copper wire production). Clean the core laminations — remove carbonized insulation, contaminated oil residues, and debris.
5. Test the core. Check for shorted laminations using a core loss test (energise the core at rated flux density and measure the loss — excessive loss indicates shorted laminations). Also check insulation resistance between core and frame.
6. Fabricate new windings. This is the skilled work. Wind the specified number of turns of copper conductor, with inter-turn and inter-layer insulation, to the correct dimensions. The winding must fit on the core with correct clearances and must be mechanically stable enough to withstand short-circuit forces.
7. Assemble. Install the new windings on the core, make internal connections (between windings, to tap positions, to bushing connections), install insulation barriers, and assemble the core-and-coil unit.
8. Re-tank. Lower the core-and-coil assembly into the tank, bolt down the cover, install bushings, and make external connections.
9. Vacuum-fill with oil. Draw a vacuum on the tank to remove moisture and air from the insulation, then fill with clean, dry transformer oil. The vacuum step is essential — air pockets in the insulation cause partial discharge and premature failure.
10. Test. Perform routine tests: turns ratio check, winding resistance measurement, insulation resistance, applied voltage test (Hi-pot), induced voltage test. These tests confirm that the rewind was done correctly and the transformer is safe to energise.

4.2 Skills and facilities required

Skills: Transformer rewinding is a recognised trade specialisation within electrical engineering. NZ has a small number of transformer repair and rewinding businesses — notably Etel Ltd (in Christchurch, which manufactures small transformers and performs rewinding), ABB’s NZ service operations, and several smaller workshops.³¹ These businesses have skilled winding technicians, testing equipment, and facilities for handling transformers up to moderate sizes. The skills census (Doc #8) should identify all NZ-based transformer repair capability.

NZ also has a broader base of electric motor rewinding shops — motor rewinding and transformer rewinding share core skills (both involve winding copper coils with precise insulation). Motor rewinders can potentially be retrained for transformer work, though transformer insulation systems and testing requirements are more demanding than for most motors.³²

Facilities: A transformer rewinding facility needs:

• Covered workshop with overhead crane (for lifting core-and-coil assemblies — distribution transformers weigh 100–2,000 kg)
• Winding lathe or winding machine (a rotating mandrel with a turns counter for winding coils)
• Oven or vacuum drying chamber (for drying insulation before oil impregnation)
• Vacuum pump and oil processing equipment (for vacuum-filling)
• Test equipment: turns ratio meter, winding resistance bridge, insulation tester (megger), hi-pot test set
• Materials: copper conductor, insulating paper (kraft paper or Nomex), pressboard, insulating varnish, transformer oil

4.3 Materials for rewinding

Copper conductor: The primary winding material. Distribution transformer windings use rectangular or round copper conductor, typically 1–10 mm² cross-section, either bare (paper-insulated) or enamel-coated. NZ does not currently draw copper wire for transformer use, but the capability exists in principle:

• Recycled copper from old windings, demolished buildings, and other sources provides the raw material (Doc #70).
• NZ Steel at Glenbrook (Doc #89) is developing wire rod capability; copper wire drawing would require a similar but separate drawing operation from copper rod.
• Copper wire drawing has been done in NZ historically and the machinery exists at some NZ electrical manufacturers. Whether it is currently operational requires verification.³³
• Australian copper via Tasman trade (Doc #70, Doc #138) is the likely long-term supplement to domestic recycling.

For distribution transformer rewinding, the copper requirement per unit is modest — a 50 kVA transformer might contain 30–60 kg of copper.³⁴ The total copper requirement for a rewinding program is manageable from NZ’s recycled copper supply, at least initially.

Insulating paper: Transformer-grade kraft paper (typically Munksjö or similar brands) is the standard inter-turn and inter-layer insulation. NZ does not manufacture this. Existing stocks at transformer manufacturers and repair shops, plus stocks held by lines companies, provide an initial supply. Over time, substitution with NZ-manufactured paper may be possible — NZ has a paper industry (Oji Fibre Solutions at Kinleith, for example) and the basic material is wood pulp. However, transformer-grade paper has specific requirements: high purity, controlled thickness, good mechanical strength, and good dielectric properties. Producing paper that meets these specifications is achievable but would require process development and quality testing.³⁵

Pressboard: Thicker insulation board made from the same cellulose base as paper but compressed to higher density. Used for structural insulation elements (barriers, spacers, end rings). Similar supply constraints to paper.

Insulating varnish/lacquer: Used to bond and protect windings after assembly. Standard electrical varnishes (polyester or epoxy-based) are imported and NZ cannot synthesise these resins. NZ-produced alternatives include linseed oil-based varnishes (linseed is grown in NZ on a small scale and the oil can be extracted by pressing, but production would need to scale significantly) and potentially shellac-based varnishes (shellac is not produced in NZ and would need to be imported or substituted). These natural varnishes have lower dielectric strength than modern synthetic varnishes — typically 5–15 kV/mm compared to 20–40 kV/mm for polyester or epoxy varnishes — and are adequate for low-voltage windings (400V class) but marginal or inadequate for 11 kV insulation systems without significantly increased insulation thickness.³⁶

Transformer oil: As discussed in Section 2. The existing national stock of transformer oil is finite and must be managed carefully.

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5. BUSHING REPLACEMENT AND TAP CHANGER MAINTENANCE

5.1 Bushings

Bushings are the insulated feedthroughs that bring electrical connections through the transformer tank wall. They must withstand both the operating voltage and transient overvoltages (lightning, switching surges) while providing a sealed oil-tight connection.

Distribution transformer bushings (11 kV and below): Relatively simple — typically porcelain or polymer insulators with a central conductor. These can potentially be manufactured in NZ:

• Porcelain: NZ has ceramic manufacturing capability (the ceramics industry is small but exists). Electrical porcelain requires high-purity clay bodies and precise firing, but the basic technology is well understood.³⁷
• Polymer bushings: Modern distribution transformer bushings are increasingly made from cycloaliphatic epoxy resin or silicone rubber. NZ cannot produce these materials — both require petrochemical feedstocks and specialised polymerisation processes not available domestically.³⁸ Polymer bushing stocks will deplete over time.

High-voltage bushings (33 kV and above): Complex assemblies — oil-impregnated paper (OIP) or resin-impregnated paper (RIP) condenser bushings with precisely graded capacitive layers. NZ cannot manufacture these. Failed high-voltage bushings on sub-transmission and transmission transformers are a serious problem — if no spare exists, the transformer is out of service until a bushing can be sourced (from another failed transformer, from spares inventory, or from trade).³⁹

Bushing inventory: Transpower and lines companies hold spare bushings, but the inventory is sized for normal replacement rates, not for isolation conditions. A national audit of bushing spares — by type, voltage rating, and physical compatibility — is a Phase 1 priority.

5.2 On-load tap changers (OLTCs)

OLTCs are the only moving parts in a transformer and the component most prone to mechanical failure. They operate by switching between tap positions on the transformer winding to adjust the voltage ratio, compensating for voltage variations in the network.

Maintenance requirements:⁴⁰

• Regular oil changes or filtration in the OLTC compartment (which is usually separate from the main tank oil)
• Contact inspection and replacement — contacts erode with each operation due to arcing
• Drive mechanism lubrication and adjustment
• Diverter switch inspection
• Operation counter monitoring — OLTCs have a defined number of operations before overhaul is required (typically 50,000–100,000 operations, depending on make and model)

NZ capability: OLTC maintenance is performed by specialist technicians employed by or contracted to Transpower and lines companies. The major OLTC manufacturers (Maschinenfabrik Reinhausen/MR, ABB) have NZ representatives or service agents. Under isolation, spare parts (particularly contacts, which are consumable) must come from existing stocks or be fabricated locally. Contact materials (typically tungsten-copper or copper-chromium alloys) are imported specialty products — NZ substitution with plain copper contacts would function but with significantly shorter contact life (perhaps 20–40% of the service life of tungsten-copper contacts, due to copper’s lower arc erosion resistance and lower melting point) and higher contact resistance, requiring more frequent overhaul intervals.⁴¹

De-energised tap changers (DETCs): Many smaller transformers have off-circuit tap changers that are adjusted only when the transformer is de-energised. These are mechanically simpler and less maintenance-intensive than OLTCs. Under isolation, where voltage regulation may be less precise than pre-event standards, some OLTCs could potentially be locked on a fixed tap and treated as DETCs, reducing maintenance requirements at the cost of less precise voltage control.

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6. THE CORE STEEL PROBLEM: GRAIN-ORIENTED ELECTRICAL STEEL

6.1 What GOES is

Grain-oriented electrical steel (GOES), also called transformer steel, is a specialty steel product with magnetic properties tailored for transformer cores. It is cold-rolled silicon steel (approximately 3% silicon by weight) in which the crystal grains have been aligned through a complex thermomechanical processing sequence so that the easy magnetisation direction (the [001] crystal axis) is parallel to the rolling direction.⁴²

This grain alignment dramatically reduces core losses (hysteresis loss and eddy current loss) when the core is magnetised along the rolling direction, as it is in a transformer. Modern GOES has core losses approximately 70–80% lower than non-oriented electrical steel of the same composition.⁴³ This is not a minor difference — it determines transformer efficiency, operating temperature, and size. A transformer built with non-oriented electrical steel would need to be physically larger, would run hotter, would waste more energy, and would have a shorter life than an equivalent transformer built with GOES.

6.2 How GOES is made

GOES manufacturing is one of the most demanding metallurgical processes in the steel industry. The sequence, in simplified form:⁴⁴

1. Steelmaking: Produce steel with precisely controlled chemistry — approximately 3% silicon, controlled levels of carbon, manganese, sulfur (or selenium), nitrogen, and aluminium. The chemistry must be very precise because the grain orientation process depends on specific precipitates (manganese sulfide, aluminium nitride) that control grain growth.
2. Hot rolling: Roll the steel to approximately 2–3 mm thickness.
3. Annealing and cold rolling: A sequence of annealing (heating in a controlled atmosphere to alter the crystal structure) and cold rolling (reducing thickness at room temperature), typically including one or two intermediate anneals and a final cold roll to approximately 0.23–0.35 mm.
4. Decarburisation anneal: Heat in a wet hydrogen atmosphere to remove carbon (which would cause magnetic aging) and form a thin glass film (forsterite/Mg₂SiO₄) on the surface.
5. High-temperature anneal: The critical step. The steel is heated to approximately 1,100–1,200°C for an extended period (hours to days) in a hydrogen atmosphere. During this anneal, secondary recrystallisation occurs — a few grains with the desired [001] orientation grow at the expense of all other grains, producing the aligned grain structure that gives GOES its magnetic properties. This process depends on the inhibitor precipitates formed during steelmaking and earlier processing to control grain growth precisely.
6. Coating: Apply a tension-inducing insulating coating (typically phosphate-based) that further reduces core losses.

6.3 Can NZ produce GOES?

Almost certainly not, for the foreseeable future. The obstacles:

• NZ Steel at Glenbrook produces flat-rolled carbon steel from ironsand. It does not produce silicon steel. Adding 3% silicon to the steel is technically possible at the melting stage, but the entire downstream processing sequence — the precise thermomechanical processing, the controlled-atmosphere anneals, the specific rolling schedule — requires specialised equipment that Glenbrook does not have and could not readily adapt.⁴⁵
• GOES production worldwide is concentrated in a small number of specialty mills (Nippon Steel, JFE Steel, POSCO, ThyssenKrupp, Baowu) that have invested decades and billions of dollars in process development. It is one of the most technically demanding steel products to manufacture. Even countries with large steel industries (India, Brazil) have had difficulty establishing GOES production.⁴⁶
• The inhibitor chemistry — the precise control of manganese sulfide and aluminium nitride precipitates that drives secondary recrystallisation — is closely guarded proprietary knowledge. Even with published metallurgical literature, replicating this process without experienced personnel and extensive development would take years.

Assessment: GOES production in NZ is rated [D] — decades of development at minimum, and quite possibly never achievable given NZ’s small industrial base and the extreme difficulty of the process. This is a fundamental constraint that shapes the entire transformer manufacturing strategy. NZ must source GOES from:

1. Existing stock: Core steel in functioning and failed transformers already in NZ.
2. Cannibalization: Cores from transformers that are beyond economical repair, or from transformers serving loads that have been decommissioned or consolidated.
3. Trade: GOES from Australia (if Australian steel mills survive and resume production) or other surviving regions. GOES is a high-value, moderate-weight product well suited to sail trade.
4. Non-oriented electrical steel (NOES) as a substitute: NOES is significantly easier to produce (it does not require the secondary recrystallisation step). NZ might eventually produce NOES at Glenbrook by adding silicon to the steel melt. NOES has higher core losses than GOES but is usable for transformer cores — the transformer would be larger, less efficient, and would run warmer, but it would function. This represents a meaningful performance gap but a workable substitution for small distribution transformers where efficiency is less critical than availability.⁴⁷

6.4 The practical GOES strategy

Triage and reuse:

• When a transformer fails with winding damage but an intact core, the core is the most valuable component. Even if rewinding is not immediately feasible, the core should be preserved for future use.
• When loads are consolidated (e.g., rural areas where population has concentrated into fewer settlements), surplus transformers can be decommissioned and their cores stored or reused.
• A national register of transformer cores — by size, voltage class, and condition — should be maintained. This is effectively a strategic materials inventory.

Core steel recovery from scrapped transformers:

• GOES laminations from scrapped transformer cores can be restacked into new cores of different dimensions, within limits. The laminations are typically 0.23–0.35 mm thick and can be sheared to new widths if narrower laminations are needed. This allows (with significant labour) building new transformer cores from salvaged material.
• The insulating coating on GOES laminations (either the original mill-applied coating or the interlaminar insulation from the stacking process) should be preserved. If damaged, replacement insulation can be applied — thin layers of varnish, shellac, or (historically) paper between laminations.⁴⁸

Implications for new construction: The GOES constraint means that NZ’s domestic transformer manufacturing capability, when it develops, will be sized and limited by the available supply of core steel. Small distribution transformers (15–200 kVA) use relatively small quantities of GOES — perhaps 50–300 kg per unit.⁴⁹ The cores from a single large failed transmission transformer might provide enough GOES for dozens of small distribution transformers. This makes the recovery strategy internally consistent: as large transformers eventually fail (over decades), their core steel enables the manufacture of smaller units to maintain distribution.

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7. SMALL DISTRIBUTION TRANSFORMER MANUFACTURE

7.1 Why this is the feasible starting point

Distribution transformers (11 kV/400V, 15–200 kVA) are the most feasible domestic manufacturing target because:

• Smaller scale: A 50 kVA pole-mounted transformer weighs approximately 200–500 kg complete, with perhaps 30–60 kg of copper and 80–200 kg of core steel.⁵⁰ These quantities are manageable with workshop-scale equipment.
• Lower voltages: The insulation requirements for 11 kV are demanding but achievable. Distribution transformers do not require the complex condenser-graded insulation systems of 110+ kV units.
• Simpler construction: Distribution transformers typically use simpler winding geometries (concentric cylindrical windings) than large power transformers.
• Existing NZ precedent: Etel Ltd in Christchurch has historically manufactured small transformers in NZ, demonstrating that the capability can exist domestically.⁵¹
• Highest unit demand: Distribution transformers are the most numerous type in the fleet, and will be needed in the largest numbers for replacement and for connecting new loads.

7.2 Staged development

Stage 1 — Rewinding (Phase 2, Years 1–3):

• Establish rewinding capability for existing distribution transformers using existing cores.
• Focus on the most common types in NZ’s fleet (identify the 5–10 most common transformer models and develop standard rewinding procedures for each).
• Train rewinding technicians — initially by up-skilling existing motor rewinders, with experienced transformer technicians as trainers.
• Use existing stocks of copper conductor, insulating paper, and transformer oil.
• Target: ability to rewind 50–200 distribution transformers per year.

Stage 2 — Core reuse and new construction (Phase 3, Years 3–7):

• Develop capability to disassemble transformer cores and restack laminations into new core geometries.
• Begin producing new distribution transformers using salvaged GOES cores and domestically wound coils.
• Develop NZ-produced insulating paper (or verify that NZ-produced paper meets minimum dielectric requirements).
• Develop NZ-produced bushing capability (porcelain bushings for 11 kV class).
• Target: ability to manufacture 20–100 new distribution transformers per year from salvaged and new materials.

Stage 3 — Material self-sufficiency (Phase 4, Years 7–15):

• Develop copper wire drawing capability specifically for transformer conductor (Doc #70).
• If feasible, develop NOES production at Glenbrook as a GOES substitute for new transformer cores.
• Scale up production as materials allow.
• Begin developing capability for larger units (33 kV class sub-transmission transformers).

7.3 Design standardisation

Under isolation conditions, NZ should standardise on a small number of distribution transformer designs rather than replicating the diverse fleet inherited from the pre-event period. Standardisation reduces the variety of parts, simplifies training, and enables interchangeable spares.

Recommended standard designs:

• 15–25 kVA single-phase: For individual rural properties and small loads. Simple and light enough for pole mounting.
• 50 kVA three-phase: For small rural communities, dairy sheds, and light commercial loads. The workhorse distribution transformer.
• 200 kVA three-phase: For larger communities and moderate commercial/industrial loads. Likely ground-mounted.

Each design should be fully documented: complete electrical design (turns, conductor sizes, insulation system, losses, impedance), mechanical drawings (core dimensions, winding dimensions, tank dimensions), bill of materials, manufacturing procedure, and test specifications. This documentation serves as both the manufacturing reference and the training curriculum.

7.4 Testing without modern instruments

Testing rewound or newly manufactured transformers requires equipment that is itself manufactured:

• Turns ratio: Can be checked with a low-voltage AC source and two voltmeters (one on each winding) — basic instruments that can be maintained or fabricated.
• Winding resistance: Requires a DC source and a millivoltmeter or Wheatstone bridge — achievable with basic instruments.
• Insulation resistance: Requires a megger (insulation resistance tester) — a hand-cranked generator producing 500–5,000V DC with a sensitive microammeter. These are robust instruments and NZ has many in service. Maintaining them is feasible.
• Applied voltage (Hi-pot) test: Requires a high-voltage AC source — typically a small test transformer. This is more challenging but test transformers exist in NZ at transformer repair facilities and electrical testing laboratories.
• Induced voltage test: Requires applying double the rated voltage at double the frequency to test turn-to-turn insulation. Requires a variable-frequency source. More difficult but achievable with a motor-generator set or purpose-built test equipment.

Key point: Maintaining and preserving existing test equipment is far easier than building new test equipment. An inventory of all transformer testing equipment in NZ — at Transpower, lines companies, manufacturers, repair shops, and electrical testing laboratories — should be compiled and the equipment designated as strategic assets.

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8. LARGER TRANSFORMERS: LIMITATIONS AND STRATEGIES

8.1 Sub-transmission transformers (33 kV class)

Rewinding sub-transmission transformers (5–60 MVA, 33/11 kV or similar) is significantly more demanding than distribution transformer rewinding:

• Higher voltages require more sophisticated insulation systems — multiple layers of paper and pressboard, graded insulation with increasing thickness toward the line end, and oil duct spacing for cooling and insulation.
• Larger physical size requires larger workshop facilities and heavier lifting equipment.
• More complex winding geometries — disc windings, interleaved windings, and tap winding arrangements require greater skill.
• Higher test voltages — type tests and routine tests for 33 kV class transformers require test equipment capable of generating 70+ kV.

NZ’s existing transformer repair businesses (Etel, potentially ABB service workshops) have some capability at this level. Developing broader capability is feasible but represents a significant step up from distribution transformer rewinding. This is a Phase 3–4 capability target.

8.2 Transmission transformers (110 kV and above)

Rewinding transmission transformers is a major industrial operation:

• Very high voltages require extremely sophisticated insulation systems that push the limits of material capability.
• Very large physical size — these units weigh 50–200+ tonnes. Handling requires heavy cranes, specialised transport, and large workshop spaces.
• Extremely demanding testing — 110+ kV test voltages require specialised high-voltage test laboratories.
• Very tight tolerances — impedance, losses, and insulation levels must meet precise specifications for network compatibility.

Honest assessment: NZ is unlikely to develop the capability to fully rewind or manufacture 110+ kV transmission transformers under isolation conditions. The insulation engineering alone requires specialised knowledge and materials that NZ lacks. The strategy for these units is:

1. Extend life through meticulous oil management and condition monitoring.
2. Reduce loading to reduce thermal stress and slow aging.
3. Maintain spares: Transpower holds some spare transformers for contingency. These spares become critically important under isolation.⁵²
4. Adapt the network: If a major transmission transformer fails and cannot be replaced, the network must be reconfigured — potentially using lower-voltage transmission, reducing loads, or bypassing the failed substation. This reduces efficiency and capacity but may maintain supply.
5. Trade: Transmission-class transformers could potentially be obtained through maritime trade with Australia, where transformer manufacturing capability exists (Wilson Transformer Company, ABB Australia).

8.3 HVDC converter transformers

The Cook Strait HVDC link uses specialised converter transformers at Benmore and Haywards. These are highly specialised, unique designs that cannot be replaced under isolation conditions. If one fails, the HVDC link operates at reduced capacity or ceases operation entirely. This would separate the North and South Island grids, requiring each island to balance generation and load independently — a significant operational challenge but not catastrophic, since each island has sufficient generation for its own needs under reduced post-event demand.⁵³

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9. WORKFORCE AND TRAINING

9.1 Existing NZ capability

NZ’s existing transformer-related workforce includes:

• Transformer manufacturers: Etel Ltd (Christchurch) manufactures small transformers and has winding, testing, and design capability.⁵⁴
• Transformer repair/service companies: Several firms offer transformer maintenance, oil testing, and repair services — often affiliated with major manufacturers (ABB, Siemens) or operating independently.
• Motor rewinding shops: NZ has motor rewinding businesses in most major centres. These technicians understand winding, insulation, and testing principles, though transformer-specific knowledge requires additional training.
• Lines company and Transpower engineers: Power system engineers with knowledge of transformer specification, protection, and application.
• Oil testing laboratories: Specialists in transformer oil analysis, including DGA interpretation.

Estimate: The total NZ workforce with directly relevant transformer repair skills is probably in the range of 50–200 people. This is a small pool and these individuals should be identified through the skills census (Doc #8) and classified as strategic personnel.⁵⁵

9.2 Training pipeline

Training new transformer technicians requires:

Apprenticeship-based training (primary pathway):

• 6 months: Basic electrical theory, safety, and workshop skills (drawing from existing trade training programs, Doc #156)
• 6 months: Supervised transformer disassembly, winding removal, core inspection, and basic rewinding under experienced technicians
• 6–12 months: Progressive skill development — winding increasingly complex transformers, learning insulation systems, performing tests
• Ongoing: Continuous development as new challenges arise

Cross-training from motor rewinding:

• Motor rewinders already understand winding mechanics, insulation handling, and basic electrical testing. Cross-training to transformer work focuses on: transformer-specific insulation systems (oil-paper vs. enamel), higher voltage testing, core handling, and transformer design principles.
• Estimated cross-training time: 3–6 months under supervision.

Rural and iwi-based training: Transformer repair training should reach rural communities, not only urban workshops. Iwi-operated training organisations and regional polytechnic programmes can incorporate transformer maintenance and rewinding into their electrical trade curricula, building local capability for communities to maintain their own distribution infrastructure.

Key knowledge holders: The most experienced transformer technicians and engineers in NZ — particularly those who have done rewinding work on medium and large transformers — hold knowledge that cannot be replaced from textbooks. Structured knowledge capture interviews (following the approach described in Doc #65 Section 2.3) should be conducted with these individuals as a Phase 1 priority.

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10. CRITICAL UNCERTAINTIES AND KEY RISKS

Uncertainty	Impact	Mitigation
NZ transformer fleet age distribution	Determines failure rate timeline	National fleet audit — Phase 1
GOES availability from salvage	Determines new transformer production capacity	Core steel inventory and conservation program
NZ paper suitability for transformer insulation	Determines whether NZ-produced insulation is feasible	Testing program — Phase 2
Copper wire availability for rewinding	Determines rewinding rate	Copper recycling program (Doc #70), wire drawing development
Transformer oil stock nationally	Determines how many transformers can be refilled after repair	National oil inventory — Phase 1
Etel and other manufacturers’ continued capability	Determines initial manufacturing base	Skills census, facility assessment — Phase 1
NOES feasibility at Glenbrook	Determines whether any domestic core steel production is possible	Engineering assessment with NZ Steel — Phase 3
Australian trade development timeline	Determines whether GOES and transformer-grade materials can be imported	Monitor trade development (Doc #138)
Lines company asset management data accessibility	Determines quality of fleet assessment	Secure data in print/digital — Phase 1

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CROSS-REFERENCES

• Doc #1 — National Emergency Stockpile Strategy (transformer oil, copper, insulating materials as strategic stocks)
• Doc #8 — National Skills and Asset Census (transformer workforce identification)
• Doc #34 — Lubricant Production (bio-based oils — not suitable for transformer insulation, but relevant to understanding oil management)
• Doc #65 — Hydroelectric Station Maintenance (transformer maintenance at generation stations; generator rewinding)
• Doc #67 — Transpower Grid Operations (network implications of transformer failure)
• Doc #68 — Rural Distribution and SWER (distribution transformer fleet management)
• Doc #70 — Copper Wire Production (conductor supply for rewinding)
• Doc #72 — Micro-Hydro Design and Construction (small transformers for micro-hydro output)
• Doc #89 — NZ Steel Glenbrook (potential NOES production; EAF transformer as critical vulnerability)
• Doc #91 — Machine Shop Operations (fabrication support for transformer components)
• Doc #105 — Fencing Wire and Nails (wire drawing capability relevant to conductor production)
• Doc #113 — Sulfuric Acid (chemical processing relevant to insulating materials)
• Doc #138 — Sailing Vessel Design (trade route for GOES and transformer materials from Australia)
• Doc #157 — Trade Training Priorities (electrician and transformer technician training pipeline)
• Doc #162 — University Reorientation (power systems engineering education)

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1. NZ transformer fleet size is not precisely documented in publicly available sources. The estimate of 30,000–50,000 units is based on NZ having approximately 2 million electricity connections (Electricity Authority data) and typical distribution transformer loading ratios. Transpower’s transmission and sub-transmission transformer fleet is much smaller — hundreds of units. Exact figures should be established through a fleet audit.↩︎

2. Transformer service life: IEC 60076 and IEEE C57.91 provide transformer loading guides that relate thermal aging of cellulose insulation to transformer life. At normal operating temperatures, paper insulation has an expected thermal life of 150,000–180,000 hours (approximately 20 years of continuous full-load operation). In practice, transformers operate below rated load much of the time, and oil-immersed transformers with good maintenance routinely achieve 40–60+ years. See: IEEE Std C57.91-2011, “Guide for Loading Mineral-Oil-Immersed Transformers.”↩︎

3. NZ transformer fleet age: Individual lines company and Transpower asset management plans, published periodically, discuss fleet age profiles. These plans are available through respective company websites and the Commerce Commission’s information disclosure requirements for regulated electricity distributors. https://comcom.govt.nz/regulated-industries/electricity-l...↩︎

4. Transformer construction: Standard power engineering references. See: Harlow, J.H. (ed.), “Electric Power Transformer Engineering,” CRC Press, 3rd edition; IEEE Std C57.12.00, “Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.”↩︎

5. Bushing failure as a proportion of transformer failures: Various industry studies report bushing failures as 10–25% of all transformer failure causes. See: CIGRE Working Group A2.37, “Transformer Reliability Survey,” Electra No. 261, 2012; IEEE Transformers Committee failure surveys.↩︎

6. On-load tap changers: See Maschinenfabrik Reinhausen (MR) technical documentation; IEC 60214, “Tap-changers.” OLTC maintenance requirements and contact life vary by manufacturer and model. https://www.reinhausen.com/↩︎

7. NZ grid voltage levels: Transpower operates the national grid at 220 kV (main transmission), 110 kV (sub-transmission in some areas), and 66/33 kV (connection to distribution networks). Lines companies distribute at 33 kV, 22 kV, 11 kV, and 400/230V (low voltage to consumers). See: Electricity Authority and Transpower public documentation. https://www.transpower.co.nz/↩︎

8. Transpower transformer fleet: Transpower owns and operates the national grid transmission assets. The estimate of 100–200 grid transformers is approximate and based on the number of Transpower substations (approximately 170+ transmission substations). Some substations have multiple transformers. Exact figures are available from Transpower’s asset management plans. https://www.transpower.co.nz/about-us/transmission-tomorrow↩︎

9. HVDC link: The Cook Strait HVDC link (Inter-Island link) connects the South Island grid (Benmore) to the North Island grid (Haywards) via submarine cables. It has a capacity of approximately 1,200 MW. The converter transformers and associated equipment are highly specialised. See: Transpower HVDC documentation. https://www.transpower.co.nz/↩︎

10. Distribution transformer fleet size: NZ’s 29 electricity distributors (lines companies) operate distribution networks serving approximately 2 million consumers. Pole-mounted transformers are the most numerous type, particularly in rural areas. The estimate of 20,000–40,000+ pole-mounted units is based on industry reports and the number of distribution transformers typically required per consumer in mixed urban-rural networks. The Electricity Authority’s asset management framework requires distributors to report on asset condition. https://www.ea.govt.nz/↩︎

11. Transpower asset management plans: Published under regulatory requirements. Available from Transpower and the Commerce Commission. These discuss transformer replacement programs, fleet condition, and life-extension strategies.↩︎

12. Lines company transformer fleet aging: Several NZ lines companies have reported in their asset management plans (AMP) that portions of their distribution transformer fleets are approaching end of expected service life. These AMPs are publicly available through the Commerce Commission’s information disclosure regime.↩︎

13. Transformer life and insulation: The “life” of a transformer is fundamentally the life of its insulation. See: IEEE Std C57.91-2011; McNutt, W.J., “Insulation thermal life considerations for transformer loading guides,” IEEE Transactions on Power Delivery, 1992.↩︎

14. Transformer oil dielectric strength: Clean, dry mineral insulating oil typically has a breakdown voltage of 30–80 kV measured under IEC 60156 or ASTM D1816 standard test conditions. The exact value depends on moisture content, particulate contamination, and test electrode geometry.↩︎

15. Moisture in transformer oil: Even small amounts of dissolved water significantly reduce oil dielectric strength. At 20°C, transformer oil saturates at approximately 55–65 ppm water; at operating temperatures (60–80°C), saturation is higher but the moisture distributes between oil and paper insulation. See: IEC 60422, “Mineral insulating oils in electrical equipment — Supervision and maintenance guidance.”↩︎

16. Oil oxidation and temperature: The relationship between temperature and oil oxidation rate is well-established in transformer engineering literature. The approximate doubling of oxidation rate per 6–8°C is a general chemical kinetics relationship (Arrhenius equation) applied to oil aging. See: IEEE Std C57.106, “Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical Equipment.”↩︎

17. Dissolved gas analysis: DGA is the most widely used diagnostic technique for oil-immersed transformers. The gases produced by different fault types are well-characterised. See: IEC 60599, “Mineral oil-filled electrical equipment in service — Guidance on the interpretation of dissolved and free gases analysis”; IEEE Std C57.104, “Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers.”↩︎

18. Oil testing methods: Standard test methods for transformer oil include IEC 60156 (breakdown voltage), IEC 60814 (moisture by Karl Fischer), IEC 62021 (acidity), IEC 60567 (dissolved gas sampling), and IEC 60422 (supervision and maintenance guidance).↩︎

19. Minimum acceptable BDV: IEC 60422 provides guidance on acceptable oil condition for different transformer voltage classes. For transformers rated ≤72.5 kV, a minimum BDV of approximately 30 kV (tested per IEC 60156 with 2.5 mm gap) is typical. For transformers rated 170–420 kV, minimum BDV of approximately 50–60 kV is typical.↩︎

20. Moisture limits in transformer oil: Guidance from IEC 60422. Target moisture levels depend on transformer voltage class and operating temperature. For large power transformers, <20 ppm is a common target. For distribution transformers, higher levels (30–40 ppm) may be acceptable.↩︎

21. DGA techniques: Gas chromatography applied to oil samples extracted from the transformer. Sampling techniques (syringe method, headspace analysis) and analysis methods are standardised in IEC 60567 and IEC 60599.↩︎

22. NZ oil testing laboratories: Transpower and major lines companies either operate their own oil testing facilities or contract to commercial laboratories. Intertek, SGS, and similar firms have offered transformer oil testing services in NZ. The current status and capability of these laboratories should be verified.↩︎

23. DGA interpretation methods: The Duval triangle (Michel Duval, 1974 onwards), Rogers ratios, and IEC 60599 methods are the most widely used diagnostic frameworks. Each uses ratios of specific gases to identify fault types. See: Duval, M., “Dissolved gas analysis: It can save your transformer,” IEEE Electrical Insulation Magazine, 1989.↩︎

24. Vacuum dehydration equipment: Mobile oil treatment plants that can filter, dehydrate, and degas transformer oil are available in NZ from specialist transformer service companies and from some lines companies. Equipment manufacturers include GlobeCore, Enervac, Micafluid. The number of units available in NZ is not publicly documented.↩︎

25. NZ bentonite deposits: Some bentonite clay deposits are known in NZ (Canterbury, Northland). Whether these are suitable for transformer oil treatment (activated clay treatment) would require testing. Fuller’s earth (attapulgite) is not known to occur in NZ; bentonite is the likely domestic substitute.↩︎

26. Marsden Point refinery closure: Channel Infrastructure (formerly Refining NZ) ceased refining operations at Marsden Point in 2022 and converted to an import-only fuel terminal. Without domestic refining, NZ cannot produce new mineral insulating oil. See: Channel Infrastructure NZ. https://www.channelnz.com/↩︎

27. Dry-type transformer maintenance: Dry-type transformers are covered by IEEE Std C57.12.01 (general requirements) and maintained through insulation resistance testing, partial discharge measurement, and visual inspection of windings and connections. They are simpler than oil-filled units but more sensitive to environmental conditions (dust, humidity, chemical contamination).↩︎

28. Transformer failure causes: See CIGRE Working Group A2.37, “Transformer Reliability Survey,” Electra No. 261, 2012, which analyses failure causes from a large international dataset. The most common cause categories are: dielectric (insulation) failure, mechanical (winding displacement from through-faults), thermal (overheating), and accessory failure (bushings, tap changers).↩︎

29. Bushing failure as a proportion of transformer failures: Various industry studies report bushing failures as 10–25% of all transformer failure causes. See: CIGRE Working Group A2.37, “Transformer Reliability Survey,” Electra No. 261, 2012; IEEE Transformers Committee failure surveys.↩︎

30. Transformer rewinding procedure: Based on standard transformer repair engineering practice. See: Fink, D.G. and Beaty, H.W. (eds.), “Standard Handbook for Electrical Engineers,” McGraw-Hill; manufacturer service manuals for specific transformer types.↩︎

31. Etel Ltd: A Christchurch-based manufacturer of small transformers (distribution transformers, instrument transformers, special-purpose transformers) and provider of transformer repair services. One of the few NZ-based transformer manufacturers. Status and capability should be verified. https://www.etel.co.nz/↩︎

32. Motor rewinding as a transferable skill: NZ has motor rewinding businesses in most major urban centres. The New Zealand Electrical Contractors Association and the Electrical Workers Registration Board maintain registers of qualified electrical workers. The number specifically experienced in transformer rewinding (as distinct from motor rewinding) is uncertain.↩︎

33. Historical NZ copper wire drawing: NZ has had copper wire drawing capability — Pacific Wire Ltd (now Pacific Steel, part of Fletcher Building) historically produced wire products including copper wire. Current capability and equipment status should be verified. See also Doc #70 (Copper Wire Production) and Doc #105 (Fencing Wire and Nails).↩︎

34. Distribution transformer material quantities: A typical 50 kVA, 11 kV/400V three-phase distribution transformer contains approximately 30–60 kg of copper winding, 80–200 kg of GOES core steel, and 100–250 litres of transformer oil. Exact quantities vary significantly with design, manufacturer, and efficiency class. See manufacturer data sheets and transformer design references.↩︎

35. Transformer-grade paper: Kraft paper for transformer insulation must meet IEC 60554 (cellulosic papers for electrical purposes). Key requirements include: controlled thickness (typically 0.05–0.20 mm per layer), minimum tensile strength, low moisture content, low ash content (indicating purity), and good dielectric strength. NZ paper mills produce kraft paper but not to transformer insulation specifications. Whether NZ-produced paper could meet minimum acceptable standards requires testing.↩︎

36. Dielectric strength of natural varnishes vs. synthetic: Polyester and epoxy electrical varnishes typically achieve dielectric strengths of 20–40 kV/mm (per IEC 60464 test methods). Linseed oil-based varnishes and shellac-based lacquers, historically used in electrical insulation before synthetic alternatives, have dielectric strengths in the range of 5–15 kV/mm depending on preparation, layer thickness, and curing conditions. See: Sillars, R.W., “Electrical Insulating Materials and Their Application,” Peter Peregrinus Ltd; IEC 60464, “Varnishes used for electrical insulation.”↩︎

37. Electrical porcelain: Porcelain bushings are manufactured from specific clay bodies (kaolin-based) fired at high temperatures (1,200–1,400°C) to achieve the mechanical strength and electrical insulation properties required. NZ has kaolin deposits and ceramic manufacturing capability, though not currently specialised for electrical porcelain. The technology is well-documented and achievable with NZ materials.↩︎

38. Cycloaliphatic epoxy resin and silicone rubber: Both materials require petrochemical precursors — epichlorohydrin (from propylene) for epoxy resins, and chlorosilanes (from silicon metal and methyl chloride) for silicone rubber. NZ lacks the petrochemical processing capability to produce either. See: IEC 61462, “Composite hollow insulators — Pressurised and unpressurised insulators for use in electrical equipment with rated voltage greater than 1 000 V.”↩︎

39. High-voltage bushing manufacturing: Condenser bushings for 33 kV and above are complex manufactured items requiring precision winding of insulating paper layers with embedded capacitive grading foils, impregnation with oil or resin, and sophisticated quality control. See: IEC 60137, “Insulated bushings for alternating voltages above 1,000 V.”↩︎

40. OLTC maintenance: On-load tap changer maintenance requirements and intervals are specified by manufacturers (MR, ABB, Hyundai). IEC 60214 provides the general standard. Contact wear rates depend on switching current and frequency of operation. Typical overhaul intervals are every 5–7 years or after 50,000–100,000 operations.↩︎

41. OLTC contact materials: Standard OLTC contacts are made from tungsten-copper, copper-chromium, or similar alloys chosen for arc resistance and low contact resistance. These are imported specialty materials. Substitution with plain copper is feasible but copper erodes faster under arcing, requiring more frequent contact replacement.↩︎

42. Grain-oriented electrical steel: The definitive reference is the work of Norman Goss, who patented the grain-orientation process in 1934. Modern GOES production uses the refinements developed by Nippon Steel (Hi-B grade, using aluminium nitride inhibitor) and others. See: Moses, A.J., “Electrical steels: Past, present and future developments,” IEE Proceedings, 1990; Matsuo, M., “Magnetic properties of highly grain-oriented Si-Fe alloys,” Transactions ISIJ, 1986.↩︎

43. GOES vs. NOES core losses: Modern GOES (Hi-B grade, 0.23 mm) has specific core losses of approximately 0.8–1.0 W/kg at 1.7 T, 50 Hz. Non-oriented electrical steel of comparable silicon content has losses of approximately 2.5–4.0 W/kg under the same conditions — roughly 3–4 times higher. See: IEC 60404-8-7 (GOES specifications); IEC 60404-8-4 (NOES specifications).↩︎

44. GOES manufacturing process: See: Arai, K.I. and Ishiyama, K., “Recent developments of new soft magnetic materials,” Journal of Magnetism and Magnetic Materials, 2006; Hayakawa, Y., “Mechanism of secondary recrystallization of Goss grains in grain-oriented electrical steel,” Science and Technology of Advanced Materials, 2017.↩︎

45. Glenbrook’s unsuitability for GOES: NZ Steel’s Glenbrook mill produces structural carbon steel from ironsand using a unique direct-reduction process (Doc #89). The mill’s casting and rolling equipment is designed for carbon steel flat products. Silicon steel production requires different steelmaking chemistry (precise silicon, carbon, nitrogen, and inhibitor element control), different casting parameters, and a completely different downstream processing sequence (multiple cold rolling passes with intermediate annealing in controlled atmospheres). Adapting Glenbrook for GOES production would require essentially building a new rolling and annealing facility.↩︎

46. GOES production worldwide: As of the 2020s, GOES production is concentrated in Japan (Nippon Steel, JFE Steel), South Korea (POSCO), China (Baowu, Shougang), Germany (ThyssenKrupp), Russia (NLMK), and a few other locations. India has been developing domestic GOES production (JSW Steel) but it has been technically challenging. The total number of GOES producers worldwide is small — perhaps 10–15. See: International Energy Agency, “The Role of Critical Minerals in Clean Energy Transitions,” 2021 (which discusses electrical steel supply chains).↩︎

47. NOES as a transformer core substitute: Non-oriented electrical steel is used in rotating machines (motors, generators) where the flux direction rotates. It is not optimal for transformers, where flux follows a fixed path along the core. Using NOES in a transformer results in higher losses, which means: (a) the transformer runs hotter, (b) efficiency is lower (more energy wasted as heat), and (c) the transformer may need to be larger to achieve the same rating without overheating. For small distribution transformers (50–200 kVA), the efficiency penalty is manageable — a few percentage points of additional loss — and the transformer functions adequately. For large power transformers, the penalty becomes prohibitive.↩︎

48. Interlaminar insulation: GOES laminations are coated at the mill with a thin insulating layer (typically a glass film from the decarburisation anneal plus an applied phosphate coating). When restacking salvaged laminations, this coating may be damaged. Replacement insulation can be applied by varnishing individual laminations (time-consuming but feasible) or by inserting thin paper between laminations (historical practice, pre-dating modern coatings).↩︎

49. Core steel quantities for distribution transformers: A 50 kVA three-phase distribution transformer core typically weighs 80–200 kg of GOES. A large grid transformer (100+ MVA) core may weigh 30,000–80,000+ kg of GOES. The core from a single large transformer could, in principle, provide enough GOES for 100+ small distribution transformers if the laminations can be cut to the required dimensions.↩︎

50. Distribution transformer material quantities: A typical 50 kVA, 11 kV/400V three-phase distribution transformer contains approximately 30–60 kg of copper winding, 80–200 kg of GOES core steel, and 100–250 litres of transformer oil. Exact quantities vary significantly with design, manufacturer, and efficiency class. See manufacturer data sheets and transformer design references.↩︎

51. Etel Ltd: A Christchurch-based manufacturer of small transformers (distribution transformers, instrument transformers, special-purpose transformers) and provider of transformer repair services. One of the few NZ-based transformer manufacturers. Status and capability should be verified. https://www.etel.co.nz/↩︎

52. Transpower spare transformers: Transpower’s asset management strategy includes maintaining strategic spare transformers for contingency — particularly for the largest and hardest-to-replace units. The number and ratings of spare units held are discussed in Transpower’s published asset management plans.↩︎

53. HVDC link: The Cook Strait HVDC link (Inter-Island link) connects the South Island grid (Benmore) to the North Island grid (Haywards) via submarine cables. It has a capacity of approximately 1,200 MW. The converter transformers and associated equipment are highly specialised. See: Transpower HVDC documentation. https://www.transpower.co.nz/↩︎

54. Etel Ltd: A Christchurch-based manufacturer of small transformers (distribution transformers, instrument transformers, special-purpose transformers) and provider of transformer repair services. One of the few NZ-based transformer manufacturers. Status and capability should be verified. https://www.etel.co.nz/↩︎

55. NZ transformer workforce estimate: The estimate of 50–200 people with directly relevant transformer repair skills is uncertain. It includes staff at Etel, transformer service companies, lines company transformer engineers and technicians, and independent contractors. The actual figure requires verification through the skills census (Doc #8).↩︎