Egypt: 66 kV/11 kV Oil-Immersed Grid Transformer

Why the specification takes this shape

At 66 kV this is a sub-transmission grid transformer rather than a distribution unit, and that distinction reshapes the specification in four connected ways.

First, it has to regulate a busbar. An 11 kV bus fed from a 66 kV network needs to stay within limits even as the upstream voltage and the downstream load both shift throughout the day, so the unit needs an on-load tap changer paired with an automatic voltage regulator — off-circuit links won't do the job. The tapping range, step size and regulation band all come from a voltage study of the network at that point, and the tap changer then becomes the transformer's only moving mechanical part and its principal item of maintenance.

Second, protection has to be unit protection. A transformer at this scale and this position in the network is protected by differential relaying with vector group and ratio compensation, restricted earth fault on the windings, and Buchholz gas and surge relays that catch developing faults inside the tank before they turn electrical. Buchholz protection isn't a backup for differential protection — it picks up slow gassing that differential relaying simply never will.

Third, the oil is as much an environmental system as an insulating one. A bunded, drained containment area sized to hold the full oil volume plus firefighting water, with adequate separation from neighbouring plant, is part of the design and part of the civil works alike.

Fourth, the desert ambient is the governing design case. IEC's standard cooling-air limits are reference values only; a site that exceeds them has to be bought with either a reduced temperature rise or extra cooling surface. Sand and dust clog radiator fins, block filters and settle on bushing surfaces, which pushes up both the required creepage class and the cooling margin.

How the system is built

Sequence of works on site

This contact sheet steps through the four stages typical of an installation of this kind: multi-axle heavy transport, placement and radiator assembly, bushing and auxiliary connection, and the finished transformer bay. The sequence follows what the tank itself will allow. The unit travels without its radiators, conservator and often its bushings, so those go on after placement. Oil work follows once mechanical assembly is complete, and the tank is never left open to desert air beyond its exposure limit. Electrical testing waits until after oil filling and the standstill period that lets entrained gas clear, and the bay is closed up and the containment finished only once the unit has been proved.

A 66 kV transformer moves as an indivisible load on a multi-axle trailer, with the route engineered before the unit ever leaves the works. Axle loading, bridge and culvert capacity, gradient, camber, overhead clearance and turning radii are all surveyed, and desert road sections are checked for bearing capacity wherever the shoulder looks soft. The tank travels braced, usually filled with dry air or nitrogen held at a positive pressure that's logged along the whole route, and fitted with impact recorders on more than one axis. On arrival, those readings are downloaded and compared against the accepted limit before the unit is off-loaded — an over-limit impact is grounds to inspect the active part rather than simply proceed. Gas pressure, dew point, and the condition of blanking plates and valves are checked and recorded at handover.

Placement uses the designated jacking pads and lifting lugs, with the lift planned around a centre of gravity that sits high and shifts further once the tank is filled. The plinth and rails are confirmed level, since an out-of-level tank throws off oil level indication and can leave a radiator bank only partly filled. Wheels or rails are then locked and the unit anchored to the seismic and wind detail specified for the bay. Radiator banks, conservator and pipework are assembled onto the tank with fresh gaskets, and valve positions are checked and logged — a closed radiator valve is one of the more common causes of an overheating transformer in service. Oil is filled under vacuum through a treatment plant, with moisture content, breakdown voltage and dissolved gas sampled before and after, and the assembly is then left to stand so entrained air can escape before any voltage is applied.

Fitting the bushings is precision work. Each one is checked for transport damage, oil level, and capacitance and tan-delta values against its test certificate, then fitted with correct gasket compression and torque sequence, with the draw-lead or connection made without twisting the internal conductor. Clearances from live terminals to earthed structure, and between phases, are measured on the as-built arrangement. Surge arresters are mounted with the shortest practicable earth lead run directly to the bay earth grid. Auxiliary work covers the Buchholz relay's mounting angle and pipe gradient — it won't function correctly on a level pipe run — the pressure relief device, oil and winding temperature indicators with their capillaries shielded from direct sun, the tap changer drive with its limit and interlock settings, and the marshalling kiosk with its heating, sealing and cable gland arrangement.

The finished configuration shows the transformer standing in a fenced bay over its containment pit, with radiators, conservator, arresters and marshalling kiosk all in place. Before energisation the unit is proved link by link: insulation resistance and polarisation index, winding resistance on every tap, turns ratio and vector group on every tap, magnetising current, and capacitance and tan-delta on windings and bushings, plus a final oil sample for moisture, breakdown voltage and dissolved gas that becomes the baseline for every future sample. Protection is proved end-to-end by injection, with differential stability checked and the Buchholz alarm and trip paths proved separately. The tap changer is run through its full range under supervision and its counter reading logged. Energisation is staged, with the unit charged unloaded, inspected, and left to soak before any load is applied.

Ways the specification can flex

For a comparable utility enquiry, Millenium can develop a grid transformer around the required capacity, impedance and impedance tolerance, vector group and neutral arrangement, with the tapping range, step size and regulation band set from the voltage study. Cooling can be ONAN, ONAF or ONAN/ONAF dual-rated, with fan staging defined and the rating declared at a stated site ambient rather than a reference ambient. On-load or off-circuit tap changing can be supplied, with an automatic voltage regulator, parallel-operation scheme and remote indication. Bushing creepage class can be matched to the site pollution level, with surge arresters coordinated to the system earthing. Protection and monitoring can extend from conventional differential, restricted earth fault, Buchholz and temperature devices up to online dissolved gas and bushing monitoring, reporting over IEC 61850, IEC 60870-5-104 or Modbus. Loss capitalisation, sound level limits, containment arrangement, and paint and corrosion class can all be specified to suit.

Information we need to quote this

• A single-line diagram of the substation showing the transformer's position in it
• Rated capacity, the ONAN and ONAF ratings required, and the loading profile including any cyclic or emergency overload duty
• The required impedance and its tolerance, vector group, and neutral earthing arrangement
• Tapping range, step size, regulation band, and whether on-load or off-circuit tap changing is wanted
• System fault level on both sides, the required short-circuit withstand duration, and the applicable insulation levels
• Protection philosophy, relay types, CT ratios, and the interlocking and alarm schedule
• Communication protocol and the substation control system's points list
• Site ambient temperature range including the design maximum, altitude, solar radiation, and sand or dust pollution class
• The loss evaluation or capitalisation formula, and any sound level limit at a stated distance
• Bay layout, containment pit arrangement, separation distances, and any firewall requirement
• Paint system, corrosion category, and any specific colour or marking requirement
• Transport route survey, permissible axle loading, crane capacity available, and delivery and site-service terms

Speak with us

Shanghai Millenium Industry Co., Ltd. (Millenium) — No. 555 Gangding Road, Lin-gang Special Area, China (Shanghai) Pilot Free Trade Zone, Shanghai, China. Tel/WhatsApp/WeChat +86 175 0213 9434 · jensen@millenium-electric.com · millenium-electric.com

Every image in this document is a representative visualization of a typical configuration — not documentary evidence of a completed project.