35kV Three Phase Power Transformer | Substation Power Transformers Supplier

Product Overview

This is an oil-immersed, three-phase, double-winding power transformer fitted with an on-load tap changer, built for networks and industrial plants where the secondary voltage has to stay inside a band no matter what the incoming supply is doing. The series covers the 10 kV and 35 kV classes as standard, with 110 kV available in the same range, and it is normally the transformer that takes the incoming supply down to the voltage the plant's distribution, drives and process equipment actually run on.

The case for regulating under load is simple. Where the grid voltage drifts, or a heavy load switches in and out through the working day, a fixed-ratio transformer passes that movement straight through to everything downstream. This unit corrects it while it stays energised — no shutdown, no manual tap change, no waiting for a maintenance window.

It is equally at home in a substation feeding a town or an industrial park, and inside a heavy plant where melting, rolling or electrolysis loads dominate. On sites that already operate an electric furnace transformer, this unit is usually the upstream stage that sets the voltage the whole plant — furnace bay included — is regulated against.

Key Features & Advantages

• Voltage held steady for the equipment that matters — because correction happens on load, process lines, control systems and instrumentation see a stable secondary voltage rather than the swings caused upstream by grid switching or a large motor starting.
• Built to arrive ready — the core and winding assembly is locked by a triple-point (top, middle, bottom) role system with anti-loosening and anti-vibration measures, so the structure is held as it was set at the works and core inspection on site is usually unnecessary before energisation.
• Oil stays inside the tank — high-strength steel plates form the tank, and every gasket and seal uses acrylate rubber chosen for ageing resistance. That combination is what keeps a joint tight through years of heating and cooling cycles instead of weeping after the first few seasons.
• Cooling sized to the duty — a self-cooled build covers base load, and a fan-cooled version gives more capacity from the same tank where the site is hot or the load sits near rating for long stretches. The model code marks which version you have.
• Three efficiency tiers, one decision — Level 1, 2 and 3 designs are available, so you buy the loss level your load profile actually justifies rather than paying for a rating that will never earn its keep.
• Configuration that fits the project — different connection and impedance arrangements are accommodated in the design, so a project is not forced into a catalogue standard that does not match its protection scheme.
• The same build discipline as the rest of our industrial range — core clamping, tank welding and oil sealing follow the practice we apply to every electric furnace transformer that leaves the same works.

On-Load Tap Changing & Voltage Regulation

The on-load tap changer is the defining feature of this series, and the regulation range is set by voltage class. In the 10 kV class the changer gives ±4x2.5% of regulation; in the 35 kV class it gives ±3x2.5%. Both step in 2.5% increments, so voltage is brought back in small corrections rather than one coarse jolt — which is what downstream electronics, rectifier feeds and drive systems need.

Operation can be manual at the transformer, motor-driven from a local control point, or fully automatic: a voltage-regulating relay watches the secondary voltage and issues raise and lower commands on its own, so nobody has to be standing at the unit when the load moves. Over a shift, that is the difference between a plant electrician chasing voltage and a transformer that holds its own.

Where regulation under load is not required, the same transformer can be supplied with an off-circuit tap changer at ±2x2.5% for the 10 kV and 35 kV classes. The tap link is then set for the site's normal supply voltage and left in position; changing it later means taking the unit out of service.

One practical point when reading the range: the taps are there to absorb movement in the supply, not to correct a badly chosen ratio. Share the expected voltage band at your point of connection — the highest and lowest levels you genuinely see, not just the nominal figure — and the tap range can be checked against it before the design is fixed.

Specifications & Parameters

Rating Basis & Project Data

• Voltage ratio and winding arrangement — give the primary voltage, the secondary voltage and any additional winding the project needs, together with the permitted deviation at each. Double-winding is the standard build; where a third winding or another arrangement is required, it is engineered for the project.
• System grounding — state whether the neutral is solidly grounded, impedance grounded or isolated. It changes the insulation coordination at the neutral end and the protection scheme, so it belongs in the design brief rather than being settled on site.
• Frequency — confirm 50 Hz or 60 Hz and the frequency deviation your grid permits. Cross-frequency operation changes the flux and loss picture and is a design input, not a nameplate detail.
• Vector group and connections — the connection group you need and the neutral arrangement must be stated, because together they decide how the unit can be paralleled and which protection functions can be applied.
• Impedance and loss guarantees — give the impedance voltage the protection study assumes and the loss values you expect to be guaranteed, so measured results can be compared against an agreed figure with the tolerance known in advance.
• Insulation level and test scope — say whether impulse or partial discharge testing is required for your market and which documents the approval file needs; the test scope is then agreed before production starts.
• Existing equipment on the same bus — list anything else sharing the bus or the protection scheme, including an electric furnace transformer already in service, so that impedance matching and protection coordination are checked together.
• Harmonic environment — if the secondary feeds a rectifier transformer bank or another DC conversion stage, give the pulse number and the expected harmonic spectrum; the winding and loss design is adjusted to suit it.
• Protection and control interfaces — tell us which indications and trip signals your protection and control system expects, and we will confirm what the standard supply provides and what is added for your project.

Key Performance Data (35 kV Series Example)

Site Conditions & Cooling

Rating figures only mean something against a stated ambient and altitude. Tell us the highest and lowest ambient temperature the site sees, the altitude above sea level and whether the transformer will stand indoors or outdoors, and the rating basis is adjusted to match. A unit specified for a mild, low-altitude site is not the same build as one that has to hold its rating through a hot summer at high altitude.

Above the usual altitude reference the air thins and carries heat away less effectively, so cooling capacity falls and the rating has to be adjusted rather than assumed. High ambient temperature does the same thing from the other direction. Both are ordinary parts of specifying a transformer; neither is a problem, provided the site data reaches us while the design is still open.

Cooling itself follows the duty. A self-cooled build suits base-load service in a moderate climate and draws no auxiliary power. Adding fans raises the usable capacity from the same tank where the site is hot, the daily peak is long, or the unit is expected to sit near its rating for hours at a stretch.

For outdoor installation, plan for solar gain, wind loading and — on coastal sites — corrosion, and the tank finish and any additional protection are agreed against the site. For indoor installation, plan the room around the unit: ventilation to carry the heat away, working clearance on every side for radiators and cable boxes, and oil containment under the tank in line with your local rules. Where the unit is over-dimensional for transport, the route becomes a design constraint in its own right — check bridge limits, turning radii and the lifting capacity available at site before the drawing is frozen.

Installation & Commissioning

1. Inspect on arrival. Check the unit and the crated accessories against the packing list, and record the condition of any transit indicators before the equipment is moved. Report damage in writing before the main tank is unloaded.
2. Set the foundation and position the unit. Place the tank on its prepared foundation using the marked lifting points, and leave the clearances the general arrangement drawing specifies for radiators, bushings and cable boxes.
3. Refit the parts that travel separately. Use new gaskets at any joint that has been opened, and torque to the figures in the manual rather than by feel.
4. Fill and settle the oil. Fill to the marked level with the grade of insulating oil specified for the unit, and let the oil stand long enough for air to clear before any electrical test.
5. Exercise the tap changer. Check the tap position indicator, run the changer through its full range and back before energisation, and set the automatic regulator to the voltage band agreed at design stage.
6. Test before energisation. Carry out the site tests your specification calls for and compare the results with the factory test report for the same unit — the report gives you the reference values, so any difference shows up straight away.
7. Energise and phase check. Confirm phase sequence and the connection group against the protection settings before the transformer is put on load, then check temperature indications and cooling control operation once it is loaded.
8. Record the commissioning result. Note the final tap position, oil level, protection settings and the site test values in the maintenance file; those figures are the baseline for every later oil sample and test.

Applications & Duty Profiles

• Distribution and industrial substations — the most common duty: taking 10 kV or 35 kV down to the plant's utilisation voltage, with 110 kV available in the same series where the site connects at a higher level.
• Melting and smelting plants — where a melt shop runs its own electric furnace transformer, this unit is the regulated supply ahead of it, holding the voltage that the furnace control is tuned against.
• Electrolysis, chlor-alkali and hydrogen production — a rectifier transformer downstream converts the AC supply into DC, and the steadier the AC voltage it is fed, the more repeatable the DC output and the cell performance.
• Rolling mills and heavy drive installations — where large drives switch in and out through the shift, regulating on load keeps the rest of the plant from seeing the dips.
• Renewable energy and grid connection points — sites connecting to a utility network are usually given a voltage band to respect, and regulating under load is how that band is held as generation and demand move.
• Plant expansion and staged projects — adding a supply stage while the existing plant keeps running, which is precisely the situation an on-load regulated transformer is built for.

Maintenance & Spare Parts

• Oil condition is the early warning — periodic oil samples track moisture and dissolved gas over time. The trend is what tells you something has changed; a single result on its own says very little.
• Renew seals whenever a joint is opened — a gasket that has been opened and refitted is no longer new, and the cost of a fresh one is small against a weeping joint on a loaded unit.
• Tap changer attention — the tap changer is the one part of the transformer with routine mechanical wear. Follow the inspection interval in the manual for the version fitted, and keep a spare contact set in the store so an inspection does not turn into an unplanned shutdown.
• Cooling system checks — on fan-cooled units, check fan operation and the control that starts them as part of the normal walk-round; a fan that has quietly stopped shows up first as an unexpected temperature rise under load.
• Temperature indications — verify the readings against the load and the ambient rather than against the gauge alone, so a drifting sensor does not go unnoticed.
• Spares worth holding — gaskets, insulating oil of the specified grade, a tap changer contact set, and the fan and control items for the cooling version you have. These are the parts that otherwise cause a wait.
• Keep the commissioning record — the final tap position, oil levels and protection settings logged at commissioning are the baseline for every later comparison; without them, a change is hard to distinguish from the original state.

Packing, Warranty & Delivery

The transformer ships fully assembled with nitrogen filling for protection. Detachable radiators, the conservator and the accessories are packed in secure crates, and the main unit is mounted on a sturdy skid base suitable for roll-on/roll-off and heavy-lift transport. Check the shipping dimensions and weight against the transport route, and confirm the lifting capacity available at site before the unit arrives.

Warranty is a standard 12 months from the date of commissioning or 18 months from the date of shipment, whichever comes first, covering defects in materials and workmanship under normal operation and service.

Several commercial points are project-specific and worth settling with your sales contact before the order is placed: the delivery term and port of loading, the packing standard your route requires, the documents your customs and approval process needs, the payment schedule, and the lead time that fits your construction programme. If the schedule is tight, say so early — production slots are far easier to align at the quotation stage than after the drawings are approved.

Technical Support & Model Matching

Every project starts with our engineering team. Share your duty conditions, drawings or existing nameplate data, and we help confirm the right rating and configuration before production begins — selection assistance, model matching, and troubleshooting before and after delivery. Backed by more than fifty years in transformer manufacturing, our engineers travel to site for installation and commissioning guidance and stay reachable remotely once the unit is energised. Functions and parameters can be customised to your project, and our quality system holds ISO9001 with IEC, CSA and CTQC testing.

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