24 Pulse Rectifier Transformer | Integrated Special Transformer for Industrial DC Conversion

Product Overview

A rectifier transformer built for continuous DC duty. This 24 pulse rectifier transformer takes medium-voltage AC from the grid and delivers clean, stable DC to rectifiers, cell lines and drive systems that cannot tolerate a distorted supply. One regulating unit and four phase-shifted rectifying units sit inside a single oil-filled tank, so the 24-pulse output is produced by the transformer itself rather than by two separate 12-pulse units wired together on site.

The core is stacked from high-permeability grain-oriented silicon steel sheets, and the phase-shift windings are immersed in insulating mineral oil. The welded steel tank is finished in light grey anti-corrosion paint and carries the conservator and the on-load tap changer assembly on the tank top, with radiator banks, porcelain bushings, a nameplate panel on the tank side and base rails for skidding the unit into its bay.

Ratings are engineered to the project rather than taken from a catalogue. Capacity runs from 1800 kVA to 28750 kVA, primary rated voltage is 10 kV with ±2×2.5% adjustment, short-circuit impedance is custom engineered, typically ≤8%, and the secondary voltage and current are set to match the rectifier they feed.

Most of these units go to four kinds of site: electrolysis plants running long cell lines, railway and industrial traction substations, large variable frequency drive installations, and metallurgical works carrying heavy dynamic loads. Whatever the process, the same three things are being bought here: a quiet DC side, low harmonic current returned to the grid, and a conversion stage that occupies one bay instead of two.

Cooling follows the site. ONAN oil-immersed cooling suits normal ambient conditions, while ONAF adds fan-assisted cooling where the duty is heavier or the ambient is hotter. Tank, winding clamping and insulation are arranged for sustained heavy load rather than short peaks, which is what continuous electrolysis and drive duty actually demand.

Harmonic Performance

• 24-pulse operation comes from the winding design — The four rectifying units are phase-shifted against one another inside the tank, so the DC side is fed from a 24-pulse waveform and the 5th, 7th, 11th and 13th low-order harmonics are effectively cancelled at source rather than filtered downstream.
• The public grid and your own equipment both benefit — Less distortion travels back into the incoming supply, which matters where a utility or a plant power-quality rule limits harmonic injection, and where drives, instrumentation and control electronics share the same busbar.
• Power factor is easier to manage — Reducing low-order current harmonics lowers the reactive burden the conversion stage places on the supply, so the compensation equipment on the project can be smaller. Agree the final harmonic and power-factor targets with your utility or plant engineer before the design is frozen.
• A calmer DC side for the process — Stable current output is what long electrolysis cycles and large drive DC links need; the smoother the current the rectifier sees, the less stress on the valves. Ripple measured at the DC busbar depends on the rectifier topology, so the ripple limit belongs in the same conversation as the transformer design.
• The pulse number is fixed once the windings are built — 24-pulse behaviour is set by the internal phase shift. If your specification calls for a different pulse count, raise it at enquiry stage; the winding arrangement is not something that can be changed after manufacture.
• When 24 pulses is the right answer — It earns its place on larger rectifier loads, on weaker incoming supplies, in plants that already carry a harmonic background, and in expansions where the utility has capped the distortion a new unit may add.

Integrated Single-Tank Construction

In a conventional 24-pulse scheme, two 12-pulse rectifier transformers stand side by side, each with its own primary termination, protection set, cooling equipment and oil system, and their outputs are combined further down the line. This unit integrates the regulating winding and the four phase-shifted rectifying windings in one tank, so the project has one primary cable termination, one protection scheme and one oil system to design, inspect and maintain. For a project engineer that is a single line item with a single set of drawings instead of two machines that have to be matched to each other at commissioning.

The difference shows up on the layout drawing first. Halving the number of transformer bays frees floor area in the substation or the process building, and it removes the inter-connections between two units that would otherwise have to be routed, supported and inspected. Fewer oil-filled units also means a smaller contained area and fewer separation provisions to satisfy in the electrical room.

Erection is shorter for the same reason. The unit ships on heavy-duty steel skid base rails with lifting lugs for hoisting, and the welded steel tank is built with reinforced clamping around the core and windings so the assembly holds its position through transport and handling. Radiators, conservator, bushings and the control cabinet travel detached and are assembled on site following the outline drawing.

In the standard arrangement the tank is light grey, the conservator sits on top of the tank, porcelain bushings stand on the lid, radiator banks run along the tank walls and the nameplate panel is fixed to the side wall with the rating data on it. Oil and pressure gauge fittings and valve tappings are positioned where the maintenance crew can reach them without breaking the oil seal.

The tank is sealed and finished in light grey anti-corrosion paint, which is the finish these installations need where heat, dust and humidity are constant. Because there is only one tank, the oil volume, the sampling routine and the leakage points a maintenance team has to watch are all reduced compared with a two-unit arrangement.

Specifications & Parameters

Rating & Configuration Options

• Capacity — 1800 kVA to 28750 kVA, built around the DC load rather than a catalogue step. Define it from the rectifier's rated DC current and DC voltage, add the losses and whatever margin your project rules require, and we check the result against your single-line diagram.
• Primary rated voltage — 10 kV with ±2×2.5% adjustment through the on-load tap changer. If your incoming level differs, or the tap range has to cover a wider band, raise it at enquiry stage: the tap winding is designed in, not adjusted later.
• Secondary voltage and current — customized to rectifier requirements. The valve group arrangement, rectifier topology and DC regulation range all feed into the secondary design, so send the rectifier data sheet with the enquiry.
• Short-circuit impedance — custom engineered, typically ≤8%. This single figure sets how fault current is limited, how parallel units share load and how the upstream protection behaves, so it should be agreed with the rectifier supplier so that both sides work to the same number.
• Winding structure — one regulating unit plus four phase-shifted rectifying units, integrated in a single tank. This is the arrangement that produces the 24-pulse behaviour; it is not a later add-on.
• Cooling mode — ONAN or ONAF oil-immersed cooling, chosen against duty cycle, ambient temperature, altitude and whether the unit stands indoors or outdoors.
• System frequency — the design follows the frequency your project operates on. If the site runs at 60 Hz rather than 50 Hz, or the operating frequency deviates from nominal, say so with the enquiry: the design and any derating are agreed at that point rather than assumed afterwards.
• Connection group, earthing and protection interfaces — vector group, earthing arrangement, relay interfaces and the conditions for parallel operation are agreed from your drawings before the winding design is released.
• DC interface data — permissible ripple and harmonic limits on the DC side, the valve group interface points and the expected load step response are agreed together with the rectifier supplier.
• Insulation level — insulation level, power-frequency withstand and partial discharge criteria are set for your system voltage and for the market the unit is going into.

Large-Scale Electrolysis Production

Electrolysis plants run cells for months without a break, and the DC current feeding those cells is close to the product itself. A transformer on this duty is sized for continuous current, not for an average over a working day, so the rating, the cooling and the winding clamping are all arranged around long, steady operation.

The 24-pulse arrangement keeps the low-order harmonic current produced by the conversion stage out of the plant's own distribution network. In a works where rectifier units, pumps, compressors and control systems share the same supply, that is the difference between a quiet network and one that needs additional filtering to stay inside its power-quality limits.

Space matters as much as power quality. Compared with a dual 12-pulse arrangement, the single integrated tank removes one transformer bay, its oil containment and the interconnecting buswork, which reduces construction and installation spending on a cell-line building that is usually built tight around the process equipment.

To size the unit we need the DC current per cell line, the number of circuits, the rectifier topology and the DC voltage range the process will run across. Where a plant is adding cell lines to an existing hall, the same information plus the existing supply fault level lets us check the harmonic position before a utility objection appears.

Because the tap changer sits on the tank top, grid voltage variations can be corrected without interrupting the DC side, and the light grey anti-corrosion finish together with the sealed oil system is chosen for the warm, humid and often chemically loaded atmosphere of an electrolysis hall.

Traction Power Supply Systems

• Designed around the duty cycle, not the average load — Traction duty repeats: a substation may sit lightly loaded and then take heavy current within seconds. The oil-immersed construction and reinforced winding clamping are arranged for that pattern rather than for a smooth industrial load.
• Stable DC for the supply point — Railway and industrial traction systems need a DC quality that vehicles and their control equipment can rely on. The 24-pulse conversion keeps the current delivered to the traction supply stable through the peaks.
• Frequent fluctuation handled with a single unit — One integrated transformer replaces the two-unit arrangement that would otherwise be installed and maintained in the traction substation, which simplifies the wiring on site and lowers the maintenance workload for a remote substation that is not staffed continuously.
• Fewer components to coordinate — One primary termination, one protection scheme and one cooling system mean fewer interfaces for the signalling and SCADA team to map, and fewer drawings to keep current through the life of the substation.
• What we need for the design — The traction duty cycle, any short-time overload requirement written into the specification, the DC voltage window, the number of feeding points and the substation's ambient and enclosure conditions. Send the requirement curve with the enquiry and the design follows it.

Heavy-Duty VFD Projects

Large variable frequency drive installations take their power through a rectifier front end, and the transformer feeding that rectifier decides how much distortion leaves the drive room and travels back into the plant. Phase-shifted windings inside the tank keep the low-order harmonic current produced by the conversion away from the upstream grid and away from the sensitive electrical devices that share it.

On the drive side, the transformer delivers stable power to a large motor drive, which is what prevents the nuisance trips and derating that unstable DC link voltage causes. The 24-pulse arrangement works together with the drive's own rectifier configuration, so the transformer design and the drive data sheet should be read together rather than separately.

Short-circuit impedance is the connecting number between the two. It shapes the fault current the drive and its protection must handle and it influences how the DC link behaves under a step load, so the figure is agreed with the drive supplier before the winding design is released.

Voltage and capacity are customized to match the drive parameters in the project, which matters when a single transformer feeds several drives or when a drive is added later to an existing installation. If more drives are planned, mention them with the enquiry so the secondary arrangement, the impedance and the cooling have room for the future load.

To prepare a design we need the drive data sheet, the DC bus voltage, the motor rating, the number of drives per transformer and the harmonic background already present on the plant supply.

Metallurgical & Industrial Power Conversion

• Heavy and dynamic loads — Metallurgical plants put a transformer through load patterns that change repeatedly through a shift. The reinforced clamping, the oil-immersed cooling and the sealing arrangement are built for that environment rather than for a steady base load.
• New build and capacity expansion — The unit suits both greenfield power conversion stations and expansion projects where additional capacity has to be added into a building that already has little room left. Outline drawings and interface data can be prepared early so the civil and electrical design can proceed in parallel.
• Working alongside other heavy loads — In a smelter the DC conversion stage rarely stands alone on the incoming supply; an electric furnace transformer feeding the melting shop may already be connected to the same busbar. Share the existing fault level and harmonic background with our engineers so the impedance and the harmonic performance are agreed against the real system rather than against an isolated calculation.
• Furnace melting duty is a separate selection — A furnace transformer feeds arc, induction or holding furnace loads directly, with its own secondary and regulation requirements. This unit feeds a rectifier that produces DC. If your project covers both duties, they should be specified as two separate items instead of one oversized machine.
• Dust, heat and a corrosive atmosphere — The welded steel tank and its light grey anti-corrosion finish are chosen for installations where the air is hot, dusty and often loaded with process fumes, which is typical of a melting or casting shop.
• Budget certainty at project level — Minimising the equipment area also reduces the civil, cabling and erection scope around it, which is usually where an expansion project's cost overrun begins. That is the practical meaning of the lower overall project budget on a single-tank design.

Installation & Commissioning

1. Check the site and the bay before the unit arrives. Confirm the foundation, the base rail positions, the clearance around the radiator banks, the oil containment and the separation for an oil-filled unit. Where the bay also holds a furnace transformer or other oil-filled equipment, keep the separation distances the local code requires.
2. Inspect the shipment against the documents. Compare the tank and the accessory crates with the packing list and the outline drawing before lifting anything, and record any transit damage on the delivery documents.
3. Place and level the unit. Hoist on the lifting lugs and set the skid base rails on the foundation; keep the lift vertical and avoid shock loading on the tank or on the bushings.
4. Assemble the detachable parts. Fit radiators, conservator, bushings and the control cabinet following the outline drawing, and check that every flanged joint and gasket is correctly seated before oil filling begins.
5. Fill and treat the oil. Evacuate, oil-fill and circulate as the manual requires so the windings are properly impregnated, then verify the oil dielectric strength before energisation.
6. Connect, earth and set the protection. Terminate the primary and secondary connections, earth the tank as the drawing requires, and set the protection relays and the tap changer controls to the agreed values.
7. Commission step by step. Carry out the pre-energisation checks, energise on no load, then bring the rectifier load on while watching current balance, temperature and tap changer operation before handover.

Factory Testing & Documentation

Every unit completes full routine and type tests before delivery, and the equipment ships with the complete factory test report, outline drawings and an operation manual. Testing follows the IEC and GB framework for industrial power conversion. Where a project requires more, third-party inspection is acceptable on request.

Insulation level, power-frequency withstand and partial discharge criteria are set for your system voltage and for the market the unit is going into, so the test programme matches the acceptance rules that your engineer or the inspecting authority will apply rather than a generic sheet.

Loss guarantees are treated the same way. No-load and load loss values, the tolerance that applies to them and the test conditions they are measured at are agreed at the design stage and recorded in the report, so the figures relate to the rated conditions of your order.

Where efficiency has to be demonstrated at part load, the 25%, 50%, 75% and 100% load points and the measurement method are agreed in advance, which means the report answers the tender question the first time and does not need re-testing.

Oil dielectric strength, ratio and winding resistance, insulation resistance and functional checks on the tap changer form part of the routine programme. If your specification calls for a special test, a witnessed run or an additional inspection stage, raise it with the enquiry so it can be planned into the production schedule instead of being added at the end.

Ordering & Delivery

• What to send with an enquiry — rated DC current and DC voltage, rectifier topology and valve group arrangement, primary voltage and required tap range, system frequency, required short-circuit impedance, cooling preference, connection group and earthing, site location and ambient conditions, and the required delivery window.
• Scope of a single project — Where the same project also covers an electric furnace transformer, a power transformer or switchgear, the units can be engineered together against one set of drawings and shipped under one shipment plan.
• Packing and transport — The unit is fixed on a heavy-duty steel skid base for hoisting and long-distance shipment, and the main tank is sealed and filled with dry nitrogen for transit. Detachable items such as radiators, conservators, bushings and control cabinets are packed separately in sturdy waterproof wooden crates, and technical documents and spare parts are packed independently to avoid transit damage.
• Warranty — 18 months after shipment, or 12 months after successful commissioning, whichever comes first. It covers manufacturing and material defects; damage caused by improper installation, overload or wrong operation is excluded.
• What to confirm before the order is placed — system frequency and any operating deviation, connection group and earthing, the agreed impedance, the tap range, the guaranteed loss values and the cooling mode. These are the items that shape the design and are difficult to change once production starts.
• Terms to confirm with your sales contact — lead time follows the rating and the amount of customization in the design; payment terms, shipping arrangement, port of loading and any inspection requirement should be confirmed with your sales contact when the technical data is settled.

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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