Technical Specifications
| Parameter | Value |
|---|---|
| Model | SZ20-31500/35 |
| Product Code | 1ZX.710.2205 |
| Rated Capacity | 31,500 kVA |
| Voltage Combination | 35±4×2.5%/10.5 kV (7-position OLTC) |
| Vector Group | YNd11 |
| Cooling Method | ONAN |
| Phases / Frequency | 3-Phase, 50 Hz |
| Service Condition | Outdoor |
| Short-Circuit Impedance | 10.0% |
| No-Load Loss / Load Loss | 14,100 W / 104,200 W |
| No-Load Current | 0.24% |
| Insulation Level — HV | LI200 / AC85 kV |
| Insulation Level — LV | LI75 / AC35 kV |
| Insulating Oil Weight | 10,700 kg |
| Core/Coil (Active Part) Weight | 28,600 kg |
| Total Weight | 52,280 kg |
| Tank Vacuum Withstand | 49 kPa |
| Standard Codes | GB1094.1,2-2013; GB1094.3-2017; GB/T6451-2023; GB20052-2024 |
This unit complies with GB20052-2024 as standard. Zhongxin General can also produce Grade 1 energy efficiency versions upon request, with technical standards equivalent to or higher than COMMISSION REGULATION (EU) No 548/2014 and Commission Regulation (EU) 2019/1783 of 1 October 2019. As a manufacturing factory, we also offer aluminium winding as an alternative to the standard copper winding configuration.
A 31,500 kVA OLTC Transformer at 35kV Class — What Sets It Apart
At 31,500 kVA, the SZ20-31500/35 sits among the larger distribution-class oil-immersed transformers we produce, typically specified for dedicated industrial substations, mining sites, and large process loads where a single high-capacity unit is more practical than multiple smaller transformers in parallel. The on-load tap changer allows the 7-position 35±4×2.5% tap range to be adjusted under full load, without any interruption to the connected process — critical for facilities where even a brief supply interruption carries real cost.
Every image on this page documents this specific 31,500 kVA unit through actual production stages — not a different capacity substituted to represent it. That distinction matters more at this scale than at smaller capacities: the difference in core dimensions, coil diameter, and tank proportions between a 31,500 kVA unit and, say, an 8,000 kVA unit is substantial and immediately visible to anyone who has handled both.
Core Assembly and Calibration
At this capacity, core lamination stacking requires a coordinated two-person team working with a spirit level and wrench to keep the stack square and the joint gaps within tolerance across the full core height. Even small cumulative misalignment across hundreds of silicon steel laminations translates directly into higher no-load loss and current on the finished nameplate — the 0.24% no-load current figure on this unit reflects careful control at exactly this stage.
Individual core components are finished and deburred on the assembly bench before final stacking, removing any sharp edges or burrs that could damage winding insulation during core-coil assembly.
Tap Changer and Winding Connections
Copper busbar connections between the winding taps and the OLTC contacts are made and torque-checked before the tap changer body is mounted on top of the active part. This is the mechanism that lets the SZ20-31500/35 shift between its 7 tap positions — from 32,375 V up to 37,625 V on the HV side — while the transformer remains energized and under load, the defining feature that separates an OLTC unit from a fixed-tap transformer at this voltage class.
Active Part and Electrical Connections
At this stage the full active part is visible — three-phase core, coil windings, and insulation cylinders — before it is lowered into the tank for oil filling. The 28,600 kg active part weight on this model reflects the scale of copper and steel involved at 31,500 kVA.
Wooden insulation supports carry the copper busbar leads at the top of the winding, maintaining electrical clearance and mechanical stability under the short-circuit forces this size of transformer must withstand.
Tank, Radiator and Structural Components
The corrugated radiator panels and welded tank shell are prepared in parallel with active-part assembly. Every tank is vacuum-tested to a withstand capability of 49 kPa before oil filling, confirming there are no leak paths before the 10,700 kg of insulating oil goes in.
Completed Unit — Four Sides
Viewing the finished unit from all four sides — HV, LV, conservator, and OLTC control cabinet — gives a far more complete picture of what a 31,500 kVA transformer actually looks like than a single front-on photo can. It is genuinely difficult to grasp the physical scale of a unit this size without seeing it from multiple angles in person; these four views are the closest substitute.
Why Zhongxin General — Our Advantages Over Other Suppliers
Real product photographs of the actual 31,500 kVA unit, at every stage. All 13 images on this page document this specific SZ20-31500/35 transformer — from core stacking through to the four-side completed view. This is fundamentally different from suppliers who substitute photographs of large-capacity dry-type transformers, or of a different oil-immersed capacity altogether, to represent products they have not actually built at this rating. What you see here is what left our factory.
Complete capacity-range production tooling. Zhongxin General holds production tooling covering the full oil-immersed transformer range from 30 kVA up to 100 MVA, including OLTC configurations at 35kV class and above. The 31,500 kVA unit is built with tooling matched to its own dimensions, not adapted from a smaller or larger capacity — this is also true of our dry-type transformer line, where we hold complete capacity-range molds that many competitors simply do not have, allowing faster tooling turnaround and delivery.
OLTC engineering in-house. On-load tap-changer integration, busbar connection work, and full-load voltage regulation testing are all performed at our Sichuan facility — not outsourced to a third-party OLTC supplier for final assembly.
Full in-house manufacturing cycle. Core stacking, winding, active-part assembly, tank fabrication, vacuum oil filling, and factory acceptance testing all take place on one site, giving us direct control over every stage that affects the final nameplate figures.
All energy efficiency grades available. Standard and Grade 1 versions can be supplied to GB20052-2024, with Grade 1 units meeting and exceeding EU Commission Regulation No. 548/2014 and 2019/1783 requirements for international projects.
Copper and aluminium winding both available. As a manufacturing factory rather than a trading company, we can configure either winding material to match project budget and specification.
Full routine testing, every unit, test report as standard. No transformer leaves our factory without passing the complete routine test programme, documented and issued as standard order documentation — not provided only on request.
Applicable Standards & Testing
Routine tests (every unit): winding resistance measurement; voltage ratio and vector group verification; no-load loss and no-load current; load loss and short-circuit impedance; induced overvoltage withstand test; separate-source AC withstand test; on-load tap changer operation test.
Type tests (reports available on request): temperature rise test; lightning impulse test.
Special tests (available on request): partial discharge measurement; sound level (noise) measurement; short-circuit withstand test.
All tests are conducted to IEC 60076 and GB1094 / GB/T 6451. Factory Acceptance Test (FAT) with client witness is supported.
Available Options & Configurations
- Winding material: copper (standard) or aluminium
- Voltage class: HV customizable to match local grid standards
- Frequency: 50 Hz (standard) or 60 Hz
- Energy efficiency grade: GB20052-2024 standard or Grade 1
- Tap changer: on-load (OLTC) as standard on this model, 7 positions
- Conservator sealing: oil-filled (standard) or nitrogen-sealed for long-distance sea freight
- Export wooden crating with full documentation package
Typical Applications
The 31,500 kVA 35kV-class OLTC transformer is specified for large dedicated industrial substations, mining and heavy-processing loads, power distribution hubs, and any project requiring stable voltage regulation under full load at this capacity — where multiple smaller parallel transformers would add unnecessary switching complexity and space requirements.
Inquire About 31,500 kVA OLTC Transformer
Need a quotation, technical datasheet, or Factory Acceptance Test arrangement? Tell us your grid voltage, required LV output, frequency, efficiency grade, and installation conditions — and we will configure the right unit for your project.
Contact Us for Price & Datasheet
About Zhongxin General — Oil-Immersed Transformer Manufacturer & Factory
Zhongxin General (Sichuan Zhongxin General Electric Energy Co., Ltd.) is a China-based manufacturer and factory specialising in oil-immersed power transformers and dry-type power transformers, HV/MV/LV switchgear, prefabricated substations, and energy storage systems.
Headquartered in Xinjin District, Chengdu, Sichuan Province, the company holds National “Little Giant” Enterprise status alongside National High-Tech Enterprise certification. Our oil-immersed transformer portfolio covers 30 kVA to 100 MVA, including OLTC configurations at 35kV class and above, in both GB and IEC standard configurations. Every transformer undergoes full routine testing before dispatch, with test reports issued as standard documentation.
Browse all case studies — including our 31,500 kVA OLTC transformer delivered to Alar Jing Teng Energy, Xinjiang — and company news to evaluate Zhongxin General as a supplier.
