You’re either the electrical engineer working on a regional grid expansion project, a procurement manager who is working on setting up the supply for a manufacturing plant or the distributor who is dealing with a rural electrification project and every equipment quote that you get has one item in common: a 1000 kVA 35kV oil-immersed distribution transformer. When you ask your suppliers for a quote, half of them will say “S11-1000/35-0.4”, while the other half will just mention the name as “1000 kVA 35 kV oil transformer S11”. If you have been wondering about the meaning of this model number, why exactly S11 is an improvement over S9 model, and what should be the price of the device, keep reading this guide.
S11-1000/35-0.4 is the distribution transformer of choice internationally in the 35 kV category. This is a three-phase, 50 Hz oil-immersed transformer of 1000 kVA, responsible for converting the voltage of 35 kV into 400/230V. In this article, we will describe the working principles of S11 model including the meaning of all components of the model number.
Short answer: The S11-1000/35-0.4 is a three-phase, oil-immersed, 50 Hz distribution transformer with a rated capacity of 1000 kVA. It has a primary voltage of 35 kV and a secondary voltage of 400 V (0.4 kV). “S” represents three-phase configuration; “11” stands for the design series number with no-load losses lowered by almost 30% compared to an older type S9; while “1000” represents the rated capacity and “35-0.4” the voltage ratio.
Decoding the Model Number: What S11-1000/35-0.4 Means
Transformer model numbers look cryptic, but in the Chinese/ISO naming convention every part carries specific meaning. For the S11-1000/35-0.4:
| Part | 의미 | 세부 사항 |
|---|---|---|
| S | Three-phase | “S” (san xiang, 三相) indicates a three-phase unit, as opposed to “D” for single-phase |
| 11 | Design series | The 11th design series, characterized by a low-loss core using cold-rolled grain-oriented silicon steel |
| 1000 | 정격 용량 | 1000 kVA apparent power rating |
| 35 | High-voltage side | 35 kV rated primary voltage |
| 0.4 | Low-voltage side | 0.4 kV (400V) rated secondary voltage |
Thus, “S11-1000/35-0.4” can be interpreted fully as a three-phase 11-series transformer with 1000 kVA power capacity operating at 35kV on the input side and 0.4 kV on the output side. Some manufacturers indicate transformers as S11-1000/35 or S11-M-1000/35; the last letter means that the transformer unit is fully enclosed and has deep electric insulation by means of oil. When the specification contains such information as “voltage combination 35 ± 2×2.5% / 0.4 kV,” it means the transformer has off-circuit tap changer with the possibility of making adjustments by means of 5% on the first side and operating at 0.4 kV on the second side of the transformer.
What Is a 1000 kVA 35kV Oil-Immersed Transformer?
A 1000 kVA oil-immersed transformer is termed as a medium-voltage distribution transformer that receives the power from the grid at 35 kV (or from a 35 kV substation) and reduces it to the low voltage level of 400 volts – 230 volts obtained at the load end. The word “oil-immersed” indicates that the winding and the core are completely immersed in an insulating oil for purposes of insulation and cooling.
In the range of distribution transformers, the 1000-kVA transformer is large enough to supply power to factories, buildings, hospitals, and groups of hundreds of houses. At the same time, it is small enough to be transported from the manufacturer as a single unit (on a truck) and installed either on a base outdoors or in a small room of the substation. It is a standard transformer for the distribution networks of 35 kV in China, South-Eastern Asia, Africa, the Middle East, and Latin America.
The most important data from any data sheet of the transformer is:
- 정격 용량 = 1000 kVA (may be also denoted as 1 MVA)
- 전압 비율 = 35kV / 0.4 kV (with off-circuit taps at 35 ± 2×2.5% kV)
- 빈도 = 50 Hz
- 벡터 그룹 = Yyn0 (or Dyn11 upon request)
- 냉각 = ONAN (Oil Natural, Air Natural)
- 임피던스 전압 = 6.5% (for this size)
Construction & Key Components
Understanding what’s inside the tank helps you judge quality when comparing quotes. A 1000 kVA oil-immersed transformer consists of these major parts:
| 구성 요소 | 기능 | Quality Indicator |
|---|---|---|
| 코어 | Magnetic circuit that transfers energy between windings | Cold-rolled grain-oriented (CRGO) silicon steel, stepped core design, low-loss grades (30Q120/30QG110 or better) |
| HV winding | 35kV primary winding | Copper or aluminum; layer or continuous-disc winding; proper insulation class (A, E, B) |
| LV winding | 0.4kV secondary winding | Foil or wire winding; copper foil preferred for high-current LV |
| 절연 오일 | Insulation + cooling | Mineral oil per IEC 60296 (e.g., 25# transformer oil); DBPC inhibited for long life |
| Tank & radiator | Houses the active part; dissipates heat | Welded steel, leak-tested, with cooling tubes/fins or radiators; good surface treatment |
| 보존기 및 호흡기 | Accommodates oil expansion; removes moisture | Silica-gel breather with oil seal; proper oil level indicator |
| 탭 체인저 | Adjusts the voltage ratio | Off-circuit tap changer (5 taps) standard; on-load tap changer optional |
| 부싱 | Bring HV/LV connections out of the tank | Porcelain or resin bushings rated for 35kV and 1kV |
| 보호 장치 | Safety monitoring | Gas relay (Buchholz), oil temperature indicator, pressure relief valve, oil level gauge |
Two material choices dominate the price difference: copper vs aluminum windings. Copper offers lower losses and better long-term performance but costs significantly more. Aluminum is lighter and cheaper and is common in budget-oriented markets. Always confirm which metal is quoted — it’s one of the biggest price drivers, and the difference can be 20–40% of the transformer price.
Technical Specifications at a Glance
Here are the representative specifications for an S11-1000/35-0.4 transformer built to IEC 60076 / GB 1094 (typical values, confirm exact figures with the manufacturer’s datasheet):
| 매개변수 | 일반 값 |
|---|---|
| 정격 용량 | 1000 kVA |
| 전압 비율 | 35 ± 2×2.5% / 0.4 kV |
| 빈도 | 50 Hz |
| 벡터 그룹 | Yyn0 (Dyn11 optional) |
| 냉각 방법 | ONAN |
| 무부하 손실 (P0) | ~1.15 – 1.45 kW |
| Load loss (Pk at 75°C) | ~10.3 – 11.4 kW (copper) / ~11.5 – 13 kW (aluminum) |
| Impedance voltage (Uk) | 6.5% – 7.0% |
| No-load current (I0) | ~0.7 – 1.0% |
| Insulation level (HV) | LI 200 kV / AC 85 kV |
| 소음 수준 | ~60 – 65 dB(A) |
| 표준 | IEC 60076, GB 1094, GB/T 6451 |
| Oil type | Mineral oil per IEC 60296 |
| Weight (total) | ~3,200 – 4,200 kg depending on materials |
| Dimensions (approx.) | ~2.2 × 1.5 × 2.4 m (L×W×H with radiators) |
According to these numbers, the S11-1000/35-0.4 falls into the standard “T2” loss category in the Chinese system of energy efficiency classification (GB 20052), which is below high-end S13/S15 amorphous models, but still much better than outdated S7/S9 models that are still present in older networks.
S11 vs S9 vs S13: How the Design Series Compare
The design ranges for Chinese distribution transformers are marked according to their generations, with each generation being more efficient than its predecessor. The following is a breakdown of the three series where efficiency at 1000 kVA at 35 kV measures.
| 시리즈 | 코어 소재 | No-load Loss (1000kVA, approx.) | Relative Loss Level | Status |
|---|---|---|---|---|
| S7 | Hot-rolled silicon steel | ~2.0 – 2.3 kW | Baseline (highest) | Phased out (banned for new grid purchases) |
| S9 | CRGO 실리콘 강철 | ~1.7 – 1.8 kW | ~25% lower than S7 | Legacy, still common in service |
| S11 | High-grade CRGO silicon steel | ~1.15 – 1.45 kW | ~30% lower than S9 | Current mainstream standard |
| S13 | High-grade CRGO + improved design | ~0.9 – 1.1 kW | ~25% lower than S11 | Energy-efficiency premium option |
| S15 (amorphous) | Amorphous alloy ribbon | ~0.55 – 0.7 kW | ~50% lower than S11 | Highest efficiency, higher cost |
S11 series has grown to be the standard choice for utility and industrial distribution due to its ideal price-quality ratio. An S11’s ~30% lower no-load loss than S9 results in USD thousands in energy savings during a 20-year service life, which is why many public service companies worldwide now require S11 units from vendors, and China has stopped allowing S7 and older units to be used in the grid connections process.
If the project is located in a country with strict energy efficiency regulations like EU Eco-design or China’s GB 20052 Level 2, an S13 or amorphous S15 would be required. For the rest of the markets, the S11 would do the job.
Applications: Where This Transformer Is Used
The 1,000 kVA, 35/0.4kV S11 transformer is used in various applications, which include:
- Industrial sector: for plants involving production lines, motors, welding and HVAC.
- Commercial buildings: it provides power to office buildings, shopping malls, hospitals, hotels, and data centers as a distribution transformer for low voltage system.
- Rural electrification: for rural village and town distribution networks fed by 35 kV lines.
- Energy Sector: it reduces voltage from PV substations and powers wind and solar plants.
- Mining industry: it provides power to isolated job sites supplied by 35 kV distribution voltage.
- Infrastructure: it is used in airports, train stations, water pumping stations, etc.
- Electrical system modernization: it replaces outdated S7/S9 units with lower losses and higher reliability.
In all of these projects, transformers power low voltage switchboards, which in turn supply electricity to 400 V power systems. As a result, failure of such electrical equipment could lead to significant problems that must be avoided at all costs.
Key Advantages of the S11 Series
Why does electricity sell so well?
- Low no-load losses: about 30% lower than that of S9, meaning that there are fewer losses with stationary transformers.
- Long lifetime: It has been used around the world for many years.
- Low maintenance: The only thing that has to be done is to change oil and conduct periodic checkups.
- High overload capability: An oil tank has a large overload capability (e.g. 120% for at least 2 hours according to IEC 60076-7).
- Cost saved: Prices for oil transformers are lower than those for dry transformers in that range.
- Standardized: The devices were made according to IEC 60076 and GB 1094 standards so that the parts could be easily found all around the world.
- Simple installation: All the devices are delivered assembled and oil-mixed.
The oil transformers are less expensive than dry-type transformers as their prices vary by 30-50% depending on their ratings. Although it has to take into account that the oil unit requires the use of a concrete container as well as compliance with the fire regulations and maintenance rules.
Price Guide: What to Expect
Transformer pricing varies widely with material, accessories, and certification. Here’s a realistic 2026 guide for an S11-1000/35-0.4 (FOB factory, excluding freight and installation):
| 구성 | Price Range (FOB) | 비고 |
|---|---|---|
| Aluminum windings, basic accessories | $11,000 – $15,000 | Budget option; higher losses |
| Copper windings, basic accessories | $14,000 – $18,000 | Recommended for most buyers |
| Copper windings + on-load tap changer | $18,000 – $24,000 | For voltage-regulation-sensitive loads |
| Copper windings + full protection package | $16,000 – $22,000 | Buchholz relay, oil temp gauge, pressure relief, level gauge |
| Premium brand (ABB/Siemens/Hitachi) | $25,000 – $45,000+ | Brand premium, local service network |
What’s included vs. additional things: the base price often covers the transformer itself, its oil, tank, bushing, and basic accessories (which include conservator, breather, and oil gauge). Additional costs usually apply to the cost of on-load tap changers, surge insulators, temperature control cabinets, specific type of painting, third-party inspection (SGS/BV/Intertek), and shipping/crating and documentation. Always demand a complete offer based on all these line items for comparative reasons.
For transportation: a unit with a power value of 1,000 kVA weighs around 3.5–4.5 tons while the shipping cost is about $1,500–$4,000 depending on the point of destination plus local customs and transport costs.
How to Choose & Verify a Supplier
Given the transformer’s life expectancy of 20 to 30 years, the selection of its manufacturer is equally as crucial as the specification choice. Here is a simple verification process to follow.
- Request the complete datasheet: its loss values(P0, Pk), impedance, vector group, dimensions, weight and check if the data obtained matches the standards for your rating.
- Ask the factory to provide the test report. Each unit is shipped with the details of the factory routine tests(ratio, losses, insulation voltage). Note that the report should comply with IEC 60076 rules in case of IEC/GB compliance.
- Check the certificates: check the compliance with IEC 60076, factory certificate ISO 9001 as well as the requested certificates specific for your market(having CE stamps etc.).
- Check the winding material. Confirm in writing that it is copper or aluminum. If the quotation has no information about the metal used in manufacturing, it is a bad sign.
- Check the core grade: find out the silicon steel grade(e.g. 30Q120), as well as whether it is CRGO.
- Specify third-party inspection(if applicable): in case of big orders, it is better to add SGS/BV/Intertek inspection during production and in the packing stage.
- Check total landed cost: think of FOB price + freight + insurance + customs + installation, not only about the FOB price.
- Look through warranty and after-sale conditions: the standard warranty period is 12-18 months; verify the availability of spare parts and support.
Why do buyers prefer Chinese factories? China, represented by Jiangsu Subian Electric Power and other state-owned companies, produces the majority of distribution transformers. The price of products from China is usually 30-50% lower if compared to products from Europe, so distributors and EPC contractors in 60+ countries buy their 35kV transformers from Chinese factories. However, the use of factory-direct manufacturing has efficiency only if all verification procedures are followed.
Installation & Maintenance Essentials
Installation Basics
- Install on a flat base, ensuring adequate circulation in the area surrounding the tank; adhere to the minimum requirements of the manufacturer on space between bushings and radiators.
- Check the oil conditions, while looking at the silica gel (blue = dry, pink = soaked), and gauges.
- Confirm the vector group and tapping settings are in alignment with the network at hand.
- Connect the tank to the earthing system appropriately; never switch the power on to the tank that has not been earthed.
- Undertake the essential tests: insulation resistance (IR), ratio, and oil dielectric strength if the unit has been out of operation for long.
Routine Maintenance Checklist
| Interval | Checks |
|---|---|
| 월간 | Visual inspection; oil level and leak check; silica-gel color; unusual noise |
| 분기별 | Check oil temperature indicator during load; confirm cooling fins are clean |
| Annually | Insulation resistance test; oil sample analysis (dielectric strength, moisture); check Buchholz relay and pressure relief device; test tap changer operation |
| Every 3–5 years | Full oil test (breakdown voltage, acidity, DGA dissolved gas analysis); inspect tank coating; check gaskets and bushings |
Dissolved Gas Analysis (DGA) is considered the best analysis available as it gives alert about the fractions of impending failures (which involve overheating, partial discharge, and arcing). It is a cost-effective method for annual testing of any essential 1,000 kVA machine.
자주 묻는 질문
What does S11-1000/35-0.4 mean?
The designation S11-1000/35-0.4 stands for a three-phase low-loss distribution transformer of series-11(S11) which is oil-immersed and has a capacity of 1000 kVA with the primary voltage of 35 kV and the secondary voltage of 0.4 kV. In series number 11, the transformers are made of cold-rolled grain-oriented silicon steel which makes them achieve a reduction in the no-load losses by about 30% compared to those of the S9 series transformers.
How much does a 1000 kVA 35kV transformer cost?
A comprehensive S11-1000/35-0.4 oil-immersed transformer usually goes for $11,000–$22,000 FOB from a Chinese production facility, according to whether the windings are made of copper or aluminum and based on the type of tap changer being used along with other accessories. Copper-wound units with normal accessories cost $14,000–$18,000; leading European brands prices are $25,000–$45,000+. Finally, add $1,500–$4,000 for maritime freight charges to most destinations.
What is the difference between S9, S11, and S13 transformers?
They refer to the recent design generations where each has less no-load loss than its predecessor. For 1000 kVA, the no-load losses are (approx): S9: 1.7- 1.8 kW, S11: 1.15-1.45 kW (30 percent less than the previous S9); S13: 0.9-1.1 kW (25 percent less than S11). New amorphous core S15 units have around 50 percent less losses, although their cost is higher than S11. In the meantime, S11 is the currently leading technology, whereas S7 and older types are gradually being discontinued on a global scale.
Can a 1000 kVA 35kV transformer be used for a factory?
Yes, it is one of the most used capacities among medium manufacturing plants. A 1000 kVA transformer at 400V has a current capacity of about 1400A per phase (roughly 800-900 kW of continuous load depending on the operating power factor). In a factory with multiple manufacturing lines, engines, and HVAC systems, this is an ideal first size; larger factories may have multiple transformers or a bigger one.
How long does an oil-immersed transformer last?
Provided that the transformer is properly maintained, S11 oil-filled transformers have a normal working life of 25 to 30 years. To this end, some of the most important factors affecting the lifetime of oil transformers are: oil quality (annual tests for oil quality), DGA, load profile (i.e. in order to prevent continuous overloads), operating temperature, and conditions of the environment. Well-installed and well-maintained transformers may serve for more than 40 years.
참고 문헌
- IEC 60076-1: 전력 변압기 — 일반 — The core international standard governing ratings, losses, impedance, and testing of power transformers, including the 35kV class.
- IEC 60076-7: Power Transformer Loading Guide — The loading guide defining overload capability and thermal limits for oil-immersed transformers.
- GB 1094 Series (Chinese Power Transformer Standard) — The Chinese national standard for power transformers, functionally equivalent to IEC 60076 and required for grid-connected units in China.
- IEC 60296: Fluids for Electrotechnical Applications — Unused Mineral Insulating Oils — The standard specifying quality requirements for transformer mineral oil.
- U.S. DOE — Distribution Transformer Efficiency — Background on why transformer efficiency matters for energy consumption and loss cost economics.
- Jiangsu Subian Electric Power Co., Ltd. — Chinese manufacturer of oil-immersed and dry-type transformers (S11/S13/S15 series, 10–110kV), IEC 60076 certified and exporting worldwide; a factory-direct reference for pricing and supply.
결론
The S11-1000/35-0.4 is an extremely reliable transformer used in the 35 kV distribution systems. The three-phase transformer is oil-impregnated and has 1 000 kVA of power. The device’s model name serves as a description. It is characterized by a three-phase low-loss design of the S11-type, being a three-phase transformer with 1 000 kVA of power.Here is a list of what the customers should follow:
- Be as specific as you can regarding the specifications. It is essential to indicate whether the windings of the transformer have been made of copper or aluminum, the type of tap changer, accessories, and certifications (IEC 60076 / GB 1094).
- Double-check everything before you make a purchase. Ask your supplier for the detailed specifications, the factory test report, and offer with a detailed list of extras.
- Estimate the money you plan to spend realistically. The price of the transformer will fluctuate from $11,000 to $22,000 with delivery from China.
- Buy what is properly maintained. Annual testing of the oil and diagnostic tests will help you to protect an asset for nearly 30 years.
- Make smart shopping as for the suppliers of the unit in China. The right suppliers will help you to save your money significantly.
The S11-1000/35-0.4 is definitely your best choice if you are replacing your old S7/S9 unit or putting S11 for a new place.