You are in the control room of a 110kV substation at 7 a.m. on a warm July morning. Demand for air-conditioning is rising in the city and the dispatcher wants to know why the busbar voltage at the 10kV level has dropped to 9.8kV rather than staying at the target voltage of 10.5kV. In a conventional transformer that lacks a tap-changing device, this situation means interruption of service or a night spent in reconnection of the circuits. However, when you have an SZ11-31500kVA 35/10.5kV oil-immersed transformer with load tap changer, everything is resolved in an automated way and the busbar voltage returns back to the required 10.5kV in seconds.
That is the reason why utilities, manufacturing enterprises, and independent power producers continue to contact the manufacturers of oil-immersed transformers with load voltage tap changers and not much cheaper models that utilize off-circuit taps. The present article examines how the SZ11-31500kVA 35/10.5kV transformer works, what its oil-immersed transformer price is, and how to acquire it without any surprises.
The SZ11-31500kVA 35/10.5kV transformer is a three-phase, oil-immersed device with a capacity of 31,500 kVA. It has a 35 kV high-voltage winding and a 10.5 kV low-voltage winding. This unit uses an on-load tap changer (OLTC) which means it can adjust the voltage while it is operating. The no-load loss is typically around 20-25 kW and the load loss is about 130-150 kW. It has a standard impedance of 8% and usually belongs to the vector group YNd11.
What Is the SZ11-31500kVA 35/10.5kV Transformer?
The designation of the SZ11 series follows the national classification system of China for power transformers. The letter “S” refers to three-phase transformers, “Z” indicates transformers with on-load tap changers, and the number “11” signifies the series of transformer designs with no-load losses that are 30% lower than the losses of the previous S9 series according to GB/T 6451 and GB 20052. The 31500 kVA transformer makes it possible to connect the 35kV medium voltage network to the 10.5kV distribution bus, and it is frequently used in 110/35/10kV substations, large industrial parks, and renewable energy plants.
The main reason for using the SZ11-31500kVA transformer instead of a fixed transformer is the improvement of the voltage quality. The voltage in distribution networks varies continually, making it necessary for the 10.5kV bus to switch from 9.5kV to 11kV. The on-load tap changer allows for the adjustment of the voltage in the bus at all times during the operation process. The estimated tapping range for such transformers is (according to IEC 60076-1) ±8 x 1.25%, thus giving the total range of ±10%. Nevertheless, usually the SZ11 transformers designated for 35kV networks operate across the range of 35 ± 3 x 2.5% or 35 ± 8 x 1.25%, depending on the specifics of the project.
In case of a transformer of 31.5 MVA, there are two sides: one can accommodate the needs of the whole district of the town or a medium industrial plant, but it is also transportable as a complete unit. The weight of 45-55 tons makes it possible to transport the device easily through many regions.
How On-Load Voltage Regulation Works
Using an on-load tap changer means that a transformer can adjust its turns ratio even while actively conducting current. This is different than any off-line tap changes because with an off-line tap change or de-energization, the transformer must not be conducting power in any way. Both types of changes involve the same switching elements and transition system based on either resistors or reactors that facilitates tap change process without incurring arc damage to the system to be changed or interruption of any power circuit during the switching sequence.
In the case of resistor-type OLTCs, a sequence of switching steps gets performed. Thus for the OLTC of a transformer operating at 35 kV and representing one of the main types in practice, the steps in a switching process are as follows:
- Diverter switch acts as a switching element in that it connects both incoming and outgoing conductors to a transition resistor.
- Introduction of the resistor into the circuit makes it possible to keep limiting the circulating current at the moment when the transformer is interconnected to the energized system.
- The switching process gets completed within a couple of milliseconds after the initiation of voltage change at the tap.
With the exception of the practical aspect of OLTC, operating the device has much to do with the functioning of the apparatus known as automatic voltage regulator (AVR). The device can manage the OLTC process remotely by monitoring the actual voltage levels.The job of the AVR controller is to control secondary voltage values using the potential transformer as a measuring device. It sends change command when the time spent in a voltage zone is exceeded which often happens during switching procedures.
OLTC maintenance is critical in terms of costs incurred in operation of any of the transformer types. In this respect, plug-in baths of oil-immersed OLTCs are the most time-consuming part of the process since these OLTCs, such as MR type VACUTAP or ABB UZ series type switchers, need that oil in their tank be monitored regularly for performance.
| Tap changer type | Typical range | 유지보수 간격 | Cost impact |
|---|---|---|---|
| Off-circuit (DETC) | ±2 × 2.5% | Inspect only | 기준선 |
| Oil-immersed resistor OLTC | ±8 × 1.25% | 50,000–100,000 operations | +$6,000–$12,000 |
| Vacuum OLTC | ±8 × 1.25% | 150,000–200,000 operations | +$10,000–$18,000 |
For a 31.5 MVA substation unit that changes taps several times per day, the vacuum type’s longer maintenance interval often justifies its premium.
Design and Construction Features
The SZ11-31500kVA 35/10.5kV transformer refers to a conservator-type oil-immersed transformer, meaning that it has a sealed expansion tank filled with silica gel rather than a completely gas-tight tank. The main building parts include the following:
- Core: The core is made of cold-rolled grain-oriented (CRGO) silicon steel of the type 30ZH120 or 27ZH110 and can have a three-limb or five-limb stepped core. The use of step-lap joints makes it possible to decrease the losses in no-load and the noise level.
- Windings: The windings are made of copper conductor, while the 35 kV HV winding is constructed as a continuous disc winding and the 10.5 kV LV winding is done in helical or spiral manner so it can endure the high forces during short-circuit according to IEC 60076-5 standard.
- Insulation: The insulation here is made of transformer-grade insulating paper and oil-impregnated pressboard with Class A type of insulation for average temperature rise of the winding 65K and hot spot temperature rise of 78 K at 40°C level.
- On-load tap changer: On-load tap changer is located inside the tank or in a separate chamber and functions with a motor mechanism and has a position indicator and remote monitoring interface.
- Attached devices: Buchholz (gas) relay, oil level gauge, pressure relief device, oil temperature indicator, winding temperature indicator and two or more silica-gel breathers.
- Cooling: The operation mode is ONAN (oil natural, air natural) for this power levels but it is possible to switch to the ONAF option in order to increase the performance by 20-30% under short-time overload.
The tank itself is made of steel plate and is covered with two-layer paint system that protects it from corrosion. The tank is designed for vacuum tank filling procedure. The size of the oil volume for 31.5 MVA type transformer is around 9,000-12,000 liters and the total weight of the device with oil is approximately 50-60 tons which is important for foundation design and transport long before the transformer is delivered to its place of work.
Key Technical Specifications
| 매개변수 | Typical Value (IEC 60076) | 비고 |
|---|---|---|
| 정격 전력 | 31,500 kVA | ONAN / ONAF ratings differ |
| 전압 비율 | 35 ± 8 × 1.25% / 10.5 kV | Range depends on spec |
| 빈도 | 50 Hz | 60 Hz options available |
| 벡터 그룹 | YNd11 | Delta LV, grounded star HV |
| 단락 임피던스 | 8% (±7.5% tolerance per IEC) | Per IEC 60076-5 |
| 무부하 손실 | ~20–25 kW | GB/T 6451 series value |
| 부하 손실 | ~130–150 kW | At 75°C reference |
| No-load current | ~0.6–0.8% | Measured at rated voltage |
| Efficiency at full load | > 99.5% | Power factor 1.0 |
| 냉각 방법 | ONAN / ONAF | IEC 60076-2 designation |
| 온도 상승 | 65K avg winding | 40°C ambient basis |
| Total weight | ~50–60 metric tons | Including oil |
| 표준 | IEC 60076 시리즈 | Also GB/T 6451 |
We should pay special attention to efficiency. When functioning at full capacity and with a power factor of 1.0, a transformer with a capacity of 31.5 MVA, a no-load loss of 20 kW and a load loss of 140 kW achieves an efficiency of about 99.49%. Although 0.5% may seem small, after 25 years of service and an average load of 60%, this may lead to about 14–18 GWh of cumulative losses costing around $1.1–$1.7 million at a typical industrial electricity price of $0.08–$0.10 per kWh. This is why that loss capitalization is an inherent part of transformer tenders in most energy companies, and this is also why the TCO of the transformer is usually far larger than the initial price of its purchase.
| Cost element | 20 kW no-load / 140 kW load loss | 25-year value at $0.08/kWh |
|---|---|---|
| No-load losses (P0) | 20 kW × 8,760 h | $350,000 |
| Load losses (Pk) at 60% load | 140 kW × 0.36 × 8,760 h | $882,000 |
| Total loss cost | — | $1,232,000 |
| Typical purchase price | FOB, Chinese export | $65,000–$110,000 |
Losses outvalue the transformer several times over its life — the core reason loss capitalization belongs in every tender.
SZ11 vs. S11 vs. Other 35kV Types
| 유형 | 무부하 손실 | 탭 체인저 | Best fit | Relative price |
|---|---|---|---|---|
| S9 (legacy) | Reference base | Off-circuit | Budget retrofit | 가장 낮음 |
| S11 | ~30% lower than S9 | Off-circuit | Fixed-ratio distribution | +10–15% vs S9 |
| SZ11 | ~30% lower than S9 | On-load (OLTC) | Voltage-critical networks | +20–35% vs S11 |
| S13 | ~50% lower than S9 | Off-circuit | Ultra-low-loss projects | +25–40% vs S9 |
| Amorphous (SBH15) | ~70% lower than S9 | Varies | High load-factor substations | +40–60% vs S9 |
The major difference between SZ11 and a fixed-ratio transformer will usually be voltage fluctuation. For example, if you have a 35kV network that is stable, the voltage levels of the feeders will remain within ±2.5% when loading changes occur very slowly, then an off-circuit S11 or S13 fitted with a manual tap switch that operates on ±2 x 2.5% would do the job for much less initial capital cost. However, if you are supplying a fluctuating industrial manufacturing activity or a long line of feeders belonging to a renewable generation facility, the use of an OLTC transformer will pay for itself in terms of avoiding under-voltage tripping and over-voltage damage.
Typical Applications
- 110/35/10kV grid substations: SZ11 offers a transform rating of 31.5 MVA serves as the 35 kV to 10.5 kV transformation stage for secondary substations that control bus voltage for the whole region.
- Industrial plants: Big plants for steel production, cement processing, chemical application or mineral extraction with engines or arcs result in drops in voltage level that makes continuous regulation beneficial
- Renewable energy collection: Solar stations or wind farms that have very big variations with generation process; OLTC is used to enable the collection bus to work legally
- Urban distribution: Mixed feeders providing both commercial and residential consumers facing peak loads in the evening having a need for some regulatory maintenance of voltage at the level of 95% of the nominal one.
- Electrified infrastructure: Data centers, transport networks, and medical campuses where quality of power is a matter of contract.
For many of the projects noted above, the transformer can operate at high voltage regulation levels as per ONAN mode of cooling and at the level of up to 100% rated load, and accept overload at the level of 130% for two hours with the help of fans, according to the IEC 60076-7 recommendations. One of the reasons the 31.5 MVA transformer is so popular among developers and engineers is great flexibility in operation
주요 브랜드 및 가격 범위
The 31.5 MVA class is a genuinely global market. Below is an indicative price comparison; actual figures vary with copper price, loss level, OLTC brand, and destination.
| 브랜드 | Origin | Typical price (USD, FOB) | 비고 |
|---|---|---|---|
| ABB | Switzerland/Global | $130,000–$190,000 | Premium, global service network |
| 지멘스 에너지 | Germany/Global | $125,000–$185,000 | Strong in utility tenders |
| 히타치 에너지 | Japan/Global | $130,000–$200,000 | High-spec, long lead times |
| Hyundai / Toshiba | Korea/Japan | $110,000–$170,000 | Industrial grade quality |
| 슈나이더 일렉트릭 | France/Global | $120,000–$175,000 | Distribution focus |
| Indian makers (TBEA, EMCO) | 인도 | $80,000–$120,000 | Competitive for regional projects |
| 장쑤 수비안 전력 | 중국 | $65,000–$110,000 | IEC 60076 certified, export-focused |
International organizations like ABB, Siemens Energy, and Hitachi Energy have lead the charge in engineering expertise and post-sale service so their prices are reflective of their status. Their main competitors in terms of technology are Chinese and Indian manufacturers, who can meet the majority of the technological requirements at significantly lower prices due to their fully established supply chains for grain-oriented steel, copper, and insulation materials.
As an example of a company producing reliable oil-immersed power transformers up to 110kV, Jiangsu Guomai Electric Co operates with compliance with IEC 60076 standards. The daily production of Jiangsu Guomai includes modern 35/10.5kV SZ11-31500kVA transformers, which are a standard product for the company and are built and tested in accordance with IEC 60076 guidelines for testing materials including losses, resistance, and temperature. The product line of the factory includes transformers of different versions for both energy suppliers and electrical equipment installation contractors in Asia, Africa, and South America including transformers built to order.Visit subian-electric.com for product ranges and test documentation.
How to Choose This Transformer
- Confirm loss specification. Losses are converted to lifetime cost. Compare bids based on TCO from local tariff and not just first price.
- Specify the tapping arrangement. Decide on whether to go for ±8 × 1.25%, ±6 × 1.67%, or ±3 × 2.5% based on the voltage profile of your network. Do not let the manufacturer dictate this process for you.
- Select OLTC brand wisely. MR and ABB OLTC are more expensive but provide higher reliability, whereas generic OLTC can save you between $5,000 and $10,000 on price at the expense of more frequent maintenance.
- Make sure to check the short-circuit withstand. Request IEC 60076-5 report or design justification if faults occur in your network.
- Check out transport dimensions. The length of the unit is around 4.5–5.5 meters and it weighs 50–60 tons. Make sure that transportation via bridges or roads is allowed.
- Confirm cooling mode. Specify ONAN only or ONAN/ONAF in case of expecting overloads.
- Request full FAT. Witness/ videotape the routine tests and ask for test certificates as a part of tender.
Installation, Testing and Maintenance
The installation process for conservator-type oil transformers involves a standard procedure. The device has been supplied with nitrogen gas dry transportation equipment or with oil, put on 300 mm or higher foundation for drainage purposes, and connected to bushing turret and cable boxes. The following actions should be taken by the site crew before the device is switched on:
- Determine insulation resistance on all windings using megger. For the unit that has been designed for 35kV the acceptable value of insulation resistance is over 1000MΩ at 20°C.
- Test oil dielectric strength of the transformer. The oil must meet the minimum dielectric strength of 50kV/2.5mm.
- Confirm from Buchholz relay and pressure relief system that alarm and trip circuits are operating properly.
- Conduct OLTC operation in the full range without load.
- Hold the device for 168 hours without load while performing DGA sampling.
Routine checks for SZ11-31500 unit may include quarterly checking of a breather if its silica gel is in the pink state as well as yearly oil sampling for DGA and OLTC working oil condition checks as per the manufacturer system requirements.
| Activity | 빈도 | Key check |
|---|---|---|
| Visual inspection | 분기별 | Breather silica gel, oil level, leaks, corrosion |
| Oil sampling and DGA | Annually | Key gas ratios per IEC 60599 |
| OLTC service | Per operation count | Contacts, oil condition, motor mechanism |
| Protective device test | Annually | Buchholz gas/surge trips, PRD, alarms |
| Insulation resistance | Every 2–3 years | Megger readings trending vs. baseline |
Ignoring DGA trends is the most common cause of premature failure, so budget for a DGA lab contract even if the transformer itself was cheap.
자주 묻는 질문
How much does an SZ11-31500kVA 35/10.5kV transformer cost?
A rough estimate indicates that one can expect the price of a Chinese-made oven to be around $65,000–$110,000 FOB, whereas European or Japanese manufacturers charge somewhere in the range of $120,000–$190,000, exclusive of the costs related to transportation, insurance, or site works. The estimated price finally depends on the loss level, OLTC brand, extra equipment, and prices of copper at the moment. Transporting a unit with a weight of 50–60 tons adds $8,000–$25,000, with the total depending on the distance between the origin and the destination ports.
What does the “SZ11” designation actually mean?
Here, S represents the three-phase designation and Z signifies the on-load tap changer, known as “有载调压” in Chinese. The number 11 indicates that this is a loss-reducing design (this has approximately 30% lower no-load loss than the S9 series). Hence, the designation of SZ11 indicates a three-phase transformer with an on-load tap changer at the modern 11-series loss levels as per specification GB/T 6451.
Can this transformer run on 60 Hz systems?
The answer is yes, due to the redesigned core and winding specifications. The selection of frequency between 50/60Hz will influence flux density and thus core dimensions should always specify the operating frequency in the inquiry for recalculating the motor ratings as per 50Hz cannot be directly shifted to the 60Hz without verifying the no-load losses and saturation limits.
How often does the on-load tap changer need maintenance?
A resistor-type oil-immersed on-load tap changer (OLTC) should be inspected after every 50,000 to 100,000 operations or 5 to 7 years, whichever is sooner. Vacuum-type OLTCs are able to reach around 150,000 to 200,000 operations before inspection. Track the number of operations using the motor mechanism’s counter and follow the OLTC manufacturer’s schedule, as this will likely be more frequent than the transformer’s.
Why choose 10.5kV rather than 10kV or 11kV on the LV side?
10.5kV has become the designated design voltage for the Chinese distribution systems 10kv class where the nominal voltage is 10kv with an allowance of 5%. The implementation of a design voltage 10.5kv takes into account the impedance drop of the transformer during the load process with the voltage in no-load condition adjustable to approximately 10.5kv and the loaded bus delivering a voltage of 10kv. In the areas applying 11kv networks, which are widely used in Asian and Middle East countries, 11kv should be mentioned instead.
참고 문헌
- IEC 60076-1: Power transformers – Part 1: General — Defines ratings, tapping, and temperature rise requirements for power transformers.
- IEC 60076-5: Ability to withstand short circuit — Specifies short-circuit withstand requirements and test procedures.
- IEC 60296: Fluids for electrotechnical applications – Mineral insulating oils — Sets quality requirements for transformer insulating oil.
- IEEE C57.12.00: Standard for Liquid-Immersed Distribution and Power Transformers — The North American counterpart to IEC 60076, useful for comparison.
- Maschinenfabrik Reinhausen (MR) on-load tap changers — Manufacturer documentation on OLTC operation and maintenance intervals.
- Hitachi Energy transformers — Reference product information for large power transformers and OLTC systems.
- Jiangsu Subian Electric Power official site — Manufacturer product ranges, certifications, and contact information.
결론
The SZ11-31500kVA 35/10.5kV oil-cooling on-load voltage regulator transformer is designed for implementation in medium voltage networks with a stable voltage under the variations of load conditions. It achieves a remarkable efficacy due to utilizing the type of oil cooling that features a conservator and the possibility to change the taps at any moment with over 99.5% efficiency.Essential things to keep in mind:
- Prepare budget of $65,000-$110,000 FOB for a unit from China; European and Japanese brands will cost more.
- Calculate losses under the lifetime cost concept, as they will exceed the cost of the equipment in 25 years.
- Deliberately choose the OLTC brand and tapping range, as they are determining the costs of operation in long-term perspective.
- Plan transportation, installation testing, and routine DGA maintenance from day 1.
In case you are looking for the manufacturers, Jiangsu Subian Electric Power will supply you with the IEC 60076-compliant SZ11 equipment for the export prices by providing the factory testing documentation at your request.Check their range at subian-electric.com before you finalize your tender shortlist.