1 Project Overview and Technical Specifications for 4 Units of 25MVA-110kV/10kV Main Transformers in Uzbekistan

This solution presents the full-process delivery of 4 SFZ11-25MVA-110kV/10kV three-phase oil-immersed power transformers, customized by Hengfengshuai Electric for the substation project of the Navoiy Industrial Park under the National Grid of Uzbekistan. With a rated capacity of 25MVA and a voltage level of 110kV, these transformers are the core power supply equipment for the industrial park substation, adapted to the local grid's operating characteristics of "significant voltage fluctuations and substantial day-night temperature differences." As required by the Uzbek side, the transformers adopt an on-load tap-changer design with a YNd11 connection group, ensuring voltage stability for industrial production equipment and regional residential power consumption. The complete cycle from design confirmation to on-site delivery is 48 days.

Main Technical Parameters of the Transformer

Parameter Name	Technical Indicator	Remarks
Model	SFZ11-25000/110	Three-phase, on-load tap-changer, oil-immersed forced air-cooled
Rated Capacity	25MVA	Continuous rated capacity
Voltage Combination	110±8×1.25%/10kV	On-load tap-changer on HV side, adapted to Uzbekistan grid fluctuations
Connection Symbol	YNd11	Standard configuration for step-down transformers
Short-circuit Impedance	Uk=10%	Complies with both GOST-R and Chinese (GB) standards
Cooling Method	ONAF	Oil-immersed forced air-cooled, adapted to -30℃ to 45℃ environment
Insulation Class	Class A	Withstands extreme temperature variations in Central Asia

2 Design and Development Phase of the 25MVA-110kV/10kV Main Transformers for Uzbekistan

2.1 Electromagnetic Scheme Design

The design team conducted specialized electromagnetic scheme design based on the Uzbek technical agreement and GOST-R, GB 1094 series standards. Using professional simulation software, core parameters such as core diameter, winding turns, and conductor specifications were precisely calculated, with a focus on optimizing short-circuit impedance, no-load loss, and temperature rise indicators to suit Uzbekistan's hot summers and cold winters. For the 25MVA capacity, the core uses high-permeability grain-oriented silicon steel (27ZH100), effectively reducing no-load loss and current, aligning with local energy-saving power supply requirements.

2.2 Structural Drawings Design

After finalizing the electromagnetic scheme, a 3-person professional technical team spent 15 days completing the full set of manufacturing drawings, including:

• Core Drawing: Determining core steps, 6-step stepped lap lamination method, high-strength clamp structure to reduce reluctance loss.

• Winding Drawing: Defining continuous winding for HV coils and helical winding for LV coils, precisely designing turn insulation thickness and oil duct distribution.

• Insulation Parts Drawing: Customizing angle rings, electrostatic shields, end rings and other insulation structures to enhance reliability under extreme environments.

• Tank Drawing: Optimizing tank structure via ANSYS finite element strength analysis to prevent leakage due to temperature difference stress.

• Component Selection: Tap-changer: Shanghai Huaming CV III 250Y/110-10193W; Bushings: Xigao brand; Gas relay and other auxiliary components are all suitable for Central Asian operating conditions.

2.3 Design Review

Upon completion of the drawings, an internal joint review was organized by the technical, process, and quality inspection teams, focusing on the rationality of the electromagnetic scheme, structural strength stability, and manufacturability. This ensured the design fully met the Uzbek technical agreement and possessed batch manufacturing and on-site adaptability. Production proceeded only after the review was passed.

3 Manufacturing Phase of the 25MVA-110kV/10kV Main Transformers for Uzbekistan

3.1 Material Procurement and Inspection

Raw materials were strictly procured according to design drawings. Key core materials include:

• Silicon Steel: High-permeability grain-oriented silicon steel; 100% magnetic property re-testing before storage to ensure low loss.

• Copper Conductor: T1 oxygen-free copper rod drawn into flat copper wire; batch-by-batch resistivity and tensile strength testing to ensure conductivity.

• Insulation Materials: Insulating pressboard, cable paper, laminated wood, etc.; strict testing of moisture content and electrical strength to prevent insulation failure in humid environments.

• Transformer Oil: Naphthenic oil (Kunlun 40#) conforming to GB 2536 standard, capable of withstanding -40℃ low temperatures, suitable for Uzbekistan's cold winters. All materials must pass Hengfengshuai Electric's SQA special inspection before storage; non-conforming materials are strictly prohibited from production.

3.2 Core Manufacturing

Silicon steel sheets were precision-processed into specified shapes by slitting and shearing lines, deburred, and evenly coated with insulating layer. Core lamination used a 6-step stepped lap process to effectively reduce joint reluctance and no-load loss. After lamination, special core tests were conducted to accurately measure no-load loss and current, comparing with design values for calibration. Upon passing, upper and lower clamps were installed, and clamping force was strictly controlled to avoid loosening during operation.

3.3 Coil Manufacturing

As the core component of the transformer, coil manufacturing was completed in a clean, temperature, and humidity-controlled workshop. During winding, precise turn count, correct transposition, and coil tightness were strictly controlled. HV coils used a continuous structure, LV coils a helical structure, suitable for the 25MVA capacity current carrying requirements. After completing each coil, DC resistance measurement and turn count verification were immediately performed to ensure compliance with standards.

3.4 Insulation Parts Processing

Insulation parts were cut and formed using CNC machining centers, ensuring dimensional accuracy error ≤0.5mm. Key insulation parts like angle rings and electrostatic shields were manually wrapped, strictly controlling the creepage path to eliminate partial discharge risks. Processed insulation parts were uniformly vacuum dried to thoroughly remove moisture, enhancing insulation stability under extreme environments.

3.5 Tank Manufacturing

Steel plates were CNC cut, welded into shape, and each tank underwent a kerosene leak test to identify weld defects. After welding, the tank was sandblasted to remove scale and rust, then primed, intermediate coated, and top-coated, achieving C4 corrosion resistance suitable for Uzbekistan's dry and dusty environment. After coating, mechanical strength tests were conducted by applying positive and negative pressure to detect tank deformation and weld reliability, ensuring no leakage during transport and operation.

3.6 Active Part Assembly

Qualified coils were precisely fitted onto the core limbs, followed by installation of insulation parts, connection leads, and tap-changer fixing. Throughout assembly, cleanliness was strictly controlled to prevent metal debris and dust residue, avoiding short-circuit faults during operation. Lead connections used silver-copper brazing, providing both mechanical strength and conductivity for long-term stable operation.

3.7 Drying Treatment

The transformer active part underwent three drying treatments, totaling 68 hours of drying time, to thoroughly remove moisture from insulation materials:

• First Drying: Pre-drying after coil winding to remove moisture absorbed during winding.

• Second Drying: Drying after HV and LV coils were assembled to eliminate moisture introduced during assembly.

• Third Drying: Vacuum drying of the fully assembled active part in a drying tank using the vapor phase drying process to ensure insulation performance meets standards.

3.8 Final Assembly

After passing drying, the active part was lowered into the tank, and accessories such as the tank cover, bushings, conservator, and radiators were accurately installed. Leads were connected precisely according to drawings, and monitoring components like thermometers and gas relays were installed. After final assembly, vacuum treatment was performed (vacuum degree ≤133Pa maintained for over 24 hours), followed by vacuum oil filling to ensure full insulation impregnation without air bubbles.

4 Testing and Verification Phase (FAT) of the 25MVA-110kV/10kV Main Transformers for Uzbekistan

4.1 Routine Tests

Before leaving the factory, each transformer from Hengfengshuai Electric must complete a full set of routine tests. All data must comply with GB 1094.3 and GOST-R standards. Core test items include:

• DC Resistance Measurement of Windings: All tap positions, three-phase unbalance ratio ≤2%.

• Voltage Ratio Measurement and Connection Symbol Verification: Ratio and polarity verification for all tap positions, error ≤0.5%.

• Insulation Resistance and Absorption Ratio Measurement: Assessing insulation moisture condition, absorption ratio ≥1.3.

• Dielectric Dissipation Factor (tanδ) Measurement: Checking overall insulation condition, tanδ ≤0.8%.

• AC Withstand Voltage Test of Windings and Bushings: Short-duration power frequency withstand voltage test for main insulation, no breakdown or flashover.

• Insulating Oil Test: Breakdown voltage ≥40kV, moisture ≤20ppm, chromatographic analysis normal.

• No-load Loss and No-load Current Measurement: Measured at rated voltage, deviation from design value ≤3%.

• Load Loss and Short-circuit Impedance Measurement: Measured at rated tap, short-circuit impedance deviation ≤±5%.

4.2 Type Tests

For the first 25MVA-110kV transformer, additional type tests were conducted to verify design rationality and extreme condition adaptability:

• Lightning Impulse Test: Full wave and chopped wave impulse tests, insulation withstands lightning overvoltage capability meets requirements.

• Partial Discharge Measurement: At 1.5Um/√3 voltage, discharge level ≤100pC.

• Sound Level Determination: Noise level ≤62dB(A), meeting low noise requirements for the industrial park.

5 Packaging and Transportation Phase of the 25MVA-110kV/10kV Main Transformers for Uzbekistan

5.1 Disassembly and Protection

After passing tests, some accessories were disassembled and protected for cross-border heavy transport:

• Accessories like bushings, conservator, and radiators were removed and individually sealed packed.

• All flange interfaces were sealed with special cover plates and filled with dry nitrogen (pressure 0.03MPa) to prevent moisture and rust.

• Residual transformer oil was retained inside the active part to maintain continuous insulation impregnation.

5.2 Packaging

The main tank was packaged with a high-strength wooden skid and double-layer waterproof/dustproof cloth. Accessories were sorted and packed into moisture-proof wooden crates with shock-absorbing foam inside. Packaging boxes were clearly marked with center of gravity, lifting points, "No Inversion," "Waterproof," "Moisture-proof," and other labels suitable for long-distance cross-border transport. Complete technical documents accompanied the shipment: factory certificate, FAT test report, installation and operation manual, and packing list.

5.3 Transportation

The 25MVA-110kV transformer main body weighed approximately 26.5 tons (excluding oil). It was transported by multi-axle semi-trailer truck by road from the production base in Shandong Province to Uzbekistan via the Khorgos port. Prior to transport, heavy transport permits from both China and Uzbekistan were obtained. The optimal route was planned jointly with local logistics teams, checking bridges and culverts for height and weight restrictions. Four 3D shock recorders were installed throughout transport to monitor vibration and shock data in real-time, ensuring no damage during transit.

6 On-site Delivery and Installation of the 25MVA-110kV/10kV Main Transformers for Uzbekistan

6.1 On-site Handover

Upon arrival at the Navoiy Industrial Park substation site in Uzbekistan, the Hengfengshuai Electric technical team, together with the Uzbek client and supervision unit, conducted joint unpacking and inspection:

• Checked the main body and accessories for damage, rust, or oil leakage.

• Verified the quantity and specifications of accessories against the packing list.

• Checked nitrogen pressure to ensure no leakage during transport.

• Read shock recorder data to confirm no excessive shock values during transport; equipment intact.

6.2 On-site Installation

Hengfengshuai Electric dispatched a professional technical team to supervise on-site installation, strictly controlling core processes:

• Positioning: The main transformer body was precisely unloaded and positioned using a large crane, leveling error ≤1mm/m.

• Accessory Installation: Radiators, bushings, conservator, oil pipes, etc., were sequentially installed, ensuring leak-proof sealing of connections.

• Vacuum Oil Filling: After vacuuming to specified value, qualified transformer oil was slowly filled at a rate ≤5m³/h.

• Standing and Air Release: After oil filling, the transformer stood for 48 hours, with multiple releases of residual air from the gas relay and piping to avoid air gaps affecting insulation.

6.3 On-site Acceptance Tests (SAT)

After installation, on-site acceptance tests were strictly conducted according to Chinese standard DL/T 5161.3-2018 and Uzbek standards. Test data were compared and analyzed against factory reports:

• Insulating Oil Test: Simplified analysis and chromatographic analysis, indicators consistent with factory results.

• DC Resistance Measurement of Windings and Bushings: Three-phase unbalance ratio ≤2%.

• Voltage Ratio and Connection Symbol Re-verification: Error ≤0.5%.

• Insulation Resistance and Absorption Ratio Measurement: No significant decrease; insulation in good condition.

• Winding Deformation Test: Waveform consistent with factory original waveform; no damage during transport/installation.

• On-load Tap-changer Operation Test: Smooth switching, precise position, no jamming.

• Cooling System Check and Switching Test: Normal automatic/manual switching, satisfactory cooling effect.

• Buchholz Relay Protection Calibration: Sensitive action, accurate signal, reliable protection function.

6.4 Energization and Commissioning

After all acceptance tests passed, the site was cleaned, and all protection devices were reconnected. Five no-load inrush current switching tests were performed to check the impact of inrush current on protection devices and listen for abnormal internal sounds. After no-load continuous operation for 24 hours with all monitoring parameters normal, no leakage, and no abnormal sound, the transformer was officially put into commercial operation with load, providing stable power to the industrial park.

7 Whole-process Quality Control

7.1 Key Control Points

• Design Review: Production proceeds only after dual review of electromagnetic scheme and structural drawings.

• Material Inspection: 100% re-inspection of core raw materials; zero tolerance for non-conforming items.

• In-process Inspection: Core testing, semi-finished random checks, complete active part cleanliness checks; full traceability.

• Factory Testing: Full-item FAT passed; report issued before shipment.

• Transport Monitoring: Continuous monitoring by 4 shock recorders; immediate investigation if data is abnormal.

• On-site Handover: 1:1 comparison of post-installation test data with factory data to ensure no anomalies.

7.2 Document Delivery

Upon project completion, a complete set of technical documentation was delivered to the Uzbek client, including:

• Transformer factory certificate.

• FAT factory test report.

• Installation, operation, and maintenance manual (Chinese-Russian bilingual).

• Instruction manuals for tap-changer, bushings, gas relay, and other components.

• Complete set of assembly drawings (electronic + paper).

• SAT on-site acceptance test report.

8 Summary of the Solution for 25MVA-110kV/10kV Main Transformers for Uzbekistan

The solution provided by Hengfengshuai Electric for the 4 units of 25MVA-110kV/10kV main transformers for the Navoiy Industrial Park substation project in Uzbekistan represents a customized achievement of "Made in China" adapted to Central Asia's extreme operating conditions while meeting international standards. From design source adaptation to Uzbekistan's grid and climate characteristics, to meticulous manufacturing control, comprehensive validation testing, cross-border transport, and full-process on-site installation services, the entire process ensures stable and reliable transformer operation.

Hengfengshuai Electric has deep experience in the Central Asian power market, with extensive cross-border project delivery expertise and technical service capability. Contact Hengfengshuai Electric (hfs@hengfengshuai.com) immediately for customized power transformer solutions that meet international standards and adapt to local operating conditions for your substation project, supporting regional grid upgrades and industrial development.