From August 28 to 29, 2025, SCOTECH hosted a crucial product acceptance event – representatives from a South African client visited the site in person to conduct a comprehensive acceptance inspection of two 20/22 MVA 33/11kV liquid-immersed power transformers. The acceptance was carried out in strict accordance with international standards and the mutually agreed test procedures. Covering everything from the verification of core transformer parameters to a full set of routine tests, every link was executed with meticulous care. Ultimately, both units passed the acceptance with excellent performance, laying a solid foundation for SCOTECH to further expand its presence in the South African power market.
The two liquid-immersed power transformers undergoing acceptance are high-quality products customized by SCOTECH to meet South Africa’s power needs, featuring flexible capacity adaptation. When operating under ONAN (Oil Natural Air Natural) cooling conditions, the maximum operating capacity reaches 20 MVA; when switched to ONAF (Oil Natural Air Forced) cooling mode, the capacity can be increased to 22 MVA, fully satisfying the load requirements of different power consumption scenarios in South Africa. The core parameters of the products fully comply with the pre-agreed terms: the rated primary voltage is 33 kV, the secondary voltage is 11 kV, the vector group is Dyn1, the rated frequency is 50 Hz, the voltage regulation method adopts ±8×1.25% on-load tap changing (OLTC), the guaranteed impedance at the principal tapping is 14.29%, the guaranteed no-load loss is 15 kW, and the guaranteed load loss is 124 kW. All key indicators have reached internationally advanced levels.
During the acceptance process, representatives from the South African client collaborated closely with SCOTECH’s technical and quality inspection teams. With international standards and the mutually confirmed test specifications as the core basis, they completed the verification of 13 routine tests one by one, conducting a comprehensive assessment of the transformers’ safety performance, electrical performance, and reliability. In the measurement of dissolved gases in dielectric liquid phase, the team took samples from each independent oil compartment (excluding the diverter switch compartment) before all tests and after insulation tests. Gas chromatography was used to detect the contents of hydrogen (H₂), methane (CH₄), ethane (C₂H₆), and other gases. The final data showed that before the test, the hydrogen content was <10 ppm, the total hydrocarbons (C₁~C₂) were <20 ppm, and acetylene (C₂H₂) was 0. After the insulation test, no abnormalities were found in the oil chromatography results.
In the measurement of voltage ratio and check of phase displacement test, the joint team measured the voltage ratio between the high-voltage (HV) and low-voltage (LV) sides across all 17 tapping positions of the transformers. The HV side tapping voltage ranges from 29,700 V to 36,300 V, and the LV side rated voltage is 11,000 V. The test results showed that the measured voltage ratio error at each tapping position was less than ±0.5% of the specified value, and the vector group was confirmed to be Dyn1, which was completely consistent with the design drawings and agreement requirements. This verified the correctness of the tap lead connections and the accuracy of the winding voltage ratio. In the check of ratio and polarity of built-in current transformers test, technicians measured the ratio of the current transformers and the DC resistance on their secondary sides. It was confirmed that the ratios (such as 400/1, 1200/1, etc.) and polarities met the drawing requirements, and the DC resistance values on the secondary sides showed no significant difference from the factory values, ensuring the accuracy of current monitoring during the transformer’s operation.
The measurement of winding resistance test strictly adhered to the requirements of Clause 11.2.3 in the IEC 60076-1 2011 standard. After the transformers were filled with insulating liquid and kept unenergized for at least 3 hours (with the temperature difference between the top and bottom liquid not exceeding 5 K), the line-to-line and phase-to-phase resistances of the high-voltage and low-voltage windings at all tapping positions were measured. The results indicated that the line-to-line resistance unbalance rate was <1%, and the phase-to-phase resistance unbalance rate was <2%, which met the acceptance standards. This fully demonstrated the excellent quality of the winding material, reliable welding quality, and tight joint connections. In the check of core and frame insulation phase, since the core and frame grounding of the transformers could be accessed after the liquid was filled, the team applied a 2500 V DC voltage for 1 minute. The measured insulation resistance between the core and tank, between the clamp and tank, and between the core and clamp was all ≥500 MΩ, with no breakdown phenomenon. This effectively prevented local overheating issues caused by poor core insulation.
In the electrical strength tests, the applied voltage test (AV) involved applying a 70 kV/50 Hz voltage to the HV side and LV side respectively for 60 seconds. There was no sudden drop in voltage during the test, verifying the ability of the line terminals and neutral terminals to withstand applied voltage against the ground and other windings. In the induced voltage withstand test (IVW), the LV side was used as the test terminal. At tapping position 9, the corresponding voltage was applied with an induction multiple of 2, a frequency of 150 Hz, and a duration of 40 seconds. No voltage collapse occurred, assessing the insulation withstand strength of the transformer’s longitudinal insulation. The operation test on on-load tap changers was divided into three stages: when the transformer was de-energized, 8 complete operation cycles were completed (from one end of the tapping range to the other and back); when the transformer was energized at no load with rated voltage and frequency, 1 cycle was completed; when one winding was short-circuited and the rated current was passed through the tapped winding as much as possible, 10 tap-change operations were completed. The tap changer showed no faults throughout the process, with stable and reliable mechanical performance.
In addition, the measurement of no-load loss and current test was conducted under 100% rated excitation voltage. The measured no-load current was ≤ +30% of the specified value, and the no-load loss was ≤15 kW. The measurement of short-circuit impedance and load loss was performed by applying voltage to the HV side and short-circuiting the LV side, with tests conducted across all tapping positions. The short-circuit impedance at the principal tapping (75℃) was 14.29%, and the load loss was 124 kW, both meeting the guaranteed values in the agreement. The insulation test of auxiliary wiring involved applying the specified voltage for 60 seconds with no sudden voltage drop, ensuring the safe operation of the auxiliary power supply and control circuits. The pressure leak test (tightness test) was carried out in accordance with the standard of “30 kPa above the normal liquid pressure” – air pressure was applied to the conservator and maintained for 24 hours. A visual inspection revealed no leakage anywhere, ensuring there was no risk of insulating liquid leakage during the transformer’s operation.
At the end of the acceptance, the representative from the South African client spoke highly of SCOTECH’s product quality and testing professionalism: “SCOTECH strictly implemented every test in accordance with international standards and the mutually agreed procedures. The data is detailed and the results are reliable, which gives us full confidence in the operational safety of the transformers.” The Quality Control Manager of SCOTECH stated: “This acceptance is a comprehensive test of our production process and quality management system. In the future, the company will continue to be guided by IEC international standards and customer needs, providing higher-quality power transformer products to global customers.”
The successful acceptance of these two 20 (22) MVA 33/11kV power transformers not only strengthens the cooperative relationship between SCOTECH and the South African client but also demonstrates SCOTECH’s technical strength in the R&D and manufacturing of liquid-immersed power transformers. In the future, SCOTECH will continue to deepen its presence in the international market, contributing to the construction of global power infrastructure and efficient energy transmission with high-standard and high-quality products.
