On August 25, 2025, Jiangshan Scotech Electrical Co., Ltd. (SCOTECH) received an important delegation of customers from South Africa. The purpose of this visit was to conduct pre-delivery inspection on one 20/22 MVA 132/11kV oil-immersed power transformer and two 20/22 MVA 33/11kV oil-immersed power transformers custom-made by SCOTECH for the customers. On the morning of the same day, the two parties first held a special negotiation meeting in SCOTECH’s office building, conducting in-depth exchanges on the core contents such as the transformer’s design concept, technical standards, and test procedures, laying a solid foundation for the subsequent inspection work.
At the meeting, SCOTECH’s technical team elaborated on the design concept of the transformers to be inspected, with “safety and reliability, high efficiency and energy saving, and compliance with standards” as the core. Both types of transformers adopt a liquid-immersed structure. Among them, when uses the ONAN (Oil Natural Air Natural) cooling method, the rated capacity is 20MVA, which is suitable for conventional load scenarios and has both energy-saving and stability advantages; when adopting the ONAF (Oil Natural Air Forced) cooling method, the rated capacity can reach 22 MVA, which can quickly dissipate heat under high-load conditions to ensure the long-term stable operation of the equipment, perfectly matching the power transmission needs of South African customers. In terms of voltage level and wiring design, the 132/11kV model has a primary side voltage of 132kV and a secondary side voltage of 11kV, with a vector group of YNyn0, meeting the precise voltage transformation needs of high-voltage power transmission scenarios; the 33/11kV model has a primary side voltage of 33kV and a secondary side voltage of 11kV, with a vector group of Dyn1, adapting to the flexible application of medium-voltage distribution systems. Both products are equipped with an on-load tap changer (OLTC) of ±8×1.25%, which can accurately adjust the voltage according to actual working conditions to ensure power supply quality.
To enable customers to fully understand the product quality control system, the person in charge of SCOTECH’s technology focused on interpreting the test procedures that strictly comply with international standards. For the 20/22 MVA transformer with a 132kV voltage level, its test process covers 18 core routine tests, starting from the measurement of dissolved gases in the dielectric liquid. Before all tests and after insulation tests, gas chromatographic analysis is conducted on the dielectric liquid in each independent oil compartment except the diverter switch compartment. Strict requirements are imposed: hydrogen (H₂) content < 10ppm, acetylene (C₂H₂) content = 0, total hydrocarbons (C₁~C₂) < 20ppm, while ensuring the moisture content of the dielectric liquid ≤ 20ppm, dielectric strength ≥ 60kV, and dielectric loss tangent tgδ at 90℃ ≤ 1%. These measures comprehensively verify the insulation performance and purity of the transformer oil, eliminating the risk of potential internal faults.
In the electrical performance testing link, the 132kV model needs to complete the verification of voltage ratio and phase displacement. The voltage ratio between high voltage and low voltage (HV-LV) is measured at all 17 tapping positions, and the allowable deviation is strictly controlled within ±0.5% of the specified voltage ratio. Moreover, the vector group must accurately match the YNyn0 agreement requirements; the inspection of the ratio and polarity of the built-in current transformers is also strict. It is necessary to ensure that the ratio (such as 600/1, 800/1, 1200/1, etc.) and polarity of different types of current transformers such as LRB-132 and LRB-46 fully meet the drawing requirements, and the DC resistance on the secondary side has no obvious deviation from the factory value; the winding resistance measurement covers all taps of the high-voltage winding and the low-voltage winding, requiring the line-to-line resistance unbalance rate < 1% and phase-to-phase < 2% to inspect the winding material, welding quality, and joint connection reliability. In addition, the model also needs to pass the insulation inspection of the core and frame (insulation resistance ≥ 500MΩ), the measurement of DC insulation resistance between windings and to earth (using a 5000V megohmmeter, absorption ratio ≥ 1.3 or polarization index ≥ 1.5), the measurement of dissipation factor (tanθ) of the insulation system capacitances (tgδ ≤ 1.5% at 20℃), the measurement of no-load loss and current at 90% and 110% of rated voltage (no-load loss ≤ 15kW at 100% rated voltage, no-load current deviation ≤ +30% of the specified value), the measurement of short-circuit impedance and load loss (HV to LV impedance 18.79% at 75℃ for the principal tapping, load loss ≤ 120kW), and other key tests. Furthermore, it is required to complete the full-wave lightning impulse test (LI) for the line terminals – applying 1 time of 50% voltage and 3 times of 100% voltage full-wave lightning impulses to the 132kV winding, requiring no significant difference between the 50% and 100% voltage waveforms, as well as the induced voltage withstand test with PD measurement (IVPD). The voltage of (1.5×Um)/√3 is maintained for 1 hour, and the partial discharge (PD) level is recorded every 5 minutes to ensure the PD value ≤ 250pC without an upward trend, comprehensively verifying the insulation strength and operational stability of the equipment under extreme working conditions.
For the 20/22 MVA transformer with a 33/11kV voltage level, the core test items are consistent with those of the 132kV model, and key parameters are optimized in combination with the characteristics of the voltage level. In terms of the performance requirements for the dielectric liquid, the dielectric strength is ≥ 55kV, and other gas chromatographic indicators are the same as those of the 132kV model; in terms of short-circuit impedance and load loss, the HV to LV impedance at 75℃ for the principal tapping is 14.29%, and the load loss ≤ 124kW; in the induced voltage withstand test (IVW), a test voltage with an induction multiple of 2 and a frequency of 150Hz is applied to the low-voltage winding for 40 seconds, requiring no sudden drop in the test voltage to fully verify the longitudinal insulation strength of the transformer.
In addition, both models need to pass the leak testing with pressure (tightness test) – applying a gas pressure of 30kPa higher than the normal liquid pressure on the conservator and maintaining it for 24 hours, and confirming no leakage anywhere through visual inspection, as well as the insulation test of auxiliary wiring (applying the specified voltage with no sudden drop in voltage during the test), ensuring the overall tightness of the equipment and the reliability of the auxiliary system.
The South African customer delegation carefully listened to SCOTECH’s report, highly recognized the strict standard basis in the test procedures (such as the IEC 60076 series of international standards, including IEC 60076-1 General, IEC 60076-3 Insulation Levels, Dielectric Tests and External Clearances in Air, IEC 60076-7 Loading Guide for Mineral-Oil-Immersed Power Transformers, etc.), and had in-depth discussions with SCOTECH’s technical team on detailed issues in the test process. The customer representative stated that SCOTECH’s professionalism and rigor in the transformer design and test links fully reflect the high-quality standards of “Made in China”, and also make them full of confidence in the application of the equipment in South African power projects.
The successful holding of this negotiation meeting not only deepened the South African customers’ understanding of SCOTECH’s product technology and quality control, but also demonstrated SCOTECH’s strong technical capabilities in the field of high-end oil-immersed transformers. As an enterprise focusing on the R&D and manufacturing of power equipment, SCOTECH has always taken international standards as the benchmark and customer needs as the orientation, pursuing excellence in every link from design, production to testing and inspection. In the follow-up, SCOTECH will fully cooperate with customers to complete the on-site inspection work, ensuring the smooth delivery of one 20/22 MVA 132/11kV transformer and two 20/22 MVA 33/11kV transformers, injecting “SCOTECH Strength” into the construction of South Africa’s power infrastructure, and laying a solid foundation for deeper cooperation between the two parties in the future.
