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I am an engineer from the transformer design team at Zisheng Electric<\/strong>, specializing in the design and manufacturing of oil-immersed power transformers, high-voltage transformers, and grid project solutions<\/strong>.<\/p>\n Over the years, I have participated in technical support for multiple 110kV\/132kV transformer projects<\/strong>, covering design review, technical clarification, manufacturing coordination, and project execution support.<\/p>\n For EPC projects<\/strong>, the manufacturing process of oil-immersed power transformers<\/strong>, Factory Acceptance Testing (FAT)<\/strong>, and compliance with IEC 60076 standards<\/strong> are among the most important evaluation factors when selecting a transformer supplier. The IEC 60076 series provides requirements and test methods related to power transformer design, performance, and verification.<\/p>\n You may be searching for a reliable supplier, comparing technical bids, or simply trying to understand one question:<\/p>\n When a transformer manufacturer claims IEC compliance, is the transformer really designed and manufactured according to the standard requirements step by step?<\/strong><\/p>\n Regardless of your reason for reading this article, I will explain the answer from three perspectives:<\/p>\n A complete oil-immersed power transformer<\/strong> goes through more than ten major manufacturing stages and hundreds of control points, from incoming raw materials to final testing, packaging, and shipment.<\/p>\n Each stage corresponds to specific requirements within the IEC 60076<\/strong> standard series, including design verification, temperature performance, insulation requirements, and routine testing procedures.<\/p>\n I will focus on the key parts that truly determine transformer reliability.<\/p>\n First, let\u2019s look at a summary table to clearly understand the key control points and acceptance criteria for the seven core manufacturing processes. Each section below will then explain these processes in detail.<\/p>\n The following sections will explain the quality control requirements of each process according to the actual manufacturing workflow.<\/p>\n During actual manufacturing, the transformer core is not simply an assembly of stacked steel sheets.<\/p>\n For high-voltage and extra-high-voltage transformers<\/strong>, the quality of core joints directly affects no-load losses and noise performance. Therefore, transformer inspectors and third-party supervision teams often treat core stacking as a key witness point during manufacturing.<\/p>\n The transformer core uses high magnetic permeability grain-oriented silicon steel sheets<\/strong>, with B30R090<\/strong> commonly used as a starting-grade material.<\/p>\n After the material arrives at the factory, incoming inspection is carried out based on the steel manufacturer\u2019s quality certificates and the IEC 60404 series standards<\/strong>.<\/p>\n Inspection items include:<\/p>\n Key magnetic performance indicators, including:<\/p>\n are verified through the steel manufacturer\u2019s factory test reports.<\/p>\n These parameters directly determine the magnetic performance of the transformer core and have a significant impact on transformer no-load losses<\/strong>.<\/p>\n For Saudi Arabia 60Hz projects<\/strong>, magnetic performance parameters should be carefully verified through frequency conversion calculations to ensure the core design matches actual operating conditions.<\/p>\n Core cutting is a precision process.<\/p>\n We use German-brand longitudinal and cross-cutting lines<\/strong>, with cutting accuracy controlled within \u00b10.2 mm<\/strong>.<\/p>\n Why is such accuracy necessary?<\/p>\n Because during core stacking, poor alignment of joints increases the effective air gap<\/strong>.<\/p>\n A larger air gap leads to:<\/p>\n In desert operating conditions, additional losses are ultimately converted into heat. Even with a strong cooling system, excessive core losses can create unnecessary thermal stress.<\/p>\n Therefore, precision cutting is not only a manufacturing requirement \u2014 it is directly related to the transformer\u2019s long-term efficiency and reliability.<\/p>\n A fully mitered three-step overlapping joint structure<\/strong> is used.<\/p>\n During the stacking process, workers use a feeler gauge<\/strong> for sampling inspection after each layer is stacked. The gap must not exceed 0.3 mm<\/strong>. After the entire core assembly is completed, a closed-circuit test<\/strong> is performed to measure the core-to-ground insulation resistance<\/strong> and check for interlaminar short circuits<\/strong>.<\/p>\n In one Saudi project, the customer\u2019s third-party inspection engineer brought a 0.02 mm precision feeler gauge<\/strong> and inspected the core joints layer by layer. What was written in the final acceptance report? \u201cThe core stacking quality exceeds IEC standard requirements.\u201d<\/strong> That is exactly the result we aim for. Although these details increase manufacturing costs, they can effectively reduce long-term operational risks.<\/p>\n Winding manufacturing directly determines the long-term operating reliability of the transformer. Especially under conditions such as short-circuit impact, electromagnetic vibration, and repeated thermal cycling, winding mechanical strength and insulation margin are the key control factors.<\/p>\n The winding conductors use oxygen-free copper conductors or copper foil. Winding conductors and insulation materials are manufactured according to the corresponding IEC material standards. Enamelled wire and paper-covered wire conductors<\/strong> comply with the IEC 60317<\/strong> series requirements, while solid insulation materials such as insulation paperboard<\/strong> comply with the IEC 60641<\/strong> series requirements. Insulation thickness is determined based on electric field simulation calculations. The thermal aging performance of the insulation system is evaluated according to IEC 60216<\/strong>, while the electrical withstand level follows the test requirements of the IEC 60076<\/strong> series.<\/p>\n For renewable energy projects in the Middle East, it is also necessary to consider the thermal-mechanical alternating stress caused by load fluctuations and large day-night temperature differences.<\/p>\n The winding process is divided into two types \u2014 layer winding<\/strong> and disc winding<\/strong>.<\/p>\n Small and medium-capacity transformers usually use layer winding<\/strong>, where the conductor is wound layer by layer, similar to winding a coil onto a spool.<\/p>\n Large-capacity and high-voltage transformers typically use disc winding<\/strong>, where individual discs are wound separately and then connected in series.<\/p>\n The advantage of disc winding is that if a problem occurs in one disc, it will not affect the entire phase winding. Disc windings can improve the mechanical strength of the winding and optimize voltage distribution.<\/p>\n During the winding process, axial compression force is a key manufacturing parameter.<\/p>\n If the compression force is too high, it may squeeze and damage the turn-to-turn insulation.<\/p>\n If the compression force is insufficient, during transformer operation, electromagnetic vibration and thermal cycling caused by load changes may cause conductor displacement and friction, resulting in insulation wear and eventually leading to partial discharge failures.<\/p>\n The production process uses a hydraulic constant-pressure compression device, with compression pressure fluctuation controlled within \u00b15%<\/strong>.<\/p>\n During the winding process, the main control parameters include conductor tension, insulation wrapping, spacer block arrangement, and compression pressure. Visual inspection and dimensional inspection are carried out.<\/p>\n The turn-to-turn impulse withstand voltage test is performed after the complete winding is finished. The dielectric loss test is conducted after winding assembly and vacuum drying are completed.<\/p>\n Active part drying is a critical process that determines transformer service life.<\/p>\n After the windings and core are assembled, the complete active part is placed into a vacuum drying chamber for vacuum drying treatment<\/strong>.<\/p>\n Insulation paper is a type of moisture-absorbing fiber material. The moisture content of the raw material is typically around 6%\u20138%<\/strong> when it leaves the factory.<\/p>\n If deep drying treatment is not performed before oil filling, the moisture remaining inside the insulation will gradually migrate into the insulating oil, reducing the electrical insulation strength.<\/p>\n More seriously, when the transformer operates under heat, moisture inside the insulation may vaporize and generate internal pressure, which can cause:<\/p>\n The generated bubbles can also trigger partial discharge<\/strong> and accelerate insulation aging.<\/p>\n Especially for Middle East projects operating under long-term high-temperature conditions, the damaging effects caused by moisture become even more significant.<\/p>\n Therefore, a strict vacuum drying process must be applied to reduce the residual moisture content in solid insulation to a very low level, ensuring the transformer\u2019s long-term operating reliability.<\/p>\n IEC 60076-14<\/strong> provides guidance on transformer insulation drying processes, but the standard does not specify mandatory numerical requirements.<\/p>\n The general industry reference target is that the residual moisture content of solid insulation should be \u22640.5%<\/strong>.<\/p>\n For Middle East projects, our internal factory control target is reduced to \u22640.3%<\/strong>, which is stricter than common industry practice.<\/p>\n The drying process is as follows:<\/p>\n The active part is first heated to 110\u2013115\u00b0C<\/strong>, followed by atmospheric pre-drying for 12 hours<\/strong>.<\/p>\n After that, the transformer enters the high vacuum stage, where the vacuum level is reduced to below 50 Pa<\/strong> and maintained for no less than 48 hours<\/strong>.<\/p>\n The complete drying cycle is normally more than 72 hours<\/strong>.<\/p>\n Note:<\/strong> The time is only the minimum process reference. The final drying completion judgment must be based on comprehensive evaluation of:<\/p>\n Drying completion cannot be determined simply by time alone.<\/p>\n During the drying process, a condenser<\/strong> is used to collect the extracted moisture.<\/p>\n The amount of released water is monitored every hour and recorded as a drying curve.<\/p>\n When the moisture release rate drops below 5 ml per hour<\/strong>, the drying process is considered to have reached the required level.<\/p>\n Different manufacturers may use different drying curves. The actual drying duration depends on:<\/p>\n Determining whether drying is complete should not rely only on time. More importantly, engineers need to evaluate:<\/p>\n After the active part drying process is completed, final assembly is carried out while the transformer is still hot<\/strong>.<\/p>\n Why assemble while hot? Because after drying, the insulation materials are in a dry and expanded state. Once the temperature drops, they may absorb moisture from the surrounding air, which would compromise the previous drying work.<\/p>\n The core and winding assembly are lifted into the transformer tank. The capacity of the overhead crane<\/strong> depends on the equipment weight. Some transformers require only several tons of lifting capacity, while others require dozens of tons.<\/p>\n During the entire process, no collision or mechanical impact is allowed. The clearance between the winding end insulation components and the tank wall must comply with the design drawings, generally controlled within 30\u201350 mm<\/strong>.<\/p>\n The next step is lead connection welding.<\/p>\n Although this process looks simple, it actually requires significant technical experience.<\/p>\n For copper conductor welding, we use medium-frequency induction brazing<\/strong>, with the temperature controlled between 750\u00b0C and 850\u00b0C<\/strong>.<\/p>\n If the temperature is too low, the welding penetration will be insufficient, resulting in increased contact resistance<\/strong>.<\/p>\n If the temperature is too high, the copper material may undergo annealing<\/strong>, reducing mechanical strength.<\/p>\n After each lead is welded, a micro-ohmmeter<\/strong> is used to measure the contact resistance and compare it with the standard value. If the deviation exceeds 5%<\/strong>, the welding must be redone.<\/p>\n In a Kuwait project, the customer required X-ray inspection records<\/strong> for all welding points. A transformer may have more than twenty welding points of different sizes, and each one was inspected individually.<\/p>\n During the factory acceptance inspection, the customer reviewed the X-ray images and commented:<\/p>\n \u201cYour welding quality control is even more rigorous than pressure pipeline manufacturing.\u201d<\/strong><\/p>\n Oil filling is the final assembly process before the transformer tank is sealed.<\/p>\n The insulating oil is processed in advance.<\/p>\n Whether it is mineral oil<\/strong> or natural ester<\/strong>, it must first undergo incoming inspection, including:<\/p>\n IEC 60296 and IEC 60422<\/strong> provide relevant guidance on insulating oil quality requirements and maintenance.<\/p>\n Unqualified oil is never allowed to enter the transformer tank.<\/p>\n The \n
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\n Process<\/strong><\/th>\n Key Control Points<\/strong><\/th>\n Acceptance Criteria<\/strong><\/th>\n Relevant IEC Standards<\/strong><\/th>\n Risks<\/strong><\/th>\n<\/tr>\n<\/thead>\n\n \n Core Stacking<\/strong><\/td>\n Silicon steel sheet joint accuracy, clamping torque, single-point core grounding<\/td>\n No-load loss shall not exceed the guaranteed design value; joint gaps shall comply with process documentation requirements<\/td>\n IEC 60076-1; IEC 60076-2<\/td>\n Excessive joint gaps increase no-load losses, noise, and no-load current<\/td>\n<\/tr>\n \n Winding Manufacturing<\/strong><\/td>\n Axial\/radial winding compression force control, winding tension, insulation wrapping process<\/td>\n Compression force meets process requirements; winding dimensions and conductor specifications comply with drawings; partial discharge is a final transformer factory test parameter and is not determined at this manufacturing stage<\/strong><\/td>\n IEC 60317-0-1 (winding conductors); IEC 60076-5 (short-circuit withstand capability)<\/td>\n Insufficient compression may cause winding deformation and displacement during short circuits, reducing short-circuit withstand capability<\/td>\n<\/tr>\n \n Vacuum Drying<\/strong><\/td>\n Solid insulation moisture content, vacuum level, temperature-time curve control<\/td>\n Solid insulation paper moisture content <0.3% (strict internal project control requirement)<\/strong><\/td>\n IEC 60076-14 (thermal performance and drying process considerations)<\/td>\n Residual moisture accelerates insulation aging and reduces transformer service life<\/td>\n<\/tr>\n \n Oil Filling (Vacuum Oil Filling)<\/strong><\/td>\n New oil filtration, degassing process, oil cleanliness control<\/td>\n New insulating oil dielectric loss (tan\u03b4@90\u00b0C) <0.005; breakdown voltage and moisture content comply with new oil requirements<\/td>\n IEC 60296 (new mineral insulating oil)<\/strong>; IEC 60156 (breakdown voltage of insulating liquids)<\/td>\n Poor oil quality may cause partial discharge and reduce insulation reliability<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n
\nI. Oil-Immersed Transformer Core Manufacturing Process and Quality Control<\/strong><\/span><\/h2>\n
1.1 Incoming Material Inspection<\/strong><\/span><\/h3>\n
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\n1.2 Silicon Steel Sheet Cutting<\/strong><\/span><\/h3>\n
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<\/p>\n\n\n\n\n\n\n\n\n1.3 Core Stacking Process<\/strong><\/span><\/h3>\n
II. Coil<\/strong><\/span><\/h2>\n
2.1 Conductors and Insulation Materials<\/strong><\/span><\/h3>\n
2.2 Winding Method<\/strong><\/span><\/h3>\n
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<\/p>\n\n\n\n\n\n\n\n\n2.3 Axial Compression Force Control<\/strong><\/span><\/h3>\n
III. Active Part Drying<\/strong><\/span><\/h2>\n
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<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n3.1 Why Drying Is Necessary<\/strong><\/span><\/p>\n\n
3.2 Drying Process Parameters<\/strong><\/span><\/h3>\n
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3.3 Moisture Release Monitoring<\/strong><\/span><\/h3>\n
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\nIV. Final Assembly<\/strong><\/span><\/h2>\n
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<\/p>\n4.1 Hot Assembly and Lifting<\/strong><\/span><\/h3>\n
4.2 Lead Connection Welding<\/strong><\/span><\/h3>\n
V. Oil Filling<\/strong><\/span><\/h2>\n
5.1 Oil Treatment<\/strong><\/span><\/h3>\n
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