Power Transformer Name Plate Details: Master It Now

The manufacturing process of power transformers usually follows 5 rigorous core stages: fully automatic step-by-step lamination (Step-Lap) core assembly, high tension continuous winding, body assembly including vapor phase drying (VPD) treatment, high vacuum oiling, and extremely strict factory acceptance test (FAT). To truly assess the manufacturing strength of a supplier, it is not enough to simply visit the factory. For technical procurement personnel and quality control (QC) engineers, the real test lies in the ability to accurately verify those small process tolerances and process control links-it is these hidden details that determine whether the transformer will have fatal problems of early insulation failure or excessive no-load loss after it is put into operation on site.

C.A.T. Evaluation System: An Audit Tool for Transformer Manufacturing

Auditing the quality of transformer manufacturing cannot be based on feeling, but requires a systematic methodology. The C.A.T. system (I. e. Core & Coil for cores and coils, Active Treatment for body processing, test and inspection Testing) provides engineers with a hard-core set of quantitative indicators for in-depth evaluation of production lines. Routine visual inspection often misses the internal mechanical weak points, and focusing on the output parameters of specific equipment is the key to ensure the long-term reliable operation of the equipment.

Step 1: Core Manufacturing And Magnetic Circuit Optimization

The cutting accuracy of the core directly determines the no-load loss and magnetostrictive noise of the transformer. At present, advanced factories will use full-automatic slitting lines to process oriented silicon steel sheets to a limit thickness of 0.23mm.

In order to minimize the eddy current loss, the industry generally adopts the “Step-Lap” lamination method. This multi-level lap mode can maximize the air gap at the seam. Not only that, the top manufacturers will run the 3D digital twin flux simulation 1 times before stacking. This software can accurately simulate the distribution trend of magnetic flux, find out the possible local hot spots in advance, and also guide how much hydraulic pressure should be used when clamping the iron core in the future. You know, if the clamping force is given unevenly, the iron core will definitely vibrate when the power consumption peaks, and local overheating will become inevitable.

The Second Step: Precision Winding And Short-Circuit Resistance

Whether the coil is strong enough or not is directly related to whether the transformer can withstand the huge impact of external short circuit. This requires that the winding machine must always maintain a large and constant tension when handling the transposed conductor (CTC).

The transposition conductor wrapped in heat-shrinkable kraft paper can not only lower the winding loss, but also fill up the space utilization. Typically, high voltage (HV) and low voltage (LV) coils are concentrically wound. During the winding process, the worker must apply a precise axial compression force. Once the winding is loose, the coil will shift when it encounters a large fault current, causing insulation breakdown instantly. Therefore, a hard rule must be added to your supplier audit list: before final assembly, a hydraulic shaper must be used to compact the coil to the dead size specified in the drawing.

Step 3: Body Assembly And Hidden Trap Of VPD

The so-called body assembly is to integrate the iron core, coil and tap changer into a solid whole. In this link, the lead wire arrangement and the insulating clamp are basically fixed by high-density laminated wood or insulating cardboard.

An insider’s guide to pit avoidance: the “over-drying” illusion of vapor-phase drying (VPD)

Dehydration of cellulose insulating materials is a dead order, but if blindly believing in conventional VPD indicators, it is easy to bury hidden dangers. In a deep vacuum environment, kerosene vapor can indeed effectively drain the water in the body. The problem is that many QC (quality inspection) experts only stare at “whether the moisture content has dropped below 0.5%”, but turn a blind eye to the heating time. Let the body bear overload thermal stress, will cause the transformer has not yet left the factory, the degree of polymerization (DP) of the insulation paper has been greatly reduced. The smart approach is: while looking at the dehydration graph, we must force the manufacturer to submit the DP (degree of polymerization) deterioration report.

Acceptable vs. Unacceptable VPD Parameters

Parameter	Standard Factory Acceptable Limit	High-End Supplier Target	Red Flag
Final Moisture Content	< 0.5%	< 0.3%	> 0.8%
Vapor Temp Limit	130°C	115°C – 125°C	> 135°C
DP Retention Rate	N/A	> 95%	< 85%

The Fourth Step: High Vacuum Into The Box And Oil Filling

The insulation strength of the transformer depends entirely on the oxygen and moisture in the insulating oil. After the body comes out of the VPD drying tank, it cannot be delayed for a second. It must be hung into the steel fuel tank immediately to prevent it from inhaling moisture in the air again.

Oil filling must be done under extremely high vacuum (usually less than 1 Torr). Vacuum the fuel tank to pull out the small bubbles that are hidden deep in the winding insulation. Next, the transformer oil that has been degassed and dehydrated by the multi-stage oil filter will be injected into the tank very slowly. Pour oil under normal pressure is definitely a big taboo in the industry, which will enclose a large number of tiny bubbles, and these bubbles are the culprit that will cause partial discharge (PD) and eventually lead to insulation scrap in the future.

Step 5: Factory Acceptance Test (FAT)

The final level of power transformer manufacturing is located in the electrical verification link in the Faraday shielding room. The core purpose of FAT is to “reconcile” the actual performance of the physical object with the design parameters “.

Routine tests can only give basic data. If you really want to find out the defects in production, you have to rely on the following specific diagnostic tests:

• Partial discharge (PD) measurement: Specially catch tiny holes in the insulation structure. At 1.5 times the maximum system voltage, the qualified bottom line must be pinched within 100 pico (pC).
• Lightning impulse test: to simulate the extreme situation of the power grid being struck by lightning, so as to test whether the wave impedance and the insulation between turns are hard or not.
• Frequency Sweep Response Analysis (SFRA): This is equivalent to entering the 1 “mechanical fingerprint” into the body “. If there is even a small deviation in the frequency response curve, it means that the winding has been physically displaced when it is hoisted into the box.

Frequently Asked Questions (FAQ)

Q: How long does it usually take to build a large power transformer?

A: The production cycle is between 6 and 12 months. In fact, the real stretch front is often the procurement of those special materials, such as oriented silicon steel and high purity copper wire. When the materials are ready, the physical assembly and testing phase will take about 4 to 8 weeks.

Q: How exactly does Vapor Phase Drying (VPD) work in a transformer plant?

Answer: VPD is in a vacuum environment, the use of vaporized kerosene to the body to do uniform heating. Kerosene vapor can penetrate into the cellulose insulation, steam out the water inside, and then condense into liquid and drain away together with moisture. This method is much faster than the traditional “hot air vacuum” drying method, and the dehydration is more thorough.

Q: What is the difference between “heart” and “shell” transformers in manufacturing?

Answer: The process of heart transformer is to wind the coil on the pillar of silicon steel core. The shell type is just the opposite, covering the entire core structure on the outside of the coil. Because of the completely different structures, the supporting and clamping equipment used in the locking magnetic circuit are also two different systems.

Q: Why do high-voltage transformers have to be vacuumed and oiled?

Answer: Vacuum is to clean up the air pockets stuck in the insulating paper. The dielectric constant of air is much lower than that of transformer oil. Once mixed in, it will cause local extremely high electrical stress, which will trigger partial discharge, which is irreversible to the transformer.

Q: What core indicators should I keep an eye on during the factory acceptance test (FAT)?

Answer: As a gatekeeper, you must check the no-load/load loss limit, impedance voltage, winding resistance, and dissolved gas analysis (DGA) report before and after the temperature rise test. Of course, the PD level must not be missed, and it is important to ensure that it is strictly controlled within 100 pC.

Q: How do manufacturers deal with the magnetostrictive noise of the iron core?

Answer: The killer mace to deal with noise is the fully automatic “Step-Lap” cutting and lamination technology. This ensures that the joints between the silicon steel sheets fit perfectly. After assembly, and then use high-pressure hydraulic equipment to the iron core dead clamping, so that the silicon steel sheet in the magnetic flux is not easy to produce resonance and noise.