Analysis of Causes of Failure in High-Voltage Foil-Wound Dry-Type Transformers

1. Overview

Epoxy resin-cast dry-type transformers are widely used in various fields such as high-rise buildings, rail transit, steel and chemical industries, offshore wind power, and distributed photovoltaic power generation. This is due to their many advantages, including excellent mechanical strength, superior resistance to impulse overvoltages, strong overload capacity, flame-retardant insulation materials, easy maintenance, and strong weather resistance.

As the application fields continue to expand, the special working conditions under different application backgrounds also pose new challenges to the operational safety of transformers. Even though dry-type transformers have good reliability and resistance to harsh operating conditions, failure incidents still occasionally occur as the number of applications increases and the application environments diversify. These failures have caused a certain degree of economic loss to both users and manufacturers of transformers. This article analyzes and summarizes several failure incidents of high-voltage foil-wound epoxy-cast dry-type transformers from different manufacturers, at different times, and in different operating locations. Through targeted testing and analysis of the failed products, valuable guidance is provided for improving product design and manufacturing processes. All the failure cases selected in this article involve epoxy-cast dry-type transformers with high-voltage foil-wound coil structures.

2. Structural Overview

The high-voltage foil-wound coil, as shown below, consists of a high-voltage conductor made of copper or aluminum foil. Due to the large longitudinal series capacitance of the high-voltage foil winding, it exhibits better inter-turn voltage distribution under impulse voltage. Additionally, it has advantages such as high production efficiency, a high overall coil fill factor, and low cost, which have led to its adoption by many manufacturers in recent years.

To ensure the cases are related, we summarized some operational and test failure cases for this type of product, which can be used as reference and discussion by industry colleagues.

3. Case Studies

3.1 Case 1: Coil Failure During No-Load Testing

In a factory, during a no-load loss test on a transformer, one phase of the current was found to be abnormally high, and the voltage could not be applied normally. After an emergency power shutdown and inspection, no abnormalities were found, but it was suspected that there was a fault in the coil. The transformer was re-energized to exacerbate the fault for easier troubleshooting. Soon after re-energizing, a ring-shaped crack appeared on the surface of the coil (following the winding direction of the turns), and power was quickly cut off.

The coil was dissected at the crack, and the short-circuit point inside the coil was found, as shown in Figure(a). The conductor is an aluminum strip, and the interlayer insulation consists of two layers of 0.036mm imported(means high-quality) polyester film. The fault occurred at the edge of two adjacent copper strips, indicating that the failure was caused by inter-turn insulation failure, leading to an internal inter-turn short circuit in the coil.

A similar failure case, dissected and shown in Figure (b), involved a copper strip conductor and H-class polyimide film as the interlayer insulation.

Both of these failure incidents occurred during routine "no-load loss testing" in the factory. Dissections of the failed coils clearly showed that the failures were caused by the failure of the inter-turn insulation films within the coils. The failure in Figure (a) can be attributed to burrs on the edge of the aluminum strip, while the failure in Figure 3(b) may have been caused by defects in the polyimide film, foreign particles in the winding process, or burrs on the conductor surface.

3.2 Case 2: Wind Farm Transformer Failure Case

Following figure shows post-failure dissection images of epoxy-cast transformers from the Horns Rev (Denmark) and Copenhagen offshore wind farms. The transformers were produced by a renowned international manufacturer. On April 11, 2006, the manufacturer analyzed operational failures in 20 epoxy-cast dry-type transformers at the Copenhagen North and South wind farms. Of the 20 transformers, 16 failed. Except for one, which failed due to a creepage breakdown caused by the misplacement of a winding temperature sensor, the other 15 failed due to partial discharges caused by resin cracking inside the epoxy-cast transformer coils.

3.3 Case 3: Substation Distribution Transformer Failure Case

Blew figure shows the dissection results of a failed transformer from a civil substation. The failure occurred shortly after the transformer was put into operation under light load, with no abnormal records in the system. There was no abnormal condition in other equipment in the system.

The dissection showed severe burn marks at corresponding positions between two adjacent sections of the coil. The resin insulation between the sections was burned and perforated, and the burn marks were near the crossover connection between the two sections.

Below figure shows the dissection of another failed transformer, used for industrial lighting supply. This failure occurred about six months after the transformer was put into operation. Before the failure, the transformer had not reached full load, and there were no abnormal system records. No other equipment in the system showed any issues.

The dissection revealed that the internal burn marks of the coil were extensive, and the short circuit was not limited to one section but spread across multiple sections. The resin insulation between the sections showed deep, tree-like burn grooves caused by electric arcing, forming arcing paths between the sections similar to wormholes in soil.

Due to the extensive internal damage in these two failure cases, it is difficult to directly determine the cause of the failures based on dissections alone. The extensive damage obscured the original failure points, leaving only traces to infer the cause and development of the failures.

4. Cause Analysis and Improvement Measures

4.1 Raw Materials

The quality of raw materials has a critical impact on the overall quality and reliability of transformers. For high-voltage foil-wound coil structures, the quality of key raw materials is particularly important. These coils are made from foil strips and inter-turn insulation films. The surface defects of the foil, such as burrs and peeling, can easily damage the inter-turn insulation films (usually 2×0.036mm polyester films). Additionally, the burrs on the edges of the foil strips must be carefully handled. Finally, defects in the inter-turn insulation films can also lead to failures.

Based on the failure image in Case 1(a), it can be concluded that the failure was caused by burrs on the aluminum foil edges, which damaged the inter-turn insulation film. In Case 1(b), the failure might have been caused by defects on the copper foil surface or flaws in the insulation film.

4.2 Internal Insulation and Structural Design

As the starting point of product quality, the design of internal insulation and structure plays a crucial role in the reliability of transformers. The inter-turn insulation of high-voltage foil-wound coils is made of polyester film. In design, it is preferable to use multiple layers of thin insulation rather than a single layer of thick insulation to minimize the risk of insulation defects. Additionally, the thickness of the inter-turn insulation must account for both the required electric field strength and the potential mechanical damage during winding.

The segment insulation between sections consists of cured epoxy resin mixtures. The insulation strength must be adequate, and adjacent polyester films between sections should not overlap. Furthermore, improvements in tap and section lead design can help reduce the risk of failure.

As shown in figure (a), the first image represents the connection design before the modification, while the second image represents the connection design after the modification. This improvement effectively avoids the risk of insufficient insulation distance between internal leads and the possibility of short-circuiting due to improper operation. Regarding the leads between the two sections, before the modification, the first and last parts of the two sections were connected via a crossover, which not only reduced the insulation distance between the two sections but also posed a risk of internal short circuits. After modifying the winding to have odd sections wound to the left (right) and even sections wound to the right (left), the handling of inter-section leads was greatly simplified, thereby reducing the risk of failure.

4.3 Casting and Curing Process

The casting and curing process is critical to the formation of the overall insulation system of the coil. In addition to the winding and insulation processes, the casting and curing of the epoxy resin are vital to the reliability of the insulation system.

4.3.1 Vacuum Level

The vacuum level during casting directly affects the removal of gases from the mixture and the quality of the cast product. If the vacuum level is too low, the degassing time must be extended. However, a vacuum that is too high can cause material volatilization, affecting the resin mixture ratio and leading to product defects.

4.3.2 Filler Sedimentation

Filler sedimentation can lead to reduced insulation performance and mechanical strength, as well as uneven distribution of filler in the cast product, increasing the risk of cracking under thermal stress.

Under the influence of an external electric field, internal cracks can lead to partial discharge, surface discharge, and penetrative discharge.

Above figure shows the test results of the filler percentage at different locations in a failed coil's casting. From the test results, it is clear that the distribution of silica micropowder filler inside the casting is highly uneven, with the highest filler content at one sampling point reaching 82%, while the lowest point was only 49%. Based on the data alone, the filler percentage distribution in this casting far exceeds the reference value, significantly increasing the risk of internal cracking and insulation breakdown in the casting.

The causes of filler sedimentation can be summarized as follows: First, the viscosity of this particular resin system is very low, and any slight mishandling can lead to severe sedimentation. Additionally, the resin system has a high glass transition temperature (Tg) and high hardness, which, when combined with uneven filler distribution, makes it more prone to internal stress concentration and cracking. Secondly, the control of the curing process plays a critical role in preventing filler sedimentation. As the temperature increases, the viscosity of the resin system continuously decreases, and during the appropriate time period, the resin system must complete the cross-linking reaction, transitioning from a liquid to a highly elastic state without causing adverse effects from the heat generated by the reaction. This is crucial to ensuring the quality of the casting. Finally, factors such as the mold, casting method, and temperature distribution in the oven also affect filler.

4.3.3 Resin Shrinkage

Shrinkage during curing is a characteristic of epoxy resin materials. Different epoxy resin systems have varying amounts of shrinkage during the curing process. Additionally, factors such as mold structure, curing temperature curves, and casting part structures all affect the degree of shrinkage to varying extents.

Improper internal shrinkage during curing can lead to internal cracking and the formation of undesirable internal stress concentrations, which may increase the risk of future cracking.

4.3.4 Stress Concentration Caused by Internal Structure

Because high-voltage coils are wound using foil strips, and the surfaces of copper/aluminum foils are very smooth and do not wet well with resin, high-voltage foil-wound coils are more prone to internal stress concentration.

The main stress concentration points are located between adjacent coil sections, around internal leads, and near tap leads. Sufficient attention should be paid to internal stress concentration. Firstly, sharp edges should be minimized as much as possible in the structural design. Additionally, effective technological measures should be adopted to enhance fiber reinforcement within the casting and improve the treatment of non-wetting surfaces, thereby minimizing the risk of internal cracking caused by stress concentration.

5. Conclusion

The causes of transformer failures are complex and multifaceted. This article provides an objective discussion and summary of the failure causes based on the analysis of several actual failure cases from a few key perspectives. It is hoped that this can serve as a useful reference for relevant personnel.

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