[Technical Analysis] Talking about non-encapsulated air-insulated three-dimensional coil core dry-type transformer

Talking about non-encapsulated air insulation three-dimensional coil core

Dry type transformer

Haihong Electric Co., Ltd.

Song Danju

【Abstract】In this paper, the author introduces the development history of non-encapsulated air-insulated dry-type transformers. By comparing the characteristics of new non-encapsulated air-insulated dry-type transformers and traditional non-encapsulated air-insulated dry-type transformers, a new type of non-encapsulated Encapsulated air-insulated dry-type transformers.

First, the history of development

In the middle of the 19th century, the world’s first distribution transformer was made using the principle of electromagnetic induction. The distribution transformers that were originally made and the transformers that were manufactured over time were air-insulated dry-type transformers. With the upgrading of the power system, the voltage level has been continuously improved. It has been found that the use of mineral oil as the insulating medium for transformers can improve the dielectric strength of the insulation and thus produce oil-immersed transformers.

In the 1960s, a large number of places requiring fire protection increased, such as high-rise buildings, subways, and mines, etc., and there was an ever-increasing demand for transformer safety. In order to meet the fire protection requirements, transformer manufacturers in various countries are looking for substitutes for mineral oil, but all have not been well applied for some reason. Then find new alternatives. This led to the production and installation of a large number of dry-type transformers.

Second, the characteristics and status of dry-type transformers

1. Features

(1) Dry-type transformers avoid the risk of fire and explosion of transformer oil due to a malfunction during operation. Since the dry-type transformer insulation is a non-flammable material, even if a transformer malfunctions during operation and causes a fire or an external fire source, the fire disaster situation will not be expanded.

(2) The dry-type transformer insulation medium is air, and there is no leakage of oil. The dry-type transformer is greatly reduced in operation, maintenance, and overhaul. It can even be maintenance-free.

(3) The dry-type transformer can be installed in the same room as the switch cabinet, and can share a shell with the switch cabinet to reduce the installation area.

2. Status

In some developed countries, it has been stipulated that transformers installed indoors should not be equipped with oil-immersed transformers. In China, the percentage of dry-type transformers in distribution transformers also increases year by year. Especially in some special fields and new energy areas, dry-type transformers occupy irreplaceable positions, such as shipboard transformers, variable-frequency phase-shifting rectifier transformers, and nuclear power transformers.

In the world, non-encapsulated air-insulated dry-type transformers and resin-insulated dry-type transformers are developed in parallel and their demands are comparable. In our country, the proportion of the two varies greatly, according to statistics, the annual demand of dry-type transformers in China is about 200,000 sets, of which the resin-insulated dry-type transformers account for 96%, and the non-encapsulated air-insulated dry-type transformers account for 4 %. The non-encapsulated air-insulated dry-type transformer technology is mainly MORA and Reliatran in China, and there are not many manufacturers introduced. Although the performance of insulating materials has been continuously improved, the overall technical level of transformers has not been correspondingly improved.

Third, the development trend

With the progress of the times, energy conservation and environmental protection have attracted increasing attention all over the world. In June 2013, China promulgated the GB 20052-2013 "Three-phase distribution transformer energy efficiency limit value and energy efficiency rating." In addition to energy efficiency standards, environmental noise levels, carbon dioxide emissions and other indicators have given limits. At present, China is implementing the reform of the power system. Safety, reliability, energy saving, environmental protection, and cost have become the five key elements of product manufacturing.

We know that oil-immersed transformers do not have the conditions for safety and environmental protection. The first resin-insulated dry-type transformer was born in 1964. The technology of the products has matured, but the partial discharge problem has not been completely solved. The facts show that most of the products on the market can burn, and a large amount of toxic gases and concentrations are generated when burning. Smoke; Epoxy-encapsulated conductors are difficult to recycle and reuse. Resin insulated dry type transformer

In the face of security risks, environmental issues have hindered its further development.

Non-encapsulated air-insulated dry-type transformers were born in the 1960s. There are dozens of manufacturers such as Toshiba and GE in the production of such transformers abroad. The first product in China was an SG dry-type transformer installed in the Beijing subway at the end of the 1970s. Later in the year, China introduced DuPont technology. The product uses Nomex paper as the wire insulation and the insulation heat-resistant grade is H class. The product is moisture-proof, anti-salt spray, anti-mold good performance, thermal shock, no cracking, no partial discharge phenomenon. Nowadays, the performance of insulating materials has been greatly improved, and the safety and reliability of products have been further improved. At the same time, transformers have been guaranteed to operate in more severe environments.

This article will introduce a new type of non-encapsulated air-insulated dry-type transformer (Fig. 1). Its product structure and process are integrated into the most advanced technology available to enable the product to have five major advantages: safety, reliability, energy saving, environmental protection and cost.

1. Safe and reliable

The safety and reliability of conventional non-encapsulated air-insulated dry-type transformers are analyzed in detail in many literatures and will not be described here. Focuses on the technical innovation of new non-encapsulated air-insulated dry-type transformers.

(1) Insulation material

The new non-encapsulated air-insulated dry-type transformers are based on aromatic polyamide insulation paper and other similar materials, and are impregnated with insulation grades of the same insulating heat-resistant paint by VPI technology. Among them, aromatic polyamide insulation paper is the main insulation material for transformer coils. Aromatic polyamide insulation paper can work at temperatures up to 220°C. It is characterized by non-flammable, flame-retardant, self-extinguishing, non-toxic and fireproof.

(2) Iron core

The new non-encapsulated air-insulated dry-type transformer uses a three-dimensional coil core (Figure 2). The three-dimensional core is composed of three identical single frames. Each single frame is made up of several material strips in sequence. The single frame after being assembled is cured and bundled to ensure that the iron core becomes a firm whole. Practice has proved that the unique structure and process of the three-dimensional coil core determine that it has a high resistance to short-circuit, and the core has no sharp corners, making zero discharge possible.

2. Energy saving and environmental protection

(1) Energy Saving

The new non-encapsulated air-insulated dry-type transformer adopts a three-dimensional coil core, and the core is seamless, which greatly reduces the magnetic resistance. The no-load current is reduced by 60% to 80% compared with the national standard, which improves the power factor and reduces the power line loss.

The cross-section of the three-dimensional core iron yoke is half of the cross section of the core column, so that the three-dimensional core can naturally reduce the no-load loss. The annealing process after the three-dimensional coil core rolling system can restore the performance of the silicon steel sheet to the maximum, ensuring the lowest no-load loss.

(2) Environmental protection

The main insulating materials for new non-encapsulated air-insulated dry-type transformers have been mentioned earlier in this article. Environmental protection has become an inherent characteristic. For detailed information on the performance parameters of insulating materials, please refer to the detailed specifications of each insulating material.

The new non-encapsulated air-insulated dry-type transformer adopts a three-dimensional coil core, and the three-dimensional coil core has the characteristics of low noise, small leakage magnetic field, zero third harmonic, etc., and reduces pollution to the environment and power grid.

3. Cost

(1) Material cost

The new non-encapsulated air-insulated dry-type transformers have obvious cost advantages. This article will give a set of comparative data (Table 1) for reference by fellow manufacturers. With regard to the principle of saving materials for three-dimensional wound core transformers, Haihong Electric made a detailed discussion in the article "Analysis of Basic Properties of S13 Three-dimensional Scroll Core Transformer", which is not described here.

From Table 1, we can see that the new non-encapsulated air-insulated dry-type transformer compared with the traditional type, the average weight of copper decreased by 15.8%, and the average weight of silicon steel decreased by 25.3%.

(2) Cost of tooling and tooling

The three-dimensional coil core is made by continuous winding of silicon steel sheet, and there is no opening in the core, so the winding needs corresponding tooling and die directly wound on the core column. For resin-insulated dry-type transformers, due to the limitations of casting size, the molds are not easily generalized, and the molding and demoulding processes are complicated. For non-encapsulated air-insulated dry-type transformers, the mold is simple and versatile. If the standard series of products, the cost of the mold can be ignored.

(3) Labor costs

Since the birth of transformers, people have been looking for ways to reduce transformer losses and noise. We know that as long as the transformer is excited, the no-load loss exists. Although the no-load loss is not large relative to the capacity of the transformer, it represents a constant energy flow vector in the power system. In order to reduce the no-load loss and noise, people have long concentrated their research efforts on the development of transformer core structures and new types of electrical steel sheets. At present, the three-dimensional coil core structure has been recognized by the world as the most reasonable core structure. The production process of electrical steel sheets is also constantly improving. In order to reduce the unit loss of electrical steel sheets, electrical steel sheets are becoming thinner and thinner. For the traditional stacked iron core, the thinning of the electrical steel sheet increases the workload on the one hand, and increases the stacking difficulty on the other hand, leading to increased labor costs. The three-dimensional coil cores are mechanically and automatically controlled from shearing to rolling, resulting in high production efficiency.