A complete analysis of the characteristics and applications of dry-type transformers

As an indispensable key component of modern power systems, dry-type transformers are rapidly replacing traditional oil-immersed transformers worldwide with their unique oil-free design and superior safety performance.

Basic Concepts and Operating Principles of Dry-Type Transformers

Dry-type transformers are power transformers that do not use a liquid insulating medium (such as transformer oil). Instead, their windings and core are either directly exposed to the air or encapsulated with solid insulating material. Compared to traditional oil-immersed transformers, dry-type transformers use solid insulating materials (such as epoxy resin and fiberglass) to achieve electrical isolation between windings, completely eliminating the risk of oil leakage and fire. They are particularly suitable for applications requiring high safety and environmental protection. Based on the insulation method, dry-type transformers are mainly divided into two categories: impregnated (VPI) and cast (CRT). The former uses a vacuum pressure impregnation process to impregnate the windings with insulating varnish, while the latter uses vacuum-cast epoxy resin to form a solid insulating protective layer.

In terms of their operating principle, dry-type transformers still adhere to the basic physical principle of electromagnetic induction. When alternating current passes through the primary winding, it generates alternating magnetic flux in the core, which in turn induces an electromotive force in the secondary winding, achieving voltage conversion. However, dry-type transformers implement this basic principle through unique structural design and material selection to optimize performance. For example, TBEA's newly developed patented dry-type transformer technology utilizes three parallel core legs with their axes perpendicular to the bottom surface. This effectively optimizes magnetic field distribution and reduces circulating and eddy current losses. This innovative core structure, combined with low-voltage windings and specially wound foil (with a winding angle controlled between 175° and 185°), significantly improves transformer energy efficiency.

Dry-type transformers have a wide range of rated capacities, ranging from tens of kVA to tens of thousands of kVA, with 1000 kVA dry-type transformers being a mainstream product in the market. These transformers typically utilize laminated high-permeability silicon steel sheets for the core. The windings are vacuum-cast, and efficient heat dissipation is achieved through natural or forced air cooling systems. In terms of voltage level, dry-type transformers have developed from the traditional 10kV and 35kV to today's 66kV and even higher.

The names of dry-type transformers generally reflect their technical characteristics. In the "SCB" series, "S" stands for three-phase, "C" for cast-type, and "B" for foil windings. The following number represents the performance level; for example, "SCB18" indicates energy efficiency that meets the Type 18 standard. With technological advances, the energy efficiency rating of dry-type transformers continues to improve. The use of new materials such as amorphous alloys has reduced both no-load and loaded losses by approximately 15%-20% compared to traditional oil-immersed transformers. These technological advances have made dry-type transformers increasingly critical in power system upgrades and the development of renewable energy.

Core Structure and Material Innovations in Dry-Type Transformers

The structural design of dry-type transformers directly determines their performance and service life. Modern dry-type transformers achieve safe, efficient, and reliable operation through sophisticated component configuration and innovative material application. A typical dry-type transformer consists of four core components: the core, windings, insulation system, and cooling system. Each component is meticulously designed and optimized to meet the demanding requirements of different application scenarios.

The iron core structure forms the foundation of a dry-type transformer's magnetic circuit. It is typically constructed by laminating high-permeability cold-rolled silicon steel sheets. The thickness and lamination process of the silicon steel sheets directly impact the transformer's no-load losses. TBEA's latest patented technology demonstrates an innovative approach to iron core design: a structure with three parallel core legs, with their axes perpendicular to the base, effectively optimizes magnetic field distribution and reduces energy loss. Even more advanced are iron cores made from amorphous alloys, which can reduce no-load losses by over 30% compared to traditional silicon steel sheets, making them particularly suitable for applications with large load fluctuations. While costly, amorphous alloys offer significant energy-saving benefits over their entire lifecycle and are becoming a standard feature of high-end dry-type transformers.

The winding system, as the circuit component of a dry-type transformer, has a direct impact on its load losses and short-circuit resistance. Modern dry-type transformer windings are primarily copper and aluminum. Copper offers superior conductivity but a higher cost, while aluminum offers a more competitive price. In TBEA's patented design, each core leg is equipped with a low-voltage winding, which is wrapped in multiple layers of foil around the outer circumference of the core leg. This structure not only improves efficiency but also reduces energy loss caused by eddy currents. The winding insulation is cast or impregnated with epoxy resin, creating a strong insulating protective layer that can withstand high voltage surges and effectively dissipate heat.

The insulation system is a key feature that distinguishes dry-type transformers from oil-immersed transformers and is a crucial factor in their safety. Modern dry-type transformers primarily use epoxy resin casting or vacuum pressure impregnation (VPI) insulation methods. Epoxy resin casting completely seals the windings in the insulating material, providing excellent moisture and dust resistance. For example, Shunte Electric uses this technology to keep transformer noise in data centers below 50 decibels. VPI technology, on the other hand, uses multiple vacuum pressure impregnations to deeply infuse the insulating varnish into the windings, forming a uniform insulation layer. Jingquanhua's latest dry-type transformers feature an optimized insulation system design, providing a safer and more reliable power supply solution for data centers.

The cooling system has a decisive influence on the load capacity and life of dry-type transformers. Since there is no oil as a cooling medium, dry-type transformers mainly rely on air convection to dissipate heat. Common cooling methods include natural air cooling (AN) and forced air cooling (AF). Large-capacity dry-type transformers are usually designed in AN/AF hybrid mode, which cools naturally under normal load and starts fans for forced cooling when overloaded. By optimizing the air duct design and heat dissipation area, 1000kVA dry-type transformers can keep the temperature rise within a reasonable range even under high load. Envision Energy's 66kV dry-type transformers for offshore wind turbines adopt an ultra-compact design, achieving efficient heat dissipation in a limited space, meeting the operating requirements in harsh offshore environments.