Understanding Transformer Core Designs: A Deep Dive into Laminations and Grounding

Transformers play a crucial role in electrical systems, and their efficiency largely depends on their core design. Different material grades have been developed over time to enhance performance, starting with non-oriented and progressing through various types such as hot-rolled grain-oriented (HRGO), cold-rolled grain-oriented (CRGO), and high permeability cold-rolled grain-oriented (Hi-B) laminations. Each grade offers unique properties that cater to specific applications, particularly in power transformers where lower eddy losses are essential.

Laminations are critical components of transformer cores, with thicknesses typically ranging from 0.23 mm to 0.35 mm for power transformers, while smaller transformers may utilize laminations up to 0.50 mm thick. Thinner laminations generally lead to improved performance due to reduced eddy current losses, but they also require a more complex manufacturing process as the number of laminations increases to maintain core integrity. This balance between thickness and efficiency is a key consideration for engineers during the design phase.

To protect the laminations from electrical stress, an inorganic coating, generally consisting of a glass film and phosphate layer, is applied. This coating, measuring between 0.002 mm to 0.003 mm, helps withstand induced voltages while ensuring the core remains grounded. Grounding is particularly critical in transformer design as it prevents the buildup of high voltages from capacitively transferred currents, which could compromise the transformer's safety and functionality.

The design of the transformer core also includes considerations for cooling and structural integrity. Cores often feature sections separated by ducts for cooling, and each section must be properly grounded. To avoid potential issues with circulating currents, careful planning is required when establishing grounding connections. Typically, grounding is achieved through a single point connection to the tank, while internal frames that support the windings are also grounded to prevent stray leakage flux.

When it comes to core construction, there are primarily two designs: the core-type and shell-type. The core-type construction, which consists of limbs and yokes, is favored for its simplicity and ease of repair. In contrast, shell-type designs allow for a more compact arrangement of windings. Each construction type offers distinct advantages and is selected based on factors such as technical specifications, manufacturing constraints, and transportation requirements.

Ultimately, the choice between a three-phase transformer and multiple single-phase transformers also impacts economic considerations. A three-phase transformer can be up to 25% cheaper than three separate single-phase units, but users often prefer single-phase transformers for their flexibility and the ease of maintenance. Understanding these various aspects of transformer core design and construction can aid in making informed decisions for optimal performance and reliability in electrical systems.