When designing large-capacity power transformers, a key decision is whether to use a three-phase three-limb core or a three-phase five-limb core. This decision is crucial for transformer design engineers and must be carefully considered during optimization.
Factors Influencing the Choice of Core Design
Customer Requirements: In some countries, customers may explicitly specify the need for a three-phase three-limb core structure when selecting a transformer. This requirement is often clearly stated in the technical specifications. However, this requirement may be implicitly expressed in some cases, and design engineers must identify these subtleties. If a transformer is designed with a five-limb core against the customer's preference for a three-limb core, the product may face rejection and return.
Transportation Height Restrictions: Many countries and regions have specific restrictions on the transportation height of transformers. The primary advantage of a three-phase five-limb core structure is that it can reduce the transportation height of the transformer, ensuring smooth delivery and potentially saving a significant amount of shipping costs. There have been instances where transformer manufacturers when designing large-capacity power transformers, did not consider transportation constraints and opted for a three-limb core structure. This ultimately resulted in the transformer being too large to transport, necessitating costly modifications and leading to substantial economic losses.
Cost Considerations: If there are no specific customer requirements and no transportation height restrictions, the choice of core structure is primarily based on overall manufacturing cost considerations. Generally, three-phase three-limb cores are more economical than five-limb cores. They are advantageous in reducing the consumption of primary materials, lowering worker labor intensity, and shortening manufacturing time.
Differences in Zero-Sequence Impedance
When customers prefer either a three-phase three-limb or five-limb transformer, it is often due to considerations of zero-sequence impedance. The magnitude of a transformer's zero-sequence impedance influences the zero-sequence short-circuit current, which is naturally related to the customer's relay protection settings.
In a three-phase three-limbs transformer, the zero-sequence magnetic flux must form a loop through the core and the tank, which presents high magnetic reluctance, resulting in smaller zero-sequence flux and consequently smaller zero-sequence impedance, typically about 90% of the positive-sequence impedance. In contrast, a three-phase five-limb transformer allows the zero-sequence magnetic flux to form a loop through the side yokes, resulting in larger zero-sequence flux and higher zero-sequence impedance, generally about 99% of the positive-sequence impedance.
Differences in Design Costs
Under normal circumstances, with all other technical parameters being equal, a three-phase three-limb core transformer is more cost-effective than a five-limb core transformer (though there are exceptions that require specific analysis). Compared to a three-limb core, a five-limb core structure requires more silicon steel sheets, resulting in higher no-load losses. Therefore, to maintain the same level of no-load loss, a five-limbs core transformer would incur additional costs, typically achieved by reducing the magnetic flux density or decreasing the core diameter. Either approach would reduce the turn voltage, increasing the amount of copper required for the windings.
In summary, the decision between a three-phase three-limb, and five-limb core structure depends on customer requirements, transportation constraints, zero-sequence impedance considerations, and overall cost efficiency.