Understanding the Load Loss Test in Power Transformers

The load loss test is a crucial procedure for assessing the performance of large power transformers. Typically, it is conducted at the rated current to ensure accurate measurement of losses. However, when the test is performed at lower current values, the extrapolated load loss may not reflect the actual performance due to the complexities involved, particularly with stray losses within structural components.

Stray losses, which arise from magnetic and electromagnetic fields, can significantly impact the total load loss in large transformers. These losses may increase at a rate greater than the square proportion as winding currents rise, especially when saturation effects in mild steel materials come into play. In cases where the electromagnetic shields are inadequately designed, the stray losses can further escalate, exhibiting an exponent ranging between 2.3 to 2.5 instead of the expected 2. This variation underscores the necessity of conducting the load loss test at rated current levels to capture the true characteristics of the transformer.

When it comes to structural elements like bushing mounting plates, the calculation of losses becomes more intricate. For example, the current exponent for stray losses in these plates is reported to be around 1.5. This means that as the current increases, the losses do not merely follow the square law, revealing the nuanced relationship between current and power loss in transformer components.

Moreover, the relationship between stray losses and current can vary based on the distance from current-carrying bars to the plates. The exponent for aluminum plates remains at 2, while for other magnetic materials, it fluctuates between 2.2 to 2.5. Understanding these nuances is essential for transformer manufacturers and users to ensure that design and operational parameters align with performance expectations.

In practice, while the load loss may seem closely related to the square of the current under normal operating conditions, this relationship can shift during overloads. Under such conditions, greater currents can lead to an increase in losses with an exponent exceeding 2, complicating the load loss dynamics further. Therefore, attention to detail in testing and design is paramount, especially under varying load conditions.