Temperature Rise
In IEC transformer standards, the temperature rise is indicated by two values: one for the top oil temperature and another for the average winding temperature. For standard ambient conditions (20°C yearly average, 30°C monthly average, 40°C maximum temperature), the temperature rise is typically 60 K for the top oil and 65 K for the windings. This is denoted as 60 K / 65 K.
In contrast, IEEE transformer standards specify a single value for the temperature rise, which applies to both the top oil and the windings. For standard ambient temperatures, the temperature rise in IEEE transformers is typically 65 K.
It is important to consider that these values are based on standard ambient conditions and may vary depending on specific operating conditions and design considerations. Proper selection and design of transformers should ensure that the temperature rise remains within acceptable limits for reliable and safe operation.
Insulation coordination / Test voltages
Both IEC and IEEE standards provide guidelines for test voltages in transformers based on the system voltage.
In IEC 60076-3, the test voltages are determined based on the highest voltage for the equipment (Um). The standard specifies two types of rated withstand voltages: the rated lightning impulse withstand voltage (LI) and the rated short duration induced or separate source AC withstand voltage (AC).
Similarly, IEEE C57.12.90 follows a similar approach. The standard also considers the highest system voltage and provides corresponding test voltages. In IEEE, the test voltage for impulse is called the basic lightning impulse insulation level (BIL).
These test voltages are important because they ensure that transformers can handle electrical stresses and operate reliably within the intended voltage range. Manufacturers and operators need to follow the appropriate standard for their region and industry to determine the right test voltages for insulation coordination in transformers.
Terminology
Indeed, there are differences in terminology between European and North American standards for certain components and tests in transformers. These differences can often be attributed to historical reasons and regional practices.