Dynamic thermal rating assessment of oil-immersed power transformers for multiple operating conditions

In this article, the dynamic thermal rating assessment method of oil-immersed power transformer with multiple operating conditions is proposed, considering the constraints of hot spot temperature (HST), top oil temperature, losses of life and the maximum allowable current of on-load tap changer (OLTC) or bushing, which can determine the dynamic load curves under different operating conditions and give the most sensitive constraints to limit the dynamic load capacity. To improve the accuracy of HST estimation, the temperature estimation model is also improved and the thermal parameters are optimised using the HST measured by optical fibre. Finally, several application examples are studied for transformers in different scenarios. The results show that the normal cyclic dynamic transformer rating (DTR) is mainly limited by the losses of life when the ambient temperature is high, and the average load factor can be increased to 0.82 with a maximum load capacity of 1.23. The main limiting factor of short-term DTR is the OLTC or bushing current constraint, and the average load factor can be increased to 1.00. The maximum load capacity of the transformer under both operating conditions is 23% and 50% higher than its rated load capacity, indicating that the transformer still has a large load potential available.