Energy limit of oil‐immersed transformers: A concept and its application in different climate conditions

Abstract

The reality of modern power grids requires the use of flexibilities from generation, load and storage. These flexibilities allow system operators to modify a transformer loading in a smart way. Therefore, power constraints of transformers can be overcome by using the appropriate flexibility. However, transformers have a physical limit of energy transfer which cannot be overpassed. This energy limit represents the unique transformer's loading profile, ensuring the highest energy transfer under a given ambient temperature profile. The paper explains how the energy limit can be calculated. Typical characteristics of an energy limit are estimated in cold continental climate of Russia and warm temperate climate in France. Maximal, minimal and mean loadings are identified for each month. Loading durations of energy limit are determined for each cooling system. It is found that winding temperatures of transformers, operating at energy limits, remain in the vicinity of design winding temperature. Therefore, transformer operation at energy limit avoids a high temperature stress and simultaneously maximizes the energy transfer. The application of energy limits for power system problems is briefly explained along the paper. Energy limit application can reduce an energy cost, maximize a renewable generation and increase a hosting capacity of distribution network. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Optimal Ageing Limit of Oil-Immersed Transformers in Flexible Power Systems

“…4b). To say precisely: such conclusion is true for Tamb ranges where a hot spot temperature remains a limiting factor of loadings (see Tamb ranges in [11] and [9] for an example where a current is a limiting factor).…”

“…Corresponding thermal constraints were calculated in accordance with IEC 60076-7 (see the annex E in IEC standard [4]). The full mathematical formulation of such optimization problem is given in [9]. To compare the optimal load profile with arbitrary load profiles in Fig.…”

“…However, the surprising observation is that the optimal transfer is higher for 0.36 MWh than the energy transfer (24 MWh) at the rated conditions: the nominal power (1000 kVA) and Tamb (+20 ℃). Such better energy transfer becomes physically possible due to the thermal inertia of transformer (see [9] for the details). Briefly, due to thermal inertias of windings their temperature changes slower than a current change.…”

“…Since the load is adjusted, the energy transfer becomes even higher (194, 77 MWh) than if transformer would operate at nominal rating (168 MWh). The reader can see [9] for more details on the impact of climates on the energy transfer of transformers. However, the most important observation in Fig.…”

Real‐time monitoring of transformer hot‐spot temperature based on nameplate data

“…The energy limit strategy provides a special loading profile for a specified ambient temperature profile, which necessitates real‐time correction of thermal ratings of the transformer [10].…”