Dynamic equivalents of tap-changing-under-load transformers in a transmission system

An equivalencing procedure is presented to account for load areas under tap-changing-under-load (TCUL) transformers in medium- to long-term dynamic studies, allowing reduction in dynamical order and number of nodes. Account is taken of specific models for interconnecting networks and loads, which are supplied by TCUL transformers equipped with an automatic tap ratio regulator. The analysis leads to an equivalent TCUL transformer, feeding a constant current load, which affects the rest of the system in the same way as the original transformers; this is acknowledged by comparing, both in steady-state and dynamic conditions, voltage and current, and therefore active and reactive power, at common specific buses in the original and equivalenced system. Two test cases have been created, showing acceptable similarities between the original system and the equivalenced system. Such promising outcomes seem to ensure the opportunity to analyse large power systems, leading to satisfactory results, for interesting quantities, without a loss of generality and excessive time-consuming procedures