Reverse Action of ULTCs: A Novel Scheme to Prevent Voltage Instability

The reverse operation of Under‐Load Tap Changers (ULTCs) poses a significant risk to system voltage stability. This study presents a straightforward method to mitigate voltage instability caused by ULTCs' reverse action. As the system approaches instability, power network equipment may fail to transmit reactive power effectively to loads, resulting in a waste of network‐generated reactive power. Transformers equipped with tap changers are particularly susceptible to this issue. To address this, a novel voltage stability index is introduced, which uses local measurements to assess variations in reactive power consumption during tap changes. This index identifies transformers that contribute to voltage instability and blocks the tap changer controllers. The effectiveness of the proposed approach is evaluated through simulations on the Nordic test system using DIgSILENT software. The proposed index has been implemented for various scenarios, including generator tripping, load increase, and line outage, under different types of static and dynamic loads and in the presence of measurement noise. It has also been compared with a state‐of‐the‐art method proposed in previous studies. The results confirm the accuracy and reliability of the proposed index in identifying tap changers with reverse performance. Furthermore, by blocking tap changers exhibiting reverse performance, voltage instability can be prevented. The main advantages of the proposed index include its low computational burden, ease of practical implementation, and dependence on local measurements only.