A New Control Method to Overcome Sympathy Between Transformers using SSSC

Abstract

Sympathy phenomenon is caused by the interaction between the transformers. Inrush current of transformers, especially in long transmission lines is unavoidable that can be as high as 10 times of the transformer rated current lasting for a few cycles or more depending on R/L in the primary side of the transformer. This inrush current causes the saturation of available parallel transformers in the line. This kind of magnetizing transient is called sympathy. Sympathy has many effects on the power system like appearance of unsymmetrical voltage sag, transient harmonics components and overvoltage, which disrupt the operation of differential relays and also coordination of transformer overcurrent relays and affects the whole protection system. In this paper, a new lateral application is proposed for a Static Synchronous Series Compensator (SSSC) to overcome the inrush current and consequently tackles the sympathy phenomena. The proposed control method detects the inrush current in the line by the Sympathy Detection Unit and determines the mode of operation to inject the high inductance series to the line. It operates independently from the switching-on instance. An 11-level Cascaded H-bridge converter is placed in series with the line acting as SSSC in the normal condition and a high impedance during energizing transformers. The simulation results show the feasibility and the proper dynamic of the proposed control method in limiting the inrush current and avoiding the sympathy for the connected transformers.