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Enhanced operation of ‘Sen’ transformer with improved operating point density/area for power flow control

In flexible AC transmission systems, conventional transformers with tap changing mechanism in ‘Sen’ transformer configuration is a cost effective approach for independent active and reactive power flow control compared to voltage source converter-based solutions. ‘Sen’ transformer regulates the power flow by injecting a series voltage in the transmission line with a specific magnitude and phase angle with respect to the line current flowing through it. The accuracy of such injected voltage depends on the number of available taps in the transformer. However, with limited number of taps the conventional ‘Sen’ transformer results in a limited number of operating points, leading to large steady state error for a given design. This study discusses different types of ‘Sen’ transformer configurations, tap control schemes and proposes an improved tap selection technique with tap reversal arrangement for enhanced operation of ‘Sen’ transformer for power flow control. The proposed method increases the number of operating points, which leads to smaller steady state error with improved copper utilisation and increased tap changing complexity. The new tap changing algorithm and its implementation techniques are described in detail. The proposed configuration is analysed against similar configurations and is verified through a 160 MVA Simulink model.