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Blocking filters (BFs) were designed to solve subsynchronous resonance (SSR) problems induced by fixed series capacitors (FSCs) of the Tuoketuo transmission system. However, during the commission of BF, unstable self excitation (SE) occurred unexpectedly, resulting in suspension of the project. This paper first analyzes the mechanism of how BF causes SE. Then, the parameters' sensitivity is investigated and the quality factor (Q) of BF reactors is discovered to have opposite impact on SE and torsional interaction (TI). Thus, by appropriately lowering Q of BF reactors with supplemental resistors, an improved BF scheme is put forward to simultaneously mitigate the different types of SSR problems. As a key issue, the determination of supplemental resistors is formulated into a multi-objective optimization problem, which is then effectively solved with our proposed searching method. Finally, the effectiveness of improved BFs is verified with field tests. For the first time, actual measurements of filed tests on BFs are offered. What is more, their comparison with model studies demonstrates a good agreement. This work paves the way for putting both BFs and FSCs into service, thereby ensuring safe operation and transfer-capability enhancement of the system.
Date of Publication: Feb. 2013