STUDY OF A COMBINED SYSTEM OF THYRISTOR CONTROLLED LC FILTER AND HYBRID ACTIVE POWER FILTER by Sut-Ian HO Thesis Supervisor: Dr. Chi-Seng LAM Thesis Co-Supervisor: Prof. Man-Chung WONG Thesis Advisor: Mr. Lei WANG Electrical and Computer Engineering A combined system of a thyristor controlled inductor capacitor (TCLC) filter with a hybrid active power filter (HAPF) in shunt (HAPF//TCLC) or series (TCLC-HAPF) connection is studied and focused in this thesis for compensating the power quality (PQ) pollution of a three-phase three-wire power system. Compare to passive power filter (PPF), active power filter (APF) and HAPF etc., HAPF//TCLC and TCLC-HAPF can provide a wide compensation range and keep a low voltage or current rating of the active inverter part for reducing the system cost in medium voltage level three-phase power system simultaneously. HAPF//TCLC is studied for compensating a system with an existing HAPF; especially when addition loads are applied and the load reactive powers become outside the original HAPF compensation range, the system compensation performances can be improved by adding an extra shunt TCLC filter. Whereas, TCLC-HAPF is suitable to be applied when the power system does not have any PQ filter installed. Simulations of HAPF//TCLC and TCLC-HAPF are carried out for providing the performance evaluation during load variations and compared their performances with PPF, APF, TCLC and HAPF.