Low power factor, harmonic pollution and unbalanced power problem are three of the major power quality (PQ) issues in nowadays, which can cause many undesirable consequences. Installation of current quality compensators is one of the solutions for the above PQ issues. Among the different compensators for medium voltage level applications (eg. 10kV), the compensation performances of passive power filters (PPFs) and static var compensators (SVCs) are not as good as traditional active power filters (APFs) and hybrid active power filters (HAPFs). However, the APFs and HAPFs require multi-level structures to reduce the voltage stress on power switches and dc-link capacitor, which increases the cost, power loss and reduce the reliability. With the comprehensive considerations of performances, cost, reliability, power loss and compensation range among other current quality compensators (passive power filters (PPFs), static var compensators (SVCs), active power filters (APFs) and hybrid active power filters (HAPFs)) in medium voltage level applications, the thyristor controlled LC-coupling hybrid active power filter (TCLC-HAPF) has been investigated and developed because it can offer the better performance than PPFs and SVC, lower cost, higher reliability, lower power loss than multi-level APFs and multi-level HAPFs and wider compensation range than HAPFs. However, based on the state-of-the-art TCLC-HAPF system (proposed at 2014), the following points have not been investigated and discussed: 1) A comprehensive comparison has not been studied yet among newly proposed TCLC-HAPF and other different power quality filters in terms of V-I characteristic, cost, power loss and reliability. 2) The TCLC part of TCLC-HAPF can inject the harmonic current during the operation. At this moment, the prevention of harmonic injection of the TCLC part has not been studied yet. 3) The existing current PWM control method for nonlinear systems like HAPF and TCLC-HAPF still suffer the high power loss problem. The nonlinear PWM control method to reduce the power loss is still lack of study. iii 4) The existing parameter of SVC and SVC based system included the TCLC-HAPF are normally designed based on maximum pre-measured load reactive power. However, when the loading is unbalanced, this would easily lead to under-compensation situation. 5) The control strategy for TCLC-HAPF to compensate reactive power and harmonic current compensation and balance active power under unbalanced loading has not been developed. 6) The minimizing inverter capacity design and comparative performance evaluation of different TCLC-HAPF structures are lack of study. In this thesis, to investigate the potential advantages of TCLC-HAPF, a comprehensive comparison among TCLC-HAPF and other different power quality compensators are provided in terms of V-I characteristic, cost, power loss and reliability. Two techniques for TCLC part and active inverter part of TCLC-HAPF have been proposed. Specifically, the solution for TCLC part to prevent harmonic current injection and nonlinear PWM method for active inverter part to reduce the power loss are proposed. Then, the TCLC-HAPF parameter design method based on the compensation range and unbalanced power is proposed, so that the under-compensation problem in the traditional parameter design method based on balanced loading can be avoided. Following that, a hybrid control method is proposed for TCLC-HAPF to compensate reactive power and harmonic current and balance the active power at same time. To further reduce total cost of TCLC-HAPF, the minimizing inverter capacity design is proposed and proved by calculation, simulations and experiments. Finally, the 110V-5kVA three-phase three-wire TCLC-HAPF experimental prototype is developed, built and tested for dynamic reactive power, current harmonics and unbalanced power compensation under different voltage and current conditions e.g. current unbalanced, voltage dip, voltage fault, etc.