A Novel Topology To Improve The Commutation Failure Immunity In LCC-HVDC And Its Control Strategy

Line commutated converter based high voltage direct current (LCC-HVDC) has been widely used in the world. However, commutation failures caused by ac system disturbances may occur in HVDC systems, which would result in overcurrent across the valves and even cause the converter block in a severe situation. The sensitivity of an HVDC inverter to commutation failures depends on the specific main circuit design and the control system. Based on the mechanism analysis of commutation failures (CFs), a few firing angle control methods have been developed to reduce the CFs, such as advancing firing angle control. The forced commutation concepts of capacitor-commutated converters (CCC) and controlled series capacitor converter (CSCC) show promising results on forced commutation, but have poor dynamic performance when recovering from unbalanced disturbances.In this paper, with thyristor based full-bridge sub-module (T-FBSM) embedded in the converter valve, an novel Line Commutated Converter is proposed to improve the commutation failure immunity of HVDC, assisting in commutating from the viewpoint of increasing the controllability of ac voltage. The operating principle of sub-module and the coordinated control strategy are designed. The current path of sub-module under different operations is shown. The voltage and current characteristics of the sub-module are also analyzed by theoretical derivation.Then, the novel LCC-HVDC and the presented control strategy is developed in PSCAD/EMTDC, the conventional LCC is adopted as the rectifier, and the LCC with semi-controlled H-bridge sub-module inserting into valve arms as inverter. The most widely used control strategy for conventional LCC-HVDC could also be adopted by the new LCC-HVDC. Then the operation performances of LCC-HVDC, with or without presented sub-module, under normal condition and different level of ac faults conditions are compared and investigated. To further evaluate the ability of the sub-module to mitigate the commutation failure, the critical inductance and commutation failure immunity index are adopted, and its dynamic performance under ac faults condition are compared with that of capacitor-commutated converter based HVDC (CCC-HVDC) and LCC-HVDC in detail. Finally, a dual-infeed HVDC system with T-FBSM applied in one HVDC link is also developed to further investigate the capability of presented T-FBSM to improve the commutation process under fault conditions. In addition, the increased capital cost of presented ELCC-HVDC is discussed. The results show that, the presented ELCC-HVDC, with T-FBSM embedded in inverter arm, has the ability to reduce the CF risk effectively and also exhibits favorable steady state and dynamic performances. It could be an interesting option to supply power to weak ac network or multi-infeed HVDC situation.