Phase Selection and Directionality Issues when Protecting Lines with Series Compensation, HVDC Devices or Non-Traditional Generation

More and more often series compensation, non-standard generation and power electronic devices co-exist in a vicinity of EHV transmission lines. A typical scenario is a relatively long transmission line already with elements of HVDC and/or series-compensation, interconnected to a new wind farm with significant capacity driven by non-standard machines. In some cases single-pole tripping is applied in order to maximize power transfer during reclosing, and to avoid synchronization of the islanded generation. During transients, in such systems complex interactions occur between series capacitors, multiple machines, and HVDC devices. This includes both the primary equipment, and the associated controls. As a result the system responds differently compared to a traditional network fed from synchronous generators. Some protection techniques that are well-recognized and successfully applied in traditional configurations may perform inadequately under such complex conditions. The paper lists protection concepts that must be carefully examined or tested before using in such a difficult application environment. It also proposes practical supervision methods that could be implemented via custom logic on a modern multi-function relay in order to solve some of the performance problems