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The robust operation of grid-connected converters under non-ideal grids is a challenging topic. Synchronising of converters requires accurate estimation of the grid vector angle which is traditionally performed by phase locked loops (PLLs). Separating the grid voltage and current sequence components is essential for controlling converters under non-ideal grids. In this study, an efficient method to separate the grid sequence components using cascaded delayed signal cancellation (CDSC) is developed. The proposed method is a reduced version of the conventional delayed signal cancellation separation technique. Implementing CDSC in the stationary frame enables for using a higher bandwidth without degrading its filtering capability which enables for using the GSS as a pre-filter stage for the traditional synchronous reference frame PLL. Therefore, the obtained grid sequence separator PLL (GSS-PLL) accurately estimates the grid vector angle under severe conditions. The performance of GSS method as well as GSS-PLL is compared to the conventional multiple second-order generalised integrator (MSOGI) method under unbalance, phase interruption and harmonically distorted grids. The accuracy of the proposed method is verified through simulation and experimental tests. The low computational effort of GSS scheme compared to the MSOGI is a significant advantage which encourages its implantation for most of the grid-connected converters.

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This article presents a design technique for reconfigurable fault-tolerant controllers acting on a flexible AC transmission system power system. A classical power system stabilizer of the generator excitation system and a controller of the thyristor-controlled series capacitor are considered. Sound operation of both controllers yields the required dynamic performance of the system. However, a faulty condition signifies a degraded control signal of one controller at a time. The proposed scheme suggests that the parameters of one controller switch to some predetermined values if the other controller is faulty with any degradation percentage. Maintaining a specific settling time of the dynamic response is assured against continual degradation of either controller as the design is carried out through the Kharitonov theorem and particle swarm optimization. The proposed design represents an indirect adaptive power system stabilizer. Simulation results denote distinct effectiveness of the proposed design in maintaining the desired system performance under sound and different faulty conditions.

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