A New Hybrid PLL for Interconnecting Renewable Energy Systems to the Grid

There is a need to develop new control strategies for interconnecting renewable energy systems (RESs) to the power grid due to the continuously increasing penetration of RES and, in particular, of wind power systems. The control strategies are typically based on a fast and accurate detection of the phase angle of the grid voltage which may be estimated by using a phase-locked loop (PLL) control circuit. The performance of the PLL under normal and abnormal operational conditions is a crucial aspect, since the RES is desired to operate accurately to support the power system under grid fault conditions. This paper investigates the performance of three different PLLs: a synchronous reference frame (SRF) PLL, a stationary reference frame PLL, and a decoupled double SRF PLL. The results of this investigation motivate the development of a new hybrid PLL which is a combination of the aforementioned PLLs and uses the advantages of each PLL. The proposed decoupled stationary reference frame PLL (d alpha beta PLL) may be an appropriate solution to use in an interconnected RES with fault ride through (FRT) capability, since it prevails the other PLLs with regard to its accuracy under unbalanced faults. Furthermore, it has a lower deviation of the estimated phase after the fault occurs. This could be depicted as a faster response of the d alpha beta PLL within the same frequency limits. The performance of the new hybrid d alpha beta PLL is verified through simulations and experiments. Furthermore, the new PLL is used in an interconnected RES through experiments under normal and FRT operations.