Decoupled DQ-PLL with positive sequence voltage normalization for wind turbine LVRT control

The behavior of PLL under usual and nonstandard operational circumstances is a vital facet, as the RES is anticipated to function to meet grid code requirement. This paper proposes a positive sequence voltage normalization method to be applied in conventional dq-PLL which is normally used as the last stage of positive-negative sequence voltage decoupled PLL scheme. The benefit of this solution is that it helps to eliminate the effect of positive voltage amplitude in PLL control loop bandwidth. Therefore, the dynamic functionality of the decoupled PLL scheme can be improved in grid voltage dip conditions. The proposed solution has been studied in DDSRF-NOM-PLL (decoupled double synchronous reference frame normalized PLL) scheme. Simulation results show that DDSRF-NOM-PLL has less frequency overshoot compared to DDSRF-PLL scheme and has identical dynamic performance as DHRF-PLL. The suggested solution can also be readily implemented to improve the dynamic response of MCCF-NOM-PLL scheme (Multiple Complex coefficient filters PLL with positive sequence normalization) or other positive-negative sequence decoupling schemes with conventional dq-PLL as its last stage for frequency and phase estimation.