Fault Ride-through Control And Short-circuit Current Calculation For Distributed Generators Integrated Into Power Systems

The increasing penetration of Distributed Generators(DG)in power system has greatly changed the original distribution of power flow and short-circuit current in power system,and the short-circuit output of DG will have a large impact on penetration and equipment operation.With the development of power electronics technology,DG with power electronics converter interface to grid are widely used in the grid.In this paper,Double-fed Induction Generator(DFIG)and Grid-following Inverter-Interfaced Distribution Generator(IIDG),which account for a high proportion of grid-connected DGs,are studied.The fault ride-through control(FRT)method and short-circuit current calculation of DFIG and IIDG are systematically carried out.Based on this,the iterative method of calculating short-circuit current in unbalanced grids with DGs is further studied based on the sequential component method.The studies are shown as follows:1)By analyzing the rotor current transient characteristics of DFIG in fault stage,the effectiveness of stator series reactance in suppressing the rotor inrush current and electromagnetic torque oscillation is verified,based on which the DFIG improved FRT control strategy(stator series asymmetric reactance + RSC improvement control)is proposed.In addition,the mechanism of short-circuit current output by DFIG is studied.The transient model of vector control method of DFIG is established.The expressions for calculating the stator and rotor currents are derived for two control models,the RSC continuous excitation control and Crowbar protection control,for both not-serious and serve faults.2)To achieve optimal voltage support control,the voltage support equation is established based on the IIDG gird-connected structure,and the active and reactive current component distribution ratios to achieve optimal voltage support control are determined.Based on this,the operating constraints of the IIDG are satisfied in a balanced manner while satisfying the voltage support control objectives by making constraint corrections to each current component.Considering the IIDG FRT control strategy,the characteristics of the IIDG fault output are investigated.To reflect the fault dynamic response of the IIDG and develop a model that is compliant with traditional short-circuit current calculation,a generic fault model considering the fault control of the converter for sub-transient,transient,and steady-state periods is proposed.The developed IIDG fault model considered the short peak current period during the sub-transient period as well as the interaction between the IIDG short-circuit current and the grid voltage during the transient period.3)The sequence-equivalent modeling method for unbalanced grids is derived,and the sequence-equivalent modeling of unbalanced grids is achieved by introducing dummy lines and dummy nodes while combining generalized Fortescue transform.Based on this,the system sequence bus voltage equation is established by combining the previously established DG equivalence model.The short-circuit current iterative calculation method is proposed since the DG fault output which is nonlinearly affected by its FRT control strategy and PCC voltage.Considering that the short-circuit current calculation must consider both computational accuracy and computational speed.The GMRES algorithm is introduced to iteratively solve the high-dimensional sparse nonlinear equations in the short-circuit current calculation,which effectively reduces the computation burden while ensuring the computational accuracy.Finally,the feasibility and effectiveness of the proposed FRT control method and short-circuit is verify by combining the PSCAD simulation results.