Study On Subsynchronous Oscillation Characteristics Of Doubly Fed Wind Farm Without Series Compensation

With the large-scale grid connection of new energy and the commissioning of multi circuit HVDC transmission project,China’s power grid is developing rapidly towards the trend of high proportion of renewable energy and high proportion of power electronic equipment.The operation characteristics of power grid are becoming more and more complex,and its safety and stability are facing new risks,such as the subsynchronous oscillation of doubly fed wind farm without series compensation.It is important for the operation of power grid and the consumption of wind power to carry out the research on the subsynchronous oscillation characteristics of doubly fed wind farm without series compensation and put forward the corresponding preventive measures.In steady-state operation,the control and operation of grid side converter and rotor side converter of DFIG are relatively independent,so AC/DC/AC converter can be divided into two parts from DC bus.Grid side converter is regarded as grid connected inverter,and rotor side converter is regarded as excitation power of induction motor rotor.Phase locked loop can extract the real-time phase of the voltage from the grid voltage and provide a reference for coordinate transformation,which is equivalent to providing a channel for grid voltage small disturbance to participate in the converter control system.This paper analyzes the small disturbance characteristics of doubly fed induction generator in frequency domain,describes the process of grid voltage harmonic small disturbance participating in doubly fed induction generator control,defines the generation mechanism of disturbance frequency current and disturbance couplingfrequency current,and establishes the small disturbance impedance of doubly fed induction generator with and without considering the dynamic characteristics of phase locked loop by using harmonic linearization method.The simulation model in PSCAD/EMTDC verifies the correctness of the small disturbance impedance model.Theoretical analysis and simulation show that the control parameters of rotor side converter and phase locked loop parameters will affect the impedance characteristics of small disturbance impedance.The small disturbance impedance of doubly fed induction generator will show"negative resistance+capacitance" under certain conditions.When the positive resistance provided by the grid is not enough to offset the negative resistance and the inductive reactance just offsets the capacitive reactance,there are weak damping or even negative damping resonance points in the system,and stable or even divergent subsynchronous oscillation current may be generated.In the control system of rotor side converter,the additional control loop can be equivalent to series virtual resistance in small disturbance impedance to counteract the negative resistance effect.STATCOM and doubly fed induction generator are connected in parallel.By setting the parameters of STATCOM,the small disturbance impedance characteristics of grid connected doubly fed induction generator can be changed to prevent the occurrence of resonance point.Theoretical analysis and simulation show that the two methods can reduce the risk of subsynchronous oscillation in grid connected doubly fed wind power system.