Research On Fault Characteristics Of Offshore Wind Power Grid-connected System With Fully Controlled Current Source Converter

As the distance of offshore wind power continues to increase,the technical and economic advantages of DC transmission solutions become more apparent.The Current Source Converter(CSC)using the RB-IGCT device of the fully controlled type has the advantages of simple structure,flexible and adjustable active and reactive power and strong fault ride-through ability,and has broad prospects in offshore wind power application scenarios.The working environment of offshore transmission equipment is harsh,and the overcurrent caused by DC line failure brings severe challenges to system safety.The control strategy and topology of CSC are quite different from the existing converter stations,and the analysis of CSC short circuit characteristics is mainly based on simulation verification,and there is a lack of quantitative analysis.In this paper,the fault characteristics and propagation mechanism of two typical CSC devices are quantitatively analyzed,and a protection algorithm based on single-ended electrical quantity is proposed.Firstly,the steady-state characteristics and control strategies of the Fundamental Frequency Modulation-based Current Sourced Converter(FFM-CSC)are analyzed.Aiming at the short circuit fault of FFM-CSC system,based on the symmetrical characteristics of three-phase electrical quantity,the dynamic components with small influence are reasonably ignored,the short-circuit fault equivalent circuit is established,the analytical calculation formula of the fault current in the initial stage of short circuit is derived,the influence of the electrical parameters of the system on the development of the fault current is analyzed,and the bipolar 12-pulsating FFM-CSC DC transmission system is established for simulation verification.The simulation and theoretical analysis results show that the short-circuit fault occurs,the fault current rises approximately rampant,the fault energy mainly comes from the AC system,the contribution of filter capacitor discharge to the fault development can be ignored,and the inductive element plays an overcurrent suppression effect to a certain extent.Secondly,the fault characteristics of the Pulse Width Modulation-Current Source Converter(PWM-CSC)are analyzed.PWM-CSC adopts PWM modulation strategy to maintain a high degree of symmetry of three-phase electrical quantity on the AC side of the fault stage,based on the principle of conservation of energy storage,simplifies and establishes the equivalent circuit,obtains a low-frequency transient mathematical model describing the faulty electrical quantity,and derives the analytical calculation formula of DC current and DC bus voltage.Based on mathematical expressions,the influence of the main parameters of the system on the fault overcurrent is analyzed.The simulation results show that the model has high accuracy,and the inductive element and control duty cycle D play an important role in the development of fault current.Finally,according to the characteristics of DC line short circuit,a protection algorithm based on single-ended monitoring is proposed,and a method for determining the setting value with clear mathematical principle is designed.Based on the PSCAD/EMTDC simulation software,the detailed model of FFM-CSC DC system is constructed,typical faults inside and outside the region are designed based on theoretical and simulation analysis,and the real application environment is simulated through simulation studies to prove the reliability and rapidity of the protection algorithm,which can protect the full length of the DC line.