In order to analyze the influence of harmonics on the stability of line commutated converter based high voltage direct current(LCC-HVDC)system.It is urgent to establish an accurate model of LCC-HVDC considering harmonic coupling.Based on the theory of harmonic state space(HSS),an impedance model of 12-pulse LCC is established considering frequency coupling effect and control system.The constructed AC/DC harmonic impedance model can match the sweep frequency results in a wider frequency band.Finally,the correctness of the proposed LCC-HSS impedance model is verified by comparing the PSCAD electromagnetic transient simulation results with the HSS impedance model calculation results.The LCC-HSS impedance modeling method improves the accuracy of mathematical modeling for LCC converter stations and can adapt to impedance modeling of LCC converter stations in various modes,providing a more accurate model for stability analysis and parameter optimization of LCC systems.
In response to the demand for green and low-carbon development, China is vigorously promoting the construction of large-scale renewable energy bases. The system of transmitting renewable energy through VSC-HVDC is receiving more and more attention. In such a system, renewable energy devices usually adopt grid-following control, while VSC-HVDC adopts grid-forming control. However, the synchronous stability mechanism of this high-proportion or even 100% power electronic system is still unclear. This paper establishes a synchronization model for grid-following and grid-forming devices, focusing on the role of phase-locked loops and virtual synchronous generator loops in synchronization. Then, the small-signal stability of the system is analyzed, and the operating area that ensures stable operation is derived. The operating states with instability risks are proposed for both renewable energy devices and VSC-HVDC. Finally, the conclusions are verified by PSCAD simulation.
Kunliulong hybrid multi-terminal UHVDC project is the first hybrid multi-terminal UHVDC project in the world, which is more flexible than the traditional two-terminal DC operation mode. Meanwhile it also increases the complexity of control and protection strategy. Since it was put into operation, there have been several abnormal operations, and the reliability of its "first set" control and protection system and equipment needs to be further improved. This paper analyzes the "6.9" valve-controlled abnormal events of Kunliulong UHVDC in detail. After that,the paper sorts out the configuration of control and protection functions and UHVDC response, and creatively proposes to increase the detection and application of valve-controlled abnormal events. This strategy can accurately and quickly detect the valve-controlled pulse loss or pulse delay fault, improve the risk identification and resistance ability of UHVDC control and protection system in this condition. The result shows that the improvement effectively increase the reliability and stability of this hybrid multi-terminal UHVDC project. It also provide reference and guidance for the subsequent functional design of this kind project.
In the context of the large-scale connection of distributed photovoltaic(PV)to the power system,in order to make use of the complementary characteristics of the distributed pumped storage power plant with other energy source output and maximize the output of combined PV-pumped storage system to meet the scheduling requirements while reducing the abandoned PV power,the output models for distributed photovoltaic and distributed pumped storage units that are more in line with the actual operation condi-tions are established.On this basis,a joint scheduling model of PV and pumped storage is developed and a deterministic transforma-tion approach is used to solve the scheduling model with random variable prediction errors in the objective function.The Euclidean distance is introduced as a measure of the matching degree of the scheduling curve.Finally,example simulation results show that the proposed strategy can make the distributed PV output better meet the scheduling demand with the cooperation of pumped storage,which promotes the system's consumption of distributed PV power.
Aiming to study the distribution characteristics of local wind fields in coastal region with transmission lines during the landfall of Typhoon Nida,the mesoscale meteorological model WRF and the microscale CFD model WindSim are used to design three simulation schemes,including mesoscale simulation based on WRF,microscale simulation based on WindSim,and mesoscale and microscale coupled simulation based on WRF and WindSim.Numerical simulation experiments are carried out and compared with measured data from LiDAR,so as to valify the applicability and accuracy of the different models.The results show that the WRF-based mesoscale simulation can better reproduce the temporal variation of meteorological fields during the landfall of Typhoon Nida period,and the WindSim-based microscale simulation can better reflect the topographic effects of the wind field at a spatial resolution of 10～100m in a specific wind direction.The mesoscale and microscale coupled simulation based on WRF and WindSim can not only correct the topographic effect bias of WRF alone,but also improve the boundary conditions and atmospheric stability of WindSim alone,and have the ability to simulate the three dimensional wind field with complex terrain during typhoon impact with higher accuracy of simulation results.
