Transient AC Overvoltage Suppression Orientated Reactive Power Control of the Wind Turbine in the LCC-HVDC Sending Grid

High-voltage direct current(HVDC) transmission is a crucial way to solve the reverse distribution of clean energy and loads. The line commutated converter-based HVDC(LCCHVDC) has become a vital structure for HVDC due to its high technological maturity and economic advantages. During the DC fault of LCC-HVDC, such as commutation failure, the reactive power regulation of the AC grid always lags the DC control process, causing overvoltage in the AC sending grid, which brings off-grid risk to the wind power generation based on power electronic devices. Nevertheless, considering that wind turbine generators have fast and flexible reactive power control capability, optimizing the reactive power control of wind turbines to participate in the transient overvoltage suppression of the sending grid not only improves the operational safety at the equipment level but also enhances the voltage stability of the system. This paper firstly analyses the impact of wind turbine's reactive power on AC transient overvoltage. Then, it proposes an improved voltage-reactive power control strategy, which contains a reactive power control delay compensation and a power command optimization based on the voltage time series prediction. The delay compensation is used to reduce the contribution of the untimely reactive power of wind turbines on transient overvoltage, and the power command optimization enables wind turbines to have the ability to regulate transient overvoltage, leading to the variation of AC voltage, thus suppressing the transient overvoltage. Finally, the effectiveness and feasibility of the proposed method are verified in a ±800kV/5000MW LCC-HVDC sending grid model based on MATLAB/Simulink.

Discharge Characteristics of Long SF6 Gas Gap with and Without Insulator in GIS Under VFTO and LI

Gas insulator switchgears(GISs),widely used in electric power systems for decades,have many advantages due to their compactness,minimal environmental impact,and long maintenance cycles.However,very fast transient overvoltage(VFTO)increases caused by a rise in voltage levels can lead to GIS insulation failures.In this paper,a generating system of VFTO and standard lightning impulse(LI)is established.The insulation characteristics of SF6 gas with and without insulators under VFTO and standard LI are investigated.Experimental results show that the 50%breakdown voltages of the inhomogeneous electric field rod-plane gap under positive VFTO and standard LI are higher than that under negative VFTO and standard LI.The research shows that the 50%breakdown voltage under VFTO could be lower than that under standard LI at 0.5 MPa for the negative polarity.Moreover,the polarity effect of the insulator without defect is different from that with defect.Similarly,the breakdown voltage of the defective insulator under VFTO could be lower than that under standard LI by 8%.The flashover channel under VFTO is seen as more than that under standard LI.Based on the analysis of discharge images and experimental results,it is concluded that the polarity effect is related to the distortion effect of ion clusters formed by SF6 on the electric field.Additionally,the steepness and front time of impulse plays an important role in the initiation and further development of discharge on insulator surface.Finally,the research shows that different discharge characteristics between VFTO and standard LI may be caused by different wave fronts and oscillation on the tails of the impulses.

Impact of Synchronous Condenser on the Dynamic Behavior of LCC-based UHVDC System Hierarchically Connected to AC System

Hierarchical connection(HC)is a very attractive mode for±800 kV line commutated converter based ultra high voltage direct current(LCC-UHVDC)system connected to different AC voltage levels because of its ability to reduce the scale factor of a converter transformer.Faults in the HC-UHVDC system can cause commutation failure(CF).In this paper,impact of synchronous condenser(SC)to mitigate CF in HC-UHVDC system is analyzed.A±800KV HC-UHVDC system along with synchronous condenser is built in PSCAD/EMTDC.Transient performance analysis of HC-UHVDC for single and three phase to ground faults is investigated.Commutation failure immunity index(CFII),commutation failure probability index(CFPI),fault recovery time(FRT),and transient overvoltage(TOV)are used as measures to evaluate the effects of SC at HC-UHVDC system design.The simulation results show that SC can make the HCUHVDC system less susceptible to CF,effectively improve fault recovery performances of the overall system,and reduce transient overvoltage when single or multiple converters are blocked.The results of this research can provide technical assistance in real world HC-UHVDC projects.

High-voltage Ride Through Strategy for DFIG Considering Converter Blocking of HVDC System

This paper presents a P-Q coordination based highvoltage ride through(HVRT) control strategy for doubly fed induction generators(DFIGs) based on a combined Q-V control and P-V de-loading control. The active/reactive power injection effect of DFIG on transient overvoltage is firstly analyzed and the reactive power capacity evaluation of DFIG considering its de-loading operation is then conducted. In the proposed strategy, the reactive power limit of DFIG can be flexibly extended during the transient process in coordination with its active power adjustment. As a result, the transient overvoltage caused by DC bipolar block can be effectively suppressed. Moreover, key outer loop parameters such as Q-V control coefficient and deloading coefficient can be determined based on the voltage level of point of common coupling(PCC) and the available power capacity of DFIG. Finally, case studies based on MATLAB/Simulink simulation are used to verify the effectiveness of the proposed control strategy.