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Lightning puncture damage on the wind turbine blades can pose a severe threat to the safe operation of large-capacity wind turbines. We investigated its formation by performing long air gap discharge experiments under positive and negative downward leaders. Upward streamers were observed to emit from the sample blade before its electrical breakdown. To explain this phenomenon, a numerical model considering the streamer discharge in air, charge transport in GFRP laminate and surface charge accumulation on air-solid interfaces was established. It is found that streamer discharges can be triggered inside the blade hollow to generate space and surface charges, which strengthen the electric field on the outer blade surface and contribute to the upward streamer initiation. The upward streamer development leads to the accumulation of surface charges with opposite polarities on the two sides of GFRP laminate, which causes the electric field on the GFRP laminate to exceed its electrical breakdown threshold. The electrical breakdown, which occurs before the thermal effect of lightning current appears, is the initial stage of lightning puncture damage of blades. The electrical discharge inside the blade hollow and the surface charge accumulation on the GFRP blade should be suppressed to prevent the puncture damage of blades. [ABSTRACT FROM AUTHOR]

• Nonuniform E-field is restored by measuring E-FISH signals along probing laser. • Convergence of inversion algorism related to measuring system parameter is given. • Accuracy of inversion algorism is validated via experimental investigations. • Robustness of inversion algorism in resisting measurement noise is confirmed. The electric field-induced second-harmonic generation (E-FISH) method can offer high spatiotemporal resolution in electric field measurement, but faces challenge in reconstructing the inhomogeneous electric field along the probing laser. This paper presents an inversion method to reconstruct the electric field profile by using E-FISH signals measured from multiple points along the probing laser. The relationship between the condition number of the inversion coefficient matrix and the parameters of E-FISH measurement system is clarified. The inversion method is validated through experimental investigations conducted in three different electrode configurations, namely the plate-to-plate gap, the rod-to-rod gap, and the rod-plate gap. With the inhomogeneity coefficient varies from 1 to 10.7, the maximum deviation between the measured electric field and the finite element simulation is less than 6%. This method does not rely on priori assumptions and electrode configurations, and exhibits good robustness in resisting the effect of measurement noise on the inversion accuracy. [ABSTRACT FROM AUTHOR]

• It is the first application of MPC in MMC-UPFC to handle the unbalanced fault. The current reference of the MPC algorithm for the shunt MMC is obtained in synchronous dq coordinate frame. Previous work has not involved this procedure. • The voltage constraint is considered in the MPC for series MMC. It enables the MMC to operate with the maximum output voltage of the converter by modifying the power reference values in the cost function. It achieves the suppression of the unbalanced current and also makes full use of the capacity of UPFC by operating it on the voltage boundary. This paper proposes a model predictive control (MPC) method to tackle the power flow control of the modular multilevel converter based unified power flow controller (MMC-UPFC) under unbalanced grid conditions. The predictive models corresponding to the discrete-time models of MMC-UPFC are derived. For the shunt MMC, the d-axis and q-axis current references for the MPC are deduced to maintain the dc-link voltage constant according to the energy balance on the dc-link side. The series MMC utilizes a cost function with the voltage constraints to suppress the faulted current and make the power flow controllable, and thus avoid the spread of the fault. Moreover, it makes full use of the capacity of MMC by operating it on the voltage boundary defined by the maximum output voltage of the converter. The model is established using the parameters from a 500 kV MMC-UPFC practical project in Jiangsu, China. Simulation results demonstrate a favorable response of the MMC-UPFC based on the proposed MPC strategy under different unbalanced grid conditions. [ABSTRACT FROM AUTHOR]