Research On Lightning Transients In Large-scale Photovoltaic Arrays

The lightning is a serious weather threat to the safe operation of photovoltaic power generation systems.With the extensive development and utilization of solar energy,much attention is paid to the lightning protection of photovoltaic power generation systems.In view of the present situation,an in-depth research is undertaken in this paper on lightning transient behavior in the bracket structures of photovoltaic arrays,which is put into effect by means of calculation analysis and experimental measurements.The research is aimed at quantitatively modeling the transient responses and electromagnetic effect in the bracket structures struck by lightning and providing a sound basis for their lightning protection design.After a bracket structure is hit by lightning,a high current is injected to the attachment point and flows through from all branches and the grounding arrangement into soil.In consideration of this transient transmission process,the circuit model of the bracket structure is built for calculating lightning transients.The circuit parameters of the branches at different spatial positions are evaluated by using Neumann integration and average potential method and their matrices are determined for the coupled branch units in the bracket structure.Subsequently,the equivalent circuit is constructed for each coupled branch unit.Around the grounding electrode,the soil ionization due to dissipating lightning current is taken into account by a practical manner.With the circuit parameters estimated for the grounding electrodes,the equivalent circuit is also constructed for the grounding arrangement.Integrating the equivalent circuits of the grounding arrangement and all coupled branch units,a complete circuit model is established for the bracket structure.The lightning stroke to the bracket structure is simulated as a lightning current source injected to the node corresponding to the attachment point.The lightning transient responses are calculated by the circuit model under the excitation of the lightning current source.The SPRIM reduced-order simplification method is also applied to the transient calculation to reduce the calculating complexity and storage capacity under conditions of guaranteeing calculation accuracy.In order to check the validity of the proposed circuit model,a reduced-scale model of the bracket structure is set up.A new type of impluse generator is developed,which has the capability of generating the impluse currents with nanosecond-grade wavefront.An approximate consideration is made in the experiment for the actual traveling wave feature of lightning current.The impluse responses are measured on the reduced-scale model.They are compared with those calculated by the circuit model and so a verification is provided for the circuit model.Thereafter the circuit model is applied to a numerical example.The basic distribution characteristics of the transient responses are explored and the influencing factors,such as lightning current waveshape,position of attachment point,soil resistivity and structure of grounding arrangement,are investigated in the bracket structure.Furthermore,the statistical analysis is also carried out for the transient responses and a pronounced difference is found between the statistical and non-statistical results.With the aid of the transient responses obtained from the circuit model,the thermal effect caused by the lightning current is analyzed for the branches.The temperature rise model is built by considering the branch material characteristic,surface current density distribution and the electrical loss of the skin effect,respectively.The temperature rise on the branches is also measured by the experiment with high impluse current.The validity of the temperature rise model is confirmed by the measured results.The influence of lightning current amplitude,waveshape and branch size on the temperature rise is discussed through a case study.On the basis of lightning current distribution in the bracket structure,the vector potential expressions are derived from Maxwell's equations for the current-carrying branches at different spatial positions.From these expressions,the differential formulas are further given to the transient magnetic field.With a discretization scheme made in time and space,an effective algorithm is proposed to perform numerical calculation for the transient magnetic field.The magnetic linkage is evaluated from the magnetic field in the bracket meshes and loop units and the induced voltages are determined according to the law of electromagnetic induction.The induced voltages are also measured with an experimental set-up and the validity of the proposed algorithm is verified by the measured results.Then,an investigation is undertaken on the influence of standard lightning current waveshape and position of attachment point on the spatial distribution of magnetic field and induced voltage level in the bracket structure.