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Wind farms are usually built in the areas with frequent lightning activities. So their collector lines are vulnerable to lightning strikes, which threaten the safe operation of wind turbines and their supporting equipment. Based on the electromagnetic transient program ATP-EMTP, the lightning current model, the tower model, the collector line model, the insulator model and the arrester model were established, which were used to simulate lightning striking at the tower of collector lines. The influence on the line overvoltage was studied and the corresponding protective measures were discussed. The results show that the position of tower struck by lightning and the grounding resistance of the tower are the key factors affecting the overvoltage of the collector lines. For wind turbines at mountainous area, the lighting-induced overvoltage can be suppressed by reducing the grounding resistance of towers near the wind turbine and installing arresters at the tower nears wind turbine.

This thesis discusses first-principles modeling of functional perovskite oxides and perovskite superlattices. In the past few decades, first-principles density functional theory has driven tremendous advances in the theoretical study of materials. However, it does not give us a conceptual understanding of the physics of these materials, which makes the first-principles modeling necessary. In the first project, we use the first-principles method to study the epitaxial strain-induced ferroelectricity in the orthorhombic CaTiO$_3$ structure and construct the energy expansion from first principles to illustrate the mechanism of the induced ferroelectricity. We also discover an unexpected polar phase of CaTiO$_3$ with in-plane polarization under compressive strain. Symmetry analysis shows that this phase is a realization of a new mechanism of geometric ferroelectricity. In the second project, we collaborate with an experimental group at SUNY Stony Brook to study the perovskite superlattices PbTiO$_3$/BaTiO$_3$. A variety of properties, including electric polarization, tetragonality, piezoelectricity and dielectric constant, have been studied from first principles. We also construct a slab model, in which different constituents are treated as bulk-like materials with appropriate electrostatic constraints, to investigate the origin of the enhanced piezoelectricity in PTO/BTO superlattices. The third project is our first-principles study of the BaTiO$_3$/CaTiO$_3$ superlattices, in which the oxygen octahedron rotations play a substantial role. We observe the phase transitions among three competing phases and enhanced piezoelectricity in all of the three phases at intermediate BaTiO$_3$ concentration. The slab models of BTO/CTO superlattices consistently underestimate the polarization, which indicates the interfacial enhancement of polarization.

The concentrated leach ate of the Chengdu Leach ate Treatment Plant was collected as a subject of case study. It was a very dark coloured liquid possessing high COD, very low BOD5 and low ammonia nitrogen concentration. The combination of coagulation and fly ash absorption was used to treat the concentrated leach ate in this study. The effects of pH, coagulant dosage and settling time on COD removal in coagulation process were investigated by single-factor and orthogonal experiments. Results showed that the optimum value was approximately pH 5, aluminium sulfate dosage 1.5g and settling time 50min. The influence of operating factors followed the order: coagulant dosage£¾pH£¾ settling time. Fly ash absorption experiments were performed after finishing the surface modification of fly ash in acid condition. The effects of pH, fly ash dosage and stirring time on COD removal were also studied by single factor. Results showed that the optimum value was about pH5, fly ash dosage 2.5g and stirring time 2.5h. After finishing the final step of fly ash absorption, the overall remova1 rate of COD was above 80%.