Grounding Grids Corrosion Diagnosis Based On Hierarchical Simplification Model

In order to diagnose the corrosion of the grounding grids, find out the defects in time and ensure the safty of power apparatus, electric power system and the personal safty, the following contributions have been made:A hierarchical simplification model for grounding grids is proposed. According to the topology and the distribution of touchable nodes, a grounding grid is simplified into four levels,such as Actual Grounding Grid (AGG),Circuit Blocks Grid (CBG),Touchable Grounding Grid (TGG) and Intrinsic Grounding Grid (IGG). The model can clearly reflect the reasons of uncertainty and the influencing ranges in corrosion diagnosis of the grounding grids. It forms the foundation of the testability evaluation and corrosion diagnosis of grounding grids.A series of novel algorithms are proposed to establish the hierarchical simplification model automatically,including the Quasi-Circuit Block dividing algorithm, the Circuit Block dividing and CBG generating algorithm, the Block Grid dividing and TGG generating algorithm and the IGG generating algorithm. The date structure which is appropriate for the proposed hierarchical simplification model of grounding grids is suggested.The branches of a grounding grid are classified into two kinds such as clear branches and uncertain branches. The resistance of a clear branch is unique whereas the resistance of an uncertain branch is not unique. A hybrid approach based on the rules and numerical methodology to evaluate the testability of branches of a grounding grid is proposed while the testing scheme is complete. The rules of transforming some uncertain branches into clear branches after diagnosis are analysed. The necessity and feasibility of the proposed approach are discussed with examples.A novel approach is presented to diagnose the corrosion of a grounding grid based on the hierarchical simplification model. The actual resistances of clear branches are obtained. The possibility resistance ranges and the corresponding probability distribution of uncertain branches are estimated within one uncertain branch group by another. The entropies of the uncertain branches, the uncertain branch groups and the grounding grid are formulated. Consequently, the uncertain degrees of the uncertain branches and the effectiveness of the diagnosis can be evaluated objectively.To determine the testing scheme for corrosion diagnosis of a grounding grid, three inferences of maximum voltage theorem are put forward and proved. It is pointed out that to make the measured voltage most sensitive to the change of resistance of a certain branch, the constant DC current exciter should be added on the corresponding branch. It is also pointed out that in case of the constant DC current exciter added on a branch, the voltage on the branch is the most sensitive to the change of resistance of the branch. Based on above theory, the theoretical best testing scheme is proposed. To avoid the influence of lead line resistances in the practice, some solutions are suggested.Based on the proposed model and approach, an experimental grounding grid with sixty branches and the grounding grid of Fenghe 330kV Substation are tested and diagnosed, respectively. The diagnose results show that the proposed approaches and algorithms are correct and feasible.