School of Electrical, Computer and Energy Engineering
M.S. Final Oral Defense
Optimal Substation Ground Grid Design Based on Genetic Algorithm and Pattern Search
by
Qianzhi Zhang
Nov.14 2014
3:00 pm-4:30 pm
GWC 487
Committee:
Dr. Daniel J. Tylavsky (chair)
Dr. John Undrill
Dr. Raja Ayyanar
Abstract
Substation ground system insures safety of personnel, which deserves considerable
attentions. Basic substation safety requirement quantities include ground grid resistance,
mesh touch potential and step potential, moreover, optimal design of a substation ground
system should include both safety concerns and ground grid construction cost. In the
purpose of optimal designing the ground grid in the accurate and efficient way, an
application package coded in MATLAB is developed and its core algorithm and main
features are introduced in this work.
To ensure accuracy and personnel safety, a two-layer soil model is applied instead of
the uniform soil model in this research. Some soil model parameters are needed for the
two-layer soil model, namely upper-layer resistivity, lower-layer resistivity and upperlayer thickness. Since the ground grid safety requirement is considered under the earth
fault, the value of fault current and fault duration time are also needed.
After all these parameters are obtained, a Resistance Matrix method is applied to
calculate the mutual and self resistance between conductor segments on both the
horizontal and vertical direction. By using a matrix equation of the relationship of mutual
and self resistance and unit current of the conductor segments, the ground grid rise can be
calculated. Green's functions are applied to calculate the earth potential at a certain point
produced by horizontal or vertical line of current. Furthermore, the three basic ground
grid safety requirement quantities: the mesh touch potential in the worst case point can be
obtained from the earth potential and ground grid rise; the step potential can be obtained
from two points' earth potential difference; the grid resistance can be obtained from
ground grid rise and fault current.
Finally, in order to achieve ground grid optimization problem more accurate and
efficient, which includes the number of meshes in the horizontal grid and the number of
vertical rods, a novel two-step hybrid genetic algorithm-pattern search (GA-PS)
optimization method is developed. The Genetic Algorithm (GA) is used first to search for
an approximate starting point, which is used by the Pattern Search (PS) algorithm to find
the final optimal result. This developed application provides an optimal grid design
meeting all safety constraints. In the cause of the accuracy of the application, the touch
potential, step potential, ground potential rise and grid resistance are compared with these
produced by the industry standard application WinIGS and some theoretical ground grid
mode.
In summary, the developed application can solve the ground grid optimization
problem with the accurate ground grid modeling method and a hybrid two-step
optimization method.