# Minimizing Electric Field Stress in Surge Arresters

Surge Arrester with ElecNet
The voltage distribution along the resistors in a surge arrester is uneven due to stray capacitances. It is possible to reduce the maximum electric field stress by a careful choice of position for the grading rings.

In this example, OptiNet is used with ElecNet to find where the rings should be placed, and what their dimensions should be, in order to minimize the total electric field stress in the resistors.

### METHODS and RESULTS

## RESISTORS ELECTRIC FIELD EQUIPOTENTIAL LINES

The electric field for the surge arrester is obtained using ElecNet. This figure displays the equipotential lines in the resistors and surrounding air.

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## IDENTIFYING the PARAMETERS

This figure shows the parameters that control the placing and the size of the grading rings. In OptiNet, the user specifies a minimum and a maximum value for those variables that can change, and OptiNet limits the searches within this range to find the optimum design.

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## OBJECTIVE FUNCTION - ELECTRIC FIELD in the RESISTORS

The goal is to minimize the electric field magnitude in the resistors. In OptiNet, basic quantities, such as the maximum field value in a component, are easily available.

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## SETTING the CONSTRAINTS

The constraints in this optimization process are pure physical ones: the two ring must not overlap and the rings cannot be less than 5mm from the surge arrester.

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## GRAPH of VARIABLES

For each iteration of the optimization process, OptiNet updates and displays the changes (in the form of graphs) for the goal, variables, objectives, and constraints -- these graphs are displayed on the Progress page. In this example, each of the six variables' graphs (in groups not exceeding five variables per window) is updated as OptiNet finds a new design.

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## OPTINET'S REPORT GENERATOR

OptiNet produces a report for each optimization run. In this report, the designs that satisfy the constraints are shown in the order that they are improved. The user can view each design individually. The report also shows the time that it took to arrive at the improved design. The values of all the variables and the optimization function are displayed in this report for each iteration. The values of each parameter can be examined to determine the sensitivity of the design to that particular parameter.

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## MAXIMUM E FIELD in the INITIAL DESIGN

This figure shows the electric field in the three resistors for the initial design that the user supplied to OptiNet, as well as the maximum field magnitude.

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## MAXIMUM E FIELD in the FINAL DESIGN

In the optimum design that OptiNet generated, the maximum electric field in the three resistors is lowered from 146558V/m to 127686 V/m, decreasing by almost 13% the maximum electric field.

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