The operational voltage or the voltage distribution along porcelain insulator string is affected by stray capacitances, which causes a non-uniform voltage distribution. The recital of insulator sometimes depends on the amount of insulator/string. In order to explain these activities and phenomena clearly in this research, an equivalent circuit that takes the insulator material properties and the stray capacitances is implemented in the TINA program package to analyze the voltage distribution along the insulator string with and without guard ring under power frequency voltage. Finally, the simulation results of voltage distribution have been compared with experimental results, which were obtained in the KKU High Voltage Research Laboratory. To investigate the voltage along across each (5, 6, 7) insulator/string in clean condition, the string efficiency has been measured by sphere gap method. From the simulation results, the voltage across (5, 6, 7 clean and contaminated insulator/string) with and without guard ring were simulated by the TINA program. From this simulation results, we found that the string efficiency of clean string insulator condition which the insulator consists of 5, 6, 7 insulator/string without guard ring equals 66.92, 58.45 and 53.71%, respectively. The string efficiency which the insulator consists of 5, 6, 7 insulator/string with guard ring equals 90.42, 87.03 and 78.35%, respectively. In the experiment, the sphere gap was used to measure the voltage along insulation string. In case of the clean string insulator condition, the string efficiency which the insulator consists of 5, 6, 7 insulator/string without guard ring equals 69.12, 62.67 and 57.04%, respectively. The string efficiency which the insulator consists of 5, 6, 7 insulator/string with guard ring equals 76.36, 66.69 and 59.76%, respectively.
