Design of grounding system to protect against lightning and fault is important for both safety and reliability especially for Metropolitan Electricity Authority (MEA) as mostly Metropolitan Electricity Authority operates on overhead networks. This thesis assesses lightning performance for 69 kV 115 kV overhead subtransmission lines 24 kV distribution line using case study from Metropolitan Electricity Authority. Because of the limitation of the right of way distribution feeders are regularly installed together with subtransmission lines in a single concrete pole. Overhead Ground Wire (OHGW) installed at top of the pole and grounding system in concrete pole protects fault current or direct lightning surge to ground. Metropolitan Electricity Authority has developed lightning protection based on an IEEE standard which uses international data for insulation coordination. As a result reliability of the system is lower than the designed value. This thesis suggests a methodology to improve the performance in order to increase the reliability of subtransmission and distribution lines feeding distribution substations. This thesis recommends installation of addition external ground wire on the outer surface of the concrete pole to reduce top pole voltage and back flashover rate or installation of bundle overhead ground wires to protect shielding failure. As all the case studies presented in the thesis which constitute this thesis are applied to existing systems of Metropolitan Electricity Authority it is expected that the proposed methodologies can be served as useful practical guidelines for lightning performance assessment subtransmission lines and distribution lines of Metropolitan Electricity Authority.

The analysis of the grounding system overvoltage in High voltage substation will be held at Bumnetnarong substation Electricity Generating Authority of Thailand ( EGAT ) by using ATP draw for grounding system’s model of substation also to model overvoltage from lightning and to read the result of overvoltage from differences positions. The advantages of the experiment are to be educated and to who design Grounding system in high voltage substation EGAT. The Study’s result of the over voltage level on Ground Potential Rise: GPR model shows that the pie will show close result to the result from the calculation by standard IEEE Std 80-2000 and the result of the Fault Current at the middle position of the grounding system will result in higher overvoltage than at the result at the corner position of Grounding system. The result of Ground Resistance: Rg shows that the increasing of overvoltage the increasing of Ground Resistance: Rg