Earthing design in power networks and analysis of the effects of faults on the system

Abstract

One of the most important aspects for every high voltage installation is adequate earthing. Earthing is required to safeguard people and equipment in the event that a fault to earth develops in the high voltage network. It is desirable that a station earthing system provides a nearly zero resistance so as to limit the earth potential rise. In practice this will primarily be limited by the station earthing system and also by the soil resistivity. In Malta the earthing system adopted for such stations consists of three bare copper wires having a cross sectional area of 70mm2, placed horizontally along a trench at a depth of 1.2m. The length of the required buried bare copper wire will be determined from the soil resistivity value obtained from a site resistivity survey. The earthing system will provide a common earth for the electrical equipment and for all metallic structures at the substation. The local power system is mainly made up of underground high voltage (HV) cables having their metallic sheaths connected to earth at each end of the cable. When an earth fault occurs, the fault current will split between the cable sheath path and the general mass. Hence, the actual value of fault current that will flow through the station earthing system must be known for effectively designing the earthing system. This thesis will show the importance of the soil resistivity survey for correct calculation of the earth resistance at site and the effect it will have on the design of the earthing system for a substation. It also explains the procedure that needs to be followed in order to conduct a soil resistivity survey. In addition this thesis will present the steps required for the correct design of a substation earth grid system for the Maltese network since this type of earthing system has never been adopted on the Maltese network. This type of earthing system will ensure that the step and touch voltage are below safe limits and that the earth potential rise (EPR) will rise uniformly within the station. For comparison, the designed grid will he analysed with a finite element analysis software for a.c. substation earthing (CYMGRD). The designed earth grid will then be implemented in practice for a 132kV distribution centre. Further, in order to quantify the amount of the fault current that would actually flow through the general mass of earth, use was made of an Electro Magnetic Transients Program (PSCAD) to model the cables. In order to estimate the actual value of the fault current which will flow through the designed earth grid system, the entire 132kV network was modelled in PSCAD. Knowing the value of the fault current that will actually flow through the earthing system will allow for its proper design with a smaller conductor cross sectional area (c.s.a) and also a possible reduction of the required area for the earthing system. This shall assist in reducing the total cost of the earthing system while still maintaining a safe working area within the substation.