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A discussion is presented on the design of a cathodically protected electrical substation grounding system in which a steel ground grid and steel ground rods were used in place of the commonly used copper ground grid and copperweld ground rods. Several electrical constraints are presented which discuss common electrical utility requirements, safety considerations, and economic factors. The grounding system ·materials chosen are discussed along with the means of cathodic protection. Finally, the design, construction, and testing considerations are presented as an aid to others who wish to design a similar cathodically protected electrical substation system.
The 138-kV system was supplied by a single 138-kV transmission line being fed from UP&L's Naughton Power Plant. The available fault current from the 138-kV system was approximately 2100 A. The secondary of the substation was supplied by two delta-resistance grounded with transformers with a normally open bus tie breaker. Either of the two 138- 4. 16-kV 12/16/20-MV A transformers could supply the entire load (Fig. 1).
Each of the grounding resistors was rated 60 and 400 A for 10 s. LOD TRAK II relaying was provided for each of the motor feeders and instantaneous ground relays were provided on each 1000-kVA auxiliary service transformer, High-speed bus differential relays were provided for 4.16-kV bus fault protection, and high-speed harmonically restrained transformer differential relays were provided for each 138-kV- 4.16-kV transformer. Bus and transformer ground backup overcurrent relays were also installed in such a manner that all 4.16-kV ground faults would be cleared in less than 1.0 s.
The proposed substation was to be placed on Utah Power and Light Company's (UP&L) 138-kV system. Although for legal purposes UP&L limits the amount of control placed on a customer-owned substation, they are quite interested in the grounding system. This is due in part to the fact that their personnel may be involved in activities within the substation · such as switching, meter reading, and inspection. Therefore, the substation grounding design had to meet the rigid requirements of the local utility.
|A Cathodically Protected Electrical Substation Ground Grid|