Approximately half of the existing 60-kV system is an open-loop network. Customers will loose power supply during single-circuit outages. Restoration of power supply is done by manual switching. Load transfer is required when any line is shut down for repair or maintenance. The proposed closed-loop system can maintain the power supply to the customers during single 60-kV circuit outages. It will also provide reliable power to the existing electrically sensitive customers like semi-conductor manufacturers, computer manufacturers, computer data center, and hospitals.
The proposed closed-loop 60-kV system can be implemented in three stages in accordance with the load sensitivity to power supply interruptions. The three stages are divided in three areas. A fiber-optic line is included in each area for relay communication (the differential relay will not operate without the fiber optic line, rendering the entire system useless):
- North Central Area Loop - This included the addition of 60-kV circuit breakers with protection and control at Pleasant Grove, Industrial and Foothills substations. This project is complete.
- Douglas, Southeast and Hardrock Loop - This involves the modifications at Douglas, Southeast and Hardrock substations.
- Cirby and Vernon Loop - This included the modification at Cirby, Vernon and Baseline substations.
- Berry Street – This is for the modifications to the Bay II feeders to become network capable.
The closed-loop system will provide the following improvements:
- Improve the overall system reliability to cater for a minimum of single-circuit outage and some of the double-circuit outages.
- To cater for system load growth and customer expansions. We need to extend our system to provide a reliable power supply to the existing customers’ expansions.
- The closed-loop system will allow the system to be operated in parallel for sharing the loads between Berry Street and Fiddyment substations with automatic backup. Also, we can operate the system for carrying out on load transfer at the 60-kV and 12-kV level with the existing system constraints during scheduled and forced system outages.
- To provide protection improvement to reduce the fault clearance time and limit the damages to the equipment.
- Provide compatible power supply system as used by other utilities for the industrial and commercial customers.
- Provide reliable circuits for Roseville Energy Park power plant output.