The Provincial Electric Authority (PEA) in the Phimai district of Nakhon Ratchasima Province in Northeastern Thailand needed to upgrade protection for a critical load — the Pimai Salt Mine. The mine is located on a branch feeder between two separate transmission substations. Downstream from the salt mine on the branch feeder, was a series of smaller less-critical residential loads. If a fault occurred at one of these residential loads, the protection scheme between the substations and the branch feeder would take the entire feeder offline, dropping the salt mine. Because the mine's process requires pressurized steam, a significant outage could take the mine one to two days to recover, losing revenue for the mine and the utility.

The utility looked for a solution that didn't rely on the upstream substation circuit breakers for loadbreak capability. Initially, PEA planned to use a mobile substation between the mine and the residential load to isolate the salt mine from downstream outages. But because of zoning and local land management policies, purchasing land to install a permanent switching station would have been difficult and costly, as would be negotiating a new right-of-way to reconfigure the way the residential load is supplied. The utility considered circuit breakers, but the size of a circuit breaker plus the structure for a separate isolating disconnect also would require a permanent land-based switching station.

The solution was a Series 2000 Circuit-Switcher, Model 2010. Using the existing right-of-way and transmission structure, a 115-kV Series 2000 Circuit-Switcher was modified by S&C Switch Products Division's Technical Services department for installation 6 m (20 ft) off the ground on a custom-built “floating” switching platform. The Circuit-Switcher was installed with a coordinating set of voltage and current sensors and a relay cabinet with distance relays for fault monitoring along the branch feeder. The Series 2000 Circuit-Switcher with integral disconnect has the full loadbreak rating the utility required, and is lightweight and compact enough to mount on the transmission structure. And the Series 2000 is a significant cost savings over both the real estate requirements for a land-based substation and equipment costs of a circuit breaker.

The utility is considering revising specifications to use the Series 2000 Circuit-Switcher and associated sensing and relaying as one of its standard design “modules” when a transmission sectionalizing station is needed, and is also considering specifying Series 2000 Circuit-Switchers in the “mini-substation (50 MVA or lower) module” to help improve the reliability of the system.

For more information, visit www.sandc.com.