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T&D World Magazine

CL&P Upgrades Distribution System for Meriden, Connecticut

As part of its ongoing efforts to improve its electrical distribution system throughout Connecticut, The Connecticut Light and Power Co. (CL&P) is upgrading the system in downtown Meriden. CL&P will invest $4 million over the next two years to reconfigure and strengthen the circuits that serve the downtown area.

"We are redesigning the electrical system serving downtown," said Robert J. Drain, director of Western Division Operations for CL&P. "The first step involved upgrading our Center Street substation and we've done that." CL&P is in the process of ordering additional materials and will begin work again in late spring, which will include the replacement of nearly two miles of underground cables with newer, more efficient technology. When the work is finished, downtown Meriden will be separated electrically into five segments, instead of one. The design is such that, if there are problems, they will be isolated and easier to locate, with fewer customers being impacted.

The Center Street substation is a 13.8/4.8-kV substation. Previously, it was fed by four 13.8-kV feeders from a nearby Connecticut Light and Power facility. At Center Street, the four supply feeders were paralleled forming a 13.8-kV node that fed two 6.25-MVA 13.8/4.8-kV transformers and a 13.8-kV outgoing circuit.

The five 13.8 kv circuits and the two 6.25 MVA transformers were connected together by five line breakers and a bus sectionalizing breaker. Problems arose when one of these breakers failed to clear for a fault on one of the 13.8-kV supply feeders, ultimately resulting in an interruption of the entire system.

In response, CL&P decided to modernize and re-design this substation to eliminate the 13.8-kV node, and turned to engineers Kris Aberg, Don Chamberlin, and Jim Harkin.

This new design consists of two independent preferred-alternate transfer schemes; one per 6.25-MVA transformer. The outgoing 13.8-kV feeder has been eliminated by adding its load directly to the incoming feeders, fed by automatic pad-mounted switchgear. The 13.8-kV oil circuit breakers have been replaced with four new Cooper NOVA reclosers with Cooper Form 6 recloser controls. Two of the four new reclosers supply one 6.25 MVA transformer. One of the reclosers will be normally closed, and the other recloser will be normally open. If the normally closed recloser loses its source, the other recloser will close and pick up the 6.25 MVA transformer.

Four 13.8-kV circuits from a nearby bulk substation presently supply the downtown Meriden 125/216-V network secondary grid through eleven network transformers. The plan is to reduce the size of the network by removing fringe ends of the network and
re-supplying them as radial systems. Five of the eleven network transformers will be eliminated, and the load removed from the network will be fed with pad-mounted or submersible three-phase transformers. These transformers will be supplied by underground 13.8-kV three-phase loop circuits fed from new or existing pad-mounted automatic transfer switchgear.

In addition to reducing the size of the network, CL&P will replace in each manhole the existing network secondary manhole bus bar connections in the remaining sections of the network with rubber-molded limitered-crab connectors configured as a ring bus. The existing 500 kcmil copper network secondary cables will be replaced with 4/0 copper secondary cables. In general, two 4/0 cables are required to match the capacity of one 500-kcmil cable, but the reduced network size gives opportunities to reduce ampacity requirements as well. CL&P expects that by using 4/0 cable rather than 500-kcmil cable, the new network system will clear secondary faults faster, if there is a problem. Use of 4/0 copper cables should allow secondary faults to burn clear, or allow the cable limiters to isolate the secondary cable fault better than has been experienced.

The crews will need to pull in the new 4/0 copper cable, install the crab ring bus, and parallel up with the existing secondary. As cables are tied into the existing secondary, the existing secondary cables will be removed. There is not enough room in the manholes to accommodate the entire new secondary system, as well as the existing system.

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