New 345-34.5 kV substation with static VAR compensation and harmonic filter bank for steel mill in Ohio

Project Description:
Total design and construction of a new 345-34.5 kV substation to supply power for a steel mini-mill. This project included installation of a 150 MVAR static VAR compensator and harmonic filter bank.

Overview:
A major American steel producer formed a partnership with an Australian conglomerate to build a new mini-mill for production of thin strip sheet steel from recycled scrap. The bulk of the product was to be furnished to the automotive industry.

We were invited to design, procure and construct a complete 345-34.5 kV substation to serve the mill load. The load consisted of a 140 MVA, three phase electric arc furnace, two 25 MVA ladle metallurgy furnaces, a continuous caster, an oxygen plant, pollution control equipment and a complete rolling mill. The owner had employed the services of a consulting engineering firm to provide the conceptual design. The technical specification for the substation consisted of a simplified single-line diagram and three or four pages of technical specifications. The rest of the design, including coordination with the servicing utility was left to us.

Design Factors:
The owner negotiated an interruptible bulk power supply rate with the utility as a means to purchase electrical energy at the lowest cost. A backup 138-34.5 kV substation was designed and constructed by the serving utility adjacent to the new 345 kV switchyard. This 138 kV station would supply backup service in the event of a 345 kV service interruption. We designed an automatic transfer/load-shedding scheme to restore 34.5 kV power to critical loads during such an event. We also furnished a plant-wide SCADA system, with inputs from the automatic transfer scheme, to provide load-shedding capability for downstream loads.

The service utility was concerned about power quality issues on their 345 kV system at the point of common coupling. Poor power quality could cause disruption of service to adjacent customers and create problems for their generation at a nearby nuclear power plant. We worked closely with one of our major subcontractors to design and furnish a static VAR compensator with suitable harmonic filter capacitor banks to be installed as a part of the substation project.

Engineering Solution:
Space considerations dictated that the substation fit into a 250 ft. by 370 ft. area bounded by railroad tracks to the north, the oxygen plant to the west, and the utility 138 kV substation to the south. Three 120/160/200 MVA power transformers were serviced through double-side break disconnect switches from a common 345 kV cross bus. The 345 kV cross bus was fed by a single 345 kV circuit breaker connected to the utility transmission line on its line side. The cross bus was designed as an overhead strain bus to allow access for transformer removal. A rigid tubular bus with appropriate switching and utility metering provisions was used to collect power at 34.5 kV from the transformers.

The owner desired metal-clad switchgear with draw out vacuum circuit breakers for 34.5 kV distribution and protection. Non-segregated phase bus duct was used to convey power to this switchgear from the outdoor rigid tubular bus. The switchgear was located in a concrete block control building along with switchboards for 345 kV line and bus protection and those for transformer protection. The thyristor valve assemblies and SVC control were located in separate rooms in the substation control building.

Underground 34.5 kV feeder cables in conduit were used inside the substation for harmonic filter banks and for the backup feeder for the utility 138 kV substation. Feeders to the mill were a combination of direct buried cables installed by the owner to an overhead tray system above the metal-clad switchgear.

Scope:
We designed, procured, constructed and commissioned a complete working substation on a turnkey basis. The installation included all structural steel, one 345 kV SF6 power circuit breaker, three 120/160/200 MVA power transformers, four 345 kV double side break switches, four 345 kV CCVTs, melt shop 34.5 kV metal-clad switchgear, rolling mill 34.5 kV metal-clad switchgear, complete static VAR compensator with thyristor controlled reactors and harmonic filter banks, EAF in-line reactor with reactor tap assembly, protection relay and control panels, plant wide SCADA system, and power line carrier protection system.