Metal Smelting

1. Lightning stroke

Metal smelting enterprises are generally located in suburban areas or outer suburbs, and occupy high buildings such as 2,000-m3 blast furnace with height up to 100 m, multilayer steel structure main workshop (for steel making, rolling, etc.), and abounding chimneys. Lightning stroke in this case will halt their production in large scale and cause vast direct economic losses.

Accident case

On April 5, 2012, a steel pipe plant in Yangzhou, Jiangsu was struck by lightning, which ignited the paint for spraying steel pipes and caused big area fire. As a consequence, the plant suffered serious losses.

2. Fault arc

Being of vast scale, complicated technologies, strict processes, metal smelting enterprises are equipped with, more electric rooms, control rooms and operation rooms, and high-temperature fault arc will easily occur in case of equipment fault or short circuit and then cause fire. As a great deal of coal, coke, fuel oil and electric energy, as well as fuel and flammable and explosive gases are needed for metal smelting, meanwhile, it will easily cause fire accident.

Accident case

On April 13, 2015, a fire occurred in a special steel plant in Dalian, Liaoning, with open fire and dense smoke on site. The plant is the largest top-grade tool and mould steel production base in China. The cause to the fire, according to investigation, was ignition of inflammable goods by the high temperature arc due to electrical equipment fault.

3. Harmonic and imbalance

High harmonic content is generally prevalent in the electric equipment with heavy load among metal smelting enterprises. And such equipment of heavy loads mainly include intermediate frequency furnace, rolling mill and electric-arc furnace, as well as finery, ferroalloy furnace, calcium carbide furnace, etc.

Loads will generate various power quality problems, e.g. heavy reactive power impact, low power factor, heavy three-phase imbalance, and high harmonic content injected into the grid, which will cause severe voltage fluctuation and flicker of grid and great electric energy loss.

3.1 Cause

Intermediate frequency furnace

Due to the impact of rectifier equipment, intermediate frequency furnace will generate a large number of harmonics during its operation, which will impact its own normal operation and enter into grid to impact the loads of other branch circuits connected to the points of common coupling.

Rolling mill

Silicon controlled rectification and variable-frequency speed control technologies are generally applied for the steel rolling, where there are a large number of higher harmonics in current.

Electric-arc furnace

Electric-arc furnace is a time-varying nonlinear load equipment and will generate higher harmonics due to the nonlinearity and transient behavior of arc resistance; AC furnace will generate negative-sequence current during its operation due to asymmetric three-phase arc.

3.2 Impacts

Both harmonic current and unbalanced current will increase the losses of grid and equipment during their flow in supply network. Due to the higher proportion of non-linear loads in total electrical loads, the electric energy losses incurred by harmonics will increase. Meanwhile, the reactive compensation capacitor bank incurred by harmonics will exit, causing more obvious increase in electric energy losses and reduction in production efficiency.