Coal Energy

1. Lightning

Located in open fields, the junction of rock and soil, and the place with sudden change in earth resistivity, most coal mining areas are generally easily struck by lightning. Coal production and transport, besides, will generate vast coal dust, which increases conductivity after suspending in the air above such coal mining areas.

When acting on the metal pipes and cables of pit, the electromagnetic pulse of lightning stroke will generate overvoltage to interfere with or damage the electronic equipment in pit and then paralyze monitoring systems such as methane monitoring system, fan monitoring system and personnel location system, hence causing production accidents and casualties.

Accident case:

On the morning of April 24, 2004, Renhua County, Guangdong was attacked by severe rain and thunderstorm. Power outage occurred in the electrical system of a coal mine due to lightning stroke, which stopped the ventilation system in pit. As the harmful gases in pit were not removed timely and mass carbon dioxide accumulated, four miners who were working under the pit did not evacuate to safety zone timely and finally died for carbon dioxide poisoning.

In the small hours of July 15, 2012, the 110-kV transformer substation of a Shanxi group company was struck by strong lightning. At 02:22., the transformer #1 was damaged, and at 06:12, the transformer #2 damaged, which caused power failure of the two loops of X and XX mines. It also caused transfinite gas, exceeding more than 40% and 4% of the maximum values of X and XX mines respectively (the maximum display value on gas sensor).

2. Fault arc

There are many production sites of wide coverage and long power supply line in pit, so generally multiple electromechanical devices with high power are adopted. And the electrical circuits of long service years and aging insulation are very likely to generate fault arc and then cause fire. The bow collector of underground locomotive for mine, moreover, will also generate arc during sliding on cable and then cause fire.

Accident case

On March 1, 2004, a major gas explosion accident occurred in a coal mine of Shanxi, causing 28 people killed, one people injured and RMB 3.187 million direct economic losses. The cause to this accident, according to investigation, was that under the circumstance of unopened mine secondary fan and transfinite gas in working face, the coal-getter Zhang X started the coal slip, which caused that the bare copper wire in the flameproof switch chamber instead of fuse was burned out and then generated arc, hence causing gas explosion.

3. Harmonic and imbalance

3.1 Cause

Surface structures

Relevant power electronic equipment is corresponding harmonic source during the operation, and gas discharge lamp is also a harmonic source during operation under nonlinear state. The massive application of such equipment increases the harmonic sources greatly. In addition, the asymmetric three-phase load distribution in surface structures causes three-phase imbalance of voltage and current, which will generate considerable impacts in severe case.

Underground system

To promote the efficiency of the electric system of coal mine, the power electronic devices are widely applied in coal mine enterprises including variable-frequency speed control system and silicon controlled rectifier, which causes a large number of harmonics injected into underground power supply system. The application of some one-phase electrical equipment for mine, on the other hand, causes a certain amount of unbalanced current in power supply and consumption system of coal mine.

3.2 Impacts

Surface structures

Both harmonic current and unbalanced current, when exceeding the limits, will increase the electric energy losses of all electric appliances in structures, and then overheat the insulation and accelerate the aging and even equipment failure in severe case.

When the harmonics in lines exceed the limit, the electric power protection devices in structure will execute false commands and cause buildings out of power.

When the harmonics in lines exceed the limit, the signal appliances, wireless receiver, computer, etc., will not operate normally, and cause poor sensitivity or incorrect operations during execution of command.

Underground system

When flowing through underground system, both harmonic current and unbalanced current will make the components in such system generate extra losses, thus increasing the electricity consumption of such components and reducing the efficiency of electrical equipment.

Unbalanced three-phase load and zero-sequence harmonic current will cause excessive current in middle wire and overload in loading wire to incur overheated supply transformer and low output capacity.

In addition, harmonics will also interfere with the underground communication, causing measuring error of personnel location system, gas monitoring system, etc., and cause malfunction of protection devices and local resonance in power supply system, hence leading to a series of severe consequences.