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Medium voltage VFD application in gas drainage system of coal mine
1. Preface
Annual production of Chongqing First Mine is 1500kt/a now. According to company plan, the production of raw coal is going to be up to 2400kt/a gradually. With increasing poduction and exploitation, gas rushing is more and more, which affects safety of production and development in mine. Draining gas by pump in goaf can control gas amount of coal face and air return system, and limit it not beyond safety regulation.
During draining gas in goaf, if draining amount is too big, fresh air will enter goaf and fire may be caused. If draining amount is too small, gas concentration of coal face will be beyond safety regulation. Therefore, after investigation and comparison, the coal mine selected Leader&Harvest VFD to adjust speed of pump. VFD can optimize gas parameters and save energy.
2. Technics and main parameters
There is a drain pipe of diameter 1000mm which is in gas lane of goaf bottom and comes to pump room. Pump is water ring vacuum pump, model SKA-720. Please see technics diagram in following:

Here are parameters:
2.1 Pump parameters
Model: SKA-720
Rated speed: 340r/min
Max drain flow: 635m3/min
Ultimate vacuum: 160MPa
2.2 Motor parameters
Model: YB63052-4
Rated power: 900kW
Rated voltage: 6kV
Rated current: 104.2A
Rated speed : 1483r/min
Power factor: 0.87
3.VFD structure
3.1 Harsvert VFD is composed of phase-shifting transformer, power cells and controller. Five power cells make up of a phase. Please see following diagram:

3.2 VFD parameters
Model: HARSVERT-A06/110
Power: 900kW
Rated output current: 110A
Input power factor: 0.95
VFD efficiency:>0.96
Frequency resolution: 0.01Hz
Overload capacity: 120% for 1min and 150% immediately
Analogue input: 0~10V/4~20mA
Analogue output: 0~10V/4~20mA
Accelerating/decelerating time: 0.1~3000s
Digital input/output: 4 routes output/4 routes input
Cooling type: air cooling

3.3 Main circuit
Gas drain pump as well as main ventilation blower is main equipment in coal mine. In the light of safety regulation of coal mine, power is supplied by double routes. There are two pumps in room, one work and one standby. So scheme of " VFD drive two" is selected. Its main diagram is following:

System can implement following operation modes:
1. With bus I supplying power, VFD drives 1# pump, consisting of QS1, QF1, QS3, QS7.
2. With bus I supplying power, VFD drives 2# pump, consisting of QS1, QF1, QS3, QS8.
3. With bus II supplying power, VFD drives 2# pump, consisting of QS2, QF2, QS4, QS8.
4. With bus II supplying power, VFD drives 1# pump, consisting of QS2, QF2, QS4, QS7.
5. With bus I supplying power, 1# pump runs in direct on line, consisting of QS1, QF1, QF5.
6. With bus II suplying power, 2# pump runs in direct on line, consisting of QS2, QF2, QS6.
3.4 VFD installation and commissioning
a. Draining gas of pump room is inflammable and explosive gas. All equipments in pump room are explosion proof. But VFD is non-explosion proof device, it should be installed in an isloated room.
b. After installation and commissioning, gas draining system puts into service. Make statistic for the system when pump runs in 30~50Hz. Here are average values:

SR frequency (Hz) Motor parameters Gas parameters
voltage (kV) Current (A) Power (kW) Draining flow (m3/min) Gas of coal face(%)
1 30 3.59 36.1 206.4 398.75 1.58
2 35 4.48 46.3 338.4 401.28 0.86
3 40 4.91 54.6 430.9 403.45 0.67
4 45 5.24 62.2 542.84 404.14 0.3
5 50 6.084 68.76 710.96 404.71 0.23

4. VFD effect in gas draining system
4.1 It meets demand of working condition
Goaf draining system is used to control gas amount not beyond safety regulation, thereby ensuring safe production of mine. In the light of survey, pump usually runs in 35~45Hz. Operation frequency is determined by distance of coal face and drain tail hole. When coal face is close to drain tail hole, frequency is low. Otherwise, frequency is high. Measure gas parameters perodically, analyse and determine operating frequency of pump. In this way, system can ensure gas amount not beyond safety regulation, and ensure fresh air not to enter goaf, avoiding natural fire accident. So raw coal production is enhanced a lot.
4.2 Soft start
Because of big power, start time is long and start current is big during direct start, which threatens motor insulation and also brings impact on power gird and mechanism. VFD can implement soft start and soft stop with following advantages:
1. Impact on motor and other equipments during start and stop is eliminated. Lifetime of pump and motor is extended.
2. VFD control limits impact on power grid during start, reduces peak start power loss.
3. With speed down, lubrication condition of equipments is improved and driving mechanism failure is reduced.
4.3 Energy saving effect
4.3.1 Energy saving calculation
Pump usually runs in 35~45Hz and mostly in 45Hz. Average work time is 350 days(maintenance needs one day per month),
1. power consumption in 50Hz:
2. power consumption in 45Hz
3. yearly energy saved:
Power price: 0.5yuan/kWh, so 707000yuan (USD103970)is saved.
4.3.2 Power factor is improved
Rated power factor of motor is 0.87. VFD can raise it up to 0.95. So power quality is heightened and power cost declines.
5. Concluding
In gas draining system of Chongqing First Coal Mine, medium voltage VFD controls gas amount of coal face and air return system effectively. It ensures safe production and saves considerable energy.

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