Medium voltage VFD application on 100MW mill fan
2010-1-4 11:26:06

System is composed of VFD and bypass cabinet. In above figure, QS1, QS2 and QS3 are all in bypass cabinet. QF and M are original equipments.

System has switch function between line frequency and variable frequency. QS2 and QS3 can not close together. They are mechanical interlock. During variable frequecy operation, QS1 and QS2 close, QS3 opens. During line frequency operation, QS3 closes, QS1 and QS2 open.

3. Technics of mill fan

Technics process of mill fan

Mill fan is installed behind of ball mill, raw powder separator and fine powder separator. In ball mill, raw coal mixes with hot air and recycle air from outlet of mill fan. Raw coal is dried and grindered into coal powder. With negative air flow produced by mill fan, fine powder separator separates air from powder. Coal powder drops in warehouse as storage. Residual gas contains 5～10% mixture of air and powder. It is delivered to furnace as tertiary air, or mixes with coal powder left as primary air. Mixture are delivered to furnace. From above processes, we see mill fan provides negative pressure for system. After reconstruction with VFD, outlet gate of mill fan is full open. Meanwhile, adjust outlet gate appropriately. It can meet requirement of providing negative pressure completely, without affection on tertiary air.

III. Energy saving

a. Power consumption when machine runs in direct on line

Pd：motor efficiency；ηd: motor efficiency；U：input voltage of motor；I：input current of motor；cosФ：power factor

Formular: Pd=1.732×U×I×cosФ

Power consumption: Pd=1.732×U×I×cosФ=1.732×6V×40A×0.87=361.64kW（including necking down and damper throttle loss）

b. Power consumption when machine runs in variable frequency

according to law of fluid mechanics, for load such as fan and pump, speed n, flow Q, pressure H and shaft power P have following relations:

Q∝n，H∝n² ,p∝n³；

That is, flow is in direct rate to speed, pressure is in direct rate to square of speed, shaft power is in direct rate to cube of speed.

Put data in
（Q¹: actual discharge flow，Q_0: rated discharge flow，P₀:rated power of fan）， we get power consumption：P¹＝144.90kW

VFD power gird side consumption:

c. Energy saving rate

（ΔP/Pd）×100%＝（210.7/361.64）×100%=58%

d. Implementation energy saving rate

during variable frequency operation, average current of motor is Ig17A,

actual energy saving rate: 1-(Ib/Ig) ×100%=1-(17/40)×100%=57.5%

e. Benefits

yearly power saved=No of mill fan × energy saving rate×power×yearly work time=

2×57.5%×361.64×5850＝2432933.1（kWh）

Power price: 0.24 yuan /kWh, yearly cost saved is 580000yuan (USD 85294)

Phase II power generating is 1439505600KWh in 2004. Power consumption rate of phase II plant lowers 0.17%.

At present, despite inlet damper is full open, inlet pipe is still necking down. We plan to take advantage of maintenance opportunity to reconstruct for the neck and small damper, to reduce throttle loss and extend variable frequency effect further.

IV. Other benefits

a. Mill fan is load which needs to start frequently. By variable frequency adjustment, medium voltage switch always closes, which prolongs maintenance period greatly.

b. Variable frequency adjustment can realize soft start, prolong lifetime of motor and fan.

Concluding:

It is the first time to make reconstruction with VFD for mill fan in China. Successful VFD application in Hepo Power Plant lays a good foundation for variable frequency control in powder production system of power plants. We hope to exchange experience further with other users in the field and contribute to build a economized society.