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Application of Medium Voltage VFD in cement plant
I. Introduction
Dalian Xiaoyetian Cement Plant is a China-Japan joint-stock enterprise, established in 1989. Its daily production is 4000t and yearly production is 1370000t. In the plant, motor of cooling fan EP fan (electrical de-dust fan) is coiled rotor asynchronous motor. Original operation mode is full speed operation. Air flow is adjusted by inlet vane. Because power grid of plant is limited, motor is not allowed to start at full voltage. It needs coiled rotor asynchronous motor. In order to save energy and implement soft start, the plant selects VFD to adjust motor speed (just short connect rotor windings).

II. Project scheme
The bypass switch cabinet of the system is a one-drive-one manual bypass system.
It is composed of 3 medium-voltage isolating switches QS1, QS2 and QS3 (see the following figure). QS3 and QS2 can not close at same time. They are interlock mechanically. In variable frequency state, QS1 and QS2 close and QS3 opens. In direct on line state, QS1 and QS2 open, QS3 closes.

QS1, QS2, QS3: Manual isolating switches
QF: Medium voltage vacuum circuit breaker
M: Asynchronous motor
Motor parameters:
Model: BST-0
rated power: 330kW
rated voltage: 6kV
rated current: 42.8A
rated speed: 590rpm
Power factor:0.8

VFD parameters:
Input voltage:6kV
Rated current:45A

III.Project effect
1. Energy-saving effect
Energy-saving is considerable after using VFD. Before using VFD, inlet vane usually opens at 40% and motor runs at full speed; after using VFD, fans runs at variable speed, VFD usually runs at 31 Hz. Power loss is reduced a lot. Data are shown in following table.

Time Adjusting mode Input current(A) Frequency(Hz) Motor average power (kW)
Before using VFD Inlet vane 32 50 266
After using VFD VFD 7.5 31 74

Motor average power=32×1.732×6×0.8=266kw
Motor average power=7.5×1.732×6×0.95=74kw (note: power factor is 0.95 when VFD works)
Energy-saving rate is (266-74)/266=72.2%
Annual workday: 300days, power price: 0.5yuan/kwh
Saving cost=(266-74)×300×24×0.5=691000yuan (USD 101617)
2. Production-improving

Before After
Start-up mode With series water resistance Soft-start
Fan noise big small
Bearing temperature-rise high low
Adjusting response quick slow
Maintenance period short long

We can see from above table the indirect effect after modification is obvious. Because VFD usually runs at 30 Hz, motor and fan speed are low, their bearing temperature is low, noise is smaller. Operators can adjust motor speed through monitoring interface at DCS. Frequency resolution precision is 0.01Hz. Adjustment precision is high. In addition, we find that motor temperature-rise is not up but down.

IV. Conclusion
Cement plant made a success at energy-saving and production-improving after reconstruction with VFD.

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