Beijing Leader & Harvest Electric Technologies Co., Ltd.
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Application of Medium Voltage VFD in Production of Blast Furnace
I. Introduction
The 750m3 blast furnace of the Iron-smelting Works of Chongqing Steel-Making Group is a new constructed blast furnace (new blast furnace). Due to a great quantity of red-brown smoke discharged from the furnace mouth during iron tapping of the old blast furnace, causing environment contamination; On other hand, the old motor operated with industry frequency and an air flow was regulated through adjusting the outlet flap of the valve to satisfy the iron tapping technology requirement. Considerable electric energy was wasted on the valve. To solve the problem, it was decided to implement speed regulation for the de-dust fan of the new blast furnace.

II. Technical Requirements for Medium Voltage VFD and Reform Scheme
The de-dust fan is the hub of the de-dust system. If the de-dust fan cannot operate normally, not only the production will be affected, leading to heavy economic losses, but also it is possible to threaten the personal safety of the working personnel at the site. In addition, the working environment of the speed regulating system is rather bad and the blast furnace performs iron tapping periodically. Therefore, the medium voltage VFD used for the de-dust fan is required to have very high reliability. Based on these working features, the main requirements are as follows:
Chinese operating interface.
High reliability that can operate for long time without failure
Bypass function that once failure happens, the motor may be switched to the operating with industry frequency.
Wide speed regulation range and high efficiency;
Logic control ability that the motor speed may automatically increase or decrease in accordance with the oxygen blowing period;
Resonance point jumping setting that makes the motor to avoid the resonance point and the fan will not surge
Remote control function of upper level computer
After investigation and comparison, the Iron-smelting Works cooperated with Beijing Leader & Harvest Electric Technology Co., Ltd. to work out a reform scheme for the de-dust fan of the 750 blast furnace commonly. The reform scheme is as follows:
1. Equipment Configuration
The bypass switching cabinet of the system is a one-draw-one manual bypassing system (for VF/industry frequency switching).
It is composed of 3 medium-voltage isolating switches QS1, QS2 and QS3 (see the following figure). It is demanded that QS1 and QS2 cannot close at same time and interlock mechanically. When operating with VF, QS1 and QS2 close and QS3 opens. When operating with industry frequency, QS1 and QS2 open and QS3 closes.
To implement protection for VFD failures, the user switch QF is interlocked. Once the VFD has failures, QF switches off. It is demanded that the user makes suitable reform to the switching circuit of QF. When the industry frequency is bypassed, QF is allowed to switch on and the switching–off signal is canceled, enabling the motor starts with industry frequency through the closed QF.

QF: Medium voltage vacuum circuit breaker of the works (providing medium voltage of 6kV for the VFD) QS1, QS2, QS3: Manual isolating switches BPQ: VFD of HARSVERT-A06/085 M: Asynchronous motor of 630kW/6kV

The total time for high speed operating for the whole iron tapping technology is about 45 min. and the frequency for high speed is set to 50Hz (may be adjusted) and the frequency for low speed is set to 0Hz (may be adjusted).

2) Monitoring and operating
The implementing of the reform of the de-dust fan with medium voltage VFD is as follows: Operating of VFD may be performed at the device itself and also may be remote control. The VFD contains a built-in PLC used for logic processing of logic signals in the cabinet and the coordination of various operating signals and status signals (such as RS485) at site. In addition, the digital signals may be expanded based on the user's requirement, improving system flexibility. The VFD may also be operated with the upper level computer or the control console in the control room. When iron tapping, the VFD operates at high speed and when not iron tapping, the VFD operates at low speed (even not operate). The speed may be set freely based on the working condition to meet the technology demand.
Upper level computer: the VFD may be remotely monitored with the upper level computer. It is convenient for the user to learn the operation of the equipment and also advantageous to the remote diagnosing and maintenance of the equipment and failure problems may be solved in time.

3) Specifications of VFD
Input voltage: Three-phase AC effective value 6kV±10%
Input frequency 50±5Hz
Output voltage Three-phase sinusoidal voltage 0-6kV
Output frequency 0-50Hz
Resolution of frequency 0.01Hz
Acceleration time may be set according to the technology demands
Deceleration time may be set according to the technology demands
Setting mode of frequency Two levels of speed, high and low, may be
adjusted in the range of 0-50Hz
Failure diagnosis and check automatic diagnosing, checking and locating
Power factor at network side 0.95 (when high speed)
Overload protection 120%, 1min (every 10 min),
150% protect instantly
Protection level IP20
Ambient temperature 0-40°C
Ambient humidity 90%, no condensation
Height above sea level Under 1,000m

III. Operation of Equipments
In the middle of 2003, the company ordered a set of VFD from Beijing Leader & Harvest Electric Technology Co., Ltd., which model of the VFD is HARSVERT-A06/085. The VFD was installed to place on June 20, 2003, the test was completed on June 24 and was put into formal operation on November 9, 2003. The period of installation and test were very short, only 5 days totally. It provided strong guarantee for the 750 blast furnace to be put into production in time.
Comparing with the old de-dust fans, the VFD made by Beijing Leader & Harvest Electric Technology Co., Ltd. has the following advantages:
1. Operating stably, safely and reliably. Since it was put into operation in November 2003, it has been continuously operated without failures for more than 1 year and provided strong guarantee for safe and reliable production of the 750 blast furnace. The HARSVERT-A VFD has features of free of maintenance. It needs to replace the venting filtering screen periodically only without stopping to ensure the production continuity.
2. Distinctive energy saving effects that decreases the electricity consumption obviously.
3. Real soft starting and soft stopping of the motor are implemented. The sinusoidal current without harmonic interference provided by the VFD reduces the failure rate of the motor. In addition, jumping frequency may be set for the resonance frequency to avoid the fan operating at the resonance point for long time, making the fan operates smoothly. This also reduces the fan bearing wearing, extends the operating life and maintenance period of the motor and fan and increases the utilizing time rate of the fan.
4. The VFD has perfect self-protective functions. Comparing with the original relay protection, the VFD has more protective functions with more sensitive. This improves the protection to the motor considerably.
5. The seamless interface between the VFD and the site satisfies the production demands. The VFD has a built-in PLC with flexible site signals. The VFD has a measuring for speed and current that may provide indicating of motor speed and current at site directly.
6. The VFD has a good suitability to the voltage fluctuating. Sometimes the network voltage is high up to 6.6kV, but the VFD can still operate normally.

IV. Analysis for Energy Savings
1. Energy savings principle

When using a VFD for speed regulation, the motor speed may be increased or decreased to change the characteristic curve of the fan to enable the parameters to meet the technology demands. Based on the similarity law of the fan, the relation between the flow, pressure, power and the speed before and after variation of the speed is:
Q1、H1、P1 -- The flow, pressure and power of the fan at the speed of n1;
Q2、H2、P2 -- The flow, pressure and power of the fan at the speed of n2 and the similar working conditions;
If the speed decreases for half, i.e.:n2/n1=1/2,then P2/P1=1/8,it may be seen that reducing the speed can decrease the axis power greatly to save energy.
When using the VFD speed regulation, for full load and frequency of 50Hz the power factor is 0.96. The current decreases to 52A. The operating current is much lower than the normal current. Since the power factor is improved by the filtering capacitor built in the VFD, the electric network capacity is saved about 20%.
2. Calculation for energy savings
Phigh speed =1.732×6×52×0.96×8600×40%=178.4
Annual electricity saving rate:(Poriginal- Phigh speed)÷Poriginal=69.8%

V. Other Effects of Application of Medium Voltage VFD Speed Regulating System
The technology is improved. After using the VFD, the flow of the de-dust fan may be adjusted very smoothly and stably. The operating personnel may adjust with ease. The operating parameters of the de-dust fan have been improved and the efficiency is improved.
The operating life of the motor is extended. De-dust fans are of centrifugal fans usually. They have long starting time and large starting current (about 6~8 times of normal current) and the starting has a strong mechanical shock to the motor and fan, affecting the operating life seriously. After using the VFD, soft starting and stopping may be performed without any shock that the operating life will be extended greatly.
It reduced the wearing of the valve mechanism and the fan impeller. So did the fan vibration and bearing wearing do. The fan overhaul period is extended and the maintenance cost and time are saved.
The maintenance cost and working load are reduced. The original de-dust fan and motor damaged frequently during operation that the working load for maintenance is heavy and the maintenance cost is high. The VFD speed regulation ensures the fan operating in normal range.
It is convenient to perform automation of de-dust system. The air flow of the de-dust system needs to vary frequently according to the technology requirements. Previously, the flow was adjusted by using the flap. There is a problem for the linearity between the opening of the flap and the flow. Too much wearing of the flap causes the valve characteristic changing and nonlinearity happens, leading to adjustment fault and the automatic control system cannot operate normally. The VFD speed regulation maintains high linearity accuracy and operating in range of 0.1~0.01Hz. It provides good conditions for the automation of the de-dust system.

VI. Conclusions
The application of HARVERT-A series VFD in the reform of the de-dust fan for 750 blast furnace of Chongqing Iron Smelting Works is successful. The advance and reliability of this series of VFD has been proved by many industrial applications. Popularization and application of the VFD speed regulation technology for medium voltage high power accessory equipment in various industries may obtain distinctive energy saving effects and also are supported by the national industry policy.

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