Application of VFD in #4 Duster Fan in Steel Plant

KM：VFD power supplied medium-voltage vacuum breaker
　　KG1、KG2、KG3：VFD Built-in vacuum contactor provided by the manufacturer
　　BPQ：HARSVERT-A06/076VFD
　　DJ：Asynchronous motor of 630KW/6KV
　　KM Original medium-voltage vacuum switch
　　DJ Reserved original asynchronous motor
　　 It is required that remote and local control can be performed. When the fan operates with high speed, VFD will stop automatically if serious VFD failure occurs. With remote controlled, the motor is switched to operate with the power supply through the Power/VFD controlon the operating console. When the blowing period finishes, service the VFD. After servicing the VFD, the motor return to operating with the VFD through the Power/VFD control on the operating console. With local control, the motor is switched to operate with the power supply through the Power/VFD control on the operating console. When the blowing period finishes, service the VFD. After servicing the VFD, the motor return to operating with the VFD through the Power/VFD control on the operating console.

2.2 Parameters of Motors and Fans
1) Motor parameters: 　　　　　　　　　　　2) Fan parameters:
　　　Model: Y4004-2　　　　　　　　　　　Model: AI850
　　　Rated power: 630KW　　　　　　　　 Inlet volume flow: 850m3/min(Mixed gas)
　　　Rated voltage: 6KV　　　　　　　　　 Pressure increment: 2600mmH2O
　　　Rated frequency: 50Hz　　　　　　　　Inlet temperature: 35℃
　　　Rated current: 73A　　　　　　　　　 　Main shaft speed: 2975rpm
　　　Rated efficiency: 92.5% 　　　　　　　 Rated power factor: 0.89
　　　Rated speed: 2970rpm　　　　　　　 　 takeoff power: 500kw

2.3 Technology requirements for duster fans
(1) Blowing technology period
　　A to B time for iron charging and scrap steel, about 1 min.
　　B to C time for speed increasing of fan, 1 min. set temporarily, may be adjusted.
　　C to D time for oxygen-blowing, about 14 min.
　　Point D fan begins to decrease, 3 min., set temporarily, may be adjusted
　　D to E time for tilting and sampling, about 2 min.
　　E to F time for discharging, about 2 min.
　　F to G time for slag escaping, about 3 min.
　　 The whole blowing technology period is about 26 min., in which time of high speed (C to D) is 12 min. High speed is set to 45 Hz, may be adjusted. Low speed is set to 20 Hz, may be adjusted.

(2) Interface between VFD and the site
　　 At point B, the signals for fore-converter, aft-converter and oxygen flow are sent to the PLC electrical station of converter #4. After processing by user programs, they are sent to a relay, which provides a pair of closed contacts (when the relay closes, VFD operates with high speed and when the relay releases, VFD operates with low speed). When operating fore the converter and there is oxygen flow, the relay operates and VFD speed rises from low to high. At point C, the operating worker at site performs blowing. At point D, preparing to discharge, the worker fore the converter sets the switch to aft converter, or the time without blowing greater than 15 min., the relay releases and VFD speed begins to decrease. The speed decreasing time is not specified, but if increasing speed is required during speed decreasing, VFD should satisfy the increasing speed requirement.
3) Technical specifications of VFD
　　Input voltage: three-phase, AC effective value of 6.3KV±10%
　　Input frequency: 50±5Hz
　　Output voltage: three-phase, sinusoidal voltage of 0-6KV
　　Output frequency: 0-50Hz
　　Frequency resolution: 0.01HZ
　　Acceleration time: May be set according to technology requirements
　　Deceleration time: May be set according to technology requirements
　　Frequency setting mode: Two levels for high and low, may be adjusted in range of 0-50Hz
　　Failure diagnosis and detecting: Automatic detect and automatic location
　　Power factor at network side: 0.95 (at high speed)
　　Overload protection: 120% l min.(every 10 min.), 150% protects immediately
　　Protection class: IP20
　　Environmental temperature: 0-40℃
　　Environmental humidity: 90%, without condensing
　　Elevation: Under 1000 m

3. VFD Speed Adjusting System
3.1 Monitoring and operating
When not blowing, the duster fan needs very low speed only and operating with full load is not necessary. The duster fan, operating with variable frequency by means of VFD based on the practical requirement ensures and improves the technology, but also achieves the purpose and effects of energy saving and loss reducing.
The specific implementation procedure for variable frequency reform of duster fans with medium-voltage VFD is as follows: VFD operation may be local or remote control. VFD includes a built-in PLC, used for logic processing of digital signals in the cabinet and coordination of various operating signals and status signals (such as RS485) at site and the controlled digital values may be expanded according to the user requirement, improving the versatility of the system. VFD may also be operated with the upper level computer or control console in the control room. When blowing (B to D), VFD operates with high speed and when not blowing, VFD operates with low speed. It may be set based on the operating mode freely and satisfy the technological requirement. The control system for VFD fans is shown in the following figure:

The operating personnel at site may perform remote operate through upper level computer or operation console and monitor VFD operating condition.
Upper level computer: Remote monitoring may be performed through upper level computer. On the one hand, it is easy for users to learn the operating condition of the equipment and on the other hand, it is advantageous for remote diagnosis and service of equipment and failures may be solved in time.
Operation console: Simple remote operation on VFD may be performed through the operation console, including Power supply/Variable frequency switching.
3.2 Principle of medium voltage VFDs
HARSVERT series VFD is designed direct “high voltage in – high voltage out’ with topology structure of cells in series. Its main part is consisted of multi groups of power modules connected in series or in parallel. So all groups of low voltage together generate medium voltage output. The input current harmonic distortion of the VFD is less than 4% and it produces less contaminant to the power network. The HARSVERT series VFD meets the most stringent IEEE 519 1992 harmonic resistance standard. It has high input power factor without the use of external power factor correction capacitors. 630kw/6kw VFD system has 21 power cells and every 7 cells make up a phase in series. Its system diagram is as the fig. (a) and (b).

(1) Power cell
Each cell is powered by one group of the input transformer’s secondaries. Power cells and transformer’s secondaries insulate each other. The secondaries are connected in the way of extended sides triangle to fulfill multi-ply so as to decrease input harmonic current.
Cell bypass function: When one module fails, it will be bypassed automatically to enable to take corresponding measures at site. A parallel bypass circuit is connected between the output terminals of each power cell. When the power cell fails, the triggering signal for the corresponding power cell IGBT will be locked and make the bypass SCR short, forming a channel for the motor current to pass. To ensure the symmetry of the three-phase voltage, when bypassing the failed power cell, two other corresponding power cells for the other two phases are also bypassed at the same time. For the VFD of 6KV, each phase has 7 power cells in series, when one cell has been bypassed for each phase, 6 power cells will be left to operate for each phase, and their highest output voltage will be 86% of the rated voltage, the output current up to 100% as well as the output power up to about 86%. The loaded pump speed may still be more than 92%. The production requirement may be basically satisfied and the system operation reliability is increased greatly.
(2) Transformer cabinet
The transformer cabinet houses the input phase shifting transformer, input voltage and current testing components, voltage and current mutual inductance and temperature controller.
(3) Power cabinet
At the output side, each cell's U terminal and the neighbor cell’s V terminal are connected in series to form a three phase star (wye) connection to supply power for the motor. By overlapping each cell's PWM waveform, we can get the multi-level SPWM waveform shown in Fig. 4. It is a excellent sine-shape, small dv/dt, no cable and motor insulation destruction, no output filter for long output cable, no motor derating, and it can be used in driving the old motor directly. Meanwhile the motor harmonics are reduced greatly, the mechanical vibration is eliminated, and the mechanical stress of shaft and vanes is reduced. Output current and voltage testing performance is attached to the cabinet.
(4) Fans
The transformer cabinet is provided with 6 cooling fans, which are controlled by a thermostat. When the transformer load increases, the temperature increase. When the winding temperature reaches to 80℃, the fans will start automatically. When the winding temperature reaches to 70℃, the fans will stop automatically. When the winding temperature continues to increase up to 110℃, the system will output over-temperature alarming signal. If the winding temperature continues to increase up to 130℃, the transformer will be switched off rapidly. A temperature display system is installed on the cabinet. Based on the power level, the power cabinet is provided with two EBM- backward centrifugal cooling fans (with large margin, capacity for single set is 50% of the total capacity)with world leading technology, which are imported from Germany. If it is required to exhaust the heat to outside, air channel may be installed.
(5) Control cabinet
The control function of the controller is performed with the coordinate computation of high-speed single-chip computer and industrial control PC. Sophisticatedly designed algorithm may ensure the motor reach to the best performance. The industrial control PC provides friendly all-Chinese WINDOWS monitoring and operating interface and may perform remote monitoring and network control. The controller also includes a built-in PLC, used for logic processing of digital signals in the cabinet and coordination of various operating signals and status signals (including DCS/RS485/Profibus/Modbus/Ether Network, etc.) at site and the controlled digital values may be expanded according to the user requirement, improving the versatility of the system.

3.3 Protective function
VFD has protective function for over voltage, over current, under voltage, missing phase, VFD overload, VFD over temperature, motor overload etc.:
1) Overload protection. 120% rated current of the motor, 1 min is allowed for every 10 min. (anti time-limit feature), protecting if surpass.
2) Over-current protection. 150% rated current of the motor, 3 sec is allowed, protecting immediately if surpass. The output current of VFD surpasses 200% of rated current of the motor, protecting in10μs.
3) Over-voltage protection. Check and measure the DC bus voltage of each power module, if the DC bus voltage surpasses 115% of rated voltage, VFD will be protected. This protection practically includes the protection for the positive fluctuation of electric network voltage.
4) Under-voltage protection. Check and measure the DC bus voltage of each power module, if the DC bus voltage is lower than the setting value, VFD will decrease the voltage and then be protected. This protection practically includes the protection for the negative fluctuation of electric network voltage.
5) Over-temperature protection, including double protection: Temperature checking and measuring are set in the VFD cabinet. When environmental temperature surpasses the preset value, alarming signal will be issued. In addition, temperature checking and measuring are set at the rectifying transformer and electric power electronic components. Once the limit temperature is surpassed (130℃ for transformer, 80℃ for heating components), VFD will be protected. If the motor has temperature contacts and the temperature analog signal is sent to PLC, motor over-temperature protection may be performed.
6) Missing phase protection. Missing phase protection is set at each power module. When the input side of VFD misses phases or the fuse of power modules is blown, alarming signals will be issued and protection acts.
7) Protection from optic fiber failure. When failure happens in the connecting optic fiber between the controller and power modules, alarming signals will be issued and protection acts.
For all the failures, the failure location is indicated on the Chinese user interface. It is easy for users to take corresponding measures.
VFD has various protections for isolated transformers.

4. Operating Condition of VFD
a. Operating stably, safely and reliably. When using hydraulic coupler originally, the bearing must be replaced about every 40 days and stop operation for about half a day for every replacement, leading to great economic losses. HARSVERT-A06/076 VFD has feature of free of service. It needs cleaning the venting filter screen regularly only and needs not to stop operation, ensuring the production continuity. Since putting in operation on January 4, the VFD has been operated normally, no failures happened.
b. Energy was save obviously. Electricity consumption per ton of steel decreases greatly.
c. Soft starting and soft stopping of the motor have been realized. VFD provides motor with sinusoidal current without harmonic interference, reducing times of motor failures. Meanwhile, VFD set a frequency for resonance jumping, avoiding the fan operating at resonance point for long time and making fans operating stably. The fan bearing wearing will decrease and operation life and service period of motors and fans extends, improving utilization efficiency of fans.
d. VFD itself has perfect protective function. Comparing with the original relay protection, VFD has more and sensitive protective functions, enhancing the protection for motors.
e. The seamless interface of VFD with the site satisfies the requirements of production. The built-in PLC makes the connection of site signals versatile. The PLC electrical station of electromagnetic station of converter #4 provides with a pair of High speed/Low speed contacts for VFD. VFD operates with high speed or low speed according to the node condition. VFD has speed measurement itself and the original tachometer connected with the motor has been removed, the VFD provide motor speed display directly for the site.
f. Good adaptation to fluctuation of voltage of the electric network. VFD may operate normally when voltage of the electric network is up to 6.7KV sometimes.
g. Comparing with hydraulic couplers, noises have been reduced during acceleration and the noise contamination is decreased. Since bearings need not to be replaced periodically, contamination of oil to environment is reduced and the site environment of the fan room has improvement considerably.

5. Analysis of Energy savings
Since the VFD for fan 4# put into operation, operating condition has been well and the expected purpose of energy saving has been achieved.
When fans operate with high speed (about 2/3 time), the current decreases to 43A, for the power factor has increased. When fans operate with low speed during blowing ring the 1/3 time, the current has decreased to 4A. The electricity meter in the transformer substation for converters of the steel works displayed (January 10, 2003 to February 10, 2003) that the electricity consumed for that month is 201,600 kWh. The annual electricity consumed may be estimated as 2,403,000 kWh.
When the fan for converter 4# has not used VFD, the operating current is 73A. The electricity meter in the transformer substation for converters of the steel works displayed (December 20, 2001 to December 20, 2002) that the annual electricity consumed was 5,412,780 kWh.
According to the present operation condition of VFD for duster fan #4, the electricity consumed may be saved more. The data are as follows:
1. For duster fan #4, before and after reform with VFD, comparing the data for this year with the data for the same period of last year, the energy saving data are shown in table 1 (Unit: kWh/ ton of steel):

　　　Table 1 Electricity consumed per ton of steel  for the steel works (Unit: kWh/ton of steel)

From the statistics in the above table, comparing the data for January 2003 - May 2003 with the data for January 2002 - May 2002, the electricity consumed per ton of steel decreased rather greatly, decreasing for 18.68%, in which the saved electricity consumed is achieved through VFD.
　　Table 2 Electricity consumed per ton of steel for fan #4 (Unit: kWh/ton of steel)

From the statistics in the table 2, comparing the data for January 2003 - May 2003 with the data for January 2002-May 2002, the electricity saving for fan #4 is 805,000 kWh, average 165,000 kWh monthly. The annual electricity saving may be estimated as: 165,000×12=1,980,000 kWh. According to the electricity cost rate of RMB0.51 /kWh, the total electricity cost savings for fan #4 is 1,980,000×0.51=RMB1,000,000(USD 147058)

6. Conclusions
From the operation condition for several months, the medium-voltage large-power VFD HARSVERT-A06/076 produced by Beijing Leader & Harvest Electric Technologies Co., Ltd. has good performance, high reliability and obvious energy-saving effects, satisfying the requirements of continuous production for speed adjusting systems. The company also used the VFD produced by Beijing Leader & Harvest Electric Technologies Co., Ltd. in duster fan #1 for speed adjustment.