1. Introduction
Iron and steel enterprises are major energy consumers in the national economy, and the steel industry has been designated as a key sector for national resource conservation. Given the current tension between energy supply and demand both domestically and internationally, steel enterprises urgently need to reduce energy consumption through various means to achieve optimal economic benefits and productivity.
For instance, in sinter production, motor-driven power consumption alone accounts for over 20% of total energy costs, and the medium voltage motors driving fans constitute a significant portion of this. For a typical sintering production line, 25% to 30% of electrical energy is used to drive various types of fans. Therefore, reducing consumption and improving efficiency for fans is an urgent task.
Sintering main exhaust fans are often driven by high-power synchronous motors. Early starting methods for synchronous motors were mostly liquid resistance starting. This starting and regulation method presented numerous issues, such as high starting current, susceptibility to out-of-step under heavy load, and severe energy waste. Additionally, for safety and stability considerations, sintering systems themselves were originally designed with significant margins for fan equipment, leading to the prevalent phenomenon of "using a sledgehammer to crack a nut" in many sintering plants.
As operating conditions and production output change, the required airflow for the system also changes. However, most fans still use traditional regulation methods, i.e., adjusting the opening of inlet and outlet dampers. This method achieves its goal by increasing wind resistance and sacrificing fan efficiency, resulting in significant energy loss. Therefore, implementing variable frequency drive (VFD) retrofits for the high-power medium voltage synchronous motors driving sintering main exhaust fans is imperative.
Figure 1: Sintering Main Exhaust Fan Site
2. Technical Solution
Leveraging its years of application experience with products in metallurgical industry sintering main exhaust fans, Nancal Electric undertook a retrofit for the sintering main exhaust system of a steel plant. This project involved a 238 m² sintering machine equipped with two main exhaust fans, driven by 10 kV / 4600 kW synchronous motors. Originally, the main exhaust fans used liquid resistance starting and operated at power frequency, meeting process requirements for wind box negative pressure and airflow by adjusting the inlet louver damper openings to ensure stable production. For this retrofit, the original two "one-drive-one" liquid resistance starting devices were removed and replaced with two VFDs for speed regulation, achieving "two-drive-two" soft starting & variable frequency speed regulation with mutual manual backup. The single line diagram of the system is shown below:
Figure 2: VFD Single Line Diagram
Operating procedure:
(1) When the sintering machine operates at full load, the sintering fans run at full speed. The VFD starts its corresponding motor as follows (using #1 VFD to start Motor #1 as an example):
a. Select constant speed operation in control system; MBL1 is open.
b. The VFD input-side medium voltage switch MBC1 and output-side medium voltage switch MBM1 are open; isolation switch QS1 is closed, isolation switch QS12 is open.
c. The current-limiting reactor bypass switch is open and prohibited from closing.
d. Close the VFD input-side switch MBC1.
e. Close the VFD output-side switch MBM1.
f. Start the motor to rated speed with the fan damper closed. When the VFD output voltage meets grid synchronization requirements (amplitude, frequency, phase), close MBL1, block the VFD trigger pulses, open MBM1, and then open MBC1.
g. Motor #1 and the sintering fan have completed starting and are now running online at power frequency.
Motor #2 is started and synchronized to the grid in the same manner.
(2) When one VFD is under maintenance or faulty, the other VFD can start both motors and switch them to power frequency operation. The procedure is as follows (using #1 VFD to start Motor #2 as an example):
a. Select power frequency operation in control system; MBL2 is open.
b. Select to start #1 VFD.
c. Select to start Motor #2.
d. The VFD output-side switch MBM1 is open; isolation switch QS2 is open; QS1 and QS12 are closed.
e. The current-limiting reactor bypass switch is open and prohibited from closing.
f. Close the VFD input-side medium voltage switch MBC1 and output-side medium voltage switch MBM2.
g. Start the motor to rated speed with the fan damper closed. When the VFD output is synchronized with the grid, close MBL2, block the VFD trigger pulses, open MBM2, and then open MBC1.
h. Motor #2 and the sintering fan have been started by #1 VFD and are now running at power frequency.
3. Basic Configuration
Based on the motor and load parameter requirements, the basic parameters of the Nancal Electric medium voltage drive are as follows:
The motor parameters are as follows:
4. VFD Functions
4.1 Excitation Synchronous Motor Control Technology
Utilizing high-performance vector control, the drive achieves decoupled control of motor torque and flux, enabling precise control of the synchronous motor's excitation current and torque current. The synchronous motor employs a field-oriented vector control strategy. The control system uses a dual closed-loop structure with a speed loop and a current loop. The current loop uses a PI regulator for good current tracking performance. The speed loop uses a PI regulator to effectively limit overshoot during dynamic response and accelerate the response speed.
4.2 Synchronous Motor Rotor Initial Position Detection
Before starting the synchronous motor, Nancal Electric's medium voltage drive applies an excitation signal to the rotor and uses the motor's mathematical model to detect the rotor position. This technology achieves an initial rotor position detection accuracy error of < 3°. The initial voltage vector angle of the VFD output starts from the rotor position angle, preventing reversal and overcurrent during startup.
4.3 Disturbance-free Variable-frequency Soft Start and Soft Stop Control
The VFD is configured with soft start functionality and incorporates disturbance-free switching technology (power frequency/variable frequency). The fully digital control system integrates synchronous grid connection functionality. Using built-in electronic detection devices for input and output voltage, it requires no external PTs or synchronizing relays, significantly improving reliability and grid connection success rates.
4.4 Low Voltage Ride-Through (LVRT)
The NC HVVF series medium voltage drives incorporate advanced Low Voltage Ride Through (LVRT) technology designed to enhance continuous operation capability. It maximizes the equipment's ability to withstand power supply quality issues, ensuring the equipment remains unaffected by external disturbances as much as possible, thereby safeguarding production continuity.
4.5 Flying Start
To accommodate grid voltage fluctuations and the need for switching the main power supply bus on-site, the system provides a restart function for rotating motors. When starting a motor that is already spinning, the VFD can automatically search for and track the motor speed, returning to normal operating status according to the set acceleration/deceleration time, ensuring safe unit operation without tripping.
Figure 3: Flying Start Logic Diagram
5. Results After Retrofit
This retrofit met the requirement for short-term production halt without fan shutdown on the sintering main exhaust line: when other on-site equipment experienced short-term failures (shutdown time less than 3 hours), the motor could be operated at a reduced frequency via the VFD, lowering the frequency to 25 Hz or even lower. During this period, fan energy savings were maximized.
Sintering process control was optimized: The sintering process requires precise control of airflow, vacuum degree, bed thickness, machine speed, and sintering end point. Satisfying all these points simultaneously ensures the quality of the finished sinter. Using the VFD speed regulation method, the optimal operating point can be found by controlling the VFD to adjust airflow and vacuum degree (mainly pressure, which is influenced by bed thickness and material fineness – coarse material offers higher permeability, requiring lower pressure; fine material offers lower permeability, requiring higher pressure) based on bed thickness and machine speed, while leaving a certain margin. This satisfies actual production process requirements while achieving maximum energy savings, achieving the goals of production assurance and efficiency improvement.
The probability of air duct deformation and air leakage was significantly reduced, improving air duct safety: Operating the sintering main exhaust fan at its optimal operating point via the VFD avoids excessive pressure on the air ducts, preventing incidents such as duct collapse or deformation, thereby reducing maintenance workload and risks for personnel.
The VFD also provides soft start functionality: By soft starting the motor with the VFD, the starting current can be controlled below the rated current. Using a medium voltage pre-charging scheme and reactor-based synchronous grid connection disturbance-free switching technology, truly impact-free smooth starting is achieved.
After the retrofit, significant energy savings were achieved through VFD speed regulation. This project retrofit saved the enterprise substantial electricity costs, reduced production costs, and improved production efficiency, yielding considerable economic benefits alongside positive social benefits. Additionally, the fan now achieves soft starting, clearly solving problems such as high starting current, heavy-load out-of-step, and high maintenance costs.
Nancal Electric remains customer-focused, providing various technical retrofit solutions for customers in metallurgical industry sintering main exhaust fan retrofit projects. These include one-drive-one soft start, two-drive-two variable frequency speed regulation with automatic mutual backup switching, and configurations where a VFD works in mutual backup with a solid soft starter (or liquid resistance starter). These solutions provide crucial technical support for enterprises to reduce costs, improve efficiency, and enhance process control.