Application of high voltage frequency converters in Kunming Salt Mine Power Plant
my country's thermal power plants, especially older ones, suffer from significant energy waste in their main electrical equipment. Typically, older plants consume 9% to 10% of their own electricity, with forced draft fans accounting for 10%, induced draft fans for 15%, and feedwater pumps for 20%. In contrast, newly built power plants consume only about 6% of their own electricity, indicating substantial potential for energy conservation and emission reduction. Therefore, all thermal power plants have prioritized energy conservation and emission reduction, starting with technological upgrades to explore various methods and approaches. Variable frequency drive (VFD) technology is undoubtedly a crucial way to save energy in power plant fans and pumps.
2. High-voltage frequency conversion energy-saving principle
Variable frequency speed regulation of asynchronous motors is achieved by changing the synchronous speed by altering the stator power supply frequency f. During speed regulation, a small slip rate can be maintained from high speed to low speed, resulting in low slip power consumption and high efficiency. It is the most reasonable speed regulation method for asynchronous motors.
As can be seen from the formula n = 60f/p(1-s), if the power supply frequency f is changed uniformly, the synchronous speed of the motor can be changed smoothly. Variable frequency speed control of asynchronous motors has the advantages of a wide speed range, high smoothness, and relatively stiff mechanical characteristics. Currently, variable frequency speed control has become the most important speed control method for asynchronous motors and has been widely used in many fields.
High-voltage variable frequency speed control has the following significant advantages:
(1) The large amount of throttling loss caused by load baffle or valve adjustment will no longer be throttling loss after frequency conversion.
(2) The grid-side power factor is significantly improved.
(3) It can achieve zero-speed start-up, avoid starting inrush current, and reduce impact and torsional vibration.
(4) The high voltage frequency converter itself has very low loss and the overall efficiency is over 97%.
For centrifugal fans, fluid mechanics holds the following principles: output air volume Q is directly proportional to rotational speed n; output pressure H is directly proportional to the square of rotational speed n; output shaft power P is directly proportional to the cube of rotational speed n; that is:
Q1/Q2＝n1/n2, H1/H2＝(n1/n2) 2 , P1/P2＝(n1/n2) 3
When the fan airflow needs to be changed, such as by adjusting the damper opening, a significant amount of electrical energy is wasted on the resistance of the valve and piping system. However, if variable frequency speed control is used to adjust the airflow, the shaft power decreases substantially as the flow rate decreases. When the fan operates below its rated speed using variable frequency speed control, the theoretical energy savings are:
E=〔1-(n′/n) 3 〕×P×T (kW·h)
In the formula: n is the rated speed, n′ is the actual speed, P is the motor power at the rated speed, and T is the working time.
It is evident that retrofitting fans with frequency converters not only saves energy but also significantly improves equipment performance. The above formulas provide a solid theoretical basis for frequency converter energy saving.
3. Technical Features of the High-Voltage Variable Frequency Speed Control System of the Wind and Solar Company
The Fengguang JD-BP38 series high-voltage frequency converter uses a high-speed DSP as its control core, employing speed-vector control technology and power unit series multi-level technology. As a high-high voltage source frequency converter, its harmonic performance is far below the IEEE 519-1992 national harmonic standard. It boasts a high input power factor and excellent output waveform quality, eliminating the need for input harmonic filters, power factor compensation devices, and output filters. It also avoids problems caused by harmonics, such as additional motor heating, torque pulsation, noise, output dv/dt, and common-mode voltage, allowing the use of ordinary asynchronous motors. In 2007, the high-voltage frequency converter was recognized as a Chinese Famous Brand Product, one of only two such products in the domestic high-voltage frequency converter industry. Specifically, in addition to the performance characteristics of ordinary frequency converters, the Fengguang high-voltage frequency converter has the following outstanding features:
(1) Using a high-speed DSP as the central processing unit, the operation speed is faster and the control is more precise.
(2) Fly-start function. It can identify the speed of the motor and start the car directly without stopping the motor.
(3) Complete automatic switching technology between power frequency and variable frequency. Current high-voltage variable frequency speed control systems generally include a power frequency bypass switching cabinet. When the frequency converter fails, it allows the high-voltage motor to switch to power frequency operation. There are two types of bypass switching: manual and automatic. Manual bypass requires manual operation and is suitable for operating conditions without backup devices or for less critical situations. Automatic bypass can automatically switch to power frequency operation directly after a frequency converter failure. The automatic bypass switching cabinet provided by Xinfengguang Company can not only automatically switch from variable frequency to power frequency operation in the event of a frequency converter failure, but also instantly switch from power frequency to variable frequency operation after frequency converter maintenance is completed. The entire switching process will not affect the operation of the user's equipment.
(4) Restart function during rotation. If the high voltage is momentarily lost during operation and then restored within 3 seconds, the high voltage inverter will not stop. After the high voltage is restored, the inverter will automatically run at the frequency before the power outage.
(5) Automatic line voltage balancing technology (star point drift technology). When a unit in a phase of the frequency converter fails, in order to balance the line voltage, the traditional method is to reduce the voltage of the other two phases to the same voltage as the faulty phase. However, the automatic line voltage balancing technology adjusts the angle between the phases to ensure the maximum line voltage balance output under the premise that the phase voltage output is maximum and unequal.
(6) Unit DC voltage detection: Real-time display of the DC voltage of the detection system, thereby realizing optimized control of the output voltage, reducing harmonic content, ensuring the accuracy of the output voltage, improving the system control performance, and enabling operation and maintenance personnel to have a comprehensive grasp of the operating status of the power unit.
(7) The electrolytic capacitors in the unit have a lifespan that can be doubled due to the adoption of the company's patented technology.
(8) The heat dissipation structure is reasonably designed, and the unit is connected in series and then in parallel. The IGBT can withstand a lower voltage and has a wider overvoltage range (≥1.15Ue), resulting in higher equipment reliability.
(9) It has the function of sudden phase-to-phase short circuit protection. If a short circuit occurs at the output due to equipment failure or other reasons, and the frequency converter does not have the function of phase-to-phase short circuit protection, it will lead to a major accident. When such a problem occurs, the frequency converter can immediately block the output of the frequency converter, protect the equipment from damage, and avoid the occurrence of an accident.
(10) Current limiting function: When the inverter output current exceeds the set value, the inverter will automatically limit the current output to avoid overcurrent protection caused by the inverter during acceleration and deceleration or by sudden load changes, and minimize the number of shutdowns.
(11) Fault self-reset function: When the inverter causes overcurrent protection of the unit or the whole machine due to sudden load change, it can automatically reset and continue to operate.
4. Application of high-voltage frequency converters in Kunming Salt Mine Power Plant
Kunming Salt Mine, completed and put into operation in June 1993, is Yunnan Province's largest salt and salt chemical raw material production base and the province's first high-efficiency, phosphorus-free, and environmentally friendly detergent additive production enterprise. The Kunming Salt Mine's self-owned power plant has a 45t boiler equipped with one forced draft fan with a 315kW/10kV motor and one induced draft fan with a 400kW/10kV motor. In recent years, the plant has experienced 60% of its operating hours being spent on load balancing and two-shift speed regulation, resulting in the forced and induced draft fans operating at reduced power. To address this energy waste, the plant underwent a major overhaul in 2009, retrofitting the 100MW unit's forced and induced draft fans with high-voltage frequency converters manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd., for energy conservation.
Figure 1 Factory Area Map
4.1 Main Circuit Control Scheme of High Voltage Frequency Converter
Figure 2 Manual bypass cabinet
Before the upgrade: The original system had a 10kV high-voltage switchgear to the motor, and the motor was directly connected to the fan. The flow rate was adjusted by the opening of the damper.
After the upgrade: The system connects the 10kV high-voltage switchgear to the wind and solar high-voltage frequency converter. The wind and solar high-voltage frequency converter is connected to the motor. The flow rate is adjusted by regulating the motor speed. No changes are needed between the motor and the fan. The damper can be fully opened during frequency conversion adjustment.
According to the site requirements, the two frequency converters adopted a one-to-one manual bypass control scheme, and its primary circuit is shown in Figure 2.
The bypass cabinet in Figure 2 contains three high-voltage disconnect switches. To ensure that no power is fed back to the inverter output, K2 and K3 are electromagnetically interlocked. When K1 and K3 are closed and K2 is open, the motor operates at the inverter frequency; when K1 and K3 are open and K2 is closed, the motor operates at the power frequency. At this time, the inverter is isolated from the high voltage, which facilitates inspection, maintenance, and debugging.
The bypass cabinet must be interlocked with the upstream high-voltage circuit breaker DL. When DL is closed, it is absolutely forbidden to operate the bypass disconnect switch and the frequency converter output disconnect switch to prevent arcing and ensure the safety of operators and equipment.
Fault Tripping: The inverter's "High Voltage Disconnect" signal and the bypass cabinet's "Inverter Engagement" signal are connected in series and then in parallel to the high voltage switch tripping circuit. When the inverter is engaged, a fault in the inverter disconnects the inverter's high voltage input; when the bypass is engaged, inverter fault tripping is ineffective.
Protection: Maintain the original protection for the motor and its setting values unchanged.
4.2 High-voltage frequency converter control logic
Figure 3 Schematic diagram of inverter control logic
Utilizing the reserved points in the DCS control system, a high-voltage frequency converter retrofit control and operation monitoring system is designed. The system includes start and stop command buttons for the mains and frequency converter; speed adjustment buttons; frequency converter reset button and emergency stop button; frequency converter operating current and motor speed display; high-voltage cabinet and frequency converter closing display; and frequency converter fault alarm display.
The DCS system outputs a 4~20mA current signal to control the operating frequency of the JD-BP38 high-voltage variable frequency speed control system, thereby controlling the motor's operating speed. The JD-BP38 high-voltage variable frequency speed control system also feeds back a 4~20mA current signal to indicate its output frequency and output current. Simultaneously, the JD-BP38 high-voltage variable frequency speed control system receives start, stop, emergency stop, and reset control signals from the DCS control system to adjust its operating status.
When the JD-BP38 high-voltage variable frequency speed control system malfunctions, the system outputs fault shutdown and alarm information to prompt the user to initiate fault handling measures. Simultaneously, the JD-BP38 high-voltage variable frequency speed control system sends a signal to the DCS (Distributed Control System), displaying the fault on the DCS for timely troubleshooting. In case of an emergency, the DCS monitoring personnel immediately press the inverter's emergency stop button. At this time, the inverter immediately blocks the output and promptly trips the high-voltage circuit breaker switch.
To ensure the operational safety of the JD-BP38 high-voltage variable frequency speed control system, the status signal of the circuit breaker needs to be connected to the JD-BP38 high-voltage variable frequency speed control system from the incoming switch cabinet of the modified motor. The fault trip signal output by the JD-BP38 high-voltage variable frequency speed control system is connected to the circuit breaker's tripping circuit, and the continuous trip high-voltage signal is connected to the safety circuit. When a serious fault occurs, the circuit breaker will trip in a timely manner to protect the JD-BP38 high-voltage variable frequency speed control system and the motor.
As shown in Figure 3, once all start-up conditions are met, the DCS issues a frequency converter operation command, and the built-in PLC of the frequency converter sends a "closing" signal to the 10kV high-voltage cabinet. The high-voltage circuit breaker switches close, and the frequency converter performs soft charging. After 5 seconds, the charging is completed, and the frequency converter is in a ready state. The DCS then sends a 4~20mA signal to control the frequency converter frequency to change the motor speed.
4.3 Operation and Testing of High-Voltage Frequency Converters
After the modification, the high-voltage frequency converter met the design requirements in simulated debugging, no-load debugging, and load debugging. The test items and results are as follows:
(1) Power factor test: The power factors of the induced draft and exhaust fan motors are both between 0.97 and 0.99.
(2) Inverter efficiency test: The efficiency of both inverters is 97%~98%.
(3) Test of harmonic impact of frequency converter on public power grid: The total distortion rate of the 10kV power grid system was tested to be 1.48%~1.84%, which meets the national standard requirement of less than 4%. The total distortion rate of the 110kV and 220kV power grid systems was tested to be 1.02%~1.42%, which meets the national standard requirement of less than 2%.
(4) Comparison of starting current of variable frequency and power frequency: the starting current of variable frequency is small and the current rises smoothly, while the starting current of power frequency is 6 to 7 times the rated current. The inrush current is large during startup, which directly threatens the safe operation of the motor and shortens its service life.
(5) Frequency range and accuracy test of inverter: The frequency adjustment range of the inverter is 0~50Hz, with a wide adjustment range and flexible adjustment, achieving ±3% of the design requirements.
4.4 Energy Saving Status of Variable Frequency Drive Retrofit
The frequency converter was officially put into operation on December 23, 2009, and has been running well ever since. The frequency converter is easy to use, reliable, stable, and requires minimal maintenance. The upgrade achieved its intended purpose. Figure 4 shows the on-site operation of the high-voltage frequency converter.
Figure 4. Field operation diagram of high voltage frequency converter
(1) Analysis of direct economic benefits
When induced draft fans and forced draft fans operate at low loads (60-100MW) for extended periods, the motor frequency is generally in the range of 30-47Hz, and often operates at around 40Hz. The Kunming Salt Mine Power Plant Energy Saving Service Center conducted actual measurements of the parameters during normal operation of the fans: the test results showed that the energy saving rate was 38% for 315kW/10kV and 40% for 400kW/10kV.
Based on the test results above, assuming a single unit operates for 5500 hours per year...
Annual electricity savings per unit = Exhaust fan capacity × Unit annual operating hours × Average power saving rate of frequency converter
=(315kW×38%+400kW×40%)×5500h=1538350kW•h;
Increased revenue from grid connection electricity price = Annual electricity savings × Grid connection electricity price = 1,538,350 kWh × 0.25 yuan/kWh = 384,587.5 yuan;
Annual standard coal savings = Annual electricity savings × Coal consumption for power supply = 1,538,350 kWh × 0.42 kg/kWh = 646,107 kg;
Annual coal purchase cost savings = Annual standard coal savings × Standard coal unit price = 646.107t × 800 yuan/t = 516885.6 yuan.
The economic benefits of using variable frequency speed control are considerable.
(2) Indirect economic benefit analysis
Starting the motor with a frequency converter eliminates inrush current to the motor, cables, and switches (when starting with power frequency, the starting current is 6-7 times the motor's rated current, often causing damage to the motor, cables, and switches, forcing the unit to reduce load or shut down). The equipment operates under varying loads according to the production process, significantly reducing the equipment load rate and extending the service life of fans, motors, and other equipment. This also reduces equipment maintenance costs and economic losses caused by unauthorized electricity usage. These indirect, hidden benefits are also considerable, saving approximately 500,000 to 1 million yuan annually.
5. Conclusion
With the separation of power generation and grid connection, and increasingly fierce competition for grid connection, reducing power generation costs, improving the competitiveness of power generation companies in bidding for grid connection, strengthening internal management, and tapping energy conservation potential are major issues that power plants must seriously study. Adopting high-voltage frequency converters to technically upgrade high-energy-consuming equipment in power plants, such as forced draft fans, induced draft fans, feedwater pumps, and circulating water pumps, can not only directly reduce plant power consumption, reduce coal consumption for power supply, and increase the direct economic benefits brought by grid-connected power, but also improve the safety and reliability of equipment and even the generating units, reducing the hidden economic benefits caused by unit failures. High-voltage frequency converter technology has broad application potential in power plants.
