Application and working principle of high voltage variable frequency speed regulation system

1. Baotou Rare Earth Aluminum Co., Ltd. is a large modern aluminum-electricity integrated factory. The first phase of the project was put into operation on October 28, 2003. At present, it has formed an annual production capacity of 880,000 tons, and the supporting thermal power units have been built one after another. Baotou Xilu's various economic and technical indicators are leading in China. It is a large-scale modern industrial enterprise with a high starting point, large capacity and low pollution. The project has been listed as a national Guangcai Program key project by the China Guangcai Program Promotion Association. The company's aluminum-electricity and bioengineering recycling industry chain has unique competitiveness and has been awarded the Inner Mongolia Autonomous Region-level circular economy park.

The total installed capacity of Baotou Hope Aluminum Thermal Power Plant is 1320MW. The 4*155MW units in the first phase and the 2*350MW units in the second phase have all been connected to the grid for power generation. The power plant currently has two 155MW condensing steam generator units in operation. With the successive commissioning of large units in the power grid, the peak-shaving task of the plant has become increasingly heavy, and the peak-to-valley difference has increased. According to statistics from on-site operation data, the daily load of the 155MW unit is generally around 73.6MW. When the system is in low-load operation, the system-related equipment must also be adjusted accordingly with the change of load. The air volume and flow must be adjusted by closing the damper or valve. At this time, a large amount of energy from the motor output power is consumed on the damper, and the throttling loss is very large. When the asynchronous motor is directly started, the starting current generally reaches 5-7 times the rated current of the motor. Excessive starting current will cause voltage fluctuations in the power grid, affecting the normal operation of other electrical equipment. At the same time, the motor's own windings will heat up severely, accelerate insulation aging, and shorten the service life. The high-voltage frequency converter speed control device can realize the soft start of the motor and change the motor speed by adjusting the frequency to meet the process requirements of different loads. It is an effective means to resolve the above contradictions.

2. High-voltage inverter speed regulation and energy-saving principle

2.1 Working principle of high voltage inverter

High-voltage frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert industrial frequency power supply into another frequency. It changes the pulse width or amplitude of the pulse train according to a certain rule to adjust the output and waveform, thereby achieving smooth changes in the voltage and frequency of the motor.

According to the principle of electromechanics, the speed of AC asynchronous motor is determined by the following formula:

Where: n, f——motor speed, power supply frequency; n, s——motor pole pair number, slip rate. It can be seen from formula (1) that the output speed of the motor is related to the input power supply frequency, slip rate, and motor pole pair number. Changing the pole pair number is called variable pole speed regulation. This type of speed regulation belongs to pole speed regulation, which is not smooth and is mainly used in places where frequent speed change is not required. The speed regulation methods that change the motor slip rate include: voltage regulation, electromagnetic speed regulation, and winding motor rotor series resistance speed regulation. This type of speed regulation requires the generation of a new slip rate and consumes slip power, which is an inefficient speed regulation. The variable frequency speed regulation that changes the input frequency of the motor is approximately stepless speed regulation. It is an efficient and energy-saving speed regulation method and is also the most widely used speed regulation method. 2.2 Energy-saving principle of high-voltage inverter According to the basic laws of fluid mechanics, it can be seen that fan (or water pump) equipment is a square torque load, and its speed n has the following relationship with flow Q, pressure (head) H and shaft power P:

Q, H - air volume and air pressure respectively; n, T - speed and torque respectively; P - shaft power. From formulas (2), (3) and (4), we can see that the flow rate of a fan (or water pump) is proportional to its speed, the pressure (or head) is proportional to the square of its speed, and the shaft power is proportional to the cube of its speed. When the fan speed decreases, its shaft power decreases with the cube of the speed, and the electric power required to drive the fan motor can also be reduced accordingly. Therefore, adjusting the speed is an important way to save energy for fans.

3. Application of high voltage variable frequency speed regulation system

3.1 Composition of high voltage inverter

Senlan SBH series high-voltage inverter adopts unit series multi-level technology, and uses multiple low-voltage frequency conversion power units in series to achieve direct high-voltage output. It belongs to the "high-high" voltage source inverter. The inverter has small harmonic pollution to the power grid and high input power factor. It does not need to use input harmonic filters and power factor compensation devices, and meets the requirements of IEEE519-1992 international standards and GB/T14549-93 national standards for input harmonics. The output waveform quality is good, and there is no additional heating and torque pulsation, noise, common mode voltage and other problems caused by harmonics. There is no need to set an output filter, and ordinary asynchronous motors can be directly driven. Its principle block diagram is shown in Figure 1:

The grid input provides multiple sets of secondary outputs through multiple secondary phase-shifting transformers to supply power to the power units respectively, and then multiple power units are connected in series to form a three-phase output in the form of a single-phase output. The main control system controls the frequency and amplitude of the inverter output voltage by controlling the PWM output of each power unit, thereby achieving the purpose of controlling the motor speed. The main control system and the units communicate through optical fibers, which not only ensures reliable signal transmission, but also ensures insulation isolation between the main control part and the high-voltage part.

3.2 High-voltage frequency conversion system configuration scheme

This time, 7 sets of SBH high-voltage inverters were applied to the 4*155MW units of the first phase of the power plant, with loads of 3 condensate pumps with a power of 200kw and 4 blowers with a power of 630kw. According to the actual situation on site, the main circuits of the 7 sets of high-voltage inverters all adopted a manual one-to-one solution, as shown in the following figure:

1. QS1, QS2 and QS3 are three high-voltage disconnectors, QF is the high-voltage switch on the user side. There is a mechanical interlock between QS2 and QS3.

2. The knife switch cannot be opened or closed with load. When operating QS1, QS2, and QS3, the upper switch must be opened before the switch operation is allowed.

3. When QS1 and QS2 are closed, QS3 is open, and QF is closed, the motor is controlled by the inverter to run at a variable speed.

When QS1 is disconnected, QS2 is disconnected, and QS3 is closed, the motor can be started and stopped directly by QF and protected, and the inverter can be completely disconnected from the power grid, making maintenance and overhaul easier.

3.3 Operation status and energy-saving benefits The installation and all on-site commissioning projects of 7 sets of high-voltage frequency converters were completed from November 10 to November 29, 2011. On November 30, all equipment was officially put into production, and all frequency converters were operating well, with obvious energy-saving effects. By recording the power consumption of the two blowers of the #5 furnace and #6 furnace after the frequency conversion transformation and commissioning, it can be concluded that compared with the #5 furnace and #6 furnace before the transformation, the power saving per week is about 49272 kwh, and the year-on-year power saving rate is 20%, with obvious power saving effect. The following table is the weekly comparison data of the #5 furnace and #6 furnace fan frequency conversion energy saving

Excluding maintenance time, calculated as 300 days per year, the unit price of electricity is 0.5 yuan/kwh, and the annual electricity saving is: 630*20%*300*24*0.5≈450,000 yuan.

4. Summary

my country has a large number of small power enterprises and self-owned power plants of large enterprises in various industries. Most of these power plants are medium and small units, with a large number and small single-unit capacity. Not only are the generators outdated, but the main auxiliary equipment they belong to is also backward, with low efficiency, improper equipment selection, and a big horse pulling a small cart. The flow and pressure regulation method of the fan pump is basically the regulation of the import and export gates, which consumes a lot of energy, has poor economic benefits, and serious equipment damage. The use of high-voltage frequency conversion technology to carry out technical transformation of high-energy consumption electrical equipment can not only directly receive direct economic benefits from reducing factory electricity consumption, reducing power supply coal consumption, and increasing online power, but also play a positive role in the safe and reliable operation of the equipment and reducing equipment failures. Senlan SBH series high-voltage frequency converters have high reliability and good input and output waveform quality. They are suitable for frequency conversion speed regulation of power plant auxiliary equipment, can improve the reliability of power plant operation and power supply, save a lot of energy, and bring greater economic and social benefits to power plants.