Research on the application of high-voltage VFD (Variable-frequency Drive) in high-power fans and pumps in Jigang

As we all know, high-voltage motors are widely used and are the main driving force in industrial and mining enterprises. In large and medium-sized factories and mines in metallurgy, steel, petroleum, chemical industry, water treatment and other industries, it is widely used to drive fans, pumps, compressors and various other large machinery. The energy consumed accounts for more than 70% of the total energy consumption of the motor, and most of them have speed regulation requirements. However, the current speed regulation and starting methods are still very backward, wasting a lot of energy and reducing the mechanical life. With the development of electrical transmission technology, especially variable frequency speed regulation technology, high-voltage variable frequency speed regulation technology as a large-capacity transmission has also been widely used. By the way, I would like to point out that the voltage of what is currently commonly called high-voltage VFD (Variable-frequency Drive) is generally 2.3-10kV. In China, it is mainly 3kV, 6kV and 10kV. Compared with the grid voltage, it can only be regarded as medium voltage, so Foreign countries often call it MediumVoltageDrive.

Jinan Iron and Steel Co., Ltd. high-pressure fan water pump speed regulating system

Most of China's high-voltage motors are 6kV and 10kV. In the old Jigang factory, the incoming power supply is 6kV. The speed regulation of high-voltage motors is mostly direct start and hydraulic coupling speed regulation. In the new factory, the incoming power supply voltage is 10kV. In the high-voltage fan speed regulation In the system, a hydraulic coupling speed regulation method is adopted. Direct starting or reduced-voltage starting not only has a large starting current, causing the grid voltage to decrease and affecting the normal operation of other electrical equipment; it also has a large mechanical impact on the spindle, which can easily cause fatigue fracture and affect the mechanical life. When the grid capacity is not large enough, starting may even fail. The hydraulic coupling adds an impeller between the motor shaft and the load shaft to adjust the pressure of the liquid (usually oil) between the impellers to achieve the purpose of adjusting the load speed. This method of speed regulation is essentially a slip power consumption method. The energy-saving effect is not very good, and as the speed decreases, the efficiency becomes lower and lower. It requires disconnecting the motor from the load for installation and maintenance. The workload is large, and after a period of time It is necessary to replace shaft seals, bearings and other components every time. The site is generally dirty, which shows that the equipment is of low grade and is an obsolete technology.

Generally speaking, the use of high-voltage (medium-voltage) frequency conversion speed regulation systems has two important characteristics for fans and water pump loads: First, it eliminates the energy loss of the valve (or baffle) and makes the working points of the fans and water pumps close to Its peak efficiency line and its overall efficiency are 25% to 50% higher than that of the hydraulic coupling; secondly, the high-voltage (medium voltage) variable frequency speed regulation starting performance is good, and the use of high-voltage VFD (Variable-frequency Drive) can achieve " Soft” start. The characteristics of the frequency conversion device ensure sufficient torque during starting and acceleration, eliminate the impact of starting on the motor, ensure the stability of the power grid, and increase the service life of the motor and machinery.

Now we will take Jinan Iron and Steel Co., Ltd. No. 3 Steelmaking as an example to analyze the energy-saving effect of high-voltage (medium-voltage) VFD (Variable-frequency Drive) in actual production. A total of 10 high-voltage dust removal motors are used in the No. 3 Steelmaking Plant of Jigang, with a total installed capacity of 23.1MW, accounting for 40% of the total installed capacity of the No. 3 Steelmaking Plant. The working current actually monitored on site has a higher proportion. The current value is shown in Table 1. The fan load accounts for 60% of the total capacity. The efficiency of high-voltage VFD (Variable-frequency Drive) can be increased by 25% to 50% compared with hydraulic coupling. Calculated based on the monthly fan energy saving of 20%, the total monthly electricity can be reduced by 8%. The monthly electricity bill of No. 3 Steelmaking is 10 million yuan. , which can reduce costs by nearly 800,000 yuan per year. Judging from the above rough calculations, the application of high-voltage (medium voltage) variable frequency speed regulation in high-pressure fans and water pumps in Jigang has broad prospects and huge energy-saving effects.

High voltage VFD (Variable-frequency Drive) application status

Although the industrialization of high-voltage VFD (Variable-frequency Drive) only began to take shape in the mid-1980s due to factors such as high voltage, high power, and complex technology, with the rapid development of high-power power electronic devices and the driving force of the huge market, High-voltage VFD (Variable-frequency Drive) has developed very rapidly in the past ten years. The devices used have developed from SCR, GTO, and GTR to IGBT, IGCT, IGET, and SGCT, and the power range ranges from hundreds of kilowatts to tens of megawatts. The technology has matured, its reliability has been guaranteed, and its use is becoming more and more widespread. High-voltage VFD (Variable-frequency Drive) can be matched with standard medium and high-power AC asynchronous motors or synchronous motors to form an AC variable frequency speed regulation system, which is used to drive fans, water pumps, compressors and various mechanical transmission devices to achieve energy saving and The purpose is to be efficient and improve product quality.

In recent years, various high-voltage VFDs (Variable-frequency Drives) have continued to appear. So far, high-voltage VFDs (Variable-frequency Drives) do not have a nearly unified topology like low-voltage VFDs (Variable-frequency Drives). According to the high-voltage composition method, it can be divided into direct high-voltage type and high-low-high type. According to whether there is an intermediate DC link, it can be divided into AC-AC VFD (Variable-frequency Drive) and AC-AC-AC VFD (Variable-AC VFD). Frequency Drive), in AC-DC-AC VFD (Variable-frequency Drive), according to the difference in the intermediate DC filtering link, it can be divided into voltage source type and current source type. The following will analyze several high-voltage VFDs (Variable-frequency Drives) that are currently widely used, and point out their respective advantages and disadvantages.

1. High-low-high type VFD (Variable-frequency Drive)

VFD (Variable-frequency Drive) is a low-voltage VFD (Variable-frequency Drive) that uses an input step-down transformer and an output step-up transformer to interface with the high-voltage power grid and motor. This was a transitional technology when high-voltage frequency conversion technology was immature. . Since the voltage of the low-voltage VFD (Variable-frequency Drive) is low, the current cannot rise indefinitely, which limits the capacity of this VFD (Variable-frequency Drive). Due to the existence of the output transformer, the efficiency of the system is reduced and the floor space is increased. In addition, the magnetic coupling capability of the output transformer is weakened at low frequencies, which weakens the load capacity of the VFD (Variable-frequency Drive) at startup. The harmonics of the power grid are large. If 12-pulse rectification is used, the harmonics can be reduced, but it cannot meet the strict requirements for harmonics. While the output transformer boosts the voltage, it also amplifies the dv/dt generated by the VFD (Variable-frequency Drive). , a filter must be installed to be suitable for ordinary motors, otherwise corona discharge and insulation damage will occur. Siemens produced VFD (Variable-frequency Drive) with this structure in the early stage, but has stopped production and only provides spare parts.

2. Current source type high voltage VFD (Variable-frequency Drive)