2. Classification and characteristics of high voltage frequency conversion scheme
2.1 high-low-high scheme
This solution takes the general low-voltage inverter as the core, that is, adding a step-down inverter to the input side of the low-voltage inverter, reducing 6KV/3KV to the input voltage of the low-voltage inverter; adding on the input side of the inverter A step-up transformer that matches the output voltage to the rated voltage of the motor. The high-low-high frequency control scheme is more common in the domestic market and is currently the most widely used one. In the market, it is mainly based on Siemens products. In this solution, according to the form of the DC power supply rectifying unit, it can be divided into a 6-pulse rectifying unit and a 12-pulse rectifying unit. The system structure of the 6-pulse rectification unit is simple, and the cost of the hardware configuration is low. The 12-pulse rectification unit is composed of a three-winding transformer with a sub-phase difference of 30 degrees and two identical rectification units, and is in the transformer. The secondary side must be connected to the line reactor, so its relative cost is higher, but from the harmonic component, the effect of 12-pulse rectification is better than 6-pulse, and the harmonic component of 6-pulse is 5, 7. 11, 13, 17, 19, 23, 25 times, the harmonic components of 12 pulsations are 11, 13, 23, 25 times.
This program is most suitable for retrofitting projects. Advantages: (1) The program is mature. Taking Siemens products as an example, the number of sets that have been applied in China has been issued. (2) The original motor cable does not need to be modified. (3) A function that facilitates switching. (4) The investment is lower than that of the high-high program. Its shortcomings: the system components are relatively high-low or high-high, and the long-term operating costs are relatively high due to the loss of filters and boost and step-down transformers.
2.2 high-low-low scheme
The core of the high-low-low scheme is still based on low-voltage inverters. A double-winding step-down transformer is installed on the input side of the inverter to turn the high-voltage motor into a usable low-voltage variable-frequency drive voltage level. This program currently has more users in China. There are many applications of Siemens products. The system is mainly composed of double-winding step-down inverters and transformers. The system scheme is relatively simple, especially suitable for new projects. For the modified projects, replacing the motors will increase the project expenses. Its harmonic components are mainly 5, 7, 11, 13, 17, 19, 19, 23, and 25 times.
The advantages of this program are: (1) The program is mature. Low-voltage IGBT or IGCT power devices are well established and have a wide range of applications. (2) The composition is simple and there are few links for maintenance. (3) Less investment and quick results. Low-voltage inverters are inexpensive and the market price is quite transparent. (4) The loss of the step-up transformer and the cost of long-term operation are low. Disadvantages of this solution: When used in the transformation project, the motor needs to be replaced, and the workload of the transformation is increased, which also increases the project investment and extends the construction period.
2.3 high-high program
The high-high inverter is a high-voltage power supply that is directly sent to the high-voltage inverter after passing through the high-voltage switchgear. After high-voltage frequency conversion, the voltage is directly supplied to the stator of the high-voltage motor.
There are four main types of medium and high voltage variable frequency speed control systems: (1) power components (GTO, SCR, SGCT) series; (2) midpoint clamps are in position, that is, three-point flat (GTO, IGBT); (3) Multi-level type, in which four-level, five-level, etc. (IGBT) are commonly used; (4) Single-phase inverters are connected in series. These high-voltage inverters have been introduced in recent years, and the technology is advanced, high efficiency, and small footprint, which is the main development direction of high-voltage inverters.
The high-high scheme adopts the unit series connection (IGBT), and utilizes the principle of phase shift superposition (phase shifting transformer) to effectively suppress harmonic interference, so that the output waveform is nearly sinusoidal. It has the following advantages: no need to add any output filtering or power compensation device; the use efficiency can be as high as 97%, the power factor is high, and the power factor is greater than 95% load when the load is >20% without any compensation device >20%; the generation of pulsating torque caused by no harmonics can avoid load resonance and prolong the service life of motor and mechanical equipment; it can protect the main motor and cable insulation from dv/dt stress damage; the motor is not affected by common mode The influence of voltage; the motor cable does not have any length limitation within the allowable range of voltage drop; no pollution to the power grid; the main device has high reliability due to redundancy design, and can adapt to extreme working conditions; the power unit has automatic bypass function. It can improve the reliability of power supply; it has the function of instantaneous power failure without stopping. When the power supply voltage is instantaneously stepped down or power is cut off (within 5s), the inverter will not stop. After the power supply voltage is restored, it will immediately re-accelerate and return to its original state. When the power supply is instantaneously stopped, the inverter will not stop running within 300ms.
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