XICHI ELECTRIC CO., LTD.
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Author: Site Editor Publish Time: 2022-08-11 Origin: Site
The high-voltage variable frequency drive we usually call, the input power supply voltage is generally above 3kV, which is a high-power VFD. At present, in the domestic market, the voltage levels range from 3kV to 10kV, and the main voltage levels are 3000V, 3300V, 6000V, 6600V, and 10000V.
According to international practice and national standards, the voltage above 10kV is called high voltage, and the voltage below 10kV is called medium voltage. So in some areas, it is also called medium voltage variable frequency drive.
However, since it is customary to call motors with rated voltages of 3kV, 6kV and 10kV as "high voltage motors", the variable frequency drives driving this voltage level are collectively referred to as high voltage VFDs.
There are many types of variable frequency drives, and the classification methods are not the same.
The three basic classifications of high-voltage VFDs are listed below, with an introduction to each type of VFD.
a. According to whether there is a DC link, it is divided into 'AC-AC' VFD and 'AC-DC-AC' VFD;
b. According to the nature of the DC link filter element, it is divided into current source type VFD and voltage source type VFD;
c. According to whether a low-voltage circuit is generated 
in the process, it can be divided into 'HV-HV' VFD and 'HV-LV-HV' VFD.
Xichi CFV9000A series of high-voltage AC Drives are designed with high reliability, easy operation and high performance to meet the needs of users for various types of load speed regulation, energy saving, and production process improvement. They are HV-HV voltage source variable frequency drives.
There is no DC link in the AC-AC VFD working process, and the thyristor can be used to directly realize the frequency conversion from AC to AC.
AC-AC frequency conversion has no DC link, high frequency conversion efficiency, simple main circuit, no DC circuit and filter part, and easy reactive power processing and active power feedback between the power supply and the power supply.
Although the AC frequency conversion double-fed system has been widely used, its application is limited due to its low power factor, many high-order harmonics, low output frequency, narrow variation range, and large number of components.
As far as the current application situation is concerned, AC-AC VFD is mainly used in high-power and low-speed AC speed control circuits of 500,000 or more than 1,000,000 W.
We call this AC to DC and then AC variable frequency drive called AC-DC-AC VFD, which is divided into two types, one is AC-DC-AC voltage type, and the other is AC-DC-AC current type. The former is widely used, and the current general-purpose inverter adopts this topology.
Its characteristics: high efficiency, no additional loss in the process of speed regulation; wide application range, available
It is suitable for cage-type asynchronous motors; the speed regulation range is large, the characteristics are hard, and the precision is high; the technology is complex, the cost is high, and the maintenance and repair are difficult. This method is suitable for occasions requiring high precision and good speed regulation performance.
The voltage source type is a VFD that converts the DC of the voltage source into an AC, and the filter element of the DC circuit is a capacitor.
Advantages:
The large capacitor is used as the intermediate DC filter link, and the dynamic voltage equalization problem and the high-order harmonic problem caused by the series device are solved through three-level control;
Using low-voltage IGBT power components, the harmonic content is low, and the voltage and current waveforms are good;
Small influence on the motor, high power factor and efficiency.
With the advancement of technology, high-voltage VFDs can realize four-quadrant operation and vector control, and have become the mainstream product of current transmission system speed regulation.
The current source type is a VFD that converts the DC of the current source into an AC, and its DC loop filter element is an inductor.
Its advantage is that it has four-quadrant operation ability, and can easily realize the braking function of the motor. The disadvantage is that the inverter bridge needs to be forced to commutate, the device structure is complex, and the adjustment is difficult. In addition, since the thyristor phase-shift rectification is used on the grid side, the harmonics of the input current are relatively large, and when the capacity is large, it will have a certain impact on the grid.
It is named after the application of low-voltage or general-purpose VFDs in medium and high-voltage environments by means of buck-boost. The principle is to reduce the grid voltage to the rated or allowable voltage input range of the low-voltage variable frequency speed governor through the step-down transformer, and then convert the variable frequency drive to form an alternating current with variable frequency and amplitude, and then convert it into the motor power through the step-up transformer. required voltage level.
The use of transformers to achieve input step-down and output step-up is essentially a low-voltage variable frequency drive.
The advantage is that it can be arbitrarily matched to different levels of power grids and motors by setting the parameters of the transformer. When the capacity is small (<500KW), the transformation cost is lower than that of direct high-voltage VFD.
One of the main disadvantages of the high-low-high VFD is that the step-up and step-down transformers are large and bulky, and the frequency range is easily affected by the transformer. In addition, the introduction of the transformer makes the system efficiency relatively low, which is not used at present.
That is, the high voltage of the power supply is directly added to the VFD, rectified by the VFD, and the inverter drives the high-voltage motor. The power device directly forms a VFD between the power grid and the motor. Since the withstand voltage problem of the power device is difficult to solve, the current international method is to increase the voltage level by connecting the devices in series.
Advantages:
The direct series VFD circuit has a simple and compact structure, saves the input and output transformers, and saves cost and floor space. Since there are no transformers with a large number of joints, the heat loss and failure rate of the joints are reduced, and the work efficiency is high (up to 98%). The direct series frequency converter can add DC braking circuit or energy feedback like a low-voltage inverter, and its dynamic performance can also be as superior as a low-voltage VFD, with a wide speed regulation range.
Disadvantage:
The input circuit adopts direct rectification, and the input current has large harmonics.
In order to reduce the impact on the input, a filter can be added at the input end or PWM rectification can be used to achieve no harmonics.
