There are many types of high-voltage inverters, and their classification methods are also diverse. According to whether there is a DC part in the intermediate link, it can be divided into AC inverter and AC AC inverter; According to the nature of the DC part, it can be divided into current type and voltage type inverter; According to whether there is an intermediate low-voltage circuit, it can be divided into high-high inverter and high-low inverter; According to the number of output levels, it can be divided into two-level, three-level, five-level and multi-level inverters; According to the voltage level and use, it can be divided into general-purpose inverter and high-voltage inverter; According to the embedding method, it can be divided into diode embedding type and capacitor embedding inverter and so on.
Current type high-voltage frequency converter
It is named because the inductive element is used in the DC link of the inverter, and its advantage is that it has the ability to operate in four quadrants, and it is easy to realize the braking function of the motor. The disadvantage is that the inverter bridge needs to be forced to commutate, and the device structure is complex, and the adjustment is difficult. In addition, due to the use of thyristor phase-shifting rectification on the grid side, the input current harmonics are large, and the capacity will have a certain impact on the power grid.
High-voltage type inverter
With the advancement of technology, the high-voltage inverter can achieve four-quadrant operation and vector control, which has become the mainstream product of current transmission system speed regulation.
High-low-high frequency converters
The method of step-up and down-voltage is adopted, and the low-voltage or general-purpose inverter is applied in the medium and high voltage environment. The principle is to drop the grid voltage to the rated or allowable voltage input range of the low-voltage inverter through the step-down transformer, and form alternating current with variable frequency and amplitude through the conversion of the inverter, and then convert it into the voltage level required by the motor through the step-up transformer.
In this way, due to the use of standard low-voltage inverter, with step-down, step-up transformer, so it can arbitrarily match the voltage level of the power grid and motor, when the capacity is small (< 500KW), the transformation cost is lower than that of direct high-voltage inverter. The disadvantages are that the step-up transformer is large and bulky, the frequency range is easily affected by the transformer, and the system efficiency is relatively low due to the introduction of the transformer.
Generally, high and low inverters can be divided into two types: current type and voltage type.
High-current type inverter
The circuit topology is named after the use of inductive components in the DC link of low-voltage inverters. The input side adopts the thyristor phase shift control rectification to control the current of the motor, and the output side is the forced commutation mode to control the frequency and phase of the motor. Four-quadrant operation of the motor can be realized.
High-voltage type inverter
A step-down transformer is introduced in the front section to step down the power grid, and then a low-voltage inverter is connected. The input side of the low-voltage inverter can be rectified by controlling the phase shift of the thyristor, or it can be rectified directly by using the diode three-phase bridge, and the middle DC part can be rectified with capacitor flat wave and energy storage. IGBT components are often used in inverter or converter circuits, and AC power with variable frequency and amplitude can be obtained through SPWM conversion, and then converted into the voltage level required by the motor through the step-up transformer. It should be pointed out that a sine wave filter (F) needs to be placed between the converter circuit and the step-up transformer, otherwise the step-up transformer will heat up due to excessive input harmonics or dv/dt, or destroy the insulation of the winding. The cost of this sine wave filter is high, generally equivalent to 1/3 to 1/2 of the price of a low-voltage inverter.
High and high frequency conversion inverter
High-speed inverters do not require step-up and step-down transformers, and power devices build converters directly between the grid and the motor. Because the power device withstand voltage problem is difficult to solve, the most direct approach at present is to adopt the method of device series connection to improve the voltage level, and its disadvantage is that it is necessary to solve the problem of device voltage equalization and buffering, and the technology is complex and difficult. However, because there is no step-up transformer, this kind of inverter has a high efficiency and a relatively compact structure.
High and high current inverters
It adopts the method of GTO, SCR or IGCT components in series to achieve direct high-voltage frequency conversion, and the voltage can reach 10KV. Because the inductive element is used in the DC link, it is not sensitive enough to the current, so it is not prone to overcurrent faults, and the inverter works reliably and has good protection performance. The input side adopts thyristor phased rectification, and the input current harmonic is large. When the capacity of the frequency conversion device is large, it is necessary to consider the pollution of the power grid and the interference to the communication electronic equipment. Voltage equalization and snubber circuits, complex technology and high cost. Due to the large number of components and the large size of the device, it is difficult to adjust and maintain. The inverter bridge adopts forced commutation, and the heat generation is also relatively large, so it is necessary to solve the heat dissipation problem of the device. It has the advantage of having a four-quadrant operation capability and can be braked.
It should be noted that due to the low input power factor and high input and output harmonics, this type of inverter needs to install high-voltage self-healing capacitors on its input and output sides.
High voltage inverter
The circuit structure adopts IGBT direct series connection technology, also known as direct device series high-voltage inverter. It uses high-voltage capacitors for filtering and energy storage in the DC link, and the output voltage can reach 13.8KV, which has the advantage that it can use lower withstand voltage power devices, and all IGBTs on the series bridge arm have the same function, which can realize mutual backup or redundant design. The disadvantage is that the number of levels is low, only two levels, and the output voltage dV/dt is also large, so it is necessary to use special motors or install common-mode voltage filters and high-voltage sine wave filters, and its cost will increase a lot. Because it has the same topology as the low-voltage inverter, it has a four-quadrant operation function like the low-voltage inverter, and can also realize vector control.
This kind of inverter also needs to solve the voltage equalization problem of the device, and generally requires special design of drive circuit and snubber circuit. There are also extremely demanding requirements for the delay of IGBT drive circuits. Once the opening and closing time of IGBT is inconsistent, or the slope difference between the rising and falling edges is too large, it will cause damage to the power device.
Embedded inverter
Clamp type inverter can generally be divided into diode clamp type and capacitor clamp type.
Diode-type inverter
It can achieve both diode midpoint embedding and three-level or more level output, which is less technically difficult than direct device series inverters. Since the DC link uses a capacitive element, it is still a voltage-type inverter. This type of inverter requires an input transformer, which is used to isolate and transform the star angle, to achieve 12 pulse rectification, and to provide an embedded zero level. The IGBT and other power devices are forcibly embedded on the middle zero level through the auxiliary diode, so that the two ends of the IGBT will not burn out due to overvoltage, and the multi-level output is realized.
With this inverter structure, the output can be without a sine wave filter. However, due to the use of transformers, the cost has increased.
Capacitive inverters
It uses the method of adding a suspended capacitor to the same bridge arm to realize the embedding of power devices, and the application of this inverter is relatively rare.
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