Features of high-low-high frequency converter

Features of high-low-high frequency converter

It is named after the low-voltage or general-purpose inverter is applied in medium- and high-voltage environments by using the step-down transformer. The principle is to reduce the grid voltage to the rated or allowed voltage input range of the low-voltage inverter through the step-down transformer, and then form alternating current with variable frequency and amplitude through the inverter, and then transform it into the voltage level required by the motor through the step-up transformer.

This method uses a standard low-voltage inverter, combined with a step-down and step-up transformer, so it can arbitrarily match the voltage level of the power grid and the motor. When the capacity is small (<500KW), the transformation cost is lower than that of a direct high-voltage inverter. The disadvantage is that the step-up and step-down transformer is large and heavy, and the frequency range is easily affected by the transformer.

Generally, high-low-high frequency converters can be divided into two types: current type and voltage type.

What are the characteristics of high-low-high current inverters?

It uses GTO, SCR or IGCT components in series to achieve direct high-voltage frequency conversion. The current voltage can reach 10KV. Since the DC link uses inductive components, it is not sensitive to current, so it is not easy to have overcurrent faults. The inverter works reliably and has good protection performance. The input side uses thyristor phase-controlled rectification, and the input current harmonics are large. When the capacity of the frequency conversion device is large, the pollution to the power grid and the interference to communication electronic equipment must be considered. The voltage balancing and buffer circuits are technically complex and costly. Due to the large number of devices and the large size of the device, adjustment and maintenance are relatively difficult. The inverter bridge uses forced commutation, and the heat generation is also relatively large, so the heat dissipation problem of the device needs to be solved. Its advantage is that it has four-quadrant operation capability and can brake.

It should be noted that this type of inverter needs to install high-voltage self-healing capacitors on its input and output sides due to its lower input power factor and higher input and output harmonics.

Features of high, low and high voltage inverters

The DC link of the low-voltage inverter is named because of the capacitor element. The input side can use thyristor phase-shift control rectification or diode three-phase bridge direct rectification. The function of the capacitor is filtering and energy storage. The inverter or converter circuit can use GTO, IGBT, IGCT or SCR elements. Through SPWM conversion, AC power with variable frequency and amplitude can be obtained, 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 or damage the insulation of the winding due to excessive input harmonics or dv/dt. The cost of the sine wave filter is very high, generally equivalent to 1/3 to 1/2 of the price of the low-voltage inverter.