High-voltage frequency converters come in a wide variety of types and can be classified in many ways. Based on the presence or absence of a DC component in the intermediate stage, they can be divided into AC-AC converters and AC-DC-AC converters; based on the nature of the DC component, they can be divided into current-type and voltage-type converters; based on the presence or absence of an intermediate low-voltage circuit, they can be divided into high-high frequency converters and high-low-high frequency converters; based on the number of output levels, they can be divided into two-level, three-level, five-level, and multi-level converters; based on voltage level and application, they can be divided into general-purpose converters and high-voltage converters; based on the clamping method, they can be divided into diode-clamped and capacitor-clamped converters, and so on.
Current-source high-voltage frequency converter
Named for the use of inductors in the DC link of the inverter, its advantages include four-quadrant operation capability and easy motor braking. Disadvantages include the need for forced commutation of the inverter bridge, resulting in a complex device structure and difficult adjustment. Furthermore, the use of thyristor phase-shifting rectification on the grid side leads to higher input current harmonics, which can have a certain impact on the power grid when the capacity is large.
High voltage frequency converter
It gets its name from the use of capacitors in the DC link of the frequency converter. With the advancement of technology, high-voltage frequency converters can achieve four-quadrant operation and vector control, and have become the mainstream product for speed regulation of current transmission systems.
High-low-high type frequency converter
It gets its name from the method of applying low-voltage or general-purpose frequency converters to medium- and high-voltage environments by stepping up and down the voltage. The principle is to use a step-down transformer to reduce the grid voltage to the rated or permissible voltage input range of the low-voltage frequency converter, which then converts it into AC power with variable frequency and amplitude. Finally, a step-up transformer converts it to the voltage level required by the motor.
This method, using a standard low-voltage frequency converter in conjunction with step-up and step-down transformers, can be matched to any voltage level of the power grid and motor. For small capacities (<500KW), the retrofit cost is lower than using a direct high-voltage frequency converter. The disadvantages are that the step-up and step-down transformers are 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-low-high frequency converters can be divided into two types: current-type and voltage-type.
High current inverter
The circuit topology is named for the use of inductive components in the DC link of the low-voltage frequency converter. The input side uses thyristor phase-shift control rectification to control the motor current, while the output side uses forced commutation to control the motor's frequency and phase. This allows for four-quadrant operation of the motor.
High voltage frequency converter
A step-down transformer is introduced at the front end to reduce the grid voltage, which is then connected to a low-voltage frequency converter. The input side of the low-voltage frequency converter can use thyristor phase-shift control rectification or direct rectification using a diode three-phase bridge. The intermediate DC section uses capacitor smoothing and energy storage. The inverter or converter circuit often uses IGBT components. Through SPWM conversion, AC power with variable frequency and amplitude can be obtained, which is then converted to the voltage level required by the motor by a step-up transformer. It should be noted 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 overheat due to excessive input harmonics or dv/dt, or damage the winding insulation. This sine wave filter is very expensive, generally equivalent to 1/3 to 1/2 the price of the low-voltage frequency converter.
High-frequency inverter
High-voltage frequency converters do not require step-up/step-down transformers; the power devices directly form the converter between the power grid and the motor. Because the voltage withstand capability of power devices is difficult to solve, the most direct approach is to use series connection of devices to increase the voltage level. The disadvantage of this method is that it requires solving the problems of voltage equalization and buffering, which is technically complex and challenging. However, because this type of frequency converter does not have a step-up/step-down transformer, its efficiency is higher than that of the low-voltage/high-voltage method, and its structure is more compact.
High-current frequency converter
It achieves direct high-voltage frequency conversion using GTO, SCR, or IGCT components connected in series, with voltages up to 10KV. Because the DC link uses inductive components, it is less sensitive to current, thus reducing the likelihood of overcurrent faults and ensuring reliable inverter operation with good protection performance. Its input side uses thyristor phase-controlled rectification, resulting in relatively high input current harmonics. For large-capacity frequency converters, pollution to the power grid and interference with communication electronic equipment must be considered. Voltage equalization and buffer circuits are technically complex and costly. Due to the large number of components, the device is bulky, making adjustment and maintenance difficult. The inverter bridge uses forced commutation, generating significant heat, requiring solutions for component heat dissipation. Its advantage lies in its four-quadrant operation capability and braking function.
It should be noted that, due to their low input power factor and high input and output harmonics, this type of frequency converter requires the installation of high-voltage self-healing capacitors on its input and output sides.
High-voltage frequency converter
The circuit structure employs IGBT direct series technology, also known as a direct-connection series high-voltage frequency converter. It uses a high-voltage capacitor for filtering and energy storage in the DC link, achieving an output voltage of up to 13.8KV. Its advantages include the ability to use lower voltage-rated power devices, and all IGBTs in the series arm function identically, allowing for mutual backup or redundancy design. Disadvantages include a lower voltage level (only two levels) and a larger output voltage dV/dt, requiring the use of special motors or the addition of common-mode voltage filters and high-voltage sine wave filters, significantly increasing costs. Because it shares the same topology as low-voltage frequency converters, it also features four-quadrant operation and vector control capabilities.
This type of frequency converter also needs to address the voltage equalization issue of its components, generally requiring specially designed drive circuits and buffer circuits. There are also extremely stringent requirements for the delay of the IGBT drive circuit. Inconsistent turn-on and turn-off times of the IGBTs, or significant differences in the rise and fall edge slopes, can damage the power devices.
Clamped frequency converter
Clamping inverters can generally be divided into diode clamping type and capacitor clamping type.
Diode type frequency converter
It can achieve diode midpoint clamping and three-level or more output levels, and its technical difficulty is lower than that of direct-device series inverters. Because the DC link uses capacitors, it is still a voltage-type inverter. This type of inverter requires an input transformer, which serves for isolation and star-delta conversion, enabling 12-pulse rectification and providing a midpoint clamping zero level. By using auxiliary diodes to force power devices such as IGBTs to be clamped at the midpoint zero level, the IGBT terminals are prevented from burning out due to overvoltage, thus achieving multi-level output.
This type of inverter structure eliminates the need for a sine wave filter at the output. However, the use of a transformer increases the cost.
capacitor inverter
It uses the method of adding floating capacitors to the same bridge arm to achieve the clamping of power devices. This type of frequency converter is relatively rare.
