Classification of AC high voltage frequency converters
AC high-voltage frequency converters are generally divided into two types based on their applications: general-purpose high-voltage frequency converters, which are mainly used for energy saving; and process applications, where the production process requires motor speed regulation, and energy saving plays a secondary role. This section mainly provides some reference opinions on the selection of general-purpose high-voltage frequency converters.
From a topological perspective, general-purpose high-voltage frequency converters can be divided into several major categories: unit series multilevel type, PWM current source type, three-level type, and load commutation type (LCI).
1. Unit series multilevel type
This type of frequency converter uses multiple low-voltage power units connected in series to achieve high voltage. The step-down transformer on the input side adopts a phase-shifting method, which can effectively eliminate harmonic pollution to the power grid. The output side adopts multi-level sinusoidal PWM technology, which can be applied to ordinary motors of any voltage.
Furthermore, if a power unit fails, it can automatically disconnect from the system, while the remaining power units can continue to run their motors, minimizing losses caused by downtime. The system employs a modular design, allowing for rapid replacement of faulty power units, and users with spare units can perform maintenance themselves.
This type of frequency converter determines the number of series units based on the user's voltage, enabling arbitrary voltage output, making it highly suitable for retrofit projects. Unit-series multilevel frequency converters constitute the vast majority of newly sold frequency converters in the Chinese market.
Implementing energy feedback in a unit-series multilevel inverter is quite complex (each unit must implement it), and some domestic manufacturers are currently attempting it, with prototypes already launched.
2. PWM current source type
The current source inverter section uses SGCTs directly connected in series to solve the withstand voltage problem, and the DC section uses reactors to store energy. The current technology can achieve an output voltage of 7.2kV, which is suitable for the current situation where most voltages in China are 6kV.
The input side of the current source inverter uses thyristor rectification, which results in a relatively low power factor. Later, a dual PWM type was developed, which uses SGCT rectification to solve this problem and replaces the transformer on the grid side with a reactor.
Current-source inverters rely on the phase of the power grid for rectification control, making them highly sensitive to grid fluctuations. Due to their current-controlled operation, output filter design is complex, and two-level inverters exhibit significant common-mode voltage, harmonic, and dv/dt issues, placing higher demands on the motor. Current-source inverters offer the advantage of energy regenerative braking, giving them a competitive edge in applications requiring rapid braking. However, current-source inverters are also more expensive.
3. Three-level frequency converter
The three-level inverter uses a clamping circuit to solve the problem of series connection of two power devices and enable the phase voltage output to have three levels.
Three-level inverters have fewer main circuit components, and although they are voltage source type, they are easy to implement for energy feedback. The biggest challenge that three-level inverters face in the domestic market is the voltage issue; their maximum output voltage cannot reach 6kV, while most of the domestic power grid is 6kV and 10kV.
4. LCI (Load-Commutated Inverter)
This type of frequency converter is actually a current source frequency converter, which uses thyristor rectification and inversion, with a reactor as the energy storage element. Since thyristors cannot turn off on their own, they must rely on the back EMF of the load motor, and synchronous motors are generally used.
This type of frequency converter is generally only suitable for ultra-high power motors. In China, it is more commonly used for starting large motors, such as soft starting of blast furnace blowers and sintering blowers. After the motor starts successfully, it is put into operation on the power grid, and the frequency converter is taken out of operation.
Purchase considerations
Users should first understand the applicability and reliability of inverters from various manufacturers, and then select a suitable inverter based on their factory's current situation, process and load requirements, financial capacity, and the skill level of their operators.
The selection of high-voltage frequency converters should be based on rational thinking, distinguishing between basic and special functions, and making multiple comparisons.
In addition, the product configuration should be fully considered, as the configuration has a significant impact on the cost; avoid being misled by some manufacturers about additional configurations besides the frequency converter, which could lead to unnecessary waste.
In addition, future maintenance should also be considered. Some manufacturers charge high maintenance fees, which users may find difficult to afford if there are many problems.
