With the continuous development of power, electronics, and computer technologies, AC motor frequency conversion technology has been increasingly improved and rapidly developed. Low-voltage frequency converters (below 690V) with small and medium capacity have been widely used. Besides energy saving and consumption reduction, they have become indispensable key equipment in machinery, papermaking, metallurgy, petrochemicals, mining, building materials, and power industries. However, for medium and large capacity, high-voltage AC motor frequency conversion speed regulation, in addition to price factors, reasonable modification or design schemes, as well as overall investment and cost-effectiveness, have become factors restricting their widespread application.
1. Current Status of High-Voltage Motor Variable Frequency Speed Control Scheme
High-voltage, high-capacity variable frequency drives (VFDs) are a complex and technologically advanced technology. Their development and production face challenges due to several factors: the high supply voltage of high-voltage VFDs, coupled with the relatively low voltage withstand capabilities of power electronic components worldwide; and the high technical difficulty and demanding manufacturing requirements of high-voltage, high-power VFDs. However, they are primarily used for energy-saving operation in applications such as fans and pumps, requiring low initial investment, with the return on investment offset by energy savings.
2. Classification and characteristics of high-voltage frequency conversion schemes
2.1 High-Low-High Scheme
This solution uses a general-purpose low-voltage frequency converter as its core. Specifically, a step-down frequency converter is added to the input side of the low-voltage frequency converter to reduce the 6KV/3KV to an input voltage suitable for the low-voltage frequency converter; a step-up transformer is added to the input side of the frequency converter to match the output voltage with the rated voltage of the motor. This high-low-high frequency conversion speed control solution is common in the domestic market and is currently the most widely used, primarily featuring Siemens products. In this solution, the DC power supply rectifier unit can be divided into a 6-pulse rectifier unit and a 12-pulse rectifier unit. The 6-pulse rectifier unit has a simple system structure and lower hardware costs. However, the 12-pulse rectifier unit consists of a three-winding transformer with a secondary side phase difference of 30 degrees and two identical rectifier units. Furthermore, an input reactor must be connected to the secondary side of the transformer, resulting in a relatively higher cost. However, in terms of harmonic components, the 12-pulse rectification is more effective than the 6-pulse rectification. The harmonic components of the 6-pulse rectification are 5th, 7th, 11th, 13th, 17th, 19th, 23rd, and 25th, while those of the 12-pulse rectification are 11th, 13th, 23rd, and 25th.
This scheme is most suitable for renovation projects. Its advantages are: (1) The scheme is mature. Taking Siemens products as an example, the number of sets currently used in China has reached a considerable scale. (2) The original motor cables do not need to be modified. (3) It is easy to realize the switching function. (4) The investment is lower compared to the high-high scheme. Its disadvantages are: The system has more components than the high-low or high-high scheme. Due to the losses of filters and step-up and step-down transformers, the long-term operating costs are relatively high.
2.2 High-Low-Low Scheme
The core of the high-low-low scheme remains the low-voltage frequency converter, with a dual-winding step-down transformer added to the input side of the converter to convert the high-voltage motor to a usable low-voltage frequency converter drive voltage level. This scheme is currently widely used in China. Siemens products are frequently used. The system mainly consists of a dual-winding step-down frequency converter and a transformer. This system scheme has a relatively simple structure and is particularly suitable for new projects. For retrofit projects, replacing the motor would increase project costs. Its harmonic components are mainly at the 5th, 7th, 11th, 13th, 17th, 19th, 19th, 23rd, and 25th orders.
The advantages of this scheme are: (1) Mature scheme. Low-voltage IGBT or IGCT power devices are already very mature and have a wide range of applications. (2) Simple structure and few maintenance links. (3) Low investment and quick results. Low-voltage frequency converters are inexpensive and the market price is quite transparent. (4) Low loss of step-up transformer and low long-term operating costs. The disadvantage of this scheme is that when used for renovation projects, the motor needs to be replaced, which increases the workload of the renovation, thereby increasing the project investment and extending the construction period.
2.3 High-High Scheme
A high-voltage to high-voltage frequency converter is a device that supplies high-voltage power directly to the high-voltage motor after passing through a high-voltage switchgear. After high-voltage frequency conversion, the voltage is directly supplied to the stator of the high-voltage motor.
There are four main types of high-voltage variable frequency speed control systems: (1) series connection of power components (GTO, SCR, SGCT); (2) neutral point clamping type, also known as three-point flat type (GTO, IGBT); (3) multi-level type, among which the commonly used ones are four-level, five-level, etc. (IGBT); (4) series connection of single-phase inverters. These high-voltage frequency converters have been launched in recent years and are technologically advanced, highly efficient, and occupy a small area, making them the main development direction of high-voltage frequency converters.
The high-frequency IGBT scheme employs a series connection of IGBT units, utilizing the principle of phase-shifting superposition (phase-shifting transformer) to effectively suppress harmonic interference, resulting in an output waveform that is nearly sinusoidal. It offers the following advantages: no need for additional output filtering or power compensation devices; efficiency up to 97%; high power factor (greater than 95% with a load >20%); no harmonic-induced pulsating torque, avoiding load resonance and extending the lifespan of motors and mechanical equipment; protection of the main motor and cable insulation from dv/dt stress damage; motor unaffected by common-mode voltage; no length limitations for motor cables within the allowable voltage drop range; no pollution to the power grid; high reliability of main components due to redundant design, adaptable to extreme operating conditions; automatic bypass function of the power unit, improving power supply reliability; and the ability to operate without stopping during momentary power outages (within 5 seconds). The inverter will not stop until the power supply voltage recovers, immediately re-accelerating and returning to its original state. When the power supply is interrupted, the frequency converter will not stop operating within 300ms.
3. Selection principles for high-voltage frequency converters
When selecting a high-voltage frequency converter, a comprehensive comparison of converters from various manufacturers should be made. Do not be misled by advertising or inaccurate articles, and do not be influenced by habitual biases. When selecting a high-voltage, high-capacity frequency converter, the following principles should be considered: First, reliability and after-sales service; second, price; third, key technical specifications; and finally, delivery time and the manufacturer's reputation.
4. Conclusion
Variable frequency speed control devices are high-tech mechatronic products. Looking at the development process and trends of frequency converters, power devices remain the primary representative. Variable frequency speed control boasts high efficiency, a wide operating range, and high precision, and is available in a complete range of specifications to meet diverse needs. Its wide applicability makes it the most promising and ideal speed control method.
