50Hz to 100kHz

In the fields of aircraft auxiliary power unit (APU) and unmanned aerial vehicle propulsion system, high-frequency motors have become an ideal choice due to their lightweight and high reliability characteristics. Micro gas turbines, flywheel energy storage systems, and other equipment utilize high-frequency motors to enhance energy conversion efficiency. High frequency motors can provide stronger suction or wind power while reducing noise and energy consumption. High frequency motor is the core component of electric vehicle drive motor, which can achieve high speed and high torque output, improve the power performance and range of the vehicle.

One of the core features of solid-state transformers nowadays is to achieve miniaturization and intelligence of the entire system by increasing the frequency, thereby reducing energy consumption losses. Solid state transformer with medium voltage input of 10/13.8kV 50/60Hz AC, low voltage output of 800VDC, intermediate frequency isolation and voltage reduction, with a capacity of up to 5000kVA.

High frequency transformers use high-frequency magnetic core materials such as ferrite to reduce volume and weight, while improving energy transmission efficiency. Suitable for high-frequency signal transmission (MHz level), with high isolation voltage. Winding voltage V=N × A × dB/dt, where N is the number of turns, A is the cross-sectional area of the magnetic core, and dB/dt is the rate of change in magnetic flux density. Under the condition of a constant winding voltage, increasing the switching frequency f leads to an increase in the time rate of change t and an increase in the magnetic flux density change rate dB/dt, resulting in a decrease in the cross-sectional area of the magnetic core and a reduction in the volume of the transformer.

When the frequency is increased from 50Hz to 100KHz, the frequency is increased by 2000 times, and the cross-sectional area of the magnetic core can be reduced to 1/2000 of the original (due to material loss limitations, the reduction ratio is usually between 1/50 and 1/200).  

A typical 60Hz 13.8kV transformer with a capacity of 1500kVA has a width of 1.65 meters, a depth of 1.2 meters, and a height of 1.5 meters. The outer shell size is relatively large, and the installation shell is equivalent to a small house. However, when the frequency is increased to 100kHz, the volume will be greatly reduced. Considering electrical insulation such as air clearance at the connection end, the size should be reduced to 0.8 meters wide, 0.4 meters deep, and 0.5 meters high.  

Although high frequency is good, switching equipment is a serious challenge. In order to increase frequency, 100/200Hz motors have been used in industry. For medium voltage and high frequency applications, the breaking capacity of vacuum circuit breakers generally deteriorates and cannot be used. Mechanical isolation switches and power electronic components are needed to operate and control high-frequency equipment. Or the circuit breaker is only used for isolation.  

The higher the frequency, the greater the electromagnetic loss. For switchgear, the high temperature and vibration caused by eddy currents should not be underestimated. Eddy currents are generated at the installation bolts of three-phase copper bars passing through plates and single-phase copper bars, and induced currents are also induced in steel plates close to conductive copper bars, resulting in eddy currents. Experimental results have shown that under 1250A 50Hz, the temperature rise of the wall sleeve installation plate is 30k, while increasing the frequency to 120Hz will result in a temperature rise of over 100k for the installation plate.

Therefore, for the copper busbar connection bolts of high-frequency and high current switchgear, it is recommended to use high-strength stainless steel bolts A4-80, and non-magnetic materials such as aluminum plates, copper plates, or stainless steel plates are recommended for the partitions, beams, etc. near the copper busbar. If iron partitions and crossbeams do not cause excessive temperature rise, they need to have sufficient strength and support to avoid vibration and noise caused by eddy currents.

Eddy current is related to frequency, and the higher the frequency, the greater the impact of eddy current. Eddy currents have a significant impact on switchgear. Even if non-magnetic material partitions are used where three-phase busbars pass through, and fake stainless steel plates are avoided, attention should be paid to the strength of the partitions. At the same time, steel partitions and cover plates made of surrounding magnetic materials should be kept as far away from the busbars as possible.

For high-frequency switching devices, considering the significant increase in skin effect, proximity effect, and eddy current effects, it is also necessary to use high current devices to replace low current applications. Usually, when the frequency increases by 1Hz, the sustained current decreases by 1%. Therefore, for 2000A@85Hz The application requires a continuous current of 2000/0.65=3077A at 50Hz, therefore it needs to be used 3150A@50Hz The circuit breaker is used for this application. For high-frequency applications, sufficient current margin must be left, and the cabinet should be made of stainless steel material to address the impact of high-frequency AC effects.

Medium voltage switchgear is designed for power frequency verification, and there is still a lack of testing measures for high-frequency applications. For example, the testing station is basically unable to conduct high-frequency breaking tests such as 100Hz, and other temperature rise tests cannot be verified.