In view of the influence of grounding capacitance to low frequency signal injection method, a new detection technology is proposed based on DC bus to grounding capacitance. This method does not affect the normal operation of DC power system. The insulation monitoring devices for DC power system is designed. A prototype test that it is able to monitor the capacitance value of the DC bus to grounding capacitance in real time, send out the alarm information with the larger grounding capacitance and improve the reliability of the DC system, which has the advantages of simple operation, accurate measurement and good stability.

As it is scant for the linear power amplifier and the two-level switching power amplifier, the principle is analyzed for the three-level power amplifier. It is proposed for the power amplifier for magnetic bearings based on the three-level PWM modulation technique. The circuit is designed for the implementation of the function. The two triangular carriers are formed by the triangular wave superimposing the positive and negative voltage bias, and then, the two drives are formed by comparing the current error signal and the two triangular carriers based on the voltage comparator. The experiment results show that the method can achieve the three-level work, reduce the output current ripple, and have good characteristics. The conclusions have some guiding significance to the design of the three-level power amplifier for magnetic bearings.