Robust active damping technique for three-phase PWM converter connected to power grid with unknown inductance

The PWM converter includes a low-pass filter in order to remove the high-frequency current that flows to the power grid. The low-pass filter is composed of two reactors and a capacitor (LCL-filter). The LCL-filter is the gain characteristic that attenuates by the high frequency band, and can eliminate the career ripple of the current. However, LCL-filter has a strong resonance characteristic. When the power grid current resonates with a high frequency, the equipments connected to the same power grid are harmfully affected. In order to control this resonance, passive damping and active damping are used. However, both methods suffer from several problems. A passive damping is a method of controlling the resonance by the damping resistance. The generation of heat by the loss of resistance becomes a problem often. The active damping is a control technology that controls the resonance without using the damping resistance. The active damping has both of the sensor method and the sensor-less method. The active damping methods with sensor require the use of an additional sensor. When active damping is used without a sensor, it becomes more sensitive to the power grid's inductance changes. Especially, in the case of an extreme increase in the power grid's inductance, problems with LCL resonance and ACR deterioration occur. When the ACR response is deteriorated, the power grid is affected by low frequency oscillation, making the power supply unstable. All equipment connected to this power grid is harmfully affected. There are few examples of low frequency oscillation in conventional active damping technologies. It is possible to control the resonance of the LCL-filter with the control approach proposed in this paper. Also, the deterioration in the current loop's response can be prevented, even if the power grid's inductance increases. And, thus, low frequency disturbances can be prevented. The proposed control method is achieved with tuning-less corresponding to the power grid's inductance.