Damping concepts of LCL filter for a multi-level medium voltage adjustable speed drive with low pulse-ratio

This paper presents the investigation of damping concepts of the resonance oscillation of an LCL-filter in a PWM inverter as a variable-speed-drive with low pulse-ratio. The system is designed for general-purpose operation and should be connected to different machines working in mediumvoltage with a power of 22 MVA. Low current THD is required on the machine side, which makes the use of a filter necessary. However, the oscillation resonance of the filter should be properly damped. The standard passive damping concept is not attractive because of high losses. Moreover, the widely-used active damping concepts using capacitor voltage or capacitor current as feedback are not applicable due to insufficient phase margin under low pulse ratios. An improved active damping concept based on a state estimator is proposed. It extends the use of active damping methods to the low-pulse ratio range. Theoretical analysis of the selected methods shows the technical trade-off regarding stability of the two active damping methods. The theory has been verified via simulations.