Sensorless active damping strategy for parallel interleaved voltage source power converters with LCL filter

Abstract

Grid-connected pulse-width modulation converters with LCL filters are widely extended as an interface for renewable energy generating systems. Those filters arise stability issues due to the filter resonance, which, must be damped, preferably by active damping methods, to avoid additional power losses. In some applications, such as offshore wind energy conversion systems, where high power converters are connected to low voltage networks, the commutation frequency is limited to low values to reduce the power losses and the resulting filter components are bulky. For this reason, in addition to its higher current handling capabilities, the use of power converters in parallel with the application of advanced modulation strategies, such as interleaving, is gaining importance. In this structure the filter components can be reduced, setting the filter resonance frequency at a higher value. As a consequence, the filter resonance frequency is close to the Nyquist frequency, complicating the implementation of some of the existing active damping solutions. In this work, an active damping approach based on a third order digital filter is proposed to stabilize the system. The robustness of the solution is investigated against grid inductance variations and the grid current harmonic content will be tested to comply with the most demanding grid codes. The proposed active damping strategy does not require extra sensors and can be easily designed by examining the open loop Bode plots. It is suitable to stabilize the LCL filter with a resonance frequency close to the Nyquist frequency, where some of the existing active damping approaches are unable to damp the system. The proposed method can be also an appropriate solution for existing grid connected power converters with stability issues due to changes in the effective grid impedance at which they are connected.