Grid-connected three-phase power electronic converters, equipped with an inductor-capacitor-inductor (LCL) filter, are widely used in energy production and consumption. The LCL filter effectively attenuates the switching harmonics of the converter; however, a drawback of the filter is its resonating behavior. The resonance can be damped by means of converter control, and the damping becomes easier and more effective, if the states of a dynamic full-order LCL-filter model are known (measured or estimated). The grid voltage is typically measured for converter control, but replacing voltage sensors with estimation may reduce system costs at low power ratings. Alternatively, a voltage estimation in parallel with the measurement increases system reliability. This thesis proposes estimation methods for grid-voltage sensorless control of converters equipped with an LCL filter. Only the converter AC currents and the DC-link voltage are measured for the control system. An adaptive observer is proposed for a combined state and grid-voltage estimation based on the full-order LCL-filter model. For unbalanced grid conditions, the observer is augmented with a disturbance model for the negative-sequence grid-voltage component. The nonlinear estimation-error dynamics of the observer are linearized and theoretically analyzed. The proposed observer is experimentally tested as a part of a grid-voltage sensorless control system, where the estimated states are applied in state-space current control. Based on the linearized dynamics, an analytic design procedure is presented for the observer in the continuous and discrete-time domains. The design procedure retains a link between the observer gains and dynamic performance, thus resulting in symbolic expressions for the gains as a function of the performance specifications and LCL-filter model parameters. The proposed observer can estimate the grid-voltage magnitude, frequency, and angle. In unbalanced grid conditions, the augmented observer can also estimate the negative-sequence component of the grid voltage. Estimated quantities can be used in the converter control system. The analytic design procedure enables the proposed estimation methods to be applied with different converters and LCL filters and further enables automatic tuning of the methods, for example, at the converter startup. The proposed methods can be applied, for example, in active-front-end rectifiers of motor drives or solar inverters.
|Publication status||Published - 2016|
|MoE publication type||G5 Doctoral dissertation (article)|
- adaptive observer, grid-connected converter, identification, LCL-filter, linearized models, sensorless, state-space control, voltage estimation