Abstract
Gridconnected threephase power electronic converters, equipped with an inductorcapacitorinductor (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 fullorder LCLfilter 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 gridvoltage sensorless control of converters equipped with an LCL filter. Only the converter AC currents and the DClink voltage are measured for the control system. An adaptive observer is proposed for a combined state and gridvoltage estimation based on the fullorder LCLfilter model. For unbalanced grid conditions, the observer is augmented with a disturbance model for the negativesequence gridvoltage component. The nonlinear estimationerror dynamics of the observer are linearized and theoretically analyzed. The proposed observer is experimentally tested as a part of a gridvoltage sensorless control system, where the estimated states are applied in statespace current control. Based on the linearized dynamics, an analytic design procedure is presented for the observer in the continuous and discretetime 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 LCLfilter model parameters. The proposed observer can estimate the gridvoltage magnitude, frequency, and angle. In unbalanced grid conditions, the augmented observer can also estimate the negativesequence 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 activefrontend rectifiers of motor drives or solar inverters.
Translated title of the contribution  Estimointimenetelmiä LCLsuodattimella varustettujen suuntaajien verkkojänniteanturittomaan säätöön 

Original language  English 
Qualification  Doctor's degree 
Awarding Institution 

Supervisors/Advisors 

Publisher  
Print ISBNs  9789526071800 
Electronic ISBNs  9789526071794 
Publication status  Published  2016 
MoE publication type  G5 Doctoral dissertation (article) 
Keywords
 adaptive observer
 gridconnected converter
 identification
 LCLfilter
 linearized models
 sensorless
 statespace control
 voltage estimation
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Aalto University Doctoral Thesis Award 2018
Kukkola, Jarno (Recipient) & Hinkkanen, Marko (Supervising professor), 2018
Prize: Award or honor granted for a specific work