Jarno Kukkola

Observer-Based State-Space Current Controller for a Grid Converter Equipped With an LCL Filter: Analytical Method for Direct Discrete-Time Design

State-space current control enables high dynamic performance of a three-phase grid-connected converter equipped with an LCL filter. In this paper, observer-based state-space control is designed using direct pole placement in the discrete-time domain and in grid-voltage coordinates. Analytical expressions for the controller and observer gains are derived as functions of the physical system parameters and design specifications. The connection between the physical parameters and the control algorithm enables automatic tuning. Parameter sensitivity of the control method is analyzed. The experimental results show that the resonance of the LCL filter is well damped, and the dynamic performance specified by direct pole placement is obtained for the reference tracking and grid-voltage disturbance rejection.

Observer-based state-space current control for a three-phase grid-connected converter equipped with an LCL filter

This paper presents a state-space current control method for active damping of the resonance frequency of the LCL filter and setting the dominant dynamics of the converter current through direct pole placement. A state observer is used, where upon additional sensors are not needed in comparison with the conventional L filter design. The relationship between the system delay and instability caused by the resonance phenomenon is considered. Nyquist diagrams are used to examine the parameter sensitivity of the proposed method. The method is validated with simulations and experiments.

State Observer for Grid-Voltage Sensorless Control of a Converter Equipped With an LCL Filter: Direct Discrete-Time Design

Synchronization with the power system is an essential part of control of grid-connected converters. This paper proposes a grid-voltage sensorless synchronization and control scheme for a converter equipped with an LCL filter, measuring only the converter currents and the dc voltage. A discrete-time pole-placement design method is used to formulate an adaptive full-order observer for the estimation of the frequency, angle, and magnitude of the grid voltage. The proposed discrete-time design method enables straightforward implementation and suits low sampling rates better than its continuous-time counterpart. The analytically derived design is experimentally validated, and the results demonstrate rapid convergence of the estimated quantities. Moreover, the experimental tests show that grid-voltage sensorless operation is possible under balanced and unbalanced grid disturbances and distorted grid conditions.

State observer for grid-voltage sensorless control of a grid-connected converter equipped with an LCL filter

AbstractThis paper proposes a simple grid-voltage sensorless alternative to the conventional PLL synchronization for a grid-connected converter equipped with an LCL filter. The grid-voltage magnitu...

State observer for sensorless control of a grid-connected converter equipped with an LCL filter: Direct discrete-time design

Synchronization with the power system is an essential part of control of grid-connected converters. This paper proposes a grid-voltage sensorless synchronization and control scheme for a grid-connected converter equipped with an LCL filter, measuring only the converter currents and the DC voltage. A discrete-time pole-placement design method is used to formulate an adaptive full-order observer for estimation of the angle and magnitude of the grid voltage. The analytically derived design is experimentally validated, and the results demonstrate rapid convergence of the estimated angle and magnitude.

Observer-based state-space current controller for a grid converter equipped with an LCL filter: Analytical method for direct discrete-time design in synchronous coordinates

State-space current control enables high dynamic performance of a three-phase grid-connected converter equipped with an LCL filter. In this paper, observer-based state-space control is designed using complex-valued modeling and direct pole placement in the discrete-time domain and in grid-voltage coordinates. The proposed observer reduces the number of measurements in control, and retained connection between the physical parameters and the control algorithms enables automatic tuning. Parameter sensitivity of the control method is analyzed, and experimental results show that the resonance of the LCL filter is well damped.

Grid-voltage sensorless control of a converter under unbalanced conditions: On the design of a state observer

This paper deals with grid-voltage sensorless synchronization and control under unbalanced grid conditions. A three-phase grid-connected converter equipped with an LCL filter is considered, and no other signals than the converter currents and the DC-link voltage are measured for control. An augmented adaptive state observer is proposed for estimation of the positive- and negative-sequence components of the grid voltage. The proposed observer is tested as a part of a sensorless control system. Experimental results show that the proposed method works well even in highly unbalanced grid conditions.

Parameter estimation of an LCL filter for control of grid converters

Model-based control techniques are frequently used with grid-connected converters. This paper proposes a method for identifying model parameters of an LCL filter connected to a grid converter for control purposes. In identification, a discrete-time autoregressive moving average with exogenous input (ARMAX) model structure is used and closed-loop current control is considered. The resulting discrete-time model parameters are translated into the continuous-time physical parameters (inductance and capacitance values of the filter) by comparing the estimated discrete-time model with the analytical discrete-time model. Simulation and experimental results show that the proposed method yields good parameter estimates that are suitable for control tuning.

State Observer for Grid-Voltage Sensorless Control of a Converter Under Unbalanced Conditions

This paper deals with grid-voltage sensorless synchronization and control under unbalanced grid conditions. A three-phase grid-connected converter equipped with an LCL filter is considered, and no signals other than the converter currents and the dc-link voltage are measured for control. An augmented adaptive state observer is proposed for estimation of the positive- and negative-sequence components of the grid voltage. Dynamic performance limitations of the proposed method and effects of parameter errors are analyzed. The proposed observer is tested as part of a sensorless control system. Experimental results show that the proposed method works well even under highly unbalanced grid conditions.

Plug-In Identification Method for an LCL Filter of a Grid Converter

This paper proposes a method for estimating two inductances and capacitance of an LCL filter, connected between a converter and a grid. Only the dc-bus voltage and converter phase currents need to be measured. An excitation signal is fed into the converter voltage reference. The fundamental and selected harmonic components are removed from the stored identification data to prevent biases in the parameter estimates. The parameters of the hold-equivalent discrete-time model are estimated recursively. The inductance and capacitance estimates are calculated from the estimated discrete-time parameters using the corresponding closed-form model. The proposed method can be added to the existing converter control algorithms. It can provide the parameter estimates for the converter control. Further, it can be run occasionally during the normal operation in order to obtain an updated grid inductance estimate. Experimental results show that the proposed method yields very good parameter estimates and that it can also detect the changes in the grid inductance.