J. Dannehl

Limitations of Voltage-Oriented PI Current Control of Grid-Connected PWM Rectifiers With

Voltage-oriented PI control of three-phase grid-connected pulsewidth-modulation rectifiers with LCL filters is addressed. LCL filters require resonance damping. Active resonance damping is state of the art to face the problem, but it is still under investigation because of the manifold solutions. It is often realized using many sensors and/or complex control algorithms. In contrast, pure PI control requires only one set of current sensors, and its implementation and design are rather simple and well known from the L filter control. PI control has already been shown to be a suitable solution also for LCL filters, but there are limitations. These are investigated in this paper. System stability is analyzed with respect to different ratios of LCL filter resonance and control frequencies. The latter are important parameters for system design and control. Both line and converter current control are analyzed. For a certain range of frequency ratios, the voltage-oriented PI control gives stable performance without additional feedback, but for ratios outside this range, stable operation is impossible. Experimental tests validate the theoretical results. In addition, an experimentally determined LCL filter transfer function is shown in this paper, which shows a lower resonance peak as expected from commonly used filter models.

LCL

Three-phase active rectifiers guarantee sinusoidal input currents and unity power factor at the price of a high switching frequency ripple. To adopt an LCL-filter, instead of an L-filter, allows using reduced values for the inductances and so preserving dynamics. However, stability problems can arise in the current control loop if the present resonance is not properly damped. Passive damping simply adds resistors in series with the LCL-filter capacitors. This simplicity is at the expense of increased losses and encumbrances. Active damping modifies the control algorithm to attain stability without using dissipative elements but, sometimes, needing additional sensors. This solution has been addressed in many publications. The lead-lag network method is one of the first reported procedures and continues being in use. However, neither there is a direct tuning procedure (without trial and error) nor its rationale has been explained. Thus, in this paper a straightforward procedure is developed to tune the lead-lag network with the help of software tools. The rationale of this procedure, based on the capacitor current feedback, is elucidated. Stability is studied by means of the root locus analysis in z-plane. Selecting the lead-lag network for the maximum damping in the closed-loop poles uses a simple optimization algorithm. The robustness against the grid inductance variation is also analyzed. Simulations and experiments confirm the validity of the proposed design flow.

Filters

This publication presents the investigation of active damping of resonance oscillations with virtual resistor for grid-connected PWM rectifiers with LCL-filter for different filter parameters. Using the voltage-oriented PI current control with converter current feedback, additional active damping of the filter resonance is necessary for stable operation. In the literature different methods are proposed that differ in number of sensors and complexity of control algorithms. If higher damping of the switching ripple current is required LCL-filters with lower resonance frequencies can be used. Resulting low ratios between resonance frequency and control frequency challenge the control with respect to damping of resonance. Moreover, some active damping methods are not suitable for these filter settings. Here the active damping concept based on virtual resistor is analyzed concerning stability for two significant filter configurations. It turns out that it is applicable for configurations with higher resonance frequency, whereas systems lower resonance frequencies can poorly be damped. Additionally the method exhibits the advantage of simple implementation but the disadvantage of additional current sensors. Theoretical analyses and of the selected method with time-discrete implementation are shown in this paper. Theory is verified by experimental results.

Systematic Design of the Lead-Lag Network Method for Active Damping in LCL-Filter Based Three Phase Converters

The control of a PWM rectifier with LCL-filter using a minimum number of sensors is analyzed. In addition to the DC-link voltage either the converter or line current is measured. Two different ways of current control are shown, analyzed and compared by simulations as well as experimental investigations. Main focus is spent on active damping of the LCL filter resonance and on robustness against line inductance variations.

Active damping of LCL-filter resonance based on virtual resistor for PWM rectifiers — stability analysis with different filter parameters

Flatness-based control is applied to the three-phase PWM-rectifier in synchronous reference frame. The DC-link voltage and reactive current are shown to be flat outputs of the full-order system. Two different approaches are presented. At first the DC-link voltage is controlled directly. The second employs inner current loops. Feed forward design based on system flatness is shown and discussed. In many applications the DC-link voltage and reactive current are controlled to constant values. In this case the direct flatness-based approach offers no advantages compared to conventional voltage-oriented PI-control whereas the second approach outperforms it with respect to the obtained control dynamic.

PWM rectifier with LCL-filter using different current control structures

This paper deals with different active damping approaches for PI-based current control of grid-connected PWM converters with LCL filters which are based on one additional feedback. Filter capacitor current as well as voltage feedback for purpose of resonance damping are analyzed and compared. Basic studies in continuous Laplace domain show that either proportional current feedback or derivative voltage feedback yield resonance damping. Detailed investigations of these two approaches in discrete z-domain taking into account the discrete nature of control implementation, sampling, and PWM are carried out. Several ratios of LCL resonance frequency and control frequency are considered. For high resonance frequencies only current feedback stabilizes the system. For medium resonance frequencies both approaches show good performance. For low resonance frequencies stability gets worse even if voltage feedback offers slighly better damping properties. Measurement results validate the theoretical results.

Flatness-based voltage-oriented control of three-phase PWM rectifiers

Pulse Width Modulated Voltage Source Converters (PWM-VSC) could be the universal interface for distributed power producers, loads, storage, power conditioning and transmission systems to the future smart grid. The differential mode filter used to connect the PWM-VSC to the electric grid has a crucial role in the proper operation of the converter and in filtering PWM switching harmonics. The paper aims to offer a complete characterization of differential mode filters with attention paid to the model of the inductors. Experimental results validate the theoretical study.

Investigation of active damping approaches for PI-based current control of grid-connected PWM converters with LCL filters

Interconnection and Damping Assignment Passivity-Based Control of grid-connected PWM converters with LCL-filter is designed and analyzed. Dynamic performance, resonance damping and robustness against grid impedance variations are studied by simulation and show good results. Experimental tests on a 22 kW induction motor drive prove the simulation results.