Christian Wessels

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

The application of a dynamic voltage restorer (DVR) connected to a wind-turbine-driven doubly fed induction generator (DFIG) is investigated. The setup allows the wind turbine system an uninterruptible fault ride-through of voltage dips. The DVR can compensate the faulty line voltage, while the DFIG wind turbine can continue its nominal operation as demanded in actual grid codes. Simulation results for a 2 MW wind turbine and measurement results on a 22 kW laboratory setup are presented, especially for asymmetrical grid faults. They show the effectiveness of the DVR in comparison to the low-voltage ride-through of the DFIG using a crowbar that does not allow continuous reactive power production.

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.

Fault Ride-Through of a DFIG Wind Turbine Using a Dynamic Voltage Restorer During Symmetrical and Asymmetrical Grid Faults

The stability of fixed-speed induction generator (FSIG)-based wind turbines can be improved by a StatCom, which is well known and documented in the literature for balanced grid voltage dips. Under unbalanced grid voltage dips, the negative-sequence voltage causes heavy generator torque oscillations that reduce the lifetime of the drive train. In this paper, investigations on an FSIG-based wind farm in combination with a StatCom under unbalanced grid voltage fault are carried out by means of theory, simulations, and measurements. A StatCom control structure with the capability to coordinate the control between the positive and the negative sequence of the grid voltage is proposed. The results clarify the effect of the positive- and the negative-sequence voltage compensation by a StatCom on the operation of the FSIG-based wind farm. With first priority, the StatCom ensures the maximum fault-ride-through enhancement of the wind farm by compensating the positive-sequence voltage. The remaining StatCom current capability of the StatCom is controlled to compensate the negative-sequence voltage, in order to reduce the torque oscillations. The theoretical analyses are verified by simulations and measurement results on a 22-kW laboratory setup.

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

A transformer based voltage sag generator to test renewable energy systems is presented. Design considerations concerning overvoltages and overcurrents during switching action are adressed. Measurement results of a 30 kW laboratory prototype testing a line side PWM converter and a DFIG are presented. Both measurement results for investigations on the transformer based sag generator at the line side converter and the DFIG show the superb functionality of the sag generator. Voltage dips of variable magnitude, duration time and fault type can be generated to perform certification of LVRT capability. The device is also able to perform HVRT tests.

StatCom control at wind farms with fixed-speed induction generators under asymmetrical grid faults

Low Voltage Ride Through is an important feature for wind turbine systems to fulfill grid code requirements. In case of wind turbine technologies using doubly fed induction generators the reaction to grid voltage disturbances is sensitive. Hardware or software protection must be implemented to protect the converter from tripping during severe grid voltage faults. In this paper two methods for low voltage ride through of symmetrical grid voltage dips are investigated. As a basis, an analysis of the rotor voltages during grid fault is given. First, the conventional hardware method using a crowbar is introduced. Then the stator current reference feedback solution is presented. Both methods are investigated and compared by simulation results using 2 MW wind turbine system parameters. Measurement results on a 22 kW laboratory DFIG test bench show the effectiveness of the proposed control technique.

Transformer based voltage sag generator to perform LVRT and HVRT tests in the laboratory

The application of a Dynamic Voltage Restorer connected to a wind turbine driven doubly fed induction generator (DFIG) is investigated. The setup allows the wind turbine system an uninterruptible fault ride through of voltage dips. The Dynamic Voltage Restorer can compensate the faulty line voltage while the DFIG wind turbine can continue its nominal operation as demanded in actual grid codes. Simulation results for a 2 MW wind turbine and measurement results on a 22 kW laboratory setup show the effectiveness of the Dynamic Voltage Restorer in comparison to the low voltage ride through of the DFIG using a crowbar which does not allow continuous reactive power production.

Fault ride through of DFIG wind turbines during symmetrical voltage dip with crowbar or stator current feedback solution

The growing amount of electric energy generated from distributed energy resources (DER), mainly of renewables, requires their appropriate integration into the electrical grid. These distributed renewable power sources are mainly feeding-in the power via converters and can be controlled well. Investigation of their grid integration means investigating their control and hardware concept in the dedicated grid environment. Measurements at downscaled systems in a laboratory environment are a time- and cost-efficient way to investigate this. The properties of the electrical grid, voltage amplitude, frequency and phase as well as the grid impedance can vary substantially by time, the grid impedance moreover is frequency dependent. These properties have to be emulated for the measurements of the grid integration. A dedicated research laboratory test-bench has been developed. The laboratory comprises special developed systems for grid voltage emulation, sag generation, grid impedance emulation, grid voltage correction and grid impedance analysis. Here, the design and realization of these systems in the laboratory test-bench for investigations on grid integration of distributed renewable energy resources are presented. Representative measurements for each system prove their functionality.

Dynamic voltage restorer to allow LVRT for a DFIG wind turbine

The stability of fixed speed wind turbines can be improved by a StatCom, which is well known and documented in literature for balanced grid voltage dips. Under unbalanced grid voltage dips the negative sequence voltage causes additional generator torque oscillations. In this paper investigations on a fixed speed wind farm with squirrel cage induction generators directly connected to the grid in combination with a StatCom under unbalanced grid voltage fault are given by means of theory and simulations. A StatCom control structure with the capability to control the positive or negative sequence of the voltage is used. The simulation results clarify the effect of positive and negative sequence voltage compensation by a StatCom on the operation of a fixed speed wind farm. The influence of different StatCom control targets is investigated that lead to an increased network voltage stabiliy and lower torque pulsations.