Alexander Micallef

Reactive Power Sharing and Voltage Harmonic Distortion Compensation of Droop Controlled Single Phase Islanded Microgrids

When paralleling multiple inverters that are capable of operating as an island, the inverters typically employ the droop control scheme. Traditional droop control enables the decentralized regulation of the local voltage and frequency of the microgrid by the inverters. The droop method also enables the inverters to share the real and reactive power required by the loads. This paper focuses on some of the limitations of parallel islanded single phase inverters using droop control. Algorithms with the aim to address the following limitations in islanded operation were proposed: reactive power sharing and reduction of the voltage harmonic distortion at the point of common coupling (PCC). Experimental results were then presented to show the suitability of the proposed algorithms in achieving reactive power sharing and in improving the voltage harmonic distortion at the PCC.

Single-Phase Microgrid With Seamless Transition Capabilities Between Modes of Operation

Microgrids are an effective way to increase the penetration of distributed generation into the grid. They are capable of operating either in grid-connected or in islanded mode, thereby increasing the supply reliability for the end user. This paper focuses on achieving seamless transitions from islanded to grid-connected and vice versa for a single phase microgrid made up from voltage controlled voltage source inverters (VC-VSIs) and current controlled voltage source inverters (CC-VSIs) working together in both modes of operation. The primary control structures for the VC-VSIs and CC-VSIs is considered together with the secondary control loops that are used to synchronize the microgrid as a single unit to the grid. Simulation results are given that show the seamless transitions between the two modes without any disconnection times for the CC-VSIs and VC-VSIs connected to the microgrid.

Secondary control for reactive power sharing in droop-controlled islanded microgrids

This paper focuses on the islanded operation of microgrids. In this mode of operation, the microsources are required to cooperate autonomously to regulate the local grid voltage and frequency. Droop control is typically used to achieve this autonomous voltage and frequency regulation. However, droop control has real and reactive power sharing limitations when there are mismatches between the microsources. This paper analyses the effect due to mismatches in the power line impedances connecting the source inverters to the microgrid. From the simulations results obtained, it was shown that the reactive power demand is unequally shared between the microsource inverters when there are mismatches between the power line impedances. To achieve equal reactive power sharing between the inverters, an external loop requiring low bandwidth communications was implemented in a central controller. Simulation results are presented showing the feasibility of the proposed solution in achieving reactive power sharing between the inverters connected to the microgrid.

Secondary control for reactive power sharing and voltage amplitude restoration in droop-controlled islanded microgrids

This paper focuses on the islanded operation of microgrids. In this mode of operation, the microsources are required to cooperate autonomously to regulate the local grid voltage and frequency. Droop control is typically used to achieve this autonomous voltage and frequency regulation. However, droop control has real and reactive power sharing limitations between the microsource inverters when there are mismatches between the output filter components and power line impedances. In this paper, secondary control loops were implemented to achieve equal reactive power sharing between the inverters and to restore the voltage deviations caused by the droop control. Primary droop control loops where implemented in the inverters to supply the real and reactive power. Simulation results are presented showing the feasibility of the proposed algorithm in achieving reactive power sharing between the inverters connected to the microgrid while simultaneously restoring the voltage deviations due to the droop control.

Mitigation of Harmonics in Grid-Connected and Islanded Microgrids Via Virtual Admittances and Impedances

Optimization of the islanded and grid-connected operation of microgrids is important to achieve a high degree of reliability. In this paper, the authors consider the effect of current harmonics in single phase microgrids during both modes of operation. A detailed analysis of the effect of the output impedance of the considered primary control loops on the harmonic output of the considered voltage source inverters is initially carried out. A virtual admittance loop is proposed to attenuate the current harmonic output in grid-connected operation that is generated due to the grid voltage distortion present at the point of common coupling (PCC) and due to local non-linear loads. This paper also considers the harmonic current sharing and resulting voltage harmonics at the PCC during islanded operation of the microgrid. A capacitive virtual impedance loop was implemented to improve the harmonic current sharing and attenuate the voltage harmonics at the PCC. Experimental results are given to validate the operation of the proposed algorithms.

Selective virtual capacitive impedance loop for harmonic voltage compensation in islanded MicroGrids

Parallel inverters having LCL output filters cause voltage distortions at the point of common coupling (PCC) in islanded microgrids when non-linear loads are present. A capacitive virtual impedance loop could be used to provide selective harmonic compensation in islanded microgrids, instead of introducing additional active or passive filters into the system that could compromise the stability of the microgrid. However, the performance of these compensation loops becomes degraded when a virtual resistance is introduced with the aim to improve the overall stability of the parallel inverters. With the capacitive virtual impedance, there is effectively a compromise between the additional stability provided by the virtual resistance and the harmonic compensation due to the virtual capacitance. This paper focuses on overcoming this limitation of the capacitive virtual impedance with additional virtual resistance for selective harmonic compensation in islanded microgrids. Simulation results were given to show the suitability of the proposed algorithms in reducing the voltage harmonics at the PCC.

Cooperative control with virtual selective harmonic capacitance for harmonic voltage compensation in islanded microgrids

This paper focuses on the islanded operation of microgrids. In this mode of operation, the microsources are required to cooperate autonomously to regulate the local grid voltage and frequency. Droop control is typically used to achieve this autonomous voltage and frequency regulation. Inverters having LCL output filters would cause voltage distortion to be present at the PCC of the local load when non-linear current is supplied to the load due to the voltage drop across the grid side inductor. Techniques to reduce the output voltage distortion typically consist of installing either passive or active filters to selectively compensate harmonic frequencies. However, by adding suitable control strategies to the inverters connected to the microgrid, the power quality of the microgrid can be improved without the installation of any additional equipment. In this paper, a capacitive virtual impedance loop, implemented in each of the microsource inverters, is proposed so as to dampen the voltage harmonics at the PCC of the local load. Simulation results are presented showing the suitability of the proposed algorithm in dampening the PCC voltage harmonics.

Performance comparison for virtual impedance techniques used in droop controlled islanded microgrids

Droop control has limitations with respect to current sharing since the output current delivered by the inverters depends on their output impedance ratios. In addition, harmonic voltage drops due to the flow of harmonic currents induce voltage distortion at the point of common coupling (PCC). Virtual impedance loops were proposed in literature to improve the current sharing between the inverters by normalizing the output impedance of the inverters. However, virtual impedance loops have constraints in this respect since the improvement in the current sharing occurs through redistribution of the current harmonics which can add to the voltage distortion at the PCC. This paper compares the performance of resistive, inductive, inductive-resistive and resistive-capacitive virtual impedance loops with respect to current sharing and voltage harmonic distortion at the PCC. Simulation results are given for a single phase microgrid setup to achieve a fair performance comparison of the different virtual impedance techniques.

Modeling of an EMC test-bench for conducted emissions in solid state applications

In the area of EMC performance of electric motor drives, research is mainly focused on the EMC performance of inverter based drives. However, in most instances the soft starter is still an appropriate choice for motor control especially when accurate speed control of the load is not a specific requirement. The EMI generation of these converters has not been given the required importance, with only a few publications regarding this issue available in the last decades. A simulation model that considers the operation of solid state devices in an EMC conducted emission measurement laboratory environment was developed. The model was implemented in MATLAB® and Simulink® using a time domain approach. Models for the LISN, EMI receiver, power cables, thyristor power modules and induction motor load were developed and implemented. These models were developed so as to reflect the actual physics of the components and, where possible, model parameters were obtained through analysis of the geometry of the system through simplifications of the surrounding environment. The simulation was then used to gain insight on the EMI generation mechanisms of the solid state system.

MPPT with current control for a PMSG small wind turbine in a grid-connected DC microgrid

This paper presents a Maximum Power Point Tracking (MPPT) system for a small wind turbine (SWT) connected to a DC Microgrid under grid-connection conditions. The system consists of a Permanent Magnet Synchronous Generator (PMSG) driven by a SWT which is interfaced to the DC microgrid through a rectification stage and boost converter. The proposed MPPT system is based on the relationship between the DC link power and voltage, which are used to obtain the required inductor current in the Boost converter to provide maximum power output at all wind speeds. The MPPT system, the Boost converter and current control are explained in detail. Simulation of the system operating under varying wind conditions is presented, showing the performance of the developed MPPT algorithm.