Ali Mortezaei

Cooperative Control of Multi-Master–Slave Islanded Microgrid With Power Quality Enhancement Based on Conservative Power Theory

Cooperative control of power converters in a microgrid offers power quality enhancement at sensitive load buses. Such cooperation is particularly important in the presence of reactive, nonlinear, and unbalanced loads. In this paper, a multi-master–slave-based control of distributed generators interface converters in a three-phase four-wire islanded microgrid using the conservative power theory (CPT) is proposed. Inverters located in close proximity operate as a group in master–salve mode. Slaves inject the available energy and compensate selectively unwanted current components of local loads with the secondary effect of having enhanced voltage waveforms while masters share the remaining load power autonomously with distant groups using frequency droop. The close proximity makes it practical for control signals to be communicated between inverters in one group with the potential to provide rapid load sharing response for mitigation of undesirable current components. Since each primary source has its own constraints, a supervisory control is considered for each group to determine convenient sharing factors. The CPT decompositions provide decoupled current and power references in abc-frame, resulting in a selective control strategy able to share each current component with desired percentage among the microgrid inverters. Simulation results are presented to demonstrate the effectiveness of the proposed method.

PQ, DQ and CPT control methods for shunt active compensators — A comparative study

This paper investigates three different control techniques for controlling shunt active compensators for harmonic, reactive power and unbalance compensation. The considered methods were the instantaneous real and imaginary power theory (PQ), the Conservative Power Theory (CPT) and the synchronous reference frame method (DQ). So, the major goal is to explore the main similarities and differences in terms of steady state performance, dynamic response and computational complexity of each compensation method. Additionally, the paper also discusses how to achieve selective compensation by means of every technique, by means of suitable power and current decompositions. A three-phase grid with unbalanced non-linear load and non-negligible line impedance was used for comparing the approaches under ideal and deteriorated voltage conditions.

Power Quality Enhancement for a Grid Connected Wind Turbine Energy System

A comprehensive control of a wind turbine system connected to an industrial plant is discussed in this paper, where an algorithm has been developed allowing a control structure that utilizes a four-leg inverter connected to the grid side to inject the available energy, as well as to work as an active power filter mitigating load current disturbances and enhancing power quality. A four-wire system is considered with three-phase and single-phase linear and nonlinear loads. During the connection of the wind turbine, the utility-side controller is designed to compensate the disturbances caused in presence of reactive, nonlinear, and/or unbalanced single- and intra-phase loads, in addition to providing active and reactive power as required. When there is no wind power available, the controller is intended to improve the power quality using the dc-link capacitor with the power converter attached to the grid. The main difference of the proposed methodology with respect to others in the literature is that the proposed control structure is based on the conservative power theory decompositions. This choice provides decoupled power and current references for the inverter control, offering very flexible, selective, and powerful functionalities. Real-time software benchmarking has been conducted in order to evaluate the performance of the proposed control algorithm for full real-time implementation. The control methodology is implemented and validated in hardware-in-the-loop based on the real time simulator “Opal-RT” and a TMSF28335 DSP microcontroller. The results corroborated our power quality enhancement control and allowed to exclude passive filters, contributing to a more compact, flexible, and reliable electronic implementation of a smart-grid based control.

Cooperative operation based master-slave in islanded microgrid with CPT current decomposition

In multi-inverter-based islanded microgrids with unbalanced and distorted loads, due to voltage drop across line, the load bus voltage is unbalanced and distorted, which shows the requirement for power quality increase at load bus. To address this problem, this paper presents master-slave-based cooperative operation of DGs with the implementation of Conservative Power Theory (CPT), in which in addition to load sharing, load voltage quality is also improved to standard limits. Sharing factors for each current component are determined by a supervisory control considering constraints for instance availability of primary sources, converter rating and structure and switching frequency. The control strategy is implemented in abc frame. This remarkably facilitates the controller design procedure, offering a very flexible, selective and powerful load current sharing scheme for the DG inverter units, and the effectiveness of this scheme is demonstrated through digital simulations conducted by means of PSIM.

Coordinated operation in a multi-inverter based microgrid for both grid-connected and islanded modes using conservative power theory

In order to address the power quality enhancement in a given microgrid, independent control of reactive, unbalanced and distorted load current components, in a multi-inverter-based approach may be required. Therefore, this paper proposes a coordinated control strategy using the Conservative Power Theory (CPT) and assuming the contribution of all grid connected interfaces, especially those related to Distributed Generators (DG). For grid-connected operation, a coordinated current-controlled operation is proposed for supplying different load current components, improving power quality and consequently, minimizing the PCC voltage disturbances. In case of islanding operation, the microgrid may suffer from serious power quality problems due to the presence of nonlinear and/or unbalanced loads. To mitigate such current disturbances, a master-slave-based cooperative operation for DGs is presented. Since each primary source has its own constraints and topology, a supervisory control is considered to determine sharing factors for each current component. The microgrid system is controlled in abc-frame, offering a very flexible, selective and powerful load current sharing scheme, as demonstrated through digital simulations. The principles supporting the proposed control schemes are also discussed.

Multifunctional strategy for four-leg grid-tied DG inverters in three-phase four-wire systems under symmetrical and asymmetrical voltage conditions

This paper investigates connecting renewable energy sources such as solar, wind and the others to the microgrids through power electronic interfaces for active power provision and also harmonic, unbalance, and reactive power compensation under both symmetrical and asymmetrical voltage sources. The four-leg converter topology is applied because of its better controllability in four-wire systems applications. Two instantaneous current decomposition techniques, namely conservative power theory and instantaneous power theory are studied to show the similarities and differences between these techniques from performance perspective in active power filter. The studied system is a three-phase four-wire grid which includes unbalanced and non-linear loads, and has negligible line impedances. The control strategy is implemented in abc-frame which allows for selective load current provision by distributed generators interface converters. The principles supporting the developed control strategy are discussed and the results are demonstrated through digital simulations conducted by means of PSIM.

Multifunctional control strategy for asymmetrical cascaded H-Bridge Inverter in microgrid applications

A multitask Asymmetrical Cascaded H-Bridge Multilevel Inverter (ACHMI), suitable for microgrid systems with possible unbalanced and nonlinear loads, is presented. The primary advantage of ACHMI is to produce a staircase output voltage utilizing unequal DC voltages such as Solar cells, fuel cells, batteries on the individual H-bridge cells. The ACHMI provide a large number of output steps without increasing the number of DC voltage sources and components where the difference between output waveform and reference sinusoidal waveform would be reduced. For grid connected mode of operation, the control strategy is based on the Conservative Power Theory (CPT), providing simultaneous functionalities for the Distributed Generation (DG) system to inject its available energy, compensate the load current distortions and allow a smooth transition between grid-connected and islanded modes of operation. For the islanded mode of operation, regulation of load voltage in a wide range of load conditions is presented. The Conservative Power Theory decompositions provides decoupled power and current references for the inverter control in abc-frame, offering a very flexible, selective and powerful strategy for the DG control system. The principles supporting the developed control strategy are discussed and analyzed and the effectiveness of the control is demonstrated through digital simulations conducted by means of PSIM.

5-level Cascaded H-Bridge Multilevel microgrid Inverter applicable to multiple DG resources with power quality enhancement capability

This paper presents a 5-level Cascaded H-Bridge Multilevel Inverter (CHMI) able to perform several tasks and applicable to a microgrid with multiple local power sources. The current control strategy is based on the conservative power theory (CPT) generating references for different modes of operation. The primary merits of multilevel cascade voltage source inverters are the possibility of having independent DC sources, such as fuel cells, solar cells and batteries and, the reduced dc link voltage compare to the traditional 2-level inverters. Hence, the voltage stresses and switching losses are also decrease. In addition to injecting the available energy from DC sources into the grid, the CHMI has also being used to compensate voltage and current disturbances and to improve the power quality at point of common coupling. Additionally, the paper presents the analysis and design of cascaded voltage control scheme based on capacitor filter current feedback to regulate load voltage in islanded operation mode. The principles supporting the proposed control strategy are presented and the inverter performance is demonstrated through digital simulations using PSIM.

A multi task microgrid inverter based instantaneous Power Theory in islanded and grid-connected modes

This paper develops a flexible control strategy for DG inverters in a microgrid application with load current decomposition based on Instantaneous Power Theory (PQ). For grid connected mode of operation, the control strategy simultaneously supply all or part of the load current components allowing the DG system to inject its available energy, as well as to work as an active power filter, mitigating load current disturbances and improving power quality. Being able to supply the total load current by inverter allows a smooth transition between grid-connected and islanded modes of operation with zero current injection by the grid to guarantee an uninterrupted power supply to the critical loads within the microgrid. For the islanded mode of operation a multi loop control method with preservation of the grid-connected control system is developed to regulate load voltage/frequency in a wide range of load conditions. The control strategy is implemented in abc-frame. The principles supporting the developed control strategy are discussed and analyzed offering a very flexible, selective and powerful strategy for the DG control system, and the effectiveness of the control is demonstrated through digital simulations conducted by PSIM.

Power quality achievement using grid connected converter of wind turbine system

In this article a comprehensive control of a wind turbine system connected to an industrial plant is developed. The algorithm utilizes a structure of the back-to-back converter where two modes of operation can be achieved. During the connection of the wind turbine (mode I), the utility side controller is designed to compensate the harmonics caused by the nonlinear load, in addition to the active and reactive power delivery. When there is no wind power available (mode II), the controller is intended to only compensate the harmonics using the dc-link with the power converter attached to the grid. The proposed control structure operates in the defined two modes with no transition required. The harmonics reference point was extracted using two different power theories for the purpose of performance comparison. Simulation was conducted to evaluate the performance of the proposed control algorithm. The results showed power quality enhancement without the use of passive filters.