Multiple Distributed Generation Systems Research Articles

A Novel Islanding Detection Method for Distributed PV System Based on μPMUs

With the rapid development of distributed PV systems in distribution grids, unintentional islanding is one major concern which maybe cause safety hazards. The existing passive islanding detection methods are mainly based on the voltage and frequency deviations during islanding conditions. Passive methods have relatively large non-detection zone (NDZ) and susceptible to power system faults. Active islanding detection methods applied in inverters have relatively small non-detection zone, but they are usually harmful to power quality. This paper presents a new islanding detection method based on equivalent inter-connection line impedance, which is normally a small value. A measured impedance is calculated with the synchronous phasors from two lPMUs which are installed in a utility substation and a PV system. Under the grid-connected condition, the measured impedance is no more than the equivalent line impedance. While under the islanding condition, the measured impedance is much larger than the equivalent line impedance. So, an islanding condition can be effectively detected by the comparison of the measured impedance with the equivalent line impedance. The non-detection zone of this proposed method is nearly zero. The detection time is normally less than 250ms. This method is applicable for PV systems connected to the power system with a distribution transformer. It is also applicable for a distribution grid with multiple distributed generation systems.

Control of Grid-Tied Multiple Distributed Generation Systems With Cooperative Compensation Capabilities

This article deals with a cooperative controlof multiple distributed generation (DG) systems with power quality (PQ)-enhancing capabilities. This control algorithm is realized using an adaptive harmonic signals extraction method based on variable weight mixed norm least mean fourth–least mean square adaptive detection. This adaptive technique extracts all the necessary information required for the development of the proposed cooperative control algorithm. With this control algorithm, the voltage source inverters (VSIs) of the DG systems contribute to PQ improvement by mitigating harmonics and reactive currents in the distribution system. This algorithm operates the VSIs in maximum power generation and total harmonic distortion (THD) improvement modes. In maximum power generation mode, the algorithm utilizes the remaining capacity of the VSIs for compensation of nonactive currents drawn by the dedicated loads. Upon finishing the dedicated loads’ compensation, the remaining capacity is spent for compensation of the loads dedicated to other DG systems that are not in the position to render full compensation services. In THD improvement mode, the algorithm prioritizes compensation of harmonic currents generated by the loads. The efficacy of the proposed cooperative control algorithm for multiple DG systems is demonstrated via simulation and hardware-in-the-loop-based experimental results.

A Procedure to Determine the Droop Constants of Voltage Controllers Coping with Multiple DG Interactions in Active Distribution Systems

In modern distribution systems, the presence of an increasing amount of Distributed Generation (DG) systems causes over-/under-voltage problems, due to the reverse power flows. To face these problems, the voltage-reactive power droop controllers of DG systems are commonly used for their simplicity and are required by international standards. On the other hand, the interaction among voltage droop controllers of different DG systems may introduce instability. The paper presents an effective procedure to determine the droop constants of voltage-reactive power controllers for multiple DG systems. Firstly, a multi-input multi-output model of the distribution system is introduced. Then, using the concept of the interaction measure under decentralized control, a simple constraint is added to the single-input single-output design of each droop controller. Such a constraint guarantees stability with respect to the interaction among the voltage droop controllers of all the DG systems. Eventually, the proposed procedure is applied to an LV test system with 24 nodes and six photovoltaic systems; the results of numerical simulations are presented, giving evidence of the effectiveness of the proposed procedure in various operating conditions of the distribution system.

Coordinated Volt-Var Control: Online Voltage-Profile Estimation in Smart Distribution Networks

The increasing penetration of intermitt ent and stochastic distributed generation (DG) sources in the existing power grid can lead to voltage rise problems. Meanwhile, the rapid development of smart grid technologies calls for effective solutions to realize the real-time measurements and coordinated volt-var control to improve the overall systems voltage profile (VP). This article presents online VP estimation and coordinated volt-var control in a smart distribution network with multiple DG systems.

Combining two techniques to develop a novel islanding detection method for distributed generation units

Renewable energy is a fast-growing sector given that it does not cause the pollution produced by nuclear and fossil generation systems. The use of renewable energy for distributed generation (DG) is important worldwide not only because it increases profits but also because integrating it into utility systems ensures power supply at points of consumption even when a DG system is disconnected from the main system. Researchers continue to exert efforts in improving islanding detection methods because current detection approaches remain unsatisfactory. Accordingly, this paper presents two new islanding detection methods that are suited for single and multiple DG units. The methods are based on the rate of change in active and reactive power, frequency, and voltage angle. They are tested on single and multiple DG systems with wind turbines and simulated using the Power World Simulator software. The simulation results show that the methods effectively reduce the time spent on islanding detection and efficiently decrease non-detection zones. To verify the accuracy and speed of the proposed methods, the results are compared with those achieved by different islanding detection approaches.

Integrated Microgrid Laboratory System

The paper presents an integrated microgrid laboratory system with a flexible and reliable multimicrogrid structure; it contains multiple distributed generation systems and energy storage systems and integrates with a diesel generator that serves as a back-up power source and flywheel energy storage for fast balancing to provide uninterruptible power-supply services in cooperation with the diesel generator. The microgrid system, by adopting the master-slave control strategy, can be transited flexibly between grid-connected and islanded modes and can be disconnected from the utility when a fault occurs or the power quality falls below specified standards. The developed bi-directional inverter which is applied in the system plays an important role. The small microgrids of this system are intended to operate separately or in the form of one large microgrid with a certain switch status. Furthermore, experiments on control, protection, and other technologies have been carried out. The results show that the operation conditions meet the related IEEE Standard 1547 and power quality requirements. The integrated microgrid laboratory system is able to operate stably and reliably under different conditions, including mode transition and fault events.

A Flexible Harmonic Control Approach Through Voltage-Controlled DG–Grid Interfacing Converters

The increased penetration of nonlinear loads and power-electronics-based distributed generation (DG) systems may introduce power quality issues to the distribution power system. However, if controlled and regulated properly, the DG-grid interfacing converters are able to improve the distribution system efficiency and power quality. This paper focuses on the distribution system harmonic control through the DG-grid interfacing converters. Two alternative DG systems, namely, current-controlled DG and voltage-controlled DG, are considered. While most of the previous works on harmonic compensation are based on the current-controlled method, a novel harmonic control scheme using a voltage-controlled method is developed in this paper. The voltage-controlled method is more flexible and has similar compensation performance compared to the conventional current-controlled method. In addition, by avoiding the implementation of a harmonic current tracking loop, the proposed voltage-based method can be seamlessly incorporated into a voltage-controlled DG unit, which is important to provide direct voltage and frequency support in a microgrid. Moreover, the possible harmonic circulating current among multiple DG systems is also investigated in this paper. Simulations and experimental results from a three-phase 5-kVA laboratory DG prototype are provided to validate the discussion.

Sandia frequency shift parameter selection for multi-inverter systems to eliminate non-detection zone

Among frequency drift islanding detection methods, Sandia frequency shift (SFS) is considered as one of the most effective methods in detecting islanding conditions for grid connected photovoltaic (PV) inverters. The performance of the SFS method during an islanding condition and its non-detection zone (NDZ) depends to a great extent on its parameters. Furthermore, the capability of the SFS method to detect an islanding condition deteriorates with multiple PV inverters. A mathematical formula is derived to aid protection engineers in determining the optimal setting of the SFS islanding detection parameters with multiple inverter-based distributed generation (DG), such as PV systems, to eliminate the NDZ. The derived formula is applied to multiple DG systems equipped with the over frequency/under frequency protection, active frequency drift and SFS islanding detection methods and is verified through NDZ analysis and simulation results on PSCAD/EMTDC. The derived formula provides an effective guideline for designing frequency drift methods in multi-inverter-based DG systems.

Operation and control of multiple Distributed Generation systems in the microgrid

This paper investigates the operation and control of multiple Distributed Generation (DG) systems interconnected to the utility in the form of microgrid. The developed model of the microgrid consists of microturbine generation system, converter-based DG system with stiff DC source, synchronous-generator-based DG system and wind power system with induction generator. The Simulation model of the microgrid is developed in the Simpowersystems of the Matlab and systematic analysis has been carried out to ascertain its operation. This paper also presents the dynamic power-sharing strategy for converter-based DG systems during autonomous mode of operation of the microgrid.

Investigation on Electromagnetic Transients of Distributed Generation Systems in the Microgrid

The increasing interconnection of distributed generation sources of diverse technologies to low-voltage grids introduces considerable complexity in its operation and control. The concept of the microgrid is emerging as a solution to this and also to take full advantage of the potential offered by distributed generation. In this article, the performance of a typical microgrid with multiple distributed generation systems in grid-connected and autonomous modes of operation has been investigated through simulation. The developed model of the microgrid consists of a converter-interfaced microturbine generation system, a synchronous-generator-based distributed generation system, and a wind power generation system with an asynchronous generator. Investigation has been carried out to study the typical electromagnetic transients of a microgrid, due to preplanned and unplanned switching events. The performance of the bidirectional grid interface topology for a microturbine generation system in a microgrid is evaluated in this work. It has been observed from the simulation results that the motoring mode operation of the microturbine generation system during starting does not cause any disturbances in the microgrid. The study also indicates that the microgrid can maintain the desired power quality at the point of common coupling.

Stability Analysis of Load Sharing Control for Distributed Generation Systems

This paper discusses the small-signal stability analysis of the combined droop and average power method for load sharing control of multiple distributed generation systems (DGS) in a stand-alone ac supply mode. The small-signal model is developed and its accuracy is verified from simulations of the original nonlinear model. It is shown that the small-signal model accurately predicts the stability of the parallel systems.