Showing papers on "Islanding published in 2004"

Design, analysis, and real-time testing of a controller for multibus microgrid system

Abstract: This paper concentrates on the design and analysis of a controller for multibus microgrid system. The controller proposed for use with each distributed generation (DG) system in the microgrid contains inner voltage and current loops for regulating the three-phase grid-interfacing inverter, and external power control loops for controlling real and reactive power flow and for facilitating power sharing between the paralleled DG systems when a utility fault occurs and the microgrid islands. The controller also incorporates synchronization algorithms for ensuring smooth and safe reconnection of the micro and utility grids when the fault is cleared. With the implementation of the unified controller, the multibus microgrid system is able to switch between islanding and grid-connected modes without disrupting the critical loads connected to it. The performance of this unified controller has been verified in simulation using a real-time digital simulator and experimentally using a scaled laboratory prototype.

An islanding detection method for distributed generations using voltage unbalance and total harmonic distortion of current

Abstract: Distributed generation (DG) units are rapidly increasing and most of them are interconnected with distribution network to supply power into the network as well as local loads. Islanding operations of DG usually occur when power supply from the main utility is interrupted due to several reasons but the DG keeps supplying power into the distribution networks. These kinds of islanding conditions cause negative impacts on protection, operation, and management of distribution systems; therefore, it is necessary to effectively detect the islanding conditions and swiftly disconnect DG from distribution network. Generally, if there are large changes in loading for DG after loss of the main power supply, then islanding conditions are easily detected by monitoring several parameters: voltage magnitude, phase displacement, and frequency change. However, in case of small changes in loading for DG, the conventional methods have some difficulty in detecting such a particular islanding condition. This paper presents a new islanding detection algorithm for DG effectively working in the most of DG loading conditions. This paper proposes two new parameters for detecting islanding operation of DG: voltage unbalance and total harmonic distortion of current. The proposed method effectively combines the conventional parameters with the newly proposed parameters for detecting the islanding conditions. The proposed methods were verified using the radial distribution network of IEEE 34 bus.

Slow coherency-based islanding

Abstract: This paper provides the analytical basis for an application of slow coherency theory to the design of an islanding scheme, which is employed as an important part of a corrective control strategy to deal with large disturbances. The analysis is conducted under varying networks conditions and loading conditions. The results indicate that the slow coherency based grouping is almost insensitive to locations and severity of the initial faults. However, because of the loosely coherent generators and physical constraints the islands formed change slightly based on location and severity of the disturbance, and loading conditions. A detailed description of the procedure to form the islands after having determined the grouping of generators using slow coherency is presented. The verification of the islanding scheme is proven with simulations on a 179-bus, 29-generator test system.

Microgrid autonomous operation during and subsequent to islanding process

Abstract: Summary form only given. This paper investigates (i) preplanned switching events and (ii) fault events that lead to islanding of a distribution subsystem and formation of a microgrid. The microgrid includes two distributed generation (DG) units. One unit is a conventional rotating synchronous machine and the other is interfaced through a power electronic converter. The interface converter of the latter unit is equipped with independent real and reactive power control to minimize islanding transients and maintain both angle stability and voltage quality within the microgrid. The studies are performed based on a digital computer simulation approach using the PSCAD/EMTDC software package. The studies show that an appropriate control strategy for the power electronically interfaced DG unit can ensure stability of the micro-grid and maintain voltage quality at designated buses, even during islanding transients. This paper concludes that presence of an electronically-interfaced DG unit makes the concept of micro-grid a technically viable option for further investigations.

Study and Development of Anti-Islanding Control for Grid-Connected Inverters

Abstract: This is a report on the development of anti-islanding control for grid-connected inverters from distributed generation sources. Islanding occurs when a distributed generation source continues to provide electricity to a portion of the utility grid after the utility experiences a disruption in service. Since the utility no longer controls this part of the distribution system, islanding can pose problems for utility personnel safety, power quality, equipment damage, and restoration of service. This report proposes a new family of anti-islanding schemes that meet IEEE 1547 interconnection standards, that can detect all disruptions in service, have minimum power-quality impact, require low-cost implementation, work for multiple distributed generators, and work for any multi-phase inverters. It also provides design guidelines for the schemes, and evaluates and validates the proposed schemes for practical applications.

A practical method for assessing the effectiveness of vector surge relays for distributed generation applications

Abstract: This paper presents a simple and reliable method for predicting the islanding detection performance of vector surge relays. The relay performance is characterized by a tripping-time versus power-imbalance curve. With the curve, one can determine the time taken by a vector surge relay to detect islanding for any generation-load mismatch level. The main contribution of this paper is the development of analytical formulas for directly determining the behavior of vector surge relays. As a result, efforts needed to asses the relay performance for a given distributed generation scheme can be simplified significantly. The accuracy of the formulas has been verified by extensive simulation study results.

Dynamic and transient analysis of power distribution systems with fuel Cells-part II: control and stability enhancement

Abstract: The objective of this paper is to analyze the dynamics of distribution systems that contain fuel cells and to enhance the stability of these systems by controlling the fuel cells. The models used in this second part of the two-part paper are the solid-oxide fuel cell (SOFC) models developed in part one. The fuel-cell control loops through the power conditioning units are first explained using a one-machine infinite bus system. Then, fuel cells are integrated into a power distribution test system, which initially contains gas turbines only. Simulation results show how a combination of fuel cells and gas turbines can help control frequency fluctuations, and supply power after islanding of the distribution system.

Development of a robust anti-islanding algorithm for utility interconnection of distributed fuel cell powered generation

Abstract: In this paper, a robust anti-islanding algorithm for distributed fuel cell powered generation (DFPG) is proposed. Three different islanding scenarios are explored and presented based on analysis of real and reactive power mismatch. It is shown via analysis that the islanding voltage is a function of real power alone, whereas its frequency is a function of both real and reactive power. Following this analysis, a robust anti-islanding algorithm is developed. The proposed algorithm continuously perturbs (/spl plusmn/ 5%) the reactive power supplied by DFPG while simultaneously monitoring the utility voltage and frequency. If islanding were to occur, a measurable frequency deviation takes place upon which the real power of DFPG is further reduced to 80%; now a drop in voltage positively confirms islanding, and the DFPG is safely disconnected. This method of control is shown to be robust, able to detect islanding under resonant loads, and fast acting (operable in one cycle). Possible islanding conditions are simulated and verified with analysis. Experimental results on a 0.5 kW fuel cell inverter connected to 120 V 60 Hz utility are discussed.

System islanding using minimal cutsets with minimum net flow

Abstract: Slow coherency has effectively proved its capability in determining sets of generator groups among weak connections in any given power system. In this paper, we provide two comprehensive approaches to deal with islanding the actual system based on the grouping information, by using the minimal cutsets technique in graph theory. The issue of minimal cutsets has been widely discussed in areas related to network topology determination, reliability analysis, etc. The results of this paper also show potential in application to power system islanding. The verification of the islanding scheme is provided based on a WECC 179-bus, 29-generator test system.

Investigation of anti-islanding protection of power converter based distributed generators using frequency domain analysis

Abstract: The anti-islanding algorithm proposed by the Sandia national laboratories is analyzed in this study because this scheme, also known as the Sandia scheme, is considered to be effective in detecting islanding of distributed generation systems. Previously, other than heuristic approaches, there has not been any quantitative analysis for tuning the control gains of the algorithm based on the power rating and bandwidth of the distributed generation (DG) power converter. The paper interprets the components of the algorithm that affect the voltage magnitude and frequency into block diagrams that can be linearized and studied using continuous time approximations. This paper uses a frequency domain approach to analyze the range for the gains required by anti-islanding algorithm to effectively determine the disconnection of the mains grid within an acceptable time duration. The analysis provides guidelines for using Sandia's national laboratory schemes under different application conditions. The results are validated using detailed time domain DG and power system simulations.

An interaction problem of distributed generators installed in a MicroGrid

Abstract: This paper investigates an interaction problem that might be induced from various kinds of distributed generators installed in a MicroGrid. One of the most important features of MicroGrid is islanding operation. MicroGrid does not include large central generators and all distributed small generators have to share all loads existing in the MicroGrid. In this case, undesirable mutual oscillation problem might appear when MicroGrid goes into islanding mode. In this paper, MicroGrid itself and several types of distributed generator are modeled by using Matlab/Simulink. A typical case is simulated and examined by using the developed model.

Proposal of a local DC distribution network with distributed energy resources

Abstract: The spread of the electronics, both for control and power applications, in the realization of apparatuses in low voltage (LV) electrical systems, requires the conversion of the electric power from alternating current to direct current by means of a rectifier stage, generally constituted by a diode bridge. Moreover, modern technologies provide various types of distributed energy resources (DER), constituted by small size generators and storage systems, that generate in direct current, such as photovoltaic arrays, fuel cells, storage, or that need DC section before the interfacing with the AC network (microturbines, variable speed wind generators, flywheels). It can be thought to integrate the various generators, storage systems and DC loads in a local DC distribution network, that is interfaced with public grid by means of one or more inverters. This solution makes easy the islanding operation of the generators. The aim of this work is to analyze a local DC distribution system both with diode and IGBT bridge interface converter in the presence of small scale generation and storage systems. After suitable modeling of the entire system, some simulation results are given.

Detection of islanded behavior and anti-islanding protection of a generator in grid-connected mode

Abstract: A method of controlling a generator system connected to an electric power system in which the output frequency characteristic of the generator system is measured, a first phase angle and frequency of the measured frequency characteristic is estimated using a first phase locked loop having a first bandwidth, and a second phase angle and frequency of the measured frequency characteristic is estimated using a second phase locked loop having a second bandwidth greater than the first bandwidth. Further, the method calculates a frequency difference between the first and second estimated frequencies, and an angle variation that is proportional to the calculated frequency difference. The estimated second phase angle is then added to the calculated angle variation so as to form an output current phase angle reference, and an output current phase angle of the generator system is controlled to be aligned with the output current phase angle reference. The method also determines whether or not the generator system is within a generation island based on the measured frequency characteristic.

Recent developments in islanding detection for distributed power generation

Abstract: The aim of this paper is to review recent developments in anti-islanding techniques for distributed resources interconnected with electric power systems. The techniques may be classified as remote and local. Remote techniques are associated with island detection on the utility side while local techniques are associated with island detection on the distributed power generation side. Local techniques may further be classified as passive and active.

Concerns generated by islanding [electric power generation]

Abstract: The purpose of this article is to discuss islanding operation and to provide end-users with items to consider when determining distributed generation plant and equipment design requirements. Islanding is operating an electric generating plant without an external voltage and frequency reference. Operating in parallel is the opposite of islanding. This article attempts to describe the valid concerns that grid operators have regarding distributed generators. Grid computing could offer an inexpensive and efficient means for participant to compete in providing reliable, cheap and sustainable electrical energy supply. With a keen awareness of the issues involved and open communication among the grid operators, generator control system manufacturers, clients and design firms, distributed generators can be operated in islanded mode without negatively impacting the grid and can continue to support facility operations.

Improvement of reliability in active networks with intentional islanding

Abstract: DERs are gaining more and more importance in distribution and they are predicted to drastically change the whole distribution system. Distribution network will be no longer a passive termination of transmission network and the concept of active network has been recently introduced to indicate a new kind of distribution with DERs actively involved in system management and operation. Intentional islanding, i.e. the use of DG to supply portions (cells, microgrids) of distribution network during upstream line faults and scheduled interruptions, can be a valuable option to reduce the number of service interruptions. In the paper, an algorithm for the optimal allocation of automatic sectionalizing switching devices has been improved for the maximum exploitation of intentional islanding. Line faults and overloads have been considered as causes of interruptions. Stochastic models have been adopted to assess the probability of overloads and of properly functioning intentional islands. The application to real world case studies has highlighted the benefits achievable with intentional islanding as well as the inability of common reliability indexes (e.g. SAIFI, SAIDI) to properly perceive advantages that are inherently local.

A microturbine generation system for grid connected and islanding operation

Abstract: The paper addresses the up-to-date issue of distributed generation (DG). In order to fully exploit DO potentialities, advanced integrated controls that include power electronics facilities, communication technologies and advanced modeling are required. Significant expectations are posed on gas micro turbines that can be easily installed in large commercial and public buildings. Modeling of an existing microturbine and its advanced controls for both grid-connected and islanding conditions with privileged loads are developed and presented. Simulations have been performed on an electric network that reproduces a test facility on the way to be completed and already partially available for validation.

An assessment of distributed generation islanding detection methods and issues for Canada

Abstract: The integration of distributed systems into the main electricity networks is a fairly new phenomenon and presents many challenges. Islanding in the context of distributed generation is the situation where a portion of the distribution system becomes isolated from the remainder of the power system, yet continues to be energized by distributed generators. Failure to trip the affected generators can lead to problems. This report reviews the status and performance of all major anti-islanding techniques. Synchronous generators present more of a problem than inverter-based ones, require different techniques and are the focus of most anti-islanding efforts. The system of passive frequency-based relays is the most attractive for synchronous generators. Islanding detection can be either remote (that is, it relies on communications) or local and built-in. The report recommends several programs to investigate, in the Canadian context, such things as the efficacy of present anti-islanding techniques, the potential of future ones, and the dynamics of island formation. 49 refs., 19 figs.

A new family of active antiislanding schemes based on DQ implementation for grid-connected inverters

Abstract: Unintentional islanding protection of distributed generation is a key function for standards compliance. For those distributed generations that use an inverter as grid interface, the function can be implemented as part of the inverter control. Existing antiislanding schemes used in inverters have power quality and nondetection zone issues. This paper proposes a new family of schemes that have negligible power quality degradation and no nondetection zone. Design guidelines based on frequency-domain analysis is also provided. Finally, both simulation and experimental results validate the proposed schemes.

Design, Analysis, and Real-Time Testing of a Controller for Multibus Microgrid System

Abstract: This paper concentrates on the design and analysis of a controller for multibus microgrid system. The controller proposed for use with each distributed generation (DG) system in the microgrid contains inner voltage and current loops for regulating the three-phase grid-interfacing inverter, and external power control loops for controlling real and reactive power flow and for facilitating power sharing between the paralleled DG systems when a utility fault occurs and the microgrid islands. The controller also incorporates synchronization algorithms for ensuring smooth and safe reconnection of the micro and utility grids when the fault is cleared. With the implementation of the unified controller, the multibus microgrid system is able to switch between islanding and grid-connected modes without disrupting the critical loads connected to it. The performance of this unified controller has been verified in simulation using a real-time digital simulator and experimentally using a scaled laboratory prototype.

Development of a robust antiislanding algorithm for utility interconnection of distributed fuel cell powered generation

Abstract: In this paper, a robust antiislanding algorithm for distributed fuel cell powered generation (DFPG) is proposed. Three different islanding scenarios are explored and presented based on analysis of real and reactive power mismatch. It is shown via analysis that islanding voltage is a function of real power alone, where as its frequency is a function of both real and reactive power. Following this analysis, a robust antiislanding algorithm is developed. The proposed algorithm continuously perturbs (/spl plusmn/5%) the reactive power supplied by DFPG while simultaneously monitoring the utility voltage and frequency. If islanding were to occur, a measurable frequency deviation takes place, upon which the real power of DFPG is further reduced to 80%, now a drop in voltage positively confirms islanding and the DFPG is safely disconnected. This method of control is shown to be robust: able to detect islanding under resonant loads, fast acting (operable in one cycle). Possible islanding conditions are simulated and verified with analysis. Experimental results on a 0.5 kW fuel cell system connected to 120 V, 60 Hz utility are discussed.

A new islanding detection algorithm for distributed generations interconnected with utility networks

Abstract: This paper introduced and proposed the voltage unbalance of distributed generation (DG) terminal output as new monitoring parameter for power islanding detection. This paper also presented a simple and novel detection algorithm, which effectively combines the detection results of the conventional parameter, voltage magnitude, and a newly proposed parameter. We tested the proposed method using several distribution network conditions including not only islanding operation conditions, but also non-islanding conditions of normal network load variations. The test results showed that the proposed parameters and algorithm are capable of correctly detecting the islanding operation not affected by variation of DG loading and also have a good selectivity for islanding conditions and non-islanding conditions.

New type of islanding detection system for distributed generation based on voltage angle difference between utility network and distributed generation site

Abstract: In this paper, we propose a new type of islanding detection system based on voltage angle difference between the utility network and distributed generation sites. Intranet and PMU (phasor measurement unit) technology are applied for easy implementation of the system. The system consists of two types of equipment. One is located in the utility substation and measures voltage angle with time stamp, while the other is located at the interconnection point of a distributed generator. The two pieces of equipment are connected by WAN and communicate with each other over TCP/IP. A prototype system has been developed to evaluate the effectiveness of the proposed system. Performance tests were conducted using the prototype system and an analog-simulator. The two units of the prototype system were connected by LAN. The prototype system was shown to be capable of detecting islanding conditions as small as 1% power imbalance, as opposed to a conventional frequency-based system that can only detect islanding conditions with more than 4% power imbalance. Test results indicate that the proposed system is superior to the original frequency-based type for reliable detection of islanding of distributed generators.

Anti-islanding protection systems for synchronous machine based distributed generators

Abstract: A system for providing anti-islanding protection of a synchronous machine based distributed generator includes a frequency sensor configured to generate a generator frequency signal, a bandpass filter configured for filtering the generator frequency signal, a governor controller configured for using the filtered frequency signal to generate a power feedback signal, and a governor summation element configured for summing the filtered frequency signal, the power feedback signal, and a reference power to provide an electrical torque signal. Another system includes a voltage sensor configured to generate a generator terminal voltage signal, a feedback power calculator configured for generating a reactive feedback power signal, a bandpass filter configured for filtering the terminal voltage signal, and a PI controller summation element configured for summing the filtered terminal voltage signal, the reactive power feedback signal, and a reference reactive power to provide an error signal.

Interconnection of distributed power to the distribution network

Abstract: The interconnection of distributed power resources, normally through a power electronic converter, to the distribution network brings challenges of its own. Due to the distributed nature of these resources, they are implemented in large numbers onto the same, sometimes remote, distribution network. These challenges include power quality issues, network stability, power balancing considerations, voltage regulation, protection protocols and unwanted islanding considerations. This paper highlights some of these issues with the aid of two case studies in large-scale PV and wind power interconnections, sensitizing some of the underlying considerations.

Safe controlled islanding of inverter based distributed generation

Abstract: When the power utility is subjected to a disturbance and part of the distribution system is approaching catastrophic failure, control actions need to be taken to limit the extent of the disturbance on the critical customers. An effective approach to secure power supply for the most important customers is by implementing safe controlled islands (SCI) in the distribution system during a utility outage. The ultimate goal when such a scheme is implemented is maintaining the voltage and frequency during islanded operation within the standard limits. This paper presents a novel control strategy to produce SCI with particular reference to inverter interfaced distributed generators (DGs).

Temporary islanded operation of dispersed generation on distribution networks

Abstract: The use of decentralized electrical energy resources may be a useful support to the main power supply. In particular, in the event of an upstream supply outage, the temporary islanding operation of dispersed generators in distribution networks might greatly improve the quality and continuity of the power supply. The aim of this work is to verify the feasibility of temporary grid-disconnected operation of autonomous portions of distribution networks that use embedded generators. The analysis focuses on the critical transients occurring at disconnection from and reclosure with the main supply. A realistic distribution network model with both rotating and static embedded generators has been considered. Suitable P-and V-f control schemes for static generators have been developed, making it possible to operate in both grid-connected and island modes. The simulation results show that the presence of static (or inverter interfaced) generators with suitable control schemes may greatly improve the steady-state as well as transient performance of the islanded generation park.

Implementation of grid-connected photovoltaic system with power factor control and islanding detection

Abstract: An approach to power factor control and islanding detection of a grid connected photovoltaic system using field programmable gate array (FPGA) is proposed. The proposed method has been tested through different situations, including the loss of the utility supply, and the deviation at the output of the inverter helps to detect islanding more effectively. Hardware design was constructed to validate the effectiveness of the approach proposed. Experimental results have been chosen to show the effectiveness of the proposed technique.

Modelling and analysis of active islanding detection methods for photovoltaic power conditioning systems

Abstract: Increasing numbers of photovoltaic arrays are being connected to power utilities through power conditioning systems (PCS). This has raised potential problems of network protection. If, due to the action of the PCS, the local network voltage and frequency remain within regulatory limits when the utility is disconnected, then islanding is said to occur. Representative methods to prevent islanding are described and a model based on PSiM (a simulation package for power electronics and motor control) and analysis of the reactive power variation (RPV) method are presented. A novel phase detector using an all-pass filter and digital phase locked loop (DPLL) is proposed, especially for the single-phase PCS. Finally, the paper provides the simulation and experimental results with a single-phase 3 kW prototype PCS. The islanding test method of IEEE Std 929-2000 was performed for verification.