Showing papers on "Islanding published in 2005"

Micro-grid autonomous operation during and subsequent to islanding process

Abstract: This paper investigates (i) preplanned switching events and (ii) fault events that lead to islanding of a distribution subsystem and formation of a micro-grid. The micro-grid 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 micro-grid. 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.

Decentralized control for parallel operation of distributed generation inverters using resistive output impedance

Abstract: In this paper, a novel wireless load-sharing controller for islanding parallel inverters in an ac distributed system is proposed. The paper explorers the resistive output impedance of the parallel-connected inverters in an island microgrid

Comparative analysis between ROCOF and vector surge relays for distributed generation applications

Abstract: This paper presents a comprehensive comparative analysis between rate-of-change-of-frequency (ROCOF) and vector-surge (VS) relays for distributed generation islanding detection. The analysis is based on the concepts of detection-time versus active power-imbalance curves and critical active power imbalance. Such curves are obtained through dynamic simulations. The performance of these devices considering different scenarios is determined and compared. Factors such as voltage-dependent loads, generator inertia constant, and multidistributed generator systems are analyzed. False operation of these relays due to faults in adjacent feeders is also addressed. Results show that ROCOF relays are more reliable to detect islanding than vector surge relays when the active power imbalance in the islanded system is small. However, ROCOF relays are more susceptible to false operation than VS relays.

A digital controlled PV-inverter with grid impedance estimation for ENS detection

Abstract: The steady increase in photovoltaic (PV) installations calls for new and better control methods in respect to the utility grid connection Limiting the harmonic distortion is essential to the power quality, but other requirements also contribute to a more safe grid-operation, especially in dispersed power generation networks For instance, the knowledge of the utility impedance at the fundamental frequency can be used to detect a utility failure A PV-inverter with this feature can anticipate a possible network problem and decouple it in time This paper describes the digital implementation of a PV-inverter with different advanced, robust control strategies and an embedded online technique to determine the utility grid impedance By injecting an interharmonic current and measuring the voltage response it is possible to estimate the grid impedance at the fundamental frequency The presented technique, which is implemented with the existing sensors and the CPU of the PV-inverter, provides a fast and low cost approach for online impedance measurement, which may be used for detection of islanding operation Practical tests on an existing PV-inverter validate the control methods, the impedance measurement, and the islanding detection

Voltage and frequency control of inverter based weak LV network microgrid

Abstract: In this paper voltage and frequency control of islanded microgrid after intentional and unintentional switching events are investigated. The weak low voltage (LV) network based microgrid consists of two inverter based distributed generation (DG) units. One unit is a storage (battery) unit and the other is a photovoltaic (PV) cell. In this case the battery inverter with rapid response is considered to act as a master and it has the main responsibility to control the voltage and frequency in microgrid when islanded from the main distribution network. The studies are performed on a PSCAD simulation software package. Simulation studies show the voltage - active power and frequency - reactive power dependency in weak LV network. The studies also show that in order to maintain frequency balance in islanded microgrid, there is need for a reference sine wave generator inside master unit which imitates the main network phase voltages and gives the input for master units' (battery storage) PLL (phase locked loop) during islanding

A simulation study of OBDD-based proper splitting strategies for power systems under consideration of transient stability

Abstract: System splitting, also called controlled system islanding, can effectively prevent blackouts. Following the OBDD-based three-phase method given in for proper splitting strategies, which satisfy necessary steady-state constraints, This work studies the feasibility of the proper splitting strategies by means of power system transient simulations on the IEEE 118-bus system. Simulation results show that a considerable proportion of proper splitting strategies can successfully split the power system into stable islands. Furthermore, considering a general knowledge that a controlled power system can easily maintain its stability after a small disturbance, This work presents "threshold value constraint" to restrict the degrees of the disturbances caused by proper splitting strategies. An approach is proposed to select threshold values for the constraint. Further simulation results show that by checking the constraint for proper splitting strategies, feasible splitting strategies can be found, which can successfully split the system into islands satisfying transient stability constraints.

Control strategies for microgrids emergency operation

Abstract: Under normal operating conditions, a microgrid (MG) is interconnected with the medium voltage (MV) network. However, planned or unplanned events like maintenance or faults in the MV network, respectively, may lead to MG islanding. In order to deal with islanded operation and even black start following a general blackout, an emergency operation mode must be envisaged. Two possible control strategies were investigated and are described in this paper in order to operate a MG under emergency mode. A sequence of actions for a well succeeded black start procedure, involving microgeneration units, has also been identified contributing for an increase in distribution network reliability

Performance of ROCOF relays for embedded generation applications

Abstract: The performance characteristics of rate of change of frequency (ROCOF) relays for embedded generation islanding detection are investigated. The concepts of detection-time versus power-imbalance curve and critical power imbalance are proposed. These concepts are employed to determine the factors that may affect the islanding detection capability of ROCOF relays. The factors analysed are voltage-dependent loads, load power factor, generator inertia constant, feeder length and X/R ratio and multi-embedded generators. In addition, false operation due to faults on adjacent feeders is investigated. The results show that ROCOF relays may become ineffective for islanding detection if the power imbalance in the islanded system is smaller than 15%. In addition, these devices have a high risk of false operation.

Application of a new sensitivity analysis framework for voltage contingency ranking

Abstract: In this paper, a new sensitivity analysis framework for voltage contingency ranking has been presented. The proposed sensitivity analysis is a combination of linear sensitivities and eigenvalue analysis. The sensitivity analysis framework can determine the voltage stability status of the power system due to the occurrence of each contingency. Moreover, stability margin or instability depth of the post-contingency state is determined in the framework. In other words, a severity index is obtained for each voltage contingency and so the contingencies can be ranked. This rank shows bottlenecks of the power system in the priority order, a property that is a key issue for both planners and operators of the power system. The proposed method can also evaluate islanding contingencies as well as the nonislanding ones. Moreover, the method can consider the generator contingencies in addition to the branch contingencies in a unique framework. The proposed method has been tested on the New Zealand test system and Iran's power network. Obtained results, discussed comprehensively, confirm the validity of the developed approach.

Intentional islanding of distributed generation

Abstract: Intentional islanding of distributed generation (DG) can support local supplies to critical customers in the event of network failure, thus increasing the system reliability. In such a case, the DG interface control is responsible for maintaining both the voltage and frequency on the islanded part of the network within the permissible operating levels. This paper presents a control strategy to intentionally island the DG during network failure with particular reference to inverter based DGs.

Secondary Load-Frequency Control for MicroGrids in Islanded Operation

Abstract: The objective of this paper is to present novel control strategies for MicroGrid operation, especially in islanded mode. The control strategies involve mainly the coordination of secondary load-frequency control by a MicroGrid Central Controller that heads a hierarchical control system able to assure stable and secure operation when the islanding of the MicroGrid occurs and in load-following situations in islanded mode.

Power generation system, and administration apparatus and administration method of power generation system

Abstract: In a power generation system which includes a plurality of sets of direct current power sources and power converters which are connected to the direct current power sources to convert a direct current power into an alternating current power, the power converters being connected in parallel to supply the alternating current power to a system power supply, from the plurality of power converters in the system, one power converter is set as a master machine which detects an islanding operation state in which power supply from the system power supply is stopped, and setting is executed so as to cause the remaining power converters not to detect the islanding operation state. When the master machine is not present among the power converters in operation, the master machine is selected from the power converters in operation in accordance with a predetermined condition.

Power signaling based technique for detecting islanding conditions in electric power distribution systems

Abstract: A signaling system for detecting power line discontinuity in a power distribution system having a local power generator in a feeder line employs a signal generator that provides a local generator control signal with a preset protocol continuously and is located in the power distribution system upstream of said local power generator. A power detector terminates the local generator control signal and a signal detector in said feeder line detects the presence of the signal and triggers the shutdown of the local generator when the signal is not detected by the signal detector for a predefined period of time.

Impact of DG interface control on islanding detection

Abstract: Within recent years, many islanding detection methods have been proposed as a result of the high penetration of the distributed generation (DG) in the distribution system. With the increasing reliance of the distribution system on DGs and knowing that DGs can provide many favorable aspects, other than providing active power, new interface controls have been proposed recently. Most of the current proposed DG interface controls are designed to supply both active and reactive power to provide voltage support and power factor improvement and this in turn could have an impact on islanding detection. Islanding detection methods are designed for DGs operating at unity power factor. The main goal of this paper is to implement some of the interface controls that allow the DG to operate efficiently and at the same time examine their effect on islanding detection.

A Novel Active Frequency Drift Method of Islanding Prevention for the grid-connected Photovoltaic Inverter

Abstract: This paper proposes a novel active frequency drift (AFD) method to improve the islanding detection performance with minimum current harmonics. To detect the islanding phenomenon of grid-connected photovoltaic (PV) inverters concerning about the safety hazards and the damage to other electric equipments, the anti-islanding methods have been described. AFD using chopping fraction (cf) enables the islanding detection to drift up (or down) the frequency of the voltage during the islanding situation. However, the performance of the conventional AFD methods is inefficient and difficult to design the appropriate cf value analytically to meet the limit of harmonics. In this paper, the periodic chopping fraction based a novel AFD method is proposed. This proposed method shows the analytical design value of cf to meet the test procedure of IEEE Std. 929-2000 with the power quality and islanding detection time. To verify the validation of the proposed method, the islanding test results are presented. It is confirmed that the proposed method has not only less harmonic distortion but also good performance of islanding detection compare with the conventional AFD method

Performance curves of voltage relays for islanding detection of distributed generators

Abstract: This letter proposes a method for systematically evaluating the voltage-based relay's capability to detect islanding of distributed generators connected to distribution systems. The method is based on a set of detection time versus reactive power imbalance curves, which can be obtained through repeated dynamic simulations. With the curves, one can determine the characteristics of voltage-based anti-islanding protection systems.

Unintentional islanding and comparison of prevention techniques

Abstract: When one or more distributed resources (DR) become isolated from the rest of the power system and inadvertently continue to serve loads separately from the utility system, the condition is known as "unintentional islanding". Utility engineers are seriously concerned about unintentional formation of islands. This is because the utility looses control of the voltage and the frequency during the islanding condition. Frequency deviation and out of limit voltage conditions can have severe consequences on utility loads. In this paper, various type of islanding is discussed. The focus is on the methods and special equipment used to detect and prevent islanding. Examples, advantages and disadvantages of each technique are also discussed.

Master/slave control of parallel connected solar battery modules

Abstract: In a power generation system (Fig. 3) which includes a plurality of sets of direct current power sources (10A - 10E) and power converters (20A - 20E) which are connected to the direct current power sources to convert a direct current power into an alternative current power, the power converters being connected in parallel to supply the alternative current power to a system power supply (30), from the plurality of power converters in the system, one power converter is set as a master machine which detects an islanding operation state in which power supply from the system power supply is stopped, and setting is executed so as to cause the remaining power converters not to detect the islanding operation state. When the master machine is not present among the power converters in operation, the master machine is selected from the power converters in operation in accordance with a predetermined condition.

Unintentional islanding in distribution grids with a high penetration of inverter-based DG: Probability for islanding and protection methods

Abstract: Unintentional islanding in distribution networks due to the presence of DG is one of the major safety concerns for the grid interconnection of generators. The present paper looks at the current protection and safety requirements and new activities in selected European countries with a focus on inverter based generation. By means of a field study carried out in a real LV network with a high DG penetration, occurrence and persistence of balanced load/generation conditions - which are the basic prerequisite for islanding - are analyzed and the according probability is calculated based on different penetration scenarios protection settings and load characteristics. The results show that under realistic conditions the probability to encounter an island is not negligible. Thus additional protection methods to the standard voltage and frequency monitoring are required in order to detect a loss of mains at the generator and ensure the safety of customers and maintenance personnel.

A New Adaptive Logic Phase-Shift Algorithm for Anti-Islanding Protections in Inverter-Based DG Systems

Abstract: Recent developments in anti-islanding techniques have demonstrated that phase shift techniques are very effective for anti-islanding protections in inverter-based distributed generation (DG) systems. Several approaches have been proposed in the previous researches such as slide-mode frequency shift (SMS), active frequency drift (AFD), and active frequency drift with positive feedback (AFDPF) etc. The automatic phase shift (APS) method is actually a modified SMS method. It can effectively reduce the non-detection zone (NDZ) of the SMS technique by introducing an additional phase shift increment each time the frequency of the terminal voltage stabilizes. However, it is very difficult to determine a stable islanding frequency, and the APS algorithm sometimes acts slowly, even fails in certain load conditions. A new adaptive logic phase-shift (ALPS) algorithm is proposed in this paper to regulate the additional phase shift at a suspicious islanding situation and evaluate the effects of the phase shift. This algorithm can yield a quick phase shift in an islanding situation yet only produce a very small phase shift when the grid is available for inverter-based DG systems. Both simulation and experiment results have proved the robustness and effectiveness of the newly proposed anti-islanding algorithm

An improved method for anti-islanding by reactive power control

Abstract: Recently, the study of the islanding phenomenon, which unfavorably maintains its operation even when the utility grid is removed, has led to the trend of utility interactive inverters in the photovoltaic generation system. In this paper, a novel anti-islanding method is presented. The proposed method can keep the real average power component constant by MPPT control and causes the load current frequency with only a change of the reactive power component under islanding conditions. Its periodical change of the reactive power component can decrease the NDZ (non-detective zone) so that the detection of various loads will be possible. Therefore, the proposed method enables islanding detection, in the worst case, no load-variation compared to before removal of the grid. It has been proven theoretically that the influence of the grid can be minimized in the case of an additional reactive power component and the islanding detection is also confirmed from the experimental results with an inverter for a 3 kW photovoltaic generation system, by applying the proposed anti-islanding method

Anti-islanding method and system for distributed power generation systems

Abstract: An electrical power supply system connected to an electrical grid, includes at least one source of electrical power, an inverter which receives power from the electrical source and delivers power to a local load, and a connection to the electrical grid. The system also includes a control unit for controlling the inverter, which interrupts the delivery of power by the inverter when at least one electrical parameter of the system exceeds a threshold value. The system also includes anti-islanding logic which detects at least one symptom of an islanding condition of the system and, when the symptom is detected, to cause a variation in the power level delivered by the inverter.

Active anti-islanding schemes for synchronous-machine-based distributed generators

Abstract: Unintentional islanding of distributed generators is considered undesirable since it exposes the personnel and the power equipment to high voltage through back energisation. Two new anti-islanding schemes for synchronous-machine-based distributed generators are proposed. The principles and design guidelines for the two new schemes are provided, and their effectiveness is validated by illustrative simulation examples conducted in PSCAD. The new proposed methods are reliable, robust to other non-islanding disturbances, and easy for implementation.

Modeling of an islanding protection method for a hybrid renewable distributed generator

Abstract: The Hydrogen Research Institute developed a hybrid renewable energy power system that uses a wind turbine, a photovoltaic array and a fuel cell. In order to connect this system to the grid, an islanding protection device is being developed. This paper presents the passive (U/O Voltage, U/O Frequency) and active (Sandia frequency shift and Sandia voltage shift) protection methods that were chosen to be added to the system. It also presents a Matlab/Simulink model of the protection device and the simulation results that were obtained using different critical operating conditions for which the clearing times can surpass those defined by the IEEE 1547 standard. Passive and active detection methods are combined in order to improve the efficiency of the islanding protection

An improved anti-islanding algorithm for utility interconnection of multiple distributed fuel cell powered generations

Abstract: This paper presents an improved anti-islanding algorithm for utility interconnection of multiple distributed fuel cell powered generations (DFPGs). A cross-correlation method is proposed and implemented in conjunction with the anti-islanding algorithm developed in the previous work (C. Jeraputra and P.N. Enjeti, 2004). While the power control algorithm continuously perturbs (plusmn5%) the reactive power supplied by the DFPG, the proposed algorithm calculates the cross-correlation index of a rate of change of the frequency deviation with respect to (plusmn5%) the reactive power to confirm islanding. If this index is above 50%, the algorithm further initiates (plusmn10%) the reactive power perturbation and continues to calculate the correlation index. If the index exceeds 80%, the occurrence of islanding can be confirmed. The proposed method is robust and capable of detecting the occurrence of islanding in the presence of several DFPGs, which are independently operating. Viability of the cross-correlation method is verified by the simulation. Experimental results are presented to support the findings of the proposed method

Reliability of distribution networks with DER including intentional islanding

Abstract: This paper presents methods for calculating the reliability of distribution systems with distributed energy resources (DER). The impact of intentional islanding of the DER units on the reliability is studied in detail. Different levels of automation are considered, starting from the existing network and continuing with increasing levels of intelligence in the network. The role of the switching devices and of the restoration process is discussed in detail

Design methodology for the frequency shift method of islanding prevention and analysis of its detection capability

Abstract: Islanding protection is one of the most important sources of discrepancy in grid-connected photovoltaic systems. Even when islanding is not very likely to happen, regulations demand the photovoltaic inverters to implement effective protection methods. Due to its several advantages, the frequency shift method of islanding prevention, commonly known as Sandia Frequency Shift, is one of the most important active methods. This method implements a positive feedback of the frequency that tends to move it outside the trip limits in case of islanding. The method shows a very high detection capability, which depends on both the values of the method parameters and the characteristics of the load that remains in the same power section after islanding. This paper develops a mathematical analysis of the Sandia Frequency Shift method and proposes a new methodology to design its parameters as a trade-off between the detection capability, which is evaluated as a function of the load characteristics, and the distortion that the method could introduce in the grid as a consequence of transitory frequency disturbances. The ability of this methodology to design the method parameters and achieve the highest detection capability is satisfactorily proved by means of both simulation and experimental results on a commercial photovoltaic inverter that implements the method once its parameters have been designed with the proposed methodology. Copyright © 2005 John Wiley & Sons, Ltd.

Adaptive corrective control strategies for preventing power system blackouts

Abstract: In order to investigate power system security and design appropriate control strategies, power systems can be conceptually classified into five operational states: Normal, Alert, Emergency, In Extremis, and Restorative. Various preventive and corrective control strategies have been studied to cope with power systems in the different operational states. Our approach primarily focuses on the corrective control strategies for power systems under emergency and in extremis states. In this paper, two cor- rective control approaches (transmission system reconfigu- ration and controlled system islanding) have been investi- gated and new algorithms have been developed. When the system is in the emergency state, two reconfiguration strategies, line switching and bus-bar switching, have been presented to solve the problems of overloads or voltage violations. Furthermore, to prevent a total system black out, a slow coher-ency grouping based adaptive power system islanding approach using minimal cutsets has been presented to deal with catastrophic events when power system vulnerability analysis indicates that the system is approaching the in extremis state. A controlled under frequency load shedding scheme with the rate of frequency decline is also proposed to balance the real power within each island and aid restoration. The verification of our approach is proven with simulations on a 179-bus, 29- generator WECC system.

A Distributed Generation Islanding Detection Method Based on Artificial Immune System

Abstract: A passive method of detecting islanding of distributed generation inspired by the information processing properties of natural immune system is presented in this paper According to the functions of the T cells and B cells of the immune system, T-module and B-module for detecting islanding are established The T-module is used to detect the islanding condition The B-module plays a role in improving the detection coverage space The frequency spectra of the inverter output voltage are used as the detecting input signals The algorithm is implemented with DSP The results of experiment are presented It illustrates the validity and efficiency of the approach

Slow coherency grouping based islanding using minimal cutsets and generator coherency index tracing using the continuation method

Abstract: Power systems are under increasing stress as deregulation introduces several new economic objectives for operation. Since power systems are being operated close to their limits, weak connections, unexpected events, hidden failures in protection system, human errors, and a host of other factors may cause a system to lose stability and even lead to catastrophic failure. Therefore, the need for a systematic study and design of a comprehensive system control strategy is gaining more attention. Among these control methods, controlled system islanding is deemed as the final resort to save the system from a blackout. In the literature, many approaches have been proposed to undertake this task. However, some of these approaches only take static power flow into consideration; others require a great deal of computational effort. It has been observed that following large disturbances, groups of generators tend to swing together. Attention has thus been drawn to the stability of inter-area oscillations between groups of machines. The slow-coherency based generator grouping, which has been widely studied in the literature, provides a potential method for capturing the movement of generators between groups under disturbance. The issue becomes on how to take advantage of slow coherency generator grouping and island the system by finding the set of lines to be tripped. Furthermore, through various simulations and analysis, it has been found that generator grouping indeed changes with respect to large changes in system load conditions. In this dissertation, a comprehensive approach has been proposed to conduct slow coherency based controlled power system islanding using the minimal cutset technique from graph theory with the transition from calculating real power imbalance within the island to calculating the net flow through the cutset. Furthermore, a novel approach has been