Disturbance detection in grid-connected distributed generation system using wavelet and S-transform

In this paper, wavelet transform and S-transform based approach is proposed for islanding detection and disturbance due to load rejection in distributed generation (DG) based hybrid system. The two types of distributed generation technology: inverter based and rotating machine based, that consists of photovoltaic, fuel cell and wind systems are considered in hybrid system configuration. The negative sequence voltage signal is analyzed through wavelet transform and S-transform for islanding detection of these resources from the grid. The above two approaches are also used in study of voltage profile at point of common coupling (PCC) with a non-linear load connected. The study for different scenarios in operation of DG system is presented in the form of time-frequency analysis. The energy content and standard deviation (STD) of S-transform contour and wavelet transform signal is also reported for both islanding detection and disturbance due to load rejection.     

Optimized restoration of combined ac/dc shipboard power systems including distributed generation and islanding techniques

Reconfiguration involves changing the status (OFF/ON) of switches, and reconfiguration for restoration involves changing the switch status to maximize the supply to loads that are left unsupplied after fault removal. Shipboard Power Systems (SPS) need automated reconfiguration for restoration schemes to restore vital loads quickly and efficiently in order to improve fight-through and survivability capabilities. The restoration in this paper is achieved using optimization with multiple objectives—maximizing the restored load and giving priority to vital loads. A restoration scheme for SPS with an integrated power system (IPS) and distributed generation (DG) involving islanding has been developed. This formulation includes a hybrid power system that has both ac and dc parts. The restoration formulation in this paper also considers the unbalanced nature of SPS operation with mutual coupling.     

Fault diagnosis in distribution networks with distributed generation

The penetration of distributed generation (DG) in distribution power system would affect the traditional fault current level and characteristics. Consequently, the traditional protection arrangements developed in distribution utilities are difficult in coordination. Also, the reclosing scheme would be affected. With the rapid developments in distribution system automation and communication technology, the protection coordination and reclosing scheme based on information exchange for distribution power system can be realized flexibly. This paper proposes a multi-agent based scheme for fault diagnosis in power distribution networks with distributed generators. The relay agents are located such that the distribution network is divided into several sections. The relay agents measure the bus currents at which they are located such that it can detect and classify the fault, and determine the fault location. The proposed technique uses the entropy of wavelet coefficients of the measured bus currents. The performance of the proposed protection scheme is tested through simulation of two systems. The first system is a benchmark medium voltage (MV) distribution system and the second system is practical 66 kV system of the city of Alexandria.     

Two level islanding detection method for distributed generators in distribution networks

Use of distributed generators in distribution networks is proved highly reliable and economical. In utility practice at the present time, accidental islanding is an undesirable mode of operation as it can harm the personnel and other connected systems. Therefore, islanding must be detected and the islanded distributed generators must be disconnected from the rest of the system. This paper proposes a new islanding detection method which provides a reliable detection in two levels. At first level wavelet energy entropy is used as an indicator to suspect islanding and activate second level method. Second level method is an active frequency drift method with positive feedback and pulsating chopping fraction that vary the output voltage frequency of distributed generators to realize the anti islanding protections. The performance of the proposed method is simulated in MATLAB environment. The proposed algorithm provides fast islanding detection with least disturbance in inverter’s grid connected operation.     

Intelligent islanding detection of a synchronous distributed generation using governor signal clustering

Detection of intentional and unintentional islanding of distributed generation units is one of the major protection issues of the distribution networks. Regarding the safety and reliable operation of a modern distribution network, an expert diagnosis apparatus is required to distinguish network cut off from variety of normal occurrences. Automatic load-frequency controller (ALFC) is an indispensable component of the synchronous generators. Simulation results show that input signal of the governor includes somewhat singular characteristics for each possible phenomenon or disturbance. Therefore, a new method based on Self-Organizing Map (SOM) neural network is proposed using input signal to the governor to cluster various occurrences into islanding and non-islanding categories. Simulation results presented in this paper shows that the input signal of the governor employed by a SOM can cluster a majority of occurrences of the system and distinguishes the islanding phenomenon with high confidence.     

Performance evaluation of WPT based islanding detection for grid-connected PV systems

Recent developments in power electronics technology have emerged towards the generation of electrical power from the renewable energy sources. Among renewable energy sources, solar energy has garnered more importance because of less maintenance and environmental friendly. In grid connected mode, distributed power generation system (DPGS) requires reliable islanding detection technique to find the electrical grid status and operate the grid connected inverter effectively. This paper investigates a comparative performance analysis of wavelet transform (WT) and wavelet packet transform (WPT) based detection in a three-phase grid connected PV inverter system under various power quality disturbances and islanding situation. The WT and WPT coefficients are determined by applying db4 wavelet basis functions, which are obtained optimally for analyzing perturbations that occur in grid connected PV system. The wavelet transform produces large errors due to spectral leakages in frequency bands. On the other hand, WPT provides uniform frequency subband with better time frequency resolution over WT. Finally, the feasibility of proposed WPT based islanding detection method is verified by simulating the system in MATLAB/SIMULINK environment. The simulated results demonstrate the better performance of WPT over WT technique and proved as an accurate, fast and reliable detection technique.     

A generalized power flow analysis for distribution systems with high penetration of distributed generation

In this paper, the element incidence matrix has been extended to develop a comprehensive three-phase distribution system power flow program for radial topology. Three-phase overhead or underground primary feeders and double-phase or single-phase line sections near the end of the feeder laterals have been considered. Unbalanced loads with different types including constant power, constant current and constant impedance are modeled at the system buses. Substation voltage regulator (SVR) consisting of three single phase units connected in wye or two single-phase units connected in open delta are modeled to satisfy the desired voltage level along the feeder. The mathematical model of distributed generation (DG) connected as PQ and PV buses are integrated into the power flow program to simulate the penetration of DGs in the distribution systems. The proposed method has been tested and compared with different IEEE test feeders result. The developed algorithm has been used to study the impact of both SVR and high penetration of DG on voltage profile and system power losses.     

Feasibility of adaptive intentional islanding operation of electric utility systems with distributed generation

As the process of deregulation of the electric utility industry proceeds, the importance of power quality is increasing. A fundamental aspect of power quality is the continuity of power supply that is the absence of momentary, temporary and sustained power interruptions. In this paper a network reconfiguration strategy is proposed which allows the distribution network to separate into a certain number of autonomous islands, supplied by local distributed generation, in case of permanent fault within the grid. Intentional islanding of portions of the grid is automatically carried out with an adaptive network reconfiguration depending upon the local generation/load state at the time of outage.     

Integrating distributed generation into electric power systems: A review of drivers,challenges and opportunities

It is now more than a decade since distributed generation (DG) began to excite major interest amongst electric power system planners and operators, energy policy makers and regulators as well as developers. This paper presents an overview of the key issues concerning the integration of distributed generation into electric power systems that are of most interest today. The main drivers behind the focus on DG integration, especially of the renewable type, in many countries around the world are discussed. A synopsis of the main challenges that must be overcome in the process is presented. Particular emphasis is placed on the need to move away from the fit and forget approach of connecting DG to electric power systems to a policy of integrating DG into power system planning and operation through active management of distribution networks and application of other novel concepts. The paper also analyses the repercussions in transmission system operation and expansion that result from the connection of large amounts of DG of different energy conversion systems focusing on issues related with impacts in steady state operation, contingency analysis, protection coordination as well as dynamic behaviour analysis. A discussion on the possibility of provision of ancillary services by DG is also included. Some results from studies performed in the interconnected Portuguese transmission system are presented and discussed. Some of the opportunities that could be exploited in support of the integration and hence greater penetration of DG into electric power systems are also explored.     

A new artificial neural network based method for islanding detection of distributed generators

This paper presents an artificial neural network (ANN) based method for islanding detection of distributed synchronous generators. The proposed method takes advantage of ANN as pattern classifiers. It is capable of identifying the islanding condition based on samples of the voltage waveform measured at the distributed generator terminals only, which is an important advantage over other ANN-based anti-islanding methods. Moreover, the proposed method is robust against false operation. In order to create a training data set for the ANN, a data selection procedure has been proposed, so that the ANN could be trained more effectively, which has contributed positively to the good performance of the method. The concept of the time-performance region has been introduced to assess the method performance, as well as the non-detection zones. A detailed discussion about the data sampling rate to feed the proposed method has also been conducted, so that the computational burden can be faced as an important factor to assess its performance.     

Direct power control of three-phase VSIs for the minimization of common-mode emissions in distributed generation systems

This paper presents two new direct power control (DPC) strategies of three-phase grid connected VSIs for distributed generation, devised for the minimization of common-mode emissions. These strategies have been called DPC-EMC 1 (electro magnetically compatible) and DPC-EMC 2. Both of them reduce the common-mode emissions of the VSI by using either even or odd voltage vectors in each of the six sectors in which the grid voltage lies, without using any null vector. DPC-EMC 2 outperforms DPC-EMC 1 in terms ripple of the active and reactive power waveforms. These approaches permit the common-mode emissions to be reduced in comparison with the classic DPC algorithm, at the expense of an increase of the harmonic content of the injected current waveform which can be further corrected by a proper power line filter. An experimental comparison among the classic DPC, DPC-EMC 1 and DPC-EMC 2 is presented in terms of dynamic performance, harmonic content of the injected current and harmonic content of the common-mode voltage.     

A Petri net based protection monitoring system for distribution networks with distributed generation

Connection of distributed generation (DG) essentially changes distribution network operation and creates a range of well-documented effects varying voltage levels and short circuit currents. Among others, DG can alter protection system operations in distribution networks, leading to failure of reclosing, disconnection of healthy feeder or prevention of protection operation. This paper proposes a procedure, based on Petri nets and supported by a centralized monitoring architecture for monitoring failures of the protection systems in radial distribution networks. Some case studies applied to a real Italian distribution network proved the effectiveness of the proposed procedure that can therefore represent an effective solution to improve distribution systems reliability in presence of DG.     

Power imbalance application region method for distributed synchronous generator anti-islanding protection design and evaluation

This paper presents a novel graphical approach to adjust and evaluate frequency-based relays employed in anti-islanding protection schemes of distributed synchronous generators, in order to meet the anti-islanding and abnormal frequency variation requirements, simultaneously. The proposed method defines a region in the power mismatch space, inside which the relay non-detection zone should be located, if the above-mentioned requirements must be met. Such region is called power imbalance application region. Results show that this method can help protection engineers to adjust frequency-based relays to improve the anti-islanding capability and to minimize false operation occurrences, keeping the abnormal frequency variation utility requirements satisfied. Moreover, the proposed method can be employed to coordinate different types of frequency-based relays, aiming at improving overall performance of the distributed generator frequency protection scheme.     

Optimization of biomass fuelled systems for distributed power generation using Particle Swarm Optimization

With sufficient territory and abundant biomass resources Spain appears to have suitable conditions to develop biomass utilization technologies. As an important decentralized power technology, biomass gasification and power generation has a potential market in making use of biomass wastes. This paper addresses biomass fuelled generation of electricity in the specific aspect of finding the best location and the supply area of the electric generation plant for three alternative technologies (gas motor, gas turbine and fuel cell-microturbine hybrid power cycle), taking into account the variables involved in the problem, such as the local distribution of biomass resources, transportation costs, distance to existing electric lines, etc. For each technology, not only optimal location and supply area of the biomass plant, but also net present value and generated electric power are determined by an own binary variant of Particle Swarm Optimization (PSO). According to the values derived from the optimization algorithm, the most profitable technology can be chosen. Computer simulations show the good performance of the proposed binary PSO algorithm to optimize biomass fuelled systems for distributed power generation.     

Optimization of distributed generation systems using a new discrete PSO and OPF

Many methods have been applied to achieve optimal site and size of distributed generation systems. This paper introduces a new hybrid method, which employs discrete particle swarm optimization and optimal power flow to overcome this shortcoming. The main technical constraints are imposed for utilities, which could apply this approach to search the best sites to connect distributed generation systems in a distribution network choosing among a large number of potential combinations. A fair comparison between the proposed algorithm and other methods is performed. For such goal, convergence curves of objective function versus number of iterations are computed. The proposed algorithm reaches a better solution than Genetic Algorithms considering similar number of evaluations.     

Reliability based optimum location of distributed generation

The increasing penetration of distributed generation and changing electricity industry necessitates that maximum benefit is obtained from distributed generators. Due to the importance of reliability of supply, the optimum location to maximise reliability has been investigated. A value based approach is used so that costs associated with levels of reliability may be used as an indicator. These are determined using probabilistic approaches which is a departure from the traditional deterministic criteria currently used.     

Control and operation of distributed generation in distribution systems

Many distribution systems nowadays have significant penetration of distributed generation (DG) and thus, islanding operation of these distribution systems is becoming a viable option for economical and technical reasons. The DG should operate optimally during both grid-connected and island conditions, which can be achieved with change in operating strategy. Many islanding detection techniques have been developed but little work has been done to detect grid re-connection. This paper presents strategies to operate DG in grid connected and islanded modes and a new grid re-connection detection algorithm, which uses average rate of change of frequency (Af5) and real power shift (RPS), in the islanded mode. RPS will increase or decrease the power set point of the generator with increasing or decreasing system frequency, respectively. Simulation results show that the proposed method can operate the DG efficiently and is effective in detecting grid-reconnection.     

Supplementing demand management programs with distributed generation options

Ever increasing electrical energy demand is forcing power serving entities around the world to use various demand management programs to help in stressful times of the electric power grid. Demand management programs aim to control electrical energy demand among customers and create load relief for electric utilities. Recently demand management contracts have been designed in which incentives are offered to customers who willingly sign up for load interruption. In recent years much technological advancement has been made in distributed generation, and the cost of using this option can bring about extra flexibility into existing demand management schemes. This paper explores the use of distributed generation technology within the existing demand management ideas. More specifically, it compares economic aspects of using demand management contracts with the use of distributed generation. A key observation of this paper is that there may be cases where it is more beneficial to use distributed generation rather than demand management contracts.     

Using gold sequences to improve the performance of correlation based islanding detection

One of the problems encountered when connecting distributed generators to a distribution system is the possibility of islanded operation. Traditionally this has been prevented through the application of passive under/over voltage and frequency relays which are triggered if the island contains mismatched amounts of active and/or reactive power, respectively. Various active techniques which reduce the power mismatch required for operation of the passive relays have been developed. These active techniques may fail to detect islanding in multiple generator islands if all the generators do not have identical active anti-islanding strategies. An islanding detection technique based on the correlation between disturbances in system voltage and a pseudo-random sequence used to perturb the generator’s output was developed for use in islands where generators may have different anti-islanding strategies. Previous investigations have always used pseudo-random sequences from the maximal length family of sequences. It is demonstrated in this paper that using either a Gold or Kasami sequence instead of a maximal length sequence can improve the performance of the correlation technique.     

Modeling and control of distributed generation systems including PEM fuel cell and gas turbine

This paper concentrates on the modeling and control of distributed generation systems including fuel cell and gas turbine. The fuel cell is connected to the power system through a dc/ac converter, which is equipped with both voltage- and power-control loops. The gas turbine is also assumed to be equipped with both voltage-control and generation (or frequency)-control loops. Moreover the gas turbine is modeled using the d–q frame of reference. The interfacing of the gas turbine with the power system is achieved by transforming its equations from the d–q frame of reference to power system frame of reference. A multivariable supplementary fuzzy logic controller is proposed for improving the dynamics of the combined fuel cell and gas turbine system. This fuzzy logic controller is designed using the Matlab Fuzzy Logic Toolbox A distribution test system including a load, a fuel cell and a gas turbine, connected to a power grid is simulated using Matlab/Simulink software package. The dynamics of the combined distributed generation plant are analyzed for the cases of with and without controller. The accuracy of the presented model and the effectiveness of the proposed multivariable supplementary fuzzy controller are deduced from the simulation results.