Optimal Placement of Distributed Generation an Overview and Key Issues
Abstract- Distributed Generation is the generation of electricity from many small energy sources and is located closer to the user, or customer. The purpose of using distributed generation is to improve voltage profile, voltage stability and to minimize power losses. This paper presents an overview of research and development work carried out in the field of Distributed Generation. This paper also discusses the key issues related to optimal placement and size of distributed generation. Types of distributed generation, technology used for distributed generation and related terms are also discussed.
Index Terms- Distributed generation, distributed generation types & technologies, purpose of distributed generation.
I.I NTRODUCTION
HE distributed generation can be defined as electric power generation within distribution networks or on the customer side of the networked[1], [2]. It may be explained in simple terms that are small-scale electricity agencies. The International Energy Agency (IEA) define distributed generation as a generating plant, transfer a customer on-site or providing support to a distribution system connected to the grid at distribution-level voltages [2]. CIGRE defines DG as the generation that has the following characteristics [3]. It is not centrally planned; it is not centrally dispatched at present; it is usually connected to the distribution network; it is smaller than 50-100 MW. Other organization like the Electric Power Research Institute (EPRI) defines a distributed generation as the generation from a few kilowatts up to 50 MW [4]. A number of technologies of DG have reached in a developed stage allowing for a large scale implementation within existing electric Utility system [5]. In general, DG means small scale generation. A lot of technologies are used for DG sources such as photo voltaic cells, wind generation, combustion engines, and fuel cells and there are some other types of generation from the natural or artificial resources that are available in the geographical area [6]-[7].The ever-increasing power demand, This work was supported by University Grants Commission (UGC), New Delhi, India under Major Research Project received vide F. No. 41-657/2012[SR] dated 26-07-2012.
Alka Yadav is with the Electrical Engineering Department, Madhav Institute of Technology & Science, Gwalior, India. (e-mail: yadav.alka20@https://www.360docs.net/doc/f011130984.html, ).
Laxmi Srivastava is with the Electrical Engineering Department, Madhav Institute of Technology & Science, Gwalior, India. (e-mail: srivastaval@https://www.360docs.net/doc/f011130984.html, ).
978-1-4799-3612-0/14/$31.00?2014 IEEE steady progress in the power deregulation and tight constraints over the construction of new transmission lines for long distance power transmission have created increased interest in distributed electricity generation. Distributed generation (DG) devices can be strategically placed in power systems for grid reinforcement, for reduction in on-peak operating costs and power losses, improvement of voltage profiles and load factors, system reliability, integrity and efficiency [8]-[9].
The DGs are increasingly in use because these are more economical than connecting a power line in remote areas. DGs are able to provide higher power quality for electronic equipments. DGs are also used for system black-start to start generation and restore a portion of the utility system without outside support after a system collapse [10].
This paper presents review of some techniques applied in DG and the approaches used by researchers for finding optimal location of DG for various purposes. In addition, this paper also discusses the key issues. No attempt is made here to prove the effectiveness of the solution technique applied by researchers for optimal placement of DG.
II.D ISTRIBUTED G ENERATION
Distributed generation is known by various names like decentralized generation, dispersed generation, embedded generation, on-site generation, distributed energy or redistributed energy. Any distributed generation creates electricity from many small energy sources. Almost all the countries generate electricity in large centralized facilities, such as fossil fuel, nuclear, hydropower plants or large plants. The classification of DG [4] has been illustrated in Fig. 1. According to their ratings, DG may be classified as micro, small, medium or large.
Distributed Generation
(Classes)
Micro
( ~1Watt < 5kW)
Small
(5 kW < 5 MW)
Medium
(5 MW < 50 MW)
Large
(50 MW < 300 MW)
Fig.1. Distributed Generation classes
The term distributed generation is sometimes used interchangeably with the term distributed resources (DR). But distributed resources is intended to include non generating
Optimal Placement of Distributed Generation: An Overview and Key Issues
Alka Yadav, and Laxmi Srivastava, Senior Member IEEE
T
technologies such as power storage devices like batteries and flywheels in addition to generators, while distributed generation is limited to small scale. Unlike central power plant generation, DG often utilizes the waste heat from the generation process as an additional form of energy for space or heating process, dehumidification, or for cooling through absorption refrigeration. Supplying peaking power to decrease the cost of electricity, decrease environmental emissions through clean and renewable technologies (Green Power), combined heat and power (CHP), high level of reliability and quality of supplied power and deferral of the transmission and distribution line investment through improved loadability are the major applications of the DG [11]. Other than these applications, the major use of DG in the deregulated environment lies in the form of ancillary services including voltage control and reactive power supply, spinning and non-spinning reserves, etc. [12].
III.T YPES OF D ISTRIBUTED G ENERATION There are different types of DGs from the constructional and technological points of view as shown in Fig.2 [5]. DGs may be broadly categorized as conventional and non-conventional generators.
A. Conventional Generator
Micro turbines are a relatively new type of combustion turbine that produces both heat and Electricity on a small scale. Micro turbines offer a clean and efficient solution to direct Mechanical drive markets such as compression and air-conditioning [13]. Micro turbines are small combustion turbines with outputs of 25 kW to 500 kW. They developed from automotive and truck turbochargers, auxiliary power units (APUs) for small jet engines, and airplanes. Micro turbines are a relatively new distributed generation technology being used for stationary energy generation uses. They are a type of combustion turbine that produces both heat and electricity on a relatively small scale [13].
Distributed Generation
Types & Technologies
Conventional Generator
Non-
Conventional
Generator
Electrochemical
Devices
Storage
Devices
Renewable
Devices
Micro Turbine
Fuel Cells Batteries Flywheels
Photovoltaic Cells Wind Turbine
Natural Gas Turbine
Fig. 2. Distributed generation types and technologies.
The conventional combustion turbines, MTs run at less pressure, temperature and faster speed, which sometimes require no gearbox [14]. The advantages of Micro turbines are compact size, light in weight, large efficient and have less emissions, lower capital and electricity costs than any other DG technology costs [15], Advance power electronic interface between the MT and the load or grid increases its flexibility to be controlled efficiently [16].
Gas turbines produce pressure gas and high temperature. Turbine shaft rotate with the help of this high pressure, which drives a compressor, an electric alternator and generator. Gas turbines are always used above 1MW, but nowadays we can generate electricity through small modular with a micro-gas turbine of 200kW size [14]. The waste heat is produce to generate steam for compound heat and power.
B. Non-Conventional Generators
The non-conventional generators include Electrochemical Devices, Storage devices and Renewable devices.
A. Electrochemical Devices
The fuel cell (FC) is a device used to generate electric power and provide thermal energy from chemical energy through electrochemical processes. It can be weigh as a battery supplying electric energy as long as its fuels are continued to supply. Unlike fuel cell, batteries do not need to be charged for the consumed materials during the electrochemical process since these materials are continuously supplied [17]. FC capacities vary from kW to MW for portable and static units, respectively. It provides clean power and heat for several applications by using gaseous and liquid fuels [18]. FCs can use a variety of hydrogen-rich fuels such as natural gas, gasoline, biogas or propane [19]. FCs operates at different pressures and temperatures which vary from atmospheric to hundreds of atmospheric pressure and from 20 to 200C, respectively [19].
B.Storage Devices
Storage device are consists of flywheels, batteries, and other devices, they are charged during low load demand and used when required. They are usually combined with other kinds of DG types to supply the required peak load demand [16]. These batteries have a charging controller for protection from overcharge and over discharge as it disconnects the charging process when the batteries have full charge. The sizes of these batteries find the battery discharge period. However, flywheels systems could be charge and provide 700kW in 5 s
[16].
C.Renewable Devices
Renewable devices commonly used for DG are Photovoltaic and Wind turbines. Photovoltaic (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the outcomes. PV power generation employs solar panels collected of a number of solar cells containing a photovoltaic material. Currently material used for photovoltaic include, mono crystalline silicon, amorphous silicon, copper indium
gallium selenide/ sulphide, polycrystalline silicon and cadmium telluride. Cells absorb solar energy with the help of sunlight, where the light photons force cell electrons to flow, and transfer it to dc electricity. Practically, each cell supply 2–4A according to its size with a 0.5V output voltage. Usually an array, cells connected in series, provides 12V to charge batteries PVs consist of modular which can be connected to provide a variety of power ranges but on the other hand there are many restrictions. Photovoltaic devices provide low output power, the cost of land is expensive where PVs installed and it is restricted to certain weather and geographic features [15].
Modern wind energy systems consist of three basic components: a tower on which the wind turbine is mounted; a rotor (with blades) that is turned by the wind; and the nacelle. Their shape was capsule-shaped component which houses the instrument, including the generator that converts the mechanical energy in the spinning rotor into electricity. Rotor blades need to be strong and light in order to be aerodynamically efficient and to withstand prolonged use in high winds. The rotor, which spins when driven by the wind, supports blades that are designed to capture kinetic energy from the wind. The advantages of Wind Turbines does not cause green house gases or other pollutants, great resources to generate energy in remote areas, take up less space in compare to power station, no fossil fuels are used to generate electricity.
IV.T ECHNOLOGY USED IN D ISTRIBUTED G ENERATION
As can be observed from table 1 current practice also shows that available technology for DG varies widely [4].There is many of the technologies utilize renewable energy resources. As per the International Energy Agency (IEA), renewable energy resources are defined as resources that are generally not subject to depletion, such as the light and heat from the sun, the force of wind, falling water, and geothermal heat [20]. As about 1000 times more energy reaches the earth as fossil fuel is currently consumed, natural energy resources can be described as easy. However, availability of the different resources varies significantly between areas and countries, as well as technology efficiency to harvest the renewable energy resources.
TABLE I
T ECHNOLOGIES FOR DISTRIBUTED GENERATION
Technologies for DG Typical Available size
per module
Combined cycle gas 33-400MW
Micro-Turbines (MT) 35KW-1MW
Renewable
Wind Turbine (WT) 200Watt-3MW
Photovoltaic arrays (PV) 20Watt-100KW
Solar Thermal 1-80MW
Gasification
Fuel cells, phosacid 200KW-2MW
Fuelcells, molten carbonate 250KW-2MW
Fuel cells, proton exchange 1KW- 2MW
Fuel cells, solid oxide 250KW-5MW
Geothermal 5-100MW
Battery storage 500 KW- 5MW
V.P URPOSE OF D ISTRIBUTED G ENERATION The main objectives of using DG in a power system are to minimize power losses, and to improve voltage profile, voltage stability, reliability and power quality. Some of the techniques/ approaches used for placement of DG optimally to achieve certain objective by various researchers have been summarized here.
A. Minimizing Power Losses
Rau et.al [8] proposed a second order algorithm with transformation of variables to optimally allocate DG resources in a meshed network. They expressed the benefits of DG as a performance index, which was the minimization of network losses. The convergence properties of the proposed algorithm have been examined with a six bus test system.
Kim et.al [21] proposed a fuzzy-GA method to resolve dispersed generators placement for distribution systems with the objective to reduce power losses of distribution systems. The constraints were the number or size of distributed generators and the deviation of the bus voltage. This objective function and constraints were transformed into multi-objectives functions and modeled with fuzzy sets. By the application of this methodology, the dispatcher can obtain the most compromised or satisfied solution among multi-objectives.
Wang et.al [22] proposed analytical methods to determine the optimal location to place a DG in radial as well as networked systems to minimize the power loss of the system. The proposed approaches are not iterative methodology, like power flow programs. Therefore, there is no convergence problem concerned, and results could be obtained. The proposed method was tested on the IEEE 6- bus and 30-bus test systems.
Acharya et.al [23] proposed an analytical expression to calculate the optimal size and an effective algorithm to identify the corresponding optimum location for DG placement for minimizing the total power losses in primary distribution systems. The methodology and analytical expression were based on the exact loss formula. The effect of location and size of DG with respect to loss in the network had been examined in detail. The proposed methodology had been tested and validated in three distribution test systems with varying size and complexity.
Moradi et.al [24] proposed an efficient hybrid method based on Imperialised Competitive Algorithm (ICA) and GA. The objective function is power loss reduction, increasing voltage stability index, load balancing, improving system voltage profile, and transmission and distribution relief capacity for both utilities and the customers. The proposed methods were implemented on IEEE-33 bus and 69-bus radial distribution systems and the result compared with GA/PSO methods. Garcia et.al [25], proposed a method which employs a Modified Teaching–Learning Based Optimization (MTLBO) algorithm to determine the optimal placement and size of a given no. of DG units in distribution systems. The objective function was to minimize power losses. The proposed
approach was a discrete version of the MTLBO algorithm. They worked on 69-bus test systems and 119-bus test system. B. Voltage Profile Improvement
Borges et.al [26] proposed an algorithm for optimal DG allocation and sizing in distribution systems. The optimization procedure was solved by the combination of GA techniques with methods to evaluate DG impacts in system voltage profile. The voltage profile rating was based on a power flow method for radial networks with the representation of distributed generators. They also proposed a method for optimal DG unit’s allocation and sizing in order to maximize a benefit/cost relation, where the benefit is measured by the reduction of electrical losses and the cost is dependent on investment and installation.
Kalantari et.al [27] proposed GA based optimal placement of DG units and proper allocation of shunt capacitors in order to improvement of voltage profile in distribution systems. The authors reported that the objective function has important index (voltage profile). The power flow had been done using backward forward sweep method and simulation had been carried out on a 28 bus test system.
Rotaru et.al [28] proposed a method to obtain the optimal size of DG sources in electrical distribution systems, taking into account the time-dependent rise of generation and load. The external stage was carried out by selecting a set of candidate nodes through a clustering-based approach based on normalized node voltages. The search of internal stage was driven by the calculation of an objective function with energy losses and voltage profile components.
C.Voltage Stability
Satpathy et.al [29] proposed a fuzzy voltage stability index indicating the location and status of critical bus bars. The input parameters were efficiently modelled in terms of fuzzy sets by assigning trapezoidal and triangular membership functions. Their results include fuzzy load flow solutions for the base case and critical conditions with and without contingencies. The case studies had been conducted on standard test systems were IEEE 14-bus, 30-bus, and 57-bus with proper substantiation of the results.
Manjunatha et.al [30] proposed a heuristic technique for allocation of DG source in a distribution network. The allocation was determined based on overall improvement in network performance parameters like improvement in voltage stability. The Network Performance Enhancement Index (NPEI) serves as an indicator for best possible choice. The designer will be able to select the optimum solution satisfying all the constraints with the help of NPEI. The developed approach was tested on 33 bus and 90 bus systems and results obtained have shown that the technique gives the better and economical solution for system improvement.
Kayal et.al [31] proposed a multi-objective PSO based wind turbine generation unit and photovoltaic array placement approach for voltage stability improvement of radial distribution system. Wind and solar based DG were operated in different active and reactive power mode and tested on four buses (12-bus, 15-bus, 33-bus and 69-bus) radial distribution system.
Roy et.al [32] proposed a static and dynamic VAR planning based on the reactive power margin for enhancing dynamic voltage stability of distribution networks with distributed wind generation. The impact of high wind penetration on the static voltage stability of the system was examined and then the effect of composite loads on system dynamics was presented through an accurate time domain analysis.
D.Reliability
Arya et.al, [33] proposed a methodology for reliability enhancement of radial distribution system by determining optimal values of repair times and failure rates of each section. Constraints on customer and energy based indicant, i.e. SAIFI, SAIDI, CAIDI and AENS were considered. Differential Evolution algorithm has been applied to evaluate optimum failure rate and repair time for each section so as to achieve desired reliability goals in terms of the mentioned indices. The problem has been solved in two stages one optimizes failure rate and other optimizes repair time.
Mohammadi et.al [34] proposed a new algorithm using PSO approach for the placement of Distributed Generators in the radial distribution systems to improve the system reliability. The effect of DG on reliability improvement of system has been considered and it is considered as one indices named as Reliability Improvement Index. They proposed method was tested on standard IEEE 12 bus test system.
E. Power Quality
Munoz et.al [35] proposed the addresses how DG, particularly when configured in Combined Heat and Power mode, can become a powerful reliability solution in highlight automated factories, particularly when integrated with complimentary power quality evaluate. The paper presented results from the PQ audit conducted at a highly automated plant. They were found that the main problems for the equipment installed were voltage sags.
Saedi et.al [36] proposed an optimal power control strategy for an autonomous micro grid operation based on a real time self-tuning method. The purpose of this work was to improve quality of power supply of the micro grid where some DG units were connected to the grid. The main performance parameters considered were voltage and frequency regulation and power sharing, especially during the transition from grid-connected to islanding operation mode and during load change.
Ipinnimo et.al, [37] presented a comprehensive review and comparison of various DG schemes used by utilities for mitigation of voltage dips in power networks.Power quality problem is a serious concern for voltage quality disturbances such as voltage dips. With the increasing usage of sophisticated sensitive electronic equipment in industrial, commercial sectors and residential, it is important to protect them from any power quality disturbance.
Basu et.al, [38] proposed performance of UPQC as a suitable interfacing equipment for enhancement of power
quality. They described the two control strategy models for UPQC, for raise power quality of sensitive non-linear loads. On the basis of two different voltage compensation strategies, two control schemes were designed. These were termed as UPQC-Q and UPQC-P. The effectiveness of the two control strategies was tested through simulation using the software SABER.
VI.K EY I SSUES
The major issues related to distributed generation are distribution process and planning, distribution network, power reserve and balancing, power quality and connection with grid.
A.Distribution Process and Planning Issues
Local protection systems need redesign efforts since DG technologies connection not only changes the power flows pattern, but also affects local voltage and fault current levels.
A revision of the distribution grids architecture may be needed due to the two-way power exchange resulting from DG deployment. Data collection needed for controlling the distribution and DG systems can be complex as the distribution system generally not controlled by SCADA (Supervisory Control and Data Acquisition) [39].
B.Distribution Network Issues
Firstly, distribution networks are often designed for a different purpose than transmission system. The main difference is that distribution systems are usually not designed for the connection of power generation devices. Moreover, distribution networks have usually a loop or radial design, and not a meshed design like transmission system. Secondly, high voltage lines, e.g. urban distribution lines or transmission lines have a low resistance compared to low voltage lines in distribution system. In transmission lines or urban distribution transmission lines networks, the effect of line or cable resistance (R) on voltage drop is small, since its magnitude is generally much less than the reactance (X), i.e. X/R>5[4].
C.Power Reserve and Balancing
Due to the intermittent output from some DG sources, the DSO (Distribution System Operator) must be able to manage fast reacting local power generation and in some cases procure the needed power reserve from the upstream transmission. By a growing penetration of intermittent renewable and CHP technologies, the cost of imbalances may consistently increase, and the application of priority dispatch mechanisms may become increasingly difficult [39].
D.Power Quality
The coming up of power quality sensitive equipment has made the provision of good power quality a real challenge across the globe. Also, the harmonics may be generated by power electronic converters used to connect DG technologies, thus causing disturbances on the grid. These may, however, be damped by properly designed filters [39]. Some of the power quality issues like harmonics, voltage variation and voltage flicker, sustained interruptions, voltage sag and voltage regulation might arise when distributed generators are interconnected to the utility distribution system [40].
E.Connection Issues
The connection of DG technologies and electricity generation technologies can be far different from conventional centralized power generation technologies. Synchronous generators are used for large DG units. These are capable of controlling the reactive power output. Large DG units utilizing natural gas for instance, also use synchronous generators. Medium-sized and small DG units are usually asynchronous generators (induction generators), as they are significantly inexpensive than synchronous generators [4].
VII.C ONCLUSION
In this paper, types of distributed generation and technologies used for distributed generation are discussed. In addition, an overview of research work carried out for optimal placement and sizing of distributed generation to achieve certain objectives by various researchers has been presented. The main aims of using DG in a power system are to minimize power losses, and to improve voltage profile, voltage stability, reliability and power quality. Also, the key issues related to distributed generation are also discussed.
VIII.A CKNOWLEDGMENT
The authors thank the Director, MITS Gwalior, India to carry out this work.
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X. B IOGRAPHIES
Alka yadav was born in Dispur in Assam, on june 20, 1988.Her graduation in Electrical Engineering from the Maharana Pratap College of Technology (MPCT) in Gwalior and pursuing M.E at Madhav
Institute of Technology and Science (MITS) in Gwalior. Alka received honorary degrees from institutions of RGPV University.
L. Srivastava (S’96-M’03) obtained her M. Tech. degree in Electrical Engineering, from the Indian Institute of Technology, Kanpur, India in 1990 and her Ph.D. degree from University of Roorkee (Presently IIT), Roorkee, India in 1998. She is a Professor in the Deptt. Of Elect. Engg., M.I.T.S. Gwalior, India. She is currently involved in research in Power System Security Analysis,
Optimization and Control, FACTS controllers and their applications, Congestion Management, ANN,
Fuzzy logic and Evolutionary programming
applications to Power Systems. She is a Fellow of
the Institution of Engineers (India) also.