Optimal planning of distributed generation systems in distribution system: A review

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Abstract

This paper attempts to present the state of art of research work carried out on the optimal planning of distributed generation (DG) systems under different aspects. There are number of important issues to be considered while carrying out studies related to the planning and operational aspects of DG. The planning of the electric system with the presence of DG requires the definition of several factors, such as: the best technology to be used, the number and the capacity of the units, the best location, the type of network connection, etc. The impact of DG in system operating characteristics, such as electric losses, voltage profile, stability and reliability needs to be appropriately evaluated. For that reason, the use of an optimization method capable of indicating the best solution for a given distribution network can be very useful for the system planning engineer, when dealing with the increase of DG penetration that is happening nowadays. The selection of the best places for installation and the preferable size of the DG units in large distribution systems is a complex combinatorial optimization problem.

This paper aims at providing a review of the relevant aspects related to DG and its impact that DG might have on the operation of distributed networks. This paper covers the review of basics of DG, DG definition, current status of DG technologies, potential advantages and disadvantages, review for optimal placement of DG systems, optimizations techniques/methodologies used in optimal planning of DG in distribution systems. An attempt has been made to judge that which methodologies/techniques are suitable for optimal placement of DG systems based on the available literature and detail comparison(s) of each one.

Introduction

Distributed generation, also called on-site generation, dispersed generation, embedded generation (EG), decentralized generation, decentralized energy, situ generation or distributed energy, generates electricity from many small energy sources. Currently, industrial countries generate most of their electricity in large centralized facilities, such as fossil fuel (coal, gas powered), nuclear, large solar power plants or hydropower plants. However, modern embedded systems can provide these traits with automated operation and renewables, such as sunlight, wind and geothermal [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98], [99], [100], [101], [102], [103], [104], [105], [106], [107], [108], [109], [110], [111], [112], [113], [114], [115], [116], [117], [118], [119], [120], [121], [122], [123], [124], [125], [126], [127], [128], [129], [130], [131], [132], [133], [134], [135], [136], [137], [138], [139], [140], [141], [142], [143]. These plants have excellent economies of scale, but usually transmit electricity long distances and negatively affect the environment. In the line of above distributed energy resource (DER) systems are small-scale power generation technologies (typically in the range from 3 kW to 10,000 kW) used to provide an alternative to or an enhancement of the traditional electric power system [2], [10].

DG has always been an attractive alternative for rural areas where transmission and distribution (T&D) costs are high, and DG is quickly becoming an attractive option for more densely populated regions due to the uncertainties associated with industry restructuring and difficulties in permitting discourage new T&D investments [3], [4], [5], [6]. The detailed comparison between distributed resource power system and conventional central station generation with T&D system, on the basis of various performance characteristics such as efficiency and losses, voltage profile improvement, reliability and power quality, investment, fuel, operation and maintenance (O&M), emissions, etc. is given in [7], [8]. The application(s) of small generators scattered throughout the power system will cope up with the growing demand for electricity in certain areas and render certain activities self-sufficient in terms of power production and achieving energy savings DG is best suited for demand side management programs [9].

The technologies adopted in DG comprise small gas turbines, micro-turbines, fuel cells, wind and solar energy, biomass, small hydro-power etc. [10], [11], [12]. DG can be used in an isolated way, supplying the consumer's local demand, or in an integrated way, supplying energy to the remaining of the electric system. In distribution systems, DG can provide benefits for the consumers as well as for the utilities, especially in sites where the central generation is impracticable or where there are deficiencies in the transmission system. Fig.1 depicted difference between the central utility of today and distributed utility of tomorrow [44].

It often offers a valuable alternative to traditional sources of electric power for industrial, commercial and residential applications so it appears as an alternative that utility planners should explore in their search for the best solution to electric supply problems. The main reasons for the increasingly widespread use of DG can be summed up as follows [2], [4], [10], [11], [13], [14], [15], [23], [25], [27], [113]:

  • DG units are closer to customers so that T&D costs are avoided or reduced.

  • T&D costs have risen while DG costs have dropped; as a result the avoided costs produced by DG are increasing.

  • The latest technology has made available plants with high efficiency and ranging in capacity from few kW to hundreds of MW of different DGs.

  • It is easier to find sites for small generators.

  • Natural gas, often used as fuel in DG stations, is distributed almost everywhere and stable prices are to be expected.

  • Usually, DG plants require shorter installation times and the investment risk is not so high.

  • DG plants yield fairly good efficiencies especially in cogeneration and in combined cycles (larger plants).

  • The liberalization of the electricity market contributes to creating opportunities for new utilities in the power generation sector.

  • DG offers great values as it provides a flexible way to choose a wide range of combinations of cost and reliability.

Parallel to the introduction of DG; when distribution system planning and DG impact are considered, the greatest attention should be paid in the siting and sizing of DG units because their installation in non-optimal locations can result both in an increasing of power losses and in a reducing of reliability levels [14], [16], [17]. Then proper tools, able to find the siting and sizing of DG units which reduces at maximum the costs while satisfying technical constraints can aid for the planner who has to face with the worldwide growth of DG penetration [15], [18]. From distribution system planning point of view, DG is a feasible alternative for new capacity especially in the competitive electricity market environment and has immense benefits such as:

  • Short lead time and low investment risk since it is built in modules.

  • Small-capacity modules that can track load variation more closely.

  • Small physical size that can be installed at load centers and does not need government approval or search for utility territory and land availability [20], [21].

  • Existence of a vast range of DG technologies [19].

Distribution systems have traditionally been designed to operate with unidirectional power flow, from the source (transmission system) to the loads. Adding DG to a distribution system imposes a different set of operating conditions on the network, namely reverse power flow, voltage rise, increased fault levels, reduced power losses, harmonic distortion and stability problems [22]. The optimal locations of distributed resources should be identified in a network in order to minimize losses, line loadings, and reactive power requirement. The foreseeable large use of DG in the future requires to distribution system engineers to properly take into account its impact in the system siting and sizing.

Section snippets

Distributed generation definition

Different definitions regarding DG are used in the literature and in practice. These variations in the definition can cause confusion. According to Electric Power Research Institute (EPRI) defines DG as generation from ‘a few kilowatts up to 50 MW’ [10]. In Refs. [10], [36], [37], [38], [39], a large number of terms and definitions are used in connection with the DG. Therefore, in [1], [3], [8], [9], [10], [14], [15], [20], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]

Impact of distributed generation

As mentioned earlier distribution systems are designed on the assumption that electric power flows from the power system to the load. Therefore, if output fluctuations or a reverse flow from generators occurs on the grid because of DG, there is likely to be some influence on the overall distribution system in terms of power losses, voltage profile, reliability, power quality or protection and safety. The potential impacts of DG are described below [70].

Various DGs planning methodologies and their comparison

Aforementioned benefits cannot be maximized except when optimal sizes of DG units are determined and are consequently placed in the best location in distribution systems. To find out the optimal size and location of DG units in power systems has been a major challenge to distribution system planners as well as researchers in the field. In tackling this problem many critical review of different methodologies employed; hence in solving this optimization problem. For ease of reference and to

Conclusion

In this paper, the effort is made towards the overview of the relevant aspects related to DG and its impact that DG might have on the operation of distributed networks. This report evolve the basics of DG, DG definition, current status of DG technologies, potential advantages and disadvantages; optimal placement of DG systems and uncertainties, optimizations techniques/methodologies used in optimal placement and impacts of DG in distribution systems. This paper also reviewed the benefits of DG

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