Critical Review of Different Methods for Siting and Sizing Distributed-generators

Due to several benefits attached to distributed generators such as reduction in line losses, improved voltage profile, reliable system etc., the study on how to optimally site and size distributed generators has been on the increase for more than two decades. This has propelled several researchers to explore various scientific and engineering powerful simulation tools, valid and reliable scientific methods like analytical, meta-heuristic and hybrid methods to optimally place and size distributed generator(s) for optimal benefits. This study gives a critical review of different methods used in siting and sizing distributed generators alongside their results, test systems and gaps in literature


Introduction
Electric power system (EPS) is one of the most complex conceptions by mankind, and it is a non-linear system.Aside this, its construction and operation are very complex and complicated because of several factors and constraints that must be considered in terms of location, type, available resources, etc.The purpose of Electrical Power System (EPS) is to generate and supply electrical energy to users [1].It comprises generation station, transmission network, distribution network and load centres.The load centres receive and consume generated power by the generation stations via the link of transmission and distribution networks.However, in a deregulated electricity market, congestion on transmission lines maybe unavoidable because of insufficient capacity of lines [2].Moreso, under voltages and over voltages in the lines lead to poor power quality and lack of stable power system [3].Not with standing, power engineers during planning stage, give a margin or forecast to accommodate future load demand on the network, however, development brings about increase in the load demand which will outgrow the specified margin at some points.Hence, there will be need for expansion when load demand equals or greater than the supply power from generation stations.
On the contrary, construction of a new generation station requires a huge capital.This has propelled several researchers to investigate alternative means to offset overshoot in load demand against the supplied power from the generation stations.One major solution discovered was the installation of distributed generator (DG) close to the load centres.Though there are some other solutions, DG gives the best option to overcome load demand, economic and environmental challenges [4] among other methods such as FACTS devices for power system improvement, network reconfiguration, capacitor compensation, static VAR etc. [5][6][7][8][9][10][11][12][13][14][15][16].Review of work done with distributed generators, will be the area of focus in this work.

What then is Distributed Generator?
Several researchers have put forward diverse definitions of distributed generator as given in [4], [6], [17][18][19][20][21][22][23][24][25][26].According to CIRED (1999), there is no consensus yet on the generally acceptable definition of DG.However, the definition given by T. Ackermann et al.  [17] will suffice for this work.It is defined as "the electric power generation source linked directly to distribution network or the meter side of customer".Other criteria for which DG can be classified are given in Figure 1 [27].Unlike conventional Central Generation (CG), distributed generation is not location bound (as the name implies).Table 1 gives comparison between CG and DG.Basically, gas and hydro turbine The technologies adopted in DG comprise small gas turbines, micro-turbines, fuel cells, wind and solar energy, biomass, small hydro-power etc.

Significance of Distributed Generator
According to the IEA (2002), there are five major factors that contribute to the advancement in distributed generation namely; developments in distributed generation technologies, constraints on the construction of new transmission lines, increased customer demand for highly reliable electricity, the electricity market liberalization and concerns about climate change.However, these factors can be summarized under these two key issues: cost effectiveness of distributed generation and friendly environmental impact.This is because DG technology is being developed (not explored) continuously because it is cost effective.Also, DG saves a huge amount of money that would have been budgeted for transmission lines.Meanwhile, consumers can only demand for a ISSN: 1693-6930  Critical Review of Different Methods for Siting and … (Shomefun TS) 2397 reliable system that is affordable.However, no investors will like to venture into any business that is not profit oriented.And lastly, advent of renewable energy sources (RES) technology, which is free from greenhouse gas emission, mitigates concerns about climate change since they are environment friendly and readily available by nature.Hence, it can be said that Installation of distributed generation permits the utilization of freely available fuel opportunities [6].Installation of DG is a short-time project and it is a less expensive alternative for electric power system expansion compared to construction of a new generation station [24,27].Employing this method will not only help to meet load demand but also, improve voltage profile, increase the system reliability level [27], minimize Total Harmonic Distortion (THD) [28], minimize cost of electricity [29], lower short-circuit level [30], relieve transmission and distribution congestion and minimize line losses.This is because it is located closer to the point of consumption than the main source for the distribution network [31].
However, with so much positive impacts which DG adds to electric power system, it must be strategically and optimally located to achieve the intended results [24].It must also be properly and optimally sized to avoid excessive generation cost, increase in the power loss, and bus voltage fluctuating in and out of the statutory limit [18,32,33].
Several researchers have used various methods to site and size distributed generator(s) ranging from analytical methods to hybrid-based optimization methods.Some of these methods are Gradient and second-order method, Hereford Ranch algorithm, Heuristic iterative method, Analytical based on 2/3 rule, Tabu search, Hybrid fuzzy nonlinear goal programming, Heuristic iterative search method, Linear programming, Sensitivity analysis, Hybrid e-constraint-based multi-objective programming, Optimal power flow, GA, Mixed integer non-linear programming, Iterative search technique with load flow [34].The details of the various methodologies that have been deployed to date are as presented in Table 2. Techniques for DG placement differ, and they are dependent on the objectives to be achieved.These techniques have their strengths and drawbacks.Table 3 gives detailed comparison of these techniques.For 38-bus system, the reduction in the active power loss was in the range of 54-67%.The reduction in the reactive power loss was in the range of 58-67%.The reduction in the total MVA intake was about 30%.For the 30-bus system, the reduction in the active power loss was in the range of 30-37%.The reduction in the reactive power loss was in the range of 26-31%.The reduction in the total MVA intake was about 62%.

Discussion
Although several researchers have considered integration of distributed generation into EPS as an alternative to construction of centralised generation station, some prevailing research problems, which require more investigations, are still open and they are listed as follows: a.One major prevailing problem in the planning of power system to incorporate DGs is to take into account various factors such as nature of DG technology, impact of DG on operating characteristics of power system and economic considerations [40].b.Another problem of integrating DG into the grid is islanding issue for which IEEE 1547 standard [41] was established: a criterion for interconnection of DG sources.The present standards do not allow islanded operation of DG [42] c.The possibility of reliability enhancement with increased penetration of RES-based DGs is another prevailing problem and it has also not been investigated.Likewise, the reliability assessment studies during islanded mode, incorporating RES-based DGs and storage has not been reported in literature.[40] d.DGs integration impact on system reliability, line losses, emissions, voltage profile and cost for an optimum system planning [40].

Conclusion
This study gave a critical, comprehensive and systematic survey of the existing methods for integrating DG(s) into EPS in order to mitigate continuous increase in load demand.Three categories of optimasation techniques i.e. analytical, meta-heuristic and hybrid optimisation methods were considered.This categorization, as well asthe representative techniques described under each category, will benefit optimisation techniques'researchers for choosing from proper state-of-the-art population-initializationbased techniques for their research.The volume of the surveyed techniques revealed that optimisation techniques havebecome an active research topic in electrical power system domain.However, some questions are yet to beresolved.Some of these questions were highlighted for future investigation.
Based on the reviewed literature, this study also gave a review of different optimisation methodologies for siting and sizing distributed generators in a distribution network.The test systems/networks as well as results obtained from these methods were also recorded.The observed gaps in the reviewed literature were also provided and finally, the strengths and weaknesses of the available methods were also included.However, most of the previous works were carried out on conventional DGs.Though many researchers did not specify the DG technology employed, their analyses prove that RES were not considered.Therefore, recommendations for further studies in this area of research will include integration of RES into the grid, consideration for islanding in integrating DG into the grid and protection coordination of a network with DG(s).

Figure 1 .
Figure 1.Criteria for DG classification

Table 2 .
Different Methods Used in Siting and Sizing DG with Test System, Result and Observed Gaps

Table 2 .
Different Methods Used in Siting and Sizing DG with Test System, Result and Observed Gaps

Table 2 .
Different Methods Used in Siting and Sizing DG with Test System, Result and Observed Gaps

Table 2 .
Different Methods Used in Siting and Sizing DG with Test System, Result and Observed Gaps

Table 2 .
Different Methods Used in Siting and Sizing DG with Test System, Result and Observed Gaps

Table 3 .
Comparison of Different Methods Used