Distributed Generation: A Critical Review of Technologies, Grid Integration Issues, Growth Drivers and Potential Benefits

Owing to liberalization of electricity market, technology evolution, energy security, environmental issues and growing concerns of energy cost, the penetration of distributed energy units in distribution network is increasingly observed worldwide. Penetratingrembedded generation, or distributedrgeneration (DG), in powerrdistribution grid requires asnumber of issues to be considered, such as definition of DG, rating of DG, the best DG technology etc. This paper presents an extensive critical review of various dimensions of distributed generation (DG) including definitions, generation technologies and their status, impact on distribution network performance etc. The study also presents comparative study between the various technologies in terms of most important technological characteristics of each DG technology. The policy makers, utility regulators and DG planning engineers can use this critical review, withoutigoing through complicatedicomputations, as guidelines to makeipolicies, standards andidecisions in DG penetration andirelated issues. ©2020. CBIORE-IJRED. All rights reserved


Introduction
SinceDthe emergenceDof AlternatingSCurrent (AC) electricalisystems and theiability to transportilarge blocks ofipower over longidistances, the top -downiparadigm has governed electricitykkgeneration andkksupply. This concerned large scale generation feeding into high voltage transmissionisystemsiwhich transferred powerito medium voltageSdistribution networksSand on to lowAvoltage customerrlevel. Considering the way inewhich thecvery earliest electricity utilitiestwere functioning to generate and distributebelectricity, it isnapparent that the idea behind the distributed generation (DG) is not entirely new at all. Network operators had their own assigned service areas, generating and delivering electricity to the consumers in their assigned geographical regions (Mehigan et al., 2018). National electric grids then were recognized and came along to develop large interconnected power networks that provided more reliable, economic and efficient power systems (Perez-Arriaga, 2016;Manfren et al., 2011).
In recent years, the electricitytsupply frameworktand its associateditransmissioniand distribution systems, has observed significant changes throughout the world. The driving forces behind these changes mainly include regulatory issues, reliability, security, technological advances (evolution of Smart Grids) and emissions reduction concerns. Thekgrowth ofkcompetition inkthe electricityncmarket, thencdevelopment ofkkrenewable electricitytgeneration technologies, concernsroverrageing infrastructure, andtcapacity constraintsthave stimulated increasing participation of DG technologies in power systems to address such key issues (Carley, S, 2009). Additionally, in the liberalized electricity market of today, the market players are not easily convinced to take investment decisions in multibillion power generation and transmission projects because of very long payback period. These key concerns, and the liberalization of the electricity sector and decentralization of power systems, along with exponentially growing consumers' demand globally has made DG technologies an attractive and viable solution to provide power generation of the future (Allan et al., 2015). Further, the integration of diverse DG sources to the utility networkswill offer aknumber of benefitsDsuch as improvedDreliability, powerDquality, efficiency, alleviationiof systemiconstraints along withithe environmentalibenefits. Withithese benefits andidue to the growingmomomentum towardsmosustainable energy developments, itsis expectedsthat large numbersof DG systemsiwill be interconnectedito the powerisystem in the comingtyears (Chmutina and Goodier, 2014;Vahl et al., 2013).
Despitepthese potentialpptheoretical benefits of distributedienergyigeneration, thereiare still complexities and constraints (technical, economic and regulatory) involvedrin its furtherrpenetration intorthe energy mix, and also restricting progress towards a new model of electric networks (Adil and Ko, 2016). However, these issues and complexities must be addressed and resolved, toipave the wayifor a sustainableienergy future basedion a large share of DG. Further, the issues pertaining to DG integration with grid; and several other factorsjsuch as which best DG technologyito beiused; the best location etc. are alsoJrequired to be considered while carrying out studies related to the planning and operational aspects of DG (Colmenar et al., 2016;CIGRE, 1998). In this context, the objective ofDthe present paper isDto provide a comprehensiveDRreview of distributed generation definitions and technologies, drivers towards DG growth, and the benefitsDoffered by DGDintegration to the distribution network.

DG Definitions
The prevailing literature on DG shows that the definitions used for DG are not consistent and yet there is no generally adopted definition of DG by the utilities (Pepermans et al., 2005;IEA, 1997). The United States Department ofiEnergy (US -DOE) definesiDG asifollows: "DistributedtGeneration is thetintegrated ortstand-alone use offsmall, modular energy (electric and/or thermal) generation resources, located near the consumption of energy. Distributed systems include wind turbines, microturbines, fuel cells, engines/generators sets, thermal solar and photovoltaic systems, combustion turbines, biomass -based generators, and storage anddcontrol techniques. Distributed technologies can eitherWbe independent of the gridtor grid connected (Ackermann et al., 2001;Hadisaid, et al., 1999). The Institute of Electrical and Electronics Engineering (IEEE) adopted the definition of DG as "the electricity generation byifacilities sufficientlyusmaller than centralizedJgenerating power plants, usuallyw10 MW or smaller, sowas to allow interconnectionjat nearly anytpoint in the electrictpower system". Nevertheless, thetfollowing definitiontfor DG is generallyIagreed in theIliterature: "A small scaleIpower generatingVsource connectedVdirectly to the grid at distributionslevel voltage or ontthe consumer sitetof the meter". This definitionWgenerally incorporatesWDG technologieswsuch as: concentrating solarwpower and photovoltaic, fuelrcells, microturbines, andrreciprocating engines. Moreover, insthe literature, a large numbereof terminologies and definitions such as decentralized generation, distributedienergy resources oriDER, dispersed generation and embedded generation have also beentused in relation to distributed generation (Willis and Scott, 2000).
In addition, in regards to the specification of DG power units, some countries define DG according to its voltage level at which it is interconnected, while other countries follow the principle that Distributed Generation is connected to circuits that feed directly to consumer loads". Although, thereiis noicommonly accepted standard or rule, the followingwratings are currently usedwin various countries andWsituations (Pepermans et al., 2005;Ackermann et al., 2001;Hadisaid, et al., 1999):

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The International Council on Large Electricity Systems (CIGRE) defines DG as generating units with rating less than 100 MW, that arerneither centrallynplanned not centrally dispatched and usuallyDconnected to theSdistribution network (CIGRE, 1998 Figure 1 depicts the various criteriatwhich can formtthe basis foridefining DG.

DG Technologies
The growing concern of governments, on a global level, to move towards low-carbon economy and sustainable growth targets has brought about the rapid proliferation of distributed power generation. DG technologies are usually classified intoDtwo broadDcategories: nonrenewable (fuel -based) andirenewable (non -fuel -based) energy sources. Non -renewable energy technologies use fossil -based fuels such as gasoline, idiesel, oil, propane, methane, natural gas, or coal source. Fossil fuel based DGs are not considered sustainable electricity generation sources because their energy source will not renew or replenish. Examples of non-renewable technologies include reciprocating engines, internal combustion gas turbines (ICGT), microturbines, and fuel cells. Distributed renewable energy technologies are in general sustainable (i.e. their primary energy source will not run out) and cause minimum or no environmental concerns. The DG technologies that fall under this category comprise wind turbines, solar thermal, solar photovoltaic, biomass and biogas, ocean and geothermal (tidal and waves) energy systems, small/mini/micro hydro power, and hydrogen fuel cells. The contribution potential of renewable energies in all countries is growing rapidly as the technology matures. Thesrenewable energyesources contribute to the diversity ofkenergy supply portfolio, mitigate the greenhouse gas emissions, and reduce the risks of continued use.of fossil fuels and nuclear power (Borbely and Kreider, 2001;Khattam and Salama, 2004).
A classification of various DG technologies is presented in Figure 2 and are discussed in following subsections.

Microturbines:
MicroturbinesWjwere initially developedjfor the transportationjsector and more recently havewfound a place inwthe power generationLsector. Microturbinetunits can use a widetrange of fuels suchtas naturaliigas, hydrogen, propanedand diesel todproduce electricity. The schematic of a typical microturbine is shown in Figure 5.

Wind Power Generation:
A windrenergyrconversion system (WECS) isipowered by windienergy andigenerates mechanicalkenergy thatksends energy to thekelectrical generator forrmaking electricity. The typical layout of a horizontal axis wind turbine is shown in Figure 12 and the block diagram of a WECS is shown in Figure 13 (Brouwer et al., 2016 A schematic of a modern biomass power plant is shown in Figure 14.
Source: [Bond and Templeton, 2011] Fig. 14 Schematic of a direct-fired biomass power plant

Small Hydro Power (SHP):
The gravitationaliforce of waterrfalling from arraised level to groundrlevel can be efficientlyTconverted toTelectric energy. ThisTform of electricitytgenerated fromthydropower is calledtas hydroelectricity. Hydropowerris a sourcerof renewablerenergy, and itsismall-scale applicationsican be used foridistributed generations. Thistform of electricitytgeneration hastbeen implementedDsince theDnineteenth century. William GeorgeAArmstrong developedAthe very firstApractical hydroelectrictsystem in 1878 intEngland. The first small hydro power plant installed in India was in Darjeeling in 1897 with a capacity of 130 kW. A typical conventional small hydro power plant (SHP) is depicted in Figure 15 (GOI, 2018). Further, therevis no globally accepted definition of SHP and asjsuch different countries have defined different capacity limits for SHP (see Table 3). Source: [Ferreira et al., 2016] The SHPHprojects do notHencounter theHproblems associatedtwith large hydeltprojects of rehabilitationtand resettlementtand do not disturbtthe ecologicaltbalance of therareas. These projectsrhave potential toomeet power requirementsWof remote andWisolated areasWin a decentralizedHmanner besidesHproviding employment opportunitykto localwpeople. The cumulative installed capacity of SHP in India as on 31st November 2019 is 4647.56 MW.
• Impact on System Line Losses: Transportingielectrical powerGjcauses lossesGjin the transmissionGjand distributionrgrid. The powerrlosses in a linercan be calculatedrusing Ohm's law. If arline is carryingra current, therpower losses in arline can betcalculated withiequation (1) Considering a sinusoidal current, i(t) = Imaxsinωt, the average power loss, PLoss over a complete cycle (T = 2π) is calculated as: which on simplification gives, Where PLoss isithe average power loss in theiline, I is rms the currentithrough the lineiand R is theiresistance of the line. Changingithe poweriflow in the lineichanges the current throughithe line and thustthe losses are affected. Theteffect of DG onigrid lossesistrongly depends onithe injected poweriand location insthe grid. ConnectingsDG to a load at aedistribution feeder, thejinjected power of theiDG will be consumediby the loadtand the power flowtin the feeder istreduced. In this casetthe losses are reducedtas well. Also, thetpower flowifrom higher voltageilevels toward the loadiis reduced. This decreases the risk of overloading at the higher voltagetlevels. Thus, distributedtgeneration is intgeneral advantageousifor overloadingiand losses. Onsthe other hand, ifsa DG system issinjecting more powerionto the distributionisystem thaniconductors weresroriginally designedsrfor, losses couldsractually increase. Moreover, intermittentigeneration sourcestwith a weaktcorrelation with thetload, such as winditurbines, can haveka negative impactkon gridklosses. Especially duringinight-time there isilow demand and in aihigh wind situationtthe distributiontgrid can start totexport power  (Delfanti et al., 2013).
• Location and Size Dependent Impact of DG: Penetratingeeembedded generation, oracdistributed generation (DG), injpower distributionjgridstpresents greattbenefits and substantialtpositive socialtimpacts toDJutilities, systemDJoperators andkkelectricity consumers. However, DG may degrade the performance of the distribution system, if it is not planned carefully. In other words, ifjthe locationtand sizeiof DGs are notiproperly determined, lossesiin the distributionksystem willkincreaseksignificantly, and also the voltage profile of the distribution network will be polluted (or distorted). In order to achieve the benefits presented by installing DG in distribution network, optimal planning of DG units are essential. Under optimal planning of DG, determination of its optimal size and location is the most notable and promising aspect to avail the prospective benefits. Thekadvantages of ankoptimal sitting andksizing of distributedRgeneration in aRdistribution networkRare many. First, anioptimal sitting ofithe availableigeneration resourcesjwill increase thejsecurity and reliabilitytof the system. Secondly, byIplanning theIoptimal locationsIfor technologiesrlike wind andrsolar power, it isrpossibletto maximizeWits penetrationWin theWsystem, thereby minimizinginot only the costsiof electricity supply, butialso secondarytobjectives liketminimization of CO2temissions. Thirdly, anroptimal sitting ofrthe electricityrgeneration portfoliorwill require of smallerrcapital expendituresron powerilines. Fourthly, distributedigeneration capacityican contributeRto the enhancementRof the overallksystem efficiency, byrfor examplerreducing power lossesrin the networkiby producingicloser to theidemand (Roy and Pota, 2015). Most of the existing work on DG siting and sizing considered various issues, such as power loss minimization; voltage profile; stability; reliability; and loading margin improvement; harmonic pollution reduction; investment minimization or profit maximization; loading margin etc., by formulating single or multi-objective problems. The impacts ofiDG allocation on the performance of distribution system are mentioned in Table 5.  (Akella et al., 2009). InDaddition, DG unitsHcan participateHinto the competitivermarket to providerancillary servicesrsuch as spinningHreserve, voltageDregulation, reactive power supporttand frequencytcontrol. The DGtplanningtstudies muststake into considerationtthe benefits thatsDGs can offer since these benefits can only be realised by selecting the optimal placement, size, DG technology type and volume within the network. However, inappropriate or sub -optimal planning can have negative impacts on the performance of distribution network. Thus, it becomes imperative to discuss the benefits of DG (Jain et al., 2017;Zahedi, 2011;Chiradeja and Ramakumar, 2004 • DGktechnology iskavailable in a wideecapacity range (i.e., from 3tkW up to 50tMW), givingtthe possibilitytto DG units totbe easily installedton distributionSnetworks, both mediumSand low voltage.

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DGs can be used as on-site standby to supply electricity in case of emergency and system outages (provide local reliability).
Source: [Jain et al., 2017]  More usage of green power Figure 16 shows that most of these benefits are interlinked
Sinceithe current andifuture trend ofielectric power systemsAis set towardsAincreased integrationAof DG, particularlyirenewableienergyisources (RES)-based units, a discussion on the relevant aspects related to distributed generation is required to be discussed or addressed. Accordingly, in this paper an extremely critical review has been made that primarily focused on DG definitions, various DG technologies and their current status, key issuesipertaining to gridiintegration (i.e. impactsiof DGion theIperformance of distributionIsystem), drivers of DG growth, and various benefits offered by deployment of DG. These perspectives of DG are of high significance for DG planning studies carried out for the optimal selection, sizing, and placement of DGs in power systems sinceiinappropriate DGilocation may result ininon-optimal use ofrDG integration, and moreover canthave negative impact onisystem performanceiin terms of increasedilosses and degradedivoltage profile. Thus, the various discussed viewpoints of DG can help the planning engineers, DG developers, policy makers and regulators toicome upiwith anAoptimum systemAplanning and enhancedAsystem performance.