Resilience of Electric Power Distribution Networks to N-1 Contingencies: The Case of Sekondi-Takoradi Metropolis

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1 IOSR Joural of Electrical ad Electroics Egieerig (IOSR-JEEE) e-issn: ,p-ISSN: , Volume 12, Issue 4 Ver. I (Jul. Aug. 2017), PP Resiliece of Electric Power Distributio Networks to N-1 Cotigecies: The Case of Sekodi-Takoradi Metropolis Godwi Diameu 1, Erwi Normayo 2 1 (Ghaa Techology Uiversity College, Faculty of Egieerig, Takoradi Campus, Ghaa) 2 (Uiversity of Mies ad Techology, Departmet of Elect. & Electroic Eg.,Tarkwa, Ghaa) Correspodig Author: Godwi Diameu Abstract:Cotigecy situatio is a abormal coditiothat do occur i a electric power distributio etworks. It puts whole system or a part of the system uder stress. It occurs due to sudde opeig of a trasmissio lie, geerator trippig, sudde chage i geeratio or sudde chage i load value. Electric power etworks are desiged to withstad sigle cotigecy, thus (N-1) criterio. However, some evets trigger others ad cascadig failures may occur. Therefore, ot all cotigecies are equal, ad the umber of compoets i a give system makes it prohibitive to evaluate all or a sigle cotigecy. The system is cosidered N-1 secure whe a sigle cotigecy will ot cause ay system limits to be violated. Typical cotigecies that ca be experieced by a electric power distributio etwork cosist of outages such as loss of feeders, distributio lies, or trasformers. This study is a attempt to determie how secure the electric power distributio etwork withi the Sekodi-Takoradi Metropolis is by modellig ad simulatios usig PowerWorld Simulator versio 19 GSO. Keywords:Cotigecies, Cotigecy Aalysis, Cotigecy Detectio, Simulatios, Mega Volts Ampere, Modellig, Violatios Date of Submissio: Date of acceptace: I. Itroductio A core missio of a electric power distributio system is to deliver electrical eergy from the supplyig poits to the ed users without ay iterruptio. The electric power distributio etwork segmet has bee the weakest lik betwee the source of supply ad the customer load poits. The greatest problem ecoutered i the area of electric power distributio system operatio ad maiteace is how to reduce the umber of iterruptios experieced by customers (Veuet al., 2014). I the moder iformatio society, requiremets ad expectatios associated with the cotiuity of electric power supply have become icreasigly importat. Electricity is ot a luxury commodity aymore as it was a few decades ago, but has become a ecessity ad a part of our everyday life. Eve short iterruptios ca be harmful at a time that the umber of computers, programmable logic cotrollers, et cetera i idustries as well as households had icreased rapidly. Large blackouts aroud the world have also aroused the cosumers iterest i electricity distributio ad reliability of distributio etworks. The reliability of supply has gaied greater iterest i recet times (Kivikko, 2010). There is always the eed to aalyse the performace of the electric power distributio etwork at aparticular operatig poit. However, for practical system operatio, apart from esurig the satisfactoryoperatio of the distributio etwork at a particular operatig coditio, it is also equally importat to makesure that the system operates with adequate level of security. Broadly, the term security impliesthe ability of the electric power distributio etwork to operate withi system costraits due to variatios i bus voltage magitudes, curretad power flow over the lies i the evet of a outage (cotigecy) of ay electric power distributio etwork compoet such as trasmissio lie, trasformer, switchgear, etc. I case the postoutage (post cotigecy) does ot ivolve ay violatio(s) ofay operatig costrait(s), the distributio etwork is said to be operatig securely or resiliet to cotigecies (Prustyet al., 2014). Otherwise, the distributio etwork is ot resiliet to cotigecies or secured. This paper seeks to ivestigate the resiliece of the electric power distributio etwork of the Sekodi- Takoradi Metropolis to N-1 cotigecies. Sekodi-Takoradi metropolis is i the Wester Regio of Ghaa which shares border with La Cote D Ivoire. II. Cotigecy Aalysis ad Cotigecy Detectio Electric power distributio etworks are seldom loaded ear their limits therefore, system adequacy is of less cocer ad reliability emphasis is o system security aalysis (Okorieet al., 2015).All over the world, DOI: / Page

2 coutries are expadig their power systems i order to meet the developmetal challeges ad this is accompaied by icreased cotigecies such as geerator outages, trasmissio or distributio elemet outages which may always cause sudde ad large chages i both cofiguratio ad the state of the system. Cotigecies may result i severe violatios of the operatig costraits. Cosequetly, plaig for cotigecies forms a importat aspect of secure operatio (Oojoet al., 2015). The electric power distributio etwork is a very complex system, cosistig of thousads of various compoets, such as coductors, isulators, posts, coectors, distributio trasformers, cables, amog others. Each of these compoets has a differet life expectacy ad failure distributio fuctio. Cotigecy aalysis is useful i uderstadig electric power system coditios i advace before takig prevetive measures. However, a accurate ad detailed aalytical method i a ear real-time maer is still a great challege due to the high oliearity ad high dimesioality of power systems (Hu, 2010). Cotigecy aalysis is a abormal coditio i a electric power etwork. It puts whole system or a part of the system uder stress. It occurs due to sudde opeig of a trasmissio lie, geerator trippig, sudde chage i geeratio or sudde chage i load value. Electric power etworks are desiged to withstad oe cotigecy thus (N-1) criterio. However, some evets trigger others ad cascadig failures may occur. Therefore, ot all cotigecies are equal, ad the umber of compoets i a give system makes it prohibitive to evaluate all or a sigle cotigecy. The system is cosidered N-1 secure whe a sigle cotigecy will ot cause ay system limits to be violated (Chaitaya et al., 2013). Typical cotigecies that ca be experieced by a electric power distributio etwork cosist of outages such as loss of feeders, distributio lies, or trasformers. Cotigecies ca occur i the form of sigle equipmet outages or i the form of multiple outages (Hu, 2010). Power Flow Study for Cotigecy Aalysis Cotigecy aalysis is the study of the outage of elemets such as geerators, trasmissio lies, trasformers etc., ad ivestigatio of the resultig effects o lie power flows ad bus voltages of the remaiig electric power system compoets. It represets a importat tool i studyig the effect of outages of elemets i power system security durig operatio ad plaig stages. Power flow aalysis is probably the most importat of all etwork calculatios. It is performed to ivestigate the magitude ad phase agle of the voltage at each bus ad the real ad reactive power flows i the electric power system compoets (Oojoet al., 2015). Variables of Load Flow Studies At each bus, two of the four quatities δ, V, P ad Q are specified ad the remaiig two are determied as give i Table 1.. Table 1 Load Flow Variables Bus Type Kow Variables Ukow Variables Slack/Swig/Referece Bus V, δ P, Q PV/Geerator/Voltage Cotrol Bus P, V Q, δ PQ/Load Bus P, Q V, δ (Source: Hu, 2010; Oojoet al., 2015) Developig a Power Relatio For the formulatio of the real ad reactive power eterig a bus, use is made of Equatio (1) through to Equatio (7) accordig to Athoy ad Chukwuma (2016) ad Oojoet al. (2015). Let the voltage at the i th bus be deoted by Equatio (1). V Vi δ Vi c o s δ i + js i δ i i i (1) Also, the self-admittace at bus iis give by Equatio (2). Y Yii θ Yii c o s θ ii + js i θ ii G + jb ii ii ii ii (2) Similarly, the mutual admittace betwee the buses iad j ca be writte as i Equatio (3). Y ij Y ij θ ij Y ij co s θ ij + jsi θ ij G ij + jb ij (3) Let the power system cotai a total umber of buses. The curret ijected at bus iis give by Equatio (4). I = Y V + Y V Y V = Yik V i i1 1 i2 2 i k (4) The assumptio here is that the curret eterig a bus is positive ad that leavig the bus is egative. Hece, the real power ad reactive power eterig a bus will also be assumed to be positive. The complex power at bus iis the give by Equatio (5). DOI: / Page

3 * P - jq = V I ( 5 ) i i i i * = Vi Yik Vk = Vi c o s δ i -js i δ i Yik V k c o s θ ik + js i θ ik c o s δ k + js i δ k Yik Vi V k c o s δ i -js i δ i c o s θ ik + js i θ ik c o s δ k + js i δ k Therefore, substitutig Equatio (5), real ad reactive power is give respectively by Equatio (6) ad Equatio (7). P = Y V V co s θ + δ -δ i ik k i ik k i (6) Q = - Yik Vk Vi si θ ik + δ k -δ i i where, Vi= voltage at bus i V k = voltage at bus k δ i = agle of deviatio at bus i δ k = agle of deviatio at bus k P i = real power at bus i Q i = reactive power at bus i Y ii = self-admittace at bus i Y ij = mutual admittace betwee buses i ad j Y ik = mutual admittace betwee buses i ad k I i = curret ijected at bus i θ ii = phase agle of self-admittace of bus i θ ij = phase agle of mutual admittace betwee buses i ad j θ ik = phase agle of mutual admittace betwee buses i ad k B ii = self-susceptace of the admittace matrix of bus i B ij = susceptace of the admittace matrix betwee buses i ad j G ii = self-coductace of the admittace matrix of bus i G ij = coductace of the admittace matrix betwee buses i ad j Cotigecy Aalysis Procedure Geerally, oce the preset workig state of the power system is kow, cotigecy aalysis ca be broke dow ito the followig steps amely, cotigecy defiitio, cotigecy selectio ad cotigecy evaluatio. Cotigecy defiitio ivolves preparig a list of probable cotigecies. This typically icludes lie outages, trasformer outages, circuit breaker outages or geerator outages. Cotigecy selectio process cosists of selectig the set of most probable cotigecies which eeds to be evaluated i terms of potetial risk to the electric power system. Usually, fast power flow solutio techiques such as DC power flow is used to quickly evaluate the risks associated with each cotigecy. Fially, the selected cotigecies are raked i order of their security, till o violatio of operatig limits is observed (Noyelu ad Madueme, 2013). For higher accuracy ad fast covergece, Newto-Raphso load flow method is employed i this work. The algorithm for a typical N-1 cotigecy aalysis is give i Figure 1 (Ao., 2012; Oojoet al., 2015). (7) DOI: / Page

4 Start Base Case Violatios?? Yes Fix Violatios Ru Primary Cotigecy Violatios? Stop No Yes Model system Adjustmets Figure 1 Procedure for N-1 Cotigecy Aalysis Types of Violatios Lie cotigecy ad geerator cotigecies are geerally the most commo types of cotigecies. These cotigecies maily result i two types of violatios amely, low voltage violatios ad lie Mega Volts Ampere (MVA) violatios. Low voltage type of violatio occurs at the buses. This suggests that the voltage at the bus is less tha the specified value. The operatig rage of voltage at ay bus is geerally betwee per uit (p.u). Thus, if the voltage falls below 0.95 p.u the the bus is said to have low voltage. If the voltage rises above the 1.05 p.u the the bus is said to be experiecig high voltage problem. It is kow that i the electric power etwork, geerally, reactive power is the reaso for the voltage limit violatio. Hece, i the case of low voltage problems, reactive power is supplied to the bus to icrease the voltage profile at the bus. I the case of the high voltage, reactive power is absorbed at the buses to maitai the system ormal voltage (Chaitaya et al., 2013). Lie MVA limit cotigecy violatios occur i the system whe the MVA ratig of the lie exceeds a give ratig. This is maily due to the icrease i the amplitude of the curret flowig i that lie. The lies are desiged i such a way that they should be able to withstad 125% of their MVA limit. Based o utility practices, if the curret crosses the 80-90% of the limit, it is declared as a alarm situatio (Chaitaya et al., 2013). III. Materials ad Method PowerWorld Simulator versio 19 GSO was used i modellig ad simulatig the electric power distributio etwork of Sekodi-Takoradi metropolis. The secodary substatios withi the metropolis are may. I modellig the etwork, the major suburbs withi the metropolis were cosidered as the load poits whilst the load poits of the mior suburbs were absorbed ito the major oes. The estimated real power requiremet of the metropolis based o istalled trasformer capacities was MW. For the modellig, the etwork of the metropolis was reduced to forty (40) buses ad iety-two (92) feeders or trasmissio lies. The modellig was doe based o the followig assumptios: 1. Base voltage = 11 kv. 2. System base MVA = 100 MVA. 3. Supply frequecy = 50 Hz. 4. Static load power factor = 0.9. DOI: / Page

5 5. Coductor type = AAC. 6. Resistace = Ohm/km. 7. Reactace = Ohm/km. The electric power distributio etwork withi the Sekodi-Takoradi metropolis cosists of 120 mm 2 ad 150 mm 2 coductor sizes. Niety-five percet (95%) beig 120 mm 2 coductor size. The modellig was carried out usig two differet coductor sizes, thus 120 mm 2 ad 150 mm 2 All Alumiium Coductor (AAC). The load ad trasformer data as well as estimated feeder legths were used for the modellig i the PowerWorld Simulator versio 19 GSO software after which simulatios were carried out. Figure 2 shows the simulated electric power distributio etwork. The sapshot of the simulatio results is also give i Figure 3. To be sure of the ability of the modelled etwork to withstad N-1 cotigecy, forty-bus ad iety-two trasmissio lie cotigecies were iserted to determie where exactly low or high voltages would be experieced withi the etwork of the metropolis. Thus, the iety-two trasmissio lies were take off oe by oe i the case of trasmissio lie cotigecy, ad the forty-buses were also take off oe by oe i the case of the bus cotigecy. For the voltage to be declared as low, the per uit (p.u.) value must fall below 0.95, ad for high voltage, it must be above 1.05 p.u for a feeder ad the brach circuits. The limits were set at ± 5% to esure optimum efficiecy for the feeder ad the brach circuits. Forty-bus cotigecies were iserted after which cotigecy simulatios were carried out resultig i a total of 154 violatios as show by the sapshot give i Figure 4. Figure 2 TheSimulated Electric Power Distributio Network of Sekodi-TakoradiMetropolis Figure 3 A Sapshot of Simulated Results of the Electric Power Distributio Network ofsekodi-takoradi Metropolis usig 120 mm2 ad 150 mm2 Coductors DOI: / Page

6 Figure 4A Sapshot of Results of Simulated Forty-bus Cotigecies of Electric Power Distributio Network of the Sekodi -Takoradi Metropolis Comprisig 120 mm 2 ad 150 mm 2 Coductors Agai, iety-two trasmissio lie cotigecies were iserted as show by the sapshot of results give i Figure 5 resultig i a total of 81 violatios. Figure 5A Sapshot of Results of Simulated Niety-two Trasmissio Lie Cotigecies of Electric Power Distributio Network of the Sekodi-Takoradi Metropolis Comprisig 120 mm 2 ad 150 mm 2 Coductors Due to the umerous violatios observed durig the simulatios of the electric power distributio etwork which comprised 120 mm 2 ad 150 mm 2 coductor sizes, all the 120 mm 2 coductors were replaced with those of 150 mm 2 AAC. The sapshot of the etwork modelled with oly 150 mm 2 coductors is give i Figure 6. The sapshot of the results of the simulated electric power distributio etwork modelled with oly 150 mm 2 coductor size is give i Figure 7. Forty (40) bus cotigecies were iserted ito the electric power distributio etwork modelled with oly 150 mm 2 coductors. A sapshot of the simulated results is show i Figure 8 with o violatio. To further ivestigate the voltage stability of the electric power distributio etwork modelled usig 150 mm 2 coductors, iety-two trasmissio lie cotigecies were iserted. A sapshot of the iety-two trasmissio lie cotigecies is give i Figure 9. DOI: / Page

7 Figure 6 The Modelled Electric Power Distributio Network usig 150 mm 2 AlumiiumCoductors Oly Figure 7A Sapshot of Simulated Results of the Electric Power Distributio Network Comprisig 150 mm 2 Alumiium Coductors Oly DOI: / Page

8 Figure 8A Sapshot of Results of Simulated Forty-bus Cotigecies of Electric Power Distributio Network Comprisig 150 mm 2 Coductors Oly Figure 9A Sapshot of Results of Simulated Niety-two Trasmissio Lie Cotigecies of Electric Power Distributio Network Comprisig 150 mm 2 Coductors Oly IV. Results ad Discussios Preseted i this sectio are the results obtaied after modellig ad simulatios of the electric power distributio etwork of Sekodi-Takoradi metropolis. Computer simulatios were doe after modellig the electric power distributio etwork with forty-buses ad iety-two feeders or trasmissio lies i the PowerWorld Simulator versio 19 GSO. Cases cosidered were the simulatios of the ormal etwork comprisig 120 mm 2 ad 150 mm 2 alumiium coductors, isertio of forty-bus cotigecies as well as isertio of iety-two trasmissio lie cotigecies. Also, the simulatios of the modelled electric power distributio etwork were doe with oly 150 mm 2 coductors i circuit, followed by isertio of forty-bus cotigecies as well as isertio of iety-two trasmissio lie cotigecies.the results are preseted i Figure 10, Figure 11, Figure 12 ad Figure 13 respectively. DOI: / Page

9 Mi. Bus Voltage Magitude (Kv) Actual Mi. Bus Voltage Magitude (Kv) Bus Voltage Magitude (P.U.) Resiliece of Electric Power Distributio Networks to N-1 Cotigecies: The Case of Sekodi-Takoradi Bus Number Figure 10Simulated Bus Voltages of the Network with 120 mm 2 ad 150 mm 2 Coductors Bus Number Figure 11Simulated Miimum Bus Voltages of the Network with 120 mm 2 ad 150 mm 2 Coductors durig Forty-bus Cotigecies Represetatio of Cotigecy Labels Figure 12Simulated Miimum Bus Voltages of the Network with 120 mm 2 ad 150 mm 2 Coductors durig DOI: / Page

10 Niety-two Trasmissio Lie Cotigecies Bus Volage Magitude (P.U.) Bus Number Figure 13Simulated Bus Voltages with oly 150 mm 2 Coductors Figure 10 showed that ot all the p.u. voltage variatios at the various buses are withi the specified rage i.e. a miimum of 0.95 p.u. ad maximum of 1.05 p.u. Twety-five (25) buses violated the miimum limit value. The miimum p.u. voltage of 0.95 was violated at bus umbers fiftee (15) through to bus umber forty (40). A idicatio that customers coected to those buses will be experiecig low voltages some of the time. No bus experieced a voltage violatio exceedig the maximum p.u. voltage of From Figure 11, twetyfour (24) of the buses experieced low voltages at their eds for a forty-bus cotigecy simulatios resultig i a total of oe-hudred ad fifty-four (154) violatios. The low voltages experieced raged betwee to 0.90 p.u. which fell below the recommeded miimum of 0.95 p.u. This is a idicatio that the existig electric power distributio etwork withi the metropolis is ot immue to N-1 bus cotigecies whe both the preset 120 mm 2 ad 150 mm 2 coductors cotiue to be used. I Figure 12, iety-two trasmissio lie cotigecies were processed successfully resultig i a total of eighty-oe (81) violatios with the p.u. miimum voltages ragig betwee to 0.90 p.u. This is also a idicatio that the existig electric power distributio etwork withi the metropolis is ot immue to N-1 trasmissio lie cotigecies. Figure 13 revealed that all the bus voltages are withi the stipulated voltage rage of 0.95 p.u. miimum ad the 1.05 p.u. maximum for the etwork havig oly 150 mm 2 coductor sizes i circuit. Whe the forty-bus ad iety-two trasmissio lie cotigecies were iserted, there was ot a sigle voltage or MVA limit violatio occurred. These are idicatios that the electric power distributio etwork modelled with 150 mm 2 coductor sizes oly i circuit is immue to N-1 bus ad trasmissio lie cotigecies. V. Coclusio The preset use of 120 mm 2 ad 150 mm 2 coductor sizes results i voltage violatios givig a idicatio that the electric power distributio etwork is ot immue to N-1 cotigecies. The electric power distributio etwork of the Sekodi-Takoradi Metropolis shall be secured ad reliable whe all existig 120 mm 2 alumiium coductors are replaced with 150 mm 2 alumiium coductors. Refereces [1]. B. Veu,C. Bhargava, adk. Sumath. Reliability Assessmet of Radial DistributioSystem by usig Aalytical Methods. Iteratioal Joural of Professioal EgieerigStudies, 4(4), 2014, [2]. K. Kivikko. Assessmet of Electricity Distributio Reliability- Iterruptio Statistics, Reliability Worth, ad Applicatios i Network Plaig ad Distributio Regulatio.PhD Thesis, Tampere Uiversity of Techology, Tampere, Filad, 2010, [3]. B. R. Prusty, B.P. Pattaik, P. K. Padey, A. S. Satosh. Power System Security Aalysis. Iteratioal Joural of Scietific ad Egieerig Research, 5 (5,), 2014, [4]. P.U. Okorie, U.O. Aliyu, B. Jimo, ad S. M. Sai. Reliability Idices of Electric Distributio Network System Assessmet. Joural of Electroics ad Commuicatio Egieerig Research, 3(1), 2015, 1-6. [5]. O.J. Oojo, K. Iyama, ad G.C. Ooiwu. Cotigecy Aalysis of the Nigeria 330 kv Post-reform Itegrated Power System usig Power World Simulator. Asia Jouralof Natural ad Applied Scieces, 4(2), 2015, DOI: / Page

11 [6]. Q. Hu. Distributio Network Cotigecy Aalysis ad Cotigecy Detectio with the Cosideratio of Load Models. PhD Dissertatio, Uiversity of Texas, Arligto USA 2010, [7]. C. K. Chaitaya, J. K. Kishore, ad G. Swapa Cotigecy Aalysis i Restructured Power System. Iteratioal Joural of Iovative Research ad Developmet,2(1), 2013, [8]. O. I. Athoy, ad U. Chukwuma. Cotigecy Aalysis of the South Easter Nigeria 330 kv Network usig PowerWorld Simulator. Iteratioal Joural of Idustrial Electroics ad Electrical Egieerig, 4(1), 2016, [9]. Ao. N-1-1 Cotigecy Aalysis usig PowerWorld Simulator Accessed: March 16, 2016 [10]. C. Noyelu, ad T.C. Madueme, Power System Cotigecy Aalysis: A Study of Nigeria s 330 kv Trasmissio Grid. Eergy Source for Power Geeratio Coferece, Electrical Egieerig Departmet, Uiversity of Nigeria, Nsukka, 4, 2013, 1-8. DOI: / Page