UPTEC-ES12011

Examensarbete 30 hpApril 2012

Wind Power and Its Impact on the Moldovan Electrical System

Joel ErikssonSimon Gozdz Englund

Teknisk- naturvetenskaplig fakultet UTH-enheten Besksadress: ngstrmlaboratoriet Lgerhyddsvgen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 471 30 03 Telefax: 018 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Wind Power and Its Impact on the MoldovanElectrical System

Joel Eriksson & Simon Gozdz Englund

The master thesis project has been executed with the cooperation of BorlngeEnergi, with the aim of reducing the high electric energy dependency which Moldovahas on Ukraine, Transnistria and Russia.

The project examines what reduction that would be possible by wind powerinstallations on the existing electrical grid of Moldova. The installations should notsurpass the capacity of the transmission lines or the voltage levels according toregulation. The southern regions of Moldova proved to have the best wind conditionsand the locations of Besarabeasca, Zarnesti, Leovo, Ciadyr and Cimislia in thesouthern region were chosen for wind power installations.

For the analysis a model over the Moldovan electrical system is constructed. Each ofthe five chosen locations is modelled with a generator symbolizing the wind powerinstallation. The power flow software PSS/E is used to construct the model. Toexamine possible wind power installations different scenarios are created. Thescenarios are executed with the southern regions 110 kV system as a focus area. Allscenarios are analysed with a contingency analysis, where transmission lines in thefocus region are tripped. The contingency analysis and the scenarios are automatedusing the programming language Python.

An economic analysis shows payback periods for wind power investments in Moldova,the analysis also shows the sensitivity of the electricity price and discount rates.

The project concludes that wind power installations are possible with the Moldovanelectric grid as it looks today. The installations would result in reducing the highdependency of imported electrical energy.

Sponsor: Forsk SidaISSN: 1650-8300, UPTEC ES** ***Examinator: Kjell Pernestlmnesgranskare: Mikael BergkvistHandledare: Ronny Arnberg

i

I Frord (in Swedish)

Bakgrunden till detta spnnande examensarbete r att frfattarna p egen hand skte en utmaning

vad gller uppbyggnad och utformning av framtida elektriska kraftnt. Deras frgestllning var vad

hnder i ett kraftnt vid en massiv utbyggnad av t.ex. vindkraft, karakteriserad av stora variationer i

effekt? Frgestllningen r hggradigt intressant i lnder med lite tillgng p vattenkraft och ur

perspektivet av utfasning av fossila brnslen. Ur den synvinkeln var det svenska kraftntet mindre

intressant. Efter kontakter med Ronny Arnberg, Borlnge Energi, kom projektet att fokusera p

elfrsrjningen i Moldavien vars kraftnt blev modell fr studien. Mlet r inte att lsa Moldaviens

energifrsrjningsproblem.

De ambitioner som frfattarna hade inledningsvis har sedan modererats p ett frtjnstfullt stt av

mnesgranskaren Mikael Bergkvist att bttre passa tillgnglighet, tid och resurser. Ronny Arnberg har

bisttt med mngder med kontakter i Moldavien som ppnat drrar fr frfattarna. Resultatet p

frfattarnas initiala frga har kanske inte blivit besvarad men vl gett stora insikter i de utmaningar

som vntar.

2012-03-16

Kjell Pernestl

Examinator

Unv.lekt.

Uppsala Universitet

ii

II Acknowledgments

This Master Thesis Project has been financed by a MFS scholarship and a Forsk scholarship without

which the project could not have been executed.

Technical support was provided by the Technical University of Moldova, Anatolie Boscneanu Main

Specialist at the National Agency for Energy Regulation and Lise Toll Project Developer at E.ON

Climate & Renewables. Technical support was also provided along with and guidance in times of

need throughout the project by our supervisors at Uppsala University; Mikael Bergkvist and Kjell

Pernestl.

Special thanks should be directed to Professor Arion Valentin and PhD student Victor Gropa at the

Technical University of Moldova who took us in with the true spirit of the Moldovan people; with

helping hands wanting nothing in return. Thank you

This project could not have been written without the help of Borlng energi and its enthusiast Ronny

Arnberg who provided contacts and a workplace in Moldova.

We would also like to take the chance to express our gratitude for the opportunity to experience

Moldova and its truly great people, out of which we now call many friends and whom we will never

forget.

iii

III Populrvetenskaplig beskrivning (in Swedish)

Moldavien r ett av de fattigare lnderna i Europa. Det finns ett starkt engagemang och en stor vilja

fr sammarbeten med vstlnder fr att lmna fattigdomen och nrma sig Vsteuropa. Ett viktigt

steg i detta r att bli av med det starka energiberoende gentemot Ukraina och Ryssland som finns

idag. Moldaviens interna politiska situation r svr. En del av Moldavien, Transnistrien, existerar idag

som en autonom republik och har en lngdragen konflikt bakom sig som nnu inte r lst.

Transnistrien och situationen dr r viktig ur en energisituation d den strsta kraftanlggningen i

hela regionen ligger dr.

Moldavien importerar idag mellan 94 och 98 % av sin totala energikonsumtion dr de stora

importprodukterna r naturgas frn Ryssland och el frn Ukraina och Transnistrien. Av sin

elkonsumtion har Moldavien idag endast mjlighet att producera ca 26 % nationellt, resten

importeras frn Ukraina och Transnistrien.

I examensarbetet utreds mjligheterna att minska detta starka beroende genom att ka intern

elproduktion genom vindkraft. Fokus ligger p elntet, allts hur mycket vindkraft som kan installeras

till dagens existerande elnt utan att elledningarna blir verbelastade eller att spnningar i elntet

kar eller sjunker utanfr gllande grnsvrden.

Vindpotentialen har underskts via tidigare studier och den visar p att potentialen r strst i den

sdra delen av Moldavien. Dessa vindkraftskarteringar r utfrda utifrn vindmtningar p

meterologiska stationer, ofta p 10 till 12 meters hjd. Genom simuleringsprogram har man sedan

kunnat uppskatta vindhastigheter fr olika omrden och hjd.

Fem platser med bra vindpotential valdes ut fr vidare studier ver hur mycket vindkraft som kan

installeras ur ntets perspektiv. Dessa platser i nrheten av stderna; Besarabeasca, Zarnesti, Leovo,

Ciadyr och Cimislia ligger alla i sdra delen av Moldavien. Den begrnsade faktorn fr hur mycket

vindkraft som kan byggas r elntet.

Fr att underska effekterna frn vindkraftsinstallationerna p elntet var det ndvndigt att bygga

upp en modell i datorprogrammet PSS/E, designat fr att berkna effektflden i elnt. Datorn kan

sedan utfra de komplexa berkningar som krvs fr att rkna ut effekter och spnningar i systemet.

Det rcker dock inte att endast se p systemet som det faktiskt ser ut, man mste ocks underska

vad som skulle hnder d en elledning i systemet kopplas bort i en s kallad n-1 analys. Att en lina

kopplas bort kan bero p behov av underhll eller rena fel som kan uppst vid till exempel olyckor.

Platserna undersktes i den frdiga modellen bland annat en och en men ven i ett scenario dr det

p alla platser samtidigt installeras vindkraft. Vindkraftparkerna symboliseras i modellen som

generatorer som genererar aktiv effekt.

Resultaten visar att den maximalt mjliga installerade effekten varierar mycket beroende p plats.

Cimislia visar sig ha mjlighet fr 100 MW, innan verfringskapaciteten blir begrnsande. De vriga

platserna begrnsas p grund av att spnningsniver stiger eller sjunker utanfr riktlinjerna. D

installation sker p alla platser samtidigt finns det mjlighet att installera omkring 260 MW, ven hr

r hga spnningar en begrnsande faktor fr ytterligare installation.

iv

Den reaktiva effekten i elntet har en stark koppling till spnningsniver och drav upprepas

scenariona d vindkraftparkerna ven har mjlighet att producera eller konsumera reaktiv effekt. D

vindkraftparkerna p detta stt har mjlighet att kompensera med reaktiv effekt hlls spnningen

konstant p basspnningsnivn.

De nya resultaten visar att detta ger en mjlighet att ka vindkraftinstallationen per plats. De platser

i tidigare scenariot som tidigt fick spnningsproblem begrnsas nu, precis som Cimislia, endast av

verfringskapaciteten. Installation vid Cimislia minskar dock ngot d reaktiv effekt ven den tar

upp plats p elntet. Maximal produktion r dock fortfarande strst i Cimislia med ca 100 MW. D

vindkraft installeras p alla platser samtidigt ges en kning till 355 MW, allts en tydlig kning av

mjlig vindkraftsinstallation.

Fr att rkna ut den totala minskningen av importerad el br man ta hnsyn till att en vindkraftpark

med installerad effekt med t.ex. 260 MW inte kommer leverera 260 MW hela tiden p grund av att

vinden inte blser hela tiden. Fr att ta hnsyn till detta anvnds tv olika utnyttjandefaktorer fr

vindkraftparkerna, 0,1 och 0,3 dr 0,1 r en relativt lg utnyttjandefaktor och 0,3 r en relativt hg

utnyttjandefaktor. Resultaten visar att vindkraftverk som endast levererar aktiv effekt kan minska

elimporten med mellan 7 % till 20 % beroende p utnyttjandefaktorerna. D vindkraftparken har

mjligheten att konsumera reaktiv effekt kan elimporten minska med mellan 8 % till 25 %.

I rapporten utfrs ven en ekonomisk analys dr terbetalningstiden fr ett vindkraftsprojekt tas

fram. terbetalningstiden berknas med nettonuvrdesmetoden och terbetalningstiden tas fram

fr ngra olika rntesatser. terbetalningstiden fr vindkraftsprojekten varierar frn 3 r till att aldrig

betala tillbaka sig vid de olika ekonomiska scenarierna.

Slutsatsen r att Moldaviens starka beroende av importerad elektricitet kraftigt kan minskas med en

utbyggnad av vindkraft i sdra delen av landet.

v

IV Executive summary

The Moldovan electrical energy imports can be reduced by as much as 25 %. This reduction is

possible by wind power installations at the suitable locations of Besarabeasca, Zarnesti, Leovo, Ciadyr

and Cimislia.

Assuming a possible good wind resource with a capability factor of 0,3 wind power installations of

355 MW would reduce the electrical energy imports by 25 % according to the model created for the

project. The model has not been verified with other models, which is of priority for future work.

For a total installation of 355 MW the installed power needs to be allocated as shown below:

Besarabasca 56 MW

Zarnesti 68 MW

Leovo 68 MW

Ciadyr 91 MW

Cimislia 72 MW

The strongest site for wind power production, one site at a time, is Cimislia with a total installed

power of 102 MW possible. With all sites together the maximum installed power is 260 MW without

reactive power compensation and 355 MW with reactive power compensation.

Economic calculations include a sensitivity analysis with different the electricity price and discount

rates. The economic analyses shows that the payback time vary from 3 years to never being paid

back and conclusions are drawn that further investigations needs to be made.

vi

V List of Acronyms and Abbreviations

ANRE National Agency for Energy Regulation

AVR Automatic Voltage Regulation

CDM Clean Development Mechanism

CER Certified Emission Reduction credits

CHP Combined Heat and Power

DSA Dynamic Security Assessment

EBRD European Banc of Reconstruction and Development

ENTSO-E European Network of Transmission System Operators for Electricity

FACTS Flexible Alternating Current Transmission Systems

HAWT Horizontal Axis Wind Turbine

HPP Hydro Power Plant

IPS Integrated Power System

MAWS Mean Annual Wind Speed

MSSR Moldovan Soviet Socialist Republic

PSS/E Power System Simulator for Engineering

p.u Per Unit

SNC Second National Communication

SSA Static Security Assessment

TUM Technical University of Moldova

UNFCCC United Nations Framework Commission of Climate Change

VAWT Vertical Axis Wind Turbine

WAsP Wind Atlas Analysis and Application Program

Table of Contents

I Frord (in Swedish) ______________________________________________________ i

II Acknowledgments _______________________________________________________ ii

III Populrvetenskaplig beskrivning (in Swedish)________________________________ iii

IV Executive summary ______________________________________________________ v

V List of Acronyms and Abbreviations ________________________________________ vi

Chapter 1 Introduction ____________________________________________________ 1

1.2 Borlnge Energi ___________________________________________________________ 1

1.3 Moldova Background _____________________________________________________ 1

1.3.1 Grid History _____________________________________________________________________ 2

1.3.2 Energy _________________________________________________________________________ 2

1.3.3 Bio Energy Potential ______________________________________________________________ 3

1.3.4 Solar Energy Potential _____________________________________________________________ 3

1.3.5 Wind Energy Potential ____________________________________________________________ 4

1.1.1 Environmental Goals ______________________________________________________________ 4

1.4 Aim and Goals ____________________________________________________________ 5

Chapter 2 Background _____________________________________________________ 6

2.1 Grid Theory ____________________________________________________________ 6

2.1.1 Active and Reactive Power _________________________________________________________ 6

2.1.2 Introduction to the Electrical Power System ___________________________________________ 8

2.1.3 Components in the Electrical Power System __________________________________________ 11

2.1.4 Per-Unit System _________________________________________________________________ 15

2.1.5 Equivalents in Electrical Power Systems ______________________________________________ 15

2.1.6 Static Modelling _________________________________________________________________ 16

2.2 Wind Power ____________________________________________________________ 19

2.2.1 Moldovas Wind Resource ________________________________________________________ 22

2.2.2 Economy ______________________________________________________________________ 23

2.3 Method ________________________________________________________________ 24

2.3.1 PSS/E _________________________________________________________________________ 24

2.3.2 Building the Model ______________________________________________________________ 24

2.3.3 Scenarios ______________________________________________________________________ 29

2.3.4 Economy ______________________________________________________________________ 30

Chapter 3 Results ________________________________________________________ 32

3.1 Base Case_______________________________________________________________ 32

3.2 Scenario I _______________________________________________________________ 33

3.2.1 Scenario I, With Reactive Power Compensation _______________________________________ 33

3.3 Scenario II ______________________________________________________________ 34

3.3.1 Scenario II, With Reactive Power Compensation _______________________________________ 35

1.1 Reduction of Imported Electric Energy _______________________________________ 36

3.3.2 Scenario I ______________________________________________________________________ 36

3.3.3 Scenario II _____________________________________________________________________ 37

3.4 Economy _______________________________________________________________ 37

Chapter 4 Discussion _____________________________________________________ 40

4.1 Scenarios _______________________________________________________________ 40

4.1.1 Scenario I ______________________________________________________________________ 40

4.1.2 Scenario II _____________________________________________________________________ 41

4.1.3 Economy ______________________________________________________________________ 42

Chapter 5 Conclusion _____________________________________________________ 43

Chapter 6 Future Work ___________________________________________________ 44

Appendix A Map of the Moldovan electrical system ___________________________ A-1

Appendix B Map over the wind potential in Moldova _________________________ B-1

Appendix C Description of WAsP __________________________________________ C-1

Appendix D Line diagram and data over the equivalent 330 kV circuit ____________ D-1

Appendix E Transmission Line Data ________________________________________ E-1

Appendix F Line diagram and data over the complete model ___________________ F-1

Appendix G General Python Script executing the contingency analysis __________ G-1

Appendix H Python Script Scenario I ______________________________________ H-1

Appendix I Python Script Scenario II, Monte Carlo Simulation____________________ I-1

Appendix J Base Case - Contingency Loading Report _____________________________ J-1

Appendix K Base Case Line Diagram with Line Capacities _____________________ K-1

Appendix L Scenario I Overload Report _____________________________________ L-1

Appendix M Scenario I Reactive Power Compensation Overload Report_________ M-1

Appendix N Scenario II - Overload Report ___________________________________ N-2

Appendix O Scenario II Reactive Power Compensation Overload Report _________ O-2

Appendix P Scenario II Contingency Loading Report _________________________ P-1

Appendix Q Scenario II Line Diagram for Line Capacities ______________________ Q-1

Appendix R Scenario II All Generators Results ______________________________ R-1

Appendix S The Contingency and Automation Process in PSS/E _________________ S-1

Appendix T Sub, Mon and Con files for the contingency analysis ________________ T-1

Appendix U Division of the Work Between the Authors ________________________ U-1

Table of Figures and Tables

Figure 1.1 Regional gropes of ENTSO-E and the IPS electrical systems [8] 2

Figure 2.1 The total power aka. the apparent power, active power and reactive power [18] 7

Figure 2.2 Real power and reactive power plotted against the load angle and voltage [20] 8

Figure 2.3 Structure of an electrical power system [20] 9

Figure 2.4 Showing the basic schematics of an on-load tap changer [20] 12

Figure 2.5 Transmission line equivalent 13

Figure 2.6 The magnetic field H between two conductors 14

Figure 2.7 Electric field E between two conductors 14

Figure 2.8 Busses connected in star and delta with line impedance Z [20] 16

Figure 2.9 An equivalent circuit of a short transmission line 17

Figure 2.10 An equivalent circuit of a medium transmission line 18

Figure 2.11 Schematic scheme over a contingency plan [23] 19

Figure 2.12 A typical arrangement for a HAWT [20] 20

Figure 2.13 A typical Cp/ curve for a wind turbine [20] 20

Figure 2.14 Turbine power as a function of the wind speed [20] 21

Figure 2.15 An investment and payback curve for a nonspecific project [25] 23

Figure 2.16 One line diagram over the PSS/E model 25

Figure 2.17 Load and generation in Moldova, rectangles represent generation and circles loads 28

Figure 2.18 The algorithm for the contingency analysis where generator G is increased 30

Figure 3.1 The dispersion of voltage levels for the base case contingency analysis 32

Figure 3.2 A histogram of the dispersion of voltage levels with a contingency analysis 34

Figure 3.3 A histogram of the dispersion of voltages levels with a contingency analysis 35

Figure 3.4 Payback time with a capability factor of 0,3 37

Figure 3.5 Payback time with a capability factor of 0,1 38

Figure 3.6 Payback time including CER:s with a capability factor of 0,3 38

Figure 3.7 Payback time including CER:s with a capability factor of 0,1 39

Figure A-1 Map over the Moldovan electrical system [31] A-1

Figure B-1 Wind Potential in Moldova at the height of 70 meters [14] B-1

Figure D-1 Line diagram from PSS/E for the equivalent circuit over the Moldovan electrical system D-1

Figure F-1 Line diagram for the equivalent circuit over the Moldovan electrical system F-1

Figure K-1 One line diagram with line capacities K-1

Figure Q-1 One line diagram with line capacities Q-1

Figure R-1 Shows the iterations with all generators in scenario I. R-1

Figure R-2 Histogram over the maximum generation without reactive power compensation R-2

Figure R-3 Results from the second iteration with a narrow interval for each generator R-2

Figure R-4 F Histogram over the maximum generation with reactive power compensation R-2

Figure S-1 Shows the recorder function within PSS/E S-2

Figure T-1 Contingency file created for the contingency analysis T-1

Figure T-2 Monitor file created for the contingency analysis T-1

Figure T-3 Subsystem file created for the contingency analysis T-1

Table 1.1 Existing transmission lines in Moldova 3

Table 2.1 Load values for the active generation and consumption 28

Table 2.2 Model load values for the active generation and consumption 28

Table 3.1 Maximum generation before violation in the contingency report 33

Table 3.2 Maximum values regarding only line capacities 33

Table 3.3 Extended generation potential until line capacity is reached 33

Table 3.4 Possible generation capacity with reactive power compensation 33

Table 3.5 Maximum generation for each location giving maximum total generation for the region 34

Table 3.6 Maximum generation for each location giving maximum total generation for the region 35

Table 3.7 Imported electrical energy reduction due to wind power installations 36

Table 3.8 Imported electrical energy reduction with reactive power compensation 36

Table 3.9 Imported electrical energy reduction due to wind power installations 37

Table D-1 Bus data for equivalent circuit over the Moldovan electrical system D-2

Table D-2 Plant data for equivalent circuit over the Moldovan electrical system D-2

Table D-3 Machine data for equivalent circuit over the Moldovan electrical system D-2

Table D-4 Load data for equivalent circuit over the Moldovan electrical system D-2

Table D-5 Branch data for equivalent circuit over the Moldovan electrical system D-2

Table E-1 Data over transmission line types E-1

Table E-2 Impedance values for the lines in the 110 kV system E-1

Table E-3 Base impedance values E-1

Table E-4 Per Unit values for the lines in the 110 kV system E-1

Table F-1 Bus Data F-2

Table F-2 Branch Data F-2

Table F-3 Machine Data F-3

Table F-4 Plant Data F-3

Table F-5 Load Data F-3

Table F-6 Switched Shunt Data F-3

Table F-7 Three Winding Data F-3

Table F-8 Winding Data, MGRAS F-4

Table F-9 Winding Data, Vulcanesti F-4

Table F-10 Winding Data, Hancesti-Straseni F-4

Table F-11 Winding Data, Chisinau F-4

Table J-1 The busses with maximum and minimum voltage levels from the loading report J-8

1

Chapter 1 Introduction The introduction starts by giving a description of the company that the project has been executed in

cooperation with. Thereafter follows a short background of Moldova with its electrical system,

energy and renewable energy potential. The introduction ends with the aim and the goals of the

project.

1.2 Borlnge Energi AB Borlnge Energi is owned by the municipality of Borlnge. Borlnge Energi provides a wide range

of services such as electricity, electricity grid, district heating, water, sewage, storm water and waste

handling. In addition to these commitments Borlnge Energi also handles the municipalitys streets

and parks [1].

Borlnge Energi has had international collaborations since the 1990th, with a primary focus on the

environment. In 1998 the local authorities in Borlnge and the Swedish embassy in Bucharest

initiated a project to establish links between the Swedish and the Romanian municipalities. This led

to collaboration between Borlnge and the Romanian city of Pietsi. In Pietsi Ronny Arnberg from

Borlnge Energi and the mayor of Borlnge Nils Persson met with representatives from Chisinau city

hall and from APA Canal, the water and wastewater company in Chisinau. This was the start for the

cooperation between Borlnge Energi and Moldova with focus on the capital, Chisinau. [2].

The municipality of Chisinau has an interest in understanding the Swedish way of thinking. From

the cooperation with the municipality of Borlnge they will try to study different ways of spreading

information to the society, working with youth and sustainable development. From the start of the

cooperation in the year 2009 several projects regarding the environment have been conducted [3].

With the cooperation as a base many master thesis projects have been written together with

Borlnge Energi in Chisinau.

1.3 Moldova Background The Republic of Moldova is a small country situated in the south-eastern part of Europe with a total

area of 33 800 m2 and 3,6 million inhabitants. Bordering countries to the north, south and east is

Ukraine and to the west Romania. The capital is Chisinau with a population of around 600 000

inhabitants, other important cities are Tiraspol (located in Transnistria, see below) and Baltsi. Around

41 % of the inhabitants live in cities. Moldova became an independent state 1991 with the

dissolution of the Soviet Union [4]. With a GDP of 1500 US dollars per capita Moldova is the poorest

country of Europe [5].

The population consists of different ethnical groups with the biggest being the Moldavians but there

are also large groups of Ukrainians and Russians. The different ethnic groups have contributed to the

violent history of the country. In connection with the dissolution of the Soviet Union an armed

conflict broke out in the eastern part of Moldova called Transnistria. The majority of the population

in Transnistria consists of Russians and Ukrainians who wanted to establish a breakaway republic of

Transnistria. The breakaway republic never gained international recognition and the armed conflict

ended in 1992. Negotiations between Moldova and Transnistria with help from Russia have ended in

a greater sense of autonomy for Transnistria, to this date the conflict is not yet solved. [6]

2

1.3.1 Grid History Before the dissolution of the Soviet Union the electrical grid and power plants were laid out to jointly

optimize the market in the south-western Soviet Union and the other countries in the region e.g.

Romania. With the fall of the Soviet Union these countries had to redesign their electrical systems.

Moldova and Ukraine stayed with the Eastern European system IPS (Interconnected Power Systems)

together with Russia while Romania chose to connect with the Western European system ENTSO-E

(European Network of Transmission System Operators for Electricity). ENTSO-E operates at the same

frequency as IPS but the two systems do not operate synchronized with each other. [7]

Figure 1.1 Regional gropes of ENTSO-E and the IPS electrical systems [8]

The ENTSO-E is the joint European transmission system operator, in Figure 1.1; all marked zones

except IPS are part of the ENTSO-E network and thus operate synchronously.

1.3.2 Energy Moldova imports 94 % to 98 % [9] of its consumed energy from Russia, Ukraine and Transnistria. The

country thus is very dependent on the eastern countries for energy supply. Striving to align itself with

the western part of Europe the energy security is an important issue. The main possibility for

improving the energy security is with new power supply within the country.

Today the electric power generation in Moldova and Transnistria consists of three CHP (Combined

Heat and Power) plants, two HPP (Hydropower Plants); MGRAS, the biggest power plant in the

region, fired with gas, and situated in Transnistria; and other minor power plants. The total capacity

in Moldova, incl. Transnistria, is 3008 MW but around 2570 MW is generated by MGRAS and is thus

not controlled by the Moldovan government. This means that Moldova only has around 438 MW of

generation capacity. This is not enough to supply the demand of baseload in Moldova [10]. The total

consumed electric energy in Moldova year 2010 was 4102 GWh out of which 1064 GWh was

produced domestically and 3038 GWh was imported i.e. Moldova imported 74 % of all electrical

energy consumed within the country. Due to the complex situation with Transnistria electrical energy

imports have mainly come from Ukraine, but recently imports from Transnistria have increased and

are now dominating. [11]

Moldova

3

The transmission grid in Moldova is interconnected with the neighbouring countries; six 330 kV

overhead power lines to the Ukrainian power system, the connections to Romania consists of four

110 kV lines and one 400 kV line with which Moldova also gets connection with the Bulgarian power

system see Appendix A. Because of the connections to the ENTSO-E system i.e. with a different

synchronization than Moldova in Romania and Bulgaria, the transmission lines can only operate in

island mode on the Moldavian side, or by using back to back frequency converters. [12]

The backbone in the Moldovan electrical system is the 330 kV line going from north to south, it is the

main connection to both Transnistria and Ukraine. Well integrated together with the 330 kV system

is the mesh of the 110 kV system which is spread out throughout Moldova. Table 1.1 shows the

existing overhead line voltages and the total length of these. [13]

Table 1.1 Existing transmission lines in Moldova

Voltage level [kV] Length of the overhead transmission lines [km]

400 214

330 532,4

110 5231,1

Total: 5977,5

1.3.3 Bio Energy Potential Moldova has no experience of large scale applications of bio energy even though its an agricultural

country. It has some experience in small scale applications in the rural area. Moldovas biomass

suitable for energy use comes from forestry, agriculture, food industry and waste from households,

where agricultural waste has the biggest potential as an energy source. At present Moldova biomass

is inefficiently used as many outdated and simple technologies are used to convert the biomass into

energy e.g. domestic fires and stoves efficiency rating rarely exceeds 50 %. There is also a lot of

biomass that today cannot be used because the lack of new, today already existing, technologies

needed for the conversion of biomass into energy. [14]

The technical potential of biomass in Moldova is 5,4TWh, where 2,1TWh comes from agricultural

waste, 1,2TWh comes from fuel wood, 1,3TWh comes from wood processing waste and 0,8TWh

comes from biogas. The potential for bio fuels is another 0,6TWh, meaning that the total potential of

bio energy in Moldova is 6TWh. [15]

Bio energy has the biggest energy potential in Moldova; both in theoretical values and in the

potential to include it in todays already existing social infrastructure and energy system

development programs. [14]

1.3.4 Solar Energy Potential There has been research about solar energy utilization in Moldova. The research where performed by

the institute of Energy of the academy of Sciences of RSSM (Moldovan Soviet Socialist Republic) in

the late 1950s. The research resulted in a greenhouse with solar installations and heat storage in the

ground. Because of the low prices for fossil fuel and lack of politic incitements for renewable energy

the project was terminated. In the 1980s the work for implementing solar installations where

restarted. [14]

4

Solar energy is received on the earths surface all the time, though the amount of energy received on

the earths surface depends on several different factors. The most important factors are the suns

brightness, duration and height above the horizon. In Moldova the theoretical duration of the sun,

when its shining unimpeded, is 4445-4452 hours per year [14] but the real duration is 2100-2300

hours per year because of clouds concealing the sun. The amount of solar energy received on the

surface of Moldova differs from 2300 kWh/ m2 year in south to 2100 kWh/m2 year in the north [14].

Other sources estimate the solar radiation in Chisinau to be 1300 kWh/m2 year. [15]

The solar energy in Moldova is primarily used for heating water using solar panels, secondarily used

to dry fruit, vegetables and medicinal plants and tertiary for converting solar energy into electricity

via photovoltaic conversion.

1.3.5 Wind Energy Potential Historically, the area that today is called the Republic of Moldova has been appreciated as favourable

wind zone for wind energy development. Statistical data from 1901, before the development of

steam engines and internal combustion engines, shows that a total of 6208 windmills were

registered in the Moldova area and its surroundings. Some of these windmills were even used during

the interwar period. During the 1950s even 350 windmills where built, exclusively to pump water for

agricultural purposes. These where later replaced by cheaper and more easy to handle electrical

systems. The electrification that occurred in Moldova during the 1950s as well as the low prices for

electrical energy where factors that wind power could not compete with at the time. Today Moldova

doesnt have any wind power.

At present day Moldova has no wind power installed; however there are plans to install wind power

plants in a near future. The south of Moldova is often mentioned as a preferable area to build wind

power. The opinions of Moldovas wind potential differ e.g. the organisation 3tier concluded MAWS

(Mean Annual Wind Speed) of 4-6 m/s at the height of 80 m [15] while a feasibility study written by

the UNDP Moldova concluded MAWS of 4,5-8,5 m/s at a height of 70 m [14]. Moldovas technical

potential for wind power is up to 1 GW installed power providing approximately 1,1TWh of electrical

energy [15]. This correlates to a capability factor of 13 %, which is very low.

1.1.1 Environmental Goals Renewable energy in Moldova would go in accordance with the goals set up in their SNC (Second

National Communication) directed to the UNFCCC (United Nations Framework Commission of

Climate Change). The national priorities to reach the goals of greenhouse gas reductions include

wider use of CDM (Clean Development Mechanism) projects, implementing a more aggressive policy

on transfer of the green technologies, intensifying the process of international cooperation. An

analysis on the possibilities to construct a wind power plant in Moldova in regards of the wind

potential and the stability of the electrical grid would facilitate and work for the Moldavian national

goals. The SNC also identifies relevant policies for the energy sector where two out of five directly

would be coherence with the intended study, assuring energy security of the country by improving

the interconnection capacity with the neighbouring countries and construction of new local sources

of power generation based of the most recent and advanced environment friendly technologies.

and increasing the share of renewable sources of energy in the energy balance of the country.

[16]

5

1.4 Aim and Goals Moldova has a high dependency of imported electrical energy from Ukraine and Transnistria. To rid

this huge dependency Moldova could look to its national resources for domestic production. The

national goals in Moldova are angled towards sustainable development with more renewable energy.

This project will investigate how much the dependency of electrical imports could be reduced by

wind power installations in the Moldovan electrical system as it looks today.

The goals of the project:

The project will conclude in how much wind power installations would be possible in

Moldova considering limiting factors of the electrical system.

The project will show how much the electrical imports can be reduced by wind power

installations in Moldova.

The project will also conclude in potential sites for wind power installation

An economic analysis will show whether it would be profitable to construct wind power in

Moldova

6

Chapter 2 Background The background describes the grid theory important for the project, a section with wind power

potential and finally the method with the creation of the model. This is followed with the scenarios

used to examine the model.

2.1 Grid Theory To understand transmission networks active and reactive power are important concepts described

below, also described are definitions of terms and components in the electrical system. The section

ends with a description of approximations needed for computerize model calculations.

2.1.1 Active and Reactive Power Power is the rate of change of energy with respect to time [17]. It is the amount of energy being

absorbed by a load during a time interval. Reactive power cannot be expressed in the same way, it

cannot be seen as a constant flow of energy from one point to another, the reactive power is flowing

back and forth in the system and when completing a cycle just as much energy that was flowing away

has flowed back. The average reactive power in any system is always equal to zero. The reactive

power is thus not measured by its average value, being zero, but by its amplitude, its maximum

value. This gives a measurement of how much reactive power that is actually flowing through the

system. [17]

In an RLC circuit, with inductance L and capacitance C, the voltage before and after the load will have

a small angular difference described by the load angle , the current will be shifted from the voltage

with the current angle . The difference between and is the power factor angle .

With a purely inductive load the current lags the voltage by and in a purely capacitive load the

current leads the voltage with . In the following equations the load angle is equal to zero.

Equation 1

Equation 2

With these expressions for voltage and current the instantaneous power can be expressed by:

Equation 3

This expression combined with trigonometric identities gives Equation 2.4.

Equation 2.4

Equation 2.4 consists of one real and one imaginary part; the real power is defined as the average

value of the real part.

Equation 2.5

The average value of reactive part, as can be seen below in the Figure 2.1, is always zero; this is the

definition of reactive power. Instead the reactive power is measured by its amplitude value, this

gives us: [18]

Equation 2.6

7

Figure 2.1 The total power aka. the apparent power, active power and reactive power [18]

P is expressed in Watts and Q in VAr (Volt Ampere reactive) both describing the same quantity but

with different units to distinguish them. The power factor angle in the cosine term in Equation 2.5

and Equation 2.6 called the power factor. For inductive loads where the current lags the voltage the

load consumes reactive power. With capacitive loads the current leads the voltage and the load

creates reactive power. [17]

According to [19] the active and reactive power in a RLC four terminal electric circuit can be

described by Equation 2.7 and Equation 2.8 if the resistance R is neglected and assuming that the

load angle is small.

| || |

Equation 2.7

and

| |

| | | | Equation 2.8

Equation 2.7 describes the dependence the active power has on the differences between the phase

voltages and the angle between these. The phase voltages in the power system may not differ much

between busses and thus the active power is highly dependent on the load angle which

is the angular difference between and . This gives us the characteristics that the active power is

strongly dependent on the load. [20]

According to Equation 2.8 even a small change in voltage causes a large change in reactive power. If

the reactive power is plotted against the voltage it corresponds to an inverted parabola, the

dependency on the reactance gives us that the smaller the reactance the steeper will the parabola

be, this means that with a low reactance small changes in voltages causes very large changes to the

reactive power. The relationship can be seen in Figure 2.2 together with the sinusoidal characteristics

of . [20]

Po

wer

Time

8

Figure 2.2 Real power and reactive power plotted against the load angle and voltage [20]

In the three phase system the power is increased by a factor of as seen in the equations below.

[17]

Equation 2.9

Equation 2.10

2.1.2 Introduction to the Electrical Power System The modern society requires energy for use in the industry, agriculture, commerce, transportation,

communications, domestic households etc. The total energy required during one year is called total

annual energy demand. About 85 % [21] of the total energy demand in the world is today supplied by

fossil fuels like coal, oil, and natural gas. A large part of these fuels contribute to the electric energy

production. Today the world is switching from these fossil fuels and more electrical energy is

produced by renewable sources like wind power, solar power, hydro power, biogas, bio energy and

geothermal energy. One of the major reasons for the increase in renewable energy is the global

warming. In the future its likely that the share of the energy market taken by renewables will

increase to high levels and play a more dominant role on the design of electrical power systems. [20]

2.1.2.1 Structure of the Electrical Power System The electrical power system can be divided into three different parts; generation, transmission and

distribution.

The transmission network is normally the network with the highest voltage, from 300 kV and above.

Transmission networks have the highest transferring capacities and are mostly built as meshed

networks to increase the security of the system. To the transmission network only very large

electrical energy consumers and producers are connected. The transmission network can also be

used as connecting lines to other systems for example tying different countries together. [20]

The sub transmission network is a part of the transmission network. It consists of a high or medium

voltage network, with the voltage levels ranging between 100 kV to 300 kV. Unlike the transmission

network the sub transmission network is built as a radial network or a weakly coupled network. To

the sub transmission network medium producers and consumers can be connected. [20]

9

Figure 2.3 Structure of an electrical power system [20]

Distribution networks are networks with medium voltages, in the range of 1 kV to 100 kV. The

distribution network is often radial built networks. To the distribution network small generation and

medium sized customers are connected. Wind power plants are often connected to the distribution

network. The classification of the different parts of the system is not a strict classification and can

vary depending on who is classifying it. [20]

2.1.2.2 Reliability of Supply One of the most important features of the electrical power system is that electrical energy cannot

easily be stored in large quantities. At any instant in time the energy demand has to be met by the

corresponding electricity generation. Fortunately the combined load pattern is pretty predictable

whilst individual loads may vary quite much. This predictable system demand can thus quite easily be

planned allowing scheduling the daily generation to be controlled in a predetermined manor. [17]

The electrical system is designed to operate within certain operational limits governed by grid codes.

These operational limits ensure that you avoid major interruption of supply that can lead to life-

threatening situations for the normal consumer, and for the industrial consumer may pose severe

technical and production problems and thus loss of income. This is why high reliability of supply is of

fundamental importance for the electrical system. High reliability can be ensured by: [17]

High quality of installed elements

The provision of reserve generation

Employing large interconnected power systems capable of supplying each consumer via

Alternative routes

A high level of system security [17]

10

2.1.2.3 Stability and Security of the Power System The stability of the power system is defined as the ability of the power system to regain equilibrium

after being subjected to a change. The most common changes that affect the stability of the system

are the variables described in the chapter on Active and Reactive Power i.e. the nodal voltage

magnitudes, which affect the reactive power, and the nodal load angles, connected with the active

power. This gives us the new terms of power angle stability, and voltage stability. [17]

The security of the power system refers to the ability of the power system to survive certain

contingencies without affecting the quality of electrical supply to the customers. The stability of the

power grid is part of the security but the concept of security is wider and also deals with other issues.

The assessment of the power system can be divided into the SSA (Static Security Assessment) and the

DSA (Dynamic Security Assessment). The DSA deals with the stability and quality of electrical supply

during a change in the system where as the SSA only considers before and after scenario and

assumes that there was no breach in stability along the way. [17]

It is in the interest of the TSO to perform the SSA in order to first evaluate the pre contingency state

i.e. determine available transfer capability of transmission links and identify network congestions.

Secondly to evaluate the post contingency states i.e. verify the bus voltages and power flow limits.

Being responsible for the grid security the TSO needs to find ways of controlling the system so that it

does not break down. Having no direct control over the generating units the only way to affect

power outputs or control settings of the power plants are the grid codes or commercial agreements.

[17]

As stated above the DSA deals with problems regarding the system stability and quality of electrical

supply, the analysis in this report strictly deals with SSA and will thus not describe the problems

regarding the dynamic simulations. A short description of some of the problems that occur follows in

the next chapter. [17]

2.1.2.4 Quality of the Electrical Supply It is not just important that there is a high reliability to the system, there also has to be a high quality

of the electrical supply. Electrical energy of high quality is provided by:

Regulated and defined voltage levels with low fluctuations

A regulated and defined value of the frequency with low fluctuations

Low harmonic content

Low content of transients and flicker

To ensure the quality of the electrical supply two basic methods can be used. Firstly the proper uses

of automatic voltage control i.e. shunt elements, tap transformers, frequency control methods and

AVR (Automatic Voltage Regulation) within the generating units. Secondly by employing large

interconnected systems because larger systems are naturally affected by load variations as well as

other disturbances. To ensure the quality of electrical supply the TSO set codes that the grid should

operate within. A common standard is that the frequency should not deviate from the base value

with more than 0,1 Hz and the nodal voltages should stay within 10 % of its normal value. These

regulations vary depending on voltage level but also depending of fault scenario. [17]

11

2.1.3 Components in the Electrical Power System The most important components of the electrical power system are generating units, transformers,

shunt elements and transmission lines. These are described below.

2.1.3.1 Generating Units Generating units are the elements in the electrical power system that produces electrical energy.

There are several different types of generators with different properties. Examples of different

generators are the synchronous generator and induction generator. The generators are converting

kinetic energy into electrical energy. Electrical energy is produced by a generator driven by a kinetic

energy source, often a turbine or diesel engine. The turbine is equipped with a turbine governor

which controls either the speed or the power output according to a pre-set power-frequency

characteristic. The generated power is then fed to the electrical power system. [17]

Traditionally the electrical power system has been operated with relatively few large power plants

connected to the transmission network. These large plants are usually either thermal or hydro based.

Concerns about global warming and sustainability have increased the interest for renewable

generation like thermal power plants which uses bio fuels, wind power and solar. This requires major

changes in the electrical power system as the generation will increasingly be based on large amount

of small producers often with the generation situated close to the energy source. Renewable energy

has lower energy density than non-renewable energy sources and therefore the renewable power

plants tend to be smaller, around hundreds of kilowatts to a few megawatts. Plants of this small size

are often connected to the distribution level of the power system, rather than the transmission level

because of the lower costs for the connection. These plants are called distributed generation. [17]

Wind turbines are a typical example of distributed generation power source. Wind turbines often use

induction generators with either fixed speed or doubly fed generators to convert the power in wind

into electrical energy. It is important to know that the rotating magnetic field in the induction

machine is produced by a magnetizing current, whether it is operating as a generator or a motor. The

magnetizing current is always supplied from an outside power source, often from the electrical

power system. This means that the induction machine always consumes reactive power and

therefore always must be connected to a power system that can provide the induction machine with

reactive power for it to function properly. The reactive power can either be provided directly from

the electrical power system or via reactive power compensation units installed together with the

wind turbine. [17]

2.1.3.2 Transformers Transformers are needed to connect parts of the power system with different voltage levels.

Generator step-up transformers are used when connecting generators to the grid. Tap transformers

are used when there is a need for voltage regulation. Transformers can also be used for reduction of

voltage to suit the low voltages needed by the consumers. This is done with distribution

transformers. Connection of different parts of the electrical network with different voltage levels is

done with transmission transformers. [20]

12

Transformers are built up by a magnetic core with windings wrapped around the core. For two

winding transformer there are two sets of windings and with the three winding transformer there are

three sets of windings. The three winding transformer can thus transform one voltage level into two

different to suit several needs at once. The relation between the phasor voltage and the number of

turns at each winding is shown in Equation 2.11. [17]

Equation 2.11

Thus the change in number of turns for the windings will affect the voltage levels proportionally.

Transformers that can control voltage levels by changing the number of turns of the windings are

known as tap changing transformers. The tap changers can operate ether as off load or on load. The

off-load tap changers have a regulation rate of generally 5 % of voltage levels. The off load tap

changers are operated manually and change is normally made to accommodate the seasons. The on-

load tap changers have a general operational range of maximum 20 % of voltage levels and change

is controlled by a regulator and can thus respond directly to disturbances such as a load change. A

basic principle of a tap changer is shown in Figure 2.4 where the selectors S1 and S2 can move

between the windings to cause small changes to the voltage. [20]

Figure 2.4 Showing the basic schematics of an on-load tap changer [20]

2.1.3.3 Shunt Elements Due to the fact that reactive power causes losses and uses the capacity in electrical lines the optimal

operation is reached if reactive power is compensated for close to the point of consumption and not

produced at the generation sources far from the consumption. One way to compensate for the

reactive power is with shunt compensation i.e. by installation of capacitors or inductors close to the

point of interest. Shunt compensation can also be used to stabilize voltage levels and thus

strengthening the stability of the electrical power system. [20]

Transmission lines are generally consuming reactive power but if the load is very low the production

of reactive power can exceed the consumption. This may lead to very high reactive power levels

which in turn may lead to very high voltage levels due to the strong correlation between reactive

power and voltages seen in Equation 2.8. Compensation for this effect is generally done for lines

longer than 200 km by installation of shunt reactors. In a loaded line, shunt capacitors may be used

to produce reactive power and compensate for voltage drops, more commonly series capacitors are

connected in series with the conductors to compensate for the reactive power consumed by the line.

[20]

13

Shunt compensation can also be supplied by a synchronous motor or generator running at no load

called synchronous compensation. Being rather expensive switched shunt capacitor banks and

reactors are often used in addition to the synchronous compensation at substations. Small such

compensators, of several MVA, are often used on the tertiary winding of transmission transformers

while larger compensation, of up to hundreds MVA, are connected to by individual step-up

transformers to high-voltage substations. [20]

2.1.3.4 Transmission Lines There are both overhead and underground transmission lines though the overhead transmission

lines are the most common. An overhead transmission line consists of three main components,

conductors, insulators and support structures. Transmission lines often also have shield wires placed

above the conductor to protect it from lightning.

2.1.3.4.1 Important Parameters The design of the transmission line determines these parameters e.g. conductor type, the space

between conductors and the size determines the series impedance and shunt admittance where the

series impedance affects the ohmic losses, line-voltage drops and the stability limits. The shunt

admittance, which is primarily capacitive, affects the line charging currents. The line charging

currents are the currents which increases reactive power in the power system. In light loaded power

systems shunt reactors often are installed to absorb this reactive power and thus reducing over

voltages. [17]

A transmission line can be described with the equivalent seen in Figure 2.5 where R is the resistance,

L is the inductance, G is the conductance and C is the capacitance.

Figure 2.5 Transmission line equivalent

2.1.3.4.2 Resistance in Transmission Lines The DC resistance in the conductors depends on the length, cross sectional area and the conductivity

of the conductor. The conductivity also depends on the temperature. The DC resistance is described

below:

Equation 2.12

where is the conductivity at temperature T, is the length of the conductor and A is the cross

sectional area. The conductivity depends on the material and common materials for conductors are

copper and aluminium. Temperature and current magnitude also affect the resistance in conductors

with AC current. The resistance is frequency dependent due to the skin effect which is the

phenomenon that the current distribution tends to be denser at the surface of the conductor. This

14

causes a conductor loss, the effect only occurs with AC currents. The higher the frequency the higher

is the real power losses due to the skin effect. losses is always bigger than losses

[17].

| |

Equation 2.13

2.1.3.4.3 Conductance in Transmission Lines The conductance can be modelled as the shunt admittance in overhead lines. The conductance

occurs because of the leaking currents due to the corona effect, damaged insulators and dirt, salt and

other contaminants. The corona effect occurs when the electrical field strength at the conductor

surface causes the surrounding air to ionize and thereby conduct. The losses from the conductance

are much lower than the ohmic losses in the conductor, and are thus normally

neglected. [17]

2.1.3.4.4 Inductance in Transmission Lines The inductance in conductors comes from the current flowing in the transmission line.

Figure 2.6 The magnetic field H between two conductors

The inductance depends on the magnetic field intensity H, the magnetic flux density B, the flux

linkages , and inductance from flux linkages per ampere

as can be seen in Figure 2.6. [17]

2.1.3.4.5 Capacitance in Transmission Lines An electric field is created between two conductors because of the difference in potential between

the conductors, represented by V in Figure 2.7.

Figure 2.7 Electric field E between two conductors

The capacitance is defined by the charge divided by the voltage

. The charge is dependent on

the electrical field and the flux. In an ideal solid cylindrical conductor the flux and electrical field is

equal to the area integral of the electric field strength and the electric flux density over the surface

area of the conductor. [17]

15

2.1.4 Per-Unit System Working with electrical systems with different voltages the per-unit system is often introduced.

Basically it reduces the risk of making calculation error when going from one voltage level to another.

If values are expressed in per-unit there can be a direct comparison from one side of a transformer to

another. The expression for calculating the per-unit value is shown in Equation 2.14.

Equation 2.14

The resistance and reactance base values are calculated using the base value of the impedance

and the base values for the conductance and susceptance is calculated with the base value for the

admittance . The connection between the two base values can be seen in Equation 2.15. [17]

Equation 2.15

2.1.5 Equivalents in Electrical Power Systems Electrical systems are generally very large with a lot of components, modelling this as a complete

system including all components is often an impossible task if even a desired one. One method of

creating an equivalent of parts in an electrical system is called model reduction methods. This

method consists first of physical reductions, where suitable models for the system are chosen

depending on how influential the system elements are to a disturbance. A component far from a

disturbance is not as affected by a disturbance and can thus be modelled more simply. Secondly

there is topological reduction where busses can be reduced to limit the size of the equivalent

network and number of components in it.

The topological reduction can be achieved by many techniques using matrix operation. The reduction

can be done with Gauss-Rutishauser elimination, also called Ward equivalent, which use the

admittance matrix as a starting point, se 2.1.6.1 for how to create the bus admittance matrix.

Reduction can also be done looking at one specific bus, a typical such reduction is reduction of a

centre bus in a star bus system creating a delta connected bus system. Equation 2.16 is describes the

new admittance derived from old admittances in the system, k here describing the centre bus in the

star system. [20]

Equation 2.16

The directly connected busses i.e. its neighbours will be affected in such a way that the admittance

needs to be changed between these busses. Busses in the system not directly connected to the bus

being removed will not be affected by the removal. With the star delta equivalent, a change of

impedance needs to be regarded in lines AB, AC and BC. [20]

16

Figure 2.8 Busses connected in star and delta with line impedance Z [20]

In the case with the star connected bus Equation 2.16 can be written as seen in Equation 2.17. When

taking into consideration that the admittance is the inverse of the impedance the equation can be

rewritten as seen in Equation 2.18. [20]

Equation 2.17

Equation 2.18

Considering a circuit which consists of only three busses connected in serial Equation 2.18 is

simplified and is expressed by Equation 2.19.

Equation 2.19

2.1.6 Static Modelling

2.1.6.1 Bus Based Equations For computing the power flow in an electrical system it is necessary to compute voltage magnitudes

and phase angles at each bus in the system. The input data for these calculations are the voltage

magnitudes V, the load angle , the net real power P and the reactive power Q. Two of these

parameters are always input data at each bus in the system and two are calculated by the power

flow program. The bus categorization is as follows: [17]

Swing bus, also known as slack bus

o The electrical model can only contain one swing bus being the reference bus for

other busses in the system. Input data are the voltage and the load angle, normally

as 1 p.u. and 0 . The swing bus is not a real bus. It is only a way to help model the

system and perform numerical calculations.

Load bus

o Normally the most common bus in a power system where P and Q are input data and

V and are calculated.

Voltage controlled bus, also known as generator bus

o Normally the bus to which a generator is connected. P and V are input data and Q

and are calculated. With this bus there are also some extra input data, one can

here also decide for example which interval a generator can operate between i.e.

QMAX and QMIN. A bus to which a tap-changing transformer is connected to should

also be designed this bus type.

17

Computer programs calculating power flows in electrical system use the bus admittance matrix

which forms Equation 2.20 together with the voltage and current. The bus admittance matrix is built

up on the diagonal by the sum of admittances connected to the specific bus in question and all off

diagonal elements are the negative sum of all admittances between the specific bus and other busses

in the system. [17]

Equation 2.20

Equation 2.20 is combined of the bus admittance matrix Y, the column vector of the bus voltages V

and the vector of current sources I. The system admittance and the bus connections can be input

data for the computations which result in the bus admittance vector. With the bus admittance vector

and the current at each bus the bus voltage can be determined. [17]

For one line these calculations can be made manually but for a system with many components this

builds up to complex matrix calculations best suited for computer computation. There are many

different programs for computing power flow problems e.g. PSS/E, PSCAD, Power World Simulator,

Aristo, etc. The solution type used to solve can also vary but the most common is the Newton-

Raphson method. [17]

Since power flow bus data consist of the real and reactive power for load busses, and real power and

voltages for generator busses. Equation 2.20 has to be rewritten while using Newton-Raphson

methods of solving matrix equations, but it is still the base for the calculations. [17]

2.1.6.2 Line Approximations Transmission lines characteristics can be modelled for calculations and depend on the length of the

transmission line. A short transmission line, while having a 50 Hz system, shorter than 100 km can be

represented as Figure 2.9 i.e. only with series resistance and inductance. The subscript S and R stands

for the sending end and receiving end voltage and current and is the length of the line.

Figure 2.9 An equivalent circuit of a short transmission line

For a medium length transmission line the admittance, Y, cannot be neglected, and is represented by

the admittance making the equivalent circuit change to a -circuit with the admittance connected in

parallel with half at each end of the circuit, as seen in Figure 2.10. It is the same equivalent seen in

Figure 2.5 but here with the admittance divided between the sending and receiving end. Medium-

length lines ranges from 100 to 300 km.

18

Figure 2.10 An equivalent circuit of a medium transmission line

The admittance is dependent on the conductance and the capacitance by:

Equation 2.21

The conductance is normally small enough to be neglected in transmission line calculations making

the admittance in Figure 2.10 and the equations below only dependent on the capacitance.

Equation 2.22 shows the relation between the sending and receiving currents and voltages for the

circuits where the parameters A, B, C and D depends on the constants R, L and C and thus changes

depending on the different length of the transmission lines.

Equation 2.22

The equation can be written in matrix format:

[

] [

] [

] Equation 2.23

For the short line equivalent circuit the A, B, C, D matrix is as shown below:

[

] [

] Equation 2.24

Equation 2.25 shows the relations for a medium length line where the more complex matrix also

includes the admittance Y.

[

] [

] Equation 2.25

The expressions above are as stated approximations where the impedance and admittance is seen as

lumped together. In reality these characteristics of the lines are uniformly distributed along the line.

To account for this one can study line section of length x which changes the relations. The relations

do not change for the short transmission lines but for medium lines with the admittance connected

in parallel we get a new A, B, C, D matrix, shown in Equation 2.26. The equation together with

Equation 2.23 makes it possible to solve for voltage and currents from one bus to another. [17]

[

] [

] Equation 2.26

19

2.1.6.3 Contingency Analysis Contingency analyses are introduced to make sure that the system maintains a certain system

security i.e. with static operation that means; operation without overloads and voltage levels within

grid code levels. The contingency refers to changes in the system that might weaken the electrical

power system and is thus one way to determine weak points in the power system in need of

upgrades. There are different types of contingency analyses from the most basic only considering the

outage of a single transmission line to more complex analyses considering multiple line outages

or/and loss or change of generators/loads in the system. Even open lines i.e. unused, can be closed in

a contingency analysis. [22]

Figure 2.11 Schematic scheme over a contingency plan [23]

The most basic contingency can be described as an N-1 contingency analysis where one component

from the model is disconnected; in the electrical system this can either be on purpose, for

maintenance, upgrades etc. or by an accident or fault. [23]

2.2 Wind Power There are several ways to extract the power of the wind but there are mainly two different types of

wind turbines are used; HAWT (Horizontal Axis Wind Turbine) and VAWT (Vertical Axis Wind

Turbines). Today the three bladed HAWT is the most common wind turbine. Three blades are

generally favoured because it has lower power pulsations, as the blade passes the tower, than a

HAWT with fewer blades. Moreover a three bladed wind turbine is more aesthetically appealing than

a wind turbine with fewer blades than three, whilst the turbines are rotating. Any number of blades

can be used on HAWT, although if too many blades are used they tend to interfere with each other

aerodynamically. Figure 2.12 shows a typical arrangement for a HAWT where Gen stands for

generator G/B for gear box and T for transformer. [20]

N-1 Contingency

Report/Fix Violations?

Final Report

N-0 Base Case

Report/Fix Violations? Yes

Yes

No

No

20

Figure 2.12 A typical arrangement for a HAWT [20]

The power of the wind is extracted by aerodynamically designed blades that produce a lift force

along the length of the blade. This aerodynamic force integrated along the length of the blade

produces the torque on the turbine shaft. The turbine shaft is connected to the gearbox which

increases the shaft speed. The gearbox and generator is placed in the nacelle at the top of the tower.

The generator is connected to the electrical power system via a transformer. [20]

The power in the wind varies with the cube of the wind speed and is described with the following

equation.

Equation 2.27

where is the power that can be extracted from the wind, is the air density, is the swept area of

the blade, is the coefficient of performance for the turbine and is the wind speed. For the wind

turbine to be able to absorb all the kinetic energy in the wind, the wind speed after the turbine has

to be zero. This is impossible because the airflow has to be continuous. The theoretical maximum of

energy that can be absorbed by the wind turbine is called the Betz limit and defined when Cp is equal

to 16/27. The Betz limit is derived from an infinitely thin rotor, which represents the turbine, and a

fluid flowing at a certain speed. In reality the coefficient of performance Cp for a wind turbine is

lower, because also varies with the tip speed ratio . A typical value for Cp is around 0,4. [20]

Figure 2.13 A typical Cp/ curve for a wind turbine [20]

21

A

curve, as seen in Figure 2.13, for a specific wind turbine helps determent at what tip speed ratio

the wind turbine extracts the maximum amount of power in the wind. This is a powerful tool when

designing wind turbines. [20]

Figure 2.14 Turbine power as a function of the wind speed [20]

Figure 2.14 shows the wind turbine power as a function of the wind speed. In order for the wind

turbine to produce power the wind speed need to be greater than vw1, which is called the cut in

speed and lies typically around 3-4 m/s. If the wind speed is lower than the cut in speed the power in

the wind is not high enough for the generator to produce energy. With increasing wind speed the

turbine produces more power until it reaches point A. At point A the generator produces its

maximum power which happens at wind speed which is the rated wind speed, more specifically

the wind speed the turbine is designed for. For higher wind speeds than the rated wind speed the

turbine is regulated with either pitch regulation or stall regulation to extract the right amount of

power from the wind preventing the wind turbine from accelerating. The power output remains

constant until the wind speed reaches , typically around 25 m/s, which is called shut down wind

speed; where the wind turbine shuts down to prevent it from breaking. [20]

The wind is the most important aspect for wind power. Therefore the wind is measured at a desired

location for building a wind power plant over at least one year. Another important aspect of wind

power is the capability factor CF that is defined as seen in Equation 2.28 for a period of one year.

Equation 2.28

CF is the ratio between actual energy production and the maximum amount of energy that could

have been produced if the plant had operated at full capacity over the designated time period. It can

be used to see how efficiently a wind power plant has been operating over one year, a typical value is

around 0,2. [20]

22

2.2.1 Moldovas Wind Resource In order to decide the wind potential for a specific location, a large amount of data for that specific

area is needed. This can be done with a variety of measuring instruments such as anemometers and

direction sensors. Especially important to investigate is the wind velocity probabilistic distribution,

daily and seasonal variations and prevalent wind directions. These are all important aspects for the

efficiency or inefficiency for utilization of the wind power. Another important aspect to account for is

the capability factor.

As described in Equation 2.27 the energy in the wind is proportional to the cube of the wind speed.

This relation is fundamental in all wind power. Statistical data with a high level of credibility is hard to

obtain because it requires systematic observations during a long period of time, at least for one year

but preferable longer, and at hub height of the wind turbine. These measurements is often

performed by companies who are specialized in determining the wind power potential, this data is

very expensive to retrieve. However there are ways to determine the wind power potential with data

measured at the lower heights, which means that data from meteorological weather stations, often

10-12 meters above ground level, can be used to determine the wind power potential. These

measurements are often influenced by the surroundings such as trees and houses.

Two different methods are mainly used to determine the wind power potential for a certain location.

One model is developed in Europe and one in USA. The American model is developed by NASA

together with the U.S.A Air Force and is based on the dynamic climate theory which means that the

model doesnt require a lot of meteorological data, but instead requires more computing processing

power. The European model is called WAsP (Wind Atlas Analysis and Application Program) and has

been used when drawing the European wind atlas. Several European countries such as Austria,

Croatia, Slovenia and Czech Republic etc. have used WAsP when drawing their wind atlases. Moldova

has several meteorological stations which has recorded the wind direction and the wind velocity

every three hours during a period of more than 10 years and have therefore chosen to use WAsP to

draw their wind atlas.

From the calculations given by the WAsP program and with the data from weather stations, a wind

atlas can be derived. The wind atlas main goal is to present the wind energy resource in the area of

the weather station, thus estimate the wind energy potential in the region and with this information

you can identify the best locations for building a wind turbine or a wind power park. A wind atlas

produced over Moldova can be seen in Appendix B [14]

The wind atlas is not very accurate and cannot be used as reference when deciding exact locations

for wind power plants, further investigations must be made. The wind atlas only gives a hint of the

wind conditions. According to the wind atlas the southern region is best suited for wind power

installations.

There are other publications of the wind potential in Moldova from the beginning of the 1990th;

these predictions give a negative picture of the wind power potential in Moldova. However these

investigations where based on wind data from the meteorological station in Chisinau, which is

located in the centre of Chisinau and is surrounded by a variety of obstacles and cannot be

considered as a good reference station. [14]

23

2.2.2 Economy The payback method is used for determining the time it takes for an investment to repay the sum of

the original investment. Its a useful tool when investigating if an investment is profitable in a

reasonable timeframe, or when comparing different investment proposals trying to determine which

one is the most profitable. Originally the payback method doesnt account for other factors such as

inflation or discount rate but there is a discounted payback method where these factors are taken

into account. Its described with the following equation:

Equation 2.29

where is the net cash flow; which is the cash inflow minus outflow, is the discount rate and is

the time. [24]

Figure 2.15 An investment and payback curve for a nonspecific project [25]

Figure 2.15 shows a typical investment and payback curve. At the start of the project money is

invested in the project, this called the investment period. Until the project reaches the self-funding

point the project just costs money. Typical cost during the investment period for wind installations

can be wind measurements, calculations of wind potential, project management and off course costs

for building the wind power plant. At the self-funding point, the investment is starting to earn money

and the investors are getting the invested money back. At the breakeven point the investor has got

all the invested money back and beyond this point all the money earned is pure profit.

24

2.3 Method Five potential locations were chosen for wind power installations. To simulate the effects new

generation would have on the electrical power system of Moldova a model for power flow

simulations is constructed. In the model different scenarios is simulated to show installation

capacities for each location separately and for all sites at the same time.

2.3.1 PSS/E Power flow simulations are done by computer programs, with approximations of transmission lines,

transformers and other components of the electrical power system. There are a number of different

programs on the market, for this project PSS/E (Power System Simulator for Engineering) is used. The

software has efficient tools for simulating static power flows, contingency analysis, and it also has the

possibility to automate these processes. The automation process in PSS/E can be executed in three

different ways, in this project Python programing was used to simulate the different scenarios

described later in chapter 2.3.3 . A description over how a contingency analysis can be executed in

PSS/E together with the creation of important files needed for the process can be seen in Appendix S.

The appendix also describes in more detail the different ways to automat in PSS/E.

2.3.2 Building the Model There were no existing models over the Moldovan electrical power systems that could be used in the

project. A model was created with the help of the Technical University of Moldova (TUM), situated in

Chisinau, specific for this project. TUM provided an equivalent circuit over the Moldovan electrical

power system. The equivalent circuit describes a 330 kV electrical power system, partially seen as the

green line in Appendix A, stretching from big cities in Moldova such as Chisinau, Baltsi and Tiraspol in

Transnistria, going in to Ukraine and finally back to Moldova again completing a full circle. The

equivalent circuit can also be seen as the green part in Figure 2.16. The model is a 7 bus system, out

of which 3 busses are situated in Ukraine, it includes 5 branches, 3 generators and 5 loads divided

between the two areas; Moldova and Ukraine. The complete model along with its specified data,

Table D-1 to Table D-5, can be seen in Appendix D.

Detailed data of the southern parts of Moldovan electrical power system was also provided, the data

was provided in the form of schematics over the grid also stating length and type of the transmission

lines. The properties of the specific lines are given in Table E-1. With this given data the initial model

was extended by 17 busses located in the southern region of Moldova. The line diagram over the

complete equivalent model is shown in Figure 2.16, detailed data over the model can be seen in

Appendix F.

25

Figure 2.16 One line diagram over the PSS/E model

The extended part is the main focus of the report and describes mainly a 110 kV system with the only

exception being one 400 kV line, the blue and yellow part in Figure 2.16. The system is connected to

the initial equivalent circuit, green in the model, at three locations. The different voltage levels also

introduces 4 three winding transformers to the system. Not all busses and thus also not all branches

are modelled but all power flow paths in the southern regions 110 kV system are accounted for.

All values in the model are expressed in per-unit values, the voltage values uses respective base

voltage value as base value i.e. 110 kV, 330 kV and 400 kV. The impedance values are expressed with

the base impedance values given for each voltage levels see Table E-3 in Appendix E. The 400 kV line

is also long enough so the admittance needs to be regarded, the relation between base impedance

and base admittance can be seen in Equation 2.15. [26]

Reactive Power Flow

400 kV Line

330 kV Line

110 kV Line

Active Power Flow

Load

Generator

Three winding

transformer

Switched shunt

Line Offline

26

2.3.2.1 Busses The model consists of 24 busses, the initial 7 have 330 kV as their base voltage, 11 busses are

connected within the 110 kV grid and 2 busses are situated on the 400 kV line between Vulcanesti

and MGRAS, in Transnistria. There are also 4 busses in the 35 kV system, each connected to a three

winding transformer, these busses have no meaning except for modelling the transformer i.e. they

have no load or generation connected to them and can thus be seen as a part of the three winding

transformer not contributing to any system losses by themselves.

The swing bus is the MGRAS bus, situated in Transnistria and is chosen as such because of the high

electrical energy imports from Transnistria. It is also an appropriate swing bus because of the

excessive generation capacity of MGRAS. The swing bus input voltage is increased a bit from

standard 1 p.u. to 1,0455 to increase the system overall voltages. Also the two generators in Ukraine

has an increased voltage to 1,0455 for the same reason.

2.3.2.2 Branches All branches in the 110 kV system are relatively short, the longest is still less than 50 km and as

described in section 2.1.6.2 the admittance is thus neglected. The 400 kV line is of medium length

type and thus also needs to take into consideration the admittance, this value can be located in Table

F-2, under the heading charging, in Appendix F

There is a branch between Tarecklia and Ciadyr that is marked in the model but it is not in use as can

be seen in Figure F-1 and Table F-2. The line is an existing one but for reasons unknown to this

project is not in use at the moment.

The extended southern system do not account for all transmission lines but all power flows paths are

accounted for, thus only transmission lines connected in series are removed. The impedance in these

lines has been accounted for by the method of Equation 2.19. Line data can be seen in Appendix

EThe total impedance values for the lines are given in Table E-2 and recalculated using Table E-3 and

Equation 2.