Comparison of existing PV models and possible integration under EU grid specifications

(1)

Degree project in

possible integration under EU grid

specifications

Ioannis-Thomas K. Theologitis

Stockholm, Sweden 2011 Electric Power Systems

(2)

Comparison of existing PV models and

possible integration under EU grid

specifications

Ioannis-Thomas K. Theologitis

Master of Science Thesis

KTH School of Electrical Engineering

Division of Electrical Power Systems EPS-2011

(3)

POSSIBLE

INTEGRATION

UNDER

EU

GRID

SPECIFICATIONS

Ioannis-Thomas Theologitis

Royal Institute of Technology (KTH), Sweden

School of Electrical Engineering Kungliga Tekniska Högskolan SE-100 44 Stockholm

Sweden

The author is officially enrolled to the Sustainable Energy Engineering Master Program (SEE) and belongs to the School of Industrial Engineering and Management and the Department of Energy Technology.

(4)

“Αποσκότισόν με” -Διογένης- “Take me out of the dark” -Diogenes-

(5)

Abstract

This master thesis investigates the capabilities of a generic grid-connected photovoltaic (PV) model that was developed by DIgSILENT and is part of the library of the new version of PowerFactory v.14.1. The model has a nominal rated peak power of 0.5 MVA and a designed power factor cosφ=0.95. A static generator component, which includes the PV array, the DC bus with the capacitor, the inverter and the control frame, is used to model the PV system. The PV array is considered to operate at the MPP and the generator with cosφ=1.

The thesis begins with a short review of the current status of the PV sector, focusing mostly on the types of PV systems and the necessary components that are used in grid-connected systems. Since the PV inverter is the key component, special reference is made to the different technologies applied and to the multifaceted role that inverters should play nowadays supporting the grid’s stability. Technical restrictions and requirements are presented highlighting primarily the German Grid Code for the MV network, which is the benchmark for the analysis of the role and behaviour of the PV model in question. Germany is regarded a very good example to base the study on due to its leading position and experience in the renewable area and its thorough grid specifications.

The main part of the report includes a detailed description of the structure of the generic model, presenting the operating procedure of its components as well as model assumptions and simplifications. Various simulations in variable solar irradiation, frequency and voltage conditions are performed in order to conclude in its capabilities. The static voltage support is investigated under cloud effect situation where the changes in active power output of the PV array can influence the voltage stability of the grid at the PCC. The active power control is examined by forcing the grid frequency to deviate beyond specified limits and observing the active power output results. At last, the dynamic voltage support capability (LVRT) is examined by simulating four different short circuit events creating four different voltage dips. The ability of the PV inverter to stay connected and to provide reactive current when necessary is seen. The external grid component is designed to represent a strong grid. The results showed that the model is capable for active power reduction and LVRT behaviour. However, the absence of reactive power control makes it inapplicable for static voltage support. Thus, a PI controller is implemented in order to supply constant reactive power in steady state operation and support the grid stability.

At last two different interconnections were built using a slightly modified version of the same generic model with a rated power 1 MVA. The control scheme remained the same. Both configurations were examined statically and dynamically and their results were compared. Small differences were found in terms of reactive power consumption/injection at the PCC.

(6)

Sammanfatning

Det här examensarbetet undersöker förmågan av en generisk nätanslutna solcell (PV) modell som utvecklades av DIgSILENT och det är en del av biblioteket av den nya versionen av PowerFactory v.14.1. Modellen har en nominell beräknat maximal effekt på 0.5 MVA och en utformad effektfaktor på cosφ=0.95. En stillastående generator beståndsdel, som innehåller PV uppställningen, DC bussen med kondensatorn, strömväxlaren och kontroll ramen, som användes för att utforma PV systemet. PV uppställningen förväntas att användas vid MPP-en och generatorn med cosφ=1.

Examensarbetet inleder med en kort genomgång av det nuvarande läget av PV sektorn, som fokus för det mesta på PV system sorter och de viktiga beståndsdelarna som användas i nätanslutna system. Eftersom PV strömväxlaren är den viktigaste beståndsdelen, är särskild hänvisning görs till de olika tillämpade tekniker och den mångfacetterade roll som växelriktare bör spela nuförtiden stödja nätets stabilitet. Tekniska begränsningar och krav presenteras för att belysa främst på den tyska GC för MV nätet, vilket är utgångspunkten för analysen av den roll och beteende av PV modellen i fråga. Tyskland anses ett mycket bra exempel att basera studien på grund av sin ledande ställning och erfarenhet inom förnybar området och dess grundliga specifikationer nätet.

Den huvuddelen av rapporten innehåller en detaljerad beskrivning av strukturen för den generiska modellen, som presenterar fungerande förfarandet av dess komponenter samt modellantaganden och förenklingar. Olika simuleringar i varierande solstrålning, frekvens och spänning villkor utförs i syfte att ingå i sin förmåga. Den statiska spänningen understödet undersökas under moln effekt situation där förändringar i aktiv uteffekt PV uppställningen kan påverka spänningsstabilitet i rutnätet på den PCC. Den aktiva effekten kontroll undersöks genom att tvinga nätfrekvens att avvika utöver angivna gränsvärden och observera det aktiva resultatet uteffekt. Äntligen är den dynamiska spänning stöd kapacitet (LVRT) undersöks med hjälp av simulerad fyra olika kortslutning händelser skapa fyra olika spänningsfall. Förmågan hos PV strömväxlaren att hålla kontakten och ge reaktiva strömmen vid behov ses. Det externa komponent i nätet utformats för att representera en stark rutnät.

Resultaten visas att modellen har kapacitet för aktiv effekt minskning och LVRT beteende. Men gör det saknas styrning av reaktiv effekt inte tillämpas under statisk spänning stöd. Därför är en PI-regulator som genomförs för att leverera konstant reaktiv effekt i konstant drift och support för stabila nät.

Äntligen två olika sammankopplingar byggdes med en något modifierad version av samma generiska modell med en nominell effekt 1 MVA. Kontrollschemat förblevs densamma. Båda konfigurationerna undersöktes statiskt och dynamiskt och resultaten jämfördes. Små skillnader fanns i form av reaktiv effekt förbrukning / insprutning i PCC.

(7)

Acknowledgements

At first I wish to thank all the people, who, in whichever way, assisted me to complete this interesting project for my master thesis and made this period an important benchmark for my future professional expectations. From Energynautics GmbH1_{, Dr Thomas Ackermann,}

who gave me the opportunity to complete the thesis in his company, Dr Eckehard Tröster, for his patience with all my questions, his valuable advices and insight that gave direction to my work, Rena Kuwahata, who was the initial contact with the company and the person that facilitated my work and life in the new environment, Dr Nis Martensen and Stanislav Cherevatskiy, who shared their experiences in the field whenever those were asked for and in general I wish to thank all the rest of the personnel, who were part of my everyday life the last five months, ensuring a friendly and highly professional environment.

Furthermore, I would like to thank Prof. Lennart Söder for the fruitful pre-presentation and his valid comments and of course Giannis Tolikas for undertaking the translation of the abstract to Swedish. Since it is likely that I forget some people that offered their helped for the completion of this project, I feel obliged to thank them as well.

Special thanks should be paid to Panagiotis Giagkalos and Kyriakos Liotsios, who were my classmates, colleagues, but most of all my friends during the last two years of this master. It is important to realize that anytime you can find people that you can count on. May this friendship lasts and don’t leave time and distance to wear it, rather strengthen it through personal or professional common experiences.

To Angela Maria Castaño Garcia, for this beautiful journey that still goes on. Her support during this time was more that I could ask for. As far as the thesis concerned, her contribution and effort to the final format of the report was significant.

At last, to my beloved family, my parents Konstantinos and Efterpi, and my brother Charalampos, who deserve my eternal gratitude for all that have offered me. Their constant support in every aspect is scarcely reflected on these few sentences here. However, any success in my life so far is mostly charged to them and consequently any success in the future will have their signature as well.

Such moments, I feel the need to give space and mention all the people that left something valuable to me. Old friends from Greece that never forget, new friends from different parts of the world, people I met for short period, all of them are the people that with one way or another made this time worth living it again. You are my personal ark. Thank you all and wish you the best.

(8)

List of Figures

FIGURE1.1:INCREASEOFRENEWABLEENERGYSOURCESINGERMANY1990-2009 ... 2

FIGURE1.2:CUMMULATIVEINSTALLEDGRIDCONNECTEDANDOFFGRIDPVPOWERIN26 COUNTRIESTHATPARTICIPATEINTHEIEAPVPS... 3

FIGURE1.3:WORLDPVCELL/MODULEPRODUCTIONFROM1990TO2009 ... 4

FIGURE1.4:ANNUALPHOTOVOLTAICINSTALLATIONSFROM2000TO2009 ... 4

FIGURE2.1:TYPESOFPVSYSTEMS ... 9

FIGURE2.2:TYPICALI-VCHARACTERISTIC ... 9

FIGURE2.3:THEEFFECTOFSOLARRADIATIONANDTEMPERATUREONTHEI-VCURVE ... 10

FIGURE2.4:THEEFFECTOFTHEINTERCONNECTIONOFPVMODULESONTHEI-VCURVE .... 11

FIGURE2.5:PRINCIPLEOFCONNECTINGPVSYSTEMSTOTHEGRIDWITHASINGLE-PHASE ANDTHREE-PHASEINVERTER ... 13

FIGURE2.6:FBINVERTERTOPOLOGY ... 15

FIGURE2.7:INVERTER’SOPERATINGRANGE ... 17

FIGURE2.8:ACTIVEPOWERCONTROLREQUIREMENTFORGRID-TIEDGENERATORS ... 22

FIGURE2.9:EXAMPLEOFCOSΦ(P)-CHARACTERISTIC ... 23

FIGURE2.10:FAULT-RIDE-THROUGHCAPABILITY ... 24

FIGURE2.11:REACTIVECURRENTINJECTIONREQUIREMENTSINTHEEVENTOFNETWORK FAULTS ... 24

FIGURE3.1:PVARRAYASDCCURRENTSOURCE ... 28

FIGURE3.2:PVMODELWITHBATTERY ... 29

FIGURE3.3:PVINVERTERASPWMCOMPONENT ... 29

FIGURE3.4:PVSYSTEMASSTATICGENERATOR ... 30

FIGURE4.1:THEBASEPVMODEL ... 31

FIGURE4.2:THEEXTERNALGRIDSETTINGS ... 32

FIGURE4.3:SIMPLEEQUIVALENTOFASHORTCIRCUITONTHEGRID... 32

FIGURE4.4:PVGENERATORPOWERFLOWCHARACTERISTICSUNDERNORMAL STEADY-STATEOPERATION ... 34

FIGURE4.5:CAPABILITYCURVEOFTHEINVERTER ... 35

FIGURE4.6:MAXIMUMREACTIVEPOWERLIMITSINTHREEVOLTAGELEVELS ... 36

FIGURE4.7:THECONTROLFRAMEOFTHEPVGENERATOR ... 37

FIGURE4.8:THESTRUCTUREOFIRRADIANCESLOT ... 38

FIGURE4.9:SOLARIRRADIATIONINCREMENT ... 38

FIGURE4.10:EFFECTOFSOLARIRRADIANCEINTHEPVCHARACTERISTICS ... 39

FIGURE4.11:EFFECTOFSOLARIRRADIANCEINTHEPVPOWEROUTPUT ... 39

FIGURE4.12:TEMPERATUREINCREMENTINTHEPVARRAY ... 40

FIGURE4.13:EFFECTOFTHEOPERATIONTEMPERATUREINTHEPVVOLTAGE ... 40

FIGURE4.14:THEPHOTOVOLTAICARRAYMODEL ... 41

FIGURE4.15:THEELECTRICALEQUIVALENTOFANIDEALSOLARCELL ... 42

FIGURE4.16:THEDCBUSBARANDCAPACITORMODEL ... 44

FIGURE4.17:THEACTIVEPOWERREDUCTIONCONTROL ... 45

(11)

FIGURE4.19:THEBASICSTRUCTUREOFAPLL ... 47

FIGURE4.20:SOLARRADIATIONDROP ... 49

FIGURE4.21:SETOFCONSTANTPOWERFACTOR ... 50

FIGURE4.22:ACTIVEANDREACTIVEPOWERCHANGEDURINGACLOUDEFFECT ... 50

FIGURE4.23:VOLTAGEDEVIATIONDURINGACLOUDEFFECT ... 51

FIGURE4.24:ACTIVEANDREACTIVECHANGEINTHELVBUS ... 52

FIGURE4.25:CHANGEINTHE“SPEED”PARAMETERTOCREATEOVERFREQUENCY ... 53

FIGURE4.26:THEOVERFREQUENCYEVENT ... 53

FIGURE4.27:THEACTIVEPOWERREDUCTIONOFTHEGENERATORDUETOOVERFREQUENCY ... 54

FIGURE4.28:THEACTIVEANDREACTIVEPOWERVALUESINTHELVBUSDURINGTHE OVERFREQUENCYEVENT ... 54

FIGURE4.29:THEACTIVEANDREACTIVEPOWERVALUESINTHEMVBUSDURINGTHE OVERFREQUENCYEVENT ... 55

FIGURE4.30:TESTSPERFORMEDFORDYNAMICVOLTAGESUPPORT ... 56

FIGURE4.31:EQUIVALENTPLANOFAGRIDWITHFAULT(A)ANDTHEELECTRICALCIRCUIT REPRESENTATION(B) ... 56

FIGURE4.32:BEHAVIOUROFTHEPVMODELIN100%VOLTAGEDIP ... 58

FIGURE4.36:POSSIBLEDYNAMICMPPCONTROL ... 63

FIGURE5.1:THECONSTANTQCONTROLIMPLEMENTATIONTOTHEMODEL ... 64

FIGURE5.2:THESWITCHINGFUNCTIONWRITTENINDSLINSIDETHECURRENTLIMITER ... 65

FIGURE5.3:THECONSTANTQSETINTHEPVGENERATOR ... 66

FIGURE5.4:THEACTIVEPOWERCHANGEOFTHEPVGENERATOR ... 66

FIGURE5.5:THEQCONTROLRESPONSETOTHEACTIVEPOWERCHANGE ... 67

FIGURE5.6:VOLTAGEVARIATIONINTHELVBUSWITHTHEQCONTROL ... 67

FIGURE5.7:THEFIRSTSETUPOFTHEPVPOWERPLANTOF20MVA ... 69

FIGURE5.8:THESECONDSETUPOFTHEPVPOWERPLANTOF20MVA3_{... 69}

FIGURE5.9:THEFIRSTCONFIGURATIONASBUILTINPOWERFACTORY ... 70

FIGURE5.10:P-QCURVE-FIRSTCASE ... 73

FIGURE5.11:THESECONDCONFIGURATIONASBUILTINPOWERFACTORY ... 76

FIGURE5.12:P-QCURVE-SECONDCASE ... 77

FIGURE5.13:P-QCURVES-BOTHCASES ... 79

FIGURE8.1:THEDSLCODEOFEACHPVMODULE ... 89

FIGURE8.2:MAINPARTOFDSLCODEINTHEACTIVEPOWERREDUCTIONBLOCK ... 90

FIGURE8.3:THEDSLCODEINTHEPICONTROLLERBLOCK ... 90

FIGURE8.4:THEDSLCODEINTHEREACTIVEPOWERSUPPORTBLOCK ... 90

FIGURE8.5:THEDSLCODEINTHECURRENTLIMITERBLOCK ... 90

FIGURE8.6:BEHAVIOUROFTHEFIRSTINTERCONNECTIONIN100%VOLTAGEDIP ... 91

FIGURE8.10:BEHAVIOUROFTHESECONDINTERCONNECTIONIN100%VOLTAGEDIP ... 94

(12)

(13)

List of Tables

TABLE 2.1: NEW REQUIREMENTS FOR GRID TIED GENERATORS ... 21

TABLE 4.1: VOLTAGE DIP TESTS FOR GENERATING UNITS TYPE-2 ... 55

TABLE 4.2: TESTS PERFORMED WITH THE PV MODEL ... 56

TABLE 4.3: FAULT CONDITIONS IN EACH TEST ... 57

TABLE 4.4: AGGREGATION OF THE RESULTS OF ALL TESTS ... 61

TABLE 5.1: PARAMETERS FOR THE CONSTANT Q CONTROL THAT ADDED IN THE ... 65

TABLE 5.2: LINES USED IN THE FIRST CONFIGURATION ... 71

TABLE 5.3: RESULTS OF THE LOAD FLOW STUDY FIRST CASE ... 72

TABLE 5.4: FAULST CONDITIONS IN EACH TEST FIRST CASE... 74

TABLE 5.5:AGGREGATION OF THE RESULTS FOR DYNAMIC VOLTAGE SUPPORT FIRST CASE 74 TABLE 5.6: LINES USED IN THE SECOND CONFIGURATION ... 75

TABLE 5.7: RESULTS OF THE LOAD FLOW STUDY SECOND CASE ... 77

TABLE 5.8: AGGREGATION OF THE RESULTS FOR DYNAMIC VOLTAGE SUPPORT SECOND CASE ... 78

TABLE 5.9: LOAD FLOW RESULTS OF BOTH CASES ... 79

TABLE 5.10: REACTIVE POWER SUPPLY OF BOTH CASES AT PCC IN SEVERAL VOLTAGE DIPS ... 80

TABLE 8.1: PARAMETERS IN PV ARRAY SLOT ... 87

TABLE 8.2: PARAMETERS IN DC BUSBAR AND CAPACITOR SLOT ... 87

TABLE 8.3: PARAMETERS IN ACTIVE POWER REDUCTION SLOT ... 87

(14)

Nomenclature

AC Alternate Current AI Anti-Islanding AM Air Mass

ASTM American Society for Testing and Materials

BDEW Bundesverband der Energie-und Wasserwirtschaft (Federal Association of Energy and Water)

CSI Current Source Inverter DC Direct Current

DIgSILENT DIgital SImuLator for Electrical NeTwork

DIN Deutsches Institut für Normung (German Standardisation System) DSL Dynamic Simulation Language (DIgSILENT Simulation Language) eEURO European efficiency

EMI Electromagnetic Interference EN European Normalization EU European Union

FB Full Bridge FiT Feed in Tariff FRT Fault ride through GCs Grid Codes

HV High Voltage

IEA International Energy Agency

IEC International Electrotechnical Commission IEEE Institute of Electrical and Electronic Engineers IGBT Insulated Gate Bipolar Transistor

IK or ISC Short circuit current

Impp Current at maximum power point

MOSFET Metal Oxide Semiconductor Field Effect Transistor LV Low Voltage

LVRT Low Voltage Ride Through MPP Maximum Power Point MV Medium Voltage NPC Neutral Point Clamped PCC Point of Common Coupling PF Power Factor

PLL Phased Locked Loop p.u. per unit

PV Photovoltaic

(15)

STC Standard Test Conditions THD Total Harmonic Distortion UL Underwriters Laboratories

VDE Verband der Elektrotechnik Elektronik Informationstechnik (Association of Electrical Engineers)

VDN Verband der Netzbetreiber (Association of network operators) Vmpp Voltage at maximum power point

VOC Open circuit voltage

(16)

1 Introduction

In the coming decades and taking into account the continuous population growth, the energy demand will probably double [1], if not more, bringing the societies to the brink of energy shortage. Even if the improvement of energy efficient technologies is significant, the future demand will not be able to be balanced if new sources would not be introduced and innovative technologies (either passive or active) would not be exploited. Due to the use of fossil fuels, major side-effects both in the environment and social life, have already caused an outburst, sounding the alarm for cleaner and carbon-free energy sources. The renewable energy sources, as carbon-free sources, appear to be a feasible alternative to conventional fuels. This shift is not a current phenomenon and mankind has already exploited the sun, the wind, the water and the earth to produce clean energy in order to address its needs and provide better quality of services.

Since 1997 and the introduction of the White Paper by the European Commission, the formation of a renewable energy policy has begun. The overall objective was to reduce the dependence on fossil fuel imports and increase the security of supply moving towards a low carbon economy [2]. Over these years the orientation of the EU has changed from indicative targets, referring to electricity and transport fuel, to specific targets that are legally established by a legislation pattern. What is more, change has occurred towards redefinition of the infrastructure policy that plays a key role to the growth of Renewable Energy Technologies (RET) [3]. Nowadays, it is common belief from all the stakeholders involved (government, producer and the end user) that the benefits of a society, where renewable sources account increasingly to the consumption needs, are multiple. Strengthening the national and local economy, jobs creation, better life quality and of course less harmful contribution to the environment are some of the strongest arguments in favour of RET. In 2001, the EU “Renewable Electricity Directive” together with the “Biofuels Directive” that was signed two years after, set quite ambitious goals for the member states by 2010. Unfortunately, the 21% of renewable electricity production was met only by very few countries (i.e. Denmark, Germany, Poland et al.), however it gave boost to many economies and the sector of renewable energies experienced significant growth with the ‘electricity production’ enjoying the biggest share [3]. In the year 2009 almost 61% of the new electricity generating capacity that was connected to the grid in the EU was from RET [4], while in 2010 the total electricity share in the EU reached 18.5%. This number, even if it is promising, is still far from the 37% that the Member States have set for 2020. Nevertheless, today the conditions are more favorable for higher and faster growth rates. There is a much more organized research field dedicated to the renewable technologies, better and more flexible legislation framework with number of incentives and support mechanisms and also a more open-minded industry.

(17)

issues of instability to the grid, which is not designed to receive such integration. The problem requires an immediate attention in order to meet the target of 2020 or even exceed it. There are researches [4] that present scenarios for 2030 and 2050, when renewable electricity supply could be 68% and 100% respectively. It is understood then that more modern electricity grid system should be adopted, which means expansion of the grid but more important implies urgent modification of the already existing one.

Improved technical specifications, the so-called Grid Codes (GCs), which will ensure the proper and safe function of the electrical grid should be introduced and facilitate the interconnection of electricity systems and the reinforcement of the grid. Problems like bottleneck in the grid should be overcome so as grid operators to exchange kWh when excess of electricity is produced from one and is needed by another one. Deregulation-based energy market for the support of the distributed generation can be the compass of the reformation of energy scenery.

1.1 The driving force

Among the RET, the lights have turned to wind turbines and solar PV technologies. Taking into consideration the fact that countries such as Germany and Spain enjoy a leading position in the renewable energy sector, tangible results can be withdrawn about the situation in Europe in general if those countries used as study cases. In figure 1.1 the aggregated renewable energy capacity in Germany is shown, proving that mainly the focus is on the wind technology and PV. As far as the PV installations concerned, there is a dramatic change since 2004, when the feed-in tariff policy mechanism and relevant subsidies have been in effect. Only in 2008 around 1.5 GWp were installed in Germany [5],

while in September of 2010 the total number of installed capacity was 15 GWp, which is the

30% of the total RET installed and the 37.5% of the minimum load of electricity in 2009 as is seen in figure 1.1.

(18)

In 2009, an improved version of the feed-in law was introduced specifying that for new PV power plants the feed-in tariffs will be reduced from 8% to 10% per year. The main reason for this change was to force the reduction of the investment cost in PV systems and lead to grid parity [5].

From the above, it is understood that the conditions are very favourable for the expansion of RET and especially of PV systems, which is the concern of this project. The Thesis focuses on grid connected PV systems and their advantage as a power generation unit. The tendency in industrialized countries is to connect the PV systems to the grid since there is almost everywhere an electrical network available. Figure 1.2 illustrates this tendency. The multifunctional role of the PV system and specifically of the grid tied inverter is highlighted in the Thesis. PV inverter is the main component of the system and is responsible for the power injection to the grid. Until now, its conventional role was to convert the DC power to AC power and feed-in the maximum possible active power to the grid. Moreover, in case of a grid failure it was designed to disconnect until the conditions stabilize again to reconnect. However, the high penetration of photovoltaics to the distribution network has raised new requirements for the modern PV inverters. Their role has become much more significant not only for the PV system but now also for the grid that is connected to.

Figure 1.2: Cummulative installed grid connected and off grid PV power in 26 countries that participate in the IEA PVPS [6]

(19)

Figure 1.3: World PV Cell/Module Production from 1990 to 2009 [7]

Figure 1.4: Annual Photovoltaic Installations from 2000 to 2009 [7]

The new setting that is being shaped because of the reasons mentioned so far makes the PV field even more interesting and every study around on-grid systems more challenging.

1.2 Overview of the Thesis report

The project in question is part of the initiative of Energynautics GmbH2_{to fulfil a study}

concerning modelling and simulating large scale PV systems in relation to their impact on

(20)

the power system. The project’s objectives are limited due to tight time constraints and are presented below in the same chapter. Nevertheless, the thesis tries to cover important aspects in theory and present realistic results through iterative simulations, aggregating some knowledge as far as the characteristics of larger penetration concerned.

In the following chapter the theoretical approach is undertaken and basic background is presented. A brief overview of PV systems and some important characteristics are included and give place to a deeper analysis of the PV inverter and its modern role. A major part of this chapter is being covered by the reference of current GCs, principally those of Germany. In the third chapter the methodology of the study is explained as well as the tool that was used to model the PV and perform simulations.

Chapter four is dedicated to the model with capacity 0.5 MVA that is used in this thesis and built by the company DIgSILENT. The control system is explained thoroughly and the choice of the configuration, inputs, outputs and parameters is justified. Its ability to be used as a generic model for PV systems that comply with the German GCs is investigated through simulations.

Next chapter is dedicated to present an interconnection of the generic model in order to achieve a higher power output. The basic model was first modified by changing appropriate parameters in terms of rated peak power and consequently active power output. The new PV model of 1 MVA was used to create two different configurations of 20 MVA each. Load flow calculations and dynamic behaviour in case of a fault are presented with pasting relevant graphs. All the important results are, almost catholically, exported graphs from the simulation program. In the same chapter and after each result, a short discussion on the findings is taking place.

In the conclusion chapter a general aggregation of the results is deployed. It is commented whether or not the objectives were met and further suggestions for future work are offered.

1.3 Objectives

The main aim of this thesis, presented also above, is to group together some basic knowledge, concerning the requirements that a grid connected photovoltaic system should fulfill in order to comply with certain codes, particularly with the German GCs, and how the integration of such decentralized power systems affect the behavior of the distribution network. More specifically, the objectives set for this project are:

(21)

and mainly the provisions of the new German Transmission Code for Medium Voltage networks.

• Give a description of the tool used to model the full PV system, meaning the network, the PV generator, the inverter and the control system. The tool used for the simulations is the PowerFactory of DIgSILENT.

• Train for the use of PowerFactory and understand number of features that are useful for modeling and simulating PV systems.

• Understand and analyze a generic PV model and its compliance with the German GCs. Furthermore, examine, through simulations the shortcomings of the model.

• Customize the PV model in order to provide higher power output.

• Create two different interconnected PV configurations with the use of the modified PV model and study their behavior in case of faults.

• Recommend related future work.

1.4 Limitations

In the thesis, due to the broadness of the subject, several limitations were delineated with a sole purpose of presenting some facts in a conceivable and precise way and not correlating general cognition without any clear purpose. Some important limitations, following the objectives above, are:

• No stand alone systems are examined. The models are grid tied PV systems, meaning that no energy storage is taking into account.

• The attention is turned more to the electrical grid, meaning that the behaviour of the grid and the role of the inverter in this part are the basic considerations. No study is carried out as far as the suitable choice of an inverter according to the PV systems arrangement (number of modules per strings, number of strings per inverter or shading phenomena).

• The PV inverter is single-stage, meaning that there is no DC-DC boost converter involved and no studies are being undertaken concerning double-stage inverters.

(22)

• No power quality studies (e.g. harmonics) are performed.

(23)

2 Background

2.1 PV systems – Overview

Before studying the particularities of grid connected systems it is thoughtful to introduce very briefly the current status and some basic terminology of the photovoltaic technology and systems, singling some known key concepts that are the basis of this sector. In figure 2.1 the different kind of PV systems are presented. As it is seen the two basic categories are: The stand-alone systems, which are usually implemented in rural and remote areas in developing countries where no access to the grid is possible. However, the low cost production and innovative ideas have led to numerous of applications in industrialized countries as well (e.g. roof top systems, PV-glazing, solar traffic lighting, solar parking ticket machines, solar chargers, telecom et al.). Stand-alone systems are usually supported by storage systems (e.g. batteries) in order to meet the load in times when the solar irradiation is not enough for the PV to cover the whole need.

(24)

Figure 2.1: Types of PV systems [8]

2.1.1 I-V Characteristics

The identity of a PV unit, either cell, module or array is the current and voltage curve or as usually found on texts I-V curve or PV characteristic curve. A typical shape of the characteristic under STC is seen in figure 2.2, showing the basic points. Those are the short-circuit current (IK), the open-circuit voltage (Voc) and the maximum power point (MPP) and

are defined below.

(25)

The maximum power point (MPP) is the point where the PV cell, module or array supplies the maximum possible power. At this point the voltage and current are defined as maximum power voltage (Vmpp) and maximum power current (Impp)

respectively. The MPP is given in peak watts (Wp) and is strongly affected by the

irradiance level as well as the operating temperature of the PV.

The short-circuit current (IK) is the maximum current that can flow from a PV when

the voltage across the terminals is zero, meaning that are either connected to each other or an abnormal low-resistance connection has occurred. The short-circuit current is strongly affected but the incoming irradiation as it is seen in figure 2.3 and is approximately 5 to 15 per cent higher than the Immp [8]. Typical values of

short-circuit current of various PV modules and under STC can be found in the specifications of the product [10].

The open-circuit voltage (VOC) is the voltage between the two terminals of the PV

when no external load in connected to it. The VOC is influenced by the operating

temperature of the PV array which is of course linked to the ambient temperature. This can also be seen in figure 2.3. Typical values of open-circuit voltage can also be seen in [10].

The STC is a standard test of uniform conditions related to IEC 60904/DIN EN 60904 standards, which categorize the PV modules according to their I-V characteristics [8]. In brief, those are: vertical irradiance E of 1000 W/m2, cell temperature T of 25°C with a tolerance of ± 2°C and a defined air mass AM =1.5. AM defines the shape of solar light spectrum, in an approximate way, at a specific position of sun in comparison to that at zenith at sea level, which considers to be 1. AM increases as the zenith - sun angle increases since the light passes through “more atmosphere” and the attenuation (scattering and absorption is greater.

(26)

A PV system, which is an interconnection of PV modules in series and in parallel, has its own I-V curve depending on how many PV modules are connected in series (strings) and how many are connected in parallel. Below it can be seen how the characteristic is formed by adding PV modules to the system. Furthermore, figure 2.4 points out that only PV modules with the same electrical characteristics are used in the interconnection in order to avoid power losses in the final system.

Figure 2.4: The effect of the interconnection of PV modules on the I-V curve

2.2 Grid-connected PV systems

(27)

The PV modules that interconnect together forming the desirable system. The PV array is basically the generator of the system and specifically the static generator as it will be presented later on, since there is no rotating part.

The mounting system, which for PV power plants is a stone or concrete pad foundation with metal or timber frames attached on it. The mounting system should above all ensure the designed angle of the PV system with the sun’s incident irradiation. When the system is implemented in an open field, sometimes the mounting should be within some requirements for environmental reasons.

The DC cabling.

The PV combiner/junction box, which is the place where all the strings are connected together and end up to the main DC cable. This box contains also important safety components as string diodes, fuses, isolations and the DC main switch to protect the system and the maintainer from accidents in case of faults. These protections together with the equivalent ones from the AC side are also found in literature as the balance of the system.

The PV inverter, which transforms the DC current to AC and supplies it to the grid (mostly distribution), fully synchronised in frequency and voltage with it. The significance of this component is high and the full modern role is described in the following section of the chapter.

The AC cabling and necessary protection.

The meter cupboard, which is the system’s data monitor involving supply and feed meter, displaying the flow energy between the PV and the grid or/and the load. In large PV power plants, there can be additional components that improve the efficiency of the system or ensure better control and monitoring. For instance, cooling pipes in the back of the PV array to reduce the operating temperature and increase the MPP or remote monitors that allow real-time performance, output values and potential faults to be displayed in a the owner’s computer are some examples of such components.

2.3 PV inverter

(28)

2.3.1 What is available – Current status

PV inverters can be categorised in various ways according to the topology, the operation principle, the type of the connection to the grid and by application. Based on the connection to the grid inverters can be:

Single-phase inverters refer to inverter structures applied in small scale roof-top systems (of until 5 kWp).

Three-phase inverters refer to larger systems, which is mostly the case for on-grid PV systems and are connected of course to a three-phase supply system. The basic three-phase inverter consists of three single line inverters, which are connected to each load terminal. So, it is not actually a true three-phase inverter and this is because a three-wire topology will require relatively high DC voltage values (around 600 V for a 400 V three-phase grid) and is limited to 1000 V due to safety reasons in installation procedures. Also the monitoring and control for islanding requirement becomes more difficult in relation to three single phase connections [14].

The inverter as an electronic oscillator is required to generate a pure sine wave synchronized to the grid as stated before.

Figure 2.5: Principle of connecting PV systems to the grid with a single-phase and three-phase inverter [8]

According to the size and the application, inverters can be central, string, multistring and module kind [11], [14].

Central inverters are connected with more than one or all the parallel strings of PV modules and can be of some kW until one MW of power range.

String inverters are connected to each string of the PV array seperately and their range of power is a few KW (0.4 - 2 kW).

(29)

together. For this reason, necessary DC-DC converters are used to provide the same output signal to the input of the multi-string inverter. Multi-string inverters increase the efficiency of the system since every string can track its own MPP. Their range varies from 1.5 kW to 6 kW.

Module-type inverters are connected to each module seperately transforming it in a PV AC module. Their use is still limited and their range is from 50 to 400 W.

Taking into consideration that the PV modules produce DC power at a low voltage, the system’s output requires some adjustment to be fed as AC power at the votage of the grid as cited before. The inverters used for this adjustment and apply diferent operation-principle are [8], [13]:

Line-commutated. Such invertrs use switching devices (thyristor bridge or IGBT) that control the switch-on time only. The switch-off time is done by reducing the circuit current to zero by using the voltage of the grid. The name line-commutated represents exactly this grid controlled dependance, meaning the inverter uses the voltage of the grid to decide the turn on and turn off time of these thyristors. One disadvantage is that they produce a square wave current output, which introduces undesirable harmonic components, which can be reduced by the use of filters. This principle is used today less especially in single phase inverters.

Self-commutated. Such inverters are more complicated and use switching devices (IGBT and MOSFET) that can control the switch-on and switch-off time and adjust the output signal to the one of the grid. The self-commutated inverters are the predominant technology in PV power sources because of their ability to control the voltage and current output signal (AC side), regulate the power factor and reduce the harmonic current distortion. Especially, since the role of PV inverter has become more vital, this operation principle is offering the capabilty to cover the multiple services and increase the resistance to the grid disturbances. Depending on the type of pulse they control, either voltage or current, self-commutated are divided to voltage source and current source inverters.

• Voltage source inverters (VSI). VSI realize the DC side as a constant voltage source and the output current is changing with the load. For this reason is normally connected to the grid with an inductance so as not to supply with current infinitely when there is not voltage or phase match between inverter and grid.

(30)

Another basic criterion for categorizing PV inverters is whether or not use galvanic isolation (transformer) to connect to the grid. There are many advantages and disadvantages in each type to be considered, with Electromagnetic Interference (EMI) being one of the most important issue. Inverters with low-frequency transformers (50 Hz) or high frequency transformers (10 kHz to 50 kHz) have the DC circuit seperated from the AC circuit, offering recuction of EMI. However, the big size especially when using low frequency transformers, the lower efficiency of the inverter due to transformer losses and the extra cost turn the attention to transformless topologies and their improvement to work in higher power ranges than today [8]. Transformless topologies still need more innovative and complicated solutions to become competitive especially in terms of electrical safety. Furthermore, in cases when the the DC output of the PV system is not as the one of the grid or higher, a step-up DC-DC converter is needed. Thus, part of the losses that were avoided from not using a transformer are compensated by the use of the converter. Nevertheless, almost all the typical applied inverter structures today need a boosting and require a DC-DC converter [14].

In general, there are numerous different topologies of inverters that could apply in grid connected systems. Today, big manufacturing companies promote their own designs and variations, which are derivatives of two main converter families [14]:

H-bridge or FB topology. Figure 2.6 shows the original structure of this topology, which is used in the most typical complete PV structures nowadays. Based on that many designs have been patented offering a wider range of choices (e.g. H5 Inverter of SMA, HERIC Inverter of Sunways, REFU Inverter etc).

NPC topology. It is a more modern topology and the one that was connected to the grid without transformer. In general and in comparison with the FB topology, NPC can produce lower switch losses and harmonics, improving the efficiency of the inverter [15]. However, is rather unbalanced and require double voltage input in comparison with the FB topology [14].

(31)

2.3.2 Issues when choosing inverter

It is obvious that selecting the right inverter technology for the PV system is an issue with many parameters, technical and economical. If there is no clear purpose from the operator of the power plant then the choice is always the one that combines the best possible efficiency output with the best possible cost (investment, operation and maintenance). However, there are PV power systems that aim to the highest energy output for covering partly or fully a load need and others that aim to support and optimize a weak grid. Therefore, the system’s basic configurations (PV array, control systems, inverter etc) should be approached and analyzed differently. Even so, no matter the objective of the system, basic considerations should be addressed when selecting a PV inverter and these considerations should be examined under the technical requirements and specifications (grid codes) of each country. Below these issues are presented [16].

Efficiency. This is a basic issue in every system, but mostly in PV systems where the highest energy yield is the priority. The current efficiency of the inverters is very high in every topologies, reaching the 92% and 94% in inverters with transformers and even higher without galvanic isolation. As rule of thumb an improvement of 1% can result in 10% more power output over a year [16]. Standby power losses during periods of negligible load need to be assessed, because they affect the overall efficiency. Since inverters operate at different efficiencies depending on the load, every inverter is expressed with different efficiency curve. A reliable method to evaluate the overall efficiency of the inverter is the European Efficiency standard or else eEURO. This standard takes into consideration the amount of time (in percentage)

that the inverter is expected to work at partial load/level of irradiation. Even if this standard is valid for irradiance levels of Central Europe, it is a sufficient way to compare different inverters [11]. The euro-efficiency is defined by (2.1)

η

EURO =0.03

η

_5%+0.06

η

_10%+0.13

η

_20%+0.1

η

_30%+0.48

η

_50%+0.2

η

_100% (2.1)

Explaining a factor of the above component e.g. 0.03, it means that the inverter is operating at 5% for a duration of 0.03 and the total operating time.

Safety. Refers mainly to Anti-Islanding (AI) protection. Unintentionally islanding takes

place when PV inverters don’t disconnect from the utility after it has been shut down. This is due to the fact that there are load circuits that happen to resonate at the frequency of the grid. So the inverter continues to put voltage on the grid making it unsafe especially for the utility workers. Isolation transformers and other AI set-ups are defined by standards (e.g. VDE 0126, IEEE 1574) [14]. Similar protection is required against over-currents, surges, under- and over-frequency and under- and over-voltages for DC input and AC output.

(32)

total harmonic distortion is given by (2.2) [17]. In general the harmonic content must be low to protect both loads and utility equipment. The waveform and power factor must be acceptable to the utility.

40 2 2 1 h h X THD X = =

∑

(2.2) 1 h X harmonics X fundamental = =

Electromagnetic Interference. It should be as low as possible in order to comply with the limits of relevant local requirements.

Compatibility with the array. Both array and inverter need to be compatible and the inverter should be able to withstand the maximum array current and voltage. The VOC of the array should also be well within the inverter’s tolerable voltage range. The

MPP range of the inverter should also match the operating voltage of the array. These compatibility issues can be seen in figure 2.7. As far as the MPP tracker requirements concerned, those should be of high efficiency during steady state, fast tracking in sudden changes of solar radiation and stable operation at very low irradiation levels [14].

Figure 2.7: Inverter’s operating range [8]

(33)

Other. Issues like size, weight, construction and materials, protection against local weather conditions, terminals, and instrumentation should be addressed in conjunction with local rules.

2.3.3 Additional requirements – Ancillary functions

The additional requirements have been “enforced” by GCs due to the increasing penetration of photovoltaics into the grid and basically impose additional functions and technical improvements concerning the grid support. Summarily these new roles are:

• Voltage control

• Active power control

• Reactive power compensation

• Harmonic compensation

• Fault ride-through

Those new services of the PV inverters will be presented more deeply later on in the “new German Grid Code” section.

2.4 Low voltage ride through (LVRT) requirement

In general, the definition of LVRT or FRT includes the requirements that a power generating unit tied to the grid should meet, in case of a voltage dip due to a fault or sudden load change in the grid. The impact of the voltage dip can be described according to the voltage level reduction and its duration.

In this case, the power generation unit is a PV system, connected with the grid through the inverter, which is in fact the device that should be capable for LVRT. The possible scenarios during a voltage dip (or power dip) are:

Immediate disconnection from the grid, when the fault occurs and throughout its duration. The inverter shall reconnect again after the fault is cleared.

Stay connected to the grid during the fault.

Stay connected but support the grid only with reactive power (reactive current) during the fault. After the clearance of the fault the unit should consume the same reactive power as before the fault.

(34)

even if they were designed to provide reactive current, during a fault they were disconnecting from the grid under technical standards as IEEE 1547 and VDE 0126-1-1 [17]. However, due the extensive PV penetration into the grid, as described in the introduction chapter, the requirements have been modified. The PV inverters should stay connected to the grid and support it with reactive power, when needed, contributing to the power quality and prevent voltage instabilities. The disconnection-reconnection scenario is aggravating for the components of the system reducing possibly their lifetime or causing even greater instabilities to the grid especially in large scale integration. Moreover, after disconnection the PV unit will be connected again when the grid is stabilized, meaning that the time that is off the grid the loss of active power could be of great significance especially on grids where the share of PV power is high.

2.4.1 Reactive power and its importance

The high importance of the reactive power was perceived after major blackouts (e.g. Ohio in 2003) that occurred due to voltage drops (and subsequent current rise) in electricity lines, when one line was cut off and the remaining ones could not bear the load. Reactive power, in general, can be seen as a tool to provide smoothly real power and has a strong effect on the voltage of the system. It is a circulating power in the grid that doesn’t do any useful work.

In PV systems the importance became obvious after the growth of PV systems connected to the LV and MV as well. The existing grid was not designed for such high penetration of interconnected PV and a violation of voltage limits in times of high solar irradiation was possible. For this reason PV inverters should be able to provide reactive power in order to reduce the voltage rise along the feeder [18]. As mentioned above, in case of voltage collapse the inverter should be able to provide reactive current and stabilize the grid within some time frames defined by grid codes. Nevertheless, reactive power compensation for the PV inverters hasn’t been part of many local GCs. German utilities, though, have defined and analysed the supply methods of reactive power, which are stated below the “the new German Grid Code” section.

The specificity of the PV systems to be close to the location where the reactive power could be needed is an advantage considering the fact that reactive power does not travel far in comparison to the active power [19].

2.5 Grid requirements for PV systems

(35)

according to some uniform guidance [20]. Photovoltaic power systems affect mostly the low and medium-voltage network and only approximately 1% of the high voltage network is fed by PV power [5], meaning that the demand for grid stability reflects the low and the medium voltage networks.

The growth of the renewable generation and the expansion of distributed generators, have aroused awareness to many countries. However the introduction of effective grid codes is a rather difficult task with problems that cause significant delays in the process [14]. Some of those problems are the different features among the different generators, the void legislation pattern and the lack of production management in the field. The PV industry is even more sensitive to such problems, because of the wide range of different PV inverter technologies and designs and their multitask role in comparison to the conventional one they had until now [14].

Due to the different grid characteristics there are many different GCs that have been introduced around the world. Countries as China, Australia and India have different requirements among them. Even inside Europe there are many differences. Concerning the PV field the requirements usually follow the requirements of wind power systems or the general provisions that apply to the generators that produce electricity close to the end users of power (distributed generation).

2.5.1 The new German Grid Code

(36)

Table 2.1: New requirements for grid tied generators [22]

Static Voltage Support

Under continuous operation and when the system operator requires it, the unit must be capable to participate in the static voltage support in order to keep the voltage within acceptable limits when slight deviations occur. The participation is has to do with reactive power injection capabilities which are described below. These voltage limits are different in every level of voltage but are usually between +12% and -13% of the nominal voltage [23]. The IEEE-1547 standard requires for the utility interactive inverters +10% and -12% at the PCC [24].

Active Power control

Active power control or active power throttling [21] or active power derating [22] or active power curtailment [14], [23] as it is found in the literature refers to the ability of the generating plant to reduce/adjust its power output as required by the network operator or even disconnect the PV plant in order to avoid potential dangers regarding the stability of the system and human personnel. Some cases of controlling the active power could be: unsafe system operation, unintentional islanding, frequency deviation or maintenance after a grid failure.

In Germany the active power is required to be changed with a ramp rate of 10% per minute, or smaller, of the rated active power capacity until any level is necessary. However, it cannot be lower than 0.1 p.u [14]. The power plant should not disconnect from the grid for any setpoint over 10%. The control is implemented normally in two ways [20][21]:

(37)

Manually, with the use of an adequate signal by the operator, which represents a setpoint (e.g. 100%, 60%, 30% and 0%). There is no physical interference in the control unit, only the use of the control signal.

As far as the first way concerned, the control unit should follow the below figure 2.8. According to this figure the PV system should reduce the power output when the frequency exceeds the value 50.2 Hz. The slope or gradient of reduction should be 40% of the instantaneous last value of power just before the 50.2 Hz. Besides the upper frequency limitation, the value 50.05 is the lower limit below which the PV system can increase again the active power feed-in. The grey areas in the figure set where the plant should disconnect from the grid, what is to say below 47.5 Hz and above 51.5 Hz.

Figure 2.8: Active power control requirement for grid-tied generators [20][21]

Reactive Power control

The reactive power was discussed in a different part of this thesis, in an attempt to emphasize its importance to the safe operation of the grid. The German GCs state that the generating units should be able to provide reactive power support in every operating point by adjusting the power factor at the PCC, at least in a value of 0.95 both leading and lagging for all the power levels. The investigated reactive power supply methods are [18][20][25]:

A fixed power factor (cosφ).

A cosφ(P) function, where the provided power factor depends on the instantaneous active power output of the inverter. In figure 2.9 an example is seen, as well as the limitations (0.95underexcited to 0.95overexcited)

(38)

A Q(U) droop function, where the provided reactive power depends on the voltage at the PCC.

Figure 2.9: Example of cosφ(P)-characteristic [20]

Nowadays, PV systems are mainly designed to provide active power, since reactive power contributes to losses in the lines, transformers and inverter. For this reason and in order to comply with the above requirement the inverter should be oversized. What is to say, taking into account the above power factor of 0.95 an inverter able to supply 475 KW should be of 500 kVA apparent power.

Dynamic Voltage Support

When referring to dynamic voltage support, it is simply implied the requirements what a PV system should fulfil under fault conditions and grid disturbances and also which should be its behaviour after the restoration of the fault. These requirements are [14], [20]-[22]:

(39)

Figure 2.10: Fault-Ride-through capability [20]

Reactive current injection requirement. During the fault and as described above the PV should support the grid (voltage support) by feeding-in reactive power or absorbing. In the German GCs the required reactive current is defined as presented in figure 2.11.

Figure 2.11: Reactive current injection requirements in the event of network faults [20]

(40)

(asymmetrical fault), the reactive current should not cause voltage increment above 10% of the nominal voltage in the non faulty phases.

2.5.2 The Situation in the Rest of Europe

There are other European countries that have developed in detail their own GCs. Scotland, Ireland and Denmark are some examples of countries that have released specific codes that determine FRT, power factor and dynamic requirements that can be found in detail in [23]. However, these directives are mostly reflect wind power systems or other renewables connected to distribution system and there is no specific reference for PV systems to comply with. The case that could be excluded is Spain where, since 1st January 2011, grid requirements are in effect covering also the photovoltaics [28]. In France the case is similar, while in Greece GCs codes are under investigation [20].

2.5.3 Further International and European requirements for PV

Beside the local GCs, there are number of worldwide standards that are being developed by international organisations in order to promote uniform-based requirements that could boost up the PV market even more and facilitate the interconnection of distributed systems among neighbour countries. Some important of those standards are presented briefly below [14]:

IEEE 1547 – Interconnection of Distributed Generation. This standard is the result of effort to establish an interconnection standard that applies to all technologies. It comes as continuity from the IEEE 929-2000 and the UL 1741 that covered recommended practices for utility interface of small-scale PV systems and listed important safety and grid performance requirements that influenced a lot the PV-inverter technologies. IEEE 1547 gives base on technical specifications and testing standards, setting mandatory provisions for power quality, dc current injection and AI requirements for interconnected generators of up to 10 MW.

IEC 61727 – Characteristics of Utility Interface. This standard is more specific for PV systems and refers to on-grid systems operating in parallel with the utility and utilize static non-islanding inverters and also to PV systems interconnected to the distribution system. A more specific standard, IEC 62116, has been implemented also, defining the testing procedures of AI measures that cited in the IEC 61727. EN 50160 – Public Distribution Voltage Quality. It defines the main voltage

parameters and the acceptable deviation ranges at the PCC in the MV and LV network and under normal operation. Those parameters affect highly the control and design of the PV inverters in order to withstand voltage disturbances. Thus, the PV inverter should be designed to comply with [14]:

• The voltage harmonic levels. Maximum THD is 8%.

(41)

• Voltage amplitude variations. Maximum ± 10%

• Frequency variations. Maximum ± 1%

• Voltage dips: duration less than 1 s at 60 % dip

(42)

3 Methodology

The method applied in this thesis and meeting the objectives was based on creating a sufficient background by continuous literature review of a number of scientific papers, articles, reports and books. The necessary information were filtered and used to provide a theoretical overview over the grid connected photovoltaic systems and support with discussion the simulation results. Furthermore, during this bibliographical research the first acquaintance with the modelling tool was made, by studying tutorials and useful parts of the technical manual.

Since the theoretical needs have been satisfied, the practical part was initiated that covers an examination of a PV model of DIgSILENT at first, a modification of it and a development of two interconnected systems.

3.1 Description of the tools

The study that was carried out in this project was a simulation study of a PV on-grid system. The model as well as the simulation was performed using the PowerFactory tool of DIgSILENT. The respective company has applied years of experience in modelling power systems and the simulation tool is considered to be one of the most powerful in the field. DIgSILENT provides the ability to the user to simulate load flow, RMS fluctuations and transient events in the same environment. PowerFactory has a quite comprehensive library of models for electrical power system components such as generators, motors, relays etc, as well as many passive network elements such as lines, terminals, transformers etc. Those built-in models can correspond to predefined types that are part of the library or user-defined data types. What is more, it is possible to create new models with DSL and by using mathematical formulas that describe the behaviour of the model.

The version used is the latest one (version 14.1), which very more adequate for distributed generation modelling. Load flow studies and RMS dynamic simulations were the functions that used in this thesis.

3.2 Ways for simulating PV with PowerFactory

(43)

depending on the user’s experience and the type of study that is performed. Below those methods are deployed in short.

DC Current Source

In this method the PV array is represented as a DC current source connected to a DC terminal. This is a simple way to simplify the PV array and focus on the grid and its behaviour. In parallel with the current source a shunt filter is used, which is the capacitor for these models. The below figure 3.1 shows the set-up as it was captured from a model.

Figure 3.1: PV array as DC current source

DC Voltage Source

(44)

Figure 3.2: PV model with battery

PWM converter

As seen in figure 3.2 and more clearly in figure 3.3, PWM converter can be used to model a PV inverter. The PV array is again modelled as a DC current source connected to a DC terminal and a PWM PV inverter is used to create a sine wave. This converter represents a self-commutated, voltage sourced converter [26].

Figure 3.3: PV inverter as PWM component

Static generator

(45)

chapters, because in this Thesis the component used for modelling the PV system is the static generator.

(46)

4 Model Description

4.1 The base model

The basic PV system that is analysed in this Thesis is developed by a static generator. It is a generic model that was built by DIgSILENT as part of a past study and is available in the newest version of the PowerFactory tool. The template consists of the PV generator with number of control systems and design features, which are integrated in it and also a LV terminal of nominal voltage 0.4 KV that the generator is connected with. The capacity of the system is 0.5 MW.

The model is being examined thoroughly and its features are being presented below. Some additional information about the DSL code and the parameters used are found in the Appendix. The model is being analyzed in accordance with the German GCs and its possibility to serve the needs of PV on-grid systems in Germany. Below in figure 4.1 the system-model is pasted and highlighted inside the red box. The rest of the configuration, which includes an external grid, a MV bus bar of 33kV nominal voltage and a step up transformer of 0.5 MVA rated power, were just built in order to serve the needs of the examination.

(47)

The external grid that is used in the system is a component of the program. The values that are used for the calculations in the study are the minimum short circuit values as seen in figure 4.2. In general, the minimum values are used to determine where to set the fault pickup level. The minimum short circuit current is the smallest current that can run at a given point and the circuit breaker should be able to sense that fault at that point [27]. The maximum values of short circuit currents are calculated to determine the breaking capacity of the circuit breakers. Both minimum and maximum values are defined by the IEC standard (IEC 60909) [27].

Figure 4.2: The external grid settings

The assumption that the short circuit power is 30 times higher than the capacity of the solar power is used [28]. This value is considered a good estimation. In order to determine how much of PV capacity can be installed in a certain grid, load flow studies are necessary to check the voltage rise at the PCC.

From this value and using (4.1) [29], the initial short circuit current is calculated automatically by the program. Figure 4.3 is a simple representation of a short circuit.

(48)

'' '' '' ''

3

kQ kQ nQ kQ kQ nQ

S

U I

I

U

=

⇔

=

_(4.1) where, 33 ( nominal voltage) nQ U = kV MV

The c-factors, or else voltage factors, are according to IEC 60038 for MV up to 35 kV [27], [29]. Voltage factors refer to voltage regulation and imply that the pre-fault voltage (nominal) would be approximately 5% lower than the no-load voltage. The voltage factors cmax and cmin define the allowance for system voltages. Here the cmin value is used, which is

used for minimum currents. As far as the R/X ratio concerned, based on the conclusions of [30], at low values (<0.4) reactive power is more effective for voltage regulation in distribution networks, while for values above 1.8 active power has a larger impact. In this case a value of 0.3 is assumed.

4.2 The PV generator

The PV generator under normal steady-state operation and flow injects 448.84 kW and 0 kVar as seen in figure 4.4, implying PF=1 at the point of connection with LV terminal. The active power is at the MPP and is defined by the parameters of the PV array and the data sheets of the PV modules used in the array. In table 9.1 the values of Vmpp and Impp of the PV

modules are given for STC and are 35V and 4.58A respectively. Taking into account that 20 modules are per strings and 140 modules in parallel then the following calculation gives the input active power result.

(49)

Figure 4.4: PV generator power flow characteristics under normal steady-state operation

Comparison of existing PV models and possible integration under EU grid specifications

possible integration under EU grid

specifications

Ioannis-Thomas K. Theologitis

Comparison of existing PV models and

possible integration under EU grid

specifications

Ioannis-Thomas K. Theologitis

Master of Science Thesis

KTH School of Electrical Engineering

Division of Electrical Power Systems EPS-2011

POSSIBLE

INTEGRATION

UNDER

EU

GRID

SPECIFICATIONS

Ioannis-Thomas Theologitis

Royal Institute of Technology (KTH), Sweden

Abstract

Sammanfatning

Acknowledgements

Table of Contents

List of Figures

List of Tables

Nomenclature

1

Introduction

1.1

The driving force

1.2

Overview of the Thesis report

1.3

Objectives

1.4

Limitations

2

Background

2.1

PV systems – Overview

2.1.1

I-V Characteristics

2.2

Grid-connected PV systems

2.3

PV inverter

2.3.1

What is available – Current status

2.3.2

Issues when choosing inverter

η

η

η

η

η

η

η

∑

2.3.3

Additional requirements – Ancillary functions

2.4

Low voltage ride through (LVRT) requirement

2.4.1

Reactive power and its importance

2.5

Grid requirements for PV systems

2.5.1

The new German Grid Code

2.5.2

The Situation in the Rest of Europe

2.5.3

Further International and European requirements for PV

3

Methodology

3.1

Description of the tools

3.2

Ways for simulating PV with PowerFactory

4

Model Description