PO
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Prof. T
Group F
A
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OWER
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Project (no.
Tore Undel
:
lvaro Basa
Max Kirchm
Rahim Malik
imo Ackerm
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3b): Power
and and Pr
a (S
mann (S
kzada (S
mann (S
ALITY
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Y FOR
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R GRI
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2.11.2012
ii
CONTENTS
List of Figures ............................................................................................................. iii
PART I: GENERAL PROJECT INTRODUCTION
1. Introduction.............................................................................................................. 1
2. Power and Voltage Quality ...................................................................................... 1
3. Photovoltaic systems and their grid connection ....................................................... 2
4. Description of the PV-Project in Kristiansand .......................................................... 4
5. Smart Grids and a short comparison to Germany ................................................... 4
PART II: POWER ELECTRONICS AND MATLAB SIMULATIONS
1. The effect of power plants and power electronics on Power Quality ....................... 5
1.1. Current harmonic basics ................................................................................. 5
1.1.1. Power electronics with effect of current harmonics and voltage distortion .. 5
1.1.2. The effect of power plants & PV-systems to current harmonics in the grid . 7
1.2. Inverters of photovoltaic systems .................................................................... 7
2. MATLAB® Simulations ............................................................................................. 9
3. Solution ................................................................................................................. 13
Literature .................................................................................................................... iv
Appendix.................................................................................................................... vii
iii
LIST OF FIGURES
PART I: GENERAL PROJECT INTRODUCTION
Figure 3.1 Equivalent circuit model of a solar cell. ................................................... 2
Figure 3.2 I(V)-curve for a PV-cell with Maximum Power Point (MPP). ................... 3
Figure 3.3 Block diagram of the grid connection of a PV module. ........................... 3
PART II: POWER ELECTRONICS AND MATLAB SIMULATIONS
Figure 1.1 Distorted line current. ............................................................................. 5
Figure 1.2 Line current for single- (left) and three-phase (right) rectifier with
harmonic distribution. ............................................................................. 6
Figure 1.3 Distorted voltage at PCC of a rectifier with a constant output
voltage .................................................................................................... 6
Figure 1.4 Different inverter types. .......................................................................... 8
Figure 1.5 Single-phase full bridge inverter (left) and PWM with bipolar
switching (right) ...................................................................................... 8
Figure 1.6 Efficiency of different inverters (SiC – Silicon Carbide, Si – Silicon) ....... 9
Figure 2.1 Thevenin equivalent circuit. .................................................................... 9
Figure 2.2 Harmonics and the superposition principle. .......................................... 10
Figure 2.3 Current harmonic is (Kristiansand data). ............................................... 10
Figure 2.4 VPCC waveform (Kristiansand data). ..................................................... 10
Figure 2.5 VPCC waveform for A = 4000 A. ............................................................ 11
Figure 2.6 VPCC waveform for A = 400000 A. ......................................................... 11
Figure 2.7 VPCC waveform for the 3rd harmonic...................................................... 12
Figure 2.8 VPCC waveform for the 9th harmonic. ..................................................... 12
Figure 2.9 VPCC waveform for the 15th harmonic. ................................................... 12
APPENDIX
Figure A.1 Most common Power Quality problems. ............................................... vii
Figure A.2 Data (1) from the project in Kristiansand. .............................................. ix
Figure A.3 Data (2) from the project in Kristiansand ................................................ x
1
PART I: GENERAL PROJECT INTRODUCTION
1. Introduction
In recent years the power production of renewable energy has strongly increased worldwide. Furthermore photovoltaic systems (PV-systems) are getting more important for power production in the private market. Especially the number of PV-systems has been excessively increased due to the Renewable Energy Law (EEG) in Germany. This causes some grid problems. Therefore the grid stability gets in risk because the renewable power plants have no grid support ability, so they cannot help to secure the grid stability. [1], [2]
Furthermore the importance of electronic equipment – especially power electronics – has strongly increased and their usage changes the electric loads in different ways. These loads are not changing linearity, so they produce problems in the Power Quality due to disturbances in the voltage waveform. On this account, Power Quality is becoming more important for the further deployment of renewable energy generation. [3], [4]
2. Power and Voltage Quality
Power Quality means the safe and reliable supply of electric power at every given time. The quality of the voltage is mainly responsible for the Power Quality. The electric equipment in the grid needs a certain good quality of power to operate in a safe mode. Light flickering, for example, is one cause of poor Power Quality because of voltage fluctuation. There are steady-state voltage quality characteristics which are described by the quality of the normal voltage (e.g. voltage magnitude, frequency, distortion, three-phase imbalance) and there are also disturbances (non-steady-state) like interruptions, voltage sags and transients. Even small photovoltaic systems can effect voltage fluctuations. These fluctuations provoke no problems in strong grids, but they may risk the power supply in weak grids. [3], [5]
Figure A.1 in the appendix shows a description of the most common Power Quality problems and their causes as well as their consequences. The criteria about the quality and the allowed limits are given in the European Standard EN 50160 [S1]. This project concentrates itself on the Power Quality problems caused by power electronics and neglects the others which are mainly caused by faults and switching operations in the network. The main problems caused on the use of power electronics are:
Harmonic distortion: Non-sinusoidal waveforms (voltage, current) because of harmonic components in the current.
Voltage fluctuation: Oscillating loads cause a voltage oscillating (can be seen by flickering of lights). There can be also voltage fluctuation at the point of common coupling (PCC) because of the unsteady solar radiation (e.g. moving clouds) over PV-systems. [3]
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4. Description of the PV-Project in Kristiansand
The project is developed by the company Eltek Valere, which produces mostly high efficiency power electronics. Different types of solar panels are installed on roofs of a few buildings. Thus the differences of these panels can be investigated. The aims of the project are: [14], [15]
Getting to know the advantages and disadvantages of the different solar panels to consider which ones are most suitable for Norwegian conditions.
Investigate suitability for solar energy production in Norway depending on weather and climate.
Gathering information about grid connection and problems of solar power production.
Testing new inverters in a real application.
5. Smart Grids and a short comparison to Germany
The main goal of the German government concerning energy policies is to increase renewable energy production. By 2020, 35 %, and by 2050 almost 80% of overall energy demand should be supplied by renewable energy sources. According to polls, nearly 93 % of the German population supports the extension of renewable energy production. [19], [20], [24] The most important instrument at this juncture is the EGG, a law introduced by the government to subsidize the development of renewable energy sources, mainly wind and solar energy. This led to a vast increase of the installed photovoltaic capacity up to 25 GW in 2011, which means 3 % of the total electric energy production. Due to rising electricity prices the EEG was changed in 2012. The subsidies for solar energy were reduced. Furthermore a limit for government-funded solar energy of 52 GW was implemented. This led to a shift towards wind energy. Consequently, in 2011 the overall capital spent on new photovoltaic sites decreased for the first time since the implementation of the EEG to 16.05 billion euros, being still on a high level. [21], [22], [23], [25] Fluctuating feeder lines as well as increasing decentralized energy production by renewable energies lead to new challenges concerning energy grids. It is estimated that in Germany 50 billion euros are necessary to modernize grids. With the use of modern information and communications technology, smart grids provide bidirectional communication between customers, appliances in general and energy generators. This leads to sustainable production and efficient distribution of electricity in the grid. Smart Grids allow greater penetration of renewable energy sources and the integration of new technologies, e.g. charging stations for electric vehicles. Another main point is the possibility of peak leveling. Owners of power generators, e.g. small photovoltaic systems, are able to dispense energy to the grid if the market prices are high, and to receive energy in times where the prices are low. This provides a more economic consumption of energy. [26], [27]
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gets less than , the diodes want to stop conducting. But this is not possible because the inductivity is loaded with energy. Therefore the current has to flow continuously until the energy of the inductor is gone. This can be seen at the voltage over the inductor . The area A must be the same as the area B. At the point , the current is zero and therefore the diodes stop conducting. Thus is again the same as , until
gets less than minus . Then the diodes D3 and D4 begin to conduct (at ). This illustrates that the use of power electronics such as rectifiers or inverters do create non-sinusoidal waveforms. [7]
1.1.2. The effect of power plants and PV-systems to current harmonics in the grid
Today, the high number of renewable power plants which are connected by power electronics to the grid require a different way of grid-operation and -management for an improving quality and reliability of the power-supplies. The negative sides of PV-systems in relation to system-instability are the use of power electronics with non-linear appliances, which generate current harmonics, and voltage fluctuations because of volatile solar radiation. These short fluctuations are caused by moving clouds over photovoltaic modules. Therefore the voltage and power is fluctuating at the PCC. The temperature and type of the modules affect the Power Quality of PV-systems. Furthermore, a low solar irradiance has a higher influence to the current distortion and therefore to the Power Quality. But the total voltage harmonic distortion is not significantly dependent on the solar irradiance. The active power of a PV-system is almost linear with the irradiance and its fluctuations. Hence, these power fluctuations can risk the grid stability, especially in a low voltage grid with a high amount of PV-systems. PV-systems with good Power Quality help avoiding the occurrence of harmonics and voltage distortion in the first place, thus leading to a better sinusoidal waveform of the output voltage. [9], [10]
1.2. Inverters of photovoltaic systems
A power inverter is an electronic device which converts DC into AC power. Switch-mode DC to AC inverters are used in order to produce a sinusoidal AC output voltage from a DC power supply while setting the desired values of magnitude and frequency. The inverters are composed of switches, diodes and capacitors. The inverters can be classified according to their input source. There are VSI’s (Voltage Source Input), which are most commonly used, and CSI’s (Current Source Input) that are rarely used (only for very high power AC motors). VSI’s have different types of inverters: [7]
Pulse-width-modulation inverters: These inverters can control magnitude and frequency of the output voltage, using a PWM module in the switch. With a diode-rectifier, the switch-mode inverter receives a constant DC input voltage.
Square-wave inverters: These inverters only adjust the frequency of the output voltage. To set the magnitude of the AC output, the input dc voltage has to be varied. Such an inverter has the advantage that each switch only changes its state twice per cycle (less switching losses).
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13
3. Solution
In general, current harmonic distortions caused by photovoltaic systems are small. This is especially the case for photovoltaic devices being quite small compared to the short-current capacity of the grid. Nevertheless, if photovoltaic sources are integrated in a big amount, current harmonics may occur, and the influence of current harmonics may lead to a violation of the voltage quality standards according to EN 50160 [S1]. Current distortion leads, due to harmonics, to a distorted output voltage. Harmonic voltages are the result of harmonic currents. To avoid further damage, there are many possibilities dealing with current harmonics. Some examples of them are explained in the following. With the integration of energy storage systems, a constant energy supply to the grid is possible without harmonic distortion, thus increasing voltage quality. The integration of energy storage systems is often necessary when using photovoltaics, even without the occurrence of harmonic distortions, due to the fact that solar cells do not provide energy at a constant rate. SMES, Supercapacitors, flywheels and Lithium-Batteries can be used as storage systems. [4]
Harmonic filtering power conditioners are often used for home or office appliances as they can deal with current harmonics in most cases. Undisruptable Power Supplies (UPS) are used for critical loads, such as medical devices in hospitals. In general they consist of a rectifier, a battery, an inverter and a filter. The rectifier keeps the battery constantly charged and provides power for the inverter. The battery provides power to the critical load in case of grave imbalances or an occurring blackout. The inverter's and filter's main function is to minimize harmonic interferences of the output voltage. [7]
Another solution is the implementation of electronic devices preventing or minimizing the occurrence of current harmonics. Passive circuits normally consist of capacitors, inductors and resistors. Active Filters, like step-up converters, provide a sinusoidal output voltage by creating a current, which cancels the generated harmonic currents. [7]
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Literature
[1] Saadat, N.; Choi, SS.; Vilathgamuwa, DM. “A Statistical Approach to Quantify the Impact on Voltage Quality Caused by PV Generators”, IEEE, 2011
[2] Troester, E. “New German Grid Codes for Connecting PV Systems to the Medium Voltage Power Grid”, 2nd International Workshop on Concerning Photovoltaic Power Plants, Darmstadt 2008
[3] Albarracín, R.; Amarís, H. “Power Quality in distribution power networks with photovoltaic energy sources”, Spain
[4] de Almeida, A.; Moreira, L.; Delgado, J. “Power Quality Problems and New Solutions”, Portugal
[5] Transmission and Distribution Committee “IEEE Guide for Identifying and Improving Voltage Quality in Power Systems”, IEEE, New-York, 2011
[6] Brückl, O.; Bäsmann, R.; Hinz, A. „Fit für mehr erneuerbare Energien“, Teil 1, ew dossier Jg. 110, book 25-26, page 62-64, 2011
[7] Mohan, N.; Undeland, T. M.; Robbins, W. P. “Power Electronics: Converters, Applications and Design”, 3. Edition, 2002
[8] Schipman, K.; Delincé, F. “The Importance of good Power Quality”, ABB, 2010
[9] Maglin, J. R.; Rames, R. “Power Quality Issues in Solar Converters: A Review”, European Journal of Scientific Research, ISSN 1450-216X Vol.61 No.2, pages 321-327, 2011
[10] Patsalides, M.; Evagorou, D.; Makrides, G.; Achillides, Z.; Georghiou, G.; Stavrou, A.; Efthimiou, V.; Zinsser, B.; Schmitt, W.; Werner, J. “The Effect of Solar Irradiance on the Power Quality Behaviour of Grid Connected Photovoltaic Systems”, International Conference on Renewable Energies and Power Quality, Sevilla, 2007
[11] Llamazares, A.; Busso, A. J.; Bajales Luna, N. “Generación fotovoltaica: caracterización de una celda comparando datos experimentales y simulados aplicando un modelo teórico simple”, Comunicaciones Científicas y Tecnológicas 2000. Argentina, 2000
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[12] Bailon Buendia, J.
“Aspectos normativos para la conexión de generación fotovoltaica a la red en España e implementación de algoritmos MPP”, PFC, Leganés, 2011
[13] National Instruments “Part III – I-V Characterization of Photovoltaic Cells Using PXI”, [online], http://www.ni.com/white-paper/7231/en
[14] Eltek Valere “Eltek Valere leads the way for solar energy production in Kristiansand, Norway”, [online], http://www.eltekvalere.com/wip4/detail_country.epl?cat=9882&id=1058115
[15] Pv magazine “Interview to Sales Director of renewable energy in the Americas, Alberto de León”, edition 12/2011, [online], http://www.solar360.com.au/files/Updated%20Datasheets/ELTEK%20INVERTERS/PV%20Magazine%20Article.pdf
[16] IBC solar ”Inversores”, [online], http://www.ibc-solar.es/inversores0.html
[17] Kerekes, T. “Transformerless Photovoltaic Inverters Connected to the Grid”, Institute of Energy Technology, Aalborg University, Denmark, [online], http://vbn.aau.dk/files/16280002/Transformerless_photovoltaic_inverters_connected_to_the_grid.pdf
[18] Yurasek , J. “Conventional and Transformerless Inverters”, 2012, [online], http://www.solarchoice.net.au/blog/conventional-and-transformerless-inverters/
[19] Deutsche Bundesregierung “Erneuerbare Energien”, [online], http://www.bundesregierung.de/Webs/Breg/DE/Themen/Energiekonzept/ErneuerbareEnergien/_node.html
[20] Deutsche Bundesregierung “Bundestag stimmt Energiekonzept 2050 zu ”, [online], http://www.bundesregierung.de/Content/DE/Artikel/2010/10/2010-10-01-energiekonzept-bt.html
[21] Koordinierungsstelle Erneuerbare Energien “Gesetz für den Vorrang Erneuerbarer Energien (Erneuerbare-Energien-Gesetz – EEG) ”, [online], http://www.bundesregierung.de/Content/DE/Artikel/2010/10/2010-10-01-energiekonzept-bt.html
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[22] Bundesregierung
“Erneuerbare Energie in Deutschland und Frankreich ”, 2012, [online], 120620_Präsentation_Koordinierungsstelle_Erneuerbare_Energien_SaarLB.ppt
[23] Agentur für erneuerbare Energien “Der Strommix in Deutschland im Jahr 2011”, [online], http://www.unendlich-viel-energie.de/de/solarenergie/detailansicht/article/37/der-strommix-in-deutschland-im-jahr-2011.html
[24] Agentur für erneuerbare Energien “Akzeptanz Erneuerbarer Energien in der deutschen Bevölkerung 2012”, [online], http://www.unendlich-viel-energie.de/de/detailansicht/article/226/akzeptanz-erneuerbarer-energien-in-der-deutschen-bevoelkerung-2012.html
[25] Agentur für erneuerbare Energien “Investitionen in Erneuerbare-Energien-Anlagen”, [online], http://www.unendlich-viel-energie.de/de/detailansicht/article/226/investitionen-in-erneuerbare-energien-anlagen.html
[26] Bundesministerium für Wirtschaft und Technologie “Energiewende”, 2012, [online], http://www.bmwi.de/BMWi/Redaktion/PDF/E/energiewende,property=pdf,bereich=bmwi,sprache=de,rwb=true.pdf
[27] U.S. Department of Energy "Smart Grid / Department of Energy", 2012
Standards
[S1] European Standard EN 50160 “Voltage Characteristics in Public Distribution Systems”
[S2] Standard IEC 60038 “IEC Standard Voltages”
Append
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Figur
re A.1: Most coommon Poweer Quality probblems. [4]
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viii
MATLAB® simulation-code for the impact of the current harmonics
clear all
%variables
f= 50;
syms t;
v_s= sqrt(2)*415/sqrt(3)*sin(2*pi*f*t);
L_s= 21.45e-6;
% current amplitude
A= 4000;
% current harmonics
i_s= A*sin(2*pi*f*t) + 1/3*A*sin(3*2*pi*f*t) + 1/5*A*sin(5*2*pi*f*t) + 1/7*A*sin(7*2*pi*f*t);
v_PCC= v_s - L_s * diff(i_s);
figure
ezplot(i_s, [0 40e-3])
figure
ezplot(v_PCC, [0 40e-3])
Figu
ure A. 2: Data (1) from the pproject in Kristiansand.
ix
x
Figu
ure A. 3: Data (2) from the pproject in Kristiansand.
x
x
