Power quality analysis of a 110 MW wind farm in a 130 kv switchyard. Elforsk rapport 13:13
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1 Power quality analysis of a 110 MW wind farm in a 130 kv switchyard Elforsk rapport 13:13 My Näslund, Elisabeth Lindberg, Anette Larsson, Urban Axelsson Jqnuary 2013
2 Power quality analysis of a 110 MW wind farm in a 130 kv switchyard Elforsk rapport 13:13 My Näslund, Elisabeth Lindberg, Anette Larsson, Urban Axelsson, January 2012
3 Preface The background for this project was that wind power manufacturers and owners meet differences in the grid connection requirements for power quality stated by different sub-transmission network owners. The different used Swedish power quality recommendations only states requirements up to 2 or 2.5 khz while the corresponding German guidelines require measurement of power quality up to 9 khz. This project was therefore carried out in order to review used recommendations and to gain experience by measurements of voltage and currents at frequencies up to 150 khz. The purpose with the project was to: Investigate whether Lillgrund fulfils measureable parts of power quality emission limit requirements and grid code stated by E.ON Elnät Sverige AB and Svenska Kraftnät. Compile the sub-transmission network owner s power quality requirements. Investigate if power quality distortion in the frequency range khz also exists in the sub-transmission grid at system voltage 130 kv. Investigate if it is possible to reach a more harmonized and common platform for the power quality requirements stated by the Swedish DSOs. Draw conclusions on measurement system requirements. Vattenfall Research and Development in Stockholm, Sweden with Urban Axelsson as project manager has carried out the project. The project was carried out as V-354 within the Swedish wind energy research programme Vindforsk III. Vindforsk III is funded by ABB, Arise windpower, AQSystem, E.ON Elnät, E.ON Vind Sverige, EBL-kompetanse, Falkenberg Energi, Fortum, Fred. Olsen Renwables, Gothia wind, Göteborg Energi, HS Kraft, Jämtkraft, Karlstads Energi, Luleå Energi, Mälarenergi, o2 Vindkompaniet, Rabbalshede Kraft, Skellefteå Kraft, Statkraft, Stena Renewable, Svenska Kraftnät, Tekniska Verken i Linköping, Triventus, Wallenstam, Varberg Energi, Vattenfall Vindkraft, Vestas Northern Europe, Öresundskraft and the Swedish Energy Agency.
4 A reference group has given comments on the work and the final report with the following members: Victor Bagge E.ON Elnät Sverige AB Heinz Hauser Jon Jensen Mats Wahlberg Christer Flood Staffan Mared Christer Kauma Siemens Wind Power A/S Siemens Wind Power A/S Skellefteå Kraft Elnät AB Fortum Distribution AB Vattenfall Vindkraft AB Vattenfall Eldistribution AB Stockholm January 2013 Anders Björck Program manager Vindforsk-III Electricity and heat production, Elforsk AB
5 Sammanfattning Den första svenska offshore vindkraftparken Lillgrund med installerad effekt 110 MW ansluts med 130 kv sjö- och landkablar till ställverket Bunkeflo som ägs av E.ON Elnät. Lillgrund i likhet med alla anslutna kunder ska uppfylla ett antal uppställda krav av elkvalitetskaraktär från nätägaren och av systemstabilitetskaraktär från Svenska Kraftnät. Avsikten med projektet var att: genom mätningar klarlägga om Lillgrund uppfyller ställda mätbara krav från E.ON Elnät och Svenska Kraftnät sammanställa hur elkvalitetskrav formuleras av olika regionnätsägare i Sverige tillsammans med en utsedd referensgrupp diskutera möjligheten att formulera mer harmoniserade anslutningskrav undersöka hypotesen om regionnät, i likhet med lågspänningsnät, har övertonsinnehåll i frekvensområdet khz dra slutsatser angående vilka krav mätsystem för elkvalitetmätning bör uppfylla Mätningar och mätsystem: Mätningarna av ström och spänning i Bunkeflo har pågått i drygt 1.5 år med dels s.k. konventionella mätgivare med begränsad bandbredd av några khz och dels mätgivare med hög bandbredd av 400 khz. Följande resultat och slutsatser kan dras från dessa mätningar och analyser. Analys: Projektet har verifierat att interaktionen mellan anläggningen och anslutande nät medför att även strömmätning innehåller bidrag från såväl anläggningen som från nätet. Mätning och analys av spänning ger systemets totala distorsion. Krav som ställts i spänning har mätts som strömvärden och därefter omräknats till spänning. För situationen i Bunkeflo har det visat sig att de ställda elkvalitetskraven på anläggningen, som mätts som strömvärden, är uppfyllda trots att även bidragen från nätet ingår i dessa strömvärden. De parametrar som analyserats är övertoner och mellantoner, obalans, flimmer (eng. flicker) och reaktiv effekt. Dessutom har transienta händelser utvärderats. Slutsatser från denna del av projektet är: Övertoner och mellantoner upp till 2.5 khz ligger under de av E.ON Elnät uppställda gränsvärdena. För toner vid högre frekvenser finns inga gränsvärden men de har mycket låga värden. En PSCAD studie för Lillgrund har visat att strömvärden vid 1 khz, 2.8 khz och 7.2 khz från vindkraftparken förstärks ca 10 gånger i Bunkeflo. De strömtoner som kan mätas i det frekvensintervallet kan alltså dels ha emitterats från Lillgrunds turbiner och dels ha sammanfallit med resonanser. De uppmätta nivåerna är dock väldigt låga. Detta har redovisats i en artikel som skrivits tillsammans med Luleå Tekniska Universitet. De dominerande övertonerna 5, 7 och 11 är oberoende av Lillgrunds produktionsnivå och har ungefär samma nivåer med som utan Lillgrund
6 ansluten. Bidraget från Lillgrund till störnivån i Bunkeflo är således lågt i hela frekvensområdet som mätningarna omfattat. Obalans och flimmer har också lägre värden än gränsvärdena. Transienta händelser har förekommit men inga som på allvar kunnat utvärdera om Lillgrunds Fault Ride Through funktion fungerar. Inte heller har frekvensavvikelser förekommit som Lillgrund skulle ha reagerat på. I frekvensområdet upp till 3 khz uppvisar mätningar med de tre olika mätsystemen mycket likartade resultat dvs även de konventionella induktiva mättransformatorerna med lämpliga mätkärnor klarar detta frekvensområde. Baserat på studier och mätningar i projektet ges följande rekommendationer: Övertoner bör mätas så högt i frekvens som det förekommer resonanser. En studie som beräknar resonansfrekvenserna bör därför ingå, åtminstone i större projekt. Spänningsmättransformatorerna bör placeras på nätägarsidan av brytaren, när så är möjligt, för att möjliggöra referensmätning när vindkraftparken inte är inkopplad och för bestämning av spänningsdip vid inkoppling. Harmonisering av krav: Projektet rekommenderar att metoden beskriven i Elforsk rapport 10:06 (Fördelning av störutrymme) används och att man utgår från planeringsnivåer i spänning för att beräkna gränsvärden, helst i ström, för de olika elkvalitetsparametrarna. Vid projektets referensgruppsmöten har flera regionnätsägare informerat att de använder denna metodik. Vidare har de informerat att de använder långtidsmätning av spänning, oberoende av eventuella framtida anslutningar av vindkraftparker, i olika platser i sina nät för att löpande ha kännedom om nätens spänningsgodhet. I samband med nya vindkraftsinstallationer används dessa mätningar som referens för att bedöma hur mycket spänningsgodheten förändras. Om spänningsgodheten förväntas att förändras påtagligt eller om den förändrats efter en anslutning - kan det vara avgörande för om distorsionsbidragen från en installation behöver mätas mer detaljerat. Vid mötena med projektets referensgrupp har det konstaterats att det ändå föreligger stora skillnader mellan de gränsvärden för olika elkvalitetsparametrar som nätägarna använder i sina tekniska krav för anslutning. Regionnätsägarna anser att kunskapen om hur större anläggningar påverkar spänningsgodheten är i ett uppbyggnadsskede. Metodik för bestämning av och nivåer för gränsvärden utvecklas löpande i takt med att de får erfarenheter via egna och externa mätningar. Ett fortsatt samarbete genom en grupp med liknande representation som i projektets referensgrupp skulle kunna effektivisera detta arbete genom t.ex. utbyte av erfarenheter och resultat från mätningar. Projektet rekommenderar därför att en sådan grupp skapas, t.ex. via Svensk Energi. Slutsatser och rekommendationer från projektet: Lillgrund uppfyller ställda elkvalitetskrav. Anslutningspunkten Bunkeflo 130 kv har ett högt kortslutningsförhållande av ca 24 vilket är en viktig faktor för att upprätthålla hög spänningsgodhet.
7 De ställda elkvalitetskraven innebär att mätdata, rådata, måste behandlas med matematiska metoder (Fourieranalys, omräkning från ström till spänning, beräkning med MatLab rutiner). Denna processning av rådata är normalt inte definierad i de tekniska kraven från nätägarna eller i standarder. Det innebär att de framräknade värdena i värsta fall kan bli olika trots att samma rådata använts. Projektet rekommenderar att den ovan föreslagna gruppen via t.ex. Svensk Energi även tittar på denna fråga med mål att ta fram riktlinjer för definition och val av beräkningsmetodik. Projektet har visat att regionnätet i Bunkeflo har mycket låga störnivåer i frekvensintervallet khz. En allmän diskussion har funnits kring oro för att modern utrustning med kraftelektronik, t.ex. omriktare, ska kunna skapa störningar i det frekvensområdet. Mätningarna i projektet visar att dessa inte, om de funnits för lägre spänningsnivåer i nätet, transporterats upp till högre spänningsnivåer där mätningarna i projektet skett. Även konventionella induktiva mättransformatorer med bra mätkärnor kan användas för mätning av elkvalitet upp till 3 khz. Regionnätsägare använder denna typ av mättransformatorer för elkvalitetsmätningar men frekvenskarakteristiken är okänd. Projektet rekommenderar att branschen efterfrågar typprov av dessa mättransformatorer så att frekvenskarakteristiken, amplitud och fasvinkel, är uppmätt och fastställd i frekvensintervallet 0 Hz till 3 khz före installation. Därmed fastställs med säkerhet vilket mätområde som kan omfattas av mätningen liksom möjligheten att vid behov korrigera amplitud eller fas mot det uppmätta mätfelet. Mätinstrument som erbjuder ett urval av färdiga analyser kan i vissa fall vara enkla och behändiga att använda. Men oftast följer inte mäteller analysmetodiken de krav som uppställts av nätägaren, åtminstone inte för samtliga parametrar som ska analyseras. Slutsatsen är att analysen kräver tillgång till rådata från mätinstrumentet. Projektets rekommendation är därför att åtminstone välja ett mätinstrument som levererar rådata utan någon som helst föregående bearbetning trots att detta utan undantag medför hantering av mycket stora datamängder.
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9 Summary The first Swedish offshore wind farm with installed power of 110 MW is connected to the 130 kv switch-yard Bunkeflo, owned by the Swedish DSO (Distribution System Operator) E.ON Elnät AB, with 130 kv sea and land cables. The wind farm, like all other grid connected customers, shall fulfil a number of power quality requirements stated by E.ON Elnät and System Stability requirements stated by the Swedish TSO (Transmission System Operator) Svenska Kraftnät. The purpose with the project V-354 was to: Investigate whether Lillgrund fulfils measureable parts of power quality requirements and grid code stated by E.ON Elnät Sverige AB and Svenska Kraftnät. Compile the sub-transmission network owners power quality requirements. Together with an appointed reference group discuss the possibility to reach a more harmonized and common platform for the power quality requirements stated by the Swedish DSOs. Investigate if power quality distortion in the frequency range khz, observed to be quickly filled up in low voltage networks, also exists in the sub-transmission grid at system voltage 130 kv. Draw conclusions on measurement system requirements. Measurements and measurement systems: The measurement of current and voltage has been running for more than 1.5 years with both conventional equipment (assumed limited frequency bandwidth) and high frequency bandwidth equipment suitable for measurements up to 150 khz. The following results and conclusions can be drawn from these measurements: Analysis: Preferably measurements shall distinguish between emission from the wind farm and background emission already existing in the grid. The measurement of voltage in Bunkeflo gives the distortion of the whole system so current measurement is used to find the wind farm contribution. The project has though verified that also current measurement has contributions from the grid due to the electrical interaction between the two parts. For the harmonics analysis the current data has been used nevertheless and the situation in Bunkeflo has shown an emission lower than the limit values. Other analysed parameters are inter-harmonics, flicker, unbalance, reactive power and transient events. The conclusions from this part of the project are: Harmonics and inter-harmonics up to 2.5 khz have values below the emission limits set by E.ON Elnät. High frequency emission has no limit values but this emission has very low values. A PSCAD study performed for Lillgrund has shown that current emission at 1 khz, 2.8 khz och 7.2 khz from the wind farm are amplified around 10 times (pu values) in Bunkeflo. Current emission in this frequency range
10 may both have originated from the turbines and coincided with resonances in the network. These frequencies are visible in the harmonics spectrum but the levels are very low. These results are presented in an article written together with Luleå University of Technology. The dominant harmonics 5, 7 and 11, measured in Bunkeflo, are independent of the Lillgrund level of production. They have about the same levels with and without Lillgrund connected. The contribution from Lillgrund to the distortion level in Bunkeflo is therefore concluded to be small in the whole frequency range that was included by the measurements. Unbalance and flicker also have lower values than the limit values. Transient events have occurred but they have not been severe enough to be able to evaluate if the Lillgrund Fault Ride Through function works in an appropriate way. No big enough frequency deviations have occurred that Lillgrund should have reacted on. In the frequency range up to 3 khz, the three different measurement systems show very similar results. This means that also conventional inductive measurement transformers, with suitable metering cores, can be used for this whole frequency range. Based on studies and measurements in the project the following recommendations are given: Harmonics shall be measured as high in frequency as there are resonances. At least for larger wind farms a resonance study should be included. The voltage measurement transformers should preferably be placed on the DSO side of the wind farm breaker when possible. That way it is possible to perform reference measurement when the wind farm is disconnected and to measure the voltage dip at energizations of the wind farm. Requirement harmonisation: The project recommends that the method described in the Elforsk report 10:06 (Distribution of disturbance space) is used and that one starts from the planning levels in voltage to calculate limit values, preferably in current, for the different power quality parameters. At the project reference group meetings several Swedish DSOs have informed that they use this method. Furthermore they have informed that they use long-term voltage measurement in different grid positions, independently of possible future wind farm connections, for continuous knowledge on distortion levels. When a new wind farm is connected the voltage measurements are used as reference to judge on distortion changes. If the distortion level is expected to be changed in an obvious way or if it has been changed after a wind farm connection it may be decisive for if the distortion contributions from the new connection needs to be measured more in detail. At the project reference group meetings it was noted that there are still quite large differences between the limit levels that the DSOs use for different power quality parameters in their respective technical specifications for wind farm connection. The DSO representatives express the opinion that the knowledge about how larger installations impact on the power quality is building up. Methodology for determination of levels and limit values is gradually
11 developed as they gain experience through own and external measurement information. A continued work through a group of similar representation as in the project reference group would make this progress more efficient through exchange of experiences and measurement methods and results. The project therefore recommends that such a group is formed, through for example Svensk Energi. Conclusions and recommendations from the project: Lillgrund fulfils stated power quality requirements. The connection point Bunkeflo 130 kv has a high short circuit ratio, SCR, of 24 which is an important factor for good power quality. All requirements demand that measured raw data (voltage and current) need to be treated in different ways (Fourier analysis, recalculation from current to voltage, calculations with MatLab routines). This processing of raw data is normally not defined, at least not in detail, in the technical requirements from the DSOs or in standards. This open the possibility that the calculated values may not be same, code dependent, even though the same measured raw data has been used. The project therefore recommends that the above proposed group through for example Svensk Energi also considers this question with the aim to create a guide line for the definition and choice of calculation algorithm. The 130 kv sub-transmission grid in Bunkeflo still has very low distortion levels in the frequency range khz. There has been a general discussion concerning an apprehension that modern equipment including power electronics, for example converters, may increase the distortion in this frequency interval. The measurements in the project show that this emission, if existing in the low voltage system, has not propagated to the sub-transmission system where the measurements have been conducted. Also conventional inductive measurement transformers with good metering cores can be used for power quality measurements up to 3 khz. The DSOs use this kind of transformers for power quality measurements but the frequency characteristics is not known. The project therefore recommends that the industry requests a type test of the transformers so that the frequency characteristics, amplitude and phase angle, is measured and established in the interval 0 Hz to 3 khz before installation of the transformers. Thereby the possible transformer measurement frequency interval is securely established as well as the possibility to, if needed, correct amplitude and phase against the known measurement error. Measurement systems that offer an assortment of fixed power quality analyses can in some cases be simple and handy to use. But often it is not possible to comply with the requirement or analysis method that has been defined by the DSO, at least not for all parameters that shall be analysed. The conclusion is that the analysis requires access to raw data from the measurement instrument. The project therefore recommends choosing at least one instrument that delivers raw data without any kind of prior treatment. The drawback of such systems is that very large amount of data need to be handled.
12 Abbreviations and definitions DSO EMC FFT Flicker FRT Harmonics HF transducers IEC IGBT PCC P lt PWM PQ P st RMS SVK TDD THD Tripping TSO VSC Distribution System Operator Electromagnetic compatibility Fast Fourier Transform Flicker is rapid visible changes of light level in lighting equipment Fault Ride Through Variations in the wave shape High frequency measurement transducers International Electrotechnical Commission Insulated-gate bipolar transistor Point of Common Coupling Long-term flicker disturbance factor Pulse Width Modulation Power Quality Short-term flicker disturbance factor Root Mean Square Svenska Kraftnät, TSO in Sweden The total Demand Distortion factor is the R.M.S value of current harmonics expressed as a percentage of the rated or maximum load current Total Harmonic Distortion Energy flow from unit to the grid is interrupted immediately Transmission System Operator Voltage source converter
13 Innehåll 1 Introduction Background Objectives Limitations Report content Project Team The power system Electromagnetic Compatibility (EMC)... 5 Network strength Planning levels Description of power quality parameters General Flicker Harmonics Inter-harmonics HF distortion > 2 khz Unbalance Transients Relevant directorial documents Sweden Technical specifications International Lillgrund and the connecting power system General Information Electrical system Measurement systems Conventional Transducer HF Transducers Measurement Instruments Measurement Results One-week measurement campaign Transients and FRT Reference measurements during 10 minutes Measurement system comparison Summary and Discussion Power quality measurements Harmonics, Inter-harmonics and HF emission Unbalance Flicker Requirement fulfilment Measurement system comparison Harmonisation of power quality requirements References 80
14 Appendix A - E.ON Elnät's restrictions 83 Appendix B - SVK's restrictions 84 Appendix C - Measurement methods 86 Appendix D - Flicker 88
15 1 Introduction 1.1 Background Our society has a growing need for electricity, it is used in almost every apparatus available and all of our fundamental infrastructure functions need power supply. As the apparatus used are becoming more technically advanced and are required for everyday life, the need for interference free electricity supply is increasing. To meet the higher electricity demand, renewable energy sources, foremost wind power, are installed. The electricity however does not only need to be supplied but also to compass a certain quality regarding frequency and voltage in order to keep up the network function and not to harm the equipment it is intended for [2]. The power quality parameters are defined with different accuracies in recommendations, standards, local grid owner s technical specifications and transmission system owner s grid codes. The power quality parameters of interest are: harmonics, inter-harmonics, resonances, flicker, voltage sags, transients and unbalance [3]. Already in 2004 German Technical Reports from BDEW and VDE stated emission limits for harmonics and inter-harmonics up to 9 khz. The reason for writing these documents was primarily that poor power quality was observed also at sub-transmission system voltage level and to be sure that emission from the quickly increasing installations of wind power were reduced as much as possible. The German emission limits follow intentions from planning levels were short circuit capacity and part of total capacity is considered for each connection point. It is shown in different PhD projects at Luleå Technical University that the distortion levels in low voltage networks are quickly increasing in the frequency range khz. One reason is that there are no standards in this frequency region. Measurement systems installed in new plants normally use conventional transducers with assumed limited useful frequency range. No measures are so far taken to be able to fulfil wind power measurement recommendations stating a minimum sampling frequency of 20 khz. The Swedish power quality standards for harmonics, on the other hand, have so far not considered increasing the emission limits frequency range above 2 khz. Wind farm developers and turbine manufacturers should appreciate more harmonized and measurable grid connection requirements and more clear definitions of the same. 1.2 Objectives The project aimed to investigate the hypothesis that harmonics in the frequency range khz can occur in the sub-transmission system. 1 (90)
16 Another ambition with the project was to investigate if a harmonization of the connection requirements between the Swedish DSOs is needed and obtainable. The project should compile the DSO methods used today and propose how the first step towards common requirements and definitions can be obtained. A third objective is to determine if the 110 MW wind power farm Lillgrund fulfils stated power quality requirements in the 130 kv connection point. In addition to this, compile experiences from used measurement systems and give recommendations on the same. 1.3 Limitations Within Vindforsk a compiling and survey report was written covering a number of Vindforsk projects addressing questions in the R&D field of power quality and system stability. [42] In the chapter trends and developments a good overview is given of parameters that could be part of a power quality investigation. The focus in this report is limited to measurable parameters as harmonics, inter-harmonics, flicker, unbalance and transients. 1.4 Report content Descriptions and presentations are given of: The power system and some important power quality related definitions The power quality parameters that have been addressed and related possible problems The Lillgrund wind farm and the Bunkeflo switch yard The measurement systems including transducers and instruments Standards and regulations, DSO requirement comparison Measurement results Short term measurements Weekly measurements Comparison of measurement systems Harmonization of requirements, first step Discussion, conclusions and recommendations 1.5 Project Team Anette Larsson, Elisabeth Lindberg and Urban Axelsson at Vattenfall Research and Development AB executed the project. My Näslund conducted her Master Thesis project as part of the main project. This project final report incorporates large parts of the Master Thesis report, thereby also including information about the different power quality 2 (90)
17 parameters and how they influence on connected equipment and the importance of controlling the levels in the power system. Two very constructive and open meetings were conducted with the reference group, which has also given information and good comments on written texts. Victor Bagge, E.ON Elnät, has given continuous support and information on short circuit data and disturbances in the 130 kv grid. Personnel from Vattenfall Wind Power in Klagshamn och Fredericia have given assistance during installation and maintenance of the measurements systems. Torbjörn Thiringer and Reinhard Kaisinger has assisted with calculation of flicker and unbalance. Math Bollen and his team at Luleå University of Technology have assisted with meetings and discussions on harmonics propagation and measurement issues. 3 (90)
18 2 The power system The power producing units, e.g. hydropower plants, nuclear power plants and wind farms, are connected to the consumers via the electrical grid. The grid in Sweden compasses three different voltage levels, the transmission network 220 kv, subtransmission network between 30 kv and 130 kv and the distribution network is < 30kV. The high voltage transmission network is used for long distance transmission of electricity in order to minimize losses. Whereas the distribution network is used for short distance transmission, close to the customers. The three network levels are constrained by various regulations and comprise different level of robustness. The Transmission System Operator (TSO), Svenska Kraftnät, is responsible for the function of the transmission network and for keeping the balance between consumption and production of electricity in Sweden, i.e. for maintaining the frequency at 50 Hz. Figure 1 Schematic outline of the Swedish power system including Lillgrund Ensuring a reliable interaction between all installations to the electrical system in Sweden is included in the responsibility. In order to keep a good function of the transmission network connected installations must fulfil specifications regarding functionality. The Distribution System Operators (DSO) must fulfil the given restrictions but may set additionally constraints on installations to their networks [4]. 4 (90)
19 2.1 Electromagnetic Compatibility (EMC) Interoperation between the electrical grid and the equipment connected is necessary to keep the network function. This is defined by IEC International Electrotechnical Commission as electromagnetic compatibility, EMC, the ability of equipment or a system to function satisfactorily in its electromagnetical environment without introducing intolerable electromagnetic disturbances to anything in that environment. On the other hand, power quality is defined in IEC as characteristics of the electricity at a given point in an electrical system, evaluated against a set of reference technical parameters, including interruptions, i.e. loss of continuous supply. The power quality is consequently determined by the supply quota (number of interruptions), the frequency and the voltage quality. In large systems the frequency is normally very stable. Variations in the Nordic grid shall be kept within 50 ± 0.1 Hz during normal operation. The ideal voltage is a sinusoidal waveform with 120 phase difference, constant amplitude and frequency. The voltage quality parameters are all affecting the system s voltage and its waveform in different ways and in different frequency ranges. The aim of the voltage quality is to achieve electromagnetic compatibility [5], [6]. Figure 2 Power Quality definition The function of a transmission grid is accordingly not only to transfer electromagnetic energy but also to provide adequate voltage quality at all connection points. All the parties connected to the power system affect the voltage quality and when the quality is not satisfactory the key question is whether the disturbance into the power system is too big or if the power system (network impedance) at the point of connection is too weak [4]. 2.2 Network strength A grid s ability to counter voltage deviations is described as the network s strength. In terms of power quality the network s strength is of great importance, since it influences the magnitude of the power quality parameters. By increasing the network s strength, all the power quality issues deriving from the customers can be reduced. This can be illustrated with Ohm s law and fluctuating current output: 5 (90)
20 Formula 1: Ohm s law This implies theoretically that the voltage would be perfect if the network impedance was zero (Z=0). Therefore, an infinitely strong network would be unaffected by large current outputs (i.e. no voltage variations, U). It is physically impossible to build an infinitely strong grid but very strong networks can be constructed, it is just an economical matter [7]. The short circuit power in the PCC, Point of Common Coupling, is also a measure of the networks strength and it generally increases with increased voltage level. It can be determined either by the operation voltage (U) and the short circuit current (I k ) or by using the short circuit impedance (Z k ). Formula 2: S k The short circuit ratio (SCR) is the ratio between the wind farm apparent power Sn and the network apparent power Sk. It determines the necessary short circuit power of the network in relation to the installed wind power in the PCC; more power needs a higher short circuit power [8]. Formula 3: SCR S k = short circuit power in the PCC S n = Wind farm apparent power 2.3 Planning levels In order to achieve good PQ, Power Quality, and EMC, tools such as compatibility levels and planning levels for PQ parameters have been introduced for the network operators. The electrical grid is not an inference free environment, thus the equipment 1 connected has to hold resilience against disturbances to be compatible with and able to operate. The method uses the resistance of the equipment against voltage deviations together with the emissions produced by the equipment and the network s impact, to provide accurate limits for power quality parameters. The immunity level is the magnitude of emission the equipment can handle without being affected. The level of emission describes the magnitude of emission created by the equipment. The background level includes all emissions from the grid, i.e. all unknown emission sources total emissions. The site disturbance level is the emission level plus the background level at 1 The term equipment is used in its broadest sense here, including both apparatus and fixed installations 6 (90)
Introduction. Harmonics and IEEE 519 Page 1 of 19
Introduction In an ideal power system, the voltage supplied to customer equipment, and the resulting load current are perfect sine waves. In practice, however, conditions are never ideal, so these waveforms
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