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Energy efficiency and harmonics in electric power systems Fredrik Kühn 2013 – 05 -05 Löpnummer: EN1318 Examensarbete för Civilingenjörsexamen i energiteknik, 30 hp Department of Applied Physics and Electronics ABSTRACT The choice of power system components is traditionally assembled with great attention to the functional performance and interaction of components. In addition, the choice of components with respect to energy efficiency today plays an increasingly important role, this is an aspect that is generally neglected, especially when taking into mind industries efforts to reduce environmental impact, the margin is, it can also improve the economy from an investment perspective. ÅF needs a thorough study on how the situation is today on the market and just how large potential there are available by selecting the right components. This thesis is intended to investigate this potential and to facilitate a comparison of the energy performance of electric power components. A harmonic is a wave whose frequency is an integer multiple of the frequency of some fundamental wave. When present in an electric power system it is added to the reference wave and the resulting wave becomes a fluctuating signal and thus the quality of the electricity is disfigured. They are generated by modern electronic devices that include adjustable speed drives and ac/dc converters. Devices such as CFLs, battery chargers, speed controlled motors and pump are all devices who contribute to the generation of harmonics in power systems. The concentration of this type of equipment increases rapidly, for example, loading of electric cars demands ac/dc converters and industries increases their usage of speed controlled motors and pumps. At a first look on the energy loss and economics in power systems the losses appears to be fairly static and on the constructor of components responsibility. However, when harmonics are present in power systems, extra heat dissipation in equipment occurs and an increase of the RMS value of current and voltage leads to higher subscription cost for energy consumers, a method for calculating the cost of harmonics with measurements and analysis of harmonic content in parts of SKF’s power system is performed and solutions for mitigation presented. Results showed large potential in improving investments and reducing enviromental impact by choosing components not only for funcional performance but also for energy performance, interrupters is the only component in this project that did not hold a significant potential. A method for calculating the economic impact of harmonics in industries was developed and showed a large economic potential in mitigation of these, however, mitigation solutions was not profitable because of high harmonic filter prices. I Spring 2013 PREFACE This master’s thesis has been written at the Department of Applied Physics and Electronics at Umeå University of Science and Technology in cooperation with ÅF consulting. It closes my Energy Engineering Master with Electrical Power as main topic. The objectives for this thesis started out quite broad with the intent of finding parts of electric power systems worth investigating from an energy optimization perspective. This resulted in three main questions to be investigated which are presented in the introduction chapter. I want to send a special thank to my supervisors Haris Mehmedovic and Robert Josefsson at ÅF Power System Analysis Group for giving me this opportunity and supporting me during the making. I also want to thank my University tutor, Johan Pålsson for the great feedback on the thesis layout and written parts. Thank you Gert Nylén, ÅF consulting for your enthusiasm and helpfulness, you contributed a lot with your expertise in measurements of electric quality and signal analyzing. Hans Moberg at SKF, thank you for giving me the opportunity of analyzing your system, the case study is an important part of this work. Finally I would like to thank my girlfriend Karoline, my mother Marie, my father Rolf and brother Daniel for your love and for listening to my concerns and ideas about this thesis. Fredrik Kühn Gothenburg, the 25th of April 2013 II Spring 2013 CONTEXT Our earth faces severe challenges; the energy consumption needs to become totally sustainable in order for us to survive in the long-term. This is a challenge which is partly political and partly technical. The Kyoto Protocol which regulated the energy policies for 191 countries worldwide from 1997 to 2012 is no longer active and negotiations about a new agreement have been staggering. The latest conference was held in Copenhagen 2009 and resulted in an agreement that we cannot let the global temperature rise reach above two degrees Celsius, although no conclusive deals were made from the participating nations. The negotiations were often characterized by short term arguments which are unfortunate for our future population. We have to start taking a larger responsibility for the long term future and it should begin with making a few charitable decisions. As for the technology we are in the middle of a historical change towards energy optimization and sustainable energy production. Fossil fuels are becoming less politically accepted and many nations are starting to replace it with green energy. This evolution leads to large changes in production sites, transmission grids and distribution facilities. In many cases it leads to some production sites closure and other, ”green”, sites replacement of it. The transmission grids have to become adapted to the changing ways of production and consumption, in some cases this means adapting to more decentralized sites like wind power and in other cases be able to manage the increasing share of electric car-charging. In distribution facilities such as industries, hospitals and railways the main focus is on energy efficiency and thus minimizing the energy loss at specific facilities. IVA, the Swedish Royal Engineering Academy informs in ”Energieffektivisering, möjligheter och hinder1” that the potential for profitable energy optimization in the industry sector is about 13 TWh/y and in Sweden as a total the potential was estimated to be about 50 TWh/y. Energy optimization is one step on the road towards total sustainability on earth. III Spring 2013 TABLE OF CONTENT 1 INTRODUCTION ......................................................................................................................................................... 1 1.1 ENERGY LOSS ...................................................................................................................................................................... 1 1.2 E NERGY EFFICIENCY ............................................................................................................................................................ 2 1.3 CHOOSING COMPONENTS .................................................................................................................................................... 2 1.4 RECTIFICATION ................................................................................................................................................................... 2 1.5 MANAGING H ARMONICS ...................................................................................................................................................... 2 1.6 QUESTIONS TO BE ANSWERED .............................................................................................................................................. 3 2 TRANSFORMERS ....................................................................................................................................................... 4 2.1 LOAD LOSS ........................................................................................................................................................................ 4 2.2 NO LOAD LOSS ................................................................................................................................................................... 4 2.3 TRANSFORMER EFFICIENCY ................................................................................................................................................... 5 2.4 AMORPHOUS METAL TRANSFORMERS – REDUCING NO LOAD LOSS ............................................................................................ 6 2.5 TRANSFORMER EFFICIENCIES IN EUROPE ............................................................................................................................... 7 2.6 E UROPEAN EFFICIENCY STANDARD EN 50464-1 ................................................................................................................... 8 2.7 ON THE MARKET ................................................................................................................................................................ 9 2.7.1 ABB TRANSFORMERS ................................................................................................................................................... 10 2.7.2 SIEMENS TRANSFORMERS .............................................................................................................................................. 14 2.7.3 ABB AMORPHOUS METAL CORE TRANSFORMER ............................................................................................................... 17 3 CABLES – A METHOD FOR COMPARISON ................................................................................................... 18 4 BREAKERS AND INTERRUPTERS .................................................................................................................... 19 4.1 INTERUPTER DESIGN ......................................................................................................................................................... 19 4.2 INTERRUPTER E NERGY PERFORMANCE ................................................................................................................................. 19 5 ADJUSTABLE SPEED DRIVES ............................................................................................................................. 20 5.1 ASD DESIGN .................................................................................................................................................................... 20 5.2 VFD E NERGY PERFORMANCE ............................................................................................................................................. 21 Spring 2013 6 HARMONICS ............................................................................................................................................................... 22 6.1 CAUSES ........................................................................................................................................................................... 23 6.1.1 RECTIFICATION ............................................................................................................................................................ 24 6.1.2 12 PULSE RECTIFICATION ............................................................................................................................................... 25 6.2 E FFECTS .......................................................................................................................................................................... 26 6.3 INCREASED FREQUENCY ⇾ INCREASED LOSSES - THE SKIN EFFECT ........................................................................................... 28 6.4 E NERGY LOSS IN COMPONENTS DUE TO HARMONICS ............................................................................................................. 31 6.4.1 TRANSFORMERS ........................................................................................................................................................... 31 6.4.2 CABLES ....................................................................................................................................................................... 31 6.4.3 CAPACITORS ................................................................................................................................................................ 32 7 SOLUTIONS TO ATTENUATE HARMONICS ................................................................................................. 33 7.1 BASIC SOLUTIONS ............................................................................................................................................................. 33 7.1.1 POSITION THE NON-LINEAR LOADS UPSTREAM IN THE SYSTEM............................................................................................ 33 7.1.2 GROUP THE NON-LINEAR LOADS ..................................................................................................................................... 33 7.1.3 CREATE SEPARATE SOURCES ........................................................................................................................................... 34 7.1.4 TRANSFORMERS WITH SPECIAL CONNECTIONS ................................................................................................................. 34 7.2 HARMONIC FILTERING ....................................................................................................................................................... 35 7.2.1 PASSIVE FILTERS ........................................................................................................................................................... 35 7.2.2 ACTIVE FILTERS ............................................................................................................................................................ 36 7.3 8 INSTALLATION OF HIGHER-PULSE RECTIFIERS ........................................................................................................................ 37 COMPUTER MODELING AND ANALYSIS: HARMONIC FLOW ............................................................. 39 8.1 FOURIER ANALYSIS ............................................................................................................................................................ 39 8.2 HARMONIC SYSTEM STUDY ................................................................................................................................................ 41 8.3 THE BUS ADMITTANCE MATRIX ........................................................................................................................................... 41 8.4 ADMITTANCE ................................................................................................................................................................... 42 8.5 MODELLING OF POWER LINES ............................................................................................................................................ 42 9 9.1 CASE STUDY: SKF POWER DISTRIBUTION SYSTEM .............................................................................. 43 SYSTEM OVERVIEW ............................................................................................................................................................ 43 9.2 SYSTEM COMPONENTS ...................................................................................................................................................... 44 9.2.1 TRANSFORMER (T5) ..................................................................................................................................................... 44 9.2.2 2 × CAPACITOR (C1, C2) .............................................................................................................................................. 45 9.3 MEASUREMENTS OF HARMONICS ....................................................................................................................................... 46 Spring 2013 9.3.1 INSTRUMENTS .............................................................................................................................................................. 46 9.3.2 METHOD ..................................................................................................................................................................... 46 9.3.3 PERIOD ....................................................................................................................................................................... 46 SUB PERIODS ............................................................................................................................................................................ 46 9.3.4 QUALITY OF MEASUREMENTS ......................................................................................................................................... 46 10 RESULTS .................................................................................................................................................................. 47 10.1 MEASUREMENTS .............................................................................................................................................................. 47 10.2 HARMONIC FILTER INVESTMENTS ........................................................................................................................................ 48 10.2.1 NO FILTER ............................................................................................................................................................... 48 10.2.2 PASSIVE FILTER ........................................................................................................................................................ 50 10.2.3 PASSIVE FILTER INVESTMENT ..................................................................................................................................... 52 10.2.4 ACTIVE FILTER ......................................................................................................................................................... 53 10.2.5 ACTIVE FILTER INVESTMENT ...................................................................................................................................... 55 10.3 E XTRA HEAT DISSIPATION DUE TO HARMONICS ..................................................................................................................... 56 10.3.1 CABLES ................................................................................................................................................................... 56 10.3.2 TRANSFORMERS ....................................................................................................................................................... 57 10.3.3 CAPACITORS ............................................................................................................................................................ 58 10.4 TRANSFORMER COMPARISON ............................................................................................................................................. 59 10.5 CABLES COMPARISON ....................................................................................................................................................... 61 11 CONCLUSIONS ...................................................................................................................................................... 62 11.1 IS THERE A POTENTIAL FOR SAVINGS BY CHOOSING HIGH EFFICIENCY COMPONENTS ?................................................................. 62 11.2 WHAT IS THE ECONOMIC IMPACT OF HARMONICS IN POWER SYSTEMS ? ................................................................................... 63 11.3 HOW MUCH ENERGY LOSS DOES HARMONICS CAUSE IN ELECTRIC POWER COMPONENTS ? .......................................................... 64 11.3.1 CABLES ................................................................................................................................................................... 64 11.3.2 TRANSFORMERS ....................................................................................................................................................... 64 11.3.3 CAPACITORS ............................................................................................................................................................ 64 12 REFERENCES ........................................................................................................................................................ 65 Spring 2013 TABLE OF FIGURES FIGURE1:SINGLE PHASE TRANSFORMER CONSTRUCTION ................................................................................................................ 4 FIGURE 2. HTTP://WWW.ENERGYRATING.GOV.AU/WP-CONTENT/UPLOADS/2011/03/200717-MEPS-TRANSFORMERS.PDF ............................... 5 FIGURE 3. OPERATING EFFICIENCY OF DISTRIBUTION SECTOR DISTRIBUTION TRANSFORMERS, EU-27 AND NORWAY ............................................. 7 FIGURE 4. BREAKDOWN OF DISTRIBUTION SECTOR DISTRIBUTION TRANSFORMER LOSSES, EU-27 AND NORWAY.................................................. 7 FIGURE 5. NO LOAD LOSS STANDARD FOR DISTRIBUTION TRANSFORMERS IN EUROPE...................................................................................... 8 FIGURE 6. LOAD LOSS STANDARD FOR DISTRIBUTION TRANSFORMERS IN EUROPE........................................................................................... 8 FIGURE 7. NO LOAD-LOSS AND LOAD LOSS FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS .......................................................... 10 FIGURE 8. EFFICIENCIES FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS .................................................................................. 10 FIGURE 9. NO LOAD LOSS AND LOAD LOSS FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS .......................................................... 11 FIGURE 10. EFFICIENCIES FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ................................................................................ 11 FIGURE 11. NO LOAD LOSS AND LOAD LOSS FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ........................................................ 12 FIGURE 12. EFFICIENCIES FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ................................................................................ 12 FIGURE 13. NO LOAD LOSS AND LOAD LOSS FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ........................................................ 13 FIGURE 14. EFFICIENCIES FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ................................................................................ 13 FIGURE 15. NO LOAD LOSS AND LOAD LOSS FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ........................................................ 14 FIGURE 16. EFFICIENCIES FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ................................................................................ 14 FIGURE 17. NO LOAD LOSS AND LOAD LOSS FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ........................................................ 15 FIGURE 18. EFFICIENCIES FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS................................................................................ 15 FIGURE 19. NO LOAD LOSS AND LOAD LOSS FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS........................................................ 16 FIGURE 20. EFFICIENCIES FOR DIFFERENT ABB POWER DISTRIBUTION TRANSFORMERS ................................................................................ 16 FIGURE 21. PRINCIPLE CHART OVER THE METHOD OF A TOC COMPARISON ................................................................................................ 18 FIGURE 22. EFFICIENCIES FOR DIFFERENT ABB SWITCH DISCONNECTORS.................................................................................................... 19 FIGURE 23HTTP://IMAGE.GREENMANUFACTURER.NET/A/DRIVING-ENERGY-EFFICIENCY-IN-MOTORS-VFD-DIAGRAM.GIF .................................... 20 FIGURE 24 HTTP://WWW.ELECTRICAL-INSTALLATION.ORG/ENW/IMAGES/9/9A/FIGM12B.JPG .................................................................... 22 FIGURE 25HTTP://WWW.ELECTRICAL-INSTALLATION.ORG/ENW/IMAGES/2/29/FIGM05.JPG ...................................................................... 23 FIGURE 26 HTTP://EN.WIKIPEDIA.ORG/WIKI/FILE:6_PULSE_BRIDGE_WITHOUT_INDUCTANCE.PNG................................................................ 24 FIGURE 27 HTTP://EN.WIKIPEDIA.ORG/WIKI/FILE:BRIDGE_RECTIFIER_AT_ALPHA%3D0_U%3D0.PNG .......................................................... 24 FIGURE 28. RESULTING VOLTAGE WITH 30 DEGREES PHASE SHIFT CREATING A SMOOTHER DC THAN A 6-PULSE RECTIFYER .................................. 25 FIGURE 29. LINE DIAGRAM OVER A 12-PULSE RECTIFYER ........................................................................................................................ 25 FIGURE 30HTTP://WWW.ELECTRICAL-INSTALLATION.ORG/ENW/IMAGES/1/14/FIGM08.JPG ...................................................................... 26 FIGURE 31HTTP://UPLOAD.WIKIMEDIA.ORG/WIKIPEDIA/COMMONS/THUMB/C/C7/SKINEFFECT_REASON.SVG/220PX-SKINEFFECT_REASON.SVG.PNG ........................................................................................................................................................................................... 28 FIGURE 32HTTP://UPLOAD.WIKIMEDIA.ORG/WIKIPEDIA/COMMONS/THUMB/6/61/SKIN_DEPTH.SVG/325PX-SKIN_DEPTH.SVG.PNG ................. 28 FIGURE 33. HTTP://WWW.ELECTRICAL-INSTALLATION.ORG/ENWIKI/BASIC_SOLUTIONS_TO_ATTENUATE_HARMONICS ...................................... 33 FIGURE 34. HTTP://WWW.ELECTRICAL-INSTALLATION.ORG/ENWIKI/BASIC_SOLUTIONS_TO_ATTENUATE_HARMONICS ...................................... 33 FIGURE 35. HTTP://WWW.ELECTRICAL-INSTALLATION.ORG/ENWIKI/BASIC_SOLUTIONS_TO_ATTENUATE_HARMONICS ...................................... 34 FIGURE 36. HTTP://WWW.ELECTRICAL-INSTALLATION.ORG/ENWIKI/BASIC_SOLUTIONS_TO_ATTENUATE_HARMONICS ...................................... 34 FIGURE 37. HTTP://WWW.MID-ISLAND.COM/DOWNLOADS/PDFS/DRIVES-BR011B-EN-P.PDF.PDF ................................................................. 35 FIGURE 38 HTTP://WWW.MID-ISLAND.COM/DOWNLOADS/PDFS/DRIVES-BR011B-EN-P.PDF.PDF .................................................................. 36 FIGURE 39. HTTP://WWW02.ABB.COM/GLOBAL/HUABB/HUABB008.NSF/0/45DF9D2440A9199BC1257A2C004404D3/$FILE/PQF+FELHARM%C3% B3NIKUS+SZ%C5%B1R%C5%91K+.PDF .................................................................................................................................. 36 TH FIGURE 40 FUNDAMENTAL SINE WAVE VERSUS ITS 5 HARMONIC ADDED, PLOTTED IN MATLAB .................................................................... 40 FIGURE 41 OVERVIEW OF THE CASE STUDY -SYSTEM. ............................................................................................................................. 43 FIGURE 42. CURRENT HARMONICS AT L05.1........................................................................................................................................ 47 FIGURE 43. VOLTAGE HARMONICS AT L05.1 ........................................................................................................................................ 47 Spring 2013 FIGURE 44. NO FILTER: FUNDAMENTAL CURRENT VERSUS DISTORTED ....................................................................................................... 48 FIGURE 45. NO FILTER: FUNDAMENTAL VOLTAGE VERSUS DISTORTED........................................................................................................ 48 FIGURE 46. PASSIVE FILTER; FUNDAMENTAL VERSUS PASSIVE FILTER CURRENT ............................................................................................ 50 FIGURE 47. PASSIVE FILTER; FUNDAMENTAL VERSUS PASSIVE FILTER VOLTAGE............................................................................................. 50 FIGURE 48. PAYOFF DIAGRAM FOR A PASSIVE FILTER INVESTMENT ............................................................................................................ 52 FIGURE 49. FILTERED: FUNDAMENTAL VERSUS ACTIVE FILTER CURRENT ..................................................................................................... 53 FIGURE 50. FILTERED: FUNDAMENTAL VERSUS ACTIVE FILTER VOLTAGE ..................................................................................................... 53 FIGURE 51. PAYOFF DIAGRAM FOR AN ACTIVE FILTER INVESTMENT. .......................................................................................................... 55 FIGURE 52. HEAT DISSIPATION IN CABLES DISTRIBUTED AT EACH HARMONIC. .............................................................................................. 56 FIGURE 53. INCREASED HEAT DISSIPATION IN CAPACITORS C1 AND C1 CAUSED BY HARMONICS ...................................................................... 58 FIGURE 54.TRANSFORMER TOC DEVELOPMENT FOR TWO ALTERNATIVES OVER 11 YEARS ............................................................................. 60 FIGURE 55. FINAL TOC FOR TWO INVESTMENTS; REGULAR GRAIN ORIENTED VERSUS AMOURPHOUS METAL CORE TRANSFORMERS. ................... 60 FIGURE 56. CABLE TOC DEVELOPMENT FOR TWO ALTERNATIVES OVER 30 YEARS. ....................................................................................... 61 Spring 2013 DISPOSTION PART 1 PART 2 PART 3 • Function and energy performance of power system components • Energy aspect of harmonics in power systems • Results/conclusions from PART 1 and PART 2 Spring 2013 1 INTRODUCTION This chapter introduces the reader by briefly describing energy loss in electrical distribution and different attenuation methods. At first we take a look at what energy loss in electric conductors really are and what causes it. Installing the right components in a system with load characteristics taken into account will increase energy efficiency and improve the investment. When driving motors, adjustable speed increase efficiency but can also cause other problems such as harmonic current injection and thus give rise to energy loss in other parts of the system, a brief review of harmonic management are presented and questions this project has explored are listed in the end. 1.1 ENERGY LOSS Electric energy loss are caused by joule heating, also known as resistive heating and ohmic heating. Joule heating is the result of the process by which the passage of an electric current through a conductor releases heat. James Prescott Joule was the first to study this phenomenon in 1841. Joule heating is caused by the interactions between the moving particles that form the current and the atomic ions that make up the body in the conductor. Charged particles in an electric circuit are accelerated by an electric field but give up some of its kinetic energy each time they collide with an ion. This kinetic energy is then converted to vibration in the particles which we refer to as heat. A lot of energy that is generated to facilities goes to waste in different ways. For example when transformers draw power to stay magnetized, even though they are not used for power distribution, this power then become waste. When lights, heating and ventilation are turned on even though no one is using it, the energy is wasted and when motors and pumps is driven with fixed speed much energy can be saved by using variable speed. This project has evaluated energy loss that occurs when: - Components hold poor efficiencies, the market research in part 1 presents today’s efficiency levels on the market and different choices are evaluated in chapter 10,”results”. - Distribution utilities create harmonics in the loads and send these through debitation meters, the signal measures higher power RMS than what is actually consumed, A Fourier analysis (see 8.1) solves this and gives what extra power is debited for in the case study. - Harmonics flow through components and give rise to increased energy loss (see 6.2). Harmonic management methods for attenuating this type of problems are described in the thesis and evaluated in chapter 10. A case study in SKF AB has been performed with measurements of the harmonic content in one of the transformer stations. 1 Spring 2013 1.2 ENERGY EFFICIENCY The ratio between useful output and the input of an energy conversion is called energy efficiency. Typically it is denoted the Greek letter small Eta (η). This ratio is used as an index of how efficient certain electric entities are. It is defined as: Eq. 1 Where Pout = output power [W] Pin = input power [W] 1.3 CHOOSING COMPONENTS In order to find out which component is optimal for some certain installation, knowledge of the load characteristics are of utmost importance. Functional conditions such as what voltages, currents and short-circuit power are critical and nowadays always taken into account. But knowledge and estimations about characteristics of the load such as max, min and mean value of the power, which periods that low resp. high power are expected and how much harmonics that are going to be present. For example transformers; if the load is expected to reduce to minimum every night, no-load loss can be assumed to be a major contributor to the total losses. Then it would be wise to choose a transformer who is constructed with extra low no-load loss. And for cables; if harmonic levels are expected to be high, to prevent the cable from overloading an increase in the cross area is necessary. When it comes to harmonics and preventing problems originating from them, choosing speed drives with higher pulse numbers is an important assessment, 6-pulse speed drives generates harmonic levels up to 30 % THD versus 24 pulse-drives who generates as low as 2.7 % THD (see section 7.3). 1.4 RECTIFICATION (AD/DC) Installing rotating machines with adjustable speeds can cause large energy savings because it allows the machines to be operated on its optimal efficiency more often. Variable speed in machines is dependent on adjustable speed drives, which are devices that consist of three steps: rectification, filtering and pulse width modulation. The first step converts the AC power to DC, the second smoothens the DC and the third creates AC with variable frequency which varies the rotating speed of machines. The problem with harmonics occurs in the first step, the rectification process which sends harmonics back into the distribution system. This process is explained in section 6.1.1. More devices that depend on rectification and thus cause harmonics are listed in section 6.1. 1.5 MANAGING HARMONICS In order to prevent or fix problems with harmonics in electric power systems various measures can be taken. Basic solutions such as designing the system in certain ways in order to protect sensitive devices, also filter solutions could sometimes prove efficient and most often, not causing the problem from start by choosing the right speed drives, low energy lamps and rectifiers could serve as a good alternative. These solutions are presented in chapter 7 and the latter two types of attenuation methods are evaluated and compared economically in ”Results”. 2 Spring 2013 1.6 QUESTIONS TO BE ANSWERED These questions are discussed in chapter 11,”Conclusions”. PART 1 1. Is there a potential for savings by choosing high efficiency components? Part 1 consists of a market research and presenting of methods with the goal of contrasting regular choices of components to more efficient. An economic comparison between these contrasting choices is presented in the result chapter to investigate if there are a significant potential to be earned in order to improve investments. PART 2 2. What is the economic impact of harmonics in electric power systems? Certain electrical devices are custom to certain frequencies e.g. 50 Hz so this can be considered as the useful frequency. The RMS, Root-Mean-Square value of a sinusoidal is the mean value of the magnitude. When a signal also contains harmonics, these adds to the RMS value. When debiting electric energy, consumers do not only pay for the useful 50 Hz RMS but also all other harmonic frequencies that are on the debiting signal. A Fourier analysis is used for solving what extra power that is paid for in the case study. 3. How much energy loss does harmonics cause in electric power components? When harmonics travel through power components such as transformers, cables and capacitors they give rise to extra energy loss, measurements of harmonics and calculation of losses caused by these are performed and evaluated for the case study. 3 Spring 2013 PART 1 2 TRANSFORMERS The Transformer loss can be divided into two main components: no-load loss and load loss. These types of losses are common to all types of transformers, regardless of transformer application or power rating. There are, however, two other types of losses; extra heat dissipation created by harmonics which is described in section 6.3 and losses which may apply particularly to larger transformers – cooling or auxiliary losses, caused by the use of cooling equipment like fans and pumps. Figure 1 to the right show a single phase transformer design with its main features. The rectangular shaped metal body is called the core and the two coiled wires are called primary and secondary windings. Three-phase transformers consist of three sets of primary and Figure 1: Single phase transformer design secondary windings and each set is separately connected to each phase. 2.1 LOAD LOSS When electric current flows in a conductor, resistance generates ohmic heating. The transformers two windings are consists of two coiled conductors and thus, the nature of the load loss is similar to the ohmic heating losses in conductors. As illustrated in figure 2 below, this loss becomes dominant when the transformer is more than about 50% loaded. Figure 2 also shows that overloading of the transformer causes significant increase in load loss and decrease in efficiency. Load loss is also dependent on frequency content of the load current, this is discussed in section 6.3. 2.2 NO LOAD LOSS The core of transformers is, even when there is no load kept charged with an alternating magnetic field. As the magnetic domains in the steel try to follow the changing orientation of the AC magnetic field they generate frictional heat in the core, this loss is called hysteresis loss. These losses can be approximated as constant as long as the primary voltage is held constant. The alternating magnetic field also induces eddy currents in the core due to the magnetic interaction. In the same way that electrical current flow generates heat, eddy currents causes additional losses. The eddy current loss depends on the electrical resistance of the core material and the AC frequency. Hysteresis loss increase linearly with frequency and eddy current loss scales as the square of frequency. 4 Spring 2013 PART 1 2.3 TRANSFORMER EFFICIENCY The efficiency of a transformer is expressed by the following equation: Eq. 2 Where Pload = Load [W] Pload loss= Load loss [W] Pno load loss = No Load Loss [W] The efficiency profile is obtained from varying the load from zero-load to max-load and subtracting the energy loss. Figure 2 below illustrates the efficiency profile of an arbitrary transformer. For low load levels you can see that the efficiency are generally poor but gets high as soon as load level passes 20%. The no load loss stays constant and load loss are exponentially dependent on the load. Figure 2. Efficiency profile of an arbitrary transformer 2 5 Spring 2013 PART 1 2.4 AMORPHOUS METAL TRANSFORMERS – REDUCING NO LOAD LOSS According to the project ”Strategies for development and diffusion of energy efficient distribution transformers”, by IEE the no-load losses account for more than 70% of total losses in Europe, as can be seen in Figure 4 below. The transformer core designs are dominated by conventional Regular Grain Oriented (RGO) silicon steel. The no load loss can be reduced by choosing amorphous metal as the material of the transformer-core. Amorphous metal transformers are energy efficient transformers with a core consisting of amorphous alloy. Amorphous alloys are different from conventional material in distribution transformer cores, RGO, in the sense that the core alloy has a structure of metal atoms that occurs in a random pattern unlike the conventional type. RGO has an organized grain structure with much higher resistance to magnetization cycles, which leads to higher core losses. Amorphous alloy has higher magnetic susceptibility, lower coercivity and high electrical resistance. The high resistance leads to decreased losses due to eddy currents when subjected to an alternating magnetic field. The higher magnetic susceptibility makes magnetization of the core more efficient and the lower coercivity leads to easier demagnetization and the magnetization becomes faster in response to the magnetic field. According to ABB´s ”Green 3 distribution transformer program”[ ] no-load losses can be reduced by up to 70 % using amorphous alloys, some of their versions are presented in section 2.7.3. 6 Spring 2013 PART 1 2.5 TRANSFORMER EFFICIENCIES IN EUROPE In the project ”Strategies for development and diffusion of energy efficient distribution transformers”, IEE have conducted a survey of losses and efficiencies in Europe where a comparison were made between already installed ”population” with transformers available on the local market. In this project they demonstrated differences for EU-27 and Norway; this is illustrated in Figure 3 and Figure 4. The efficiency of installed transformers compared to what’s avilable on the market appears to differ quite variably in Europe and Scandinavia is in the top percentile of effectiveness. Figure 3. Operating efficiency of distribution sector distribution transformers, EU-27 and Norway As can be seen in Figure 4 the no load losses generally dominates as contributor to the transformer losses in Europe and UK, DE and FR have the highest losses in commensurate to their installed power. No load losses especially can be reduced, in some cases by as much as 70% with some of today’s transformers on the market, it is easy to see a potential for efficiency improvements. Figure 4. Breakdown of distribution sector distribution transformer losses, EU-27 and Norway 7 Spring 2013 PART 1 2.6 EUROPEAN EFFICIENCY STANDARD EN 50464-1 Unlike many countries around the world, Europe has no mandatory standard on energy efficiency of distribution transformers. The main document which describes losses in transformers are the European Standard EN 50464-1. This is a superseded document from the standard HD428 for oil cooled transformers which is still valid. The values are divided into two types of losses: no load-loss and load-loss respectively. They are categorized by three efficiency levels; A, B, C for different power-ratings, primary voltages and short circuit impedances. Figure 5 illustrates No Load loss standard for distribution transformers in Europe. Drytype 24kV No load loss standard 4 3,5 3 [kW] 2,5 A, Noload-loss 2 B, Noload-loss 1,5 C, Noload-loss 1 0,5 0 50 100 160 250 400 630 1000 1600 2500 [kVA] Figure 5. No load loss standard for distribution transformers in Europe Figure 6 illustrates load loss standard for distribution transformers in Europe. Drytype 24kV load loss standard 35 30 [kW] 25 20 A, Load-loss 15 B, Load-loss C, Load-loss 10 5 0 50 100 160 250 400 630 1000 1600 2500 [kVA] Figure 6. Load loss standard for distribution transformers in Europe 8 Spring 2013 PART 1 2.7 ON THE MARKET A market research for distribution transformers has been performed and is presented in this chapter. It should be stated that data below are represented for full-load operation and phase angle=0°. In cases of lower load levels, the efficiencies decrease since the no load loss stays constant while the load loss decreases. Because the no load loss stays constant the load loss does not decrease in the same rate as the load, and thus get a larger share. The condition that the phase angle are equal to zero means that a straight conversion between active and reactive power are possible and thus the efficiencies can be calculated straight off with values given in both VA and W. The data are grouped by power rating [kVA] so that comparisons in loss between transformers with the same power rating may be performed. No load and load loss are combined to create a range of products that differs in efficiency as shown in figure 7 below. As can be seen, products with the same power-rating differ about 0.2% in efficiency. For higher power ratings the difference increases. Compare transformers with the same power rating. 9 Spring 2013 PART 1 2.7.1 ABB TRANSFORMERS Dry-isolated transformers ABB, 12kV 12kV Dry-isolated transformers, ABB 30 25 [kW] 20 15 Noload-loss 10 Load-loss 5 3150 3150 3150 2500 2500 2500 2000 2000 2000 1600 1600 1600 1250 1250 1250 1000 1000 1000 800 800 800 630 630 630 0 [kVA] Figure 7. No load-loss and load loss for different ABB Power Distribution Transformers 12kV Dry-isolated transformers, ABB 0,994 0,992 0,99 0,988 Efficiency 0,986 0,984 3150 3150 3150 2500 2500 2500 2000 2000 2000 1600 1600 1600 1250 1250 1250 1000 1000 1000 800 800 800 630 630 630 0,982 [kVA] Figure 8. Efficiencies for different ABB Power Distribution Transformers 10 Spring 2013 PART 1 Dry isolated transformers ABB, 24kV 24kV Dry-isolated transformers, ABB 25 [kW] 20 15 Noload-loss 10 Load-loss 5 3150 3150 3150 2500 2500 2500 2000 2000 2000 1600 1600 1600 1250 1250 1250 1000 1000 1000 800 800 800 630 630 630 0 [kVA] Figure 9. No load loss and load loss for different ABB Power Distribution Transformers 24kV Dry-isolated transformers, ABB 0,993 0,992 0,991 0,99 0,989 0,988 0,987 0,986 0,985 0,984 0,983 3150 3150 3150 2500 2500 2500 2000 2000 2000 1600 1600 1600 1250 1250 1250 1000 1000 1000 800 800 800 630 630 630 Efficiency [kVA] Figure 10. Efficiencies for different ABB Power Distribution Transformers 11 Spring 2013 PART 1 Liquid filled transformers ABB, 10kV 10kV Liquid filled transformers, ABB 25 [kW] 20 15 Noload-loss 10 Load-loss 5 2000 1600 1600 1250 1250 1000 1000 800 800 630 630 630 630 500 400 400 400 250 250 200 200 160 160 100 100 50 50 0 [kVA] Figure 11. No load loss and load loss for different ABB Power Distribution Transformers 10kV Liquid filled transformers, ABB 0,995 0,99 0,985 0,98 0,975 Efficiency 0,97 0,965 2000 1600 1600 1250 1250 1000 1000 800 800 630 630 630 630 500 400 400 400 250 250 200 200 160 160 100 100 50 50 0,96 [kVA] Figure 12. Efficiencies for different ABB Power Distribution Transformers 12 Spring 2013 PART 1 Liquid filled transformers ABB, 20kV 20kV Liquid filled transformers, ABB 25 [kW] 20 15 Noload-loss 10 Load-loss 5 2000 1600 1600 1250 1250 1000 1000 800 800 630 630 630 630 630 500 400 400 400 250 250 200 200 160 160 100 100 50 50 0 [kVA] Figure 13. No load loss and load loss for different ABB Power Distribution Transformers 20kV Liquid filled transformers, ABB 0,995 0,99 0,985 0,98 0,975 Efficiency 0,97 0,965 2000 1600 1600 1250 1250 1000 1000 800 800 630 630 630 630 630 500 400 400 400 250 250 200 200 160 160 100 100 50 50 0,96 [kVA] Figure 14. Efficiencies for different ABB Power Distribution Transformers 13 Spring 2013 PART 1 2.7.2 SIEMENS TRANSFORMERS Dry-isolated transformers, Siemens 10kV Dry-isolated transformers, Siemens 10kV 30 25 kW 20 15 Noload-loss 10 Load-loss 5 6300 5000 3150 4000 2000 2500 100 100 160 160 250 250 315 315 400 400 500 500 630 630 800 800 1000 1000 1250 0 Figure 15. No load loss and load loss for different Siemens Power Distribution Transformers Dry-isolated transformers, Siemens 10kV 1 0,995 0,99 0,985 0,98 0,975 0,97 0,965 6300 6300 5000 5000 4000 3150 3150 2500 2000 1600 1250 1000 1000 800 800 630 630 500 500 400 400 315 315 250 250 160 160 100 100 Efficiency Figure 16. Efficiencies for different Siemens Power Distribution Transformers 14 Spring 2013 PART 1 Dry-isolated transformers, Siemens 20kV Dry-isolated transformers, Siemens 20kV 60 50 kW 40 30 Load-loss 20 Noload-loss 10 4000 5000 5000 6300 6300 8000 8000 10000 10000 12500 12500 16000 16000 1000 1250 1600 2000 2500 3150 630 800 315 400 500 160 250 100 0 Figure 17. No load loss and load loss for different Siemens Power Distribution Transformers Dry-isolated transformers, Siemens 20kV 1 0,995 0,99 0,985 0,98 0,975 0,97 0,965 16000 16000 10000 10000 12500 12500 6300 6300 8000 8000 2500 3150 3150 4000 4000 5000 5000 800 1000 1000 1250 1600 630 315 400 400 500 250 100 100 160 Efficiency Figure 18. Efficiencies for different Siemens Power Distribution Transformers 15 Spring 2013 PART 1 Dry-isolated transformers, Siemens 30kV Dry-isolated transformers, Siemens 30kV 60 50 [kW] 40 30 Noload-loss 20 Load-loss 10 16000 16000 12500 12500 10000 10000 8000 8000 2500 2000 1600 1250 1000 800 630 500 500 400 315 250 0 [kVA] Figure 19. No load loss and load loss for different Siemens Power Distribution Transformers Dry-isolated transformers, Siemens 30kV 1 0,995 0,99 0,985 Efficiency 0,98 0,975 16000 16000 12500 12500 10000 10000 8000 8000 2500 2000 1600 1250 1000 800 630 500 500 400 315 250 0,97 Figure 20. Efficiencies for different Siemens Power Distribution Transformers 16 Spring 2013 PART 1 2.7.3 ABB AMORPHOUS METAL CORE TRANSFORMER Conclusions from the market research on Regular Grain Oriented (RGO) distributions transformers above is that differences are not significant in the regular product portfolio from ABB or Siemens. They have however one type of product available on the market and that is transformers with amourphous metal cores (AMC). Table 1 below show comparisons in the no load loss between these two types of transformers for several power ratings. Table 1 Energy performance comparison between a RGO and an AMC for different power ratings4. Rating No-Load No-Load Loss (kVA) Losses (W) Losses Reduction Regular Amorphous Grain Metal Core Oriented Single Phase 15 55 20 64% 25 65 30 54% 50 105 35 67% 75 155 55 65% 100 200 75 63% 167 235 95 60% 300 505 200 60% 500 725 220 70% 750 1125 355 68% 1500 2170 725 67% 2500 2750 745 73% Three Phase 17 Spring 2013 PART 1 3 CABLES – A METHOD FOR COMPARISON This chapter concerns cables and presents a method for choosing the best cable-energy-alternative. When comparing cable investments with energy losses related to them, the sectional area of the cable is of utmost importance. The energy loss due to heat dissipation depends on the cross area as follows5: Eq. 3 Where I = current [A] = electrical resistivity of the material [Ω ] 2 A = cross area [m ] l = length [m] As can be concluded from the equation, increased cross area leads to decreased losses. From an investment point of view the decreased energy loss must be weightened against a higher price of investment. To compare these variables and optimize the investment, linear graphs over the total ownership costs should be executed and compared. The best investment is the one having the lowest TOC after a chosen period of time. Total Ownership Cost Comparision Higher price, lower loss Cost Lower price, higher loss Time Figure 21. Principle chart over the method of a TOC comparison To obtain a graph over the TOC, the cost due to energy loss is gradually added to the capital of investment which is where the lines cross the y-axis. The cost of the annual losses is estimated by choosing a certain price for electricity and then multiplying this by the energy loss from the chosen time period. 18 Spring 2013 PART 1 4 BREAKERS AND INTERRUPTERS This chapter describes as well as the function, also based on a market research and qualitative interview discusses energy performance of electric power interrupters. 4.1 INTERUPTER DESIGN Interrupters have the task of breaking emerging currents. This requires interrupter designs based on mediums that can quench the upcoming arc. The two most common types are: - SF6 interupters This construction works according to the puffer principle where a compression chamber – puffer chamber - follows the contact movement, compresses the SF6 gas and via a nozzle, length blows the channel of the arc. - Vacuum interrupters A competitor to SF6 interrupters at voltages up to 36 kV is the vacuum interrupter. It is slightly more expensive but in return it has significantly lower audit interval. It is particularly suitable for installations where the breaking frequency is high. Because of the closed structure, it is suitable in corrosive and explosive atmosphere. It operates at high vacuum – which interrupts the current arc. 4.2 INTERRUPTER ENERGY PERFORMANCE An interview with Per Johnsson, interrupter-specialist at ABB was performed on april 11, 2013. The conclusions from that survey were that the biggest difference in interrupter performance is mostly a question of functionality and endurance. Construction-differences lies in which method that is used for breaking the current; oil-, SF6- or vacuum-based. During operation all interrupters hold good connectivity to the conductors and sometimes even lower resistance than the cables connected to the interrupter, because of a larger conductor cross area in the interrupter. The energy Switch-disconnectors ABB, 750 V current for a fix power rating. Since resistance 0,9998 2 0,99975 lower current. Energy loss and efficiencies of 0,9997 interrupters with different power ratings are shown in figure 22 to the right. As can be seen, the interrupters hold high efficiencies. Efficiency 24000 energy loss depends on R*I the loss is reduced by 93750 0,99985 86250 conductors increased voltage enables a lower 60000 0,9999 47250 loss, and thus to decrease energy losses, as for all 30000 loss in interrupters are dominated by resistivity 0,99995 [W] Figure 22. Efficiencies for different ABB switch disconnectors 19 Spring 2013 PART 1 5 ADJUSTABLE SPEED DRIVES This chapter describes function and energy performance of adjustable speed drives. 5.1 ASD DESIGN An adjustable speed drive (ASD) is a device that controls the rotational speed of motor-driven equipment. Variable frequency drives (VFDs), the most common type of ASD, are solid-state electronic motor controllers that efficiently meet varying process requirements by adjusting the frequency and voltage of power supplied to an alternating current (AC) motor to enable it to operate over a wide speed range. External sensors monitor flow, liquid levels, or pressure and then transmit a signal to a controller that adjusts the frequency and speed of the motor to match process requirements. As illustrated in figure 23 below variable frequency drives consists of three steps: rectification, filtering and inverting DC to AC. The first step converts the AC power to DC, the second smoothens the DC and the third creates AC with adjustable frequency which varies the rotating speed of machines. It is the first step, rectification which converts AC line voltage to DC voltage output by superimposing non-linear half-phase current pulses thus creating harmonic current distortion, and hence voltage distortion, of the AC line input. More about this in section 6.1.1 (Rectification). Figure 23. Illustration of a Variable Frequency Drive three step process. 20 Spring 2013 PART 1 5.2 VFD ENERGY PERFORMANCE Typical efficiencies of VFDs are presented in table 2. These efficiency values may be considered representative of “typical” PWM VFD performance. As can be seen, the VFD efficiency decreases with decreasing load and the decline in efficiency is more pronounced with drives of smaller horsepower ratings. In this thesis, the connection between VFDs and harmonic issues has been of major concern. The rectification process is presented in section 6.1.1 and the impact of harmonics in powersystems is discussed in chapter 6. Table 2 Typical efficiencies of VFDs6 Variable Efficiency % Frequency Load, Percent of Drive Rated Power Output Drive 1.6 12.5 25 42 50 75 100 5 35 80 88 91 92 94 95 10 41 83 90 93 94 95 96 20 47 86 93 94 95 96 97 30 50 88 93 95 95 96 97 50 46 86 92 95 95 96 97 60 51 87 92 95 95 96 97 75 47 86 93 95 96 97 97 100 55 89 94 95 96 97 97 200 61 81 95 96 96 97 97 Hp rating 21 Spring 2013 PART 2 6 HARMONICS This chapter describes basic characteristics about harmonics in electric power systems; causes, effects and mitigationtechniques are presented. Harmonics are disturbances in the voltage and current of the grid, which has a frequency which is a multiple to the fundamental frequency. (The Swedish electricity network has a fundamental frequency of 50 Hz.) Usually, harmonic amplitude decreases with increasing frequency and is specified as fraction of the fundamental amplitude according to the diagram below: Figure 24 Harmonic presentation; as a fraction of the fundamental current7. THD - Total Harmonic Distortion is a measurement of the general level of harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. It is expressed mathematically as: Eq. 3 Were Y is a signal of the current, voltage or power. Y 1 is the amplitude of the fundamental signal and Yi is the amplitude of each harmonic in a chosen frequency band. THD is useful to get an index over just how distorted the signal are and it is tempting to try to use if for calculations of different entities such as energy loss, level of stress on equipment and draw conclusions from that. But the fact is, certain harmonics affect certain entities in different ways. This will be evaluated in the following sections. 22 Spring 2013 PART 2 6.1 CAUSES The distortion of the current or voltage wave means that the distribution of electrical energy is disturbed and power quality is not optimum. Harmonic currents are caused by non-linear loads connected to the distribution network. The flow of harmonic currents causes harmonic voltages via distribution-network impedances and consequently distortion of the supply voltage. Examples of devices that cause harmonics: - Industrial equipment (welding machines, arc furnaces, induction furnaces, rectifiers) - Adjustable-speed drives for asynchronous or DC motors - UPSs - Office equipment (computers, photocopy machines, fax machines, etc.) - Home appliances (television sets, micro-wave ovens, fluorescent lighting) - Certain devices involving magnetic saturation (transformers) When harmonics are generated in some parts of a power system, every other part of the system is affected. Motors experience counter electromotive force, transformers, cables, and capacitors experience increased heat-loss. Figure 25 show the harmonics spreading and adding up from the load branches back into the distribution-system through the transformer. 23 Spring 2013 PART 2 Figure 25. Harmonics spreading through the distribution system8 24 Spring 2013 PART 2 6.1.1 RECTIFICATION The common factor for most of harmonic emitting devices is rectifiers9. This is a power electronic component that is used for converting alternating currents (AC) to direct current (DC). The process in which it operates is called rectification. When alternating current is connected to a tyristor, it only lets through current and voltage in one direction, creating a pulsating direct current. This technology can be extended to three phases and connected as a six pulse converter as illustrated in Figure 26 below. Figure 26 6 pulse AD/DC rectifyer10. The resulting signal Ud shown in Figure 27 drives a pulsating DC current Id forward, but also backward –Id due to V4, V6 and V2. This backward signal - Id is in this construction pushed back into the AC-signals and becomes harmonics in the original waveforms ILN1, ILN2 and ILN3. Figure 27 Resulting currents and voltages11. Rectification can be performed by any number of pulses for integers that are multiples to the number 6, the most common are 6 as described above, 12, 18, 24 and 48 are mostly used for higher power levels because of increased ivestment cost. 25 Spring 2013 PART 2 6.1.2 TWELVE PULSE RECTIFICATION Six pulse rectifiers produce high levels of harmonics, about 30% THD on both the DC and AC side. 12-pulse rectifiers are often used in installations that demands lower levels of harmonics, about 12% THD. As shown in figure 30 a 12-pulse rectifier consists of two 6-pulse bridge circuits connected in series. It has its AC connections fed from two supply transformers with a 30 degree phase shift between the two bridges, this is illustrated in figure 31. In this way, many of the characteristic harmonics produced by the six-pulse bridges are cancelled. The 30-degree phase shift is usually achieved by a transformer with two sets of secondary windings, one with D connection and one with Y connection. Figure 28. Line diagram over a 12-pulse rectifyer Figure 29. Resulting voltage with 30 degrees phase shift creating a smoother DC than a 6-pulse rectifyer Lower harmonics such as the 5th and 7th, which normally contributes the most to the THD are due to the phase shift eliminated because the Y current are opposite to the D current for the specific harmonics and this consequently enables them to cancel out each other. Power system converters have a practical limitation of 12 pulses because of the large expense of producing high-voltage transformers with the appropriate phase shifts. 26 Spring 2013 PART 2 6.2 EFFECTS The effects of harmonics can be divided into three components: joule losses caused by additional current waveforms, joule losses caused by the skin effect and RMS increase of the waveform due to harmonics. In addition to the joule losses certain harmonics cause negative torque in motors and pumps. The measured current is not the same as the useful and this means that when utilities measure their active power consumption and the signal contains harmonics, they pay for unused energy. The measured power is a function of the fundamental component I1 with all the harmonics added. When the current contains harmonics, the RMS value of the current Irms is greater than the fundamental I1. This leads to a higher level of power consumption than what would be measured if the signal contained no harmonics. Filtering the signal leads to lower current RMS. In the case study in this thesis such an investment is evaluated. In the same way that the fundamental current provokes joule losses the harmonic currents also provoke an increase in the joule losses in all conductors in which they flow and additional temperature rise in transformers, devices cables, etc. Figure 30 below shows as a function of the total harmonic distortion: - The increase in the current RMS for a conductor containing a given fundamental current - The increase in joule losses, not taking into account the skin effect Figure 30. Increase in current RMS and increase in joule losses as a function of THD 12. Equipment connected to the current network is designed to operate with an ideal sinusoidal 50 Hz fundamental frequency voltage. This means that when there is multiples of the fundamental current frequency the network device cannot take up the energy that contains in these harmonics. In other words, when the equipment is not adapted to take advantage of the harmonic energy will lead to untapped power. Because it is unused energy but get debited anyway it is said to be a loss. 27 Spring 2013 PART 2 The increased joule heating due to harmonics leads to over dimensioned components because the heating causes temperature rise. Harmonic currents flowing in the transformer produce an increase in the load loss due to the joule effect and increased no load losses due to eddy currents. The harmonic voltages are responsible for "iron" losses due to hysteresis. It is generally considered that the losses in the windings increases as the square of the THD and the core losses increase linearly with THDu. The utility distribution transformers, where distortion levels are limited, the losses increase between 10 and 15%. In capacitors the harmonic voltages applied to capacitors provoke the flow of currents proportional to the frequency of the harmonics. These currents cause additional losses. The 5th and 11th harmonics are also of particular concern to industry today [13]. Although the 5th harmonic is much more prevalent, both have a negative sequence. This means that when distorted voltage containing the 5th or 11th harmonic is applied to a 3-phase motor, it will attempt to drive the motor in reverse, creating a negative torque. In order to compensate for this negative torque, the motor must draw additional current. This, in turn, can cause overheating and/or the tripping of over-current protection devices. 6-Pulse adjustable speed drives are a major source of the 5th, 7th and 11th harmonics. 12Pulse drives are significantly more expensive, and are a source of the 11th and 13th harmonics, but through their design are able to eliminate the 5th and 7th. The negative torque created by the 5th and 11th can be calculated as an economic loss, such an analysis would require harmonic values at each pump and motor in a facility, due to a lack of time and resources it is left out of this thesis. 28 Spring 2013 PART 2 6.3 INCREASED FREQUENCY ⇾ INCREASED LOSSES - THE SKIN EFFECT Harmonics are per definition steady state waveforms with frequencies that are integer multiples of a fundamental waveform frequency, the order of each harmonic are the integer multiple that corresponds to each harmonics frequency. The attribute that separates each harmonic from the other are thus the frequency. Higher order means higher frequency. This aspect is one of the reasons for harmonics to cause increased losses in electrical equipment. The skin effect causes the current to experience higher effective resistance for higher frequencies. Skin effect is the tendency for an alternating electric current AC to become distributed within a conductor such that the density is largest near the surface of the conductor, and thus decreases with greater depths in the conductor. The cause of the skin effect is electromagnetic induction. As shown in figure 32 a Figure 31. Cross section of a conductor. Current flows mainly outside of δ. time-varying magnetic field H around the main current I is accompanied by a time-varying induced electric field, which in turn creates secondary time-varying currents Iw and a secondary magnetic field. These induced currents produce a magnetic flux which is opposite to the external flux that produced the induced currents. This opposing magnetic flux is called counter electromotive force. The counter EMF is strongest at the center of the conductor, and forces the electrons to the skin of the conductor. A higher frequency intensifies this phenomenon and causes a thinner skin depth δ. The skin depth is defined as the depth below the surface of the conductor at Figure 32. Current flow I in a conductor which the current density has fallen to 1/e, about 0.37 of Js. Skin depth can be with counter EMF illustrated, resulting in the skin effect. expressed as14: Eq. 4 where = resistivity of the conductor [Ω ] = frequency of the current [Hz] = absolute magnetic permeability of the conductor [ ] 29 Spring 2013 PART 2 Energy loss as heat dissipation in a conductor is dependent of effective resistance and current according 15 to Joules Law : Eq. 5 where I = current [A] Reff = effective resistance [Ω] Effective resistance in a conductor can be expressed according to Pouillet’s law as: Eq. 6 where length [m] = resistivity of the material [Ω ] A = cross-area [m2] The effective resistance Reff in a conductor due to the skin effect can be solved by approximating that the current flows uniformly through a layer of thickness corresponding to the skin depth and thus approximate it as a cylindrical flow. This changes the cross area and Eq. 6 derives to: Eq. 7 where length [m] = resistivity of the material [Ω ] D = diameter of the conductor [m] = skin depth [m] 30 Spring 2013 PART 2 Combining Eq. Eq. and Eq. derives the expression: Eq. 8 This equation gives energy losses due to heat dissipation in a conductor for a certain frequency when taking into account the skin effect. As can be seen in the equation, increased frequency leads to increased energy loss. These equations and especially Eq. 8 are used when calculating the energy loss caused by harmonics in the case study. 31 Spring 2013 PART 2 6.4 ENERGY LOSS IN COMPONENTS DUE TO HARMONICS 6.4.1 TRANSFORMERS Energy loss due to harmonics in transformers is expressed as: Eq. 9 Where Pn = nominal load loss [W] Kw = iron loss coefficient = 0.04 Ih = current of harmonic order h (in p.u.) h = harmonic order Kh1 = 6.4.2 CABLES Energy loss due to harmonics in cables is expressed as: Eq. 10 Where L = length of cable [m] Rh = resistance of cable to harmonic order h [Ω] Ih = current of harmonic order h (in p.u.) 32 Spring 2013 PART 2 6.4.3 CAPACITORS Energy loss due to harmonics in capacitors is expressed as: Eq.11 Where PNC = losses in the capacitor excluding loss caused by harmonics [W] h = harmonic order Vh = voltage of harmonic order h (in p.u.) 33 Spring 2013 PART 2 7 SOLUTIONS TO ATTENUATE HARMONICS In order to eliminate problems in facilities due to harmonics, attenuation is necessary. This chapter describes different solutions for managing harmonics. An economical analysis of harmonic filter solutions are presented in chapter 10. 7.1 BASIC SOLUTIONS 7.1.1 POSITION THE NON -LINEAR LOADS UPSTREAM IN THE SYSTEM As the short-circuit power decreases the overall harmonic disturbances increase. Not taking economic considerations into account, the preferable solution is to connect the non-linear loads as far upstream as possible. Figure 33 below illustrates a system with the non-linear loads connected above the sensitive loads. Figure 33. Power system with the non-linear loads connected above the sensitive loads 16 7.1.2 GROUP THE NON -LINEAR LOADS Create systems with non-linear loads separated from linear loads. Figure 34 show connections that should be avoided in order to prevent harmonic disturbances on sensitive loads. Figure 34. Connections that should be avoided in order to prevent harmonic disturbances on sensitive loads 17 34 Spring 2013 PART 2 7.1.3 CREATE SEPARATE SOURCES One solution can be to separate non-linear loads from sensitive loads via separate transformers and the linear loadtransformer-impedance will protect the linear loads from harmonics emitted from the non-linear loads. Figure 35 show this kind of connection. This solution is appropriate only if it the cost for the extra transformer are weighed against the positive impact on the system. Figure 35. Separation of non-linear loads from sensitive loads via separate transformers 18 7.1.4 TRANSFORMERS WITH SPECIAL CONNECTIONS Elimination of certain harmonic orders can be performed by different types of transformer connections: - A Dyd connection suppresses 5th and 7th harmonics (see Figure 36) - A Dy connection suppresses the 3rd harmonic - A DZ 5 connection suppresses the 5th harmonic Figure 36. A Dyd connection which suppresses 5th and 7th 19 35 Spring 2013 PART 2 7.2 HARMONIC FILTERING 7.2.1 PASSIVE FILTERS Passive filters are LC circuits which are tuned to each harmonic order to be filtered. When installed in parallel with the nonlinear load, it provides a low impedance path for the major harmonic currents demanded by the drive. This reduces the amount of harmonic current flowing through the distribution system and results in improved power factor, lower RMS currents, lower harmonic current distortion, lower harmonic voltage distortion, and increased system capacity. Figure 37 below illustrates the setup of a passive filter and how it is connected to the system. The drive generates the harmonics and sends it back into the system and is then absorbed by the filter due to the lower impedance for chosen frequencies. Without the filter, these frequencies would pass through the transformer and cause increased heat dissipation and increased RMS value of the power. This solution are good for systems with harmonic levels which is easily predicted, e.g. in a system with 6-pulse rectifiers (AC/DC converters) to all of its drives can be predicted to have high levels of 5th and 7th harmonics, and thus a filter tuned for absorbing these harmonics can be connected. Figure 37. The setup of a passive filter and how it is connected to the system. 20 36 Spring 2013 PART 2 7.2.2 ACTIVE FILTERS Active filters measure each harmonic level by induction and inject the inverse harmonic signal Iharm to attenuate the harmonic distortion. Figure 38 illustrates setup of an active filter and how it is connected to the system. The drive generates the harmonics and sends it back into the system and when it passes through the filter measuring point it is counteracted by the injected current Iharm. Without the filter, these frequencies would pass through the transformer and cause increased heat dissipation and increased RMS value of the power. This is a simple solution because it does not require detailed knowledge regarding the nature of the load, or the type of harmonics present. Although it is generally considered as a more expensive and energy-consuming solution than passive filters, for higher load levels and unpredictable waveforms it is often the preferable solution. Figure 38. Setup of an active filter and how it is connected to the system 21 Figure 39 below describes how an active filter injects the opposite signals to the harmonics present and creates a clean fundamental signal without significant distortion. Figure 39. Active filter opposite signal injection22 37 Spring 2013 PART 2 7.3 INSTALLATION OF HIGHER-PULSE RECTIFIERS The source of problems with harmonic generation is primarily the rectification (AC to DC) process in the loads which sends harmonic back into the system. 6-pulse converters (described in section 6.1.1) are the most commonly used rectification technique in distribution facilities today; 6-pulse rectification generates high levels of 5th, 7th, 11th and 13th harmonics due to the low pulse number. A typical harmonic current spectrum with resulting THD for a 3 phase 6-pulse rectifier is presented in Table 3. Table 3. A typical harmonic current spectrum with resulting THD for a 3 phase 6-pulse rectifier h 1 5 7 11 13 17 19 THD Ih6p,% 100 20 14.3 9.1 7.7 5.9 5.3 28.45% 23 Instead of treating the symptoms, sometimes treating the sources is a more effective method of attenuating problems. One way of doing this in electric power systems, with distorted waveforms as symptoms and rectification as the source; improving the rectification process is a solution. If the pulses in the rectification process are increased in number, the waveform becomes less distorted. A typical harmonic current spectrum with resulting THD for a 3 phase 12-pulse rectifier is presented in Table 34. Table 4 A typical harmonic current spectrum with resulting THD for a 3 phase 12-pulse rectifier24 h 1 5 7 11 13 17 19 23 25 29 31 35 37 THD Ih12p,% 100 1.8 1.6 6.6 5.4 0.33 0.3 1.5 1.3 0.25 0.2 0.8 0.4 9.14% When increasing the number of pulses per period to 24, an even better result can be achieved in decreasing the harmonic generation. A typical harmonic current spectrum with resulting THD for a 3 phase 24-pulse rectifier is presented in Table 5. Table 5 A typical harmonic current spectrum with resulting THD for a 3 phase 24-pulse rectifier25 h Ih24p,% 1 100 5 1.88 7 1.23 11 0.36 13 0.22 23 1.09 25 0.87 47 0.22 49 0.22 THD 2.7% 38 Spring 2013 PART 2 In a comparison between the THD generated by the 6-pulse versus the 12-pulse, the THD generated by a 6-pulse is about 3 times the THD generated by the 12 pulse. The 24 pulse rectifier is superior to the 6 and 12 with its low THD level. A summary of the rectifiers is listed in Table 6 below. Table 6. Summary of rectifiers with corresponding THD Pulse number 6 12 24 THD% 28.45 9.14 2.7 In the choice of new devices like frequency drives to pumps and motors or rectifiers to for example charging equipment it is important to weighted different pulse numbers for the rectification process against price, the resulting harmonics affect the system in many ways. 39 Spring 2013 PART 2 8 COMPUTER MODELING AND ANALYSIS: HARMONIC FLOW This chapter describes the method of how to deal with harmonics in power systems. How to simulate mixing and splitting of waveforms and how to model the system’s equivalent impedances. 8.1 FOURIER ANALYSIS The fourier transform are a transform that is often used to transfer a function from the time-domain to the frequency-domain. The function is expressed as the sum of its sinusoidal basic functions. When applied on periodic functions the results are referred to as fourier series. Every continuous periodic function can be expressed as the sum of a number of sinusoidal functions with varying amplitudes and frequencies that are integer multiples of a fundamental waveform frequency, i.e. the same characteristics as for harmonics. A fourier series takes a signal and decomposes it into a sum of sines and cosines of different frequencies. Definition is as follows26: Eq. 12 Where is the signal in the time domain and are the amplitude of each harmonic. The integer, n, has units of Hertz (1 Hz =1/s) and corresponds to the frequency of the wave. When analyzing harmonic content on a current or voltage the harmonic content on the fundamental waveform are most often represented by the amplitudes in the frequency domain. In some cases, only the percentage of total harmonic content on a wave is of interest and not the specific level for each harmonic. The general level is referred to as THD - Total Harmonic Distortion and is expressed for a signal Y as follows27: Eq. 13 Where Y1 is the amplitude of the fundamental signal and Yi is the specific amplitude for each harmonic, Yi corresponds to the coefficient an and bn in above. The signal Y can be voltage, current or power etc. When modeling harmonic flow in power systems you have to start with a distorted signal. One way of doing this is to represent the signal with a fourier series and add on sinusoids representing harmonics on a fundamental wave. This is one of the applications where fourier series is applicable. 40 Spring 2013 PART 2 Let us take a look at a fundamental sinusoid versus the same wave with its 5th (5 times the fundamental frequency) harmonic added on having 20% of the fundamental amplitude: Fundamental sinusoid with its 5:th harmonic separated Fundamental with its 5:th harmonic added Figure 40 Fundamental sine wave versus its 5th harmonic added, plotted in MatLab So, when a sinusoid with higher frequency is added to the fundamental, the result becomes a fluctuating reference wave. As can be seen from the functions above an infinite number of harmonics can be simulated using fourier analysis just by adding them to the fundamental sinusoid function. The RMS - Root Mean Square of an oscillating signal represents the mean value of the magnitude. In contrast of the amplitude which represents the maximum magnitude of the oscillation. RMS is the most usual way of referring to a certain level of voltage, current or power in AC. It is calculated as: Eq. 14 Where S is an oscillating signal, usually voltage, current or power and n equals the number of magnitude samples for the signal. This equation is used for calculating the RMS difference between a signal with harmonics and one without harmonics in the case study. 41 Spring 2013 PART 2 8.2 HARMONIC SYSTEM STUDY Harmonic studies are used to analyze harmonic situations. They are aimed at detecting resonance and calculating harmonic currents, voltages, phases and distortion levels. For simple systems with bus-number lower than a dozen, spreadsheet calculation are possible. In realistic systems which contain a large amount of buses, the Y matrix which represents the system is large and requires a computer-based approach. The general method for a computer program to perform such a study is as follows: 1. Build the bus admittance matrix Ybus for each harmonic 2. Get the bus impedance matrix for each harmonic, 3. Given the harmonic source current spectrum I(h), find the bus harmonic voltages as 4. Calculate the line harmonic currents as 5. Calculate the voltage and current total harmonic distortion factors (see 8.1) 8.3 THE BUS ADMITTANCE MATRIX The admittance matrix, also known as Y Matrix or Ybus is a matrix describing a power system with n buses. It represents the nodal admittance of the busses in a power system and is constructed as described below: 1. The system single line diagram is converted to an impedance diagram A single line diagram is a simplified notation for representing a three-phase power system. Electric elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols. These symbols can be replaced by impedances and hence it becomes an impedance diagram. 2. All voltage sources are converted to their equivalent current source representations Using Ohms law I=V/Z and converting the voltage sources in the impedance diagram to current sources makes the system more manageable. 3. The impedance diagram is converted to an admittance diagram Invert all impedances: admittance = Y = 1/Z 4. The Y Matrix itself is created 42 Spring 2013 PART 2 8.4 ADMITTANCE The admittance (Y) is a measure of how easily a circuit or device will allow a current to flow. It is defined as the inverse of the impedance (Z). The SI unit of admittance is the siemens (S). Mathematically expressed as: Eq. 15 Where Y = admittance [S] Z = impedance [Ω] 8.5 MODELLING OF POWER LINES When constructing the admittance matrix to represent the system, the admittance of each branch of the bus system is calculated and grouped. To calculate a power line’s admittance, because of the skin effect (6.3) the impedance are specific for each harmonic and increases with the root of the harmonic number. It is calculated according to28: Eq. 16 Furthermore, a power line’s total impedance and admittance are given by: Eq. 17 Where Z(h) = specific harmonic impedance [Ω] h = harmonic number, integer R = resistance [Ω] X = reactance [Ω] L = length of cable [m] This equation is used for calculating the specific admittances in the admittance matrix for each harmonic travelling through the power system. 43 Spring 2013 9 CASE STUDY: SKF POWER DISTRIBUTION SYSTEM In this chapter the case study system: SKF´s distribution system is described. 9.1 SYSTEM OVERVIEW The case study system is a part of SKF´s power distribution system which consists of several stations of 11000V/420V transformers. The station this analysis is going to be performed in consists of 5 such transformers. Figure 41 Overview of the case study system. to the right illustrates components connected below T5, one of the transformers in the station to be examined. Four feeding lines L51, L52, L53 and L54 supplies equipment such as fluorescent lamps, frequency controlled motors and pumps and other harmonic emitting loads. L55.2 and L55.3 contains two capacitors for phase compensating purpose. Energy losses occur in each of these components and are increased by the order of the harmonics. When calculating the energy loss caused by harmonics in power components, the levels of harmonics have to be known at each single component before qualified results can be delivered. In this case, measurements at L05.1 have been used for calculating increased RMS value of current and voltage in the debiting meter; these values are also used calculating T5 and C1-2 heat loss. Measurements on the feeding lines L51-54 has been Figure 41 Overview of the case study system. used for calculation of cable heat loss. 44 Spring 2013 9.2 SYSTEM COMPONENTS 9.2.1 TRANSFORMER (T5) Table 7 show nominal values of the transformer T5 in the case study system. Table 7. Nominal data for transformer (T5) Rated power kVA 1600 Un1 In1 Un2 In2 Hz V A V A 50 11000 83,98 420 2199,43 No-load losses Un2 Io Io% Po V A % W 420 6,62 0,30 1879 Load-losses Usc In1 Psc V A W 614,45 83,98 12988 Rated frequency High voltage Low voltage 45 Spring 2013 9.2.2 2 × CAPACITOR (C1, C2) Table 8 show nominal values of the capacitors C1 and C2 in the case study system. Table 8. Nominal data for each capacitor; C1 & C2 Rated power kvar 300 Rated voltage Rated frequency Rated current Rated capacitance Rated inductance Power loss Tuning frequency Icw Conncection Discharge time Temperature category Insulation level Ip code Standard Serial no V Hz A W Hz kA 400 50 482 120 141/12.6 % 25 D 60 0/+40 3/IP20C EN60439-1 F0300550 s Cables: 4 x 1000A cables: L51, L52, L53 and L54 Splints: Splint “Skena A5-0,4” collecting signals from L51, L52, L53 and L54 46 Spring 2013 9.3 MEASUREMENTS OF HARMONICS A summary of information about the instruments involved, measurement period and method. 9.3.1 INSTRUMENTS Type: Datalogger Product: Dranetz PX5 Power Explorer 4 x Probes: Flex probes 0-3000A Software: Dran-View, analyzing measurement results. 9.3.2 METHOD Harmonic levels have been measured on the feeding lines L51-54 and also on the debiting meter L05.1. RMS-value of voltage, current, power, harmonics and also phases are measured with 10 minutes interval. For every interval phase angle, RMS average-, RMS maximum and RMS minimum is registered. All odd, mild and severe events are directly recorded for thorough analyzes. 9.3.3 PERIOD 25.03.2013 – 22.04.2013 Total time: 29 days SUB PERIODS L51 & L52 was measured 25/3 – 02/4 L53 was measured 02-11/4 L54 & L05.1 was measured 11-22/4 9.3.4 QUALITY OF MEASUREMENTS Even thought the measurements were performed in sub periods, the calculations that were made based on them were separately performed and presented. No assumptions of even load levels had to be made. 47 Spring 2013 10 RESULTS This chapter presents the results for measurements, calculations derived from the measurements and economic analysis of energy efficient components. 10.1 MEASUREMENTS Harmonic levels in SKF´s distribution facility has been measured at L05.1. These measurements are the basis for calculation of signal RMS increase, filter investments and extra heat dissipation in transformer T5, cables L51-54 and capacitors C1 and C2. Measurement results of current harmonics are given in figure 42, and voltage harmonics in figure 43. Phase A is represented by red bars, B by green and C by blue. THD for each parameter is given to the left in each figure. The fundamental components 50Hz is left out of the presentation to enable a more detailed view of the harmonics, they are listed in table 9. Figure 42. Current harmonics at L05.1. Figure 43. Voltage harmonics at L05.1. 48 Spring 2013 10.2 HARMONIC FILTER INVESTMENTS This chapter show how filtering of harmonics affect the RMS value of the current, voltage and hence the active power which are debited for in the transformer station at SKF. The signals have been represented by fourier series in Matlab. 10.2.1 NO FILTER Figure 44 below are illustrating comparisons between the fundamental 50Hz, 3-phase current waveform and the distorted current based on the measured harmonic levels presented in section 10.1. Figure 45 illustrates the voltage. Figure 44. No filter: Fundamental current versus distorted. Figure 45. No filter: Fundamental voltage versus distorted. 49 Spring 2013 Table 9 below show values for calculation of active power with harmonics. The RMS values of the fourier represented signals has been calculated using Matlab command “rms()”. Table 9. Distorted, nonfiltered values [Deg] Irms [A] Vrms [V] Phase 1 451.82 166.46 15.8 Phase 2 444.97 166.58 16.73 Phase 3 423.36 166.62 18.83 W Table 10 below show values for calculation of active power with no harmonics. Table 10. Fundamental values [Deg] Irms [A] Vrms [V] Phase 1 448.88 166.4 15.8 Phase 2 443.11 166.62 16.73 Phase 3 421.88 166.55 18.83 209184 W This gives the power increase caused by the harmonics; the difference between the active power with respectively without harmonics: 974 W 50 Spring 2013 10.2.2 PASSIVE FILTER The figures below are illustrating comparisons between the fundamental and filtered, 3-phase current waveform. It simulates a passive filter tuned to eliminate the 5th, 7th, 11th and 13th which are the dominating current harmonics. Figure 46. Passive filter; fundamental versus passive filter current. The voltage harmonics are created by the current harmonics, so when filtering the current, implicitly the voltage is also filtered. Figure 47 below illustrates a small difference between the fundamental and the distorted voltage. Figure 47. Passive filter; fundamental versus passive filter voltage. 51 Spring 2013 Table 11 below show values for calculation of active power with less harmonics; the 5th, 7th, and 11th and 13th are eliminated by a passive filter. The RMS values of the fourier represented signals has been calculated using Matlab command “rms()”. Table 11. Distorted, passive filter values [Deg] Irms [A] Vrms [V] Phase 1 450.20 166.40 15.8 Phase 2 443.30 166.62 16.73 Phase 3 421.70 166.64 18.83 W The harmonic active power is the difference between the active power with a passive filter respectively without harmonics; 122 W 52 Spring 2013 10.2.3 PASSIVE FILTER INVESTMENT The difference between the harmonic power with respectively without a passive filter gives what savings is made by installing the filter; 852W Table 12. Investment conditions for the payoff analysis Electricity price [SEK/kWh] Interest rate [%] Investment Cost [SEK] 0,8 5 103000 Earning per year: 5971 SEK/year Payoff time with 5% interest then becomes: 35 years Passive filter payoff diagram 20000 0 -20000 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 -40000 -60000 -80000 -100000 -120000 Cashflow Figure 48. Payoff diagram for a passive filter investment. 53 Spring 2013 10.2.4 ACTIVE FILTER The figures below are illustrating comparisons between the fundamental, 3-phase current waveform and the distorted. It simulates an active filter that eliminates 97% of all harmonics. Figure 49. Filtered: Fundamental versus active filter current. The voltage harmonics are created by the current harmonics, so when filtering the current, implicitly the voltage is also filtered. Figure 26 below illustrates a very small difference between the fundamental and the distorted voltage. Figure 50. Filtered: Fundamental versus active filter voltage. 54 Spring 2013 Table 13 below show values for calculation of active power with 97% of all harmonics eliminated by an active filter. The RMS values of the fourier represented signals has been calculated using Matlab command “rms()”. Table 13. Distorted active filter values. [Deg] Rrms [A] Arms [V] Phase 1 449.88 166.40 15.8 Phase 2 443.11 166.62 16.73 Phase 3 421.52 166.64 18.83 W The difference between the active powers with respectively without harmonics gives us what power the harmonic contributes: 0.1W 55 Spring 2013 10.2.5 ACTIVE FILTER INVESTMENT The difference between the harmonic powers with respectively without a passive filter gives what savings is made by installing the filter; 974 W Table 14. Investment conditions for the payoff analysis Electricity price [SEK/kWh] Interest rate [%] Investment Cost [SEK] 0,8 5 168000 Earning per year: 6833 SEK/year Payoff time with interest 5% then becomes: More than 50 years Active filter payoff diagram 0 -20000 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 -40000 -60000 -80000 -100000 -120000 -140000 -160000 -180000 Cashflow Figure 51. Payoff diagram for an active filter investment. As can be seen, the active filter may give an increase in the earnings per year, but this increase is not significant to the higher investment cost. The passive filter investment is better, but none of the investments are profitable in this case. 56 Spring 2013 10.3 EXTRA HEAT DISSIPATION DUE TO HARMONICS 10.3.1 CABLES The heat dissipation in cables L51, L52, L53 and L54 have been calculated by equation 8 and measurement values presented in section 10.1. Table 15 show calculation input. Table 15. Calculation input for increased heat dissipation in cables due to harmonics Calculation input: Length Area Resistivity Magnetic permeability [m] [mm2] [Ω/m] [H/m] 200 15,35 17,5E-9 1,256E-6 Calculation results are presented in figure 52. 80 Heat dissipation in cables caused by harmonics 70 Power [W] 60 50 40 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 Harmonic number Figure 52. Heat dissipation in cables distributed at each harmonic. The 5th harmonic is the dominating contributor to the total heat dissipation caused by harmonics in the cables. The non-linear loads that cause the harmonics are mainly 6-pulse frequency drives which as you know from section 7.3 emits the 5th harmonic to a large extent. The total harmonic heat dissipation in cables L51 + L52 + L53 + L54 adds up to: 105 W. This is about 3,2% of the heat dissipation caused by the fundamental 50 Hz current. 57 Spring 2013 10.3.2 TRANSFORMERS When calculating the increase in heat dissipation in the transformer T5, equation 9 and measurement values presented in section 10.1 has been used. The nominal values used in the calculation are listed in table 16. Table 16. Calculation input Calculation input: Nominal load loss [W] Iron loss koefficient 1636 0,04 And the results in table 17 below. Table 17. Increased heat dissipation in transformer T5 due to harmonics Results Power loss [W] 411 Percenage of load loss [%] 8,38 58 Spring 2013 10.3.3 CAPACITORS The heat dissipation in capacitors C1 and C2 have been calculated by equation 11 with measurement values presented in section 10.1. Calculation input: Nominal capacitor loss [W/capacitor] 120 Calculation results are presented in figure 52. Heat dissipation in capacitors caused by harmonics 0,14 0,12 Power [W] 0,1 0,08 0,06 0,04 0,02 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 Harmonic number Figure 53. Increased heat dissipation in capacitors C1 and C2 caused by harmonics The 5th harmonic is the dominating contributor to the total heat dissipation caused by harmonics in the capacitors. The nonlinear loads that cause the harmonics are mainly 6-pulse frequency drives which as you know from section 7.3 emits the 5th harmonic to a large extent. The total harmonic heat dissipation in capacitors C1 and C2 adds up to: 14,4 W. This is about 6% of the heat dissipation caused by the fundamental 50 Hz voltage. 59 Spring 2013 10.4 TRANSFORMER COMPARISON This section presents a comparison between a ”regular” oil cooled distribution transformer with a ”green alternative” i.e. an amorphous metal core distribution transformer. In the comparison between a ”regular” Regular Grain Oriented (RGO) silicon steel distribution transformer with a ”green alternative” i.e. an amorphous metal core distribution transformer table 18 shows transformer data that has been used. Table 19 shows the investment conditions used in the analysis. Table 18 Transformer nominal data Transformer data: Primary voltage [V] Secondary voltage [V] Power rating [kVA] Load level [%] 11000 420 1600 60 Table 19 Investment conditions for ownership cost comparison Investment conditions: Electricity price [kr/kWh] Life length [y] Interest rate [%] 0,8 30 5 60 Spring 2013 The development of the TOC over 11 years for an RGO and an amorphous metal core alternative are illustrated in figure 54. The TOC is equal after 5 years and after that the amorphous alternative has the lower TOC. Transformer TOC plan 11 years kr 900000 Accumulated ownership cost 800000 700000 600000 500000 RGO 400000 Amourphous 300000 200000 100000 0 1 2 3 4 5 6 7 8 9 10 11 [Year] Figure 54.Transformer TOC development for two alternatives over 11 years The final TOC is calculated assuming the transformer service life is 30 years and the result is presented in figure 55. kkr Total Ownership Cost, 30 years Total Ownership Cost 2500 2000 1500 RGO Amourphous 1000 500 0 1 2 TOC Savings 433 Capitalized loss cost 1914 1373 Purchase price 136 244 Figure 55. Final TOC for two investments; Regular Grain Oriented versus Amourphous Metal Core transformers. As can be seen, the amorphous metal core transformer outperformed the Regular Grain Oriented (RGO) silicon steel transformer in this case. The amorphous alternative leads to 20 % less TOC. 61 Spring 2013 10.5 CABLE COMPARISON In figure 56 below, a comparison has been made between two copper cables that differ in two parameters; price and cross area, chapter 3 explains the method. Higher cross area leads to lower energy loss, but is the price of a higher cross area worth the investment? Equation 2 has been used calculating Ploss. Table 20. Cable characteristics for the comparison between alternative 1 and 2. Cable Data Current Length Area Resistivity Price Loss cost [A] [m] [mm2] [Ω/m] [SEK/m] [SEK/y] 1 374 200 185 17,5E-9 966 18564 2 374 200 240 17,5E-9 1270 14310 TOC plan of cables 200m 900000 800000 700000 600000 500000 400000 300000 200000 100000 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 Higher price, lower loss Lower price, higher loss Figure 56. Cable TOC development for two alternatives over 30 years. As can be seen, the best alternative is the one having the higher cross area. 62 Spring 2013 11 CONCLUSIONS Based on the results and market research the “questions to be answered” that was presented in the introduction chapter are discussed and answered. 11.1 IS THERE A POTENTIAL FOR SAVINGS BY CHOOSING HIGH EFFICIENCY COMPONENTS? Variable frequency drives, breakers, transformers and cables has been investigated with energy performance as main subject. Part 1 of the thesis consists of a market research and presenting of methods with the goal of contrasting regular choices of components to more efficient. An economic comparison between these contrasting choices is presented in the results chapter to investigate if there is a significant potential to be earned in order to improve investments. Variable Frequency Drives Chapter 5: Variable Frequency Drives (VFD´s) has been investigated with energy performance and especially VFD´s connection to harmonics in electric power systems. They are the largest contributor to harmonics in industries today. Industries increase their usage of them and this leads to higher harmonic level and lower reactive power. The first step of VFD´s, the rectification can be performed by a 6-pulse bridge or a 12-pulse bridge, (24-pulse bridges is rarely installed in distribution facilities today). The majority of VFD´s in industries today use 6-pulse bridges which causes harmonic levels of about 30%. Instead, choosing a 12-pulse bridge for the VFD´s will half the harmonic level to about 12%. Conclusion: In the choice of VFD´s, always evaluate the alternative of 12-pulse variable frequency drives. The economic aspect of harmonic levels is discussed in section 11.2. Interrupters and breakers Chapter 4: During operation interrupters and breakers hold high efficiencies. The market research resulted in efficiencies close to 100%. Some of these components hold higher levels than the cables connected to them because of a higher conductor cross area. The conclusion is that this is not a component with potential to increase efficiency in electric power systems. Transformers Chapter 2: According to EU´s IEE, and a large survey in the project “Strategies for development and diffusion of energy efficient distribution transformers” the dominating type of loss in distribution transformers today are the no load loss. Further, the most common type of transformer has a core consisting of Regular Grain Oriented silicon steel which in comparison to transformers with amorphous metal cores hold significantly higher no load loss. In some cases, choosing an amorphous metal transformer leads to 70 % less no load loss. The case in the results chapter evaluates the total ownership cost (TOC) for two transformers: one with RGO and one with amorphous metal core, The TOC is 21% less for the amorphous metal transformer. The conclusion is that in the choice of transformers, always evaluate an alternative with amorphous metal core. 63 Spring 2013 Cables A method for comparison of cables which differs in energy loss and price is presented in chapter 3 and an application of this in the results section 10.4.1. The case in the results evaluated what the cross area of a 200m power cable meant economically with energy loss as a operation cost and the price of the cable as an investment cost. A larger cross area means lower operation cost but higher investment cost. The conclusion is that the larger cross area´s impact on a lower operation cost is significant to the higher price of investment. 11.2 WHAT IS THE ECONOMIC IMPACT OF HARMONICS IN POWER SYSTEMS ? The economic impact of harmonic presence appears different for different parties; distribution facility owners, grid owners and electricity suppliers respectively view the problem from different perspectives. This makes the economic impact of harmonics a quite complex question. For distribution facility owners, the signal which the debiting meter measures is essential. Most debiting meters installed today measures the RMS value of the wave regardless of the form. This means that if harmonics are on the signal, these get debited for. This is because a fluctuating wave has a higher RMS-value than a ideal wave of the same fundamental frequency and amplitude. Most of Sweden´s electronic equipment is custom to AC with 50 Hz, which means that signals consisting of other frequencies added to the fundamental 50 Hz cannot be consumed. When energy distribution companies measure the energy that consumers use and the signal contains harmonics, the consumers pay for the useful, fundamental frequency which in Europe is 50 Hz, but also all of the other frequencies, the harmonics. To calculate this increase in RMS and thus the economic impact of harmonics it is appropriate to let Fourier series (see section 8.1) represent the signal and manage each harmonic separately comparing the resulting distorted wave to the fundamental and calculating the RMS difference between these. In section 10.2 “Harmonic Filter investment” this method is used for estimating the harmonic cost in the case study, which proves to be significant. However, two attenuation methods; active and passive filter investments has been economically evaluated, the conclusions are that investments of attenuating harmonics in the case study is not profitable (10.2). A larger study “The cost of harmonic losses and mitigation in distribution systems” by Mohamed Ashour, Kamelia Youssef and Salah El Sobki made similar conclusions for 10 case studies in Alexandria in Egypt which confirms the results of this thesis; significant potential, expensive solutions. 64 Spring 2013 11.3 HOW MUCH ENERGY LOSS DOES HARMONICS CAUSE IN ELECTRIC POWER COMPONENTS ? It is important to separate the increase of heat dissipation in components to the increase in current and voltage RMS. Increased heat dissipation in certain components does not necessarily mean increased economic loss for distribution facilities. As an example; assume that a load emits 100% harmonic energy. Further, assume that 20% of this energy converts to heat in the network. Then 80% of the energy in the harmonics is debited for. Therefore, using the energy in heat dissipation as a base for economic analysis could lead to wrong conclusions. The increased heat development in components requires component over sizing to cope with the increased stress. It also causes a reduced service life. 11.3.1 CABLES 3,2% increased resistive loss in cables L51-54 due to harmonics. The current magnitude is more important than a higher frequency for the harmonics when it comes to energy loss in cables. Because the main current is so much larger than the harmonic current, harmonic loss becomes a small part of the total resistive losses. This can also be observed by the distribution of the power loss for different harmonics in figure 52; even though the skin effect makes harmonics of a higher order more contributing to the total loss, the current magnitude is much more significant to the total contribution of resistive loss. Equation 6 is used for calculating the energy loss for cables and the conclusion is verified by analyzing the relationship between power loss, frequency and current magnitude; it depends to the square of the current but only to the square root of the frequency. 11.3.2 TRANSFORMERS 8,38% increased load loss in transformer T5 due to harmonics. The increase in heat dissipation due to harmonics in transformers is calculated with the harmonic currents. The load loss depends on the current and its frequency, because of the skin effect which creates eddy currents in the windings, increased frequency leads to higher load loss. The no load loss depends on voltage harmonics and is negligible in this context. 11.3.3 CAPACITORS 6% increased loss in capacitor C1 and C2 due to harmonics. The increase in heat dissipation due to harmonics in capacitors is calculated with the harmonic voltages. There is no significant current flowing into the capacitor except the one that replaces heat loss; this is neglected in the heat loss due to harmonics calculations. 65 Spring 2013 12 REFERENCES 1 . Kungliga ingenjörsvetenskapsakademien (2009) Vägval Energi. Available at: http://www.iva.se/Documents/Publikationer/Projekt/4_ENERGIEFFEKTIVISERING_web.pdf Accesed: May 17, 2013 2. The ministerial Council on Energy of Australia and New Zealand (2007) Equipment Energy Efficiency Program. Available at: http://www.energyrating.gov.au/wp-content/uploads/2011/03/200717-meps-transformers.pdf Accesed: May 17, 2013 3. ABB (2000) ABB green distribution transformer program. Avaliable at: http://www05.abb.com/global/scot/scot252.nsf/veritydisplay/47f61603dac5fb168525773e0078254f/$file/1luj460910lte_partnership_in_sustainable_environement.pdf Accesed: May 17, 2013 4. ABB (2000) ABB green distribution transformer program. Avaliable at: http://www05.abb.com/global/scot/scot252.nsf/veritydisplay/47f61603dac5fb168525773e0078254f/$file/1luj460910lte_partnership_in_sustainable_environement.pdf Accessed: May 17, 2013 5. William Francis Magie (1911). Principles of Physics: Designed for Use as a Textbook of General Physics. 6. U.S. Department of Energy (2012) Energy Efficiency & Renewable Energy. Available at: http://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/motor_tip_sheet11.pdf Accessed: May 17, 2013 7. Schneider electric (2010) Wiki- EIG Available at: http://www.electrical-installation.org/enw/images/9/9a/FigM12b.jpg Accessed: May 17, 2013 8. Schneider electric (2010) Wiki- EIG Available at: http://www.electrical-installation.org/enw/images/2/29/FigM05.jpg Accessed: May 17, 2013 66 Spring 2013 9. George J. Wakileh, Power System Harmonics - Fundamentals, Analysis and Filter design, page 74, ISBN: 3-540-42238-2 10. 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Rockwell Automation (2009) Variable Frequency Drive Solutions for Low Harmonics Available at: http://www.mid-island.com/downloads/pdfs/drives-br011b-en-p.pdf.pdf Accessed: May 17, 2013 22. ABB (2008) Power Quality filters Available at: http://www02.abb.com/global/huabb/huabb008.nsf/0/45df9d2440a9199bc1257a2c004404d3/$file/PQF+felharm%C3%B3nik us+sz%C5%B1r%C5%91k+.pdf Accessed: May 17, 2013 23. George J. Wakileh, Power System Harmonics - Fundamentals, Analysis and Filter design, ISBN: 3-540-42238-2 24. George J. Wakileh, Power System Harmonics - Fundamentals, Analysis and Filter design, ISBN: 3-540-42238-2 25. George J. Wakileh, Power System Harmonics - Fundamentals, Analysis and Filter design, ISBN: 3-540-42238-2 26. Brian D. Storey Computing Fourier Series and Power Spectrum with MATLAB Available at: http://faculty.olin.edu/bstorey/Notes/Fourier.pdf Accessed: May 17, 2013 27. Schneider electric (2010) Wiki- EIG Available at: http://www.electrical-installation.org/enwiki/Total_harmonic_distortion_(THD) Accessed: May 17, 2013 28 George J. Wakileh, Power System Harmonics - Fundamentals, Analysis and Filter design, page 74, ISBN: 3-540-42238-2 68 Spring 2013