Biomass combustion is an interesting alternative to fossil fuel. Modeling and simulation is used for design optimization of biomass boilers and furnace. It is difficult to develop a sufficiently accurate and computationally efficient model because the combustion system is highly complicated multi-scale, multi-phase and multi-physics problem. The study of biomass combustion in different scales allows engineers to understand the combustion process and tochoose necessary simplification to develop a computationally efficient model.The chemical and physical properties of fuels are altered during different fuel preparation methods (i.e. pretreatment and pelletization), and as a result the fuel conversion is also affected. The aim of this thesis is to understand thermal conversion of those chemically or physically altered fuels. Both experimental and modeling techniques were chosen to addressthe aim. Experiments were performed in thermogravimetric analysers, isothermal macro thermogravimeters (iTG), and a pot furnace to account fuel conversion in micro-, meso- and macro scale. In addition, three different types of mathematical model were developed. They are (i) a simplified particle pyrolysis model, (ii) two detailed numerical models that simulate particle pyrolysis and char oxidation and (iii) finally a computational fluid dynamic (CFD) model of combustion of biomass particles in a bed. The results indicate that both the intrinsic and the apparent conversion of the fuel wasinfluenced by the process conditions of fuel preparation methods. Intrinsic pyrolysis reactivitywas reduced due to mild pretreatment; however, it was increased with further increase in pretreatment severity. In contrary, severity of pelletization tends to reduce the apparent reactivity of pellets combustion. It was also investigated that how each physical and chemical parameter should be modelledfor a untreated biomass (i.e. wood logs) and a densified biomass (i.e. pellets) through parametric studies with a detailed particle simulation. The result shows that a model for wood logs should exclude convective heat transfer by volatiles if the fibers align to longitude direction while it is important part in the models for pellets. Devolatilization of wood logs was expressed as endothermic reactions while the model results showed best agreement withexperimental data of wood pellets when the heat of reaction was assumed to be zero, possibly due to the secondary reactions. Then, it was demonstrated that a constitutive equation, i.e. analytical solution of the shrinking core model, is sufficient to express devolatilization rate of thermally-thick particles at the temperature of 1173 K. While studying apparent oxidation of wood pellet char , it was found that change in intrinsic char oxidation reactivity due to different pyrolysis conditions does not influences the model prediction at high temperature. In addition, at high temperature, the reaction front became thin and reaction rate was hardly affected by temperature.It was also found by the simulation of pellet bed combustion that the apparent density of the particle significantly affected the flame velocity.

This paper presents how pelletizing die temperature and moisture content affect combustion behaviour of single wood pellet. Pine wood particles with two different moisture contents (i.e. 1 wt.% and 12 wt.%) were pelletized in a laboratory-scale single pelletizer (single die pellets) at die temperature of 20, 100, 150 and 200 °C. The pellets were combusted in a laboratory scale furnace at 800 °C. Time required for single pellet combustion generally increased with both increase of pelletizing temperature and moisture content of biomass. In addition, combustion behaviour of single die pellets was significantly different than those produced in a pilot scale pelletizing plant (semi-industrial scale pellet). That difference was due to variation in physical properties of pellets (e.g. density, and morphology).

Many phenomena affects devolatilization of relatively large wood particles, e.g. wood pellets and logs, including mass and heat transfer, chemical reactions and physical transformation such as shrinkage. Many studies investigated the importance of these phenomena through detailed mathematical models at particle scale, but the models need to be simplified at a certain degree to be implemented into large-scale simulation for gasifiers and boilers. This paper first presents how each physical and chemical parameter should be modelled for wood logs (low density and anisotropic) and wood pellets (high density and isotropic) through parametric studies with a detailed particle simulation. They required different sub-models for effective thermal conductivity and heat of reactions due to the difference in isotropy of particles between pellets and logs. Then, we demonstrated that a constitutive equation, i.e. analytical solution of the shrinking core model, is sufficient to express devolatilization rate of thermally-thick particles at the temperature of 1173 K with proper sub-models of physical and chemical parameters. The constitutive equation agreed better with experimental data of wood log than wood pellets, mainly because of the error caused during the consideration of the effect of convective cooling of char layer on thermal conductivity. Both detailed and simplified particle models were validated with the experimental data in an isothermal macro thermogravimeter allowing devolatilization of large particles

Electronic household appliances are non-linearloads and emit harmonics into the low voltage networks.Usually, these loads are simply modelled by constantcurrent source models, which only represent the harmonicemission of the appliances for a single supply voltagedistortion, mostly sinusoidal conditions.Measurements have shown that the harmonic currentsemitted by electronic devices can significantly depend onthe circuit topology and the existing supply voltagedistortion. This paper studies the impact of supply voltagedistortion, which can be typically found in public lowvoltage networks (so-called flat-top voltage waveform), onthe harmonic current emission of individual devices withdifferent circuit topologies as well as the impact of supplyvoltage distortion on the harmonic summation of thesedevices.

Consumer electronic devices mostly get their energy from the electric power grid. Such devices might be continuously connected to the grid (like televisions) or only connected to charge the batteries (like cell phones). The amount of energy taken from the grid is not reduced by using devices powered by batteries. Instead, the electrical energy consumption is more likely increased due to the losses in the conversion process and because there are more opportunities to use the device.

This paper contains some thoughts on the relations between power quality and smart grids. It includes some of the, in our opinion, important research and development activities that are needed within power quality as part of the transition to the smart grid.

This paper introduces methods to quantify the impact of wind power and other types of distributed generation on the overvoltage risk. The so-called hosting capacity approach, introduced in the very first issue of this magazine, is used as the basic approach. Both a deterministic and a statistical approach are introduced. The deterministic approach is suitable for generation with a constant production most of the time. However, it is shown that a deterministic approach could easily result in an unnecessary barrier against the introduction of wind power. For any method used to determine the hosting capacity, a serious discussion is needed about overvoltage indices and objectives.

This book links the challenges to which the electricity network is exposed with the range of new technology, methodologies and market mechanisms known under the name "smart grid." The main challenges will be described by the way in which they impact the electricity network: the introduction of renewable electricity production, energy efficiency, the introduction and further opening of the electricity market, increasing demands for reliability and voltage quality, and the growing need for more transport capacity in the grid. Three fundamentally different types of solutions are distinguished in this book: solutions only involving the electricity network (like HVDC and active distribution networks), solutions including the network users but under the control of the network operator (like requirements on production units and curtailment), and fully market-driven solutions (like demand response). An overview is given of the various solutions to the challenges that are possible with new technology; this includes some that are actively discussed elsewhere and others that are somewhat forgotten.Linking the different solutions with the needs of the electricity network, in the light of the various challenges, is a recurring theme in this book.

This document covers methods to improve the voltage-dip immunity of installations, starting from a detailed description of voltage dips, an overview of the knowledge on equipment immunity, and global voltage-dip statistics. The economics of voltage-dip immunity are discussed. Recommendations for immunity testing are given. A systematic method for designing and improving the immunity of an industrial process is introduced.

This paper summarizes the state of discussions in CIGRE/CIRED joined working group C4.24, concerning expected impacts on the power quality of future methods for volt-var-control in the distribution grid. The positive impacts include the reduction of the number of overvoltage and undervoltages and also a reduction of voltage unbalance, when some control schemes are applied. Potential negative impacts include an increased number of short-duration undervoltages, rapid voltage changes, flicker, and voltage transients; a higher risk of harmonic resonances; and increased emission of supraharmonics. All these potential impacts are discussed in the paper.

The Council of European Energy Regulators has been publishing Benchmarking Reports on the Quality of Electricity Supply since 2001. For the 2011 edition of the Benchmarking Report the 29 member countries of CEER were joined by the 9 NRA's from the Energy Community and the NRA from Switzerland. This paper contains the main results, findings and recommendations on voltage quality from the 2011 edition.

Guidelines are given for setting up and running a voltage quality monitoring programme. These guidelines are published jointly by CEER and ECRB and contain among others recommendations on the number and location of monitors, on disturbances to be monitored and indices to be calculated, on reporting of the results and on financing of the programme. It is concluded that voltage quality monitoring programs are important tools for voltage quality regulation and that all other possible applications should be kept in mind when setting up such a programme. It is also concluded that such programmes should be funded through the network tariffs; that the results should be made available regularly and that diversification of indices and methods is to be avoided.

The main purpose of the electric power system and the electricity network (power grid) is to supply electrical energy to the consumers. The interest of those consumers is in the price of electricity and in the performance of the delivery of the electrical energy. For the electricity producers, the aim of the grid is to enable the transport of electricity from them to the consumers. Also for the producers the importance is in costs and performance. Performance of the grid is typically divided into two parts: continuity of supply and voltage quality.The appearance of the smart grid, introduced as the use of new technology, methodology or market principles, to address new challenges, impacts continuity of supply and voltage quality. Such challenges include new types of production, new types of consumption, and electricity markets, but also increasing demands by customers on continuity of supply and voltage quality.This paper discusses a number of examples of new thinking for addressing the challenges that the power system has to cope with.An alternative approach for overload protection of subtransmission grids will be proposed, where the “smartness” is in the fact that the overload protection does not remove the overloaded component but the cause of the overload. Upon detection of an overload, the protection disconnects part or whole of curtailable customers to reduce the current through the lines to a level below the overload limit. As a result subtransmission lines can be operated without any reserve, so that more customers can be connected for the same costs while at the same time the continuity of supply for the non-curtailable customers is not impacted.The limits set to the hosting capacity by the risk of overvoltages due to distributed generation can be removed either by new technology (curtailment of production) or by allowing occasional overvoltages. An example will be provided to show the probability of an overvoltage occurring with a low-voltage customer when increasing amounts of wind power are connected to a medium-voltage feeder.An example of the measured emission from wind turbines will be presented. Allowing higher levels of non-characteristic harmonics is a possible alternative for strict emission limits on new installations.The provided examples of the use of new technology and/or new ways of thinking are part of the transition to the smart grid. Such new thinking in combination with new technology will be an important element in the transition to the future electricity network i.e. the “smart grid”. With any design or operation issue of the power system it remains important to always keep the main aims of the power system in mind: to maintain acceptable continuity of supply and voltage quality for all network users at a reasonable price.

This paper presents some of the results from an international working group on voltage-dip immunity. The working group has made a number of recommendations to reduce the adverse impact of voltage dips. Specific recommendations to researchers and manufacturers of powerelectronic equipment are: considering all voltage dip characteristics early in the design of equipment; characterize performance of equipment by means of voltage-dip immunity curves; and made equipment with different immunity available

This paper presents some of the results from an international working group on voltage-dip immunity. The working group has made a number of recommendations to reduce the adverse impact of voltage dips. Specific recommendations to researchers and manufacturers of power-electronic equipment are: considering all voltage dip characteristics early in the design of equipment; characterize performance of equipment by means of voltage-dip immunity curves; and made equipment with different immunity available.

This paper shows measurements of the emission of harmonics by a small windpark. The spectrum consists of the characteristic harmonics associated with six-pulse converters and a broadband spectrum covering frequencies at which emission normally is not present. These frequencies are caused by the switching pattern of the power-electronics converters. Similar spectra have been reported by other authors and for other types of modern converters. Whereas the actual emission is small, the presence of emission at non-characteristic frequencies could result in voltage distortion limits being exceeded.

This paper maps the expected and possible adverse consequences for power quality of introducing several smart distribution-grid technologies and applications. The material presented in this paper is the result of discussions in an international CIGRE-CIRED joint working group. The following technologies and applications are discussed: microgrids; advanced voltage control; feeder reconfiguration; and demand-side management. Recommendations are given based on the mapping.

This paper introduces three examples of new thinking for addressing the challenges that the power system has to cope with. Such new thinking in combination with new technology will be an important element in the transition to the future electricity network (the —). With overload protection it is important to not remove the overloaded component but the cause of the overload so as to protect the other network users against an interruption. The limits set to the hosting capacity by potential overvoltages can be removed either by new technology (curtailment of production) or by allowing occasional overvoltages. Allowing higher levels of non-characteristic harmonics is a possible alternative for strict emission limits on new installations. In all cases it is essential that the interests of the network user are considered.

Signal processing has been used in many different applications, including electric power systems. This is an important category, since a wide variety of digital measurements is available and data analysis is required to deliver diagnostic solutions and correlation with known behaviors. Measurements are taken at numerous locations, and the analysis of data applies to a variety of issues in ¿ power quality (PQ) and reliability ¿ power system and equipment diagnostics ¿ power system control ¿ power system protection. This article focuses on problems and issues related to PQ and power system diagnostics, in particular those where signal processing techniques are extremely important. PQ is a general term that describes the quality of voltage and current waveforms. PQ problems include all electric power problems or disturbances in the supply system that prevent end-user equipment from operating properly. Examples of voltage and current variations that can result in PQ problems include voltage interruptions, long- and short-duration voltage variations, steady-state research opportunities that use the measured voltages and currents to indicate possible equipment and system problems (referred to as equipment diagnostics).

This letter presents a method for characterizing the fast voltage variations that occur on a time scale between the subsecond fluctuations covered by the flickermeter standard and the 10-min values covered by standards like EN 50160 The method is fully compatible with IEC 61000-4-30 class A. The new characteristic is correlated to small switching actions, such as domestic load switching and transformer tap-changer operation.

This paper proposes a method for analyzing measurements of voltage transients in three-phase systems. The method is based on the Clarke transform introduced in 1950 for calculations of travelling waves along three-phase transmission lines. The proposed method also shows close similarities with the classification of three-phase unbalanced voltage dips. After extracting the actual transient (e.g., by using a notch filter centered on the power-system frequency), the three signals are decomposed into seven components. From the relation between these seven components, the dominant component is identified. The method is successfully applied to a number of measured transients. The paper also identifies the limitations of the method and gives suggestions for future work

This paper presents the two main types of classification methods for power quality disturbances based on underlying causes: deterministic classification, giving an expert system as an example, and statistical classification, with support vector machines (a novel method) as an example. An expert system is suitable when one has limited amount of data and sufficient power system expert knowledge; however, its application requires a set of threshold values. Statistical methods are suitable when large amount of data is available for training. Two important issues to guarantee the effectiveness of a classifier, data segmentation, and feature extraction are discussed. Segmentation of a sequence of data recording is preprocessing to partition the data into segments each representing a duration containing either an event or a transition between two events. Extraction of features is applied to each segment individually. Some useful features and their effectiveness are then discussed. Some experimental results are included for demonstrating the effectiveness of both systems. Finally, conclusions are given together with the discussion of some future research directions.

Distributed Generation (DG) reduces the amount of energy lost in transmitting electricity because the electricity is generated very near where it is used. This book introduces systematic and transparent methods for quantifying the impact of DG on the power grid. It emphasizes systematic and transparent calculation methods, allowing for a quantification of the amount of DG that can be integrated at a certain location of the grid or in the grid as a whole. It also provides an overview of the different energy sources, with emphasis on wind power, solar power and combined heat and power in the power grid.

This paper introduces a reliability index that is directly linked to the satisfaction of individual customers with the experienced reliability of supply. The definition of the index is based on the observation that customers are satisfied as long as they have less than 3 interruptions per year, none of which lasts longer than 8 hours. The customer dissatisfaction index (CDI) is defined as the probability that this condition is fulfilled for a given customer. Mathematical expressions are obtained for the CDI; its relation with existing indices is studied; and the results of a case study in a medium-voltage distribution network are presented.

This paper investigates the spread of the high frequency current emission between devices of different size and the grid. The impact of the EMC filter, either LCL or CLC configuration, has been considered from a simplified model. The high frequency current emission, produced by a large device, can potentially cause a relatively large current flowing through a nearby small device. An important conclusion from the study is that current amplification can occur due to harmonic resonances between different types of filters.

This paper presents the three aspects of power quality concerning distributed energy resources (DER). The voltage quality experienced by a DER unit impacts the performance of the unit: bad voltage quality may reduce the life length of the unit and lead to incorrect operation or tripping. The DER unit's current (the "current quality" impacts the system and through the system other customers. The hosting-capacity concept is proposed as a systematic method for quantifying the impact of DER units. The third aspect of power quality only appears with large (local or global) penetration of DER. The tripping of DER units on voltage dips or frequency swing endangers the reliability, stability and security of the system.

This paper presents a method for considering the voltage-dip performance of the power system in the design of subtransmission lines. The shielding performance of the lines is expressed in terms of the number of faults for different number of phases involved in the fault. Relations between fault type and voltage dip at the terminals of sensitive equipment are used to determine the resulting number of equipment trips. The method is illustrated by applying it to three different tower designs used in the Swedish subtransmission networks. It is shown that the installation of shield wires significantly reduces the number of equipment trips, especially in combination with the installation of additional capacitance in adjustable-speed drives.

This paper presents the status of the work, by April 2008, in C4.110, a joint working group by CIGRE, CIRED and UIE. The scope of the working group is to gather technical knowledge on the immunity of equipment, installations and processes against voltage dips, and to use this knowledge in the further development of methods and standards. The activities of the working group are divided in seven "chapters". Chapters 1 and 7 are introduction and conclusions, respectively. Chapter 2 gives a general description of voltage dips as they appear at the terminals of sensitive equipment. Chapter 3 describes the performance of equipment and processes during voltage dips. This chapter also includes recommendations on the design of processes. In Chapter 4 the results from Chapter 2 and Chapter 3 are combined to set requirements for the dip characteristics that should be included in immunity testing. Chapter 5 is the data gathering chapter, covering data on voltage-dip statistics at different locations, but also data on the economics of equipment immunity and testing. Finally, in Chapter 6, recommendations for immunity objectives will be given. Important contributions of the working group are: a check-list of voltage dip characteristics to be used early in the design of equipment; a methodology to assess the performance of a complete installation and to include voltage-dip performance in the design of the installation; recommendations for characterization testing of equipment against voltage dips; recommendations for voltagedip immunity of equipment.

A workshop on power system harmonics was organized in Stockholm in January 2014. On the agenda was among others a discussion on what are the main issues on harmonics at the moment and in the near future. The results of this discussion are summarized in this paper and some of the issues are discussed in more detail in this paper and in its companion paper. This paper discusses emission from wind and solar power as well as advantages and disadvantages of active and passive filters.

This paper gives some examples of harmonicissues that can occur when long ac cables are connected in thetransmission grid. The main impact is that resonances can occurat much lower frequencies than when only overhead lines arepresent. Two illustrative case studies are presented: one for a275-kV cable, one for a 400-kV cable in combination with a 132-kV capacitor bank. A simple rule-of-thumb is given, to decide ifa detailed harmonic study is needed. Some guidelines for such astudy are given as well.

This paper discusses the voltage rise due to PV installations connected to a low-voltage network. The connection of individual installations is studies for both single-phase and three-phase connection. A stochastic method is presented to estimate the hosting capacity. This method is illustrated for random and coordinated connection of single-phase installations. It is shown, in a number of ways, that the installation of large (e.g. 6 kW) single-phase connected units can easily result in unacceptable overvoltages.

Consumer electronic devices mostly get their energy from the electric power grid. Such devices might be continuously connected to the grid (like televisions) or only connected to charge the batteries (like cell phones). The amount of energy taken from the grid is not reduced by using devices powered by batteries. Instead, the electrical energy consumption is more likely increased due to the losses in the conversion process and because there are more opportunities to use the device.

There is a serious interest from the international standard-setting community in knowledge about voltage and current distortion in the frequency range 2 to 150 kHz, referred to as supraharmonics. At the same time, research is ongoing at a number of locations, but the knowledge about supraharmonics remains limited. This paper compares some of the properties of harmonics and supraharmonics. An increase in supraharmonics for individual devices is observed in association with a decrease in harmonic emission (i.e. below 2 kHz). A proposal is made for setting limits in this frequency range. The proposal is based on existing standards and is meant to be for discussion. The authors strongly encourage an open discussion about the proposed limits. There are a number of differences between harmonics and supraharmonics that are not covered by the proposed set of limits. A substantial amount of further research is needed to extend the standard framework for supraharmonics such that this can be covered by standards

This paper outlines a number of possible research directions in power quality. The introduction of new sources of generation will introduce the need for new research on voltage-magnitude variations, harmonic emission and harmonic resonance. Statistical performance indicators are expected to play an important role in addressing the hosting capacity of the power system for these new sources. The quickly growing amounts of power-quality data call for automatic analysis methods. Advanced signal-processing tools need to be developed and applied to address this challenge. Equipment with an active power-electronic interface generates waveform distortion at higher frequencies than existing equipment. The emission, spread, consequences and mitigation of this distortion require more research emphasis. The growing complexity of the power system calls for remote identification of system events and load transitions. Future DC networks, at different voltage levels, require the research on DC power quality next to AC power quality. Research on methods to describe and analyse time-varying harmonics has applications in a number of the above-mentioned issues. So does the use of hardware-in-the-loop (HIL) and real-time-digital simulation. Existing power quality standards should not form a barrier against future research; instead research should result in improved standards as well as completely new concepts. Examples are: voltage dips in three-phase systems, flicker due to non-incandescent lamps, and voltage variations on the timescale between 1 second and 10 minutes. All together, it is concluded in this paper that sufficient important and interesting research challenges and opportunities remain in the power quality area.

This paper addresses voltage distortion in the frequency range 2 to 9 kHz, above what is normally considered in harmonic studies. By extrapolating the voltage-distortion limits that exist in international standards for distortion up to 2 kHz, it is concluded that 0.5% of nominal voltage per 200-Hz band is a safe limit. This limit is next used to estimate the number of small generator units (1-10 kW) that can be connected to a low-voltage grid. It is concluded that in some cases the connection of one or just a few units already leads to a distortion level above the limit.