Showing papers by "Vattenfall published in 2013"

Active distribution system management

Abstract: The paper highlights the need for an Active System Management (ASM) of distribution networks as a key tool for the efficient and secure integration of a high share of Distributed Energy Resources (DER). The paper provides technical and regulatory recommendations that mainly focus on distributed generation but are also largely applicable to flexible loads, electric vehicles and storage. (4 pages)

An Empirical Analysis of Liquidity and its Determinants in The German Intraday Market for Electricity

Abstract: This paper presents a theoretical and empirical analysis of liquidity in the German intraday market for electricity. Two models that aim at explaining intraday liquidity are developed. The first model considers the fundamental merit-order and intraday adjustment needs as the drivers of liquidity in a perfectly competitive market. The second model relaxes the assumption of perfect competition in the intraday market and assumes that the trading behavior of profit maximizing market participants influences the liquidity provision. The relevance of commonly used liquidity indicators like the bid ask-spread, resiliency, market depth, price variance, delay and search costs as well as trading volume and the number of trades are analyzed with respect to both models of liquidity. The empirical findings indicate that liquidity in the German intraday market can be explained by the trading model while the purely fundamental model is rejected.

Control Power and Variable Renewables A Glimpse at German Data

Abstract: Control power (regulating power, balancing power) is used to quickly restore the supply-demand balance in power systems. Variable renewable energy sources (VRE) such as wind and solar power are often thought to increase the reserve requirement significantly. This paper provides a comprehensive overview of balancing systems in Europe, discusses the role of VRE, and presents empirical market data from Germany. Despite German VRE capacity doubled during the last five years and has surpassed 70% of peak load, contracted control power decreased by 20%, and procurement cost fell by 50%. Today, control power adds only 0.4% to household electricity prices. Nevertheless, we identify several sources of inefficiency in control power markets and imbalance settlement systems and propose a number of policy changes to stimulate the participation of VRE in control provision and to improve the incentives to forecast accurately.

Modeling Crosslinking Polymerization in Batch and Continuous Reactors

Abstract: A new pseudo-distribution approach is applied to the modeling of crosslinking copolymerization of vinyl and divinyl monomer and compared to Monte Carlo (MC) simulations. With the number of free pending double bonds as the main distribution variable, a rigorous solution of the three leading moments of the molecular size distribution becomes possible. Validation takes place with data of methyl methacrylate with ethylene glycol dimethacrylate. Well within the sol regime perfect agreement is found, but near the gelpoint larger discrepancies do appear. This is probably due to the existence of multiradicals that are not taken into account in the population balance approaches.

The Decreasing Market Value of Variable Renewables: Integration Options and Deadlocks

Abstract: Wind and solar power are variable renewable energy sources (VRE) in the sense that their output is variable, subject to forecast errors, and that they are bound to certain sites. These three inherent properties reduce the income that VRE generators earn on markets (market value), especially at high penetration rates. The resulting lack of competitiveness could become a major barrier for the transition to renewable energy systems. This chapter explains the mechanisms that cause the decreasing market value, provides quantifications from model and market data, and discusses options to mitigate this value drop. Such integration options can be technological innovations, such as new storage technologies, based on investments of existing technologies, such as a shift in the generation mix from base load to peak load generators, or institutional, such as an appropriate design of spot and control power markets. We also indicate a few deadlocks, decisions that could turn out costly once high VRE penetration is reached.

A Methodology for Protective Vibration Monitoring of Hydropower Units Based on the Mechanical Properties.

Abstract: Different methods are used to monitor and protect the hydropower unit from harmful operation modes. Eccentricities and shape deviations in generators [1], mass unbalances in rotors [2], dynamic flow properties in turbines [3], etc. cause radial loads on the bearings and supporting structures. Figure ​Figure11 presents the components in a hydropower unit. Demands on the unit's instrumentation are dependent upon the importance of the unit and the cost of outage. Some older units do not have an installed vibration monitoring system, but most units are equipped with shaft displacement sensors or accelerometers to monitor vibrations in the unit. There are also units equipped with air gap sensors that measure the distance between the generator's rotor and stator. In some hydropower units, load cells are installed inside the bearing or bearing bracket in order to monitor radial loads on the structure. Several different types of sensors and methods can be used to monitor vibrations in a hydropower unit; the key issue with vibration monitoring is to protect the machine and avoid outage. Fig. 1 Components in a hydropower unit The recommendations in various standards for permitted vibration level values are often used as an aid to determine and decide if a unit is to be stopped for maintenance. The standards ISO 7919-5 [4] and ISO 10816-5 [5] divide vibration levels into classes with increasing levels from Class A to Class D, where Class A is a good machine that does not need attention while Class D is a machine that should be stopped for immediate corrective action. The permitted levels for each class vary with the unit's rotational speed; a low speed permits higher values of vibration levels in each class, compared to high speed. According to Totir et al. [6], ISO 7919-5 and ISO 10816-5 are not sufficient as vibration monitoring standards. The recommended vibration levels for each class are based on the unit's rotational speed and statistical data; consideration of the physical properties of bearings and brackets is not taken, which strongly affect the relationship between the radial load and vibration levels and which vibration levels the components can withstand. The objective of this paper is to present a vibration monitoring methodology based on the physical properties of the hydropower units, i.e., based on the design criteria of the hydropower unit and the measured radial loads in the machine during operation. The structure of this paper is to first describe the theoretical background regarding how to determine vibrations and forces in hydropower units. Following this, an example of design criteria for hydropower units is presented. Subsequently, Secs. 4 and 5 present a proposed methodology for vibration monitoring and an example of how the methodology is applied to a hydropower unit. Finally, discussions and conclusions regarding the methodology are presented.

Price Determinants in the German Intraday Market for Electricity: An Empirical Analysis

Abstract: This paper presents a first investigation of hourly price determinants in the German intraday market for electricity. The influence of power plant outages, forecast errors of wind and solar power production, load forecast errors and foreign demand and supply on intraday prices are explained from a theoretical perspective. Furthermore the influences of the non-linear merit-order shape, ramping costs and strategic market behavior are discussed. The empirical results from different regression analysis with data from 2010 and 2011 show that most price determinants increase and decrease intraday prices as expected. Nevertheless, only a minor share of power plant outages and solar power forecast errors are traded on the electronic intraday trading platform, thus influencing prices not as strongly as expected. Furthermore the price determinants influence intraday prices differently over the course of the day which may be explained by an alternating liquidity provision.

Characterization and quantification of deposits build up and removal in straw suspension fired boilers

Abstract: An increased use of biomass in large suspension-fired power plants can be a relatively economic and potentially also efficient way to utilize biomass for heat and power production. However, large deposit formation problems limit the electrical efficiency by limiting the maximum applicable superheater temperature, and the deposits may also cause boiler stops where different parts of the boiler have to be cleaned. This project aims at providing scientifically based knowledge on understanding of ash deposit formation and shedding in biomass suspension-fired boilers. Deposit probe measurements have been conducted in different biomass suspension-fired boilers by using advanced ash deposition probes. Two kinds of ash deposition probes have been used. A horizontal probe that has been developed further, which can register probe temperature, deposit mass uptake, heat uptake and video monitoring. First steps have also been taken for the development of a vertical probe that was employed for ash deposit formation measurements on a boiler furnace wall. In the first series of probe measurements, the influence of straw firing technology (grate and suspension) on ash transformation, deposit formation rate and deposit characteristics has been investigated. Full-scale probe measurements were conducted at a 250 MWth suspension boiler, firing straw and wood in suspension, and the results were compared with measurements conducted at a 105 MWth straw-fired grate boiler. Bulk elemental analysis of fly ashes revealed that fly ash from suspension firing of straw has high contents of Si, K and Ca, while fly ash from straw grate firing was rich in the volatile elements, K, Cl and S. SEM-EDS analysis showed that the fly ash from straw suspension firing consists of three kinds of particles: 1) flake type Si rich particles, 2) molten or partially molten particles (> 20 μm) rich in Si, K and Ca with small amounts of Mg, P, and potassium salts on the outer surface, and 3) small particles rich in K, Cl and S (aerosols, between 0.1 and 5 μm). Ash deposition data were compared with data from previously conducted deposit probe measurements in biomass-fired grate boilers. The comparison showed an increasing trend in deposit formation rate with increase in flue gas temperature. At a flue gas temperature of 650 ◦C, the deposit formation rate is typically from 5 to 30 g/m2/h and at 900 ◦C, the deposit formation rate is typically 20 to 110 g/m2/h. At higher fuel alkali vi ABSTRACT contents (K > 0.9 wt.%), the increase in deposit formation rate with flue gas temperature was more significant, compared to the increase in deposit formation rate at lower fuel alkali contents (K ≤ 0.9 wt.%). An increased flue gas temperature probably increases the fraction of molten ash as well as provides an increased content of gas phase alkali species, and both will lead to an increased deposit formation rate. The deposit formation rates during straw suspension firing and straw grate firing were on similar levels. This was observed even though the concentration of fly ash in the flue gas was higher during straw suspension firing. The objective of the second probe measuring series was to investigate the influence of fuel type (straw share in wood), probe exposure time, probe surface temperature (500 ◦C, 550 ◦C and 600 ◦C), and flue gas temperature (600–1050 ◦C) on the transient deposit build-up and shedding in a 350 MWth suspension boiler, firing straw and wood. Two different measures of deposit formation rate are used in the analysis of the data. The first is the integral deposit formation rate (IDF-rate) found by dividing the integral mass change over integral time intervals (of order several hours) by the time interval. The IDF-rate is then the result of both the deposit formation rate and shedding events in a given period. In this work IDF-rates were determined using 12 h intervals. The IDF-rate is similar to deposit formation rates determined from previous full-scale measuring data. A second measure, the derivative-based deposit formation rate (DDF-rate), was determined by calculating the local values of the time derivative of the deposit mass uptake, removing large negative values signifying major shedding events, and finally time smoothing the derivatives to remove excessive noise. The DDF-rate excludes the major shedding events. The overall DDF-rates were measured to be between 234 and 3105 g/m2/h. The IDF-rates were measured between 1 and 95 g/m2/h. The results showed that the DDF-rate was influenced by flue gas temperature and straw share, while changes in probe surface temperature had no significant influence. The IDF-rate, qualitatively related to the ratio between the time-integrated DDF-rate and the integration time, followed the same trends. Quantification of naturally occurring deposit shedding and deposit shedding during plant sootblowing was made via deposit mass uptake signals obtained from the deposit probe. The deposit shedding process was characterized by calculation of the amount of deposit removed at a shedding event [g/m2] and the frequency of the shedding events [h−1]. The results showed that the shedding process is stochastic and the amount of deposit removed varies even at constant local conditions. However, the deposit shedding rates showed an increasing trend with increase in flue gas temperatures and probe deposit mass loads. The deposit shedding rate was in most of the cases higher at a probe temperature of 500 ◦C than at a probe temperature of 600 ◦C. A possible reason for this is partial melting and/or sintering of the innermost deposit layer (rich in K, Cl and S) at higher probe viicontents (K > 0.9 wt.%), the increase in deposit formation rate with flue gas temperature was more significant, compared to the increase in deposit formation rate at lower fuel alkali contents (K ≤ 0.9 wt.%). An increased flue gas temperature probably increases the fraction of molten ash as well as provides an increased content of gas phase alkali species, and both will lead to an increased deposit formation rate. The deposit formation rates during straw suspension firing and straw grate firing were on similar levels. This was observed even though the concentration of fly ash in the flue gas was higher during straw suspension firing. The objective of the second probe measuring series was to investigate the influence of fuel type (straw share in wood), probe exposure time, probe surface temperature (500 ◦C, 550 ◦C and 600 ◦C), and flue gas temperature (600–1050 ◦C) on the transient deposit build-up and shedding in a 350 MWth suspension boiler, firing straw and wood. Two different measures of deposit formation rate are used in the analysis of the data. The first is the integral deposit formation rate (IDF-rate) found by dividing the integral mass change over integral time intervals (of order several hours) by the time interval. The IDF-rate is then the result of both the deposit formation rate and shedding events in a given period. In this work IDF-rates were determined using 12 h intervals. The IDF-rate is similar to deposit formation rates determined from previous full-scale measuring data. A second measure, the derivative-based deposit formation rate (DDF-rate), was determined by calculating the local values of the time derivative of the deposit mass uptake, removing large negative values signifying major shedding events, and finally time smoothing the derivatives to remove excessive noise. The DDF-rate excludes the major shedding events. The overall DDF-rates were measured to be between 234 and 3105 g/m2/h. The IDF-rates were measured between 1 and 95 g/m2/h. The results showed that the DDF-rate was influenced by flue gas temperature and straw share, while changes in probe surface temperature had no significant influence. The IDF-rate, qualitatively related to the ratio between the time-integrated DDF-rate and the integration time, followed the same trends. Quantification of naturally occurring deposit shedding and deposit shedding during plant sootblowing was made via deposit mass uptake signals obtained from the deposit probe. The deposit shedding process was characterized by calculation of the amount of deposit removed at a shedding event [g/m2] and the frequency of the shedding events [h−1]. The results showed that the shedding process is stochastic and the amount of deposit removed varies even at constant local conditions. However, the deposit shedding rates showed an increasing trend with increase in flue gas temperatures and probe deposit mass loads. The deposit shedding rate was in most of the cases higher at a probe temperature of 500 ◦C than at a probe temperature of 600 ◦C. A possible reason for this is partial melting and/or sintering of the innermost deposit layer (rich in K, Cl and S) at higher probe vii ABSTRACT surface temperature. This could cause the adhesion strength of the deposit to the probe to increase at the higher probe temperature. Quantification of the necessary Peak Impact Pressure (PIP) needed to remove the deposit was also made by use of a sootblowing probe in conjunction with the deposit probe. Results of deposit removal by artificial sootblowing showed that the deposits formed on a 500 ◦C probe temperature and at exposure time less than 91 h can be removed with a PIP of less than 55 kPa. However, an increase in probe exposure time and/or probe surface temperature (to 600 ◦C) significantly increases the PIP needed to remove the deposits. As a part of the third probe measurement series, full-scale probe measurements of deposit build-up and shedding were conducted at a 800 MWth suspension boiler, firing wood and natural gas, with the addition of coal fly ash. The results showed that the addition of coal fly ash can significantly affect the ash deposition behaviors and the deposit properties. At the location of the probe measurements with flue gas temperature of 1250– 1300 ◦C, although the addition of coal fly ash increases the

Balancing Power and Variable Renewables: A Glimpse at German Data

Abstract: Balancing power (regulating power, control power) is used to quickly restore the supply-demand balance in power systems. Variable renewable energy sources (VRE) such as wind and solar power, being stochastic in nature, ceteris paribus increase the need for short-term balancing. Their impact on reserve requirements is heavily discussed in academic and policy circles and often thought to be large. The paper contrasts a literature survey and model results with descriptive statistics of empirical market data from Germany, providing surprising insights: all models predict VRE to increase balancing reserve requirements - however, despite German VRE capacity doubled during the last five years, balancing reserves decreased by 20%, and procurement cost fell by 50%. Other factors, such as increased TSO cooperation and the recession, must have overcompensated for the growth of renewables. To the extent this specific German experience can be generalized, we interpret this as an indication that balancing power is not necessarily a major barrier to VRE integration at moderate penetration rates. Next to reserve requirements, the paper discusses two additional links between renewables and balancing systems: the supply of balancing power by renewables; and the role of the imbalance price as incentive for forecast improvements. Reviewing these three links, the paper also provides a comprehensive overview of balancing systems.

Torsional Stability of Hydropower Units Under Influence of Subsynchronous Oscillations

Abstract: Hydropower units are known to be comparatively insensitive to subsynchronous power oscillations. During a startup test of an electrical island in the Nordic power system, a series capacitor tripped due to a subsynchronous oscillation within the normal frequency range of hydropower unit torsional modes. Since no thermal units were connected, it is motivated to question the traditional view. In this paper, the small-signal and transient torsional mode stability of hydropower units are assessed through time-domain simulations. The model is based on the first IEEE benchmark model for subsynchronous resonance which has been tuned to fit one of the startup test system units for which detailed measurements are available. The stability conditions are investigated for several load conditions and machine configurations. It is found that the damping in the startup test system is sufficient to prevent growing oscillations. A fault however could expose the machines to high transient torques.

In situ corrosion testing of various nickel alloys at Måbjerg waste incineration plant

Abstract: The majority of waste in Denmark is disposed via waste to energy (WTE) incineration plants which are fabricated from carbon steel. However, due to the increasing corrosiveness of waste over the years, more corrosion resistant alloys are required. In Denmark, Inconel 625 (UNSN06625) is the weld overlay material currently being used to give improved corrosion resistance. In order to assess the use of alternative nickel alloys, test panels have been manufactured and inserted into Mabjerg waste incineration plant. Inconel 625 as a 50% weld overlay, two layered weld overlay and as a spiral weld overlay was exposed. Other nickel materials exposed were weld overlay Alloy 686, Alloy 50 and Sumitomo Super 625 coextruded tube. Exposure has been undertaken from 2003 to 2009 in the first pass and 2005–2009 in the second pass, and sections have been removed and investigated during this period. The composition of the deposits from the exposed waterwall panels was also analysed each time sections were removed. This paper will compare the various nickel alloys in the two areas and assess the results of the long-term testing project.

Propagation of harmonic emission from the turbines through the collection grid to the public grid

Abstract: This paper addressed the harmonic emission from a large off-shore wind farm. An overview is given of the issues, where a distinction is made between frequencies below and above 2 kHz. Three different approaches are presented: a simplified mathematical model; a more detailed mathematical model; and measurements at the point of connection for an off-shore wind farm. It is concluded from both models and measurements that the emission is small for frequencies above a few kHz. However, specific resonances at higher frequencies involving the power transformers, when coinciding with switching frequencies or harmonics of switching frequencies, could result in high emission even at these high frequencies. Studies, including the propagation through the collection grid, are needed with the connection of any wind park to the grid. (4 pages)