<p>Examensarbetet har utförts på uppdrag av NEA Gruppen och är en elkvalitetsanalys av VBG Groups maskinhall. Målet med analysen var att uppvisa elkvaliteten i maskinhallen.</p><p>Perfekt elkvalitet kan definieras som total frånvaro av elektriska störningar. Av elektriska störningar är det framförallt spänningsstörningar, såsom kortvariga spänningsvariationer, spänningsosymmetrier, spänningstransienter och likspänningskomponnenter, som orsakar mest besvär för elnät och anslutna laster. Även övertoner, som kan delas i spännings- och strömövertoner, påverkar elsystem på ett negativt sätt.</p><p>Analysen visar att maskinhallens spänning är stabil. Detta var väntat då kortslutningseffekten är hög jämfört med anläggningens storlek. Det matande nätet kan alltså anses starkt.</p><p>Maskinhallens laster alstrar strömövertoner av framförallt ordningstalen 5 och 7 men eftersom det matande nätet är starkt ger dessa inte upphov till några spänningsövertoner som ligger utanför rekommenderade gränser.</p><p>I maskinhallens ena inmatningspunkt, T1A15, finns ett kondensatorbatteri anslutet för reaktiv effektkompensering. Vid den andra inmatningspunkten, transformatorstation T3, finns idag ingen kompenseringsutrustning. Behovet är inte heller stort då belastningsgraden av T3 idag är under 10%. Man kan dock se att den reaktiva effekten redan nu är lika stor som den aktiva vilket medför att effektfaktorn är ca 0,7 och inom en nära framtid kommer fler laster att anslutas till T3 vilket innebär att ett kondensatorbatteri kan vara en lönsam investering.</p><p>Maskinhallens belysning alstrar strömövertoner av framförallt ordningstal tre. Övertoner av detta ordningstal har egenskapen att de ger upphov till strömmar i neutralledaren, i värsta fall så stora att neutralledaren blir överbelastad. Någon risk för överbelastning av neutralledaren för maskinhallens belysningsgrupp finns dock inte.</p><p>Totalt sett är anläggningens elkvalitet god och inga akuta åtgärder behöver vidtas.</p>

<p>För att öka skyddet mot elektriska fält i processnätet på Stora Enso Skoghalls bruk utreds det i denna rapport om det går att direktjorda processnätet som i dagens läge endast är skyddsjordat. Eftersom det tidigare endast funnits skyddsjord vill företaget veta om kabeldimensioneringen är tillräcklig så att de skydd man har mot kortslutningsströmmar även fungerar i ett direktjordat nät. I ett direktjordat nät blir den minsta kortslutningsströmmen cirka gånger mindre än icke direktjordat nät då kortslutning kan ske mellan fas och nolla (fasspänning). Nollan finns inte i ett icke direktjordat nät och kablarna är kan alltså vara för långa eller för klena så att en fasspänning inte klarar att driva tillräckligt stor ström vid kortslutning genom kabelns impedans förr att skydden skall lösa ut. Eftersom en nydragning av alla de kilometervis utlagda kablarna skulle bli en så ofantligt kostnad och att tiden för stoppet av processen skulle bli för lång är detta inte ett alternativ. Den här utredningen kommer att visa maximala kabellängden mellan de olika ställverken och motorerna i det befintliga nätet för att man skall kunna direktjorda det.</p> / <p>To increase the protection against electrical fields in the distribution net on Stora Enso’s mill in Skoghall it will be investigated if it’s possible to use direct earth instead of only protective earth as of today. Because the net only has protective earth the company wants to know if the dimensions of the cables are enough to meet the lesser shortcut voltage in a net with direct earth. The smallest shortcut current when using direct earth occurs between one phase and the zero and will be aproximately times less than when using non direct earth where there is not a zero and the shortcut can onlyu occur between two phases. This means that when building a non direct eart net you can use longer or thinner cables with more resistance than you could using direct earth. New cables are not an alternative when the cost would be to great and the downtime of the machines would be too long. This inquiry will show the maximum cabel length from the different protections to the different engines can be if you want to use direct earth and be sure that the fuses will burn when an error occurs.</p>

<p>Harmonics are a well-known problem that has to be dealt with in the design of thegenerator. Internationally accepted standards limits the amount of harmonics allowedin the no-load voltage. These limits can be difficult to fulfill with integral slot winding,where the number of slots per pole and phase is an integer. The presence of thedamper winding often makes the problem with harmonics even worse and harmonicswith frequencies of order 6q±1 arise with significant amplitude. How the damperwinding is designed affect the content of harmonics in the no-load voltage and the aimwith these … has been to investigate in which extent design of the damper windingaffects the contents of harmonics.Simulations with the 2-D finite element method have showed that if the damper barsis centered in the pole shoe, the amplitude of the harmonics of order 6q±1dependsboth on the ratio between the stator slot pitch and damper bars slot pitch and if thedamper bars are connected between the poles or not.If the damper bars is displaced with ¼ stator slot pitch alternately, the amplitude ofthe harmonics of order 6q±1 is reduced and the influence of the ratio between thestator slot pitch and the damper bars slot pitch will vanish. To minimize the loss inthe damper bars the distance between the damper bars should then be equal thedistance between the stator slots.</p>

There has been a pressure to operate power systems closer to their security limits. This has partially been due to financial imperatives following the deregulating of markets. Other practical difficulties have been obtaining authorization from regulatory bodies to build power plants and transmission lines. In this situation it is essential to monitor the system and to have tools that can predict the distance to the point of collapse (PoC). Much effort has been put into research of the phenomenon voltage collapse, and many approaches have been explored. Both dynamic and steady-state behavior have been studied thoroughly, though very few protection and control schemes have been implemented. In this dissertation the possibility of an index based on local area measurements have been explored. Voltage stability can be classified as either a transient or a long-term stability problem, and the index proposed in this dissertation is based on long-term dynamics.

The focus of this work has been on hybrid insulation in inhomogeneous electric fields under lightning impulse voltage stress. The principal idea behind hybrid insulation is the intentional use of surface charges to re-distribute the electric field within an insulation system. This allows a significant part of the electric stress to be transferred from the dielectric weaker gas to the dielectric stronger solid insulation thus increasing the total electric strength of the insulation system. The concept has been theoretically and experimentally addressed by means of a hemispheric rod covered with a layer of solid insulation. Discharge activity and surface charge accumulation have been studied in an air gap by measuring the voltage and discharge current and recording the discharge activity using a high-speed digital camera. New methods have been introduced and evaluated for the evaluation of surface charge measurements. The experiments found that the increase in positive inception voltage was considerable compared to uncovered rods. This increase varied from 35% up to 100% depending on the electrode distance. The increase in breakdown strength is higher than the increase in inception voltage and dependent on the covered length of the rod. During the application of a lightning impulse, the discharge activity spreads upwards along the rod and out into the air gap. Positive discharges form numerous branches and bridge the air gap in most cases. Negative discharges are more diffuse, less light intensive and only form a few branches around the tip of the rod where the electric field is the strongest. Discharge activity along the insulating surface has been observed where the background field is lower than the critical electric field strength. Visible discharge activity is observed where the background field is higher than 2.3 kV/mm and 2.5 kV/mm for positive and negative impulses respectively. During the application of lightning impulses, discharge activity starts in the air gap around the tip where the electric field is highest and spreads upwards along the rod. As expected, negative charges accumulate on the surface in the case of positive impulse voltage and vice versa. However, after more powerful discharges during negative impulse voltage application, surface charges of both polarities have been observed. Accumulated surface charges decay exponentially with a time constant τ varying from micro-seconds to hours depending on the material properties of the solid insulation. The dominating relaxation mechanism is found to be conduction through the solid insulation. Improved methods to calculate surface charges based on probe response for a 2D axial symmetric case have been developed and evaluated. The method that is best suited for this purpose is the λ-method with truncated singular value decomposition (TSVD) as regularization. Surface charge calculations show that the accumulated surface charges for the used configuration typically have a maximum value of 0.6 to 1.5 µC/m² and 0.4 to 1 µC/m² after positive and negative impulses respectively. The surface charge density in the areas with the highest discharge activity is relatively uniform. Further upwards along the rod, the surface charge density is reduced relatively fast towards zero, and in some cases, it changes polarity before approaching zero.

In this master thesis the theory and practical use of modal analysis is explained, giving an introduction to the possibilities of modal analysis. The master thesis starts with a look at wind power and the design of a modern wind turbine. Two models, one for constant wind speed wind turbines and one for variable speed wind turbines, are presented. An example shows how modal analysis can be utilized to evaluate a network's dynamic stability. Simulations are performed on a two-area network where different wind power models are tested and compared. A two-mass model is used to model a constant wind turbine. The model consists of an asynchronous generator, a turbine, and a low speed shaft with a tensional stiffness. The model representing the variable speed wind turbine is based on a DFIG model included in the simulation software. The two-area network consists of two areas connected together through a long line between Bus 5 and Bus 6. Area 1 has two production sources, one placed in Bus 1 and one placed in Bus 2. The second area represents a large network modelled as a very large synchronous generator with a high inertia. The calculations have showed how modal analysis can be used to evaluate a system by using linearized differential equations and how the systems robustness against small disturbances can be altered by changing the systems parameters. Simulations have verified that a two-mass model must be used when modelling a constant speed wind turbine. The inertia of the turbine will greatly influence the model's behaviour and must therefore be included in the model. Eigenvalues analysis performed during different wind speeds have documented that wind power will not become less stable towards small disturbances when operated at low wind speed conditions.

With the recent developments in semiconductors and control equipment, Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) has attracted the growing interest of researchers. The use of VSC technology and Pulse Width Modulation (PWM) has a number of potential advantages: short circuit current reduction; rapid and independent control of the active and reactive power, etc. With such highly favourable advantages, VSC-HVDC is definitely going to be a large part of future transmission and distribution systems. HVDC technology based on VSC technology has been an area of growing interest recently because of its suitability in forming a transmission link for transmitting bulk amount of wind power. This thesis deals with the control of VSC-HVDC. The objective of the work is to understand the control structure of the VSC-HVDC system, and establish the tuning criteria for the PI controllers of the converter controllers. A model of a VSC based dc link using PWM Technology is developed. A mathematical model of the control system based on the relationships between voltage and current is described for the VSC. A control system is developed combining an inner current loop controller and outer dc voltage controller. The vector control strategy is studied and corresponding dynamic performance under step changes and system fault is investigated in PSCAD/EMTDC simulation package. The simulation results verify that the model can fulfill bi-directional power transfers, fast response control and that the system has good steady state performance. The controller parameters tuned according to the developed tuning criteria is found to provide acceptable system performances.

The North Sea has a vast amount of wind energy with largest energy per area densities located about 100-300Km of distance from shore. Should this energy be tapped by offshore wind farms, HVDC transmission would be the more feasible solution at such long subsea distances. On the other hand Norwegian oil/gas platforms in the North Sea use electricity from gas fired turbines at offshore sites. These gas turbines have much less efficiency than onshore generation of electricity and also release large amounts of green house gases. Therefore supplying the platforms with power from onshore transmitted by HVDC will result in benefits both from economic and environmental protection perspectives. Given these two interests for HVDC in the Norwegian offshore, the use of Multiterminal HVDC (MTDC) is a potential solution for the integration of the wind farms and oil/gas platforms into the onshore grid system. Hence, this thesis focuses on the operation and control of MTDC systems. The MTDC system is desired to be capable of interfacing with all kinds of AC grids namely: stiff, weak and passive grid systems. Compared to the classical thyristor based converter, VSC has several features that make it the most suitable converter for making of MTDC, the most decisive being its ability of bidirectional power transfer for fixed voltage polarity. VSC-HVDC is also suitable for implementing control of active and reactive current in synchronously rotating d-q reference frame which in turn results in decoupled control of active and reactive power. In the first two chapters of the thesis literatures are reviewed to understand operation of VSC and its use in HVDC systems. Afterwards controllers are developed for different AC connections (stiff, weak and passive) and for different DC parameter (power, DC voltage) control modes. DC voltage and active power control are implemented by active current control and AC voltage and reactive power control are achieved by reactive power compensation. Tuning techniques for the PI controllers are discussed and used in the simulation models. Finally control techniques for reliable operation of MTDC are developed. In order to validate theoretical arguments, each of the control schemes was developed and simulated in PSCAD/EMTDC simulation software. Simulation results indicate that satisfactory performance of VSC-HVDC was obtained with the proposed active/reactive power controllers, AC/DC voltage controllers, frequency and DC overvoltage controllers. For coordinated multiterminal operation, voltage margin control method and DC voltage droop characteristic were used. These are control methods based upon realization of desired P-UDC characteristic curves of converter terminals. Four-terminal MTDC system with different AC grid connections was used to study the multiterminal operation. Simulations have shown that voltage margin control method results in reliable operation of MTDC during loss of a terminal connection without the need for communication between terminals. The use of DC voltage droop control along with voltage margin control enabled load sharing among VSC-HVDC terminals in DC voltage control mode according to predetermined participation factor.

Direct electrical heating (DEH) of pipelines is a flow assurance method that has proven to be a good and reliable solution for preventing the formation of hydrates and wax in multiphase flow lines. The technology is installed on several pipelines in the North Sea and has become StatoilHydros preferred method for flow assurance. Tyrihans is the newest installation with 10 MW DEH for a 43 km pipline. However, the pipeline represents a considerable single-phase load which makes the power system dependent on a balancing unit for providing symmetrical conditions. This limits the step out distance and is not suitable for subsea installation. Aker Solutions has proposed several specially connected transformers for subsea power supply of DEH systems, Scott-T being one of them. The Scott-T transformer is a three-to-two-phase transformer which provides balanced electrical power between the two systems when the two secondary one-phase loads are equal. By implementing this transformer, it can be possible to install the power supply subsea as there is no need for a balancing unit. In addition, the system may be applicable for long step out distances. This is because the pipeline is inductive and can use the reactive power produced by the long cable which also can increase the critical cable length. There are however some limitations on this system using the Scott-T transformer. There is a large variation in the magnetic permeability between individual joints of the pipeline. This can result in different load impedance of the two pipe sections connected to the Scott-T transformer. The result is unbalance in the power system. The method of symmetrical components is applied to investigate the behavior during unbalanced loading of the Scott-transformer. The relationship between the negative- and the positive sequence component of the current is used to express the degree of unsymmetry. For the simulations in SIMPOW, the Scott-T transformer is modelled by the use of Dynamic Simulation Language. The simulations on the DSL model give correct and reliable results for analysing the the degree of unsymmetry in the Scott-T transformer. When the load impedance of one pipe section is varied, simulation proves that it can change between 0.75 and 1.34 per unit of the other pipe impedance. The Scott-T transformer does still provide electrical power between the two systems which is below the limit for the degree of unsymmetry (15%). Case 1 and Case 2 introduce two possible configurations for a subsea DEH system with the Scott-T transformer implemented. The configurations include an onshore power supply which is connected to a subsea power system for direct electrical heating and a subsea load at the far end of the subsea cable. The pipeline in Case 1 is 100 km long and is divided into two pipe sections of 50 km which are connected to a Scott-T transformer. The pipeline in Case 2 is 200 km long and is divided into four pipe sections of 50 km each. There are two Scott-T transformers in Case 2. For normal operation of the subsea load (50 MW, cosfi=0.9) and heating the pipe content from the ambient sea emperature, the results indicate that tap changers are necessary to keep the Scott-T transformers secondary terminal voltage at 25 kV. This meets the requirement in both cases for heating the pipe content from 4 to 25 degrees celsius within 48 hours after a shutdown of the process. The degree of unsymmetry is zero for both cases when the system is operated as normal. However, all system simulations indicate that reactive power compensation has to be included for Case 1 as well as for Case 2 in order to have a power factor of unity at the onshore grid connection. The fault scenarios indicate that the degree of unsymmetry is dependent on both the type of fault and the power supply in the system. For Case 1, the relationship (I-/I+) is only of 3.3% in the subsea cable when there is a short-circuit at DEHBUS3, but as much as 87% at the grid connection. The degree of unsymmetry in the Scott-T transformer is then 67%. This is far beyond the limit for maximum negative sequence component of 15%. The significant unsymmetry in the line between the grid and BUS1 is most likely due to the large power delivered to the fault. During the fault, the reactive power delivered to the system increases from 10.6 Mvar to 131.9 Mvar after the fault, but the active power increases only from 75.2 MW to 87.1 MW. This means that it is most likely the reactive power that contributes to the consequent unsymmetry and negative sequence component of the current. There are two Scott-T transformers installed in Case 2. If the DEH system is only heating the pipe section closest to shore (at DEHBUS33), simulations show that the three-phase power system becomes unsymmetric which results in different phase currents. The degree of unsymmetry at the grid connection is 32% when only the pipe section at DEHBUS33 is heated. In addition, the unbalance in the three-phase system caused by SCOTT1 involves unbalance in the SCOTT2 transformer as well. The load voltages are not equal in magnitude and dephased of 90 degrees for this mode, but are 32 kV and 35 kV respectively and dephased of 88 degrees. This concludes a very important behavior of the Scott-T transformer. The simulations conclude that the Scott-T transformer provides symmetrical conditions for both configurations when the two load impedances are equal. However, Case 2 shows an important result when installing two Scott-T transformers in the same system. Unbalanced loading of one of the specially connected transformers gives unsymmetrical conditions in the three-phase system which results in unbalanced load voltages for the other Scott-T transformer. The analysis is limited to the configurations given for Case 1 and Case 2, but shows typical results when an alternative transformer connection is implemented in a DEH system.

There is an increasing amount of power processed through power electronics in the areas of generation interface, energy storage and loads. This increment enables possibilities for improved solutions for efficient generation and use of electric power. Traditionally loads in AC distribution systems have been seen as passive and individual elements rather than active components of the system. An AC distribution system with high percentage of power electronic loads can be susceptible for instability under abnormal operation conditions if the converter controls are not designed for such conditions. This thesis introduces a solution of reactive current control for a constant power load (CPL) in an AC distribution system. A CPL is a load that draws a constant amount of active power without regard to any drops in system voltage. The resistance seen from the AC distribution system to the CPL is known as a negative input resistance which is characteristic for this kind of load. A drop in AC system voltage will result in increased current and vice versa. Hence the resulting resistance will be negative. Negative resistance behaviour is not favourable for system stability and the work has focused on a control of the converter that reduces the instability effect of the load. By controlling the reactive current component to inject current for support of the AC distribution system voltage during faults and other interferences, the load becomes an active participant in the AC distribution system. An AC distribution system has been built in PSCAD/EMTDC. An asynchronous generator and a fixed capacitor are used as distributed generation. The CPL consists of an AC/DC converter and a voltage dependent current source in order to keep constant DC power at the load. Focus has been on the converter control implementing a voltage oriented vector control based on a two phase rotational reference frame. Measurements of current and voltage with different voltage drop levels have been done for comparison of a CPL with and without reactive current control. The simulation example has verified the negative resistance phenomenon of the CPL. Results shows that the AC system voltage is less vulnerable in case of faults if the converter control is designed to inject reactive current into the AC distribution system.