| 1. |
- Aaro Jonsson, Catherine
(författare)
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Long-term cognitive outcome of childhood traumatic brain injury
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There is limited knowledge of cognitive outcome extending beyond 5 years after childhood traumatic brain injury, CTBI. The main objectives of this thesis were to investigate cognitive outcome at 6-14 years after CTBI, and to evaluate if advancements in the neurosurgical care, starting 1992, did influence long-term outcome and early epidemiology. An additional aim was to study the relationship between early brain injury parameters and early functional outcome. Study 1 evaluated cognitive progress during 14 years after CTBI, over three neuropsychological assessments in 8 patients with serious CTBI. Study 2 used patient records to investigate early epidemiology, received rehabilitation and medical follow up in two clinical cohorts, n=82 and n=46, treated neurosurgically for CTBI before and after 1992. An exploratory cluster analysis was applied to analyse the relation between early brain injury severity parameters and early functional outcome. In Study 3, participants in the two cohorts, n=18 and n=23, treated neurosurgically for CTBI before and after 1992, were subject to an extensive neuropsychological assessment, 13 and 6 years after injury, respectively. Assessment results of the two cohorts were compared with each other and with controls. Data were analysed with multivariate analyses of variance. Results and discussion. There were significant long-term cognitive deficits of similar magnitude and character in the two cohorts with CTBI, treated before and after the advancements in neurosurgical care. At 6-14 years after injury, long-term deficits in verbal intellectual and executive functions were found, and were discussed in terms of their late maturation and a decreased executive control over verbal memory-functions after CTBI. Visuospatial functions had a slightly better long-term recovery. The amount of rehabilitation received was equally low in both cohorts. The length of time spent in intensive care and the duration of care in the respirator may have a stronger relationship to early outcome than does a single measure of level of consciousness at admission. Main conclusions are that cognitive deficits are apparent at long-term follow up, 6-13 years after neurosurgically treated CTBI, even after advancements in the neurosurgical care in Sweden. Measures of verbal IQ, verbal memory and executive functions were especially low while visuospatial intellectual functions appear to have a better long-term recovery.
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| 2. |
- Aas, Randi Wågö
(författare)
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Workplace-based sick leave prevention and return to work : exploratory studies
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Background: Earlier research have revealed risk factors for sick leave in the workplace, and thus the workplace has become an important arena for sick leave prevention and return to work (RTW). Despite that, some of these aspects have received little attention in exploratory studies. Simultaneously, there is a need to translate and implement the growing knowledge base in this field in order to develop evidence-based practice (EBP). Aim: The aim of the present research was to explore some aspects of workplace-based sick leave prevention and RTW, such as workplace interventions (studies III, IV, and the appendix), leadership qualities (study I), and work demands (study II), and also to reveal challenges to translating scientific knowledge into intervention decisions in the RTW process, and possible solutions to these challenges (study III). Methods: Content analysis methods were applied on data from interview transcripts and documents. In addition, a Cochrane systematic review of the literature was conducted. Results: Study I identified 78 distinct leadership qualities and seven leadership types (n = 345 meaning units) perceived by 30 employees on long-term sick leave and their immediate supervisors. The three most valued leadership qualities were “ability to make contact”, “being considerate”, and “being understanding”. The three most valued leadership types were the Protector, the Problem-Solver, and the Contact-Maker. The subordinates gave more descriptions of the Encourager and the Recognizer, whereas the supervisors most often described the Responsibility-Maker and the Problem-Solver. The combination of leadership types reported most frequently was the Protector together with the Problem-Solver. In study II, eight employees on long-term sick leave due to musculoskeletal diseases and disorders described 51 work demands they had experienced. The demands were perceived in some cases as having only a negative or a positive impact on work performance, but in others as both. Only seven of the demands were physical in nature, and most involved emotional and cognitive challenges in mastering the work tasks. It was also experienced that most demands came from the employee (n = 36) and only a few from the employer/work environment (n = 7) or both those sources (n = 8). Study III was a hypothetical case study aimed at revealing the challenges associated with translating scientific evidence into intervention decisions in the RTW process. This investigation was performed according to EBP frameworks. The evidence seemed to differ depending on whether it came from preventive, curative, or rehabilitative interventions. Moreover, it appeared that evidence in some cases originated from “good-for-all” interventions but in others from “tailored-type” interventions. Thus, a need to differentiate the roles of evidence was revealed in terms of whether it inspired, challenged, enlightened, informed, or determined the intervention decision. In general, the evidence-based framework seemed to construct a confined decision process. Possible solutions, and revised EBP steps were suggested. In study IV, 15 workplace interventions were identified (n = 306 meaning units), which were intended to reduce sick leave rates in 12 municipalities. The interventions were divided into two groups according to their targets in the organizations: nine organizational-workplace interventions targeted structures, processes, and culture (n = 220 descriptions, 72%); six employee-workplace interventions targeted persons (n = 86 descriptions, 28%). Examples of organizational-workplace interventions were developing routines/systems, establishing cooperation/ collaboration, providing information/education, building culture/anchoring, and recruiting/staffing. Employee-workplace interventions involved well-being/lifestyle interventions, physical activity/exercise, redeployment, adaptation, follow-up of employees on sick leave, and RTW programmes. The intervention profiles varied considerably between the municipalities. In the appendix (study V), a Cochrane systematic review of the literature was conducted to reveal the content and effectiveness of workplace interventions for employees with neck pain. Of 1,995 references found, 10 randomized controlled trials (RCTs) were included. Two of the RCTs had low risk of bias, and eight of them examined office workers. Few were on sick leave. Only three of the ten studies assessed the outcome of sick leave. The workplace interventions varied considerably regarding complexity and content. Overall, evidence was of low quality and showed no significant impact of workplace interventions on pain reduction (seven RCTs, 2,368 workers). Furthermore, one RCT, with 415 workers revealed that workplace interventions were significantly more effective in reducing sick leave in the intermediate term (OR 0.56, 95% CI 0.33–0.95), but not in the short or the long term. Conclusions: The results reported in this thesis revealed a variety of terminology related to workplace interventions, leadership qualities, and work demands, which might contribute to more in-depth understanding of sick leave prevention and RTW at workplaces. It was a challenge to trying to use evidence from randomized controlled trials in the RTW process, and the results call for new EBP approaches to translate evidence into decisions concerning complex workplace interventions. The current research also revealed that knowledge about the effectiveness of workplace interventions is still limited.
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| 3. |
- Aasa, Mikael
(författare)
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Reperfusion therapy in acute ST-elevation myocardial infarction : a comparison between primary percutaneous intervention and thrombolysis in a short- and long-term perspective
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Approximately 35,000 people suffer from a heart attack in Sweden annually. Among them, approximately 8000 are diagnosed with a ST-elevation myocardial infarction (STEMI) where timely reperfusion has been shown to save lives. Previous studies that have compared the existing reperfusion strategies, thrombolysis (TL) and primary PCI (PPCI), made use of treatment regimens that since have been improved with the use of mechanical and medical adjunctives. The objective of this thesis was to compare both of these strategies employing updated regimens in accordance to current guidelines with respect to; 1) efficacy in restoring blood flow and myocardial perfusion, 2) clinical outcome and 3) cost-effectiveness. Methods and results: Between November 2001 and May 2003, 205 patients with STEMI were randomized to PPCI with adjunctive abciximab or TL. The low molecular weight heparin enoxaparin was used as anticoagulant in both groups. In 42% treatment was initiated in the pre-hospital phase. The primary end points were the rate of STsegment resolution (STRES) ≥ 50% 120 minutes after inclusion and the rate of normalized (TIMI 3) flow in the infarct related vessel 5-7 days after treatment, serving as surrogates for a beneficial outcome. Secondary end points were the ability to restore myocardial perfusion evaluated angiographically by TIMI Myocardial Perfusion Grade (TMPG) 5-7 days after inclusion in the study, clinical events at 30 days and one year cost-effectiveness. The patients were followed prospectively for one year and, in addition, information on survival status and major clinical events was collected from national registries for an extended follow up period of a median of 5.3 years. STRES≥ 50% was achieved in 68% following PPCI and 64% after TL (n.s.). However, the TIMI 3 rate was higher after PPCI compared to TL (71% vs. 54%, p=0.04). TMPG tended to be better in the PPCI group than in the TL group. An analysis of the evolution of TMPG in the PPCI cohort revealed that there was a significant improvement of myocardial perfusion in the week following PPCI. Thirty day mortality rates were low and similar in the groups. At one year PPCI was tended to be less costly ($-2,505) than TL ($-2,505; n.s.), mainly due to higher costs for re-hospitalizations in the TL group. Primary PCI also lead to an insignificant gain in quality-adjusted survival (0.031 QALYs). A bootstrap analysis indicated that PPCI has a high probability of being cost-effective when a threshold value of $50,000 is employed. A survival analysis at 5.3 years showed a significant benefit from PPCI in terms of the combination of all-cause death and recurrent infarction (p=0.03) as well as for cardiac mortality alone (p=0.02). Conclusion: Primary PCI is more efficient than thrombolysis in re-establishing antegrade flow in the infarct- related artery and offers a better long term clinical outcome with respect to major cardiac events without an increase in societal costs. Thus, based on the conditions under which this study was performed, primary PCI is a more efficient alternative than thrombolysis for the treatment of ST-elevation myocardial infarction.
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| 4. |
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| 5. |
- Abbas, Haider, 1979-
(författare)
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Options-Based Security-Oriented Framework for Addressing Uncerainty Issues in IT Security
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Continuous development and innovation in Information Technology introduces novel configuration methods, software development tools and hardware components. This steady state of flux is very desirable as it improves productivity and the overall quality of life in societies. However, the same phenomenon also gives rise to unseen threats, vulnerabilities and security concerns that are becoming more critical with the passage of time. As an implication, technological progress strongly impacts organizations’ existing information security methods, policies and techniques, making obsolete existing security measures and mandating reevaluation, which results in an uncertain IT infrastructure. In order to address these critical concerns, an options-based reasoning borrowed from corporate finance is proposed and adapted for evaluation of security architecture and decision- making to handle them at organizational level. Options theory has provided significant guidance for uncertainty management in several domains, such as Oil & Gas, government R&D and IT security investment projects. We have applied options valuation technique in a different context to formalize optimal solutions in uncertain situations for three specific and identified uncertainty issues in IT security. In the research process, we formulated an adaptation model for expressing options theory in terms useful for IT security which provided knowledge to formulate and propose a framework for addressing uncertainty issues in information security. To validate the efficacy of this proposed framework, we have applied this approach to the SHS (Spridnings- och Hämtningssystem) and ESAM (E-Society) systems used in Sweden. As an ultimate objective of this research, we intend to develop a solution that is amenable to automation for the three main problem areas caused by technological uncertainty in information security: i) dynamically changing security requirements, ii) externalities caused by a security system, iii) obsoleteness of evaluation. The framework is general and capable of dealing with other uncertainty management issues and their solutions, but in this work we primarily deal with the three aforementioned uncertainty problems. The thesis presents an in-depth background and analysis study for a proposed options-based security-oriented framework with case studies for SHS and ESAM systems. It has also been assured that the framework formulation follows the guidelines from industry best practices criteria/metrics. We have also proposed how the whole process can be automated as the next step in development.
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| 6. |
- Abbas, Qaisar, 1975-
(författare)
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Weak Boundary and Interface Procedures for Wave and Flow Problems
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, we have analyzed the accuracy and stability aspects of weak boundary and interface conditions (WBCs) for high order finite difference methods on Summations-By-Parts (SBP) form. The numerical technique has been applied to wave propagation and flow problems.The advantage of WBCs over strong boundary conditions is that stability of the numerical scheme can be proven. The boundary procedures in the advection-diffusion equation for a boundary layer problem is analyzed. By performing Navier-Stokes calculations, it is shown that most of the conclusions from the model problem carries over to the fully nonlinear case.The work was complemented to include the new idea of using WBCs on multiple grid points in a region, where the data is known, instead of at a single point. It was shown that we can achieve high accuracy, an increased rate of convergence to steady-state and non-reflecting boundary conditions by using this approach.Using the SBP technique and WBCs, we have worked out how to construct conservative and energy stable hybrid schemes for shocks using two different approaches. In the first method, we combine a high order finite difference scheme with a second order MUSCL scheme. In the second method, a procedure to locally change the order of accuracy of the finite difference schemes is developed. The main purpose is to obtain a higher order accurate scheme in smooth regions and a low order non-oscillatory scheme in the vicinity of shocks.Furthermore, we have analyzed the energy stability of the MUSCL scheme, by reformulating the scheme in the framework of SBP and artificial dissipation operators. It was found that many of the standard slope limiters in the MUSCL scheme do not lead to a negative semi-definite dissipation matrix, as required to get pointwise stability.Finally, high order simulations of shock diffracting over a convex wall with two facets were performed. The numerical study is done for a range of Reynolds numbers. By monitoring the velocities at the solid wall, it was shown that the computations were resolved in the boundary layer. Schlieren images from the computational results were obtained which displayed new interesting flow features.
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| 7. |
- Abbasi, Abdul Ghafoor, 1977-
(författare)
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CryptoNET : Generic Security Framework for Cloud Computing Environments
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The area of this research is security in distributed environment such as cloud computing and network applications. Specific focus was design and implementation of high assurance network environment, comprising various secure and security-enhanced applications. “High Assurance” means that - our system is guaranteed to be secure, - it is verifiable to provide the complete set of security services, - we prove that it always functions correctly, and - we justify our claim that it can not be compromised without user neglect and/or consent. We do not know of any equivalent research results or even commercial security systems with such properties. Based on that, we claim several significant research and also development contributions to the state–of–art of computer networks security. In the last two decades there were many activities and contributions to protect data, messages and other resources in computer networks, to provide privacy of users, reliability, availability and integrity of resources, and to provide other security properties for network environments and applications. Governments, international organizations, private companies and individuals are investing a great deal of time, efforts and budgets to install and use various security products and solutions. However, in spite of all these needs, activities, on-going efforts, and all current solutions, it is general belief that the security in today networks and applications is not adequate. At the moment there are two general approaches to network application’s security. One approach is to enforce isolation of users, network resources, and applications. In this category we have solutions like firewalls, intrusion–detection systems, port scanners, spam filters, virus detection and elimination tools, etc. The goal is to protect resources and applications by isolation after their installation in the operational environment. The second approach is to apply methodology, tools and security solutions already in the process of creating network applications. This approach includes methodologies for secure software design, ready–made security modules and libraries, rules for software development process, and formal and strict testing procedures. The goal is to create secure applications even before their operational deployment. Current experience clearly shows that both approaches failed to provide an adequate level of security, where users would be guaranteed to deploy and use secure, reliable and trusted network applications. Therefore, in the current situation, it is obvious that a new approach and a new thinking towards creating strongly protected and guaranteed secure network environments and applications are needed. Therefore, in our research we have taken an approach completely different from the two mentioned above. Our first principle is to use cryptographic protection of all application resources. Based on this principle, in our system data in local files and database tables are encrypted, messages and control parameters are encrypted, and even software modules are encrypted. The principle is that if all resources of an application are always encrypted, i.e. “enveloped in a cryptographic shield”, then - its software modules are not vulnerable to malware and viruses, - its data are not vulnerable to illegal reading and theft, - all messages exchanged in a networking environment are strongly protected, and - all other resources of an application are also strongly protected. Thus, we strongly protect applications and their resources before they are installed, after they are deployed, and also all the time during their use. Furthermore, our methodology to create such systems and to apply total cryptographic protection was based on the design of security components in the form of generic security objects. First, each of those objects – data object or functional object, is itself encrypted. If an object is a data object, representing a file, database table, communication message, etc., its encryption means that its data are protected all the time. If an object is a functional object, like cryptographic mechanisms, encapsulation module, etc., this principle means that its code cannot be damaged by malware. Protected functional objects are decrypted only on the fly, before being loaded into main memory for execution. Each of our objects is complete in terms of its content (data objects) and its functionality (functional objects), each supports multiple functional alternatives, they all provide transparent handling of security credentials and management of security attributes, and they are easy to integrate with individual applications. In addition, each object is designed and implemented using well-established security standards and technologies, so the complete system, created as a combination of those objects, is itself compliant with security standards and, therefore, interoperable with exiting security systems. By applying our methodology, we first designed enabling components for our security system. They are collections of simple and composite objects that also mutually interact in order to provide various security services. The enabling components of our system are: Security Provider, Security Protocols, Generic Security Server, Security SDKs, and Secure Execution Environment. They are all mainly engine components of our security system and they provide the same set of cryptographic and network security services to all other security–enhanced applications. Furthermore, for our individual security objects and also for larger security systems, in order to prove their structural and functional correctness, we applied deductive scheme for verification and validation of security systems. We used the following principle: “if individual objects are verified and proven to be secure, if their instantiation, combination and operations are secure, and if protocols between them are secure, then the complete system, created from such objects, is also verifiably secure”. Data and attributes of each object are protected and secure, and they can only be accessed by authenticated and authorized users in a secure way. This means that structural security properties of objects, upon their installation, can be verified. In addition, each object is maintained and manipulated within our secure environment so each object is protected and secure in all its states, even after its closing state, because the original objects are encrypted and their data and states stored in a database or in files are also protected. Formal validation of our approach and our methodology is performed using Threat Model. We analyzed our generic security objects individually and identified various potential threats for their data, attributes, actions, and various states. We also evaluated behavior of each object against potential threats and established that our approach provides better protection than some alternative solutions against various threats mentioned. In addition, we applied threat model to our composite generic security objects and secure network applications and we proved that deductive approach provides better methodology for designing and developing secure network applications. We also quantitatively evaluated the performance of our generic security objects and found that the system developed using our methodology performs cryptographic functions efficiently. We have also solved some additional important aspects required for the full scope of security services for network applications and cloud environment: manipulation and management of cryptographic keys, execution of encrypted software, and even secure and controlled collaboration of our encrypted applications in cloud computing environments. During our research we have created the set of development tools and also a development methodology which can be used to create cryptographically protected applications. The same resources and tools are also used as a run–time supporting environment for execution of our secure applications. Such total cryptographic protection system for design, development and run–time of secure network applications we call CryptoNET system. CrytpoNET security system is structured in the form of components categorized in three groups: Integrated Secure Workstation, Secure Application Servers, and Security Management Infrastructure Servers. Furthermore, our enabling components provide the same set of security services to all components of the CryptoNET system. Integrated Secure Workstation is designed and implemented in the form of a collaborative secure environment for users. It protects local IT resources, messages and operations for multiple applications. It comprises four most commonly used PC applications as client components: Secure Station Manager (equivalent to Windows Explorer), Secure E-Mail Client, Secure Web Browser, and Secure Documents Manager. These four client components for their security extensions use functions and credentials of the enabling components in order to provide standard security services (authentication, confidentiality, integrity and access control) and also additional, extended security services, such as transparent handling of certificates, use of smart cards, Strong Authentication protocol, Security Assertion Markup Language (SAML) based Single-Sign-On protocol, secure sessions, and other security functions. Secure Application Servers are components of our secure network applications: Secure E-Mail Server, Secure Web Server, Secure Library Server, and Secure Software Distribution Server. These servers provide application-specific services to client components. Some of the common security services provided by Secure A
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| 8. |
- Abbott, Allan, 1978-
(författare)
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Physiotherapeutic rehabilitation and lumbar fusion surgery
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the last two decades, the economic costs and rates of lumbar fusion surgery for chronic low back pain has risen dramatically in western industrialized countries. Data from the Swedish National Spine Register suggest that 25% of patients experience unimproved pain and up to 40% are not satisfied with the outcome of lumbar fusion surgery. Thus, there is a definite need to optimize the selection and management of patients to improve lumbar fusion outcomes. Aim: To investigate the role of biopsychosocial factors in explaining disability and health related quality of life in chronic low back pain patients before and after lumbar fusion surgery and to evaluate the effectiveness of post-operative rehabilitation regimes. Methods: At total of 107 patients were recruited, aged 18 to 65 years, selected for lumbar fusion due to 12 months of symptomatic back and/or leg pain due to spinal stenosis, degenerative/isthmic spondylolisthesis or degenerative disc disease. Measures of disability, health related quality of life, pain, mental health, fear of movement/(re)injury, self-efficacy, outcome expectancy, pain coping styles, work status, health care use, analgesic use and sickness leave were collected with self-rated questionnaires at baseline (Studies I-IV), 3, 6, 12 months (Study II) and 2-3 years after surgery (Studies II-III). In Studies II-IV, patients were randomised to psychomotor therapy (N=53) or exercise therapy (N=54) implemented during the first 3 post-operative months. Semi-structured interviews were conducted 3-6 months after surgery on 20 patients including 10 from each rehabilitation group to investigate experiences of back problems before and after surgery, post-operative recovery and expectations of rehabilitation analysed in terms of the International Classification of Functioning, Disability and Health (Study IV). Results: Approximately 50% of the variability in baseline disability and 40% of the variability in baseline health related quality of life could be explained by psychological variables. In particular, catastrophizing, control over pain, self-efficacy and outcome expectancy had significant mediation roles (Study I). For the short and long term outcome of lumbar fusion surgery, post-operative psychomotor therapy is significantly more effective than exercise therapy with approximately 10-20% better outcome in measures of disability, fear of movement/(re)injury, pain catastrophizing, self-efficacy, outcome expectancy sickness leave, health care utilization and return to work (Study II). A model with good outcome predictive performance which significantly predicts disability, back pain and health related quality of life outcomes 2-3 year after lumbar fusion surgery, was shown to involve pre-operative screening of disability, leg pain intensity, mental health, fear of movement/(re)injury, outcome expectations, catastrophizing, control over pain and the implementation of post-operative psychomotor therapy (Study III). Lumbar fusion patient s experiences of back problems before and after the operation as well as experiences of recovery and outcome expectations correspond well with the content of outcomes measures used in the study suggesting good content validity (Study IV). Conclusion: Psychological factors strongly influence levels of disability and health related quality of life in lumbar fusion candidates as well as predicts post-operative outcomes. Early post-operative rehabilitation focusing on cognition, behaviour and motor control is recommended for improved lumbar fusion outcomes.
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| 9. |
- AbdGawad, Mohamed
(författare)
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Proteinase 3 and Neutrophil Apoptosis in ANCA-Associated Systemic Vasculitis
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- ANCA-Associated Systemic Vasculitis (AASV) is characterized by leukocytoclasis- dying neutrophils surrounding small blood vessels. Previous studies have shown increased plasma PR3 and increased proportion of neutrophils expressing plasma membrane-associated PR3 (mPR3+). Neutrophils were isolated by polymorphprep and membrane expression was measured by FACS. Plasma PR3, pro-PR3 and cytokines were measured by ELISA. Neutrophil apoptosis was measured by FACS after 20h in vitro culture. Expression of pro/anti-apoptotic factors and transcription factors C/EBP-α, C/EBP-β and PU.1 was measured by real-time PCR. Plasma PR3, pro-PR3, and proportion of mPR3+neutrophils were elevated in AASV. mPR3 and CD177 were co-expressed on a subset of neutrophils both in AASV and controls. mPR3 expression was not correlated directly to circulating PR3 or PR3 gene transcription, but was dependent on transcription of the CD177 gene. AASV neutrophils had a significantly lower rate of apoptosis than neutrophils from healthy blood donors (HBD). Lower apoptosis rate did not correlate with clinical data, plasma PR3, cytokine levels, mPR3 expression or level of pro/anti-apoptotic factors. However, C/EBP-α and C/EBP-β were significantly higher in neutrophils from AASV patients than in HBD neutrophils. These results indicate an altered neutrophil phenotype in AASV and suggest that the rate of granulopoiesis is higher, while the rate of neutrophil apoptosis is lower. This may predispose for the presence of dying neutrophils in regions of inflammation in AASV. Additional studies are required to determine why the rate of neutrophil apoptosis is lower in AASV patients and if dysregulation of neutrophil apoptosis contributes directly to the pathogenesis.
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| 10. |
- Abdulahovic, Tarik, 1976
(författare)
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Analysis of High-Frequency Electrical Transients in Offshore Wind Parks
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, a study of high frequency electromagnetic transient phenomena is performed. Models of various components needed for simulations of high frequency transients, such as for transformers, cables and breakers are developed. Measurements were performed in Chalmers research laboratory as well as at ABB Corporate Research in Västerås, Sweden for the purpose of parameter estimation of models and for verification of simulations. Critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the voltage distribution along the winding during very fast transients is studied in order to estimate turn-to-turn voltages and the critical voltage envelope. In the work it was found that simulations of the high frequency transients are in a very good agreement with the measurements obtained in the laboratory. Simulations predicted accurately critical surges with the highest magnitude and matched with good accuracy surge waveforms recorded during the measurements. The accuracy of the rise times is within 10%, while the magnitudes during the critical cases are within a 5% margin. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of dry-type transformers specified by IEEE standards, both in magnitude and rise time are observed at the transformers even when surge arresters are used to protect the transformers. Furthermore, obtained voltage surges exceeded the proposed critical voltage envelope. It is shown that the most critical voltage strikes are obtained with dry-type transformers, where the rise time is five to ten times shorter compared to oil-insulated transformer with the same rating. It was confirmed both in simulations and measurements that the use of additional transient protections devices such as surge capacitors and RC protections, decreased the magnitude of surges to be below the critical level. The analysis of the voltage during very fast transients showed that the rise time of the transients directly influences the magnitude of the turn-to-turn voltages. Furthermore, during the breaker closing transient, turn-to-turn voltages, measured in delta connected dry-type transformers, were 2.5 times higher than the voltages obtained during the same transient with a wye connected dry-type transformer, or during the stress with a 4.4 p.u. lightning impulse voltage.
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