Delta STATCOM with partially rated energy storage for intended provision of ancillary services

This thesis presents research on two distinct areas, where the work carried out in the first half highlights the challenges posed by the declining system inertia in the future power systems and the potential capability of the energy storage systems in bridging the gap, supporting a safe and reliable operation. A comparison of various energy storage technologies based on their specific energy, specific power, response time, life-cycle, efficiency, cost and further correlating these characteristics to the timescale requirements of frequency and RoCoF services showed that supercapacitors (SC) and Li-ion batteries present the most suitable candidates. Results of a network stability study showed that for a power system rated at 2940 MVA with a high RES contribution of 1688 MVA, equating to 57% of the energy mix, during a power imbalance of 200 MW, an ESS designed to provide emulated inertia response (EIR) in isolation required a power and energy rating of 39.54 MW and 0.0365 MWh respectively. Similarly, providing primary frequency response (PFR) on its own required a power and energy rating of 114.52 MW and 2.14 MWh respectively. ESS providing these services in isolation was not able to maintain all the frequency operating limits and similar results were also seen in the case of the recently introduced Dynamic Containment service. However, with the introduction of a combined response capability, a significantly improved performance, comparable to that of the synchronous generators was observed. In order to maintain the RoCoF and the statutory frequency limit of 0.5 Hz/s and ±0.5 Hz respectively, an ESS must be able to respond with a delay time of no more than 0.2 seconds and be able to ramp up to full response within 0.3 seconds (0.5 seconds from the start of contingency) for a frequency deviation of ±0.5 Hz. The second half of the thesis focused on investigating the current state-of-the-art power conversion system topologies, with the objective of identifying a suitable topology for interfacing ESSs to the grid at MV level. A delta-connected Modular Multilevel STATCOM with partially rated storage (PRS-STATCOM) is proposed, capable of providing both reactive and active power support. The purpose is to provide short-term energy storage enabled grid support services such as inertial and frequency response, either alongside or temporarily instead of standard STATCOM voltage support. The topology proposed here contains two types of sub-modules (SM) in each phase-leg: standard sub-modules (STD-SMs) and energy storage element sub-modules (ESE-SMs) with a DC-DC interface converter between the SM capacitor and the ESE. A control structure has been developed that allows energy transfer between the SM capacitor and the ESE, resulting in an active power exchange between the converter and the grid. A 3rd harmonic current injection into the converter waveforms was used to increase the amount of power that can be extracted from the ESE-SMs and so reduce the required ESE-SMs fraction in each phase-leg. Simulation results demonstrate that for three selected active power ratings, 1 pu, 2/3 pu, & 1/3 pu, the fraction of SMs that need to be converted to ESE-SMs are only 69%, 59% & 38%. Thus, the proposed topology is effective in adding real power capability to a STATCOM without a large increase in equipment cost. Furthermore, modifying the initially proposed topology with the use of Silicon Carbide (SiC) switching devices and interleaved DC-DC interface converter with inverse coupled inductors resulted in similar efficiencies when operated in STATCOM mode.Open Acces

Physical and numerical investigation of conglomeratic rocks

This thesis investigates the mechanics of clast supported conglomerates, through physical and numerical simulations on idealised specimens with spherical clasts and a homogeneous cement matrix. In the physical experiments synthetic conglomerate specimens were prepared from steel spheres as clasts and Portland cement paste as the cement matrix. The mechanical parameters of these specimens were measured in ISRM standard tests. Numerical specimens were prepared in PFC3D using measured and known micro parameters, and tested in conditions as equivalent as possible to the physical experiments. In order to validate the numerical simulations, the responses of the numerical and synthetic conglomerates were compared. Although the numerical tests reproduced many features of the physical tests, some significant differences were observed which were attributed to the presence of cement matrix in the synthetic conglomerate. After a achieving a reasonable calibration between the physical and numerical conglomerates, the simulations were extended to investigate the sensitivity of the cement matrix and the clast properties, effect of specimen size and size distribution of the clasts in controlling the mechanical response of a clast supported conglomerate. The study showed that the mechanical response is sensitive to the strength and stiffness of the cement matrix in uniaxial conditions only. Similarly, clasts’ strength and stiffness was found to significantly influence the mechanical response in triaxial loading but not in uniaxial loading. The specimen size was found to influence the mechanical response of conglomerates, similar to natural rocks. In the clast size distribution study, the peak strength and stiffness of the conglomerate was observed to decrease as the maximum to minimum clast size ratio is increased. A micro mechanical investigation using PFC2D was conducted to explore the clast-cement interaction by modelling the cement matrix as an aggregate of micro particles. It was observed that the properties of the clast-cement interface significantly affect the failure mechanism and peak strength in various modes of deformation. Similarly, the role of the cement matrix was also investigated. It was found that the cement matrix acts as a stress riser and a relation was proposed to estimate the cement induced stress effect, named, the Cement Wedge Effect

Numerical Verification of Empirically Designed Support for a Headrace Tunnel

In this paper, we used two empirical rock classification systems of rock mass rating (RMR) and rock quality tunnelling index (Q-system) for the support design of a tunnel in District Battagram, Khyber Pakhtunkhwa, Pakistan. Along the tunnel route, the rocks of Precambrian namely Gandaf Formation, Karora Formation and Besham Complex were exposed. During the field investigations, two shear zones were marked in the schist of Karora Formation. The discontinuities parameters collected during the field investigations, results of laboratory testing and material constants determined from RocData version 5.0 software were used during the empirical classification and numerical modelling. The support was designed for the rock mass units from RMR and Q. The quantification of the thickness of plastic zone and total displacement around the tunnel were achieved by the numerical modelling of RS2 9.0 software in both unsupported and supported conditions. The empirically designed support was installed in the model prepared in the RS2 software. According to the results, the empirically designed support when installed in models prepared in RS2 significantly reduced the plastic zone around the tunnel. The reduction in the plastic zone and displacement around the tunnel verified the support design by empirical methods. The present research concludes that empirical designed support can be used for the complex geology of Pakistan

Connection between the South and East Asian Monsoons: Comparing Summer Monsoon Rainfall of Pakistan and South Korea

This study investigates the tele-connection of the southeast Asian monsoon systems by comparing the summer monsoon (June to September) rainfall variability between Pakistan and south Korea. The daily data sets (19812014) of rainfall of Pakistan and south Korea are utilized to explore the possible link. The data products of the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) were also used for the understanding of the large-scale atmospheric environments. The patterns of summer monsoon rainfall on a daily basis between Pakistan and south Korea followed to each other throughout the year. Sub-seasonal differences of the summer monsoon revealed that July is the wettest month in both countries. The large-scale atmospheric environment of higher geopotential height revealed that the Tibetan high and the western north Pacific subtropical high are showing positive anomalies during positive phases over south Asia and east Asia, respectively. The anomalies of zonal wind are negative during positive phase and adverse in the negative phase between 20-40oN. The reduced westerly is interpreted as the seasonal variation and moving of jet streams from the east Asian route. The Tibetan high, northwestern Pacific subtropical high and the east Asian jet stream have reliable and sufficient linkage between the Pakistan and south Korea summer monsoon system

Seasonal and Regional Variations in Rainfall Distribution Over the Punjab-Pakistan

The purpose of present study is to investigate district and division wise annual rainfall variations over theprovince of Punjab, which is the largest in terms of population size as well important contributor in agricultural producein Pakistan. The results revealed that the rainfall trend has shifted from upper and lower Punjab towards the south,west, north and east respectively. The statistical analysis has inferred an overall increasing trend for the period 1990 to2000 and a decreasing trend during period 2001 to 2010 in Punjab province. The Z test value differences in the averagerainfall for each district level meteorological station have detected three increasing and two decreasing trends duringsummers of 1981-2015.The data revealed a significant changed seasonal trend observed in Murree and Sialkot(northern Punjab), Faisalabad and Lahore (central Punjab). Particularly, the changes have been observed in the southPunjab and over the central Punjab, while the same rainfall variations have shown a southward shift. The trend of therainfall had shifted in the Faisalabad division with the significant positive trend. In the Central Punjab, the positivetrend has also been found in all the districts. In the lower Punjab, Multan, Dera Ghazi Khan districts has shown anincreasing trend of rainfall. The findings are significant in changing agro-climatic zones in the Punjab and consequentshifting patterns of agriculture therefore can have net impact on the food security situation in the Punjab-Pakistan

SecureSurgiNET:a framework for ensuring security in telesurgery

The notion of surgical robotics is actively being extended to enable telesurgery, where both the surgeon and patient are remotely located and connected via a public network, which leads to many security risks. Being a safety-critical application, it is highly important to make telesurgery robust and secure against active and passive attacks. In this article, we propose the first complete framework, called SecureSurgiNET, for ensuring security in telesurgery environments. SecureSurgiNET is primarily based on a set of well-established protocols to provide a fool-proof telesurgical robotic system. For increasing the efficiency of secured telesurgery environments, the idea of a telesurgical authority is introduced that ensures the integrity, identity management, authentication policy implementation, and postoperative data security. An analysis is provided describing the security and throughput of Advanced Encryption Standard during the intraoperative phase of SecureSurgiNET. Moreover, we have tabulated the possible attacks on SecureSurgiNET along with the devised defensive measures. Finally, we also present a time complexity analysis of the SecureSurgiNET through simulations. © The Author(s) 2019

Outcome of patients operated for depressed skull fracture with dural tear

Objective:  The objective of this study was to determine the outcome of patients operated for depressed skull fracture with a dural tear. Material and Methods:  A descriptive case series (n = 155) was carried out in the Department of Neurosurgery, Hayatabad Medical Complex Peshawar for six months. Results:  The mean arrival GCS was 10.64 ± 2.33. About 21.9% (n = 32) patients presented with a GCS of ? 8, while the remaining 78.1% (n = 123) presented with a GCS of ? 8. About 8.4% (n = 13) patients died due to the complications of the brain injury. The most common postoperative complication was found to be progressive neurologic deficit (PND) occurred in 21 (13.5%) patients. Penetrating injury to the head was also associated with unfavorable outcomes after surgery (p = 0.046), which shows that penetrating injury is associated with increased brain damage and hence consequently poor outcomes. Conclusions:  The neurologic status as denoted by the Glasgow coma scale is one of the most important factors which predicts the outcome. Surgical management of depressed skull fractures with dural tear has favorable outcomes in about two-thirds of patients. The remaining one-third patient remains in the severely disabled group. Every effort should be made to reduce the occurrence of complications as they are directly related to postoperative functional outcomes

Outcome of the Traumatic Extradural Hematoma on the Basis of Size of Extradural Hematoma

Objective:  This study aimed to determine the outcome of traumatic extradural hematoma and to compare the outcome of small and large size extradural hematoma. Materials & Methods:  A descriptive case series was carried out in the Department of Neurosurgery, Hayatabad Medical Complex Peshawar for six months and included 188 patients. The detailed history and clinical examinations were noted through a pre-designed proforma. Patients were followed post-operatively till the time of discharge and for either favorable outcome or unfavorable outcome according to the GOS. Results:  73% of patients had extradural hematoma volume between 25 – 50 mL, while 27% patients had extradural hematoma >50 mL. 12% of patients had Glasgow Outcome Score (GOS) scores 1 – 3, while 88% of patients had GOS scores: 4 – 5. The mean GOS was 3. Patients with favorable outcomes were 88% while 12% had an unfavorable outcome. Conclusion:  The size of the extradural hematoma has a strong correlation with outcome. The greater the size of the hematoma, the poorer the outcome

Temporary epicardial pacing wires in isolated coronary artery bypass graft: Necessity or force of habit?

Objective: To determine the frequency of Temporary epicardial pacing wires usage and its predictors in the immediate postoperative period in isolated coronary artery bypass graft surgery. Methods: The longitudinal study was conducted at the Aga Khan University Hospital, Karachi, from September 2019 to August 2020, and comprised adult patients of either gender who underwent isolated coronary artery bypass graft in the Department of Cardiothoracic Surgery. Demographic, peri-operative and post-operative Temporary Epicardial Pacing Wires use data was extracted from patient\u27s files and the institutional electronic database. Logistic regression models were built to explore predictors of Temporary epicardial pacing wires usage. Data was analysed using SPSS 22. Results: Of the 322 cases evaluated, 27(8.4%) required the use of Temporary Epicardial Pacing Wires. Mean age of the patients requiring temporary epicardial pacing wires was 66.3±8.9 years compared to 58.7±8.9 years in those who did not require it (p\u3c0.001), while the left ventricular ejection fraction percentage was 44.1±12.8 and 48.9±12.8 respectively (p=0.032). After adjusting for clinically plausible demographics and peri-operative variables, increasing age and low left ventricular ejection fraction were significantly associated with the use of temporary epicardial pacing wires in post-operative period of isolated coronary artery bypass graft patients (p\u3c0.05). Conclusions: The frequency of temporary epicardial pacing wires usage in the post-operative period of coronary artery bypass graft was found to be low