Academic literature on the topic 'Research Subject Categories – TECHNOLOGY – Electrical engineering, electronics and photonics'

The exploitation of nonlinear dynamics behavior in ferroelectric material toward the realization of innovative transducers for the detection of weak and low frequency electric fields is the focus of this thesis. A nonlinear dynamical system based on ferroelectric capacitors coupled into a unidirectional ring circuit is considered with particular interest for developing novel electric field sensors. The focused approach is based on the exploitation of circuits made up by the ring connection of an odd number of elements containing a ferroelectric capacitor, which under particular conditions exhibits an oscillating regime of behavior. For such a device a weak, external, target electric field interacts with the system thus inducing perturbation of the polarization of the ferroelectric material; this, the target signal can be indirectly detected and quantified via its effect on the system response. The conceived devices exploit the synergetic use of bi-stable ferroelectric materials, micromachining technologies that allow us to address charge density amplification, and implement novel sensing strategies based on coupling non-linear elemental cells. Advanced simulation tools have been used for modeling a system including electronic components and non linear elements as the conceived micro-capacitors. Moreover, Finite Element Analysis (FEM) has allowed us to steer the capacitor electrodes design toward optimal geometries and to improve the knowledge of effects of the external target E-field on the electric potential acting on the ferroelectric material. An experimental characterization of the whole circuit, including three cells coupled in a ring configuration has also been carried out.

Printed antennas play an significant role in satellite, mobile, and other wireless communications networks, military systems and several more emerging applications in- cluding radar sensing and imaging. Several of these new applications not only have special demands for antenna designs but also require incorporation of smart features into it. In addition, research for high gain, wideband and compact antennas for several emerging technologies like 5G and IoT are rapidly progressing. In this work, design and analysis of several variants of wideband microstrip patch antennas are taken up. We begin with the design of a single layer suspended antenna where a small coplanar capacitive strip is used as the feed. Following a recent research from the group, di erent radiating patch shapes like rectangular, triangular and semi- elliptical patch geometries are compared with each other for various performance parameters. It is important that these wideband antennas must be able to transmit and receive short time pulses without any distortion and dispersion. Thus, parameters like group delay(GD) and fidelity factor(FF) are studied along with other parameters like re ection loss (S11), gain, polarisation, e ciency and radiation pattern. Full wave electromagnetic simulations using CST microwave studio is used to analyze these antenna characteristics. Triangular and semi-elliptical patches are fed in two di erent configurations (edge-feed and vertex-feed). Simulation studies showed that among the five configurations the vertex fed triangular antenna has the best performance in terms of at gain and low group delay variation over the frequency band of operation. Fabricated antenna has a return loss bandwidth from 3.1 GHz to 4.2 GHz at S11 better than -10 dB where the group delay variation is less than 2 ns. This antenna has a peak gain of 7 dBi and a beamwidth of 60 in both principal planes at the center frequency. Pulse propagation characteristics of this antenna is also studied using modulated sinusoid waveforms as the transmit signal. The degradation of pulse shape with angle is studied through simulations. The transmitted and received pulses are compared with two identical antennas at boresight to experimentally analyze the impact on time domain waveforms. A similar performance has been obtained for an antenna with the feedstrip embedded within the patch by providing a slot surrounding the strip. While retaining most of the performances the footprint of the antenna could be reduced by this approach. Surface wave generation is considered a common drawback in relatively thick microstrip configurations including suspended microstrip patch antennas (MPA) as this may cause less gain, asymmetry in the radiation pattern at higher frequencies within the operational band and increased mutual coupling when these antennas are used in arrays. To address these issues, conducting walls are introduced surrounding the above suspended MPA. The resulting configuration is a wideband cavity backed microstrip patch antenna (WCMPA). The proposed configuration has the widest impedance bandwidth reported for a cavity backed MPA. Measured S11 band- width at -10 dB return loss is from 2.89 GHz to 5.18 GHz (% BW > 55%) with peak gain of 7.6 dBi. The radiation pattern is symmetrical throughout the operational frequencies. The group delay variation is < 1ns and FF is above 0.9. In addition to these performance improvements, the addition of cavity walls provides physical stability to the antenna. Further, a modification to this antenna is investigated to increase its gain. By increasing the lateral dimensions of the cavity the gain is increased from 7.6 dBi to 10.2 dBi. However, this results in an overall footprint of about 1.3 for the entire antenna. The impact of having conducting sidewalls on mutual coupling is also studied for two element arrays in both E-plane and H-plane. A comparison of arrays with and without cavity showed that, the isolation between elements improves by 14 dB with cavity walls for a similar distance between elements in the E-plane. Furthermore, these two-element arrays can be designed for combined peak gain of 11 dBi by choosing appropriate inter-element spacing. Another modification to this antenna is proposed here, where a short horn is mounted on the substrate to increase the overall gain. The gain throughout the frequency range of the antenna can be improved by appropriately choosing the dimensions and are angle of the horn-like structure. Extensive parametric studies have been conducted on this wideband quasi-planar antenna (WQA) to analyze the impact of various design parameters. Measured peak gain for this antenna is 13.2 dBi. This low profile antenna o ers the best gain for similar quasi-planar configurations. The measured impedance bandwidth is 46.7% from 3.1 GHz to 4.6 GHz and the beamwidth is around 35 in both principal planes. While the calculated aperture efficiency is comparable to any other horn antenna, the proposed antenna geometry has a lower height compared to horn antennas and the coaxial transition is similar to those used in planar technologies. In these respects, the proposed quasi-planar an- tenna eliminates the complexity involved in integrating conventional horn antennas for compact wireless terminals. Furthermore, it has been shown that the design can be appropriately modified for di erent frequencies of operation and similar performance is achieved for these designs. One of these designs have been employed in a passive radar developed by the group. The wideband high gain antennas proposed in this research may find several applications in wideband RADAR and imaging. As a case study, we integrated this with a wideband frequency modulated continuous wave (FMCW) RADAR. A WCMPA is used as the transmitter and a two-element array of WCMPA is used as the receiver. A rat race hybrid circuit is integrated with the receiver antennas to obtain sum and di erence patterns in a monopulse configuration. This information can be used to measure the angular position to track a target. 2D and 3D tracking antenna integrated circuits are designed and the results are validated experimentally. Another case study dealt with application of high gain antenna WQA for passive radar since this wideband antenna with large beamwidth helps locate the target with good res- olution. Thermal radiations emitted by the human body in the microwave range are sensed using radiometric system. Experimental results demonstrated capability of the system to detect the presence of a person upto a distance of 2 m where the range improvement may be attributed to the high gain of the WQA used. In summary, the work reported in this dissertation enables the design and analysis of several variants of wideband microstrip antennas with low profile. In addition to extensive simulation studies, some of these designs are experimentally validated and employed in practical wideband radar applications.

Use of Drones for communication is picking up the pace and expected to grow exponentially in the coming years. These drones will be used as access points for providing WiFi services in the near future. It is well known that WiFi was designed for low mobility scenario and the conventional transceivers do not work when drones move resulting in significantly higher Doppler frequency. This motivates us to design a modified physical (PHY) layer characteristics for a drone to ground communication in the WiFi band. Wireless Channel modeling of such drone-to-ground links is very important for this objective. The 2.4 GHz and 5 GHz bands (typically known as the WiFi bands) are unlicensed bands and hence most of the wireless devices use this band for communication. Developing a channel model for this band with a transmitter in motion is a challenging task. In this project, we have designed and developed a channel measurement system at 2.4 GHz link by conducting measurement campaigns to collect data and post-processing it. We model the channel as a finite impulse response (FIR) filter having tap coefficients that are stochastic in nature. The transmitter device HackRF/BladeRF carried by the drone, continuously transmits pilot data and the Universal Serial Radio Peripheral (USRP) receives the pilots. The drone is made to hover at a given distance and data is collected by USRP placed on the ground. The received data is post processed and the channel is estimated using known estimators like Maximum Likelihood (ML) and Minimum Mean Square Error (MMSE) estimators. The estimated channel is represented by 20 tap FIR filter. The histograms of the magnitude of estimated filter tap are fitted to known distribution with suitable parameters. The above procedure is repeated for different terrains and for varying distances between the drone and the ground at several different locations in IISc campus. A draft channel model is presented based on the measurement data

Driven by the exponential growth of data tra c, current infrastructure and standards are evolved to meeting the requirements. In long haul communication, 1550/1310 nm based singlemode ber technology is a commercially viable platform. For short-reach optical interconnects for rack-to-rack communication and within buildings, the matured 850 nm VCSEL based multimode ber (MMF) technology is an industry-standard. IEEE has recently proposed a 400 Gbps roadmap for data centers to scale up short-reach infrastructure. However, the current shortreach datacom infrastructure is not scalable to support a 400 Gbps data rate. Integration of all the functional components of an optical interconnect on a single platform can meet the requirement of a scalable, energy-e cient, and a ordable system. Additionally, CMOS compatibility can leverage electronic and photonic circuits' co-existence on a single platform and low-cost mass manufacturing. Integrating optical functionalities on a single-chip also o ers application in sensing as well. Due to the low absorption in water, the 850 nm wavelength window is also attractive for realizing Lab-on-a-chip biosensors. Integrated photonic circuits at 850 nm band can therefore be useful for a lab-on-a-chip biosensor platform as well. This thesis presents an integrated photonic platform comprising silicon nitride (SiN) waveguide, SiN surface grating coupler, silicon photodetector, and wavelength lters integrated monolithically on the SiN-on-SOI platform at 850 nm wavelength. Our primary focus is to overcome the limitation of lower responsivity and bandwidth of silicon photodetector. We extensively study various techniques to integrate silicon photodetector with passive SiN waveguides e ciently suitable for future short-reach datacom and lab-on-a-chip biosensors. In the rst part, we realize a single-mode SiN waveguide along with high-e ciency surface grating couplers. We have demonstrated a uniform and apodized grating coupler with a bottom Bragg re ector. Apodized gratings provide higher coupling e ciency than uniform gratings due to better mode pro le matching between Gaussian-shaped ber mode and the apodized grating eld pro le. Distributed Bragg re ector (DBR) reduces the optical loss due to high order di racted light directed towards the bottom substrate. SIN apodized grating coupler with DBR as the bottom re ector achieves the highest ever coupling e ciency of 2.19 dB/coupler and 3dB bandwidth of 40 nm at 876nm wavelength. In the second part, we demonstrate various architectures to integrate high-speed silicon photodetector with SiN waveguide. First, we demonstrate the integration of SiN waveguide with high-speed, lateral silicon pin photodetector. Compared to the silicon photodetector realized on bulk silicon, photodetector on an SOI has higher bandwidth due to the lower cross-section. We use silicon inverse taper to improve the coupling from SiN to silicon, which results in better responsivity of silicon photodetector. We have achieved the highest responsivity of 0.44 A/W and bandwidth of 15 GHz for the integrated silicon pin. Bandwidth improvement without degradation of responsivity is attributed to the lateral collection of photocarriers transverse to the propagation direction, and low RC time-limited bandwidth due to the thin silicon. To enhance the photodetector responsivity further, we propose a SiN ring resonator enhanced silicon metal-semiconductor-metal (MSM) photodetector. Compact, cavity-enhanced silicon- MSM photodetector responsivity is estimated to be 0.81 A/W at 5 V, which is 100 times higher than the conventional waveguide photodetector. Moreover, the photodetector's compact size (6X6 m2) can o er high bandwidth due to reduced RC time-limited bandwidth. In this section, we also discuss the integration of SiN waveguide with a thin silicon-MSM photodetector (70 nm thick). In this con guration, the SiN waveguide is placed on top of the silicon-MSM. Since the silicon's thickness is low SiN, the waveguide does not su er from mode mismatch losses between silicon and SiN. Such con guration is attractive due to its high responsivity and bandwidth, along with ease of fabrication. We have shown the DC measurements with a maximum responsivity of 0.56 A/W at 10 V bias. Finally, we have demonstrated the integration of wavelength division multiplexer with silicon photodetector since the shortwave wavelength division multiplexing (SWDM) at 850 nm wavelength band is considered one of the viable solutions to attain 400 Gbps roadmap. We have realized the WDM using SiN Echelle gratings and integrated the output channel waveguides with a silicon-MSM photodetector. Experimentally, we have shown that the Echelle grating has the insertion loss of 4.3 dB and adjacent channel cross talk of 22 dB for the channels having wavelength separation of 10 nm. Future exploration of the demonstrated device can lead to precise wavelength ltering with on-chip detection useful for both high-speed short-reach datacom and lab-on-a-chip biosensors. In summary, we have demonstrated the capability of realizing a scalable, energy-e cient, and cost-e ective silicon nitride based integrated photonic receiver in the 850 nm wavelength band.

Organic photovoltaics is a fast-developing technology and has the potential to revolutionize the energy market. In the recent years, high photoconversion efficiency has been achieved through continuous efforts on design and synthesis of newer organic semiconductors. However, enormous scope of development and research in this area remains. BODIPY based organic semiconductors for examples have only recently started catching up with other class of organic materials for solar cell applications. In this thesis, therefore we focus our efforts on designing newer BODIPY materials and more importantly investigating the structure-function correlation. Understanding the structure-function correlation in this class of materials will help provide guidelines for designing newer materials with better performance. In Chapter 1, Introduction, we discuss the development of organic semiconductors and their application in solar cells reported in the literature. We find both small molecules and polymers used for both donor and acceptor applications in OPVs. While historically more research have been done on donor materials, in recent years development of acceptors to replace fullerenes is catching up. In the same chapter, we also discuss some of the examples of BODIPY based OPV materials and decide on the different aspects of structure-function correlation that we would be looking into in this thesis. In Chapter 2, we have discussed the major characterization techniques we use in the thesis, to study the new BODIPY molecules synthesized here. The results of this thesis are laid down in the subsequent 4 chapters. In Chapter 3, we look into the effect of trifluoromethyl substituent at the meso position of BODIPY on the properties of BODIPY based A-D-A molecules. We designed and synthesized two pairs of A-D-A molecules with terminal BODIPY (with either a 4-methylphenyl meso group or a 4-trifluoromethylphenyl at the meso position) and Fluorene (S1 and S2) and Benzodithiophene (S3 and S4) as the central donor unit. The effect of the trifluoromethyl group on the photophysical properties of these molecules were thoroughly characterized using steady state and femtosecond transient absorption spectroscopy. We further look into charge transfer from these molecules into PC60BM using Time resolved microwave conductivity (TRMC) measurements and study the effect of the trifluoromethyl group on the charge transfer mechanism. Based on our finding we could suggest that incorporation of the trifluoromethyl group in BODIPY based donor small molecules leads to a poorer charge transfer into fullerene acceptor. In Chapter 4, we look into the effect of trifluoromethyl substituent at the meso position of BODIPY on the properties of BODIPY based D-A polymers. We could synthesize a set of BODIPY-fluorene polymers (P7 and P8) which had sufficiently low HOMO and low band gap allowing absorption throughout the visible region of the solar spectra. Through TRMC measurements we could demonstrate that these polymers could be used as non-fullerene acceptors with PTB7-Th donor polymer for all polymer solar cells. The trifluoromethyl group is seen to clearly render better electron acceptor property to the polymer. Organic solar cells are also fabricated with the pair of BODIPY polymer (with and without trifluoromethyl group) as electron acceptors and the trifluoromethylated polymer (P8) is seen to perform better than its counterpart. In Chapter 5, we design and synthesize another set of BODIPY based polymers (P9-P12). The BODIPY subunits in these polymers have been modified by fusing thiophene to the core and hence the conjugation length is extended. This leads to a narrower band gap and the resulting polymers absorb in the NIR region up to 1200 nm. Using TRMC measurements we demonstrate that these polymers could be used as non-fullerene acceptors. These polymers (P11 and P12) can be used for development of fullerene free polymer photodetectors with NIR response. In Chapter 6, we study the effect of structural isomerism in BODIPY based D-A polymers. BODIPY can be connected to the polymer backbone through either the α or the β position. However, in literature most of the BODIPY based D-A polymers used for OPV applications are β-connected. In this chapter we design and synthesize a pair of structural isomeric BODIPY polymers (α-connected and β-connected). The properties of the α-connected (P1 and P3) and β-connected polymers (P2 and P4) are compared. The optoelectronic properties change drastically without affecting the charge carrier mobility. However, the α-connected polymers perform better than the β-connected polymers as electron acceptors. This is shown by the photovoltaic performance of all polymer solar cells fabricated with P3HT donor and the BODIPY polymers a well as by Time resolved photoluminescence spectroscopy. In summary (Chapter 7), we have designed and synthesized a library of new BODIPY based organic semiconductors. We have also demonstrated for the first time, the possibility of using BODIPY based polymers as non-fullerene acceptors. More importantly, we have investigated three major aspects of structure function correlation in this class of materials, previously overlooked. The results from this thesis help us conclude that incorporation of trifluoromethyl group improved the electron mobility in BODIPY polymers. Also, we see that α-connected polymers and not β-connected polymers could perform better for photovoltaic applications

Real-time Magnetic Resonance Imaging (rtMRI) is a tool used exhaustively in speech science and linguistics to understand the dynamics of the speech production process across languages and health conditions. rtMRI has two advantages over other methods which capture articulatory movement, like X-ray, Ultrasound and Electromagnetic articulography - it is non invasive, and it captures a complete view of the vocal tract including pharyngeal structures. The rtMRI video provides spatio-temporal information of speech articulatory movements, which helps in modeling speech production. For this purpose, a common step is to obtain the air-tissue boundary (ATB) segmentation in all frames of the rtMRI video. The accurate estimation of ATBs of the upper airway of the vocal tract is essential for many speech processing applications like speaker verification, text-to-speech synthesis, visual augmentation for synthesized articulatory videos, and analysis of vocal tract movement. Thus, it is necessary to have an accurate air-tissue boundary segmentation in every frame of the rtMRI videos. The best performance in ATB segmentation of rtMRI videos in speech production, in unseen subject conditions, is known to be achieved by a 3-dimensional convolutional neural network (3D-CNN) model. In seen subject conditions, both 3D-CNN and 2-dimensional deep convolutional encoder-decoder network (SegNet) show similar performance. However, the evaluation of these models, as well as other ATB segmentation techniques reported in literature, has been done using Dynamic Time Warping (DTW) distance between the entire original and predicted boundaries or contours. Such an evaluation measure may not capture local errors in the predicted contour. Careful analysis of predicted contours reveals errors in regions like the velum part and tongue base section, which are not captured in a global evaluation metric like DTW distance. In this thesis, such errors are automatically detected and a novel correction scheme is proposed for them. Two new evaluation metrics are also proposed for ATB segmentation, separately for each contour, to explicitly capture errors in these contours. Moreover, the state-of-the-art models use overall binary cross entropy as the loss function during model training. However, such a global loss function does not give enough emphasis on regions which are more prone to errors. In this thesis, together with global loss, the use of regional loss functions has been explored, which focus on areas of the contours which have been analyzed as error prone in the analysis. Two different losses are considered in the regions around velum and tongue base - binary cross entropy (BCE) loss and dice loss. It is observed that dice-loss based models perform better than their BCE loss based counterparts.

Grounding the frame of photovoltaic (PV) panel is a necessity for the safety of humans. This leads to the formation of large capacitance between PV cells and ground. Hence, to reduce leakage current due to parasitic capacitance between PV cells and ground, the DC output voltage of a photovoltaic panel is normally kept below 1 kV. Conventionally, for medium voltage (3.3 kV-66 kV) AC grid integration of PV panel, the DC output of PV is rst converted to 400V AC and is connected to a 400V collection grid through a line frequency transformer (LFT). This LFT provides isolation and limits circulating current among the PV modules. Another step-up LFT is used to connect 400V AC grid to the medium voltage (MV) transmission grid. These line frequency transformers are bulky and expensive. The line side lters are placed on the low voltage side of the rst LFT and hence, experience high currents leading to higher copper losses. To avoid the limitations of LFTs, power converters with high frequency transformer (HFT) are becoming popular. The HFT is fed from a DC side inverter (DSI) and the output of HFT (which is high frequency AC) is converted to line frequency AC using power electronic converters. This type of converter is known as high frequency link (HFL) DC to AC converter. State-of-the-art HFL DC to AC converters mostly employ a multi-stage power conversion technique where an isolated DC to DC converter is cascaded with an inverter. The stages are controlled independently. The inter-stage voltage sti DC-link is maintained with large electrolytic capacitor. But such an approach requires higher amount of ltering and use of electrolytic capacitor a ects long-term reliability. Moreover, the capacitor voltage needs to be tightly regulated to protect the devices. The grid interfaced inverter is high frequency hard-switched resulting in reduced e ciency. These drawbacks are overcome in a single-stage power conversion approach where the inter-stage lter capacitor is removed and all the power devices are either soft or line frequency switched resulting in reduction in switching loss and improvement in e ciency. In literature, to replace the step-up LFT and to directly integrate the converter to the medium voltage grid, a popular solution is the usage of cascaded multilevel power conversion. Generally, the above discussed multi-stage converter is employed as modules in a cascaded multilevel con guration to produce medium voltage. Moreover, some existing topologies use single-stage converters in a cascaded multilevel con guration to produce medium voltage, but the grid side converters are high frequency switched, leading to higher loss. In this thesis, a new topology is proposed to overcome the drawbacks of existing cascaded multilevel power conversion topologies. In the thesis, a new single-stage high frequency link cascaded multilevel converter topology is proposed for MV grid integration of solar power. A single-stage high frequency link DC to AC converter is used as a module. The DC side of each module is connected to a PV source. The AC sides of multiple such modules are connected in series in a cascaded fashion to interface with the MV AC grid. Proposed modulation of the DC to AC module results in zero voltage switching (ZVS) of the DC side converter and line frequency switching of the AC side converter. ZVS happens for most part of the line cycle. Over a switching cycle, the operation of this module is similar to a phase-shifted full bridge (PSFB) DC to DC converter. In the PSFB converter, during switching transition, the parasitic capacitance of AC side diode bridge along with leakage inductance of HFT forms a resonating circuit. This resonating circuit leads to high voltage stress on the secondary side devices. An active snubber is designed to restrict the voltage overshoot. The operation of PSFB converter, considering all parasitics, is not explored in literature. In this thesis, a detailed analysis of the operation of the PSFB and step-by-step design methodology is given. The hardware is designed and tested with DC input voltage of 400 V, DC output voltage of 1240 V, output power of 1.5 kW and switching frequency of 20 kHz. Experimental results validate the analysis. A method is proposed to observe medium voltage waveforms with the standard low-voltage probe. A method to remotely control the medium voltage converter is developed to ensure safety.

Opacity is notion of privacy that is well-studied in computer science and discrete-event systems. In our work, we extend the opacity notion to linear dynamical systems. Opacity describes an eavesdropper’s inability to estimate a system’s “secret” states by observing the system’s outputs. We consider four opacity classes - initial-state, current-state, K-step and infinite-step opacity, and show that they are fundamentally connected with two subspaces of the linear system - the weakly unobservable subspace and the weakly unconstructible subspace. Further, we establish that a trade-off exists between opacity and security in the system. We show this in two ways – (i) we prove that an opaque system always permits undetectable attacks, (ii) we show that expanding the set of opaque states in the system always expands the set of undetectable attacks. We also propose optimization algorithms to minimally perturb a non-opaque system to make it opaque. We demonstrate our results on a smart grid system. Our work is the first to study opacity in such generality for linear dynamical systems, and provides necessary mathematical foundation for system designers to develop and build opaque systems, while ensuring adequate security.

Shunt capacitor banks (SCBs) are usually used for providing reactive power support to power systems. Reactive power support leads to power factor correction, voltage regulation and reduction of network losses. Outage of the SCBs may affect the power system. Thus, SCBs should be well protected for efficient operation of power system. SCBs consist of many capacitor elements connected in series and parallel combinations. Failure of capacitor elements leads to cascaded failures within the SCB if left undetected. Thus, early detection of internal failures in SCBs is crucial. Double wye SCB con figuration is commonly used for reactive power support in high voltage transmission systems. Unbalance protection methods are mostly used for protecting SCBs against internal faults. Among them, neutral current unbalance methods are sensitive and are commonly used to locate the internal faults in double wye SCBs. Locating the internal faults helps in speeding up the repair process and reducing the outage time. One of the limitations of existing neutral current unbalance methods is that they fail to detect simultaneous faults. Simultaneous faults are those faults which happen at the same time or happen between two consecutive protection passes. During some simultaneous fault conditions, existing neutral based methods may misinterpret the fault condition as a healthy condition. In some other cases, one type of fault may be misinterpreted as another type of fault. The severity of fault may also be misinterpreted during simultaneous faults. Misinterpretation of fault type and severity delays the repair process. There is a need for a fault location method which overcomes the drawbacks of the existing methods. In this thesis, a novel method is proposed to detect different types of internal faults in grounded and ungrounded double wye SCBs. The proposed method detects different types of single and consecutive faults in double wye SCBs. The proposed method can also locate simultaneous faults happening in any of the two legs of the bank. This method uses compen- sated negative sequence quantity and compensated neutral currents to locate the fault. In the proposed method, compensated quantities are obtained by subtracting the pre-fault quantities from the quantities during fault. This helps in cancelling the effect of pre-fault conditions. The advantage of the proposed method compared to the existing methods is that it can lo- cate simultaneous faults. The proposed method is analyzed under different practical scenarios such as system voltage unbalances, temperature effects, switching transients, external faults and manufacturing unbalances. Severity of fault is indicated by the number of failed elements. Proposed method can also calculate the number of failed elements accurately. Simulation studies have been performed on a test system developed in PSCAD software which validates the proposed method. Fuseless ungrounded bank and grounded fuse type bank con gurations have been simulated. It has been found that the proposed method performs satisfactorily under conditions like load switching, external faults and voltage unbalance. Per- formance of the proposed method during internal faults like single faults, simultaneous faults and consecutive faults has been tested. The proposed method performs effectively even during external fault and load switching conditions. A laboratory scale test setup consisting of fuseless ungrounded SCB has been developed. It has been found that proposed method locates different fault types like single and simultaneous faults accurately. Both software and hardware results validate the successful working of the proposed method

In recent years, stringent restrictions on greenhouse gas emission due to the present global warming scenario is driving governments and power utilities worldwide behind electricity generation using renewable energy sources. Conventionally, for grid integration of a large scale photovoltaic (PV) system, a three-phase voltage source inverter followed by a line frequency transformer (LFT) is used. The inverter generates line frequency (50/60 Hz) AC from the DC output of PV. The LFT provides galvanic isolation and thus reduces the circulation of leakage current, and ensures safety. Few limitations with the conventional system are a) huge volume as the LFT is bulky, (b) quite expensive due to large amount of iron and copper used in LFT and (c) the inverter is hard switched. The converter topologies with high-frequency galvanic isolation have attractive features like high power density and are less expensive. Hence these converters are promising alternatives to the conventional solution. The three-phase inverter topologies with high-frequency transformer are generally of two types- a) multi-stage and b) single-stage. In multi-stage, interstage DC link is voltage sti as lter capacitor is used. In a single-stage solution, the intermediate DC link is pulsating as lter capacitor is avoided to improve reliability. Though these converters have high power density, they employ large number of active switches on both the sides of the transformer to process power and hence have relatively lower e ciency compared to the conventional solution. The active switch count can be reduced in case of unidirectional applications like grid integration of PV, fuel-cell where the active power ows from DC source to AC grid. The converter e ciency can be further improved by reducing the switching loss. In this work, we have investigated four new unidirectional single-stage three-phase inverter topologies with low or negligible switching loss. To reduce the switching loss, the active switches of the introduced topologies are either line frequency switched or high-frequency soft-switched. The soft-switching is achieved without additional snubber circuit. The pulse width modulation is implemented on the input DC side converters which are soft-switched. The active switches of the grid interfaced converter are low frequency switched and thus enabling the use of high voltage blocking inherently slow semiconductor devices for direct medium voltage grid integration. The topologies are gradually improved to achieve soft-switching of the DC side converters throughout the line cycle. The conditions on dead time to ensure soft-switching are derived through detailed circuit analysis. The operations of these topologies are experimentally veri ed on hardware prototypes with power range 2-6kW. Out of four introduced topologies, three topologies can support only unity power factor operation. An additional shunt compensator is needed for any reactive power support. The fourth topology can support up to 0.866 power factor operation though it has relatively higher conduction loss. The performances of the introduced topologies are compared with multi-stage and conventional solutions. Though the new topologies have relatively higher switch counts, the converter power losses, lter requirements are comparable with the conventional solution with line frequency transformer, and have high power density.