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With the increase in number of VSC based HVDC systems, it is inevitable that there would be interconnections with weak ac grids. The paper presents the analytical basis for identifying coupling in the power loops which is dependent on grid inductance and operating point by utilizing the impedance-based modeling technique. The formulation is used to define an inverse-based decoupling outer-loop controller for VSC-HVDC system. The concept is extended to HVDC grids and it is shown that the gains obtained using simple inverse may lead to instability. Robust controller tuning with pole-placement and minimization of $H_\infty$ norm is presented for HVDC grids, which form a restricted feedback system. The results show that the proposed method of decoupling results in effective decoupling of the power loops over a large range of SCR values as compared to that with traditional vector control. The system stability is also maintained over a large range of SCR values and PLL gains when a sudden variation of parameters is considered. The stability margins can be enhanced by pole-placement. The observations from the small-signal models are confirmed using detailed time-domain simulations. The proposal is expected to be useful in controller design for HVDC grids.

Electrical discharge machining (EDM) is a non-conventional machining technique that requires pulse generators to create sparks between the tool and the workpiece. Classical texts describe only two types of EDM pulse generators (EPGs), although there are several different types. This work presents a classification and detailed comparative study of 13 important EPG topologies from the power electronics perspective. The operation of each topology is explained using simulation studies for EDM load with spark current of 10 A and pulse repetition frequency of 1 kHz. Control schemes of each topology are explained from the viewpoint of mechanical performance parameters, viz. material removal rate, surface finish, and tool electrode wear ratio. A general analytical methodology to compute efficiency based on machining duty is illustrated considering the existence of two distinct switching frequencies. Overall comparison between EPG topologies is presented considering control aspects, size, efficiency, voltage stresses and suitability for any required application. As an example, a case of investigation of cutting silicon ingots with wire EDM is considered and experimental results are provided for the same. This work can be used as a guideline to understand and develop EPG based on particular target specifications.

Pulse generators for wire electric discharge machining (WEDM) are derived from standard dc–dc converters. However, their design and analysis need to be done carefully by considering the pulsed nature of the load. In this article, these issues are discussed for a dual-converter-based topology considering the application of WEDM for slicing silicon ingots in the solar photovoltaic industry. It is shown that appropriate sizing can reduce the overall size of the pulse generator significantly. The efficiency computation procedure is outlined in detail as it is an important parameter, particularly when WEDM becomes a part of solar panel manufacturing chain. The present control scheme for the pulse generator yields around 31% efficiency. A new pulsed current reference scheme is proposed, which can improve the efficiency to 62%. Frequency-domain modeling is presented by considering distinct stages of operation of the pulsed load. The models are compared with the standard converters from which the topology is derived. This modeling method can be applied to other pulse generators as well. Hardware of the pulse generator is built with a maximum open-circuit voltage of 100 V, a maximum spark current of 10 A, and a pulse repetition frequency of 1 kHz. The novel control scheme is verified with experimental results.

Using HVDC grids for interconnection of ac grids across countries posses questions regarding interactions and stability. Important aspects that may be considered are the different short circuit ratio (SCR) values of the corresponding ac grids and outages. The analysis of such mixed SCR will be time-consuming since; detailed models and multiple simulations of the entire system will be required in order to develop a complete understanding of the modes of interactions and existing stability margins. Use of small signal model will allow for easy parametric variation after the initial barrier of model development is overcome. The article presents an analysis technique that uses the mode identification and participation factor augmented with eigen-sensitivity to analyze modes of interactions and stability margins of the HVDC grids when system parameters and controller gains are varied. The modes are classified as local and inter-area modes based on the participation of controller states for HVDC grid. The method does not require detailed switched models to be developed. The paper also proposes an adaptive control technique which results in a reduction of interactions and prevents instability when system operating conditions are changed. The results obtained using the proposed analysis technique are verified with a detailed time-domain simulation of the HVDC grid. The interaction modes and participation of states are dependent on the converter operation modes, loading characteristics, SCR values, PLL gains and loading conditions.

Wire Electric Discharge Machining (WEDM) is being explored as an efficient alternative for cutting silicon wafers for photovoltaics. This is because WEDM results in lesser kerf loss as compared to traditional abrasive cutting techniques. While the commercially available WED machines are developed for cutting metals, it is necessary to develop a customized WEDM for cutting semiconductor materials such as silicon. To this end, this paper discusses design aspects of a miniature WED machine (of size 1200 × 600 × 600 mm) customized for cutting of silicon. Design and topology of pulse generator are also presented here. Voltage and current waveforms while cutting silicon are compared with those obtained while cutting steel. It is seen that there are significant differences in the two, such as the spark current is more pulsating in cutting silicon than that in cutting steel. Such observations point out some fundamental differences in the nature of sparking while cutting semiconductors vis-a-vis metals, which could be attributed to difference in their physical properties.

Multi-Wire EDM (MWEDM) is being investigated for cutting silicon wafers from ingots in solar industry and IC manufacturing industry. However, practical realization of MWEDM is limited by the forces exerted on the wires. This paper explains nature of forces in case of MWEDM for slicing of silicon ingots, which is distinct from the case of single wire EDM for metal cutting. In particular, the material properties of semiconductor are different from those of the metals and the geometry of the problem is also different. It is shown that the wires in MWEDM exert forces on each other, in addition to those exerted by the workpiece on wires. Quantification of dominant electromagnetic forces is done by analytical means and the results are validated by Finite Element Analysis (FEA) for the cases of 5-wire EDM and 10-wire EDM. Experimentally determined material properties and spark current profiles are used as input to FEA. An algorithm has been developed to calculate the required wire tension considering number of wires, radius of wires, spark current, and wafer thickness. Validation of the algorithm is provided by comparing the results with the experimental data for 40-wire EDM published earlier.

The recent advances in electric vehicles (EVs) technologies like batteries, increase in greenhouse gas emissions and increase in fossil fuel prices have caused the more interest in using EVs. But the charging of these EVs may put an additional requirement on the existing electricity grid. Therefore, the power grid must be ready for these challenges. In order to know the type of EVs charging infrastructure required for India, the distribution feeder ratings and different charging levels should be studied. The impact of number of EVs i.e. penetration impact on Indian distribution grid will be analysed. This will lead to identification of different issues due to charging. A typical Indian distribution network will be modelled in OpenDSS. The critical infrastructure issues like voltage profile at different nodes, transformer loading, peak load and power losses will be investigated for different penetration levels. Finally, the results will be analyzed to mitigate the impacts and to suggest various measures.

There is an increase in fault current levels seen by utilities due to grid expansion and the addition of new resources. Installation of superconducting fault current limiter (SCFCL) at transmission voltage levels is being explored to reduce the fault levels. The SCFCL limits the fault current by inserting an additional impedance in the network, which may impact the protection system. This paper discusses the impacts of SCFCL on the operation of line distance protection. It is observed that operation of SCFCL may not impact all the distance relays in the substation and near-by substation in the same way, thus a boundary definition method is proposed in this study to identify the affected relays. The definitions of boundary condition based on steady-state fault current values and critical current of the SCFCL are proposed in this study. The impact of SCFCL on the line distance relays which manifest as underreaching of the relays and may also lead to coordination issues and erroneous operation. An implementation method for mitigation which is compatible with existing substation relays is proposed and demonstrated to ensure proper operation of the relays. The proposed mitigation ensures that the system integrity is maintained during faults.

A dc series–parallel wind farm was proposed as an alternative to an ac wind farm. But, the operation of such a wind farm results in significant wind turbine (WT) output voltage variation, which may lead to severe stress being seen by the turbine capacitors. The output voltage variation problem is demonstrated and rigorously analyzed with proposed power flow calculation. In order to mitigate the voltage variation a dc WT with integrated storage along with strategies for control and storage sizing is proposed. The time domain simulation results for 120 MW 2 $\times$ 6 dc series–parallel wind farm shows that a large voltage variation, with maximum value reaching close to 2 p.u and low voltage of about 0.2 p.u, is obtained when WT output voltage is not limited. The power input variation of a single turbine affects voltages and currents of all the WTs in the wind farm. A 48 kWh storage integrated with the WT appears to significantly alleviate the WT voltage variation.

The interconnection of ac grids using HVDC grids across countries posses questions regarding interactions and stability. Such analysis with mixed short circuit ratio (SCR) will be time-consuming since; detailed models and multiple simulations of the entire system will be required in order to develop a complete understanding of the modes of interactions and existing stability margins. In addition, stability and interaction are impacted by the cable and generator model. The paper investigates the impact of cable models on the interaction and stability margins. In addition, the synchronous generator model is also investigated. A lower-order frequency-dependent model of cable is shown to be effective in predicting the interaction, when frequency-dependent shunt capacitance and conductance are used. It is observed that the cable model has a significant impact on controller interaction and instability.

Breaking of fault current in dc microgrid has been a challenge due to lack of dc current zero crossing. The fault current in case of pole to pole fault is high and even higher in the presence of multiple generators as all the generators feed current to the fault. This results in very high current in the faulted line. Clearing such high fault current is difficult. Many methods have been proposed in literature to maintain low current during fault for ease of fault clearing. This paper presents a fault ride through (FRT) techniques for the generator to limit the current fed during fault. The proposed FRT scheme limits the fault current to a minimum value by feeding a minimum current into the network. The minimum current is derived based on the characteristics of the source and operating strategies of converter. The proposed FRT scheme ensures fast recovery of the network voltage. The proposed FRT scheme is verified through detailed simulations in Simulink.

The multi-terminal high voltage dc (HVdc) networks require isolated dc-dc converters for voltage matching, power flow control and fault protection. The dc-dc converters based on modular multilevel converter (MMC) are continuously gaining popularity. The trapezoidal modulation method in dc-dc converters provides controlled dv/dt stress, high dc voltage utilization and power transfer capability. The simulation based models are gaining interest in recent literature. However, more focus is required on the mathematical modeling aspect of these converters. This paper presents an averaged dynamic model of MMC working in the trapezoidal modulation. The developed model effectively captures both ac and dc side dynamics along with the internal dynamics of sub-module capacitors. The presented average model provides a simplified method of analyzing large converter system. The comparison between simulation and analytical model validates the performance of the developed model. A laboratory prototype is also developed to observe MMC operation in the trapezoidal modulation scheme.

There is increasing interest in development of magnetic energy storage with conduction cooling. In contrast with the liquid- and gas-cooled coils, the conduction-cooled coils are expected to have significant amount of thermally conducting structures. These thermal structures are usually made of copper, which is also a good electrical conductor. Thus, it is expected that the support structures would modify the resistances and inductances seen by the front-end converters connected to these coils. The paper, thus, presents detailed analysis and experimental results investigating the impact of temperature and frequency variations on conduction-cooled coils. The frequency variations are considered because the front-end converters are likely to produce high-frequency currents in the coil. The results indicate that there is a significant reduction in the inductance of the coil when temperature is reduced and remarkable change is also observed when operating frequency is increased. The increase in frequency is known to increase the resistance, but significant increase in resistance is also observed at low temperature when frequency is increased. The results indicate that beyond a certain frequency, the frequency effects dominate the observed resistance values of the coils.

This study uses electrical characterization of sparks to advance the understanding of complex mechanism involved in wire electrical discharge machining (WEDM) of photovoltaic grade silicon. Specifically, WED machining of silicon is compared with that of steel as a base, with two approaches of experimentation — (a) one factor at a time (OFAT) and (b) design of experiments (DoE). The responses are measured in terms of four pulse parameters — maximum current I max , maximum voltage V max , average voltage V avg , pulse energy E p ; and three summary statistics. It was evident that the current required for steel workpiece is more by ≈ 10–15 A than that for silicon, whereas the spark voltage is more by ≈ 10 V for silicon than that for steel. Analysis of variance (ANOVA) showed that, current setting primarily affects the mean of I max , V avg , E p and the effect is > 60 % for both silicon and steel. Moreover, quantitative effect of input factors on the pulse-to-pulse variation was also obtained for the first time. The pulse-to-pulse variation in V avg for silicon is influenced by several parameters: current, pulse period, ON-time, and wire speed (cumulative effect ≈ 55 % ), whereas that for steel only current setting influences and the effect is > 51 % . The role of workpiece thickness (height of cut) has not been studied in detail in the available literature and is important for slicing cylindrical ingots as the length of cut changes continuously while slicing the cylindrical ingots. Therefore, the effect of workpiece thickness was studied, especially from perspective of the current and voltage pulse parameters during slicing. For silicon, an increase in workpiece thickness results in an increase in V avg , and a decrease in I max , whereas for steel, such effect was not seen. The equivalent circuit of spark as observed from the pulse generator showed that besides electrical conductivity of the materials, the junction between spark plasma and silicon plays a dominant role in deciding the current drawn by the spark-erosion process.

The undispatchability of the wind farms poses lot of grid integration challenges like power balancing, increased ancillary service requirements and maintains grid security as well as reliability. The new grid regulation emphasises the wind farm to be dispatch like the conventional power plant, hence the wind farm operators have to deploy energy storage systems (ESS) to absorb the wind variability and to make wind farm dispatchable. The size of the storage can be determined from the statistical profile of the wind speed. To manage these ESS, different operating strategies have been reported in different paper, literature survey indicates that the impact of the performance of these operating strategy on the reliability of the wind farm has not be studied. In this paper we are evaluating the performance of well known method like minute-by-minute (MBM), min-max (MM) and moving horizon predictive control (MHPC) methods by evaluating reliability indices like energy served (ES), energy not served (ENS) and energy not utilised (ENU). The impact of operating methods on the lifetime of the BESS has been also been presented in this paper. Persistence method which is a well-established wind forecasting method is used for accurate short term wind power prediction.

Due to absence of natural zero crossing and fast rise in dc current during fault, the fault current clearing in dc microgrid has been a challenge. The fault current is high in pole to pole fault. It is even higher in the presence of multiple dc generators as all the generators feed the maximum possible current in the network during the fault. This results in very high currents in the lines. In order to clear the fault in such a case, the dc breaker needs to break high fault current. However, the dc breaker operation at high fault current level is difficult and unsafe. Therefore, in this paper, a control scheme is proposed for dc-dc converter which reduces the fault current level at which the dc breaker operation is effective and safe. It uses an additional current control loop which is activated during the fault event. This modified control scheme ensures that the current fed by the converter into the network during the fault is low and thus resulting in low line currents in the network. The proposed control scheme is validated using a detailed simulation.

Short circuit protection remains one of the major technical barriers in DC power system because of the high rate of rise of fault currents and the absence of current zeros. Circuit breaker based on power semiconductor devices have the potential of fast interruption of fault currents. The paper analyses the performance of various solid state circuit breakers. Wideband devices such as silicon carbide offers better performance than silicon devices in terms of on-state conduction loss, faster fault clearing time and higher temperature handling capability. Among the SiC device normally on JFET has better performance than SiC BJT and SiC MOSFET, in terms of avalanche and short circuit rating. A self powered normally-on circuit breaker was analyzed and designed. The overload protection feature which is essential for a circuit breaker was incorporated. Experimental tests were conducted to verify the performance of the breaker under overload and short circuit conditions. The results shows that the breaker operation is completed in 17 microseconds and the recovery voltages rises to 520 V. The designed breaker is capable of interrupting short circuit current at 27 A.

Frequency response analysis (FRA) is a mature technique to characterize the behaviour of apparatus like transformers, and is commonly used for condition monitoring and diagnostics. FRA is also used for sub-synchronous resonance/control interaction analysis of rotating machines and power electronic converters. The frequency responses (transfer functions) can be experimentally obtained from tests at the apparatus terminals. These are generally done offline, i.e. when the equipment is out of service. Online testing is attractive as it can be done frequently and periodically, but it requires auxiliary equipment for injecting probing signals into an energized high voltage system. The alternative online approach uses high bandwidth measurements of “naturally” occurring external transients (due to faults, switching and other disturbances) to obtain the frequency response. The key challenge in online FRA of multi-port apparatus is in the extraction of the “open-loop” transfer function of the apparatus, while connected to the rest of the system, essentially in a “closed-loop” configuration. This paper proposes a technique to address this challenge by utilizing data from multiple independent events, and presents proof-of-concept simulations. The technique can be applied not only to passive components like transformers and transmission lines, but also to power electronic systems and rotating machines.

The processes of inception, growth, and propagation of Partial Discharge (PD) along the dielectric are complicated. Their analysis requires a thorough understanding of underlying physics. A software simulation tool has been developed to study typical PD processes. The simulator generates pulses of varying amplitudes, shapes and time of occurrence of pulses observed in practice using National Instruments LabVIEW software as a base platform. The rise time of PD pulses plays an important role in the characterization of PD. The rise time formulation has been attempted based on the approach using the dipole moment development and law of conservation of energy. PD phenomenon is usually studied with the assumption of an air-filled void being energized by parallel plate/ coaxial cylinder electrode configuration. An SF 6 -filled spherical void inside a dielectric between two coaxial cylinder electrode plates is used as another configuration. Phase-resolved partial discharge (PRPD) pattern and the characteristics of PD pulses simulated are in agreement with results reported in literature. The contribution of this paper is versatility of the simulator for detailed study and comprehensive analysis of PD without the use of high voltage (HV) setup.

The use of medium frequency allows the transformer size to be reduced at the higher power level. Reduction in size of the transformer leads to a reduction in installation area which is of prime concern in many renewable energy resources. But the reduction in size at higher power level comes with the cost of higher loss density and lower thermal stability. Therefore, this paper deals with the various constraints and considerations that are required to be taken care of while designing the medium frequency high power transformer (MFHPT). This paper also provides the optimal design of transformer w.r.t. frequency and power. For analysis and comparison purpose, both core and shell type design is developed in Ansys Maxwell (Ver. 16) for 3 MVA transformers at 1000 Hz of frequency using nanocrystalline core material.

The North East Region-Agra HVDC link is the world's first Multi-Terminal Project at ±800 kV, linking rectifier stations at Biswanath-Chariali and Alipurduar, with inverter station at Agra. Multi Terminal HVDC (MTDC) systems require complex actions for their operation and control. Hence a comprehensive study of the entire system is necessary. The main aim of this paper is to analyze the basic operation of the North East-Agra MTDC scheme and suggest appropriate control measures for it. A simulation of the system is done in MATLAB-Simulink to understand how the system operates. Along with it, the details of the proposed controllers that can ensure proper control of each station of the HVDC link are also included. The results of the simulations show that the proposed controllers can be effectively incorporated in the system.

The Modular Multilevel Converter (MMC) has modular and redundant structure, which makes it more reliable during certain fault conditions. Hence MMC technology has received considerable attention among various topologies for interconnecting High Voltage Direct Current (HVDC) grid. Existing literature has devoted limited focus on the behavior of the MMC during internal fault condition. This paper presents the switching as well as approximate voltage source model of MMC with analysis of its internal fault current behavior. It also presents an analytical approach for calculating harmonic components in arm current. The simulation studies with both MMC models rated at 161 kV, 625 MVA are performed using MATLAB/Simulink. For this study the power transfer is controlled by changing modulation signal. Presented results emphasize on understanding fault current behavior for various internal faults in MMC.

Electrical Discharge Machining (EDM) is a manufacturing process in which erosion takes place due to sparking between a workpiece and a tool electrode. The EDM circuit consists of DC-DC converters, the EDM load and an interfacing circuit to regulate the machining. The DC-DC converter topologies are modified to suit requirements of the EDM load. Such modifications along with the nature of load and the interfacing circuit, give rise to issues specific to the EDM power supplies. For a particular topology with two DC-DC converters, such issues in modelling and control of the converters are addressed in this work. A new methodology to determine sizing of passive components is discussed. Pulsed current reference scheme is also illustrated, which can improve input to the EDM load efficiency by more than 30%.

This paper presents a simple modulation index-dependent Thevenin equivalent circuit model of the voltage-source converters (VSCs) and the associated anti-parallel diode rectifier. The model derivation uses orthogonal transformation and a transformer analogy to transfer impedances to the dc side of the converter. The paper proposes a switched equivalent representation of a three-phase diode rectifier formed due to the antiparallel diodes when VSC switching is inhibited. The proposed model is implemented in Alternate Transients Program–Electromagnetic Transients Program and simulation results are compared with the switched model. The proposed model is shown to under predict the fault currents for pole-ground (PG) faults and to provide a nearly exact match for pole-pole faults. The rectifier equivalent provides increased accuracy in the transient region with the inclusion of the source inductance. A multiterminal dc (MTDC) system model is used to study effects of aggregation which gives increased accuracy of the equivalent circuit. It is shown that the MTDC system simulation with the proposed Thevenin equivalent is about 3 times faster than full-switched circuit simulation for a time step of 5 $\mu$ s. The MTDC system simulation with the Thevenin equivalent circuit is still accurate and 126 times faster when the time step is increased to 750 $\mu$ s.

The important requirement of High-Voltage Direct Current (HVDC) multi-terminal system is the mature Circuit Breaker (CB). In this paper, the systematic mathematical modeling is implemented to design resonance circuit parameters of Active type resonance HVDC CB. The designed model of CB is tested on three cases, namely, fault current, load current and system inductance variation. These cases are compared based on total time required to reduce fault current to residual current and to de-energize breaker system. The simulation results for these conditions validate the performance of designed HVDC CB.

The impact of the system parameters and controller gain on the dynamic and steady state behavior of the multi-terminal direct current (MTDC) system is discussed using small signal model. The small signal model (SSM) of VSC is derived including controllers and measurement filters and it is also validated with non-linear detailed switched model in PSCAD simulator. The SSM of DC grid and VSCs are augmented into single SSM to study the stability of the MTDC system. The limitation of the PLL gain caused by the strength of AC grid (short circuit ratio) is illustrated using eigenvalues analysis. This analysis is done for different control scheme as well as operating mode. The range of stable region for different SCR and loads are checked by varying the PLL gain. The effect of the controller gain on the AC resonance is also discussed in this paper. As the VSC terminals are increased, the DC resonance appears and its impact on the bandwidth of the direct voltage controller is explained briefly.

The dc series and series-parallel wind farms have been recently proposed as alternatives to ac for offshore applications. A dc wind turbine for use in dc series and series-parallel wind farms has been proposed in this paper. The operation of the dc series and series-parallel wind farm results in continuous variation of the output voltage with possibility of losing maximum power point tracking capability (MPPT). A wind turbine (WT) design with back-to-back connection of voltage source converter (VSC) and dual active bridge (DAB) dc-dc converter is proposed. An integrated storage connected to internal dc bus of the turbine with a DAB is also proposed. Simulation results show that the proposed design ensure maximum energy capture during undervoltage and overvoltage operating conditions. The output voltage variation is limited to a range of 0.7 to 1.5 p.u.

There is an increasing interest in voltage source converter (VSC) based HVDC grids. The development of DC grids is likely to take place in phased manner with several voltage levels similar to that of the AC grids. These different high voltage DC levels will have to be interconnected with each other which will require DC-DC transformers. The paper presents a concept of DC-DC converter based on modular multilevel voltage source converters (MMC) and high frequency transformer. The MMC are connected as front ends on the DC side with interposed high frequency transformer. The design principle, selection of components and operation of the converter are discussed. The paper also presents open loop simulation results of a ±400/±400 kV, 2250 MVA DC-DC converter with the calculation of losses. The power transfer is controlled with simple phase shifting operation.

The integration of renewable energy resources has increased possibility of application of multi terminal direct current (MTDC) system. In this paper, the primary level control of voltage source converter (VSC) and their design method is discussed. The control schemes for the MTDC system such as DC voltage control, voltage margin, droop control, coordinated and distributed DC voltage control are discussed and compared. A four terminal VSC based MTDC system is modeled in PSCAD. The performance of the controllers are compared under step load changes and fault on the DC grid.

There has been significant capacity addition to the power system leading to increased fault current at substations buses. This call for costly equipment upgrades and replacements. In order to defer investments utilities are exploring use of superconducting fault current limiters (SCFCL) and current limiting reactors (CLR). The SCFCL can be built with either resistive or inductive shunts. Also, it might be possible to include a recovery switch in series with the superconducting element. The choice of shunt is likely to have significant impact on the current limiting capability of the SCFCL. Also, for correct application design the circuit breaker transient recovery voltage (TRV) should be within its capability limits. It is expected that the TRV will be impacted by the choice of shunt and use of the recovery switch. The paper analyzes the effect of type of shunt on efficacy of limiting fault current and impact on TRV using EMTP-ATP. It is observed that the use of resistive shunt may be better for limiting first peak of the current and limiting TRV values. The inductive shunt reduces the steady state fault current effectively. The use of recovery device has adverse impact on the TRV characteristics of the system.

In this paper, the concept of an all-dc wind park with series-connected turbines is investigated as an alternative to the classical ac parallel or radial wind park. This paper presents a literature overview of all-dc wind park concepts with series connection. A three-phase conversion system with permanent magnet machine, ac-ac converter, high-frequency transformer, and diode bridge rectifier is suggested in this paper for the series connection of dc turbines. The dc series park with the suggested conversion system is compared in terms of losses, cost, and reliability to the state-of-the-art park configuration which is the ac radial park with HVDC transmission. It is found that the dc series park becomes comparable with the ac radial design for high ratings of the dc turbines. Furthermore, the comparison shows that emphasis must be put on reducing the losses in the conversion system of the dc turbine and, particularly, the ac-ac converter. Therefore, the efficiency of the ac-ac converter is compared for three different topologies: the direct matrix converter, the indirect matrix converter, and the conventional back-to-back converter. The direct matrix converter is found to be the most efficient, suitable for the suggested conversion system.

This paper presents an overview of importance and significance of controlled switching in complex power networks. The main objective of controlled switching is to avoid transients in the system. In a power system network transients are the redistribution of stored energy among the system capacitance and inductance whenever there is a change of state. So controlled switching is the method to optimize the instant of switching the circuit breaker resulting in minimal energy redistribution.

During the last decade there has been an increasing interest in the High Voltage DC grids amongst researchers and policy makers. Protection has been identified as a critical component in the realization of these multi-terminal direct current (MTDC) systems. The fault currents in the DC systems are limited by the circuit resistance and inductance and thus are likely to reach very high values. This may also lead to voltage collapse. In order to limit the fault current magnitudes and to prevent voltage collapse use of superconducting fault current limiters (SCFCL) in MTDC system is proposed in this paper. The effectiveness of SCFCL in limiting the fault current along with sizing of SCFCL resistor are explored in this paper. The paper also discusses the effect of initial discharge and critical current setting of SCFCL.

With the fast load growth, the grid is pushed to operate near to the limits. The fault current levels at critical substations can overshoot the limits on various equipments like busbars, circuit breakers and transformers. The various conventional remedial measures to reduce fault currents are reported in literature. However, there are time critical situations which can be better tackled by recent devices like fault current limiters (FCLs). Super Conducting FCL (SCFCL) can limit the fault current below the prospective peak values. The paper presents a simple steady state model for SCFCL. An analytical approach to determination of SCFCL impedance is presented. An utility test system is selected to demonstrate the effectiveness of the SCFCL. The location and sizing issues of the SCFCL to minimize cost and obtain desired fault current reduction are also addressed.

The share of wind energy is growing day by day and asynchronous machines like doubly fed induction generators are finding their place in the grid. These asynchronous generators under transient conditions like fault behave in a different manner than synchronous ones. In order to achieve fault ride through capabilities for such generators special protection scheme is needed. One of the solution technique is to use crowbar resistances on rotor winding to protect the rotor side converter. During a fault, the current profile of DFIG is different compared to a conventional generator. This paper focuses on the short circuit current profiles of DFIG for wind turbines. An analytical expression is described for an asymmetrical fault current which is slip and initial condition dependent. Crowbar protection scheme and its effect on short circuit characteristics has been discussed. A brief analysis on the effects of these fault currents on generator circuit breaker is considered. Simulation results compared with theoretical analysis is presented for verification.

Current-limiting reactors are placed in series with capacitor banks to limit the rate of rise of current to the values specified in the circuit breaker (CB) standards. But this arrangement has created capacitor bank failures when attempting to clear faults in between the reactor and the capacitor bank. After detailed analyses of failures, solutions have been proposed by researchers: 1) Add a surge capacitor to ground on the capacitor bank side of the breaker and 2) add a surge capacitor across the reactor. These surge capacitors are sized based on the stray capacitances of the bus, the reactor, the circuit breaker, and on the maximum-available fault current at the substation. This paper presents a simplified means of sizing the surge capacitors for method 2), based only on the CB's interrupting current rating and reactor size. This eliminates the need for and uncertainty of stray capacitance values. Also, the design does not need to be revisited when grid enhancements increase the available fault current at a substation. A standard surge protection package, which can also be applied to existing installations, is proposed. This new approach has been verified with studies using Electromagnetic Transients Program/Alternative Transients Program.

With the inclusion of renewable energy sources, the variability of the power supply has increased. To reduce this variability, energy storage equipments such as superconducting magnetic energy storage (SMES) are required. The energy is stored in SMES coil, making its design very crucial. This paper presents the effect of cryogenic temperature on the resistance and inductance of air core solenoid. The experiment is performed on the copper wire wound over teflon and copper formers. The published mathematical model for the calculation of the coil inductances is taken and results are compared with measurements performed with an LCR meter. The difference between the measured and calculated inductance value for both coils is found to be less than 10% at 300 K. The cryogenic temperature is maintained by first using a liquid nitrogen bath and later by cryocooler.

New offshore wind farms are being proposed to be located at distances of 50-100 km from the shore. These large distances make HVDC connection to grid a feasible option. DC collection grids have been proposed to eliminate offshore platforms in wind farms. Two main topologies previously proposed for DC offshore wind farms with series connection of wind turbines to eliminate offshore platforms have been studied. The aspects of reliability of wind farm collection system are important in order to attract investors and make the designs physically realizable. Wind farms are large economic investments and in order to make good economic decisions a very important factor is the amount of additional income that can be generated over the life time of the wind farm. Thus the reliability calculations have to be included as a part of design process. The collection system reliability for DC Series and Series-Parallel topologies is mainly addressed in this paper. Various reliability indices like the interruption frequency, interruption duration and expected energy not supplied are calculated using an analytical method. The largest contributors to unreliability are identified. Alternative designs with added redundancy are proposed and evaluated. The important factor of possible additional income that can be generated is calculated to select and improve designs to be implemented. The evaluation results indicate that the DC Series-Parallel wind farms offer a great promise in terms of feasibility and reliability. It is observed that the DC Series design can be used as base system for calculations and understanding of the important factors in reliability.

Increasing energy demand and environmental factors are driving the need for the green energy sources. The trend, in general, with respect to wind farms is to increase the number and the size of wind farms. The wind farms are also being located offshore with the prospect of more consistent and higher energy capture. The offshore wind farms are likely to move farther off from the shores to reduce visual impacts and increase the size. But, this has implications in terms of design of the collection grid and grid interconnection. Farms of 1 GW size and at distances of about 100 km are envisaged [1]. The design of collection system and turbine interconnection will become very important as the farms move farther offshore. Proper choice of collection system topology is important from the point of view of maximum energy capture while ensuring a high reliability of the design. Different collection system topologies have been proposed by researchers before, with the radial system being most popular. One of the key factors in selection would be the losses in the farm. In order to select the most suitable topology a comparison of different topologies with respect to losses, reliability and costs has to be done. Comparisons of calculations indicate that the DC series and series-parallel wind farm design may be options for future wind farm designs. The DC series and series-parallel design have lower reliability, but can be improved by providing redundancies. The designs have equipment costs almost equal to the AC wind farm costs. The losses in DC series-parallel wind farm are higher by about 12 % when compared to AC wind farms. The DC series design is also very attractive design, but has restrictions with respect to insulation. Also, the required turbine ratings may be significantly and unrealistically high when it comes to designing large wind farms. It can also be concluded that the novel designs require significant amount of work before these can be used in real wind farms.