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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.

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.

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.

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.

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.