Abstract: No-insulation (NI) high temperature superconducting (HTS) coils possess much higher thermal stability than similar traditionally insulated HTS coils. Some NI coils are self-protecting in the sense that they fully recover after a quench without any external protection mechanism to dissipate the stored energy. The underlying mechanisms that make NI coils highly stable or even self-protecting, however, remain unclear. To answer this question, a numerical multiphysics quench model for NI pancake coils is built to study the electrical, thermal and magnetic behavior of NI coils subjected to local heat disturbances. The multiphysics model is built from an electric network model, tightly coupled to a two-dimensional thermal coil model and a three-dimensional magnetic field coil model. The results show that when heat disturbance initiates a local normal region on a turn, the transport current is redistributed not only from the local normal region, but also along the entire turn. The redistributed current flows in the form of radial current across the turn-to-turn contact resistance along the entire turn to the neighboring turns which are still in the superconducting state, driving these turns to an overcurrent state. This full-turn current sharing and overcurrent operation accelerate the redistribution of current away from the hot-spot, reducing localized Joule heating that would otherwise cause a sustainable quench. The results also show that the magnetic field generated at the coil center drops rapidly and the coil voltage changes dynamically during the early stage of normal zone formation. These phenomena can be utilized as effective methods for quench detection in NI coils by monitoring the magnetic field and coil voltage.

Abstract: For a large multiterminal HVDC system, it is important for a dc fault on a single branch to not cause significant disturbance to the operation of the healthy parts of the dc network. Some dc circuit breakers (DCCBs), for example, mechanical type, are low cost and have low power loss, but have been considered unsuitable for dc fault protection and isolation in a multiterminal HVDC system due to their long opening times. This paper proposes the use of additional dc passive components and novel converter control combined with mechanical DCCBs to ensure that the healthy dc network can continue to operate without disruption during a dc fault on one dc branch. Two circuit structures, using an additional dc reactor, and a reactor and capacitor combination, connected to the dc-link node in a radial HVDC system, are proposed to ensure that overcurrent risk at the converters connected to the healthy network is minimized before the isolation of the faulty branch by mechanical DCCBs. Active control of dc fault current by dynamically regulating the dc components of the converter arm voltages is proposed to further reduce the fault arm current. Simulation of a radial three-terminal HVDC system demonstrates the effectiveness of the proposed method.

Abstract: The integration of converter-interfaced distributed generation in microgrids has raised several technical issues, including the successful operation of protective devices during faults. The protection issue is associated with the lack of large current injection during a fault, due to limits imposed by the semiconductor switches. This paper proposes a fault-detection and clearing control strategy method for symmetrical and asymmetrical line faults in a looped microgrid. The protection devices are simple overcurrent devices with the same settings, due to the looped microgrid topology. The proposed method is applied without using any kind of physical communication. The fault is detected by measuring indirectly the microgrid impedance. After the fault identification, the distributed energy resources (DERs) adjust their control in order to inject a current proportional to the measured microgrid impedance, according to a droop curve. This means that the DER closer to the fault injects a relatively larger current, achieving by this way a selective coordination of the protection means. The effectiveness of the proposed control strategy is evaluated through detailed simulation and experimental tests.

Abstract: A modified Z-source breaker topology is introduced to minimize the reflected fault current drawn from a source while retaining a common return ground path. Conventional Z-source breaker topologies do not provide steady-state overload protection and can only guard against extremely large transient faults. The Z-source breaker can be designed for considerations affecting both rate of fault current rise and absolute fault current level, analogous in some respects to a thermal-magnetic breaker. Detailed analysis and design equations are presented to provide a framework for sizing components in the Z-source breaker topology. In addition, the proposed manual tripping mechanism enables protection against both instantaneous current surges and longer-term overcurrent conditions. The fault operation intervals of the proposed Z-source breaker topologies are both demonstrated in SPICE simulation and validated in experimental characterization.

Abstract: Conventional overcurrent protection schemes for radial distribution systems usually attempt to coordinate a recloser at the beginning of the feeder with the fuses on the laterals. The integration of distributed generation in distribution systems leads to problems related to protection coordination that are difficult to be solved by applying conventional protection techniques. This paper proposes an efficient communication-based protection scheme that implements common directional overcurrent relays instead of reclosers at the line, assisted by intertripping and blocking transfer functions. The proposed protection strategy guarantees selectivity regardless of whether the generating units are connected to the network or not, and can be designed retaining either the fuse-blowing or fuse-saving philosophy. Meaningful conclusions are derived from the application of the scheme on a test distribution system.

Abstract: This paper presents a 15kV silicon carbide (SiC) MOSFET gate drive, which features high common-mode (CM) noise immunity, small size, light weight, and robust yet flexible protection functions. To enhance the gate-drive power reliability, a power over fiberbased isolated power supply is designed to replace the traditional design based on isolation transformer. It delivers the gate-drive power by laser light via optical fiber over a long distance (>1 m), so a high isolation voltage (>20 kV) is achieved, and the circuit size and weight are reduced. More importantly, it eliminates the parasitic CM capacitance coupling the power stage and control stage, and thus eradicates the control signal distortion caused by high dv/dt in switching transients of the high-voltage SiC devices. In addition, the gate-drive circuit design integrates comprehensive protection functions, including the overcurrent protection, undervoltage/overvoltage lockout, active miller clamping, soft turn off, and fault report. The overcurrent protection responds within 400 ns. The experimental results from a 15kV double-pulse tester are presented to validate the design.

Abstract: Introduction of distributed generation (DG) to the power system may lead to nonselective protection actions. For every future DG installation, the relay settings need to be modified to guarantee protection coordination that can lead to numerous changes in relay settings. This paper presents a novel approach to plan relay protection coordination considering future DG installations. Thus, this paper aims at proposing a method capable of optimally identifying one set of relay settings valid for all possible future DG planning scenarios. The proposed algorithm is formulated as a linear programming problem and the simplex algorithm is utilized to solve it. The proposed approach is tested on the distribution part of the modified meshed IEEE 14-bus system and the IEEE 13-bus radial test system. Comparative studies have been conducted to highlight the advantages of the proposed approach under various planning scenarios considering application of fault current limiters.

Abstract: This study presents a hybrid gravitational search algorithm-sequential quadratic programming (GSA-SQP) algorithm based on the GSAand SQP to optimal coordination of directional overcurrent relays (DOCRs). The SQP routine is incorporated in GSA as a local search mechanism to improve the performance of the conventional GSA algorithm. The coordination between DOCRs is formulated as a non-linear and highly constrained optimisation problem in which two settings namely time dial setting and pick up current setting of each relay are considered as decision variables. The main objective is to minimise the sum of operating times of all the primary relays, which are expected to operate in order to clear the faults of their corresponding zones. Performance of this approach is studied and evaluated on three different test systems with 6, 24 and 38 relays. Simulation results show that the proposed approach provides effective and robust high-quality solution of the problem.

Abstract: The modern power system networks are very complex and often consist of multiloop structures with increased penetration of renewable energy sources-based distributed generations. Directional overcurrent relays (DORs) are the key protection devices in such networks and their coordination has a profound impact on the overall protection of networks. Optimisation of DOR settings is an important concern in protection coordination in power systems. In this study, optimisation of DOR settings, namely time dial setting and pickup current (I p) setting is achieved by adopting symbiotic organism search technique which is a recently proposed proficient optimiser imitating biological give and take policy while searching for an optimum. Computational ability of the technique to coordinate DORs is validated in IEEE 6-bus and WSCC 9-bus test systems. Results show that the new technique causes for notable reduction in relays’ operating time, with maintaining reliable coordination margin for each primary/backup relay pair, in comparison with other techniques. Also, computation time to find optimum solution is less using the new method.

Abstract: SiC MOSFETs are a leading option for increasing the power density of power electronics; however, for these devices to supersede the Si insulated-gate bipolar transistor, their characteristics have to be further understood. Two SiC vertically oriented planar gate D-MOSFETs rated for 1200 V/150 A were repetitively subjected to pulsed overcurrent conditions to evaluate their failure mode due to this common source of electrical stress. This research supplements recent work that demonstrated the long term reliability of these same devices [1] . Using an RLC pulse-ring-down test bed, these devices hard-switched 600 A peak current pulses, corresponding to a current density of 1500 A/cm2. Throughout testing, static characteristics of the devices such as $B_{{\rm VDSS}}$ , $R_{{\rm DS}({\rm on})}$ , and $V_{{\rm GS}({\rm th})}$ were measured with a high power device analyzer. The experimental results indicated that a conductive path was formed through the gate oxide; TCAD simulations revealed localized heating at the SiC/SiO2 interface as a result of the extreme high current density present in the device's JFET region. However, the high peak currents and repetition rates required to produce the conductive path through the gate oxide demonstrate the robustness of SiC MOSFETs under the pulsed overcurrent conditions common in power electronic applications.

Abstract: Unbalanced load currents not only give rise to unbalanced voltages but also adversely affect the performance of the conventional current-limiting mechanisms. The latter might result in overcurrent stress on the distributed energy resources (DERs) or current harmonics. In this paper, a novel decentralized control method is proposed to improve the power quality and protect DERs from overload. The proposed controller makes use of the model predictive control (MPC) technique to minimize the voltage unbalance, improve current limiting, and prevent active power overload. The MPC is combined with the ${\rm V}$ - ${\rm I}$ droop method to realize coordinated operation with fast dynamic response. The proposed method is tested on the CIGRE benchmark microgrid. Simulation results demonstrate that the proposed method improves power quality but also allows for operation close to the maximum load capacity without imposing DERs to overload.

Abstract: For doubly fed induction generator (DFIG)-based wind turbine, the main constraint to ride-through serious grid faults is the limited converter rating. In order to realize controllable low voltage ride through (LVRT) under the typical converter rating, transient control reference usually need to be modified to adapt to the constraint of converter's maximum output voltage. Generally, the generation of such reference relies on observation of stator flux and even sequence separation. This is susceptible to observation errors during the fault transient; moreover, it increases the complexity of control system. For this issue, this paper proposes a scaled current tracking control for rotor-side converter (RSC) to enhance its LVRT capacity without flux observation. In this method, rotor current is controlled to track stator current in a certain scale. Under proper tracking coefficient, both the required rotor current and rotor voltage can be constrained within the permissible ranges of RSC, thus it can maintain DFIG under control to suppress overcurrent and overvoltage. Moreover, during fault transient, electromagnetic torque oscillations can be greatly suppressed. Based on it, certain additional positive-sequence item is injected into rotor current reference to supply dynamic reactive support. Simulation and experimental results demonstrate the feasibility of the proposed method.

Abstract: Current-source converters (CSCs) have a promising potential to interface the large-scale photovoltaic (PV) generators to electric grids. In order to overcome several drawbacks associated with the conventional vector-controlled CSC-based PV systems, this paper presents for the first time, the power synchronization control (PSC) scheme for the single-stage grid-connected CSC-based PV generator. A detailed small-signal model for the complete CSC-based PV system with the PSC strategy is developed to investigate the system stability, characterize the converter interactions with the electric grid, and design the controller parameters. The PSC of the CSC-based PV system reflects a stable performance under different operating conditions and following sudden variations in the insolation level. Due to the inherent synchronizing feature of the PSC, the phase-locked loop is eliminated and, hence, the integration to the very weak grids is seamlessly achieved. Although there is no direct control on the injected ac current to the grid, the inherent self-current-limiting feature of the CSC through the modulation of the regulated dc-choke current offers an overcurrent protection. Detailed nonlinear time-domain simulations results validate the theoretical analysis and show the effectiveness of the PSC strategy.

Abstract: Concerning that a short-circuit fault happens inside a microgrid system, parts of distributed generation (DG) units may not meet the fault ride-through (FRT) requirements and will be enforced to disconnect. This paper suggests a modified flux-coupling-type superconducting fault current limiter (SFCL) to improve the DG's FRT capability, and investigates the relay protection coordination in the microgrid. The SFCL's structural principle is introduced, and according to the fault characteristic of the microgrid under its grid-connected and islanded statuses, the modified SFCL's application fields are conducted. Furthermore, the directional overcurrent protection and differential protection are proposed for the microgrid's two statuses, respectively. Based on the transient simulations in the MATLAB, the SFCL's positive effects on enhancing the DG's FRT capability can be confirmed, and also it will not affect the relay protection coordination. A comparison of the conventional distance relay protection and the proposed protection scheme is performed, and the latter has a better applicability than the former to match the SFCL. Finally, regarding the impacts of fault type and fault resistance on the FRT operation and the proposed protection, technical discussions are performed preliminarily, and the application value of the SFCL integrated with the proposed protection is clarified further.

Abstract: The three-phase series-connected modular multilevel converter (SC-MMC) is presented as one option for high-voltage direct current (HVDC) tap to reduce the cost of an HVDC station. Compared with conventional three-phase parallel-connected MMC, the number of semiconductor switches in SC-MMC is reduced by one-third to withstand the same dc voltage. First, the paper studies the operating principle of SC-MMC, and then presents an interspacing phase-shifted pulsewidth-modulated method which could improve the waveform of the output ac voltage for a given switching frequency. Next, the control strategy based on the identical real power input of each phase is proposed to balance the dc voltage of each phase under an unbalanced grid condition. The current limit of switching devices is also taken into consideration to protect SC-MMC from overcurrent. Furthermore, a universal zero-sequence current controller is developed as one part of the control strategy. In addition, an innovative method to suppress the third harmonics of ac current is presented. With the third harmonic suppression strategy, the SC-MMC can operate with much smaller SM capacitors without deteriorating the ac current, which contributes to the cost reduction further. Finally, simulation results obtained in PSCAD/EMTDC are provided to validate the proposed control strategies.

Abstract: This paper proposes the dynamic voltage and current assignment (DVCA) strategies for doubly fed induction generator (DFIG) wind power system, which is fed by nine-switch converter (NSC). With the proposed DVCA strategies, the voltage and current capacities of switches in NSC are dynamically assigned to the rotor- and grid-side branches, depending on different grid conditions. With dynamic voltage assignment, the rotor-side gets more voltage to suppress the overcurrent when the symmetrical grid voltage dips, and with dynamic current assignment, the grid-side gets more current capacity for reactive current compensation. Based on the proposed DVCA strategies and configuration, NSC-based DFIG system can be globally optimized, so that the rating capacities designed in the normal condition can also be fully used during low-voltage ride-through (LVRT) period, and no extra capacity margins or auxiliary circuits are needed. The proposed DVCA strategies and global optimization are designed, and also compared with those typical B2B-based DFIG systems. The experimental results from a 1.5-kW NSC-based DFIG system verify the validity of the analysis.

Abstract: Microgrid is regarded as a new form to integrate the increasing penetration of distributed generation units (DGs) in the extensive distribution systems. This paper proposes an adaptive overcurrent protection strategy for a microgrid network. The protection coordination of the overcurrent relays is treated as a linear programming problem for the different operation states. In the control center, an artificial neural network (ANN) model is trained with real-time measurements to identify the states whether there is a fault on the line segment. Fault location is estimated further with the same measurements in another neural network model. Reconfigurations can be performed to modify the settings of the on-field relays to enhance the reliable operation for the different operational situations. The test results show that the adaptive overcurrent protection scheme with the assistance of estimation model can modify the protective settings for the new operation state accurately and intelligently.

Abstract: PROBLEM TO BE SOLVED: To provide a power conversion device which gives the opportunity of normally driving an inverter at a side of a motor for vehicle drive while appropriately attaining overcurrent protection in each of inverters by temporarily improving voltage resistance of an inverter at a side of an electric oil pump in accordance with voltage resistance of the inverter at the side of the motor for vehicle drive without enlarging the inverter at the side of the electric oil pump.SOLUTION: The power conversion device comprises: a DC power source; the inverter for drive which includes a pair of first switching elements connected in series across the DC power source, converts DC power of the DC power source into AC power and supplies AC power to the motor for drive; the inverter for pump which includes a pair of second switching elements connected in series across the DC power source, converts DC power of the DC power source into AC power and supplies AC power to the electric oil pump; means which performs shutdown for turning off the second switching elements if a voltage across the inverter for pump is equal to or higher than a first threshold; and means which performs shutdown for turning off the first switching elements if a voltage across the inverter for drive is equal to or higher than a second threshold that exceeds the first threshold.SELECTED DRAWING: Figure 4

Abstract: Control of three-phase grid-connected voltage source converter under unbalanced grid faults greatly depends on the active and reactive powers processed by the converter. The instantaneous active power theory with sequence decomposition is employed to analyse the instantaneous power components, especially the second-order oscillation power. Study shows that the second-order oscillation power comprises two quadrature components, the cosine and sine terms, which are contributed by the average active power and the average reactive power, respectively. This finding sheds insight on the regulation of oscillation power under unbalanced grid conditions. Based on this observation, the authors propose a positive and negative sequence conductance and susceptance control scheme, which enables simple regulation of the active power or reactive power oscillation with the average active power and reactive power control. In addition, the authors investigate the relationship between the positive/negative sequence conductance and susceptance distribution factors with power oscillation and peak current. A maximum current limitation scheme is embedded into the current reference generation block for overcurrent protection. Numerical simulations and prototype measurements verify the accuracy of the analysis and the effectiveness of the control scheme.

Abstract: A coordinated secondary control approach based on an autonomous current-sharing control strategy for balancing the discharge rates of energy storage systems (ESSs) in islanded ac microgrids is proposed in this paper. The coordinated secondary controller can regulate the power outputs of distributed generation (DG) units according to their states-of-charge and ESS capacities by adjusting the virtual resistances of the paralleled voltage-controlled inverters. Compared with existing controllers, the proposed control strategy not only effectively prevents operation failure caused by overcurrent incidents and unintentional outages in DG units, but also aims to provide a fast transient response and an accurate output-current-sharing performance. A complete root locus analysis is given in order to achieve the system stability and parameter sensitivity. Experimental results are presented to show the performance of the whole system and to verify the effectiveness of the proposed controller.

Abstract: AC–DC–AC converter-fed induction motor (IM) drive is mainly realized by back-to-back three-phase converters. However, fault in a single semiconductor switch will make it inoperative. To enable continued controllable operation in case of the faults occurring in the converter, the five-leg converter with a shared leg between the grid and load sides is a possible solution. However, this topology poses an inherent two-objective control problem, because its grid and load sides should be controlled simultaneously. More importantly, the potential increase of the shared-leg current may destroy the converters. In this paper, a new control scheme based on the FCS-MPC combined with intrinsic characteristic of the five-leg converter is proposed for independent control of the rectifier and inverter subsystem with the shared-leg overcurrent constraint. The condition for independent control is analyzed. In order to give a complete evaluation of the proposed control scheme, the conventional pulsewidth modulation (PWM) control scheme is conducted for comparison. Experimental results are provided to validate the effectiveness of the proposed scheme.

Abstract: In this paper, a new procedure based on a backward/forward sweep (BFS) algorithm for solving power flows in weakly meshed dc traction networks is presented. The proposed technique is able to consider the trains as nonlinear and nonsmooth (nondifferentiable) voltage-dependent loads or generators. This feature permits the inclusion of the trains’ overcurrent protection and the squeeze control. With the use of the mentioned controls, the conventional power flow problem becomes a voltage constrained power flow problem, and the interaction between the trains and the network can be accurately modeled. However, the train control induces a highly nonsmooth voltage-dependent load characteristic, causing convergence problems in most of the derivative-based algorithms. The proposed algorithm is faster, more robust, and more stable than the derivative-based ones. In addition, the authors present all of the formulation in a compact matrix-based form by means of the graph theory application and the node incidence matrix.

Abstract: Incorrect responses from protective devices weaken the reliability of microgrids. This paper investigates the effects of the protection system on the reliability performance of a microgrid integrated with weather-dependent microsources. It relies on a stochastic model to simulate the challenges existing in current protection schemes designed for microgrids. Abnormal operating conditions, including overcurrent, overvoltage, and undervoltage, which are detected by a deficient protection scheme, are analyzed by a defined fuzzy virtual setting value. Their contributions, along with the conventional random failure rate achieved by statistical results, are jointly used to evaluate the short-term outage rate of a feeder. A series of fuzzy reliability indices is further proposed for the short-term and periodical reliability of both microgrid system and customers. Subsequently, an evaluation strategy, combined with fuzzy simulation and load restoration, is presented as a solution tool. Simulations on a test system are provided to support the necessity of considering the effects of protection operation in evaluating microgrid reliability.

Abstract: The placement of the superconducting fault current limiter (SFCL) can be utilized to accommodate the increasing integration of distributed generations (DGs) in a power system. The installation of DGs in a power system can affect the power flow and the fault current patterns. Substantial changes in fault currents can cause coordination problems among the overcurrent relays. SFCL placement is a possible approach for accommodating the DGs by locally limiting the fault currents resulting from the DGs, while maintaining the existing relay settings. This paper presents a systematic procedure for analyzing the impacts of SFCL placement on DG expansion, considering the relay coordination in power systems.

Abstract: Weak low voltage ride-through (LVRT) ability and unstable output power are two major problems faced by the doubly-fed induction generator (DFIG). To solve these two problems simultaneously, a commercially available fault current limiter-battery energy storage system (FCL–BESS), which is suitable to be applied in a microgrid, is proposed in this study. During normal operation, the FCL–BESS stabilises the output power of DFIG by compensating the fluctuating component of DFIG output power with energy buffering capability provided by the battery energy storage system (BESS). On occurrence of a grid fault, the FCL–BESS enhances the LVRT ability of DFIG by inserting the fault current limiting inductor into the stator, which weakens the rotor back electromagnetic force voltage and limits the rotor overcurrent. The FCL–BESS also stabilises the DC-link voltage by the BESS, which further strengthens the controllability of grid-side converter and rotor-side converter. Moreover, the FCL–BESS is able to help grid voltage recover fast and smoothly by providing power support to the grid. Experiments under different conditions have been carried out with a 2 kW prototype to evaluate the performance of the proposed FCL–BESS. Experimental results show that the FCL–BESS have much better performance than using single fault current limiter or BESS and can solve the two problems faced by the DFIG simultaneously and effectively.

Abstract: In digital protection systems, discrete Fourier transform (DFT) technique is the most widely used frequency analysis tool for extraction of fundamental and harmonic components of fault currents. However, some fault currents are accompanied by decaying direct current (DC) offset which affects the accuracy of the DFT-based methods as the polluted fault current with decaying DC offset is not a periodic signal. This matter may cause mal-operation of some protection schemes such as overcurrent and distance relays. In this study, to address this issue and decrease the estimation error, a novel technique is proposed to eliminate the effects of DC offset on DFT-based methods. In the proposed method, the decaying DC offset is estimated using one-cycle data windows of the fault current signal. By subtracting the estimated decaying DC offset from the fault current, the remaining signal becomes periodic. Thereafter, the DFT method can be effectively used for extraction of fundamental and other harmonic components. The performance of the proposed method is evaluated comprehensively by investigating different signals even in noisy conditions. Also, a prototype is conducted to validate the applicability of the proposed method in the real power system by real-time simulation.