Search

Summary: Based on the analysis of the short current of the fixed speed induction generator (FSIG) under symmetrical system faults, a strategy using the dynamic voltage restorer (DVR) is proposed to improve the low‐voltage ride through (LVRT) capability of FSIGs. During the system fault, LVRT‐DVR is connected in series between the grid and the generator by switching‐off the by‐pass bidirectional thyristors and provides a compensating voltage to isolate FSIG from the system fault. To increase the switch‐in speed of LVRT‐DVR, an antivoltage technique is used to shut the thyristors off immediately. After the fault is cleared, the phase‐shifting control is applied to avoid phase jumps of the FSIG terminal voltage. The PSCAD/EMTDC simulation and field tests have been carried out to prove the effectiveness of the proposed LVRT strategy, and related conclusions and comments are summarized. [ABSTRACT FROM AUTHOR]

Summary: Penetration of distributed generations in distribution networks (active distribution networks) has resulted in some problems such as change in short circuit level and direction of fault current flow. Such conditions may cause some relays to malfunction. In this paper, a method for coordinating directional overcurrent relays in active distribution networks taking uncertainties into consideration is proposed. These uncertainties include changes in operation conditions, changes in fault conditions, error in measuring equipment, and outage of distributed generations. Therefore, an optimization frame with new variables is presented, which can be used to create different characteristic curves. Taking uncertainties into account causes the number of constraints to increase and the problem of coordination of directional overcurrent relays to become more complicated. Thus, a two‐step approach is used to solve this problem. In the first step, an algorithm is used to form the constraints of the problem, then, in the second step, a hybrid optimization algorithm consisting of the cuckoo search algorithm and linear programming is proposed. The simulation results for active radial and meshed distribution networks are presented to demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

In this article, an adaptive fault line selection method based on atomic comprehensive measures for a distribution network is proposed. Firstly, wavelet denoising is introduced to remove the noise contained in the transient zero-sequence current; then, the matching pursuit algorithm is utilized to decompose the denoising transient zero-sequence current. Moreover, based on singular value decomposition and information entropy, the atomic singular entropy values are computed, and the actual energy of feature atoms are obtained. Furthermore, according to the fuzzy mathematics, the fault comprehensive measurement function is built. Finally, the line that has the maximum of the fault comprehensive measure value is chosen as the fault line. A large number of simulation experiments and comparing results proved that the proposed method has high selection accuracy and reliability, as well as a powerful ability to resist noise. [ABSTRACT FROM AUTHOR]

The shunt flexible AC transmission system devices affect the protection schemes because of reactive power injection. Therefore, robust protection schemes are required. This study presents a fault detection scheme, which is based on estimated reactive power factor (ERF) and differential current with modified setting. The ERF is defined as sine of phase difference between the static synchronous compensator tap voltage and differential current. The static synchronous compensator tap voltage can be estimated using local end voltage and current. The ERF is measured at both the relay ends. In capacitive mode, if both ERFs are more than upper threshold, the fault is external, and if any ERF is less than upper threshold, the fault is internal. In inductive mode, if ERF is between upper and lower threshold, fault is internal else, if differential current is more than modified differential current setting, the fault is internal, or else, the fault is external. The performance of presented scheme is investigated for various system configurations with PSCAD/EMTDC simulations. Further, the scheme is successfully validated in real-time digital simulator. It is found that the scheme is robust and selective for various faulty conditions, compensation level variations, and location variations. [ABSTRACT FROM AUTHOR]

This paper presents a subsea DC collection grid with renewable power parks which inherently has robust DC fault protection and flexible expansion capability. By virtue of the applied DC/DC converter and the proposed DJ-Index Clamping control, different sections of the subsea grid are functionally interconnected and in the meantime decoupled in the event of DC faults. The faults are selectively isolated without the need for fast DC circuit breakers and system recovery from the faulty DC intertie branch is achieved by a coordinated operation strategy on DC/DC converters and DC disconnectors (or standard AC circuit breakers). A 200-MW test system is presented which consists of two 100-MW renewable energy sources (tidal stream and wind) and DC collection/transmission grids. The simulation results confirm the N-1 security of the presented topology and good control performance over a range of DC faults. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

DFIG is generally used in wind turbines because of its many advantages. However, DFIGs are sensitive to the network faults. When a fault occurs, the DFIG rotor current is increased and its converters may be damaged. A general solution to protect the DFIG converters is crowbar protection. However, when the crowbar is activated, the rotor-side-converter is disconnected and DFIG control may be lost. Therefore, reducing the crowbar operation time is essential. The SFCL can present an efficient protection against severe faults. However, recovery time of SFCL has undesirable effects. In this study the impacts of SFCL on fault-ride-through capability of DFIG are investigated in detail by considering the thermal model of resistive-type SFCL. The simulation results illustrate that by applying the SFCL, the rotor current is decreased and fault-ride-through capability of DFIG is enhanced. Finally, a novel approach to suppress unpleasant influences of nonzero recovery time of the SFCL is proposed. [ABSTRACT FROM AUTHOR]

Ground impedance measuring elements (ground elements), which are responsible for removing single-phase-to-ground faults in distance relays, are highly prone to underreach when facing a significant resistance in the fault path. This paper proposes a new ground element with ensured dependability in high fault resistance conditions for the first-zone digital distance relaying of parallel lines. The proposed ground element functions on the basis of the fault distance and fault resistance it estimates, online during the faults, through the equation derived from the voltages and currents of the distributed-parameters line model. Hence, it will be particularly applicable to the parallel lines of long length in which the distributed parameters effect is significant. The proposed element requires only the data acquired at the relaying location. Further, its function is independent of the structural configuration of the system. Unlike conventional ground elements, the proposed element preserves its security in case of irrelevant-circuit as well as external faults. Simulation studies conducted in MATLAB and PSCAD/EMTDC examine the proposed ground element in a variety of influencing situations, and the results reveal that it accurately trips the faults in the first zone with no incidence of underreach in case of high resistance faults. The accuracy of the proposed element is over 99.4% and its tripping time within 0.3-1.3 cycles. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper analyzes the operation characteristics of the modular multilevel converter when the arms are asymmetrical with different sub-module number caused by sub-module faults. The research reveals the problems caused by asymmetrical arms, including the asymmetrical capacitor voltages, the asymmetrical direct current (dc) components of the arm currents, the asymmetrical circulating currents, the dc current fluctuation, and the voltage offset at the dc or alternating current (ac) side. Then, it is demonstrated by the theoretical analysis that the series of problems can be solved by rebalancing the fundamental frequency components of the upper arm voltage and the lower arm voltage. Therefore, a fault-tolerant control strategy is proposed by introducing a fundamental frequency resonant controller to the traditional second-order frequency proportional resonant circulating current controller. Besides, the research shows that the coordination of the proposed control strategy and the negative sequence current suppressing control can keep the output voltages of the three phases balanced. A 201-level modular multilevel converter-high-voltage direct current time-domain simulation model is established based on PSCAD/EMTDC, and the simulation studies confirm the efficiency of the proposed controller under balanced and unbalanced ac grid conditions. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A reliable protection system is needed for transmitting large amount of power over long distances. An alternating current-direct current mix network in offshore wind farm (OWF) interconnection with onshore grid offers a major challenge for faulty section identification. The transient faults in OWF system are simulated in MATLAB 7.10.0(R2010a). The fault signals, voltage and current, are retrieved at common collector platform and at voltage-source converter-high-voltage direct current sides of network in the OWF system. The detection capability of signal-processing techniques, Stockwell transform, empirical mode decomposition, and Hilbert transform, is explored for localization of different types of faults. In addition, fault events at onshore grid and those developed at the time of change in operating parameters are also simulated and analyzed. Further, the quantitative analysis for detection of fault events with the computation of maximum and mean indices and instantaneous time indices of the fault signal is presented in the study. Coordination of overcurrent relays is another challenging task in OWF interconnection with onshore grid system. The objective function of relay coordination problem is to minimize the operation time of associated relays. Time multiplier setting is used as the test variable in the objective function. An optimal coordination of overcurrent relays by using genetic algorithm, particle swarm optimization, and gravitational search algorithm is discussed in this paper. Further, these techniques are compared with hybrid meta-heuristic optimization method, a combination of particle swarm optimization and gravitational search algorithm to achieve relay coordination more efficiently. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents a novel formulation of impedance-based fault location method for overhead unbalanced distribution systems. The proposed method uses a general fault location equation that is applicable to all possible single and compound shunt fault types. This is in contrast with existing impedance-based methods which use an equation for each fault type and do not cover all types. The proposed method also overcomes the limitation of the conventional method, which requires the identification of fault type. Only the fundamental values of voltages and current measured at the substation terminal are required for the proposed method. The single-phase and three-phase laterals and the capacitive effect of distribution line are considered in this paper. The proposed method was evaluated and tested on a modified IEEE 34-bus distribution system using PSCAD/EMTDC software. Test results verify the accuracy and robustness of the proposed method. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY Induction motors condition monitoring is a very important process for immediate detection of the incipient faults. This prevents the spread of the faults up to risky degrees and avoids catastrophic stop of the motor and relevant production line. The stator winding fault is one of the most commonly occurring faults in alternating current machines. This paper is a comprehensive survey on the stator winding faults causes, diagnosis methods, indexes, and modeling and simulation methods. An efficient method is used to model and simulate the squirrel-cage induction motor performance under inter-turn fault. The results are used to obtain the performance of some common indexes of the fault and recognize the most proper index. Finally, some simulation results are compared with the corresponding experimental results and good agreements are achieved. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In this paper, the application of the hyperbolic S-transform is proposed for detecting internal incipient faults of transformers during the impulse test. This test is performed on the under consideration transformer and during this test, the input current and applied voltage are recorded. The S-transform is applied to the recorded input current. An index is proposed and computed considering absolute deviations of the S-matrix. Faulty and non-faulty windings are discriminated by using the suggested index. The data obtained from field tests of a 66 kV/25MVA interleaved transformer winding and computer simulations, are recorded. Then, these data are fed to the proposed method, and faulty and normal conditions are discriminated, accordingly. Also, a wavelet-based method is implemented in MATLAB environment and the proposed method is compared with the wavelet-based method. According to simulations and experimental results, the proposed method is superior to wavelet transform-based method. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Current differential protection is widely used in transmission line due to its inherent simplicity and good performance. In order to strive to increase sensitivity of protection without compromising stability on external faults, phase correlation current differential protection is presented in this paper. Current phase difference is introduced to the differential current principle so as to improve the sensitivity and reliability of differential protection simultaneously. Comprehensive analysis and comparisons of performance characteristics in Alpha Plane demonstrate that phase correlation current differential protection has preferable performance than that of conventional current differential protection. As far as CT saturation is concerned, the proposed protection scheme has flexible adaptability to ensure significant improvement in sensitivity and dependability for internal faults while still improving stability for external faults. The performance of the proposed protection is investigated for various faults of transmission line simulated by Electro-Magnetic Transients Program including DC and record fault data from real power system. The test results show that the proposed protection has satisfactory operation performance. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Summary: A wide‐area backup protection (WABP) method is proposed in this paper based on the zone status of distance relays. The time difference between the operation of zone‐3 and wide‐area based methods makes it possible to employ wide‐area data for improvement of zone‐3. In case of zone‐3 is directly backup for zone‐1, the proposed method can decrease the time delay of zone‐3 with a complete reliability. Moreover, the proposed method is capable of discriminating between the actual faults and power system stressed conditions. Considering the operational time of wide‐area methods, the algorithm could also be applied for zone‐2 backup protection. The proposed method is also developed to consider the availability of wide‐area data for all busses within the disturbance area, as well as data missing for one or two adjacent busses, which are remarkable advantages over the existing WABP methods. Meanwhile, the wide‐area data used in the proposed method are only the pickup/release status of distance relays zones which are binary data and can be communicated readily through existing wide‐area infrastructures. Simulation results on the IEEE 57‐bus test system demonstrate the effectiveness of the proposed method as a promising WABP in improving the performance of zone‐3 in distance protection schemes. [ABSTRACT FROM AUTHOR]

Summary: Compared with conventional AC transmission systems of PV generation, its DC counterpart is more resilient for improving the PV utilization. However, in the clamped double submodule‐modular multilevel converter (CDSM‐MMC) and DC/DC converter‐based PV grid‐connection system, pole‐to‐ground and pole‐to‐pole fault characteristics still need investigation. The equivalent model of PV stations and current circuits of the CDSM‐MMC, before and after the fault, are studied first. Secondly, the influence of different connection groups of the converter transformer on the DC fault characteristic and that of DC faults on AC‐side electrical quantities are discussed. Subsequently, development stages of DC faults are divided according to the charging and discharging processes of the CDSM‐MMC and the DC/DC converter, and corresponding equivalent models are established and solved. Finally, the DC transmission system of the PV generation is modeled in Power Systems Computer Aided Design/Electromagnetic Transients including DC to verify the theoretical analysis. The DC fault analysis can provide reference for the subsequent protection system design. [ABSTRACT FROM AUTHOR]

Summary: Distance relay is susceptible to maloperation during power swing. Power swing blocking function is integrated in distance relays to ensure security during power swing. However, if a fault occurs during power swing, it must be detected and cleared by the relay as quickly as possible. Since power swings and three‐phase faults are both balanced phenomena, discriminating them is very challenging. This paper proposes a method to detect three‐phase fault during power swing by computing the Hilbert envelope of the current waveform followed by zero‐frequency filtering. The average rate of change in amplitude of the zero‐frequency filtered current is used as the fault detector index. The discrimination capability between three‐phase fault and power swing of the extracted feature is the main contribution of this paper. Results of various possible test cases simulated through EMTDC/PSCAD show that the proposed method is fast and reliable in detecting three‐phase faults during power swings. [ABSTRACT FROM AUTHOR]

Summary: This paper was interested in flexible control of an active generator (AG) that includes a wind turbine, battery‐supercapacitors hybrid storage system, and loads. This AG can operate according to grid stability in islanded, synchronization, and grid‐connected modes. The developed control strategy comprises two principal control tasks: the first one is a two‐layer power management algorithm (PMA). The response of two‐layer PMA enables to detect the islanding mode in case of grid fault and to monitor the renewable power generation into nine operating modes according to the state‐of‐charge of each storage system. The second task is droop control. This droop control aims to reduce grid voltage and frequency variations. It controls exchanged powers with grid to ensure its stability. It also ensures a continuous supply of load in case of grid fault. The system is simulated using MATLAB software, and results are provided in order to show the feasibility of this control strategy. [ABSTRACT FROM AUTHOR]

Summary: This paper presents the design and analysis of a hybrid controller for a doubly fed induction generator to be employed in a wind energy system. Proposed controller gives the quick dynamic and good transient response during asymmetrical faults as well as sudden changes in wind speed. The proposed technique also help in minimizing DC‐transients during grid faults and is capable to extract the maximum power from wind. Further performance of proposed hybrid controller has been analysed independently with sliding mode control (SMC) and proportional and integral (PI) control as its components. It is observed that SMC gives better performance compared with PI in terms of flux ripples, torque ripples, settling time, and DC‐transients. The proposed technique is implemented in hardware in loop‐based OPAL‐RT and MATLAB/SIMULINK environment at a rating of 5.5 KW system under single‐line‐to‐ground and line‐to‐line fault and with variations of wind speed. [ABSTRACT FROM AUTHOR]

Summary: Prompt detection and isolation of faults is crucial in improving the reliability and safety of Wind Turbines (WT) and extending their lifespan. This paper proposes a model‐based robust Fault Detection and Isolation (FDI) scheme for the blade pitch positions of a WT. The approach is able to detect both sensor and actuator faults in a noisy environment. An Unscented Kalman Filter (UKF), with a newly proposed de‐correlation approach for process and measurement noises, is developed to simultaneously estimate the states and parameters of the WT's pitch position. The effectiveness of the proposed approach was assessed using actual data derived from a 2.5 MW wind turbine and simulated using Matlab/Simulink environment. Besides its accuracy and effectiveness in detecting faults, the proposed FDI algorithm considers admissible parameter uncertainties as well as the correlation between process and measurement noises. [ABSTRACT FROM AUTHOR]

Summary: The use of series compensation in a transmission line with capacitors may result in new problems like torsional oscillations in shaft sections of turbine‐generator due to subsynchronous resonance (SSR) when system frequency matches with any one of torsional mode frequency of shaft system during fault or disturbances occur in the transmission line. The subsynchronous current due to SSR during fault also affects the reach of a distance protection system. The diminution of SSR is enhanced by series voltage injection into the power transmission line with static synchronous series compensator (SSSC) from the knowledge of subsynchronous components, extracted from the system. The use of SSSC in transmission line reduces the subsynchronous oscillations, but it adversely affects the distance protection system which leads to an imprecise assessment of fault location, ie, over or under‐reach. This paper investigates the SSR mitigation in the presence of SSSC and its effect on distance protection scheme. A new approach is proposed for mitigating the problems associated with distance protection by using the synchrophasor measurement. The entire study of simulation is investigated by using a 48‐pulse SSSC carried out with Bergeron model of transmission line in PSCAD. The simulation results represent that the suggested adaptive scheme is truthful, reliable, and secure with a change in compensation level of SSSC. [ABSTRACT FROM AUTHOR]