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In substations with an inner bridge connection, the two main transformers are generally connected in parallel. The differential protection of the transformer, under normal operating conditions, may maloperate when the other transformer is switching on through the bridge breaker after scheduled maintenance. This paper analyzes the factors that may lead to the maloperation of transformer differential protection and concludes that local transient saturation of the current transformer (CT) caused by the decaying dc component in the magnetizing inrush is the main cause of the maloperation of the transformer differential protection. By detecting the time difference between the instant of sudden changes occurring in the transformer wye-side current and the instant of the differential current increasing, a new algorithm based on the time differential method is proposed in this paper, which can prevent the maloperation of transformer differential protection in substations with an inner bridge connection. On the basis of detailed analysis and simulation work, the new algorithm is verified to be applicable under the conditions of sympathetic inrush and CT saturation. [ABSTRACT FROM AUTHOR]

This paper presents a new two-terminal impedance-based fault-location algorithm, which takes into account the distributed parameter line model. The algorithm utilizes unsynchronized measurements of voltages and currents from two ends of a line and is formulated in terms of the fundamental frequency phasors of symmetrical components of the measured signals. First, an analytical synchronization of the unsynchronized measurements is performed with use of the determined synchronization operator. Then, the distance to fault is calculated as for the synchronized measurements. Simultaneous usage of two kinds of symmetrical components for determining the synchronization operator makes that the calculations are simple, noniterative, and at the same time highly accurate. The developed fault-location algorithm has been thoroughly tested using signals of Alternate Transients Program-Electromagnetic Transients Program versatile simulations of faults on a transmission line. The presented evaluation shows the validity of the developed fault-location algorithm and its high accuracy. [ABSTRACT FROM AUTHOR]

This paper describes the capability of artificial neural network for predicting the critical clearing time of power system. It combines the advantages of time domain integration schemes with artificial neural network for real time transient stability assessment. The training of ANN is done using selected features as input and critical fault clearing time (CCT) as desire target. A single contingency was applied and the target CCT was found using time domain simulation. Multi layer feed forward neural network trained with Levenberg Marquardt (LM) back propagation algorithm is used to provide the estimated CCT. The effectiveness of ANN, the method is demonstrated on single machine infinite bus system (SMIB). The simulation shows that ANN can provide fast and accurate mapping which makes it applicable to real time scenario. [ABSTRACT FROM AUTHOR]

It is very important to maintain supply reliability in the deregulated environment. The transient stability problem is one of the major concerns in studies of planning and operation of power systems. Time domain simulation is currently the best available tool for allowing the use of detailed models and for providing reliable results. The main limitations of this approach involve long computation times. Therefore, this paper proposes a transient stability evaluation method using minimum transient kinetic energy of postfault trajectories. This method can estimate critical clearing times with only two points of the minimum transient kinetic energy, regardless of whether the trajectories after the clearing time are stable or unstable. The validity of the proposed method is shown by simulation studies with the IEEJ EAST 10-machine 47-bus system. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 174(3): 18-24, 2011; Published online in Wiley Online Library (). DOI 10.1002/eej.21018 [ABSTRACT FROM AUTHOR]

This paper presents a novel morphological scheme for the identification of transformer magnetizing inrush. The scheme decomposes a current signal into multiresolution levels based on synthesis and analysis operators of mathematical morphology. It is able to discriminate between inrush and internal fault currents even in the case of an inrush with a low second harmonic component and an internal fault current with a high second harmonic component. Simulation studies have been undertaken to evaluate the scheme based on a three-phase high-voltage power transformer operating under different conditions, which is simulated with an Alternative Transients Program. The testing results show that the proposed scheme is able to identify inrush currents reliably and provides an alternative solution for transformer differential protection. [ABSTRACT FROM AUTHOR]

The concept of an optimal and flexible control (OFC) strategy for active filtering has been reported in the literature. The paper demonstrates that, under nonstiff-voltage conditions, due to the source impedance, the OFC strategy performs as a closed-loop adaptive system. This closed-loop system may exhibit poor transient response or even unstable behaviour. Based on source-impedance identification, the paper proposes an improved OFC (IOFC) algorithm to overcome the shortcomings of the OFC. Desirable performance of the IOFC algorithm, based on digital-time-domain- simulation studies in a MATLAB/SIMULINK environment, is also presented. Based on the IOFC algorithm, a new and important harmonic compensation strategy to minimise load-voltage harmonics is introduced and verified. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a neural network predictive controller for the UPFC to improve the transient stability performance of the power system. A neural network model for the power system is trained using the backpropagation learning method employing the Levenberg–Marquardt algorithm for faster convergence. This neural identifier is then utilized during predictive control of the UPFC. The damped Gauss–Newton method employing ‘backtracking’ as the line search method for step selection is used by the predictive controller to predict the future control inputs. The 4- machine 2-area power system which is a benchmark power system is used to demonstrate the performance of the proposed controller. The system under consideration is simulated for different transients over a range of operating conditions using Matlab/Simulink. The proposed neural network predictive controller exhibits superior damping performance in comparison to the conventional PI controller. The simulation results also establish convergence of the minimization algorithm to an acceptable solution within single iteration. [Copyright &y& Elsevier]

Abstract: Single-phase-to-ground faults are the most frequent faults likely to occur in power distribution networks. As for a neutral un-effectively grounded system (NUGS), the low fault current is very common in the case of occurrence of a single-phase earth fault, leading to hard identification of the faulty feeder. In conventional way, this target can be possibly achieved by the comparison of the polarities, magnitudes, or the phase angles of the zero-sequence currents of all feeders connected to the same busbar. However, it becomes a difficult task to implement this functionality into the protection of feeder. In this paper, a novel single-phase-to-ground fault protection for NUGS is put forward. Different from conventional centralized-comparison protection, this protection can detect single-phase-to-ground fault on the feeder individually, and can be realized on FTU (Feeder Terminal Unit). It is based on the measurement of the zero-sequence voltage and zero-sequence current. The tripping strategy follows the characteristic of the modified inverse-time delay curve. By means of analyzing the characteristics of the zero-sequence transient currents, it is disclosed that the magnitude of the zero-sequence current of the faulty feeder should be greater than that of any sound feeder. Then a composite compensated voltage is formed based on the zero-sequence voltage and zero-sequence current to evaluate a revising factor for every feeder respectively. Using this revising factor to modify the identical standard inverse-time delay curve adopted by all the feeder zero-sequence overvoltage protections, the tripping time of the zero-sequence overvoltage protections of all feeders will differ from each other. In this case, the selectivity of the protection can be guaranteed. This proposed algorithm is validated with the EMTP simulation tests. [Copyright &y& Elsevier]

Efficient transient simulation of large power grid is of great importance for system operation and dispatching, fault repetition, operator training etc. Online stability estimation and testing on system control and protection require hard real-time performance of transient simulation. The conventional real-time simulation, which splits an entire grid into sub-grids and solves them in parallel, lacks generality for various case systems. This study proposes a new efficiency enhancement method without grid partitioning. Three key techniques are involved. First, parallel solving format is proposed to distribute two-layer computational burdens into central processing unit cores in balance. Second, to calculate the fault changing the grid structure, the inverse current compensation is employed instead of re-forming impedance matrix and repeating lower-upper (LU) decomposition. Third, dual-loop forward and backward substitutions to solve grid equations are modified to a single-loop iteration, which further improves the computational efficiency. At last, the validity and effectiveness of the proposed method are verified by tests. [ABSTRACT FROM AUTHOR]

A comprehensive procedure for the frequency domain evaluation of transient finite element simulations of induction machines is presented. The proposed algorithm allows the determination of the magnetic air-gap flux density for arbitrary operational points under consideration of the rotor movement. The flux density harmonics can be determined on the stator as well as on the moving rotor surface. For skewed induction machines, an interpolation algorithm for the axial behavior of the flux density is included. The procedure is demonstrated for a skewed induction machine operating at nominal load. [ABSTRACT FROM PUBLISHER]

The knowledge about the behavior of cables is substantial in cases of transients or in cases of faults. However, there are only a few models that are tailored to the current requirements for calculations of transient phenomena in three-phase cable systems. These models are based on complex structures. PI-section cable models with simple structures were previously qualified only for calculations in the frequency domain. A new full frequency-dependent cable model to simulate transient phenomena is introduced and validated. The model is based on lumped parameters with cascaded frequency-dependent PI-sections. For the implementation and the integration in simulation tools, it is important to investigate the impact of the PI-section parameters to the accuracy, the stability and the mathematical robustness. In this work, the impact of the frequency dependence of cable parameters, the length distribution and the number of PI-sections on the results of the developed three-phase cable model have been discussed. For simulations in the time domain, two algorithms have been presented to optimize the number of PI-sections based on a specified accuracy. [ABSTRACT FROM AUTHOR]