Post-fault Conditions Research Articles

Improved low-voltage-ride-through capability of fixed-speed wind turbines using decentralised control of STATCOM with energy storage system

The design and implementation of a new control scheme for reactive power compensation, voltage regulation and transient stability enhancement for wind turbines equipped with fixed-speed induction generators (IGs) in large interconnected power systems is presented in this study. The low-voltage-ride-through (LVRT) capability is provided by extending the range of the operation of the controlled system to include typical post-fault conditions. A systematic procedure is proposed to design decentralised multi-variable controllers for large interconnected power systems using the linear quadratic (LQ) output-feedback control design method and the controller design procedure is formulated as an optimisation problem involving rank-constrained linear matrix inequality (LMI). In this study, it is shown that a static synchronous compensator (STATCOM) with energy storage system (ESS), controlled via robust control technique, is an effective device for improving the LVRT capability of fixed-speed wind turbines.

Intelligent Control For Locating Fault in Transmission Lines

This paper presents a new approach for locating fault in transmission line using intelligent control relaying. Fault must be detected at its inception by issuing an output signal indicating this condition. Neural network approach for locating fault can be posed as a pattern-recognition to recognize pure sinusoidal signals as indicators of a normal system condition; abrupt changes of amplitude, phase, or the presence of transient components as indicators of fault. This method uses the fundamental frequency components of voltage and current basically current at pre-fault and post fault condition, measured at each phase from any one end of the selected power system. In this approach the data sets were trained using the available data from the system which comprises of different fault types data, and fault inception angles. This approach of locating fault using intelligent control can be used for supporting a new generation of very high speed protective relaying system.

Inproving profitability of the network distribution by protecting power transformers with a current limiting technology device

This paper presents the improvement of the p rofitability of the Network Distribution, when power transformers are protected only with Fault Current Limiting Technologies (FCLT). In assessing the total benefits of the Network Distributions from normal to post-fault conditions, we will compare two kinds of profitability taking in account the impact of Circuit Breakers (CBs) and FCL used separately as protection devices during the life span of power transformers. This study will be performed using MATLAB/SIMULINK as software and it was concluded that the FCL perfectly improves the profitability of the Network Distribution.

Improvement of transient stability of power systems with STATCOM-controller using trajectory sensitivity

This paper discusses the use of trajectory sensitivity analysis (TSA) in determining the transient stability margin of a power system compensated by a shunt FACTS device. The shunt device used is static synchronous compensator (STATCOM). It is shown that TSA can be used for the design of controller for the STATCOM. The preferable locations for the placement of the STATCOM for different fault conditions are also identified. The effects of STATCOM in maintaining different bus voltages in the post-fault condition are studied. The STATCOM is modeled by a voltage source connected to the system through a transformer. The systems used for the study are the WSCC 3-machine 9-bus system and the IEEE 16-machine 68-bus system.

Impact of Stator Winding of a Five-Phase Permanent-Magnet Motor on Postfault Operations

The performance of a five-phase permanent-magnet (PM) motor is analyzed under postfault conditions. Proper current control strategies are adopted so as to guarantee safe drive operation after any fault occurrence. This paper covers three fault types: the open circuit condition of a single phase, the open circuit condition of two nonadjacent phases, and the open circuit condition of two adjacent phases. Two motors with two different windings (with double and single layers, respectively) are compared under each fault type. This paper aims to highlight the difference in the motor performance of motors adopting these two different windings. A further novelty of this paper is that the proper current control strategies are derived analytically, including not only the fundamental harmonic of the flux-density distribution but also the higher harmonics. It is shown that these harmonics cause high torque oscillations. Owing to this analytical approach, the strategy can be applied to a variety of PM motors. In addition, the postfault current waveforms remain sinusoidal, making the current control easier. For each fault type, the results of both simulations and experimental tests are included. A good match between analytical predictions and experimental tests validates the proposed current control strategies.

IMPROVEMENT OF POWER SYSTEM PERFORMANCE THROUGH WAVELET NEURAL NETWORK STATIC VAR COMPENSATOR CONTROLLER

This paper aimed to apply a nonlinear hybrid wavelet neural network, WNN controller for static VAR compensators SVC. The proposed WNN control the amount of the reactive power consumed or delivered to the network by controlling the TRC firing angle. Single layer wavelet neural network, WNN model technique is used in the present paper. The proposed controller tracks the power systems response to zero error in the post fault conditions. The proposed controller is applied for sample single machine infinite bus power system. The time simulations indicate the effectiveness, robustness and fast response of the proposed controller in comparison with the conventional one's. The studied system is modeled by nonlinear differential and algebraic equations which solved by the Matlab Software.

Application of a new multi-variable feedback linearization method for improvement of power systems transient stability

This article investigates the application of a multi-variable nonlinear controller to the model of a power system. The proposed controller is based on the feedback linearization concept and its main goal is improvement of transient stability and voltage regulation at post-fault conditions. It will be shown that the controller enhances first swing stability and system damping under large disturbances as well as improves terminal voltage regulation. The response of the system is simulated on a test system in the presence of three-phase short circuit at terminal of the synchronous generator and compared to the performance of the standard IEEE type-1 voltage regulator, PSS and PID speed regulator.

Analytical Approach to Internal Fault Simulation in Power Transformers Based on Fault-Related Incremental Currents

A new method for simulating faulted transformers is presented in this paper. Unlike other methods proposed in the literature, this method uses the data obtained from any sound transformer simulation to obtain the damaged condition by simply adding a set of calculated currents. These currents are obtained from the definition of the fault. The model is fully based on determining the incremental values exhibited by the currents in phases and lines from the prefault to the postfault condition. As a consequence, data obtained from simulation of the sound transformer may be readily used to define the damaged condition. The model is described for light and severe faults, introducing this latter feature as a further add-on feature to the low-level faults simulation. The technique avoids the use of complex routines and procedures devoted to specially simulate the internal fault. Of prompt application to relay testing, the proposed analytical model also gives an insight into the fault nature by means of the investigation of symmetrical components. In contrast with its low complexity, the method has shown to present large accuracy for simulating the fault performance.

Diagnosis of a turn-to-turn short circuit in power transformers by means of zero sequence current analysis

What to do after a transformer is tripped out is a difficult question, and it is desirable to make certain that the transformer is not faulted before getting it into service in order to avoid additional consequential damage. In this paper, a novel analysis of the currents arising during a turn-to-turn fault in transformer in which a winding is delta-connected is done, so that data acquisition pre- and post-fault conditions may lead to a correct diagnosis. The approach given in this paper is based in the analysis of current sequences which appear in the fault state, and mainly in the nature of a zero sequence current (ZSC) in a delta winding, which is thoroughly discussed. The paper examines a faulted transformer performance by analyzing the influence of four variables: load, fault location, fault severity and load power factor. A set of rules is given to determine the currents unbalance in coils as a function of such variables. Results are obtained from finite element analysis and validated in laboratory by means of a specially built transformer. The conclusion obtained is that, even under unbalanced conditions, the comparison of currents in phases identifies the existence of an interne transformer fault, thus warranting a correct switch back and/or disconnection.

Power system stabilization by switching-over control of transmission lines

New technologies such as synchronized switching and power electronics will make it possible to change the configuration of a transmission network in future electric power systems. It is called VIPS (Variable Impedance Power System) by the authors who have been studying the effects of those technologies on power system stabilities. This paper considers the improvement of transient stability by means of switching-over control of transmission lines. It is clear that in some power systems, the simultaneous energization of the line de-energized in the normal state with the fault clearing better improves first-swing stability than the pre- and post-fault energization of the line. The energy function-based index to select such a transmission line is derived. In addition, this paper shows that multiple switching of a transmission line improves damping of post-fault conditions, where minimization of time derivative of energy functions is used. The switching-over control is applied to multimachine power systems and its effectiveness is demonstrated.

送電線の緊急切換え制御による電力系統の安定化に関する基礎的考察

New technologies such as synchronized switching and power electronics will make it possible to change the configuration of a transmission network in future electric power systems. It is called VIPS (Variable Impedance Power System) by the authors who have been studying the effects of those technologies on power system stabilities. This paper considers the improvement of transient stability by means of switching-over control of transmission lines. It is clear that in some power systems, the simultaneous energization of the line de-energized in the normal state with the fault clearing better improves first-swing stability than the pre- and post-fault energization of the line. The energy function-based index to select such a transmission line is derived. In addition, this paper shows that multiple switching of a transmission line improves damping of post-fault conditions, where minimization of time derivative of energy functions is used. The switching-over control is applied to multimachine power systems and its effectiveness is demonstrated.

Swivelling characteristic for the protection of series compensated lines

Abstract The necessity for series compensation of long EHV lines is evident. Such lines are difficult to protect using either the conventional impedance or directional comparison relaying schemes. This work presents a layout of hardware with a swivelling quadrilateral characteristic to cater for the switching of the series capacitor spark gap. The characteristic swivelling through appropriate logic will encompass the measured impedance with the proper contour, according to the working conditions of the spark gap. A check for accommodating the instantaneous fault impedance at post-fault conditions is included as well.

Artificial Neural-Net Based Dynamic Security Assessment for Electric Power Systems

In the post-fault dynamic analysis of interconnected power systems, the critical fault clearing time (CCT) is one of the parameters of paramount importance. Critical clearing time is a complex function of pre-fault system conditions (operating point, topology, system parameters), fault structure (type and location) and post-fault conditions that are in part dependent on the protective relaying policy. To define analytically such a relationship would be highly desirable but diversity of variable involved makes this task extremely complicated. Our efforts focus on examination of that complex mapping and investigation of the influence of the various parameters on CCT. The evaluation of CCT involves elaborate computations that often include time-consuming solutions of nonlinear on-fault system equations. Existing conventional pattern recognition techniques are incapable of synthesizing such complex and transparent mappings. Thus, when a human operator tells the machine learning unit (that is the pattern recognizer) that system state belongs to a certain class, say emergency, the pattern recognizer merely records that classification mindlessly and is not able to look at the pattern with insight and discover what underlies the emergency nature of pattern. It is, therefore, highly desirable to have a mechanism which when presented with a sequence of class labeled patterns not only learns an internal structure which allows it to generalize and to classify other and to classify other patterns correctly, but also is able to shed some light on what combination of features give rise to the particular class membership.

Iterative Algorithm for Critical Energy Determination in Transient Stability of Power Systems

Abstract In this paper an algorithm for determination of critical energy for analysis of power systems transient stability is presented, being des cribed by an electrical network with constant impedances and with generators represented through the classical model, aiming to obtain critical fault clearing times for short circuits eliminated through transmission line disconnection. The concepts of potential energy boundary surface (PEBS) and system energy function are both used, similar to Liapunov's function represented by integral form due to the electrical network renre sentation. with no reduction to the internal busses of feneration, and the trajectories of states are obtained through series Taylor expansion. Taking into account an algorithm nreviouslv developed, the compensation theorem is applied to determine a submatrix of the bus impedance matrix for the system configurations in pre. under and post-fault conditions. This aims better computational efficiency, besides the expansion of the computational program for large power systems. One example consisting of the analysis of 22 contingencies in a system with 45 generators. 340 busses and 684 transmission lines is included, within just one sinple processing with duration of about 9 minutes in the computer used.

Improved Reliability in Solid-State AC Drives by Means of Multiple Independent Phase Drive Units

Phase redundancy is a proposed technique for improving the reliability of large solid-state adjustable-frequency ac drives. Based on the more general concept of parallel redundancy, this new technique relies heavily on the inherent capability of a general n-phase ac motor to continue operation with (n -1) or less of its stator phases excited. Motor performance degradation which inevitably accompanies such operation is sensitive to a variety of system parameters including the number of stator phases and the type of excitation source. A thorough evaluation of (n - 1)-phase excitation performance characteristics of an n-phase squirrel-cage induction motor is presented in this paper. Under favorable conditions, a large percentage of the motor's balanced excitation rating can be retained during postfault conditions of steady-state (n -1)-phase excitation. The n-phase-drive units must be conservatively designed to withstand transient stresses associated with fault-induced transitions from balanced n-phase to (n-1)-phase excitation. Key analytical results have been experimentally verified by tests conducted on a 5-hp six-phase squirrel-cage induction motor. Results of this investigation support the basic soundness of the phase-redundancy concept.