Unbalanced Fault Conditions Research Articles

Implementation of a Non-Linear Adaptive Filter Based Sag Detection Method for Dynamic Voltage Restorers under Unbalanced Fault Conditions

The most common power quality problems in distribution systems are related to unbalanced voltage sags. Voltage sags must be detected quickly and corrected in a minimum amount of time. One of the most widely used methods for sag detection is based on the d-q transformation. This method has the disadvantage of missing the detection of unbalanced faults, because this method uses a voltage sag level signal obtained from the average of 3 phases for sag detection. In this paper, an adaptive filter sag detection method is proposed for Dynamic Voltage Restorers (DVR) under unbalanced fault conditions. The proposed DVR controller is able to detect balanced, unbalanced and single phase voltage sags. A novel reference voltage generation method is also presented. To validate the proposed control methods, a 3-phase DSP controlling a DVR prototype with a power rating of 1.5-kVA has been developed. Finally, experimental results are presented to verify the performance of the proposed control methods.

Novel Fault Ride-Through Configuration and Transient Management Scheme for Doubly Fed Induction Generator

This paper proposed a novel fault ride-through (FRT) configuration and transient management scheme to enhance the FRT capability of doubly fed induction generator-based wind turbines. The new configuration of the grid-side converter introduces shunt and series compensation for normal operation and voltage dips, respectively. A braking resistor is added to smooth switching transients from shunt to series interfaces and dissipate excessive power from the grid-side converter. To attain a flexible control solution for balanced and unbalanced fault conditions, the proposed transient management scheme employs positive and negative sequence controllers. A small-signal linear model is developed and examined to analyze the system dynamics for the series compensation topology. Based on the mathematical model, the controller is tuned to balance both voltage regulation performance and transient stability margins with consideration of various operating conditions. The combination of shunt and series interfaces demonstrates a low component count, simple protection structure, and improved performance of FRT with effective compensation to the electric grid. A comprehensive simulation verified the capability of the new configuration and transient management scheme.

Support Vector Machine-Based Algorithm for Post-Fault Transient Stability Status Prediction Using Synchronized Measurements

The paper first shows that the transient stability status of a power system following a large disturbance such as a fault can be early predicted based on the measured post-fault values of the generator voltages, speeds, or rotor angles. Synchronously sampled values provided by phasor measurement units (PMUs) of the generator voltages, frequencies, or rotor angles collected immediately after clearing a fault are used as inputs to a support vector machines (SVM) classifier which predicts the transient stability status. Studies with the New England 39-bus test system and the Venezuelan power network indicated that faster and more accurate predictions can be made by using the post-fault recovery voltage magnitude measurements as inputs. The accuracy and robustness of the transient stability prediction algorithm with the voltage magnitude measurements was extensively tested under both balanced and unbalanced fault conditions, as well as under different operating conditions, presence of measurement errors, voltage sensitive loads, and changes in the network topology. During the various tests carried out using the New England 39-bus test system, the proposed algorithm could always predict when the power system is approaching a transient instability with over 95% success rate.

Dynamic Performance of a Back-to-Back HVDC Station Based on Voltage Source Converters

Dynamic Performance of a Back-to-Back HVDC Station Based on Voltage Source Converters The recent developments in semiconductors and control equipment have made the voltage source converter based high voltage direct current (VSC-HVDC) feasible. This new DC transmission is known as "HVDC Light or "HVDC Plus by leading vendors. Due to the use of VSC technology and pulse width modulation (PWM) the VSC-HVDC has a number of potential advantages as compared with classic HVDC. In this paper, the scenario of back-to-back VSC-HVDC link connecting two adjacent asynchronous AC networks is studied. Control strategy is implemented and its dynamic performances during disturbances are investigated in MATLAB/Simulink program. The simulation results have shown good performance of the proposed system under balanced and unbalanced fault conditions.

Robust coordinated design of excitation and STATCOM-based controller using genetic algorithm

Power system stability enhancement via robust coordinated design of a power system stabiliser (PSS) and a static synchronous compensator (STATCOM)-based stabiliser is thoroughly investigated in this paper. The coordination among the proposed damping stabilisers and the STATCOM internal AC and DC voltage controllers as well as the current regulator has also been taken into consideration. The coordinated design problem is formulated as an optimisation problem with a time-domain simulation-based objective function and genetic algorithm (GA) is employed to search for optimal controller parameters. The proposed stabilisers are tested on a weakly connected power system with different disturbances and loading conditions. The non-linear simulation results show the effectiveness and robustness of the proposed control schemes over a wide range of loading conditions and disturbances. Further, the proposed design approach is found to be robust and improves stability effectively even under small disturbance and unbalanced fault conditions.

Fault Analysis on Distribution Feeders With Distributed Generators

This paper shows that the current an inverter interfaced distributed generator (IIDG) contributes to a fault varies considerably, due mainly to fast response of its controller. This paper proposes a method to extend the conventional fault analysis methods so that IIDG contribution can be estimated in the fault analysis. The proposed method gives rms profiles of the fault currents of interest (IIDG contribution and the fault currents the protective device will see). Test results, based on a prototype feeder, show that the proposed approach can estimate the fault current's contributions under both balanced and unbalanced fault conditions.

Characterising the power system at a load busbar by measurement

The paper presents the Theévenin equivalent network models for 3-phase 3- or 4-wire power systems and indicates how these models can be approximately determined by simple measurement. A simple measurement technique is defined as one requiring measurement of RMS quantities only. It is shown that the system impedance matrix is inherently non-reciprocal and that it is not possible to determine such non-reciprocity by simple measurement. Simple measurement methods for determining the phase and line impedances of the system at supply frequency and the terminal voltage sequence description are given. These methods require RMS voltage measurements alone; phase angle measurement is not required. The theory and measurement methods presented enable the voltage disturbance due to unbalanced loading at a load point in the system to be predicted a priori, and facilitate investigation of power system unbalance. The sequence network models for unbalanced load or fault conditions may also be determined.

General fault analysis using phase frame of reference

Traditionally, the analysis of power systems under unbalanced fault conditions is carried out using symmetricalcomponent theory. In the paper, an alternative approach, using phase frame of reference, is presented. Full network representation is used, which allows for network asymmetry arising from unbalanced generator impedances or nontransposition of transmission lines. Any type of fault can be represented singly or in combination. Sparsity methods are used to reduce the computation time and storage to a minimum. A special method is incorporated for quadrature-boosting transformers that provides direct solution and retains symmetry of the network equations

D.W.R.-synchronous-machine response to unbalanced faults

This short paper describes an investigation into the behaviour of a divided-winding-rotor (d.w.r.) synchronous generator, equipped with regulators for controlling the terminal voltage, torque angle and speed, when subjected to unbalanced short circuits at its terminals. The performance of the machine has been compared with that of a conventionally wound rotor arrangement. It has been observed that the d.w.r. arrangement exhibits exceptionally superior transient-performance characteristics compared with the c.w.r. machine, even under unbalanced-fault conditions, which the machine has to encounter more often in practice than balanced faults.

Comparative steady-state performance of crossbonded cable systems

The paper gives a universal system of steady-state analysis for any crossbonded and/or transposed cable system with unequal lays. It first introduces a nomenclature which enables any conductor or sheath to be identified in geometric and/or longitudinal position in any lay and/or section both for the general case and for four specific cases of crossbonding. It is then shown that the effect of unequal lays on sheath voltages, currents and losses may always be analysed, provided that there is rigorous adherence to the system of nomenclature. The paper is restricted to cases of unbalanced conductor currents in the steady state and does not deal with the additional variables and complexities when the various crossbonded systems have multiple-earthed points and are under external or internal unbalanced fault conditions. A large selection of graphical results is presented for sheath currents, losses and longitudinal sheath voltages of the three main cases of crossbonding. In the particular case of trefoil spacing, taken as an example, the influence of variation of lay length between 0.6 and 1.4 p.u. on the sheath currents and longitudinal induced voltages is shown. The results are discussed in detail since comparative performances for these various systems have not previously been presented. The conclusions extend also the analytical methods, it being shown that all crossbonded systems of whatever complexity can be analysed by the use of rotation matrices and the notation for nonuniform lay length; it follows en passant that normal symmetrical-component methods do not yield a solution, owing to the crossbonded systems having multiple unbalances and six degrees of freedom.

Study of power-system transient stability by a combined computer

A power-system computer is described that combines an analogue computer for the representation of the synchronous generator with a network analyser for the representation of the transmission network. These two forms of computer are coupled to give an overall representation that facilitates the study of the transient performance of a synchronous power system in unbalanced fault conditions. In the studies described, the system represented is that of a synchronous generator connected to an infinite busbar through a double-circuit transmission line, one circuit of which is subjected to a series of unbalanced and balanced faults. Studies of the effect of fault type, fault resistance, system neutral earthing and single-phase autoreclosing are included. For purposes of comparison, and in view of their ubiquity on a modern power system, automatic-voltage-regulator and speed-governor representation are included in some of the studies.

Regulating Transformers in Power-System Analysis

IN THE analysis of power systems it is customary to represent generators, transformers, and transmission lines by their equivalent circuits. The resulting circuit network is solved either by representation on a network calculator, from which the solution is read directly, or analytically by successive simplification of the network. When the network has been simplified as far as possible, it is usually relatively easy to solve the simplified network, by the application of Kirchhoff's laws, for the currents in the remaining circuits and the voltages at the remaining terminals. When this solution has been obtained, the steps of simplification are reversed until the original network is restored, the currents and voltages being calculated at each step, until the desired quantities are finally obtained for the original network. When the system is unbalanced, as by an unbalanced fault condition, the method of symmetrical components is widely used. Equivalent networks are derived for the positive-, negative-, and zero-sequence components of current and voltage. Interconnections between the sequence networks are made, corresponding to the unbalanced condition, and the resulting network is solved by the methods just described.

Regulating transformers in power-system analysis

IN THE analysis of power systems it is customary to represent generators, transformers, and transmission lines by their equivalent circuits. The resulting circuit network is solved either by representation on a network calculator, from which the solution is read directly, or analytically by successive simplification of the network. When the network has been simplified as far as possible, it is usually relatively easy to solve the simplified network, by the application of Kirchhoff's laws, for the currents in the remaining circuits and the voltages at the remaining terminals. When this solution has been obtained, the steps of simplification are reversed until the original network is restored, the currents and voltages being calculated at each step, until the desired quantities are finally obtained for the original network. When the system is unbalanced, as by an unbalanced fault condition, the method of symmetrical components is widely used. Equivalent networks are derived for the positive-, negative-, and zero-sequence components of current and voltage. Interconnections between the sequence networks are made, corresponding to the unbalanced condition, and the resulting network is solved by the methods just described.