In this paper, a hierarchical control scheme for improving transient stability of powersystems by thyristor controlled braking resistor is proposed. Based on three-level co- state prediction method of large-scale sys...
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In this paper, a hierarchical control scheme for improving transient stability of powersystems by thyristor controlled braking resistor is proposed. Based on three-level co- state prediction method of large-scale system principle, the control structure is organized into a two-layer controller. The lower layer is composed of decentralized local feedback controllers, each of which takes local feedback signal and controls its connected generator. In the upper layer a global controller is designed to enhance the effectiveness of decentralized local controllers. The global controller coordinates local controllers by generating remote feedback signals which ensuring the global optimal solution of the overall system transient performance. The proposed control scheme can guarantee fast and accurate control actions of local controllers when a powersystem is subjected to large disturbances. Simulation results show the effectiveness of the proposed control scheme.
The paper presents the dynamic model of doubly fed induction generator (DFIG) including the mechanical model, the generator model and the PWM voltage source converter model. The mechanical model uses two-mass model, a...
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The paper presents the dynamic model of doubly fed induction generator (DFIG) including the mechanical model, the generator model and the PWM voltage source converter model. The mechanical model uses two-mass model, and the generator model includes both the stator side and the rotor side dynamics. The PWM voltage source converter is modeled as an average model in which the action of power electronic switches is neglected. The paper features vector control of DFIG. On the rotor side converter, the control scheme enables the independent control of speed and reactive power from the stator. On the grid side converter, the control scheme is implemented to regulate the DC voltage and reactive power from the grid as well. In order to study the influence of the grid disturbances on the DFIG, grid voltage dip and quick frequency change are simulated with the engineering software, MATLAB/Simulink. Meanwhile, in the paper the pole-placement method and internal model control method are implemented for designing the PI controllers
The phase angle of the utility voltage plays a most important role for the operation of FACTS (flexible AC transmission system) and renewable energy systems, and PLL (phase locked loop) should be accurate, fast and ro...
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The phase angle of the utility voltage plays a most important role for the operation of FACTS (flexible AC transmission system) and renewable energy systems, and PLL (phase locked loop) should be accurate, fast and robust. Operation of a vector-based soft phase locked loop system under distorted utility conditions is presented in this paper. An intelligent PI controller is proposed to solve the conflict between tracing speed and filtering characteristics, which is to regulate the parameters of PI controller based on the phase error input. All analytical results are verified by simulations and experiments. The merits of high-speed, robust and strongly restraining ability to distorted voltage make it could be widely used in FACTS and renewable energy controlsystems
In this paper, a novel method for perturbed trajectory prediction of post-fault rotor angle is presented based on PMU (phasor measurement unit) measurements and trigonometric function curve fitting technique. A mathem...
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In this paper, a novel method for perturbed trajectory prediction of post-fault rotor angle is presented based on PMU (phasor measurement unit) measurements and trigonometric function curve fitting technique. A mathematical model about the relationship of post-fault rotor angle and time is derived from a speed-governing system model, which reveals that the perturbed trajectory of the post-fault rotor angle can be expressed by trigonometric functions. By curve fitting technique, a least square method to predict the perturbed trajectory using trigonometric function is proposed. Numerical simulations are conducted on IEEE 9-bus system to test the effectiveness of this method. The proposed method can give more reasonable and more promising prediction results as compared with polynomial curve fitting technique
Traditional voltage injection methods for DVR are in-phase strategy which injects voltage in phase with the sag voltage and missing voltage technique which compensates for the difference between the sag and pre-sag vo...
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Traditional voltage injection methods for DVR are in-phase strategy which injects voltage in phase with the sag voltage and missing voltage technique which compensates for the difference between the sag and pre-sag voltages. Since a certain component of injected voltage is in-phase with the load current, DVR has to provide some amount of energy to the grid. This paper use DQ analyzing method based on instantaneous reactive power theories to detect and obtain the parameters of source side voltage for balanced faults of three-phase three-wire system. Through circumgyrating the phasor and injecting voltage with an appropriate advance phase relative to source side voltage, the minimum energy injection can be achieved. For unbalanced faults, fictitious three-phase voltages which come from a single- phase source can be constructed mathematically, so unbalanced faults can be dealt with easily in a balanced way. Simulation results are presented to demonstrate the validity of the proposed method.
Nowadays worldwide, distributed generation plays a more and more important role in world's energy supply system. But distributed resources' innate weaknesses cause some problems such as wavy output power and i...
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Nowadays worldwide, distributed generation plays a more and more important role in world's energy supply system. But distributed resources' innate weaknesses cause some problems such as wavy output power and instability of power generation. These innate weaknesses are hard to overcome only through improving one single distributed generation plant or adding energy storage equipment etc. In this article, a new concept of connecting several AC distributed generation plants (such as wind farm) via VSC-converter, and DC distributed generation plants (such as solar cell) via DC/DC converter and forming a low voltage DC network is given. This low DC network will be integrated into AC grid in some proper way. Namely equalize these small distributed generation plants to a big power generation farm to improve stability of electricity generation. Some simulations based on PSCAD/EMTDC are also given.
Determining loadability margins to various security limits is of great importance for the secure operation of a powersystem. A novel approach is proposed in this paper for fast prediction of loadability margins with ...
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Determining loadability margins to various security limits is of great importance for the secure operation of a powersystem. A novel approach is proposed in this paper for fast prediction of loadability margins with respect to small-signal stability based on neural networks. Small-signal stability boundaries are constructed by means of loading the powersystem until the stability limits are reached from a base operating point along various loading directions. Back-propagation neural networks (BPNN) for different contingencies are trained to approximate these stability boundaries. A search algorithm is then proposed to predict the loadability margins from any stable operating point along arbitrary loading directions through an iterative technique based on the trained BPNNs. The simulation results for the IEEE two-area benchmark system demonstrate the effectiveness of the proposed method for on-line prediction of loadability margins.
The paper investigates the features of separately clearing the energy in the day-ahead market and then doing the same for the real-time market from the system operator's perspective. The analysis will show that th...
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The paper investigates the features of separately clearing the energy in the day-ahead market and then doing the same for the real-time market from the system operator's perspective. The analysis will show that this separate clearing mechanism is not always the most economical. An alternative approach based on co-optimization of both energy markets is presented. Considering stochastic electric demand, a novel integrated energy dispatching model (IEDM) is proposed for system operators that schedules day-ahead energy trading taking possible real-time demand and prices into account to find an optimal day-ahead dispatching scheme so as to achieve the total purchase cost minimum. Mathematical analysis is provided to illustrate the IEDM is more economical than the conventional separate dispatching approach. The IEDM solution integrates Monte Carlo simulation and linear programming with Hooke and Jeeves pattern search method. A 6-unit system example is applied to illustrate that the IEDM provides a more economical approach
A novel scheme based on mathematical morphology to distinguish the inrush current from the internal fault current in power transformers is proposed, which is derived from high non-linearity in exciting cores of transf...
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A novel scheme based on mathematical morphology to distinguish the inrush current from the internal fault current in power transformers is proposed, which is derived from high non-linearity in exciting cores of transformers. Transient current signals are extracted by use of the novel morphological gradient and morphological opening and closing transform, meanwhile various kinds of additive white noises are effectively suppressed by adaptive filtering. Based on different characteristics of transients between the short circuit current and the inrush current, the transformer criterion is developed. The proposed technique is immune to CT saturation. A total of 120 Electrical powerdynamiclaboratory (EPDL) simulation cases are used to test the performance of the technique. This method has stability during the symmetrical inrush current and can be implemented in real time with very high accuracy. The results of different EPDL testing systems verify the feasibility and practicability of the proposed scheme
A new method of fault classification based on transient signals for high voltage power transmission line is presented. Based on the wavelet analysis and the idea of entropy, the concept and definition of wavelet energ...
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A new method of fault classification based on transient signals for high voltage power transmission line is presented. Based on the wavelet analysis and the idea of entropy, the concept and definition of wavelet energy entropy (WEE) for feature extraction are introduced. First, appropriate wavelet decomposition of acquired post-fault signal is conducted and the WEE is calculated. ; then the characteristic vectors for post-fault signal are built; the characteristic vectors containing fault information are used as the input of adaptive nerve-fuzzy inference system; finally, the fault classification is realized according to the output of adaptive nerve-fuzzy inference system. The simulation results of fault classification have been analyzed on a 500 kV transmission line PSCAD/EMTDC model. The result shows that the variation regularity of WEE with time can effectively reflect fault features. With the help of adaptive nerve- fuzzy inference system, it achieves good classification effect.
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