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Effectiveness of an artificial neural network (ANN), functioning as a power system stabilizer (PSS), in damping multi-mode oscillations in a five-machine power system environment is investigated in this paper. Accelerating power of the generating unit is used as the input to the ANN PSS. The proposed ANN PSS using a multi-layer neural network with error-backpropagation training method was trained over the full working range of the generating unit with a large variety of disturbances. The ANN was trained to memorize the reverse input/output mapping of the synchronous machine. Results show that the proposed ANN PSS can provide good damping for both local and inter-area modes of oscillations. Keywords - Power System Stabilizer, Artificial Neural Network, Inverse Plant, Multi-layer Network, Error Backpropagation, Multi-machine, Multi-mode Oscillation.

Fuzzy Excitation Control System (FECS), based on Fuzzy theorem and having the function of both AVR and PSS, was established to improve power system stability. This paper includes the results of experiments of the FECS on AC/DC Power System Simulator of the CRIEPI. The FECS is compared with the conventional quick-response excitation system with optimized PSSs on a 4-machine long distance fish-bone type power system and various system configurations. The results show that the FECS increases the power transfer limit more than 20 (%) without any parameter tuning.

Application of a neural adaptive power system stabilizer (NAPSS) to a five-machine power system is described in this paper. The proposed NAPSS comprises two subnetworks. The adaptive neuro-identifier (ANI) to dynamically identify the non-linear plant, and the adaptive neuro-controller (ANC) to damp output oscillations. The back-propagation training method is used on-line to train these subnetworks. The effectiveness of the proposed NAPSS in damping both local and inter-area modes of oscillations and its self-coordination ability are demonstrated. Keywords--Power System Stabilizer, Neural Networks, Multi-Machine Power System, Indirect Adaptive Control.

This paper investigates a self-tuning steam turbine control scheme designed to improve the quality of control of power system frequency. The recursive least-squares estimation algorithm and minimum-variance control technique are implemented to synthesize the turbogenerator speed control loop. Computer simulations of a 5-unit power system show that employing this type of control is effective in curbing excessive over- and under-frequency deviations and alleviating generator and load sheddings for major disturbances. Keywords: Least-squares estimation, Minimum-variance control, Load/generator sheddings, Large frequency deviation

Due to the complexity of detailed models, the single-generator equivalent model (SEM) of a wind farm is commonly used to facilitate the stability analysis. However, the adequacy of the SEM for stability analysis in direct-drive wind farms with VSC-HVDC systems is still uncertain. Therefore, this article analyzes the SEM adequacy in two aspects: the oscillation modes analysis and the sub-synchronous oscillation (SSO) stability enhancement by optimizing wind farm parameters. Firstly, various critical oscillation modes are characterized according to the frequency, and a comparison of these modes between the two-generator equivalent model and the SEM is presented. The effect of single-generator aggregation on the critical oscillation modes is discussed. Then, the impact of permanent magnet synchronous generator (PMSG) parameters on the inside-wind-farm/wind-farm-grid SSO modes is explored. It is revealed that with the change of PMSG parameters, the damping effect of the two SSO modes is identical. This implies that the SEM can be used to improve the SSO stability of the multi-machine system by optimizing the PMSG parameters. PSCAD/EMTDC simulations are further performed to verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

Independent flux and torque control of an ac machine can be achieved by means of vector control, utilizing only two stator d-q current components. Consequently, in ac machines with a phase number greater than three, there exist additional degrees of freedom. Although they can be used to enhance the torque production of a multiphase machine through injection of higher stator current harmonics, an entirely different purpose is possible as well. The additional degrees of freedom can be utilized to control independently other machines within a multimotor drive system. In order to do so, it is necessary to connect stator windings of all the multiphase machines in series, with an appropriate phase transposition, apply a vector control algorithm to each machine separately, and supply the stator windings of the multi-machine system from a single current controlled voltage source inverter (VSI). Inverter current control is performed in the stationary reference frame, using inverter phase currents. The foundations of the concept are set forth in the paper, for an arbitrary odd n-phase case, using the general theory of electrical machines. Further analysis is performed for all the theoretically possible odd phase numbers and it is shown that the number of machines connectable in series depends on the properties of the phase number. Connection diagrams are illustrated next for some selected phase numbers and vector control, including the inverter reference current generation, is detailed for the multimotor drive system. The main advantages and drawbacks of the concept are discussed and verification is provided by simulation of a nine-phase four-motor drive system. [ABSTRACT FROM AUTHOR]

Describes a design for a neural network-based Power System Stabilizer (Neuro-PSS) using power flow characteristics. Connection of a generator to a multi-machine power system utilizing the nonlinear power flow dynamics; Power system stabilizers; Low-frequency oscillation; Power flow characteristics.

The air-cooled turbo-generator often works in the more frequent start-up and shut-down operation. Either the misoperation or the failure of synchronizing controllers can cause the out of phase synchronization fault. The fault can disturb the power angle and cause a strong impact electromagnetic force for the generator. With the traditional numerical calculation method, it is difficult to calculate the variations of key physical parameters and the electromagnetic field in the end region of the generator under complex working conditions, especially the out of phase synchronization fault. In order to solve the problem, a calculation method of the nonlinear transient electromagnetic field with multi-domain field-circuit-grid coupled is proposed. The correctness of the model and method is validated by comparing the calculated and test values of the magnetic density flux at the key points of the generator under the no-load condition. Based on this method, the power angle variation, the magnetic flux leakage distribution in the end region, and the electromagnetic force on the end winding in the transient process of the fault are calculated and analyzed. [ABSTRACT FROM AUTHOR]

With widespread application of voltage source converter (VSC) as a key energy-conversion power electronic device by using the phase-locked loop (PLL) technique for synchronization, the system dynamics has become much complicated. In this paper, the nonlinear dynamics and transient stability of the PLL-based VSC system are investigated, within a unified framework of the (normalized) generalized swing equation. It is found that there are three different types of bifurcation, including the generalized saddle-node, Hopf, and homoclinic bifurcations. Within the coexistence parameter region, the basin boundary of the stable equilibrium point shows either a closed-loop or a fish-like pattern. With the help of the equal area criterion (EAC), the transient stabilities under different transient disturbances including short circuit, voltage dip, and power rise are analyzed. Because the equivalent damping of the VSC is state-dependent, the theoretical results based on the EAC are examined. Furthermore, based on the analytical results from the generalized swing equation, the principle for all major transient stability enhancement methods is uncovered. All these findings are well verified by extensive electromagnetic transient simulations. Therefore, the generalized swing equation provides a deeper physical insight and plays a crucial role in transient stability problems in power-electronic-dominated power systems, similar to the swing equation in traditional power systems. [ABSTRACT FROM AUTHOR]

This paper presents a dual-hysteresis band control method for multi-drive systems based on a nine-switch inverter. The drive system contains two independent three-phase induction motors fed by a nine-switch inverter. Independent control of torque, speed, and stator current of two load stages is achieved. The proposed strategy offers a reliable control operation of both induction machines under individual load profiles. Instead of using independent sets of three-phase voltage source converters, the nine-switch inverter is used to drive separate machines. Thus, the volume and size of the power stage are noticeably reduced due to the use of fewer semiconductors. The theoretical concept and design steps of the proposed method are presented. The performance of the closed-loop independent motor control scheme is experimentally validated using a 3.2 kW lab-scaled nine-switch inverter prototype and Altera Cyclone IV Field-programmable gate array. [ABSTRACT FROM AUTHOR]

An aggregation method is proposed that transforms the multiple DFIGs into an equivalent DFIG model that retains the major collective dynamic characteristics of a group of DFIGs. It is intended for Electromagnetic Transients Simulation (EMT). The aggregated machine can take into account different speeds and parameters of each of the individual DFIGs and the connecting impedance of individual DFIGs. Starting with a State Variable (SV) model of an individual DFIG, aggregation is carried out recursively, by combining two DFIGs at a time and then reducing the order of the aggregate to match the state variable equations of a single DFIG so that the steady state performances are identical. Validation is carried out by comparing the detailed electromagnetic transient (EMT) simulation of the unreduced system with the reduced aggregate system. It is shown that the proposed aggregation method accurately matches the steady state response and also accurately reproduces the dominant transient response of the unreduced system. As the aggregate DFIG has the same number of state variables as a single DFIG, the overall wind farm's order is reduced significantly to increase the modelling and simulation efficiency. [ABSTRACT FROM AUTHOR]

This paper investigates the transient stability of power systems co-dominated by different types of grid-forming (GFM) devices. Synchronous generators (SGs and VSGs) and droop-controlled inverters are typical GFM devices in modern power systems. SGs/VSGs are able to provide inertia while droop-controlled inverters are generally inertialess. The transient stability of power systems dominated by homogeneous GFM devices has been extensively studied. Regarding the hybrid system jointly dominated by heterogeneous GFM devices, the transient stability is rarely reported. This paper aims to fill this gap. It is found that the synchronization behavior of the hybrid system can be described by a second-order motion equation, resembling the swing equation of SGs. Moreover, two significant differences from conventional power systems are discovered. The first is that the droop control dramatically enhances the damping effect, greatly affecting the transient stability region. The second is that the frequency state variable exhibits a jump at the moment of fault disturbances, thus impacting the post-fault initial-state location and stability assessment. The underlying mechanism behind the two new characteristics is clarified and the impact on the transient stability performance is analyzed and verified. The findings provide new insights into transient stability of power systems hosting heterogeneous devices. [ABSTRACT FROM AUTHOR]

Impedance modeling of a permanent magnet synchronous generator (PMSG)-based wind energy conversion system (WECS) usually ignores the machine-side dynamics, instead, the machine-side system is simplified as a constant power source, while merely keeping the detailed grid-side converter model. This simplification is mainly based upon the intuition that the dc-bus capacitor is large enough so that the machine- and grid-side dynamics can be decoupled. However, the simplification probably does not hold in practice since there is always a tendency to minimize the capacitor size to achieve a smaller package for installation. Therefore, the machine-side system should not be ignored, otherwise maybe leading to error for stability analysis. To address this issue, this paper developed a complete ac-side impedance model of the PMSG-based WECS. Then, impact factors about the coupling characteristics between machine- and grid-side systems of the PMSG-based WECS are discussed. Finally, a case study of sub/super synchronous oscillation is carried out on a practical offshore wind farm integrated with a modular multilevel converter based high-voltage dc (MMC-HVDC) transmission system in China. The results show that ignoring the machine-side system would lead to an optimistic stability judgement, which confirms the necessity of the complete model of PMSG-based WECS. Furthermore, the system can be stabilized by either changing the couplings between machine- and grid-side systems of PMSG-based WECS or implementing active damping in the wind farm side MMC. [ABSTRACT FROM AUTHOR]

The paper makes an indepth analysis of the excitation of shaft torsional vibrations in steam turbine-generator-exciter shafts by variable-frequency ripple currents superimposed on DC currents in asynchronous Links using finite element and reduced models of the machine shafts. Frequencies at which sympathetic shaft torsional vibrations would be excited by modulation product harmonics in 50Hz/50Hz and 50Hz/60Hz asynchronous Links as a function of deviation in system frequency are illustrated. It is shown that amplitude of shaft torque due to steady resonant torque excitation is a function of initial rate of increase of vibrations at adjacent cells, the time constant for decay of the vibration, and stiffness between adjacent shaft cells. Time constants for decay of shaft torsional vibrations due to electrical and steam viscous damping at full-load and no-load for finite element and reduced shaft models are then evaluated and compared. Modal torque correlation factors for finite element and reduced shaft models which relate modal torque at sections of shaft are also evaluated and compared. Amplitudes of modal torques for finite element and reduced shaft models for given steady state resonant torque excitations are illustrated. The effect of generator load on shaft torque due to resonant excitation is also illustrated. Typical shaft torques for generators connected directly to HVDC Link rectifiers due to resonant excitation for the generator at full-load and no-load are also compared. The paper then shows that shaft torques in multi-machine networks may be estimated by proportioning HVDC Link disturbance current to each machine at risk using system network data, generator data, fault analysis data, and load flow data, considering frequency dependence of the system parameters. This scaling factor is calculated for different scenarios of system operation and load. Equivalent circuits for the synchronous generator are employed appropriately to correlate HVDC Link disturbance current impressed on the generator stator with steady-state torque excitation from which magnitude of turbine-generator-exciter shaft torque is found. It is concluded that onerous Shaft torsional vibrations may be set up in large steam generator shafts by DC Link ripple currents and that torques evaluated using finite element and reduced shaft models can differ significantly. Finite element shaft models, therefore, should be used to evaluate modal torque correlation factors, initial rate of increase of modal vibrations due to application of steady resonant torque excitation, and time constants for decay of vibrations from which modal torque at shaft sections due to steady resonant excitation is found. Detailed (2d,3q) rotor equivalent circuits should be used to evaluate steady-state torque excitation due to injected harmonic currents, and to proportion injected harmonic current to each machine of a multi-machine network which is at risk. Machines rated at 500 MW, 660 MW, 1000MW and 1300 MW are considered in the analyses that are made. KEYWORDS: Generator shaft torsionals, generator harmonics, HVDC converter stations, HVDC Links, excitation of torsional vibrations, non-characteristic harmonics, asynchronous HVDC connections, damping of generator vibrations, generator shaft torques.

Ripple currents on the DC side of both HVDC synchronous and asynchronous Links together with cleared HVDC and AC system disturbances can excite in some circumstances onerous torsional vibrations in large steam generator shafts. The problem has assumed importance in recent months on account of the HVDC Link between Scotland and Northern Ireland going ahead, on account of the proposed Eire/Wales Link, and because AC/DC/AC couplers are to be installed extensively to interconnect the East and West European Grid Systems. This paper discusses and analyses excitation of shaft torsional vibrations in steam turbine-generator-exciter shafts in close proximity to HVDC converter stations by (i) variable-frequency ripple currents superimposed on the DC currents in asynchronous Links, and (ii) disturbances at bi-polar converter stations. In (i) shaft torques in multi-machine networks are evaluated by proportioning HVDC Link disturbance currents to each machine at risk using system network data, generator data, fault analysis data, and load flow data, considering frequency dependence of the system parameters. This scaling factor is calculated for different scenarios of system operation and load. Equivalent circuits for the synchronous generator are employed appropriately to correlate HVDC Link disturbance current impressed on the generator stator with steady-state torque excitation from which magnitude for turbine-generator-exciter shaft torque is deduced. In (ii) shaft torques are evaluated by time domain computation using converter station phase currents and a faithful single machine-infinite busbar equivalent of the multi-machine power system where converter station currents are proportioned to the particular machine which is under investigation. Generator air-gap torque is evaluated by solution of the converter station/synchronous machine/infinite system differential equations. Turbine-generator-exciter torque is computed from transient generator airgap torque either in the time domain using differential equations for the turbine-generator-exciter shaft or in the frequency domain using stiffnesses, inertias and damping of modal vibrations of the shaft. Detailed (2d,3q) and reduced (1d,2q) and (1d,1q) phase-variable models for the synchronous-generator are employed. The time responses and tables show that for the systems studied variable-frequency ripple currents superimposed on the DC current in asynchronous Links can excite shaft torsional vibrations, that very small noncharacteristic currents could result in onerous shaft torques which might damage the machine, and that DC line faults at converter stations in close proximity of steam turbine-generator units can excite onerous turbine-generator shaft torsional response. Detailed simulation of the HVDC converter and generator is necessary for precise assessments of shaft torsional response following HVDC converter station faults. 500MW, 660MW, 1000MW and 1300MW machines are considered in the analyses that are made.

The lack of inertial response from non-synchronous, inverter-based generation in microgrids makes the power system vulnerable to a large rate of change of frequency (ROCOF) and frequency excursions. Energy storage systems (ESSs) can be utilized to provide fast-frequency support to prevent such large excursions in the system. However, fast-frequency support is a power-intensive application that has a significant impact on the ESS lifetime. In this paper, a framework that allows the ESS operator to provide fast-frequency support as a service is proposed. The framework maintains the desired quality-of-service (limiting the ROCOF and frequency) while taking into account the ESS lifetime and physical limits. The framework utilizes moving horizon estimation (MHE) to estimate the frequency deviation and ROCOF from noisy phase-locked loop (PLL) measurements. These estimates are employed by a model predictive control (MPC) algorithm that computes control actions by solving a finite-horizon, online optimization problem. Additionally, this approach avoids oscillatory behavior induced by delays that are common when using low-pass filters as with traditional derivative-based (virtual inertia) controllers. MATLAB/Simulink simulations on a test system from Cordova, Alaska, show the effectiveness of the MHE-MPC approach to reduce frequency deviations and ROCOF of a low-inertia microgrid. [ABSTRACT FROM AUTHOR]

A new DC current-voltage droop control method for a hybrid high-voltage direct-current (HVDC) system featuring a voltage-sourced converter (VSC) rectifier and a line-commutated converter (LCC) inverter is presented. The proposed method utilizes DC voltage droop control to reduce the AC voltage fluctuation at the inverter side, to exploit the hybrid HVDC system for connecting offshore wind farms to weak grids without necessitating continuous reactive power compensation facilities. Additionally, an optimal droop coefficient calculation method is proposed to keep the AC voltage constant for wind power fluctuations. To evaluate the stability of the proposed method, a small-signal state-space (SS) model of the target system is derived and the root locus analysis is given. Simulation case studies are performed using an electromagnetic transient model in PSCAD and an SS model in MATLAB. The results demonstrate that a hybrid HVDC system with the proposed method successfully integrates offshore wind energy and weak grids without additional facilities because the AC voltage can be maintained at a near constant during wind fluctuations. [ABSTRACT FROM AUTHOR]

In this article, we present a new approach to the detection of Loss-Of-Excitation (LOE), a typical failure of synchronous generators. Unlike most of the algorithms proposed in the literature, which only use the information available at the point of connection, we also take advantage of prior knowledge of the generator model. To track the field voltage and the other state variables, we have chosen the Constrained Unscented Kalman Filter (CUKF) as the core estimation technique, with phasor measurements as the input for this filtering algorithm. Detection of LOE is then performed by using the Faulty Modes Detection and Diagnosis (FMDD) algorithm, which combines the normal operation and a LOE-based model to decide in real-time whether an LOE has occurred or not. Results of simulations using a small two-area power system and the IEEE 39-bus system show that LOE detection times can be significantly reduced as compared to conventional and state-of-the-art approaches. Moreover, we observe that the new fault detection signal used to trip the generator can avoid short-term voltage stability problems. [ABSTRACT FROM AUTHOR]

Power buffers are power electronic converters, with large capacitors, that decouple volatile loads and a low-inertia distribution network in a DC microgrid. In this work, a set of distributed optimal control policies enable power buffers to reciprocally assist each other during abrupt load changes. While the majority of existing control paradigms are localized, enabling communication among buffers extends their effective range of assistance and helps them minimize a shared objective in a cooperative fashion. The control law's weights surfaces are learned for a mesh of reference loads of each power buffer. Hamilton-Jacobi-Bellman equation is solved by a continuous-time adaptive dynamic programming (ADP) approach with off-policy learning to directly provide a feedback controller, instead of existing approaches that obtain open-loop policies via Pontryagin's minimum principle. This paper presents the first attempt in using ADP techniques for the control of power buffers that respects their original nonlinear dynamics, overcoming the limitations of previous approaches based on small-signal analysis. Compared to the current literature, the proposed approach provides trained controllers that are known a priori, avoiding player-by-player solutions or real-time optimization procedures that could degrade performances or become computationally intensive. Hardware-in-the-loop emulations of a low-voltage DC microgrid validates the proposed approach. [ABSTRACT FROM AUTHOR]

If multi-phase machines equipped with tooth-concentrated winding with half a slot per pole and per phase offer interesting characteristics (simplified manufacturing, no space subharmonic, fault-tolerant ability), their low fundamental winding factors make their designs and controls challenging. The paper addresses the case of a seven-phase Surface-mounted Permanent Magnet (SPM) machine which has a fundamental winding factor lower than the third. This so-called bi-harmonic specificity is considered in order to achieve good torque quality (average value and ripples). Regarding the design, the magnet layer is segmented into two identical radially magnetized tiles that cover about three-quarters the pole arc. Regarding the control, the rated Maximum Torque Per Ampere (MTPA) supply strategy (h1h3 control) aims at generating a third harmonic current component greater than the fundamental. A prototype has been manufactured: the ability of the machine to provide smooth torque is experimentally confirmed through the implementation of a simple MTPA control which copes with high distortion in no-load voltage. [ABSTRACT FROM AUTHOR]

Benchmark models provide a common platform that facilitates comparison of different methods and strategies that can be further validated in a real representation of the power system. Nevertheless, when using RMS test cases, transients present in modern power systems may be neglected. Modern energy conversion systems require an enhanced level of detail in order to properly represent their behavior. Moreover, due to the steady increase of non-synchronous generation, new operation and management strategies need to be tested to ensure power system security. One way to overcome these constraints is to use EMT real-time digital simulations which allows to obtain high precision results in a timeframe suited to a specific study. This work presents an EMT real-time model for integration studies of advanced energy conversion systems that can be flexibly expanded to higher renewable penetration. The model, based on the Simplified Australian 14-Generator test system, has been expanded to represent in an improved way the dynamics of power system components as well as the high penetration of non-synchronous power electronics generation. Transient simulations are undertaken to illustrate system's response due to non-synchronous generation and when faults are applied. This work also addresses the current gap in benchmark models for real-time simulations. [ABSTRACT FROM AUTHOR]

Dual stator-winding induction generator (DWIG) is one kind of emerging multiport electrical machine. It has become a promising direction in the last two decades, especially for stand-alone power systems and renewable energy power systems. This paper mainly focuses on the DWIG with similar-poles stator windings and cage-type rotor. An overview of DWIG and its system is given. Recent advances of this generator in system topologies, mathematical model, optimal design, control strategies, and performance analysis are presented. The developments and potential applications of the DWIG-based system are also discussed. [ABSTRACT FROM AUTHOR]

For power converter-interfaced wind-generation systems, low-frequency instability issues arising from the interactions among converter outer power flow control (PFC) loops have got increased attention recently. This paper explores the possible origin of such instability in grid-tied doubly fed induction generator systems. To comprehensively analyze this multiloop interacted system, a reduced transfer matrix model is first constructed allowing for the PFC dynamics. Then the individual channel analysis and design approach is employed to decompose such closed-loop multivariable system into components such as local subsystem and loop interactions. Subsequently, frequency-response behavior analyses of these components are individually conducted at various operating points to study their relative contribution to overall stability. Results indicate that local active power control loop will induce a right-half-plane zero (RHPZ) for certain high-power operating conditions, and the resulting inverse response leads system unstable. Meanwhile, the critical operating point at which the RHPZ arises is analytically derived, and demonstrated to be close to the actual dynamic stability boundary. [ABSTRACT FROM AUTHOR]

This paper presents a sensor fault-tolerant control (SFTC) strategy, combining perturbation observer-based direct power control (PODPC) and two-stage Kalman filter (TSKF) to enhance the fault tolerance of a doubly fed induction generator-based wind turbine (DFIG-WT) subject to rotor and stator current sensor faults. In the PODPC scheme, the interactions between active and reactive power control loops are represented by newly introduced perturbation states, and the feedback linearization control is realized with the state estimations derived from perturbation observers to achieve the decoupled power control. No rotor current sensors nor parameters of DFIG-WT are required in the implementation of PODPC. Stator current TSKFs are designed to generate residuals for fault detection and isolation, and provide current estimations to replace the faulty current measurements for system reconfiguration under stator current sensor faults. Simulation studies undertaken on a grid-connected DFIG-WT system reveal that the proposed SFTC strategy is immune to rotor current sensor faults, and it provides strong fault tolerance to stator current sensor faults. [ABSTRACT FROM AUTHOR]

In future power systems, reduced overall inertia caused by an increased dominance of asynchronous generation and interconnections would make frequency control particularly challenging. As the number and power rating of voltage source converter (VSC) HVDC systems increases, network service provision would be expected from such systems and to do so would require overload capacity to be included in the converter specifications. This paper studies the provision of frequency services from modular multilevel converter (MMC)-based VSC HVDC interconnections using temperature-constrained overload capability. Overload of the MMC-based HVDC system is achieved through controlled circulating currents, at the expense of higher losses, and subject to a control scheme that dynamically limits the overload available in order to keep the semiconductor junction temperatures within operational limits. Two frequency control schemes that use the obtained overload capacity to provide frequency response during emergency conditions are investigated. The controllers’ performance is demonstrated in the context of the future Great Britain transmission grid through a reduced equivalent test system. Simulation results show that even modest temperature margins which allow overload of MMC-based HVDC systems for a few seconds are effective as a primary frequency reserve and also reduce the loss of infeed requirements of such interconnections. [ABSTRACT FROM AUTHOR]

Loss-of-field (LOF) phenomenon in synchronous generators may cause equipment damage and voltage drop in the power system, which can be terminated to a blackout. Conventional schemes, which detect LOF on the basis of the measured impedance from the generator terminal viewpoint, have some major drawbacks such as low speed, complicated setting procedure, and unsecure performance in the face of similar phenomena, e.g., stable power swing. In this paper, different electrical quantities of the generator including terminal voltage (V), current (I ), active (P) and reactive power (Q), and power angle (δ) variations are studied to achieve a secure technique for LOF protection in synchronous generators. Comprehensive studies show that a simple criterion based on polarities of V, Q, and δ variations is accessible to achieve this goal, which does not need a complicated procedure to set it. To evaluate the performance of the proposed algorithm, some cases are simulated on sample power systems under various operating conditions. Also, superiority of the proposed algorithm in comparison with the other schemes is proven by the obtained results, as well. Moreover, the proposed technique is experimentally tested by using some 2-kVA laboratory generators to detect LOF. [ABSTRACT FROM AUTHOR]

This paper presents a dynamic modeling methodology of a DFIG-based wind turbine (WT) for small-signal stability analysis in DC-link voltage control (DVC) timescale in power electronized power systems. The DVC timescale (around 100 ms) is determined by DVC, terminal voltage control, active power control, and phase-locked loop in DFIG WT. Motion equation concept is introduced and extended to describe DFIG WT external characteristics in the concerned timescale. The relation between the active/reactive power imbalances and phase/magnitude dynamics of defined synthetic internal voltage (inner potential) vector are developed. The model in DVC timescale is similar to synchronous generator rotor motion equation that is in electromechanical timescale (around 1 s) and familiar to power engineers. With the developed model, characteristics of equivalent inertia, damping and synchronizing coefficients of DFIG WT in DVC timescale can be understood, and the dynamic interactions among multiple DFIG WTs, as well as between DFIG WT and other grid-connected devices in DVC timescale can be fully interpreted. Comparisons of eigenvalues show that the proposed model can hold the main behaviors of concern. Applications on the stability analyses of DFIG WT interconnected with VSC-HVDC system and two-DFIG WT system are taken as examples to validate the feasibility of the proposed model. [ABSTRACT FROM PUBLISHER]

High penetration of wind power into existing grid can be attributed largely to the doubly fed induction generators (DFIG). However, their sensitive nature to the grid faults has created concern for their mass integration into the existing power system. The grid code has been imposed by the regulatory bodies and the transmission system operators to ensure the low-voltage ride through (LVRT) capability of the wind farms for stable operation. DFIGs with their popular topology of partially rated converters are unable to provide the LVRT capability alone. In this work, a nonlinear control-based modified bridge-type fault current limiter (NC-MBFCL) is proposed to enhance the LVRT capability of DFIG-based wind farms. The efficacy of the NC-MBFCL is evaluated through performance comparison with that of the conventionally controlled bridge-type fault current limiter (BFCL) and the modified BFCL (MBFCL). Extensive simulations executed in MATLAB/Simulink environment for both the symmetrical and unsymmetrical faults reveal that the proposed NC-MBFCL is very effective in enhancing the LVRT capability of DFIG-based wind farms and outperforms the conventionally controlled BFCL and MBFCL. Also, it was found that the BFCL demonstrates better performance than the MBFCL. [ABSTRACT FROM AUTHOR]

Recent focus on component-network interfacing has greatly increased the maximum step size at which machine models can be simulated accurately and stably in nodal-analysis-based electromagnetic transients programs (EMTP-type). However, considerably smaller step sizes are often used in practice due to the presence of fast modes in external power networks. To further improve simulation speed, this paper proposes a multirate framework for EMTP-type machine models connected to dynamically faster power systems. In particular, two multirate induction machine models based on the voltage-behind-reactance (VBR) formulation are presented. These new models can be interfaced directly with commercial EMTP-type programs with no modifications to their computational engines. The presented computer studies demonstrate the superior combination of numerical accuracy and efficiency of the proposed multirate models compared with the existing models. [ABSTRACT FROM PUBLISHER]

Transient stability is a major concern for doubly fed induction machine (DFIM). A DFIM-based wind generator is readily affected by faults at the grid side as its stator windings are interfaced to grid. However, the wind generators need to remain connected and continue operation during faults at the grid side according to the grid code requirements. Therefore, it is important to enhance the transient stability of the DFIM-based wind generators. To achieve enhanced transient stability of the DFIM, a bridge-type fault current limiter (BFCL) is proposed in this study. Symmetrical as well as unsymmetrical faults were applied to the test system to check the efficacy of the BFCL in transient stability enhancement. Simulations were carried out in Matlab/Simulink environment. To demonstrate the effectiveness of the proposed BFCL, its performance is compared with that of the series dynamic braking resistor (SDBR). Simulation results show that the BFCL is a very effective device to attain better stabilization of the DFIM and outperforms the SDBR in all aspects. [ABSTRACT FROM AUTHOR]