Power System Transients Research Articles

Software & Applications: Power System Transients and Dynamics [Book Reviews

Software & Applications: Power System Transients and Dynamics [Book Reviews

A New Adaptive Mother Wavelet for Electromagnetic Transient Analysis

Abstract Wavelet Transform (WT) is a powerful technique of signal processing, its applications in power systems have been increasing to evaluate power system conditions, such as faults, switching transients, power quality issues, among others. Electromagnetic transients in power systems are due to changes in the network configuration, producing non-periodic signals, which have to be identified to avoid power outages in normal operation or transient conditions. In this paper a methodology to develop a new adaptive mother wavelet for electromagnetic transient analysis is proposed. Classification is carried out with an innovative technique based on adaptive wavelets, where filter bank coefficients will be adapted until a discriminant criterion is optimized. Then, its corresponding filter coefficients will be used to get the new mother wavelet, named wavelet ET, which allowed to identify and to distinguish the high frequency information produced by different electromagnetic transients.

Multivariate Statistics and Supervised Learning for Predictive Detection of Unintentional Islanding in Grid-Tied Solar PV Systems

Integration of solar photovoltaic (PV) generation with power distribution networks leads to many operational challenges and complexities. Unintentional islanding is one of them which is of rising concern given the steady increase in grid-connected PV power. This paper builds up on an exploratory study of unintentional islanding on a modeled radial feeder having large PV penetration. Dynamic simulations, also run in real time, resulted in exploration of unique potential causes of creation of accidental islands. The resulting voltage and current data underwent dimensionality reduction using principal component analysis (PCA) which formed the basis for the application ofQstatistic control charts for detecting the anomalous currents that could island the system. For reducing the false alarm rate of anomaly detection, Kullback-Leibler (K-L) divergence was applied on the principal component projections which concluded thatQstatistic based approach alone is not reliable for detection of the symptoms liable to cause unintentional islanding. The obtained data was labeled and aK-nearest neighbor (K-NN) binomial classifier was then trained for identification and classification of potential islanding precursors from other power system transients. The three-phase short-circuit fault case was successfully identified as statistically different from islanding symptoms.

Simplified variable frequency induction-motor drive model for power system stability studies and control

Abstract: Powerful variable frequency drives (VFD) are becoming more widely used in power systems. However, there are only a few works addressed to VFD motor dynamics that are suitable for large-scale power system stability studies. The paper proposes a fundamental frequency simplified model of the VFD, which provides an adequate modeling for the study of electromechanical transients in large-scale power systems. Its implementation was carried out in Power System Toolbox (PST), an open source software available for both scientific and commercial purposes. The model was complemented by a number of protection and control strategies that allow to perform adequate modeling for severe disturbances.

Reachability-based Control Synthesis for Power System Stability

Abstract: We present a nonlinear reachability framework for control of power system transients. The power system consists of synchronous generators, coupled by transmission lines and controlled HVDC links. The approach consists of characterisation of the region of attraction as a backward reachable set of a stable equilibrium point of the power system. Furthermore, the optimal HVDC control as a feedback map is synthesised. As such, the set of the generators’ disturbed states that return to a stable equilibrium point under an optimal HVDC control input are computed. The numerical examples include a two area power system with a HVDC link.

Fault location scheme for multi-terminal transmission lines using unsynchronized measurements

Abstract In this paper, a simple alternative fault location algorithm for multi-terminal transmission lines using unsynchronized measurements is proposed. The developed data synchronization procedure is employed to identify the faulted leg before the fault location is calculated. The fault location algorithm is independent of the fault resistance and source impedance variations. The proposed faulted leg identification and location algorithm is extensively tested for all major fault types and different high fault resistances. The results show that the proposed multi-terminal fault location algorithm is fast, accurate and immune to power system transients.

Validation Limits of Quasi-TEM Approximation for Buried Bare and Insulated Cables

For the analysis of power system transients in overhead transmission lines and underground cables, a common assumption is related to the quasi-TEM (transverse electromagnetic) approximation. In this paper, we investigate the applicability of this approximation throughout a wide frequency band when both conduction and displacement currents in ground are considered, typically only the former is considered. A full-wave model is established to assess the behavior of the propagation constant for bare and insulated underground cables in a wide frequency band and verify the adequacy of the quasi-TEM approximation. The results demonstrate that it is possible to retain quasi-TEM approximations in the high-frequency range as well. One has only to consider different assumptions for the propagation constant. For the evaluation the adequacy of the quasi-TEM approximation, a simple, single-phase buried cable is considered. The effect of the cable coating is also taken into account and the results indicate that the quasi-TEM approximation is rather distinct for bare or insulated cables. The accuracy of the approximation is evaluated from comparison of step and impulse voltage tests considering different soil models.

A Practical Exciter Model Reduction Approach For Power System Transient Stability Simulation

Explicit numerical integration methods for power system transient stability simulation require very small time steps to avoid numerical instability. The EXST1 exciter model is a primary source of fast dynamics in power system transients. In case of the EXST1, the required small integration time step for entire system simulation increases the computational demands in terms of running time and storage. This paper presents a practical exciter model reduction approach which allows the increase of the required step size and thus the method can decrease the computational demands. The fast dynamics in the original EXST1 are eliminated in the reduced exciter model. The use of a larger time step improves the computational efficiency. This paper describes the way to eliminate the fast dynamics from the original exciter model based on linear system theory. In order to validate the performance of the proposed method, case studies with the GSO-37 bus system are provided. Comparisons between the original and reduced models are made in simulation accuracy and critical clearing time.

Impact of emulated inertia from wind power on under-frequency protection schemes of future power systems

Future power systems face several challenges. One of them is the use of high power converters that decouple new energy sources from the AC power grid. This situation decreases the total system inertia affecting its ability to overcome system frequency disturbances. The wind power industry has created several controllers to enable inertial response on wind turbines generators: artificial, emulated, simulated, or synthetic inertial. This paper deals with the issues related to the emulated inertia of wind turbines based on full-converters and their effect on the under-frequency protection schemes during the recovery period after system frequency disturbances happen. The main contribution of this paper is to demonstrate the recovery period of under-frequency transients in future power systems which integrate wind turbines with emulated inertia capability does not completely avoid the worse scenarios in terms of under-frequency load shedding. The extra power delivered from a wind turbine during frequency disturbances can substantially reduce the rate of frequency change. Thus it provides time for the active governors to respond.

Improvements in the modeling of wind turbines in power system studies

Several methods to improve the current wind turbine modeling techniques for power system studies are discussed in this paper. Some of the modeling improvements which are discussed are dynamic initialization of models, differences between wind turbine models developed in electromagnetic transient program and bulk power system simulation software, and techniques to avoid numerical instability with minimum errors. Algorithms to provide automatic initial conditions of all variables for any given load flow condition, wind speeds, and wind farm control strategies are developed in this paper. This approach is an improvement to the traditional method of providing constant initial values for given operating conditions. Several physical components such as harmonic filters, cable impedances, and complex control algorithms are either simplified or excluded in the power system models. The fault response of the power system model under extreme conditions leads to a numerically unstable condition. The instability might be a result of simplification discussed above and does not occur in the real turbine. Additional modeling techniques that can be used to achieve numerical stability, dynamic and fault response close to the actual wind turbine are discussed. These techniques are used to compensate for the simplifications made in power system models. Finally, the issue of nuisance tripping faced very frequently by the Independent System Operators, while working on the studies involving wind power plants, is addressed. Various methods to reduce the deviations and prevent nuisance tripping are implemented in the paper.

A Constant-Parameter Voltage-Behind-Reactance Synchronous Machine Model Based on Shifted-Frequency Analysis

Recently, the concepts of dynamic phasors and shifted-frequency analysis (SFA) have received renewed attention as a possible solution framework for the modeling of power system components and transients, as opposed to using instantaneous time-domain variables or conventional phasors. In this paper, a new voltage-behind-reactance (VBR) synchronous machine model based on SFA is presented. Using dynamic phasors, the proposed model permits the use of a much larger range of step sizes to efficiently simulate electromagnetic and electromechanical transients. Moreover, the proposed model has a constant-parameter (CP) stator interface, which is simple to implement and numerically more efficient compared with the prior state-of-the-art models with rotor-position-dependent stator inductance matrices. Rigorous transient case studies demonstrate that the new model requires significantly fewer time steps than the conventional time-domain models, and is more efficient than the previously established variable-parameter SFA model.

Accurate Impedance Calculation for Underground and Submarine Power Cables Using MoM-SO and a Multilayer Ground Model

An accurate knowledge of the per-unit length impedance of power cables is necessary to correctly predict electromagnetic transients in power systems. In particular, skin, proximity, and ground return effects must be properly estimated. In many applications, the medium that surrounds the cable is not uniform and can consist of multiple layers of different conductivity, such as dry and wet soil, water, or air. We introduce a multilayer ground model for the recently-proposed MoM-SO method, suitable to accurately predict ground return effects in such scenarios. The proposed technique precisely accounts for skin, proximity, ground and tunnel effects, and is applicable to a variety of cable configurations, including underground and submarine cables. Numerical results show that the proposed method is more accurate than analytic formulas typically employed for transient analyses, and delivers an accuracy comparable to the finite element method (FEM). With respect to FEM, however, MoM-SO is over 1000 times faster, and can calculate the impedance of a submarine cable inside a three-layer medium in 0.10~s per frequency point.

Fast algorithm for estimating power frequency phasors under power system transients

Power frequency phasors are widely used in the measurement, analysis, control and protection of power systems. When disturbances occur in such systems, power frequency phasors must be estimated quickly to ensure an accurate restoration to normal operation. Hence, a fast and accurate estimation algorithm would be a significantly useful tool. In consideration of the composition of transient signals in power systems, a novel algorithm for power frequency phasor estimation is proposed. Based on the double matrix pencil method, a unitary reference matrix of complex-valued exponential signals is formed. The power frequency phasor estimation is then transformed to a brief multiplication of matrices. Thus, the QR decomposition, generalised eigenvalue calculation, and secondary estimation can be avoided and no complex multiplication is introduced. Therefore, the computational efficiency of the proposed method is improved compared with previous estimation techniques. Analytical and experimental investigations show that the proposed algorithm can extract power frequency phasors accurately, stably and quickly under power system transients.

Implementation of an integrated online instantaneous discrete wavelet transform decomposition toolbox in ATP-EMTP

Abstract Although wavelet transform decomposition has wide applications in the analysis of power system transients due to its multi-resolution analyzing nature, conventional transient simulation programs do not provide an effective inbuilt wavelet transform toolbox for online studies. This paper proposes the development and implementation of an Online Instantaneous Discrete Wavelet Transform Decomposition Toolbox (IWTD toolbox) in ATP-EMTP software by its MODELS programming language. Integration of a powerful online discrete wavelet transform toolbox with alternative transient program makes this software highly functional especially with selectable different full and reduced order mother wavelets with definable reduction degree. The proposed toolbox provides a user-friendly interface with additional capabilities and features for ATP users. Numerous cases simulated and compared with the results of the well-known MATLAB Wavelet Toolbox have indicated that the proposed toolbox works reliably and accurately in the ATP-EMTP environment.

Considerations for packet delivery reliability over polling-based wireless networks in smart grids

Supervisory control and data acquisition (SCADA) systems currently use the polling technique for monitoring electric utility networks. Unfortunately, conventional SCADA systems do not suit the needs of smart grids in terms of the required data rate. Polling-based wireless networks can extend the capabilities of SCADA systems as they provide low cost transceivers and bounded packet delay. However, the harsh environment of power stations negatively impacts the performance of wireless links. This paper introduces a field measurement-based study that focuses on the effect of power system noise and transients on packet delivery reliability of Zigbee and WiFi polling-based wireless networks. Extensive experiments show that the electromagnetic interference emitted from high voltage substations, during normal operation conditions, do not significantly affect wireless communication in the gigahertz range. Moreover, we analytically and experimentally demonstrate that abnormal operation conditions of power systems may negatively impact the reliability of packet delivery in polling-based wireless networks. Furthermore, we show that this negative impact can be mitigated by following some proposed technical considerations regarding the wireless standard, the operating frequency, the location, and the number of wireless transceivers used.

Proximity-Aware Calculation of Cable Series Impedance for Systems of Solid and Hollow Conductors

Wide-band cable models for the prediction of electromagnetic transients in power systems require the accurate calculation of the cable series impedance as function of frequency. A surface current approach was recently proposed for systems of round solid conductors, with inclusion of skin and proximity effects. In this paper we extend the approach to include tubular conductors, allowing to model realistic cables with tubular sheaths, armors and pipes. We also include the effect of a lossy ground. A noteworthy feature of the proposed technique is the accurate prediction of proximity effects, which can be of major importance in three-phase, pipe type, and closely-packed single-core cables. The new approach is highly efficient compared to finite elements. In the case of a cross-bonded cable system featuring three phase conductors and three screens, the proposed technique computes the required 120 frequency samples in only six seconds of CPU time.

Electromechanical wave in power systems: theory and applications

The continuum model is a key paradigm describing the behavior of electromechanical transients in power systems. In the past two decades, much research work has been done on applying the continuum model to analyze the electromechanical wave in power systems. In this work, the uniform and non-uniform continuum models are first briefly described, and some explanations borrowing concepts and tools from other fields are given. Then, the existing approaches of investigating the resulting wave equations are summarized. An application named the zero reflection controller based on the idea of the wave equations is next presented.

Method to Prevent the Malfunction Caused by the Transformer Magnetizing Inrush Current using IEC 61850-based IEDs and Dynamic Performance Test using RTDS Test-bed

The digital substations are being built based on the IEC 61850 network. The cooperation and protection of power system are becoming more intelligent and reliable in the environment of digital substation. This paper proposes a novel method to prevent the malfunction caused by the Transformer Magnetizing Inrush Current(TMIC) using the IEC 61850 based data sharing between the IEDs. To protect a main transformer, the current differential protection(87T) and over-current protection(50/51) are used generally. The 87T IED applies to the second harmonic blocking method to prevent the malfunction caused by the TMIC. However, the 50/51 IED may malfunction caused by the TMIC. To solve that problem, the proposed method uses a GOOSE inter-lock signal between two IEDs. The 87T IED transmits a blocking GOOSE signal to the 50/51 IED, when the TMIC is detected. The proposed method can make a cooperation of digital substation protection system more intelligent. To verify the performance of proposed method, this paper performs the real time test using the RTDS (Real Time Digital Simulator) test-bed. Using the RTDS, the power system transients are simulated, and the TMIC is generated. The performance of proposed method is verified in real-time using that actual current signals. The reaction of simulated power system responding to the operation of IEDs can be also confirmed.

On fixed-point iterations for the solution of control equations in power systems transients

This paper contributes toward the establishment of a formal analysis method of control system equations solved through fixed-point iterations. The success of fixed-point iterations relies on contraction properties of the function to be iterated. A convergence criterion is presented and accuracy is not sacrificed over gain in computational performance.The presented algorithms are illustrated in EMTP-RV for practical control systems used in wind power generation and for a user defined model case. Limitations and performances are discussed in relation to the Newton method.

A sparse representation based approach for recognition of power system transients

The present work shows how a new genre of problem solution techniques called compressive sensing (CI) can be utilized for solving power quality problems. Specifically speaking, this work undertakes the power system transient recognition problem and solves it using sparse representation based classification (SRC) technique, a popular, recently emerging technique in CI. The method employs SRC based classification in combination with a feature extraction procedure, carried out using dual tree complex wavelet transform (DTCWT). The proposed method uses statistical characteristics to extract features from the DTCWT coefficients obtained from each signal and then these extracted features are used as input arrays for the SRC based classifier. The PQ disturbance events considered in this work include four different transient conditions, namely, transients due to capacitor switching, transformer inrush currents, transients due to motor switching and transients due to short circuit faults. The proposed algorithm could achieve perfect classification accuracy of the order of 100% and could comfortably outperform several similar contemporary methods known for identical power system transient classification problems.