Stability Margin Of Power System Research Articles

Coordinated optimization and control of SFCL and SMES for mitigation of SSR using HBB-BC algorithm in a fuzzy framework

Abstract Increasing demand for electrical power and expensive expansion of power systems to meet this demand leads the planners to find techno-economical solutions to overcome these challenges. Using series capacitors as reactive power compensation devices in a long transmission lines is a well-known and suitable way to face these challenges in power systems. Transmitted electrical power and the stability margin of power systems are economically improved by the proposed series devices. However, employing the series capacitors in long transmission lines causes a well-known problem in power systems as Sub Synchronous Resonance (SSR). In the present work, the mitigation of SSR is formulated as a Multi Objective Problem (MOP) in a fuzzy framework by using optimal coordinated control of Superconducting Fault Current Limiter (SFCL) and Superconducting Magnetic Energy Storage (SMES). Objective functions of MOP includes the minimization of the initial energy stored in the SMES unit, the energy loss of SFCL, the kinetic energy in generator rotor, the active power deviations in faulted bus, and the rotor speed deviations. These five objective functions are scaled by a fuzzy operator, then the scaled objective functions are aggregated by the “max-geometric mean” operator to generate the multi objective function. To solve the proposed multi-objective problem, the Hybrid Big Bang-Big Crunch (HBB-BC) as a meta-heuristic optimization algorithm is used. In order to evaluate the efficiency of the proposed algorithm, it is implemented on the IEEE First Benchmark Model (FBM) for SSR studies. According to the simulation results of the present study, the proposed algorithm is more effective in reducing the SSR problem in comparison with other algorithms.

Modal analysis of active distribution networks using system identification techniques

Mode identification from post-disturbance “ringdown” responses can provide vital information concerning the dynamic performance and the stability margins of power systems. Therefore, several measurement-based identification techniques have been proposed in the literature to analyze ringdown responses of transmission systems and provide close to real-time estimation of the modal content. However, the applicability of these methods has not been thoroughly investigated for the analysis of active distribution networks (ADNs). Scope of this paper is to evaluate the applicability and the performance of eight measurement-based system identification techniques for the modal analysis of ADNs. The examined methods are used to identify the dominant oscillatory modes contained in ringdown responses of different types of signals. The Monte Carlo method is applied to investigate the influence of several parameters on the accuracy and efficiency of the identification procedure, while laboratory measurements are used to further demonstrate the accuracy of the examined methods. Practical issues encountered in the application of the identification techniques for the analysis of ADNs are discussed and potential solutions are proposed. Results reveal that although most of the examined techniques perform satisfactorily enough and thus can be readily employed for the modal analysis of ADNs, the Vector Fitting and the Hybrid FD/TD seem to be the most effective methods in terms of accuracy, robustness and computational efficiency.

Searching severest VSM basing on CCPF among multiple dispatch centres

It is important to search voltage stability margin (VSM) of an interconnected power system in order to determine secure operation constraints. Traditionally, centralised continuation power flow (CPF) is used, which may fail to model the varied load growth directions of different regional power grids. In this study, a distributed VSM searching method is proposed grounding on a coordinated CPF (CCPF) performed by neighbouring dispatch centres. First, a distributed power flow model for VSM is established, which considers the automatic allocation of network loss. Then, a critical bus of the power system is used to indicate the system-level voltage stability. It is chosen according to the sensitivity of the minimum singular value of Jacobi matrix to nodal voltage amplitudes, and the sensitivity of boundary power injection to nodal voltage amplitudes. Moreover, the gradient of critical bus voltage amplitude to regional loading parameters is computed, which is set as the load growth direction for the current step. The severest VSM is obtained by solving the CCPF iteratively along the worst load growth direction. Tests on IEEE 118 bus system and a real power grid of China validate the proposed method in the enhancement of the security for interconnected power systems.

An Improved Multi-Infeed Effective Short-Circuit Ratio for AC/DC Power Systems with Massive Shunt Capacitors Installed

The multi-infeed effective short-circuit ratio (MESCR) is widely used in indicating the strength of multi-infeed AC/DC power systems. However, when the widely used MESCR was adopted to evaluate the stability margin of the Eastern China Grid including three infeed ultra-high-voltage DC (UHVDC) and five high-voltage DC transmission lines in 2016, the MESCR result indicated the system was strong enough but in fact occasionally collapses after the N-1 contingency. To determine the reason for this conflict, this paper theoretically analyzes the limitations of the existing MESCR. The theoretical analysis reveals that when a large amount of capacitor compensations are concentratively installed in the system, the conventional MESCR will not be able to reflect the capacitor compensations’ influence on the system stability, and no matter how many capacitors are installed or where the capacitors are installed, the MESCR almost retains the same value; namely, the MESCR is saturated in such systems. To address the saturation problem of conventional MESCR, this paper proposes an improved multi-infeed effective short-circuit ratio (IMESCR) which considers the influences of all capacitor compensations by converting all capacitors installed throughout the system to virtual capacitors at the DC inverter station. Case studies are carried out based on the New England 39-bus system and the Eastern China Grid, respectively. The simulation results verify the theoretical analysis of the MESCR’s limitations in evaluating the stability of power systems with massive capacitors installed, and proves that the proposed IMESCR could accurately indicate the strength of AC/DC power systems. Therefore, the proposed IMESCR provides a new index for evaluating the stability margin of power systems with massive capacitor compensations installed.

Псевдомодальное оценивание запасов статической устойчивости электроэнергетических систем

Псевдомодальное оценивание запасов статической устойчивости электроэнергетических систем

Impact of Overexcitation Limiters on the Power System Stability Margin Under Stressed Conditions

This paper investigates the impact of the overexcitation limiters (OELs) on the stability margin of a power system which is operating under stressed conditions. Several OEL modeling approaches are presented and the effect of their action has been examined in model power systems. It is realized that, more often than not, OEL operating status goes undetected by existing dynamic security assessment tools commonly used in the industry. It is found that the identification and accurate representation of OELs lead to significantly different transient stability margins. Unscented Kalman filtering is used to detect the OEL activation events. In the context of stressed system operation, such quantitative assessment is very useful for system control. This understanding is further reinforced through detailed studies in two model power systems.

A New Iterative Identification and Control Method of Closed-Loop Power System Based on Ambient Signals

Identification and control are important problems of power system based on ambient signals. In order to avoid the model error influence of the controller design, a new iterative identification and control method is proposed in this paper. This method can solve model set and controller design of closed-loop power system. First, an uncertain model of power system is established. Then, according to the stability margin of power system, stability theorem is put forward. And then controller design method and the whole algorithm procedure are given. Simulation results show the effective performance of the proposed method based on the four-machine-two-region system.

A preventive control strategy for static voltage stability based on an efficient power plant model of electric vehicles

With the increasing integration of wind farms and electric vehicles (EVs) in power systems, voltage stability is becoming more and more serious. Based on vehicle-to-grid (V2G), an efficient power plant model of EVs (E-EPP) was developed to estimate EV charging load with available corresponding response capacity under different charging strategies. A preventive control strategy based on E-EPP was proposed to maintain the static voltage stability margin (VSM) of power system above a predefined security level. Two control modes were used including the disconnection of EV charging load (‘V1G’ mode) and the discharge of stored battery energy back to power grid (‘V2G’ mode). A modified IEEE 14-bus system with high penetration of wind power and EVs was used to verify the effectiveness of preventive control strategy. Simulation results showed that the proposed strategy can not only improve the static voltage stability of power system with considerable wind generation, but also guarantee the travelling comfort for EV owners.

Probabilistic assessment of oscillatory stability margin of power systems incorporating wind farms

With the increase in penetration of wind power which is essentially intermittent and random, the dynamic performance of the power system will change significantly, and so will the characteristics of the oscillations and their stability margins. This paper provides an attempt to include probabilistic character of wind power into the power system oscillatory stability margin (OSM) analysis. Nataf transformation approach is applied to generate the wind speed samples, which models the correlation between the various wind farms. The mathematical model of OSM for wind farm integrated power system is formulated and is calculated by the integration-based eigenvalue tracing approach. Considering the uncertainties of the wind power, several statistical indices are presented to evaluate OSM. Monte Carlo simulation (MCS) is used to calculate these statistics. The impact of wind power uncertainty on OSM restricted by inter-area mode is investigated in four-machine two-area test system and 16-machine five-area test system, respectively, for different wind farm locations, wind power penetration levels and wind speed correlation (WSC) degrees. Appropriate conclusions are finally drawn.

A sensitivity based approach to assess the impacts of integration of variable speed wind farms on the transient stability of power systems

The impact of large penetration of wind power on the transient stability of multi-machine power systems is discussed in this paper. Variable speed wind farms with doubly fed induction generators (DFIG) are considered. A new index based on the sensitivity of the DFIG terminal voltage is used to assess the transient stability margin of power systems, both for constant wind speed condition and during a sudden change in wind speed. Suitable locations for connecting the DFIG to an existing power system are identified with the help of this index. The effect of increase in wind power penetration on the transient stability condition of the existing power system is studied. The proposed index is also used to select the optimum crowbar resistance during fault ride-through operation.

Regression tree for stability margin prediction using synchrophasor measurements

A regression tree-based approach to predicting the power system stability margin and detecting impending system event is proposed. The input features of the regression tree (RT) include the synchronized voltage and current phasors. Modal analysis and continuation power flow are the tools used to build the knowledge base for offline RT training. Corresponding metrics include the damping ratio of the critical oscillation mode and MW-distance to the voltage collapse point. The robustness of the proposed predictor to measurement errors and system topology variation is analyzed. The optimal placement for the phasor measurement units (PMUs) based on the importance of RT variables is suggested.

On-line monitoring of eigen-frequency and stability of power system by use of SMES

It is useful to grasp power system stability margin from on-line data in real time. We have proposed to estimate the system operating conditions from on-line data by use of superconducting magnetic energy storage (SMES) system. In this paper, we developed the proposed method to monitor the eigen-frequency of the power system in real time, which changes with time due to the system operating conditions (such as, generator output change), by use of a SMES and a lock-in amplifier. Basic experiments were carried out on an analogue-type power system simulator and a model SMES for the simulator. By use of the SMES, the eigen-frequency change according to the operating condition change was successfully measured, tracked and monitored on-line. The stability margin can be estimated by the monitored eigen-frequency. The availability of the proposed method is discussed on the experimental results.

Optimal reactive power dispatch for improving voltage stability margin using a local voltage stability index

Management of reactive power resources is vital for stable and secure operation of power systems in the view point of voltage stability. This paper deals with the management of on-load tap changers (OLTCs) and dynamic VAR sources (including synchronous generators, synchronous condensers, and shunt reactive power compensators) to improve voltage stability margin (VSM) of power systems. This problem is usually called optimal reactive power dispatch (ORD) in the literature. The main contribution of the paper is to introduce a new objective function for the ORD problem. The proposed objective function is derived based on a local voltage stability index, called DSY, and has a strong correlation with VSM. This strong correlation makes the objective function effective for improving VSM, which is the main purpose of ORD. The proposed objective function is tested on the New England 39-bus test system and its performance is compared with some of the most common objective functions used in ORD. The obtained results show that solving ORD problem using the proposed objective function yields considerable increase in VSM.

Multi-objective congestion management by modified augmented ε-constraint method

Congestion management is a vital part of power system operations in recent deregulated electricity markets. However, after relieving congestion, power systems may be operated with a reduced voltage or transient stability margin because of hitting security limits or increasing the contribution of risky participants. Therefore, power system stability margins should be considered within the congestion management framework. The multi-objective congestion management provides not only more security but also more flexibility than single-objective methods. In this paper, a multi-objective congestion management framework is presented while simultaneously optimizing the competing objective functions of congestion management cost, voltage security, and dynamic security. The proposed multi-objective framework, called modified augmented ε-constraint method, is based on the augmented ε-constraint technique hybridized by the weighting method. The proposed framework generates candidate solutions for the multi-objective problem including only efficient Pareto surface enhancing the competitiveness and economic effectiveness of the power market. Besides, the relative importance of the objective functions is explicitly modeled in the proposed framework. Results of testing the proposed multi-objective congestion management method on the New-England test system are presented and compared with those of the previous single objective and multi-objective techniques in detail. These comparisons confirm the efficiency of the developed method.

Application of artificial neural networks to improve power transfer capability through OLTC

On load tap changing (OLTC) transformer has become a vital link in modern power systems. It acts to maintain the load bus voltage within its permissible limits despite any load changes. This paper discusses the effect of different static loads namely; constant power (CP), constant current (CI) and constant impedance (CZ) on the maximum power transfer limit from the generation to the load centre through the OLTC branch and in turn on the static stability margin of power systems. Then the paper introduces a novel approach for the on-line determination of the OLTC settings using artificial neural network (ANN) technique in order to improve the power transfer capability of transmission systems. The proposed approach is tested on a sixbus IEEE system. Numerical results show that the setting of OLTC transformer in terms of the load model has a major effect on the maximum power transfer in power systems and the proposed ANN technique is very accurate and reliable. The adaptive settings of OLTC improve the power transfer capability according to the system operating condition. Keywords: OLTC, Static Load, Voltage Stability, ANN.

A novel line drop secondary voltage control algorithm for variable speed wind turbines

AbstractThis paper addresses the design and implementation of the line drop secondary voltage control (LDSVC) for the doubly fed induction generator‐wind turbine (DFIG‐WT) complemented with reactive power allocation algorithm to achieve more efficient voltage regulation, reactive power compensation and to enhance the transient stability margin of the electric power system. The LDSVC is used to generate the local voltage reference, providing an improvement for overall voltage profile. The paper presents the influence of the integration of variable speed wind turbines‐based doubly fed induction generator (DFIG) while employing LDSVC for increasing the transient stability margin. This paper proposes an improved voltage control scheme, based on a secondary voltage controller complemented with an automatic gain controller (AGC). The scheme is applied to a wind energy system incorporating DFIG‐based wind turbines. The controller structure is developed and the performance of the self‐tuning AGC scheme is developed and analysed. The proposed controller is tested in response to system contingencies for different short circuit ratios. The performance of the secondary voltage control without and with AGC is verified. The influence of the AGC in improving the transient response and damping of voltage oscillations is verified. Copyright © 2010 John Wiley & Sons, Ltd.

Automatic Gain Controller for Variable Speed Wind Turbines

This paper presents a novel controller for DFIG based wind parks, designed to achieve more efficient voltage regulation, reactive power compensation and to enhance the transient stability margin of the interconnected power system. The supervisory-secondary voltage control is used to generate the local voltage reference, providing an improved overall voltage profile, while combining an automatic gain controller (AGC) to improve the transient response of the primary control loop. The controller is implemented and tested with a power system comprising of a lumped, fundamental frequency model of a DFIG based wind park, and hydro and diesel generators connected to the electric grid. The performance of the controller was investigated for both steady-state improvements as well as under extreme contingencies to demonstrate its benefits.

A method for computing minimum voltage stability margins of power systems

A new and fast method for computing the minimum voltage stability margin (VSM) of power systems is presented. The computation of the VSM is required for power systems planning and operation. Usually, it is assumed that loads increase along a predefined direction (e.g. with constant power factor, followed by a proportional MW generation increase) up to the system's maximum loading point is reached. Situations may occur where variations from the predefined load increase direction, for example an unexpected load increase at some bus or area may result in smaller VSM, taking the system to an insecure state. The computation of the minimum VSM (mVSM) allows forecasting the load increase worst scenario. The information regarding the mVSM and the corresponding load increase direction for which it occurs, along with the usual VSM, allows operators to take measures like preventive control actions to move the system to securer operating points. Simulation results for IEEE test systems and for realistic systems are shown. Also, the proposed method is compared with other methods found in the literature.

A robust adaptive LQG/LTR TCSC controller applied to damp power system oscillations

This paper investigates the use of self-tuning Linear Quadratic Gaussian control with Loop Transfer Recovery (LQG/LTR), applied in Thyristor Controlled Series Capacitors (TCSC) installed in interconnected power systems. The proposed robust adaptive controller can improve power system stability margins, damping properly the system's dominant electromechanical modes. Although this TCSC controller is designed mainly to identify and damp inter-area modes, it does not affect negatively the local modes damping, therefore enhancing the overall system performance. The proposed controller properties can be verified using non-linear simulations for a four-machine power system, in different operation conditions.

VOLTAGE STABILITY ASSESSMENT AND ENHANCEMENT OF THE THAILAND POWER SYSTEM

This paper presents voltage stability study of the Thailand power system by considering the existing Static Var Compensators, generation and load directions. An approach to determine appropriate voltage settings of existing Static Var Compensators is proposed in order to maximize static voltage stability margin of the system. Two most recent methods, namely Maximum Loading Margin and realistic load direction methods, are applied to the Thailand power system in order enhance loading margin of the system in a practical way. With the applications of the proposed and existing techniques, the highest and practical voltage stability margin of Thailand power system is obtained. This provides a guide for electric power utilities to enhance voltage stability margin of power systems in a practical way.