Loading Margin Research Articles

00/01908 Comparison of feature selection techniques for ANN-based estimation

Voltage stability problems have been one of the major concerns for electric power utilities due to increased interconnections and loading of the present day power system. Fast estimation of loadability margin is essential for evaluating on-line voltage stability condition of a power system. In this paper, an approach based on parallel self-organizing hierarchical neural network is presented to predict a maximum loadability margin which is an indication of the power system’s proximity to voltage collapse. Parallel self-organizing hierarchical neural network (PSHNN) are multi-stage neural networks in which stages operate in parallel rather than in series during testing. The revised back-propagation algorithm is used, along with forward–backward training of stage neural networks. Input features for PSHNN are selected on the basis of entropy concept in one method while in the other method, real and reactive loads at critical buses are considered as the inputs for PSHNN. The proposed PSHNN based methods are compared by estimating the maximum loadability margin at different loading conditions in IEEE 30-bus and a practical 75-bus Indian system. Entropy based PSHNN learns faster, at the same time it provides more accurate loadability margin estimation as compared to that based on critical buses. It is found to be suitable for on-line applications in energy management systems.

Crust‐mantle decoupling by flexure of continental lithosphere

Mechanical decoupling between the crust and upper mantle has been proposed as an explanation for anomalously low effective elastic thicknesses (<20 km) locally associated with thermally mature (∼1 Ga) continental lithosphere. The processes and consequences of crust‐mantle decoupling are investigated with a fully dynamic elastic‐viscous‐plastic (EVP) finite element model of orogenic loading and foreland basin subsidence. The basic dependencies of continental flexure on lithospheric thermal state, crustal thickness, and load magnitude are determined and are characterized by the best fit elastic thickness for the simulated deflections. Additional variables such as loading rate, viscoelastic stress relaxation, the boundary condition at the underthrust edge of the plate, and the lithospheric rheology also influence the flexural signature, and these sensitivities are likewise defined by EVP simulations. Decoupling can result in substantial reductions in the effective elastic thickness‐up to a factor of 2 for lithosphere with a thermal age of 1 Ga. Elastic‐viscous‐plastic models show that lower crustal weakening occurs by locally enhanced creep driven by high shear stresses in the middle lithosphere at the load margin. Crust‐mantle decoupling in these models is fundamentally controlled by the temperature and the rheological contrast at the Moho. Previous studies of flexural decoupling have employed simplified multilayered elastic or elastic‐plastic methods, the latter using a yield strength envelope with aprescribed strain rate and an assumption of complete slip at the Moho. However, lithospheric rheology is inherently stress and time dependent, with the deformation rate varying both spatially and temporally. Comparison of elastic‐plastic solutions to the EVP simulations indicates that the former method is satisfactory for oceanic and coupled continental lithosphere but performs poorly with decoupled lithosphere. The EVP model is marginally successful at achieving effective elastic thicknesses <20 km for ∼l‐Ga lithosphere; additional reductions in the thickness may be achieved by a more thorough treatment of the thermal evolution of underthrust continental lithosphere.

Voltage collapse risk assessment

Abstract Several approaches have been proposed in the literature to analyze system voltage collapse, based on indices that can efficiently evaluate the load margin for which the system remains stable. All this discussion, including possible methods of solution, is being carried out under deterministic assumptions; load, generation and transmission are defined for each operating condition. The basic idea is to obtain the maximum amount of load the system can support, and what distance it is from this condition. In this work, the voltage collapse problem is reviewed, considering the stochastic nature of the load. A probabilistic methodology is developed to provide risk indices of voltage collapse, based on a combination of the tangent vector and Monte Carlo simulation methods. It is also shown how a probabilistic load flow analysis can be adapted to evaluate the proposed risk indices. In order to illustrate the proposed approach, a modification of the IEEE reliability test system is used and the applicability of the results to operation and expansion planning is discussed.

Estimation of loadability margin using parallel self-organizing hierarchical neural network

Voltage stability problems have been one of the major concerns for electric power utilities due to increased interconnections and loading of the present day power system. Fast estimation of loadability margin is essential for evaluating on-line voltage stability condition of a power system. In this paper, an approach based on parallel self-organizing hierarchical neural network is presented to predict a maximum loadability margin which is an indication of the power system’s proximity to voltage collapse. Parallel self-organizing hierarchical neural network (PSHNN) are multi-stage neural networks in which stages operate in parallel rather than in series during testing. The revised back-propagation algorithm is used, along with forward–backward training of stage neural networks. Input features for PSHNN are selected on the basis of entropy concept in one method while in the other method, real and reactive loads at critical buses are considered as the inputs for PSHNN. The proposed PSHNN based methods are compared by estimating the maximum loadability margin at different loading conditions in IEEE 30-bus and a practical 75-bus Indian system. Entropy based PSHNN learns faster, at the same time it provides more accurate loadability margin estimation as compared to that based on critical buses. It is found to be suitable for on-line applications in energy management systems.

A critical evaluation of step size optimization based load flow methods

In this paper, a critical evaluation of three Newton load flow methods based on step size optimization is carried out. These methods are based on the computation of a scalar multiplier at each iteration, which is multiplied by the voltage correction vector so as to minimize a power mismatch based quadratic function. Special attention was paid to evaluating the performance of these methods for ill-conditioned, heavily loaded and overloaded systems. The possibility of using them as general load flow tools is also discussed. These methods can also play an important role in voltage stability analysis methods for the determination of load margins to voltage collapse.

Minimization of series reactive power loss for the voltage instability problems

Abstract This paper introduces a method to find the location of series capacitors to minimize the total series reactive power loss (SRPL) of a power system. Since the increase in reactive power loss has been used as an indicator for the voltage instability, a newly developed indicator d Q loss i − j /d K s is introduced to calculate the decrease in SRPL as a result of series compensation. The proposed method is simulated on one− and two-area systems and the results are discussed and found to be consistent. The divergence of the power flow solution is taken to check the increase in load margin, when the resulting lines are compensated.

Contingency ranking for voltage collapse via sensitivities from a single nose curve

The change in the loading margin to voltage collapse when line outages occur is estimated. First a nose curve is computed by continuation to obtain a nominal loading margin. Then linear and quadratic sensitivities of the loading margin to each contingency are computed and used to estimate the resulting change in the loading margin. The method is tested on a critical area of a 1390 bus system and all the line outages of the IEEE 118 bus system. The results show the effective ranking of contingencies and the very fast computation of the linear estimates.

Sensitivity of the loading margin to voltage collapse with respect to arbitrary parameters

Power system loading margin is a fundamental measure of a system's proximity to voltage collapse. Linear and quadratic estimates to the variation of the loading margin with respect to any power system parameter or control are derived. Tests with a 118-bus system indicate that the estimates accurately predict the quantitative effect on the loading margin of altering the system loading, reactive power support, wheeling, load model parameters, line susceptance and generator dispatch. The accuracy of the estimates over a useful range and the ease of obtaining the linear estimate suggest that this method will be of practical value in avoiding power system voltage collapse.

Look-ahead voltage and load margin contingency selection functions for large-scale power systems

Given the current operating condition (obtained from the real-time data), the near-term load demand at each bus (obtained from short-term load forecast), and the generation dispatch (say, based on economic dispatch), we present in this paper a load margin measure (MW and/or MVAr) to assess the system's ability to withstand the forecasted load and generation variations. We also present a method to predict near-term system voltage profiles. The proposed look-ahead measure and the proposed voltage prediction are then applied to contingency selections for the near-term power system in terms of load margins to collapse and of the bus voltage magnitudes. We evaluate the proposed look-ahead measure and the voltage profile prediction on several power systems including a 1169-bus power system with 53 contingencies with promising results.

Some effects of lateral heterogeneities in the upper mantle on postglacial land uplift close to continental margins

We investigate the effects of lateral heterogeneities in the upper mantle on the calculation of postglacial land uplift. For the model calculations we use a commercial finite-element code, which enables us to solve the equations governing a layered, isotropic, incompressible, Maxwell-viscoelastic half-space with laterally varying layer thicknesses and physical properties. Following previous investigations performed by Sabadini, Yuen & Portney (1986) and Gasperini & Sabadini (1989), we extend their results using a more realistic loading history and different earth models. We then focus our attention on the question whether lateral heterogeneities in the upper mantle can be modelled correctly using a set of homogeneous earth models. To this end, a comparison of model calculations using both laterally homogeneous and heterogeneous earth models is performed. We find that lateral heterogeneities in the upper mantle significantly influence the calculated postglacial land uplift. The resolving power of relative sea-level observations for the prescribed lateral heterogeneities used in this study is mainly focused on observations around the load margin and outside the glaciated areas, where differences in predicted land uplift between individual models are large enough to be resolved by observations. We can qualitatively determine lateral heterogeneities in the upper mantle using a set of laterally homogeneous earth models, if the geological structure, for example a continental margin, is known. However, in order to infer the correct values of lithospheric thickness and asthenospheric viscosity, we need to use laterally heterogeneous models.

A method to approximate a closest loadability limit using multiple load flow solutions

A new method is proposed to approximate a closest loadability limit (CLL), or closest saddle node bifurcation point for power systems, using a pair of multiple load flow solutions. More strictly, the obtainable points by the method are the stationary points including not only CLL but also farthest and saddle points. An operating solution and a low voltage load flow solution are used to efficiently estimate the node injections at a CLL as well as the left and right eigenvectors corresponding to the zero eigenvalue of the load flow Jacobian. They can be used in monitoring loadability margin, in identification of weak spots in a power system and in the examination of an optimal control against voltage collapse. Most of the computation time of the proposed method is taken in calculating the load flow solution pair. The remaining computation time is less than that of an ordinary load flow.

Optimizing valve actuator parameters to enhance control valve performance

The market demands for higher quality products have increased the need for reduced process variation. One factor in the quality of the final end product is the improvement of the control loop performance. A critical component in the loop is the final control element, the control valve package. Optimized actuator parameters play a vital role in the dynamic performance of the control valve. This paper reviews the pneumatic actuator and positioner parameters that effect the control package performance. This is done through the use of a control valve package computer model to assess the dynamic performance. The attributes of spring return versus double acting actuators are illustrated. The effects of supply pressure, step size, load margin, and flow, actuator volume and design style are investigated through the use of mathematical simulations of control valve dynamic performance.

Fracture analysis of an all-ceramic bearing system

This paper summarizes the fracture mechanism of an all-ceramic duplex spin bearing which was tested to failure to establish a design load margin. This bearing is part of a gyro-optics assembly being developed for use in an infrared seeker. The analysis revealed that machining-induced microcracks grew in the inner raceway beneath a ball. These microcracks coalesced to form macrocracks which led to fracture of the inner race and the failure of the bearing system.

95/03697 Wonkey waves — A sine of the future

This study proposed an improved continuation power flow (CPF) model to calculate the load margin for voltage stability. A self-adjusting step-size controlling scheme of CPF, which is based on the iterations in the corrector and the reactive power reserve of generator buses, is presented. The load margin here is defined either by encountering the saddle node bifurcation (SNB) or the limit-induced bifurcation (LIB). To confirm the LIB, a rapid approach is proposed to calculate the sensitivity of voltage magnitude to reactive power injection change for load buses. Because CPF with distributed slack bus (DSB) could generate different load margin results corresponding to various allocations of imbalanced power, optimization techniques should be adopted in the search for maximizing the load margin. Therefore, the improved CPF is integrated with an evolutionary mechanism-based particle swarm optimization (PSO) method via coordinate transformation. Furthermore, parallel processing is deployed in the programming for high-performance computing. The case studies for the IEEE 5-bus and IEEE 14-bus test systems demonstrate the efficiency of the proposed approaches.

System loadability and load shedding

Abstract System loadability is evaluated using a nonlinear programming technique, and a loadability margin is determined for purposes of static voltage stability assessment. The margin is either in terms of voltampere load or VAR load. Generation participation is adjusted in order to produce higher system loadability. The effect of the forced outage of transmission elements on system loadability is studied. The method is also used to obtain the complete nose curve (upper and lower sides) for particular load buses. The curves indicate the system's strength in terms of potential voltampere or reactive system load as a function of voltage magnitude at a particular bus. A load shedding strategy that maximizes the system reactive security margin is introduced.

Point of collapse methods applied to AC/DC power systems

The authors describe an extension of the point of collapse method developed for studies of AC systems to the determination of saddle-node bifurcations in power systems including high voltage direct current (HVDC) transmission. Bus voltage profiles are illustrated for an AC/DC test system. They significantly differ from the profiles of pure AC systems for typical system models. In particular, voltage dependent current order limits are shown to affect the voltage profiles and the loadability margin of the system. It is also shown that Hopf bifurcations, which are possible in purely AC lossless systems with second-order generator models, become plausible when the dynamics for the HVDC system are included. >

大規模電力系統の電圧安定性解析と運用指標

In order to prevent the voltage collapse on bulk power system considered to be originated by long distance and heavily loaded transmission line between generating site and load center accompanied with the increase of voltage-independent type of load, it is needed to clarify the dynamic behavioures of collapse and recovery process of system voltage, and to establish a suitable guide for system operation against the voltage collapse possibility. This report describes the newly developed calculation method to clarify the phenomena which gives not only the simulation of dynamic behavioures of system voltage but also the evaluation of load margin for any given condition of power system as to the basic index to keep the system voltage stable.

An Exploratory Study of Shear Fatigue Behavior of Prestressed Concrete Girders

T he use of pretensioned concrete beams in bridge superstructures has increased dramatically in the past 30 years. At the same time, strength reduction factors and load limits have been increased, effectively reducing load margins for existing bridges. As a result, the limit state of fatigue failure is now a more important consideration in the evaluation of existing bridges and the design of new bridges. At the present time, engineers have few guidelines to follow for fatigue design of pretensioned concrete members. This paper describes the results of an exploratory investigation undertaken in the Phil M. Ferguson Structural Engineering Laboratory at The University of Texas at Austin to gain a better understanding of the shear frtigue behavior of pretensioned concrete girders. At present, little experimental research has been conducted on shear fatigue. The authors are unaware of any previous shear fatigue tests on full scale specimens designed according to present design specifications. The present investigation was very limited in scope, involving tests of only three full scale girder specimens. Obviously, the wide range of variables actually present in pretensioned bridge girder usage could

Assessment of critical voltage and load margins in VAR-compensated power transmission systems

An electrical power network is examined with respect to steady state limits on power transfer and voltage stability as affected by shunt compensation. Subject to certain assumptions concerning system structure and operation, expressions are derived for critical voltage and power margins. Typical results are illustrated. The approach is intended primarily for use in connection with long-term planning of VAR support in transmission systems.