Loading Margin Research Articles

A Maximum Loading Margin Method for Static Voltage Stability in Power Systems

In this paper, the maximum loading margin (MLM) approach is proposed in finding generation directions to maximize the static voltage stability margin, where the MLM is evaluated at various possible generation directions in the generation direction space. An approximate and simple model representing the relationship between the generation direction and the LM is used to obtain the MLM point. The proposed method is validated in the modified IEEE 14-bus test system and applied to the Thailand power system. LMs of the system with the generation directions are compared for different generator combinations using the proposed technique.

Oscillatory Stability Limit Prediction Using Stochastic Subspace Identification

Determining stability limits and maximum loading margins in a power system is important and can be of significant help for system operators for preventing stability problems. In this paper, stochastic subspace identification is employed to extract the critical mode(s) from the measured ambient noise without requiring artificial disturbances (e.g., line outages, generator tripping, and adding/removing loads), so that the identified critical mode may be used as an online index to predict the closest oscillatory instability. The proposed index is not only independent of system models and truly represents the actual system, but it is also computationally efficient. The application of the proposed index to several realistic test systems is examined using a transient stability program and PSCAD/EMTDC, which has detailed models that can capture the full dynamic response of the system. The results show the feasibility of using the proposed identification technique and index for online detection of proximity to oscillatory stability problems.

An economical generation direction for power system static voltage stability

This paper proposes an economical generation direction for static voltage stability. The proposed generation direction minimizes the total operating cost at any loading level, up to the point of collapse. The proposed approach, named as EGD approach, is based on the economic load dispatch with load flow. Two alternative methods to identify the economic generation direction are proposed. Economical generation direction is used in the continuation power flow process to obtain the static voltage stability margin. Static voltage stability margin with economical generation direction is compared with the margins resulting from other existing generation directions in the modified IEEE 14-bus test system under various system operating conditions. Cost of providing loading margin as well as PV curves and losses under different generation directions resulting from various generation direction approaches are also studied and compared. The proposed approach provides an alternative way for utilities to obtain the lowest total operating cost with voltage stability constraint by using any existing commercial software.

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.

A corrective load shedding scheme to mitigate voltage collapse

Voltage stability is concerned with the ability of a power system to maintain acceptable voltages at all buses. A measure of the power system voltage stability is the distance to the saddle node bifurcation of the power flow equations, which is called the load margin. When the power system load is very high, and/or there exists a large generation-demand imbalance in the power system areas the load margin to the saddle node bifurcation may be too low, and the power system may become close to voltage collapse. In case that active and reactive power generation resources in the importing areas are exhausted, corrective load shedding may become the last option. This paper presents a LP-based optimization load shedding algorithm to improve the load margin. The objective function consists of minimizing the total system demand decrease. First order sensitivities of the load margin with respect to the load to be shed are considered. The performance of the method is illustrated with a scenario of the Spanish power system.

Acoustical Analysis of Snoring: Can the Probability of Surgical Success be Optimized?

This paper describes an algorithm for rescheduling reactive power control variables so as to have an adequate loadability margin for current operating point. Co-ordinated aggregation based PSO (CAPSO) has been used as optimization technique. Two objective functions have been selected for load margin enhancement. One is minimization of total reactive power loss and other considers minimization of incremental reactive power loss. The developed algorithm has been implemented on 14-bus and 57-bus standard test systems. Results obtained have been compared with those obtained using Davidon–Fletcher–Powell’s (DFP) optimization technique. CAPSO performance is better over DFP method. It has been observed that incremental loss minimization formulation does not require additional voltage stability related constraints hence computational time required is less as compared to reactive power loss minimization. Reactive power loss minimization formulation is of significance when limited reactive power source is available.

Available transfer capability calculation considering voltage stability margin

To make the electricity trades carry out successfully, the calculation of available transfer capability (ATC) must coordinate the relationship between the security and economic benefits. In this paper, a model for ATC calculations accorded with trade-off mechanism in electricity market was set up. The impact of branch outage contingency on the static voltage stability margin was analyzed, and contingency ranking was performed through sensitivity indices of branch flows with respect to the loading margin. Optimal power flow based on primal–dual interior point method was applied to obtain ATC when the N − 1 security constraints were included. The calculation results of IEEE 30-bus and IEEE 118-bus systems show that the proposed model and method are valid.

Loadability margin calculation of power system with SVC using artificial neural network

Voltage stability has become a major concern among the utilities over the past decade. With the development of FACTS devices, there is a growing interest in using these devices to improve the stability. In this paper a method using parallel self-organizing hierarchical neural network (PSHNN) is proposed to estimate the loadability margin of the power system with static var compensator (SVC). Limits on reactive generations are considered. Real and reactive power injections along with firing angle of SVC and bus voltage at which SVC is connected, are taken as input features. To improve the performance of network, K-means clustering is employed to form the clusters of patterns having similar loadability margin. To reduce the number of input features in each cluster, system entropy information gain method is used and only those real and reactive power injections, which affect the loadability margin most, are selected. Separate PSHNN is trained for each cluster. The proposed method is implemented on IEEE-30 bus and IEEE-118 bus system. Once trained, the network produces the output, with accuracy and speed. The computation time is also independent of the system size and the load pattern.

Maximizing Static voltage stability margin in power systems using a new generation pattern

In this paper, the maximum loading margin approach for generation directions in static voltage stability margin is proposed based on the maximum loading margin at various possible generation directions in the generation direction space. An equation representing the relationship between generation directions and loading margin is used to obtain the maximum loading margin point. The proposed method is validated in the modified IEEE 14-bus test system. Loading margins of the system with the generation directions are compared for different generator combination using the proposed technique.

An effective method to locate series compensation for voltage stability enhancement

Series compensation of transmission lines is one of the most effective means to increase the loading margin of an interconnected power system. In this paper, a methodology for the identification of the critical transmission lines with respect to voltage stability issues via sensitivity analysis is proposed. The methodology is applied to the Hellenic interconnected transmission system in normal operation and in ( N−1) configurations taking into account its future expansion plans.

Real Power Losses Reduction and Loading Margin Improvement via Continuation Method

This letter presents an alternative approach for reducing the total real power losses by using a continuation method. Results for two simple test systems and for the IEEE 57-bus system show that this procedure results in larger voltage stability margin. Besides, the reduction of real power losses obtained with this procedure leads to significant money savings and, simultaneously, to voltage profile improvement. Comparison between the solution of an optimal power flow and the proposed method shows that the latter can provide near optimal results and so, it can be a reasonable alternative to power system voltage stability enhancement.

04/01862 Improving the steady-state loading margin to voltage collapse in the North-West control area of the Mexican power system: Calderón-Guizar, J. G. et al. International Journal of Electrical Power & Energy Systems, 2003, 25, (8), 643–649

04/01862 Improving the steady-state loading margin to voltage collapse in the North-West control area of the Mexican power system: Calderón-Guizar, J. G. et al. International Journal of Electrical Power & Energy Systems, 2003, 25, (8), 643–649

Steady-State and Dynamic Approaches for the Evaluation of Loadability Margins in the Presence of Secondary Voltage Regulation

The electricity markets are changing the power system operation. The increasing power exchanges make it necessary to operate the transmission grids closer and closer to security limits. A problem posed to power system engineers is therefore to find a suitable methodology to combine the results of both steady-state and dynamic tools: this can reduce the overall computational effort and the difficulty in the interpretation of results. The paper presents a detailed comparison of the computations that can be performed through steady-state and dynamic procedures regarding the power system security. In particular, the analysis of the loadability margins available on the corridor between the Italian power system and the Union for the Coordination of the Transmission of Electricity (UCTE) grids is carried out using both steady-state and dynamic tools; the results are compared, pointing out also the security enhancement given by a hierarchical voltage control.

Comparison of Shunt Capacitor, SVC and STATCOM in Static Voltage Stability Margin Enhancement

This paper compares the shunt capacitor, SVC and STATCOM in static voltage stability improvement. Various performance measures are compared under different operating system conditions for the IEEE 14 bus test system. Important issues related to shunt compensation, namely sizing and installation location, for exclusive load margin improvement are addressed. A methodology is also proposed to alleviate voltage control problems due to shunt capacitor compensation during lightly and heavily loaded conditions.

Enhanced look-ahead load margin estimation for voltage security assessment

An enhanced method for estimating look-ahead load margin to voltage collapse, due to either saddle-node bifurcation or the limit-induced bifurcation, is proposed. The enhanced method contains improved schemes for accurately estimating the look-ahead load margins to bifurcation points. In addition, the enhanced method proposed in this paper can resolve the difficulty associated with the missing alarm problem. Two new strategies, sensitivity-based selection scheme and the reselection scheme are developed and incorporated into the enhanced method. The enhanced method is applied to a practical 3000-bus power system with promising results.

Multiobjective optimal power flows to evaluate voltage security costs in power networks

In this paper, new optimal power flow (OPF) techniques are proposed based on multiobjective methodologies to optimize active and reactive power dispatch while maximizing voltage security in power systems. The use of interior point methods together with goal programming and linearly combined objective functions as the basic optimization techniques are explained in detail. The effects of minimizing operating costs, minimizing reactive power generation, and/or maximizing loading margins are then compared in both a 57-bus system and a 118-bus system, which are based on IEEE test systems and modeled using standard power flow models. The results obtained using the proposed mixed OPFs are compared and analyzed to suggest possible ways of costing voltage security in power systems.

Improving the steady-state loading margin to voltage collapse in the North-West Control Area of the Mexican Power System

This paper reports the application of a static approach for assessing the steady-state loading margin to voltage collapse of the North-West Control Area (NWCA) of the Mexican Power System. The approach uses both optimal load flow (OLF) and conventional load flow (LF) solutions, and singular value decomposition of the load flow Jacobian matrix ( J). Additionally, the approach allows to determine suitable locations for corrective actions such as, the addition of new equipment or load shedding. The results shows that the combination of OLF and LF resulted in a steady-state loading margin to voltage collapse of the NWCA 7.2% higher than the case when only conventional load flow solutions were considered.

A New Approach for Preventive/Corrective Control Strategy to Avoid Voltage Collapse Based on FACTS Devices Investment

This paper proposes a new method of optimal FACTS devices allocation to be used in the preventive/corrective control (PCC) against voltage collapse. The new formulation takes into consideration the transition states that the power system may expose such as normal state and contingency states. For the severe contingencies that drive the system to voltage collapse, the corrective control is initiated by means of fast response of FACTS and load shedding to restore the system with small positive load margin, then the preventive control is carried out using all the fast and slow devices ensuring security margin and voltage feasibility. For this control strategy, FACTS allocation are determined based on the minimum cost for the installations and the operations.. The problem is formulated as a mixed integer nonlinear programming problem, which is solved using hybrid genetic algorithm/successive liner programming (GA/SLP) approach. The IEEE 57 bus system is tested to demonstrate the effectiveness of the proposed method.

Contingency ranking for voltage collapse using parallel self-organizing hierarchical neural network

On-line monitoring of the power system voltage security has become a vital factor for electric utilities. This paper proposes a voltage contingency ranking approach based on parallel self-organizing hierarchical neural network (PSHNN). Loadability margin to voltage collapse following a contingency has been used to rank the contingencies. PSHNN is a multi-stage neural network where the stages operate in parallel rather than in series during testing. The number of ANNs required is drastically reduced by adopting a clustering technique to group contingencies of similar severity into one cluster. Entropy based feature selection has been employed to reduce the dimensionality of the ANN. Once trained, the proposed ANN model is capable of ranking the voltage contingencies under varying load conditions, on line. The effectiveness of the proposed method has been demonstrated by applying it for contingency ranking of IEEE 30-bus system and a practical 75-bus Indian system.

State-of-the-Art Methods for Electric Power Systems Voltage Stability Analysis

This paper presents a literature survey on the subject of voltage stability analysis of power systems. The survey describes several published methods and techniques used to determine voltage stability indices. These indices predict proximity to voltage instability and collapse problems. The Q-V and P-V curves; singular value decomposition; modal analysis; test function; reduced determinant; loading margin by multiple power flow solutions; local load margins; thevenin/load impedance; and energy function are the methods which have been decribed in the paper. The methods described are based on the original work that first proposed them. They are based on the power-flow system model, where the variation of real and reactive powers are assumed to be the main parameters driving the system to voltage instability. Some of the described methods were applied on the IEEE 30-bus power system.