Voltage Stability Criteria Research Articles

Preventive control approach for voltage stability improvement using voltage stability constrained optimal power flow based on static line voltage stability indices

Voltage stability improvement is a challenging issue in planning and security assessment of power systems. As modern systems are being operated under heavily stressed conditions with reduced stability margins, incorporation of voltage stability criteria in the operation of power systems began receiving great attention. This study presents a novel voltage stability constrained optimal power flow (VSC-OPF) approach based on static line voltage stability indices to simultaneously improve voltage stability and minimise power system losses under stressed and contingency conditions. The proposed methodology uses a voltage collapse proximity indicator (VCPI) to provide important information about the proximity of the system to voltage instability. The VCPI index is incorporated into the optimal power flow (OPF) formulation in two ways; first it can be added as a new voltage stability constraint in the OPF constraints, or used as a voltage stability objective function. The proposed approach has been evaluated on the standard IEEE 30-bus and 57-bus test systems under different cases and compared with two well proved VSC-OPF approaches based on the bus voltage indicator L - index and the minimum singular value. The simulation results are promising and demonstrate the effectiveness of the proposed VSC-OPF based on the line voltage stability index.

Voltage Stability Criterion in New and Key Nodes and Regional Research

In recent years， as the load increasing gradually，problems in voltage stability of regional power network which is the terminal of power grid is serious. From the voltage stability machine rational analysis to roll out small interfering voltage stability criterion of new, and the modal method is used to determine the key nodes and areas of the system theory basis.

Voltage Stability Criterion in New and Key Nodes and Regional Research

In recent years， as the load increasing gradually，problems in voltage stability of regional power network which is the terminal of power grid is serious. From the voltage stability machine rational analysis to roll out small interfering voltage stability criterion of new, and the modal method is used to determine the key nodes and areas of the system theory basis.

A Synchrophasor Assisted Frequency and Voltage Stability Based Load Shedding Scheme for Self-Healing of Power System

This paper presents a new scheme for load curtailment in the power system required for its self-healing under critical contingencies, which may pose a threat to frequency as well as the voltage stability of the system. The load shedding requirement in the system has been calculated based on the computed disturbance power as well as the voltage stability condition of the system, with the aid of real-time data, assumed to be available from the synchrophasor based wide area monitoring and control system (WAMCS). This scheme assesses voltage stability based on a dynamic voltage stability criterion, formulated using a Voltage Stability Risk Index (VSRI). Suitable locations for the load curtailments have been chosen according to the VSRI, calculated at each load bus. Performance of the proposed scheme has been tested on New England 39-bus system and a practical 246-bus Indian system. The results are compared with a conventional under frequency load shedding scheme and an adaptive load shedding scheme.

Market-clearing for pricing system security based on voltage stability criteria

This paper presents a novel technique for representing system security in the operation of decentralized electricity markets, with special emphasis on voltage stability. The market-clearing algorithm is modeled as voltage stability constrained-optimal power flow (VSC-OPF) problem for providing market solutions by means of a function of complying with the required voltage stability criteria. Benders’ decomposition is applied for solving the VSC-OPF incorporating post-contingency control actions, which is motivated by the improvement of computational efficiency using parallel processing. The proposed VSC-OPF framework also takes into consideration the bilateral contract information, which is integrated into the market-clearing process and, at the same time, the optimal pricing expressions through computing local marginal prices (LMPs) with respect to ensuring voltage stability are derived. VSC-OPF is tested on the IEEE 14-bus benchmark system and the results obtained, when compared to those obtained by means of a conventional OPF, show that the proposed technique is able to improve system security while yielding better market solutions and total transaction levels.

Adaptive Scheme for Minimal Load Shedding Utilizing Synchrophasor Measurements to Ensure Frequency and Voltage Stability

This article presents a new scheme for load curtailment in the power system under critical contingencies that may pose a threat to the frequency and voltage stability of the system. The proposed scheme is based on a two-stage load-shedding strategy. In the first stage, disturbances have been classified based on the computed value of the disturbance power, and the load-shedding requirement in the system has been assessed based on the frequency stability, as well as dynamic voltage stability, conditions. The scheme assumes the availability of real-time data from a synchrophasor-based wide area monitoring and control system. A dynamic voltage stability criterion has been developed in this stage using a voltage stability risk index to assess the voltage stability in real time. Suitable locations for the load curtailments have been chosen according to the voltage stability risk index, which is calculated at each load bus. After the system attains transient stability, the second stage of the proposed scheme determines the optimal redispatching of generators for recovering the loads as much as possible, which had been curtailed in the first stage following the contingency. Performance of the proposed scheme has been tested on a 10-machine, New England 39-bus system, and the results are compared with a conventional under-frequency load-shedding scheme and an adaptive load-shedding scheme.

A global Particle Swarm-Based-Simulated Annealing Optimization technique for under-voltage load shedding problem

In this paper, a new approach based on hybrid Particle Swarm-Based-Simulated Annealing Optimization technique (PSO-B-SA) is proposed for solving under-voltage load shedding (UVLS) problem. Under-voltage load shedding (UVLS) is one of the most important tools for avoiding voltage instability. In this paper, the UVLS problem is formulated based on the concept of the static voltage stability margin and its sensitivity at the maximum loading point or the collapse point. The voltage stability criterion is modeled directly into the load-shedding scheme. In any UVLS scheme finding the global point is very important for having cost effective economy. The proposed PSO-B-SA methodology is implemented in the under-voltage load shedding scheme for IEEE 14 and 118 bus test systems. In addition to having better solution, the global property of the proposed approach plays an important role in on-line applications. Simulation results show the efficacy and advantage of the proposed scheme.

Optimum Reactive Power Dispatch and Identification of Critical On-load Tap Changing (OLTC) Transformers

The voltage stability problem has become a major concern in planning and operation of today's power system as a result of stressed conditions. It is known that the following disturbances, on-load tap changers (OLTCs), act to restore the load bus voltage and thus restore the load power to near pre-disturbance level. But the operation of OLTCs has a significant influence on voltage stability under some operating conditions. This article discusses the effect of OLTCs with different objectives of reactive power dispatch and identify critical OLTCs based on voltage stability criteria. Results obtained on an equivalent system of 24-node EHV system are presented.

Congestion management in open access based on relative electrical distances using voltage stability criteria

This paper presents an approach for alleviation of network over loads in the day-to-day operation of power systems under deregulated environment. The control used for over load alleviation is real power generation rescheduling based on relative electrical distance (RED) concept. The method estimates the relative location of load nodes with respect to the generator nodes. The contribution of each generator for a particular over loaded line is first identified, then based on RED concept the desired proportions of generations for the desired overload relieving is obtained, so that the system will have minimum transmission losses and more stability margins with respect to voltage profiles, bus angles and better transmission tariff. Results obtained for network overload alleviation of suitably modified IEEE 39-bus New England system are presented for illustration purposes.

An enhanced under-voltage load-shedding scheme to provide voltage stability

Under-voltage load shedding is one of the most important tools for avoiding voltage instability. In this paper, an optimal load-shedding algorithm is developed. This approach is based on the concept of the static voltage stability margin and its sensitivity at the maximum loading point or the collapse point. The traditional load-shedding objective is extended to incorporate both technical and economic effects of load shedding. The voltage stability criterion is modeled directly into the load-shedding scheme. The proposed methodology is implemented over the IEEE 14 and 118 bus test systems and solved using a mathematical (GAMS/CONOPT) and two heuristic (PSO and GA) methods. The heuristic techniques are employed only to validate the results.

Interior point models for power system stability problems

In this paper, a detailed analysis of the use of optimization techniques in the study of voltage stability problems, leading to the incorporation of voltage stability criteria in traditional Optimal Power Flow (OPF) formulations is presented. Optimal power flow problems are highly nonlinear programming problems that are used to find the optimal control settings in electrical power systems. The relationship between the Lagrangian Multipliers of the OPF problem and the classification of the maximum loading point level of the system is given. Finally, the paper presents a sequential OPF technique to enhance voltage stability using reactive power and voltage rescheduling with no increase in real (active) generation cost.

BTI characteristics and mechanisms of metal gated HfO/sub 2/ films with enhanced interface/bulk process treatments

Significant deviations in BTI characteristics for metal gate HfO/sub 2/ films compared to silicon oxide based films prove that conventional reliability models based on SiO/sub 2/ films can no longer be directly applied to HfO/sub 2/ based MOSFETS. This study shows the use of conventional accelerated reliability testing in the Fowler-Nordheim tunneling regime to extrapolate time to failure at operating voltages (direct tunneling regime) overestimates device lifetimes. Additionally, unlike conventional gate oxides, the slope of /spl Delta/V/sub t/ versus time (or the rate of charge trapping) in HfO/sub 2/ MOSFETS is dependent on stress voltage. The HfO/sub 2/ based metal gated nMOSFETS show poor PBTI characteristics and do not meet the 10 year lifetime criterion for threshold voltage stability. On the other hand, HfO/sub 2/ based pMOSFETS show superior NBTI behavior and meet the 10 year lifetime criterion. These results are contrary to the observations with conventional gate dielectrics. This paper explores the anomalous charge trapping behavior and provides a comprehensive study of the PBTI characteristics and recovery mechanisms in metal gated HfO/sub 2/ films.

Reactive reserve-based contingency constrained optimal power flow (RCCOPF) for enhancement of voltage stability margins

This paper presents a new concept of reactive reserve-based contingency constrained optimal power flow (RCCOPF) for voltage stability enhancement. This concept is based on the fact that increase in reactive reserves is effective for enhancement of voltage stability margins of post-contingent states. In this paper, the proposed algorithm is applied to voltage stability margin of interface flow. Interface flow limit, in the open access environment, can be a main drawback. RCCOPF for enhancement of interface flow margin is composed of two modules, modified continuation power flow (MCPF) and optimal power flow (OPF). These modules are recursively performed until satisfying the required margin of interface flow in the given voltage stability criteria.

Dynamic and static voltage stability enhancement of power systems

The static voltage stability and the dynamic voltage stability are integrated. To consider a shunt compensator and/or a more general load which is dependent on voltage, a modified criterion for the static voltage stability is developed and the system is controlled to satisfy this static stability criterion. For an accurate analysis of the dynamic voltage stability, the system model includes excitation systems, tap-changers, capacitors and power system stabilizers in addition to network equations. A parameter optimization technique with a modal performance measure is developed to determine optimal control parameters for dynamic voltage stability enhancement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Voltage stability as affected by static load characteristics

Voltage stability is analysed as a dynamic phenomenon associated with the excitation field dynamics of synchronous machines. It is shown that, for constant impedance loads, a single machine powersystem is voltage stable for all reasonable operating conditions. For constant power loads, however, a simple system has a voltage controllability limit that can be close to nominal voltage. The voltage stability region as a function of real and reactive load dependence on voltage is calculated for a single machine system with and without automatic voltage regulation. The analysis is then extended to the multimachine case, for which approximate voltage stability criteria are developed using the extended linearisation coefficient matrices. The linearised model analysis is found to be absolutely consistent with the results obtained by nonlinear simulation of a multimachine power system.< >

New contingency ranking technique incorporating a voltage stability criterion

A new technique for ranking outages in power systems is presented. Traditional methods for ranking are limited to cases where the system state under the respective contingency condition can be simulated. Cases where a load-flow solution is unobtainable are unexplained. The new method presented in the paper can similarly rank all cases of contingencies, and cases that can cause voltage instability and system collapse are revealed. The information given by this new method is important because it can then attract the attention of the planning engineer to the problem of voltage instability that may exist in a power system. Thus power systems can now be designed to take into account the voltage instability problem as well as the normal security requirements.

Deterministic and Probabilistic Approach to Voltage Stability of Series-Compensated EHV Transmission Lines

The paper describes a method for evaluation of voltage stability of EHV lines with series es compensation taking into consideration uncertainty of the load parameters. The results obtained are based on a newly def ined dV/dK criterion for voltage stability. Voltage stability of system with different types of loads is considered. The method of analysis and practical application are discussed and an example is included.

The voltage transfer curve and stability criteria in the theory of the AC plasma display

At an ac plasma display discharge site the voltage transfer curve and the locus of the equilibrium points establish conditions which govern the change of wall voltage toward or away from equilibrium levels. Analysis based on simple geometric ideas leads to a precise description of these conditions, and in the neighborhood of the equilibrium levels, the conditions reduce to known results.

Voltage stability of radial power links

The general phenomenon of voltage collapse or instability at the load end of transmission links is introduced with reference to previous work, and the induction motor is seen to be the critical constituent of system loads. The power/voltage and reactive-power/voltage characteristics of power-system loads are predicted and accurately represented by polynomial expressions which are used in the analysis of radial transmission links fed from infinite busbars. Graphs summarising the critical load voltages and the corresponding short-circuit levels for various transformer tap ratios are given and are general in application; these are based on the dV/dZ criterion for voltage stability. It is shown that, although tap changing to raise the load voltage increases the critical length of the link, it also reduces the margin between normal operating voltage and the voltage at the onset of instability. The effects of the injection of reactive power by static capacitors are discussed, together with the effect of the induction-motor content of the loads. The use of the dEs/dVr criterion as advocated in the Russian literature is discussed.