Overload Alleviation Research Articles

Enhanced Genetic Algorithm Approach for Security Constrained Optimal Power Flow Including FACTS Devices

� Abstract—This paper presents a genetic algorithm based approach for solving security constrained optimal power flow problem (SCOPF) including FACTS devices. The optimal location of FACTS devices are identified using an index called overload index and the optimal values are obtained using an enhanced genetic algorithm. The optimal allocation by the proposed method optimizes the investment, taking into account its effects on security in terms of the alleviation of line overloads. The proposed approach has been tested on IEEE-30 bus system to show the effectiveness of the proposed algorithm for solving the SCOPF problem. Keywords—Optimal Power Flow, Genetic Algorithm, Flexible AC transmission system (FACTS) devices, Severity Index (SI), Security Enhancement, Thyristor controlled series capacitor (TCSC). preventive approach for security enhancement, the post contingency state corrective action can also be used for security enhancement. The resulting stage has the same security level as the usual security - constrained optimal power flow case with lower operating cost. The power electronics-based FACTS devices can also be employed for corrective action due to its high speed of response. Thyristor controlled series capacitor (TCSC) is one such device which offers smooth and flexible control of the line impedance, with much faster response compared to the traditional control devices. TCSC can be used effectively in maintaining system security in case of a contingency; by eliminating or alleviating overloads along the selected network branches. It is important to ascertain the location for the placement of these devices due to their considerable costs. In this paper, the location of the TCSC is

Overload Alleviation With Preventive-Corrective Static Security Using Fuzzy Logic

This paper presents a concept overview of an automatic operator of electrical networks (AOEN) for real-time alleviation of component overloads and increase of system static loadability, based on state-estimator data only. The control used for this purpose is real-power generation rescheduling, although any other control input could fit the new framework. The key performance metrics are the vulnerability index of a generation unit (VIGS) and its sensitivity (SVIGS), accurately computed using a realistic ac power flow incorporating the AGC model (AGC-PF). Transmission overloads, vulnerability indices and their sensitivities with respect to generation control are translated into fuzzy-set notations to formulate, transparently, the relationships between incremental line flows and the active power output of each controllable generator. A fuzzy-rule-based system is formed to select the best controllers, their movement and step-size, so as to minimize the overall vulnerability of the generating system while eliminating overflows. The controller performance is illustrated on the IEEE 39-bus (New England) network and the three-area IEEE-RTS96 network subjected to severe line outage contingencies. A key result is that minimizing the proposed vulnerability metric in real-time results in increased substantial loadability (prevention) in addition to overload elimination (correction).

Transmission line overload alleviation due to contingency based on DAG assisted PSO method

Transmission line overload initiate the cascading outages, which forces the system to collapse. The most practiced techniques for overload alleviation is generator rescheduling and/or load shedding. The appropriate selection of generators and load buses to perform the control action for overload alleviation is an important task for the operators. Local optimisation based overload alleviation requires appropriate selection of generator and load buses for minimum control action. This paper presents a new technique for selection of participating generators and load buses using direct acyclic graph (DAG). PSO solved the generator rescheduling and/or load shedding optimisation problem with in security constraints. The effectiveness of the proposed approach is demonstrated for different contingency cases in IEEE 30 and 57 bus systems. The result shows that the proposed approach is computationally fast, reliable and efficient, in restoring the system to normal state after a contingency with minimal control actions.

HYBRID-NEURAL-NETWORK-BASED ESTIMATION OF LINE OVERLOADING

Since the modern power systems are being operated under heavily stressed conditions, the cases of voltage limit violation and power system line overloading are occurring frequently. These events are responsible for several incidents of major network collapses leading to partial or even complete blackouts. Alleviation of line overloads is the suitable corrective action in this regard and for this, fast and accurate identification of overloaded lines is essential along with the estimation of the overloading amount in these lines. In this paper, an approach based on hybrid neural network (HNN) is presented for identification and estimation of line overloading in an efficient manner. The effectiveness of the proposed HNN-based approach is demonstrated by estimation of line overloading for different loading conditions in IEEE 14-bus system. The developed HNN provides accurate and quick results for previously unseen operating condition during testing phase.

Prediction of transmission line overloading using intelligent technique

With the worldwide deregulation of power system, fast line flows or real power (MW) security assessment has become a challenging task for which fast and accurate prediction of line flows is essential. Since last few years, limit violation of voltage and line loading has been responsible for undesirable incidents of power system collapse leading to partial or even complete blackouts. Accurate prediction and alleviation of line overloads is the suitable corrective action to avoid network collapse. The control action strategies to limit the transmission line loading to the security limits are generation rescheduling/load shedding. In this paper, an intelligent technique based on cascade neural network (CNN) is presented for identification of the overloaded transmission lines in a power system and for prediction of overloading amount in the identified overloaded lines. The effectiveness of the proposed CNN based approach is demonstrated by identification and prediction of line overloading for different generation/loading conditions in IEEE 14-bus system. Once the cascade neural network is trained properly, it provides accurate and quick results for previously unseen loading scenarios during testing phase.

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.

Corrective action planning using RBF neural network

In recent years, voltage limit violation and power system load-generation imbalance, i.e., line loading limit violation have been responsible for several incidents of major network collapses leading to partial or even complete blackouts. Alleviation of line overloads is the suitable corrective action in this regard. The control action strategies to limit the line loading to the security limits are generation rescheduling/load shedding. In this paper, an approach based on radial basis function neural network (RBFN) is presented for corrective action planning to alleviate line overloading in an efficient manner. Effectiveness of the proposed method is demonstrated for overloading alleviation under different loading/contingency conditions in 6-bus system and 24-bus RTS system.

A fuzzy control for network overload alleviation

This paper presents a prototype of a fuzzy system for alleviation of network overloads in the day-to-day operation of power systems. The control used for overload alleviation is real power generation rescheduling. Generation Shift Sensitivity Factors (GSSF) are computed accurately, using a more realistic operational load flow model. Overloading of lines and sensitivity of controlling variables are translated into fuzzy set notations to formulate the relation between overloading of line and controlling ability of generation scheduling. A fuzzy rule based system is formed to select the controllers, their movement direction and step size. Overall sensitivity of line loading to each of the generation is also considered in selecting the controller. Results obtained for network overload alleviation of two modified Indian power networks of 24 bus and 82 bus with line outage contingencies are presented for illustration purposes.

Line switching for alleviating overloads under line outage condition taking bus voltage limits into account

This paper presents an interactive line switching algorithm for overload alleviation under line outage condition. Line outage distribution factors are used to determine line flows. It has the advantage that a contingency from a base case can be computed for an actual flow on a line in a real time situation. A relation has been derived for calculating load bus voltages under post-contingency condition and after line switching. In the event of bus voltage limit violation the algorithm explores the possibility of the other line to be switched. The algorithm also investigates the possibility of overload rotation amongst two disjoint sets of overloaded lines obtained after line switching, if overload elimination is not possible. The algorithm has been tested on the IEEE 25 bus system.

Alleviation of transmission system overloads using fuzzy reasoning

Power flows on a large inter-connected power system must be kept within limits to prevent thermal overloading leading to excessive sagging of lines or over-heating of transformers and cables. Flows must also be limited to prevent transient instability. Such control essentially requires the co-ordination of sources of active power and controls of its flow such as quadrature boosters. This paper describes a new approach to this co-ordination problem which utilises a fuzzy expert system. A set of rules is developed to model operators' decisions based on criteria concerning the extent of a limit violation, the sensitivity of a candidate control device, the margin available at the control device, the cost of the required control move and the maximum number of control settings to be moved. Co-ordination of control actions using fuzzy inference and composition is shown to succeed in resolving conflicts while use of fuzzy cardinality enables effective control devices to be ranked. The work is extended to address the duration of overloads so that actions are taken only as and when needed, the scheduling of preventive actions and the meeting of group constraints.

FAST METHOD TO ALLEVIATE LINE OVERLOADS BY CORRECTIVE GENERATION RESCHEDULING AND LOAD SHEDDING

This paper presents simple, efficient and fast technique for the alleviation of line overloads by corrective generation and/or load shedding. The developed method ensures that alleviation of the existing violations does not create any new violations. The proposed algorithm is superior to existing methods and, in some cases, significant smaller amount of load shedding is required. The sensitivity matrix and the Lagrangian function, which are needed in almost all of the existing methods, it is not used in the proposed method. The reconstruction of Jacobian-like matrices, and determination of their inverse at each new operating point during the optimization procedure are not required. It gives less computation time for the proposed method as compared to the existing methods. The proposed technique will find extensive applications in operational planning, security and contingencies studies, and the reliability evaluation of power systems. To validate the proposed method results for 6-bus and 14-bus sample test systems have been presented.

An expert system for alleviation of network overloads

This paper presents a prototype of an expert system for alleviation of network overloads in the day-to-day operation of a power system. The expert system provides a solution in real time for network overload alleviation using phase shifting transformers, and if overload still persists by rescheduling generation and/or load curtailment. The knowledge base of the expert system is obtained by carrying out extensive off-line analysis. An operational load flow (OLF) algorithm is used in the off-line analysis which incorporates a more realistic representation of the generator model and load characteristics. The off-line analyses are carried out using sensitivity factors (phase shifter distribution factors, generation shift sensitivity factors) and a linear programming (LP) optimization technique. The performance of the expert system is compared with the conventional LP optimization method of network overload alleviation and concurrent results have been obtained. The proposed expert system has been tested with the simulated conditions of a few practical Indian power systems. Results obtained for two Indian power networks of 24 nodes and 82 nodes are presented for illustration purpose. The expert system is found suitable for application in Energy Control Center as the solution is obtained very fast with a minimum of numerical computations.

Alleviation of line overloads and voltage violations by corrective rescheduling

This paper presents simple and efficient algorithms for the alleviation of line overloads and voltage violations by corrective rescheduling. The proposed approach utilises the decoupling of real and reactive power and the decomposition between optimisation without security constraints and optimisation to satisfy security constraints. Highlights of the proposed approach are: (i) a choice of performance index which ensures that alleviation of some of the existing violations does not create any fresh violations, thus avoiding the need for cycling in optimisation and, (ii) the use of a classical optimisation technique for faster solutions. Results for two sample test systems have been presented to validate the proposed algorithms.

Extended security enhancement algorithm for power systems

An enhanced formulation of a linear method for transmission line overload alleviation is described, based on the relationship between line current and voltage parameters. This new development ensures adequate modelling of real power injections in buses without specified loads, inclusion of the swing bus from those generators with the capability of alleviating line overloads, and incorporation of the dependence of load on voltage during load shedding.

New techniques for secure power system operation

This paper describes a security algorithm that incorporates load flows, contingency selection, AC outage studies and security control. The load flow method is based on the superdecoupled load flow method of Haley and Ayres, which gives rapid and exact load flow solutions. Separate contingency selection methods for real power flow and voltage security analyses have been developed. For real power analysis the line overload contingency selection technique of Stott et al. has been adopted, but for voltage security analysis a new voltage contingency selection technique has been developed. Critical outages selected from the contingency selection functions are analysed in detail with AC outage simulations of line and generator outages. For security control a range of feasible corrective actions are provided which inclued phase shifter control, generation rescheduling and/or load shedding to alleviate the line overload problem and transformer taps to adjust the bus voltage deviations. New and fast line overload alleviation techniques have been developed by using injections at the terminating buses of the overloaded line without having to compute the sensitivity and Jacobian line flow matrices. The proposed security algorithm has been validated by testing on several standard test systems.

ON THE COMBINED USE OF THE POLAR AND RECTANGULAR COORDINATES APPROACH FOR TRANSMISSION LINE OVERLOAD ALLEVIATION

An alternative formulation of a linear method for transmission line overload alleviation is described based on the relationship between the line currents and voltage parameters defined in rectangular coordinates; the fast‐decoupled loadflow method is used for base calculations. Tests are carried out on the Saskatchewan Power Corporation 6‐bus network and the IEEE 14‐bus system.

Realistic power system security algorithm

The paper presents a security algorithm which consists of the superdecoupled loadflow technique for loadflow solution, outage studies, contingency selection and line overload alleviation. A superdecoupled loadflow method with a distributed slack busbar has been applied to line and generator outage studies as well as for line overload alleviation. The accuracy and reliability of the line and generator outage contingency selection method, as previously developed by comparison of its ranking results with the correct ranking of AC outage simulation, has been investigated. A fast computation technique in real power line overload alleviation by generation scheduling and/or load shedding has been proposed by using injections at the terminating busbars of the overloaded line. These injections are determined by the amount of overload only without having to compute the sensitivity and Jacobian line flow matrices and pseudoinverse of the product of these matrices. The proposed algorithm has been validated in tests on the IEEE 24 busbar test system.

Generation Rescheduling and Load Shedding to Alleviate Line Overloads-System Studies

A basic mathematical model to describe the problem of line overload alleviation has been developed in a companion paper. Algorithms for solving the modeland suitable for incorporation in the Newton-Raphson and decoupled load flow programs are also described. In this paper, the effectiveness of the proposed techniques is examined using a six bus model of the Saskatchewan Power Corporation system, and the IEEE 14, 30 and 57-bus test systems. A single pass of generation rescheduling and load shedding has been found to be adequate for alleviating line overloads in almost all the systems studied. Computational time of each pass of these techniques is approximately 1/2 to 3/4th of the computational time required for the initial load flow. Storage requirements are slightly more than the requirements of the load flow technique used. The proposed techniques identify and determine the existence of a physically realizable operating condition to alleviate the line overload condition. The techniques appear attractive for security and reliability studies, and for operational planning

Generation Rescheduling and Load Shedding to Alleviate Line Overloads-Analysis

This paper deals with the problem of alleviating line overloads in a power system by generation rescheduling and load shedding. Mathematical models based on linearized relationships between line currents and state variables, and bus injected powers and state variables are systematically developed to considerthe line overload problem. Two algorithms are described to solve the models in the Newton-Raphson and decoupled load flow programs. The techniques developed can beused to determine the generation rescheduling and load curtailment pattern to alleviate line overloads. The approaches presented in this paper shouldprove useful in system security studies and reliability studies for examining line overload alleviation with minimum computational expense. The analytical results can also serve as an operating guide to the system dispatcher. In a companion paper, the effectiveness of these techniques in alleviating line overloads in several test systems is investigated.