D. Thukaram

ANN applications in fault locators

Recent developments indicate that Artificial Neural Networks (ANNs) may be appropriate for assisting dispatchers in operating electric power systems. The fault location algorithm being a key element in the digital relay for power transmission line protection, this paper discusses the potential applicability of ANN techniques for determination of fault location and fault resistance on EHV transmission lines with remote end in-feed. Most of the applications make use of the conventional Multi Layer Perceptron (MLP) model based on back propagation algorithm. However, this model suffers from the problem of slow learning rate. A modified ANN learning technique for fault location and fault resistance determination is presented in this paper. A reasonably small NN is built automatically without guessing the size, depth, and connectivity pattern of the NN in advance. Results of study on a 400 kV transmission line are presented for illustration purposes. Performance of the modified ANN is compared with the analytical algorithms and conventional MLP algorithm for different combinations of pre-fault loading condition, fault resistance and fault location. The results are found to be encouraging.

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.

Coordination of Reactive Power in Grid-Connected Wind Farms for Voltage Stability Enhancement

Summary form only given. This paper studies the feasibility of utilizing the reactive power of grid-connected variable-speed wind generators to enhance the steady-state voltage stability margin of the system. Allowing wind generators to work at maximum reactive power limit may cause the system to operate near the steady-state stability limit, which is undesirable. This necessitates proper coordination of reactive power output of wind generators with other reactive power controllers in the grid. This paper presents a trust region framework for coordinating reactive output of wind generators with other reactive sources for voltage stability enhancement. Case studies on 418-bus equivalent system of Indian southern grid indicates the effectiveness of proposed methodology in enhancing the steady-state voltage stability margin.

An expert fuzzy control approach to voltage stability enhancement

The voltage stability control problem has become an important concern for utilities transmitting power over long distances. This paper presents a new approach using fuzzy set theory for reactive power control with the purpose of improving the voltage stability of a power system. Voltage stability index and controlling variables are translated into fuzzy set notations to formulate the relation between voltage stability level and controlling ability of controlling devices. Control variables considered are switchable VAR compensators, OLTC transformers and generator excitations. A fuzzy rule based system is formed to select the controllers, their movement direction and step size. The developed fuzzy system is tested on a few simulated practical Indian power systems and modified IEEE 30 bus system. The performance of the fuzzy system is compared with conventional optimization technique and results obtained are encouraging. Results obtained for a 82 node Indian power network and modified IEEE 30 bus system are presented for illustration purposes.

Improved algorithm for optimum reactive power allocation

An improvised algorithm is presented for optimal VAr allocation in a large power system using a linear programming technique. The proposed method requires less computer memory than those algorithms currently available.

Curtailed number and reduced controller movement optimization algorithms for real time voltage/reactive power control

This paper addresses the problem of curtailing the number of control actions and minimizing controller movements for real-time voltage/reactive power control. Algorithms are proposed to identify the most effective subset of control actions and to minimize controller movements. An algorithmic objective function appropriate for the treatment of system security and economy is also proposed. It eliminates trial and error adjustments of weightage factors for a combined objective. A single parameter decides the priority between movement of controls and gains in security as well as loss reduction. The algorithms are compared and contrasted with sequential linear programming and sequential quadratic programming techniques by means of two practical systems. >

Approach for Multistage Placement of Phasor Measurement Units Based on Stability Criteria

Identification of optimal locations to deploy the phasor measurement units (PMUs) is a preliminary task in wide area monitoring and control of power systems. Given the cost and other associated restrictions, it is highly unlikely for any utility to install all the required number of PMUs to ensure the complete observability of the network. This necessitates judicious identification of locations in the network to deploy the PMUs. Ideally, the choice of location must depend on the intended purpose. In this paper, an approach for placement of PMUs considering both the stability and topological aspects is presented. First, a probabilistic framework based on proximity to instability is proposed to rank the generators/load buses in the order of vulnerability. Subsequently, a binary integer linear programming formulation is proposed to identify the optimal locations considering both stability and topological aspects. The optimal locations obtained using the proposed approach ensures a priori observability of most vulnerable buses in addition to enhancing the network observability. Results on IEEE 39-bus system and 246-bus equivalent system of Indian southern grid are presented to illustrate the proposed approach.

Optimal static voltage stability improvement using a numerically stable SLP algorithm, for real time applications

A numerically stable sequential Primal–Dual LP algorithm for the reactive power optimisation (RPO) is presented in this article. The algorithm minimises the voltage stability index C 2 [1] of all the load buses to improve the system static voltage stability. Real time requirements such as numerical stability, identification of the most effective subset of controllers for curtailing the number of controllers and their movement can be handled effectively by the proposed algorithm. The algorithm has a natural characteristic of selecting the most effective subset of controllers (and hence curtailing insignificant controllers) for improving the objective. Comparison with transmission loss minimisation objective indicates that the most effective subset of controllers and their solution identified by the static voltage stability improvement objective is not the same as that of the transmission loss minimisation objective. The proposed algorithm is suitable for real time application for the improvement of the system static voltage stability.

Accurate Steady-State Representation of a Doubly Fed Induction Machine

This letter presents an accurate steady-state phasor model for a doubly fed induction machine. The drawback of existing steady-state phasor model is discussed. In particular, the inconsistency of existing equivalent model with respect to reactive power flows when operated at supersynchronous speeds is highlighted. Relevant mathematical basis for the proposed model is presented and its validity is illustrated on a 2-MW doubly fed induction machine.

A numerically stable decomposition based power system state estimation algorithm

Abstract This paper proposes anew approach for solving the equality constrained power system state estimation (PSSE) problem. The proposed approach decomposes the linear least squares with the linear equality constraints (LSE) problem encountered at each PSSE iteration into two unconstrained linear least squares (LS) problems. The solution of the LSE problem is the sum of the solutions of the individual LS problems. The proposed approach avoids giving large weights to equality constraints and hence results in a more stable formulation, that can be solved within the framework of unconstrained least squares. Based on the proposed decomposition, a Givens rotations based PSSE algorithm is proposed and developed. It is shown that the proposed implementation is numerically more stable than the conventional NE, normal equations with constraints (NE/C), Givens rotations and Hachtels methods for PSSE. The first LS problem to be solved in the proposed implementation is identical to that encountered in conventional constrained weighting approaches like NE approach and Givens rotations approach, except for the fact that equality constraints (zero injection constraints) are not assigned large weights. The resulting error due to the lack of large weights for equality constraints is corrected by the solution obtained from the second LS problem. Simulation results on 23, 89 and 319 node systems show the desirable stability characteristics of the proposed implementation in comparison with NE, Givens rotations, NE/C, Hachtels approach and NE based decomposition (DNE). The proposed approach has better speed characteristics compared to NEIC and Hachtels approaches. .