Abhinandan De

A Study on the Impact of Low-Amplitude Oscillatory Switching Transients on Grid Connected EHV Transformer Windings in a Longitudinal Power Supply System

Switching events in inter connected power systems have significant effects on EHV transformers. Standard high-voltage lightning and switching impulse tests are performed on EHV transformers during manufacture, to ascertain breakdown strength of insulation used. But, during their service, transformers encounter numerous voltage transients of complex and varying wave shapes which do not necessarily resemble these standard surge type voltages. Transformers are complex assembly of coils around magnetic core having inductances and capacitances and therefore, exhibit unique frequency response characteristics with several natural resonate frequencies. Oscillatory overvoltages originating in power systems due to switching, faults and other phenomena often traverse through transformers and can excite one or more of the windings natural resonate frequencies resulting severe internal voltage amplification and damage to insulation even when the terminal excitation voltages have relatively low amplitude. Adequate consideration must therefore be given to such low amplitude oscillatory transient voltages while designing insulation for grid connected EHV transformers. In this paper, the authors have analyzed the voltage stresses developed in the windings of a 400/220/33-kV grid connected transformer operating in the Eastern Regional Power Grid of India, when natural frequencies of the transformer winding are triggered by oscillatory switching transients. The developed voltage stresses have been compared with those under standard laboratory dielectric tests including newly incorporated fast front-long tailed switching surges.

Voltage stability assessment in power network using self organizing feature map and radial basis function

Self-organizing feature map (SOFM) in conjunction with radial basis function (RBF) has been applied in this paper to determine and classify the voltage stability states of a multi-bus power network. Simulations were carried out on a real 203-bus system of an Indian power utility considering load changes and contingencies. The data collected from simulations are then used as inputs to the SOFM which acts as a classifier to classify the voltage stability states of the system under test. To augment the effectiveness of the proposed method, the initial classification results were improved with the application of RBF technique. Studies show that the SOFM-RBF combination delivers high classification accuracy in the order of almost 100% and can be considered an effective soft-computing tool to ease the operation of large-multi bus power network under variable operating conditions.

Kruskal's Maximal Spanning Tree Algorithm for Optimizing Distribution Network Topology to Improve Voltage Stability

Abstract—Alteration of power network topology is often required to meet important objectives, such as restoring connectivity, minimizing power losses, maintaining stability, maximizing power transfer capability etc., and is achieved by switching of circuit breakers and other switching devices in the power network. Primary power distribution networks are often interconnected and meshed but should be transformed to radial topology to achieve various operational advantages. Distribution networks also need to be reconfigured after faults to restore power at all the load points. Reconfiguring a power network, however, is a complicated multi-constrained optimization problem, as there may exist many feasible switching combinations in a large power network. This article proposes a novel application of graph theory, supported by Kruskals maximal spanning tree algorithm, to search for the optimal network topology and to optimally convert an interconnected meshed network into a radial system to achieve best operational characteristics, cost, and control. The proposed technique has been demonstrated on a 30-node primary distribution network originally having a mesh topology, and the results indicate significant performance improvement after transformation into optimal radial topology.

Response of EHV Grid Transformers to System-Originated Oscillatory Switching Transients

This paper investigates the conditions for transformer winding resonances with an emphasis on oscillatory excitation voltages originating from various switching events in the power grids. A section of the Eastern Regional Power Grid in India has been simulated in this paper by using the Alternative Transient Program for the purpose of defining high-frequency transient voltages which may occur during various switching events. The investigation reveals that such oscillatory voltages could excite natural frequencies of a 400/220/132-kV power transformer, which closely matches the terminal excitation frequencies. Short time Fourier transform (STFT) has been used to analyze the frequency contents of the oscillatory terminal excitation as well as the voltage response of the windings. The results of the STFT confirmed the presence of resonant frequency in the oscillatory terminal voltage and in the 400-kV main and tap changer windings of the transformer. It has been demonstrated that the voltage stresses in the windings under certain critical switching operations can exceed the stresses developed under the standard 1.2/50- μs lightning impulse at the transformers basic insulation level voltage. The reduction in winding voltage stresses under resonance is possible by deploying some remedial measures involving winding design modifications which have also been suggested in this paper.

Alleviation of line congestion using Multiobjective Particle Swarm Optimization

This paper presents a methodology based on Particle Swarm Optimization technique for rescheduling of generation patterns to manage congestion in contingent power networks. In deregulated systems, line congestion attract additional penalties which add on to the overall operational cost to be incurred by the Independent System Operators (ISO), apart from causing limit violations, and stability problems. Thus, limiting the congestion level of lines and restricting power flows within the safe limits is important from stability as well as economy point of view. The algorithm proposed in the present paper uses a Standard Sensitivity Index to identify the congested zone(s) in a large power network and then adopt corrective actions for limiting line congestion at the cost of a nominal rescheduling cost without any load curtailment and installation of FACTS devices. It has been demonstrated that the proposed method can reduce congestion even below the minimum level obtained from the conventional cost optimization results. It has been depicted that the methodology on application can provide better operating conditions in respect of improvement of bus voltage profile. The efficiency of the proposed methodology has been tested on a IEEE 30 bus benchmark system and the results look promising.

Investigation on the voltage stresses developed on transformer insulation under non-standard terminal excitations

Grid connected EHV transformers experience different types of high voltage surges when in service. Transients generated due to switching operations, lightning strokes, power system faults etc have significant impact on the transformer windings and their insulation. To investigate the effect of developed voltage stresses on the winding insulation of the transformers under various types of terminal disturbances, a number of standard and non-standard wave shapes like lightning impulse, chopped lightning impulse, steep-front long tail switching surge and oscillatory transient over voltage have been simulated. A comparison of the stress conditions in the main and tap changer windings and their insulation structures have been made when the transformers are subject these terminal disturbances. The results of the present study should provide comprehensive idea about how transformer insulations are subject to voltage stresses under different kinds of terminal excitation and this knowledge should be helpful in optimizing the insulation design of high voltage transformers.

Resonant overvoltages produced in EHV transformer windings due to power system transients

Electrical transients generated in power systems due to switching operations comprise of voltages and currents of complex waveshapes ranging over broad spectrum of frequencies. Such transients when travel through transformers can excite the windings natural resonate frequencies. The resulting winding resonance leads in severe internal voltage amplification and abnormal stresses on the insulation. In the present paper the authors have conducted studies to ascertain how the winding insulations of a 400 KV EHV power transformer are stressed when one or more of the windings natural frequencies are triggered by power system transients.

An intelligent energy management system to optimise demand response in Smart Micro Grids

This paper proposes the development of an intelligent Energy Management System (EMS) at consumer end to utilize the sensitivities of the local renewable generation, battery backup and Plug In Electric Vehicle resources to the changes in grid power input and price of electricity for utility and Demand Response optimization in Smart Micro Grids. In Demand Response usually the price elasticity of demand is considered as per statistical data. In this work the development of an optimized Demand Response(DR) has been depicted for reliable and efficient operation of Micro grids with Smart Communication facility. A housing complex, in the proposed work has been viewed as a future micro-grid and it has been demonstrated that the EMS developed can be effectively programmed to minimize the consumer payment by efficient utility and load management with the assistance of pricing signals from the Smart Grid. The choice of Particle Swarm Optimization(PSO) was compelling for the nonlinear nature of the optimization surface. The results obtained from simulation looked promising from context of future Smart Micro Grids.

Transmission line fault detection and classification using wavelet analysis

The paper proposes a new technique for fast and accurate detection as well as classification of power system faults using modern signal processing techniques The technique involves wavelet analysis of faulty voltage and current waveforms, which are recorded at a suitable monitoring location in multi-bus power system to gather valuable information required in detection and classification of faults. The application of wavelet analysis helps in accurate classification of the various fault patterns. In MATLAB Simulink a multi bus system has been modeled for case study and various possible fault types and their combinations were simulated. The results indicate that the proposed technique is capable of classifying all fault categories including multiple faults.

Waveform Analysis-Based Intelligent Fault Classifier with Novel Fault-Zone Segregations for Real-Time Application in Large Power Networks

Abstract The article proposes a power system fault-recognition technique founded on waveform analysis to identify and categorize short circuit faults in large power systems. The main criticism against waveform-based approaches is their huge computational burden and relatively slow processing, making them unsuitable for on-line applications. This article efficiently handles the computational load by applying a novel zone partitioning concept, where a large power system is partitioned, forming smaller hypothetical zones with fewer number of buses and lines. Crude determination of fault zone is first performed by a neural network (NN)-based zone classifier. A succeeding module of classifier, then determines the exact fault nature and its precise location within a hypothetical zone. Transient recorders installed at optimal monitoring buses, in each partitioned zone, collect the preliminary wave-data, which is then passed-on to an extended Kalman filter (EKF)-based feature extraction module, to filter-out relevant fault features. The EKF-NN combination is found to be highly efficient and precise in determining fault nature and location when tested on WECC-200-bus practical system. Hypothetical-zone partitioning has been established as a highly efficient technique, reducing the computation burden and CPU usage considerably, and rendering the scheme suitable for on-line implementation. The method also improved the fault-recognition accuracy phenomenally.