Transformer Outages Research Articles

Winding deformations detection in power transformers by tank vibrations monitoring

There is an increasing interest in applying monitoring systems and on line diagnosis techniques to power transformers, as these are one of the most expensive and critical components of a power system. Detecting failures in an incipient stage can avoid major transformer damages, allowing important savings in investing, and giving time enough to plan transformer outages. In recent years, a great number of monitoring systems have been reported. Some of them, make use of models as a tool to assess transformer condition. In this paper the use of a vibration model to be applied to transformer monitoring is proposed. In the paper, a review of the basic theory of transformer vibration is performed and the results of an experimental set of tests carried out in order to establish the model basis are reported.

Transfer function-based partial discharge localization in power transformers: a feasibility study

Statistical studies have shown that failures of bushings, winding insulation, and online tap changers are the main causes for long-duration outages of transformers. This article investigates the development of an instrument for supervising the conditions of transformer units. The use of sectional winding transfer functions (SWTFs) for online PD evaluation in power transformers has several advantages: localization of PD sources, discrimination between PDs inside or outside of the transformer, and evaluation of actual PD amplitudes along the winding. Direct measurement of SWTFs-when different points along the coil are accessible-and use of these SWTFs for PD evaluation and localization has shown excellent results. In order to use this method at transformers on site, an appropriate modeling method is needed for the computation of the SWTFs using only measurements at the transformer terminals. A detailed modeling approach based on discrete RLC circuit elements has been studied and different algorithms used for parameter estimation and optimization. This model is applicable in practical cases for a limited frequency range if genetic algorithms (GAs) are used for parameter optimization. A new method based on traveling wave theory has been investigated using genetic algorithms to search for the optimum parameters of a partial differential equation that describes the transient behavior of the coil. This method has shown potential in solving SWTF calculation problems for different transformers.

A versatile tool for electric substation reliability assessment

Performance evaluation of a combined generation and transmission system normally includes the independent outages of generators, transformers and transmission lines. However, substation originated outages due to failure of breakers, transformers and busbars can also have a significant effect on the bulk power system performance indices in terms of outage frequency and duration. The studies described in this paper utilized various substation components such as breakers, busbars and transformers to develop a general-purpose software to assess the reliability of the system. It performs failure modes and effect analysis and provides a concise and orderly description of the various combinations of occurrences within a system that could result in an interruption of supply. Reliability analysis technique using the minimal cut set method has been implemented. In addition the software package is able to define and incorporate 15 types of system configurations in the analysis and at the same time, comprehensive analysis tools such as print, save, sensitivity studies, tutorial guide, etc., are also included. The concept associated with the reliability analysis is illustrated by application to a simple but comprehensive system configuration.

AC train emergency fuzzy logic control using an event driven approach

The paper proposes a fast, versatile and easy-to-use technique for control of train despatch schedules in an electrified railway system for improving its performance following outage of a station transformer. Using the technique of event driven approach and fuzzy logic, it is shown how a set of performance indices which represent the requirements on passengers, headway, regularity, and electrical loading, are optimised during rescheduling following such outage. Without the proposed emergency control technique, the railway system under consideration exhibits unstable buildups of unserved passengers/transformer overloads. Such buildups severely restrict the system's capabilities to return to normal operation.

Open loop distribution system design

The ability to supply consumers of an urban area without prolonged interruption during a feeder segment or substation transformer outage, is assured by a uniform cable size of the feeder segments along the entire loop. Based on the criterion of the uniform cable size, a loop configuration is obtained first (by minimizing the installation costs), and then an open loop solution is found (by minimizing the power losses). Heuristic rules are proposed and used to obtain an initial solution, as well as to improve current solutions. >

An expert system for comprehensive on-line economic-security controls of distribution systems

This paper presents an architecture which assists the system operator to perform comprehensive controls in a distribution SCADA environment. A typical zone substation and open-ended multiple outgoing feeder group is used as an example for testing the validity of the approach. The paper first reviews a prototype expert system which was developed by the authors to help operators provide sufficient security margins against overloading by meeting certain heuristic operating guidelines. Next, the paper describes new functions of the expert system aiming for comprehensive controls of each zone substation: (1) generalized operating guidelines for meeting security constraints for all loads; (2) additional procedures for removing any existing busbar/feeder overload, restoring customer loads after each feeder fault and/or repair, reconfiguring busbar connections after each supply transformer outage, and reducing distribution losses on one feeder group at a time; (3) tracking substation load variations for continuous security monitoring and assessment of substation security; (4) additional numerical modules for minimizing overall distribution losses, and knowledge-based assessment of overvoltages due to inter-zone substation switching.

Distribution factors of reactive power flow in transmission line and transformer outage studies

The authors present a fast and accurate method to calculate a set of distribution factors of reactive power flow for transmission line and transformer outage studies. The transmission branches are broken into two categories: transmission lines and transformers. Separate formulas are derived to calculate the distribution factors of the reactive power flow. A transmission line model is developed. The reactive power flow in a line is divided into two parts: the transmission part and the loss part. Based on this model, transmission line outages were simulated by power injections which cancel the reactive power flow in the outage lines. Formulas to calculate the distribution factors of the reactive power flow are derived using the decoupled load flow characteristic, network sensitivities, and the principle of superposition. The formulas to calculate the distribution factors of the reactive power flow in transformers are also derived. >

Lightning-caused distribution transformer outages on a Florida distribution system

The data obtained from the Electric Power Research Institute (EPRI) Research Project (RP) 2542-1 on the characteristics of lightning surges on distribution lines are presented. This project was conducted to measure the lightning-caused currents and voltages that occur on distribution lines and to quantify the distribution system damage that occurs because of lightning. During the 3 yr data-gathering period, 2448 distribution transformers were monitored. The estimated lightning-caused transformer failure rate was 0.25%, and the estimated lightning-caused transformer fuse operating rate was 0.43%. Although these data are useful in the operation of distribution systems, further reducing the transformer failure rate will require that the exact failure mechanisms be established. It is noted that the research to determine the failure mechanisms will require a long measuring period and thus can be expected to be expensive. >

Detection of topology errors by state estimation (power systems)

Errors in the telemetered data of breaker and switch status, through the network topology processor in the EMS (energy management system) computer, may result in errors in the determination of the current network topology of the system. The use of normalized residuals that result from state estimation is proposed for the detection of topology errors. Three types of topology errors are considered: line or transformer outage, bus split, and shunt capacitor/reactor switching. Conditions for detectability of topology errors are presented. The conditions are tested on the IEEE 30 bus system, and the results confirm the theoretical predictions. The problem of topology error identification is also discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A technique for simulating outages of lines and transformers in load flow studies

A technique for simulating outages of lines and transformers is described in this paper. The technique makes use of the fact that the power demands at the buses to which the equipment is connected are met by the power flows in the branches connected to the buses. This concept is used to calculate the power injections to the buses necessary to simulate the outage of the branch. The calculated power injections and the sensitivity matrix are used to obtain the load flow solution. Results from sample systems are also presented.

Simulation of line and transformer outages in power systems using constant-sensitivity matrix

A simple and fast method for simulation of line and transformer outages in a power system is presented. The method is based on a constant-Jacobian load-flow model which results in a constant-sensitivity matrix. Some of the elements of the constant-sensitivity matrix are used in calculating the required changes in injections at the terminal buses of the outaged element during the simulation. The corrections in the state vector are also computed using the constant-sensitivity matrix. The proposed method has been validated by testing with different IEEE test systems and the 26-bus Saskatchewan transmission network, and the relevant results are presented. Multiple outages have also been simulated using the new technique.

An Aggregate Induction Motor Model for Industrial Plants

A technique is developed for deriving an equivalent load model to represent the dynamic and steady state effects of a system consisting of induction motors and constant impedance loads interconnected by a network. Motor parameters are calculated from standard specifications, and together with network and load data are incorporated into an admittance matrix which is used to define most of the parameters of the load equivalent. The inertia and running slip are chosen to minimize the error between the transient response of the system and its equivalent. Step responses are calculated using a state approach. Adequacy of the equivalent is validated by comparing the response of the original system and the equivalent to simulated bus voltage change, transformer outage, and remote and local balanced faults using a transient stability program.

Efficient Simulation of Line and Transformer Outages in Power Systems

Efficient Simulation of Line and Transformer Outages in Power Systems

Efficient Simulation of Line and Transformer Outages in Power Systems

A method of simulating single and multiple transmission element (line and transformer) outages is discussed in the paper. The method simulates the outaged system states assuming the scheduled demands of the end buses of the outaged elements are still met. Sensitivity relationships of power systems are used both to determine the required power injections into the end buses, and to determine the corresponding corrections to the phase angles and magnitudes of bus voltages. The proposed method has been applied to the 30-bus and 118-bus IEEE standard test systems. Relevant results are presented and discussed.

Interactive computer program for planning the expansion of meshed transmission networks

A scheme is described for the interactive development of expansion strategies for transmission networks in which stability and voltage considerations are not of prime importance. According to the planning methodology adopted, the transmission/generation system is to be reinforced when it can no longer feed the peak demand without equipment overload in a single contigency. Outages of generators are considered, as well as the outages of transmission lines and transformers. Generation can be redispatched in order to eliminate overloads. If this is not possible, the model minimizes the load curtailment and identifies the weak parts of the system. It also ranks the possible investments according to their effectiveness in reducing the load curtailment. The planning engineer can build, interactively, several expansion policies by successive investment decisions. The model computes the discounted costs of the various policies developed. The files containing the expansion policies created are also automatically managed by the interactive package. The scheme has been implemented in a package called Tranex which consists of two separate interactive computer programs. An example illustrating the application of the method via this package to the Yugoslav network is given, along with computer performance details and an overview of the program structure.

A Modified Newton Raphson Load Flow Technique and Its Use in Simulating Line and Transformer Outages

This paper presents a modified formulation of the Newton-Raphson load flow using the Cartesian coordinates to represent the bus voltages and system parameters. The suggested modification reduces the computer storage required for the Jacobian matrix and the cpu time required for obtaining a load flow solution. The proposed load flow formulation is then used in a technique which simulates the outages of lines and transformers. The outage simulation technique determines the bus power injections whose effects are similar to the outage and uses this information to calculate, upto a prespecified accuracy, the voltages and power flows in the post outage states. Some results of line outage simulations are included.

Correction to "A simulation technique for studying real and reactive power flow patterns" and "An approach for preventing system insecurities arising from line and transformer outages"

Correction to "A simulation technique for studying real and reactive power flow patterns" and "An approach for preventing system insecurities arising from line and transformer outages"

An approach for preventing system insecurities arising from line and transformer outages

This paper presents an algorithm that improves steady state security of a power system. An objective function which includes power flows in the overloaded system elements and bus voltages that exceed the permissible limits is formulated. Weighting factors, obtained from the magnitudes and frequencies of the violations, are also used in this function. An optimal combination of controlling variables, such as voltage magnitudes at generator buses, real power generation, transformer and phase shifter tap settings, is determined by minimizing the objective function. Instead of the non-linear load flow equations, sensitivity analysis approach has been used to reduce computation time and effort. This technique has been applied to a 6 bus model and a 26 bus version of the Saskatchewan Power Corporation transmission system; and some of the more interesting results are presented.

A Fast Approximate Technique for Outage Studies in Power System Planning and Operation

This paper presents a technique that simulates load and generation changes and outages of transmission lines and transformers. The modification of power injected into the system buses, which would simulate the outage of a transmission element, is calculated using the sensitivity matrix of the basic system state. The real and reactive power flowing in the system elements and voltages at all the system buses are then computed. The proposed technique has been applied to the Saskatchewan Power Corporation system andto the combined SPC and Manitoba Hydro systems. The power flows and bus voltages calculatedby the proposed technique are compared with those obtained from the Newton Raphson load flows.

Automatic Evaluation of Power System Performance-Effects of Line and Transformer Outages

A method is given, and a computer program described, for the automatic testing of a power system under critical line and transformer outages. The technique permits the estimate of voltage drops and line loadings for each network element out of service and performs those load flows for conditions which exceed the allowable operating limits. Results from the use of this technique in a number of actual planning studies are presented.