Effects of direct currents in power transformers Part II. Simplified calculations for large transformers

In a previous paper, a general theoretical method was developed to calculate the effects of direct current circulating in single-phase transformers of any rating. The method is now applied to HV and large distribution transformers. Simplified calculation formulae are deduced to provide magnetizing current and voltage along with their harmonic contents. The effects of DC saturation and over-excitation are compared. A method of core saturation and magnetizing current calculation is presented for the case of a rectified load or a DC source between two transformers.     

Peak-switching currents of power transformers

The switching conditions of nonloaded transformers have been investigated to understand the effect of current inrushes and limitations by considering the connecting circuit of the windings, magnetization performances, contact parameters, type of switcher drive, and the value and sign of residual magnetization. The peak-switching currents have been calculated, simulated, and measured and the results have been compared. The obtained results make it possible to increase the power efficiency of the facility by decreasing the losses and maintainign the quality factor of electric power in transient modes.     

Transient node admittance matrices of three-phase power transformers. Part 2. Multiwinding transformers

In Part 2 of this paper we present a generalization of the basic theory described in Part 1. Thereby it becomes possible to cover windings with more complicated interconnections, e.g. transformers containing stabilizing as well as zigzag-connected windings. Starting with a multiwinding phase model, the specific connections are introduced by incidence matrices.     

Analytical approach to internal fault simulation in power transformers based on fault-related incremental currents

A new method for simulating faulted transformers is presented in this paper. Unlike other methods proposed in the literature, this method uses the data obtained from any sound transformer simulation to obtain the damaged condition by simply adding a set of calculated currents. These currents are obtained from the definition of the fault. The model is fully based on determining the incremental values exhibited by the currents in phases and lines from the prefault to the postfault condition. As a consequence, data obtained from simulation of the sound transformer may be readily used to define the damaged condition. The model is described for light and severe faults, introducing this latter feature as a further add-on feature to the low-level faults simulation. The technique avoids the use of complex routines and procedures devoted to specially simulate the internal fault. Of prompt application to relay testing, the proposed analytical model also gives an insight into the fault nature by means of the investigation of symmetrical components. In contrast with its low complexity, the method has shown to present large accuracy for simulating the fault performance.     

Transient node admittance matrices of three-phase power transformers. Part 1. Two-winding transformers

In the past, asymmetrically operating three-phase transformers could only be analytically described in special cases. In part 1 of this paper three-phase node admittance matrices which cover both stationary and transient behaviour are derived for two-winding transformers. In part 2 of this paper transformers with more general connection symbols are treated. For this reason, the underlying modelling is done for n-winding transformers. This method only requires their identification plate data as well as generally accessible recommended configuration values. If necessary, the modelling can therefore be accomplished without knowing the construction data.     

Cellulose insulation in oil-filled power transformers: Part I - history and development

This paper presents a brief discussion on the development of electrical grade paper/pressboard for transformer use from the raw materials, improvements made, and particularly the use of thermal upgrading agents to extend the useful life of transformers.     

Probability approach to installation studies of power transformers

A probability approach to transformer installation studies which have traditionally been based on very conservative deterministic criteria is described. The Monte-Carlo technique is used to evaluate the distribution function of the insulation deterioration with respect to the stochastic nature of load, ambient temperature and emergency loading. A statistical analysis is made to guarantee that a preestablished risk (R₀) associated with the operation is not exceeded. Owing to the fact that the transformer insulation is not regenerative, the concepts of ‘life expectancy’ and ‘rate of insulation deterioration’ are used in order to calculate the permissible peak loads.     

Electrification of a major steel mill. Part I. early use of direct current power

This paper discusses the shift of Bethlehem Steel Company from steam to electrical power for its process machinery. Part I of this article covers the early use of direct current (DC). Subsequent parts will cover the development of the 25-Hz power system and the electrification of the rolling mill drives.     

The effect of geomagnetically induced currents on the thermal conditions of power transformers

Methods to calculate the thermal processes and temperature of the most heated points of the tank and windings of a power transformer in conditions of flow of geomagnetically induced currents in grounded windings are developed. Dependences of additional loss in the tank and windings are obtained for a TRDN-63000/115/6.3/6.3 power transformer. It is shown that additional losses in power transformer windings are caused by the geomagnetically induced currents and depend on the first and nth harmonic components of magnetizing current, as well as on the load factor of a power transformer. The dependences of excess temperature of the most heated points of windings over ambient temperature on the geomagnetically induced current and load factor are determined. It is found that the excess temperature of the most heated point of winding over ambient temperature is also affected by the total additional loss of active power in the tank caused by the eddy currents at a geomagnetic storm. The acceptable load capacity of power transformers in geomagnetic storms with different intensities is determined in dependence on the ambient temperature. An increase in the load capacity of power transformers above set values in geomagnetic storms can lead to overheating of transformer windings and triggering of transformer gas protection, which will cause an electric power system to malfunction.     

Space-phasor model of 3-phase AC systems with current-source power converters. Part I: theoretical background

Contents The presented method for the calculation of 3-phase AC systems with DC link indirect converters is based on the perception that the stationary operation of converter systems can be treated as a periodic sequence of similarly transient processes. It combines the advantages of space phasor and Laplace transformations and leads to simple equivalent circuits using the standard theories for quadripole networks. Therefore, all network functions can be determined analytically. The developed method is presented on the example of a 6-pulse current link indirect converter. Based on the analytic calculation of the current in the DC link all other network variables can be determined analytically. Furthermore, the calculation rules for the harmonics are specified so that the interactions of the converter and the surrounding network can be characterized and evaluated exactly. The entire method can be extended to systems with two or more current converters (or flexible AC transmission systems) using the multiport theory.     

Diagnosis of incipient faults in power transformers using CMAC neural network approach

Dissolved gas analysis (DGA) is one of the most useful techniques to detect the incipient faults of power transformer. However, the identification of the faulted location by the traditional method is not always an easy task due to the variability of gas data and operational natures. In this paper, a novel cerebellar model articulation controller (CMAC) neural network (NN) method is presented for the fault diagnosis of power transformers. By introducing the IEC standard 599 to generate the training data, and using the characteristic of self-learning and generalization, like the cerebellum of human being, a CMAC NN fault diagnosis scheme enables a powerful, straightforward, and efficient fault diagnosis. With application of this scheme to published transformers data, the diagnoses demonstrate the new scheme with high accuracy and high noise rejection ability. Moreover, the results also proved the ability of multiple incipient faults detection.     

电力变压器铁芯磁场和损耗分布的三维数值方法与实施 第一部分:三相三柱变压器

基于用等效拼接间隙考虑搭迭效应的新构想,本文建立了三柱变压器铁芯磁场的三维标量位各向异性非线性时变分析模型,提出了由瞬时磁密幅值和磁化角计算铁耗的新方法,并通过对实际模型的有限元模拟证实了分析模型及计算方法的合理性和可靠性。     

An experimental approach for investigating low-level interturn winding faults in power transformers

Interturn faults are one of the most prevalent and potentially destructive electrical faults in power transformers. In spite of an extensive interturn fault detection literature, the issue still constitutes an open problem. The difficulty is mainly caused by lacking of enough information on the characteristic signatures associated with interturn faults which were not clearly brought out by the previous studies. This contribution is aimed at obtaining a better understanding of the physical behavior of the power transformers in the presence of interturn faults as well as extracting several features which would be useful to specify the transformers interturn faults. The approach keeps at disposal a 100 kVA distribution transformer on which interturn faults were imposed and an experimental setup consisting of an instrumentation system and suitable transducers which enables monitoring selected characteristics of the transformer. The paper examines the faulted transformer performance under various fault and transformer operating conditions. Unique characteristic fault-signatures derived from the experiments will provide an important basis for developing more reliable and sensitive methods to detect interturn faults on the transformer windings well before such faults lead to a catastrophic failure with serious damage to the windings.     

Flow electrification in power transformers. Explanation of thewall-current measurements

The aim of our work is to understand better the mechanisms of flow electrification due to oil circulating inside a rectangular pressboard channel. This research has been done because of technical incidents in power transformers in which oil is flowing past pressboard with a velocity in the range of several meters per second. A possible explanation of the wall current observed along the channel is proposed     

Application of a general regression neural network for health index calculation of power transformers

A power transformer is one of the most important components in a transmission network. To assess the overall condition of this valuable asset, health index calculations are recently gaining more attention from the utility companies that operate networks. Only limited research has been conducted on health index calculations of transformers. Most of the past approaches are based on the linear combination of weighted scores of measurements following the industry standards such as IEEE, IEC and CIGRE. A few previous methods based on artificial intelligence and statistical approaches such as fuzzy logic, multivariate analysis and binary logistic regression have been published in recent years. In this paper, a General Regression Neural Network (GRNN) which has a nice nonlinear property and can work with measurements without quantization has been evaluated. The GRNN allows multi-dimensional measurements to be combined through an optimal weighting and scoring system to compute a quantitative health index of power transformers. The weighting of each test was assigned based on a smoothly interpolated continuous function. The efficacy of the model has been validated against expert classifications and data sets published in the literature. The comparative results demonstrate that, the proposed method is reliable and very effective for condition assessment of transformers through an automated health index calculation.     

A wavelet-based technique for discrimination between faults and magnetizing inrush currents in transformers

This paper presents the development of a wavelet-based scheme, for distinguishing between transformer inrush currents and power system fault currents, which proved to provide a reliable, fast, and computationally efficient tool. The operating time of the scheme is less than half the power frequency cycle (based on a 5-kHz sampling rate). In this work, a wavelet transform concept is presented. Feature extraction and method of discrimination between transformer inrush and fault currents is derived. A 132/11-kV transformer connected to a 132-kV power system were simulated using the EMTP. The generated data were used by the MATLAB to test the performance of the technique as to its speed of response, computational burden and reliability. The proposed scheme proved to be reliable, accurate, and fast.     

Integral approach to the protection of power transformers by means of a microprocessor

The development of an integral digital approach to transformer protection is presented. Algorithms are developed for differential protection including inrush and overexcitation restraint, earth-fault protection, primary-to-secondary winding-fault protection and time-current protection. All these protections are implemented by means of a single microprocessor of the 68000 type. A prototype has been developed to protect a two-winding-three-phase transformer. The protection is tested online. Results of the online tests are presented.     

A novel direct search approach for combined heat and power dispatch

A novel approach based on the direct search method (DSM) is proposed for the solution of combined heat and power (CHP) dispatch problem. To deal with the mutual dependency of multiple-demand and heat–power capacity of cogeneration units, the penalty functions should be considered in DSM to enforce the corresponding violated constraints from the infeasible region into the feasible region. Many nonlinear characteristics of the generator can be handled properly in the direct search procedure. To increase the possibility of exploring the search space where the global optimal solution exists, another effective strategy based on a successive refinement search technique is also proposed to guarantee a possibly complete examination of the solution space. Numerical experiments are included to demonstrate the proposed direct search approach can obtain a higher quality solution than many existing techniques.