Three-phase Transformer Banks Research Articles

Accurate Fault Diagnosis in Transformers Using an Auxiliary Current-Compensation-Based Framework for Differential Relays

This article proposes an auxiliary framework to address the challenges of transformer differential protection for single-phase transformers or three-phase transformer banks. This framework enables the differential scheme to: 1) work properly if transformers or current transformers (CTs) saturate; 2) detect internal and cross-country faults; 3) detect internal faults while energizing transformers; and 4) detect inrush currents. Unlike the existing methods in the literature, this framework addresses the above-mentioned challenges without sacrificing the sensitivity and/or the speed of differential relays. The proposed method models a transformer and its CTs with linear parameter varying (LPV) state-space equations, and uses the polytopic form of these equations and LPV observers to estimate the states of the transformer and its CTs. To address the CT saturation problem, it accurately estimates primary currents of CTs using their secondary currents. Thus, the differential scheme uses the estimated primary currents of its CTs instead of their distorted secondary currents. Additionally, the proposed framework detects inrush currents of transformers and differentiates them from internal faults by estimating the primary current of the transformer and comparing the estimated and measured primary currents. A discrepancy between the measured and estimated primary currents signifies an internal fault. The results of electromagnetic transient simulations in Electromagnetic Transient Program (EMTP) platform corroborate the effectiveness of the proposed method.

Modelling and analysis of delta-connected distribution transformers with symmetrical neutral-grounding structure for microgrid networks

The conventional grounding methods for the delta-connected secondary windings, such as corner grounding and mid-tap grounding, result in unbalanced phase-to-ground voltages. To overcome this issue, the neutral-grounding method can be employed, but when it operates under unbalanced load conditions, the neutral current flowing through the grounding winding may cause overheating and failures. We then propose a symmetrical method for the grounding of three-phase transformer windings, based on neutral grounding of the delta-connected configuration that can help to distribute the neutral currents. In addition, the proposed method provides a multi-voltage solution relevant for microgrid networks, reduces the costs, and makes it more convenient for planning and design. We formulate the mathematical model of a three-phase neutral-grounded transformer bank with three grounding windings based on the nodal admittance matrices and the coupling-free equivalent circuits. A function block of the equivalent circuit of the model is implemented in MATLAB/Simulink and is verified by field testing. The proposed method can provide a reference for engineers for the design and operation of microgrid networks.

Investigation of transformer banks with delta‐connected secondary windings and various grounding methods for delta service

Grounding at source transformers or generators in a distribution system can help to stabilise system voltages and enhance overall reliability and safety. However, the operational characteristics of a distribution system depend on the type of grounding method employed. This study examines the operation characteristics of three-phase transformer banks with delta-connected secondary windings using a variety of grounding methods for three-phase three-wire or three-phase four-wire delta secondary services. The grounding methods investigated in this study include two conventional methods and a neutral grounding structure, called grounding winding. The authors began by developing coupling-free equivalent circuits of three-phase transformer banks with delta-connected secondary windings in accordance with the network-overlapping technique and circuit theory. These equivalent circuits faithfully reflect the features of the transformer banks with the various grounding methods. Then they implemented coupling-free equivalent circuits using the software package MATLAB/Simulink to verify the suitability of the equivalent circuits for this study. The authors’ analytical results reveal that this neutral grounding structure provides a solid system neutral through a grounding winding as well as a voltage steadier than that obtained under ungrounded conditions or using corner-grounding or mid-tap grounding. Their results provide a valuable reference for power engineers seeking a suitable grounding method for delta service systems.

Harmonic Propagation from the Low Voltage Four-Wire Delta Systems

The electric networks for the distribution to low voltage costumers can be configured in different layouts. Two main approaches are used: the European system composed by three-phase distribution transformers or the North American system composed by single-phase distribution transformers and three-phase transformer banks of single-phase transformers. With respect to harmonic analysis, much more attention has been focused on the three-phase balanced systems arrangements than on the unbalanced four-wire delta system extensively used to supply low voltage loads of 120/240 V. Different authors have shown the three-phase power systems modeling on a phase-coordinates frame. However, the presence of significant asymmetries in the network forces the need of adding a new phase-coordinates model to represent the three-phase transformers banks of two or three single-phase transformers in its various connections. Several papers treat the use of harmonic analysis programs based on a phase-coordinates frame to study the Wye or Delta connected three-phase systems. However, the commonly used four-wire delta connected systems are not fully treated in literature. This paper presents a phase-coordinates model for the representation of the commonly used three-phase transformer banks of three or two single-phase transformers, and single-phase distribution transformers for the harmonic analysis of the four-wire delta connected systems. The harmonic analysis method based on the presented model is used to examine the characteristics of this kind of distribution system with respect to the penetration of harmonics currents from loads to the primary system.

NSGA-II phase balancing of primary distribution circuits by the reconnection of their circuit laterals and distribution transformers

The primary distribution circuits are unbalanced not only by the spread use of unbalanced three-phase transformer banks and single-phase distribution transformers, but also by the use of two-phase three-wire and single-phase two-wire circuit laterals. The unbalance of the phase currents produces two fundamental problems in the circuit: the energy losses in the primary conductors are increased and the presence of a high neutral current on normal operation make it difficult the detection of earth faults by the neutral over current relays. The present work implements an NSGA-II application for the phase balancing problem, which search for the minimal reconnection needed for the two-phase three-wire and the single-phase two-wire circuit laterals, as well as for the three-phase transformer banks and the single-phase distribution transformers, to minimize both: the energy losses and the maximum neutral current of the circuit. The successful performance of this application is achieved by the new definition of the set of independent variables, which permits the NSGA-II to optimize the selection of the elements to be reconnected. A test example with all the needed data is solved to shows the features of the presented approach.

Correction to the phase-coordinate model of the single-phase transformer with mid-tap on the secondary side

It is available a phase-coordinate model for the single-phase transformer with mid-tap on the secondary side. This model allows the representation of 120 V single-phase loads on each section of the secondary of the lighting transformer in a three-phase transformer bank that supplies a four wire delta system. However, the available model represents the entire transformer's impedance in the secondary winding. It differed from the traditional equivalent transformer's circuit, which always represents some portion of the impedance in the primary winding. The presented work develops a new phase-coordinate model for the single-phase transformer with mid-tap on the secondary side to be used in Power Flow and Short-circuits programs. The new model is general and adaptable to be used with transformers with interlaced and non interlaced secondary windings. The developed model is proved with examples of the literature showing a good agreement with the available model for the Power Flow Studies, and a great disagreement for the Short Circuit Studies.

Frequency scan on a phase-coordinates frame for unbalanced systems

The unbalanced electric networks analysis has upgraded from the sequence components frame models to the phase-coordinates frame models. The electric power distribution systems for industrial plants, commercial buildings or residential costumers can contain several network unbalances that can be easily represented by phase-coordinates models. The phase-coordinates models can be used for the analysis of distribution systems at fundamental frequency as well as for other frequencies and are well suited for the harmonic analysis of unbalanced networks. The formulation of the frequency scan method in a phase-coordinates frame, besides allowing for a more accurate modeling of unbalanced networks, allows the specification of harmonic injections of any type: single-phase or three-phase of the desired sequence. Therefore, it is possible to study the effect of the non-linear loads according to its features and connection to the circuit. This paper develops the phase-coordinates model of an unbalanced distribution power system, considering the representation of three-phase transformer banks of three or two single-phase transformers, as well as other asymmetries that can be found in the electrical systems of industrial plants, commercial or service facilities. The present model improves the representation of the single-phase transformer with mid-tap on the secondary side. The frequency scan based on the present model is tested with a simple example of a four-wire delta system that highlights the possibilities of the model in phase-coordinates.

Evaluation of distribution transformer banks in electric power systems

SUMMARY This article compares the total mass and the total owning cost (TOC) of three-phase distribution transformer banks with standard three-phase distribution transformers. The comparison is based on the minimum TOC. This is achieved through a field-validated distribution transformer design program that automatically minimises the objective function (TOC). In particular, 12 oil-immersed distribution transformers are designed: 6 three-phase transformer banks and 6 three-phase transformers; these designs meet all the requirements of a given transformer standard. As a result, curves of minimum TOC versus transformer rating are obtained for three-phase transformer banks and three-phase transformers. Moreover, similar curves from seven transformer manufacturers are collected; the advantage of this collection is that these different manufacturers have different types of transformers: oil immersed or dry type, core or shell type, various voltage classes and power ratings, and so on, and consequently more general conclusions can be drawn regarding the comparison of three-phase transformer banks and three-phase transformers. From these investigations, it was found that from the viewpoint of minimum total mass and minimum TOC, three-phase transformer banks should be recommended in case of small-size transformers (rating lower than 45 kVA). This is an important finding that is not emphasised in recommended practices reported in transformer textbooks. Copyright © 2011 John Wiley & Sons, Ltd.

00/02622 Modelling and analysis of asymmetrical three-phase distribution transformer banks with mid-tap connected to the secondary neutral conductor

00/02622 Modelling and analysis of asymmetrical three-phase distribution transformer banks with mid-tap connected to the secondary neutral conductor

Evaluation of load division among transformers of different capacities in the grounded wye–delta and open wye–open delta banks under balanced loading and various power factor conditions

This paper evaluates the load division among transformers of different capacities in the grounded wye–delta and open wye–open delta banks under balanced loading and various power factor conditions. In some contingency cases two or three single-phase transformers of different kVA ratings are connected as a three-phase transformer bank to provide three-phase service for critical customers. In these cases, the load division among transformers becomes unequal and results in an unbalanced distribution system. The results of this study corroborate power company experience. Even when both three-phase sources and loads of the bank are balanced, the load division among transformers is unequal if transformers of different capacities are used in a bank, or an open wye–open delta connection is adopted.

Modelling and analysis of asymmetrical three-phase distribution transformer banks with mid-tap connected to the secondary neutral conductor

Abstract This paper presents phase co-ordinate models for five asymmetrical three-phase distribution transformer banks with mid-tap connected to the secondary neutral conductor. On the basis of the proposed models, these three-phase transformer banks are investigated in this paper. To simplify the derivation of the proposed models, the network-overlapping technique was used. The major considerations in the derivations include copper and core losses, phase angular displacement between primary and secondary voltages, and off-nominal tapping. Using appropriate three-phase power-flow programs or commercial simulation software, the inherent characteristics of these five specific transformer banks under a steady-state condition could be correctly retrieved. The comparison of results of field tests and computer simulations have demonstrated the correctness of these proposed models. The models are of value to distribution engineers for improving the simulation accuracy of a distribution system especially under unbalanced circumstances.

Modeling and analysis of unsymmetrical transformer banks serving unbalanced loads

It is a common practice to serve combination three-phase and single-phase loads from an unsymmetrical three-phase transformer bank and a four-wire secondary. Depending upon the loads and company standards, nine different transformer connections can be considered along with either an open four wire or a quadraplex secondary. The selection of the proper connection and transformer ratings can he achieved by the correct modeling and analysis of a system consisting of an equivalent source, transformer bank, secondary, and loads. This paper develops the models for the nine transformer connections, the secondary, and the loads. A computer program has been written that allows the analysis of the system for loading and short-circuit studies. The paper includes several examples of different normal and abnormal operating conditions on some of the transformer connections.

FFT algorithms for the harmonic analysis of three-phase transformer banks with magnetic saturation

New iterative procedures using FFT (fast Fourier transform) algorithms are proposed for harmonic studies of three-phase transformer banks with various connections under steady-state operating conditions. Three-phase transformer banks with different connections produce different harmonic distortions, which are analyzed and compared. Good convergence is achieved for each iterative procedure of the corresponding transformer connection, and the resulting current waveform and its spectrum are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Faraday electrical machines laboratory

The author had the major responsibility for developing a new concept for an electrical laboratory with the aid of industrial grants at the Rensselaer Polytechnic Institute. A description of the laboratory designed to meet specific objectives is covered in the referenced literature. Emphasis is placed on the teaching of the physical understanding of the internal behavior of transformers and rotating machines. Information windings designed into the small machines, accompanied by analog computing elements, provide the basis for the shape measurement philosophy of the laboratory. Details of many unique experiments with typical recorded results dealing with transformers, induction and synchronous machines, and some system phenomena are included. For example, the two components of the total excitation current of a transformer are isolated and recorded individually under steady-state and transient conditions for any line voltage and load. The rotating field is extensively explored and other experiments relate to the slip between the rotatiag field and the rotor of an induction motor. The sudden loading of synchronous machines reveals the transient behavior of internal power, instantaneous speed, and rotor position. The most complex experiment involves ferroresonance in a system consisting of a synchronous generator and a capacitance compensated transmission line terminated in a three-phase transformer bank.

Asymmetrical loading of three-phase three-winding transformer banks

Asymmetrical loading of three-phase three-winding transformer banks

Asymmetrical loading of three-phase three-winding transformer banks

It is accepted USSR practice to carry three-phase industrial load and single-phase traction load on a single three-phase three-winding transformer. For a given combination of loads it is necessary to determine the proper capacity relationships of the three windings.

Asymmetrical Loading of Three-Phase Three-Winding Transformer Banks

It is accepted USSR practice to carry three-phase industrial load and single-phase traction load on a single three-phase three-winding transformer. For a given combination of loads it is necessary to determine the proper capacity relationships of the three windings. This paper presents an analysis of the problem by the methods of symmetrical components. The data obtained from this analysis were used in setting up the standards for selection of transformer size for combined three-phase industrial and single-phase traction loading.

Open-delta transformer banks — Analysis of circuit

WHEN a three-phase transformer bank is operated in open delta, an unbalanced secondary voltage results, and the transformers themselves may be overloaded. The unbalanced voltage will cause a circulating current to flow which may cause overheating of the equipment in the secondary circuit to an extent depending upon the amount of the voltage unbalance and the load being carried. Wolf and Mattison <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> have described a method of overcoming the voltage unbalance, and this paper is written to mention briefly the effect of unbalanced voltages on the operation of induction motors and to present an analysis of the circuit, together with some correlated material.

Open-Delta Transformer Banks - Analysis of Circuit

WHEN a three-phase transformer bank is operated in open delta, an unbalanced secondary voltage results, and the transformers themselves may be overloaded. The unbalanced voltage will cause a circulating current to flow which may cause overheating of the equipment in the secondary circuit to an extent depending upon the amount of the voltage unbalance and the load being carried. Wolf and Mattison1 have described a method of overcoming the voltage unbalance, and this paper is written to mention briefly the effect of unbalanced voltages on the operation of induction motors and to present an analysis of the circuit, together with some correlated material.