Three-phase Power Transformer Research Articles

Measurements and Correction of No-Load Losses of Power Transformers

This paper investigates the practice of correcting the results of no-load loss measurements of power transformers for the effect of distorted supply voltage waveform. A three-phase 700-MVA power transformer is tested twice: first with its low-voltage windings connected line-to-neutral, with nearly sinusoidal test voltage conditions, and then for the second test with its windings connected line-to-line with more distorted voltage waveforms. The test data obtained during the measurement was reproduced at the National Research Council (NRC) Laboratory to demonstrate the limits of the application of the loss correction formula currently used.

A New Transient Model for Three-Phase Power Transformers Using a Wavelet Filter Bank

This paper presents a model for three-phase power transformers for electromagnetic transient studies, which employs a wavelet filter bank. In this model, both the primary and the secondary windings are represented by stages of a wavelet filter bank, where each turn is represented by one complete stage. The sum of responses of all stages forms the transformer total response. Using the wavelet filter bank model eddy currents, leakage flux and inrush currents have been simulated. In addition, the proposed model has been found to be suitable for simulating different currents with various transient components including inrush and high frequency effects.

Numerical Modeling of Magnetic Devices

We present a general approach to directly couple finite-element models with arbitrary electric circuits for application to electromagnetic devices. We describe both two-dimensional (2-D) and three-dimensional (3-D) transient finite-element models, with emphasis on 3-D using a T-/spl Omega/ formulation. For 3-D transient and circuit coupling, the derivation of the induced voltage is an integral part of the coupling approach, and the induced voltage links the magnetic field and the electrical circuit together. The system of electric circuits is created automatically. Then graph theory is used to deduce the circuit by tree/cotree and loop analysis. The resulting field equations and circuit equations are coupled together and solved simultaneously at each time step in the time domain. We give three examples of applications: a brushless dc motor drive, a permanent-magnet synchronous motor drive, and a three-phase power transformer with rectifier and load circuit.

Hybrid Numerical Techniques for Power Transformer Modeling: A Comparative Analysis Validated by Measurements

The paper presents alternative numerical techniques implemented in power transformer analysis and design focusing on the short circuit impedance evaluation. The proposed method adopts a particular reduced scalar potential formulation enabling a three-dimensional (3-D) magnetostatic problem solution. This method, requiring no source field calculation, in conjunction with a mixed finite-element/boundary-element technique, results in a very efficient 3-D numerical model for power transformer design office use. This model is used to develop an effective computational tool, enabling the accurate transformer characteristics prediction, thus increasing its reliability and reducing its production cost. The computed results of the proposed methodology are validated through measurements in the case of a three-phase wound core power transformer.

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

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

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.

COMBINATION OF FINITE AND BOUNDARY ELEMENT METHODS IN INVESTIGATION AND PREDICTION OF LOAD-CONTROLLED NOISE OF POWER TRANSFORMERS

A recently developed calculation scheme for the computer modelling of the load-controlled noise of oil-insulated three-phase power transformers is presented. This modelling scheme allows the precise and efficient computation of the coupled electromagnetic, mechanical and acoustic fields. The equations are solved using the finite element method (FEM) as well as the boundary element method (BEM), resulting in a separation of the calculation of the winding and tank surface vibrations (using FEM) and the computation of the acoustic free-field radiation (using BEM). The complex dynamic behaviour of the loaded transformer can then be studied and, furthermore, an appropriate computer-aided design including an investigation and optimization of design parameters can be established.The validity of the computer simulations has been verified by means of appropriate measurements. Simulated and measured values for winding and tank surface vibrations as well as sound power levels of the loaded transformer are found to be in good agreement. The applicability of the calculation scheme with respect to the computer-aided design of power transformers is demonstrated by reporting two practical applications: the influence of the stiffness of winding supports and the influence of the tap changer positions.

HTS in electric power applications, transformers

With the discovery of the high temperature superconductors the prospect to find widespread applications of these conductors also in electric power applications grew very fast. The advantages that could be anticipated was reduced losses, reduced weight and volume plus less environmental impact. During the last five-six years techniques have been developed to produce long lengths of flexible conductors that can be used in cables or windings. Most manufacturers of high temperature superconductors concentrated on traditional DC applications. However, much larger markets will open up when good enough AC conductors become available for power applications. In 1994 ABB started a project aiming at a 630 kVA three-phase power transformer with windings made of high temperature superconductors. This transformer was taken into operation in the spring of 1997 and operated perfectly for one year. I will discuss the performance and price demands the high temperature superconductors have to fulfill to be useful in power applications and review the above mentioned transformer project and also give some insight in a running project with the goal to design, build and put in operation a 10 MVA power transformer with high temperature superconductors with low AC-losses in the windings.

Improved design of cover plates of power transformers for lower eddy current losses

Due to the heavy current passing through the steel cover plates of power transformers, some areas are subject to serious local overheating. This affects the safety and reliability of the expensive equipment and power delivery. In this paper, the improved design of cover plates is presented to reduce eddy current losses and to protect them from local overheating. For the accurate calculation of eddy current losses and power loss density distribution in the cover plates, the indirect BIEM (boundary integral equation method) is applied to the 3D open boundary problem caused by the heavy current of leads in a three-phase power transformer. The numerical results by the indirect BIEM are compared with experimental values.

Behavior of three-phase transformers supplying non linear loads using time domain representation and finite element analysis

This paper aims to present and discuss the magnetic and electrical results derived from three-phase power transformers modeling under nonlinear load conditions. The approach uses time-domain techniques and the finite element method. The transformer internal electromagnetic equations and the external equivalent circuit are simultaneously solved through a well-established computational package. The nonlinear load is a 6-pulse rectifier supplied by a nonlinear modeled power transformer. The waveform for different currents and voltages, as well as the flux line distribution are given and fully described. The results obtained from the simulation are compared to experimental tests.

A self-organizing fuzzy logic based protective relay-an application to power transformer protection

In the paper the fuzzy logic based multi-criteria protective relay for a three-phase power transformer is presented. Twelve criteria used to stabilize the relay are introduced and integrated by means of multi-objective decision-making methods. The introduced protection scheme contains several internal functions and coefficients which are subject to off-line pre-installation self-setting. Three unique procedures are delivered for self-adjusting the relay, and thus, making it self-organized. The results of testing show significant gains in both sensitivity and selectivity of the self-organized relay compared with traditional approaches.

Power loss in electrical steel under elliptically rotating flux conditions

The power loss of electrical steel sheet given in the data sheets of the steel manufacturers is related to linearly alternating flux conditions, measured with an Epstein frame or a single sheet tester, respectively. In the application of the material in electrical machines, we find large areas with rotational flux conditions, i.e., in the T-joint region of three-phase power transformers or above the stator teeth of three-phase motors and generators. The most general description of the magnetization process in this case is an elliptically rotating flux. The paper outlines the magnetic behavior of steel sheet under these flux conditions, and finally defines a simple method to predict the total power loss under elliptically rotating flux from data measured under linearly alternating and circularly rotating flux conditions only.

Reactive power demand of transformers with DC bias

It has been observed that the presence of DC bias currents/fluxes in power transformers generates harmonic currents and voltages with an associated increase in reactive power (kVAr) demand resulting in a voltage drop. Such DC biases can occur: in transformers near high-voltage DC (HVDC) terminals; due to imperfect or half-controlled rectification resulting in DC, even and odd harmonics; because of geomagnetically induced currents; as a consequence of nuclear explosions changing the Earth's magnetic field; and due to rectifiers working in a normal (e.g., half-wave) mode. The actual network conditions causing such kVAr demand and its suppression, canceling, or mitigation are not very well understood, particularly for transformers with three legs. This article explains for the first time this kVAr demand and its suppression based on experimental data. Tests were performed for three-phase power transformers with three legs, with or without the influence of a magnetic tank.

Transient model and simulation in three-phase three-limb transformers

When a three-phase three-limb power transformer, whose primary windings are Y-connected with isolated neutral, is energized or reclosed by asynchronous closure of the circuit breaker poles, the transient voltage at the still disconnected transformer terminal and the transient current flowing from the connected windings may reach considerably high values. This can cause serious damage to equipment connected to the transformer. This transient phenomenon is closely related to the switching sequence of the circuit breaker poles, switching-on angles, remanent flux and the magnetic saturation characteristics of the transformer's iron core. This paper presents a simple and effective method to predict the transient phenomenon of power transformers. The proposed method formulates a model to represent the transient under various operating conditions. Simulations and experiments are both demonstrated to support the proposed method. >

Development and practical operation of perfluorocarbon immersed 275 kV transformers with compressed SF/sub 6/ gas insulation

A perfluorocarbon (PFC) immersed 275 kV power transformer with compressed SF/sub 6/ gas insulation has been under development. This paper clarifies the AC partial discharge inception voltage and time characteristics of PFC immersed insulation and also clarifies that a prototype 275 kV 100 MVA three-phase transformer could be operated without any trouble during long-term overvoltage tests. This prototype proved that it had AC partial discharge inception strength of higher than 1.5 times the AC test voltage and lightning impulse breakdown strength of 1.5 times the test voltage. A 275 kV, 250 MVA three-phase power transformer was developed and operated at the outdoor substation of the Chubu Electric Power Co., Japan. This transformer has been successfully operated to date and a detailed internal inspection of the transformer was carried out after one year and 9 months of successful practical operation. No significant abnormal condition was recognized. >

Application of the coenergy model to different electrotechnical devices: comparison of different magnetic sheets

Models taking into account the anisotropy and nonlinearity of doubly oriented, grain oriented and nonoriented electrical sheets, were implemented in finite element CAD software. These models based on the coenergy density have been used to model different electrical power devices and to compare their performance. The doubly oriented sheets proved to be more suitable than the conventional soft magnetic materials. The power devices modelled were: a large generator, a small synchronous motor, a T-joint of three-phase power transformer, and a small punched transformer.

Integrated digital power transformer protection

In the paper the hardware and software prototype realisation of the integrated digital three-phase power transformer protection for selective operation with all kinds of short circuits, ground faults and turn-to-turn faults is presented. The proposed integrated protection concept was developed on the basis of a nonlinear mathematical transformer model which includes nonlinear effects of saturation, hysteresis and eddy currents. The protection algorithm includes a percentage differential relay with harmonic restraint at inrush and overexcitation conditions, an overcurrent relay with instantaneous and time-lag operation, a sensitive relay for selective operation at high-impedance ground faults and a fault recorder. The protection algorithm was implemented on powerful signal processor. Presented are the results of real-time laboratory and field testing which include the comparison of the standard static protection performance with the proposed digital one in relation to speed and accuracy.

Dynamic model of a three-phase power transformer

An adequate mathematical model of a three-phase power transformer is one of the important elements in the programs for the computer analysis of power system transients. Featured in this paper is the simulation model of a three-phase, three-limb core-type power transformer. Nonlinear effects of saturation, hysteresis and eddy currents are considered. Two ways of creating major and minor hysteresis loops are presented. The transformer model, described by a system of time dependent differential equations, is solved by an efficient numerical algorithm. The behaviour of the transformer model during switching-in and fault transients, as well as other types of transients, has been tested. The computed transient waveforms are compared with the measured ones and there exists very close agreement between them.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A stand-along digital protective relay for power transformers

The complete design of a stand-alone prototype digital protective relay for three-phase power transformers is described. The major emphasis is on the detailed description of the hardware and software of the prototype relay. The protection functions implemented include: a percentage differential protection with a second-harmonic restraint for magnetizing inrush and a fifth-harmonic restraint for overexcitation conditions, and a separate protection for high-impedance primary and secondary ground faults. The relay design is tested with the Fourier algorithm, and any other relay algorithm can be used by replacing only one subroutine. The relay hardware consists of a data acquisition board and a digital processing board which is based on the TMS320E15 processor. Sample real-time test cases are discussed. The results show that the relay never misoperated and correctly identified all the faults.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Stand-alone Digital Protective Relay for Power Transformers

The complete design of a stand-alone prototype digital protective relay for three-phase power transformers is described. The major emphasis is on the detailed description of the hardware and software of the prototype relay. The protection functions implemented include: a percentage differential protection with a second-harmonic restraint for magnetizing inrush and a fifth-harmonic restraint for overexcitation conditions, and a separate protection for high-impedance primary and secondary ground faults. The relay design is tested with the Fourier algorithm, and any other relay algorithm can be used by replacing only one subroutine. The relay hardware consists of a data acquisition board and a digital processing board which is based on the TMS320E15 processor. Sample real-time test cases are discussed. The results show that the relay never misoperated and correctly identified all the faults. >