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1

Transients in Transformers with Non-Uniform Inductance and Capacitance Distributions

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2017

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Vol. 20, no. 1

7--14

EN

The electromagnetic transients in transformer windings exhibiting location–dependent inductances and capacitances are investigated in the time domain. Analytical functions describing this dependence are assumed and incorporated in the two integro–differential equations governing the transient voltage and current distributions. The boundary conditions are available from the source initiating the transients and the winding’s end termination. A numerical procedure is applied in order to get frequency domain solutions for the voltage and current in the form of Interpolating and Parametric Functions. The numerical Laplace inversion is then applied to these s–domain expressions. Results pertinent to transients initiated by step- and double-exponential impulse sources are presented and discussed. All possible transformers’ neutral connections are considered. The possible error introduced by neglecting either or both of the inductance and capacitance non-uniformities is addressed. Results indicate that the main error is attributed to neglecting the inductance non-uniformity, whereas the impact of the capacitance non-uniformity is relatively small. In most cases, the winding’s copper and insulation losses have a small effect on the transient response.

2

Modifying the Transient Overvoltages in Mixed Power Networks by Inserting Cable Sections

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2015

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Vol. 18, iss. 1

7--14

EN

The paper deals with the analysis of the electromagnetic transients in mixed power net-works. Special emphasis is made on assessing the effectiveness of using cable sections in reducing the transients in the power network components such as transformer substations. A distributed parameter modeling of the overhead lines, underground cables and transformer windings is applied in the Laplace domain. The simulation can handle the different time waveforms of the sources initiating the transients, the lengths of the cable sections as well as the transformers’ neutral treatment. The direct analytical s-domain solution is numerically inverted in order to get the corresponding time domain results. The affecting parameters such as the line and cable surge impedances, the length of the cable section (or its time delay) and the transformer data, are investigated. A case involving multiple-pulse lightning surges is also addressed. The results of four case studies of known solutions are presented in order to validate the developed mathematical model and computer program.

3

Transformer Modelling for the Frequency and Transient Analyses, with Non-Uniform Inductance Emulating the Inter-Turn Magnetic Coupling

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2014

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Vol. 17, iss. 1

1--11

EN

A new procedure for analyzing power transformer windings with location-dependent circuit parameters, such as the series inductance, is presented. This dependence is introduced in order to take the inter-turn mutual inductive coupling into consideration. It can be expressed either by analytical expressions or even in a tabulated form. The paper addresses both the frequency and time domain analyses. They are based on replacing the winding by an adequate number of equivalent cascade connected two-ports. In contrast to the usual practice of applying the simple medium line representation, each of these two-ports is treated as a long transmission line. Their A, B, C, D generalized circuit constants will be therefore generally location-and frequency-dependent. The analyses will be conducted in the complex s-domain. The corresponding time-domain results can then be obtained by applying a numerical inverse Laplace transform. Expressions for the winding’s input impedance with different treatments of the transformer’s neutral point will be derived for any assumed number of the equivalent two-ports. Results pertinent to the frequency characteristics including the resonance frequencies are presented. The paper also describes the winding’s transient response to the application of two standard voltage stimuli. The suggested approach is validated by its application to a case study for which an analytical closed-form solution is available. The analysis of windings exhibiting nonuniformities in more than one equivalent.

4

A Study on the Low Frequency Transients in Power Transformers and Some Protection-Related Issues

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2014

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Vol. 17, iss. 2

9--16

EN

This paper presents typical results for the non-linear low frequency transients following the energization of power transformers. The three operating conditions of no-load, full-load and internal faults are considered. A direct analytical procedure is applied for solving the corresponding set of differential equations describing the transformers equivalent circuit. The core representation is based on the use of curve fitting applied to their magnetization curves. The results include plots versus time for the supply current, the core flux, the magnetizing current as well as the internal induced voltage. Moreover, graphs for the hysteresis loops relating the instantaneous values of both the excitation current and the core flux are given. An approach is also presented for the numerical determination of the amplitudes of the different harmonics existing in any of these signals utilizing the corresponding equidistant samples. As examples, the DC offset as well as harmonics up to the fourth order are considered. It is shown that several useful features can be extracted from the results in both the time and harmonic domains that can assist in differentiating between overcurrents resulting from short-circuits or inrush phenomena.

5

Corona Modeling for the Transient Analysis, Steady State and Corona Loss Performance of Transmission Lines

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2013

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Vol. 16, iss. 2

11--20

EN

This paper addresses the steady state analysis and the electromagnetic transients of overhead power transmission lines exhibiting corona. After a brief survey of the currently available modeling techniques, an alternative approach to the simulation of the corona discharge is presented. It is based on adding voltage-dependent current sources at prespecified nodes within the line’s lumped, ladder-shaped, equivalent circuit including an aduqate number of sections. The characteristics of these current sources can be determined using the usually available corona power loss measurements under power frequency sinusoidal voltage conditions. The line model is used for finding its transient response using time domain techniques. Several voltage excitation waveforms and magnitudes as well as different line’s loading conditions are investigated. As a further application of the suggested approach, the minimizion of the total corona losses of a long shunt compensated line will be considered as an additional criterion in the process of identifying the appropriate rating and location of the compensating coil primarily used for improving the line’s voltage profile.

6

Circuit for Balancing Harmonic-Polluted Three-Phase Networks

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2013

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Vol. 16, iss. 1

19--24

EN

This paper presents a new circuit capable of balancing harmonics-polluted three-phase power networks supplying single-phase loads. It takes into account the negative and positive sequences exhibited by the fifth and seventh harmonics, respectively. The concept extends the relations of the Steinmetz circuit, which are based exclusively on fundamental frequency considerations. The Steinmetz circuit does not, therefore, guarantee the balancing of the different harmonics in the line currents. Furthermore, it can lead to resonances between the capacitive and inductive elements in the two added balancing branches. A procedure for identifying the topology and the circuit parameters of the proposed connection is given. The results of a detailed case study are presented in order to demonstrate the superiority of the proposed circuit over the conventional Steinmetz connection.

7

The Impact of the Load Characteristics on the Assessment and Optimization of the Shunt Compensation of Distribution Feeders

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2009

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Vol. 15, iss. 1

9--14

EN

The paper gives the results of applying an alternative procedure for the economical as-sessment and optimization of the shunt compensation of radial power distribution feeders with con¬centrated and/or distributed loading, taking into account the feeder copper losses as well .is the installation cost of capacitors. In addition to the energy and capacitor specific costs, the objective function depends on the si/e and location of the capacitor, the feeder resistance, the voltage level, and the load current. The results <»f a detailed parameter study on the impact of the load parameters are presented. The study focuses primarily on the impact of the load factor, the load power factor as well as the feeder's loading profile.

8

Methods for Canceling the Power Frequency Magnetic and Electric Fields at Pre-Specified Points on the Lines’ Corridors

Saied M. M.

Electrical Power Quality and Utilisation. Journal

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2009

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Vol. 15, iss. 2

13--19

EN

Methods are presented for the cancellation of the power line’s magnetic and electric fields at pre-specified points close to the line corridor. For canceling the magnetic field, three auxiliary conductors constituting two mitigating loops are used. Their configuration should be geometrically similar to the arrangement of the 3-phase power line’s conductors, with respect to the point at which a zero magnetic field is required. A procedure is suggested for the optimal selection of the scaling factor relating the power conductors’ and the mitigating conductors’ geometrical dimensions. In a case study, the magnetic field reduction varies between 100% near the zero-field point to 35% to the right of that point, with an optimal scaling factor of 0.15. Similar procedure can be used for cance¬ling the electric field at a pre-specified point. The three auxiliary conductors should carry charges proportional to those on the active conductors, but of opposite signs. Results show that the field at the specified point will be exactly zero, instead of the original value of 2.6kV/m. The three voltages required to supply the auxiliary conductors are 76.62, 62.53 and 51.50kV, with almost equal phase angles. The fact that the potentials of the three auxiliary conductors are almost in phase suggests the possibility of using one transformer with appropriate taps to supply them.