On the significance of recent EHV transformer failures involving winding resonance

McElroy, A. J.

doi:10.1109/t-pas.1975.31968

Cited by 37 publications

(11 citation statements)

References 11 publications

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“…The relationship between the currents and the voltages of the branches can be described by the following equation: (4) where (order ) is the vector of the voltages at the corresponding (series or auxiliary) inductances of the Foster circuits . .…”

Section: Equivalent Circuitmentioning

confidence: 99%

New Equivalent Circuit of Transformer Winding for the Calculation of Resonance Transients Considering Frequency-Dependent Losses

Eslamian

Vahidi

2015

IEEE Trans. Power Delivery

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In this paper, a new equivalent circuit for the transformer winding is presented by which the eddy current losses in the transformer can be represented accurately at high frequencies.The new model enables representing the frequency-dependent impedances of a coil system using the idea of mutually coupled series Foster circuits. An effective method for the determination of model parameters is also provided which involves using the vector-fitting (VF) method for the rational approximation of the system impedance matrix using a certain number of common stable poles. A set of nonlinear equations is then established using the VF results that can be solved with the method of fixed-point iteration to obtain the parameters of the equivalent circuit in a fast and accurate way. The new model can be used for the calculation of resonance transients within the transformer, the situation wherein the maximal value of overvoltages is highly dependent to the damping effects or the loss phenomena in the transformer. Since the model only contains constant lumped parameters, it can be used for transient calculations in the time domain directly using the electromagnetic transients programs, such as the Alternative Transients Program or Electromagnetic Transients Program.

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Section: Equivalent Circuitmentioning

confidence: 99%

New Equivalent Circuit of Transformer Winding for the Calculation of Resonance Transients Considering Frequency-Dependent Losses

Eslamian

Vahidi

2015

IEEE Trans. Power Delivery

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“…The developed voltage should not be of much concern as the frequency of the transient voltage is different form the resonate frequencies of the main and tap winding coils of the 400/220kV substation transformer connected to this bus. To analyze the phenomena in depth, a wide range of capacitor values from 0.42µF to 1 21.4kHz respectively. The results of simulation have been presented in Fig.…”

Section: (A) Transformer Energizationmentioning

confidence: 99%

“…Such power system transients comprise of voltages and currents of very complex wave shapes, embodying broad spectrum of frequencies and due to their comparatively lesser amplitude they generally remain undetected by surge protectors and are impressed on the transformer without much attenuation. Oscillatory transient voltages generated out of perturbations in electrical grids are therefore considered to be source of threat as they often overstress transformer insulations due to development of high internal voltage triggered by internal resonance [1,2] in the windings.…”

Section: Introductionmentioning

confidence: 99%

Study of the voltage stresses developed in grid connected EHV transformers under switching conditions in power grids

Debnath¹,

De²,

Chakrabarti³

2006

2006 IEEE Power India Conference

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For many years high voltage lightning and switching impulses simulated in the laboratory have been used to ascertain breakdown strength of insulation materials used in the transformer. On the contrary, a transformer in actual field operation encounters voltage transients of rather complex and varying wave shapes. Transformers are complex assembly of coils around magnetic core having inductances and capacitances and thus, exhibit unique frequency response characteristics with a number of natural resonate frequencies. Electrical transients generated in power systems due to switching and other operations often traverse through transformers and can excite one or more of the transformer's natural resonate frequencies resulting severe internal voltage amplification. It is important for the transformer designer to anticipate the problem and to know how the windings will respond to such system transients that can trigger internal resonances.In the present paper the authors have explored the occurrences of oscillatory switching transients in the Eastern Regional Grid of India and analyzed the behavioral characteristics of a 400/220kV grid connected transformer under such transients. The results have indicated that the voltage stresses developed in the windings under certain switching conditions can exceed the stresses developed under 1.2/50 µ µ µ µs standard lightning impulses at transformer's BIL voltage.

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“…Due to the close spacing of coils, transformers have significant inductance and capacitance, which make the transformer windings oscillatory structures, having a number of internal resonate frequencies [1][2][3]. Surge-type voltages, generated out of the lightning strike, switching events or other system transients can enter and travel through a transformer winding and excite these resonances, causing voltage amplification inside the winding [3][4][5]. It is important for the transformer designers to anticipate these internal phenomena and adopt design techniques to ensure that the voltage disturbance entering the winding does not result in excessive voltage stress at any point of the winding.…”

Section: Introductionmentioning

confidence: 99%

Self‐capacitance grading approach to suppress winding resonance in EHV transformers under oscillatory terminal disturbances

De¹,

Bandyopadhyay

2018

High Voltage

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Interleaving of the high-voltage winding turns is a widely-accepted method for restraining the surge voltage stresses in extra high voltage (EHV) transformers. Reduction of voltage stresses under external surge is attributed by the increase in the self-capacitance of the interleaved winding disks. However, the apparent benefit of interleaving may be counterpoised by a significant decrease of natural frequency of windings, which in turn, may force a winding to resonate under external surges. Winding resonance is particularly noticeable when uniform interleaving is adopted for the whole winding, uniform interleaving attributes a unique natural frequency to all interleaved coils and consequently, the collective resonance of the winding disks is plausible. Uniform interleaving in large EHV transformers therefore leads to complications and the windings reportedly exhibit an unexpectedly poor surge voltage response because of such internal resonances. This study describes a novel concept termed as 'graded interleaving' for EHV transformer windings to overcome resonance and to achieve an improved response against surge voltage. The performance of the proposed winding was tested on a 132/33 kV two-winding transformer with a tap changer using EMTP(ATP) simulations with encouraging results. Manufacturing and real-time implementation feasibility of the winding were verified with transformer manufacturers.

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On the significance of recent EHV transformer failures involving winding resonance

Cited by 37 publications

References 11 publications

New Equivalent Circuit of Transformer Winding for the Calculation of Resonance Transients Considering Frequency-Dependent Losses

New Equivalent Circuit of Transformer Winding for the Calculation of Resonance Transients Considering Frequency-Dependent Losses

Study of the voltage stresses developed in grid connected EHV transformers under switching conditions in power grids

Self‐capacitance grading approach to suppress winding resonance in EHV transformers under oscillatory terminal disturbances

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