A severity factor is established as a
coefficient
In mathematics, a coefficient is a multiplicative factor in some term of a polynomial, a series, or an expression; it is usually a number, but may be any expression (including variables such as , and ). When the coefficients are themselves var ...
to assess the
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
severity supported by a
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
winding considering the incoming transient
overvoltage
When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous. Depending on its duration, the overvoltage event can be transient—a voltage spike—or perm ...
(
voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage (voltage spikes), current (current spikes), or transferred energy (energy spikes) in an electrical circuit.
Fast, short duration electrical transients ...
). It determines the safety margin regarding to the standard acceptance tests either in the
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
or
time domain
Time domain refers to the analysis of mathematical functions, physical signals or time series of economic or environmental data, with respect to time. In the time domain, the signal or function's value is known for all real numbers, for the cas ...
.
Severity factors are a newly concept for analyzing the dielectric severity supported along transformer windings when a transformer is submitted to a non-standardized transient
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
waveform
In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electronic ...
induced
Induce may refer to:
* Induced consumption
* Induced innovation
* Induced character
* Induced coma
* Induced menopause
* Induced metric
* Induced path
* Induced topology
* Induce (musician), American musician
See also
* Inducement (disambiguation ...
from the
power system
An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The ...
.
Two are the new factors considered for evaluating the severity supported by the insulation windings both in factory and in service. One factor is called Time Domain Severity Factor (TDSF) and another one is the Frequency Domain Severity Factor (FDSF).
Background
One first approach to the concept of severity factor was made by Malewski et al. Later, Asano et al. applied the Malewski's idea for further analysis but including the concept of Energy Spectral Density (ESD) associated to the transient voltage wave. A step forward was given by Rocha et al., whom introduced a new coefficient called Frequency Domain Severity Factor (FDSF).
[Joint Working Group SC A2 CIGRÉ,]
Electrical Transient Interaction between Transformers and Power Systems
", Technical Brochure JWGA2/C439, Part-1 Expertise&Part-2 Case Studies, April 2014 For those situations where an internal assessment is necessary a new coefficient named Time Domain Severity Factor (TDSF) was proposed by Casimiro Alvarez-Mariño & Xose M. Lopez-Fernandez.
[Xose m. Lopez-Fernandez and Casimiro Alvarez-Mariño,]
Iknduced Transient Voltage Performance Between Transformers and VCB. Severity Factors and Case Studies
"IEEE Transactions on Power Delivery, Issue 99., April 2015.
Frequency Domain Severity Factor (FDSF)
The FDSF is calculated at transformer
terminal
Terminal may refer to:
Computing Hardware
* Terminal (electronics), a device for joining electrical circuits together
* Terminal (telecommunication), a device communicating over a line
* Computer terminal, a set of primary input and output devic ...
s and it is mathematically defined as
:
where ''ω'' is the
angular frequency
In physics, angular frequency "''ω''" (also referred to by the terms angular speed, circular frequency, orbital frequency, radian frequency, and pulsatance) is a scalar measure of rotation rate. It refers to the angular displacement per unit tim ...
, ESD
noStd(''ω'') is the maximum
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
spectral density
The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, o ...
of the input no-standard transient voltage applied at transformer terminals and ESD
envol(''ω'') is the energy spectral density
envelope
An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.
Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a shor ...
for all standards dielectric tests at terminals.
Time Domain Severity Factor (TDSF)
The TDSF gives further detailed information on the severity supported by the transformer windings due to the transient event coming from the power system, regarding to the internal transient response due to dielectric tests in the time domain. The mathematical expression of this factor is
:
where ∆V
noStd(''i'') is the maximum voltage drop along the ''i''th dielectric path due to the no-standard transient events and ∆V
envol(''i'') is the maximum
voltage drop
Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirable ...
along the same ''i''th dielectric path for all standards dielectric tests.
See also
*
Transformer oil testing
Transformer oil, a type of insulating and cooling oil used in transformers and other electrical equipment, needs to be tested periodically to ensure that it is still fit for purpose. This is because it tends to deteriorate over time. Testing sequ ...
References
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{{Electric transformers
Electric transformers