A severity factor is established as a coefficient to assess the dielectric severity supported by a transformer winding considering the incoming transient overvoltage (voltage spike). It determines the safety margin regarding to the standard acceptance tests either in the frequency or time domain.
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 waveform induced from the power system.
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).
One first approach to the concept of severity factor was made by Malewski et al. [1] 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. [2] A step forward was given by Rocha et al., whom introduced a new coefficient called Frequency Domain Severity Factor (FDSF). [3] [4] 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. [4] [5] [6]
The FDSF is calculated at transformer terminals and it is mathematically defined as
where ω is the angular frequency, ESDnoStd(ω) is the maximum energy spectral density of the input no-standard transient voltage applied at transformer terminals and ESDenvol(ω) is the energy spectral density envelope for all standards dielectric tests at terminals.
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 ∆VnoStd(i) is the maximum voltage drop along the ith dielectric path due to the no-standard transient events and ∆Venvol(i) is the maximum voltage drop along the same ith dielectric path for all standards dielectric tests.
An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors. The most common examples are non-metals.
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This is an alphabetical list of articles pertaining specifically to electrical and electronics engineering. For a thematic list, please see List of electrical engineering topics. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.
In electric power, a bushing is a hollow electrical insulator that allows an electrical conductor to pass safely through a conducting barrier such as the case of a transformer or circuit breaker without making electrical contact with it. Bushings are typically made from porcelain, though other insulating materials are also used.
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VLF cable testing is a technique for testing of medium and high voltage cables. VLF systems are advantageous in that they can be manufactured to be small and lightweight; making them useful – especially for field testing where transport and space can be issues. Because the inherent capacitance of a power cable needs to be charged when energised, system frequency voltage sources are much larger, heavier and more expensive than their lower-frequency alternatives. Traditionally DC hipot testing was used for field testing of cables, but DC testing has been shown to be ineffective for withstand testing of modern cables with polymer based insulation. DC testing has also been shown to reduce the remaining life of cables with aged polymer insulation.
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Condition monitoring of transformers in electrical engineering is the process of acquiring and processing data related to various parameters of transformers to determine their state of quality and predict their failure. This is done by observing the deviation of the transformer parameters from their expected values. Transformers are the most critical assets of electrical transmission and distribution systems, and their failures could cause power outages, personal and environmental hazards, and expensive rerouting or purchase of power from other suppliers. Identifying a transformer which is near failure can allow it to be replaced under controlled conditions at a non-critical time and avoid a system failure.
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 sequences and procedures are defined by various international standards, many of them set by ASTM. Transformer oil testing consists of measuring breakdown voltage and other physical and chemical properties of samples of the oil, either in a laboratory or using portable test equipment on-site.
