Severity factor

Last updated

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.

Contents

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).

Background

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]

Frequency Domain Severity Factor (FDSF)

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.

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 ∆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.

See also

Related Research Articles

<span class="mw-page-title-main">Insulator (electricity)</span> Material that does not conduct an electric current

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.

<span class="mw-page-title-main">Transformer</span> Device to couple energy between circuits

In electrical engineering, 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's core, which induces a varying electromotive force (EMF) across any other coils wound around the same core. Electrical energy can be transferred between separate coils without a metallic (conductive) connection between the two circuits. Faraday's law of induction, discovered in 1831, describes the induced voltage effect in any coil due to a changing magnetic flux encircled by the coil.

<span class="mw-page-title-main">Dielectric</span> Electrically insulating substance able to be polarised by an applied electric field

In electromagnetism, a dielectric 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 material as they do in an electrical conductor, because they have no loosely bound, or free, electrons that may drift through the material, but instead they shift, only slightly, from their average equilibrium positions, causing dielectric polarisation. Because of dielectric polarisation, positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarised, but also reorient so that their symmetry axes align to the field.

<span class="mw-page-title-main">Alternating current</span> Electric current that periodically reverses direction

Alternating current (AC) is an electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans and electric lamps into a wall socket. The abbreviations AC and DC are often used to mean simply alternating and direct, respectively, as when they modify current or voltage.

<span class="mw-page-title-main">Surge protector</span> Protects electrical devices from voltage spikes

A surge protector (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD), transient voltage suppressor(TVS) or transient voltage surge suppressor (TVSS)) is an appliance or device intended to protect electrical devices in alternating current (AC) circuits from voltage spikes with very short duration measured in microseconds, which can arise from a variety of causes including lightning strikes in the vicinity.

In electrical engineering, partial discharge (PD) is a localized dielectric breakdown (DB) of a small portion of a solid or fluid electrical insulation (EI) system under high voltage (HV) stress. While a corona discharge (CD) is usually revealed by a relatively steady glow or brush discharge (BD) in air, partial discharges within solid insulation system are not visible.

<span class="mw-page-title-main">Synchronous motor</span> Type of AC motor

A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. Doubly fed synchronous motors use independently-excited multiphase AC electromagnets for both rotor and stator.

<span class="mw-page-title-main">Isolation transformer</span> Electrical component

An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons or to reduce transients and harmonics. Isolation transformers provide galvanic isolation; no conductive path is present between source and load. This isolation is used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.

<span class="mw-page-title-main">Current transformer</span> Transformer used to scale alternating current, used as sensor for AC power

A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.

Electric power quality is the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady AC frequency close to the rated value, and smooth voltage curve waveform. In general, it is useful to consider power quality as the compatibility between what comes out of an electric outlet and the load that is plugged into it. The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality, and many more causes of such poor quality power.

Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled wet transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

<span class="mw-page-title-main">Arcing horns</span>

Arcing horns are projecting conductors used to protect insulators or switch hardware on high voltage electric power transmission systems from damage during flashover. Overvoltages on transmission lines, due to atmospheric electricity, lightning strikes, or electrical faults, can cause arcs across insulators (flashovers) that can damage them. Alternately, atmospheric conditions or transients that occur during switching can cause an arc to form in the breaking path of a switch during its operation. Arcing horns provide a path for flashover to occur that bypasses the surface of the protected device. Horns are normally paired on either side of an insulator, one connected to the high voltage part and the other to ground, or at the breaking point of a switch contact. They are frequently to be seen on insulator strings on overhead lines, or protecting transformer bushings.

<span class="mw-page-title-main">Electrical treeing</span>

In electrical engineering, treeing is an electrical pre-breakdown phenomenon in solid insulation. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, in a path resembling the branches of a tree. Treeing of solid high-voltage cable insulation is a common breakdown mechanism and source of electrical faults in underground power cables.

<span class="mw-page-title-main">Capacitor</span> Passive two-terminal electronic component that stores electrical energy in an electric field

In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term still encountered in a few compound names, such as the condenser microphone. It is a passive electronic component with two terminals.

<span class="mw-page-title-main">Gyrator–capacitor model</span> Model for magnetic circuits

The gyrator–capacitor model - sometimes also the capacitor-permeance model - is a lumped-element model for magnetic circuits, that can be used in place of the more common resistance–reluctance model. The model makes permeance elements analogous to electrical capacitance rather than electrical resistance. Windings are represented as gyrators, interfacing between the electrical circuit and the magnetic model.

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.

<span class="mw-page-title-main">Korndörfer autotransformer starter</span>

In electrical engineering, the Korndorfer starter is a technique used for reduced voltage soft starting of induction motors. The circuit uses a three-phase autotransformer and three three-phase switches. This motor starting method has been updated and improved by Hilton Raymond Bacon.

<span class="mw-page-title-main">Film capacitor</span> Electrical capacitor with an insulating plastic film as the dielectric

Film capacitors, plastic film capacitors, film dielectric capacitors, or polymer film capacitors, generically called film caps as well as power film capacitors, are electrical capacitors with an insulating plastic film as the dielectric, sometimes combined with paper as carrier of the electrodes.

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.

References

  1. Malewski, R.; Douville, J.; Lavallee, L. (1988). "Measurement of switching transients in 735 kV substations and assessment of their severity for transformer insulation". IEEE Transactions on Power Delivery. 3 (4): 1380–1390. doi:10.1109/61.193935.
  2. Asano, R., Rocha, A., Bastos, G. M., “Electrical Transient Interaction Between Transformers and the Power System”, CIGRÉ A2-D1 Colloquium, Brugge, Belgium, October 2007.
  3. A. C. O. Rocha, “Electrical Transient Interaction Between Transformers and the Power Systems”, CIGRÉ Session 2008, pp. 1-10, Paris France, August 2008.
  4. 1 2 Joint Working Group SC A2 CIGRÉ, "Electrical Transient Interaction between Transformers and Power Systems Archived 2014-11-29 at the Wayback Machine ", Technical Brochure JWGA2/C439, Part-1 Expertise&Part-2 Case Studies, April 2014
  5. Álvarez-Mariño, Casimiro; Lopez-Fernandez, Xose M.; Jacomo Ramos, Antonio J.M.; Castro Lopes, Ricardo A.F.; Miguel Duarte Couto, José (2012). "Time domain severity factor (TDSF)". COMPEL - the International Journal for Computation and Mathematics in Electrical and Electronic Engineering. 31 (2): 670–681. doi:10.1108/03321641211200644.
  6. Lopez-Fernandez, Xose M.; Alvarez-Marino, Casimiro (2015). "Induced Transient Voltage Performance Between Transformers and VCB. Severity Factors and Case Studies". IEEE Transactions on Power Delivery. 30 (3): 1137–1144. doi:10.1109/TPWRD.2014.2352112.

Further reading