Condition monitoring of transformers

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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 failures can occur due to various causes. Transformer in-service interruptions and failures usually result from dielectric breakdown, winding distortion caused by short circuits, hots spots caused by localized deviations in winding and electromagnetic fields, deterioration of insulation, effects of lightning and other electrical disturbances, inadequate maintenance, loose connections, overloading, or failure of accessory components (e.g.: OLTCs, bushings, etc). [1] Accounting for these causes through monitoring can allow for the determination of the overall condition of the transformer.

Aspects

The important aspects of condition monitoring of transformers are: [2]

Thermal modelling – The useful life of a transformer is partially determined by the ability of the transformer to dissipate its internally generated heat to its surroundings. [3] The comparison of actual and predicted operating temperatures can provide a sensitive diagnosis of the transformer condition and might indicate abnormal operation. Consequences of temperature rise include gradual deterioration of insulation, damage which is very costly. To predict this, thermal modelling is used to determine the top transformer oil temperature and hot spot temperature (the maximum temperature occurring in the winding insulation system) rise.[ citation needed ]

Dissolved gas analysis – The degradation of transformer oil and solid insulating materials produces gases, which are generated at a more rapid rate when an electrical fault occurs. [4] By evaluating the concentration and proportion of hydrocarbon gasses, hydrogen, and carbon oxides present in the transformer, it is possible to predict early stage faults in three categories: corona or partial discharge, thermal heating, and arcing. [5]

Frequency response analysis – When a transformer is subjected to high currents through fault currents (abnormal currents), the mechanical structure and windings are subjected to severe mechanical stresses causing winding movement and deformations. It may also result in insulation damage and turn-to-turn faults. [6] Frequency response analysis (FRA) is a non-intrusive and sensitive technique for detecting winding movement faults and assessing the deformation caused by loss of clamping pressure or by short-circuit forces. FRA technique involves measuring the impedance of the windings of the transformer with a low-voltage sine input varying in a wide frequency range. [7]

Partial discharge (PD) analysis – Partial discharge occurs when a local electric field exceeds a threshold value, partially breaking the surrounding medium. Its cumulative effect leads to the degradation of insulation. [8] PDs are initiated by defects during manufacture or higher stress dictated by design considerations. Measurements can be collected to detect these PDs and monitor the soundness of insulation. PDs manifest as sharp current pulses at transformer terminals, whose nature depends on the types of insulation, defects, measuring circuits, and detectors used. [9]

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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

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.

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">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">Electric arc</span> Electrical breakdown of a gas that results in an ongoing electrical discharge

An electric arc is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An arc discharge is initiated either by thermionic emission or by field emission. After initiation, the arc relies on thermionic emission of electrons from the electrodes supporting the arc. An arc discharge is characterized by a lower voltage than a glow discharge. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".

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Condition monitoring is the process of monitoring a parameter of condition in machinery, in order to identify a significant change which is indicative of a developing fault. It is a major component of predictive maintenance. The use of condition monitoring allows maintenance to be scheduled, or other actions to be taken to prevent consequential damages and avoid its consequences. Condition monitoring has a unique benefit in that conditions that would shorten normal lifespan can be addressed before they develop into a major failure. Condition monitoring techniques are normally used on rotating equipment, auxiliary systems and other machinery like belt-driven equipment,, while periodic inspection using non-destructive testing (NDT) techniques and fit for service (FFS) evaluation are used for static plant equipment such as steam boilers, piping and heat exchangers.

<span class="mw-page-title-main">Predictive maintenance</span> Method to predict when equipment should be maintained

Predictive maintenance techniques are designed to help determine the condition of in-service equipment in order to estimate when maintenance should be performed. This approach promises cost savings over routine or time-based preventive maintenance, because tasks are performed only when warranted. Thus, it is regarded as condition-based maintenance carried out as suggested by estimations of the degradation state of an item.

<span class="mw-page-title-main">Bushing (electrical)</span>

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.

<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.

In Electrical Power Systems and Industrial Automation, ANSI Device Numbers can be used to identify equipment and devices in a system such as relays, circuit breakers, or instruments. The device numbers are enumerated in ANSI/IEEE Standard C37.2 "Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations".

Dissolved gas analysis (DGA) is an examination of electrical transformer oil contaminants. Insulating materials within electrical equipment liberate gases as they slowly break down over time. The composition and distribution of these dissolved gases are indicators of the effects of deterioration, such as pyrolysis or partial discharge, and the rate of gas generation indicates the severity. DGA is beneficial to a preventive maintenance program.

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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Sweep frequency response analysis (SFRA) is a method to evaluate the mechanical integrity of core, windings and clamping structures within power transformers by measuring their electrical transfer functions over a wide frequency range.

A severity factor is established as a coefficient to assess the dielectric severity supported by a transformer winding considering the incoming transient overvoltage. It determines the safety margin regarding to the standard acceptance tests either in the frequency or time domain.

<span class="mw-page-title-main">Current sensing</span>

In electrical engineering, current sensing is any one of several techniques used to measure electric current. The measurement of current ranges from picoamps to tens of thousands of amperes. The selection of a current sensing method depends on requirements such as magnitude, accuracy, bandwidth, robustness, cost, isolation or size. The current value may be directly displayed by an instrument, or converted to digital form for use by a monitoring or control system.

References

  1. ↑ Arvind Dhingra, Singh Khushdeep and Kumar Deepak, "Condition monitoring of power transformer: A review." Transmission and Distribution Conference and Exposition, 2008. T&D. IEEE/PES. IEEE, 2008.
  2. ↑ W. H. Tang and Q. H. Wu, “Condition Monitoring and Assessment of Power Transformers Using Computational Intelligence”, Springer, 2011
  3. ↑ Tang, W. H., Q. H. Wu, and Z. J. Richardson. "Equivalent heat circuit based power transformer thermal model." Electric Power Applications, IEE Proceedings-. Vol. 149. No. 2. IET, 2002. ISSN 1350-2352.
  4. ↑ Emsley, A. M., and G. C. Stevens. "Review of chemical indicators of degradation of cellulosic electrical paper insulation in oil-filled transformers." Science, Measurement and Technology, IEE Proceedings-. Vol. 141. No. 5. IET, 1994.
  5. ↑ Wang, Dian. Ontology-based fault diagnosis for power transformers. Diss. University of Liverpool, 2011.
  6. ↑ Abu-Elanien, Ahmed EB, and M. M. A. Salama. "Survey on the transformer condition monitoring." Power Engineering, 2007 Large Engineering Systems Conference on. IEEE, 2007.
  7. ↑ Gonzalez, Carlos, et al. "Transformer diagnosis approach using frequency response analysis method." IEEE Industrial Electronics, IECON 2006-32nd Annual Conference on. IEEE, 2006.
  8. ↑ Bartnikas, R. "Partial discharges. Their mechanism, detection, and measurement." Dielectrics and Electrical Insulation, IEEE Transactions on 9.5 (2002): 763-808.
  9. ↑ Stone, G. C., et al. "Practical implementation of ultrawideband partial discharge detectors." Electrical Insulation, IEEE Transactions on 27.1 (1992): 70-81.
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