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Reference Number NIA_NGTO0050
Title New online tools for Assessment of Bushing Condition (Not-ABC)
Status Completed
Energy Categories Other Power and Storage Technologies(Electricity transmission and distribution) 100%;
Research Types Applied Research and Development 100%
Science and Technology Fields ENGINEERING AND TECHNOLOGY (Electrical and Electronic Engineering) 100%
UKERC Cross Cutting Characterisation Not Cross-cutting 100%
Principal Investigator Project Contact
No email address given
National Grid Electricity Transmission
Award Type Network Innovation Allowance
Funding Source Ofgem
Start Date 01 March 2020
End Date 30 June 2020
Duration ENA months
Total Grant Value £7,000
Industrial Sectors Power
Region London
Programme Network Innovation Allowance
Investigators Principal Investigator Project Contact , National Grid Electricity Transmission (100.000%)
  Industrial Collaborator Project Contact , National Grid Electricity Transmission (0.000%)
Web Site
Objectives Solid type bushings such as, ERIP (epoxy resin impregnated paper) and SRBP (synthetic resin bonded paper), are attractive due to being maintenance free and pose no environment risk from oil leakage. Nevertheless, solid dielectrics can degrade over time in the presence of a high internal electric field and thermal ageing. The most common manifestation of degradation is electrical treeing as a result of partial discharge (PD) activity. In the specific case of bushings the trees grow between the stress grading foils progressively shorting these out; this changes the capacitance of the bushing. When too many foils have become shorted the bushing is liable to fail.Due to the degradation mechanism two complementary detection techniques are available:Monitor for partial discharge activity – this tells the asset owner that degradation is taking place but not the severity or how much damage has already occurred. Radiometric techniques, such as RFI (radio frequency interference) monitoring, provide spot data during routine substation inspections and can detect internal PD activity; this is the first chance to detect a developing problem. RFI data can be used as the trigger for more detailed monitoring to track the development of the PD through a permanent monitoring solution.Monitor the capacitance and power factor of the bushing – these measurements can either be performed as a relative measurement, comparing all three phases, or by using a voltage reference (from a Voltage Transformer on site), the true power factor can be determined. Relative measurements have shown success, but suffer from variability in system voltage; having a voltage reference provides a means to provide more consistent and meaningful data which can be more readily compared with factory, nameplate or previous field test results. The project will assess the advantages and disadvantages of relative and absolute measurements, as well as the practicalities of installing the additional equipment required for a voltage reference. Additionally, consideration will be given to the benefits of permanently installed systems versus the concept of installing the necessary sensors and safety box, and using a portable system to acquire data for a limited time period i.e. a week or month. In the latter case, reproducibility of results will be a key consideration.An adapter will be attached to the bushing tap (Bushing Tap Adapter or BTA) to measure the leakage current from which the capacitance and relative power factors can be determined.The BTA can also be used to detect PD signals. Any measured PD may originate either from inside the bushing or from outside the bushing i.e. from inside the transformer tank, on the bars connected to the bushing, or on equipment in proximity to the bushing. On one hand this is a disadvantage in terms of assessing the health of the bushing, but the presence of PD is an indicator of some developing problem, so detection of an issue inside the transformer is advantageous; appropriate signal processing is required to discriminate and identify the source of PD in detail. The project will therefore also assess the potential advantages of both asynchronous and simultaneous PD monitoring along with the capacitance measurements.In addition, the leakage current sinusoidal waveforms may be captured over a short time period so as to allow for Fourier analysis and detection of harmonic content present in the current; the presence of harmonics may indicate a system issue, including an overvoltage condition, or may indicate the presence of a DC offset related to geomagnetically induced currents. In addition, the capture of fast transients on the system, picked up through the bushing tap, will provide indiciation of possible root causes of deterioration. The role of fast transients, related to lightning and switching activity, is poorly understood in terms of bushing reliability in the field; traditional but infrequent offline testing provides little value in this area. Online monitoring is a means to improve situational awareness with respect to the bushing population. The project will take place in two initial phases.Phase 1An initial assessment of an online continuous system will be undertaken by:Installing continuous online bushing monitoring at a number of National Grid sites.Collecting data with and without a voltage reference.Rotating monitoring systems between sites and assess reproducibility of the data.Contextual data such as load, MW/MVAr, tap position etc will also be captured as a means to understand possible variations in the raw diagnostics data.Phase 2Trial newly launched bushing monitoring system with integrated PD monitoring.Benchmark against Phase 1 results.Assess benefits of simultaneous power factor and PD measurements.After sufficient confidence has been gained in the reliability and repeatability of the data generated during Phases 1 and 2, a potential follow-on project utilising the facilityies at Deeside is envisaged. Here the power frequency waveform could be distorted by injecting a DC offset and/or harmonics, and the effect on the output of the monitoring system investigated. To provide National Grid with the knowledge to be able to undertake condition assessment programmes of solid type bushings based on onsite condition monitoring measurements, and to provide tools to detect early bushing deterioration and prevent catastrophic failure.
Abstract Bushings pose a challenge in assessing their condition and determining a meaningful asset health index, which would assist in the management of this asset class. Even when oil sampling can be performed this is carried out on a very infrequent basis as they are typically inaccessible even during outages without scaffolding for the purpose of taking a sample. For dry-type bushings there is no oil to sample. Consequently alternative methods of assessment are required, which would ideally allow more frequent assessment of condition. This project will apply online bushing monitoring on a continual and temporary basis to assess the relative benefits of each. The project will further consider the advantages simultaneous partial discharge (PD) monitoring brings to the assessment of the bushing but more broadly to other connected assets.Although this project will focus on transformer bushings, the technology may applied to through wall bushings.
Publications (none)
Final Report (none)
Added to Database 02/11/22