A review on machinery diagnostics and prognostics implementing condition-based maintenance

As transformers age, their internal condition degrades, which increases the risk of failure. To prevent these failures and to maintain transformers in good operating condition is a very important issue for utilities. Traditionally, routine preventative maintenance programs combined with regular testing were used. The change to condition-based maintenance has resulted in the reduction, or even elimination, of routine time-based maintenance. Instead of doing maintenance at a regular interval, maintenance is only carried out if the condition of the equipment requires it. Hence, there is an increasing need for better nonintrusive diagnostic and monitoring tools to assess the internal condition of the transformers. If there is a problem, the transformer can then be repaired or replaced before it fails. An extensive review is given of diagnostic and monitoring tests, and equipment available that assess the condition of power transformers and provide an early warning of potential failure.

Review of condition assessment of power transformers in service

Infrared thermography for condition monitoring – A review

Determining whether power transformers are suffering from internal arcing or dangerous levels of partial discharge (PD) is important because failure without warning can result in damage to neighboring equipment, customer dissatisfaction, disruption to economic activity, and the imposition of regulatory fines. This paper provides an overview of the excitation of UHF signals by PD inside transformers. The use of externally mounted sensors has been outlined, and the structure of a dielectric window that can be constructed on an inspection hatch has been outlined. Attenuation of UHF signals propagating inside a transformer tank has been shown to be relatively low. A new approach to locating PD sources in three dimensions has been presented, based on using a numerical model of the transformer materials, which defines electromagnetic propagation velocities on a mesh of 5-cm sub-cells. The concepts of the propagation-velocity matrix (PVM) and the propagation-time matrix (PTM) were thereby introduced. Once the PD source has been located to a specific region of the transformer, the PVM might be used to index a database of physical descriptions of the transformer that could provide additional information to assist with diagnosis.

Partial discharge monitoring of power transformers using UHF sensors. Part I: sensors and signal interpretation

The Health Index represents a practical tool that combines the results of operating observations, field inspections, and site and laboratory testing to manage the asset and prioritize investments in capital and maintenance plans.

An approach to power transformer asset management using health index

FRA tests in the frequency range up to 10 MHz were investigated using simulations and measurements. It was found that the FRA test was more sensitive to changes in the transformer at frequencies above 1 to 3 MHz than at lower frequencies. However, the effects of the conventional test circuits mask the changes at the higher frequencies. A new test method is proposed that is effective up to at least 10 MHz.

Improved detection of power transformer winding movement by extending the FRA high frequency range

This paper presents data from a study conducted to identify the key factors that contribute to frequency response analysis (FRA) measurement results. The results show that the lower the shunt impedance, the higher the sensitivity of the FRA measurement for the detection of winding movement. On transformers with large bushings, the bushings can mask the detection of winding movement. To detect minor winding movements, the FRA comparison has to be made using the same connections each time. With a typical test circuit, the FRA measurement sensitivity range of the standard lead arrangement is up to approximately 2 MHz only. For optimum measurement, the lead lengths need to be as short as possible, and the test configuration must remain constant for repeated tests. The higher the frequency range used, the more critical this becomes. From the results, it is apparent that minor winding movement or winding looseness generally shows in the high frequency region of the FRA admittance. All of the factors investigated impact on the ability of FRA measurements to detect winding movement. To optimize the sensitivity of FRA measurement method, these factors need to be taken into account.

Transformer winding movement monitoring in service - key factors affecting FRA measurements

Low voltage impulse testing (LVI) on power transformers can be used to detect winding movement, such as that caused by short circuits or by loss of winding clamping pressure. The technique is being developed to perform tests with transformers online. The techniques are described and results presented.

Condition monitoring of transformers in service by the low voltage impulse test method

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and BC Hydro. V/sub 50/ (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 /spl mu/s), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The V/sub 50/ results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns. >

M. Wang

Papers

Review of condition assessment of power transformers in service

Improved detection of power transformer winding movement by extending the FRA high frequency range

Transformer winding movement monitoring in service - key factors affecting FRA measurements

Condition monitoring of transformers in service by the low voltage impulse test method

The electrical breakdown characteristics of oil-paper insulation under steep front impulse voltages