<p class="MsoNormal">At Amperis we know that predictive maintenance is the basis for increasing productivity. By predictive maintenance we understand the technique, based on the measurement of different parameters, that forecasts the future failure of a machine component so that it can be replaced before it fails.</p>
Monitoring techniques measure these parameters, which are indicators of the machine's condition. By analysing these variables and comparing them with reference values, we know the degree of deterioration, since these parameters show a predictable relationship with the equipment life cycle, as is the case with insulation resistance.
In short, predictive maintenance seeks to achieve high power quality through suitable corrective measures in order to guarantee reliable operation and extend transformer life (the aim is to avoid negative consequences such as excessive energy costs or production stoppages).
With Amperis equipment it is possible to monitor the power quality system, accurately detect problems and extend the service life of transformers, reducing their downtime, in short: increasing productivity.
Diagnostic tests on power transformers
As we know, during commissioning and operation it is essential that the power transformer is in good condition. Different factors adversely affect the life expectancy of a transformer, such as: ageing, thermal, mechanical and electrical influences, etc.
- General dielectric tests, which make it possible to establish the general condition of the insulation of the inspected equipment, are:
- Tan δ, which provides information about the condition of the insulation. High tan δ values imply that the dielectric is deteriorated or undergoing a degenerative process.
- Capacitance (pF), which mainly provides information about the geometric configuration of the transformer.
- Current in the dielectric.
- Dielectric loss power.
Capacitance and power factor (PF) or dissipation factor (DF) measurements are carried out to determine the condition of the transformers and the bushings.
Bear in mind that insulation degradation produces ageing, high oil conductivity or high water content. These cause an increase in losses. The loss power value mainly reflects the degree of surface contamination and moisture (a high power value indicates a large resistive current)
- TIP-UP tests. This is carried out on the insulating section with a suspect power factor and consists of comparing the results obtained in the general dielectric tests performed at different voltage levels, which gives us the severity or criticality level of the insulation deterioration (poor insulation condition if different tan δ values are observed in the measurement at 2kV and the measurement at 10kV)
- Internal insulation measurements on bushings. Bushings are used to insulate the conductors carrying high-voltage current through an earthed enclosure; they are tested to verify the condition of their insulation (tan delta, capacitance, current, loss power and tip-up tests) and thus detect low liquid or filling levels.
- Reduced-voltage excitation test (10Kv). Applying voltage to each of the transformer windings separately allows us to obtain magnetising current and short-circuit loss power readings. This is essential for detecting problems in the transformer such as: turn-to-turn faults, a loose core, short-circuited iron laminations or changes in its characteristics. Detecting this damage or these changes in the geometry of the core and windings, as well as short-circuited turns and poor-quality terminals from the time of construction, makes it possible to reduce losses in transformers.
- Transformer turns ratio test (TTR). These are carried out to verify the fundamental operating principle of a power transformer. By measuring the ratio and the phase angle from one winding to another, open circuits and short-circuited turns can be detected. It makes it possible to detect small insulation defects in the windings and in the tap changer, and also to confirm the nameplate ratio and detect turn-to-turn short circuits in the coils. This test is carried out at the factory during acceptance testing and routinely when the transformer is in service. It is important to note that the results are compared with the nameplate values and the measured values must not deviate by more than 0.5 % from the rated ratio between phases (in accordance with standards IEC 60076-1 and IEEE C57.152)
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- Winding resistance tests. As with the TTR test, the diagnosis is made by comparing the measurements (in this case the 3 phases of each winding with a resistance meter) with the manufacturer's data. By means of this test we detect faults in connections, hot spots in the winding, turn-to-turn short circuits and defects in tap changers.
- Insulation resistance tests in D.C. This is the measurement par excellence for assessing the condition of insulation; it is carried out with a megohmmeter, which reports the insulation resistance value and the polarisation index (which indicates the cleanliness and dryness of the insulation.
- Frequency response analysis (FRA) test. This test establishes the transformer's fingerprint. By studying the resulting curves and comparing them with the reference curves, it makes it possible to detect mechanical or electrical problems in the windings such as winding movement, deformation of internal components and turn-to-turn defects. This is one of the most common electrical tests since standard IEC 60076-18 was introduced. SFRA is a non-invasive method. It allows reliable assessment of the integrity of the power transformer without applying high voltage.
- Leakage reactance test. This is a technique used on transformers liable to suffer high mechanical stress due to inductive loads, consisting of calculating the leakage reactance in order to compare it with the transformer's short-circuit impedance.
- Dynamic resistance test on tap changers. By means of this test, defects in the contacts of the on-load tap changer are detected. The test method is simple. A constant voltage is injected into the transformer while recording the current variations that occur when a tap change is made.
- Oil analysis. Different analyses are carried out when assessing transformer oil during maintenance work; these fall into 3 groups.
a. Physical-chemical analyses, which report on the condition of the oil and its possible contaminants. The tests carried out are: dielectric breakdown voltage, acidity (which determines the degree of ageing of the oil), water content (amount of water in parts per million contained in an insulating oil according to UNE 21-384/92) and tangent of the loss angle (a parameter with an exponential dependence on temperature that reports contamination by colloidal products, contamination with undissolved water and contamination by polar products)
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b. Gas chromatographic analysis, tests that make it possible to diagnose thermal defects in the transformer, arcing, partial discharges, etc. Through oil degassing and chromatographic analysis, the qualitative and quantitative content of the gases is determined.
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c. Analysis of furanic compounds, to assess the condition of the paper by indirect methods.
Amperis Products S.L. is an international company based in Lugo, Spain, with many customers who trust in our quality and commitment in more than 50 countries. Amperis, a benchmark in substation maintenance, has been serving the electrical power industry for more than 10 years, offering our customers personalised advice, fast assistance from highly qualified engineers and solutions created from experience, aiming for comprehensive customer care
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