9.3.1. General (AST READY)
“Most transformers are equipped with protection systems to avoid damage to the transformers, the network or both, in the event of non-normal operation of the transformer or the network. Examples of such protection systems are the network and transformer protection current transformers mounted usually at the transformer terminals feeding relays (overcurrent, differential), the gas and oil activated relay (Buchholz) and the winding and oil temperature indicators (WTI and OTI). These systems are common to most power transformers, including some distribution transformers. However, monitors are mostly confined to medium to large power transformers, in particular those that are strategically important in a network or, as in the case of generator step-up transformers and HVDC converter transformers, where the transformers are the only means of power transfer in networks. An unplanned transformer outage in these networks can have severe technical and economic consequences for the network operator. The main purpose of fitting monitors to these transformers is for condition assessment with the purpose of mitigating some of the above difficulties.” [Draft IEC 61378-3 Convertor transformers - Part 3: Application guide]
Monitoring of transformers can roughly be divided in high and low level monitoring. • High level
o On-line monitoring • Low level
o Periodic recording of a few parameters, visual inspection o Oil condition monitoring
Transformer handbook. Draft. Rev. 02Q Page 110 of 197
9.3.2. High level monitoring (AST READY)
The major goals of an on-line monitoring system are to prevent major failures, to achieve better utilisation of load capacity, optimise maintenance and to extend the remaining lifetime. Transformers of strategically importance should be equipped with on-line monitoring systems in
order to ensure reliable operation, a minimum of maintenance (condition based maintenance) and low life cycle cost.
The on-line monitoring system receive and archive all information needed from just a few sensors, other necessary parameters are calculated based on the compiled data. The on-line monitoring system receives typically the following information from sensors:
• Temperatures • Gas in oil • Moisture • Partial discharge • Currents • Voltages • Tap Changer
The measured and calculated parameters are used in algorithms used in models for calculating and recording:
• Cooling/overload forecast • Real-time status/availability • Lifetime
• Event recording
• Condition based maintenance • Operation and updates
E.g., by knowing the current at each OLTC operation and the number of operations, the wear of the contacts can be calculated. Maintenance can then be performed as condition based instead of periodically. The advantage of condition based maintenance, is that the maintenance is performed when necessary and not to early or long overdue. With such systems, early warnings are given and necessary precautions with regards to maintenance can be taken in order to avoid failures that may have large economical consequences.
9.3.3. Low level monitoring (AST READY)
Most transformers have a minimum of monitoring. Low level monitoring can be divided into the following:
• Periodic recording of a few parameters, visual inspection • Oil condition monitoring
• Off-line monitoring
Periodically recording is normally done when the transformer is energised. It is important to take the necessary safety precautions when the inspection is performed. The parameters recorded can be oil level, top oil temperature, winding temperature, load current and the number of operations since last maintenance of the tap-changer. The amount of equipment will vary depending type of transformer, importance of transformer, etc. Depending on the importance of the transformer the recording of these parameters could be done daily, weekly, monthly or even more rarely. At the same time as the above mentioned parameters are recorded a visual inspection of the transformer for leakages and cracks in bushings should be performed.
“Periodic analysis of the oil in a transformer (oil condition monitoring) is by far the most widely used method for monitoring the general condition of a transformer. The condition of the oil and in particular, of the transformers insulation systems, can be determined by taking oil samples from the transformer for dissolved gas analysis (DGA) and other tests. The tests are comprehensively described for example, in the IEC Standard 60296 and the Guides 60422 and 60567. The presence of an incipient fault in a transformer will normally be detected by this means. Though, the interpretation of the fault mechanism and its location could be more difficult to determine.” [Draft IEC 61378-3 Convertor transformers - Part 3: Application guide].
Normally oil samples are taken when the transformer is energised and necessary safety precautions should be taken. The frequency of how often the oil samples should be taken will vary depending on the importance of the transformer. The most common frequency for oil condition
monitoring is annually. When there is suspicion of an incipient fault in progress, the oil samples should be taken more frequently, e.g. every three months.
“Improvements in the test procedure and analysis have occurred from time to time as the laboratory test equipment and the understanding of the chemical processes occurring in transformers and their analysis improve. An example of this is the measurement of the furfuraldehyde (FFA) content in transformer oil samples. The magnitude of the FFA constituent present in transformer oil samples is an important factor for assessing the ageing of the transformer insulation and indirectly offers an assessment of transformer life expectancy.” [Draft IEC 61378-3 Convertor transformers - Part 3: Application guide].
“Off-line condition monitoring takes place during a transformer outage, for maintenance or when an outage has occurred as the result of a transformer or network fault. Tests are on the transformer at site with the transformer de-energised.
The general condition of the transformer and of the insulation systems can be determined by taking oil samples from the transformer for dissolved gas analysis (DGA) and other tests, as described above.
“In addition, specific site tests can be performed to determine the electrical and mechanical status of the transformer. The parameters that can be measured and used for this purpose include:”. [draft IEC 61378-3 Convertor transformers - Part 3: Application guide].
• Winding resistances • Magnetising currents • Impedance voltages • Dielectric loss factor
• Insulation resistance, including core and yoke clamps to earth, • Inter-winding and winding to earth capacitance measurements. • Dielectric response, measurement of moisture in solid insulation.
All the above tests are non-invasive with the exception possibly of the core insulation resistance measurements. Unless the core and clamps earthing connections have been brought out for external grounding, these particular tests may have to be made by opening the transformer to obtain access to the connections.”
9.3.4. Inspection in energised condition (AST READY)
Inspection during operation shall only be performed after taking safety measures into consideration as applicable:
• If there is a maximum indicator on the thermometer the maximum temperature should be recorded,
• Oil levels, transformer, tap-changer, • Check oil / water flow,
• Pressure gauges, leakage detectors, • Surge arrester counters,
• Motor drive units counters, • Surge arrester leakage currents, • OLTC pressure oil filter manometers,
• Inspection for contamination, especially on bushings, • Inspection of surface condition,
• Dehydrating breather. The silica gel shall be changed when approx. 2/3 of the silica gel has changed from blue to red colour (old type), or from pink to white, respectively. (Conservator type only),
• Inspection for leakages,
• Irregular noise from transformer, oil pumps, motors and cooling fans equipment • Other irregular conditions; alarms, trips etc.
Transformer handbook. Draft. Rev. 02Q Page 112 of 197
9.3.5. Inspection in de-energised condition (AST READY)
After the transformer is disconnected and properly earthed, inspection in de-energised condition may be performed. In addition to the items listed in section 9.3.4 it is possible to inspect items that not are visible in energised condition, typically equipment located on the transformer cover. Further, if necessary, internal inspection of the transformer may be performed after draining the oil.