High transmission losses are the biggest cause of revenue losses for utilities. In the transmission and distribution (T&D) system, transformers play an instrumental role of scaling down high voltage energy and making it usable for customers. Hence, maintaining transformer operations is crucial for ensuring T&D system efficiency. Moreover, dealing with transformer failures is both costly and time-consuming, and unscheduled transformer outages due to unexpected failures can have serious implications. Diagnosis and proper monitoring are, therefore, important to increase the life expectancy of a transformer.
Transformer testing methods adopted at factories are broadly classified as routine tests, type tests and special tests. Routine tests are conducted to evaluate the operational performance of individual units in a large production lot. There are different types of routine tests such as ratio and polarity tests, load tests, impedance measurement tests, insulation resistance tests, winding resistance tests, voltage tests and core insulation tests.
Type tests are done mainly in a prototype unit and not in all manufactured units. Type tests conform to the main and basic design criteria of a production lot, and are conducted to demonstrate that the transformer complies with specified requirements that are not covered by routine tests. There are three type tests that are widely used – the temperature rise test, the impulse voltage test and the noise level test. Special tests are those that are conducted for specific parameters. These include partial discharge, radio interference, vibration, short circuit withstand and tan delta tests.
All transformers are subjected to pre-commissioning tests at site such as the protective relay test, the transformer oil test and the magnetic balance test. The protective relay test involves testing of the communication path between relays to ensure that it is not broken and the signal strength is within the specified limits. Transformer oil testing is done prior to charging to assess the dielectric strength of oil. The oil has to be tested at a 2.5 mm gap. Three readings are taken and an average of these readings is the breakdown voltage of the oil, which should be greater than 60 kV. The magnetic balance test is conducted only on three-phase transformers to check the imbalance in the magnetic circuit. The test result indicates the uniform distribution of flux insulation.
Best practices in O&M
To provide long and trouble-free service, a transformer must receive regular maintenance. The methods that can be considered are time-based condition monitoring (TBCM), condition-based monitoring (CBM), online condition monitoring (OLCM) and time-based maintenance (TBM). TBCM requires that the measurement or inspection intervals are shorter than the fail point to allow detection before failure occurs. Meanwhile, CBM involves monitoring the change in the condition of any item on an as and when basis. The governing principle of CBM is long-term experience coupled with perceived usage. However, a fail condition can occur in CBM. OLCM requires that data, measurements or samples are collected when the subject transformer is energised by a visit to the site. It allows continuous monitoring by capturing real-time data to provide information on developing trends in transformer condition. OLCM can help develop smart transformers and smart systems. TBM is the most popular method of maintenance, mainly because most substations operate in isolation. TBM consists of regular, programmed inspection, followed by testing and reconditioning when necessary. A fail condition may occur with this approach also.
The best way to protect a transformer is to have a good preventive maintenance schedule. Various O&M measures have evolved based on past experiences, which can enable utilities to enhance the performance and life of transformers. In the case of liquid fillings for transformers, any leakage of traditional insulating oil must be contained to prevent contamination of the surrounding area. Some liquids other than traditional insulating oil have gained traction as they have a very high flame ignition point and are considered environment friendly. These alternative fillings are used when the transformer has to be installed inside buildings. These fillings do not require strict fire precautions, resulting in potential reduced costs.
The mineral insulation oil functions as a coolant, and helps extinguish any currents arcs in the transformer. Oil deteriorates gradually with time as contact with air causes oxidisation. If a given unit operates at higher than normal temperature, chemical reactions such as decomposition and polymerisation occur, producing solids that are not dissolved and get collected within the core/winding structure, leading to sludge formation. Keeping the oil in good condition will help prevent or slow down the deterioration of solid insulation materials immersed in it. Another cause of deterioration is moisture or humidity. For this, a dehydrating breather is fitted into the transformer to limit the entry of moisture into a free breathing tank.
It is recommended that the core and windings be removed from the tank for visual inspection, if absolutely necessary – for example, if an internal fault involving coil is detected. On replacement, a full test procedure must be performed to ensure transformer connections are satisfactory for further service. Outdoor porcelain insulators and rain sheds should be cleaned at regular intervals. A metallic scrubber can be used effectively to remove dirt/stains provided the surface is not damaged. Intervals between cleaning will be dependent on the degree of contamination in the immediate area.
The compound-filled cable box should be checked regularly for leakage in the weatherproof plugging/sealing with a bituminous compound. The box should be free from any cracks. In an air-filled cable box, no maintenance is required, but it is advisable to check regularly for cleanliness, damage to bushings, and ingress of moisture.
Cooling radiators are produced by a number of manufacturers. Although similar in style, they tend to differ in detail. It is preferable that the radiators fitted in a transformer come from the same source or supplier. Cooling radiators should be frequently checked for any oil leakages along all the welded joints, gasket joints, plugs, etc. Any bend or dents should be rectified as soon as possible.
In an explosion vent, the diaphragm is fitted at the exposed end of the vent, which should be inspected at frequent intervals and replaced if found damaged. Failure to replace the defective diaphragm quickly may allow the ingress of moisture, which will contaminate the oil. Surge arresters may be provided if outdoor terminal bushings are fitted to prevent sudden voltage surges. The protection fuses should be checked to ensure serviceability. Any fuse appearing unserviceable must be replaced with identical or equivalent units and correctly clamped into position.
Challenges in O&M and the way forward
Some challenges in the O&M of distribution transformers (DTs) occur when the oil level of transformers is not maintained, which may lead to a flashover, the oil is not filtered, and the breaker for LV protection is removed or not provided at the time of installation. Inadequate protection from overloads and short circuits, non-maintenance of silica gel breather leading to ingress of moisture, and no factual data on loading of transformers also lead to ineffective O&M. Further, there could be other challenges like improper earthing of transformers, broken bushings, unbalanced loading on different phases, long LT lines, loose LT lines, trees touching LT lines, causing frequent short circuits and overload. Besides, the non-provision of lightning arresters, improper HV protection, frequent blowing off of HG fuses used for HV protection due to improper fitting, and wind action also result in mechanical damage.
While power transformers are usually provided with adequate protection and skilled staff is available for their periodic maintenance and day-to-day operations, DTs are located in far-flung areas throughout the country. The large scale electrification of villages and agricultural pump sets has resulted in the installation of a large number of DTs. Thus, their performance needs to be monitored remotely by retrofitting intelligent tools, and through load survey for accurate distribution planning. Intelligent DTs enable monitoring, connectivity mapping, reporting of anomalies such as rapid changes in voltage, current, temperature and pressure, smart metering, and distributed control and protection applications.