Regardless of the type of service-entrance connection, it is generally necessary to perform regular transformer maintenance. Transformers are usually thought of as stationary objects with no moving parts. Because of this misconception, transformers are often neglected and left out of routine preventive maintenance schedules. This could prove to be a very expensive omission. Transformers must be inspected and maintained on a regular schedule to get maximum performance and life from them. This applies to all transformers no matter how large or small they are. Environmental conditions such as changing temperatures caused by varying loads and changing ambient temperatures affect the operation and life of the transformer. Dust, moisture, and corrosive chemicals in the air surrounding the transformer greatly affect its operation and life. The type of maintenance procedures and intervals between procedures are governed by the type, size, location, and application of the transformer.
Power Transformer Maintenance Procedures
As with any electrical equipment, the primary consideration when working on or near transformers must be the safety of personnel. Before working on any transformer, establish whether it is energized and whether the work can be done safely with power on the transformer. Most maintenance procedures require that power be disconnected and locked or tagged out. On larger transformers with high-voltage connections, it is usually advisable to prepare a written switching procedure detailing each step of the process of de-energizing the equipment.
By following a written procedure and initialing each step as it is taken, errors in switching can be avoided. In many larger installations, grounds are placed on each side of the transformer after it is de-energized, to protect the workers. If these grounds are not removed before the transformer is energized, the windings could be severely damaged. A written switching procedure includes the placement and removal of these grounding connections. This helps avoid energizing a transformer with the grounds still in place.
After the power to the transformer has been disconnected and before doing any work, it is advisable to test all exposed connections for voltage. Be sure to use the proper test instrument with a voltage rating at least as high as the voltage rating of the connection. This is especially important when there is more than one source of power, as in a double-ended substation.
Double-ended substations permit power to be supplied from another source in the event of equipment failure. Although the circuit kVA capacity is reduced, power can be maintained until the defective equipment is repaired or replaced. This can, however, cause a back feed to the secondary side of a transformer that has the primary disconnected. Extreme care must be taken when working with double-ended systems to ensure that power is not being applied to either the primary or secondary windings.
Entering a transformer tank
In some of the maintenance procedures, it is necessary to enter a transformer tank. When this is part of the maintenance procedure, the atmosphere in the tank must be tested for the presence of combustible and/or toxic gases and also for the presence of sufficient oxygen. Oxygen is normally present in the atmosphere at about 21.2%. If this concentration is less than about 20%, it could be a health threat to the worker. If there are dangerous gases present or if there is insufficient oxygen in the tank, it must be ventilated with fresh air until safe conditions are met. When anyone is inside the tank, there must be a person outside the entrance to observe the worker in the tank and be alert for any problems encountered.
The first step in any preventive maintenance procedure is the inspection of the equipment. Inspection of the transformer will reveal the presence of rust or corrosion. Dirt or dust buildup or rust and corrosion should be noted at this time.
The outside of the transformer should be cleaned with an approved solvent or cleaner. Rust and corrosion should be removed and the housing painted if necessary.
All connections and mounting bolts should be tightened. Any corroded connections should be replaced.
A megger test from each winding to the housing or core will show any insulation weakness in this area.
Use a megger with voltage ratings close to the rating of the transformer winding; for example, a 500-volt megger would be used to test the insulation on a transformer with a 480-volt-rated winding.
After testing the windings with a megger, each winding should be tested for continuity with an ohmmeter.
This can be accomplished by connecting the ohmmeter leads across the terminals or leads connected to the ends of each winding. This test will determine whether any of the windings are open, but it will probably not determine whether they are shorted. In some instances, the insulation of the wire breaks down and permits the turns to short together. When this occurs, it has the effect of reducing the number of turns for that winding. If these shorted windings do not make contact with the case or core of the transformer, a megger test will not reveal the problem. This type of problem is generally found by connecting the transformer to power and measuring the current and voltage values. Excessive current draw or a large deviation from the rated voltage of a winding is a good indicator of a shorted winding. When making this test, however, be aware that it is not uncommon for the secondary voltage to be higher than the rated value under a no-load condition. Voltage ratings are listed for a full load, not no load. It would not be uncommon for a transformer winding rated at 24 volts to measure 28 or 30 volts with no load connected.
Pad-Mounted Oil-Cooled Transformers
The first step in maintaining these transformers is a thorough external inspection. Look for evidence of leaks in the housing or cooling radiators. Inspect the housing for rust, corrosion, or damage, and note the general condition of the paint. Inspect the bushings for cracks or chips. Look for loose, corroded, or discolored connections. Inspect the housing ground connection. Make sure it is tight and free of corrosion. Most pad-mounted transformers are contained in vaults. Many are equipped with temperature and pressure gauges to measure the coolant temperature and the pressure inside the transformer.
If the transformer utilizes external cooling fins, make sure that they are free of debris and clean. If the transformer is equipped with auxiliary cooling equipment, it should be checked for proper operation. Check radiator connections to the tank for leaks, and make necessary repairs. Automatic cooling fans should be operated manually to be sure they work. Temperature and pressure switches and gauges are removed and calibrated once a year to ensure their proper operation. On transformers with a gas blanket (usually dry nitrogen) over the oil, the gas pressure should be checked at least once a week. These transformers often have external temperature and pressure gauges because the transformer is in a sealed container. The pressure regulator and gauges should be removed and calibrated once a year.
Transformer Protective Relaying
Transformers that have a gas blanket on top of the insulating oil have pressure switches that actuate an alarm system if the gas pressure on the blanket drops below a certain point. These switches should be tested frequently along with any temperature or pressure alarm devices on the transformer windings or tank. Protective relaying usually includes overcurrent relays, sudden pressure relays, reverse-current relays, and winding and oil over-temperature relays of various types. These devices should be tested and calibrated by qualified technicians on a regularly scheduled basis, but at least once a year.
Internal Inspection and Maintenance
On larger transformers, it is necessary to open manholes or inspection covers to determine the condition of the windings, connections, and other parts inside the housing. Before removing any covers, it is advisable to have new gaskets available for replacement when the opening is reclosed. Relieve any internal pressure in the transformer before loosening the flange bolts. No tools or equipment must be left inside the housing. Inventory all tools, parts, and equipment brought to the work area before opening the transformer and also before reclosing it. Anything left in the transformer could cause a short circuit or interfere with the normal circulation of the cooling medium, and destroy the transformer.
Make sure all safety precautions are followed, and the atmosphere is tested before entering the transformer. Look for loose, corroded, or discolored connections; distorted or damaged windings; and broken or missing spacers between windings. Check the general condition of the insulation for deterioration. Clean and tighten connections where necessary. Be sure to follow the manufacturer’s recommendations for torque when tightening any connections. Check and tighten any mounting bolts. Look for deposits of sludge on windings core material or other structures. Sludge deposits indicate contamination of the oil and reduce the dielectric strength of the insulation. Sludge can also act as thermal insulation, thus decreasing the transfer of heat from the internal parts to the cooling oil. While inspecting the internal parts of the transformer, it is good practice to look for any evidence of rust on the inside of the housing or covers. This might indicate condensation of moisture on these parts, which could be caused by a leaky gasket that allows ambient air to be admitted to the housing.
As with any other transformer, the dielectric strength of the insulation must be tested at least once a year. Megger testing can be done on the lower voltage transformers. Special high-voltage equipment is necessary to test the insulation on higher-voltage units. A high-voltage tester is generally referred to as a “HiPot,”. This unit develops a high voltage and measures any leakage current caused by weak or defective insulation. For voltages above 13,800 volts, it is usually advisable to contract high-voltage insulation tests to a company that specializes in this type of test and has trained technicians and the proper equipment available. As with any insulation testing, a record should be kept of test results to establish any trends in insulation dielectric strength.
Transformer oil testing should be conducted at least once a year, and more frequently in cases of frequent overloads, or if there is a history of marginal oil test results. Oil samples are drawn into clean dry containers. Label each container with the identity of the transformer. After the sample is drawn, it should be allowed to stand for a short period of time to allow any free water to settle to the bottom of the sample. If glass containers are used, it will be easier to see any free water in the sample.
Testing for dielectric strength is done in a special device that has a cup for the sample, and electrodes placed 0.1 inches apart. Thoroughly clean and dry the sample cup, and then rinse it with a portion of the sample. Fill the cup and allow it to settle for at least three minutes to eliminate air bubbles. Turn the device on and gradually increase the voltage until it arcs across the sample. Record the voltage and repeat the test five times on each of the three samples from each transformer. Calculate the average of the fifteen tests done in this manner to get the representative dielectric strength of the oil. The average dielectric strength of 26 kV to 29 kV is considered usable. 29 kV to 30 kV is good. Less than 26 kV is poor, and the oil should be replaced or filtered to increase dielectric strength. Special equipment is required to filter transformer oil, and the transformer must be de-energized. This process is usually contracted to companies specializing in transformer maintenance.
Other tests conducted on transformer oil include water content, gas content, and color. The water content of fewer than 25 parts per million is usually acceptable for units operating at voltages up to 228 kV. Excess water can come from condensation or leaks in the housing or cooling system, and it reduces the dielectric strength of the insulation and oil. Filtering removes excess water from the oil.
Arcing or overheating can cause combustible gases such as acetylene, hydrogen, methane, and ethane to be formed in the oil. The presence of these gases can only be detected by specialized test equipment and should be done by qualified technicians. Samples should be sent to laboratories specializing in this type of testing. Most transformer consulting firms prefer to have their technicians collect the samples to ensure uniform sampling procedures. In most cases, the companies doing this type of testing submit a report listing the conditions found, probable causes, suggested remedies, and suggested frequency of retesting.