IEEE STD C57.93-1995: GUIDE FOR INSTALLATION OF LIQUID-IMMERSED POWER TRANSFORMERS TUTORIAL

IEEE STD C57.93-1995: GUIDE FOR INSTALLATION OF LIQUID-IMMERSED POWER TRANSFORMERS

This IEEE standard aims to show the appropriate way of shipping, handling, inspecting, installing, and maintaining liquid-immersed power transformers. Power transformers are defined in this guide to be above 501kVA and which its secondary voltage should be equal or above 1000V. Two sizes of transformers are basically discussed in this guide, those which are 10MVA and above with a primary voltage winding of 69kV and above and transformers which are 501kVA to 10MVA (oil or ail cooled) having a primary voltage winding of 69kV and below.

501kVA TO 10 MVA (OA) LIQUID-IMMERSED POWER TRANSFORMERS

This type of transformers are said to be usually a station or pad-mount installed transformers. Its tanks are tightly sealed to preserve the liquid or inert gas used as insulation. Radiators which are usually provided by manufacturer for cooling can be welded directly to the tank. Transformers with lower capacity are typically shipped filled with insulating fluid while transformers rated 7.5 to 10MVA are usually shipped without liquid and its primary bushing detached.

Proper inspection should be done at the time of delivery from the manufacturer which should include the checking of any damages in the transformer tank and bushings like dents, nicks and scratches. Leaks should also be searched and one way of finding it besides ocular inspection is through testing, a positive or negative pressure indicates that the tank did not leak.

Manufacturers should be immediately be notified if damages are found and internal inspections should only be done if the tank openings are available. Otherwise, the transformer supplier should first be contacted to arrange a field inspection by a representative from the manufacturer.

Before energizing the transformer, a number of things that shall be checked to ensure that the correct internal connections should have been made, first is that the transformer is properly grounded, and that proper precautions should have been taken.

Installation of these transformers does not end upon the energization rather proper maintenance should always be observed. Negligence of treatment to a transformer in service may result to serious consequences and careful regular inspection is of grave importance. The frequency of the inspection is determined by climatic conditions and severity of loading. Spare transformers should be given the same care as a transformer in operation. Check with the manufacturer for recommendations regarding special maintenance for the specific unit in question.

Transformer units must be regularly be tested as included in the maintenance procedures. Transformer testing result must strictly follow the test values found and specified in IEEE Std C57.104-1991 or IEEE Std C57.106-1991. Critical transformers should have their insulating liquid tested frequently for the first few weeks of operation to make sure that no gas or moisture is being driven from the solid transformer insulation into the liquid.

10 MVA AND ABOVE (OA) LIQUID-IMMERSED POWER TRANSFORMERS

The recommendations presented in this clause apply to modem large liquid-immersed power transformers with high-voltage windings rated 69 kV and above. However, appropriate subclauses may be applied to lower-voltage transformers when similar conditions and similar transformer construction exist, and may also be used in servicing older transformers that have been opened for maintenance or repair work.
The transformer shipping is intended to apply not only for original shipment of transformers from the factory to the user, but also as a guide for later shipment by the user, whether to another station, another utility, or to the factory for repair. Large power transformers are normally shipped on railroad cars. When the distance involved and weight and dimensions of the transformer permit, shipment may be done by truck.

Similar to any transformers, testing and inspection should also be strictly be observed in this kind of transformer. When a transformer is received, a thorough external inspection should be made before the unit is removed from the car. Inspect carefully for any apparent damage or shifting of the transformer during transit. If there is evidence of damage or rough handling in transit, an inspector representing the carder and the manufacturer should be notified. In all cases, the manufacturer's instructions should be followed. For shipments equipped with impact recorders, representatives from the purchaser and carrier should be present to inspect the transformer and examine the impact recorder chart at the site location. For smaller transformers in this size range, this may not be considered necessary.

Perform the selected tests of those listed in TRANSFORMER TESTS. Set cooling controls to automatic (if possible), energize the transformer, and hold at rated voltage and no-load for at least 8 h. Test gas blanket for oxygen and combustible gas. Check operation of LTC mechanism and auxiliary equipment during this time. Note excessive audible noise and vibration. The transformer is now ready for service. However, observe the transformer carefully (particularly in critical low ambient temperature areas) for the first few hours after load is applied. After several days, retest the dielectric liquid for moisture and dissolved gas, and the gas blanket for oxygen, carbon dioxide, and combustible gas.

Because of their electrical location in a power system, transformers are often subjected to heavy electrical and mechanical stresses. To avoid failures and problems, it is essential to conduct a program of careful supervision and maintenance. The life of a transformer is highly dependent upon the heat prevailing in the windings and core of the unit; therefore, it is important that the temperature be periodically monitored. Also, as is the case with all liquid-filled electrical apparatus, the integrity of the dielectric liquid is extremely important and should be maintained at a high quality. If the liquid is removed so that the level falls below the top of the core and coil assembly, all of the liquid should be removed and the transformer vacuum-filled.

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