Oil Level Indicators in Transformer
- After 1 month of service, and every 3 to 5 years, check the tank oil level indicators. These are float operated, with the float mechanism magnetically coupled through the tank wall to the dial indicator. As level increases, the float rotates a magnet inside the tank. Outside the tank, another magnet follows (rotates), which moves the pointer. The center of the dial is normally marked with a temperature of 25 ºC (77 ºF). High and low level points are also marked to follow level changes as the oil expands and contracts with temperature changes.
Figure (a) - The proper way to determine accurate oil level is to first look at the top oil temperature indicator. After determining the temperature, look at the level gauge. The pointer should be at a reasonable level corresponding to the top oil temperature.
- Calibrate or replace the conservator oil level indicator if needed, but only after checking the top oil temperature indicator as shown in the above section. If the transformer is fully loaded, the top oil temperature will be high, and the level indicator should be near the high mark. If the transformer is de-energized and the top oil temperature is near 25 ºC, the oil level pointer should be at or near 25 ºC. See figure (b). Reference also IEEE 62-1995™ [20], section 6.6.2.
Figure (b) - To check the level indicator, remove the outside mechanism for testing
without lowering transformer oil. After removing the gauge, hold a
magnet on the back of the dial and rotate the magnet; the dial indicator
should also rotate. - If it fails to respond or if it drags or sticks, replace
it. As mentioned above, defective units can be sent to the factory for
repair. - There may also be electrical switches for alarms and, possibly, for
tripping off the transformer when the tank level falls. - These switches should be checked with an ohmmeter for proper operation. The
alarm/tripping circuits should also be tested to see if the correct
annunciator points and relays respond. See the transformer instruction
book for information on your specific indicator. - If oil has had to be lowered in the
transformer or conservator for other
reasons (e.g., inspections), check the oil
level float mechanism (figure a). Rotate
the float mechanism by hand to check for
free movement. - Check the float visually to make sure it is
secure to the arm and to ensure that the
arm is in the proper shape. Some arms
are formed (not straight).
