Magnetic Oil Gauge (MOG) in Power Transformer

Magnetic Oil Level gauge or MOG is provided in a Power Transformer to indicate the level of insulating oil in the Conservator. A decrease in level of Transformer Oil in Conservator is an indication of leak and therefore MOG level is an important parameter to be monitored.

Normally all Transformers are provided with an expansion vessel called Conservator, to take care of expansion in the oil volume due to rise in temperature, when the load on the Transformer increases or due to increase in ambient temperature. The oil level in the Conservator consequently goes up and conversely it fall when temperature or load reduces. It is essential that oil level in the conservator tank to be maintained at a pre-determined minimum level. Therefore all large Transformers are fitted with a Magnetic Oil Level Gauge, which are incorporated with mercury switches which operates when oil level in the Conservator goes below minimum level and generate an alarm or Trip contact.

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Construction and Working of MOG:

This Magnetic Oil Gauge or MOG is composed of a disc indicator (or a red pointer) behind a dial frame and a float linked to said indicator by means of a magnetic field of force. The dial of this device has scale from empty to full. It has some intermediate divisions such as 1/4, 1/1, 3/4. The prescribed oil level at either 30°C or 35°C ambient temperature may also be indicated on the dial depending upon the design of MOG.


In some design of MOG, the indicator disc is for one half enamelled white and, for the other half, red. Alternatively the colour proportion visible through the dial glass indicates the respective level position, while the dial face is marked at minimum and maximum oil level and at level corresponding to 20°C and 85°C.

The float of Magnetic Oil Gauge, changes its position as the level of oil in conservator changes. Because of the change in position of the float, the gear and pinion also moves which in turn produces a movement in the connected Driving Magnet. Due to this change in position of Driving Magnet, the magnetic field distribution changes which forces the driven magnet to change its position and hence a movement in the pointer of MOG results. Note that this driving magnet of MOG instrument remains inside the conservator and driven magnet remain outside the tank and directly connected to the pointer of the Magnetic Oil Gauge or MOG

The gauge may be provided with one, two or four microswitches for signaling of minimum and/or and maximum oil level in the control room.

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