Recommended locations of oil connections - OPT100

OPT100 Installation Guide

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DGA monitor needs to pump oil out of the transformer and return it after each measurement cycle. It is important that the oil is clean and from a representative location in the transformer, and the same oil volume is not measured repeatedly.

The oil volume needed for a single measurement cycle is slightly more than one liter. To make it possible to pump a new sample and return the measured oil without mixing them up, you must build two oil lines between the DGA monitor and the transformer. Consider the following when selecting the locations of the oil connections in the transformer and preparing for the installation:

  • The recommended arrangement is to use two valves in different locations on the transformer. This improves oil circulation inside the transformer and prevents the returned oil from immediately mixing with the next sample.
  • Use the same valve for oil intake that has been previously used to take oil samples. This makes the measurements results of the DGA monitor more comparable with previous laboratory analysis results. After the oil lines are connected, you will not be able to take oil samples from the valves. Consider adding a new sampling valve to the intake oil line if you need to continue taking oil samples.
  • Check if the transformer circulates its oil by forced convection. Oil flow inside the transformer makes the sampled oil more representative of the overall oil volume. It is also useful if you have to use two valves that are close together, as it carries the returned oil away from the valve.

  • It is possible to use a T-joint to connect both lines to a single valve but the T-joint must be close to the transformer − no farther than one meter (3 ft) from the valve. If a T-joint is used, the transformer should be of a type that circulates its oil.

  • Check the type of connection thread on the valves, and make sure you have appropriate adapters for connecting the oil lines to the valves.
  • The maximum allowed length of an oil line is 10 m (33 ft).

Some typically available locations of oil connections are presented in Figure 1.

Figure 1. Possible locations of oil connections

Oil reservoir. Do not use.


Side of the oil tank, top level. Suitable location for oil connection if the valve is under oil level at all times.


Side of the oil tank, high enough from the bottom to enable proper oil movement. Suitable location for oil connection.


Radiator and its pipes. Suitable location if there is guaranteed continuous oil flow in the radiator. Not recommended if there is no forced circulation or circulation is not continuous.


Drain valve of the oil tank. Not recommended due to risk of sludge and free water. Measurement from the bottom of the tank is also not representative for moisture measurement.

CAUTION Operate the selected valves and make sure the oil is clean with no sludge or free water present.
CAUTION Before installing, check the insulation liquid type from the DGA monitor type label or tag on the oil connectors. The transformer liquid type must correspond to the DGA monitor liquid type.

DGA monitor needs to pump oil in both directions during initialization and maintenance. To make sure this is possible, check that:

  • Intake and return valves are both below oil level in the transformer. There should be no risk of drawing gas into the oil lines even when the flow is reversed.
  • There are no flow direction control valves on the oil lines.
Figure 2. Oil connections below oil level