POWER TRANSFORMER ACCESSORIES

There are many different accessories used to monitor and protect power transformers, some of which are considered standard features, and others of which are used based on miscellaneous requirements. A few of the basic accessories are briefly discussed here.

LIQUID-LEVEL INDICATOR

A liquid-level indicator is a standard feature on liquid-filled transformer tanks, since the liquid medium is critical for cooling and insulation. This indicator is typically a round-faced gauge on the side of the tank, with a float and float arm that moves a dial pointer as the liquid level changes.

PRESSURE-RELIEF DEVICES

Pressure-relief devices are mounted on transformer tanks to relieve excess internal pressures that might build up during operating conditions. These devices are intended to avoid damage to the tank. On larger transformers, several pressure-relief devices may be required due to the large quantities of oil.

LIQUID-TEMPERATURE INDICATOR

Liquid-temperature indicators measure the temperature of the internal liquid at a point near the top of the liquid using a probe inserted in a well and mounted through the side of the transformer tank.

WINDING-TEMPERATURE INDICATOR

A winding-temperature simulation method is used to approximate the hottest spot in the winding. An approximation is needed because of the difficulties involved in directly measuring winding temperature. The method applied to power transformers involves a current transformer, which is located to incur a current proportional to the load current through the transformer. The current transformer feeds a circuit that essentially adds heat to the top liquid-temperature reading, which approximates a reading that models the winding temperature. This method relies on design or test data of the temperature differential between the liquid and the windings, called the winding gradient.

SUDDEN-PRESSURE RELAY

A sudden- (or rapid-) pressure relay is intended to indicate a quick increase in internal pressure that can occur when there is an internal fault. These relays can be mounted on the top or side of the transformer, or they can operate in liquid or gas space.

DESICCANT (DEHYDRATING) BREATHERS

Desiccant breathers use a material such as silica gel to allow air to enter and exit the tank, removing moisture as the air passes through. Most tanks are somewhat free breathing, and such a device, if properly maintained, allows a degree of control over the quality of air entering the transformer.