Transformer’s intrinsic protections against overtemperatures

In the previous article, we talked about the protections of a transformer against short circuits and overloads, by means of circuit breakers or fuses.

There is a type of intrinsic protection against overtemperatures, by means of probes and control units or control relays.

The probes are dimensioned at a temperature below the limit of the insulation class applied on the transformer. Two probes can be arranged per phase, one for alarm temperature and the other for triggering. If only one is placed it must be set to the maximum trigger temperature gauge.

Probe gauge

Class

Alarm

Trigger

B-130ºC

100ºC

120ºC

F-155ºC

120ºC

150ºC

H-180ºC

150ºC

180ºC

They can be of three types:

1. By bimetal (1xphase)

NC (normally closed) 250V 1A is usually set, to activate a protection circuit with minimum coil (0V).

If NO (normally open) is chosen it is to activate a protection circuit with a maximum coil (239V).

2. By means of PTC probes (1 x phase)

The operation is like a bimetal NC, which has a high impedance drive temperature behaving like an open contact.

A relay is available that can control three PTC probes at the same time, facilitating the safety maneuver circuit.

3. By means of PT100 probes (1xphase) + CTM4 control unit

To measure the temperature of the winding and to be able to connect a CTM4 control unit.

This type of protection does not require a gauge, as it records the actual temperature of the winding. The programming of the control unit will allow selecting the alarm and trip temperatures that are required according to the class of the transformer.

The control unit allows up to four probes to be connected. Three for the winding, If it is three-phase and one for the magnetic core or for the ambient temperature of the enclosure.

This control unit can be supplied loose on it can be attached to the enclosure.