Protection of transformers

The most common reason of transformer failure is damage to winding insulation. This can be caused by over-current or over-voltage. Over-current occurs when the transformer is overloaded, such as from a short circuit. This results in overheating of the winding which leads to the total destruction of the insulation in a short time.

Overheating of windings can be caused by electrical phenomena as discussed above or non-electrical phenomena, for example by insufficient air exchange, increase of ambient temperature, decrease of water flow (in case of water-cooled windings), fan failure or soiling of fan protective mesh in the case of air-cooled transformers.

Short-circuits cause, in addition to overheating, the occurrence of a very large electrodynamic force between the windings and their elements.  
 


Example of short-circuit current run
u, i – current and voltage before short-circuit, iknu – non-fixed component of short-circuit current, iku – fixed component of short-circuit current, ik – short-circuit current, iud – surge current
Description: chwila zwarcia – moment of short-circuit
 

These forces are proportional to the square of the current and are taken into consideration at the design stage of transformer. Average durations of non-fixed short-circuit (time after which non-fixed component of short-circuit fades) amount to 0.02 s to 0.2 s. The value of fixed short-circuit current depends on circuit parameters, including transformer short-circuit voltage. The lower short-circuit voltage of the transformer, the higher value of fixed short-circuit current.
Example: if the transformer short-circuit voltage is 5%, then fixed short-circuit current is 20 times higher than rated current.
In order to reduce the fixed short-circuit current and dynamic short-circuit forces our company applies special constructions of transformer windings. 

Protections.
Mostly commonly transformers are protected against overloading and short circuit by applying appropriate fuse elements. Fuse current rating and I2t characteristic are the parameters most important in selection. The value of fuse element rated current on the primary side should close to the rated current of transformer primary winding and equal to the upper value of the closest standard fuse elements. Important transformer parameter deciding on time-current characteristics of fuse element is the starting current.Transformer starting current can reach the value 20÷40 times higher than rated value. The transformers produced by our company are characterized by starting current which is higher by 15÷30 times than the rated current, and we can reduce it even more on customer’s request. For example in the transformers intended for supply of medical rooms it does not exceed 12-fold value of the rated current. Therefore the best solution for transformers is to apply fuse-elements with gTr delay characteristics or gL/gG characteristics (they are more available on the market). In case of low-power transformers miniature fuses of TT or T characteristics may be installed. 


Run of starting current for ET1MED-6,3 230V//230V (I1n=28A) transformer produced by ELHAND

It is not recommended to protect the transformers with popular overcurrent switches of S type, although their version with D characteristics (band 10-20xIn) is available, because the transformer starting current may trigger an electromagnetic release when the transformer is switched on.
The best solution for transformer protection is the application of switch with adjustable parameters of time-current characteristics separately for short-circuit and overload modules. In addition to that such switches allow to set the delay, so we can ”put to sleep” its reaction, e.g. during the start-up which lasts usually for first 5-7 half-periods. The disadvantage of such switches is their high price.
Also the protections on secondary side can be applied but they protect more the receiver. In this case the value of fuse-elements rated current cannot exceed the value of rated current on transformer secondary side and the fuse-element time-current characteristics depends on receiver character.

Temperature sensors.
Fuses and switches will protect against overloading, but there are many other factors which may lead to overheating of the transformer. For this reason temperature sensors are installed in the windings, depending on the application and requiremnets of the user.

It is possible to apply the following sensors:

  • resistance sensors (Pt type),
  • semiconductor sensors (e.g. Posistors PTC),
  • miniature bi-metal switches NO – (normally open) or NC – (normally closed).

The sensors are placed inside the winding. In some applications, e.g. mining or medical, it is required to install two types of sensors on one winding.
 
Example: Pt100 type sensor to constantly monitor the winding temperature and additionally a NO or NC bi-metal switch which signals when a temperature limit is the exceeded.  

Monitoring temperature is important from a system operational point of view, as well as for the life of the transformer, since constant overheating by 5ºC will halve the service life of the transformer.

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