There is a rule that says that all transformers with a capacity of 1 MW or more should be equipped with protection. To perform this task, the gas protection of the transformer was chosen.
Transformer protection
The direct purpose of the protective device for a high power transformer is to save it from internal damage. These internal threats include the following:
- Revolutions in such windings as VN and NN.
- Fire steel transformer.
- Oil leakage from the transformer tank.
The principle of the gas protection of the transformer is based on the fact that the system controls the decomposition of transformer oil, which occurs due to the influence of very high temperatures on the gases. An increase in temperature is a local problem that arises due to a short circuit in the winding of the described device or when a steel fire occurs. When this problem occurs, the place where the failure occurred will become very hot, because of which the temperature of the gases will also increase.
Transformer gas protection operation
If the temperature is too high anywhere inside the transformer, the oil will generate gas. Gases formed as a result of this will tend to enter the expansion tank of the device, and in order to penetrate there, they will pass through the gas relay housing.
It is through him that the gas protection of the transformer is carried out. If the gas pressure in the relay housing is too high, the oil level will inevitably fall, which, in turn, causes the cups to tip over. At this moment, the gas relay is triggered.
Gas relay
The named type of protection in the transformer is represented by a mechanical relay, which is supplemented by two pairs of contacts. It is worth noting that the intensity of gas formation inside the transformer will directly depend on the degree, as well as the nature of the damage that caused this very gas formation.
It is thanks to this that it is possible to create such a gas protection for the transformer that will be able to determine the degree and nature of damage and, depending on the data received, send a signal or immediately turn off the unit. The main element of protection in such devices is a gas relay of class KSG. Its installation is carried out in an oil pipe, which is located between the tank and the expander.
Elements of a gas relay
We repeat that the gas protection of the transformer is carried out by means of a relay. In other words, the principle of protection is based on the operation of this device. One of the fundamental elements of protection are flat-bottomed aluminum cups that rotate in time with the contacts of the movable type around the axis of the cup.
These wires can short circuit with immovable if cups begin to lower. And during the correct operation process (when the oil volume in the relay housing is at an acceptable level), these basic protection elements are held in a certain position in which they do not close any of the contacts.
With a decrease in the oil level in the casing, the cups also begin to sink along with the contacts, which close with the other, motionless. Moreover, with minor damage, only the upper cup will fall, and the closure of its contacts will result in the gas protection device of the transformer giving only a signal about a breakdown.
If the intensity of gas generation is high, then the flow of oil and gas will also affect the blade, which, when the contacts are closed together with the lowered cup, will cause the shutdown of the working transformer.
During normal operation of the unit, the oil speed inside has a value of 0.6 / 0.9 / 1.2 m / s. This indicator depends on the quality of the cooling facility. If a malfunction occurs, the response speed of the gas protection of the transformer takes from 0.05 s to 0.5 s. We can add that in the territory of the Russian Federation the most widely used gas relay with two ball-shaped plastic floats BF80 / Q.
Transformer gas protection action
It is worth mentioning that gas protection has become widespread due to the special sensitivity of this system - it responds well to any internal damage that occurs in the facility.
In case of any malfunction, either an electric arc or heating of the parts will form inside the unit, and this will lead to a strong decomposition of oil and insulating materials, which, in turn, causes the formation of gases. Since volatile gases are much lighter than oil, they will rise into an expansion tank located at the very top of the transformer. If gas generation is too intense, then the pressure of this substance will start to move the oil, which will begin to move towards the expander through the casing.
From this it follows that the gas protection of the transformer responds to intense gas generation and oil movement inside the object, since in total these two actions signal that a breakdown has occurred inside the unit.
Power transformers
In power transformers, in addition to the presence of a gas relay, which reacts to gas formation and the movement of oil in the direction of the expander, there are also jet type relays. The purpose of these relays is to protect contactors of oil-filled transformer branch switches from damage that can occur due to too fast oil flow from the contactor tank to the expander.
It is also worth mentioning that the gas protection of power transformers works even with the slightest damage, that is, even with a low intensity of gas formation and at a low speed of oil movement. By the way, the electrical protection of the jet relay is not able to catch such minor violations.
Security features
It is important to note that by its principle of operation, gas protection is multifaceted. It reacts not only to the formation of gases, but also to the presence of atmospheric air in the transformer, to the movement or tremors of oil inside the casing, as well as to mechanical damage that may occur due to vibration of the unit casing.
In order to avoid false triggering of gas protection and unnecessary shutdown of the transformer, the lower float of the gas relay is most often set so that it responds to the speed of the oil in the range from 50 to 160 cm / s.
Protection benefits
Among the main advantages of the gas protection of the transformer, the following points can be distinguished:
- The simplicity of this protection device, as well as the high sensitivity of the relay.
- The amount of time that the defense needs to make a decision is very small. There is a choice between a signal and a trip, depending on information about damage inside the object.
- It is gas protection that is considered the most sensitive when protecting the transformer windings, as well as when closing its turns.
In addition to the above, it can be added that all transformers with a power of 1,000 kW or more are already supplied with this type of protection. However, there is a small minus, which consists in the fact that the gas protection does not react in any way to damage to the terminals of the unit, and therefore should be equipped with a second protection against internal malfunctions. For example, in low power transformers, MTZ steel and current cutoffs became such a protection system.