What is Vacuum Drying of Power Transformer? Why is it required and How it is done?
Here in this article we will address vacuum drying process/method , principle of vacuum dryer of power transformers and how it is done.
Let us start with basic concepts of insulation and its importance.
Insulation materials are those materials that act as a barrier for electricity conduction. Insulation materials or simply electrical insulation is of prime importance in area of utility of Electric appliances, be it a home appliance operating on 110VAC/230VAC or an industrial equipment operating on 380VAC/415VAC or electrical high voltage devices like the transformers, transmission lines, circuit breakers and like.
As the power levels scale up, the internal insulation levels of devices, (say like insulation between two phases, phase to ground insulation etc.) become more crucial and decisive factor for the life of the device. Devices like transformers and motor-generators require a good insulation for robust electrical performance.
Water is abundantly present in the atmosphere in form of water vapour. Insulation material like paper, insulating oils, wood etc. have natural tendency to absorb water. Water is a good conductor of electricity and if absorbed by the insulation of the device weakens the insulation.
When a device with a weak insulation is put in operation, the electric pressure (voltage) may break the insulation barrier for the current resulting in heavy inrush of electric current which the device may not be able to withstand and eventually break down. In case the device happens to be a part of the system, this may result in failure of the system.
We thus need to ensure a proper insulation of the electrical devices to assure non-failure of the devices.
How can we ascertain a good insulation? What are parameters that decide that the insulation is really in good condition?
Insulating materials say like a plastic handle of electric iron, rubber shoes of wire-man, insulating oils in a Transformer oil filter or say chemicals used in motor windings, all of them require three properties to be defined as good insulators.
First property of the insulator is its resistance offered to the electrical current. The higher the resistance better is the insulation. Second property, the dielectric strength or breakdown strength of insulation, defines strength of the insulation to withstand electric pressure the stress that is developed in the insulation. The third property, the dielectric constant, or the dielectric power factor or also called tan delta, states the amount of electrical energy consumed by the insulator itself in order to act as a barrier to the electrical potential.
By judging these three properties, one can evaluate the quality of insulation.
How can we build good insulation properties?
The parameters defined above which give evidence of good insulation are sharply affected by the water content in the insulation material. Thus if are able to develop insulation that has least water content we are able to achieve better insulation properties. Removal of water from the insulating material is called the process of drying. Drying of insulation to some extent can be done by heating the insulating material and vapourizing the water from the insulation. However, heating of insulation material is limited activity as it may damage the physical and chemical properties of the insulation material. In case of transformers, one cannot simply elevate temperature in order to evaporate more and more water from the insulation. This damages the physical properties of the transformer and also may be hazardous.
We have seen a generalized scenario of insulation and drying. In the discussion to follow we shall now concentrate on drying of transformers.
How can we achieve drying without elevated heating of the insulation material for transformers?
Drying of insulation materials can be achieved at low temperature and low pressure levels. The pressure levels and temperature levels for drying vary for distinct material. At low pressure or under vacuum the rate of water vapour evaporation is accelerated if sufficient heat is provided to the insulation material. Thus we can achieve a good drying of insulating material if the material is suitably heated and subjected to vacuum.
What are the components/equipments used in vacuum drying process of power transformers?
Vacuum drying process requires a closed vacuum tight chamber, a heating system that can heat the insulation in stipulated time, a vacuum system and a controlling mechanism. We shall discuss these one by one below
Vacuum Tight Chamber:
This is generally a rectangular chamber of size suitable to place a transformer or a multiple set of transformers. The construction of the chamber is made rigid to withstand heat and vacuum. The chamber is provided with trolley to maneuver the transformer in/out of the chamber.
Inside the oven, on the walls, tubes are provided wherein a hot fluid flows to transfer heat to the object placed in the oven. This fluid is externally heated by electrical heaters or by a gas/fuel fired boiler. A fan is provided in the oven to uniformly distribute the temperature and accelerate the heating of the transformer insulation.
The vacuum system consists of vacuum pumps, condensers and booster pumps. The vacuum pump and booster pumps evacuate the chamber and extract the moisture/gas (air) from the chamber. Water cooled Condensers placed in series of the vacuum pumps aid in condensing the water vapour extracted from the transformer’s insulation.
With the advent of PLC and SCADA the whole process of drying can be monitored and controlled to desired levels. Vacuum levels can be monitored and controlled. The temperature level in the vacuum drying oven can be controlled. The time taken for drying, amount of drying done and end of power transformer drying process can be determined and monitored.
What is vacuum drying of transformers and how vacuum drying of transformers is done?
In the process of vacuum drying the insulation material of transformer, the transformer is placed in vacuum drying oven. A vacuum drying oven consists of heating facility and a vacuum system to develop low pressure in the oven/chamber. The transformer is first heated to a desired level. The insulation material should be uniformly heated and its temperature should be elevated to acceptable levels. After heating the material, it is subjected to vacuum. Vacuum in the oven is developed by a vacuum system consisting of vacuum pumps. Due to low gas pressure in the vicinity of the material, i.e. in the oven, the moisture absorbed by the insulation is released and extracted by the vacuum system. The vacuum system consists of the water condensation system where this water vapour can be condensed and also measured.
What decides the efficiency of the drying process?
The factors that result a good drying process are
(a) insulation material temperature and its uniform distribution,
(b) property of the material to release water also called the diffusion coefficient,
(c) difference of pressure exerted by the water vapour on the insulation and surrounding area (i.e. inside of the oven) and
(d) the time required for drying out the insulation.
At NACH we have expertise in developing the vacuum drying ovens of required sizes for drying of power transformers. Our installations of large drying system are a state of art. We understand the requirement of the drying process and its influence on quality of the transformer. We believe that with our drying process techniques, the client shall be benefited in quality and costing of the transformer.