COOLING SYSTEMS OF TRANSFORMERS A Presentation on

Presented By:

•Malik Armughan Bhutta SP14-BEE-012 B1B

TABLE OF CONTANTS• Introduction•Methods of Cooling•Applications•Protection

INTRODUCTION: Power transformers are key components for electricity supply

systems. It convert energy at one voltage level to another voltage level.

During the process of energy transfer ,losses occur in the winding of the transformer. This losses appear as heat that may burn the winding of the transformer.

So to make the transformer healthier cooling is needed.

WHAT IS COOLING ?When the Electric Machine is operating because or losses i.e coper losses , eddy current losses coils of Machine heated up if the rated temperature exceeds machine will burn out , to avoid this have to cool it down and that process is called cooling

METHODS OF COOLING There are two basic types of cooling for transformers :•Air type cooling •Oil immense cooling

Air Type cooling

• Air natural (AN) Cooling• Air blast cooling

Oil immense cooling

• Oil natural (ON) cooling• Oil natural air forced (ONAF)cooling• Oil natural water forced (ONWF)

Cooling• Oil forced air natural (OFAN) cooling• Oil forced air forced (OFAF) Cooling• Oil forced water forced (OFWF)

Cooling

AIR NATURAL COOLING• The transformers having smaller output(5-10KVA) are cooled by this method.In this method the external surface is sufficent to dissipate the heat produce by the losses.• Such transformers are cooled by the combination of air convection currents within the enclose case and by natural radiation from the case itself.

AIR BLAST COOLING • This type of cooling is used in dry type transformers.• In this method ,heat dissipation is improved by a continuous blast of air forced through the core and windigs.• The air blast is produced by means of external fans and blowers.• This type of cooling is limited to transformers of voltage rating not exceeding 25kv.

DISADVANTAGES• In this process the increase in insulation strength is not obtained due to non-immersion in oil.•A regular maintenance is a must for this kind of transformers.(i.e filter maintenance)

OIL NATURAL(ON) COOLING• This is the most usual method of cooling.•Here the assembly of core and windings is immersed in insulating oil contained in iron tank.• The heat produced in core and windings passed on to oil by conduction. Oil in contact with heated parts rises and its place is taken by cool oil from the bottom. • The heated oil transfers its heat to the tank surface which dissipates it to the surroundings

OIL NATURAL AIR FORCED COOLING (ONAF)

•Here the assembly of core and windings is immersed in insulating oil and cooling is improved by forced air over the cooling surfaces• The air is forced over external surfaces such as the case,tubes and radiators, usually by means of fans mounted external to the transformer.•Medium to large capacity transformers are cooled by this method.

ONAF PRACTICAL VIEW:

OIL NATURAL WATER FORCE COOLING

• In this type of cooling,the core and windings is immersed in an insulating oil and cooling is improved by circulation of water through copper cooling coils mounted above the transformer core but below oil surface.• The heated water is cooled in a spray pond or cooling

tower.• This method will be cheaper where a natural water head

is already available.

OIL FORCE AIR NATURAL COOLING (OFAN)

• In this method, oil is circulated through the transformer with the help of pump and cooled in a heat exchanger by natural circulation of air.•This method proves very useful where coolers have to be well remove from the transformer.•This method is not very common.

OIL FORCED AIR FORCED COOLING (OFAF)

• In this method,the oil is cooled in external heat exchanger using air blast produced by fans.• At light load, say upto 50% of rated load,(when the losses

are small) natural circulation of air may be sufficient to cool the transformer.• At higher loads,the pump and fans may be switch on by

temperature sensing elements.• This arrangement results in higher efficiency of the

system.

OFAF

OIL FORCED WATER FORCED COOLING (OFWF)

• In this method,the heat oil pumped out from the main tank to the radiator where the oil is cooled by the water passing through copper tubes.• The pressure of oil is kept higher than that of water therefore,any leakage that occurs is from oil to water.• There is no condensation problem.

NITROGEN COOLING• Heat transfer analysis on a cryogenic cooling system is performed

for HTS transformers to be operated at 63–66 K. • HTS windings are immersed in a liquid nitrogen bath where the

liquid is cooled simply by colder copper sheets vertically extended from the coldhead of a cryocooler.

Liquid nitrogen in the gap between the windings and the copper sheets develops a circulating flow by buoyancy force in subcooled state close to the normal freezing point.• Nitrogen functions as a heat transfer medium and an electrical

insulating fluid at the same time.

COUN’THTS windings in a main cryostat filled with subcooled liquid

nitrogen at around 65 K, and located an iron core through room-temperature bore of the cryostat.

The subcooled liquid is continuously chilled by two sets of cryocoolers in a secondary cryostat and circulated through transfer lines to the main cryostat by

a pump.

ADVANTAGES

•Faster Thermal Cycles•Lower Temperature Limits•Minimal Maintenance Costs•  Smaller Equipment•  Lower Electricity Usage

APPLICATIONS• Air natural (AN) cooling is used for smaller output transformer having rating 5-10kva.• Air blast cooling is used for dry type transformer. The voltage rating should not be exceed 25kv.• The oil forced air forced (OFAF) cooling is usually used for power transformer of rating 30mva and higher.

COUNT’D•Oil forced water forced (OFWF) cooling is used for transformers designed for hydro-electric power plant.•Cooling radiator system with natural cooling is used for power transformers with a capacity of 10Mva

PROTECTION:The power transformer protection is realized with two different kinds of devices, namely the devices that are measuring the electrical quantities affecting the transformer through instrument transformers and the devices that are indicating the status of the physical quantities at the transformer itself

TYPES OF PROTECTIONThe following discusses protection devices typically delivered as a part of the power transformer delivery.

•Buchholz (Gas) Relay•Pressure Relay•Oil Level Monitor Device•Winding Thermometer

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