STEP DOWN TRANSFORMER

1- MAIN COMPONENTS OF OIL IMMERSED TRANSFORMER

1-1- The Core

• In a transforming voltage from a primary coil induced to the secondary coil of the production of magnetic flux in the core of the transformer.
• In all types of construction, high-permeability-grade core, high-rolled grain-oriented laminated silicon steel. Laminations are assembled together to provide minimum magnetic loss.
• The resistivity of the steel sheet is made by making the laminations very thin and thus
• Thickness of the lamination is between 0.25mm to 0.5mm, the laminations are electrically insulated from each other by a very thin coating of insulating varnish.

1-2- The Winding

• Transformer windings (coil) are made from copper wire and wound on the core of the transformer. The low voltage winding is placed on the core and high voltage winding is placed on the low voltage winding.
• The coils are paper insulated of insulation class 'grade A'.
• The winding is connected to the voltage source and creates a magnetic flux called the primary winding, and the other winding called the secondary in which voltage is induced as a result of mutual induction.

1-3- Tank and conservator

• The core and coil assembly are immersed in an Oil Tank. The oil acts as an insulating and cooling medium for the transformer.
• The Tank is connected to a Conservator, placed above the Tank and connected with the Tank by pipes. The conservator is in the process of expanding oil. It also acts as a reservoir for transforming insulating oil.
• The conservator is connected to the atmosphere with a connection to the top of the conservator. The pipe is connected to the atmosphere through a Silica gel breather.
• During expansion and contraction of the air is the breath of the Silica gel breather.
• Silica Gel in the breather absorbs moisture content of the air taken inside which ensures no deterioration of insulating oil properties due to atmospheric moisture ingress.
• The storage tank should be mounted above the highest point of the equipment oil circulation system and, in the case of on-load tap-changer transformers, two (2) oil-tight compartments. and the appropriately sized gas, one for the main tank and one for the bypass switch compartment must be provided.
• The preservative must not allow contact between the oil in the main transformer tank and the switch compartment (if available) and outside air.
• This must be achieved either by the use of a diaphragm between the oil and the air, or by the use of a multi-compartment preservative with internal baffles.

1-4- The Cooling system

• When a Transformer is loaded, the current starts flowing through its windings and produces heat that transforms the temperature of Transformer Oil. The oil tank is connected to the outer end of the radiator with isolating valves. The radiator accelerates the cooling rate of transformation.
• In order to increase the frequency of cooling, they are widely used to transform along with the radiators. Fans operate automatically with rise of oil / winding temperature.
• MVA rating of transformer provided with external cooling fan on the radiator is given for the Oil Natural Air natural cooling (ONAN i.e. fan not running) and Oil Natural Air forced cooling (ONAF, i.e. with fan running). Start and stop the fans are controlled by oil / winding temperature.
• The above types of cooling systems are mainly used, but for very large transformers, in addition to the cooling fans, an oil pump is also installed in the pipe between the tank and the radiator.
• This type of cooling is called Oil Forced Air Forced (OFAF).
• Regardless of the ONAN and ONAF rating for forced and forced air (OFAF, that is both the fan and the oil pump), this is also mentioned for this type of transformer.
• Starting and stopping of the oil pumps is controlled by the oil / winding temperature.

1-5- Voltage Regulation Equipments

• Transformers are generally provided with Taps for adjustment of voltage ratio by adding or removing tapping turns.
• Two types of tap changer are used viz. No Load Tap Changer (NLTC) and ON Load Tap Changer (OLTC). Taps are provided on the HV winding.
• The NLTC is simple type and can be used only when the transformer is de-energised whereas OLTC can be operated when the transformer is loaded without interrupting the current. 
• Transformer with OLTC is provided with a Remote Tap Changer Control (RTCC) panel from which tap can be changed manually/electrically and also in auto mode to maintain the pre-set voltage of the system to which transformer is feeding power

1-6- Terminals

• The winding connections are brought out to insulated bushings where they connect with the supply and the load. 
• Generally in all transformers, Terminals are suitably designed for connecting bare conductor, cables or bus-duct depending on the requirement. 
• In most cases terminals are air-insulated but transformer with cable terminal box can be air insulated or oil-insulated.

1-7- Buchholz relay

• A Buchholz relay is located in the connection pipe between the main tank and conservator. The relay has two functions:
• Collect free gas on their way to conservator.
• Sensitive when oil flow between main tank and conservator exceeds a pre set value.
• When minor amount of gas is collected in the relay an alarm actuated. If an additional amount of gas is collected tripping contact actuated. 
• In case of heavy fault with in the transformer arcing may occur within the transformer, this will lead to sudden push a burst of oil in the conservator from the main tank. Such fault causes actuation of tripping contact.
• The Buchholz relay has gas release pockets on top to collect the gas in the relay for testing to understand the nature of fault in the transformer.

1-8- Temperature Indication

2- Oil and Winding Temperature Measurement

2-1- Oil temperature indicator (OTI)

• These  devices  are  designed  to  measure  the  temperature  of  the  insulating  oil  inside  of power transformer tanks. 
• The bulb  of thermometer detects  the transformer oil temperature variations generating a contraction  or  expansion  of  the  capillary  connected  to  the  pointer shaft.  
• The  adjustable switching system connected to the pointer shaft provide the desired alarm/trip signals.

2-2- Winding temperature indicator (WTI)

• The  winding  is  a  “high  temperature”  component  in  oil  transformers,  subjected  to  fast temperature  changes.  Thus,  an  indirect  system (Called Thermal Image Principle)  is  used  to  measure  the  winding temperature because it is dangerous to place a sensor close to the winding. 
• This instrument is designed to measure the temperature of winding by using a special bulb surrounded  by  a  heating   resistance  through  which  passes  a  current  proportional  to  the winding current under certain load and temperature conditions. 
• The bulb  detects  the  temperature  variations  generating  a contraction  or  expansion  of  the  capillary  connected  to  the  pointer  shaft.  The  adjustable switching system connected to the pointer shaft provide the desired alarm/trip signals.
• The top oil temperature is measured directly by sensor kept in oil pocket in the oil top layer in the tank. 
• Winding temperature is measured in indirect way; by connecting the Secondary of the Current Transformer (CT) through a shunt resistor (heater coil) inside the Winding Temperature Indicator. 
• The Heater Coil heats the operating bellow of the temperature indicator to give additional temperature rise of the winding over the oil temperature.
• Both winding and oil temperature have two output contacts, one for pre trip alarm and other for tripping, if the temperature rises to a set value or above.
• For ONAN and OFAF transformer apart from alarm & trip contact, output contacts are provided for auto start of cooling fan and oil pump.

OIL LEVEL  GAUGES

• The oil    level  gauges  serve  the  purpose  of  visually  indicating  the  oil  level  in  a  tank  or  in  a conservator of oil filled transformers.
• It is normal practice to use magnetic oil level gauges with low level alarm and trip contacts. This can also be of the plain glass or prismatic type. 
• The oil level gauge is marked to indicate minimum, normal and maximum oil levels in the conservator (as a function of conservator capacity). 
• The conservator (or partition) for on-load tap changer carries its own oil level gauge. 
• For bigger units, oil level gauge is fitted on the tank also.

Dehydrating breathers

• During the breathing process, the incoming air may consist of moisture and dirt which should be removed in order to prevent any damage. 
• Hence the air is made to pass through the silica gel breather, which will absorb the moisture in the air and ensures that only dry air enters in to the transformer.
• Silica gel in the breather will be blue when installed and they turn to pink colour when they absorb moisture which indicates the crystals should be replaced. 
• These breathers also have an oil cup fitted with, so that the dust particles get settled in the cup.