TRANSFORMERS
What Is A Transformer ?
Transformer is an electrical machine that can converts electrical energy from one voltage level to another voltage level.This convertion can be done in two ways,either steph up or steph down.
Normaly the efficiency of a transformer is a very high value since there is no any rotating parts in a transformer.
Classification Of Transformers
We can classified transformers in many ways .
1 - According To Insulations
Liquid Immersed Transformers
Transformers whose core and coils immersed in an insulating liquid.
Liquid should be non flammable
it can be either mineral or synthetic.
1 - Dry Type Transformers
Dry transformers requier less maintenace.
Negligible smoke generation.
No hazards of fire.
2 -Acording To The Number Of Phases
Single phase transformers
Three phase transformers
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| three phase transformer |
3 -Acording To The Purpose
Current transformers (CT)
In general ,current transformers are used to take measurements of electrical current.
Constant voltage transformers
Auto transformers
4 -Acording To The Construction
Core type transformers
Shell type transformers
5 -Acording to the servise
Distribiution Transformers
Power Transformers
Construction Of Transformers
There are mainly three parts in a transformer.
Primary winding - conected to the suply
Secondary winding - conected to the load
Core-core is the medium that link the primary winding flux to the secondary winding.
How does the transformer works ?
When a AC suply is conected to the primary winding , coil will prodiuse a changing magnetic flux .This flux is directed to the secondary by the means of the core. So there will be an indiused voltage in the secondary.Out put voltage can be steph up or steph down according to the ratio of the number of turns that woun in each side.
Protection Of The Transformer
Transformers are the critical and expensive equipment of a power system.
Faliures in a transformer can be occured as ,
Winding failiers due to short circuits.(phase -phase faults,phase -ground faults,turn-turn fault,open winding)
Core faults (core insulation failiers,shorted laminations)
Terminal failiers (loose conections , short circuits)
On load tap changer failures.
high current surges ( due to lightnings . etc )
How to keep a smooth functioning of a transformer ?
For this it is essential to moniter the situation inside of the transformer regularly.
In oil immersed transformers variation of transformers properties can identified by the monitering the oil situation inside it. To moniter this there are some equipments like Buchhoz relay ,DGPT 2
Buchholz Relay
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| Buchholz Relay |
Buchholz Relay is designd to detects faults as well to minimize the propagation of any damage which might occur within oil-filled transformers, capacitors and reactors supplied with oil
conservator.
Buchholz relay is located in the midle of pipe between the concervator and the transformer tank.
The devise has two elements. The upper element consists of a murcurry type switch attached to a float.The lower element containeds a murcurry switch mounted on a hinged type flap located in the direct path of the flow of oil (coservator to the transformer).
Operation of the Buchholz relay
1- Incase of slowly devolping faults within the transformers,some heat wil generate due to the fault.This heat decomposite the transformer oil of the main tank.The product of decomposition mainly contain of 70% of hydrogen.Hyfrogen is a very light and tries to move in to the conservator tank.On the way they are traped in the upper part of the relay chamber.When pre determined amount of gas gets accumilated it exerts sufficient pressure on the float to close the murcurry switch.This completes the alarm circuit to sound an alarm.
2 -If serious fault occur in a transformer, considereble amont of gas will generated in the main tank.So the oil in the main tank rushes towards the conservator due to the pressure generated by the accumilated gas.So this cause to tilts the flap to close the contacts of mercurry switch(lover valve).This completes the trip circuit to open the circuit breaker controlling the transformer.
The relay is therefore particularly effective in case of;
- Short circuited core laminations
- Bad contacts
- Over heating of the some parts of windings
- Short circuit between phases
Furthermore the relay can prevent the development of conditions leading to a fault in the transformer,
such as the falling of the oil level owing to leaks, or the ingress of air as a result of defects in the oil circulating
system. Disadvantages of the Buchholz relay
It can only use with the oil immersed transformers Iquiped with conservators.
Buchholz relays do not moniter the temperature
Buchholz relay only can ditect faults below the oil leveling the transformer, therefor seperate protection is need for conecting cables.
DGPT 2
DGPT 2 is also a protecting equipment which have to mount directly mounted on the tank at its highest point and with no bends. So gas bubles can traped in the devise.
DGPT 2 do all things that bucholz relay do .
And also DGPT 2 can mesure the temperature of the transformer.
DDLO
DDLO (Drop Down Lift On ) also use as a major protective equipment for the transformer.
DDLO is cnect in series with the input.
Maintaining the moisture inside of the transformer
There are three ways that moisture can come in to a transformer tank .
1 Moisture left in the transformer by Manufacture.This amount can be 0.5 % to 3 % when the transformer leaves the factory.
2 MOisture comes through with Oil
3Through week points and during maintainus period.
Why its essential to keep moisture out from the transformer
- Expansion of the paper insulations and making them weak.
- Loss of insulating ability ( water break down the dielectric)
- Increase the corrosion of the core and tank.
- Break down the dielectric strength of the transformer oil.
Losses Of A Transformer
If the arrester can be fixed and then the DDLO ,then there can be less DDLO failiers. (As shown in figure B )
