All About Auto-Transformer

All about Electrical Transients

900 MVA , 845 MVA Transformers

What is the purpose of step up transformer in electrical power system?

What is the purpose of step up transformer in electrical power system?

The main use of transformer in electrical power system is to produce the needed voltage level. Some time voltage is needed to be increased or decreased than the available voltage level. Depending on the operation, transformers can be divided into two types,

(i) Step up transformer,
(ii) Step down transformer.

As the name suggests, the step up transformers are used to step up the voltage level. These are also termed as power transformer when used in power plants. Similarly step down transformers are used to step down or bring down the voltage.

Now, coming to the step up transformer, they are used in electrical power system at generation level. When power is generated at power plants the voltage level is pretty low.
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So when they are transmitted from generation station to distribution substation, power is reduced due to the different types of losses and the voltage of the receiving end decreases radically and thus the economic purpose is also not served.

That’s why step up transformers are introduced at the power plants to step up the voltage level before transmission. So, the receiving end voltage is not decreased too much.
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Again as this high voltage cannot be used directly, so voltage is again stepped down by step down transformer before distribution.
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What is LVDT?

What is LVDT?

LVDT or Linear variable differential transformer is a passive transducer and is commonly employed to measure force (or weight, pressure and acceleration etc. which depend on force) in terms of the amount and direction of displacement of an object.

Construction :

It consists of one primary and two secondary windings S₁ and S₂. These are place on either side of the primary mounted on the same magnetic core. The magnetic core is free to move axially inside the coil assembly and the motion being measured is mechanically coupled to it.
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The two secondary winding have equal number of turns but are connected in series opposition. Emf induced in them are 180° out of phase. The primary is fed form an a.c source.

Working Principle: 

When the core is in the centre (reference position) and induced voltages E₁ and E₂are equal and opposite. Hence they cancel out and output voltage is zero. When the external applied force moves the core towards coil S₂, E₂ is increased but E₁ is decreased in magnitude though they are still anti phase with each other. The net voltage available is (E₂ - E₁) is in phase with E₂.
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Core balanced current transformer

Core balanced current transformer

Core balance current transformer or CBCT is a special variation of current transformer, where the main current coil is constructed in ring style. This type of current transformers are specially used for the measurement of three phase current. They are installed in substations in order to protect the line from earth faults.
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So, an earth fault relay is connected with the CBCT and after sensing the fault current, CT gives the fault signal to the relay. Energised phase cables are inserted through the hollow section of ring coil of CBCT, and automatically the primary of current transformer built, and the air section in between the cable and ring coil acts like the low reluctance flux path.
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In this way, the ring coil becomes the secondary of the Core balance current transformer. In three phase power system, the current in phase cables are balanced. So, during the normal operation conditions, no current is there in the secondary ring coil of CT.
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But, when an earth fault is arised, that is any of the phase cable subjected to the direct contact with earth, then huge amount of unbalanced fault current starts flowing through the conductors.
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When this residual fault current travels through the phase cable, then automatically there will be a considerable current in the secondary coil of CBCT.
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This fault current signal is received by the relay coil, and automatically trips the contacts. In this way, the line is kept protected from the effect of local earth fault.
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Admin:- Bellapuri saikumar
                 ( Sai Saikumar Jn)

Types of Current's in a transformer... ?

What is Magnetizing Current ?

Magnetizing current is a current component which is seen in electrical machines likes transformers and motors. No load current of transformer has two components, one is the magnetizing component and other is the active component.

Magnetizing component is also called the reactive or wattles component of no-load current. The active component or watt full component supplies the hysteresis and eddy current loses. Magnetizing component of no load current magnetizes the core of the transformer. Thus this component sets up flux in the transformer core.

Magnetizing current is in phase with the flux and is 90 ° to applied voltage and active component. Magnetizing current is associated with the primary winding of the transformer. It is the current that flows in primary when voltage is applied across it and in this condition secondary is unloaded. Magnetizing current value is related to supply voltage, frequency and primary inductance value.
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Primary side always has a current irrespective of the load condition of transformer which is called magnetizing current and this current adds on to the load current. So primary of transformer draws a peak current that is known as magnetizing inrush current. Magnitude of this current is high and it lasts for a very less time and it is transient in nature.
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What is Eddy Current?

Magnetic circuits are subjected to two kinds of losses- these are Hysteresis loss and Eddy current loss. Eddy current is responsible for eddy current loss. Core is composed of conducting materials so that the voltage induced in it by the varying flux produces circulating currents in the iron core. These are called eddy currents and are accompanied by I²R loss in the core. Since eddy current depends upon the rate of change of flux as well as the resistance of the path, eddy current loss varies to the square of both maximum flux density and the frequency. Eddy current loss is given by –

P_e = K_e f² (B_max) ² t²v Watts or joules/second.

K_e is the eddy current coefficient and it depends upon the type of core material, f is the number of complete magnetization cycles per second, B _max is the maximum flux density in Tesla (WB/m²), t is the thickness of laminations in metres and v is the volume of core material in metre cube. To increase the core resistance and thereby minimizing eddy currents, magnetic cores subjected to alternating fluxes are assembled from thin sheets with an insulating layer between successive laminations .
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Eddy currents have a magnetic effect tending to make the flux density at centre lower than at the surface. Hysteresis and eddy current losses taken together are known as core or iron loss.
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Admin :- Bellapuri saikumar

                 ( sai saikumar jn)

TRANSFORMER-MCQ with Answers

TRANSFORMER-MCQ
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1.Which of the following does not change in transformer ?

A. current
B. voltage
C. frequency
D. all the above

Ans: C

2. The efficiency of transformer is maximum when

A. copper loss=hysteresis loss
B. copper loss=eddy current loss
C. hysteresis loss=eddy current loss
D. core loss=copper loss

Ans: D

3.The transformer ratings are usually expressed in

A. ampere
B. KVA
C. KW
D. voltage

Ans: B

4. The transformer ratings are usually expressed in

A. ampere
B. KVA
C. KW
D. voltage
Ans: B

5. Which of the following does not change in a transformer ?

a.current
b. voltage
c. frequency
d. all the above

Ans: C

6. For parallel operation of single phase transformers it is necessary that they should have

A. same efficiency
B. same polarity
C. same KVA rating
D. same no.of turns on secondary side

Ans: B

7. A certain transformer has 400 turns in the primary winding and 2,000 turns in the secondary winding. The turns ratio is

A. 0.2
B. 0.4
C. 5
D. 25

Ans: C

8. The main purpose of performing open-circuit on a transformer is to measure its

a) copper loss
b) core loss
c) total loss
d) insulation resistance

Ans: B

9. Secondary current of a step down transformer is

A. Lower than primary current

B. Higher than primary current

C. Equal to primary current

D. Double than primary current

Ans: B

10. Principle of statically induced emf is used in

A. transformer
B. motor
C. generator
D. battery

Ans: A

11. A buchholz relay can be installed on

A. auto-transformers
B. welding transformers
C. air-cooled transformers
D. oil-cooled transformers

Ans: D