Transformers
-
Upload
bhuvaneswari-veeravaghu -
Category
Education
-
view
4.141 -
download
0
description
Transcript of Transformers
Prepared by
V.BHUVANESWARI
TRANSFORMERS
• Transformer works on the principle of electromagnetic induction.
• Transformer Is an electrical device having no moving parts, it transfers
electric energy by mutual induction from one circuit to another at same
frequency with changed values of V and I.
• It consists of two winding insulated from each other and wound on a
common core made up of magnetic material.
• Alternating voltage is connected across one of the winding called primary
winding.
• In both the windings emf is induced by electromagnetic induction.
• And the second winding is the secondary winding of transformer.
WORKING PRINCIPLE OF TRANSFORMER
• When primary winding is connected to an ac source an exciting current flows through
the winding. As the current is alternating it will produce an alternating flux in the core
which will be linked by both the primary and secondary windings.
E1=V1
• Where ,E1 is induced emf in primary
V1 is applied voltage in primary
• Power transferred from the primary to the secondary circuit by electromagnetic
induction.
• Flux in the core will alternate at same frequency of the supply voltage.
• Frequency of induced emf in secondary is the same as that of the supply voltage.
• Magnitude of emf induced in secondary winding will depend upon its number of turns.
In a transformer if the number of turns in secondary winding is less than that in the
primary winding it is called step-down transformer.
In a transformer if the number of turns in secondary winding is higher than that in the
primary winding it is called step-up transformer.
CLASSIFICATION OF TRANSFORMERS
Transformers are classified on basis of
1. Duty they perform
2. Construction
3. Voltage output
4. Application
5. Cooling
6. Input supply
• Duty they perform
I. Power transformer – from transmission and distribution
II. Current transformer- instrument transformers
III. Potential transformer- instrument transformers
• Construction
I. Core type transformer
II. Shell type transformer
III. Berry type transformer
• Voltage output
I. Step down transformer(Higher to Lower)
II. Step up transformer(Lower to Higher)
III. Auto transformer(Variable from ‘0’ to rated value)
• Application
I. Welding transformer
II. Furnace transformer
• Cooling
I. Duct type transformer
II. Oil immersed transformer
self cooled
Forced air cooled
Water cooled
Forced oil cooled
• Input supply
I. Single phase transformer
II. Three phase transformer
star- star Delta-Star
Star-delta Open- Delta
Delta-delta Scott connection
Constructional details
Transformer is a static device and has no moving parts.
Main components of a transformer are:
• Magnetic core
• Primary and secondary windings
• Insulation of windings
• Expansion tank or conservator
• Tank , oil , cooling arrangement , temperature gauge, oil gauge
• Buchholz relay
• Silica gel breather
(1) MAGNETIC CORE
• Magnetic core consists of an iron core. The core is laminated and made of silicon steel.
• Thickness varies from 0.35mm to 0.5mm.
• Laminations are insulated from each other by coating then with a thin coat of varnish.
• Various types of stampings and laminations employed in the construction of transformers.
There are two types of transformer cores ,they are
(a) Shell type (b)Core type
Shell type – Two windings are carried by central limb. Core is made up of E and I stampings
and has three limbs. Has two parallel paths for magnetic flux.
Core type- Has two limbs for two windings and is made up of two L-type stampings. Has only
one magnetic path.
(2) Winding
• There are two windings in a transformer.
• They are primary and secondary windings.
• Made up of copper.
(3) Insulation
• Paper is still used as basic conductor insulation.
• For power transformers enamelled copper with paper insulation is also used.
(4) Insulating oil
• The coil used in transformer protects the paper from dirt and moisture and removes the heat
produced in the core and coils,
• It also acts as insulating medium.
Oil must possess following properties:
• High dielectric strength
• Free from inorganic acid , alkali and corrosive sulphur.
• Low viscosity to provide good heat transfer.
• Good resistance to emulsion so that the oil may throw down any moisture entering the tank
instead of holding it.
(5)EXPANSION TANK or Conservator
• A small auxiliary oil tank mounted above the
transformer and connected to main tank by a pipe.
• Its function is to keep transformer tank full of oil.
(6)TEMPERATURE GAUGE
• Is to indicate hot oil or hottest spot temperature.
• It is self contained weather proof unit made of alarm contacts.
(7) OIL GAUGE
• Every transformer is fitted with an oil gauge to indicate the oil level
present inside the tank.
• It is provided with an alarm contact which gives an alarm when the
oil level drops beyond permissible height due to oil leak or due to any
other reason.
(8) BUCHHOLZ RELAY
• First warning that fault is present is given by presence of bubbles in oil.
• It gives an alarm in case of minor fault and to disconnect transformer
From the supply mains in case of severe faults.
(9) Breather
• It is a chamber which prevents entry of moisture
inside the transformer tank.
• It is filled with drying agent such as calcium chloride
or silica gel.
This absorbs moisture and allows dry air to enter
transformer tank. It is replaced regularly.
EMF EQUATION OF TRANSFORMER
Wkt, T=1/f
Average emf (e)= dΦ/dt
dΦ = Φm
dt = 1/4f
Average rate of change of flux= Φm / (1/4f) = 4f Φm volts
Average emf induced per turn = Average rate of change of flux= 4f Φm volts
Form factor = RMS value/ Average value = 1.11
RMS value= Form factor x Average value
=1.11 x Average value
RMS value of emf induced/turn = 1.11 x 4f Φm = 4.44 f Φm volts
Primary and Secondary winding having N1 and N2 turns.
RMS value of emf induced Primary winding, E1 = 4.44 f Φm N1 volts
RMS value of emf induced Secondary winding, E1 = 4.44 f Φm N2 volts
TRANSFORMER RATIO
For an ideal transformer, E1 = V1 and E2 = V2
There is no voltage drop in the windings.
V1 I1 = V2 I2
V2 / V1 = E2 / E1 = I1 / I2 = N2 / N1 = K
V2 / V1 = K -> Voltage ratio
E2 / E1 = K -> Transformation ratio
N2 / N1 = K -> Turns ratio
I1 / I2 = K -> Current ratio
(i) If K > 1 , then the transformer is called step-up transformer.
(ii) If K < 1 , then the transformer is called step-down transformer.
RATING OF TRANSFORMER
• The rating of transformer is expressed by Volt- Ampere (VA)
• Cu loss depends on Current (A)
• Iron loss depends on Voltage (V)
• Total loss depends upon Volt- Ampere (VA)
• It is independent of Load power factor cos Φ.
Ideal transformer
Ideal transformer has following properties :
• No winding resistance i.e., purely inductive
• No magnetic leakage flux
• No cu loss
• No core loss
Ideal transformer secondary is open. Ac supply is connected to primary winding.
Current flows through primary winding. This current is called MAGNETISING CURRENT
(Iμ).
Value of Magnetising current is small. The Magnetising current produces an alternating
flux (Φ).
Iμ and Φ are in-phase. This changing flux links primary with secondary winding .
Due to alternating flux a self-induced emf (E1) is produced in primary winding which is
equal to and in opposition with V1. It is known as counter emf or back emf of primary
winding.
Induced emf E2 is produced in secondary winding because of alternating flux linking
with secondary winding. This emf is known as mutually induced emf.
PRACTICAL TRANSFORMER ON NO-LOAD
If the primary winding is connected to alternating voltage and secondary winding is left open then transformer is said to be on NO-LOAD. Since secondary is open this current is called no-load primary current (Io).
No load input power, P0= V1 I0 cos Φ0
Active or working or iron loss or wattful component (Iw) which is in-phase with ‘V1’ and
supplies iron loss and small amount of primary cu loss.
Iw = I0 cos Φ0
Where, cos Φ0 is no load power factor.
Reactive or magnetizing or wattles component Iμ
which is in quadrature with V1 and its function is
to sustain flux in core.
Iμ= I0 Sin Φ0
I0 = 𝐈𝐰𝟐 + 𝐈𝛍
𝟐
TRANSFORMER ON LOAD
When the secondary winding is connected to load then the transformer is said to be
on load.
Phase angle between V2 and I2 depends on type of load.
Resistive = I2 in-phase with V2
Load Inductive = I2 will lag V2
capacitive= I2 will lead V2
When transformer is loaded,
Flux is constant at no-load as well as at loaded condition, therefore transformer is
called as constant flux apparatus.
Total primary current will be vector sum of I0 and I2’
Transformer winding resistance
In practical transformer the winding have some resistances. Primary winding has
primary resistance(R2).
Transformer winding leakage reactance
Primary leakage flux(ΦL1) – All the flux generated by the primary winding does not
link with secondary winding. Some part of flux passes through air rather than
around the core. This flux is in-phase with I1.
Secondary leakage flux(ΦL2) – Leakage flux is set up in secondary winding. This
flux induces eL2 in secondary winding. This flux ΦL2 does not link with primary is
also in-phase with I2.
VOLTAGE REGULATION
Regulation of transformer is defined as reduction in magnitude of terminal
voltage due to load wrt no-load terminal voltage.
% Regulation = |V2 on no-load| - |V2 when loaded|
|V2 on no-load|
LAGGING POWER FACTOR
% Regulation = [I1 R01 cos Φ+ I1X01sin Φ ] X 100
V1
LEADING POWER FACTOR
% Regulation = [I1 R01 cos Φ - I1X01sin Φ ]
V1
UNITY POWER FACTOR
% Regulation = I1 R01
V1
EQUIVALENT CIRCUIT OF TRANSFORMER
Where V1 = Voltage applied to the primary winding. I0 = No load primary current. Im = Magnetizing component of no load primary current Ic = Core loss component of no load primary current I1 = Primary current on load I2 = Secondary load current V2 = Voltage across the secondary load terminals
R0 =Resistance representing the core loss
R1=Resistance of the primary winding
X1=Reactance of the primary winding
R2 =Resistance of the secondary winding
X2=Reactance of the secondary winding
E1 = E.M.F. induced in the primary winding
E2=E.M.F. induced in the secondary winding
• The equivalent circuit consists of two circuits, one representing the primary winding and
another is the secondary winding.
• The transfer of power from one circuit to other takes place due to mutual induction.
APPROXIMATE EQUIVALENT CIRCUIT OF TRANSFORMER
The equivalent circuit can be simplified by transferring the exciting branch to left position of
circuit. This circuit is known as approximate equivalent circuit of transformer.
EQUIVALENT CIRCUIT OF TRANSFORMER REFFERED TO PRIMARY
If all the secondary parameters are transferred to primary side we get equivalent circuit of
transformer referred to primary.
Here, Resistance and Reactance > Divided by K2
Voltages > Divided by K
Currents > Multiplied by K
Where, K is transformation ratio.
EQUIVALENT CIRCUIT OF TRANSFORMER REFFERED TO SECONDARY
Why is transformer rating in KVA?
• Cu loss depends on Current and Iron loss depends upon voltage.
• Hence total loss in transformer depends upon Volt- Ampere(VA) only and
not on the phase angle between voltage and current ie., it is independent
of load power factor.
TESTING OF TRANSFORMER
The performance of transformer can be calculated by following equivalent
parameters:
R01 – Equivalent Resistance referred to primary side
R02 – Equivalent Resistance referred to secondary side
X01 – Equivalent Leakage reactance referred to primary side
X02 - Equivalent Leakage reactance referred to secondary side
Rm or R0 – core loss resistance
Xm or X0 – Magnetising reactance
TESTING IN TRANSFORMER
TESTING
INDIRECT
OPEN CIRCUIT
TEST
SHORT CIRCUIT
TEST
DIRECT
LOAD TEST
OPEN CIRCUIT / NO-LOAD TEST ON TRANSFORMER
PURPOSE OF THIS TEST IS TO DETERMINE
• Core loss or Iron loss Or Magnetic loss (Pi)
• No load current (I0)
• Shunt branch parameters R0 and X0
One of the winding is kept open.
Rated voltage at rated frequency is applied to other(LV) winding.
A voltmeter, wattmeter, and an ammeter are connected in LV side of the
transformer.
Ammeter > Reads No-Load Current, I0
Voltmeter > Reads Applied Voltage, V0
Wattmeter> Reads No-Load Input Power, W0 or P0
DETERMINATION OF EQUIVALENT CIRCUIT CONSTANTS THROUGH NO- LOAD TEST
No load power factor, CosΦ0 = W0 / V0 I0
Core loss component, Iw = I0 CosΦ0
Magnetising component, Im = I0 SinΦ0
Core Loss, Pi = No load power (W0)
Core loss resistance, R0 = V0 / Iw = V0 / I0 CosΦ0
Magnetising reactance, X0= V0 / Im = V0 / I0 SinΦ0
SHORT CIRCUIT / IMPEDANCE TEST ON TRANSFORMER
PURPOSE OF THIS TEST IS TO DETERMINE
• Z01 or Z02 – Total impedance referred to either primary or secondary side
• R01 or R02- Total resistance referred to either primary or secondary side
• X01 or X02- Total reactance referred to either primary or secondary side
• Full load cu loss I22 R02
In this test one of the winding is short circuited by thick conductor.
Current rating of HV side is low compared with LV side.
Power input gives total cu loss at rated load.
Unity power factor wattmeter is used for measuring power in SC test.
DETERMINATION OF EQUIVALENT CIRCUIT CONSTANTS THROUGH LOAD TEST
SC power factor, CosΦsc = Wsc / Vsc Isc
Resistance of transformer referred to primary side , R01 = Wsc / (Isc)2
Reactance of transformer referred to primary side , X01= Z01 SinΦsc =
𝐙𝟎𝟏𝟐 − 𝐑𝟎𝟏
𝟐
Impedance of transformer referred to primary side, Z01 = Z01 Cos
Φsc= Vsc / Isc