Contents Introduction 2. Working principle 3. Emf equation 4. Turns ratio 5. Equivalent circuit & phasor diagram 6. Voltage regulation 7. Losses & efficiency 8. Cooling 9. Tests on transformers 10. Three phase transformers 11. Other types of transformers 1.
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Contents Introduction 2. Working principle 3. Emf equation 4. Turns ratio 5. Equivalent circuit & phasor diagram 6. Voltage regulation 7. Losses & efficiency 8. Cooling 9. Tests on transformers 10. Three phase transformers 11. Other types of transformers 1.
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1)
A static device, which transfers electrical energy from one circuit to another circuit, without change of frequency
Introduction
Types of transformers:
Power transformers (Single-phase or 3-phase) - Used in power transmission and distribution systems - Step-up or step-down
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Current transformers
Voltage transformers
Auto-transformers
2) Working Principle (Single-phase power transformers) 2 windings
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Primary
- connected to the alternating voltage source
Secondary
- delivers the power to the load
Ideal Transformer
Secondary is open circuited
It does not supply any energy to the load
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Transformer Taps Voltage drops in transmission lines
Secondary voltage is considerably less
Taps are provided on the primary winding
Taps enable to change the turns ratio -> to raise the secondary voltage
Automatic taps
Maintain the secondary voltage within ±2% of its rated value
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3)
emf Equation
Sinusoidal primary current produces a sinusoidal flux
emf induced in the primary winding
maximum value of e1
effective value of the primary emf
Similarly, the effective value of the secondary emf
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4)
Turns Ratio
Stepped up and down transformer depends on the turns ratio
For an ideal transformer, Magneto-motive force due to primary current
= Magneto-motive force due to secondary current
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5)
Equivalent Circuit and Phasor Diagram
Practical Transformers
Windings of practical transformers have resistance
Core is not infinitely permeable
Flux is not completely transferred to the secondary
Iron cores produce eddy-current and hysteresis losses
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Practical Transformer
Rm (core loss resistance) - represents iron losses
Xm (magnetizing reactance) - measure of the permeability of the transformer core
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Equivalent Circuit & Phasor Diagram
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Referring Impedances
Resistance in the primary becomes the secondary
Resistance in the secondary becomes to the primary
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, when referred to
, when referred
Equivalent Circuit
Referred to primary
Referred to secondary
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Approximate Equivalent Circuit
Referred to primary
Referred to secondary
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Example 1 A 100kVA single phase transformer has 400 turns in the primary and has 80 turns in the secondary. In primary side X1=1.1Ω and R1=0.3Ω. In secondary side, X2=0.035Ω and R2=0.01Ω. Supply voltage is 2200V. Determine the equivalent impedance referred to primary.
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6)
Voltage Regulation
When the transformer is loaded -> secondary voltage changes due to drop in the internal impedance
Voltage regulation is a term that is used to identify this variation
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For leading power factor, the sign is negative.
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Example 2 A 100kVA single phase transformer has 400 turns in the primary and has 80 turns in the secondary. In primary side X1=1.1Ω and R1=0.3Ω. In secondary side, X2=0.035Ω and R2=0.01Ω. Supply voltage is 2200V. Calculate the voltage regulation and secondary terminal voltage at full load at power factor of 0.8 lagging.
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7)
Losses & Efficiency
Efficiency is a function of secondary current For maximum efficiency;
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8)
Cooling
Indoor transformers below 200kVA Cooled by the natural flow of the air Placed inside a metallic housing which is having ventilating louvers
Larger transformers Built the same way Forced circulation of clean air is provided
Distribution transformers below 200kVA Immersed in mineral oil and enclosed in a steel tank Heat is dissipated by radiation and convection to the outside air
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Large distribution transformers
External radiators are added to increase the cooling surface of the oil filled tank
Oil circulates around the transformer windings and moves through the radiator
Heat is released to the surrounding air
For still higher ratings
Cooling fans blow air over the radiators
For transformers in the megawatt range
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Cooling may be effected by an oil-water heat exchanger
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Tests on Transformers
Open Circuit Test
High voltage side kept open
Wattmeter reading is the core loss
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Short Circuit Test
Low voltage side short circuited
Wattmeter reading is the copper loss
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Example 3 A 2400V/240V, 24kVA transformer gave the following results.
High voltage side open
- 240V, 2A, 200W
Low voltage side closed
- 100V, 10A, 150W
Calculate the voltage regulation if it is supplying an load of 0.8 power factor lagging at full load. Calculate operating efficiency and maximum efficiency.
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10) Three Phase Transformers Made either by:
3 single phase transformers connected in a 3 – phase bank
3 windings are wound on a common core
1st approach is cheaper, smaller and lighter
Three-phase transformer connections
Depending on the connections of primary and secondary coils; Y-Y Y-Δ Δ-Y Δ-Δ
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Y-Y connection
Rarely used for large amount of power transmission
Can be used for special types of applications
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Y-Δ
connection
Used at substations where the voltage is stepped down
Secondary coil current is 57.7% of the load current
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Δ-Y
connection
Used at the generator station, where the voltage is stepped up
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Δ-Δ connection
If one transformer is removed, the remaining two are able to supply three-phase power to the load terminals
Known as open delta or V-V connection
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11) Other types of Transformers Auto Transformer
1)
A small rating transformer with variable output
Used in educational laboratories
Has one winding -> common for primary and secondary
Advantages:
Cheaper initial investment Low leakage reactance
Low losses
Requires low excitation current
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2)
Instrument Transformers
Voltage Transformer Ratio of primary to secondary voltage is known
Used to measure the voltage on transmission lines
Current Transformer Ratio of primary to secondary current is known
