Transformer

• Transformer: Static device transferring electrical power from one circuit to another through a magnetic field without altering frequency.
• Transformers only work with AC

Transformer Materials

• Active materials in transformer construction are magnetic materials and conductors
• Magnetic materials consist of a laminated iron core. and carries flux linked to windings
• The iron core provides a low reluctance path to magnetic flux, which reduces magnetizing current

Why Transformers are Needed

• Changing voltage and current levels in electrical power systems
• Stepping up generated voltage for power transmission
• Stepping down transmission voltage for distribution and power utilization
• Matching source and load impedances for maximum power transfer
• Providing electrical isolation between circuits

Transformer Classification

• Single-phase transformer
• Poly-phase transformer

Transformer Function

• Power transformer
• Distribution transformer
• Measuring transformers, which include voltage and current transformers
• Autotransformer- Tapped autotransformer

Power Transformers

• Connect to a generator's output, stepping up voltage for transmission
• Higher voltage and lower current decreases transmission line sizes, costs, and losses

Power Transformer specifics

• Used between generators and distribution circuits
• Rated at 500 kVA and above
• Used for step-up operation at the generator
• Also known as generator step-up (GSU) transformers
• Generator step-up transformers in power plants receive electrical energy at generator voltage and increase it for transmission lines
• Power is transmitted at high voltages like 132kV, 220kV, 230kV, 400kV or 500kV, and more

Distribution Transformers

• Convert high distribution voltage to final levels
• Increase current
• Smaller than 500 kVA and located to residences and small businesses

Distribution Transformer: step-down

• Step-down transformers take in higher voltage and lowers it for distribution to various loads
• Consumer voltage requirement is usually 220v or 400v

Measuring Transformers

• These measure high voltage using a low-range voltmeter if the voltage of an AC circuit exceeds the range of the voltmeter
• It steps down the voltage to the level of voltmeter

Current Transformer

• Measures high current with low-range ammeters in HV AC circuits
• They step down current to a known ratio enabling use of ammeters, voltmeters, watt meters, and energy meters
• Instrument transformers are generally used for measuring and control purposes

Autotransformers

• Have only one winding
• An autotransformer contains a typical magnetic core but only one winding is common to both the primary and secondary circuits

Transformer Design

• Shell type transformers have laminated steel cores surrounding the copper windings
• Core type transformers include copper windings surrounding the laminated steel core

Transformer Choice

• Choice of core or shell-type transformer is decided by cost, voltage ratings, weight, KVA ratings, and heat distribution
• Similar performance can be obtained with each type
• Shell types are perferred for high voltage transformers and multi winding designs
• Mean coil turn length in shell-type is longer than in core-type designs

Transformer Cooling

• Cooling system and application is determined by transformer application
• Transformers for indoor use are typically dry type, but can be liquid immersed
• Outdoor transformers are generally liquid immersed

Air Cooled Transformers

• For dry-type transformers usually with voltages below 25KV
• Air natural type (A.N.) cooling methods are common in dry type small rating transformers
• Power ratings increase, transformers are often cooled by forced-air cooling

Air Forced Cooling

• Forces air onto the tank surface to increase heat dissipation
• Fans switch on when the winding temperature rises above a permissible level

Oil Immersed Transformers

• Oil Natural Air Natural Type (O.N.A.N.) cooling is used for oil-filled transformers up to 30MVA
• Heat is transferred from transformer windings/core to the oil
• Heated oil is cooled by the natural air, and area is increased by adding cooling tubes

Oil Natural Air Forced Cooling

• Applied to higher rating transformers with difficult heat dissipation
• Fans help remove heat from core and windings

Oil Forced Air Forced Cooling

• Used if oil natural air forced cooling is not adequate to remove heat caused by losses
• Transformers above 60 MVA use a combination of forced oil and forced air cooling

Oil Forced Water Forced Cooling

• Intended for very large transformers with ratings of hundreds of MVA
• These are typically used in large substations and power plants

Transformer Elements (Parts)

• Two coils with mutual inductance, wound on a laminated steel core
• Consists of primary and secondary windings
• Coils are insulated from each other and the core
• Paper and wood are used for support and insulation
• Container houses core and windings
• Container has a medium for insulation
• Bushings insulate and bring out winding terminals from the container

Transformer Accessories

• Conservator: Expansion tank, keeps the transformer tank full of oil during oil expansion/contraction
• It is placed above a transformer and connects to the tank by a pipe
• Temperature gauge: Indicates oil temperature and is connected to an alarm
• Oil gauge: The oil level in the tank. If the oil drops too low, contacts close and trigger an alarm
• Buchholtz relay: Gas-operated relay in pipe to the conservator
• If a fault releases gas bubbles, the bubbles trigger a relay to create an alarm signal
• Breather: Prevents moisture from entering tank using silica gel
• Silica gel absorbs moisture, allowing dry air to enter the tank

Transformer Working Principle

• Works on the principle of electromagnetic induction between two magnetically coupled coils
• Alternating flux will depend on the Primary voltage (Vp) and number of primary turns (Np)
• Mutual flux will link the secondary winding and changes, inducing an EMF
• The value of the EMF depends on number of secondary turns (Ns) and the magnitude of the mutual flux

Magnetic Flux

• Magnetic flux within the secondary coil induces an e.m.f in the secondary winding
• Electrical energy is transferred magnetically from primary to secondary winding when the secondary circuit is closed

Transformer EMF equation

• Proportional to the turns N and rate of flux change

Flux

• Increases from zero to maximum in one-quarter of a cycle

Rate of Change

• Average rate of flux change is equal to 4 * flux * f

Induced EMF

• Average emf/turn equals to 4f * flux
• If flux is sinusoidal, RMS value of induced EMF equals form factor times the average value

RMS value of EMF

• With the r.m.s value of e.m.f/turn given by 1.11average value = 1.11 4fm = 4.44f volt

RMS and Winding

• Now r.m.s value of the induced e.m.f in the whole of primary winding(E₁)
• E₁ = induced e.m.f/turn*No turns of primary winding
• This means E₁ = 4.44f N₁ * flux = 4.44f N₁ B * A
• Finally the relation to RMS value of EMF for secondary
• E₂ = 4.44f N₂ * flux = 4.44f N₂ B * A

Ideal Transformers

• Lossless with input and output windings

Properties of ideal Transformers

• No iron and copper losses
• No winding resistance
• No leakage fluxes
• Voltage equations
  • E₁=V₁
  • E₂= V2
• V₁I₁ =V2I2, equal power
• Turns ratio E₁/E2 =V1/V2 =N1/N2 =I2/I₁
• and E₁/N₁ = E2/N2, equal EMF per turn

Ideal Transformers

• Time varying flux, Om(t), established in the iron core
• Flux linkages equations for primary and secondary side in Ideal transformer
  • λp = NpΦm(t)
  • λs = NsΦm(t)
• Voltages will be induced in these two coils:

Turn Ratio

• Derived from voltage and current equations
• Ratio connects voltage, current, and number of turns

Real Transformer

• Turn ratio (a) relates voltages and currents with series and parallel impedances

Transformers

• An AC source connects to the primary where a current flows through the primary winding, before the secondary opens
• On a system on no load, if secondary current (IS) is at zero then the primary current should be zero to
• If not on no load then an excitation current is seen on the primary
• Flux produced in ferromagnetic core requiring excitation core

Excitation Current

• Excitation current consists of two components:
• Loss(Ic) and Magnetization (Im) Current,
• Includes hysteresis, and losses for the setup
• Magnetization, which can be used to core the flux

Increasing Load

• When load is increases to a transformer then the currents for both the secondary winding and supplied sources increase
• Following the first increase, the voltage drop for primary winding increases as well from more current
• This leads to an EMF drop, as mutual flux decreases from decreased magnetizing current

Impedance

• Calculations to simplify the impedance are transferred, using and eliminating the transformers to provide equivalent electrical circuit
• There can be some inconvenience when working on a winding instead

Equivalent Circuit

• The transformer is diagrammatically shown when the circuits can determine resistance and leakage
• This circuit can then function via parameter transform
• Can also be diagramed in magnetization through current in a real system

Magnetization Current

• Circuits for the current through real transformers can be resolved by the winding (Even before opening) through a primary circuit
• If on no-load, and the secondary circuit is zero, then there is no primary current
• However if on no-load, current begins through excitation and core
• The current is also used to form a magnetization for the ferromagnetic core, and the excitation circuit

Series and Shunt for transformers

• Increase of load across can shift components, to be better measured from series or parallel

Transformer Loss

• Comes from I^2R
• Eddy Currents can cause a loss in heat, following resistivity within the materials
• This proportional to the squared voltage when setting circuits, and cores, and circuits

Eddy Currents

• Transformers with Eddy Currents also follow hysteresis loss and frequencies, relating to:
  • Maximum flux density
  • Core materials
  • Supply
  • Thickness
• Hysteresis can be used to rearrange domains throughout half of a cycle

Hysteresis

• Hysteresis loss includes applied voltage with frequencies, and a second
• Hysteresis loss depends upon
• The material.
• flux/Density
• the frequency
• Core material volume

Heat

• As heat losses rise they must remain lower then a limit
• Exceeding limits can reduce lifespan, and cause damage when voltage in currents drops
• Limits also the magnetic and resistances of transformers and inductance

Rating Transformers

• Dependent on Volts and AMPS (VA) and (KV)
• The is regardless from phase angles
• Therefore, the rating comes from KVA instead of (KW)

Regulations of Voltages

• A "real" transformer has voltages that varies the results, based of what is connected to load and inputs
• Thus you can the find magnitude and the changes when you reduce the load
• %voltage regulations = Vs( no load ) - Vs(full load )*100/Vs at full load
• Finding "b/n on load and off" can give the percent between the changes

Lagging

• Terminal voltage can fall then a load can apply more through heat
• Voltage is increased when power factors lag/lead
• There must have a power factor depending on what the voltages and loads are as it transfers
• Positives have a factor that follows the negative when that voltage regulates

Performance of a transformer

• For any test there are several main components including
• equivalent testing
• resistance and Leakages
• conduction of the core
• The 2 main tests are
• Open/Short

To determine transformers testing

• Some measurements must be in place
• Also some components need to be off and other to be connected to apply it

Tests include

• The coil being cut or connected
• And applying certain Volts (Depending on the circuits)
• Wattmeters are key components to measure

Circuit

• Short circuits can then be measured, or "full Loads" with certain components working
• These can the be represented with measurements, charts, equations and values

Three-phase systems

• Three-phase systems dominates electric power systems with a high degree of voltages, phases and power
• Transformers step to the generated voltage levels
• Three phase can have:
  • 3 sets to create 1 -Or Y types for core balance and efficient materials

Limb

• Single units may use a three-limb for cores
• This uses "LV windings"" with H.V
• To counter balance this insulation in placed

Transformers Connections

• Most connections must form
• Y/Delta, or vice verse
• All connections form:
  • Higher or lower phases
  • Transfers from connections from phase units