Embed Size (px)
Citation preview
EN 2061/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
EN 206 - Power Electronics and MachinesTransformers
Suryanarayana DoollaDepartment of Energy Science and Engineering
Indian Institute of Technology, [email protected]
EN 2062/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Lecture Organization - Modules
Introduction and Power Semiconductor Switches
Module 1: Transformers
Module 2: AC/DC converter / Rectifier
Module 3: DC machines and Drives
Module 4: DC/DC converter
Module 5: Induction Machine
Module 6: DC/AC converter / Inverter
Module 7: AC/AC converter / Cyclo converter
Module 8: Synchronous Machine
Module 9: Special Topics: Machines, HVDC, APF
EN 2063/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Review of Last Class
Transformers
Phasor diagram for ideal transformer and no loadcondition, impedance transformationEquivalent circuitSC and OC Test of a Transformer
EN 2064/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Voltage Regulation
Definition:
It is defined as change in voltage from no load to full load,as a percentage of rated terminal voltage at full load.
Voltage Regulation = VNL−VFL
VFL
EN 2065/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Problem-1
Short circuit measurements done on a 2200/220V transformerrated 500 kVA provide the following values of equivalentresistance and reactance, respectively, referred to low-voltage(secondary) side:
Re2 = 2mΩ, xe2 = 6mΩCalculate(a) Rated secondary current(b) Full-load equivalent resistance voltage drop I2Re2, Full loadequivalent reactance voltage drop I2Re2.(c) Induced voltage when the transformer is delivering ratedcurrent to a unity PF load.(d) Voltage regulation at UPF. (e) Voltage regulation for ratedload at 0.8pf lagging. (f) Voltage regulation for rated load at0.8pf leading.
EN 2066/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
VR - Analysis
It is practically impossible to determine voltage regulationfor a transformer. In such case the results of SC test shallbe used to determine the voltage regulation.
It is possible to have zero percent or negative voltageregulation depending on magnitude of leading pf.
EN 2067/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Losses in a Transformer
Constant loss:
Core loss: Assumed to be constant for rated voltage andfrequency. Depends on flux density of the core, magneticproperties of the material, resistivity of core material,volume of core material.Stray load loss: Leakage flux present in a transformerinduce eddy currents in the conductors, tanks, channels,bolts etc. These eddy currents give rise to stray load loss.Dielectric loss: This loss occurs in the insulating materials(transformer oil, solid insulation of HV transformers)
Variable losses: These are due to ohmic losses in theprimary and secondary of transformer when it is loaded(no load neglected).
Under normal operating conditions, the variation of mutual fluxis not more than 2 to 3%, consequently, core losses areassumed to be constant throughout.
EN 2068/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Efficiency
Both SC and OC test data is required to calculate efficienyof a transformer.
Efficiency (η):OutputPower(kW )
OutputPower(kW )+Losses(kW ) = Po
Po+Losses = V2I2cosθ2
V2I2cosθ2+(Pc+I 22 Re2)
Maximum efficieny of a transformer occurs at load whenfixed losses are equal to variable losses. “Try the proof athome”.
Pc = I 22 Re2 =⇒ I2 =
√PcRe2
All Day Efficiency: In a transformer where the loadchanges continuously with time (ex. distributiontransformer), all day efficieny is defined as
ADE=1- DailyLosses(kWh)DailyOutput(kWh)+Dailylosses(kWh)
EN 2069/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Autotransformer
A transformer in which a part of winding is common toboth the primary and secondary circuits, is called anauto-transformer.
In an autotransformer primary and secondary winding arenot isolated.
Working mmf in a transformer remains substantiallyconstant at its no load value. The primary must drawadditional current from the source, in order to neutralizethe effect of load current.
EN 20610/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Autotransformer - Analysis
mmf of winding AB= I1(N1 − N2) = (I2 − I1)N2 = ICBN2
= mmf of winding CB
Transformer action takes place between winding AB andBC
Transformed VA = VAB IAB = (V1 − V2)I1.
Transformed VA/Input VA = V1−V2V1
= 1− k
EN 20611/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Autotransformer - Advantages & Disadvantages
Advantages
High efficiency
Low leakage impedance and has superior voltageregulation when compared to a two winding transformer ofsame output
Requires less conductor compared to a two windingtransformer (k < 1).
Disadvantages
Isolation or direct electrical connection
If the ratio of transformation is far from unity, theeconomic advantages of auto transformer over twowinding transformer decreases.
EN 20612/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Three Phase Transformer
Three phase supply for rectifiers, less ripple occurs in therectified output
Simplicity, low cost, reliability, efficiency and safety
Three phase to two phase is mutually convertible.
Six and twelve phase circuits are obtained from Z circuits.
Most commonly used circuits ∆ or Y . Others include interconnected Y arrangement, Open Delta (V connection), Teeconnections, Scott-Connection.
EN 20613/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Delta - Delta connection
The transformation ratio of all the transformers must bethe same, otherwise circulating currents round the windingwill be present even under no load condition
The impedance of all the units should be the same toensure equal load sharing
Secondary voltages are in phase with primary
EN 20614/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Delta - Delta connection
The voltages does not get unbalanced due to unbalancingin the loads.
Cross section of the wire is reduced as Iph = IL√3
If one of the three transformers gets burnt, the delta-deltasystem continues functioning uninterrupted.
EN 20615/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Star - Star connection
Line voltage =√
3× phase voltage and line current =phase current
No phase shift between primary and secondary voltages
With an unbalanced load on the secondary winding, thephase voltage on the load side changes unless the neutralpoint is earthed.
The neutral is also unstable because of third harmoniccomponent in the exciting currents, and also due tounequal magnetizing currents for the three units.
EN 20616/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Star - Delta connection
The coil in star winding will carry full current but voltageacross them will be 1√
3of line voltage
Delta coils, carry only 1√3
of the line current but whole of
line voltage
Generally used at substation end of the transmission linewhere the incoming voltage is to be stepped down forconsumption purposes.
30o phase shift is observed between corresponding primaryand secondary terminal voltages.
Cannot be paralleled with ∆−∆ and Y-Y.
EN 20617/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Delta - Star connection
Generally used for step-up the voltages
Most generally employed (both single phase and threephase circuits)
Low voltage distribution systems.
300 phase shift is observed between corresponding primaryand secondary terminal voltages.
Cannot be paralleled with ∆−∆ and Y-Y.
EN 20618/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Open Delta and Scott Connection
Open Delta:
It is extensively used in distribution systems
Initial stages of rural electrification
The two transformers will carry an overload of 15.5% as awhole which is acceptable.
Scott Connection:
A poly phase system may be transformed to anotherpolyphase system.
General application of Scott connection involves 3∅ to 2∅and 2∅ to 3∅ transformation.
EN 20619/19
Prof. Doolla
Introduction
VotlageRegulation
Efficiency
AutoTransformer
Three PhaseTransformer
Summary
TransformersVoltage regulation and efficiency of a transformer. All dayefficiency of a distribution transformer.Auto Transformer (principle of operation, equivalentcircuit, advantages and disadvantages)Three Phase Transformer
Next ClassAC/DC Converter
Thank you!!
For Further Reading:
Transformer Engineering: Design and Practice Authors:S.V. Kulkarni and S.A. Khaparde Publisher: MarcelDekker (Taylor & Francis Group), New York, May 2004ISBN: 0-8247-5653-3
Electric Machinery: A. E. Fitzgerald, C. Kingsley, S. D.Umans. Publisher: TMH, New Delhi, India, 2009
