Department of Electrical Engineering Lecture Basic Operating Principles of Transformers
In this Lecture Basic operating principles of following transformers are introduced Single-phase Transformers Three-phase transformers Auto-transformers Instrument transformers
Introduction Transformer is a machine that has no moving parts but is able to transform alternating voltages and currents from high to low (step-up transformer ) and vice versa (step-down transformer). Transformers are used extensively in all branches of electrical engineering from the large power transformer employed in the T&D network to the small transformer of an electronic amplifier. 3
45VA -Ø Transformer 4
Single-phase transformer A simple single-phase transformer consists of two coils wound on a closed iron core as shown: Primary winding Secondary winding 5
Construction of Single-phase transformer Basically, a transformer has two windings: primary winding Secondary winding Each winding consists of many turns and are wound on a laminated iron core. The iron core is insulated with the windings. The core itself forms a closed iron magnetic circuit. Consequently, the windings encircle the core and the core encircles the windings. 6
Core type windings are wound around two legs of a magnetic core. Shell type windings are wound around the center leg of a threelegged magnetic core 7
Transformer action - Ideal Transformer Assumptions:. The winding resistances are negligible (R = 0). All fluxes are confined to the core and link both windings (leakage flux = 0) 3. Permeability of the core is infinite (I φ = 0) 4. o eddy current and hysteresis losses 8
Transformer action - Ideal Transformer (cont) When an A.C. voltage v applied to the primary winding: dφ v = e = () dt The core flux also links the secondary winding, then: dφ v = e = () dt From equations () and (): v v = = a 9
Transformer action - Ideal Transformer (cont) when a load is connected to the secondary winding, i will flow and will provide an mmf i for the core. i would immediately flow to establish another mmf i to oppose i since no mmf is required to establish a flux in the ideal core i i = 0 i = i Then: i i = = a 0
Transformer action - Ideal Transformer (cont) If the supply voltage is sinusoidal, then in rms values: Since V V = = a and I I = = a V = I V I input VA = output VA
Transformer action - Ideal Transformer (cont) Emf Equation of a Transformer (/) Φ = Φ m sinπft instantaneous value of induced emf/turn = dφ/dt volts = πfφ m cosπft volts = πfφ m sin(πft π/) volts
Transformer action - Ideal Transformer (cont) Emf Equation of a Transformer (/) rms value of induced emf / turn = 0.707 x πfφ m volts = 4.44 x fφ m volts Hence E = 4.44 fφ m volts E = 4.44 fφ m volts E = E 3
Phasor Diagram of Ideal Transformer action under o-load EEE344 Electrical Machines V = E I m Φ E E =( / ) x E 4
Example A 00kVA, 6600/400V, 50 Hz single-phase transformer has 80 turns on the secondary. Calculate: a) the approximate values of the primary and secondary currents; b) the approximate number of primary turns; and c) the maximum value of the flux. Ans: 30.3A, 500A; 30; 0.05Wb 5
Features of Practical Transformer the windings have resistances not all windings link the same flux permeability of the core material is not infinite, and core losses occur when the core material is subjected to time-varying flux Losses in Practical Transformer Copper losses ( I R losses ) Iron losses ( Core losses ) 6
Transformer Losses Copper losses ( I R losses ) Power losses due to the resistances in the primary and secondary windings P C = I R + I R Iron losses ( Core losses ) a. Hysteresis loss Power loss due to the alignment of magnetic dipoles in the iron core b. Eddy current loss Power loss due to the induced eddy current circulating in the iron core 7
Parameters of Transformer In order to analyze transformer at different loading conditions, based on the principles of transformer action, equivalent circuit of practical transformer is worked out for calculation. R eq X eq R C X m Equivalent Circuit of Transformer The four essential parameters of the transformer equivalent circuit are then measured by Open-circuited test, and Short-circuited test on transformer 8
Three-Phase Transformer A set of three similar single phase transformers may be connected to form a three-phase transformer (three-phase transformer bank). The primary and secondary windings may be connected in either star or delta configurations. 9
Characteristics of Three-phase Transformer Bank Ease of transportation Inefficient magnetic circuit, less efficient Higher capital cost than a single one -phase of the transformer at fault, the other two are not affected 0
Construction of Three-phase Transformer Usually 3-limb core structure 5-limb core may be used to reduce the overall height of a 3-limb core Magnetic flux shares the magnetic circuit Fault on one-phase very likely affects the other two phases
Three-phase Transformer 3 limb core structure 3- phase transformer
Three-phaseTransformers in T&D System 5kV Alternator Customers 3
3-Ø kv/380v Tx LV Side HV Side 4
Pole-mounted 3-Ø kv/380v Tx kv/380v 3-phase transformer fixed at a wooden pole 5
Turn ratio/voltage ratio/current ratio of 3-phase transformer (/4). STAR/delta ( Yd ) EEE344 Electrical Machines V V = 3.a = 3. ; I I = 3.a = 3. 6
Turn ratio/voltage ratio/current ratio of 3-phase transformer (/4). DELTA/star ( Dy ) EEE344 Electrical Machines V V = a 3 = 3. ; I I = 3 a = 3. 7
Turn ratio/voltage ratio/current ratio of 3-phase transformer (3/4) 3. DELTA/delta ( Dd ) EEE344 Electrical Machines V V = a = ; I I = a = 8
Turn ratio/voltage ratio/current ratio of 3-phase transformer (4/4) 4. STAR/star ( Yy ) EEE344 Electrical Machines V V = a = ; I I = a = 9
Auto-transformer transformer having a part of its windings common to the primary and secondary when a load is connected across b and c, then a current I will flow through the load. The current I will produce an m.m.f. in the core which will be balanced by a current I flowing in the complete winding 30
Auto-transformer The voltages and currents are related by the same turns ratio as in a two-winding transformer: V V = = a I I = = a 3
Single-phase Auto-transformer 3
Three-phase Auto-transformer 33
Example EEE344 Electrical Machines A φ, 00 kva, 000/00 V two-winding transformer is connected as an autotransformer as shown such that more than 000 V is obtained at the secondary. The portion ab is the 00 V winding, and the portion bc is the 000 V winding. Compute the kva rating as an autotransformer. 34
Example (cont) The current rating of the winding are: 00,000 I ab = = 500A 00 00,000 I bc = = 50A,000 Therefore, for full-load operation of the autotransformer, the terminal currents are: I H = 500A I L = 500 + 50 = 550A 35
Example (cont) ow, V L = 000V and V H = 00V Therefore, 000 550 kva L = = 00 000 00 500 kva H = = 00 000 (ans) (ans) ote: A φ, 00 kva, two-winding transformer when connected as an autotransformer can deliver 00 kva. 36
Advantages of Auto-transformer It effects a saving in winding material (copper or aluminum), since the secondary winding is part of the primary current. Lower copper loss, therefore efficiency is higher than in the two winding transformer. Lower leakage reactances, lower exciting current. Variable output voltage can be obtained. 37
Disadvantage of Auto-Transformer There is a direct connection between the primary and secondary sides. Should an open-circuit develop between points b and c, the full mains voltage would be applied to the secondary. The short-circuit current is much larger than for normal two-winding transformer 38
Application of Auto-transformer Boosting or bucking of a supply voltage by a small amount. (The smaller difference voltage between the output and input voltages the greater is the saving of winding material.) Starting of a.c. machines, e.g. induction motor, where the voltage is raised in two or more steps from a small value to the full supply voltage. Continuously variable a.c. supply voltages, normally connected between a low voltage supply in and a high voltage supply out. Production of very high voltages, e.g. 75kV and 400kV grid system 39
Instrument Transformers Instrument transformers are used to reduce the voltage or current magnitudes so that the measuring ranges of measuring instruments can be extended. Measuring Instruments are connected to the secondary of the transformers. The measured values should then be multiplied by the appropriate turns ratio to get the actual primary values.. Voltage Transformer VT (or PT) / is large and standard 0 V at the secondary.. Current Transformer CT / is small and standard 5 A or A at the secondary. 40
Voltage Transformer (VT) Fundamentally similar in principle to power transformers but with rated outputs in VA rather than kva or MVA. Operating at full supply voltage, the secondary current is very small so that the transformers may be regarded in the same way as a power transformer on no load. Current Transformer (CT) Transforms large primary current value in terms of its magnitude and phase to a smaller secondary current value. the secondary value is directly proportional to the primary value. ominal CT ratio (60~3000A/5A, 30~000A/A) 4
Protection CT ominal CT ratio (000/5, 00/5) Required to operate at many times full load current. Linearity under these conditions is not of great importance. The essential point is that saturation must be high enough to drive the magnetizing current and the secondary current under fault condition. P Primary Insulatio n S Iron area S P Ring type CT terminal Markings Ring type CT Secondary 4
Metering CT For non-protection purpose Metering CTs need perform very accurately but only over the normal range of load up to about 0% full load current. 43
Reasons for using Instrument Transformer Do not require special designed instruments for h.v. or heavy current measurements (the instrument ranges could be standardized) Electrical isolation from the primary (h.v. side) is achievable For safety reason, one terminal of the secondary winding can be earthed 44
About the Use of Instrument Transformer may have turns ratio error phase shift may exist between primary and secondary measuring quantities. CT is actually a step-up transformer with very large turns ratio (eg 500/), excessive voltage will exist between the secondary terminals if they were left open. EVER LET THE SECODARY OF A C.T. OPE-CIRCUITED 45
Department of Electrical Engineering ED of Lecture Basic Operating Principles of Transformers