A Transformer May Look Like This

Transformer Basics

A Transformer May Look Like This

Any transformer consists of the following three basic parts in it. Primary coil Secondary coil Transformer (Magnetic) core

1. Primary coil The primary coil is the coil to which the source is connected. It may be the high voltage side or low voltage side of the transformer. An alternating flux is produced in the primary coil. 2. Secondary coil The output is taken from the secondary coil. The alternating flux produced in the primary coil passes through the core and links with there coil and hence emf is induced in this coil. 3. Magnetic core The flux produced in the primary passes through this magnetic core. It is made up of laminated soft iron core. It provides support to the coil and also provides a low reluctance path for the flux.

Construction of Power and Distribution Transformers Construction Core Type High voltage Shell Type Less leakage flux Type of Cooling Ventilated Dry-Type Transformers They are cooled by natural air convection. Gas-Filled Dry-Type Transformers Cooled with nitrogen or other gases Liquid-Immersed Transformers Hermetically sealed tanks with insulated liquid (mineral oil, silicone oil)

Transformer Types In a step-down transformer, the secondary voltage is less than the primary voltage and Nsec < Npri In a step-up transformer, the secondary voltage is greater than the primary voltage and Nsec > Npri Step Up Transformer In an isolation transformer, the secondary voltage is equal to the primary voltage and Nsec = Npri https://www.youtube.com/watch?v=gmepe7nzcxw

Faraday s Law Principle of Transformer Action d e N e N dt 1 1 2 2 d dt The same flux (mutual flux) exists in both coils. Flux is generated by i 1 (right-hand rule). The induced emf e 1 and e 2 are generated to oppose the buildup of flux in its window. i 2 is generated by the induced emf e 2. In DC, the induced emfs are transients, in steady state: AC Assuming: Core permeability constant No leakage flux p p max s s d 0 e 1 e 2 0 dt E 4.44N f E 4.44N f E E p s N N p s max

Transformer V P V S N P N S - is the Primary Voltage - is the Secondary Voltage - is the Number of Primary Windings - is the Number of Secondary Windings Φ (phi) - is the Flux Linkage

Example A ideal transformer having 90 turns on the primary and 2250 turns on the secondary is connected to a 120 V, 60Hz source. What is the effective voltage across the secondary terminals? Solution: N 2 N1 = E 2 E1 2250 90 = E 2 120 25 = E 2 120 E 2 = 25 120 = 3000 V

Example An ideal transformer having 90 turns on the primary and 2250 turns on the secondary is connected to a 200 V, 50 Hz source. The load across the secondary draws a current of 2 A. What is the effective value of the primary current? Solution: N 2 N1 = I 1 I2 2250 90 = I 1 2 25 = I 1 2 I 1 = 25 2 = 50 A

Single Phase Transformer information: HV 115 kv LV 7200 V Capacity 16,670 KVA Primary amps 16,670,000 115,000 = 145 amps primary Secondary amps 16,670,000 7,200 = 2315 amps secondary Turns Ratio 115,000 7200 = 15.97/1 Current ratio Check 2315 145 = 15.97/1 Calculating single phase transformer MVA Voltage X Current 115,000 X 145 amps = 16,675,000= 16.67MVA 7,200 X 2315 amps = 16,668,000 = 16.67 MVA

Transformer efficiency The efficiency of a transformer is the ratio of power delivered to the load (P out ) to the power delivered to the primary (P in ). That is ƞ = P out P in 100% P out = V S 2 R = 152 100 = 2.25 P in = V p I = 120 20 10 3 = 2.4 ƞ = P out P in 100% = 2.25 2.4 100% = 93.75% 94%

In-Rush current When connecting an AC source to a transformer circuit the current called the transient component or in-rush current Although the in-rush component to a transformer decays rapidly, dropping to the normal no-load current within 5 to 10 cycles, it may exceed 25 times the full-load rating during the first halfcycle. This high in-rush must be taken into consideration when selecting fuses and/or circuit breakers. The magnitude of the in-rush depends on the magnitude and phase angle of the voltage wave at the instant the switch is closed.

Transformer Polarity Transform polarity refers to the relative phase relationship of transformer leads On power transformers the terminals are designated by the symbols H1, and H2 for the high-voltage (HV) winding and by X1, and X2 for the low-voltage (LV) winding. By convention, H1 and X1 have the same polarity. The transformer has either additive or subtractive polarity. A transformer is said to have additive polarity when terminal H1 is diagonally opposite terminal X1 A transformer has subtractive polarity when terminal H1 is adjacent to terminal X1

Three-Phase Systems Three-phase power is preferred over single-phase power for several important reasons: Three-phase motors, generators, and transformers are simpler, cheaper, and more efficient Three-phase transmission lines can deliver more power for a given weight and cost The voltage regulation of 3-phase transmission lines is inherently better

Three Phase Transformers A three-phase transformer can be built by constructing a threephase transformer on a common magnetic structure or can be built by suitably connecting a bank of similar three singlephase transformers Three-phase transformers use much less material than three single-phase transformers for the same three-phase power and voltage ratings. Three-phase transformers have all three phases wound on a single magnetic core or in shell-type and core-type construction. The principal disadvantage of a three-phase transformer, compared with its three-transformer counterpart, is that failure of one phase puts the entire transformer out of service

Balanced Three-Phase System Three-phase transformers are required to step up or step down voltages in the various stages of power transmission. The primary and secondary windings may be connected in either wye (Y) or delta (Δ) configurations. There are therefore four possible connections for a three-phase transformer:» Y -Δ» Δ -Y» Δ - Δ» Y-Y

Three Phase connections of three Single Phase Transformers

Connections Wye connected windings: Have a common connection point Have two voltages available L-L & L-N Has only one current : Line current

Connections Delta connected windings: Have no common connection point Have only one voltage available L-L Delta can be closed connected in more than one way Has two currents Line current Phase or winding current

Three-phase Voltage and Current Connection Phase Voltage Line Voltage Phase Current Line Current Star V P = V L 3 V L = 3 V P I P = I L I L = I P Delta V P = V L V L = V P I P = I L 3 I L = 3 I P P = 3I L V L or P = 3I PH V PH turn ratio = V PH primary/v PH secondary

HS Line 230 kv LS Line 23 kv Transformer 3 Ø 230/13280/23000 Gnd. Wye 25 MVA 230 kv A B C H1 H2 H3 W1 W2 W3 W4 W5 W6 N X1 X2 X3 23 kv a b c

HS Line 115 kv LS Line 12 kv Transformer: 3 Ø 115/12000/6928 Gnd. Wye 40MVA Voltage on HS 115 kv. Only one voltage because it is delta Voltage on LS 12,000 & 6,928 because it is wye connected A B C H1 H2 H3 16.6/1 W1 W2 W3 Transformer Ratio 16.6/1 N W4 W5 W6 X1 X2 X3 a b c

Transformer Solutions Information: Δ-Υ 230/23kV 25 MVA transformer What is the voltage on High voltage line? What is the voltage on low voltage line? What is the voltage on low voltage phase? What is the voltage across W1, W2, W3? 230 kv (Given) 23 kv (Given) 13,280 kv ( V L L 3) 230 kv (Same as the line for Δ winding) What is the voltage across W4, W5, W6? What is the current in line A, B, C? What is the current in W1, W2, W3? What is the current in W4, W5, W6? 13,280 kv ( L-N voltage) 25 MVA 230,000 3 = 62.75 25 MVA 230,000 3 3 = 36.23 25 MVA / 23 kv / 3 = 627.56 amps

Transformer Solutions What is the current in line a, b, c? Same as winding W4, W5, W6= 627.556 amps On the delta side of the transformer The line current 3 = Winding Current ; 62.76 3 = 36.23 Calculating three phase transformer MVA Voltage X Current X three windings 230,000 X 36.3 amps = 8,349,000 x 3 (windings)= 25,047,000 = 25 MVA 13,280 X 627.57 amps = 8,334,129 X 3 (windings)= 25,002, 388 = 25MVA

Transformer Ratio 3 Phase 230/23 kv 25 MVA Transformer is 230/ 23 kv Ratio is NOT this ratio but is the ratio of the voltages across the windings. W1 230 kv, W4 13,280 Ratio is 230,000 / 13,280 = 17.32/1 A B C H1 H2 H3 W 1 W2 W3 Note: W4 current/ W1 current = 627.56/36.3 = 17.3 627.56 amps X 13,280 volts = 8.3 MVA 36.3 amps X 230,000 volts = 8.3 MVA 8.3 MVA X 3 (windings) =25 MVA N W4 W5 W6 X1 X2 X3 a b c

How Are They Connected? H H H H primary primary H primary H H secondary H H secondary H H secondary H

A B C H1 H2 H3 W1 W2 W3 W4 W5 W6 N X1 X2 X3 a b c

Both transformers are Δ-Υ but notice the Δ windings are not closed the same A B C H1 H2 H3 W 1 W 2 W 3 N W 4 W 5 W 6 X1 X2 X3 A B C H1 H2 H3 a b c W 1 W 2 W 3 N W 4 W 5 W 6 X1 X2 X3 a b c

Transformer Information For Practice Calculations 3Ø, Δ-Υ 115/12 kv 25MVA 3Ø, Δ-Υ 115/23 kv 40 MVA 3Ø, Δ- Δ 23/ 13.8 kv 5 MVA 3Ø Υ- Υ 4160/ 2400 5 MVA 3Ø, Δ- Δ 115/ 13.8 kv 10 MVA 1Ø Υ- Δ 230/22 kv 350 MVA Delta and Wye 3-phase circuits : Electronics Worksheet