List of Participants · Web view6.0 TAP CHANGER It is essential to use tap changers so as to vary...

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TRANSFORMERS by Er. Y.S. JADAUN 1.0 PRINCIPAL Laminated core Primary winding Secondary winding If V 1 = V im sin2ft then = Hence e 1 = N 1 m 2f cos2ft Maximum value (r.m.s) Similarly, E 2 = 4.44 m fN 2 = 4.44 B m AfN 2 1.1 IDEAL TRANSFORMER where; N 1 & N 2 = No. of turns in Prim. Sec. m = Max. flux in core, weber B m = Flux density in web/sqm A = net cross section area of

Transcript of List of Participants · Web view6.0 TAP CHANGER It is essential to use tap changers so as to vary...

Page 1: List of Participants · Web view6.0 TAP CHANGER It is essential to use tap changers so as to vary turns ratio to maintain the system voltage within prescribed limits. Tap changers

TRANSFORMERS

byEr. Y.S. JADAUN

1.0 PRINCIPAL

Laminated core Primary winding Secondary winding

If V1 = Vimsin2ft then = msin2ftHence e1 = N1 m 2f cos2ftMaximum value (r.m.s)

Similarly, E2 = 4.44m fN2 = 4.44 Bm AfN2

1.1 IDEAL TRANSFORMER

Actual Transformer:(i) iron loss (hysteresis loss) and eddy current losses.(ii) copper loss.

(i) Iw = Iron loss and small copper loss(ii) I = wattles component(iii) I = I0 sin 0 I0 small Hence no load input

Iron loss

where;N1 & N2 = No. of turns in Prim. Sec.m = Max. flux in core, weberBm = Flux density in web/sqmA = net cross section area of core (Sq. Tesla)F = Frequency

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I2’ = I2 , 2 = 2, but in opposite direction.I2 = load component of primary current.

Hence, magnetic losses are same.N1I2 = N1I1 = N2I2

Or,

(a) At No Load (b) On Load

1.2 Transformer Having Winding Resistance But No Magnetic Leakage

V2 = E2 – I2R2 , E1 = V1 – I1R1

1.3 Magnetic Linkage

Ideal case: all the flux linked with primary also linked with secondary winding. Practically: not possible, hence self excitation. Leakage could be avoided if primary and secondary occupy the some space.

Physically not possible. Minimization of leakage may be done by placing windings concentrically.

1.3.1 Regulation

If primary voltage V1 constant when transformer loaded, V2 drops (log PF)

where: V2, = Secondary voltage at no loadV2 = secondary voltage at full load

% Regulation at any P.F. = (R Cos  + X Sin)

where, R = percentage resistive drop

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X = percentage reactive dropCos = P.F. log (Note: for leading P.F., will change to -)

1.4 Transformer Losses

1.4.1 No Load Losses:

Hysteresis loss, Wh = Kh f Bm2 watts. Eddy current losses, We = Ke Kf Bm watts

Where:Kh = Hysteresis constantKe = Eddy current constantKf = Form factor

To reduce losses: Use Core steel with high silicon constant. Thin lamination.

Core losses = No load input power of a transformer

1.4.2 Load Losses

Mainly due to ohmic resistance of transformer and include stray losses (due to stray flux in mechanical structure and winding conductor).

Measured by short circuit test.

1.5 EFFICIENCY

Condition of maximum efficiency: Occurs when Iron loss = copper loss Load corresponding of maximum efficiency

2.0 TWO WINDING TRANSFORMER

Total weight is proportional to N1I1 + N2I2

Working: as above.

3.0 AUTO TRANSFORMER

Single continuous winding. Secondary side voltage is obtained by tapping the winding. Used where transformer ratio differs slightly from unity. Part winding carries current I1 and remaining portion I2 – I1 where I2 > I1.

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Total weight of copper is proportional to {(N1 – N2) I1 + N2 (I2 – I1)}. Weight of copper compared to that in two winding Transformer.

i.e. Weight of copper in Auto-transformer = (1 – K) . W0

where W0 = weight of copper in two winding transformer.Therefore, Saving = W0 – Wauto

= W0 – W0 (1 – K) = KW0

Higher the value of K, higher is the saving.

4.0 THREE WINDING TRANSFORMER

Two main windings and a tertiary winding.

Used for: Supply of load which is not to be connected to secondary winding for

some reason. In star-star transformers, for allowing sufficient zero sequence current for

protection; to suppress harmonic voltages, to limit unbalanced current; to limit voltage unbalance when main load asymmetrical winding is delta connected.

To interconnect 3 supply systems at different voltages (tertiary generally delta winding).

As voltage coil in testing transformer. Delta tertiary used in star-star transformer to limit disadvantages (when

loads unbalanced, third harmonic distortion) by circulating induced currents in delta winding.

Rating of tertiary: If for additional load, as per load and 3 phase dead short at terminals with

power from other 2 windings. If for stabilization as per thermal/mechanical stresses for short duration fault

currents. (1 Ph, line-ground fault most harmful).

5.0 PARALLEL OPERATION

For Satisfactory Operation: Same phase sequence and zero relative displacement (essential) Same polarity (essential)

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Same voltage ratio (to close degree) Same p.u. impedance (desired for better load division).

Phase Sequence: Phase sequence decides the order in which phases reach their maximum

positive and negative voltages. If not identical, each pair of phases will be short circuited in each cycle.

Following transformers may be paralleledTransformer 1: Yy Yd Yd

Transformer 2: Dd Dy Yz

Polarity: wrong polarity will mean dead short circuit, n).

Voltage Ratio: If not same, circulating currents in secondaries and therefore in primaries also. I2R losses, undesirable loading condition occur.

Impedance Currents proportional to ratings, if impedance inversely proportional to ratings

and per unit impedance identical. If difference in quality factor (x/R ratio) of per unit impedance, divergence of

phase angle to two currents; hence power factor will be different.

6.0 TAP CHANGER

It is essential to use tap changers so as to vary turns ratio to maintain the system voltage within prescribed limits. Tap changers are of types, viz.

Off Load tap Changer On Circuit Type

6.1 Off Load Tap Changer

Comprises of three parts

Operating handle projecting outside the transformer. Fixed contact with connecting terminal. Moving contact system with insulating shaft.

Winding Circuit Arrangements

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Double Compartment Type For large transformers.

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Separate tap selector and divertor switches.

a. M1 opens.b. A1 opens: interruption of circulating current.c. A2 closes: circulating current through R1 and R2.d. M2 closes.e. Complete operation in 40 to 80 ms.f. For tap changing in opposite direction: sequence is reversed.

Tapping on neutral end of High Voltage winding. Only on 3 pole tap changer required per transformer (3 ).

Large regulating transformer 3 single phase type required, coupled together, driven by 1 shaft.

OLTC with delta connected windings

A mechanical lock provided: prevent unauthorized operation. Electromagnetic latching device/micro switch for interlocking C.B.

operation.

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6.2 On Load Tap Changer

Changing turn ratio while load is being delivered. Operating efficiency improved. These possess an impedance: prevents short circuiting during changing operation.

Classified as: Reactor Transition Type:

a. Center tapped reactor.b. Large number of taps.c. Shorter contact life due to long arching time.d. Used in USA but not other countries in general.

Resistor Transition Type:a. Longer life of contact due to shorter arcing time (unity P.F.).b. Resistance on transition tap.

Single Compartment Type

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6.3 Mannual and Electrical Operation

6.4 Automatic Control

With voltage relay. Time delay to prevent hunting during transients. Line drop compensation arrangements provided.

Voltage at some distance point can be made constant irrespective of load.

6.5 Tap Changer Selection

Voltage class of transformer winding and its rating. % voltage variation required. Maximum through current.

7.0 TYPES OF TRANSFORMERS

7.1 Power Transformersi. Two windingii. Three windingiii. Autoiv. Step upv. Step down

7.2 Instrument Transformersi. Potential Transformer (P.T.) – also called voltage Transformer (V.T.).ii. Current Transformer (C.T.).

8.0 CONSTRUCTION

8.1 CoreCore diameter is adjusted to meet the guaranteed parameters and it depends on:

i. Rating.ii. % impedance between windings.

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iii. Basic insulation level (BIL).iv. Transport height.v. Over-fluxing requirement.vi. Type of core and quality of steel.

8.2 Windingsi. Spiral: Medium current, low voltage (Tertiary winding of star/star/delta is

generally of this type).ii. Helical: High current, low voltage (generally for L.V. coils of large generator

transformer).iii. Reversed section (disk winding): low to medium current, high voltage (usually

up to 132 kV class windings and not for higher voltages due to impulse distribution characteristic).

iv. Parallel layer: for star connected transformers having graded insulation & for voltages above 132 kV.

v. Tapered layer: use a number of concentric spiral coils arranged in layers.vi. Interleaved discs: for improved behaviour against impulse voltage.

8.3 Insulationi. Minor: Generally paper insulation between different parts of one winding e.g.

between turns, layers etc.ii. Major: Generally press board cylinders separated by oil ducts. Insulation of

windings to earth & transformer to core, other windings of same phase (H.V. to L.V.) and phase to phase.

8.4 Tap Changeri. Off – Circuit.ii. On – Load.

8.5 Tank

8.6 Bushings:i. Porcelain (concentration of electric stress, up to 36 kV – other bulky).ii. Condenser (insulation wall thickness divided in to a number of capacitors by

concentric cylinders, outside porcelain).

8.7 Insulating oil

8.8 Cooling System and Arrangementi. Cooling system

a. ONAN (Oil natural, Air natural).b. ONAF (Oil natural, Air forced).c. ONAF (OFWF (Oil forced, water forced).d. ODAF (Oil directed, Air forced).e. ODWF (Oil directed, Water forced).

ii. Cooling Arrangementa. With radiators.b. Use of coolers.

8.9 Conservator

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8.10 Buchholz relay8.11 Temperature indicator8.12 Pressure relief valve8.13 Oil level indicator8.14 Cable sealing box

9.0 RATING

i. Type of transformer.ii. Number of phases.iii. Frequency.iv. Rated power (kVA or VA & for taping ranges exceeding + 5%, required

power on extreme tapings).v. Rated voltage for each winding.vi. Connection symbol.vii. Requirement of on-load/off-load tap changers or links – number of tapings,

taping range, location of tapings, particular voltage required to be varied, whether constant flux, variable flux/combined voltage variation.

viii. Impedance voltage at rated current and principal taping for different pairs of windings and at least on extreme tapings in case of parallel operation, if required.

ix. In door or out door type.x. Type of cooling.xi. Temperature rises and ambient temperature conditions including altitude and

in case of water cooling, chemical analysis of water.xii. No. of cooling banks, spare capacity if any, with no. of stand by cooling

fans/pumps.xiii. Highest system voltage for each winding.xiv. Method of system earthing for each winding.xv. Whether windings shall have uniform or non-uniform insulation, if non-

uniform – then power frequency with stand voltage of neutral and impulse with stand level if an impulse test on neutral is required.

xvi. For windings having system highest voltage greater 300 kV, the method of dielectric testing.

xvii. With stand voltage values constituting insulation of line terminals.xviii. Limitation of transportation weight, moving dimensions and special

requirement, if any, of installation, assembly and handling.xix. Whether stabilizing winding is required.xx. Over fluxing conditions/any other exceptional service conditions.xxi. Loading combinations in case of multi-winding transformer and when

necessary, active and reactive outputs separately, especially in case of multi-winding autotransformer.

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