Six months training AT

33/11 KV SUBSTATION

BySrajan bharti

E103052

the Transformer

"the heart of the alternating current system" - William Stanley Jr.

POWER TRANSFORMERS

1. Transformer Tank 2.High Voltage Bushing 3. Low Voltage Bushing 4.Cooling Fins/Radiator 5. Cooling Fans 6. Conservator Tank 7. System Ground Terminal 8. Drain Valve 9.Dehydrating breather10. Oil Temperature/Pressure gauges 11. Bushing Current Transformers 12. Control Panel 13. Surge Arresters 

PARTS OF THE TRANSFORMER

Main components of a transformer are: •Magnetic core •Primary and secondary windings •Insulation of windings •Expansion tank or conservator •Tank , oil , cooling arrangement , temperature gauge, oil gauge •Buchholz relay •Silica gel breather

(1) MAGNETIC CORE •Magnetic core consists of an iron core. The core is laminated and made of silicon steel. •Thickness varies from 0.35mm to 0.5mm. •Laminations are insulated from each other by coating then with a thin coat of varnish. •Various types of stampings and laminations employed in the construction of transformers.

There are two types of transformer cores ,they are (a) Shell type (b)Core type Shell type – Two windings are carried by central limb. Core is made up of E and I stampings and has three limbs. Has two parallel paths for magnetic flux. Core type- Has two limbs for two windings and is made up of two L-type stampings. Has only one magnetic path.

(2) Winding •There are two windings in a transformer. •They are primary and secondary windings. •Made up of copper.

(3) Insulation •Paper is still used as basic conductor insulation. •For power transformers enamelled copper with paper insulation is also used.

(4) Insulating oil •The coil used in transformer protects the paper from dirt and moisture and removes the heat

produced in the core and coils, •It also acts as insulating medium.

Oil must possess following properties: •High dielectric strength •Free from inorganic acid , alkali and corrosive sulphur. •Low viscosity to provide good heat transfer. • Good resistance to emulsion so that the oil may throw down any moisture entering the tank

instead of holding it.

(5)EXPANSION TANK or Conservator •A small auxiliary oil tank mounted above the

transformer and connected to main tank by a pipe. •Its function is to keep transformer tank full of oil.

(6)TEMPERATURE GAUGE •Is to indicate hot oil or hottest spot temperature. •It is self contained weather proof unit made of alarm contacts.

(7) OIL GAUGE •Every transformer is fitted with an oil gauge to indicate the oil level

present inside the tank. •It is provided with an alarm contact which gives an alarm when the

oil level drops beyond permissible height due to oil leak or due to any other reason.

(8) BUCHHOLZ RELAY •First warning that fault is present is given by presence of bubbles in oil. •It gives an alarm in case of minor fault and to disconnect transformer

From the supply mains in case of severe faults.

(9) Breather •It is a chamber which prevents entry of moisture

inside the transformer tank. •It is filled with drying agent such as calcium chloride

or silica gel. This absorbs moisture and allows dry air to enter transformer tank. It is replaced regularly.

CLASSIFICATION OF TRANSFORMERS Transformers are classified on basis of 1.Duty they perform 2.Construction 3.Voltage output 4.Application 5.Cooling 6.Input supply Duty they perform I.Power transformer – from transmission and distribution II.Current transformer- instrument transformers III.Potential transformer- instrument transformers

Construction I.Core type transformer II.Shell type transformer III.Berry type transformer

Voltage output I.Step down transformer(Higher to Lower) II.Step up transformer(Lower to Higher) III.Auto transformer(Variable from ‘0’ to rated value)

Application I.Welding transformer II.Furnace transformer Cooling I.Duct type transformer II.Oil immersed transformer self cooled Forced air cooled Water cooled Forced oil cooled Input supply I.Single phase transformer II.Three phase transformer

star- star Delta-Star Star-delta Open- Delta Delta-delta Scott connection

EMF EQUATION OF TRANSFORMER Wkt, T=1/f Average emf (e)= dΦ/dt dΦ = Φm dt = 1/4f Average rate of change of flux= Φm / (1/4f) = 4f Φm volts Average emf induced per turn = Average rate of change of flux= 4f Φm volts Form factor = RMS value/ Average value = 1.11 RMS value= Form factor x Average value =1.11 x Average value RMS value of emf induced/turn = 1.11 x 4f Φm = 4.44 f Φm volts Primary and Secondary winding having N1 and N2 turns. RMS value of emf induced Primary winding, E1 = 4.44 f Φm N1 volts RMS value of emf induced Secondary winding, E1 = 4.44 f Φm N2 volts

TRANSFORMER RATIO For an ideal transformer, E1 = V1 and E2 = V2 There is no voltage drop in the windings. V1 I1 = V2 I2 V2 / V1 = E2 / E1 = I1 / I2 = N2 / N1 = K V2 / V1 = K -> Voltage ratio E2 / E1 = K -> Transformation ratio N2 / N1 = K -> Turns ratio I1 / I2 = K -> Current ratio (i)If K > 1 , then the transformer is called step-up transformer. (ii) If K < 1 , then the transformer is called step-down transformer.

RATING OF TRANSFORMER •The rating of transformer is expressed by Volt- Ampere (VA) •Cu loss depends on Current (A) •Iron loss depends on Voltage (V) •Total loss depends upon Volt- Ampere (VA) •It is independent of Load power factor cos Φ.

Ideal transformer Ideal transformer has following properties : •No winding resistance i.e., purely inductive •No magnetic leakage flux •No cu loss •No core loss •Ideal transformer secondary is open. Ac supply is connected to primary winding. Current flows through primary winding. This current is called MAGNETISING CURRENT (Iμ). •Value of Magnetising current is small. The Magnetising current produces an alternating flux (Φ). •Iμ and Φ are in-phase. This changing flux links primary with secondary winding.

•Due to alternating flux a self-induced emf (E1) is produced in primary winding which is equal to and in opposition with V1. It is known as counter emf or back emf of primary winding. •Induced emf E2 is produced in secondary winding because of alternating flux linking with secondary winding. This emf is known as mutually induced emf.

PRACTICAL TRANSFORMER ON NO-LOAD If the primary winding is connected to alternating voltage and secondary winding is left open then transformer is said to be on NO-LOAD. Since secondary is open this current is called no-load primary current (Io). No load input power, P0= V1 I0 cos Φ0 Active or working or iron loss or wattful component (Iw) which is in-phase with ‘V1’ and supplies iron loss and small amount of primary cu loss. Iw = I0 cos Φ0 Where, cos Φ0 is no load power factor. Reactive or magnetizing or wattles component Iμ which is in quadrature with V1 and its function is to sustain flux in core. Iμ= I0 Sin Φ0

TRANSFORMER ON LOAD When the secondary winding is connected to load then the transformer is said to be on load. Phase angle between V2 and I2 depends on type of load. Resistive = I2 in-phase with V2 Load Inductive = I2 will lag V2 capacitive= I2 will lead V2 When transformer is loaded, Flux is constant at no-load as well as at loaded condition, therefore transformer is called as constant flux apparatus. Total primary current will be vector sum of I0 and I2’

Transformer winding resistance In practical transformer the winding have some resistances. Primary winding has primary resistance(R2).

Transformer winding leakage reactance Primary leakage flux(ΦL1) – All the flux generated by the primary winding does not link with secondary winding. Some part of flux passes through air rather than around the core. This flux is in-phase with I1. Secondary leakage flux(ΦL2) – Leakage flux is set up in secondary winding. This flux induces eL2 in secondary winding. This flux ΦL2 does not link with primary is also in-phase with I2.

OPEN CIRCUIT / NO-LOAD TEST ON TRANSFORMER PURPOSE OF THIS TEST IS TO DETERMINE •Core loss or Iron loss Or Magnetic loss (Pi) •No load current (I0) •Shunt branch parameters R0 and X0

One of the winding is kept open. •Rated voltage at rated frequency is applied to other(LV) winding. •A voltmeter, wattmeter, and an ammeter are connected in LV side of the transformer. Ammeter > Reads No-Load Current, I0 Voltmeter > Reads Applied Voltage, V0 Wattmeter> Reads No-Load Input Power, W0 or P0

DETERMINATION OF EQUIVALENT CIRCUIT CONSTANTS THROUGH NO- LOAD TEST No load power factor, CosΦ0 = W0 / V0 I0 Core loss component, Iw = I0 CosΦ0 Magnetising component, Im = I0 SinΦ0 Core Loss, Pi = No load power (W0) Core loss resistance, R0 = V0 / Iw = V0 / I0 CosΦ0 Magnetising reactance, X0= V0 / Im = V0 / I0 SinΦ0

SHORT CIRCUIT / IMPEDANCE TEST ON TRANSFORMER PURPOSE OF THIS TEST IS TO DETERMINE •Z01 or Z02 – Total impedance referred to either primary or secondary side •R01 or R02- Total resistance referred to either primary or secondary side •X01 or X02- Total reactance referred to either primary or secondary side •Full load cu loss I22 R02

In this test one of the winding is short circuited by thick conductor. Current rating of HV side is low compared with LV side. Power input gives total cu loss at rated load. Unity power factor wattmeter is used for measuring power in SC test.

DETERMINATION OF EQUIVALENT CIRCUIT CONSTANTS THROUGH LOAD TEST SC power factor, CosΦsc = Wsc / Vsc Isc Resistance of transformer referred to primary side , R01 = Wsc / (Isc)2 Impedance of transformer referred to primary side, Z01 = Z01 Cos Φsc= Vsc / Isc