Transformer is one of the most important equipments in a power transmission and distribution system. It does stepping up or stepping down the voltage and transfer power from one A.C. voltage to another A.C. Voltage at the same frequency.
Transformer has Primary & Secondary windings housed in main tank filled with insulated oil. Oil is used for providing insulation as well as cooling of windings.
1) The capacity of Transformer is expressed in Volt-ampere (KVA / MVA) 2) The transformation ratio K (constant) = Vs/Vp = Ns/Np
Where Vp, Np denote primary voltage & turns respectively. And Vs, Ns denote secondary voltage & turns respectively.
If K > 1, then transformer is called step-up transformer If K < 1, then transformer is called step-down transformer For an ideal transformer, Input VA = Output VA
i.e. Vp x Ip = Vs x Is or Is/Ip = Vp/Vs = 1/K (where Ip & Is are Primary and secondary current respectively). Hence currents are in the inverse ratio of the (voltage) transformation ratio.
To calculate current of Primary & Secondary winding of 132 / 33 KV, 50 MVA Transformer:-
a) Primary Current in amp = Ip = VA / √3 x Vp, where Vp & Ip are primary voltage and current respectively.
Hence Ip = (50 x 10*6) / (√3 x 132 x 10*3) = 218.69 Amp
b) Secondary Current = Is = VA / √3 x Vs, where Vs & Is are secondary voltage and current respectively.
Hence Is = (50 x 10*6) / (√3 x 33 x 10*3) = 874.77 Amp
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General view of Power Transformer :-
Main fixtures of Power Transformer and their functions are listed below: - a) Buchholz Relay - This relay is designed to detect transformer internal fault in the initial stage to avoid major breakdown. Internal fault in transformer generates gases by decomposition of oil due to heat & spark inside the tank. These gases pass upward towards the conservator tank, trapped in the housing of the relay, thereby causing the oil level to fall. The upper float rotates & switches contacts close & thus giving alarm.
In case of a serious fault, gas generation is more, which causes operation of lower float & trips the circuit breaker. The gas can be collected from a small valve at the top of relay for Dissolved Gas Analysis (DGA).
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Checking the floats operation manually: -
a) Close the both valves. (From Transformer and main conservator side) b) Drain oil from the buchholz relay.
c) Top float makes contact as the oil gets lowered and gives Alarm.
d) If oil is further drained, bottom float makes contact and gives trip signal.
After testing, both valves must be opened without fail and released the air from relay. Alarm & Trip circuit can also checked by shorting contacts externally by link.
b) Oil Surge Relay - It is similar to Buchholz relay with some changes. It has only one float & operates when oil surges reach and strike the float of OSR. It is used with OLTC for detection of any damage or fault inside the tap changer and prevents tap changer from damages in case of low oil level in OLTC tank.
Checking the float operation manually: -
a) This relay can be checked by pressing test switch provided on top side. Here only one contact is provided which gives trip signal on operation of float. By shorting contact externally by link, trip circuit can also be checked.
c) Explosion Vent - It consists of a bent pipe with bakelite diaphragm at both ends. A protective wire mesh is fitted on the opening of transformer to prevent the pieces of ruptured diaphragm from entering the tank. The wire mesh is also provided at the upper end to protect upper diaphragm from any mechanical damages. At the lower end, there is a small oil level indicator. When the lower diaphragm ruptures due to excess internal pressure, the oil level rises in the vent pipe & is visible through the indicator. In case the internal pressure developed is not reduced to safe value after the bursting of lower diaphragm, upper diaphragm gives away throwing the gas and oil outside and prevents further mechanical damages.
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d) Pressure Relief Valve - When the pressure in the tank rises above pre-determined safe limit, this valve operates & performs the following functions: - 1) Allows the pressure to drop by instantaneously opening the port.
2) Gives visual indication of valve operation by raising a flag.
3) Operates a micro switch, which gives trip command to breaker.
Checking the PRV operation manually: -
a) The operation of PRV can be done by lifting the plunger (Plunger operates switch). By shorting contact externally by link, trip circuit can also be checked.
e) Oil Temperature Indicator - It is dial type thermometer, works on the vapour pressure principle. The bulb, which is known as ‘Probe’ is exposed to the temperature to be measured, is connected by a length of flexible tubing to a borden gauge tube, which is known as 'operating bellow'. This bellow is filled with a volatile liquid. The change in bulb temperature causes change in the vapour pressure of the liquid & pointer moving on a dial calibrated in degree centigrade indicates the consequent movement of the operating bellow. It has two pair of contacts, one for Alarm & another for Trip. In general, oil temperature alarm is set at 80°- 85° C and oil temperature tripping is set at 85°- 90° C.
Checking the OTI operation manually: -
a) The operation of OTI can be checked by tilting the float position. The first float S1 is used for alarm and another float S2 is for trip signal.
Alarm & Trip circuit can also checked by shorting contacts externally by link.
f) Winding Temperature Indicator - It is also similar to OTI but has some changes. It consists of a probe fitted with 2 capillaries. Capillaries are connected with two separate bellows (operating/compensating). These bellows are connected with temperature indicator. Operating bellow is surrounded by heater coil, which gets current from one WTI CT, when load on transformer increases, corresponding current passes to the heater coil mounted on operating bellow. The heater coil heats the operating bellow, which is filled with volatile liquid.
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Due to this heat, vapour pressure of volatile liquid increases hence WTI shows more temperature as compared to OTI. There are four mercury switches, 1 contact for Alarm, 2 for Trip circuit and 3 for cooler control and 4 as a spare.
In general, winding temperature alarm is set at 85°- 90° C and winding temperature tripping is set at 90°- 95° C. The fan Auto ON operation is set at 60°
C and Fan auto OFF is set at 55° C.
Checking the WTI operation manually: -
a) The operation of Winding Temperature Indicator can be done by tilting the float position. The first float S1 is used for alarm and another float S2 is for trip signal.
Fan auto operation can also be checked by float movement. Alarm / Trip circuit can also be checked by shorting contacts externally by link.
g) Conservator - As expansion and contraction occurs in transformer main tank, consequently the same phenomena takes place in conservator as it is connected to main tank through a pipe. Conservator communicates with the atmosphere through a breather, incorporating a dehydrator, which is connected to the breather pipe. Other end of this pipe opens at the top in the conservator, just below the conservator upper wall.
h) Breather - This is a special air filter incorporating a dehydrating material, called, Silica Gel. It is used to prevent the ingress of moisture and contaminated air into conservator. It consists of an inner metal cylinder filled with silica gel. Both ends of this cylinder are enclosed by wire mesh screen. This cylinder is enclosed in an outer casing of cast iron. Casing has 2 parts. The upper part is cover; where as lower part is attached with an oil seal. When transformer breathes in, the air enters which passes into the oil seal. The contamination, if any, is observed in this oil. Then air passes through silica gel, where the moisture, if any, is observed by the silica gel and pure and dry air goes to conservator tank of transformer. Normal colour of Silica Gel is blue. If it turns to pink, then Silica Gel is to be reactivated / replaced by fresh Gel.
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i) Oil Level Indicator - It is also known as magnetic oil gauge (MOG). It has a pair of magnet. The metallic wall of conservator tank separates magnets without any through hole. Magnetic field comes out and it is used for indication. This eliminates any chances of leakage. The driving magnet rotates and acquires the position corresponding to height of oil level, as it is linked with a float. The float is hinged & swings up and down with oil level. This rise or fall rotates driving magnet with the help of bevel gear and pinion. Follower (Driven) magnet moves accordingly and operates a pointer & a cam. The pointer reads oil level & cam is set to operate a mercury switch to give low oil alarm as per the oil position.
Checking the MOG operation manually: -
a) Operation of MOG can be done by tilting the float position which gives alarm signal. Alarm circuit can also checked by shorting contacts externally by link.
j) Radiators - Small Transformers are provided with welded cooling tubes or pressed sheet steel radiators. But large transformers are provided with detachable radiators plus valves. For additional cooling, exhaust fans are provided on radiators. The hot oil in main tank goes up and enters in the radiators. After cooling in radiators, either by natural air or forced air, oil again goes to main tank from the lower valve and circulates continuously.
k) Bushings – It comprises a central conductor surrounded by graded insulation.
Bushing is necessary when a conductor is taken out through metallic tank or wall.
Oil filled bushing is used for 33 KV applications. For making bushing compact, synthetic resin bonded condenser bushing is used for 33 and 66 KV applications.
For 132 KV & above voltages, oil impregnated paper condenser bushing is used.
It consists of a central conductor surrounded by alternate layers of insulating paper & tin foil. The capacitor formed by alternate layers of tin foil and paper insulation results in uniform electric stress distribution between conductor surface and earthed flange. The bushing core is coated with suitable resin.
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The assembly is enclosed in hollow porcelain and is provided with support flange and top hood. The porcelain is filled with oil.
Creepage Distance (CD) – It is the shortest distance between two conductive parts along the surface of the insulating material. CD requirement depends upon rated phase to ground voltage and degree of atmospheric pollution.
Degree of Pollution Recommended Min. CD
1) Clean area 16 mm / KV
2) Moderately polluted area 20 mm / KV
3) Industrial area 22 mm / KV
4) Heavily polluted/coastal area 25 mm / KV
l) Tap Changer - As load on the transformer increases, secondary terminal voltage decreases. To maintain the secondary voltage, tap changers are used.
Tap changers are connected with H.V. winding (Primary winding).
Therefore in tap changers transformer, there are two windings in H.V. side, 1) Main winding and 2) Tap winding. There are two types of tap changers.
A) Off Load Tap Changer - In this type, before moving the selector, transformer is made OFF from both ends. Such tap changers have fixed brass contacts, where taps are terminated. The moving contacts are made of brass in the shape of either roller or segment.
B) On Load Tap Changer - In short we call it as OLTC. In this, taps can be changed manually by mechanical or electrical operation without making off the transformer. For mechanical operation, interlocks are provided for non-operation of O.L.T.C. below lowest tap position and above highest tap position. Similarly for electrical operation, limit switches are provided in circuit for non-operation of tap change below lowest tap position and above highest tap position. For mechanical operation, one hand interlock switch is provided in the circuit.
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As soon as we insert handle, hand interlock switch opens out the electrical circuit and no one can operate O.L.T.C. electrically.
RTCC (Remote tap change control cubicle) is used for tap changing by manually or automatically through Automatic Voltage Relay (AVR) which is set +/- 5% of 110 Volt (Reference taken from secondary side PT voltage). During Auto tap changing, Bell / Hooter will ring up thus gives information to substation operator for tap changing.
Transition resistances are used in OLTC for avoiding momentarily interruption of power supply during tap changing. At the time of tap changing, load current passes through the transition resistances & no power interruption occurs during tap changing.
Transformer Tap: - Tapping is provided in Primary winding. Hence by changing the tapping, we can change secondary voltage as per requirement.
The transformer equation is: - V2/V1 = N2/N1 i.e. V2 = (N2 x V1)/N1
There is an Inverse relationship exists between secondary voltage & primary turns. When primary turns are decreased i.e. Tap position is shifted from 3 to 4, secondary voltage gets increased and if primary turns are increased i.e. Tap position is shifted from 4 to 3, then secondary voltage gets decreased.
Parallel Operation of Transformers:
Before paralleling two or more transformers, the four principal characteristics of those transformers should match as given below:
1) Same voltage ratio
2) Same percentage impedance 3) Same polarity
4) Same vector group
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If two transformers of same output operating in parallel, the % impedance must be identical, if Transformers are to share equally. If % impedance is not identical, suppose T/F 'A' is having 4% impedance and T/F 'B' is having 2% impedance, then load sharing will be,
Load A = L x ( Z2 / Z1+Z2 ) Where L is total combined output Load B = L x ( Z1 / Z1+Z2 ) and Z is percentage impedance So that A transformer will share only 1/3rd load & B transformer will share 2/3rd load. Hence operating transformers in parallel, the output of the smallest transformer should not be less than 1/3rd of the output of largest one.
When operating two transformers in parallel, one of the RTCC panels is kept on Master mode and another one is kept on Follower mode so that simultaneously tap changing is possible on both transformers. If transformers are not running parallel, the control switch is kept on Independent Mode i.e. both transformers taps can be separately changed.
Site Testing of Transformer:
1) Insulation Resistance Test –