bhel report 1
TRANSCRIPT
BHEL, BHOPAL
Bharat Heavy Electrical Plant, Bhopal is the mother plant of Bharat Heavy Electrical
Limited, the largest engineering and manufacturing enterprise in India in the energy-related
and infrastructure sector, today. It is located at about 7 kms from Bhopal Railway station,
about 5 kms from Habibganj Railway station and about 18 kms. From Raja Bhoj Airport.
With technical assistance from Associated Electricals (India) Ltd., a UK based company; it
came into existence on 29th of August, 1956. Pt. Jawaharlal Nehru, first Prime minister of
India dedicated this plant to the nation on 6th of November, 1960.
BHEL, Bhopal with state-of-the-art facilities, manufactures wide range of electrical
equipments. Its product range includes Hydro, Steam, Marine & Nuclear Turbines, Heat
Exchangers, Hydro & Turbo Generators, Transformers, Switchgears, Control gears,
Transportation Equipment, Capacitors, Bushings, Electrical Motors, Rectifiers, Oil Drilling
Rig Equipments and Diesel Generating sets.
BHEL, Bhopal certified to ISO: 9001, ISO 14001 and OHSAS 18001, is moving towards
excellence by adopting TQM as per EFQM / CII model of Business Excellence. Heat
Exchanger Division is accredited with ASME ‘U’ Stamp. With the slogan of “ Kadam kadam
milana hai, grahak safal banana hai”, it is committed to the customers.
BHEL Bhopal has its own Laboratories for material testing and instrument calibration which
are accredited with ISO 17025 by NABL. The Hydro Laboratory, Ultra High Voltage
laboratory and Centre for Electric Transportation are the only laboratories of its in this part of
the world.
INDEX
INTRODUCTION
FORMULAE USED IN DESIGN
DESIGNING OF TRANSFORMER (THEORITICAL ASSUMPTIONS MADE)
CORE DESIGN
TYPES AND FEATURES OF DIFFERENT WINDINGS
INSULATION
TAPPING AND TAP CHANGER
TRANSFORMER TESTING
Transformer Engineering Division
1. General
BHEL is the foremost transformer manufacturer in India today and to improve its position
further in India and International market, it is necessary that the engineers coming to work in
this division quickly absorb the specified knowledge relating to transformer design
manufacture, testing, maintenance and operation. The following pages give the information
required start giving useful output at an early date and also prepare him to improve quickly
his ability for gaining more in-depth knowledge for the particular assignments given to him.
In addition to the notes given in the following pages, he studies the following documents
within the first six months of his posting in this division.
1) Indian standard for pourer transformers T3: 2026.
2) Indian standard for current transformers 13 2705
3) Indian standard for voltage transformers IS 3156.
4) CHIP Documents on Transformers -
i) Section A - General
ii) Section B - Distribution transformers up to 100 kVA/11 kV
iii) Section C - Transformers up to 3.15 MVA and up to 33 kV
iv) Section D - Transformers up to 20 MVA of 132 kV
v) Section E - Generator transformers for thermal stations.
vi) Section F - Transformers above 20 MVA and 220 kV
vii) Section G -- Transformers of 400 kV class
viii) Section H - Earthing Transformers.
ix) Section I —Reactors
x) Section J - Tests
xi) Section K - Erection, maintenance and commissioning manual.
5) I.S. for Transformer oil: IS: 335.
1.1 TRANSFORM RS CAN BE CATTEGORISED IN THE FOLLOWING
TYPES
1) Power transformers -
(a) Generator - Two winding
(b) Transmission transformers
i. Two winding
ii. Three winding .iii Auto connected
iii. Auto connected
Three are of various voltage classes depending on voltage of the high tension
winding as below
33 KV
66 KV
132 KV
220 KV
400 KV
2. Rectifier Transformers
3. Furnace Transformers
4. Earthing Transformers
5. Dry type transformers
6. Current Transformers
7. Voltage Transformer 1
The types 2 to 5 are generally referred to as special transformers and 6 & 7 are called
instrument transformers.
FORMULA USED IN DESIGN
Voltage per turn =
Where B = Max. Flux density in web / m2
A = Nett core area in aq.cm
F = frequency
Mean turn = TT (Coil inside dia + Radial dimension of coil)
Resistance/phase at 75°C =
Copper weight = 8.9 x 10-6 x Mean turn x CSa(mm2) of copper x no-of turns.
R1 = Ro
where t0 = initial temperature °C
t1 = final temperature °C
Cu Loss =
Impedance
Where R = % Cu Loss; x = % Reactance
Core and winding wt = 1.2 (Core punching Wt. + winding copper wt.) approx.
Oil displaced = Core + winding wt. (kg) / 7.5 liters approx
Oil absorbed = Paper + insulation wt. (kg) x .3/.85 liters
Weight of insulation as of copper weight
= K x t (Semi.-perimeter of conductor + 0.8t) / A
Where K = 9 for paper
K = 14 for enamel wit vinyl acetyl base.
t = total increase of conductor for dimensions due to covering.
A = area in mm2 of bane conductor.
Copper wt. = L x A x 8.9 x 10-6 Kg.
Where L = Length in mm and A= A in mm2
Eddy current loss: te in mm2
Eddy current loss: te =
Where
te = percentage eddy loss
t = thickness in mm of the individual strands in the direction perpendicular to the
leakage flux.
m = No. of strands in conduct e in the direction perpendicular
to the leakage flux and reckoned from the position of minimum flux.
w = width of bare conductor in the direction parallel with the flux.
w1 = Pitch of turn i.e. width of co gered conductor and - inter-turn insulation.
(See sheets H.7.8 to 10 for relationship between
DESIGNING OF TRANSFORMER
(Theoretical assumptions made)
ASSUMPTIONS
Various assumptions such as leg length, core area etc. is made before the start-of design.
Using the assumed values, further parameters are calculated.
1. LEG LENGTH:
As the rating of the transformer increases, leg length has to increase to meet the
demand. One in leg length increases the overall height of the transformer. Due to the
transportation limitation max length of the lore is limited.
Transportation length has been given by the railway authorities and leaver with no
choice other than taking maximum possible leg length for high rating transformers.
Maximum possible length may be deduced from the transportation length taking into
consecration - the top and bottom yokes, feel, tank cover, bottom plate thickness, top oil
clearance up to inside tank cover and clearance for the air release valve.
Leg length also helps in the variation of reactance.
2. CORE DIAMETER:
Core diameter is one of the most important parameter to be designed as core diameter
is an imp factor leading to the net reactance and net weight of the system. Increase or
decrease in the core diameter creates two effects which counteract each other compensating
for each other's advantage.
When core clam increases the core area inc. and in turn voltage per turn increases,
this reduces the no. of turns and hence the amount of copper used and also the weight of
system. But the inc. core area also inc. the amount of steel required due to which the iron
losses of the system are increased.
Both of these compensate for the other. Therefore for the economic design of the core
proper estimates are to be made. So that both copper wt. and iron losses are limited to the
specified value.
3. FLUX DENSITY:
Flux density or B is usually assumed as per the following basis.
As the flux density in creases the iron losses increases but it reduces the amount of
conductor used and also cot and the overall size.
Therefore in practice a nominal value of B is to be assumed and the value taken is
1.12 wb/m2. There is an upper limit to the value of B due to the saturation effect.
Usually high flux density is the choice as size of the transformer is a major concern.
The iron losses may be overcome by the material uses (Usually CRGO).
4. CURRENT DENSITY:
Current density increases the copper losses in the conductor thereby the amount of the
conductor used is decreased. Similarly when the current density decreases, amount of Cu.
and cot of the system increases.
Therefore for the economic design proper selection is to be made. In practice the
values of current density are taken from 2.2 A/sq.mm to 3.5A/sq. mm.
For LV winding high current density is needed and for HV, low density is prettied.
Low current density makes winding more mechanically strong.
5. DECIDING THE COIL LENGTH (CD) & MAJOR CLEARANCES:
This necessitates the over potential circuit diagram to be made. Such a diagram gives
the ids or potentials to which the winding terminals, neutral etc. will be raised during
the test. The requisite electrical clearances for these voltages may be
seen from the design data, other and ducts between various coils and between sect
soil (if it is helical or RS) can be taken based on previous design and/or from the
design data. The paper insulation thicknesses on different coils are decided by making
impulse calculation.
The clearances from HV natural and line end decide the first and last layer Cds and
CDs of other layers of the shielded layer type winding.
Suitable dimensions of the conducers are chosen. The assumed current density will
give the area Axial dimension and No. of parallel wires are selected from the tables of
copper strips and cables. It should not be more than 5 times and less than 2 times the
radial dimension. .
Next course of calculations is to calculate the effective length of the AV coil because
the other coils are going to be of the same effective length. This is from balancing
point of view.
In the case when windings have unequal lengths or unevenly distributed mmfs along
the winding height (i.e. the coils are un-balanced), axial components of external
forces also appear which have either opposite or the same directions to axial internal
forces and hence decrease or increase the pressure on space Vs between coils.
The external forces acting on the conductors of the winding are those which are
caused due to the interaction between the currents in the different windings, and,
internal forces appear due to interaction of the currents in the same winding.
The internal forces lead to an all-round squeezing of each winding because the
currents in the conductor of the winding con-tide in phase resulting in force of
attraction, Current in separate winding of a two circuit transformer are approximately
opposite to each other in phase and therefore the windings are repelled from each
other with a tendency for the distance between them to increase.
Coming back to external axial forces occurring due to unbalance conditions, these
forces tend to bend the winding turn in a vertical direction. The whole coil may bend
in the axis direction and may break the supporting insulation and inseevete cases the
forces ray be partially tending to tear them off the limbs.
If the windings carry the rated currents, mechanical are not large, but when short circuit
current pass through these forces increase many times and can be dangerous for the
transformer.
Apart from the force considerations, the uneven distribution of mmfs due to net lengths of
coil gives rise to radial component of leakage flux. This flux enters the core perpendicular to
its surface. The total leakage flux, which is the sum total of axial and radial components of
leakage flux increase which further increases the leakage reactance of the transformer which
may not be desirous.
In view of foregoing discussion the balancing is very important. The effective length of HV
coil will decide the pitch and the conductor dimensions of other windings.
6. PERCENTAGE REACTANCE CALCULATION: PERCENTAGE IMPEDANCE
It is the impedance voltage expressed as a percentage of the rated voltage of the primary
winding, where, Impedance no. is the voltage required to be applied at rated frequency
between the line terminals of the primary winding, to cause the rated current to flow into
these terminals when the terminals of the secondary winding are short circuited the measured
value are corrected to an average temperature of 775°C of the winding.
PERCENTAGE REACTANCE:
As the percentage resistance is negligible, the percentage, impedance may be termed as
percentage reactance.
Percentage Reactance calculation is the main part of the design. It is adjusted for the exact
value by trial and error method. If the, reactance does not come as required, then, the
complete design, altogether changed.
Diameter of the core is the main parameter which is chant for getting the specified value of
reactance; of course, leg length, clearances between the windings may also be changed but
these effects very less.
(The formula for the reactance calculation of a two win-ding transformer is given as -
% X = 3.74 x 77 x AT x
x Reactance length (HV radial immersion
+ LV LD x Mean dia of LV 3
+ Gap between LV and HV x Mean gap dia
7. PERFOEMANCE EVALUATION: Calculations, of, Iron weight, Iron loss copper
weight copper-rose and gradients etc, are the main things to be done. These evaluate
the efficiency, regulation etc of the transformer.
All the impulses have a wave shape of 1.2/50 micro seconds. Chopping is produced
by the fieshover of a standard rod gap connected in parallel with the transformer
under test. For each application of test voltage, photographic record of the impulse
voltage wave shapes and also currents are obtained from a cathode ray oscillograph.
CORE DESIGN
The transformer core is a closed magnetic circuit through the mutual flux i.e. the flux which links with both the windings passes. The core material and construction should be such that both the magnetizing current and the core losses are minimum. The cores or transformers are laminated in order to reduce the eddy current losses. The eddy current loss is proportional to the square of thickness of lamination. This apparently implies that the thickness of the laminations should be extremely small in order to reduce the eddy current losses to a minimum. However, there is a practical limit beyond which the thickness of the laminations cannot be decreased further on account of mechanical consideration. This practical limit of thickness is 0.3 mm. The laminations are made 0.33-0.5 mm thick. The thickness should not be reduced below 0.3 mm because in that case, the lamination become mechanically weak and tend to buckle. These laminations are made of the so called transformer grade steel containing 3-5 % silicon. The higher content of silicon increases the resistively of the core, thereby reducing the eddy current core loss. High content silicon steel is a soft iron material having a narrow hysteretic loss is also small. This material has a high permeability and hence the magnetizing current is also small. The steel used for transformer cores may be hot rolled or cold rolled. The hot rolled steel which permitted a maximum flux density of 1.45 Wb/m2 was in use for a considerable length of time. In recent years this type of steel has completely been superseded by 0.33 mm (or 0.35 mm) thick cold rolled steel slowing much higher flux densities up to 1.8 Wb/m2 to be used. Although. Could rolled steel is 25-35 % more expensive than the hot rolled steel, the increase in value of maximum flux density makes in possible to reduce the amount of core material.
Cold Rolled Grain Oriented (CRGO) – steel sheet with an approximate silicon content of 3 % is typically used for magnetic circuits of transformer. CRGO steel offers following advantages:
i. Magnetic induction is maximum and the loop of BH curve is low. ii. Core loss during no load operation of the transformer is low. iii. Reactive power input at no load operation of the transformer in low. iv. Magnetostriction is low. v. Good mechanical properties.
CORE CROSS- SECTION
Small core type transformers have rectangular section limbs with rectangular coils as shown. However, in large capacity transformer, the economic use of core material requires that the cross-section of the core should ideally be a circle since a circle has the minimum periphery for a particular area and hence, the windings which are put around the core have a minimum length of mean turn resulting in reduced amount of conductor material thereby reducing the costs. A circular core, however, involves the use of an unmanageable large number of lamination of different sizes. The use of laminations of different sizes is possible but is highly time-consuming and uneconomical on account of the obvious difficulties in core
assembly and increased labor costs. A compromise is achieved by arranging the core section in steps in such a way that the net sectional area is maximum for the number of steps employed and the corners of the steps are so arranged that they lie on a circle known as a circumscribing circle of predetermined diameter. However as the number of steps increases, the number of different sizes of lamination also increases. With large number of steps, there is a reduction in the length of mean turn of windings and consequently there is a reduction in the cost of conductor material and the I2R losses but there is extra cost involved in shearing and assembling of different sizes of lamination. Therefore, while designing a core section, balance should be struck between the cost of conductors and core and also the labor charges. Cores for shell type transformers are usually of simple rectangular cross-section.
EFFECTS OF JOINTS:
Joints in a magnetic path can greatly inc. the total excitation requirements, especially operating at inductions where permeability of material is normally be very high.
In case where the flux is forced to pass perpendicularly to the laminations as in the areas adjacent to the gap formed by abutting laminations, core losses may inc. appreciably.To reduce such losses care should be taken to minimize the no. of joints and the reluctance associated with them. When lowest exciting current is of prime importance and where the path length is relatively short, as in small transformer cores, the designer can profit by giving major considerations to improving the joints in this design.
VARIOUS JOINTS:
INTERLEAVED JOINTS:
In this type of core construction also fluxes are bound to travel approximately perpendicular to the direction of the orientation of grains i.e. perpendicular to direction of rolling. Hence facing the problem of excessive core loss.
MITRED JOINTS:
Quarter, semi and fully mitered joints are the latest developments and practice followed in the design of joints of core construction. To achieve this type of construction stampings are cut at 45 degrees.
Due to this type of joint construction, flux passes more or less, along with the grains even at the joints, hence decrease the cross grain losses.
FIVE LIMBED CORES:
This type of core construction solves the problem due to transport height limitations. In this type of construction the top and the bottom yokes are only 57% of the limbs. The result is that of a net saving in overall height. The rest 43% area of the yoke are covered by designing auxiliary limbs and yokes. Their prime function is to give a return path for the fluxes fully mitered, five limbed core construction.
CLAMPING OF THE CORE: In small types of transformer, the limitations are held together by a string or by strong cotton webbing. But this method does not give much strength and rigidity. Cores can also be clamped between the iron frames .in big transformers, cores are kept in position by side plates bolted together at intervals along the limbs and the yoke. Holes are punched in the laminations to accommodate the bolts .these bolts which necessarily pass through the core must be insulated from side plated and lamination .while the side plates are insulated from lamination .the isolation is necessary as the bolts would short circuit the lamination and would provide path for eddy currents .in order to provide more rigidity to the core and to prevent bulging of core between the bolts flitch plated are used.
COOLING OF THE CORE
For small and medium sized oil immersed transformers the dissipation of core loss is simple, as the surface o the lamination is large compared to the volume and losses .very large cores on the other hand ,have relatively small surface / volume ratio, so that the cooling surface must in some way be augmented by additional ducts.
The magnetic circuit is therefore, divided into packets insulated from each other and to ensure good electrical continuity between packets ,tinned copper strips bridging pieces are used.
TYPE & FEATURES OF DIFFERENT WINDINGS
There are four main types of windings currently used in the High voltage transformers.
a. Spiral winding (SPI) b. Helical winding (HEL)c. Continuous Disc (CD) or Reversed Section Windings (RS) d. Shielded Layer type Winding (LAYER)
As a general rule spiral is a low voltage low current winding, HEL is low voltage high current and RS is high voltage medium to high current winding. The later type coil is exclusively used for 220 kV winding. Tapping coils may be either spiral or helical but mostly they are intertwined spiral or intertwined helical.
2. INTERTWINED SPIRAL AND HELICAL WINDINGS – Toppings of a big transformer like the extra High voltage are wound as a separate coil. In such coils conductors of all steps are taken together and wound from top to bottom of the leg such that turns of each step occupy the whole length. There is therefore a high capacitance between the intertwined sections of the winding improving the surge voltage distribution. In fact the complete tapping coil acts as a shield and thus eliminates the necessary of a magnetic neutral shield.
Also from mechanical strength point of view such coils are strong. Ampere turn balance is achieved by keeping the effective length of the coil same as that of HV main. Such coils generally require more insulation.
Where possible leads are arranged so that the voltage between conductors is minimum. All intertwined coils are tested for an appropriate voltage between leads before assembly or during winding in dry condition. The voltage chosen is 100 v/mil of redial insulation on the conductor or 5 kV whichever is less.
Generally, the first endeavor is to make an intertwined spiral. If a spiral is not feasible; this will be the case when LD of the coil is more than 15-16 mm (because beyond that the coil is no suitable from cooling point of view); the next course is to go in for the intertwined helical.
3. HELICAL WINDING – Helical winding being a high current coil finds its use for LV of large power transformers when the number of winding turns is comparatively small but the current is large.
As the cross section is much more, a term of such winding consists of several parallel conductors of rectangular cross-section wound on the flat side, side by side with each other in radial direction. The turns are separated by dovetail blocks forming the radial ducts.
It is accomplished more often as a single layer or as a double layer type, the layers being separated by an axial oil duct. Special purpose may require 4 layers or more also.
It may be noted that the calculated helical winding pitch, compressed paper covered axial dimension of the turn (i.e. the conductor) and compressed inter-turn insulation are to be taken into account.
When multi-rectangular conductors per turn are used transposition becomes very necessary. Transpositions are introduced throughout the winding at regular intervals to ensure that the leakage flux linkages of each strand will be the same. This will help reduce the stray loss, or to be exact, circulating current loss.
The modern way is to use continuously transposed cables (CTV) which avoids the mechanical transposition as is done for the rectangular strip cables. The coil is more mechanically strong but the transposed cable in costly.
It is to be seen that the ratio of bare copper width to thickness should preferably be in the rage 2:1 to 5:1 and shall not exceed 6:1.
The radial ducts throughout the turns may not be same but may require be increasing or thinning in accordance with HT winding so as to achieve a more uniform distribution of both winding mmfs, along the winding height. The electrical length of LT winding, therefore, becomes equal to that of the HT winding.
4. REVERSED SECTION WINDING – where numbers of turns preclude the use of helical type of winding, the reversed section or continuous disc coil is used. Generally in case of 66 kv or 132 kv class winding number of terms are more, such coils therefore to be made by RS winding.
The coil consists of several discs radially wound with strip conductors so that there may be several turns in each disc.
This type has been termed “continuous disc” because the winding is wound with a continuous conductor without a single soldered joint. For continuous inter disc connections every alternate disc is wound with its turns reversed while the other discs are wound in the normal manner. So that the inter disc connections are always at the back or the front of the winding and do not cross from back to front.
A coil turn may consist of one or more parallel conductors, its number does not usually exceed four, as otherwise forming the winding become difficult. The conductors may be transposed but within each disc at the inter disc connections.
The winding is wound on a SRBP cylinder and separated from it by pressboard vertical dovetail spacers. Pressboard blocks are fixed on the spacers and form one coil to another is possible only between the spacers.
The continuous disc winding permit easy arrangement of voltage-control taps which are usually taken from the outer and normally not from the inner inter disc connections so that two adjacent taps will give one step of voltage variation.
To permit the turns to be tightened satisfactorily on the built-up sections the winding should not contain more than 25 turns per section. Also the ratio of width to thickness of conductor should not be less than 2:1. The radial depth of the coil is generally not exceeded by 5”
It is recommended that number of strands in parallel should not exceed 4.
Generally the coil has reinforced sections at both the ends but more at line end. The reinforced sections may have conductors of different cross sectional areas. In such case separate reinforced discs are made and connected to the main continuous part of the winding by soldering.
5. LAYER TYPE WINDING - Layer type windings find application in high voltage power transformers when the neutral terminal of the high voltage windings is solidly earthed.
This type of winding consists of a series of concentric layers of progressively reduced lengths. As ratio shield is wound round the outside of the outermost layer and is connected to the line terminal. This is called the line shield. To reduce the high electrostatic voltages which may appear in low voltage winding, a neutral shield similar to the connected to the neutral lead which to the neutral layer and is connected to the neutral lead which is solidly earthed. However, the neutral shield is unnecessary. Where an intertwined tapping coil find place between LV and HV coils.
The series capacitance of this winding is inherently very high resulting in a sow value of Alpha = where Cg = capacitance to ground, Cs= series capacitance and hence a good impulse voltage distribution. The layers of the winding resemble the layer of foils in a condenser bushing and the electro stations
The conductor cross section is chosen to provide the necessary mechanical strength to withstand redial short circuit forces developed in the individual layer. Generally for the first 2 or 3 layer multi wire transposed conductors are utilized so as to reduce the eddy-loosed. Remaining layers may contain ordinary conductors of rectangular cross section.
Shields provided outside the line layer take the form of concentric cylinders. Since the voltage stress at the ends of the shields is very high, end rings are provided. The paper wrap under the shield is petalled round the ends rings are also provide at the ends of layers others other than line layer.
AUX
ILIARIES FOR TRANSFORMERS
SYNOPSIS:
Disturbances, which up-set the satisfactory operations of the transformers, can be caused due
to effects of faults, either occurring on any part of the system or arising in the transformer
itself. These disturbances may even cause failure of the transformer. Under such conditions
auxiliaries are the devices which give the visual indication as well as alarm annunciation.
Simultaneously, they actuate certain relays which, in their action, protect the trans-former.
In paragraphs to follow, some important auxiliaries have been discussed along with their
operating mechanism.
1. RELIEF VENT:
In case of severe fault in the transformer, the intez: l pressure bay be built up to a very
high level which may result in an explosion of the tank. To avoid such contingency a
relief vent is fitted.
The relief vent consists of a pipe fitted on the tank cover. The pipe has a bakelite
diaphragm at the top, which breaks and relieves the pressure in the event of excessive
pressure built up. The bakelite diaphragm has a perforated sheet underneath it to
ensure that broken pieces of the bakelite diaphragm are not drawn in the tank.
'The top of the explosion vent is connected with the conservator by means of an
equalizer pipe to equalize the pressure in the conservator vent.
2. BREATHERS
A dehydrating breather is used to dry the air that enters a transformer as the
volume of oil decreases because of a fall in temperature.
Air entering the breather is first drawn through an oil seal and passes upwards
through the silica gel crystals to the connecting pipe at the top. During this upward
passage of air, any moisture present is absorbed by the dry oil.
The oil seal that the gel absorbs moisture only when the transformer is
breathing.
I .B. t If the oil, in the transformer i.e. more than the capacity of one breather
to breath.* then two breathers (ganged together that both breathe through the same oil
seal and also connected to the conservator by a common pipe) are used.
3. Winding temperature Indicators (Acc. Controls Ltd.)
There are precision instruments designed for the protection of the transformers. One
number instrument with three for number of contacts can be provided on each
transformer, to indicate the temperature of hottest part of the windings (the hot spot
temp.).
It will perform the following functions:
(a) It will indicate the maximum winding temperature irrespective of the conditions of
loading, temp. of cooling media and altitude. Thus the load on the transformers can
be kept within the limit.
(b) It will close alarm Strip contacts at predetermined temperatures.
(c) It will operate the automatic starter to introduce the fans when the winding
temperature reaches a prearranged level.
(d) It will also operate the automatic starter to introduce the pump for OFB cooling when
the winding temperature is reached a prearranged value (level)...
The operating mechanism consists of a sealed hydraulic system comprising a liquid filled
bellows, capillary tube and a sensing element. This system is balanced, by a compensating
bellows and second capillary at the head of the sensing element. The two bellows are linked
together via a bell crank lever in such a manner that temperature changes on the capillary line
or head are automatically compensated for and in no way effects the indication. Temperature
changes on the sensing element motivates the operating bellows only which is linked to the
indicating pointer mechanism and the rotary switch plate-form.
The instrument has a maximum indication pointer fitted to the instrument window and
operated by a peg driven by the indicating pointer. It can be reset with a standard screw-
driver blade after removing the knurled weather proof knob. The switches are of the dry
electrode mercury and glass type rated at few amps. They are fully adjustable over the range
of the instrument to individual scales and all the switches are adjustable.
A simple potentiometer system is employed to give remote temperature indication.
4. OIL TEMPERATURE INDICATOR:
Oil temperature indicator is a distance thermometer operating on the principle of
liquid expansion. It provides local indication at the marshalling box of the top oil
temperature. The connection between the thermometer bulb and the dial indicator is
made by flexible steel capillary tube. The bulb is enclosed in a pocket and the pocket
is fixed in the transformer at the hottest oil region. The pocket has to be filled with
Transformer oil.
The oil temperature indicator is provided with a maximum Pointer and two mercury
epitome. The mercury sari; ah s ire adjustable to make or break contacts- between
500C and 1200C the have a differential of 10°Cs The trip contact is of normally open
and the alarm contact is of normally closed type.
5. CONSERVATOR
As the temperature of transformer oil increases or decreases, there is a
corresponding rise or fall in the oil volume to account for this an expansion vessel
(called conservator) is connected to the transformer tank. The conservator has got
a capacity between the minimum and maximum oil level equal to 7t of the total
oil volume in the transformer.
The conservator is provided with detachable end covers on either side. On one
cover an oil gauge is provided to indicate the level of oil in the conservator. The
oil gauge indicated empty, i5°C oil level and full. The oil gauge is provided with a
low—oil level alarm giving an indication when oil is low. Conservator is also
provided with a filling hole and drain valve.
The conservator is connected to outside atmosphere through dehydrating—
breather to make sure that air entering in it is dry, The pipe that connects the tank
with the conservator i.e. the feed pipe, projects above the lowest point of the
conservator, such that portion below acts as a sump. An isolating valve is
provided in the feed pipe.
6. MAGNETIC OIL LEVEL INDICATOR;
The magnetic oil gauge is fitted on an end cover of conservator. It is provided with a
low oil level alarm (LOLA) and a safety cut out at emergency low levels.
The operation is by simple float arm mechanism: the rotary drive to the switch cams
being transmitted via a magnetic completing thereby ensuring that no liquid can enter
the weatherproof s switch compartment.
In some requirements mechanical type oil level gauge is used.
7. BUCHHOLZ RELAY
A double float Buchholz relay is fitted in the feed pipe from conservator to tank and
is provided with two sets of contacts. The device comprise of cast iron housing
containing two liquid floats, one in the upper part and the other in the lower part.
Each float is fitted with a mercury switch, leads of which are connected to terminal
box for external connections. The alarm contact is normally closed to open for alarm
and trip contact is normally open to close for tripping.
The buchholz is also provided with a petcock for collection of gas and also a petcock
for testing. An arrow is provided to indicate the direction of conservator. A pipe
connection is provided from the top petcock to a small valve situated at an easily
choose able point to draw off any gas collected in the Buchholz.
The top most point in the bushing turrets and inspection ovens are piped to the main
pipe on which the Buchholz is counted, so that the gases generated in the transformer
are directed to the Buchholz relay without being trapped anywhere.
Dividing the LV voltage by V/T we get the LV turns. This value is rounded off to full
turn. Corresponding to this rounded value of LV turns; V/T and Bm are adjusted.
Further with the new V/T, HV turns; Tertiary/stabilizing (if any) turns and Tapping
turns are calculated.
INSULATION DESIGN
MATARIALS – CLASS OF TRANSFORMER INSLATION
Chief insulating medium used for insulation in transformers in transformer oil, in combination with solid dielectrics which include press boards, paper, wood, cotton, tape, synthetic resin bonded paper tubes and cylinders and porcelain all these constitute class ‘A’ of insulation which has a maximum permissible operating temperature of 105oc (also called hot spot temperature)
The oil serves the double purpose as in insulation and a cooling agent transforming heat from hot parts of a transformer (the oils and the core) to the tank and radiator walls. As an insulator, oil backs up the solid insulation between HV and LV windings and raises the insulation level of the transformer as a whole.
TYPE OF INSULATION:
The insulation of transformer winding is divided into-
a. Major insulation, b. Minor insulation,c. Insulation relative to a tank, and d. Insulation between phases.
Winding insulation, relative to the grounded transformer core and to the other winding of the same phases is called major insulation. Insulation between different parts of one winding i.e. Insulation between coils, turns, layers etc. as well as between the winding and the elements of a shielding, is called minor insulation.
The oil-barrier insulation is mainly used in oil immersed power transformers. Such insulation consists of oil gaps, barriers and coverings. Transformer parts at a potential, are separated from grounded parts, and from other parts at a potential too, not only by oil gaps but barriers and coverings as well. Partitions of solid insulating material placed inside an oil gap are called barriers. For example, bakelite or press board cylinders located between HT and LT windings, between a winding and core, angular washers etc. are barriers.
Covering differ from barriers. They cover closely transformer parts which are at potential. Additional insulation on coil, bakelite tubes etc. are known as coverings. The aim of barriers and coverings is to increase the electrical strength of an oil gap. This is chiefly reached by the fact that barriers and coverings avoid the formation of the bridges of conductive impurities in oil.
ANGLE RINGS
At higher voltages, non uniformity of field at the edge of the high voltage winding increases, due to which the construction of the corner of the insulation becomes considerably complicated. Such unfavorable conditions at the edge of a high voltage winding, forces one to use corner washers or angle rings. This helps in insulating against more stresses by increasing oreepage & puncture value at the edge of the winding. For higher voltages, larger no. of such washers has to be used.
The coils are insulated from top and bottom yokes by what is known as yoke insulation. It is in the form of press board washers & blocks to form necessary oil ducts.
PRESSURE RING
In the transformer of high capacity a clamping ring of permawood is used for pressing the windings.
MINOR INSULATION
This is the insulation of the individual turns and between layers. The tow principal materials used for inter layer insulation are press board & paper.
TAPPING AND TAP CHANGERS
This chapter deals with brief descriptions of the location of tappings, various tapping arrangements and “OFF/ON” load tap changers etc. 1. LOCATION OF TAPPINGS:
The physical location of the tapping in the transformer winding is based on the electro magnetic forces developed and the change in the reactance when the tappings are cut in out of the circuit.
In large transformers of 220 kV class and above it is a common practice to provide the tappings in a separate coil so that each tappings section occupies the full length of the winding. This provides a complete balance. If amp turns in all case of tappings, minimum/normal/maximum.
Normally it is a standard practice to provide tappings in the neutral end of the HV winding in a double wound transformer due to insulation considerations.
The HV side is preferred for the tappings due to followings reasons-
2. LV side has got less number of turns and hence for getting a fine voltage variation, toppings may not be done at integral number of turns. Against it, HV side has got sufficient no. of turns, enabling the tappings to be done at integral number of turns.
3. Generally, in power transformers, the low voltage winding is located adjacent to the core. This brings a large number of leads to the tap changer from these winding calls for greater clearances.
4. The lager current involved in the low voltage winding leads to difficulties bringing out leads. Also, high current leads will cause additional problems like high stray losses and cooling of the leads etc.
2. VARIOUS TAPPING ARRANGEMENTS :
There are three different tapping arrangements in common use-
a. Linear type arrangement b. Coarse/fine arrangement c. Reversing type arrangement
All the three types are shown
a) LINEAR TYPE :
It is the simplest types of arrangement. The tap selector moves progressively forward of backward to out in or out the tapping turns.
b) COARSE/FINE TYPE :
This arrangement involves the provision of two sets of tapping windings (one for coarse variation and other for fine variation) know as coarse and fine tapping windings. This arrangement needs a change over selector in addition to a selector switch with the result that mechanical design of the tap changer becomes more complicated in camping to linear type. Coarse tapping winding has got number of turns more by one step turns than fine tapping winding turns.
c) REVERSING TYPE :
Like coarse/fine type, it also needs change over selector in this type of arrangements, the selector runs through two revolutions for the one complete runs of tap changing. The reversing switch changes position after on run. With the change over of the revering switch, the direction of flux also reverses.
This type of arrangement is preferred due to the reduced number of leads to be-brought out to the tap changer for large number of steps and low impulse voltages appearing across the switch.
2.1 FIELD OF USES CHOICE FOR PARTICULAR TYPE :
Linear type arrangement is used when the numbers of tapping positions are less. This is generally used in off-circuit generator transformers.
Coarse/fine and Reversing types are used where the numbers of steps, in which variations are required, are large. Reversing type is preferably used in auto transformers.
3. TAP CHANGERS :
There are two types of tap changers-
A) “Off load” tap changer B) “On load” tap changer
The choice of a tap changer is limited by a number of factors vis.
a) No. of steps required b) Design of tapping winding
c) Tapping range d) Insulation Level e) Step voltage
The customer specifies the percentage variation of voltage desired the number of steps in which the variation will take place. Depending upon the specifications as discussed earlier, the type of tapping coil as whether linear, coarse/fine of reversing is selected. After the winding has been designed, the tap changer is selected.
Both the types of tap changers are described in brief as follows –
A) OFF LOAD TAP CHANGER:
If the customer wants to change the voltage after long duration the off circuit tap switch is provided normally this is case of generator transformers. The contacts are mounted on bakelite board and by lever arrangements, which is hand operated, the desired position is selected. The whole thing is enclosed in transformer tank in self and is immersed in oil. The handle is outside the tank. Generally with off circuit tap switch, linear type of tapping coil is used. The normal range is 10 % but in special cases it can be made up to 15 %
B) ON LOAD TAP CHANGER:
The on-load tap changer, transfer/ load under “supply on” condition, with full load on transformer.
There are two main considerations – a) A duplicate circuit is to be provided to carry load current when the tapping is
changed. b) An impedance of resistance is to be provided to limit the short circuit current
which results when the winding turns between tapping in service and the selected tapping get short circuited.
4. ELEMENTS OF ON LOAD TAP CHANGERS
Basically an ‘on-load’ tap changer consists of –
4.1 TAP SELECTOR :
These are provided to select tapings under on load condition. It consists of fixed contacts of copper mounted on boards, in a circle or segment of circle. Two boards are arranged, one below the other and each board carries half the number of contacts. A pair of moving contacts, on for each board engages the fixed contacts alternatively.
4.2 DIVERTER SWITCH :
To break the load current and the circulating current, while transferring load from one tapping to another.
The switch consists of fixed and moving contacts of copper of adequate cross-sectional area. There are two pairs of contacts per phase. These contacts are tipped with tungsten copper alloys tips to minimize the wear due to arcing during diverter operation.
Out of the four contacts, two are main contacts and two are auxiliary contacts. The main contacts break the main load current whereas the auxiliary contacts break either the sum of half the load current and circulating current or their difference.
4.3 TRANSITION RESISTORS :
These are used to reduce circulating current during tap change.
During a tap change, the director switch bridges the selected tapping and the tapping in use and the resistors are used to restrict the short circuit current.
4.4 DRIVING GEAR :
To supply the driving force to the tap changer mechanism, driving gear is used.
Besides the manual operation features provided for applying, the motive force for a tap change the driving gear employee motor for the facility of remote control of the tap changer.
TRANSFORMER TESTING
All the power transformers after their manufacture are tested in accordance with IS: 2026, to
ensure their electrical and mechanical roundness and also to make certain that they meet the
guaranteed performance. Tests conducted are divided into three categories:
i. Routine tests
ii. Type tests
iii. Supplementary Tests
ROUTINE TESTS:
These tests are conducted on all the transformers manufactured. Normally following tests are
included in routine tests:
a. Measurement of windings resistances
b. Ratio polarity and phase relationship tests
c. Impedance voltage test
d. Load losses test
e. No load loses and no load current test
f. Insulation resistance measurement
g. Induced over voltage withstand test
h. Separate source voltage test
TYPE TEST
These tests are conducted on first unit of a particular design, to prove the design. The
following tests come under these headings:
a. Impulse voltage withstand test
b. Temperature rise test
Impulse voltage withstand test
The impulse testing of transformers has been standardized as a type test as a means of
ensuring a satisfactory level of insulation and the ability of the transformer to withstand
within certain limits, such over voltage a urges as may occur in the service.
Impulse voltage testing circuits is shown in the figure No. (ii) The test voltage to be
applied to transformer under test is laid down in IS 2026 and the impulses are required to be
applied in the following order:
a. Adjustment and calibration shot at between 50 percent and 75 percentage and
75 percent of the insulation level.
b. One full wave shot at the standard insulation level.
c. Two chopped wave shots at 115 percent of the standard insulation level.
d. One full wave shot at standard insulation level.
INTEPRETATION OF TEST RESULTS:
Evidence of insulation failure arising from the test may be given by sufficient
variations of wave shape (apart from in ended amplitude changes) indicated by the records of
applied voltage and of supplementary current or voltage for 311 applications at the test level
and at reduced level.
Temperature –RISE TEST
The losses in the transformer result in generation of heat, which ultimately results a
rise in temperature of oil, wise-ding and core, depending upon the method of cooling
employed, ambient of the place where transformer is to be installed, the winding and oil
temperature rises are guaranteed. To check whether the temperature rises are within the
specified limits, temperature rise test is performed.
The temperature rises test is conducted by short circuit method, supplying HY
winding with LV winding short circuited (Tertiary, if any, open circuited). Power supplied is
sum of iron and copper losses at 75°C, when steady state condition is reached the current is
reduced to full load value and afar about one hour shut down is made for H.V. winding
resistance measurements. This routine is repeated for all types of cooling provided for the
transformer.
Ambient temperature, oil temperature, winding temperature. When winding is
cold and also when it is heated is logged every half an hour. With the help of these readings
oil temperature rise and winding temperature rise are evaluated as per IS 2026.
SUPPLEWNTARY TESTS
Apart from routine end type teats mentioned earlier, some additional tests, if customer
desire are conducted, which are known as supplementary tests. These include measurement
of iron loss at 90% or 110% excitation, zero phase sequence test noise level tests etc.