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TRANSFORMERS
1.1 NECESSITY: Electrical energy generated at generating stations is transported to remote load centers. Between generating station and consumers we have transmission, sub transmission and distribution levels of voltage. Since the long distance transmission at high voltage is cheap and low voltages are required for utility purpose, the voltage levels goes on decreasing from the transmission system to the distribution system. For this high voltages and low voltages transformer is necessary in transmission and distribution system.
1.2 PRINCIPLE: A transformer is a static piece of apparatus used for transferring power from one circuit to another at a different voltage, but without change in frequency. It can raise or lower the voltage with a corresponding decrease or increase of current.
A Current in the primary winding produces a magnetic field in the core. The magnetic field is
almost totally confined in the iron core and couples around through the secondary coil. The
induced voltage in the secondary winding is also given by Faraday’s law.
1.3 Constructional types:
Constructionally, transformers are of two general types, distinguished from each other merely by the manner in which the primary & secondary coils are placed around the laminated core.
The two types are known as
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1. Core-type
2. Shell-type
In the core type transformers, the winding surrounded a considerable part of the core whereas in shell type transformer, the core surrounds a considerable portion of the windings as shown in fig (a) and (b) respectively.
1.4 Core type transformers:
In the small size core-type transformers, a simple rectangular core is used with cylindrical coil which are either circular or rectangular in form. But for large size core-type transformers, round or cylindrical coils are used which are so wound as to fit over a cruciform core section. The circular cylindrical coils are used in most of the core-type transformers because
Of their mechanical strength. Such cylindrical coils are wound in helical layers with the different layers insulated fromeach other by paper, cloth,micarta board or cooling ducts.
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1.5 Shell-type transformers:
In these case also, the coils are form-would but are multi-layer disc type usually wound in the form of pancakes. The different layer discs are insulated from each other by paper. The complete winding consist of stacked discs with insulation space between the coils the spaces form.
The very commonly used shell-type transformer is one known as berry transformer so called after the name of its designer and is cylidrical in form. The transformer core consists of laminations arranged in groups which radiate out from the centre.
The choice of core or shell –type construction is usually determined by cost, beacause similar charecteristics can be obtained with both types. For very hiogh-voltage transformers or for multi winding design,shell-type construction is preferred by many manufacturers. In this type, usually the mean length of coil turn is longer than in a comparable core-type design. Both core and shell forms are used and the selection is decided by many factors such as voltage rating, kVA rating, weight, insulation stress, heat distribution etc.
1.6 Operation:
When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core.
The changing magnetic field induces an electromotive force (EMF) across each
winding. So the voltages VP and VS measured at the terminals of the transformer are equal to the
corresponding EMFs.
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1.7 No load operation:
If the secondary of a transformer is left open-circuited, primary current is very low. No-load current produces the magnetic flux and supplies the hysteresis and eddy current losses in the core.
The no-load current (IE) consists of two components magnetizing current (Im) and core loss (IH). Magnetizing current lags applied voltage by 90°, while core loss is in phase with the applied voltage. VP and VS are shown 180° out of phase. IH is very small in comparison with Im and Im is nearly equal to IE. No-load current, IE, is also referred to as exciting current
1.8 Loaded operation:
The transformer under loaded condition differs in its characteristics of type of load say resistive, inductive and capacitive loads.
1.9 Transformer on load:
When the secondary is loaded, the secondary current I2 is set up. The magnitude and phase of I2
with respect of V2 is determined by the characteristics of the load. Current I2 is in phase with V2
if load is non-inductive, it lags if load is inductive and it leads if load is capacitive.
The secondary current sets up its own m.m.f. (=N2I2) and hence its own flux Φ2 which is in opposition to the main primary flux Φ which is due to I0. The secondary ampere-turns N2 I2 are known as demagnetizing amp-turns. The opposing secondary flux Φ2 weakens the primary flux Φ momentarily, hence primary back e.m.f. E1 tends to be reduced. For a moment V1 gains the upper hand over E1 and hence causes more current to flow in primary.
Let the additional primary current be I21. It is known as load component of primary
current. This current is anti-phase with I21. The additional primary m.m.f N1 I2
1 sets up its own flux Φ2
1 which is in opposition to Φ2 (but is in the same direction as Φ) and is equal to it in magnitude. Hence, the two cancel each other out. So, we find that the magnetic effects of secondary current I2 are immediately neutralized by the additional primary current I2
1 which is brought into existence exactly at the same instant as I2. The whole process is illustrated in figure,
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Hence, whatever the load condition, the net flux passing through the core is approximately the same as at no-load. An important deduction is that due to the constancy of core flux at all loads, the core loss is also practically the same under all load conditions.
As Φ2=Φ21 Therefore, N2I2=N1I2
1 Therefore, I21=N2/N1×I2 = KI2
Hence, when transformer is on load, the primary winding has two currents in it; one is I0 and the other is I2
1 which is anti-phase with I2 and K times in magnitude. The total primary current is the vector sum of I0 and I2
1.
In figure are shown the vector diagrams for a load transformer when the load is non-inductive and when it is inductive (a similar diagram could be drawn for capacitive load). Voltage transformation ratio of unity is assumed so that the primary vectors are equal to the secondary vectors. With reference to figure (a), I2 is secondary current in phase with E2 (strictly speaking it should be V2). It causes primary current I2
1 which is anti-phase with it and equal to it in magnitude (K=1). Total primary current I1 is the vector sum of I0 and I2
1 and lags behind V1 by Φ1.
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In figure (b) vectors are drawn for an inductive load. Here I2 lags E2 (actually V2) by Φ2. Current I2
1 is again anti-phase with I2 and equal to it in magnitude. As before, I1 is the vector sum of I21
and I0 and lags behind V1 by Φ1.
It will be observed that Φ1 is slightly greater than Φ2. But if we neglect I0 as compared to I21 as in
figure (c), then Φ1=Φ2. Moreover, under this assumption,
N1I21 = N2I2 = N1I1 Therefore, I2
1/I2=I1/I2=N2/N1=K
It shows that under full-load conditions, the ratio of primary and secondary current is constant. This important relationship is made the basis of current transformer-a transformer which is used with a low-range ammeter for measuring current in circuits where the direct connection of the ammeter is impracticable.
1.10 Energy losses in transformer:
Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, termed iron loss.
Copper loss: Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer windings
Iron loss: Core losses or iron losses are caused by two factors: hysteresis and eddy current losses.
1. Hysteresis loss: Hysteresis loss is that energy lost by reversing the magnetic field in the core as the magnetizing AC rises and falls and reverses direction.
2. Eddy current loss: Eddy current loss is a result of induced currents circulating in the core.
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1.11 Emf Equation of transformer:
The voltage developed by transformer action is given byE = 4.44× f × N× Bmax ×Acore
Where E = rated coil voltage (volts),f = operating frequency (hertz),N = number of turns in the winding,Bmax = maximum flux density in the core (tesla), andAcore = cross-sectional area of the core material in Sq. meters.In addition to the voltage equation, a power equation expressing the volt-ampere rating in terms ofthe other input parameters is also used in transformer design. Specifically, the form of the equation isVA = 4.44× f × N× Bmax ×Acore × J ×Acond
Where, N, Bmax, Acore and f are as defined above, J is the current density (A/ sq. mm), and Acond is thecoil cross-sectional area (2mm) in the core window of the conducting material for primary winding. Jdepends upon heat dissipation and cooling.
1.12 Voltage Transformation Ratio (k):
From the e.m.f equation,
VS/VP=NS/NP=k
This constant K is known as voltage transformation ratio.
If Ns>Np i.e. K>1, then transformer is called step-up transformer.
If Ns<Np i.e. K<1, then transformer is called step-down transformer.
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1.13 Losses in Transformers:
The losses in a transformer are as under.1. Dielectric Loss2. Hysteresis Losses in the Core3. Eddy current losses in the Core4. Resistive Losses in the winding conductors5. Increased resistive losses due to Eddy Current Losses in conductors.6. For oil immersed transformers, extra eddy current losses in the tank structure
1.14 Equivalent circuit:
The transformer shown below is an equivalent circuit in which the resistance and leakage reactance of the transformer are imagined to be external to the winding whose only function is to transform the voltage
Where
Vp – primary side voltage Ep – e.m.f induced at primary side
Vs – secondary side voltage Es – e.m.f induced at secondary side
Ip – primary side current Rp – primary side resister
Is – secondary side current Xp – primary side reactance
Io – No load current Rs – secondary side resister
Ic – working component Xs – secondary side reactance
IM – magnetizing component Rc –non inductive resistance
Np – primary side turns XM –pure inductance
Ns – secondary side turns
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1.15 Efficiency of a transformer:
1.16 All-day efficiency:
The ordinary or commercial efficiency of a transformer is given by the ratio
Output in watt / Input in watt.
But there are certain types of transformers whose perfomance cannot be judged by this efficiency. Transformers used for supplying lighting and general network i.e., Distribution transformers have their primaries energized all the 24 Hrs although their secondary supplies little or no-load much of the time during the day except during the house lighting period. It means that whereas core loss occurs throughout the day, the copper loss occurs only when the transformers are loaded. The performance of such is compared on basis of energy consumed during a certain time period, usually a day of 24Hrs.
η all-day = output in kWh/input in kWh (for 24 Hrs)
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2.1 Introduction:
The role of a transformer is to convert high-voltage electricity supplied from a power station into lower-voltage electricity. The transformer is a constant frequency and power device .The primary and secondary are electrically isolated but magnetically coupled with one another .Apart from the cold rolled grain oriented steel and copper many of ferrous, on ferrous and insulating materials are employed in the manufacturing process of the transformer .The manufacturing process undergoes many stages for completion of some specified job.
However here are the steps regarding the process of building up of a transformer.
1. Core manufacturing section
2. Coil or Winding section
3. Core Coil assembly section
4. Transformer tanking section
5. Painting and Finishing section
Hence each of the sections are assigned a set of job to in order to attainmaximum profit and all these jobs are said to carry in parallel way in order to meet higher production levels. The general block diagram for the process of manufacturing is as shown below.
Fig: Manufacturing process sequence
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2.2 Core section:
Here in this section the amorphous metal sheets are first transformed into core as per the design which houses the windings of the transformer. Transformers operate 24 hours a day during which time they undergo constant losses of 2 to 4% of the electricity that passes through them. This loss is divided into two different categories mainly load losses caused by the load on the transformer during the use of electricity and no-load losses caused regardless of whether a load is present. Amorphous core transformers significantly reduce no-load losses by using an amorphous alloy for the iron core, which the transformer windings that carry the electricity are coiled.
Amorphous transformers use amorphous steel, instead of grain oriented steel. This change allows a significant decrease of losses in Distribution Transformer, resulting in energy savings. Metals have a crystalline structure with an orderly arrangement of atoms. Amorphous metal is said to be as metallic glassy structures.
A metal in a liquid state at a high temperature can retain its liquid structure upon solidifying if it is cooled rapidly. The resulting non-crystalline alloy has a random arrangement of crystals and is called an amorphous alloy. Amorphous alloys have excellent strength and electrical characteristics, but require advanced techniques for machining.
There are several other ways in which amorphous metals are produced which include the physical vapour deposition, ion irradiation, extreme rapid cooling and mechanical alloying.
The available ribbons of the amorphous core type material are varied in lengths as 5.6mm, 6.7mm and 8.4mm.
Amorphous Metal typical composition is in such a way that it include the composition of different types of metals and ferrous, boron and silicon forms the major part. This allows building transformers with very low no-load losses. Because of the flexible structure of the core, the capacity of amorphous core transformers is currently limited to 10 MVA. Amorphous core transformers are 5 to 20 % heavier than that of the conventional type of transformer.
2.3 Amorphous metal process:
A proprietary molten alloy of iron, boron and silicon is cooled rapidly at a rate of 1 millionth degree per second such that crystals are not formed. The metal can be drawn very thin (0.025mm-0.05mm) and so exhibit very low eddy current losses.
When the a.c magnetic field is applied the random atomic structure causes less friction and hence less hysteresis loss. However it has less space factor than that of CRGO
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type of core. Space factor is defined as the ratio of core cross section area to the area available for the core. Here the weight and cost is more.
It can be seen that losses in amorphous metal core is less than 25% of that in CRGO. This material gives high permeability and is available in very thin formations (like ribbons) resulting in much less core losses than CRGO. The tradeoff between the both types is interesting. The use of higher flux densities in CRGO (up to 1.5 T) results in higher core losses. However, less amount of copper winding is required, as the volume of core is less. This reduces the copper losses.
In amorphous core, the flux density is less and thinner laminations also help in reducing core losses. However, there is relatively a larger volume to be dealt with, resulting in longer turns of winding, i.e. higher resistance resulting in more copper losses. Thus iron losses depend upon the material and flux densities selected, but affect also the copper losses. The steps in amorphous core manufacturing process are as follows.
1. Core pre spooling
2. Core cutting and Core stacking
3. Core lacing and Core forming
4. Core annealing
5. Core testing and Finishing.
2.4 Core pre spooling:
The core pre spooling process is that the thin ribbons of amorphous metal are joined together to improve the mechanical strength. However the amorphous ribbons are available at the thickness of 0.025mm.This thickness should not be exceed because the metal forms a crystalline structure leading to increase in losses which is said to be undesirable.
Even though the thickness cannot be increased the net cross sectional area can be increased. The widths available in the market are 142.23mm, 170.18mm and 213.35mm.
Since the design proposed in the report is single phase 15KVA the width chosen is 142.23mm.The width for higher ratings that is 200KVA to 3MVA is to be as 213.35mm.
In the process of manufacturing the thin ribbons of core are joined to form a core of thickness 250 micro meters. This process of joining the ribbons of amorphous metal to increase the thickness is called core pre-spooling.
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2.5 Core Cutting and Core Stacking:
2.5.1. Core Cutting:
Core cutting is a process in which the core sheets after pre-spooling are cut to length as per the design of the transformer. The cutting is done in such a way that the first layer has more length than last layer. This is because, when the core layers are bent to form the rectangular shaped core, the outer layer must have more diameter than inner layer. This can be obtained only when the first layer has more length than last layer.
2.5.2. Core Stacking:
Core stacking is the process in which the layers after cutting will be arranged in such a way that the layer with long length is at the top and the layer with the short length is at the bottom. This arrangement allows the manufacturer to make a rectangular core with rounded edge.
2.6 Core lacing and Core Forming:
2.6.1. Core Lacing:
Core lacing is the process in which the stacked core is bent in the shape of rectangle with rounded edge. After the formation of rectangular core, one side of the core will have more thickness than the other three sides .This is because the stacked core from the two legs will be joined on this side .This joining of stacked core on one side of the rectangular core is called core lacing.
2.6.2. Core forming:
After lacing, CRGO SHEETS are added to the inner and outer loops of laced core to provide mechanical strength .Also plates of mild steel are added on four sides of the core and are fixed by a package wire. After this process the core finally looks as a single piece. This process is core forming.
2.7 Core Annealing:
Annealing is a process of heating and subsequent cooling gradually. During the process of prespooling, cutting, stacking, lacing, forming, handling etc mechanical stresses are developed inside the lamination that disturbs the original randomness of atoms inside the core and there by increases the iron loss. Also there is a possibility for the presence of moisture.
This problem is eliminated by annealing the laminations in roller hearth furnace are batch annealing furnace at the temperature of 810 degrees centigrade with a tolerance of 10 degrees preferably in a neutral atmosphere zone and subsequently cooled by blast air.
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2.8 Core Edge Finishing:
It is the process in which the cooled core is painted with epoxy paint not only serves as a insulation material but also used for joining the layers of core. Finally the mild steel plates are removed and the core is sent to core coil assembly section.
2.9 Winding Section:
Winding forms the electrical circuit in the transformer. These produce magnetic flux in the core .These are mainly divided into two parts one is primary winding other is secondary winding .In terms of voltages we classify them as LV winding and HV winding .In the positioning of HV and LV windings with respect to the core is also important from the point of view of insulation requirement. The LV winding is placed nearer to the core in case of concentric windings and on the outside positions in case of sandwiched windings on account of easier insulation.
For core type transformer the windings are cylindrical and are arranged concentrically. For shell type the coils are usually rectangular .The conductors used in transformer windings may be Cu or Al. Small transformer with aluminum is cheaper when compared to Cu winding .However with the increase of rating and voltage the transformer with Cu winding is much cheaper in overall cost as compared to Al winding. Commonly the following types of windings are mostly employed.
2.9.1 Single Coil Winding:
In this winding first the LV is wound on the mandrel and then the HV is wound on LV winding with some of the layers of insulation between them, in such a way that both the windings form a single coil. This type of winding is generally used in amorphous core distribution transformers.
Fig: windings of the transformer
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The sequence of manufacture of single coil winding is given below
Loading of modules on the winding machines
Loading of the conductor reels on stands
Dressing of mould i.e., assembly of insulation spacers and blocks on the mould
Manufacture of the winding on horizontal or vertical winding machines depending on design ,number of conductors to be handled at a time and the type of conductor .
Preparation of leads
Dismantling of winding from the machine
Preparation of joints between the conductors.
After removing the windings from the winding machine, each winding is clamped between the top and bottom plates through tie rods a kept in an Owen for heating. The windings are individually shrunk to require axial dimensions by heating in the steam heated Owens and by applying the required pressure. Heating ensures removal of moisture from the insulation items and this proceed is called stabilization of windings.
2.10 Core Coil Assembly (CCA):
CCA consists of assembling the windings to the core .Core from the core section is moved to CCA .Now the core is unlaced on one side and the LV/HV coil is placed on the limb of the core .Bottom insulation items like press board rings ,petals ,perms wood rings are placed on the bottom Yoke on each core limb.
Fig: The core coil assembly section
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Windings are lowered on the core limb in such a way that LV leads should come to the LV side of the core channels and HV leads should come to the HV side of the core channels. After placing the coils in the position the core which is unlaced is replaced and welded. Insulation items like fiber tubes, empire sleeves and top insulation arrangement is completed. During insulation assembly, various leads are properly positioned and fully insulated phase barrier assembly is then completed. Top yoke laminations are assembled back in positions starting with centre step top core channels are then clamped and tightened .Coils are kept under pressure by tightening tie rods.
2.11 Fabrication of Transformer Tank:
All the transformer tanks are made of high quality mild steel and can withstand vacuum as specified by international standards and the customer. The tank is provided with the pressure releasing devices so that the oil in the transformer tank should not burst in extreme conditions.
The oil level is also found out through the help of oil level gauges. The provision for the circuit breaker is also given depending on the customer specifications.
All the welds are tested in such a way that it assures 100 percent leak proof. To provide mechanical strength these are provided with the cold rolled steel fins. The height of the fin is according to the customer specifications. All the transformer tanks are given a smooth finishing by using the shot blasting process.
2.12 Painting and finishing:
This process is done in 3 stages as follows
2.12.1 Cleaning of tanks:
The cleaning of tank is done by chipping/grinding.
The outside is short blast to achieve fine and smooth finish.
2.12.2 Painting of tanks:
After cleaning a coat of hot oil resistance paint say varnish is applied on the internal surface of the tank.
The external surface is coated with red oxide and subsequently by enamel
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2.12.3 Finishing:
Fittings and accessories as per customer's specification and drawing are checked.
Air Pressure test is subjected to avoid any leakage on all transformers.
Transformers are filled with oil up to the minimum level marking, wherever necessary.
2.13 Tanking:
Tanking means assembling the core coil-assembling job into transformer tank with all necessary accessories according to bill of material and design diagram. Here are the stages that undergoes during the tank up process.
First the CCA jobs are loaded into preheating oven and kept for 8 to 10 hours at a temperature setting of 110°c for removal of moisture.
Then the jobs are tested for insulation check up using 2000 volts megger and then tightened.
After this job o.c test is applied through 25% of the rated voltage on LV side on each phase, to check the inter turns shorting if any.
Then fittings like HV & LV bushings drain valves, oil level indicator and explosion vents are fitted to this tank.
Purely filtered transformer oil is then filled in the transformer tank and this oil is maintained at the temperature of 60 degrees centigrade to avoid further moisture check up.
This completes the tank up process and rest of the procedure is carried out by the testing department.
Hence in this way the manufacturing process undergoes the above series of steps and the transformer is made to dispatched to the required customer by the maintenance department.
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2.14 comparison in properties among CRGO and amorphous materials:
SL.NO PROPERTIES AMORPHOUS METAL CRGO STEEL
1. Density 7.15 7.65
2. Specific resistance 130 45
3. Saturation flux density 1.56 2.03
4. Typical core loss Watt/kg at 50 Hz 1.4tesla
0.20 0.90
5. Thickness 0.025 0.27
6. Space factor 0.80 0.97
7. Brittleness Higher Down
8. Available from Ribbon/foil
Standard sizes
106mm,142.2mm
170.2mm,213.4mm
Sheet / roll
9. Annealing temperature 360 8/0.1
10. Annealing atmosphere Inert gas Inert gas
11. Special annealing requirement1
Magnetic field annealing -----
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3.FAILURE OF DISTRIBUTION TRANSFORMERS
3.1 Introduction
DT (Distribution Transformer) is a static device without any rotating or moving parts and is
considered as most rugged device among all electrical equipment & should be least prone to
failure. Unfortunately for various reasons, the rate of failure of DT is high (20 to 30% annually)
in a number of Indian Power Utilities. This chapter examines the factors responsible for high
failures and steps to be taken to minimise it.
3.2 Factors responsible for failure of DT
The factors responsible for failure of DT can be broadly classified into three categories namely
manufacturing defects, Utility defects and natural calamities.
3.3 Design Aspects
3.3.1 Tank Size: Inadequate clearance to free circulation of oil in many Makes of Transformer,
have led to abnormal temperature rise, causing sufficient damage to the HV winding insulation
and consequently premature failure of transformers. Therefore Tank size and Quantum of oil
should be adequate.
3.3.2 Percentage of Impedance (Mechanical Strength of Coil): Most of the Distribution
Transformers are located in remote areas and special attention cannot be given to the operating
conditions of the transformers. The solution to this problem is to design transformers with
increased impedance to increase the short circuit withstand capacity of the transformers.
The Percentage of impedance depends upon 2 factors namely Size of Wire used in HV Coils and
Radial Distance between HV & LV Coils.
3.3.3 Size of Wire used in HV Coils: Economical size of coil will yield lower size gauge wire,
but this will reduce mechanical capability of coils as a result the coils may not be able to
withstand higher current densities which occur during the short circuit conditions.
3.3.4 Radial Distance between HV & LV Coils: Increasing the radial distance between HV and
LV coils increases the percentage of impedance and this will lead to higher cost. But this will
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lead to better mechanical strength of the coil to withstand higher short circuit stresses developed
during short circuit conditions.
3.3.5 Effect of Impedance on the Short Circuit Stresses: The short circuit stresses are
proportional to the square of the short circuit current. The impact of keeping the impedance at
4.5%, 5% & to 5.5%, on the short circuit stresses developed in the transformer is illustrated here.
3.3.6 Improper Use of Aluminium Wires: This leads to HV Coil failure. REC recommends use
of Aluminium conductors for windings upto 200 kVA transformers. Use of higher cross section
wires with paper cover insulation instead of enameled wire may be specified, the super enamel
covering aluminium wire tends to crack during asymmetrical condition that leads to coil failure
and this can be avoided with double paper covering conductor.
3.3.7 Improper Use of Inter Layer Papers: Major portion of coil failures are seen as electrical
failures. These electric failures occur once when interlayer insulation breaks down or at the end
of the turn and creeps to the next layer. This type of insulation failure can be avoided by using
folding papers and reinforcing the end turn insulation with proper sleeves Further HV coil, has to
be separated uniformly along with the inner coil spacers so as to avoid pressing of only end turns
as well as to avoid any further shrinkage during service.
3.4 Improper Workmanship
Improper Alignment of HV Windings: When the transformer is loaded, the primary and
secondary ampere turns acts in magnetic opposition with respect to the core and coils and the
space between these are magnetically excited. In case of a small error in the alignment of either
of the coils, an asymmetrical ampere turn balancing, leads to production of cross fluxes resulting
in mechanical failure of the coils.
3.4.1 Improper Clamping Arrangement: Inadequate clamping arrangements of the HV -Coils
lead to vibration and move the coils during short circuit conditions resulting in failure of HV
Coils.
3.4.2 Improper Connections: In many cases the connecting delta leads to the bushing are not
properly supported on the frame work, resulting in breaking during transhipment or at the first
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charge. Improper soldering of leads will result in open circuit due to even normal full load
conditions also such transformer may fail during encountering the first fault or a few faults.
3.4.3 Inadequate Tightening of Core: Even with proper fuse protection on the HV side,
inadequate tightening will result in failure of transformer due to collapse of the windings. Even
under minor fault condition in the LT distribution, due to mechanical vibration in the core and
windings due to the above fault, the transformer will fail.
3.4.4 Continuous Over Heating and Higher No Load Losses: In addition to normal full load,
continuous over heating and higher no load losses may reduce the life of the transformer, due to
reduction in life of insulating papers, oil etc.
The quantity and grade of input materials of core and windings furnished in the tender may be
verified with the design calculation of the bidder and this will ensure that the losses and
impedance furnished in the tender would be achieved in the transformers.
3.5 Operational Defects:
The following operational defects cause failure of DT.
3.5.1 Over Loading of Distribution Transformer: Inadvertent connecting of extra load more
than capacity of DT, unauthorized load can be identified by periodical testing of current in
distribution transformer using Tong Tester at peak hours and other times of the day. Transferring
the load to the near by transformer or enhancement of the existing transformer capacity or
proposing a new transformer will avoid transformer failure even though total load may be within
limits of DT capacity. Unequal loading in three phases may also cause over loading in one phase.
This also can be detected by Tong tester readings. Redistribution of the loads will avoid
transformer failure.
3.5.2 Usage of Improper Size of HG Fuse: HG fuse is the only main reliable protection for the
DT other than CSP type under condition of fault in LT distribution. The importance of HG fuse
protection can be appreciated considering the following facts
The most reliable protection for the transformer is the HG fuses only as the LT fuses
being of a heavy size for dealing high currents on the LT side, the chance of LT fuses
blowing in short time is less.
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Higher size HG fuses will sustain the faults for longer period causing damage leading to
failure.
Usage of proper size HG fuse will minimise damage in case of fault within the
transformer.
3.6 Improper Maintenance
3.6.1 Tree Branches Touching LT Lines: Tree branches touching LT Lines will reflect as high
impedance earth fault, resulting in sustained over loading off resulting in failure. It may result in
conductor snapping or cracked LT pin insulators. This in turn causes heavy earth fault current
which may lead failure, if protective devices do not act promptly.
3.6.2 Tree Fouling on HT Lines: This may cause failure of transformers due to flow of earth
fault current since the primary of all the transformers are delta - connected and all the 3 windings
from cluster of transformers connected to this HT distribution line will feed the fault apart from
the source of substation.
3.6.3 Non Maintenance of Breather: Non provision of fly-nuts for the breather container will
create a gap through which moisturized air wilt enter into the transformer tank. To arrest this
gap, neophrine gaskets are to be provided instead of rubber gaskets. Rubber will damage, if it
comes in contact with transformer oil. If oil is not filed in the breather, then the dust particles
will not be absorbed from the air entering the transformer tank causing transformer failure.
3.6.4 Oil Leak in Bushings or any Other Weak Part of the Transformer: Oil leak may be
due to excessive heating or pressure developed in the bushing. This will bring down the oil level
in the tank and also moisture will find access into the tank through the aperture from which oil is
oozing. Because of this oil in tank will be contaminated resulting in deterioration of HV/LV
insulation and ultimately to transformer failure. To avoid these bimetallic clamps with proper
size of bolt and nuts connected to the LT bushing will reduce excessive heating and damage to
bushing rods.
3.6.5 Low Oil Level: The non-visibility of the oil level in the gauge glass level due to
accumulated dust may not show the exact level in the tank. The oil is likely to go below the core
level, the jumper wire from core winding assembly to the bushing rod will not be covered with
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oil, this leads to excessive temperature rise and the failure of inter turn insulation and flash over
the windings.
3.6.6 High Oil Level: The Oil should be filled upto the marking in the conservator tank. There
should be space in the conservator tank for expansion of oil when the transformer's loaded, if the
conservator tank is filled with oil, then the transformer may fail due to high pressure by
explosion of vent pipe.
3.6.7 Low BDV of the Oil: Oil contamination may cause the BDV of the transformer oil to be
very low. This results in the increase of the carbon content and decrease of resistance in the oil.
The temperature of the oil will not be reduced and hence aridity of the oil increases resulting in
deterioration of insulation of the windings and transformer failure.
To avoid this oil has to be filtered to remove the dust and processed through the reclamation
plant to reduce the acidity and improve the BDV value of the oil.
3.6.8 Low IR Value: This may be due to moisture content in the oil and in the winding
insulation. This may cause transformer failure. To avoid this, the transformer core with windings
should be placed in Hot air chamber till the moisture content is removed from the core and
windings insulation.
3.7 Due to Natural Calamities:
3.7.1 Heavy Lightning: If HT LA'S fails to divert the direct stroke or surges due to discontinuity
in earthing system, this will result in either failure of HV winding due to surge voltage or
bursting of HT lightning Arrestor itself.
3.7.2 Bushing Flash Over: Dust and chemicals carried away with air and deposited on the
bushings will reduce the electric leakage distance causing flashover. To avoid this, clean the
bushings (both HT & LT) periodically with banian waste. If cotton waste is used for cleaning
this may cause scratches in the bushing & subsequently leading to flashover to bushing.
3.7.3 Failure due to Bird Fault: To avoid failure of DT due to squirrel crossing the DT or due
to the birds sitting on the transformer, the HT/LT bushing and HT/LT jumper leads from the
bushing may be covered with yellow tape insulation. This yellow tape insulation will also
23
indicate the overloading phase of the transformer by the colour of the tape changing from Yellow
to Black.
3.8 Determining Cause of Failure
The determination of cause of failure of DT is important to plan strategy for reduction of failure.
But concluding cause of failure from preliminary reports and maintenance record of field is
difficult for various reasons. The best method to arrive at causes in by examination of damages
caused in failed units. The Table gives the probable cause of failure based on damages observed
on failed unit.
3.8.1 Determination of Cause of Failure
SI. No.
Damages Found in Examination Probable Cause of Failure
1. Top coils in all 3 phases got charred and all coils below them are 6,K
Lev.- oil level
2. Internal Jumpers got cut Bad soldering and/or surge
3. End coils got damaged Severe external short circuit not cleared
by fuses in. time
4. Insulation on all coils in all phases turned brittle
Obvious case of continuous overloading
5. Inter turn short Poor insulation due to:
- Ageing
- Bad Oil
- Moisture
- Continuous overloading, etc
6 Puncturing of Coils
7. Strips forming L.V star point got sheared Unbalance in loading coupled with bad earthing
8. L.T Coil damages Insulation failure on LT, repeated test
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charges as fault
9. Moisture inside transformer Bad breather maintenance, loose top gaskets, puncture on vent pipe diaphragm
10. High magnetizing current and flash to core Core bolt insulation failure
The damages responsible for failure of DT in a utility are listed in Table
3.8.2 Damages Responsible for Failure
D. Ts. Failed Due to Failure
Damage to HV Coils 65%
Damage to LV Coils 06%
Damage to Both 10%
Other Reasons 20%
4.COMPLETE SELF PROTECTION TECHNOLOGY FOR HIGH PERFORMANCE DISTRIBUTION TRANSFORMERS
4.1 INTRODUCTION
25
The high rate of failure of secondary distribution transformers in power systems may
perhaps be described as one of the tragedies of distribution system management of present times,
especially in developing countries like India.
The advent of CSP technology has encouraged progressive manufactures to go in for high
performance distribution transformers which mitigate the operation and maintenance problems
associated with conventional transformers.
4.2 Why do Distribution transformers fail in such large numbers?
Every year distribution transformers worth nearly 200crore rupees fail in power
distribution companies in India. The average period before a new distribution transformer comes
back to repair shop is estimated to be a mere 3-4 years. Even a conservative estimate puts the
failure rate at over 20% compared to less than 1-2% in many utilities in advanced countries.
Why should these simple, static, silent and efficient pieces of electric equipment fail in
such large numbers causing enormous loss to electric utilities?
The reasons are not far to seek.
Distribution transformers in rural areas form the bulk of the transformers in service. They
are very much exposed to changing weather conditions and more dangerously to lightning.
Distribution transformers in low load density rural areas feed lengthy low voltage lines
which are themselves prone to faults, not merely because they are with, bare conductors and in
exposed environment but ate also carelessly constructed.
Faults on low voltage lines constitute considerable means to distribution transformers.
Short circuits caused on account of clashing of loosely strung LV lines and high impedance
faults where bad tree conditions exist are typical faults, small is beautiful where LV system is
concerned since we have lesser loop resistance to contend with and protective gear operates
positively.
Overloading of transformers without looking into their overload capacity is another
reason for early failure. It has become the practice to connect additional loads on the basis of
maximum demand recorded at some point of time without reference to seasonal variations and
26
assuming unrealistic diversity factors. Unauthorized loads result in unforeseen overloading.
Periodical checks are not made for overloading and corrective measures are not taken.
Wide variation in load levels and ambient temperature makes undesirable breathing and
ingress of moisture even more intense in the case of rural distribution transformers. The
interchange of air brings oxygen from the atmosphere into contact with oil it is well known that
moisture weakens the dielectric strength of oil to form sludge and finally causes a deposit to
form on the windings. The deposit may in time be sufficient to obstruct the ducts placed in the
windings for the purpose of oil circulation resulting in temperatures higher than those for which
the transformers are designed. Ultimately the insulation of the winding may become carbonized
to such an extent as to cause failure. The dehydrating breather is more often than not in a
deteriorated condition to be of any use for want of timely check and reconditioning/replacement
which is not practicable because of the ever increasing number of these transformers in the
distribution network.
4.3 Unreliable protection in conventional transformers:
Power utilities in India provide horn gap fuse (HG Fuse) on primary side and a rewirable fuse with fuse holders set on LV side of a distribution transformer for system protection.
4.4 The conventional protection system has the following demerits:
Both the HV and LV fuse sets provided externally are exposed to all weather conditions like wind and rain. They become mechanically weak very soon and blow frequently.
27
They are vulnerable for tampering by eager consumers especially in rural areas who would replace the blown fuses with the available fuse wires mostly with higher size wires which cannot protect the transformer from over loads.
4.5 CSP TECHNOLOGY
CSP Technology shows the way out of this distressing situation. Unfortunately, the
advantages of CSP Technology are yet to be fully appreciated by a majority of power utilities in
developing countries.
CSP technology enables a transformer to protect itself from faults.
The transformer is protected from persistent overloads not cleared by conventional
protective gear and causing dangerous temperature rise.
The distribution system to which it is connected is protected from a transformer that has
failed. The faulty transformer is isolated and only consumers served by the transformer
are affected.
Uneconomic overloading on a regular basis is avoided.
Protection from lightning is most effective with the surge arrester mounted to the
transformer tank and directly connected to the HV bushing, reducing to the minimum the
impedance of the ground connection.
The transformer is completely sealed. There is no scope for ingress of moisture and
pilferage of oil which is very common, with disastrous consequences. The space above
the oil level is filled with nitrogen. The volume of the space above the oil level is not less
than 55% of the volume of oil. Thus-expansion of oil is taken care of as well as the
condition of the oil. A pressure relief device takes care of undue pressure rises which
should be rare.
4.6 Components of the CSP System
CSP System has essentially three components. They are:
Primary Fuse: Internal expulsion fuse (other than oil filled) for system protection.
Secondary Circuit Breaker: for overload and secondary fault protection. Signal Light,
The Emergency Control, Magnetic trip.
Surge Arrester: for lightning protection.
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4.6.1 Primary (High Voltage Fuse)
Power Utilities in India provide a HG Fuse on primary side of a distribution transformer
for system protection, it has the following demerits.
It is exposed to wind and rain and becomes mechanically weak very soon and blows
frequently.
It is vulnerable to tampering by eager consumers especially in rural areas who would
replace a blown fuse, with the available fuse wire.
It is not ensured that it does not blow for secondary faults and inrush current
surges.
Ideally the primary side fuses for an outdoor for distribution transformer should be
internally mounted (tamper proof) and the rating is determined on the basis that it should not
blow for secondary faults and exciting current surges. British Electricity Authority have found
from experience that when the fuse is rated to stand 12 times the full load current for 10ms it
meets the requirement.
29
In a CSP transformer, the primary fuse which fulfills the above requirement is placed in
series with the primary winding. This fuse is normally mounted inside of the primary bushing
and is connected via a terminal block to the high voltage winding.
The purpose of this expulsion fuse is to protect the part of the electrical distribution
system, which is ahead of the transformer from faults which occur inside of the distribution
transformer. If a fault occurs in the windings or some other part of the transformer, it will cause
abnormally large currents to flow and the flow of these currents will cause the fuse to melt open
and clear the circuit. In this way, the fault is limited only to those customers who are served by
this particular transformer and service is maintained on the rest of the system. When this type of
fault exists, the transformer is no longer usable and must be removed from service for repair.
Any fault ahead of the transformer will not be seen by any of the transformer's internal protective
devices and will have to be cleared by some other protective device upstream from the
transformer.
4.6.2 Secondary (Low Voltage) Circuit BreakerThe low voltage circuit breaker is the central component of the CSP protection package.
It is this circuit breaker which provides the entire over current protection to the transformer. In
order to perform this critical function its thermal characteristics and the time response to the
thermal changes must match those-of the transformer.
4.7 Thermal Protection of the Transformer
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The average temperature of the transformer winding at any time, is given by the average
oil temperature plus the average winding temperature rise due to the instantaneous load current.
In general, these will be or maximum value of average winding temperature which should not be
exceeded if the transformer is to function satisfactorily over its normal product life. One of the
functions of the circuit breaker is to make sure that this predetermined value of average winding
temperature is not exceeded.
Maximum oil temperature could also be the limiting constraint. In many cases oil
temperature limits are established recognizing the inflammability of insulating oil and these can
be the limiting thermal parameters (instead of average winding temperature) in certain
transformer designs.
The CSP circuit breaker in order to be universally applicable to all transformers and all
thermal constraints, has protective characteristics which are sensitive to the same thermal inputs
as the transformers.
4.8 Secondary Fault Protection: The Other Important Function of the CSP Circuit Breaker
The CSP circuit breaker will respond to secondary faults external to the transformer by
tripping open, and in most cases, this action will prevent any thermal damage occurring to the
transformer. This feature is particularly important for the installations where un-insulated
Secondary distribution and service lines are used. The use of bare conductors increases the risk
of faults especially in areas where there is large growth of trees and vegetation.
If the circuit breaker does trip in response to even a temporary secondary fault,
service can be restored easily by clearing the fault and reclosing the circuit breaker.
When the simple action of reclosing the CSP circuit breaker is compared to the action
required in the case of a non CSP transformer where either a primary fuse or secondary fuse
must be replaced, the benefit of CSP technology is apparent.
4.9 Constructional Features
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For all that it does the circuit breaker is of relatively simple construction. It is an electro
mechanical device with three major elements. These elements are:
Temperature sensing,
Latching and tripping,
Current interrupter.
The temperature sensing function is accomplished through the use of bimetallic strips
which are built into the breaker such that the load current flows through them.
The circuit breaker is mounted inside the transformer, so that these bimetallic strips are within
the top layer of the transformer in this way, the critical thermal modeling of the transformer by
the circuit breaker is accomplished because the bimetallic strips are responding thermally to the
temperature of the transformer oil and also to the temperature changes created by the flow of the
load current through them.
The latching and tripping functions of the circuit breaker are carried out with an assembly
of parts quite similar to those used in industrial type air circuit breakers. Other features that are
built into the latching and tripping functions are
The signal light latch,
The emergency control assembly,
The magnetic trip device.
The last major element of a circuit breaker is the current interruption element. The
current interruption element consists of copper carrying parts plus a set of copper tungsten
current interrupting contacts, once the "hold-close" latch is released the contacts spring open and
interrupt the circuit.
4.10 The Signal Light:
A signal light is mounted on the wall of the transformer tank. It gives a visual external
indication that the transformer has reached a specified level of overload and overload duration at
least once, and thus alerts the power utility about the need to change out transformer for a longer
size in time.
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SIGNAL LIGHT
The signal circuit is mechanically connected to the circuit breaker latching and bimetal
systems through an auxiliary contact. The signal light circuit consists of an auxiliary transformer
winding (one turn) which generates about 3 volts and signal light contacts set within the circuit
breaker. Signal light is mounted on the wall of this transformer tank.
The signal light contacts will close at a preset thermal condition. This occurs before the
main latching system opens the main contacts.
The signal light mechanism does not reset itself when the load drops off. The signal light
remains lighted once the signal light contacts close and can only be turned off by manually
operating the external handle of the circuit breaker. However if the overload has persisted the
signal light will relight as soon as the operating handle is restored to its normal position
indicating the need for a larger size transformer.
4.11 The Emergency Control:
This device is provided when the power utility wants the facility of immediate restoration
of service in an emergency by closing the circuit breaker even when the preset overloading limit
is reached.
Once the emergency control is activated the circuit breaker is no longer thermally
protecting the transformer and significant insulation deterioration can occur if these high loads
reoccur.
Once it becomes necessary to activate the emergency control, the power utility should
plan to change out the transformer for a larger size as soon as possible.
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The emergency control linkages can be externally activated to increase the amount of
engagement of the main and signal light latches within the circuit breaker which has the effect of
requiring more bimetal strip movement to trip the circuit breaker open and more bimetal
movement requires higher temperature.
4.12 Magnetic Trip:
Certain circuit breakers are furnished with an instantaneous magnetic trip element in addition to
the standard bimetallic thermal trip element. The magnetic trip element increases the opening
speed of the circuit breaker under high fault current conditions. This increased opening speed
permits the circuit breaker to interrupt larger values of fault current than would normally be
possible. The response of the circuit breaker to thermal activity is unchanged by the addition of
the magnetic trip element.
4.13 Primary Fuse Vs Secondary Breaker
One of the most important design tasks which are done by the CSP transformer design
engineer is the coordination between the primary fuse and the secondary circuit breaker as
mentioned earlier, in performing this coordination task, the design engineer must use the
minimum melt time current, characteristic curves of the primary expulsion fuse and the average
clearing time current characteristic curves for the CSP Circuit breaker. Coordination should be
such that the circuit breaker clears the circuit for any fault on the load side of the transformer
before the primary fuse melts. In order to achieve this coordination, the calculations are made for
the worst case.
34
The maximum secondary current that can flow under any fault condition is that current
created by a bolted fault on the secondary terminals of the transformer. Usually, when this
calculation is made, an infinite bus is assumed on the primary side of the transformer and the
transformer's own impedance is taken as the only current limiting impedance. Coordination is
achieved by selecting the expulsion fuse's minimum melt curve and the circuit breaker's average
clearing curve so that under this worst case situation, the circuit breaker will clear the circuit
without the expulsion fuse melting.
If the coordination is not properly done, the expulsion fuse can melt when the fault is on
the secondary side of the transformer thus bypassing the protective function of the circuit
breaker. When coordination is properly done, the melting open of the primary fuse, generally,
can only occur when a fault is inside the transformer. When this type of fault occurs, the
transformer is no longer usable and must be removed from service and taken to a repair shop. If
the fault had been on the load side of the transformer, the circuit breaker would have interrupted
the circuit.
Of course, any fault ahead of the transformer will not be seen by any of the transformer's
internal protective devices and will have to be cleared by some other protective device upstream
from the transformer.
4.14 SURGE ARRESTER
The closer the surge arrester can be mounted to the transformer, the shorter will be the
ground lead connection between the arrester and the transformer. The shorter this connection, the
less will be the lightning surge induced voltage stress on the transformer winding. When the
35
surge arrester is mounted directly to the transformer tank (as in the case of CSP transformer) the
ground lead length is effectively zero and maximum transformer protection is obtained.
4.15 How CSP Technology Facilitates Optimum use of the Transformer's Capability?
In the case of cyclic loads (containing peaks and valleys) where peak load is of relatively
short duration, transformers considerably smaller than the peak loads can be safely installed
without any concern for rapid loss of transformer life due to overload. The CSP Circuit Breaker
will permit the transformer to function within cyclic loads up to the point where the amount and
duration of the peak load begins to cause significant loss of transformer life. When this point is
approached, the signal light will light with the first indication that the loads on this particular
transformer have grown to the point when significant insulation deterioration can occur.
With the signal light indication mentioned, several options are open to the power utility.
They are
A change out of the transformer for a larger size can be planned for at a future convenient
date.
The signal light may be reset to determine if it will light again indicating that the
overload condition has become a normal load condition at this site and then a change out
can be planned-
Nothing can be done except to wait and see if the breaker itself will trip open at some
future date.
If nothing is done, and the load continues to grow at the location, eventually a condition
of peak load and load duration will be reached which will cause the circuit breaker to open. At
this point the lineman from the power utility must be sent to the transformer location in order to
restore electric service to these customers.
Several courses of action are open now.
1. The transformer can be immediately changed out for a larger size.
2. If it is not possible to change because of time of day factor and availability of personnel
for change over, it may be possible to close the circuit breaker manually and restore
service and then plan a change out.
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3. Not plan a change out and see if the load reaches these levels again and causes the circuit
breaker to trip open.
4. Finally, it may not be possible to close the circuit breaker at all because the connected
load has not dropped off and the circuit breaker will trip open as soon as it is closed.
The CSP circuit breaker thus provides several types of warning to the power utility of the
existence of overload conditions long before it becomes mandatory to change out the
transformer.
To overcome the problem indicated at (4) and to permit the power utility to rapidly restore
services without having to perform an immediate transformer change out, most CSP transformers
contain the emergency control element.
It is these early warning features which permit the power utility to effectively plan its
transformer loading so that maximum use of the transformer's capability is obtained without
sacrificing significantly the life expectancy of the transformer.
4.16 Benefits of CSP technology:
Lower Installed Cost: Less external mounting arrangement and connections there is no
need for separate mounting arrangement for primary fuse, surge arrester, low voltage
circuit breaker and connecting leads.
Less time for Installation: A non CSP installation takes twice as long as a CSP
installation.
Easier and Simpler Installation: Less external connections and spacing for electrical
clearances. Transformer, surge arrester, H.V.Primary Fuse and secondary circuit breakers
are one compact unit.
4.17 SAFER OPERATION
When a distribution transformer becomes severely overloaded, the temperature of the
insulating oil becomes dangerously hot. A non CSP transformer which is protected by fusing can
reach excessive oil temperatures before the fuse operates in response to the flow of overload
current. By the time the fuse does operate, not only is the oil very hot, but the transformer's solid
insulation has been severely damaged. When the primary fuse finally does operate, service must
37
be restored quickly, and safely. One procedure commonly used is to send the lineman to the
location. The lineman inspects the installation for any obvious secondary faults and finding none
places a new fuse in the cut out to restore service. If the fuse operates again, he replaces it and
tries again. In some cases there were failures causing harm to person (s) and property. The
chances of this kind of failure occurring can be reduced significantly in the CSP transformer
because the circuit breaker provides the type of protection which will prevent excessive oil
temperatures and/or severe damage to the transformer insulation system, When severe failure
does occur within the CSP transformer, the internal primary fuse operates - operation of this fuse
is a signal to the lineman that a severe fault has taken place and the transformer must be
replaced. There is no provision in the CSP transformer installation for the replacement of any
primary side fuse because the fuse operates only when the transformer itself has been damaged.
4.18 More Reliable Service
The early warning feature of the CSP transformer via the signal light helps the power
utility to increase the reliability of the electrical service which it provides to its consumers.
In the case of the non CSP transformer installation, there is no early warning of
increasing load on a particular transformer. The load will Increase until either the transformer
completely fails or the cut-out fuse operates. When this happens, the consumer is suddenly
without electric service, generally during peak load time, and a lineman must be sent out to try
and restore service and alert the power utility to the potential overload problem at the
installation. As stated before, once the potential load problem is identified, it can be corrected on
a planned basis through planned transformer change out. A planned transformer change out
creates much less of a problem for the consumer because the consumer can be informed of the
time of change out, the consumer will be without electric service for a very much shorter period
of time and the change out can be scheduled for a time of day when the demand for electric
power is minimal.
If the transformer has been severely damaged, the serviceman must call out
repair over to replace the transformer which will create a service outage of several hours
duration.
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The CSP transformer, as load builds up, will light, the signal light and alert the power
utility to the potential overload problem at the installation. As stated before, once the potential
load problem is identified, it can be corrected on a planned basis through planned transformer
change out. A planned transformer change out creates much less of a problem for the consumer
because the consumer can be informed of the time of change out, the consumer will be without
electric service for a very much shorter period of time and the change out can be scheduled for a
time of day when the demand for electric power is minimal.
Conventional Transformer CSP Distribution Transformer
cleaning of bushings and external surface of tank cooling pipes
As and when necessary
As and when necessary
Checking of oil levels in the conservator and gauge glass
Monthly not necessary
Checking of Silica gel in the breather and replacement if necessary
Monthly not necessary
Checking of H.G. Fuses and L.T Fuses and renewing with correct gauges if necessary
Monthly not necessary
Checking of vent pipe diaphragm Monthly At the time of other checks
Checking of loose terminal connections if any and tightening the same
As and When necessary
As and when necessary
Checking for any oil leaks and rectification (including replacement of oil seals if required)
Replacement of oil seals does not arise as there is no breather. The transformer is hermetically sealed and ingress of air & moisture affecting oil is ruled out.
Tanking long tester reading during peak load hours and remedial action whenever load exceeds 80% rated capacity
Quarterly The CSP Transformer method of optimizing loading of individual transformers relies upon the information provided by the
39
signal light. Normally the signal light is calibrated to operate when the temperatures effect of the instantaneous toad current plus the steady state oil temperatures as above 80 percent of the value which will trip open the circuit breaker.
Noting down the neutral currents and load balancing in all the three phases
Quarterly Quarterly
Measurement of 1R Values Half yearly Half yearly
Testing of Oil for 80V and acidity Quarterly Not necessary
Checking of Surge Arrester and replacement if required
Preferably be-fore monsoon
Preferably before monsoon
Measurement of earth resistance, checking of earthing system, and rectification if required
Half yearly Half Yearly
Overhaul of transformer once in 5 years once in 5 years
Automatic Load Management: The CSP Signal light at each transformer provides
information about loading conditions. This can be used by the power distribution
company to manage the loading on the transformers to insure the best economic use of
each size transformer. This concept is further explored under Benefit - 6.
4.19 Lower Cost of Operation
There are some very sophisticated analytical techniques available which compare the cost
of operating two different sizes of transformers with a given load profile. As the load
increases, a point is reached where the best economic decision is to replace the smaller
size transformer with the larger size-Use of this type of analysis requires continuous
knowledge of the loading on individual transformers and a thorough knowledge of the
system economics involved.
40
The CSP technology method of optimizing loading on individual transformer relies upon
the information provided by the signal light. Normally, the signal light is calibrated to
operate when the temperature effect of the instantaneous load current plus the steady state
oil temperature is about 80% of the value which will trip the breaker.
Lower Maintenance Cost and Time From the comparative statement of schedules of
maintenance for conventional secondary distribution transformers and CSP secondary
distribution transformers, it will be seen that CSP transformers offer great, advantage to
power utility in reducing the time and cost of maintenance of the ever increasing
population of distribution transformers in Power utilities.
Neater Appearance
CSP transformer installation presents a much cleaner and uncluttered appearance. Unlike
the non-CSP transformer installation with mounting arrangements for externally fixed
protective equipment like primary fuse, surge arrester and secondary circuit breaker and
electrical connections between them.
D J Ristuccia, the Westing House Engineer aptly describes the CSP transformers as
"beautiful in concept and in physical appearance".
.
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5.1 Necessity of H.V.D.S:
The existing rural distribution system in India consists of largely 3 phase 11 KV main distribution feeders with 3 phase and 11/0.4 KV three phase distribution transformers. The distribution system on low voltage side is done by 3 phase 4 wire, 3 phase 5 wire, single phase 3 wire, and single phase 2 wire LT lines. This system involves nearly 2:1 ratio of LV and HV line lengths. Large LT network results in high occurrence of LT faults leading to frequent interruptions in supply and high incidence of distribution transformer failures due to LT fault currents. This system is unsuitable to cater certain areas like villages, desert, tribal and forests where the load density is very low and the development of load in these areas is slow. Heavy capital investment on 3 phase 11 KV lines with higher rating 3 phase transformers is not economically justified. To improve the quality of supply, one of the recommendations is the implementation of "Single phase HT distribution system with small capacity single phase transformers”. Under this system HT line is extended up to or as near the load as possible and to erect small capacity distribution transformers i.e. 10 KVA, 16 KVA and to extend supply to the consumer through a short length of LT lines, preferably insulated overhead cable (Aerial Bunched Cables) system.
5.2 Demerits of L.V.D.S: High I2R losses: As the network mainly comprises of L.T lines carrying very high
currents, the losses are very high. Unauthorized connections and energy thefts by illegal tapping or hooking of loads. Fluctuations in voltages: occurs as more number of consumers is connected to a single
transformer. Over loading of transformer occurs and in certain events the transformer may fail to
function properly leading to inconvenience of many consumers. High voltage drop at the consumers end. Frequent motor burn-outs due to low voltage. Frequent jumper cuts and fuse may blow out. Nobody owns the transformers since everybody thinks that other will take care of
transformers. Poor tail voltages, huge L.T line losses. Damages to the standing crops due to the abnormal delay in replacement of failed DTRs.
5.3 Technical Superiority Of H.V.D.S Compared To L.V.D.S: For distribution of same quantum of power, the comparison of losses and voltages drop is given below (per 100 as LT base).
S. No Parameters 1 Phase 6.35 3 Phase 4 Wire,
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KV HVDS 415V LVDS1. Current (Amps) 11.0 1002. Losses (KW) 8.5 1003. Voltage drop (KV) 12.7 100
The LT lines have to be laid using aerial bunched cables (ABC) of size 16 Sq. mm with a bearer wire. The length of LT lines has to be kept minimum level to reduce LT losses. It is seen that the cost of line is cheaper compared to conventional LT 3 phase line used in LT distribution system.The major advantage of ABC is that the fault on the LT lines are totally eliminated thereby improving the quality of supply, besides elimination of theft of energy/ conductors reduced height of supports and elimination of isolators/ associated hardware, etc.
5.4 Advantages of High Voltage Distribution System:
The HVDS offers the following direct advantages.
1) In single phase system only few numbers of consumers are connected to a particular transformer, as a result of which chances of unauthorized connections and energy thefts are reduced.
2) Reduction of system faults because of low length of LV lines.3) Distribution losses are reduced by 75% or more depending on the load factor4) The HVDS is cost effective to electrify remote villages where bringing of long 3
phase lines is costly due to low demand.5) The single phase line can be upgraded to 2 phase or 3 phase circuits in future, if the
load growth warrants it. The power utilities can keep the investment low and cut down the expenses during the initial period of low demand and electrifying remote rural areas.
6) In the event of failure of transformers, it will affect only a small number of consumers, whereas failure of large sized distribution transformers will affect large number of consumers.
7) In view of less LT system and usage of ABC, which has tough insulating cover, direct tapping by unscrupulous consumers is avoided.
8) Since losses are reduced considerably, power can be supplied to additional loads without any further investment on infrastructure.
9) No additional generation capacity is needed for giving power to new loads as there is a reduction in power drawn.
43
10) Single phase motor up to 5 HP can operate efficiently on single phase lines. The power factor of these motors is nearly unity. And thus the system efficiency also gets improved.
5.5 Merits and Demerits in comparison to 3 phase LV systems:
1. Line Losses: The losses in H.V.D.S for distribution of same power are much less when compared to that of L.V.D.S. Thus, the losses in LV network are negligible bringing down the total energy losses considerably.
2. Voltage Drop: The voltage drop for distribution of same power is less than that of L.V.D.S and thus ensures proper voltage profile at consumer points.
3. System Power Factor: The single phase motors have built in capacitors and PF is more than 0.95. This high PF causes low energy losses and better voltage profile.
4. Failure of Distribution Transformers: The failure of distribution transformers due to LV line faults is eliminated as the length of LT lines is minimized and usage of Aerial Bunched Cables (ABC) system. The over loading is prevented as each single phase transformer caters 2 to 3 consumers.
5. Theft of Energy: The LT lines are virtually eliminated and even short LT lines required will be with AB Cables. This makes direct tapping very difficult.
6. End Use Equipment: Due to better voltage profile, the efficiency of end use equipment is high, bringing in considerable benefit by way of energy conservation.
7. Reliability of Supply: The failure of transformer will affect only a small number of consumers served by it, thus the reliability of supply is high.
8. Voltage Fluctuations: The voltage drop on LV lines is negligible and voltage profile is very stable. Any voltage fluctuations occurring can be remedied by installations of Automatic Voltage Regulators on H.V line.
5.6 Connection Diagram of L.V.D.S and H.V.D.S:
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From the figure (1) it can be seen that power is directly tapped from the parallel lines by the 3-phase transformers and then is supplied to the various loads. Also we can observe the presence of many L.T lines which accounts for very high losses.
Fig (1) Connection Diagram of L.V.D.S
The figure(2) below is that of a typical high voltage distribution system where the loads are fed by small capacity individual transformers. A noticeable difference is the reduction in the L.V lines and corresponding increase in the number of H.V lines which eventually reduce the losses.
Fig (2) Connection Diagram of H.V.D.S5.7 Restructuring Present L.V.D.S to H.V.D.S:
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To implement H.V.D.S, the present system i.e. L.V.D.S need not be completely removed. But the same system could be restructured to H.V.D.S with minimal changes and used with good results.Existing L.V lines are restricted to H.V lines by
1) Replacing large 3-phase distribution transformer with small capacity 1-phase or 3-phase transformers (5, 10, 15, 25 kVA).
2) Same conductors could be used because H.V lines carry less current than L.V lines.3) L.V 3-phase cross arms are replaced by 11KV cross arm.4) Replacement of 3 numbers LT pin insulators with 3 number 11KV pin insulator.5) Replacement of 3 numbers LT shackles with 3 number 11KV strain insulator.6) Erection of additional support where ever clearances are inadequate.
The restructuring also depends on the type of HVDS being implemented.
In case 1, the existing 3-phase, 4-wire L.V lines is restricted to 1-phase and neutral. Here the 1-phase are connected through small D.T.R to the 3-phase H.V lines. The D.T.R is tapped between any phase and neutral.
In case 2, the restructuring is done in the same way as case 1, except for the D.T.R which is connected between the phases. In this case the neutral wire is not needed but the D.T.R turns ratio would be more, hence even the cost would be more.
In the case 3, the L.V.D.S is directly given to smaller capacity 3-phase D.T.R and thus power is tapped.
5.8 Actual Implementation Of H.V.D.S:
H.V.D.S is not an abstract concept; it is implemented in our very own state by A.P.C.P.D.C.L in Kotturu, Murakambattu, Patnam and Bangaru Palam.
Let us consider the case of Kotturu, where 15KVA transformers were erected and operated for 15 days. The results are tabulated below.
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S.NO.
PARTICULARS
KOTTURU
1 N.O of 15KVA distribution transformers erected 11 numbers
2 Number of days 15 days
3 Input 5310 units
4 Output 5019.2 units
5 Losses 290.8 units
6 % of line loss on H.V.D.S 5.47%
7 % of line loss on earlier LT distribution system 18.63%
8 % net reduction in line losses 13.16%
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6.Conclusion:
The scope of our project includes the manufacturing of a single phase amorphous core transformer and we also added an introduction to high voltage distribution system.
The amorphous core is the latest technological advancement in the development of distribution transformers with which decreases no load and core losses when compared to that of the conventional transformer.
The CSP transformer is hermetically sealed (top covered welded to the tank). There is no scope for ingress of moisture during breathing due to variations in ambient
conditions and loading pattern. Pilferage of oil from the transformers can be eliminated which is very common by the unscrupulous persons.
Built in protection is available for external and internal faults. The space above the oil level is filled with nitrogen. Thus expansion of oil is taken so that chances of failure of transformer is very remote.Hence CSP technology provides protection against internal faults, lightning and external faults (over loads).
The H.V.D.S is technically superior to that of the LV distribution system with regard to the quality of supply, better voltage profile, reduced losses and reliability. However this system is costlier, but it is more economical when compared with the other.
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7. BIBLIOGRAPHY:
1. Study material of M/s Vijai electrical Ltd .
2. Complete self protection technology for high performance distribution transformers
-Mr.T.Sugunakara Rao
Advisor,VEL
3. Electrical Machinery -P. S. Bimbra
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