Rev Asg 1 Distribution Transformers

Materials and Components

DISTRIBUTION TRANSFORMERS

Introduction:

The distribution transformer forms vital equipment in the power network. It forms the majority of the population in the network. Hence the reliabi-lity is all the more important. Unlike other power transformers, distribu-tion transformers will be energized through out the day, which means No load loss plays prominent role. Hence, all day efficiency is to be given due importance.

Design of distribution transformer is intricate to the extent that it has to accommodate all the necessary fittings (including LV & HV Cable boxes wherever applicable) and accessories with necessary external clearances.

Since distribution transformers forms the major portion of power net work, energy efficiency distribution transformers are gaining its importance recently. Already Bureau of Energy Efficiency (as per the directions of Central Government), are formulating guidelines of energy efficiency transformer with different star ratings.

The Motto of Star ratings is ” MORE STARS MORE SAVINGS”.

Components of DT:

The important components of a Distribution Transformer are listed below

i. Core for the magnetic circuit.

ii. Primary and secondary windings for the electric circuits.

iii. Off-load tap-change of the primary side (Optional). Normally Small capa-city DT may not have tap changer.

iv. Main tank to house the core and windings with cooling fins or pipes, breather and explosion vent.

v. Conservator tank with associated Buchholtz relay and piping (Optional) Small capacity DT may not have conservator.

vi. Insulating oil, filled up in the tank which also acts as a cooling medium for conduction of heat, generated in the windings.

vii. H.V. and L.V. bushings to bring out the terminal connections of the windings.

Types of Transformers:

Conventional 3 phase DT:

These DT are generally delta (11 kV) - star (LV) vector group. Star-star vector group DT are also used. The standard ratings are 25, 50, 63, 75, 100, 160, 250, 315, 500, 630, 1000 as per IS.Distribution & Rural Electrification Manual 1


Single Phase DT:

The Single phase DT are two types namely Phase-Neutral (P-N) DT are of Voltage ratio 6.35 kV (11/√3) / 240 volts and Phase-Phase (P_P) DT are of Voltage Ratio 11 kV/240Volts. The merits and demerits of the two systems are dealt in Line Manual.

In case of phase to neutral system, a continuous earth wire is required to be drawn on LT insulators from the neutral of 33/11 kV substation. This earth wire has to be earthed at all the poles The P- N single phase DTs are to be connected between one phase wire and a neutral wire. The neutral current will flow through the earth and neutral conductor. The transformers can be hooked on single PCC poles and LT service connections can be given from the secondary side of the distribution transformers with a fuse switch unit/circuit breaker on the secondary side. The single phase transformers could be oil filled type or encapsulated type. The cost of resin encapsulated transformer is high and therefore these have not been popular in India. The failure of single-phase distribution transformer is reported to be less as compared to conventional distribution transformers because only two or three connections with ABC cable are given and no over-loading of the transformers is reported.

Self Protected (CSP) DT:

The salient features of CSP transformers are:

A HV cartridge fuse link is provided in the HV bushings for protection of transformer against internal faults.

A LV side circuit breaker, which is sensitive to current and also temperature is provided immersed in oil to protect the transformer against short circuits and also over loads.

The transformer has a signal light which glows when the transformer is over-loaded condition arise.

LV circuit breakers and HV fuse links, which are internally mounted, prevent consumers meddling.

The LV circuit breakers and HV fuse link are well coordinated suchthat the HV fuse link does not blow for a secondary fault, but only theLV circuit breakers acts

Since it is completely scaled, there is no scope for ingress of moistureor pilferage of oil.

Requirement of maintenance is minimum.

Cast Resin Distribution Transformers:

Distribution & Rural Electrification Manual 2


The coils are put in the resin bath under vacuum conditions which helps to impregnate the resin into the small voids of the coil. When cooled, the coil becomes one solid part. This prevents the inter coil vibrations due to axial forces produced during short circuit. These transformers are 2.5 to 3 times expensive as compared to the conventional transformers.

The main advantages of Resin cast transformers is that they are fire- resistant and are ideally suited for installation in the multi-storeyed buildings and mines where protection against fire is to be achieved even at a higher cost. In buildings these DT can be used without a transformer tank. But a tank is necessary when exposed to sun as ultra violet rays affect the Resin. Another advantage of DT is that they have higher short circuit withstand capability as compared to the conventional transformers.

Amorphous Core Transformers:

The conventional transformer core, being metal, possesses crystalline structures in which atoms form ordered, repeating patterns. The amorphous alloy, being metallic glass, differs from conventional metal in that the atoms are arranged in a near random way. This novel, non-crystalline structure lends the material unique material properties. Metallic glass alloys combine the strength and hardness characteristics of a silicate glass with the plasticity and toughness common to metals. Perm-magnetic metallic glass alloys magnetize and de-magnetize more easily than any other known material. Use of such materials for transformer cores thus brings down the core loss.

Properties of Amorphous Metals:

Amorphous alloys for electromagnetic applications are based on alloys of iron, nickel and cobalt. Their properties may vary from alloy to alloy but in general they have:

Higher electrical resistivity.

More flexibility without loosing hardness.

Higher tensile strength.

Ease of Magnetization.

Ability to change magnetic orientation with small expenditure ofenergy.

Ability to be produced from non strategic metals at very high speed.

Some Aspects of Amorphous Core:

Thickness of the core is 0.025 mm.

It is available in the form of ribbon and so wound core is made which covers large surface area.

Advantages of AMT:



The main advantages of AMT are less iron (No load) losses, due to better magnetic properties of amorphous core. The Comparison of no load loss of AMDT with normal cored (CRGO) is shown in Table 1.

Table 1: Load Loss of CRGO Core and Amorphous Core Transformer

Rating of Transformer in

kVA

No Load Loss in (Watts) %reduction No Load Loss by

Amorphous CoreCRGO CoreAmorphous

Core63 180 45 75

100 260 66 74.6

200 500 160 68

315 580 200 65

500 650 250 61.5

Economic Analysis:

As seen from Table 1.6.1 transformers with amorphous alloys in their core have losses around 25 to 30% of their equivalent CRGO steel cored transformers. The adoption of AMDT to improve efficiency will involve drastic change in manu-facturing process which will no doubt involve investment.

The decision to purchase more or less efficient transformer should be based on a long range view of future energy generation and capacity costs over 25 + years that the equipment will be in service. Economic analysis, based on incremental generation costs, projected capacity additions to meet future demand, reserve margin requirements and the current cost of capital are used by the worlds leading utilities to judge the cost effectiveness of more efficient and costlier versus less efficient and cheaper transformers.

An expert committee consisting of experts from SEBS, CBIP, and IERMA with REC as convener was formed to evolve a scientific basis for utilities to compare different offers of energy efficient transformers having higher cost with present day less expensive low efficient transformers. The expert committee decided following formula to arrive at capitalized cost of transformer.

Capitalized cost of transformers = Ic + (A Wi) + (B Wc)

Where Ic = Initial cost of transformer quoted by manufacturer in Rs.

A =

H = No. of service hours per year

Ec = Energy cost in Rs/kwh

r = Rate of interest



n = Life of transformer in years

B = A L.S.

L.S. = Loss load factor

= 0.2 LF + 0.8 (LF)2

L.F. = Load Factor

Wi = No load loss in kW.

Wc = Load loss in kW.

Based on above formula, the capitalized cost of CRGO steel core and Amorphous Metal Core transformers for various rating is given in Table 2.

Table 2: Capitalized Cost of CRGO Core and Amorphous Core Transformer

Capacity of Transformer

(kVA)

Initial cost (Rs.)Capitalized Cost

(Rs.) Break evenPoint

CRGOSteel Core

Amorphous Metal Core

CRGOSteel Core

Amorphous Metal Core

63 30,000 35,500 1,01,412 78,807 21.5

100 40,000 50,000 1,42,495 1,12,107 28.1

200 70,800 96,890 2,57,337 2,12,643 45.1

315 1,11,247 1,61,609 3,47,416 3,18,666 96.7

500 1,18,000 2,12,555 4,12,711 4,23,991

It is seen that the capitalized cost of AMT is much less than CRGO for smaller ratings. However, the difference reduces with increase in rating and the trend reverses for transformers above of 315 kVA.

Transformer Construction:

Core

Windings

Insulation

Core coil assembly

Tank

Fittings and accessories

Core:

Types: 1) CRGO 2) Amorphous

CRGO: (Cold Rolled Grain Oriented Silicon Steel)

Available grades : M3, M4, M5, MOH etc.

Thicknesses: 0.23mm; 0.27mm; 0.30mm etc



Annealing Temp.: 8000 C

Saturation Flux Density: 1.92 Tesla

Amorphous Metal:

Available Ribbons: 5.6”; 6.7” & 8.4”

Thicknesses: 0.023mm

Annealing Temp: 3600 C

Saturation Flux Density: 1.58 Tesla.

Advantages of Amorphous Metal Transformers:

No-load loss reduced to a rock bottom low of 75% as compared to CRGO transformers.

Less magnetizing current.

Cumulative saving of energy cost.

Total owning cost to customer is much less.

Better “demand side management”.

Less temperature rise of core.

Less noise level due to less magnetostriction.

As the sheet thickness of the material is 10 times lesser to that of conven-tional grade material, the eddy current losses in the amorphous core are less.

Core Construction:

Stacked Core (CRGO)

Three Limbed core type

Wound core (CRGO & Amorphous)

Shell type

Core type

Stacking Factor:

The lamination factor depends on

Flatness of the steel lamination.

Surface roughness.

Stack pressure.

Thickness variation (non uniform)

Steel thickness (thicker lamination increase the lamination factor).

Type of insulation coating.

Assembly method (interleaved, or butt stacks).



ASTM A – 719 standard defines the lamination factor, S, of a stacked laminated core as the ratio of solid volume of steel stack (calculated from weight and density) to the actual volume of the compressed stack (determined from its dimension).

S = 100 (Solid Volume by wt and density)/(Measured Volume).

Three Limbed Stacked Core:

Shell Type Wound Core Construction:

Core Type Wound Core:



Windings:

Winding Materials:

Copper

Aluminum

Covering:

Super Enamel

Double Paper

Selection of Conductor Size mainly depends upon Current Density.

Parameters Effecting Current Density:

Load Losses.

Short Circuit Thermal ability.

Winding Temperature Gradients corresponding to the guaranteed winding temperature rise.

Any Customer’s stipulation.

Further, a maximum current density

3.8 Amp/mm² for copper and

2.2 Amps/mm² for aluminum

can be taken meeting the above mentioned considerations.

Types of Windings:

LV Winding: LV winding will be predominantly Spiral winding with two layers or foil winding.

HV Winding: HV winding will be generally of multilayer helical for 11 kV transformers and cross over coils in case of 22 kV and 33 kV transformers.

CCA of Stacked Core Construction & Single Coil Winding:



CCA of

Stacked Core Construction & Cross over Coils:

Shell Type Wound Core Transformer:



Tank:

Tank shall be designed maintaining the required internal clearances from the CCA depending on the Voltages.

Normally for distribution transformers, rectangular tanks will be preferred.

The tank is manufactured with mild steel; and shall be robust in construction. Proper stiffening shall be provided wherever necessary.

Tank shall be provided with sufficient cooling area by using different types of cooling arrangements like cooling tubes, pressed steel radiators and corrugated fins etc.

Transformers with Cooling Tubes:



Transformer with Radiators:

Corrugated Tank:



Advantages:

Compact in size.

Improved heat dissipation.

Elegant appearance.

Due to vertical oil channel greater stability is achieved.

Oil expansion during service can be absorbed by expansion in corrugation.

No blockage of oil flow into the fins.

Oil cleaning is easier and less expensive.

Fittings & Accessories:

Rating & diagram plate,

VE Logo plate,

Off-circuit tap switch,

Earthing terminals,

Tank lifting lugs,

Top cover lifting lugs,

Drain cum sampling valve & filter valve,

Oil level gauge,

Pressure relief device or explosion vent,

Thermometer pocket,

Silicagel breather,

Bottom channels,

Conservator,

Oil filling plug and

Bushings.


Documents

Rev Asg 1 Distribution Transformers