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Why Core Type Transformers Have Small Kva Ratings And Shell Type Transformers Have Large Kva

Where are shell type and core type transformer used? Why?

It is a choice made by the designer, mostly. There is no hard and fast rule. But there are some trends that we see are based on mere practicalities.  Mostly, Core type transformers are popular in High voltage applications like Distribution transformers, Power transformers, and obviously auto transformers. reasons are,High voltage corresponds to high flux. So, for keeping your iron loss down you have to use thicker core. So core type is better choice.At high voltage you require heavy insulation. In core type winding putting insulation is easier. In fact LV(low voltage) winding it self acts as an insulation between HV (high voltage) winding and core.Where as, Shell type transformers are popular in Low voltage applications like transformers used in electronic circuits and power electronic converters etc. Reasons are,At low voltage, comparatively you require more volume for the copper wires than that of iron core. So the windows cut on the laminated sheets have to be of bigger proportion with respect to the whole size of the transformer. So, shell type is a better choice.Here you don't care about the insulation much and insulation is thin and light. So you can put the winding anyway you want in the shell.

Why is core type transformer preferred for high voltage?

High voltage transformers mean that the kVA rating would also be more, in most cases.More voltage requires thicker insulation and hence the winding becomes bulky.In core type design, both of the LV and HV windings are distributed in two limbs, which becomes necessary at times to accomodate the larger windings, otherwise the shell type design would be more in height to accomodate the same winding.Also higher the voltage, higher would be the heat loss, which is very crucial to dissipate or the machine would fail. Core type construction has exposed windings, which means better cooling.Also due to easily accessible windings, the tapping and repairing is easier in core type.Insulation breakdown is the most feared cause of failure of an HV transformer, and prolonged heating causes insulation degradation at a speedy pace, so universally core type design is accepted.

What type of connection is used at transformers used at generation side and distribution side (start, delta)? Why?

Generator side: The transformers are generally step up transformers with LV winding (i.e. primary) connected in Delta and HV winding (i.e. secondary) connected in Star.-The advantages of using transformer's primary in delta are:1) The line current gets divided by √3 and hence the cross-sectional area of the conductor to be used in each of the three phases of the primary winding will be reduced. Thus saving in copper.2) Delta connection provides a path for the third order harmonic current and hence no distortion because of it.-The advantage of using transformer's secondary in star is:1) On the secondary side, the line voltage is high and thus using star connection, the phase voltage will be 1/√3 times line voltage. Thus the cost of insulation is saved. Also, the cost of insulation throughout the transmission line is reduced.Distribution Side: The transformers in distribution side are step down transformers. The HV winding is connected to source and LV winding is connected to load. Here the LV winding (i.e secondary) is always star connected to provide a neutral point to consumers by which 3-phase can be converted to 1-phase. The HV winding (i.e primary) may be delta or star based upon the the KVA rating of transformers and economical aspects of insulation and cross-sectional area of conductor to be used.

What is the difference between dry type and an oil type transformer?

Simply, dry type TR use solid & air as insulation, but oil TR use paper & oil as insulation.More differences derived from above:Air is worse insularion than oil. So the dry TR’s rated voltage is limited up to 35kV (66kV dry TR had been finished in lab, but not be lauched.) Oil TR canreach very high voltage, even 1000kV.Dry TR is larger and lower efficiency generally, because same insulation level requires larger air gap than oil gap.Oil TR have oil tank to save oil and winding&core. There are more sensors to monitor temperature, pressure, oil level, oil humidity and gas flow and also some safe device to avoid explode. Dry TR just have one temperature monitor.Oil TR rated power can be very large because oil is very good coolant.From fire behavious, oil TR is more danger because oil is flamble material and easy flow in site.

What is the difference between a core type and a shell type transformer?

Difference Between Shell And Core Type TransformersIn core type transformer winding is placed on two core limbs, whereas in case of shell type transformer winding is placed on mid arm of the core. Other limbs will be used as mechanical support.2 . Core type transformers have only one magnetic flux path but Shell type transformers have two magnetic flux path.3. Core type has better cooling since more surface is exposed to atmosphere but in case of shell type transformer, cooling is not effective.4. Core type is very useful when we need large size low voltage but shell type transformer is very useful when we need small size high voltage.5. In core type output is less, because of losses. In shell type transformer output is high because of less loss, thus efficiency will be more in case of shell transformer.6. In core type winding is surrounded considerable part of core whereas in shell type Core is surrounded considerable part of winding of transformer.7. Shell type has less mechanical protection to coil but Core type has better mechanical protection to coil.8. Core type is easy to repair and maintain but shell type is not easy to repair. We need a skilled technician to maintain it.9. In core type transformer concentric cylindrical winding are used. In shell type transformer sandwiched winding are used.

What is percentage of each loss from total losses in a transformer?

The ratio of winding loss to core loss varies depending on the size of the transformer. In general, smaller transformers are winding loss dominant. As they become larger and heavier, the proportion of core loss to winding loss increases.Here are some numbers gleaned from an old design file of mine for single phase transformers up to 7.5kVA. These are at full rated load. Keep in mind that at reduced loads, the winding losses reduce as the square of the load reduction, whereas core losses stay constant. They are for silicon steel laminated cores, 50Hz. 60Hz designs should be much the same.Smaller transformers use stamped E & I laminations, making up the so-called “shell core”. Up to 200VA, losses are around 80:20 winding to core losses.Between 250 - 450VA , losses are around 70:30 winding to core losses.Between 500 - 950VA , losses are around 60:40 winding to core losses.Between 1kVA- 1.5kVA , losses are around 50:50 winding to core losses.Above 1.5kVA, losses are around 40:60 winding to core losses.For the above generalisations, there are certain outliers. As the stack height for a particular lamination size increases, the core loss increases with increasing core weight, but the allowable winding loss cannot increase because the exposed surface area stays the same. So the winding loss proportion of total losses reduces.At ratings greater than around 2.5kVA, E & I laminations are not a good choice because the core becomes heavy and the winding cooling is impaired fro the reason given above. The design becomes “core dominant”. So the trend is to switch to limbed cores, as shown in the drawing. This enables a more economical design and a better balance between winding and core losses. The aim is to get to as close to 50:50 winding to core losses, as it is thought this cost-optimises the design.For limbed cores, the loss proportions are heavily dependent on the stack profile. For under-square stacks (stack height less than limb width), losses are around 55:45 winding to core losses.For square stacks (stack height equal to limb width), losses are around 50:50 winding to core losses.For over-square stacks (stack height greater than limb width), losses are around 40:60 winding to core losses.For three-phase transformers, the optimal design will usually be with 50:50 winding to core losses. Although in general, large transformers will probably become core loss dominant and move towards 40:60 winding to core losses.

Why as frequency increases the size of transformer decreases?

While designing a transformer, the designer starts with an equation called the output equation of the transformer. This equation relates the KVA rating of the transformer and the dimensions of the transformer.The output equation for a single phase transformer is:KVA Rating = 2.22 x frequency x magnetic flux density x window space factor x Area of the window x cross-sectional area of the limb x current density x 10^(-3)The area of the window and cross-sectional area of the limb are the main dimensions of a transformer; the product of which is directly proportional to the size and weight of the transformer. Larger this product, bigger and heavier is the transformer. In the output equation, the flux density depends on the type of material used to construct the transformer core; the current density depends on the type of cooling provided; and the window space factor is a constant. Therefore, from a designers perspective, the KVA rating is directly proportional to the product of frequency, area of the window and cross-sectional area of the limb. Or more concisely, the KVA rating is directly proportional to the product of frequency and the size of the transformer. For a given transformer rating, as the frequency increases, the product of window area and cross-sectional area of the limb decreases; which means the size of the transformer core and the amount of iron required for the core decreases. Therefore as the frequency increases, the transformer becomes lighter and smaller in size. An example where frequency is increased to reduce the size and weight of the transformer is in aircrafts where the transformers are designed for 400 Hz.Note: The output equation is true for core type and shell type transformers. For a three phase transformer, the equation is same except that instead of 2.22, it is 3.33. The derivation for the output equation is very simple and can be found in Electrical Machine Design by A.K. Sawhney.