R.M.U

R.M.U – Ring main unit is an Electrical power distribution system, A ring main unit is a factory assembled,metal enclosed set of switchgear used at the load connection points of a ring – type distribution network. A Ring Main Unit (RMU) is a totally sealed, gas-insulated compact switchgear unit. The primary switching devices can be either switch disconnectors or fused switch disconnectors or circuit breakers.

Auto-Transformer

AUTO-TRANSFORMERS – A transformer, in which a part of the winding is common to both the primary and secondary circuits, is called an auto-transformer. In a two winding transformer, primary and secondary windings are electrically isolated, but in an auto-transformer the two windings are not electrically isolated.

Advantages of auto-transformer over two winding transformer.

(1) The weight of conductor (copper or aluminium) for any winding depends upon the cross-sectional area and
length of the conductor. Now the conductor area is proportional to the current carried by it,whereas the length of conductor in a winding is proportional to the number of turns in the winding. Thus the weight of conductor in a winding is proportional to the current and number of turns in the winding.

(2) Owing to the reduction in conductor and core materials, the ohmic losses in conductor and the core loss are lowered. Therefore, an auto-transtormer has higher efficiency than a two-winding transformer of the same output.
(3) Reduction in the conductor material means lower value of ohmic resistance. A part of the winding being common, leakage flux and, therefore, leakage reactance is less. In other words,an Auto-Transformer has lower value of leakage impedandce and has Superior voltage regulation than a two-winding transformer of the same output.

Disadvantage of Auto-Transformer over two Winding transformer.

(1) If the ratio of transformation k differs far from unity, the economic
advantages of auto-transformer over two-winding transformer decrease.
(2) The main disadvantage of an auto-transformer is due to the direct electrical connection between the low-tension and high-tension sides. If primary is supplied at high voltage,
then an open circuit in the common winding BC, would result in the appearance of dangerously high voltage on the L.V. side. This high voltage may be detrimental to the load and the persons working there. Thus a suitable protection must be provided against such an occurrence.
(3) The short-circuit current in an auto-transformer is higher than that in a corresponding two-winding transformer.

Transformer

A transformer is a electrical device that transfers electrical energy from one electrical circuit to another cuircuits. A varying current in any one coil of the transformer produces a varying Magnetic flux, which in turn induces a varying Electromotive force across any other coils wound around the same core.

1.1 Transformer construction –

TRANSFORMER CONSTRUCTION
There are two general types of transformers, the core type and the shell type. These two types differ from each other by the manner in which the windings are wound around the magnetic core.
The magnetic core is a stack of thin silicon-steel laminations about 0.35 mm thick for 50Hz transformers. In order to reduce the eddy current losses, these laminations are insulated from one another by thin layers of varnish. For reducing the core losses, nearly all transformers have their magnetic core made from cold-rolled grain-oriented sheet steel
(C.R.G.O.). This material, when magnetized in the rolling direction, has low core loss and high permeability.

In the core-type, the windings surround a considerable part of steel core as shown in Fig. 1.1 (a). In the shell-type, the steel core surrounds a major part of the windings as shown in Fig. 1.1 (b). For a given output and voltage rating, core-type transtormer requires less iron but more conductor material as compared to a shell-type transformer. The vertical portions of the core are usually called limbs or legs and the top and bottom portions are called the yoke.This means that for single-phase transformers, core-type has two-legged core whereas shell-type has three-legged core.

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In iron-core transformers, most of the flux is confined to high permeability core. There is,however, some flux that leaks through the core legs and non-magnetic material surrounding the core. This flux, called leakage flux, links one winding and not the other. A reduction in this leakage flux is desirable as it improves the transformer performance considerably. Consequently, an effort is always made to reduce it. In the core-type, transformer this is achieved by placing half of the low voltage (L.V.) winding over one leg and other half over the second leg or limb. For the high voltage winding also, half of the winding is over one leg and the other half over the second leg, Fig. 1.1 (a). L.V. winding is placed adjacent to the steel core and H.V. winding outside, in order to minimise the amount of insulation required.

In the shell type transformer, the L.V. and H.V. windings are wound over the central limb and are interleaved or sandwiched as shown in Fig. 1.1 (b). Note that the bottom and top L.V. coils are of half the size of other L.V. coils. Shell-type transtormers are preferred for low- voltage, low-power levels, Whereas core-type construction is used for high-voltage, high-power transformers.

In core-type transformer, the flux has a single path around the legs or yokes, Fig. 1.1 (a). In the shell-type transformer, the flux in the central limb divides equally and returns through the outer two legs as shown in Fig. 1.1 (b).

There are two types of windings employed for transformers. The concentric coils are used for core type transformers as shown in fig.1.1(a) and interleaved (or sandwiched) coils for shell type transformers as shown in figure 1.1(B).

one type of laminations for the core and shell type of transformers is illustrated in figure 1.2( a )and (b)respectively. The steal core is assembled in such a manner that the butt joints in adjacent layers are staggered as illustrated in fig. 1.2(c)The staggering of the butt joints avoids continuous air gap and therefore the reluctance of the magnetic circuit is not increased. At the same time,a continuous air gap would reduce the mechanical strength of the core, and therefore, the staggering of the butt joints is essential.

During the transformer construction, first the primary and secondary windings are wound, then the laminations are pushed through the coil openings, layer by layer and the steel core is prepared. The laminations are then tightened by means of clamps and bolts.

Low power transformers are air-cooled whereas large power transformers are immersed in oil for better cooling. In oil-cooled transformers, the oil serves as a coolant and also as an insulating medium.

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For power frequency range of 25 to 400 Hz, tranformers are constructed with 0.35 mm thick sllicon-steel lamination. For audio-frequency range of 20 to 20,000 Hz, iron core with suitable refinements is uaed.For high frequencies employed in communication circuits, core is made up of powdered ferromagnetic alloy. In special cases, the magnetic circuit of a transformer may be made of non-magnetic material and in such a case, the transformer is referred to as an air-core transformer. The air-core transformer is primarily used in radio devices and in certain types of measuring and testing instruments. Cores made of soft ferrites are also used for pulse transformers as well as for high frequency electronic transformers.

Three Phase Transformer

Three Phase Transformer

Generation, transmission and distrbution of electric energy is invariably done through the use of three-phase* systems because of its several advantages over single-phase systems. As
such, a large number of three-phase transformers are inducted in a 3-phase energy system for stepping-up or stepping-down the voltage as required. For 3-phase up or down

transformation, three units of 1-phase transformers or one unit ot 3-phase transformer may
be used. When three identical units of 1-phase transformers are used, Fig. 1.75 (a) the arrangement is usually called a bank of three transformers or a 3-phase transformer bank.
A single 3-phase transformer unit may employ 3-phase core-type construction, Fig. 1.75 (b) or 3-phase shell-type construction (not shown). A single-unit 3 phase core-type transformer uses a three-limbed core, one limb for each phase winding as shown in Fig. 1.75 (b). Actually, each
limb has the l.v. winding placed adjacent to the laminated steel core and then h.v. winding is placed over the L.v. winding. Appropriate insulation is placed in between the core and L.v. winding and also in between the two windings.

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A 3-phase core-type transformer costs about 15% less than a bank of three 1-phase transformers Also, a single unit occupies less space than a bank.

Three-phase Transformer Connections

Three-phase transformers may have the following four standard connections:

(a) star-delta (Y-∆)

(b) delta-star (∆-Y)

(c) delta-delta (∆-∆)

(d) star-star (Y-Y)

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