Parallel operation of three phase transformer

Two three phase transformers are said to be connected in parallel if their primary windings are connected to supply bus bars and secondary windings are connected to the load bus bars.

While connecting two or more than two three phase transformers for parallel operation, it is essential that their terminals of similar polarities are joined to the same bus bars because the improper connections may cause a short-circuit and the primary side of the transformers may get damaged unless protected by fuses or circuit breakers.

There are some principal reasons for connecting the three phase transformers in parallel.

 

• If any transformer damaged, the continuation of supply can be maintained through other transformers.

• When the load on the substation becomes more than the capacity of the existing transformers, then another transformer can be added in parallel.

• Any transformer can be taken out of the circuit for repair or routine maintenance without interrupting supply to the consumers.

Conditions for proper parallel operation of three phase transformers:

 

In order that the transformers work satisfactorily in parallel, the following conditions should be satisfied:

 

• Transformers should be properly connected according to their polarities.

• The voltage ratings and voltage ratios of the transformers should be the same.

• The per unit impedances of the transformers should be equal.

• The reactance or resistance ratios of the transformers should be identical.

Skin Effect in Transmission Lines

The phenomena arising due to unequal distribution of current over the entire cross section of the conductor being used for long distance power transmission is referred as the skin effect in transmission lines. Such a phenomena does not have much role to play in case of a very short line, but with increase in the effective length of the conductors, skin effect increases considerably. So the modifications in line calculation needs to be done accordingly.

The distribution of current over the entire cross section of the conductor is quite uniform in case of a dc system. But what we are using in the present era of power system engineering is predominantly an alternating current system, where the current tends to flow with higher density through the surface of the conductors (i.e skin of the conductor), leaving the core deprived of necessary number of electrons. In fact there even arises a condition when absolutely no current flows through the core, and concentrating the entire amount on the surface region, thus resulting in an increase in the effective resistance of the conductor. This particular trend of an ac transmission system to take the surface path for the flow of current depriving the core is referred to as the skin effect in transmission lines

Why skin effect occurs in transmission lines ?

Having understood the phenomena of skin effect let us now see why this arises in case of an a.c. system. To have a clear understanding of that look into the cross sectional view of the conductor during the flow of alternating current given in the diagram below.

Let us initially consider the solid conductor to be split up into a number of annular filaments spaced infinitely small distance apart, such that each filament carries an infinitely small fraction of the total current.

Like if the total current = I

Lets consider the conductor to be split up into n filament carrying current ‘i’ such that I = n i .

Now during the flow of an alternating current, the current carrying filaments lying on the core has a flux linkage with the entire conductor cross section including the filaments of the surface as well as those in the core. Whereas the flux set up by the outer filaments is restricted only to the surfaceitself and is unable to link with the inner filaments.Thus the flux linkage of the conductor increases as we move closer towards the core and at the same rate increases the inductance as it has a direct proportionality relationship with flux linkage. This results in a larger inductive reactance being induced into the core as compared to the outer sections of the conductor. The high value of reactance in the inner section results in the current being distributed in an un uniform manner and forcing the bulk of the current to flow through the outer surface or skin giving rise to the phenomena called skin effect in transmission lines..

 

The Ferranti effect

In electrical engineering, the Ferranti effect is an increase in voltage occurring at the receiving end of a long transmission line,to the voltage at the sending end. This occurs when the line is energized but there is a very light load or the load is disconnected. The capacitive line charging current produces a voltage drop across the line inductance that is in-phase with the sending end voltages. Therefore both line inductance and capacitance are responsible for this phenomenon.

 

The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length.

The Ferranti effect is much more pronounced in underground cables, even in short lengths, because of their high capacitance.

It was first observed during the installation of underground cables in Sebastian Ziani de Ferranti's 10,000 volt distribution system in 1887.

String Effeciency and methods to improve String Effeciency

The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency i.e.,

where n = number of discs in the string.
String efficiency is an important consideration since it decides the potential distribution along the string. The greater the string efficiency, the more uniform is the voltage distribution. Thus 100% string efficiency is an ideal case for which the volatge across each disc will be exactly the same. Although it is impossible to achieve 100% string efficiency, yet efforts should be made to improve it
as close to this value as possible.

Methods of Improving String Efficiency

The maximum voltage appears across the insulator nearest to the line conductor and decreases progressively as the crossarm is approached. If the insulation of the highest stressed insulator (i.e. nearest to conductor) breaks down or flash over takes place, the breakdown of other units will take place in succession. This necessitates to equalise the potential across the various units of the string i.e. to improve the string efficiency.
The various methods for this purpose are :

1.        By using longer cross-arms. The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution. The value of K
can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross-arms. In practice, K = 0·1 is the limit that can be achieved by this method.

2.        By grading the insulators. In this method, insulators of different dimensions are so chosen that each has a different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalise the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required. However, good results can be obtained by using standard insulators for most of the string and larger units for that near to the line conductor.

3.        By using a guard ring. The potential across each unit in a string can be equalised by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units.

 

Step & Touch potential

In electrical engineering, earth potential rise (EPR) also called ground potential rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high potential may be a hazard to people or equipment.

The change of voltage over distance (potential gradient) may be so high that a person could be injured due to the voltage developed between two feet, or between the ground on which the person is standing and a metal object. Any conducting object connected to the substation earth ground, such as telephone wires, rails, fences, or metallic piping, may also be energized at the ground potential in the substation. This transferred potential is a hazard to people and equipment outside the substation.

Low voltage systems

Requirements for Electrical Installations, more commonly known as the Wiring Regulations, nominal a.c. voltages are defined as follows:

·         Extra-low voltage: not exceeding 50 V a.c., whether between conductors or to earth.

·         Low voltage: exceeding extra-low voltage, but not exceeding 1000 V a.c. between conductors, or 1500 V a.c. between conductors and earth.

·         High voltage: normally exceeding low voltage.

 

New harmonized color code

Old color code

 

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