Skin Effect of Conductors In Transmission Lines

Skin Effect of Conductors In Transmission Lines

When a conductor is carrying steady direct current (d.c.), this current is uniformly distributed over the whole X-section of the conductor. However, an alternating current flowing through the conductor does not distribute uniformly, rather it has the tendency to concentrate near the surface of the conductor as shown in Fig. This is known as skin effect in transmission lines.

The tendency of alternating current to concentrate near the surface of a conductor is known as skin effect.

                                

Due to skin effect, the effective area of cross-section of the conductor through current flows is reduced. Consequently, the resistance of the conductor is slightly increased when carrying an alternating current. The cause of skin effect can be easily explained. A solid conductor may be thought to be consisting of a large number of strands, each carrying a small part of the current. The inductance of each strand will vary according to its position. Thus, the strands near the center are surrounded by a greater magnetic flux and hence have larger inductance than that near the surface. The high reactance of inner strands causes the alternating current to flow near the surface of conductor. This crowding of current near the conductor surface is the skin effect.

Skin effect depends upon the following factors :

(i) Nature of material

(ii) Diameter of wire − increases with the diameter of wire.

(iii) Frequency − increases with the increase in frequency.

(iv) Shape of wire − less for stranded conductor than the solid conductor.

It may be noted that skin effect is negligible when the supply frequency is low (< 50 Hz) and conductor diameter is small (< 1cm).

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Why We Can’t Store AC Voltages ?

Why we can not store AC like DC

Before going to discussion on "why we can't store ac current in batteries" understand what is AC & DC voltages.AC voltage changes it's polarity 50-60 times per second (50Hz for India,UK & 60Hz for USA).But DC is constant voltage with respect to time.If we consider charging mechanism of electro-chemical cell or battery,it is continuous process of injecting electrons on negative side(-Ve) plate and protons positive side(+Ve) plate.

AC voltage changes it's polarity 50-60 times per second battery cannot change their terminals with 50-60 Hz speed to maintain AC output.so that’s why we can’t store AC in Batteries.

We cannot store AC in batteries because AC changes their polarity up to 50-60(depends on frequency) times in a second.Storing of charges will not happen as in a cycle charging and discharging happens  when we connect a battery with AC supply,.so average current stored in battery is zero,that's why we can't store AC in batteries.However AC can be stored in capacitor or inductor but this is not much efficient.so we have only DC batteries in our power system.

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Brake Test on DC Shunt Motor:Efficiency By Direct Loading

In this method, the d.c. machine is loaded and output and input are measured to find the efficiency. For this purpose, two simple methods can be used.

Brake test on dc machine:-

In this method, a brake is applied to a water-cooled pulley mounted on the motor shaft as shown in Fig.(6.1). One end of the rope is fixed to the floor via a spring balance S and a known mass is suspended at the other end. If the spring balance reading is S kg-Wt and the suspended mass has a weight of W kg-Wt,then,

Net pull on the rope = (W - S) kg-Wt = (W - S)*9.81 newtons
If r is the radius of the pulley in metres, then the shaft torque Tsh developed by

the motor is

Tsh = (W - S)*9.81*r N - m

If the speed of the pulley is N r.p.m., then,

Let V = Supply voltage in volts
I = Current taken by the motor in amperes
Input to motor = V I watts

(ii) In another method, the motor drives a calibrated generator i.e. one whose efficiency is known at all loads. The output of the generator is measured with the help of an ammeter and voltmeter.

Output of motor =Generator output/Generator efficiency.

Let V = Supply voltage is volts

I = Current taken by the motor is amperes
Input to motor = VI
Thus efficiency of the motor can be determined.
Because of several disadvantages (See Sec. 6.1), direct loading method is used

only for determining the efficiency of small machines.

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Sumpner's Test (Back to Back Test) on Transformer

Well, there are some methods to find out efficiency of transformers like OC & SC test on transformers,but they require direct loading on secondary.but in Sumpner's Test or Back to Back Test we apply phantom loading on transformer,so that we can save large amount of power from wasting.
Direct loading means the transformer is to be loaded to its rated capacity for specified number of hours to find out temperature rise.Hence, direct loading method of testing is normally not recommended especially for large capacity transformers as it involves huge amount of energy to be wasted only for testing of the transformer.

Sumpner's Test on Transformers :

In Sumpner's Test test two transformers are connected back to back means,primaries of two transformers connected in parallel and secondaries  side connected in series. 

Sumpner's test on transformer

So,one transformer is loaded on the other in Sumpner's Test.We connect AC supply at primary side of transformers.And one more low voltage supply is connected in series with secondaries to get the readings, as shown in the circuit diagram shown below.

1. Refer above diagram,Name the two transformers as T1 and T2,and they are identical.Secondaries of them are connected in voltage opposition, i.e. E2 of first and E2 of secondary emf's cancel each other because transformers are identical. 
2. In this case, as per superposition theorem,I1 & I2 vales are equal and in opposite direction,so resultant no current flows through secondary. And thus the no load test is simulated. The current drawn from V1 is 2I0, where I0 is equal to no load current of each transformer. Thus input power measured by wattmeter W1 is equal to iron losses of both transformers.

Also Read:   Buchholz Relay - Construction, Working    Different Types Of Transformers       Why Transformer Rating In KVA Not In KW?

i.e. iron loss per transformer Pi = W1/2.

1. Now, a small voltage V2 supplied to secondary side from low voltage transformer. By adjusting voltage V2 make rated current I2 flows through the secondary. In this case, both primaries and secondaries carry rated current.
2. Thus short circuit test is simulated and wattmeter W2 shows total full load copper losses of both transformers.

i.e. copper loss per transformer PCu = W2/2.

equation of full load efficiency of each transformer in Sumpner's Test is 

The indirect method or phantom loading method requires only iron and copper losses to be supplied corresponding to full load and still temperature rise corresponding to rated capacity of the transformer can be obtained. The only drawback is that it requires an additional and identical transformer for the transformer to be tested.These are the main advantages of sumpners or back to back test 

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E.M.F. Equation of a D.C. Generator

We shall now derive an expression for the e.m.f. generated in a d.c. generator.

Let Ø= flux/pole in Wb

Z = total number of armature conductors

P = number of poles

A = number of parallel paths = 2 ... for wave winding

= P ... for lap winding

N = speed of armature in r.p.m.

Eg = e.m.f. of the generator = e.m.f./parallel path

Flux cut by one conductor in one revolution of the armature,

dØ = PØ webers

Time taken to complete one revolution,

dt = 60/N second

e.m.f generated/conductor = dØ/dt=PØ/{60/N}=PØN/60

e.m.f. equation of generator

Eg = e.m.f. per parallel path= (e.m.f/conductor) ´ No. of conductors in series per parallel path.

Eg=PØN/60*(Z/A)

Also Read:   EMF Equation of Transformer    How Does DC Generator Works?   Unseen DC Windings

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D.C. Machine Armature Windings

D.C. Machine Armature Windings

The different armature coils in a d.c. armature winding must be connected in series with each other by means of end connections (back connection and front connection) in a manner so that the generated voltages of the respective coils will aid each other in the production of the terminal e.m.f. of the winding.This windings are same in both DC motor and DC generator.

Two basic methods of making these end connections are:

1. Simplex lap winding.
2. Simplex wave winding.

1. Simplex lap winding.

lap winding diagram

For a simplex lap winding, the commutator pitch YC = 1 and coil span YS ~ pole pitch. Thus the ends of any coil are brought out to adjacent commutator segments and the result of this method of connection is that all the coils of the armature .ire in sequence with the last coil connected to the first coil.Consequently, closed circuit winding results. This is illustrated in Fig. where a part of the lap winding is shown. Only two coils are shown for simplicity. The name lap comes from the way in which successive coils overlap the preceding one.

2. Simplex wave winding

wave winding diagram

For a simplex wave winding, the commutator pitch YC ~ 2 pole pitches and coil span = pole pitch. The result is that the coils under consecutive pole pairs will be joined together in series thereby adding together their e.m.f.s [See Fig. 1.22].After passing once around the armature, the winding falls in a slot to the left or right of the starting point and thus connecting up another circuit. Continuing in this way, all the conductors will be connected in a single closed winding. This winding is called wave winding from the appearance (wavy) of the end connections.

Above we discussed difference between lap winding and wave winding briefly.

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Type of Electrical Poles : Overhead Line Supports

Type of Electrical Poles : Overhead Line Supports 

Overhead transmission lines requires some support to carry the conductors.The supporting structures for overhead transmission line conductors are various types of poles and towers called line supports or electrical poles.

Properties of Line Supports :-

(i) Line Supports should have high mechanical strength to withstand the weight of conductors and wind loads etc.

(ii) Light in weight without the loss of mechanical strength.

(iii) Poles should be cheaper in cost and eco-friendly .

(iv)High life time because frequent replacement of poles may economical .

(v) Easy accessibility of conductors for maintenance.

Various types of poles:-

We have different types of electrical poles to carry various types of conductors,we can't use any one of the below line support for all types of conductors. 

1.Wooden poles
2.Steel poles
3.R.C.C. poles 
4.Lattice steel towers.

1. Wooden poles:-

Wooden poles are made of wood (sal or chir) and are suitable for lines short distance (up to 50m) and less area lines.wooden pole supports are cheap in cost,easily available, provide natural insulating properties and moreover economical.The wooden poles generally tend to rot below the ground level, causing foundation failure. In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil.but now a days these poles are out of use,they are replaced by R.C.C poles because of their long life.

Disadvantages of wooden supports are :

(i) tendency to rot below the ground level
(ii) comparatively smaller life (20-25 years)
(iii) cannot be used for voltages higher than 20 kV
(iv) less mechanical strength and 
(v) require periodical inspection.

2. Steel poles:

The steel poles are used as a replacement for wooden poles.Because steel poles possess greater mechanical strength, longer life than wooden poles and able to withstand longer spans.These poles are generally used for distribution purposes in the cities. This type of supports need to be galvanised or painted in

order to prolong its life.

The steel poles are of three types viz., 

(i) rail poles

(ii) tubular poles and

(iii) rolled steel joints.

3.RCC poles: 

RCC stands for reinforced concrete.These poles have become very popular as line supports in recent years.They have greater mechanical strength, longer life and permit longer spans than steel poles. Moreover, they give good outlook, require little maintenance and have good insulating properties.Expect high weight and manufacturing cost they are very good alternative for all types of line supports.

4.Steel towers: 

In practice, wooden, steel and reinforced concrete poles are used for distribution

purposes at low voltages, say upto 11 kV.To transmit higher voltages than 11kv we use steel tower.Steel towers have greater mechanical strength, longer life, can withstand most severe climatic conditions and permit the use of longer spans and we can increase or decrease length of the pole by assembling some more cross arms.these are more efficient but requires more space to establish.

These are the different types of electric poles which we can see in daily observations.

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