STANI MEMORIAL COLLEGE OF ENGINEERING & TECHNOLOGY

POWER SYSTEM DESIGN LAB

EXPERIMENT NO. 3

Aim: - Distribution system Design: Design of feeders & distributors. Calculation of voltage

drops in distributors, Calculation of conductor size using Kelvin’s law.

Apparatus Requirements: - Feeder, Conductors, Distributors, Service mains.

Theory: - Distribution system may be divided in to two systems known as High voltage or

Primary distribution system and Low voltage or Secondary distribution system.

Primary Distribution System: - It is that part of A.C. distribution system which operates at

voltages (such as 3.3, 6.6, 11kV or higher than) that of general utility and handle larger block of

power. Primary distribution system is carried out by 3-ϕ, 3-Wire system.

The 33/11 kV secondary substations are usually located in the area having load requirement

5MVA and normally a primary distribution line or a feeder is designed to carry a load of 1-2

MVA. So the number of feeder emanating from a secondary substation of 33/11 kV is 3-4. For

load exceeding 8MVA the secondary transmission is carried out at 66 kV so as to reduce the line

losses and therefore secondary substations is 6-8. The feeders may be radial, parallel, ring or

interconnected.

1) Radial Feeder: - A radial feeder connects between a source and a load point, and it may

supply one or more additional load points between the two. Each load point can be

supplied from one direction only. Radial feeders are most widely used by the Navy

because the circuits are simple, easy to protect, and low in cost. Radial feeder shown in Fig

3.1.

2) Parallel Feeder: - Parallel feeders connect the source and a load or load centre and

provide the capability of supplying power to the load through one or any number of the

parallel feeders. Parallel feeders provide for maintenance of feeders (without interrupting

service to loads) and quick restoration of service when one of the feeders fails. Parallel

feeder shown in Fig. 3.2.

3) Loop Feeder: - A loop feeder has its ends connected to a source (usually a single source),

but its main function is to supply two or more load points in between. Each load point can

be supplied from either direction; so it is possible to remove any section of the loop from

service without causing an outage at other load points. The loop can be operated normally

closed or normally open. Most loop systems are, however, operated normally open at some

point by means of a switch. The operation is very similar to that of two radial feeders.

Loop feeder shown in Fig 3.3.

4) Interconnected Network System: - A distribution system with multiple available power

sources that can loop throughout the network. If one source goes down, a different source

can be activated to maintain service.

Secondary Distribution System: - The secondary distribution system is that portion of the

network between the primary feeders and utilization equipment. The secondary system consists of

step-down transformers and secondary circuits at utilization voltage levels.

Residential secondary systems are predominantly single-phase, but commercial and industrial

systems generally use three-phase power.

1) 3-Φ, 4-Wire Distribution System

2) 1-Φ, 2-Wire Distribution System

Distributors:-

1. Distributor fed at one end: - In this type of feeding, the distributor is connected to the

supply at one end and loads are taken at different points along the length of the distributor.

Fig.3.4 shows the single line diagram of a D.C. distributor AB fed at the end A (also

known as singly fed distributor) and loads I1 , I2 and I3 tapped off at points C, D and E

respectively.

2. Distributor fed at both ends: - In this type of feeding, the distributor is connected to the

supply mains at both ends and loads are tapped off at different points along the length of

the distributor. The voltage at the feeding points may or may not be equal. Fig.2 shows a

distributor AB fed at the ends A and B and loads of I1, I2 and I3 tapped off at points C, D

and E, respectively. Here, the load voltage goes on decreasing as we move away from one

feeding point say A, reaches minimum value and then again starts rising and reaches

maximum value when we reach the other feeding point. The minimum voltage occurs at

some load point and is never fixed. It is shifted with the variation of load on different

sections of the distributor.

Kelvin’s law:-

A transmission line can be designed by taking into consideration various factors out of which

economy is the most important factor. The conductor which is to be selected for a give

transmission line must be economical. Most of the part of the total line cost is spent for conductor.

Thus it becomes vital to select most economic size of conductor.

The most economic design of the line is that for which total annual cost is minimum. Total annual

cost is divided into two parts viz. fixed standing charges and running charges.

The fixed charges include the depreciation, the interest on capital cost of conductor and

maintenance cost. The cost electrical energy wasted due to losses during operation constitutes

running charges.

The capital cost and cost of energy wasted in the line is based on size of the conductor. If

conductor size is big then due to its lesser resistance, the running cost (cost of energy due to

losses) will be lower while the conductor may be expensive. For smaller size conductor, its cost is

less but running cost will be more as it will have more resistance and hence greater losses.

The cost of energy loss is inversely proportional to the conductor cross section while the fixed

charges (cost of conductor, interest and depreciation charges) are directly proportional to area of

cross section of the conductor. Mathematically we have,

Annual interest and depreciation cost = S1

S1 α a

a is area of cross section of conductor

S1 = K1 a

Annual cost of energy loss in line = S2

S2 α

S2 = K2/a

Here K1 and K2 are constants

S = Total annual cost

S = S1 + S2

... S = K1 a + K2/a

For economical design of line, the cost will be minimum for a particular value of area of cross-

section 'a' of the conductor.

Thus for economic design, dS/da = 0

K1 - K2/a2 = 0

... K1 = K2/a

2

... K1a = K2/a

... S1 = S2

The most economical conductor size, a = √ (K2/K1)

Thus the most economical conductor size is one for which annual cost of energy loss is equal

to annual interest and depreciation on the capital investment of the conductor material. This is

known as Kelvin's law.

Limitation of Kelvin's Law: - Following are limitations in applying Kelvin's law.

a. The amount of energy loss cannot be determined accurately as the load factors of the

losses and the load are not same. Also the future load conditions and load factors cannot be

predicated exactly.

b. The cost of energy loss cannot be determined exactly. The cost of losses per unit is more

than the generating cost per unit.

c. The cost of conductor and rates of interest are changing continuously.

d. 4. If economical conductor size is selected then voltage drop may be beyond the

acceptable limits.

e. The economical size of conductor may not have the enough mechanical strength.

f. The cost of conductor also includes to some extent cost of insulation which changes with

change in cross section of conductor. The cost of insulation is difficult to express in terms

of cross section of conductor.

g. Due to problem of corona, leakage currents, the economic size of conductor cannot be

used at extra voltages.

h. Due to change in rates of interest and depreciation continuously, even if other parameters

are same, application of Kelvin's law will give economical conductor size different at

different time and in different countries.

Circuit Diagrams:-

Fig 3.1 Radial feeder Fig 3.2 Parallel Feeder Fig 3.3 Loop Feeder

Fig. 3.4 Distributor fed at one end

Fig. 3.5 Distributor fed at both ends

Fig. 3.6 Kelvin’s Law

Precautions:- 1. System design should be proper and reliable

2. System design must be in such a way that fault probability very less.

3. Operating voltage should be taken according to load requirements and losses.

Procedure:-

Observation Table:-

Calculation:-

Result: - We have successfully studied about distribution system design, design of feeders &

distributors. Calculation of voltage drops in distributors, Calculation of conductor size using

Kelvin’s law.

Viva- Voice Questions:-

Ques. 1 What is the Function of feeder?

Ques.2 How many types of Feeder? And Name of all feeders.

Ques. 3 Discuss any two limitations of Kelvin’s Law.

Ques. 4 Write final Expression of Kelvin’s Law.

Viva- Voice Answers:-

Ans. 1 Feeders are the conductors which connect the stations to the areas to be fed by those

stations. Generally from feeders no tapping is taken to the consumers, therefore, current loading of

a feeder remains the same along its length. It is designed mainly from the point of view of its

current carrying capacity.

Ans. 2 Mainly 3 types of feeder

Radial Feeder

Parallel Feeder

Loop Feeder

Ans. 3

a. The amount of energy loss cannot be determined accurately as the load factors of the

losses and the load are not same. Also the future load conditions and load factors cannot be

predicated exactly.

b. The cost of energy loss cannot be determined exactly. The cost of losses per unit is more

than the generating cost per unit.

Ans. 4

K1a = K2/a