Schemes of Distribution System

Schemes of Distribution System

AU : May-07, 12, 16, 17, Dec.-05, 07, 09, 10, 12, 13

The various schemes of distribution system are,

i) Radial distribution system

ii) Ring main distribution system

iii) Ring main distribution system with interconnector

iv) Interconnected system 

 

1. Radial Distribution System

The Fig. 7.6.1 shows a radial distribution system.

When the distributor is connected to substation on one end only with the help of feeder, then the system is called radial distribution system. The feeders, distributors and service mains are radiating away from the substation hence name given as radial system. There are combinations of one distributor and one feeder, connecting that distributor to the substation. In the Fig. 7.6.1, distributor 1 is connected only at one end to substation through a feeder at point A. Similarly the other feeder is feeding the distributor 2, only at one point B.

Due to such system, if the fault occurs either on feeder or a distributor, all the consumers connected to that distributor will get affected. There would be an interruption of supply to all such consumers. Similarly the end of the distributor nearer to the substation will get heavily loaded than the end which is too far away from the substation. Similarly the consumers at the distant end of the distributor would be subjected to the voltage variations and fluctuations, as the load on the distributor changes. The system is advantageous only when the generation is at low voltage level and the substation is loaded at the centre of the load.

The fault on a feeder or a distributor causes interruption in supply to all the consumers connected to the distributor. This can be avoided by modifying the radial system as shown in the Fig. 7.6.2. In this system, the distributor is fed at number of points with the help of feeders. In Fig. 7.6.2, the feeders from the substation are feeding to a single distributor at points A, B and C.

a. Advantages of Radial System

The various advantages of radial system are,

1. Simplest as is fed at only one end. 

2. The initial cost is low.

3. Useful when the generation is at low voltage.

4. Preferred when the station is located at the centre of the load.

b. Disadvantages of Radial System

Apart from its advantages, this system is suffered from the following disadvantages,

1. The end of distributor near to the substation gets heavily loaded.

2. When load on the distributor changes, the consumers at the distant end of the distributor face serious voltage fluctuations.

3. As consumers are dependent on single feeder and distributor, a fault on any of these two causes interruption in supply to all the consumers connected to that distributor.

 

2. Ring Main Distribution System

Another system of distribution which eliminates the disadvantages of the radial system is used in practice called ring main

In such system, the feeder covers the whole area of supply in the ring fashion and finally terminates at the substation from where it is started. The feeder is in closed loop form and looks like a ring hence the name given to the system as ring main system. This is shown in the Fig. 7.6.3.

The feeder in the ring fashion is divided into number of sections as AB, BC, CD, DE and EA. The various distributors are connected at A, B, C, D and E. Each distributor is supplied by the two feeders and hence the design is similar to the two feeders in parallel on different paths.

Hence if there is any fault on any part of the feeder, still the consumers will keep on getting the continuous supply. For example, if the fault occurs at point P in the section AB of the feeder, still the consumers connected to the distributors at A and B will get supply from the sound feeder sections AE and BC. The part AB of the feeder can be isolated and repaired. The feeder can be fed at one or more feeding points. Thus the disadvantages of radial system are eliminated in this system. The great saving in copper is another major advantage of the ring main system. 

a. Advantages of Ring Main Distribution System

1. The feeders get equally loaded.

2. If fault develops on one of the feeders then consumer gets continuous supply from the other part of the feeder.

3. It eliminates the possibility of the voltage fluctuations.

4. Easy from the maintenance and repair point of view without interrupting the supply to the consumers.

5. Great saving in copper required.

 

3. Ring Main Distributor with Interconnector

It has been mentioned that in ring main system, the cable is arranged in the loop or ring fashion. In most simple case, the ring distributor is fed at only one point.

But sometimes the ring main system is used to supply a large area and hence voltage drop across the various sections may become large in such case. Hence to compensate for such excessive voltage drops, the distant points of ring distributor are joined together by a conductor. This is called an interconnector. The Fig. 7.6.4 shows a ring main system with an interconnector.

The points D and G are joined by an interconnector.

Such a case is generally analysed using Ihevenin's theorem.

Let us briefly revise the steps to use the Thevenin's theorem.

The steps to use Thevenin's theorem :

1. Remove an interconnector DG.

2. Find the voltage V_DG without an interconnector, which is Thevenin's voltage denoted as E_o.

3. Determine the equivalent resistance as viewed through the terminals D and G, i.e. where an interconnector is to be connected. This is Thevenin's equivalent resistance denoted as R_TH.

4. Knowing the resistance of an interconnector DG, the Thevenin's equivalent can be drawn as shown in the Fig. 7.6.5.

5. The current I through an interconnector then can be obtained as,

  I = (E_o / R_TH + R_DG) amp

Once this current is known, current in all the sections and the voltages at load points can be determined.

 

Example 7.6.1 A 2 wire d.c. ring main is fed at A and load is tapped at points B, C and D. The distributor length is 400 m long and points B, C and D are 150 m, 250 m and 375 m from point A. The loads are 150 A, 40 A and 200 A respectively. If the resistance per 100 m of single conductor is 0.04 Ω and voltage at feeding point A is 240 V, calculate: 1. Current in each section of distributor 2. Voltages at points B, C and D.

Solution: The distributor is shown in the Fig. 7.6.6.

The resistances of various sections are,

Let I_A be the current flowing through section AB, so other currents in various sections are as shown in the Fig. 7.6.6.

Applying KVL, total voltage drop along loop is zero.

Hence the actual distribution of currents are as shown in the Fig. 7.6.7.

The negative sign of current in any section indicates that the actual direction of flow of current is opposite to what is assumed.

The various voltages are,

It can be seen that, point B is the point of minimum potential and is fed by currents in both the directions as shown in the Fig. 7.6.7.

Example 7.6.2 A two wire dc ring main distributor ABCDEA is fed at point A with 230 V supply. The resistances of go and return conductors of each section AB, BC, CD, DE, AE are 0.1 ohm. The main supplies the loads of 10A at B, 20A at C, 10A at D, 30A at E. Find the voltage at each load point.

Ans. :

The distributor is shown in the Fig. 7.6.8.

R_AB = R_BC = R_CD = R_DE = R_AE = 01Ω

Applying KVL, the total voltage drop along the loop is zero.

The actual current distribution is as shown in the Fig. 7.6.8 (a)

The negative sign of the current in any section indicates that the actual direction of flow of current is opposite to what is assumed. The voltage at each load point is, 

 

Example 7.6.3 The Fig. 7.6.9 shows a ring distributor with interconnector BD. The supply is fed at point A. The resistances of all the sections are indicated in the Fig. 7.6.9.

Calculate : 1. Current in the interconnector 2. Voltage drop in the interconnector

Solution: Use the Thevenin's theorem. Remove the interconnector BD.

The current in section AB is I A. So other currents are as shown in the Fig. 7.6.9 (a). The total voltage drop around a loop is zero.

The negative sign indicates that current I_CD flows from D to C and not from C to D.

The drop BCD = I_BC R_BC + I_CD × R_CD = 1.304 × 0.06 - 8.695 × 0.06 = - 0.4435 V

This is Thevenin's voltage E_o and B is - ve w.r.t. D, as indicted by negative sign.

E_o = 0.4435 V

Now calculate RTH as viewed through the terminals BD.

Hence the Thevenin's equivalent is as shown in the Fig. 7.6.9(b).

The drop along the interconnector is,

V_BD = I_BD × R_BD = 4.5538 × 0.04 = 0.1821 V

 

4. Interconnected Distribution System

When a ring main system is supplied by two or more than two generating stations then it is called interconnected system.

The Fig. 7.6.10 shows the single line diagram of an interconnected system.

The closed ring is supplied from two sources S₁ and S₂ at points E and D respectively. The distributors are connected at the points A, B and C, through distribution transformers. The advantages of such an interconnected system are,

1. Reliability of supply increases, continued with the help of other In case of fault on one source, supply can be sources.

2. Additional load demand in one area can be fed from other source where load demand is less. This reduces the reserve power capacity and improves the efficiency of the distribution system.

Review Questions

1. Explain radial distribution system in detail.

AU May-07, Dec.-lO, 13, Marks 8

2. Explain about ring main distribution system. State its advantages.

AU : Dec -05, 10, 12, 13, May-12, Marks 8

3. Explain ring main distribution with interconnector.

AU : May-07, Dec.-09, 10, 13, Marks 8

4. Explain the interconnected system of distribution. State its advantages.

AU : Dec.-13, May-17, Marks 4

5. Discuss and compare radial and ring main distribution systems. What is the role of interconnectors ?

AU : Dec.-07, May-17, Marks 4

6. A ring distributor is loaded as shown in the Fig. 7.6.11. The resistances indicated are for go and return both.

a. Current in the interconnecter b. Drop in the interconnector.

[Ans.: 9.8 A, 0.49 V]

7. A DC ring main A, B, C, D, A is fed from point A from 250 V supply and resistance (including go and return) cf various sections are given below.

AB = 0.2 ohms, BC = 0.018 ohms, CD = 0.025 ohms and DA = 0.02 ohms.

The main supplies a load of 150 A, at B, 300 A at C and 250 A at D.

i) Determine the voltage at each load point.

ii) If the points A and C are linked through an inter connected of resistance 0.02 ohms, determine the new voltage at each point.

8. The points B and D of a D.C. ring main ABCDEA are linked through an inter connector. The supply is given at point A. The resistance of go and return conductors cf various sections are as shown in Fig. 7.6.12. Calculate

i) Current in the interconnector

ii) Voltage drop in interconnector.

[Ans.: 9.7705 A, 0.488 V]