The electric energy produced at the generating station is conveyed to the consumers through a network of transmission and distribution systems. It is often difficult to draw a line between the transmission and distribution system of a large power system. In general, distribution system is that part of power system which distributes power to the consumers for utilization.
“That part of power system which distributes electric power for local use is known as distribution system.”
In general, the distribution system is the electrical system between the sub station fed by the transmission system and the consumers meters. It generally consists of feeders, distributors and the service mains.
A feeder is a conductor which connects the substation to the area where power is to be distributed. Generally, no tapping are taken from the feeder so that current in it remains the same throughout. The main consideration in the design of a feeder is the current carrying capacity.
A distributor is a conductor from which tapping are taken for supply to the consumers. The current through a distributor is not constant because tapping are taken at various places along its length. While designing a distributor, voltage drop along its length is the main consideration since the statutory limit of voltage variations is approx. 6% of rated value at the consumers terminals.
A service mains is generally a small cable which connects the distributor to the consumers terminals.
Classification Of Distribution Systems
A distribution system may be classified according to:
Nature Of Current
According to nature of current, distribution systems may be classified as
- D.C Distribution System
- A.C Distribution System
Generally, A.C system is universally adopted for distribution of electric power as it is simpler and more economical than direct current method.
Type Of Construction
According to type of construction, distribution systems may be classified as
- Overhead System
- Underground System
The overhead system is generally employed for distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the underground system is used at places where overhead construction is impracticable or prohibited by the local laws.
Scheme Of Connection
According to scheme of connection, the distribution systems may be classified as
- Radial System
- Ring Main System
- Inter Connected System
Each scheme has its own advantages and disadvantages.
Now a days electrical energy is generated, transmitted and distributed in the form of alternating current. One important reason for the wide spread use of alternating current in preference to direct current is the fact that alternating voltage can be conveniently changed in magnitude by means of a transformer. Transformer has made it possible to transmit A.C power at high voltage and utilize it at a safe potential. High transmission and distribution voltages have greatly reduced the current in the conductors and the resulting line losses.
There is no definite line between transmission and distribution according to voltage or bulk capacity. However, in general, the A.C distribution system is the electrical system between the step down station fed by the transmission system and the consumers meters. The A.C distribution system is classified into
- Primary Distribution System
- Secondary Distribution System
Primary Distribution System
It is that part of A.C distribution system which operates at voltages somewhat higher than general utilization and handles large blocks of electrical energy than the average low voltage consumer uses. The voltage used for primary distribution depends upon the amount of power to be conveyed and the distance of the sub station required to be fed. The most commonly used primary distribution voltages are 11 KV, 6.6 KV and 3.3 KV. Due to economic considerations, primary distribution is carried out by 3 phase 3 wire system.
Electric power from the generating station is transmitted at high voltage to the sub station located in or near the city. At this sub station, voltage is stepped down to 11 KV with the help of step down transformer. Power is supplied to various sub stations for distribution or to big consumers at this voltage. This forms the high voltage distribution or primary distribution.
Secondary Distribution System
It is that part of A.C distribution system which includes the range of voltages at which the ultimate consumer utilizes the electrical energy delivered to him. The secondary distribution employs 400/230 V, 3 phase, 4 wire system.
The primary distribution circuit delivers power to various sub stations, called distribution sub stations. These sub stations are situated near the consumers localities and contain step down transformers. At each distribution sub station, the voltage is stepped down to 400 V and power is delivered by 3 phase, 4 wire A.C system. The voltage between any two phases is 400 V and between any phase and neutral is 230 V. The single phase domestic loads are connected between any one phase and the neutral, whereas 3 phase, 400 V motor loads are connected across 3 phase lines directly.
It is a common knowledge that electric power is almost exclusively generated, transmitted and distributed as A.C. However, for certain applications, D.C supply is absolutely necessary. For instance, D.C supply is required for the operation of variable speed machinery (D.C Motors) for electro chemical work and for congested areas where storage battery reserves are necessary. For this purpose, A.C power is converted in to D.C power at the substation by using converting machinery e.g mercury arc rectifiers, rotary converters and motor generator sets. The D.C supply from the sub station may be obtained in the form of
- 2 wire
- 3 wire for distribution.
2 Wire D.C System
As the name implies, this system of distribution consists of two wires. One is the outgoing or positive wire and the other is the return or negative wire. The loads are connected in parallel between the two wires. This system is never used for transmission purposes due to low efficiency but may be employed for distribution of D.C power.
3 Wire D.C System
It consists of two outers and a middle or neutral wire which is earthed at the substation. The voltages between the outers is twice the voltage between either outer and neutral wire. The principal advantage of this system is that it makes available two voltages at the consumers terminal between any outer and the neutral and 2V between the outers. Loads requiring high voltage are connected across the outers, whereas loads requiring less voltage are connected between either outer and the neutral.
An underground cables essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover.
Electric power can be transmitted or distributed either by overhead system or by underground cables. The underground cables have several advantages such as less liable to damage through storms or lightning, low maintenance cost, less chances of faults, smaller voltage drop and better general appearance. However, their major drawback is that they have greater installation cost and introduce insulation problems at high voltages compared with the equivalent overhead system. For this reason, underground cables are employed where it is impracticable to use overhead lines. Such locations may be thickly populated areas where municipal authorities prohibit overhead lines for reasons of safety.
The chief use of underground cables for many years has been for distribution of electric power in congested urban areas at comparatively low or moderate voltages. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltages. This has made it possible to employ underground cables for transmission of electric power for short or moderate distances.
Connection Schemes Of Distribution System
All distribution of electrical energy is done by constant voltage system. In practice, the following distribution circuits are generally used for D.C as well as A.C distribution.
- Radial System
- Ring Main System
- Inter Connected System
In this system, separate feeders radiate from a single sub station and feed the distributors at one end only. The radial system is employed only when power is generated at low voltage and the sub station is located at the center of the load. This is the simplest distribution circuit and has the lowest initial cost. However, it suffers from the following drawbacks:
- The end of the distributor nearest to the feeding point will be heavily loaded.
- The consumer are dependent on a single feeder and single distributor. Therefore, any fault on the feeder or distributor cuts off supply to the consumers who are on the side of the fault away from the sub station.
- The consumers at the distant end of the distributor would be subjected to serious voltage fluctuations when the load on the distributor changes.
Due to these limitations, this system is used for short distances only.
Ring Main System
In this system, the primaries of distribution transformers form a loop. The loop circuit starts from the sub station bus bars, makes a loop through the area to be served, and returns to the sub station. The ring main system has the following advantages:
- There are less voltage fluctuations at consumers terminals.
- The system is very reliable as each distributor is fed via two feeders. In the event of fault on any section of the feeder, the continuity of supply is maintained.
Inter Connected System
When the feeder ring is energized by two or more than two generating stations or sub stations, it is called inter connected system. The inter connected system has the following advantages:
- It increases the service reliability.
- Any area fed from one generating station during peak load hours can be fed from the other generating station. This reduces reserve power capacity and increases efficiency of the system.
Types Of D.C Distributors
The most general method of classifying D.C distributors is the way they are fed by the feeders. On this basis, D.C distributors are classified as:
- Distributor fed at one end
- Distributor fed at both end
- Distributor fed at the center
- Ring distributor
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. The following points are worth nothing in a singly fed distributor:
- The current in the various sections of the distributor away from feeding point goes on decreasing. Thus current at nearer end is more than the current at the farthest end.
- The voltage across the loads away from the feeding point goes on decreasing.
- In case a fault occurs on any section of the distributor, the whole distributor will have to be disconnected from the supply mains. Therefore, continuity of supply is interrupted.
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. The load voltage goes on decreasing as we move away from one feeding point 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. The following are the few advantages of that type of distributors.
- If a fault on any feeding point of the distributor, the continuity of supply is maintained from the other feeding point.
- In case of fault on any section of the distributor, the continuity of supply is maintained from the other feeding point.
- The area of X section required for a doubly fed distributor is much less than that of a singly fed distributor.
Distributor Fed At The Center
In this type of feeding, the center of the distributor is connected to the supply main. It is equivalent to two singly fed distributors, each distributor having a common feeding point and length equal to half of the total length.
In this type, the distributor is in the form of a closed ring. It is equivalent to a straight distributor fed at both ends with equal voltages, the two ends being brought together to form a closed ring. The distributor ring may be fed at one or more than one point.
3 Wire D.C System
The great disadvantage of direct current for general power purposes lies in the fact that its voltage cannot readily be changed, except by the use of rotating machinery, which in most cases is too expensive. The problem can be solved to a limited extent by the use of 3 wire D.C system which makes available two voltages. Motor loads requiring high voltages are connected between the outers whereas lighting and heating loads requiring less voltages are connected between any one outer and the neutral. Due to the availability of two voltages, 3 wire system is preferred over 2 wire system for D.C distribution.
In this system, if the loads applied on both sides of the neutral are equal. The current in the neutral wire will be zero. Under these conditions, the potential of the neutral will be exactly half way between the potential difference of the outers.
If the load on the positive outer is greater than on the negative outer, then out of balance current will flow in the neutral wire from load end to supply end longer be midway between the potentials of the outers.
If the load on the negative outer is greater than on the positive outer, then out of balance current will flow in the neutral from supply end to load end. Again, the neutral potential will not remain half way between that of the outers.
As the neutral carries only the out of balance current which is generally small, therefore, area of X-section of neutral is taken half as compared to either of the outers.
It may be noted that it is desirable that voltage between any outer and the neutral should have the same value. This is achieved by distributing the loads equally on both sides of the neutral.
In the beginning of electrical age, electricity was generated, transmitted and distributed as direct current. The principal disadvantage of D.C system was that voltage level could not readily be changed,except by the use of rotatory machinery, which in the most cases was too expensive. With the development of transformer, A.C system has become so predominant as to make D.C system practically extinct in the most parts of the world. The present day large power system has been possible only due to the adoption of A.C system.
Now a days. electrical energy is generated,transmitted and distributed in the form of alternating current as an economical proposition. The electrical energy produced at the power station is transmitted at very high voltage by 3 phase 3 wire system to step down sub substations for distribution. The primary distribution circuit in 3 phase 3 wire and operates at voltages somewhat higher than general utilization levels. It delivers power to the secondary distribution transformer steps down the voltage to 400 V and power is distributed to ultimate consumer’s by 400/230 V, 3 phase, 4 wire system.
A.C Distribution Calculations
A.C distribution calculations differ from those of D.C distribution in the following respects:
- In case of D.C system, the voltage drop is due to resistance alone. However, in A.C system, the voltage drops are due to the combined effects of resistance, inductance and capacitance.
- In a D.C system, additions and subtractions of currents or voltages are done arithmetically but in case of A.C system, these operations are done vectorially.
- In an A.C system, power factor has to be taken into account. Loads tapped off from the distributor are generally at different power factors.
- It may be referred to supply or receiving end voltage which is regarded as the reference vector.
- It may be referred to the voltage at the load point itself.
There are several ways of solving A.C distribution problems. However, symbolic notation method has been found to be most convenient for this purpose. In this method, voltages, currents and impedance’s are expressed in complex notation and the calculations are made exactly as in D.C distribution.
3 Phase Unbalanced Loads
The 3 phase loads that have the same impedance and power factor in each phase are called balanced loads. The problems on balanced loads can be solved by considering one phase only, the conditions in the other two phases being similar. However, we may come across a situation when loads are unbalanced i.e. each load phase has different impedance and/or power factor. In that case, current and power in each phase will be different. In practice, we may come across the following unbalanced loads:
- Four-wire star-connected unbalanced load
- Unbalanced Δ-connected load.
- Unbalanced 3-wire, Y-connected load.
The 3 phase 4 wire system is widely used for distribution of electric power in commercial and industrial buildings. The single phase load is connected between any line and neutral wire while a 3 phase load is connected across the 3 lines. The 3 phase, 4 wire system invariably carries unbalanced loads.
“The assembly of apparatus used to change some characteristic (voltage, A.C to D.C, frequency, power factor) of electric supply is called substations.”
Sub stations are important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub station. It is, therefore, essential to exercise utmost care while designing and building a sub station. The following are the important points which must be kept in view while laying out a sub station:
- It should be located at a proper site. As far as possible, it should be located at the center of gravity of load.
- It should provide safe and reliable arrangement. For safety, consideration must be given to the maintenance of regulation clearances, facilities for carrying out repairs and maintenance, abnormal occurrences such as possibility of explosion or fire etc. For reliability, consideration must be given for good design and construction, the provision of suitable protective gear etc.
- It should be easily operated and maintained.
- It should involve minimum capital cost.
Classification Of Sub Stations
There are several ways of classifying sub stations. However, the two most important ways of classifying them are according to
- Service Requirement
- Constructional Features
A substation may be called upon to change voltage level or improve power factor or convert A.C power into D.C power etc. According to the service requirement, sub stations may be classified as:
- Transformer Sub Station: Those sub station may be called upon to change voltage level of electric supply are called transformer sub stations. These sub stations receive power at some voltage and deliver it at some other voltage. Obviously, transformer will be the main component in such sub stations. Most of the sub stations in the power system are of this type.
- Switching Sub Station. These sub stations do not change the voltage level i.e incoming and outgoing lines have the same voltage. However, they simply perform the switching operations of power lines.
- Power Factor Correction Sub Station. Those sub stations which improve the power factor of the system are called power factor correction sub stations. Such sub stations are generally located at the receiving end of transmission lines. These sub stations generally use synchronous condensers as the power factor improvement equipment.
- Frequency Changer Sub Station. Those sub stations which change the supply frequency are known as frequency changer sub stations. Such a frequency change may be required for industrial utilization.
- Converting Sub Station. Those sub stations which change the A.C power into D.C power are called converting sub stations. These sub stations receive A.C power and convert it in D.C power with suitable apparatus to supply for such purposes as traction, electroplating, electric welding etc.
- Industrial Sub Station. Those sub stations which supply power to individual industrial concerns are known as industrial sub stations.
A sub station has many components (circuit breakers, switches, fuses, instruments etc) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub stations are classified as:
- Indoor Sub Stations. For voltages up to 11 KV, the equipment of the sub station is installed indoor because of economic considerations. However, when the atmosphere is contaminated with impurities, these sub stations can be erected for voltages up to 66 KV.
- Outdoor Sub Stations. For voltages beyond 66 KV, equipment is invariably installed outdoor. It is because for such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor.
- Underground Sub Stations. In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the sub station is created underground.
- Pole Mounted Sub Stations. This is an outdoor sub station with equipment installed overhead on H pole or 4 pole structure. It is the cheapest form of sub station for voltages not exceeding 11 KV. Electric power is almost distributed in localities through such sub stations.