Electrical Power- Distribution System

The National Grid Network

A national grid network is a network of cable that connects all the power stations in a country to transmit electricity to the consumers throughout the country. Electricity is in demand just about everywhere in the civilized world, and in Great Britain the network for supplying this electrical power is known commonly as the National Grid. The National Grid is an electric power transmission network which connects the substations and power stations.

This is so that any electrical energy generated in Great Britain, can be utilized and help meet energy demands elsewhere. This grid system also includes interconnections that run under the sea to Northern Ireland HVDC Moyle, the Isle of Man and France HVDC Cross-Channel. The electrical energy generated for the National grid needs to be moved around all parts of the country to supply the demand. There are two methods available for the transmission and distribution of electric power and these are:

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• Underground Insulated Cables
• Overhead Cables (Bare Conductors Suspended at a Safe Height Above Ground)

The overhead lines are generally used for high-voltage long distance transmission, because the cost is lower than underground cables, especially at higher voltages. In British practice, high voltage transmission lines carry voltages ranging from 66kv to 132kv, and extra high voltage lines carry voltages from 220kv to 380kv. In all cases the power is transmitted in the form of three-phase alternating current at 50cycles/sec, and the cost of an overhead line depends largely on conductor size and voltage [Cheesman, 2007].

Structure of distribution grids The structure or "topology" of a grid can vary considerably. The physical layout is often forced by what land is available and its geology. The logical topology can vary depending on the constraints of budget, requirements for system reliability, and the load and generation characteristics.

Figure represent a classic electricity distribution grids-simple radial tree, sending power from a source (point A representing power generation or a substation) to delivery points (other points representing homes, businesses, or other sub networks). The cheapest and simplest topology for a distribution or transmission grid is a radial structure. This is a tree shape where power from a large supply radiates out into progressively lower voltage lines until the destination homes and businesses are reached. Most transmission grids require the reliability that more complex mesh networks provide.

Other topologies used are looped systems and tied ring networks. National grids are composed of many smaller electrical networks that are linked together into a larger network called a Wide Area Synchronous Grid, also known as an interconnection. A Wide Area Synchronous Grid allows all the independent electrical networks in a particular area to be connected by synchronizing the electrical frequency between them. United Kingdom interconnections are synchronized at 50Hz.

Radial Feeder System

In a radial configuration, lines branch out sequentially and power flows strictly in one direction, only one path is connected between each customer and the substations. The electrical power flows from the substation to the customer along a single path. If this path is interrupted, it will result in a complete loss of power to the customer. The loading of a distribution feeder is inherently unbalanced because of the large number of unequal single-phase loads that must be served. An additional imbalance is introduced by the non-equilateral conductor spacing of the three-phase overhead and underground line segments.

Some of the advantages of this system include: minimum initial cost and simplicity of planning, design and operation.

Disadvantages include: low reliability factor, distributor nearer to the feeding end is heavily loaded. Ring or Loop system: In a ring, any two points are usually connected by more than one path, meaning that some lines form loops within the system. This distribution system consists of two or more paths between the substations and the customers. It is selected to carry its normal load plus the load of the other half of the loop also.

Therefore the size of the feeder conductor in a loop distribution system is the same throughout the loop. Figure 4 below shows the ring or loop feeder system. Figure 4: Loop distribution system Advantages of this system include: Less conductor material is required as each part of the ring carries less current. Less voltage fluctuations. It is more reliable.

Disadvantages include: It is difficult to design compared to the radial system.

The efficiency of the system

The efficiency of the transmission line is given by: Efficiency =100% * Power supplied / (power supplied + power loss) PP+PL*100%= 2040020400+1558. 5*100%=92. 9% Task 4: Write a short report on distribution systems. The report should include the following content:

1. The most common LV distribution systems used.
2. A diagram of the single phase 3 wire 240/120 topology
3. Description of the single phase 3 wire 240/120 development
4. The effect of unequal loading
5. The advantages of the single phase 3 wire 240c/120v system
6. A diagram of the three phase 4 wire 208v/120 topology
7. three phase 3 wire 600v topology,
8. three phase 4 wire 480v/277v topology.

Examples of where this would be used Solution

Distribution Subsystem

The distribution system connects the distribution substations to the consumers’ service-entrance equipment. There are two types of distribution system: the primary and secondary distribution system.

The Primary Distribution System

The primary distribution lines range from 4 to 34. 5 kV and supply the load in a well-defined geographical area. The transmission system voltage is stepped-down to lower levels by distribution substation transformers. The primary distribution system is that portion of the power network between the distribution substation and the utilization transformers.

The primary distribution system consists of circuits, referred to as primary or distribution feeders that originate at the secondary bus of the distribution substation. The distribution substation is usually the delivery point of electric power in large industrial or commercial applications Primary distribution system voltages range from 2,400 V to 69,000 V. The distribution voltages in widest use are 12,470 V and 13,200 V, both three and four wire.

Types of Primary Distribution System. There are two fundamental types of primary distribution systems; Radial and Network.

Simply defined, a radial system has a single simultaneous path of power flow to the load. A network has more than one simultaneous path. Each of the two types of systems has a number of variations. There are other more complex systems, such as the primary network (interconnected substations with feeders forming a grid) and dual-service network (alternate feeder to each load). These systems, however, are simply variations of the two basic feeder arrangements.

Some small industrial customers are served directly by the primary feeders.

The secondary Distribution System

The secondary distribution network reduces the voltage for utilization by commercial and residential consumers. Lines and cables not exceeding a few hundred feet in length deliver power to the individual consumers. The secondary distribution serves most of the customers at levels of 240/120 V, single-phase, three-wire; 208Y/120 V, three-phase, four-wire; or 480Y/277 V, three-phase, four-wire.

The power for a typical home is derived from a transformer that reduces the primary feeder voltage to 240/120 V using a three wire line. Distribution systems utilize both overhead and underground conductors. The voltage levels for a particular secondary system are determined by the loads to be served. The utilization voltages are generally in the range of 120 to 600 V. In residential and rural areas the nominal supply is a 120/240 V, single-phase, three-wire grounded system. If three-phase power is required in these areas, the systems are normally 208Y/120 V or less commonly 240/120 V.

In commercial or industrial areas, where motor loads are predominant, the common three-phase system voltages are 208Y/120 V and 480Y/277 V. The preferred utilization voltage for industrial plants, however, is 480Y/277 V. Three-phase power and other 480 V loads are connected directly to the system at 480 V and fluorescent lighting is connected phase to neutral at 277 V. Small dry-type transformers, rated 480-208Y/120 or 480-120/240 V, are used to provide 120 V single-phase for convenience outlets and to provide 208 V single- and three-phase for small tools and other machinery.

Types of Secondary distribution Systems. Various circuit arrangements are available for secondary power distribution. The basic circuits are: simple radial system, expanded radial system, primary selective system, primary loop system, secondary selective system, and secondary spot network.

References

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