power delivery system

Power Delivery SystemThe primary mission of Power Delivery System is to deliver power to electrical consumers at their place of consumption and in ready to use form. That means, It must;

Place of consumptionBe dispersed throughout the service territory in rough proportion to customer location.Have sufficient capacity to meet the customer peak demand and energy demand.provide highly reliable delivery to its customerready to use formDeliver the power at the utilization voltage required for electrical appliances and equipment.provide stable voltage quality to its customer

And of course to meet the above missions at least possible cost.

AKM/Distribution System

As the power is moved from generation (large bulk source) to customer (small demand amounts) it is first moved in bulk quantity at high voltage and as power is dispersed throughout the service territory, it is gradually moved down to lower voltage levels. See fig. 1.


Natural question is why so?It is more economical to move power at high voltage.

The high voltage lines, while potentially economical, cost a great deal more than low voltage lines, but have a much greater capacity. They are only economical if one truly needs the giant size.

Utilization voltage is useless for the transmission of power. The application of these lower voltages for anything more than very local distribution at the neighborhood level results in unacceptably high electrical losses, severe voltage drops, and sky-high equipment cost.

It is costly to change the voltage level which does nothing to move the power any distance in and of itself.


While each level varies in the types of equipment it has, its characteristics, mission, and manner of design and planning all share several common characteristics:

Each level is fed power by the one above it.Both the nominal voltage level and the average capacity of equipment drops from level to level, as one move from generation to customer.Each level has many more pieces of equipment in it than the above. As a result, the net capacity of each level (No. of units times average size) increases as one moves towards the customers. (Greater capacity at every level is required for both for reliability & to accommodate coincidence of the load.)Reliability drops as one moves closer to the consumer.


In essence there are only two types of equipment that perform the power delivery function.

Transmission & distribution lines (Transmission lines, sub transmission lines, feeders, laterals secondary and service drops ie. Conductors)Transformers (Grid substations, area substations, service transformers etc.)

Added to these basic equipment types;Protective equipment ( Provide safety, reliability and fail safe operation)Voltage regulation equipment (maintain the voltage within the acceptable range as the load changes for both safety and economic reason)


What is Transmission And What is Distribution?By voltage Class: Transmission: 66 kV -1100kVDistribution:33kV and below (some country up to 132 kV is categorized as distribution.)

By Function: Transmission: Transmission of bulk powerDistribution:includes utilization voltage and sources for these including transformer & their source.

By Configuration: Transmission: often designed and operates in networkDistribution:often designed and operates radially.


The Distribution System Equipments & Terminology:Service Wire (Or Secondary Distribution): The conductor which route power at utilization voltage within very close proximity to customers.Service Transformer (Distribution Transformer Also Called Distribution Substation):The transformer which lowers the voltage at utilization voltage level.Load Center:Area served by a particular service transformer.Primary Distribution (Feeder): The line (conductor) which feds power at to the primary of service transformer.Substation( Area Substation):Meeting point between transmission & distribution the primary distribution lines receives power at this juncture. Service Area: Service area of a particular substation or feeder is the area served by that particular substation or feederSub-transmission Line:The line (conductor) which feds power to the primary of transformer at substations.


Feeder Layout Types

There are three fundamentally different ways Radial, LoopsNetwork


Radial System The biggest advantage of the radial system configuration, in addition to its lower cost, is the simplicity of analysis.

On debt side, radial feeder systems are less reliable than loop or network systems because there is only one path between the substation and the customer. Thus if any elements along the path fails, a loss of power delivery results.

But a cleaver design and planning of radial distribution system can achieve a fair degree of reliability even without much addition of cost (discus later).


Loop system

loop system consists of a distribution design with two paths between the sources (substations, service transformer) and customer. This layout is often called European because this configuration is preferred in Europe. Equipment is sized and each loop is designed so that service can be maintained regardless of where an open point is on the loop.In terms of complexity, a loop feeder system is only slightly more complicated than a radial system The major disadvantage of loop systems is capacity and cost. A loop must be able to meet all power and voltage drop requirements when fed from only one end, not both.


NETWORK

The distribution network involves multiple paths between all points in the network. Power flow between any two points is usually split among several paths, and if a failure occurs it instantly and automatically re-routes itself.Rarely does a distribution network involve primary voltage level network design, in which all or most of the switches between feeders are closed so that the feeder system is connected between substations. They are much more complicated, than other forms of distribution, and thus more difficult to analyze. Loadings and power flow, fault currents and protection, must be determined by network techniques such as those used by transmission planners. In densely populated area networks are not inherently expensive the radial system designed to serve the same loads. It only increases the complexity of the design. But in other areas a network configuration has definite increment to meet the excess capacity cost. The excess capacity cost has to be justifiable on the basis of reliability.


The alternative to this in urban area is several loop crossing each other as shown in fig. below.


Urban Vs Rural Electrification

The special characteristics of the urban distribution system are;Capacity limits design: Voltage drop & losses costs are seldom a major concern requiring large no. of feeders.Loads are large & often 3-phasesReliability requirements are above averageRoute are restricted ie. land problemFor these reasons following there are some common adaptations to work within these design constraints. UG practice: Maximum size cable is often installed: Very grid like planning:


And reinforcing is often done by:


Rural DistributionThe special characteristics of the Rural distribution system are;

Sparse loadLoads vary from small single phase to medium sized three phase.Distances are tremendousLosses are highVoltage drop limits designReliabilty requirements below averageOften not profitable


For these reasons following there are some common adaptations to work within these design constraints. Application of higher voltage than its typically used in urban or sub urban distribution to meet higher load reach.Use of single phase feedersExtreme and innovative measures are some time need to apply eg. Use of very high voltage with earth return.No provision for contingency back up of feedersRadial feeders layouts are normally the rule.


power delivery system

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