Distribution System

DISTRIBUTION 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 consumption Be 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 form Deliver 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.Distribution system is the electrical system between the transmission system and the consumers meter. The integral part of a distribution system is Distributor:- A distributor is a conductor from which tapings are taken for supply to the consumers. The current through a distributor is not constant.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. 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 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.

Fig 1: power system overviewWhat is Transmission And What is Distribution?By voltage Class: Transmission: 66 kV -1100kV Distribution:33kV and below (some country up to 132 kV is categorized as distribution.)By Function: Transmission: Transmission of bulk power Distribution:includes utilization voltage and sources for these including transformer & their source.By Configuration: Transmission: often designed and operates in network Distribution:often designed and operates radially.By Design point of view: Transmission: grid status mainly the power and distance Distribution: Consumers demand and location

The Distribution System Equipments & Terminology: Service Wire (Or Secondary Distribution or LT/LV lines): 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,HT/HV Lines): The line (conductor) which feds power at to the primary of service transformer.

SERVICE MAINSFEEDER

SUB-STATION

DISTRIBUTOR

Generally from feeders no tapping is taken to the consumers, therefore loading of current of the feeders remains the same along its length. Substation( Area Substation):Meeting point between transmission & distribution line. 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 feeder Sub-transmission Line:The line (conductor) which feds power to the primary of transformer at substations. For distribution system, three-phase segments have a tremendous advantage: The current only has to be sent out to the load. There are no losses or voltage drop associated with bringing the current back to the source in a completely balanced ckt. By contrast, if serving a load from a single-phase Y-circuit (one-phase and neutral) or two-phase delta ckt. Current flows goes out of the load & then must return and there are losses associated with each direction. Exactly how and where the transition from 3-phase to 1-phase ckt. Takes place in a distribution system depends on planner preferences and other situations.

Primary & Secondary distribution system Primary Distribution System:- It is the part of distribution system which operates at voltages somewhat higher than general utilization Handles large blocks of electrical energy than the average low voltage consumers uses. Criterion for primary voltage level?? The most commonly used primary distribution voltages are 33kv, 11 kv, 6.6 kv, and 3.3kv. Due to economic consideration, normally primary distribution is carried out by 3-phase 3-wire system ?? Secondary Distribution System:- It is the part of distribution system at which the ultimate consumers utilizes the electrical energy delivered to them. It employs 400/230 V,3-phase 4-wire system in our case. The supply for this secondary distribution is obtained from distribution transformer.

Feeder Layout TypesThere are three fundamentally different ways Radial, Loops Network

Radial SystemIn this system, separate feeders radiate from a single substation and feed the distributors at one end only. 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. The end of the distributor nearest to the feeding point will be heavily loaded. The consumers at the distant end of a distributor would be subjected to serious voltage fluctuations when the load on the distributor changes.Loop SystemLoop system consists of a distribution design with two paths between the sources (substations, service transformer) and customer. The system is very reliable as each distributor is fed via two feeders. 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 A loop must be able to meet all power and voltage drop requirements when fed from only one end, not both. There are less voltage fluctuations at consumers terminals. The major disadvantage of loop systems is capacity and cost.Network system 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. It increases the service reliability. Any area fed from one generating station during peak load hours can be fed from the other generating stations. This reduces reserve capacity and increases efficiency of the system.

Urban Vs Rural ElectrificationThe major criterions for urban and rural electrification are voltage regulation, current carrying capacity of conductor and power loss in distribution system. 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-phases Reliability requirements are above average Route are restricted ie. land problemThe load density is very high in urban. Thus the current flowing in the conductor is very high. Before the criteria of voltage drop and power loss violation, the current carrying capacity of the conductor may be less than the current in the feeder. Thus, current carrying capacity of conductor is the guiding factor for the load center size. If the load center is chosen such that it doesnt exceed the current carrying capacity of the conductor then it automatically doesnt violate voltage regulation and power loss criterion. For these reasons there are some common adaptations to work within these design constraints as follows Use of Interlaced feeder ie two feeder in parallel. ( but there can be limits in number of possible gate way due to space limit in the junction due to higher insulation for more feeder.) Maximum size cable is often installed: Very grid like planning: The special characteristics of the Rural distribution system are; Sparse load Loads vary from small single phase to medium sized three phase. Distances are tremendous Losses are high Voltage drop limits design Reliability requirements below average Often 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 feeders Extreme and innovative measures are sometime need to apply eg. Use of very high voltage with earth return. No provision for contingency back up of feeders Radial feeders layouts are normally the rule.