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DESIGNING OF PRIMARY AND SECONDARY distribution SYSTEMS

DESIGNING OF PRIMARY AND SECONDARY distribution SYSTEMS. INTRODUCTION:. Customers install all types of energy consuming devices which can be connected in all sorts of combination and at times of their choice.

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DESIGNING OF PRIMARY AND SECONDARY distribution SYSTEMS

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  1. DESIGNING OF PRIMARY AND SECONDARY distribution SYSTEMS

  2. INTRODUCTION: • Customers install all types of energy consuming devices which can be connected in all sorts of combination and at times of their choice. • The size and locations of the loads are decided by the customers not the distribution engineers so the power distribution systems should concentrate on gathering functions rather than distributing. • Starting from the individual customers and loads distribution proceeds through several stages with increasing no. of customers with each stage. • services →secondaries→distribution transformers→ primary feeders→distribution substation→ subtransmission→bulk power stations→ transmission lines→generating stations.

  3. One line diagram of distribution system

  4. Definitions: PRIMARY SYSTEMS: • The part of the electrical utility lying between the distribution substation and distribution transformer. • It is made up of circuits known as primary feeders or primary distribution feeders. SECONDARY SYSTEMS: • The part of the electrical utility lying between the primary system and consumer’s property. • Its includes step down transformers,secondary circuits(sec. mains),consumer’s services(service drops),meters to measure consumer’s energy consumption.

  5. Feeders: • Any of the medium-voltage lines used to distribute electric power from a substation to consumers or to smaller substations are called feeders. • A FEEDER INCLUDES : • Main or main feeder – A 3-Φ 4-wire circuit. • Branches or laterals - 1-Φ or 3-Φ circuits tapped off the main(sublaterals may be tapped off the laterals). • A given feeder is sectionalized by reclosing devices at various locations in such a manner that only a very small part of the faulted circuit is removed such that only few consumers are affected. UNDERGROUND PRIMARY FEEDERS: • They are usually radial three conductor cables. • Advantages: • Improved appearance • Less frequent trouble expectancy • Even though they are more expensive and the repair time is longer than overhead systems they are used in heavily loaded and congested metropolitan areas.

  6. Factors affecting selection of primary feeder rating: • The nature of the load connected. • The load density of the area served. • The growth rate of the load. • The need of providing spare capacity for emergency operation. • The type & cost of circuit construction employed. • The design and capacity of substation involved. • The type of regulating equipment used. • The quality of service required. • The continuity of service required.

  7. PRIMARY DISTRIBUTION SYSTEM

  8. One line diagram of primary distribution feeders:

  9. Simple radial type primary feeder:

  10. Radial type primary feeder: • Simplest ,lowest cost,most common form. • The main primary feeder branches into various primary laterals which in turn separates into several sublaterals to serve all the distribution transformers. • The main feeder and sub feeders are 3-Φ 3 wire or 4 wire circuits and the laterals are 3-Φ or 1-Φ. • In such arrangements the current magnitude is greatest in the circuit conductors that leave the substation and it goes on decreasing as we move out towards the end of the feeder as the laterals and sublaterals are tapped off the feeder. • As the current lessens the size of the feeder also gets reduced. • However, the permissible voltage regulation may restrict any feeder size reduction which is based only on the thermal capability,i.e.,current–carrying capacity,of the feeder.

  11. Modified radial-type primary feeder • The reliability of service continuity of simple radial type primary feeder is very low. • A fault occurrence at any location on the radial primary feeder causes a power outage for every consumer on the feeder unless the fault can be isolated from the source by a disconnecting device such as fuse,sectionalizer,disconnecting switch or recloser. • Modified radial type primary feeder with tie and sectionalizing switches provides fast restoration of service to customers by switching unfaulted sections of the feeder. • The faulted sections are isolated by opening the associated disconnecting device on each side of the faulted section. • Express feeder-the section of the feeder between the substation and the load center of the service area. No subfeeders or laterals are allowed to be tapped off. • A subfeeder is allowed to provide a backfeed towards the substation from the load center.

  12. Modified Radial type primary feeder figure:

  13. Radial type primary feeder with express feed and backfeed figure:

  14. Loop type primary feeder

  15. Loop type primary feeder: • Loops through the feeder load area and returns back to the bus. • They are provided with tie disconnect switches or tie breakers normally open or closed. • The size of the feeder conductor is kept constant throughout the loop. • Feeder is designed so as to carry its normal load plus the load of the other half of the loop. • It provides 2 parallel paths from the substation to the load when the loop is operated with N.O. tie breakers or disconnecting switches. • In case of a primary fault the feeder breaker opens and remains open until the fault is isolated from both the directions. • The parallel feeder paths can also be connected to separate sections of the bus and supplied from separate transformers. • Provides high service reliability.

  16. Primary feeder voltage levels: • It is the most important affecting the design,cost & operation. • Design & operation aspects getting affected are: • Primary feeder length • Primary feeder loading • No. of distribution substations • Rating of distribution substations • No. of subtransmission lines • No. of consumers affected by a specific outage • System maintenance practices • Extent of tree trimming • Joint use of utility poles , type of pole design , appearance of pole line

  17. Certain design considerations: • Primary feeders located in low load density areas are restricted in length and loading by permissible voltage drops rather than by thermal limits. • For those located in high load density areas may be restricted by thermal limitations. • Voltage square rule:According to this the feeder length and loading are a direct functions of feeder voltage level for a given voltage drop. • If the feeder voltage is doubled then for the same voltage drop it can supply the power 4 times the distance. • Feeder with increased length feeds more load.

  18. Primary network:

  19. Primary networks: • Primary network is a system of interconnected feeders supplied by a no.of substations. • It supplies load from several directions. • Radial primary feeders can be tapped off the interconnecting tie feeders. They can also be tapped off directly from the substations. • Each tie feeder has two circuit breakers at each end so that there is less service interruptions due to tie-feeder fault. • Losses are less in comparison to radial type feeder due to load division. • Reliability and the quality of service is better than radial type. • Complex and more difficult design.

  20. One line diagram of a radial Sec0ndary network:

  21. Secondary banking:

  22. SECONDARY BANKING: • BANKING: Parallel connection or interconnection of secondary side of two or more distribution transformers which are supplied by the same primary feeder. • ADVANTAGES: • Improved voltage regulation • Reduced voltage dip or light flicker due to motor starting,by providing parallel supply paths for motor starting currents. • Improved service continuity or reliability. • Improved flexibility in accommodating load growth,at low cost.

  23. Secondary network:

  24. SECONDARY MAINS: 3-Φ 4 wire Y-connected with solidly grounded neutral conductor. • Proper size & arrangement of secondary mains should provide for: • Proper division of normal load among network transformers • Proper division of fault current among network transformers • Good voltage regulation to all consumers. • Burning off short circuits or grounds at any point without interrupting service. • The conductor size depends on: • the network transformers. • burning faults clear • Voltage drop criterion LIMITERS: • High capacity fuse with a restricted copper section, and installed in each phase conductor of secondary main at each junction point. • Distribution engineer’s decision of using limiters should be based upon 2 considerations: • Minimum service interruption • Whether the saving in damage to cables pays more than the cost of the limiters.

  25. NETWORK PROTECTOR: • A network transformer is connected to the secondary network through the NP. • It consists of an air C.B. with a tripping and closing mechanism controlled by network master,phase relay,and back up fuses all enclosed in a metal case. • FUNCTIONS: • To provide automatic isolation of faults occurring in the network transformer and primary feeder. • To provide automatic reclosure under the predetermined conditions,i.e.,when the primary feeder voltage magnitude and the phase relation w.r.t. the network voltage are correct. • To provide protection against reverse power flow in some feeders connected to separate buses. • NETWORK transformer: • May be mounted on the poles(75 or 150 KVA) or platforms(300 KVA)depending on their sizes. • 1-Φ or 3-Φ distribution transformers.

  26. HIGH VOLTAGE SWITCH Position 2-normal operation,Position 3-disconnecting the network transformer, Position 1-grounding the primary circuit.

  27. THANK YOU

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