EDS UIT I 1. Introduction Distribution systems serve as the link from the distribution substation to the customer. This system provides the safe and reliable transfer of electric energy to various customers throughout the service territory. Typical distribution systems begin as the medium-voltage three-phase circuit, typically about 30–60 kV, and terminate at a lower secondary three- or single-phase voltage typically below 1 kV at the customer’s premise, usually at the meter. Distribution feeder circuits usually consist of overhead and underground circuits in a mix of branching laterals from the station to the various customers. The circuit is designed around various requirements such as required peak load, voltage, distance to customers, and other local conditions such as terrain, visual regulations, or customer requirements. These various branching laterals can be operated in a radial configuration or as a looped configuration, where two or more parts of the feeder are connected together usually through a normally open distribution switch. High-density urban areas are often connected in a complex distribution underground network providing a highly redundant and reliable means connecting to customers. Most three-phase systems are for larger loads such as commercial or industrial customers. The part of power system which distributes electric power from sub transmission system to local use is known as distribution systems. Power systems are comprised of 3 basic electrical subsystems. • Generation subsystem • Transmission subsystem • Distribution subsystem The distribution system may also be divided into three distinct subsystems. • Distribution substation • Primary distribution system Secondary distribution system 1.2Distribution substations:The distribution substation receives power from one or more transmission or sub transmission lines at the corresponding transmission or sub transmission voltage level and provides that power to one or more distribution feeders that originate in the substation and comprise the primary network. Most feeders emanate radially from the substation to supply the load. 1.2.1Primary Distribution:The primary distribution system consists of the feeders emanating from the substation and supplying power to 1 or more secondary distribution systems. Such feeders are usually 3-phase circuits. • • • Feeders are almost always radial from substation to loads (i.e., one way flow of power) in rural areas, usually radial in residential neighborhoods, and they are often radial even in urban areas. In densely populated urban areas, particularly commercial and business districts where reliability is critical, feeders may be looped. The prices to pay for such a reliability benefit are as follows: Protection is more costly since a given fault on the loop will require at least two protective devices to operate, and to obtain the reliability benefits, multiple switching devices must be installed along the feeder. The fault currents tend to be lower, closer to normal load currents, and therefore there is less margin between breaker trip current and normal load current. Voltage control is complex since there are 2 control points. One way to obtain the reliability benefit of a looped configuration while avoiding some of the above difficulties is to operate a looped configuration in open-loop, i.e., employ a normally open switch mid-way in the loop. Then when the loop is faulted, the normally open switch can be closed while a switch just downstream of the fault can be opened, and all of the de-energized loop up to the downstream switch can be supplied. This is illustrated in Fig. 4.
fault N.O. As indicated previously, the standard primary distribution voltage levels include 4.16kV, 7.2kV, 12.47kV, 13.2kV, 14.4kV, 23.9kV, and 34.5kV. However, equipment is specified in terms of voltage class. Equipment of one voltage class may be utilized in at any operating voltage assigned to that class. For example, an insulator of voltage class 15 kV may utilized in a 12.47kV, 13.2kV, and 13.8kV system. There are four major distribution-level voltage classes: 5kV, 15kV, 25kV, and 35kV. The 15kV voltage class is the most prevalent. 1.2.2Secondary Distribution:Branching from the main feeder are laterals, also referred to in the industry as taps or branches. The laterals may be three-phase, two-phase (two phases of the three-phase feeder with a neutral), or single-phase (one phase from the single phase feeder and a neutral). The laterals are usually protected with fuses so that faulted laterals do not cause interruption at the feeder level. • • • Standard secondary voltage levels are 120/240 single phase 120/208 3 phase 277/480 3 phase The 120/240 configuration is obtained from the low-side of a HV/240 volt transformer, where HV is the rated voltage on the high voltage side, and the 240 is the rated voltage on the low voltage side. Then a center tap is connected to the low voltage winding and grounded along with the low side of the primary winding. This provides three wires on the low voltage side. One is +120V, one is -120V, and one (the center tap) is 0. Thus, two are “hot,” one is ground (neutral). The 240V connection is obtained by connecting across the two hot wires. The 120V connection is obtained by connecting from either hot wire to the neutral wire. 1.3.ELEMENTS OF A DISTRIBUTION SYSTEM:In general, the distribution system is the electrical system between the sub-station fed by the distrubution system and the consumers meters. It generally consists of feeders, distributors and theservice mains. (i) Feeders. A feeder is a conductor which connects the sub-station (orlocalised generating station) to the area where power is to be distributed. Generally, no tappings 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.
(ii) Distributor. A distributor is a conductor from which tappings are taken for supply to the consumers. In Fig. 12.1, AB, BC, CD and DA are the distributors. The current through a distributor is not constant because tappings 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 ± 6% of rated value at the consumers’ terminals. (iii) Service mains. A service mains is generally a small cable which connects the distributor to the consumers’ terminals. 1.4.Classification of loads:A load or power requirement (also kVA) of a consumer varies widely. But in general the consumers can be grouped into a few categories as their needs and demands are the same. Broad classifications of loads are; (i) Residential loads (ii) Commercial loads (iii) Industrial loads (iv) Agricultural loads (v) Municipal loads Domestic and Residential Loads The important part in the distribution system is domestic and residential loads as they are highly variable and erotic. These consist of lighting loads, domestic appliances such as water heaters, washing machines , grinders and mixes ,TV and electronic gadgets etc. The duration of these loads will be few minutes to few hours in a day. The power factor of these loads in less and may vary between 0.5 to 0.7. In residential flats and bigger buildings, the diversity between each residence will be less typically between 1.1 to 1.15 . The load factor for domestic loads will be usually 0.5 to 0.6. Industrial Loads Industrial loads are of greater importance in distribution systems with demand factor 0.7 to 0.8 and load factor 0.6 to 0.7. For heavy industries demand factor may be 0.9 and load factor 0.7 to 0.8 Typical power range for various loads Cottage and small-scale industries : 3 to 20 kW. Medium industries (like rice mills, oil mills, workshops, etc.) : 25 to 100 kW
Large industries connected to distribution feeders (33 kV and below) : 100 to 500 kW Muncipal loads Most of the panchayats , small and medium municipalities have protected water system which usepumping stations. They normally operate in off peak time and use water pumps ranging from 10 h.p to 50 h.p or more, depending on the population and area. Agricultural and Irrigation Loads Most of the rural irrigation in India depends on ground water pumping or lifting water from tanks or nearby canals. In most cases design and pump selection is very poor with efficiencies of the order of 25%. Single phase motors are used (up to 10 h.p.) for ground water level 15 m in depth or less with discharge of about 20 l/sec while multi stage submersible pumps with discharge of 800 to 1000 l/m may require motors of 15 to 20 h.p. Sensitive and important Loads With computer applications in every area, computer loads and computer controlled process loads are often non-linear and sensitive. They require close tolerance limits for voltage and frequency (voltage limit ± 5% and frequency ± 0.5 Hz with unbalance and wave form distortion less than 3%. This requires special attention while providing the distribution of electric power. 1.5. DISTRIBUTION SYSTEM PLANNING: System planning is essential to assure that the growing demand for electricity can be satisfied by distribution system additions that are both technically adequate and reasonably economical. Even though considerable work has been done in the past on the application of some types of systematic approach to generation and transmission system planning, its application to distribution system planning has unfortunately been somewhat neglected. In the future, more than in the past, electric utilities will need a fast and economical planning tool to evaluate the consequences of different proposed alternatives and their impact on the rest of the system to provide the necessary economical, reliable, and safe electric energy to consumers. The objective of distribution system planning is to assure that the growing demand for electricity, in terms of increasing growth rates and high load densities, can be satisfied in an optimum way by additional distribution systems, from the secondary conductors through the bulk power substations, which are both technically adequate and reasonably economical. All these factors and others, for example, the scarcity of available land in urban areas and ecological considerations, can put the problem of optimal distribution system planning beyond the resolving power of the unaided human mind. Distribution system planners must determine the load magnitude and its geographic location. Then the distribution substations must be placed and sized in such a way as to serve the load at maximum cost effectiveness by minimizing feeder losses and construction costs, while considering the constraints of service reliability. In the past, the planning for other portions of the electric power supply system and distribution system frequently has been authorized at the company division level without the review of or coordination with long-range plans. As a result of the increasing cost of energy, equipment, and labor, improved system planning through use of efficient planning methods and techniques is inevitable and necessary. The distribution system is particularly important to an electrical utility for two reasons: (1) its close proximity to the ultimate customer and (2) its high investment cost. Since the distribution system of a power supply system is the closest one to the customer, its failures affect customer service more directly than, for example, failures on the transmission and generating systems, which usually do not cause customer service interruptions. Therefore, distribution system planning starts at the customer level. The demand, type, load factor, and other customer load characteristics dictate the type of distribution system required.