LESSON NO. 1.pdf

DISTRIBUTION SYSTEM AND SUBSTATION DESIGN BY ENGR. JORGE P. BAUTISTA EE, MEP-ECE COURSE DESCRIPTION This course deals with the study and design of primary and secondary distribution networks, load characteristics, voltage regulation, metering techniques and systems and protection of distribution systems. COURSE OUTLINE 1. Fundamentals of distribution system 2. Load characteristics 3. Overhead distribution lines 4. Underground distribution lines 5. Distribution transformer applications 6. Voltage regulators 7. Substation layout REFERENCES Electric Power Distribution Handbook, by Short, T.A. CRC Press, 2004 Electric Power distribution System Engineering, by Gonen, T., 3rd ed, CRC Press, 2014 Electric Power Generation, Transmission and Distribution, by Grigsby, L., 3rd ed, CRC Press, 2012 LESSON NO. 1 FUNDAMENTALS OF DISTRIBUTION SYSTEM LESSON OBJECTIVES: AT THE END OF THE LESSON, THE STUDENT WOULD BE ABLE TO 1. Explain the parts of power distribution system 2. Understand the power generated from the supply system to users Distribution System The electrical energy produced at the generating station is conveyed to the consumer through a network of transmission and distribution systems. It is the part of power system which distributes electric power for local use In general, the distribution system is the electrical system between the sub station fed by the transmission system and the consumers meters It generally consists of feeders, distributors and the service mains 1.Feeders A feeder is a conductor which connects the sub-station (or Localized generating station) to the area where the power is distributed. Generally, no tappings are taken from the feeder so That the current remains the same throughout. The main Consideration in the feeder is the current carrying capacity. 2. Distributor A distributor is a conductor from which tappings are taken For supply to the consumers. The current through the Distributors 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. 3. Service Mains A service mains is generally a small cable which connects the distributor to the consumer’s terminals. A distribution system may be classified according to; 1. Nature of Current. According to nature of current, distribution system may be classified as (a) DC distribution system (b) AC distribution system. 2. Type of Construction. According to type of construction, distribution system may be classified as (a) overhead system (b) underground system. The overhead system is generally employed for distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the underground system is used at places where overhead construction is impracticable or prohibited by the local laws. 3.Scheme of Connection. According to scheme of connection, the distribution system may be classified as (a) radial system (b) ring main system (c) inter connected system. Types of Delivery Systems The delivery of electric energy from the generating plant to the consumer may consist of several more or less distinct parts that are nevertheless somewhat interrelated. The part considered “distribution” i.e., from the bulk supply substation to the meter at the consumer’s premises, can be conveniently divided into two subdivisions: 1. Primary Distribution which carries the load at higher than utilization voltages from the substation (or other source) to the point where the voltage is stepped down to the value at which the energy is utilized by the consumer. 2. Secondary Distribution which includes that part of the system operating at utilization voltages, up to the meter at the consumer’s premises. Primary distribution systems include three basic types: 1. Radial systems, including duplicate and throwover systems 2. Loop systems, including both open and closed 3. Primary network systems 1.RadialSystem.In this system, separate feeders radiate from a single substation and feed the distributors at one end only. The radial system is employed only when power is generated at low voltage and the substation is located at the center of the load. This is the simplest distribution circuit and has the lowest initial cost. However, it suffers from the following drawbacks: (a)The end of the distributor nearest to the feeding point will be heavily loaded. (b)The consumers are dependent on a single feeder and single distributor. Therefore, any fault on the feeder or distributor cuts off supply to the consumers who are on the side of the fault away from the substation. (c)The consumers at the distant end of the distributor would be subjected to serious voltage fluctuations when the load on the distributor changes. 2. Ring Main System. In this system, the primaries of distribution transformers form a loop. The loop circuit starts from the substation bus-bars, makes loop through the area to be served, and returns to the substation. The ring main system has the following advantages: (a) The are less voltage fluctuations at consumer’s terminals. (b) The system is very reliable as each distributor is fed via two feeders. In the event of fault on any section of the feeder, the continuity of supply is maintained. Secondary Distribution Secondary distribution systems operate relatively low utilization voltages and, like primary systems, involve considerations of service reliability and voltage regulation. The secondary system may be of four general types: 1.An individual transformer for each consumer ,i.e., a single service from each transformer. 2. A common secondary main associated with one transformer from which a group of consumer is supplied. 3. A continuous secondary main associated with two or more transformers, connected to two or more primary feeders, from which a large group of consumers is supplied. This is known as a low-voltage or secondary network. 1.Individual Transformer–Single Service Individual transformer service is applicable to certain loads that are more or less isolated, such as in rural areas where consumers are far apart and long secondary mains are impractical, or where a particular consumer has an extraordinarily large or unusual load even though situated among a number of ordinary consumer. 2.Common Secondary Main The most common type of secondary system in use employs a common secondary main. It takes advantage of diversity between consumer’s loads and demands. Moreover, the large transformer can accommodate starting currents or motors with less resulting voltage dip than would be case with small individual transformers. 3.Banked Secondaries This type of system may be viewed as a single-feeder low-voltage network, and the secondary may be along section or grid to which the transformers are connected. Fuses or automatic circuit breakers located between the transformer and secondary main serve to clear the transformer from the bank in case of failure of the transformer. Fuses may also be placed in the secondary main between transformer bank. 4.Secondary Networks In general, the secondary network is created by connecting together the secondary mains fed from the transformers supplied by two or more primary feeders. Automatically operated circuit breakers in the secondary mains, known as network protectors, serve to disconnect the transformer from the network when its primary feeder is de-energized; this prevents backfeed from the secondary in to primary feeder. This is especially important for safety when the primary feeder is de-energized from fault or other cause. The circuit breaker or protector fail to operate, the fuse will blow and disconnect the transformer from the secondary mains. Electric power distribution is the portion of the power delivery infrastructure that takes the electricity from the highly meshed, high-voltage transmission circuits and delivers it to customers. Primary distribution lines are “medium-voltage” circuits, normally thought of as 600 V to 35 kV. At a distribution substation, a substation transformer takes the incoming transmission-level voltage (35 to 230 kV) and steps it down to several distribution primary circuits, which fan out from the substation. Close to each end user, a distribution transformer takes the primary-distribution voltage and steps it down to a low-voltage secondary circuit commonly 120/240 V. From the distribution transformer, the secondary distribution circuits connect to the end user where the connection is made at the service entrance. an overview of the power generation and delivery infrastructure and where distribution fits in. Functionally, distribution circuits are those that feed customers. Some also think of distribution as anything that is radial or anything that is below 35 kV. The distribution infrastructure is extensive; after all, electricity has to be delivered to customers concentrated in cities, customers in the suburbs, and customers in very remote regions; few places in the industrialized world do not have electricity from a distribution system readily available. Distribution circuits are found along most secondary roads and streets. Urban construction is mainly underground; rural construction is mainly overhead. Suburban structures are a mix, with a good deal of new construction going underground. Elements of a substation A: Primary power lines' side B: Secondary power lines' side 1. Primary power lines 2. Ground wire 3. Overhead lines 4. Transformer for measurement of electric voltage 5. Disconnect switch 6. Circuit breaker 7. Current transformer 8. Lightning arrester 9. Main transformer 10. Control building 11. Security fence 12. Secondary power lines 13. Wave traps Video A https://www.youtube.com/watch?v=7Q-aVBv7PWM https://www.youtube.com/watch?v=R6aAVEmtO0c Types of substation 1. Transmission substation 2. Distribution substation 3. Collector substation 4. Converter substations 5. Switching station 1. Transmission Substation A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. Transmission substations can range from simple to complex. In such cases, substation contains high-voltage switches that allow lines to be- connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control/power factor correction devices such as capacitors, reactors or static VAR compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems. The largest transmission substations can cover a large area with multiple voltage levels, many circuit breakers. Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages, such as 66 kV and 33 kV, are usually considered sub-transmission voltages, but are occasionally used on long lines with light loads. Voltages above 765 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages. 2. Distribution Substation A distribution substation transfers power from the transmission system to the distribution system of an area. The input for a distribution substation is typically at least two transmission or sub transmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution substation typically operates at medium voltage levels, between 2.4 kV-33 kV. The feeders run along streets overhead (or underground, in some cases) and power the distribution transformers at or near the customer premises. In addition to transforming voltage, distribution substations also isolate faults in either the transmission or distribution systems. Distribution substations are typically the points of voltage regulation, although on long distribution circuits (of several miles/kilometers), voltage regulation equipment may also be installed along the line. The downtown areas of large cities feature complicated distribution substations, with high-voltage switching, and switching and backup systems on the low-voltage side. A distribution substation is a combination of switching, controlling, and voltage step-down equipment arranged to reduce sub-transmission voltage to primary distribution voltage for residential, farm, commercial, and industrial loads. What is a feeder? All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device. Distribution substation is generally comprised of the following major components: Supply Line Transformers Bus-bars Switchgear Out-coming feeders Switching apparatus Switches Fuses Circuit breakers Surge voltage protection Grounding 1. Supply line Distribution substation is connected to a sub-transmission system via at least one supply line, which is often called a primary feeder. However, it is typical for a distribution substation to be supplied by two or more supply lines to increase reliability of the power supply in case one supply line is disconnected. A supply line can be an overhead line or an underground feeder, depending on the location of the substation, with underground cable lines mostly in urban areas and overhead lines in rural areas and suburbs. Supply lines are connected to the substation via high voltage disconnecting switches in order to isolate lines from substation to perform maintenance or repair work. 2. Transformer Transformers “step down” supply line voltage to distribution level voltage. See Figure next slide. Distribution substation usually employs three-phase transformers. However, banks of single-phase transformers can also be used. Transformer can be classified by the following factors: a. Power rating Which is expressed in kilovolt-amperes (kVA) or megavolts- amperes (MVA), and indicates the amount of power that can be transferred through the transformer. Distribution substation transformers are typically in the range of 3 kVA to 25 MVA. b. Insulation Which includes liquid or dry types of transformer insulation. Liquid insulation can be mineral oil, nonflammable or low-flammable liquids. The dry type includes the ventilated, cast coil, enclosed non-ventilated, and sealed gas- filled types. Additionally, insulation can be a combination of the liquid, vapor, and gas-filled unit. c. Voltage rating Which is governed by the sub-transmission and distribution voltage levels substation to which the transformer is connected. Also, there are standard voltages nominal levels governed by applicable standards. Transformer voltage rating is indicated by the manufacturer. For example, 115/34.5 kV means the high-voltage winding of the transformer is rated at 115 kV, and the low voltage winding is rated at 34.5 kV between different phases. Voltage rating dictates the construction and insulation requirements of the transformer to withstand rated voltage or higher voltages during system operation. Transformer Voltage Ratings The following is a list of some conventions for specifying transformer voltage ratings: U-W The dash between the voltages U and W indicates they are on different sides of the transformer. For example 480—120 tells us the primary winding is rated 480 V and the secondary is rated 120 V. U/W The slash indicates the two voltages are from the same winding and that both voltages are available; Ex. 120/240 can indicate a 240 volt winding with a center tap. U×W The cross indicates a two-part winding that can be connected in series or parallel to give higher voltage or current, respectively. Only one voltage is available at a time; e.g., 120×240 indicates the transformer can operate at 120 V or 240 V, but not both simultaneously. U Y/W The Y indicates a three-phase winding connected in a wye conﬁguration. The ﬁrst letter (U) is the line voltage and the second letter (W) is the phase voltage (line to neutral). Clearly, U=√3 W. Examples include 208Y/ 120 and 480Y/277. d. Cooling Which is dictated by the transformer power rating and maximum allowable temperature rise at the expected peak demand. Transformer rating includes self-cooled rating at the specified temperature rise or forced-cooled rating of the transformer if so equipped. According to cooling methods 1. AA – dry type self cooled 2. AA/FA – dry type, force air 3. OA – oil immerse, self cooled 4. OA/FA – oil immerse, force air 5. OA/FOA – oil immerse, force oil and air 6. OA/FOW – oil immerse, force oil and water Air forced (blast) For transformers rated more than 3 MVA, cooling by natural air method is inadequate. In this method, air is forced on the core and windings with the help of fans or blowers. The air supply must be ﬁltered to prevent the accumulation of dust particles in ventilation ducts. This method can be used for transformers up to 15 MVA. e. Winding connections Which indicates how the three phases of transformer windings are connected together at each side. There are two basic connections of transformer windings: Delta (where the end of each phase winding is connected to the beginning of the next phase forming a triangle); and Star (where the ends of each phase winding are connected together, forming a neutral point and the beginning of windings are connected outside). Typically, distribution transformer is connected delta at the high-voltage side and wye at the low voltage side. Delta connection isolates the two systems with respect to some harmonics (especially third harmonic), which are not desirable in the system. A wye connection establishes a convenient neutral point for connection to the ground. f. Voltage regulation Which indicates that the transformer is capable of changing the low voltage side voltage in order to maintain nominal voltage at customer service points. Voltage at customer service points can fluctuate as a result of either primary system voltage fluctuation or excessive voltage drop due to the high load current. To achieve this, transformers are equipped with voltage tap regulators. Those can be either no-load type, requiring disconnecting the load to change the tap, or under-load type (on-load type), allowing tap changing during transformer normal load conditions. Transformer taps effectively change the transformation ratio and allow voltage regulation of 10–15% in steps of 1.75–2.5% per tap. Transformer tap changing can be manual or automatic. However, only under-load type tap changers can operate automatically. Video B https://www.youtube.com/watch?v=Ly1rm2pbq_E https://www.youtube.com/watch?v=sxZFUCy1qoc 3. Bus bars can be found throughout the entire power system, from generation to industrial plants to electrical distribution boards. Busbars are used to carry large current and to distribute current to multiple circuits within switchgear or equipment Plug-in devices with circuit breakers or fusible switches may be installed and wired without de-energizing the busbars if so specified by the manufacturer. 4. Switchgear Switchgear is a general term covering primary switching and interrupting devices together with its control and regulating equipment. Power switchgear includes breakers, disconnect switches, main bus conductors, interconnecting wiring, support structures with insulators, enclosures, and secondary devices for monitoring and control. Power switchgear is used throughout the entire power system, from generation to industrial plants to connect incoming power supply and distribute power to consumers. Switchgear can be of outdoor or indoor types, or a combination of both. Outdoor switchgear is typically used for voltages above 26 kV, whereas indoor switchgear is commonly for voltages below 26 kV. 5. Outcoming feeders A number of outcoming feeders are connected to the substation bus to carry power from the substation to points of service. Feeders can be run overhead along streets, or beneath streets, and carry power to distribution transformers at or near consumer premises. The feeders’ breaker and isolator are part of the substation low voltage switchgear and are typically the metal-clad type. When a fault occurs… When a fault occurs on the feeder, the protection will detect it and open the breaker. After detection, either automatically or manually, there may be one or more attempts to reenergize the feeder. If the fault is transient, the feeder will be reenergized and the breaker will remain closed. If the fault is permanent, the breaker will remain open and operating personnel will locate and isolate the faulted section of the feeder. 6. Switching apparatus Switching apparatus is needed to connect or disconnect elements of the power system to or from other elements of the system. Switching apparatus includes switches, fuses, circuit breakers, and service protectors. Switches are used for isolation, load interruption, and transferring service between different sources of supply. Isolating switches are used to provide visible disconnect to enable safe access to the isolated equipment. 7. Fuse Switching apparatus is needed to connect or disconnect elements of the power system to or from other elements of the system. Switching apparatus includes switches, fuses, circuit breakers, and service protectors. 8. Surge voltage protection Transient overvoltage are due to natural and inherent characteristics of power systems. Overvoltages may be caused by lightning or by a sudden change of system conditions (such as switching operations, faults, load rejection, etc.), or both. Generally, the overvoltage types can be classified as lightning generated and as switching generated 9. Grounding Grounding is divided into two categories: power system grounding and equipment grounding. Power system grounding means that at some location in the system there are intentional electric connections between the electric system phase conductors and ground (earth). The main purposes of equipment grounding are: 1. To maintain low potential difference between metallic structures or parts, minimizing the possibility of electric shocks to personnel in the area 2. To contribute to adequate protective device performance of the electric system, and safety of personnel and equipment 3. To avoid fires from volatile materials and the ignition of gases in combustible atmospheres by providing an effective electric conductor system for the flow of ground-fault currents and lightning and static discharges to eliminate arcing and other thermal distress in electrical equipment 3. Converter Substations Electrical machines or equipment operated on DC voltages from home to industrial applications. A converter station converts electricity between Alternating Current (AC) and Direct Current for sending electricity. Converter substations may be associated with High Voltage DC (HVDC) converter plants, traction current, or interconnected non-synchronous networks. These stations contain power electronic devices to change the frequency of current, or else convert from alternating to direct current or the reverse. 4. Collector Substation To build a wind farm, a solar farm or hydroelectric plants need a collector substation to tie all the generators and connect them to the power grid. It looks like a distribution substation although power flow is in the opposite direction, from many wind turbines or solar cells up into the transmission grid. Usually for economy of construction the collector system operates around 35 kV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation can also provide power factor correction if it is needed, metering, and control of the wind farm. 5. Switching Substation A switching station is a substation without transformers and operating only at a single voltage level. Switching stations are sometimes used as collector and distribution stations. Distribution Planning System 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 system from the secondary conductors through the bulk power substation, which are both technically adequate and reasonably economical. 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 the feeder losses and construction costs, while considering the constraint of service reliability. Factors affecting system planning 1. Load forecasting 2. Substation expansion 3. Substation site location 4. Other factors: primary voltage selection, feeder route selection, number of feeders, conductor size selection , total cost. The acceptability criteria, representing the company’s policies, obligations to consumers and additional constraint can include 1. Service continuity 2. The maximum allowable peak load voltage drop to the most remote customer on the secondary 3. The maximum allowable voltage dip occasioned by the starting of a motor of specified starting current characteristics at the most remote point on the secondary 4. The maximum allowable peak load 5. Service reliability 6. Power losses

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