In electricity distribution networks, spot network substations (network transformers) are used in interconnected distribution networks. They have the secondary network (also called a grid network) with all supply transformers bussed together on the secondary side at one location. Spot networks are considered the most reliable and most flexible arrangement of connecting power to all types of loads. [1] Switching can be done without interrupting the power to the loads.
Electricity distribution networks are typically of two types, radial or interconnected. A radial network arranges the station and branches like a tree with no connection to any other supply. This is typical of long rural lines with isolated load areas. In general, the radial distribution network has more power failures than the interconnected distribution networks.
In a secondary network the transformers are distributed across an area (e.g. in streets) and have multiple supplies. The transformers are wired together on the secondary side. The system is arranged so that nearby transformers do not use the same feeder. In case of an issue with a feeder (or transformer) the load is fed by nearby transformers, so there is no interruption, although a voltage drop for said load may then be present. A fault on the secondary side may cause damage to the transformers before the primary-side protection system detects and clears the fault. [2]
A spot network is basically a secondary network condensed to a point. Several transformers have multiple supplies and their secondaries are bussed together. Besides a region-wide blackout, they are vulnerable to a bus fault, which is extremely rare. The simplest case is where each transformer connects to one feeder and vice versa ("unit system"). High-voltage switching can be used to handle more cases, e.g., working transformer but faulty feeder or the reverse.
Network protectors, (reverse current relays), are used to detect any open circuits that are letting the electrical current flow back towards its source.
In large cities, many electric utility companies use grid feeders to make interconnected distribution networks to serve the downtown core. The interconnected network has multiple connections to the points of supply. Some of New York City's downtown areas are powered by submersible network transformers of 500 to 2,500 kVA. Usually these transformers are in vaults below metal grates in the sidewalks. [3]
Urban (spot) network transformer substations can be used to make interconnected distribution networks to serve a single facility. These substations may consist of two to eight or more primary transformers connected to the same secondary bus to provide reliable facility power. Examples of such single facilities include airports, hospitals, major data processing centers (especially those using uninterruptible power supplies), and sports arenas that regularly broadcast nationally televised events. St. Jude Children's Research Hospital in Memphis, Tennessee has eight primary transformers that are connected to the same secondary bus. The FedExForum in Memphis has a network of four primary transformers connected to the same secondary bus. In some arrangements with four transformers, any of the transformers can carry all of its connected loads. The Toronto Pearson International Airport is electrically fed by four grid feeders, each capable of carrying the entire 20+ MW load. [4]
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines which facilitate this movement are known as a transmission network. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. The combined transmission and distribution network is part of electricity delivery, known as the electrical grid.
In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the earth.
Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment and household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.
Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power from an electrical grid to remote areas at low cost. Its distinguishing feature is that the earth is used as the return path for the current, to avoid the need for a second wire to act as a return path.
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation; no conductive path is present between source and load. This isolation is used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.
In electrical engineering, particularly power engineering, voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation describes the ability of a system to provide near constant voltage over a wide range of load conditions. The term may refer to a passive property that results in more or less voltage drop under various load conditions, or to the active intervention with devices for the specific purpose of adjusting voltage.
An autotransformer is an electrical transformer with only one winding. The "auto" prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which have no metallic conducting path between them.
A zigzag transformer is a special-purpose transformer with a zigzag or "interconnected star" winding connection, such that each output is the vector sum of two (2) phases offset by 120°. It is used as a grounding transformer, creating a missing neutral connection from an ungrounded 3-phase system to permit the grounding of that neutral to an earth reference point; to perform harmonic mitigation, as they can suppress triplet harmonic currents; to supply 3-phase power as an autotransformer ; and to supply non-standard, phase-shifted, 3-phase power.
A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.
A distribution transformer or service transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. The invention of a practical efficient transformer made AC power distribution feasible; a system using distribution transformers was demonstrated as early as 1882.
In electric power distribution, automatic circuit reclosers (ACRs) are a class of switchgear which is designed for use on overhead electricity distribution networks to detect and interrupt momentary faults. Also known as reclosers or autoreclosers, ACRs are essentially high voltage rated circuit breakers with integrated current and voltage sensors and a protection relay, optimized for use as an overhead network distribution protection asset. Commercial ACRs are governed by the ANSI/IEEE C37.60, IEC 62271-111 and IEC 62271-200 standards. The three major classes of operating voltage are 15.5 kV, 27 kV and 38 kV.
An earthing system or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary considerably among countries, though most follow the recommendations of the International Electrotechnical Commission. Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.
Power system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the disconnection of faulted parts from the rest of the electrical network. The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation. Thus, protection schemes must apply a very pragmatic and pessimistic approach to clearing system faults. The devices that are used to protect the power systems from faults are called protection devices.
A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.
A network protector is a type of electric protective device used in electricity distribution systems. The network protector automatically disconnect its associated distribution transformer from the secondary network when the power starts flowing in reverse direction. Network protectors are used on both spot networks and grid networks. The secondary grid system improves continuity of service for customers, since multiple sources are available to supply the load; a fault with any one supply is automatically isolated by the network protector and does not interrupt service from the other sources. Secondary grids are often used in downtown areas of cities where there are many customers in a small area.
An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids vary in size and can cover whole countries or continents. It consists of:
In the recent years, utilization of electrical energy increased exponentially and customer requirement and quality definitions of power were changed enormously. As the electric energy became an essential part of the daily life, its optimal usage and reliability became important. Real-time network view and dynamic decisions have become instrumental for optimizing resources and managing demands, thus making a distribution management system which could handle proper work flows, very critical.
A low-voltage network or secondary network is a part of electric power distribution which carries electric energy from distribution transformers to electricity meters of end customers. Secondary networks are operated at a low voltage level, which is typically equal to the mains voltage of electric appliances.
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In some cases, the feeder relays may not clear a low-voltage fault until sufficient transformer damage occurs to involve the high-voltage winding
for the Ontario Energy Board