[MUSIC] After transmission, transmission, let's discuss electric power distribution. This 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 and 35 kilovolts 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 or households. 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. Distribution lines have lower voltage ratings such as 69, 34.5, and 13.8 kilovolts. For convenience, many in the industry refer to ratings of 115 kilovolts and above as transmission. Things are not that simple. Because lower voltages are often used for transmission in rural areas, where power transfer requirements are less. A functional definition is also used. Typically, transmission lines serve the bulk power system, and distribution lines serve retail customers. This distinction is also compromised as large industrial customers often receive retail services over high voltage lines. Let's talk about distribution networks. Distribution networks are usually one of two different types, radial or interconnected. Radial distribution networks are systems with a single power source for a group of distribution customers. In radial systems, distribution lines stem from a single power source and continue through the service area without a conduction to additional power supplies. This is the cheapest type of distribution network. But also the least reliable as there are no redundant or back up power sources. This type of system is more common in remote locations or in locations where a low population density. Interconnected distribution networks are composed of multiple connections to power supply sources. Interconnected systems might be configured in the loop with power sources located in various locations along the loop. They may also be configured in a web with power sources interconnected within the complex framework. Interconnected systems are more expensive than radial systems, but they offer a much higher level of reliability due to the redundancy of power sources. The transition from transmission to distribution happens in the power sub-station. As we discussed before, sub-stations are fenced in areas that contains switches, transformers, and other specialized electrical equipment that convert electric power from the transmission system to distribution voltage level. Distribution substations are where distribution circuits ordinate, are monitored, and are adjusted. Distribution substations have the following functions. Circuit breakers and switches enable the substation to be disconnected from the transmission grid or for distribution lines to be disconnected. Transformers step down transmission voltages, 34.5 kilovolts or more, down to primary distribution voltages. These are medium voltage circuits, usually 600 to 34,500 volts. From the transformer, power goes to the busbar that can split the distribution power off in multiple directions. The bus distributes power to distribution lines, which fan out to customers. Urban distribution is mainly underground, sometimes in common utility ducts. Rural distribution is mostly above ground with utility poles and suburban distribution is a mix. Closer to the customer, a distribution transformer steps the primary distribution power down to a low voltage secondary circuit usually 120, 240 volts in the US for residential customers. The power comes to the customer via a service drop and electricity meter. The final circuit in an urban system may be less than 50 feet, but may be over 300 feet for a rural customer. The electricity that comes to homes and businesses must be metered. Once the electricity reaches its final destination, it runs through a meter for billing purposes. These meters have traditionally been electromechanical devices that measure the electricity as it passes through. Historically, an employee of the distribution company, a so-called meter reader, would come to read how much power had been used during the billing cycle. Today, meters are frequently more high tech, and can communicate with the distribution company without a meter reader going to the trouble of checking each meter individually. These new technologies are commonly referred to as Smart Meters. Smart meters use advances in information technology to allow the various pieces of the power grid, generators, distributors, and consumers, to communicate more effectively, and in real time. Collectively, these technology-enabled communications between different parts of the grid are referred to as the smart grid. As electric utilities convert analog features to digital, the grid is becoming smarter and allowing for new types of communication. For example, the smart grid allows customers with smart meters to change their consumption patterns, if they choose, by reacting to real time prices in the wholesale power market. It also allows power companies to better detect grid abnormalities or outages. However, replacing the existing infrastructure with a smarter one is expensive and can make the grid more susceptible to cybersecurity threats.
