Distribution includes all the parts of the electricity system between the power substations supplied from high-voltage transmission lines and the consumer’s switch.
Electric power is received from substations and distributed to the consumers at the voltage levels and with the degree of continuity that are acceptable to the various types of consumer. In large metropolitan systems both over head and underground distribution methods are used. Although underground distribution is more expensive than an overhead system, it is virtually a necessary in heavily urbanized areas. In smaller towns and in the less congested districts of large cities, the entire distribution system is usually overhead.
Electric System Monitoring
A modern electric power system is an assembly of many components each of which influences the behavior of every other part. Proper functioning of the system as a whole makes it necessary to monitor conditions existion at many different points on the system in order to assure optimum operation.
The concern of the customers is primarily that the frequency and voltage of the supply are held within certain rather narrow limits. Since frequency of the system is the same everywhere, it may be monitored by a single frequency meter located at any convenient point. In contrast the voltage of the system may be quite different at different points. Consequently, it is necessary to make continuous
observation of the voltage at certain key points on the system in order to provide acceptable service.
Efficient operation of the system is obtained by assigning proper load schedules to each of the generators on the system. Newer plants, although individually more efficient, may be located at points on the system where their loading occasions large system losses. It is desirable to operate with a division of the load between generators so that the total cost of fuel consumed is minimized. To provide
reliability of the power supply in tile event of unexpected conditions, it is desirable to have the total kilowatt rating of all machines in operation somewhat greater than the total load plus losses. This excess of generation, known as spinning reserve, is then available for picking up suddenly applied customer loads or to pick up the load dropped by a generator that must be removed from service for emergency maintenance.
Instrumentation is necessary to permit billing of Customers for energy used. Many interconnections exist between different power systems. Instruments must be provided at interchange points to permit billing for energy transferred from one system to another. The continuous monitoring of energy transfer is necessary to assure that interchanged power is within the limits of contract agreements.
The continuous measurement of conditions on major pieces of equipment is necessary to avoid damage due to overload. As load increases from month to month,
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points at which additional capacity of equipment is required may be recognized and provision made for the installation of additional equipment. Thus instrumentation serves as a guide for future construction in a growing power system.
Occasionally, under emergency conditions, a system operator observes that his system load exceeds the ability of the available generating and transmission equipment. He is then faced with the problem of load shedding or ,more properly, load conservation. It is then necessary to drop selected loads where service interruption is least objectionable. In such an event, he relies on the many instruments which provide information relative to system-operation conditions.
Instruments may sound alarms as advance warnings of conditions requiring action to avoid damage to equipment operating beyond its design limitation. In the event of extreme conditions such as power-system faults, defective equipment is switched out of service automatically. Instruments that continuously monitor current voltage, and other quantities must be able to identify the faulted equipment and to bring about operation of the circuit breakers which remove it from service, while leaving in service all other equipment on the operating system.
The many different electrical devices on a power system and those owned by the customers are designed for operation within certain specified ranges. Operation, excessive deterioration, or (in extreme cases ) the destruction of the device. Careful attention to the
conditions under which equipment is operating may indicate corrective action that must be taken.
Overcurrent on all electrical devices is undesirable, as it produces excessive temperatures, inefficient operation, and reduced service life. Overcurrent in residential circuits may bring about disconnection of the circuit by fuse or breaker action. Overcurrent in motors may damage insulation, with possible early insulation failure.
Undervoltage considerably reduces the efficiency of incandescent lamps and may result in nonoperation of fluorescent lamps. Undervoltage of the power supply to motors may result in excessive currents in the motors, with possible damage to windings.
Overvoltage increases the light output of lamps but in many
instances seriously shortens useful life. Overvoltage applied to motors and transformers may result in excessive losses within the iron, with possible damage to the iron or to the adjacent winding insulation.
Overspeed of rotating machines may result in structural damage to rotating parts. The overspeed of the customers’ production equipment may result in an inferior
quality of the product.
An out-of-step condition existing between two generators or between a generator and a synchronous motor results in an interruption of useful power transfer between the two machines. An out-of-step condition should be recognized promptly and the machines separated from each other. They may then be resynchronized and brought back into service.
Instruments of many different types must be installed at many locations on a power system and on the premises of the many customers. With such instruments, conditions existing on the system may be continuously monitored.
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