Specialized hardware is necessary to interconnect the elements of a power-distribution system. Utility control and switching systems operate under demanding conditions, including high voltage and current levels, exposure to lightning discharges, and 24-hour-a-day use. For reliable performance, large margins of safety must be built into each element of the system. The primary control and switching elements are high-voltage switches and protection devices.
High-voltage switches are used to manage the distribution network. Most disconnect switches function to isolate failures or otherwise reconfigure the network. Air-type switches are typically larger versions of the common knife switch device. To prevent arcing, air switches are changed only when power is removed from the circuit. These types of switches can be motor driven or manually operated.
Oil-filled circuit breakers are used at substations to interrupt current when the line is hot. The contacts usually are immersed in oil to minimize arcing. Oil-filled circuit breakers are available for operation at 500 kV and higher. Magnetic air breakers are used primarily for low-voltage indoor applications.
Protection devices include fuses and lightning arresters. Depending upon the operating voltage, various types of fuses can be used. Arc suppression is an essential consideration in the design and operation of a high-voltage fuse. A method must be provided to extinguish the arc that develops when the fuse element begins to open. Lightning arresters are placed at numerous points in a power-distribution system.
Connected between power-carrying conductors and ground, they are designed to operate rapidly and repeatedly if necessary. Arresters prevent flashover faults between power lines and surge-induced transformer and capacitor failures. The devices are designed to extinguish rapidly, after the lightning discharge has been dissipated, to prevent power follow on damage to system components.
A fault in an electrical power system is the unintentional and undesirable creation of a conducting path (a short circuit) or a blockage of current (an open circuit). The short-circuit fault is typically the most common and is usually implied when most people use the term “fault.” The causes of faults include lightning, wind damage, trees falling across lines, vehicles colliding with towers or poles, birds shorting out lines, aircraft colliding with lines, vandalism, small animals entering switchgear, and line breaks resulting from excessive ice loading. Power system faults can be categorized as one of four types:
• Single line-to-ground
• Line-to-line
• Double line-to-ground
• Balanced three-phase
The first three types constitute severe unbalanced operating conditions.
It is important to determine the values of system voltages and currents during fault conditions so that protective devices can be set to detect and minimize their harmful effects. The time constants of the associated transients are such that sinusoidal steady-state methods can typically be used.
High-voltage insulators permit all of the foregoing hardware to be reliably interconnected. Most insulators are made of porcelain. The mechanical and electrical demands placed on high-voltage insulators are stringent. When exposed to rain or snow, the devices must hold off high voltages. They also must support the weight of heavy conductors and other components.
FAULT PROTECTION DEVICES
Fuses are designed to melt and disconnect the circuit within which they are placed should the current in the circuit increase above a specified thermal rating. Fuses designed to be used in circuits operating above 600 V are classified as fuse cutouts. Oil-filled cutouts are mainly used in underground installations and contain the fusible elements in an oil-filled tank. Expulsion-type cutouts are the most common devices used on overhead primary feeders. In this class of device, the melting of the fusible element causes heating of a fiber fuse tube, which, in turn, produces deionizing gases to extinguish the arc. Expulsion type cutouts are classified as:• Open-fuse cutouts
• Enclosed-fuse cutouts
• Open-link-fuse cutouts
The automatic re-closer is an over current device that automatically trips and recloses a preset number times to clear or isolate faults. The concept of reclosing is derived from the fact that most utility system faults are temporary in nature and can be cleared by de-energizing the circuit for a short period of time.
Re-closers can be set for a number of operation sequences, depending on the action desired. These typically include instantaneous trip and reclose operation followed by a sequence of time-delayed trip operations prior to lockout of the re-closer. The minimum pick-up for most re-closers is typically set to trip instantaneously at two times the nominal current rating.
An automatic line re-closer is constructed of an interrupting chamber and the related contacts that operate in oil, a control mechanism to trigger tripping and reclosing, an operator integrator, and a lockout mechanism. An operating rod is actuated by a solenoid plunger that opens and closes the contacts in oil. Both single-phase and three-phase units are available.
The line sectionalizer is yet another over current device. It is installed in conjunction with backup circuit breakers or re-closers. The line sectionalizer maintains coordination with the backup interrupting device and is designed to open after a preset number of tripping operations of the backup element. Line sectionalizers are installed on poles or cross arms in overhead distribution systems. The standard continuous current rating for sectionalizers ranges from 10 to 600 A. Sectionalizers also are available for both single-phase and three-phase systems.
The function of a circuit breaker is to protect a circuit from the harmful effects of a fault, in addition to energizing and de-energizing the same circuit during normal operation. Breakers are generally installed on both the incoming sub-transmission lines and the outgoing primary feeders of a utility substation.
These devices are designed to operate as quickly as possible (less than 10 cycles of the power frequency) to limit the impact of a fault on the distribution and control system. At the same time, the arc that forms between the opening contacts must be quenched rapidly. Several schemes are available to extinguish the arc, the most common being immersion of the contacts in oil. Some circuit breakers have no oil, but quench the arc by a blast of compressed air. These are referred to as air circuit breakers. Yet another type encloses the contacts in a vacuum or a gas, such as sulfur hexafluoride (SF6).
Air circuit breakers are typically used when fault currents are relatively small. These devices are characteristically simple, are low cost, and require little maintenance. The fault current flows through coils, creating a magnetic field that tends to force the arc into ceramic chutes that stretch the arc, often with the aid of compressed air. When the arc is extinguished through vacuum, the breaker is referred to as a vacuum circuit breaker. Because a vacuum cannot sustain an arc, it can be an effective medium for this application.
However, owing to imperfections present in a practical vacuum device, a small arc of short duration can be produced. The construction of vacuum circuit breakers is simple, but the maintenance is usually more complex than with other devices.
LIGHTNING ARRESTER
A lightning arrester is a device that protects electrical apparatus from voltage surges caused by lightning. It provides a path over which the surge can pass to ground before it has the opportunity to pass through and damage equipment. A standard lightning arrester consists of an air gap in series with a resistive element. The resistive element is usually made of a material that allows a low-resistance path to the voltage surge, but presents a high-resistance path to the flow of line energy during normal operation.This material is known as the valve element. Silicon carbide is a common valve element material. The voltage surge causes a spark that jumps across the air gap and passes through the resistive element to ground.