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In order to distribute electrical power, it is necessary to use many types of specialized equipment. The electrical power system consists of such specialized equipment as power transformers, high-voltage fuses and circuit breakers, lightning arresters, power-factor-correcting capacitors, and power-metering systems. Some types of specialized power distribution equipment will be discussed in this Section.
TERMINOLOGY
In this section (Section 9), power distribution equipment is discussed. After studying this Section, you should have an understanding of the following terms:
- Substation
- High-Voltage Transmission Line
- High-Voltage Fuse
- High-Voltage Circuit Breaker
- High-Voltage Disconnect Switch
- Lightning Arrester
- High-Voltage Insulator
- High-Voltage Conductor
- ACSR Conductor
- Voltage Regulator
- Plug Fuse
- Cartridge Fuse
- Time Delay Fuse
- Renewable-Element Fuse
- Low-Voltage Circuit Breaker
- Protective Relay
- Fault Current
- Bimetallic Overload Relay
- Melting Alloy Overload Relay
- Undervoltage Protection
- Raceway
- Feeder Circuit
- Branch Circuit
- Underground Distribution
- Safety Switch
- Panelboard
- Low-Voltage Switchgear
- Load Center
- Service Entrances
- Grounding Electrode
- Ground
- Service Entrance Conductors
- Uninterruptible Power Supply
- Power Line Filter
- Power Conditioner
- Floor-Mounted Raceway
- Conduit Connectors
- Wire Connectors
- Plastic Conduit and Enclosures
- Power Outlet Design
- International Power Source
EQUIPMENT USED AT SUBSTATIONS
Substations are very important parts of the electrical power systems.
The link between the high-voltage transmission lines and the low-voltage power distribution systems is the substation. The function of a distribution substation, such as the one shown in FIG. 2, is to receive electrical power from a high-voltage system and convert it to voltage levels suitable for industrial, commercial, or residential use. The major functional component of a substation is the transformer, whose basic characteristics were discussed previously. However, there are many other types of specialized equipment required for the operation of a substation.
High-voltage Fuses
Since power lines are frequently short circuited, various protective equipment is used to prevent damage to both the power lines and the equipment. This protective equipment must be designed to handle high voltages and currents. Either fuses or circuit breakers may be used to protect high-voltage power lines. High-voltage fuses (those used for over 600 volts) are made in several ways. An expulsion-type fuse has an element that will melt and vaporize when it is overloaded, causing the power line it is connected in series with to open. Liquid fuses have a liquid-filled metal en closure that contains the fuse element. The liquid acts as an arc-suppressing medium. When the fuse element melts from an excessive current in a power line, the element is immersed in the liquid to extinguish the arc.
This type of fuse reduces the problem of high-voltage arcing. A solid-material fuse is similar to a liquid fuse, except that the arc is extinguished in a chamber filled with solid material.
Ordinarily, high-voltage fuses at substations are mounted adjacent to air-break disconnect switches. These switches provide a means of switching power lines and disconnecting them for repair. The fuse and switch enclosure is usually mounted near the overhead power lines at a substation.
High-voltage Circuit Breakers
Circuit breakers that control high voltages are also located at electrical substations. In this type of circuit breaker, the contacts are immersed in an insulating oil contained in a metal enclosure. Another type of high-volt age circuit breaker is the magnetic air breaker in which the contacts separate, in the air, when the power line is overloaded. Magnetic blowout coils are used to develop a magnetic field that causes the arc (which is produced when the contacts break) to be concentrated into arc chutes where it is extinguished. A modification of this type of circuit breaker is the com pressed-air circuit breaker. In this case, a stream of compressed air is concentrated on the contacts when the power line is opened. The compressed air aids in extinguishing the arc that is developed when the contacts open.
It should be pointed out that large arcs are present whenever a high-volt age circuit is interrupted. This problem is not encountered to any great extent in low-voltage protective equipment. There are two major types of high-voltage circuit breakers-oil filled and oilless.
These circuit breakers are designed to operate on voltages of 1000 volts to over 500,000 volts. Oil-filled circuit breakers are used primarily for outdoor substations, except for very high voltages in the range of 500,000 volts and higher. Oilless circuit breakers are ordinarily used for indoor operation.
High-voltage Disconnect Switches
High-voltage disconnect switches are used to disconnect electrical equipment from the power lines that supply the equipment. Ordinarily, disconnect switches are not operated when current is flowing through them. A high-voltage arcing problem would occur if disconnect switches were opened while current was flowing through them. They are opened mainly to isolate equipment from power lines for safety purposes. Most disconnect switches are the "air-break" type, which is similar in construction to knife switches. These switches are available for indoor or outdoor use in both manual and motor-operated designs.
Lightning Arresters
The purpose of using lightning arresters on power lines is to cause the conduction to ground of excessively high voltages that are caused by lightning strikes or other system problems. Without lightning arresters, power lines and associated equipment could become inoperable when struck by lightning. Arresters are designed to operate rapidly and repeatedly, if necessary. Their response time must be more rapid than that of the other protective equipment used on power lines.
Lightning arresters must have a rigid connection to ground on one side. The other side of the arrester is connected to a power line. Sometimes, they are connected to transformers or the insides of switchgear. Lightning is a major cause of power-system failures and equipment damage, so lightning arresters have a very important function. Lightning arresters are also used at outdoor substations. The lightning arrester is used to pro vide a path to ground for lightning strikes or hits. This path eliminates the flashover between power lines, which causes short circuits. Valve-type lightning arresters are used frequently. They are two-terminal devices in which one terminal is connected to the power line, and the other is connected to ground. The path from line to ground is of such high resistance that it is normally open. However, when lightning, which is a very high voltage, strikes a power line, it causes conduction from line to ground. Thus, volt age surges are conducted to ground before flashover between the lines occurs. After the lightning surge has been conducted to ground, the valve assembly then causes the lightning arrester to become nonconductive once more.
High-voltage Insulators
All power transmission lines must be isolated so as not to become safety hazards. Large strings of insulators are used at substations, and at other points along the power distribution system, to isolate the current carrying conductors from their steel supports or any other ground-mounted equipment. Insulators may be made of porcelain, rubber, or a thermoplastic material.
Power transmission lines require many insulators in order to electrically isolate the power lines from the steel towers and wooden poles that support the lines. Insulators must have enough mechanical strength to support power lines under all weather conditions. They must also have sufficient insulating properties to prevent any arcing between the power lines and their support structures. High-voltage insulators are usually made of porcelain. Insulators are constructed in "strings," which are suspended from steel or wooden towers. The design of these insulators is very important, since design affects their capacitance and their ability to withstand weather conditions.
High-voltage Conductors
The conductors used for power distribution are, ordinarily, uninsulated aluminum wires or aluminum-conductor steel-reinforced (ACSR) wires for long-distance transmission, and insulated copper wires for shorter distances.
Voltage Regulators
Voltage regulators are an important part of the power distribution system. They are used to maintain the voltage levels at the proper value, as a constant voltage must be maintained in order for the electrical equipment to function properly. For instance, motors do not operate properly when a reduced or an excessive voltage is applied to them. Transformer tap-changers, illustrated in FIG. 1 may be used as voltage regulators. The secondary tap can be changed, either manually or automatically, to change the volt age output, in order to compensate for changes in the load voltage. As load current increases, line loss (I × R) also increases. Increased line loss causes the secondary voltage (Vs) to decrease. If the secondary tap is initially connected to tap No.4, the secondary voltage can be boosted by reconnecting to either tap No. 3, No. 2, or No. 1. This can be done automatically with a motor-controlled tap changer. There are various other types of automatic volt age regulators that can be used with electrical power distribution systems.
FIG. 1. Transformer tap-changer voltage regulator
POWER SYSTEM PROTECTIVE EQUIPMENT
There are many devices that are used to protect electrical power systems from damage due to abnormal conditions. For instance, switches, fuses, circuit breakers, lightning arresters, and protective relays are all used for this purpose. Some of these devices automatically disconnect the equipment from power lines before any damage can occur. Other devices sense variations from the normal operation of the system and make the changes necessary to compensate for abnormal circuit conditions. The most common electrical problem that requires protection is short circuits. Other problems include overvoltage, undervoltage, and changes ill frequency. Generally, more than one method of protection is used to protect electrical circuits from faulty conditions. The purpose of any type of protective device is to cause a current-carrying conductor to become inoperative when an excessive amount of current flows through it.
Types of Fuses
The simplest type of protective device is a fuse. Fuses are low-cost items and have a fast operating speed. However, in three-phase systems, since each hot line must be fused, two lines are still operative if only one fuse burns out. Three-phase motors will continue to run with one phase removed. This condition is undesirable, in most instances, since motor torque is greatly reduced, and overheating may result. Another obvious disadvantage of fuses is that replacements are required. All protective devices, including fuses, have an operating-characteristic time curve, such as the one shown in FIG. 2, prevent any possible damage to equipment, circuit protection should be planned utilizing these curves. They show the response time required for a protective device to interrupt a circuit, when an overload occurs.
Plug Fuses--Fuses are used in safety switches and power distribution panels. The plug fuse is a common type of fuse. Standard sizes for this fuse are 10, 15, 20, 25, and 30 amperes at voltages of 125 volts or be low. These fuses have a zinc or metallic-alloy-fusible element enclosed in a case made of an insulating material. Their most common use is in safety switches and fuse panelboards.
FIG. 2. Typical operating-characteristic time curves for three different
types of fuses (Littelfuse, Inc.)
Cartridge Fuses--Cartridge fuses are commonly used in power distribution systems for voltages up to 600 volts. They have a zinc- or alloy-fusible element, which is housed in a round fiber enclosure. One type has a nonrenewable element, while another type has a renewable element. Cartridge fuses may be used to protect high-current circuits, since they come in sizes of 60,100,200,400,600, and 1000 amperes.
Time Delay Fuses--A modification of the plug or cartridge fuse is called a time delay fuse. This type of fuse is used to delay the circuit-interrupting action. It is useful where momentary high currents exist periodically, such as motor-starting currents. The fuse element melts only when an excessive current is sustained over the time-lag period; thus, sufficient circuit protection is still provided.
Time delay fuse are used to limit current on systems including electric motors, which draw higher currents during their start cycle than during normal operation. These devices allow the system to start up at a higher than normal current; they then protect the system during normal operation without disrupting the distribution system.
Fuse Metals--The type of metal used in fuses is ordinarily an alloy material or, possibly, aluminum. All metals have resistance, so when cur rent flows through metal, heat energy is produced. As the current increases, more heat is produced, causing the temperature of the metal to in crease. When the melting point of the fuse metal is reached, the fuse will open, causing the circuit to which it is connected to open. Metals that de compose rapidly are used, rather than ones that produce small metallic globules when they melt. This reduces the likelihood of any arc-over occur ring after the fuse metal has melted. The current rating of fuses depends upon the melting temperature of the fuse metal, as well as its shape, size, and the type of enclosure used.
