Electrical Power Distribution An AC System Louis E. Frenzel A presentation of e. Syst. org
Summary • Course use: AC circuits, combined DC/AC course. Inclass presentation. • Objective: To give a relevant system example early in the electronics curriculum. • Content: Provides a general introduction to AC power generation, transmission, and distribution. Introduce conventional AC house wiring and related hardware as a system. A presentation of e. Syst. org
An Introduction to AC Power Wiring • Most electronic equipment gets its power from the AC power lines. • The AC line provides the input for power supplies that generate one or more DC voltage to operate the circuits in electronic equipment. • Most technicians will encounter AC power distribution in their work and at home. • It is useful to understand the fundamentals of AC wiring. • AC wiring in a home is a good example of an AC system. A presentation of e. Syst. org
The AC Power Distribution Module • This module is designed to be used as part of an AC circuits course. • The prerequisite to this module is a basic understanding of AC theory including transformers. A presentation of e. Syst. org
Objectives • • • Understand the basic process of AC power generation. Understand the basic process of AC power distribution. Explain the rationale for the use of high voltage distribution. Name the contents of the service entrance in a typical home. Indicate the voltage levels in home power distribution. Trace or wire the connections in an AC outlet, switch, and light fixture. Indicate common wire sizes, color codes, and current capacity. State the basic conditions for electrical shock in a human. Explain how the separate ground wire protects humans and equipment. State how a ground fault interrupter works. Name the source of the rules and regulation of electrical wiring. A presentation of e. Syst. org
Part 1 POWER GENERATION AND TRANSMISSION A presentation of e. Syst. org
AC Generation • AC power is generated by huge electromechanical generators at a utility’s power station. • The power comes from heat generated by coal, natural gas, oil, nuclear, or water energy. • The heat produces steam that drives a turbine. A presentation of e. Syst. org
AC Generation • Refer to Fig. 1. The heat turns the turbine that, in turn, rotates the generator. • A DC generator is also rotated to create the power for the AC generator field coils. A presentation of e. Syst. org Figure 1
AC Generator • Fig. 2 shows how the AC generator works. • Fixed stator poles and coils generate the 3 phase voltages. • A center rotating magnetic field induces voltages into the stator coils as it turns. • The field magnet gets its power from the DC generator. A presentation of e. Syst. org Figure 2
3 -Phase Voltages • Fig. 3 shows the three sine wave voltages produced by the generator. • There is a 120 degree time difference between the three sine wave outputs. Figure 3 A presentation of e. Syst. org
Generator Connections • There are two ways to connect the three generator output coils: delta (Δ) and wye (Y). See Fig. 4. A presentation of e. Syst. org Figure 4
Generator Connections • With the delta connection, each phase output (Vo) is the same as the generator coil voltage (Vg). • With the Y connection, each coil is in series with another coil so each pair of coil voltages (Vg) add. However, because the voltages are out of phase, the coil voltages do not add directly. • The voltage between any two Y output connections (Vo) is equal to the voltage of one coil multiplied by 1. 732. • If the voltage from one coil is 16, 000 volts, the voltage between each Y connection is 16, 000 x 1. 732 = 27, 712 volts. A presentation of e. Syst. org
Long Distance Transmission • The AC generator produces a very high output voltage in the 13 k. V to 40 k. V range. • For long distance transmission, the voltage is further stepped up with a transformer to an even higher voltage. See Fig. 5 on the next slide. A presentation of e. Syst. org
Long Distance Transmission A presentation of e. Syst. org Figure 5
Long Distance Transmission • Common values are 138 k. V, 550 k. V and 765 k. V. Newer systems use mega volt levels. • This step up in voltage is done to minimize power loss in the long lines. • To illustrate how power is lost in the transmission lines, assume we wish to transmit 100 volts one mile to a 10 ohm load. See Fig. 6. A presentation of e. Syst. org Figure 6
High Loss at Low Voltages • With no line resistance, the load would receive 100 volts and produce a current in the line of V/R = 100/10 = 10 amperes. • A power of V 2/R = 1000 watts is transmitted. • However, the transmission lines have resistance. A presentation of e. Syst. org
High Loss at Low Voltages (continued) • Assume a size #0 wire with a resistance of 0. 528 ohms per 1 mile. With two wires, the total line resistance is 1. 056 ohms. • Total circuit resistance is 10 + 1. 056 = 11. 056 ohms. • With 100 volts applied, the current in the lines and load is 100/11. 056 = 9. 04 amps. • There is a voltage drop across each line of 9. 04 x 0. 528 = 4. 77 volts. • The total line voltage loss is 9. 54 volts. • The voltage at the load is only 100 – 9. 54 = 90. 46 volts. • The power delivered to the load is only 90. 46 x 9. 04 = 818 watts. • 182 watts is lost as heat in the transmission lines. A presentation of e. Syst. org
Low Loss at High Voltages • The secret to minimizing line voltage losses is to step the voltage up to a very high voltage first. • Assume the 100 volts is stepped up to 10, 000 volts as shown in Fig. 7. Figure 7 A presentation of e. Syst. org
Low Loss at High Voltages • The current in the lines is only 0. 1 amp. • The voltage drop across each line is only 0. 1 x. 528 = 0. 0528 volts or 0. 1056 total out of 10, 000. The loss is negligible. • The total power lost is only 0. 1056 x 0. 1 = 0. 01056 watts. • The voltage is stepped down to 100 volts at the load so virtually all of the original power is received. A presentation of e. Syst. org
Sub-Stations • As the transmitted power gets near the area to be served, it is stepped down to a lower voltage at a sub-station by a transformer. • Two or more substations may be used resulting in even lower voltage. See Fig. 5. • The resulting lower voltage will then be distributed to the local neighborhoods and other areas. • The voltage will be in the 2000 to 7000 volt range, 4100 volts here. A presentation of e. Syst. org
Local Distribution • The lower voltage is sent on wires to local areas. There it is stepped down to the desired voltage. • One arrangement is shown in Fig. 8. A transformer on a pole steps the 4100 volts on the line down to 240 volts. A presentation of e. Syst. org Figure 8
Local Distribution • Some local distribution is by underground wiring and a transformer on a concrete pad. See Fig 9. A presentation of e. Syst. org Figure 9
Source Distribution • Fig. 10 shows how the local high voltage is distributed. A presentation of e. Syst. org Figure 10
Source Distribution • The voltage to homes is from a 240 volts center-tapped secondary winding on the transformer. This provides two 120 volt circuits. • Some offices or factories get different voltages depending on their need. Voltages or 208 and 480 volts are typical. A presentation of e. Syst. org
Part 2 IN-HOME DISTRIBUTION A presentation of e. Syst. org
Service Entrance • The service entrance refers to the connections to a central distribution and connection point where the power line enters the home. A presentation of e. Syst. org
Service Entrance • The service entrance includes the line from the transformer, a kilowatt hour meter and the service box that contains all of the circuit breakers and connections to all wiring throughout the house. See Fig. 11. • Note the bus bars to which are screwed all of the wiring circuits. A presentation of e. Syst. org Figure 11
Service Box Wiring • Another view of the service box is given in Fig. 12. A presentation of e. Syst. org Figure 12
Service Box Wiring • Note that the center tap of the incoming transformer connections is connected to earth ground. This becomes the neutral connection. • The voltage is then distributed to multiple circuits throughout the house or building. Each circuit feeds several AC outlets and lights that receive 120 volts. • There are several special circuits that receive 240 volts for a water heater, clothes dryer, or heating and air conditioning system. A presentation of e. Syst. org
Circuit Breakers • Each circuit is protected by a circuit breaker, a switch that connects in series with the hot wire of each circuit. See Fig. 13. • When used as a switch the circuit breaker provides a way to turn off the voltage to a circuit while work is being done. • The circuit breaker is designed to pass a specific maximum current. If the current exceeds that value, the switch opens and turns off the voltage. • The circuit breaker protects against current overloads that can produce sufficient heat to start a fire. A presentation of e. Syst. org Figure 13
Circuit Breakers (continued) • When the circuit breaker switch is on, current passes through an electromagnet. If the current exceeds the rated current value, the electromagnet has sufficient pull to disengage the switch turning off the voltage to that circuit. • Circuit breakers are available in current ratings of 15, 20, 30 and 40 amperes. Large breakers of 100 to 200 amps are used as the main switch in each service box. A presentation of e. Syst. org
Circuit Breakers (continued) • Fig. 14 shows the typical circuit breaker wiring symbol and connections. A presentation of e. Syst. org Figure 14
Wire • The circuit wiring is done with cable designed for the purpose. It contains three insulated solid copper wires contained in an insulating sheath. See Fig. 15. The wire is often referred to as Romex. • The wires are color coded for specific purposes. The neutral wire has white insulation. The hot wire insulation is usually black. A separate ground wire has green insulation or may be bare. • The specifications are typically printed on the outer plastic covering. A presentation of e. Syst. org Figure 15
Wire Current Rating • The size of the wire determines its maximum current carrying capacity. • Size #14 wire is the most common for most circuits. • Larger wire like #12 or #10 is used on circuits that must handle higher current. • For copper wire the maximum current is: – – #14 #12 #10 #8 15 amperes 20 amperes 30 amperes 40 amperes • Such specifications ensure that high currents do not produce heat sufficient to start a fire. • Special armored cable like that in Fig. 16 is used for outdoor wiring and for wiring in harsh environments. A presentation of e. Syst. org Figure 16
Outlet Wiring • Fig. 17 shows the wiring of a common AC outlet. • The wire from the cable attaches with screws to the right side. The neutral screw is usually silver colored and the hot wire screw is usually brass colored. • Note that the neutral wire connects to the slot that is slightly larger to help ensure correct alignment of the plug. • The holes below the slots connect to the green copper wire. A presentation of e. Syst. org Figure 17
Switch Wiring • Common switch wiring is shown in Fig. 18. The switch itself and its wiring are contained in a box like the AC outlet. • The switch is connected in series with the hot wire (black). • Note that wire connections are by wire nuts. These are screwed down on the solid wires that have been twisted together first. • The ground wire connects to the switch metallic frame. A presentation of e. Syst. org Figure 18
Light Fixture Wiring • Fig. 19 shows the wiring for a typical ceiling light fixture. A presentation of e. Syst. org Figure 19
Light Fixture Wiring • Then trace the switch wiring to the fixture. See Fig. 20. Figure 20 A presentation of e. Syst. org
240 Volt Connections • Fig. 21 shows typical plugs and outlets for use with high current 120 volt circuits and 240 volts circuits. • From left to right: – A. 120 V, 20 A – B. 240 V, 30 A – C. 240 V, 40 A – D. 240 V A presentation of e. Syst. org Figure 21
Part 3 SAFETY A presentation of e. Syst. org
Safety First • All electrical wiring is designed and installed to rigid specifications and standards. The reason for this is safety. • Safety precautions are taken to prevent electrical shock which can kill a person and for protection against fire. • Wiring specifications are dictated by the National Fire Protection Agency (NFPA) via their extensive National Electrical Code (NEC). • Most states and local governments follow the NEC guidelines but many have their own special rules and regulations. • A copy of the NEC book is a useful reference for anyone doing or working with electrical components and wiring. A presentation of e. Syst. org
Electrical Shock • 120 and 240 volt power is sufficient to kill a human if he/ she comes into direct contact with the voltage. • The contact resistance of dry skin is in the 100 K to 500 K ohm range. Wet skin has much lower resistance. • It is the amount of current that flows through a person that determines the degree of lethality. • At a current of 1 m. A a human can feel a tingling. At a current of 5 m. A mild shock occurs. • Usually any current above 50 m. A is sufficient to cause heart stoppage. • As a rule of thumb, any voltage over about 40 volts can cause such current levels. A presentation of e. Syst. org
How Grounds Protect • Since the neutral side of the AC voltage is connected to ground, any contact between the hot side of the line and ground will cause shock. See Fig. 22. • The separate ground wire (green) is included to prevent shock and electrical overload. • This separate ground wire is connected to the housing of most appliances or tools with metal enclosures. A presentation of e. Syst. org Figure 22
How Grounds Protect (continued) • Most appliances contain some kind of AC motor. If a short or other malfunction in the motor causes the hot side of the line to touch the metallic housing of the motor or appliance, a person will be shocked if he/she touches the appliance (hot side) while standing on ground (neutral). A presentation of e. Syst. org
How Grounds Protect (continued) • The separate ground wire will cause a short to the circuit and trip the breaker thereby preventing a shock. See Fig. 23. Figure 23 A presentation of e. Syst. org
Ground Fault Interrupter • Another potential shock hazard is where a low current path is created between the hot side of the line and ground. Such a high resistance path, which could be through a human, may not be lethal but it could cause a hurtful shock. Fig. 24 show such a path. A presentation of e. Syst. org Figure 24
Ground Fault Interrupter • This path is usually created when there is a source of moisture present. • A device designed to protect against such conditions is the ground fault interrupter (GFI). • Fig. 25 shows the basic idea of a GFI. A presentation of e. Syst. org Figure 25
Ground Fault Interrupter (continued) • The AC line to the load usually connected to an AC outlet is passed through a magnetic core. • If there is no fault, the current in the hot and neutral leads are the same so no magnetic field is produced in the magnetic core of a transformer. • If a high resistance path is created between the hot side and ground, the currents in the two AC leads will be unequal. A magnetic field will be produced in the transformer core inducing a voltage in the coil. • The coil voltage is converted to DC and used to control a relay that opens the connections to the AC line. • Most GFI units are installed in AC outlets where moisture is present as in the kitchen, bathrooms, garage and outdoors. • Most GFI’s are very sensitive and trip with a current of 1 or 2 m. A. A presentation of e. Syst. org
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