Power Distribution Systems

Transmission Systems

The central-stationsystem of power generation and distribution enables power to be produced at one location for immediate use at another location many miles away. Transmitting large amounts of electric energy over fairly long distances is accomplished most efficiently by using high voltages. Figure 3-1 illustrates the typical transformation stages through which the distribution system must go in delivering power to a commercial or industrial user.

Without transformers the widespread distribution of electric power would be impractical. Transformers are electrical devices that transfer energy from one electrical circuit to another by magnetic coupling. Their purpose in a power distribution system is to convert AC power at one voltage level to AC power of the same frequency at another voltage level.

• High voltages are used in transmission lines to reduce the amount of current flow.
• The power transmitted in a system is equivalent to the voltage multiplied by the current. If the voltage is raised, the current can be reduced to a smaller value, while still transmitting the same amount of power.
• Because of the reduction of current flow, the size and cost of wiring are greatly reduced.
• Reducing the current also minimizes voltage drop (IR) and amount of power lost (I 2R) in the lines

The circuits of Figure 3-2 illustrate how the use of high voltage reduces the required amount of transmission current required for a given load. Their operation is summarized as follows:

• 10,000 W of power is to be transmitted.
• When transmitted at the 100 V level, the required transmission current would be 100 A:

P = V × I = 100 V × 100 A = 10,000 W

• When the transmission voltage is stepped up to 10,000 V, a current flow of only 1 A is needed to transmit the same 10,000 W of power:

P = V × I= 10,000 V × 1 A = 10,000 W

• When the transmission voltage is stepped up to 10,000 V, a current flow of only 1 A is needed to transmit the same 10,000 W of power:

P = V × I= 10,000 V × 1 A = 10,000 W

There are certain limitations to the use of high voltage in power transmission and distribution systems. The higher the voltage, the more difficult and expensive it becomes to safely insulate between line wires, as well as from line wires to ground. The use of transformers in power systems allows generation of electricity at the most suitable voltage level for generation and at the same time allows this voltage to be changed to a higher and more economical voltage for transmission. At the distribution points, transformers allow the voltage to be lowered to a safer and more suitable voltage for a particular load.

Power grid transformers, used to step up or step down voltage, make possible the conversion between high and low voltages and accordingly between low and high currents (Figure 3-3). By use of transformers, each stage of the system can be operated at an appropriate voltage level. Single-phase three-wire power is normally supplied to residential customers, while three-phase power is supplied to commercial and industrial customers.

Unit Substations

Electric power comes off the transmission lines and is stepped down to the distribution lines. This may happen in several phases. The place where the conversion from transmission to distribution occurs is in a power substation. A substation has transformers that step transmission voltage levels down to distribution voltage levels. Basically a power substation consists of equipment installed for switching, changing, or regulating line voltages. Substations provide a safe point in the electricity system grid for disconnecting the power in the event of trouble, as well as a convenient place to take measurements and check the operation of the system.

The power needs of some users are so great that they are fed through individual substations dedicated to them. These secondary unit substations form the heart of an industrial plant’s or commercial building’s electrical distribution. They receive the electric power from the electric utility and step it down to the utilization voltage level of 600 V nominal or less for distribution throughout the building. Unit substations offer an integrated switchgear and transformer package.

A typical unit substation is shown in Figure 3-4. Substations are factory assembled and tested and therefore require a minimum amount of labor for installation at the site. The unit substation is completely enclosed on all sides with sheet metal (except for the required ventilating openings and viewing windows) so that no live parts are exposed. Access within the enclosure is provided only through interlocked doors or bolted-on removable panels.

The single-line diagram for a typical unit substation is illustrated in Figure 3-5. It consists of the following sections:

• High-voltage primary switchgear: This section incorporates the terminations for the primary feeder cables and primary switchgear, all housed in one metal-clad enclosure.
• Transformer section: This section houses the transformer for stepping down the primary voltage to the low-voltage utilization level. Dry-type, air-cooled transformers are universally used because they do not require any special fireproof vault construction.
• Low-voltage distribution section: This switch-board section provides the protection and control for the low-voltage feeder circuits. It may contain fusible switches or molded-case circuit breakers in addition to metering for the measurement of voltage, current, power, power factor, and energy. The secondary switchgear is intended to be tripped out in the event of overload or faults in the secondary circuit fed from the transformer; the primary gear should trip if a short circuit or ground fault occurs in the transformer itself.

Before attempting to do any work on a unit substation, first the loads should be disconnected from the trans-former and locked open. Then the transformer primary should be disconnected, locked out, and grounded temporarily if over 600 V

Distribution Systems

Distribution systems used to distribute power throughout large commercial and industrial facilities can be complex. Power must be distributed through various switchboards, transformers, and panelboards (Figure 3-6) without any component overheating or unacceptable voltage drops. This power is used for such applications as lighting, heat-ing, cooling, and motor-driven machinery.

The single-line diagram for a typical electrical distribution system within a large premise is shown in Figure 3-7. Typically the distribution system is divided into the following sections:

• Service entrance: This section includes conductors for delivering energy from the electricity supply system to the premises being served.
• Feeders: A feeder is a set of conductors that originates at a main distribution center and supplies one or more secondary or branch circuit distribution centers. This section includes conductors for delivering the energy from the service equipment location to the final branch circuit overcurrent device; this protects each piece of utilization equipment. Main feeders originate at the service equipment location, and subfeeders originate at panelboards or distribution centers at locations other than the service equipment location.
• Branch circuits: This section includes conductors for delivering the energy from the point of the final overcurrent device to the utilization equipment. Each feeder, subfeeder, and branch circuit conductor in turn needs its own properly coordinated overcurrent protection in the form of a circuit breaker or fused switch.

Correct selection of conductors for feeders and branch circuits must take into account ampacity, short-circuit, and voltage-drop requirements. Conductor ampacity refers to the maximum amount of current the conductor can safely carry without becoming overheated. The ampacity rat-ing of conductors in a raceway depends on the conductor material, gauge size, and temperature rating of the insulation; the number of current-carrying conductors in the raceway; and the ambient temperature.

The National Electrical Code (NEC) contains tables that list the ampacity for approved types of conductor size, insulation, and operating conditions. NEC rules regarding specific motor installations will be covered throughout the text. Installers should always follow the NEC, applicable state and local codes, manufacturers’ instructions, and project specifications when installing motors and motor controllers.

All conductors installed in a building must be properly protected, usually by installing them in raceways. Race-ways provide space, support, and mechanical protection for conductors, and they minimize hazards such as electric shocks and electric fires. Commonly used types of raceways found in motor installations are illustrated in Figure 3-8 and include:

• Conduits: Conduits are available in rigid and flexible, metallic and nonmetallic types. They must be properly supported and have sufficient access points to facilitate the installation of the conductors. Conduits must be large enough to accommodate the num-ber of conductors, based generally, on a 40 percent fill ratio.
• Cable trays: Cable trays are used to support feeder and branch circuit cables where a number of them are to be run from a common location. They support conductors run in troughs or trays.
• Low-impedance busways (bus duct): The busways are used in buildings for high-current feeders. They consist of heavy bus bars enclosed in ventilated ducts.
• Plug-in busways: These busways are used for overhead distribution systems. They provide convenient power tap-offs to the utilization equipment.

Power Losses

The unit of electric energy generated by the power station does not match with that distributed to customers. Some percentage of the units is lost in the power grid. This difference in the generated and distributed units is known as transmission and distribution loss.

There are two types of transmission and distribution losses: technical losses and nontechnical losses. Technical losses are losses incurred by physical properties of components in the power grid. Common examples of technical losses include:

• Power that is lost in the form of heat caused by current passing through the resistance of transmission lines and cables. These losses are equal to the square of the electric current (I) flowing through any line times the value of the resistance of the line (R): Losses = I^2 × R
• Power that is lost in transformers, which can be categorized into two components: no-load losses and load losses. No-load losses are caused by the magnetizing current needed to energize the core of the transformer, and do not vary according to the loading on the transformer.
• Power that is lost due to poor power factor. If the power factor of a load is low, the current drawn will be high and the losses proportional to the square of the current will be greater. Line losses due to poor power factor can be reduced by the application of capacitors into the system.

Nontechnical losses consist of administrative losses, losses due to incorrect metering, nonpaying customers, and theft/fraud. Common examples of such losses include:

• Losses due to metering inaccuracies are defined as the difference between the amount of energy actually delivered through the meters and the amount registered by the meters.
• Theft of power is energy delivered to customers that is not measured by the energy meter for the customer. This can happen as a result of meter tamper-ing or by bypassing the meter.

Switchboards and Panelboards

The Code defines a switchboard as a single panel or group of assembled panels with buses, overcurrent devices, and instruments. Figure 3-9 shows a typical combination service entrance and switchboard installed in a commercial building.

• The service entrance is the point where electricity enters the building.
• The switchboard has the space and mounting pro-visions required by the local utility for its metering equipment and incoming power.
• The switchboard also controls the power and protects the distribution system through the use of switches, fuses, circuit breakers, and protective relays.
• Switchboards that have more than six switches or circuit breakers must include a main switch to protect or disconnect all circuits.

A panelboard contains a group of circuit breaker or fuse protective devices for lighting, convenience receptacles, and power distribution branch circuits (Figure 3-10). Panelboards (sometimes referred to as load centers) are placed in a cabinet or cutout box, which is accessible only from the front, and have dead fronts. A dead front is defined in the Code as having no exposed live parts on the operating side of the equipment. The panelboard is usually supplied from the switchboard and further divides the power distribution system into smaller parts. Panelboards make up the part of the distribution system that provides the last centrally located protection for the final power run to the load and its control circuitry. Panelboards suitable as service equipment are so marked by the manufacturer.

Figure 3-11 shows the typical internal wiring for a 277/480 V, three-phase, four-wire panelboard equipped with circuit breakers. This popular system used in industrial and commercial installations is capable of supplying both three-phase and single-phase loads. From neutral (N) to any hot line, 277 V single-phase for fluorescent lighting can be obtained. Across the three hot lines (A-B-C), 480 V three-phase is present for supplying motors.

The proper grounding and bonding of electrical distribution systems in general and panelboards in particular are very important. Grounding is the connection to earth, while bonding is the connection of metal parts to provide a low-impedance path for fault current to aid in clearing the overcurrent protection device and to remove dangerous current from metal that is likely to become energized. The main bonding jumper gives you system grounding. If a transformer is immediately upstream of the panelboard, you must bond the neutral bus or neutral conductor to the panel enclosure and to a grounding-electrode conductor, as illustrated in Figure 3-12.

The Code requires the panelboard cabinets, frames, Panelboard and the like to be connected to an equipment grounding conductor, not merely grounded. A separate equipment grounding terminal bar must be installed and bonded to the panelboard for the termination of feeder and branch circuit equipment grounding conductors (Figure 3-13). The equipment grounding bus is noninsulated and is mounted inside the panelboard and connects directly to the metal enclosure.

A busbar can be defined as a common connection for two or more circuits. The Code requires that busbars be located so as to be protected from physical damage and held firmly in place. Three-phase busbars are required to have phases in sequence so that an installer can have the same fixed phase arrangement in each termination point in any panel or switchboard. As established by NEMA, the phase arrangement on three-phase buses shall be A, B, C front to back, top to bottom, or left to right as viewed from the front of the switchboard or panelboard (Figure 3-14).

Panelboards are classified as main breaker or main lug types. Main breaker–type panelboards have the incoming supply cables connected to the line side of a circuit breaker, which in turn feeds power to the panelboard. The main breaker disconnects power from the panelboard and protects the system from short circuits and overloads.

A main lug panelboard does not have a main circuit breaker. The incoming supply cables are connected directly to the busbars. Primary overload protection is not provided as an integral part of the panelboard. It must be externally provided. Normally panelboard circuit terminals are required to be labeled or to have a wiring diagram. One scheme (sometimes called NEMA numbering) uses odd numbers on one side and even on the other, as illustrated in Figure 3-15.

Motor Control Centers (MCCs)

At times a commercial or industrial installation will require that many motors be controlled from a central location. When this is the case, the incoming power, control circuitry, required overload, and overcurrent protection are combined into one convenient center. This center is called the motor control center.

A motor control center is a modular structure designed specifically for plug-in type motor control units. Figure 3-16 illustrates a typical motor control center made up of a compact floor-mounted assembly, composed principally of combination motor starters that contain a safety switch and magnetic starter placed in a common enclosure. The control center is typically constructed with one or more vertical sections, with each section having a number of spaces for motor starters. The sizes of the spaces are determined by the horsepower ratings of the individual starters. Thus, a starter that will control a 10 hp motor will take up less room than a starter that will control a 100 hp motor.

A motor control center is an assembly primarily of motor controllers having a common bus. The structure supports and houses control units, a common bus for distributing power to the control units, and a network of wire troughs for accommodating incoming and outgoing load and control wires. Each unit is mounted in an individual, isolated compartment or bucket having its own door. Motor control centers are not limited to housing just motor starters but can typically accommodate many unit types as illustrated in Figure 3-17. These may include:

• Full-voltage nonreversing NEMA and IEC starters • Full-voltage reversing NEMA and IEC starters
• Soft starters
• AC variable-frequency drives
• Programmable logic controllers (PLCs) • Solid-state motor controllers
• Transformers
• Analog or digital metering • Feeder circuit breakers
• Feeder fusible disconnects
• Contactors