Back To Basics

An electrical distribution system is a series of electrical circuits that delivers power in the proper proportion to homes, commercial businesses and industrial facilities. Regardless of the size and applications, the ultimate goal remains universal: the economic and safe delivery of adequate electric power to electrical equipment.

In general, there are three types of distribution systems: Radial Distribution System, Loop Distribution System and Network Distribution System. The type used by the utility company depends on the services required, location and economics.

Radial Distribution System

The Radial Distribution System has one power source for a group of customers. If there is a power failure, the entire group loses power. In addition, a circuit failure somewhere in the system could mean a power interruption for the entire system.

Figure 2. Simple Radial Distribution System

This is the most economical and widely used system. It is used for residential homes where the supply of electricity is not as critical if the power is disrupted.

Loop Distribution System

The Loop Distribution System loops through the service area and returns to the point of origin. The strategic placement of switches permits the electric company to supply power to customers from either direction. If one power source fails, switches are opened or closed to obtain power source.

Figure 3. Simple Loop Distribution System

Obviously, the loop system provides better continuity of service than the radial system, with only short interruptions of service during switching. Because the system requires additional equipment for switching, it is more expensive than the radial system. As a result, it is used for commercial buildings and shopping centers where it is necessary to minimize interruptions.

Network Distribution System

The Network Distribution System is the most expensive and the most reliable in terms of continuity of service. This system consists of a number of interconnecting circuits operating at the same utilization voltage. The customer is connected to two or more power supplies. If one power source fails, the customer gets power from the other sources, without interruption.

Figure 4. Simple Network Distribution System

It is utilized in areas with high and/or critical demand, such as hospitals, large critical manufacturing process complexes, and centralized computer installations.

Electrical Utility Systems

To understand the Electrical Distribution System, you need to understand the flow of electricity from generation to the end user. To do this let's follow the simple electrical distribution system in Figure 5 in steps:

Figure 5. Electrical Distribution System

STEP 1: The flow of electricity begins at the utility company where it is created at the generating station .

STEP 2: The voltage is then stepped up (increased) by a generator transformer at the Station Switchyard . This is done to minimize the cable size and electrical losses.

STEP 3: The Transmission Substation increases the voltage by a Step-Up Transformer from 69,000 to 765,000 volts. The voltage increase depends on the distance it will go and the type of facilities it will ultimately supply. The power is then distributed in multiple directions to the proper subtransmission station .

STEP 4: The subtransmission station is located closer to its end customer and as a result the voltage is decreased by a Step-Down Transformer to between 22,000-69,000 volts.

STEP 5: The electricity is then sent to the Distribution Substation where the voltage is stepped down by the Step-Down Transformers to useful voltages. The power is then distributed to homes and facilities that the Substation supplies.

STEP 6: At or near each home and facility are transformers that adjust the voltages down to the proper level for use. For example, a large industrial plant will receive voltage at levels from 2400 – 15,000 volts. It will use its own on-site step-down transformers to produce the different voltage levels needed in the facility.

Cabling

One of the most important parts of the electrical distribution system is the conductor or cable that carries the power from its source to its destination. The cables going across the country are relatively small in diameter compared to the high voltages they carry. How can that be? To understand why, we need to look at the formula for power:

P = V x I

V= voltage I = current P= power

Because power is equal to the voltage times current and the equation is always equal on both sides, we cannot change the voltage without changing the current. So when voltage is increased, current has to be decreased. Reducing current allows the power to be transmitted through smaller diameter (gauge) conductors. Reducing the conductor size reduces the cost and makes the system more efficient.

Power System Types

Now that you know about how power is transmitted, we need to discuss the two types of AC power: Three-Phase and Single-Phase.

Figure 6. Types of AC Power

Single-Phase System : This system is standard for residential service. It can consist of two or three wires entering the home where the power will be used. The three-wire system (1f3W) is most common today and will be discussed here. (The two-wire system (1f2W) is common in older construction.)

Figure 7. Single-Phase, Three-Wire (1f3W) System

Although the modern single-phase system uses three wires, it is single-phase, not three-phase. It actually consists of three wires coming into the house: two hot wires and one neutral wire.

Use of the three wires in different electrical combinations can provide different voltages. For example, one circuit can be made up of one of the two hot wires and the neutral wire. This circuit provides 120 volts AC, the voltage required for most household lights and small appliances.

Another circuit can be made up of both hot wires to provide 240 volts AC, the voltage required for larger appliances like a clothes dryer.

Three-Phase System : The electric company produces three-phase power, by rotating three coils through a magnetic field within a generator. As they rotate through the magnetic field, they generate power. Each phase is 120 degrees apart (out of phase) from each other.

Each phase flows from the generator in a separate cable. These phases are delivered to end users as either a three-phase power or a single-phase power.

Figure 8. Three-Phase Sine Wave

Commercial and industrial facilities use three-phase power because it is efficient and makes the equipment run more smoothly than single-phase. The reason is because each phase is 120 degrees apart; therefore, the equipment does not see a zero point. This is especially important when running motors because each new phase keeps the motor turning.

In addition, single-phase power is available from a three-phase system by use only one of the phases. This is beneficial for supplying power to lights, receptacles, heating and air conditioning.

There are two types of three-phase systems: three-phase, three-wire (3f3W) and three-phase, four-wire (3f4W). The main difference between the two is the 3f4W system has a neutral.

Figure 9. Three-Phase Three Wire

Figure 10. Three-Phase Four Wire

These systems offer a wide range of possible voltages, including 208, 240, 277, 480 and 600 volts. The voltages available are determined by the wiring configuration within the transformer. A number of these wiring configurations will be covered in Module 4, Transformers. Three-phase power systems have a number of advantages: