111 Transmission and Distribution System Characteristics 1111 Physical Characteristics

The transmission system connects the generating stations and loads together through nodes called substations (see Figure 11.1). The substations contain switches and circuit breakers, transformers to connected different voltage levels, and other ancillary substation equipment (voltage control capacitor banks, reactors, metering and control equipment, etc.). Substation layout and complexity vary widely depending on the application.

Typical transmission voltages in the U.S. include extra-high voltages of 765 kV, 500 kV, and 345 kV. Other common voltages include 230 kV, 161 kV, 138 kV, and 115 kV. Lower voltages, such as 115 kV and 69 kV, are sometimes called subtransmission voltages. The difference between transmission and subtransmission has little to do with actual voltages — subtransmission refers to a lower-level grid hierarchy that interfaces with the bulk transmission backbone.

TRANSMISSION LINE

TRANSMISSION LINE

RADIAL FEEDERS (SECONDARY DISTRIBUTION)

FIGURE 11.1

Typical transmission and distribution system (courtesy of Nicoara Graphics).

The distribution system provides the infrastructure to deliver power from the substations to the loads. Figure 11.2 shows the most common designs for distribution feeders. Typically radial in nature, the distribution system includes feeders and laterals. Typical voltages are 34.5 kV, 14.4 kV, 13.8 kV, 13.2 kV, 12.5 kV, 12 kV, and sometimes lower voltages. The distribution voltages in a specific service territory are likely similar because it is easier and more cost effective to stock spare parts when the system voltages are consistent.

FIGURE 11.2

Typical distribution feeder configurations. The radial design is the most common in the United States (courtesy of Nicoara Graphics).

FIGURE 11.2

Typical distribution feeder configurations. The radial design is the most common in the United States (courtesy of Nicoara Graphics).

Power flow on a line is a function of voltage and current. Because the current itself is inherently bidirectional, power can typically readily flow in either direction. However, other operational constraints such as circuit breakers and other control devices may not be able to accommodate reversal of power flow without replacement or modification.

The electric power grid operates as a three-phase network down to the level of the service point to residential and small commercial loads. Feeders are usually three-phase overhead pole line or underground cables. As one gets closer to the loads (many of which are single phase), three-phase or singlephase laterals provide spurs to the various customer connections.

Three-phase electricity refers to voltage waveforms (and corresponding current) 120° out of phase with each other. This provides advantageous characteristics for rotating machines (both generators and motors) by inducing a smooth rotating magnetic field with which the rotating magnetic field can be coupled. Also, this offers a significant advantage for electric power transmission and distribution because each of the three phases will cancel each other out when combined. This makes it possible to string three conductors carrying voltage and rely on the mathematical cancellation of this power to provide a virtual neutral. Without a metallic return wire, significant cost savings can be achieved. Thus, nearly all transmission and distribution power lines have only three conductors.

To ensure efficient operation, it is important to balance the phases so that they are approximately equal. This is achieved through load balance, and also obtained through transformer configurations (see Figure 11.3) depending on whether the transformer terminals are configured with a three-conductor delta, a three-conductor wye, or a four-conductor wye (with this fourth connection optionally grounded). It is also common for the primary and secondary terminals of a transformer to have different configurations (hence a delta-wye transformer). These transformer connections are important for

FIGURE 11.3

Single-phase transformer configurations for delivering three-phase power (courtesy of Nicoara Graphics).

FIGURE 11.3

Single-phase transformer configurations for delivering three-phase power (courtesy of Nicoara Graphics).

balancing phases. These considerations are also important for how ground fault current will flow.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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