As load increases throughout the day, how do two adjacent lines, one overhead and one underground, respond?

• A. The overhead line will carry more of the load increase.
• B. The underground line will carry more of the load increase.
• C. The line with the higher impedance will carry more of the load.
• D. The load increase will be distributed evenly over the two lines.

Answer: (B)

In the context of power systems, when load increases, different types of transmission lines respond based on their characteristics. An underground line typically has a higher thermal capacity and less reactance compared to an overhead line. This means that underground lines can handle greater loads effectively, especially as temperatures rise and demand increases during the day.

As the load rises, the overhead line, which is more susceptible to environmental factors and has higher line losses due to its design and exposure, may not be as efficient in carrying additional load compared to the underground line. Consequently, the underground line tends to take on more of the load increase, effectively managing the demand with less impedance impact. This results in the underground transmission line being the preferred path for load increases as it operates more efficiently under these conditions.

This understanding is crucial for system operators who need to optimize power flows and ensure reliability while minimizing losses and potential issues associated with overloading lines, making the selection of transmission line types a significant consideration in planning and operation.