When and why is DC used instead of AC for long-distance electric power lines?

Answer:

DC is used instead of AC for long-distance electric power lines in some cases because it has some advantages over AC transmission. Some of the advantages are:

1- DC transmission has lower losses than AC transmission, as it does not have the effects of capacitance, inductance, and skin effects that cause power loss in AC transmission¹²³.

2- DC transmission can transmit more power over the same transmission line, as it can use both conductors for carrying current, while AC transmission uses only one conductor at a time due to phase differences.

3- DC transmission has better voltage regulation than AC transmission, as it does not have a voltage drop due to reactance¹³.

4- DC transmission can avoid the problems of synchronization, stability, and power factors that affect AC transmission¹²³.

However, DC transmission also has some disadvantages, such as:

1- DC transmission requires more complex and expensive equipment to convert AC to DC and vice versa, such as rectifiers, inverters, and choppers¹²⁴.

2- DC transmission is not suitable for applications that require varying voltage levels, such as motors and transformers¹²⁴.

3- DC transmission is more prone to faults and corona losses than AC transmission¹³.

Therefore, the choice of DC or AC transmission depends on various factors, such as the distance, the load, the cost, and the availability of technology. Generally, DC transmission is preferred for long-distance, high-voltage, point-to-point transmission, such as submarine cables or interconnecting grids. AC transmission is preferred for short-distance, low-voltage, multi-point transmission, such as distribution networks or local grids²⁴.