What is the primary purpose of a series reactor in a power system?

A series reactor in a power system is primarily used to limit fault current. When a fault occurs, such as a short circuit, the current can increase dramatically, leading to equipment damage and operational issues. By incorporating a series reactor into the power system, the impedance of the circuit is increased, which subsequently reduces the magnitude of the fault current. This protective measure helps safeguard both the infrastructure and the reliability of the entire grid during fault conditions.

While the other options address various aspects of power system performance, they do not accurately capture the primary function of a series reactor. For instance, although series reactors can influence reactive power flow, their main role is not solely to decrease reactive power demand or improve voltage regulation, which may be more pertinent to other devices such as shunt reactors or capacitors. Additionally, while increasing power flow is important in certain configurations, it does not align with the primary protective purpose of a series reactor.