(1) The Role of Series Capacitors

Improve the voltage at the end of the line. Capacitors, when connected in series in the circuit, use their capacitive reactance (Xc) to compensate for the inductive reactance (Xl) of the line, reducing voltage drops and thereby increasing the voltage at the end of the line (the receiving end). The voltage at the end of the line can generally be increased by up to 10% to 20%.

Reduce voltage fluctuations at the receiving end. When the receiving end of the line is connected to loads with large variations, such as arc furnaces, welding machines, and electric railways, series capacitors can eliminate severe voltage fluctuations. This is because the compensating effect of series capacitors on voltage drop in the line changes with the load passing through the capacitor, offering instantaneous adjustment capabilities with load changes, automatically maintaining the voltage level at the load end (receiving end).

Increase the transmission capacity of the line. As the line is compensated with the capacitive reactance (Xc) of the capacitor, both voltage drop and power loss in the line are reduced, which correspondingly increases the transmission capacity of the line.

Improve the power flow distribution of the system. By connecting capacitors in series on certain lines within a closed network, the line reactance is partially altered, directing the current flow through specified routes to achieve an economical distribution of power.

Enhance the stability of the system. The introduction of capacitors into the line increases its transmission capacity, inherently enhancing the static stability of the system. In cases where a line fault leads to partial disconnection (e.g., one of the double-circuit lines is cut off, or one phase of a circuit is grounded and cut off), the system's equivalent reactance increases sharply. At this point, forcibly compensating with series capacitors—temporarily changing the number of series and parallel capacitors and momentarily increasing the capacitive reactance (Xc)—reduces the total equivalent reactance of the system, increasing the limit of power transmission (Pmax=U1U2/(Xl-Xc)), and thereby enhancing the dynamic stability of the system.

(2) The Role of Shunt Capacitors

Shunt capacitors, connected in parallel to the system's bus, act like a capacitive load on the system bus, absorbing the system's capacitive reactive power, which is equivalent to the shunt capacitors supplying inductive reactive power to the system. Therefore, shunt capacitors can provide inductive reactive power to the system, improving the power factor during system operation, increasing the voltage level at the receiving end bus, and reducing the transmission of inductive reactive power along the lines. This reduces voltage and power losses, thereby increasing the transmission capacity of the line.