How to improve system stability without risking subsynchronous resonance

Various static devices for raising the stability limit of a power system are examined with respect to their employment at or near the electrical center of a two-machine system. Among these devices are series capacitors, shunt capacitors, combined series and shunt capacitors, and shunt static reactive compensators consisting of a fixed capacitor and a variable reactor. New concepts of current gain, voltage gain, and power gain are defined and illustrated. The natural frequencies of each circuit are examined, and it is shown that with series capacitors these frequencies are subsynchronous and hence produce a risk of subsynchronous resonance, whereas all shunt-connected devices give supersynchronous natural frequencies and therefore do not involve such a risk.