The role of static VAr compensators in staving off voltage collapse

Continuous operating voltages in a power system are kept typically within +5% of the nominal voltage levels for power transmission and sometimes a slightly wider range for distribution. Standards generally provide for equipment to be able to operate correctly within the range of +10%. It is vital that voltages do not fall too far below these lower limits for any significant period of time because, in general, reactive current demand increases as the voltage falls. The increase in reactive demand causes the voltage to fall further and the system tends towards complete collapse. Poor power-factor loads, induction motors running at a high slip after a disturbance, and transformers changing taps to raise the secondary voltages but inadvertently increasing VAr absorption, have been known to have precipitated such unstable operating conditions. This contribution illustrates how static VAr compensators (SVCs) help to stave off voltage collapse by improving the stiffness of the voltage at a given power level or by permitting more power to be transmitted without increasing the risk of voltage collapse