The main difference between a STATCOM and an SVC is the way they operate: a STATCOM works as a controllable voltage source while an SVC works as a dynamically controllable reactance connected in parallel.
Compared with an SVC, a STATCOM offers the possibility of feeding the grid with the maximum available reactive current even at low voltage levels, this is possible because in every equilibrium condition the injected reactive power varies linearly with the voltage at the Point of Common Coupling (PCC). In contrast, for an SVC there is a quadratic dependence of the reactive power on the voltage at the PCC which means that to inject the same reactive power it is necessary to install an SVC with a nominal capacity higher than that of a STATCOM.
With regard to the maximum transient capacitive current it is observed that in an SVC the capacitive current is limited by the size of the capacitor and by the magnitude of the AC voltage. In the case of a STATCOM the maximum capacitive current that can be injected is limited by the maximum current capacity of the semiconductors used and is independent of the voltage level at the PCC.
Another feature of a STATCOM is that the DC-link capacitor serves as storage for active power. Therefore in certain situations, depending on the capacitor size, it is possible to regulate the interchange of active power with the grid also.
STATCOM devices are capable of much faster dynamic reaction (1/4-1 cycle) than an SVC. In a STATCOM the speed of response is limited by the commutation frequency of the IGBT’s (normally 1 kHz).