In traditional high-voltage direct current systems, alternating current system faults can lead to commutation failures, which may further induce the blocking of converters. After the faults are cleared, the fast recovery of commutating voltage, as one key factor to avoid converter blocks, is closely related to dynamic voltage support. The dynamic voltage behaviour considering the reactive power characteristic of converters is analysed, where the polynomial approximation is applied to facilitate our studies. Focusing on the synchronous compensator, the transient voltage support ability is analysed, and then the steady state output of synchronous compensators is regarded as a parameter to take parametric analysis. Case studies show that the steady state VAR output of synchronous compensators has influence on the countering of commutation failures, and an optimal allocation can be found by analysing the results in the form of polynomials.