The ITER experiment is an international project that aims to prove the viability of energy production by a nuclear fusion reactor. The fusion reactions are achieved by heating a plasma up to million of Celsius degree and confining it by means of appropriate magnetic fields, which interact with the charged particles of the plasma. In ITER the magnetic fields will be produced by the currents flowing in the superconducting coils supplied by ac/dc converters based on thyristor technology. The ITER power supply system may consume a total active and reactive power respectively up to 500 MW and 950 Mvar; a Static Var Compensation system with nominal power of 750 Mvar based on Thyristor Controlled Reactor + Tuned Filter (as fixed capacitor) was selected to reduce the reactive power demand from the grid. In this paper an analytical approach has been considered to study the interactions between the ITER power supply system and the grid, taking the starting points from the methods developed for the HVDC applications, which is a large scale thyristor based converter system usually installed with reactive power compensation equipment. This model aims to evaluate the strength of the electrical network feeding the ITER power supply system by sensitivity analysis allowing the calculation of some index as the Critical Short Circuit Ratio and the Voltage Sensitivity Factor.