Recent advances in the large-scale wind turbine industry have seen many manufacturers develop and introduce designs incorporating permanent magnet generators and full power electronic converters. These systems provide benefits in terms of maintenance and efficiency, however, their fixed rotor excitation, provided by the permanent magnets, causes specific control challenges. Many transmission system operators require, in grid codes, that wind turbines stay connected and transiently stable through severe voltage disturbances on the networks. This requirement has typically determined the key system design parameters, particularly relating to the over-speed capability of a wind turbine. This paper demonstrates why such systems are highly likely to be fitted with a full rating chopper and... resistor to absorb the machine's power during faults. But grid codes may soon also introduce a requirement for wind turbines to provide an inertial response to frequency deviations on the National Grid. This paper demonstrates that inertial response poses a potential challenge to wind turbines with Permanent Magnet Generators and that therefore their synthesized inertial response may have to be reduced compared to their mechanical inertial constant.