G99 / Grid Code – Fault Ride Through / Fast Fault Injection

Fault Ride Through studies are a basic requirement of Type B and C / D generators, to demonstrate ride through for a fault lasting 140ms. The basic process is the same in all cases, but the retained voltages and recover rate differs slightly between the cases and generation type. For Grid Code studies, there are some extra requirements for longer duration events, for differing levels of voltage depression on the network.

At the moment the requirement is to carry these out as RMS (ideally unbalanced) simulations. However, NG ESO are moving to require these to be carried out as detailed EMT studies, to ensure that the FRT response is a true reflection of the generator capability.

Fast Fault Current Injection is part of this process, and specifically associated with 3-phase faults. It should meet a specific magnitude in order to satisfy the requirement.

There are a couple of interesting anomalies in both G99 and the Grid Code, that are ‘known’ about, but have not been corrected. 1) For 2-phase faults it is impossible to get a retained voltage of less than 50%. 2) For an impedance earthed MV system a bolted fault on the MV, usually wont cause a big enough voltage depression on the LV system to trigger the FRT response.