Which action must system operators take based on predetermined GMD conditions?

System operators are tasked with responding to geomagnetic disturbances (GMD) in a manner that ensures the reliability and stability of the power system. When predetermined GMD conditions are identified, the appropriate action is to execute specific mitigation measures. These measures can include adjusting transformer settings, monitoring grid conditions more closely, and potentially reconfiguring the power system to reduce the impact of GMD on the infrastructure.

Mitigation measures are crucial because geomagnetic disturbances can induce currents in power systems that can lead to equipment damage and operational issues. By having specific actions predefined based on known GMD conditions, operators can quickly and effectively manage the risks posed by these events, thereby protecting the grid from the potential negative impacts of geomagnetic disturbances.

The other options, while potentially relevant to other situations, do not align specifically with the mandated response to predetermined GMD conditions. For instance, shutting down all connected transformers might not always be necessary or effective, and initiating emergency communication procedures or performing routine maintenance may not address the immediate threats posed by GMD. Executing specific mitigation measures is the proactive strategy designed to specifically counteract the effects of geomagnetic disturbances.