Impact of over-frequency events on Nordic NPPs

The following faults are considered:

• Disconnection of HVDC link

• Reverse power flow of HVDC link

• Disconnection of several HVDC links

• System split

Using the simulation model the resulting frequency change are calculated. For HVDC link faults, RoCoF of up to 0.82 Hz/s; and frequency deviation up to 51.1 Hz. For the System split scenario, RoCoF of up to 1.7 Hz/s and frequency deviation of up to 52 Hz. The findings of the study indicate that the NPPs should be robust to most of the studied cases related to large disturbances related to HVDC links. This is further highlighted from the experience of the NPPs, as there has been no significant impact on the recent historical over-frequency events on the operation of the NPPs. The most extreme system split scenario might result in extreme over-frequency deviations beyond the system and NPPs capabilities. Reference is made to the 1983 historical event where over-frequency of 54.0 Hz after 5 seconds was reached.

Measures undertaken to limit the impact of over-frequency events in other synchronous areas have also been reviewed. This includes the introduction of market mechanisms, such as Fast Frequency Response/Reserve, and technical requirements, such as provision of synthetic inertia.