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Under-frequency Load Shedding Schemes (UFLS) are a well-established part of any under-frequency defence plan. Their future effectiveness in systems with increased penetration of renewable energy sources without rotating mass should however be questioned: lower inertia implies higher frequency gradients; more power electronics implies higher harmonics. Both factors will concur to make frequency measurements and short relay operation times more challenging: In this contribution, the impact of the penetration of renewable sources on the rate of change of frequency (RoCoF) in continental Europe has been simulated with the dynamic ENTSO-E model for different penetration levels, confirming the higher RoCoF and lower frequency nadir to be expected in case of large disturbances. The operation of the current UFLS scheme is included in the model and has been compared with a scheme incorporating a RoCoF criterion. The authors have also performed laboratory tests in order to assess the use of RoCoF-based load shedding as an improvement to frequency-only based load shedding schemes. Central European transmission and distribution system operators have established joint bodies for agreeing on the best evolution of underfrequency defence plans: challenges include frequency measurement and correct coordination of several control schemes and approaches within the CE transmission system community.

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