There was a power system outage in Spain/Portugal on April 28, 2025.
Still not knowing the reason at the time of writing this - Mai 29, 2025 - one month after the outage - there is increased concern regarding the solidity of the design. Possible outage reasons could be
a) Incorrect prediction of solar and wind feed-in. Basically: In the event of a power deficit, if the rotating reserve of conventional power plants is too low => frequency drop. In the event of oversupply - either due to too much feed-in of renewable energies or too little power consumption => frequency increase. In both cases, frequency-dependent automatic power control would be required: throttling of feed-in, load shedding, fast-acting storage. If renewable energies are not or insufficiently involved in the automatic power-frequency control system, no or too a little adjustment of generation and consumption is made. Essential for stability is that there is sufficient inertia in the system, and at the proper places so that the rate of frequency change is sufficiently low and inverters - whether GFM or GFL - have a clean proper signal to act on.
The coordination between active power and voltage/reactive power control is also important here. Voltage control takes precedence over power coverage. Reason: If voltage is not controlled then there is no clean way to handle an active power deficit.
b) Insufficient synchronizing power from wind turbines and photovoltaics – possibly due to a lack of grid forming capability, insufficient rotating masses of synchronous machines or incorrect locations. A distinction is made between GFL (grid following) and GFM (grid forming) inverters. Grid forming inverters shall compensate for the loss of rotating masses of synchronous machines. However, they are not equivalent to synchronous machines. Line lengths and their reactances, connection points of inverters and consumers, and transient overcurrent capacity are important factors. Inverters are also responsible for reactive power supply and voltage regulation. As said, voltage control has priority over active power control. However, this can lead to a power deficit and frequency collapse, with and without inter-area power and/or rotor angle oscillations. Eigenvalue sensitivity studies can improve insight. This requires closed system descriptions – see Daniel Müller's doctoral thesis at DTU. However, he has not yet taken power converters into account.
Eigenvalue calculations and transient load flow calculations with regulated power converters are necessary during operation in order to determine the stability margins required for stable operation.
Oversizing of power inverters, which is expensive, is necessary for controlling transient current distribution and to split power between inverters distributed in the grid according to their respective capability. Otherwise, different lengths of transmission lines would have to be compensated for mathematically. However, this is difficult to achieve reliably at changing switching states.
All in all, depending on the degree of expansion – i.e. the use of RES based inverter feed-in and the dismantling of conventional power plants – it may be necessary to retain synchronous machines, at least as synchronous condensors without external power supply at the shaft, simply running as idle machine. This would provide the necessary rotating mass and thus frequency stability. As far as it is known, this is the case at the dismantled Biblis nuclear power plant.
The problem of frequency stability in connection with inverters operating on weak grids is not new. It has always existed for classic HVDC operating on weak grids. There have been various studies on this topic over the last few decades, some of which date even back to the 1940s and deal with considerations for the use of solar thermal energy by means of HVDC long-distance transmission (Henning/Sattler). By the way: the Inga-Kolwezi HVDC transmission (1700 km long) has synchronous condensors at the receiving transmission end to cope with almost electric island conditions.
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