Moneyweb reports that the reality is that Eskom is skating a very fine line when it comes to managing its operating reserve these days. Eskom requires an operating reserve of 2 200MW. This means it requires 2 200MW more electricity generated than demanded.
Sudden loss of supply
On Saturday 6 November 2021 Eskom rapidly lost 1 000 megawatts (MW) of supply from the Cahora Bassa hydro scheme in Mozambique.
Power systems operate at a specific frequency. In Eskom’s case, this is 50 Hertz. The system can tolerate small swings in either direction (between 49.5Hz and 50.5Hz), but if there is a large loss of generation, the remaining generators in the system cannot carry the load. Without any response, the result of this is that the remaining operating generators will slow down and then trip.
With the loss of the Cahora Bassa supply, the frequency of the pool dropped to 49.31 Hz. This is noticeably below the normal range.
If Eskom did not react immediately, all the units could potentially have tripped.
Sequence of other events occurring can trigger a crisis
This means that the sequence of events following a sudden loss of generation capacity is critical.
If Eskom is already loadshedding at Stage 2 or higher, the addition of a series of small events occurring concurrently, or a significant event outside of Eskom’s control such as a blackout occurring in Zambia or Zimbabwe, will quickly lead to loadshedding increasing to higher Stages,
Although pumped storage can quickly be ramped up – except that the downstream water must again be pumped back – the large amount of diesel needed to fuel the OCGTs and Eskom’s ability to finance it is of greater concern.
Risk management
The diesel shortage is a major concern because the OCGT plants are effectively useless if they can’t be called on immediately when a unit in Eskom’s coal fleet trips. The lead time for diesel procurement is at least six weeks.
Eskom had managed to recover its pumped storage scheme dams, which are now full. However, eating into the operating reserve is simply not an option until the diesel supply issue is resolved. This could take several weeks.
Until then Eskom remains on a knife edge.