Monday, January 24

For the first time, there is a real concern about network stability.


Sometime between 6 am and 9 am on Saturday morning, Eskom rapidly lost 1,000 megawatts (MW) of supply from the Cahora Bassa hydroelectric scheme in Zambia.

Typically, the company receives around 1,500MW of imported power.

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In a statement Sunday afternoon, he described this as a “major incident in Zambia [which] affected the entire South African energy group [SAPP]”.

According to publicly available information published by Eskom, the amount of imported energy received was reduced from:

  • 1,491MW at 6 a.m. M.
  • 1045MW at 7 a.m. M.
  • 729MW at 8 a.m. M.

Presumably this was further reduced after 8 a.m. M. (Eskom only publishes data every hour).

This doesn’t seem so sudden, but this offering is relatively stable and Eskom is betting on it.

By 9 a.m., the amount of energy supplied by the plan had returned to normal (1,498 MW). It has been stable ever since.

How, then, did a temporary loss of imported electricity on Saturday morning plunge the country into phase 4 load shedding for this entire week?

Read:

A full week of cargo shedding is coming
Freight shift: stage 4 is back

The reality is that the system operator is skating a fine line when it comes to managing Eskom’s operational reserve.

Eskom requires an operating reserve of 2,200MW. This means that it requires 2,200MW more electricity generated than demanded. This buffer allows you to balance the power system when the units fire. For example, a 794MW unit at Medupi that could be generating 700MW at the time could explode and the operating reserve gives Eskom some breathing space to react to the trip, using pumped storage or open cycle gas turbines (OCGT). ) to supplement short-term supply. notice.

Tolerance and travel

Power systems operate at a specific frequency. In Eskom’s case, it’s 50 Hertz. The system can tolerate small oscillations in any direction (between 49.5 Hz and 50.5 Hz), but if there is a large loss of generation, the remaining generators in the system cannot support the load. Without any response, the result of this is that the remaining operating generators will drop down and then trip.

The sequence of events that followed the alleged sudden loss of generation capacity in Zambia on Saturday is critical.

Energy expert Chris Yelland has reported that the pool’s frequency dropped to 49.31 Hz. This is notably below the normal range, which can tolerate drops of around 49.5 Hz, as long as there are responses from Eskom ( but above the threshold of 49 Hz).

Preliminary investigation

Eskom Group broadcast executive Segomoco Scheepers says a preliminary investigation shows this was the “culmination of a series of small incidents.”

The investigation is being led by the SAPP coordination center in Harare.

Scheepers noted that Zambia experienced a blackout over the weekend. At this point, it lost 2,000MW of supply. He said “power shifts were observed between the Zambian and Zimbabwean grids”, resulting in “overfrequency”, which at one point reached 54 Hz in Zambia. This caused the generators to go off in Zambia and the interconnector to go off between Zambia and Zimbabwe.

This then caused an “overfrequency” in Zimbabwe, which then affected the interconnector to South Africa, resulting in the loss of 1000MW of imports.

Scheepers notes that by the time these events occurred, Eskom was already losing charge in Stage 2.

Earlier this week, the system operator had thought the drop in frequency triggered a unit (600MW) at Tutuka. Typically this would be a material event where the stability of the network is in serious doubt.

Scheepers says it has been established that the trip to Tutuka was not caused by the incidents north of our borders.

Safeguards

Eskom, like other utility companies, has several security measures in place to prevent networks from becoming unstable.

Given the size of the system (SAPP is approximately 50,000 MW), Eskom’s main safeguard is its pumped storage schemes. These can generate a significant amount of energy instantly, as the gates open (if, of course, the storage dams are full).

One of the main risks with Eskom when using storage schemes as base load capacity is that these dams are not full and therefore this capacity is not instantly available.

Diesel OCGT they are also a useful answer.

Eskom can also instantly cut supply to large customers through its instant and complementary demand response programs (compensates these customers for these events).

The last option is to reduce the load, in other words, implement load shedding or move to a higher stage of load shedding, as happened on Monday when Eskom announced that it would move to stage 4 of load shedding (from Stage 2 ) at 1pm – with just a few minutes’ notice.

Going back to what happened on Saturday

Yelland reports that on Saturday, the utility’s systems automatically started two generators at the Drakensberg, Ingula and Palmiet pumped storage systems, as well as the Acacia and Port Rex OCGT plants.

Eskom confirmed this in a briefing on Tuesday.

This can be seen in the public generation data, which shows that pumped storage schemes increased from 122MW at 7am to 1,435MW at 8am. Their contribution was likely even greater during this chaotic period.

But it is OCGT it contributed just 283MW at 7am and 269MW at 8am. This is a direct consequence of the large amount of diesel used to keep the lights on. Simply put, Eskom had very little diesel available to operate peak plants.

The gravity of the situation

This is the first major event of this nature that Eskom has publicly disclosed in recent years.

On the one hand, this level of transparency is commendable, but it clearly shows how fragile the network is today.

Scheepers says the incident “was resolved in about two hours, or a little less.” It says that “some other incidents occurred at the same time” but that these were unrelated.

Read: Eskom remains on the razor’s edge

Eskom has to keep some pumped storage generating capacity available at all times, and after Saturday one can be sure it is probably being a bit more conservative.

The diesel shortage, because Eskom burned that a lot of diesel last week – it’s a big concern because OCGT plants are effectively useless if they can’t be called up immediately when a unit in Eskom’s coal fleet travels.

The rapid switch from Stage 2 to Stage 4 on Monday at 1pm was effectively necessitated by the fact that Eskom simply cannot generate between 1000 MW and 2000 MW of power at the push of a button by using peak plants. .

Ironically, the performance of the base load generating fleet improved for the remainder of Monday. Eskom needed headroom to restore stability to the grid. And rolling the dice and consuming the operating reserve is simply not an option until the diesel supply problem is solved.

Read:

Eskom is rebooting: CEO exposes how
Blaming Eskom staff for cargo shedding is unfair, union says


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