Massive inverter problem brewing for South Africa

 ·2 Oct 2023

South Africans who can’t afford to go full solar to escape load shedding are turning to easier solutions like inverters and battery backups – but researchers say that these systems are basically undoing any ‘good’ that load shedding is trying to achieve.

As load shedding shows no signs of stopping, more South Africans are turning to power backups to keep outages at bay. This has seen a boom in alternatives like solar and generator use in the country.

The use of solar is being incentivised by the government, with the introduction of solar tax incentives for the 2023/24 financial year. A big point of contention with the incentive, however, is that it only applies to solar panels – not the batteries, inverters or installation costs that go with it.

The reason for this, according to the National Treasury, is that the incentive’s goal is to boost alternative generation and take pressure off the national grid. Inverters and batteries do not do this, even though they may be vital to an effective solar setup.

New research from the Departments of Electrical and Electronic Engineering and Industrial Engineering at Stellenbosch University shows that the situation may be worse than simply not being net generators of power.

An inverter and battery setup without solar is actually undoing the work of load shedding altogether, and may pose a greater risk to the grid, they said.

According to the researchers, this is because non-PV-linked inverters have to charge batteries of various sizes, and this charging kicks in as soon as load shedding ends. Suddenly, the draw from the grid is the normal load and the added load of having to charge the batteries.

To test the extent of this problem, the researchers simulated a large group of residential households in South Africa to determine the aggregated electricity usage. They then evaluated the impact of load shedding on the grid and the effect of users installing inverters.

What they found is that the installation of inverters ultimately increased the amount of energy demand on the grid and negated the impact of load shedding. This negation differed depending on the level of penetration of the systems, as well as the charging rates.

At a base level, even with a 15% penetration level, the load peak is closely matched to the normal load, meaning that load shedding is negated.

“This is caused by the fact that when the power is returned for a zone that just experienced load shedding, all the inverters will begin to charge at the same time, which, consequently, pulls additional electricity from the grid.

“Most importantly, a peak is formed at the start of the charging period,” the researchers said.

This peak can range from matching the load shedding load to surpassing the normal load by adjusting the inverter penetration level and varying the charging rate.

By comparing the results of inverters for the various penetration levels, battery charging can undo approximately 85% and 90% of load shedding in summer and winter, respectively, at a PV-free inverter
penetration level of 25%, the researchers said.

“Even with a penetration level of 15%, these values are still as high as approximately 70%,” they said.

This is a problem that Eskom itself is acutely aware of, with the utility calling for inverters to be switched off and charging to be delayed when load shedding returning.

The researchers offered some more solutions.

They concluded that the impact of allowing users to charge at 0.5 C (half the battery capacity equivalent), without solar augmentation, will have a dramatic impact on the domestic load, even with only 15% penetration.

Therefore, they said it is imperative that charging batteries from the grid is restricted to protect the potency of load shedding as a grid-balancing tool.

“The default charge rate on inverters can be as high as 1 C (e.g. 5 kW for a 5 kWh battery). We recommend that the charging rate of battery backup solutions is restricted to 0.15 C (e.g. 0.75 kW for a 5 kWh battery) to prevent the high curtailment after a zone is switched on,” they said.

“This lower charge rate should be ample to recharge the battery between bouts of load shedding.”

The full research paper can be found here.

The research was conducted by the Departments of Electrical and Electronic Engineering; Industrial Engineering at Stellenbosch University. The papers were published in the South African Journal of Science.

Read: Big changes for inverters and solar in Cape Town next month

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