Nationwide power blackout explained

At about 20 minutes past midday on Friday, the state-run power grid operator—the Uganda Electricity Transmission Company Limited (UETCL)—posted on its X (formerly Twitter) platform an explanation about what triggered the power outage.

“UETCL informs the general public that a national (power) blackout has been registered this morning following a load rejection test at Karuma Hydropower Plant. System restoration is underway by our technical team; all inconveniences are highly regretted,” the statement reads.

While the state-run power grid operator had little to cheer about, UEGCL was elated by the amount of power generated. It described the test as “one of the last checks before the commencement of operations at the power plant.” It added that the “milestone” heralded “light at the end of the tunnel.”

On May 30, engineers successfully synchronised onto the national grid all six units (turbines) of the dam for the first time. The exercise was part of the capacity tests of the plant that were completed last year, bringing the plant’s commissioning to the closest.

What really happened?

Subsequent (social) media debates following the blackout that lasted several hours suspected a possible catastrophic impact on the transition and generation infrastructure. Whereas under normal circumstances, a load rejection test should not cause a blackout to a given electricity grid, Sunday Monitor has learnt that the blackout resulted from the country’s small electricity grid and low consumption of the generation of electricity overall.

While the other electricity generation plants were asked, during the test, to cut down their generation output to enable the grid to accommodate the 600MW from Karuma, we understand that there was a largely insufficient pool of megawatts to sustain the grid. We have also established that, because the country’s consumption at the peak is currently low, it would have proven a technically risky gamble to keep the rest of the plants generating at maximal and normally while Karuma dam is also tested.

In an interview, Mr Edgar Kansiime, the UEGCL acting communication and corporate affairs manager, explained that while UEGCL successfully conducted the tests, the impact could not be avoided on the transmission infrastructure. He added that because the rest of the electricity generation plants were asked to cut down their generation output to accommodate the huge output that was to be loaded onto the grid from Karuma Hydropower Dam, the national grid could not shoulder the shock and impact that resulting from the abrupt switch-off of Karuma Hydropower Dam.

Is bigger better?

For the 600MW from Karuma to fit into the grid during the tests, Mr Kansiime said the off-taker (i.e. the state-run power grid operator) communicated to the other plants to reduce their output.

“When that happens and you pull out the 600MW abruptly, if the grid is very big, you find that on withdrawal of 600MW, you can easily stabilise the grid,” he reasoned.

This is because the other plants would continue producing and offsetting at maximal and normally. However, “when [the grid] is small like Uganda and it is just growing, it has a big impact on the grid that would cause the loss in frequency.”

Mr Kansiime further offered: “In other countries, it is not that when they are doing a load rejection test, there must be a national blackout; it depends on the size of the grid; sometimes the impact is very big such that it results in a national blackout.”

He added: “That abrupt loss of load (blackout), when you see the other plants that we have been testing, it wasn’t causing a national blackout because the load is not so much; for example, when we did it to Isimba, we just loaded 180 MW, and this is 600 MW; the challenge is that, when you look at our grid, our grid is not so stable to be able to load-reject such a big amount of electricity at the same time.”

According to him, the grid’s strength is determined by the consumption size.

“When you have many people connected to the grid, the bigger the grid, the more stable it is; the smaller the grid, the less stable it is,” he said, adding, “With our grid, it is not as big as you would want it to be, so when we load-rejected the 600MW, it caused the frequency disturbance and jam that made us experience that temporary national blackout.”

Why the test?

The turbine load rejection test, per Mr Kansiime, is one of the pre-commissioning tests of an electricity generation plant to confirm that the system can withstand the sudden loss of load and return to normal condition using its governors. After that test, there is one critical test called the load rejection test. This test, it is said, is when you load the turbines (unit) with maximum electricity load. In the case of Karuma, it meant that each turbine discharged at maximum 100MW.

“All of a sudden, you remove the load and switch off abruptly. This is intentional; it is not like an accidental loss of load,” Mr Kansiime explained.

He added: “You turn off the switch; remember the turbines are spinning, and when you abruptly turn off the switch, you want to test and see whether the turbines will keep spinning normally at maximum or if it will result in an overspin, which can lead to catastrophic incidents, for example, the turbines malfunctioning and popping out of the powerhouse.”

Mr Kansiime said the test enabled their engineers to comprehend if the units can endure the burden and mitigate over-speed tripping, steady speed control at no-load operation, and remain in service without any adverse effect on the turbine.

“Based on international standards, when you have a contractor you secured to build for you a power plant, there are checks and standards. You are required to conduct certain tests that we have to do to ensure this plant is healthy for operations, and one of them is the load rejection test,” he said.

He added: “One of those tests is the capacity test, when you run the plant at maximum load, meaning when the contractor tells you that I have built for you a 600MW plant, you run the plant at maximum load (600MW) and maintain it for two hours nonstop to be sure it has passed the capacity test, and that was done.

UETCL responds

UETCL said in a statement: “We did expect the interruption to the grid. We put out radio announcements to inform the general public on the likely impact of the Karuma tests. Even Umeme [power distributor] did.

Karuma power plant has six units each rated 100MW. Since May 2023, a number of load rejection tests have been conducted at the dam as part of the commissioning tests, and at progressively high levels.

In comparison with all the other large plants (Bujagali 250MW, Isimba 183MW, Kira 200MW, Nalubaale 180MW), there has never been a load rejection test during commissioning of any plant, of the magnitude that was tested on Friday in the history of Uganda.

In fact, the 330MW load rejection test, which again was higher than rated capacity of all other large plants, was successfully undertaken.

There were messages from Umeme to customers on the same.”

What next?

Mr Jonan Kiiza, the Uganda Electricity Distribution Company Ltd (UEDCL) spokesperson, in a comment, said the tests bore no impact on the distribution infrastructure.

“I do not think it is something that Ugandans must have to worry about or get scared about because it is just that time when a turbine goes off until the time it is restored to the grid,” he said.

He added: “The engineers at this time are monitoring the shortness and the length of time it takes to restore normalcy; it is like a fireman’s experience; you set the alarms on and monitor how long it takes to evacuate everyone out of the house.”

According to Mr Kiiza, the load rejection test is one critical and risky test that a plant must undergo.

“It is a critical test; first of all, it is a worrisome test because you might destroy the turbines once they are not properly built when you do that test, but the fact that it was done successfully and it remained healthy means the system is fit,” he said.

According to UEGCL, the completion of the load rejection test sets the Karuma Hydropower Dam ready for technical commissioning.

“As far as the dam is concerned, these are the major two tests that we have conducted and passed successfully on Karuma Hydropower Dam, and we are looking at the technical commissioning, which we shall communicate to the public in a very short while,” Mr Kansiime said.