A preliminary company memo suggests that problems with at least two 500-kilovolt transmission lines were the proximate cause of nationwide blackouts in Argentina on Sunday 16 June. The lines connect a pair of hydroelectric dams to Argentina’s grid. Parts of Brazil, Paraguay, and Uruguay also experienced power outages, though the total number of people affected is not yet clear.
Government authorities have not yet determined what caused the disconnect and investigations are ongoing. Officials are expected to issue a more comprehensive report within 10 days.
In a statement on Sunday morning, the Secretariat of Energy attributed the blackouts, which began at 7:07 AM local time, to the “collapse of the Argentine Interconnection System (SADI).” The SADI is a high-voltage transmission network operated by Transener that transports electricity from generators, including power plants and dams, to distribution networks that serve tens of millions of customers.
According to a public statement by Edesur, one of Argentina’s largest electricity distributors, the failure occurred along a critical route of Argentina’s interconnection system that supplies the nation’s grid with power generated by the Yacyreta Dam in Paraguay and the Salto Grande Dam on the Uruguay River.
“I can only tell you that it is an event that no one could imagine would happen, given that there are multiple resources to avoid it,” says Raúl Bianchi Lastra, an electrical engineer at the National University of La Plata.
Details about what may have happened appeared in a memo circulated on Sunday to electricity distributors by Cammesa, the company that coordinates the sale and delivery of electricity throughout Argentina. In this memo, Cammesa, in which the Argentine government holds a 20 percent stake, passed on information from its initial review of the matter. (That memo was shared with IEEE Spectrum by multiple sources.)
According to the company, a short circuit disconnected a 500-kV transmission line that runs from the city of Colonia Elía to Belgrano, a neighborhood in Buenos Aires. And it appears that, at the same time, an automated system disconnected another 500-kV line that runs from the city of Mercedes to Colonia Elía. The cause of that second disconnection remains under investigation. A third 500-kV line, which typically connects the towns of Colonia Elía and Nueva Campana was taken out of service on 18 April in order to relocate a tower.
Ricardo Rubio Barros, a lecturer in electromechanics at the National University of San Juan, says the SADI transmission network has experienced some instability in the past, but not to this degree. “There have been many SADI areas disconnected with partial blackouts due to transmission faults, but never causing a complete SADI blackout,” he says.
Water levels have recently been high on the Paraná and Uruguay rivers, where Yacyreta Dam and Salto Grande Dam are located, respectively. As a result, these facilities were producing close to their maximum output early on Sunday morning, while nationwide demand was low. Meanwhile, Argentina was also importing about 1,000 megawatts of electricity from Brazil through Garabi, a high-voltage, direct-current link. With one major transmission line down for service, high levels of power were being transferred south on the remaining lines from Argentina’s northeast region on the morning of the blackouts.
Santiago Barbero, an electrical engineer at the National University of La Plata, says Argentina’s grid has automated systems in place to shed load or generation to suit current demand, which should have isolated any issues associated with a short circuit and prevented those problems from spreading throughout the system. “It is currently under investigation why these mechanisms did not isolate the event that led to the total blackout of the Argentinean electrical system,” he said.
One such mechanism is an automated system called the Generation Automatic Disconnection (known by the acronym DAG for its Spanish name) that can automatically disconnect generators in 200 milliseconds if it detects a problem. It’s also capable of reducing the rate at which electricity is fed into the grid. It is unclear what role, if any, this system played in the blackouts. The DAG consists of a master station at a secret spot within the SADI network and several programmable logic controllers located at substations throughout the grid.
A consultant for electrical power systems in Buenos Aires, who wished to remain anonymous because they were not authorized to speak on the matter, speculated on a possible chain of events that may have caused Sunday’s blackout:
“In operational conditions like this, against a line tripping in this corridor, the DAG scheme carries out generation disconnection to maintain the transient stability of the Northeast. For this reason, I assume that at first, a single contingency occurred (or double contingency, I don’t know yet) and the associated DAG scheme made a fast first-generation disconnection in Yacyretá (it could have been six or seven machines); the active power flow through the HVDC link should also have been lowered in the form of a ramp. The adjustment of these special protection schemes is typically studied using off-line simulations—typically with higher demand and relatively typical dispatches (outside Northeast region). Dynamic online contingency analysis is not performed.
I suppose that for this particular operating condition, the amount of generation disconnection was insufficient to maintain transient stability, and then transient instability of generators in the Northeast occurred. Possibly, the machines in Yacyretá and Salto Grande that were in service were tripped by some protection. Then some short-term equilibrium point (with respect to the transient stability) could have been reached, and then the problem of frequency stability began.
After the disconnection of the Northeast generators, the whole system was left with a large generation deficit, and the under-frequency load shedding schemes (which should shed up to 40 percent of the demand supposedly, though it is still not known if they acted well) were insufficient to re-establish the proper frequency. [Once the system starts to disconnect] generators—among them the Embalse Nuclear Power Plant, which was the largest generator in service—[as a countermeasure to] under-frequency, the system collapses.”