Measure increases industry agents’ suspicion of miscalculation in system operation
Luiz Carlos Ciocchi — Foto: Leo Pinheiro/Valor
Brazil’s national grid operator ONS confirmed Thursday in a preliminary analysis report that the failure of the Quixadá II-Fortaleza II transmission line, operated by Eletrobras’ subsidiary Chesf, caused a technical effect that split the country into three electrical areas during the power outage occurred on Tuesday.
According to the document, the phenomenon occurred 600 milliseconds after the line went offline. While investigating the causes of the outage, the ONS adopted a more conservative stance in the operation of the system and, according to agents in the sector, one of the measures was to limit the sending of renewable energy from the Northeast region to the rest of the country.
The measure reinforces the suspicions of industry players that a momentary surplus of renewable power and a possible miscalculation in the operational planning of the National Interconnection System (SIN) caused transmission failures that led to the outage that affected much of the country.
The excess of renewable power is an operational situation that requires specific technical procedures to avoid imbalances in the electricity system. So far, the government points to the occurrence of a failure on the Quixadá II–Fortaleza II line as the root cause of the outage. A second fault was initially considered, but ruled out by the ONS.
The ONS reduced the limit for transmitting 13,000 megawatts of wind and solar power from the Northeast to 8,000 MW, according to sources who spoke on condition of anonymity because of the sensitivity of the situation. The cut was confirmed by ONS’s Preliminary Daily Operation Report (IPDO), a technical bulletin with data on the operation of the power system, as part of the operating procedure to ensure the safety of the system, but without informing the new ceiling for dispatch.
In the preliminary report on the outage, the ONS stated that the system was operating “under more conservative conditions” to ensure the safety of the service, and did not rule out adopting more restrictive criteria. As a result, the energy produced by the power plants now is being lost, with possible financial consequences that have not yet been measured.
By reducing energy exports from the Northeast while maintaining hydroelectric and thermal generation, the ONS technically increased the so-called “inertia” of the power system, a situation that did not exist at the time of the outage, sources say. Wind and solar power are sources classified in the electricity sector as variable or intermittent, meaning they only generate energy when the wind is blowing or the sun is shining. When the wind stops blowing or the sun stops shining on the panels (because of a cloud, for example), hydro and thermal power plants must be brought online quickly.
This backup prevents the frequency of the power system from dropping. Frequency is a physical concept that corresponds to the number of cycles of an alternating electric current. By default, Brazil has adopted a frequency of 60 Hertz (Hz). Frequency is a phenomenon that must remain stable. Below 59.5 Hz, instabilities in the electrical system become critical.
Due to the generation characteristics of wind and solar power farms, these units are called asynchronous generators, which means that if consumption is greater than the generation of these units, or if there is a sudden reduction in generation (due to a sudden lack of wind or sun), these units will stop working immediately.
To prevent this from happening, there must be other sources of energy that can come online quickly and keep the power system running. Hydroelectric and thermal power plants are able to provide this support: in the jargon of the sector, they are called synchronous machines, as they are able to maintain the frequency of the networks, which guarantees the “inertia” of the system.
In other words, if hydroelectric and thermal plants are not operating at a time when renewable energy production is fluctuating, the frequency is reduced, the transmission network is destabilized, the power supply is interrupted, and a domino effect can occur in the national electricity system.
For this reason, protection systems are activated to “isolate” the fault, cutting off power to some areas until consumption and generation are back in balance, preserving the rest of the system. In the report, the ONS said a “sudden voltage drop” after Chesf’s line left the system. “After 600 milliseconds, the loss of synchronism protection was activated, causing the controlled opening of the lines that make up the North/Southeast, Northeast/Southeast, and North and Northeast interconnections, separating the SIN into three electrical zones.”
Agents in the sector suspect that the ONS programming area did not foresee a minimum “inertia” for Tuesday sufficient to support a safer operation – with more hydroelectric and thermal plants scheduled to operate.
According to the sources, on Monday, the team tasked with the daily programming of the operation miscalculated the amount of generation needed for the following day, Tuesday, the day of the outage.
The team that monitors the operation in real time possibly failed to notice the problem in the programming, especially in the control of the wind generation, mainly because of a change in the wind scenario, which would have become stronger. ONS Director General Luiz Carlos Ciocchi did not immediately reply to a request for an interview.
The oversupply of wind generation would have decompensated the frequency. And without “inertia”, the SIN was unable to withstand a disturbance, as the frequency dropped from 60 Hz to as low as 58 Hz.
When it reached this level, the ONS activated the Regional Load Relief Scheme (ERAC) protection system, which isolated the fault and prevented the outage from spreading more widely across the country.
But the lower frequency may have caused a disturbance that shut down the Quixadá II–Fortaleza II transmission line, sources say. However, according to the Ministry of Mines and Energy, ONS, and Eletrobras, this problem would not be enough to “bring down” the national power system.
“The isolated outage would not cause the impact seen in the SIN, and this is a point that is still under investigation,” said the ONS.
“It should be emphasized that the shutdown of the aforementioned transmission line, in isolation, would not be sufficient for the magnitude and systemic impact of what happened,” Eletrobras said.
The ONS also said in the preliminary report that it is analyzing the equipment shutdowns, which include about 3,000 files with information from more than 250 substations. “More documents are currently being received and assessed to provide a deeper and more detailed diagnosis of the situation,” the ONS said.
First, it was suspected that the Xingu substation on the Belo Monte transmission line was the cause of the second outage, but Valor found that the line’s protection systems were functioning properly. Renewable generation has attracted the attention of experts in grid operations because the country has a high potential for wind and solar production, which is now impossible to ignore, but which has its own generation characteristics that require updating of operating procedures.
Eletrobras pointed out that the transmission networks of the National Interconnected System are designed according to the so-called “N-1” reliability criterion. In other words, in the event of a component failure, the electrical system must be able to continue operating.
The 19 GW blackout at 8:31 a.m. represented about 27% of the country’s total load, of 73 GW, affecting 25 states and the Federal District.
*Por Fábio Couto — Rio de Janeiro
Source: VAlor International