Dive Brief:

• Puerto Rico was left without grid power on Wednesday after a line fault tripped two of the island's main power plants offline. 
• A fault on a 230kV transmission line between the Aguirre Generation Station in Salinas and the AES Generation Station in Guyama tripped the two power plants offline due to a lack of generation adequacy, PREPA Sub-director Justo González said. The two plants are located in the south of the island about 5 miles apart. 
• Less than a week ago, a similar line fault tripped the Palo Seco and Aguirre generators offline, cutting power to 840,000, and in early March another fault plunged 800,00 into darkness. The Puerto Rico Electric Power Authority (PREPA) said power should be restored from the latest outage within 36 hours.  

Dive Insight:

Puerto Rico's latest blackout shows that while nearly all the island's generation is back online after Hurricane Maria, the grid that supports it remains fragile and the territory lacks sufficient reserves to resist minor disruptions.

"The grid is suffering from gross instability," said Jorge Camacho, a former D.C. regulatory staffer now working with the National Institute of Standards and Technology on power restoration. "Right now it is very frail."

Wednesday's outage was caused by an excavator operated by Cobra Acquisitions, an Oklahoma-based company working under a $445 million contract with PREPA.

Camacho said that individual faults can have a ripple effect on the grid because Puerto Rico lacks sufficient generation reserves — known as resource adequacy — to continue operation when one plant goes offline.

"When you have a generator that comes offline due to a localized fault, due to the lack of resource adequacy, that has a domino effect on the other generators that have to supply it to make up for the fault," Camacho, who has worked closely with PREPA, told Utility Dive. "The other generators cannot supply the island because we don't have the reserves, so the other generators will try to meet that demand and will trip offline."

The islands of Culebra and Vieques, located to the east of Puerto Rico, retained power throughout the Wednesday outage, Camacho said. Typically, they receive power from the main island through sub-sea cables, but since the Hurricane have switched to distributed generation — mostly diesel generators supplied by the government.

Distributed generation could have a major role to play in the island's future. In January, the Puerto Rico Energy Commission, the island's utility regulator, laid out a new slate of rules for microgrid development after comments from industry stakeholders. But days later Gov. Ricardo Rosselló released a plan to overhaul the regulatory body that critics say would diminish its authority.

The Puerto Rico Energy Commission is also currently wrestling for control of PREPA with a federal oversight board appointed in 2016 to oversee the utility's finances. Federal government agencies plan to wind down their restoration operations on the island next month, leaving Puerto Rican officials to oversee the rebuild of the territory's grid.