Blackout After Blackout – How critical loads become your worst enemy!

As it was left after hurricane María, Puerto Rico’s Electric Power Authority (PREPA) bulk electric system and the limited load connected to it in the San Juan metro area is not a stable system. It blacks out frequenty and it will continue to do so in the next few months until its load footprint grows and becomes connected with other power islands within Puerto Rico.
Electric systems need to maintain a balance between generation and load, energy cannot be stored in this system. PREPA’s power system runs at 60 cycles per second or hertz (Hz). Like any other AC system, when you have more generation than load the whole system’s frequency goes over 60 Hz and under 60 Hz when you have less generation than load.
With a large combined cycle unit and some CT peakers being the sole source for a peak of a 150 MW power island, any time one of those units comes forced offline, there is no system in place to prevent the frequency decrease resulting off of this imbalance of load and generation. System frequency will keep falling to the point where it cannot recover resulting in a complete blackout. Likewise, any sudden loss-of-load event will result in an irrecoverable overspeed condition ending in a blackout too.
Most of the under-frequency load-shedding (UFLS) blocks are possibly still un-energized, critical loads are the ones in service but they were never intended to be sheddable in the first place! They used to be critical, now food for thought.
I’m sure PREPA is already taking measures to reconfigure their UFLS posture to very agressive while they remain in their current topology.

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