Operating Committee hears results of July 10 low-frequency event study

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A PJM Apparent Cause Analysis was initiated to examine the July 10 low-frequency event, resulting in the identification of several causal factors, Donnie Bielak, manager – Reliability Engineering, told the Operating Committee at its Sept. 11 meeting.

Those factors included multiple unit trips, non-approved cases from Real-Time Security Constrained Economic Dispatch (RTSCED), an unexplained drop in Eastern Interconnection frequency and poor synchronized reserve response, Bielak said in delivering the ACE/Frequency Excursion Apparent Cause Analysis Report.

With the scheduled frequency at 59.98 Hz, there was a series of events that negatively impacted the Eastern Interconnection frequency, culminating with a pseudo-tied generator tripping at 3:46 p.m. that day. A low frequency of 59.903 Hz subsequently occurred. The PJM tie error was about 900 megawatts low when PJM called on synchronized reserves. At its lowest point, the tie error dropped to 1,800 megawatts low.

Area control error (ACE) is a measure of the mismatch between the generation needed to meet load, or the demand for electricity, and what is actually supplying load.

Such frequency drops, while not normal, do occur every few years. The RTO saw a similar frequency deviation in 2014.

Bielak said that about the same time on the previous day, the generation dispatcher who validates real-time data and approves RTSCED cases every 5 minutes had seen cases solving incorrectly because bad data was being fed into the software. These cases were correctly not approved. On July 10, similar scenarios were appearing, and it took some time before the dispatcher was able to properly validate the data, he said, resulting in carrying forward the existing approved cases in RTSCED.

As a result of the investigation, PJM has rolled out a new display for dispatchers in the control room showing an operational representation of the RTSCED data that will help them validate cases. New training for dispatchers also has been developed.

Poor synchronized reserve response also contributed to the event, Bielak said. About 67 percent of Tier 1 resources responded, and 62 percent of Tier 2 resources responded.

The NERC Resources Subcommittee is investigating the unexplained drop in Eastern Interconnection frequency.

Bielak also delivered the PJM Operations Summary for August, noting that the peak load for the summer – and the year – has been adjusted down from the 151,478 MW originally reported for Aug. 28 to 150,646 MW.

In addition, the committee unanimously endorsed Revision 39 of Manual 1: Control Center and Data Exchange Requirements.

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