February 1, 2010

Widespread Voltage Collapse Demonstrates the Importance of Generator Acceptance Testing

Robert Castro

Analyzing the Sequence of Events for Preventive Equipment Measures

The outage would have been avoided had the utility worker been trained to treat all existing wires as energized and cut them one at a time instead of in a bundle. Cutting three of the cables at the same time caused two breaker live wires to be connected to a grounded wire and opened the breakers to two busses at RS-E.

Thirty minutes into the disturbance, T-O Line 3 relayed after sagging into a tree. This demonstrates that even if a specific line is thermally rated to handle emergency loading, this capability is lost if appropriate tree trimming precludes the requisite sagging of the overloaded line.

Attempts to restore transmission at RS-E failed due to a large power angle. Load dispatchers were not initially aware of the large phase angle between two of the generators. Accurate knowledge of the phase angle at RS-E would have allowed them to more quickly analyze the situation and develop a remedial plan. This problem has subsequently been corrected by the installation of a phase angle monitoring device (a phasor measurement unit) at critical receiving and generating stations.

Fifteen transmission circuits terminate at RS-E. This is too many for reliable operations; the loss of RS-E will black out large portions of the service area. This situation was mitigated by conducting comprehensive planning studies that evaluated and implemented a method of bypassing RS-E with transmission so that any future loss of the station will be less likely to cause blackouts.

The H Generation Station was tripped out of service seven minutes into the disturbance. If this generation had remained online, it might have supplied enough reactive support to avoid the voltage collapse incident. What is troubling is that the unit that caused the H Generation Station to trip off, H Unit 10, was operating within its capability curve. More on the capability on H Unit 10 later.