Investigations into the H Unit 10 Trip
The generating units at H must remain online during and after disturbances to relieve loading on overloaded circuits and to provide dynamic voltage support. At the time it tripped off, 7 minutes into the disturbance, H Unit 10 was generating 156 MW and 104 MVAR, which is within its reactive capability.
A review of the records showed that the relay for the feeder to the excitation system tripped, and when this relay tripped it left the remainder of H’s units without a source of steam, thus tripping the entire plant.
Before getting into the specifics of this particular generating unit, some background on how the excitation system is supposed to function. A voltage sensor measures the output voltage of the generator. If the voltage is too low, the voltage regulator and power system stabilizer work to increase the excitation current, which is fed onto the field windings to increase the output voltage. The increase in output voltage provides extra reactive power to provide dynamic voltage support.
In H Unit 10, the exciter is a 380-V static exciter unit and is fed from a transformer that lowers the voltage from 4,160 V. It turns out that the 380-V feeder line relay tripped because of too much current on the feeder. To provide 104 MVAR output on the unit, the excitation feeder required 157 A. The feeder relay was set to accommodate the size of the feeder cables, which were rated at 100 A. As a result, the feeder cables were undersized. Had the feeder cables been sized correctly, say at 200 A, then the excitation feeder relay would not have tripped, and H Unit 10, along with the other H units, would have been able to supply reactive power to help support the collapsing system voltage. This problem has since been resolved by increasing the ampacity of the exciter feeder on this unit.
The Importance of Acceptance Testing
Besides the mitigation measures outlined above to avoid such cascading failures in the future, an important lesson on acceptance testing was learned.
The undersizing of the excitation feeders was a design flaw that should have been corrected during the acceptance testing of the generation equipment. Acceptance testing is a process in which all the generation systems are tested across their design range to ensure operation not only during normal operation but also during abnormal operations — like the disturbance causing this voltage collapse.
It is evident that the testing protocol followed during the acceptance testing of H Unit 10 did not include the full range of testing on the excitation system. Had this been done, the excitation system would have failed and a proper redesign would have been implemented.
— Contributed by Robert Castro (robert.castro@alumni.usc.edu) who teaches graduate-level power classes at the University of Southern California and negotiates wind generation contracts for a local utility.
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