Grant: Engage Grants Program

The power system at Calgary International Airport (YYC) incorporates the use of utility power paralleled withlocal generation. There are total four generators, which are connected through two 5 MVA ratedautotransformers to step-up the voltage from a 13.2 kV local generation bus to a 25 kV utility bus. The poweris further distributed to the load from the 25 kV utility bus to downstream through six 3 MVA ratedtwo-winding transformers. Because this is new equipment, no significant loads are connected to the secondaryof the six 3 MVA transformers, i.e., the system is running at a light loading condition.If the system is supplied by the utility only, the operation is normal. However, when the system with bothutility and local generation is energized through two parallel autotransformers, the zero-sequence harmonicissue occurs. Recent voltage and current monitoring data indicate that there are very rich 3rd and 9th orderharmonic content in the current traces. These zero-sequence harmonic currents circulate in the two parallelautotransformers during the system's start-up and energizing, which causes the zero-sequence ground relay(51G) to trip. The ground detection threshold is set at 5 A. The circulating current builds over several cyclesand reaches the threshold (5 A) in about 10 cycles after the circuit breaker closes, it continues to increase forseveral cycles before reducing below 5 A after about 1 second. Presently, the temporary solution is to set 51Grelays with 2 second delays in order to allow the circulating current to dissipate without tripping 25 kV circuitbreakers. However, this is only fixing the symptom.The proposed research project aims to investigate fundamental theory and root cause of the problem inorder to better understand the phenomenon, and find an effective mitigation technique. The findings andsolution through this research can significantly improve future design of the equipment and the system, andgreatly benefit the company's business in similar areas.