Papers discussing DFA Technology
DFA Technology Puts End to Three-Month Problem
In September 2004, lights went out at several residences fed from a single-phase 50-kVA pole-top transformer on a 13-kV circuit in Staten Island, New York. Con Ed field personnel found that the secondary breaker of the CSP transformer at this location had tripped. At this location, secondary service cable from the transformer went down the pole and into a buried connection box at the base of the pole. A "crab" in the buried box connected this supply cable to three direct-buried service cables, each of which fed a customer meter. Personnel found no evidence of a problem, and reset the transformer breaker. The unit remained closed with all affected customers in service and no reports of flickering lights or partial service.
High-impedance, arcing faults (HiZ faults) are a perennial problem for distribution systems. They typically occur when overhead conductors break and fall, but fail to achieve a sufficiently low-impedance path to draw significant fault current. As a result, conventional protection cannot clear them, resulting in situations that are hazardous both to personnel and to property.
Texas A&M researchers spent two decades characterizing HiZ faults and developing and testing algorithms for detecting them. In the mid 1990's, General Electric commercialized the algorithms in a relay for detecting a large percentage of these faults, while maintaining security against false operations.
In an effort to mitigate problems associated with these faults, Potomac Electric Power Company (Pepco) installed the HiZ relays. They evaluated the performance of these relays on 280 feeders over a period of two years and gained significant operational experience with them. Being the first utility to apply high impedance fault detection technology on such a widespread basis makes Pepco's experience valuable to other utilities that are struggling with decisions regarding their own response to the problem of high-impedance faults.