Until recently, TriMet traction power substations (TPSs) utilized a low resistance grounding system for the dc switchgear and rectifier equipment enclosures. The TPS negative bus was connected to a dc ground mat through a grounding diode. In addition to the basic dc overcurrent and rate-of-rise relays, each substation is provided with a low resistance frame fault current relay (device 64C) and a negative-to-ground potential relay (device 64V). Recently, TriMet has conducted several field tests: a) Bolted frame fault tests, by applying 750Vdc to the dc switchgear enclosure with the grounding diode connected and then with the grounding diode removed, b) Ground fault tests, by connecting the OCS contact wire to an OCS pole ground rod (the ground fault tests were done with and without the diode), c) Ground fault tests simulating an arcing ground fault, with a 9 ohm resistor inserted in series between the OCS contact wire and the OCS pole ground rod. This paper provides the test data and discusses the results of these frame fault and ground fault field tests. Technical analysis including personnel safety considerations are performed for various test configurations. The analysis will be focused on ground fault detection since most ground faults are undetected because of the very low ground fault current magnitudes, which are normally not detected by the rate-of-rise relay.

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