Ground Fault Protection of Microgrid Interconnection Lines Using Distance Relay with Residual Voltage Compensation

Protection of utility feeders that supplies an ungrounded/impedance-grounded Microgrids (MGs) or Distributed Energy Resources (DERs) is a challenging task, especially for Single-Line-to-Ground (SLG) faults. Normally, the interconnection line can be protected using: (i) a Direct Transfer Trip (DTT) from the utility station and/or (ii) an over-voltage relay (59G) energized by a broken-delta potential transformer on the utility side of the interconnection transformer. However, the cost associated with the installation of a communication system to enable DTT scheme can be excessive. Further, proper settings of the 59G relay to ensure selectivity is not always possible, that is, longer operating times will be required to make sure the protection will not operate for an external fault. More importantly, an SLG fault on the utility feeder is seen as a phase-to-phase fault by the interconnection relay at the MG/DER side of the connection with lower fault current, which makes the interconnection protection more challenging.This paper studies the use of phase distance relay at the MG/DER side of the interconnection transformer to provide coordinated protection against ground faults on the utility side. The apparent impedance measured by the phase distance relay is not accurate if traditional methods are used. The study shows the closer the fault to the relay, the higher the apparent impedance seen by the relay. Therefore, it is proposed to utilize residual voltage compensation method to solve this issue such that the phase distance relay can correctly identify the SLG fault, accurately measure the apparent impedance, determine the fault location, and reliably isolate the fault without jeopardizing the stability of downstream system (i.e., MG/DER) and/or causing dangerous overvoltage/arcing conditions.