Jeff Glenney, P.Eng. has a Bachelor of Science degree in electrical engineering from the University of Saskatchewan and is a registered Professional Engineer in Saskatchewan. He has worked in the electrical protection relay market with various manufacturers since 1995. For more information, visit www.benderinc.com.
Continuous NGR monitoring avoids the potential dire effects of inadvertently operating with an ungrounded system. In mining, for example, the NGR is recognized as an integral part of the ground-return path and part of the control mechanism for touch potential on moveable and mobile equipment. NGR failures occur regularly enough that the Canadian Standards Association and others mandate continuous monitoring of impedance grounded systems.
A well-designed NGR monitor confirms the electrical path from the transformer or generator neutral through the NGR to the station ground. When powered from a separate source, the monitor will perform active monitoring whether or not the system is energized to ensure a trip or alarm if an NGR failure occurs.
The failure mode of an NGR is usually open circuit. An NGR can be short-circuited accidentally during construction or maintenance. Unlike an open-circuited NGR, a short-circuited NGR results in a grounded and stable electrical system that is not subject to the transient overvoltage hazard. Ground-fault current will flow during a ground fault, and the fault will be cleared by ground-fault or overcurrent protection.
A shorted resistor, while rare, has similar consequences to a phase-to-phase failure. It brings higher likelihood of interrupting power to a large section of a facility due to the higher-level ground-fault currents. A shorted resistor increases the potential for an arc flash hazard and fault clearing equipment that interrupts the high-level faults may need servicing after interrupting one fault (similar to the phase-to-phase interruption issue mentioned earlier).
NGRs can be impacted by factors such as lightning, extreme temperature changes or other harsh environmental conditions, corrosive atmospheres, harmonic currents, manufacturing defects and vibration. Using NGRs beyond their time rating can also cause them to fail. Many consider monitoring more important on noncontinuous duty-rated resistors because of their somewhat weaker construction.
Failure of the ground path in the open condition means that a resistance-grounded system is transformed into an ungrounded system. Without continuous NGR monitoring, operators have no way of knowing that the current-sensing ground-fault protection has failed and there is a risk of transient overvoltages. An open-circuited NGR may not show external signs of failure or it may be mounted in a location where it is difficult to view. The system usually continues to operate until the open resistor is discovered following an event.
Open resistors may also be discovered during regular maintenance involving measurement of the NGR resistance —if such testing is being performed. However, periodic measurement of NGR resistance during maintenance only provides confirmation that the NGR was good at the time when the resistance was measured. The NGR could fail at any time after the measurement is taken, or in some cases it may not be reconnected after the measurement.
Where maintenance procedures involve testing ground-fault relays by using an intentional ground fault, an open NGR is more likely to be discovered only as the result of an investigation into why ground-fault relays failed to operate. There is enhanced danger of intentionally grounding a conductor when another phase may already be faulted.
When primary current injection through the current transformer opening is used for ground-fault relay testing, it confirms ground-fault relay operation and wiring. Ground-fault relays cannot operate as designed on a system with an open NGR. The ground-fault relay testing maintenance procedure could falsely confirm that ground-fault protection is operational. If there is a rectified ground fault or a DC fault, this current can flow through the system and may be undetected by less sophisticated ground-fault relays. The NGR is an ideal location to use full frequency detection capable devices to give better knowledge of the faults downstream.
An advanced continuous NGR monitor can detect an open NGR when the failure occurs and detect DC and non-60 hertz faults. It is active when control power is applied and indicates NGR health whether or not the system is energized, with or without a ground fault.
It is the foolproof method of safeguarding the integrity of electrical system grounding.