Continuous Monitoring of Neutral Grounding Resistors Maximizes Plant Availability

Faults on electrical systems are particularly difficult to locate and rectify in arduous production environments such as oil and gas, petrochemical, pulp and paper, water treatment and mining. Ground faults are a common failure in these systems and can impact a wide range of equipment including pumps, drives, generators and substations. High resistance grounding (HRG) is designed to control fault current—reducing point of fault damage and arc-flash hazards, controlling ground-fault voltage and transient overvoltages. Failure of a neutral grounding resistor (NGR) can occur due to the harsh environment, lightning, mechanical abuse, vibration or abnormal harmonic or direct current (DC) flow. Whether by mandate or best practice, monitoring for abnormal NGR currents and voltages combined with continuous resistor monitoring for both opens and shorts can help maintain safe operations. Early detection of the change in the ground path resistance also provides increased system data, identifying potential future issues and giving the operator the ability to rectify faults before they become critical. NGR monitors are emerging as the preferred solution to continuously measure voltage, current, continuity of the system grounding path and phase-to-ground voltage. This continuous monitoring provides real-time insight into the health of the electrical system and helps plan predictive and preventive maintenance to enhance system availability. In a resistance-grounded system, the transformer or generator is neutral connected to ground through a current-limiting resistor. There are many industrial applications that have used resistance grounding on three-phase power successfully for decades. Properly designed resistance grounding eliminates many of the problems associated with ungrounded and solidly grounded systems while retaining many of their benefits. Transient overvoltages, while rare, can be prevented when a properly sized grounding resistor is selected, because the NGR provides a discharge path for the system capacitance. The NGR limits ground-fault current and minimizes point-of-fault damage and controls ground-fault voltage. In some cases, a resistance-grounded system can be allowed to operate with one phase faulted. The fault can be located without de-energizing with installed ground-fault sensors and portable zero-sequence current measuring devices. A common complaint is a ground fault that is only detected at the main and causes a plant-wide outage. Whether the system is tripping or alarm only, a combination of resistance grounding and time-coordinated downstream ground-fault detection equipment can help prevent this waste of production time. However, resistance grounding has a critical element that is often overlooked—the NGR. An NGR should be continuously monitored. A single phase-to-ground fault results in current flow from the transformer or generator winding through the faulted-phase conductor to the fault and to ground. The current does not take the path of least resistance. It takes all paths back to the source. This current flow could cause secondary problems to the system. The current returns to the source winding through the ground-return path and the NGR. Failure of an NGR is usually open circuit, leaving the ground-return path open and the system floating. The current-sensing ground-fault protection in a resistance-grounded system will not operate with an open grounding resistor. If a continuous NGR monitor is employed, ground-fault protection can be maintained. There are many documented cases of failed resistors. The danger comes from running with one phase grounded and a second phase going to ground, causing a phase-to-phase fault through earth. This type of fault has the potential to release more energy, do more damage and require longer downtime for repair or replacement of associated equipment. A phase-to-phase fault is typically cleared by a high-current protection device. However, the downtime can be more extensive, and the clearing device may not be rated for more than one high-level fault. Continuity of service is often the justification for implementing high resistance grounding. It is not surprising that an often-overlooked option for system ground-fault protection is to trip or de-energize faulted noncritical loads. Indication of ground-fault location and communication of the fault to maintenance staff is not always enough to have maintenance go and remove the fault. The solution to trip noncritical loads when ground faults occur can help prevent running continuously with one fault on the system for extended periods of time. Advanced ground-fault systems are available with prioritized tripping of the lowest priority feeder if there is a second high-resistance fault on a system. This can be another tool to keep as much equipment as operational as possible.

Value of Monitoring

Properly applied resistance grounding has several advantages. It can limit point-of-fault damage, eliminate transient overvoltages, reduce the potential for an arc flash and allow continuity of service with a ground fault. It can also provide adequate current for ground-fault detection and allow selective coordination on tripping systems. Selective coordination is the ability of the system to trip only the faulted section with no unnecessary downtime on other unfaulted feeders or loads. Unfortunately, when an NGR fails, all of the advantages inherent in resistance grounding disappear. In fact, the result may be that the resistance grounded system is less protected than an ungrounded system if the HRG relies only on current-based protection or passive means of monitoring the neutral grounding resistor. 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.

Conclusion

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.