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.