HRG stands for high resistance grounding – an electrical supply system that is used frequently in applications that cannot afford a shutdown, or that must control ground-fault voltage on driven equipment.
When a ground fault occurs, ground-fault current will flow as in a solidly grounded system, but its magnitude is severely restricted (to a few Amperes, typically 10 or less) by a neutral-grounding resistor. This limited current has several advantages—it is sufficient to detect and locate ground faults; it will not cause escalating point-of-fault damage; it will not escalate to an arcing ground fault, and it limits touch potential (the voltage between equipment frame and earth) on portable or mobile loads to a safer level.
It is important to note that the line-to-ground voltage of the unfaulted phases will increase during a ground fault, which increases the probability of a second ground fault. This requires a reliable insulation system not only line-to-ground but also line-to-line. Resistance grounding reduces the probability of a line-to-ground arc flash making systems safer, but it does not limit line-to-line arc-flash energy.
High resistance grounded systems cannot depend on overcurrent protective devices such as circuit breakers and fuses to protect against ground faults. In fact, in many cases, ground faults are allowed to remain on the system until they can be repaired in an orderly and planned manner.
It is necessary to install a ground-fault detection system since the ground-fault current is not interrupted due to overcurrent. When properly designed, such systems can also quickly allow locating the faulted branch feeder, switchgear or load. Such systems however do rely on the integrity of the neutral grounding resistor.
These are also stated in the IEEE Standard 142-2007: Recommended Practice for Grounding of Industrial and Commercial Power Systems (Greenbook).
HRG utilizes the best attributes of both solidly grounding and ungrounded systems while remaining cost efficient. Solidly grounded systems might be the most common form of power supply system, but in terms of hazards it is also a very susceptible one. The ungrounded system is more rarely used in some industries and countries, even though it has some advantages.
Because neutral voltage is elevated during a ground fault, line-to-neutral loads must not be served when using a high resistance grounded system. For most industrial facilities that is about 15 % of the total system load. These can be accommodated by installing an isolation transformer to power these loads while otherwise utilizing the benefits of an HRG system.
There is a concern about the ability to detect intermittent ground faults. Since most ground-fault meters only show constant ground faults, they do not react if the fault current is intermittent and it is very unlikely for maintenance personnel to be present at the exact moment an intermittent ground fault occurs. The solution is to upgrade to a more advanced monitoring system like the NGRM700, which can detect any ground fault, intermittent or not.
Another thing many engineers worry about is HRG system maintenance. The neutral grounding resistor is the key. If the resistance of the neutral-to-ground path (where the NGR is located) sinks below 75% of the desired value, the system tends toward being solidly grounded.
If the resistance is too high, for example beyond 125% of the desired value, the system tends toward being ungrounded. Reliability of the ground-fault detection and ability to control touch potential may be limited by such occurrences. It is recommended and in certain jurisdictions code required to monitor the NGR resistance continuously.
The NGRM700 does this, in addition to other functions (ac/dc ground-fault detection, neutral-voltage and -current measurement, phase-voltage monitoring, data logging and communications).
Similar to ungrounded systems, there is a myth that it is difficult and time-consuming to locate a ground fault in HRG systems. Even though in some industries one ground fault may not require a shutdown it is advisable to clear the fault before a second one occurs.
But just like in Bender EDS equipped floating systems, Bender RCMS technology used on HRG systems allows fast location of ground faults and offers a trip delay for isolating the faulty feeder at any chosen timeframe, enabling the user to plan maintenance actions.
In the rare case of a second ground fault occurring before the first one could be eliminated the scenario would be a phase-to-ground-to-phase fault and could cause damage.
Bender can supply advanced monitoring systems that detect the second fault and trip the least important faulted feeder, allowing continued activity of the more important one. Certain non-critical loads could also be programmed to trip on first fault to assist in prevention of phase-to-phase fault occurrence.
It can safely be said that – just like the ungrounded (floating) system – the HRG system is a very effective choice with almost no negative side effects as long as the proper monitoring technology is installed. While the ungrounded (floating) system uses the iso685 and EDS series to detect and locate ground faults, the HRG system has NGRM700 and RCMS series to provide these functions.
There are many kinds of systems that typically use a high resistance grounded power supply. Most of them usually require a high availability, reliability and thorough safety for their personnel and equipment.
One of the most common industries to use HRG is the mining sector.
Dirt, moisture, vibration, extreme temperatures, elevation, large systems and heavy machinery are sources for weakening insulation and damage that can cause critical hazards. Many applications in mining are mobile or movable equipment.
HRG is the ideal choice for mining applications on every level. Unlike in typical industrial installations, mining electrical Codes and Standards require both HRG systems and first-fault tripping. To minimize the equipment taken off line, each feeder, and some individual loads, are protected with ground-fault relays to provide selective coordination of tripping. The faulted equipment is de-energized while the rest remains in operation.
Other installations rely on HRG to allow continued operation with one ground fault. Refineries, saw mills, petrochemical facilities, factories, pipelines, food processors, and pulp mills are just some of the industries that have adopted HRG for their standard electrical systems.
Not only do the systems remain operational in case of a first ground fault (when controlled to 10 A or less), it is also easy to locate the fault and prevent higher level faults.
In a solidly grounded system the only limitation to ground- fault current is system impedance. Ground-fault current is similar in magnitude to short-circuit current. Overcurrent protective devices will operate. They shut down the energy if there is fault current, no matter where it flows.
This convenient method of avoiding harmful effects of ground faults will not work in floating or HRG systems, because a high fault current is needed for overcurrent protection to operate. An ungrounded system requires an alternative approach to current sensing—ideally, insulation monitoring.
In an HRG system, current-sensing protective relays can be used to detect and locate ground faults. These relays can be used in either tripping or alarm-only scenarios—it is a question of how their outputs interact with the rest of the power system.
No matter what the system configuration, rest assured, it can be guaded by Bender.
|System Grounding in the Americas, Australia, South Africa and Other Regions
|Guideline on NGR/Safety Ground Monitoring