Benefits of HRG systems
Resistance grounding is used for several purposes:
It reduces burning or melting effects in faulty equipment
It reduces electrical shock hazards for personnel
It reduces the dip in line voltage while dealing with a ground fault
It reduces mechanical stresses in circuits that carry ground-fault currents
It can avoid shutdowns in the event of a first ground fault
Personnel close to the ground fault are safer
Equipment destruction is reduced
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.
Characteristics of HRG systems
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 and capture alarms on a system with time and date stamped information to assist troubleshooting.
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 occurance.
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.