HRG and LRG -Impedance grounding theory/operation

[Dale001289]

We have a HRG @10A at a 81.250MVA steam generator (neutral grounding transformer 13.8kV/240V) and a LRG (resistor) @ 400A downstream from the generator at the 13.8kV switchgear bus.

One of our Leads is telling me fault current at any point on the grid - including EGC's - will be limited to 410A max due to the existence of these two impedance grounding systems. This include lightning strikes, ground faults to conduits, equipment, structural steel, cable tray i.e., at any point downstream from the two impedance grounds throughout the entire plant. I tried to explain, fault current is reduced only AFTER it passes through the impedance grounding systems - not before when fault current could be in the thousands of amps, such as a 100KA lightning strike to the electrode systems, such as steel tower or ground fault within a major piece of equipment.

This is very basic stuff - but how do I convince this guy he's wrong?

 

[Smart $]

Hmmm... I'm not really seeing the argument here. The whole point of an impedance grounded system is to reduce the potential ground fault current... and when there is a ground fault, it is monitored in a manner similar to an ungrounded system, but it will (or should I say can) trip a relay on the first fault compared to an ungrounded system.

As to convincing the guy, since he is the lead, you draw the diagram with all pertinent elements and current pathways (at least representative when it comes to ground fault current) and have him mark the complete current path including through your fault detection sensors.

If he does manage to show you a current path through the sensors, perhaps he doesn't understand how HRG/LRG systems reduce fault current while being just as sensitive to a ground fault as non-impedance grounded systems (at least on the front end). How to get around that can get a bit touchy.... :huh:

 

[Dale001289]

Hmmm... I'm not really seeing the argument here. The whole point of an impedance grounded system is to reduce the potential ground fault current... and when there is a ground fault, it is monitored in a manner similar to an ungrounded system, but it will (or should I say can) trip a relay on the first fault compared to an ungrounded system.

As to convincing the guy, since he is the lead, you draw the diagram with all pertinent elements and current pathways (at least representative when it comes to ground fault current) and have him mark the complete current path including through your fault detection sensors.

If he does manage to show you a current path through the sensors, perhaps he doesn't understand how HRG/LRG systems reduce fault current while being just as sensitive to a ground fault as non-impedance grounded systems (at least on the front end). How to get around that can get a bit touchy.... :huh:

Smart - thanks for the suggestions. Its frustrating dealing with someone that obviously doesn't understand the fundamentals of grounding.

 

[gadfly56]

Smart - thanks for the suggestions. Its frustrating dealing with someone that obviously doesn't understand the fundamentals of grounding.

As Smart says, draw a picture. I'm very visually oriented. I've had my share of "dawn breaks over Marble Head" moments once someone put it on paper.

 

[Phil Corso]

Dale...

Paraphrasing your post, "Why is the MV system-neutral grounded, especially when a 4th or neutral-conductor is never required?" If it were possible to insulate electrical equipment with glass, this discussion would hardly be necessary.

Early in electrical history ungrounded poly-phase systems were used somewhat optimistically, that is, it was possible to maintain operation with one-phase grounded. That is, until such operation leads to over-voltages, both transient and continuous, resulting in a substantial increase in subsequent damage. Although somewhat loosely connected to this discussion it is interesting to note that LV ungrounded systems are mandated by the USA NEC (National Electric Code), but only in specific situations.

The premise behind system-neutral grounding, at any voltage level, is to mitigate the effects of ground-fault current flow, i.e., burned-insulation, and melted magnetic-iron. A LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of tens of thousands of kVA) but solid-grounding eliminates the need to install additional ground-fault detectors or breakers. The phase-over-current protective device is sufficient. Repair or replacement of LV equipment is easily handled, both in material and cost!

Any MV system-neutral can be also be solidly-grounded, but the resultant ground-fault current is much greater (of the order of hundreds of thousands of kVA.) than in LV systems. Thus, if damage is kept low, repair usually involves just wire replacement. If, however, current magnitude is very large, subsequent damage to the magnetic-structure may preclude repair, but instead will require total replacement.

Thus, impedance-grounding of a MV system-neutral is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.

HGR or LGR does not reduce the impact of lightning-strikes!

Regards, Phil Corso

 

[Dale001289]

Dale...

Paraphrasing your post, "Why is the MV system-neutral grounded, especially when a 4th or neutral-conductor is never required?" If it were possible to insulate electrical equipment with glass, this discussion would hardly be necessary.

Early in electrical history ungrounded poly-phase systems were used somewhat optimistically, that is, it was possible to maintain operation with one-phase grounded. That is, until such operation leads to over-voltages, both transient and continuous, resulting in a substantial increase in subsequent damage. Although somewhat loosely connected to this discussion it is interesting to note that LV ungrounded systems are mandated by the USA NEC (National Electric Code), but only in specific situations.

The premise behind system-neutral grounding, at any voltage level, is to mitigate the effects of ground-fault current flow, i.e., burned-insulation, and melted magnetic-iron. A LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of tens of thousands of kVA) but solid-grounding eliminates the need to install additional ground-fault detectors or breakers. The phase-over-current protective device is sufficient. Repair or replacement of LV equipment is easily handled, both in material and cost!

Any MV system-neutral can be also be solidly-grounded, but the resultant ground-fault current is much greater (of the order of hundreds of thousands of kVA.) than in LV systems. Thus, if damage is kept low, repair usually involves just wire replacement. If, however, current magnitude is very large, subsequent damage to the magnetic-structure may preclude repair, but instead will require total replacement.

Thus, impedance-grounding of a MV system-neutral is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.

HGR or LGR does not reduce the impact of lightning-strikes!

Regards, Phil Corso

Thanks Phil, noted. Lots of great discussion here gentlemen. The Forum allows us to tap into some very serious brain-power!