It seems to me that a system of multiple parallel sources which are separately derived and connected as a HRG neutral bond should be single point grounded. If not, all HRG bonds at each source would see the ground fault and draw current through each of the resistors resulting in a system wide alarm and initiate each of the timers to trip the mains. Would four pole breakers be effective? Are four pole breakers only for isolation for maintenance? ref. link at http://www.criticalpowernow.com/pdfs/ClPwrAOLCase.pdf
parallel generators and HRG
- Thread starter wieand
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tryinghard
Senior Member
- Location
- California
I might be missing something obvious here but what's a "HRG??
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
High Resistance Ground.
The four pole breakers are definitely required for safety; you cant trust what potential the neutral will be at with respect to real ground.
A ground fault will never "trip the mains", it just raises an alarm stating that something is wrong.
The four pole breakers are definitely required for safety; you cant trust what potential the neutral will be at with respect to real ground.
A ground fault will never "trip the mains", it just raises an alarm stating that something is wrong.
wirenut1980
Senior Member
- Location
- Plainfield, IN
According to the IEEE Green Book:
1.6.3 Multiple Power Sources
For installation with multiple power sources (Le., generators or power transformers) interconnected that are or can be
operated in parallel, the system ground can be accomplished in one of two ways:
1) Each source grounded, with or without impedance (Fig 7).
2) Each source neutral connected to a common neutral bus, which is the grounded, with or without impedance
(Fig 8).
(My words) It seems from the diagrams, which I am having problems posting...If you impedance ground multiple separately derived systems through a common point, then yes, the neutral should be switched (a procedure must be followed so that the neutral is not switched under load). However, if you impedance ground each SDS separately through its own impedance, a 4 pole switch is not necessarily the best way.
I guess my point is it doesn't have to be grounded at a single point.
1.6.3 Multiple Power Sources
For installation with multiple power sources (Le., generators or power transformers) interconnected that are or can be
operated in parallel, the system ground can be accomplished in one of two ways:
1) Each source grounded, with or without impedance (Fig 7).
2) Each source neutral connected to a common neutral bus, which is the grounded, with or without impedance
(Fig 8).
(My words) It seems from the diagrams, which I am having problems posting...If you impedance ground multiple separately derived systems through a common point, then yes, the neutral should be switched (a procedure must be followed so that the neutral is not switched under load). However, if you impedance ground each SDS separately through its own impedance, a 4 pole switch is not necessarily the best way.
I guess my point is it doesn't have to be grounded at a single point.
"If you impedance ground multiple separately derived systems through a common point, then yes, the neutral should be switched "
....but only if the switch is between the XO and the HRG, which is not likely.
Since a HRG system can only supply 3ph3w loads, is this a moot point? That is, on a fault, the neutral (XO) will experience a neutral shift voltage to earth ground potential but will be limited to the XO bushings and any insulated neutral conductor used to bond the multiple sources. None neutral circuit conductor is used to serve a load.
....but only if the switch is between the XO and the HRG, which is not likely.
Since a HRG system can only supply 3ph3w loads, is this a moot point? That is, on a fault, the neutral (XO) will experience a neutral shift voltage to earth ground potential but will be limited to the XO bushings and any insulated neutral conductor used to bond the multiple sources. None neutral circuit conductor is used to serve a load.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
coulter said:
The L-N loads are the only prohibited ones. The 277V is not a usable connection even though you can see it with your meter.
I got that part. Been clear on that for a long time. I've got some people telling me that 480HID lighting is a code violation on HRG.jim dungar said:
I could probably even find and old post where Don told me it was against code. But it may have changed in the last few cycles. I want to get the code references lined up for a yea or nay.
i had no intent to hijack. i was hoping for a kick start on this issue from wieand.
carl
Last edited:
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
coulter said:
I have never considered single phase L-L loads as an issue in any HRG I ever designed. Most people hear "single phase" and immediately shift into referencing N. The transformer doesn't care and the HRG doesn't care, and I can't recall the NEC ever addressing L-L single phase.
Excellent choice. And 210.6.B, C, and D And Art 100 definitions, Voltage to ground. - Yep, seen those too. But thanks anyway.benaround said:
Ben -
As I said, I'm thinking this has changed in the last few code cycles. My hope was that someone was aware of the change and could kickstart my research. This is not critical, but I will eventually run it down.
carl
multiple HRG's
multiple HRG's
"Each of the 6,000 amp engine-generator paralleled switchboards and the 4,000 amp UPS parallel switchboards, all utilizing Masterpact breakers, are separately derived power sources, which are individually grounded with the unique high resistance neutral grounding system. These systems require 4-wire switchboards with neutral buses to properly parallel the multiple units. Because the entire distribution system serves only 3-phase, 3-wire loads, the NEC allows that these neutral buses may be grounded via high-resistance bonds. As with the service entrance equipment, ground faults on the
engine-generator and UPS systems are through the neutral resistors, but the neutral bus may have elevated voltages during the fault -- hence the need for the four-pole breakers."
per link http://www.criticalpowernow.com/pdfs/ClPwrAOLCase.pdf
I have drawn some variations but only #5 makes sense to me. A HRG system is just a variation of a secondary delta or wye ungrounded system where the equipment is bonded to ground but a GF does not trip the OCD.
multiple HRG's
"Each of the 6,000 amp engine-generator paralleled switchboards and the 4,000 amp UPS parallel switchboards, all utilizing Masterpact breakers, are separately derived power sources, which are individually grounded with the unique high resistance neutral grounding system. These systems require 4-wire switchboards with neutral buses to properly parallel the multiple units. Because the entire distribution system serves only 3-phase, 3-wire loads, the NEC allows that these neutral buses may be grounded via high-resistance bonds. As with the service entrance equipment, ground faults on the
engine-generator and UPS systems are through the neutral resistors, but the neutral bus may have elevated voltages during the fault -- hence the need for the four-pole breakers."
per link http://www.criticalpowernow.com/pdfs/ClPwrAOLCase.pdf
I have drawn some variations but only #5 makes sense to me. A HRG system is just a variation of a secondary delta or wye ungrounded system where the equipment is bonded to ground but a GF does not trip the OCD.
wieand -
Disclaimer: i'm not a Grade A wizzard at designing power systems. I have install/commissioned/rebuild/redesigned several impedance grounded power systems for 480V as well as 13.8kv range.
Several things look odd to me:
1. I've never seen an HRG system with 4pole CBs.
2. I've never seen a multiple sourced HRG with the seperate sources having the neutrals solidly connected - nor connected through switches or CBs. The idea that the neutrals have to be connected to parallel the sources is unknown to me.
3. There really isn't any difference between 1 and 5.
4. As db said (adding: if the neutrals are to be connected) you will need a switch/CB to isolate the neutral for safe maintenance. Picture 4 would fit this. Usually this is seen with grounded systems, multiple sources
I've attached two sketches showing a 13.8kv and a 480v system. These would be typicals, certainly many other ways to meet customers specific needs.
I'm still completely baffled by AOL/SQD insisting the neutrals have to be tied together. Something they are doing that we aren't privy to.
Whoops - add a GF relay to the 13.8 utility NGR.
carl
Disclaimer: i'm not a Grade A wizzard at designing power systems. I have install/commissioned/rebuild/redesigned several impedance grounded power systems for 480V as well as 13.8kv range.
Several things look odd to me:
1. I've never seen an HRG system with 4pole CBs.
2. I've never seen a multiple sourced HRG with the seperate sources having the neutrals solidly connected - nor connected through switches or CBs. The idea that the neutrals have to be connected to parallel the sources is unknown to me.
3. There really isn't any difference between 1 and 5.
4. As db said (adding: if the neutrals are to be connected) you will need a switch/CB to isolate the neutral for safe maintenance. Picture 4 would fit this. Usually this is seen with grounded systems, multiple sources
I've attached two sketches showing a 13.8kv and a 480v system. These would be typicals, certainly many other ways to meet customers specific needs.
I'm still completely baffled by AOL/SQD insisting the neutrals have to be tied together. Something they are doing that we aren't privy to.
Whoops - add a GF relay to the 13.8 utility NGR.
carl
weressl
Esteemed Member
dbuckley said:
Depends on its setting. There COULD be Zero Sequence trip installed that is set low. ALthough the most common reason to employ HRG is to avoid tripping on a single ground fault, it could also be used just to LIMIT the ground faults magnitude.
weressl
Esteemed Member
wieand said:
For the life of me I can't understand why would they bother bringing the neutral into the gear at all.
See if this makes any sense, the paralelled systems operate thur transfer
switches, as do the Gen. and the UPS. The neutrals from each transfer thru
4 pole switches, the neutrals all land in the switchboard ( when ATS is closed)
neutral bus and the HRG is made from there. So, witchever power source is
in use, for whatever reason, that system will still be a HRG neutral system.
switches, as do the Gen. and the UPS. The neutrals from each transfer thru
4 pole switches, the neutrals all land in the switchboard ( when ATS is closed)
neutral bus and the HRG is made from there. So, witchever power source is
in use, for whatever reason, that system will still be a HRG neutral system.
weressl
Esteemed Member
benaround said:
We agreed that when HRG is used the neutral CAN NOT be utilized. So why bring it into the grear, why switch it?
Laszlo, To have a HRG neutral system you need to utilize the neutral, correct?
'Why bring it into the gear' . To use only one HR for all systems to use. The
gear is a central point for GE, HR, and whatever neutral is being utilized, thru
the closed ATS ( 1 of 3 ) of the source.
' Why switch it '. First thing I can think of is two of the sources are Utility,
and from seperate sub-stations. That's enough reason for me. Would you
advise to solidly connect these neutrals together, along with the three Gen.
sets??
'Why bring it into the gear' . To use only one HR for all systems to use. The
gear is a central point for GE, HR, and whatever neutral is being utilized, thru
the closed ATS ( 1 of 3 ) of the source.
' Why switch it '. First thing I can think of is two of the sources are Utility,
and from seperate sub-stations. That's enough reason for me. Would you
advise to solidly connect these neutrals together, along with the three Gen.
sets??
weressl
Esteemed Member
benaround said:
HRG systems do not have a 'neutral'. You do not utilize the 'neutral' as any grounded point becomes a 'neutral' WHEN it is directly connected to the earth. The word 'neutral' is to indicate that that point of the systems has no potential difference, it is neutral in relationship to the earth. Since there is a(n intentional) resistor between the grounded point and the earth, there is a potential difference, therefore it is no longer a neutral point.
There should be one resistor per system.
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