in a HRG system if I understand you place a resistor in the neutral to allow for reduced ground fault current to flow allowing continunity of service until the fault can be located (using a pulsing contactor) and repaired, my question is if you have a HRG system feeding a delta-wye transformer the primary of the transformer is seeing 277V per phase upon a ground fault the phase to phase voltage is increased to 480V? if that is the case dosent this effect the seconday voltage based on the turns in the Xfmr??
High resistance ground?
- Thread starter Mike01
- Start date
- Status
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The delta windings do not care what the voltage to ground is, they will always see 480V L-L.
ohmhead
Senior Member
- Location
- ORLANDO FLA
Well i think the NEC does not allow NGR with phase to neutral single phase loads .
But when a fault happens current is limited or held to a low point and a relay senses with some delay time the excessive voltage drop on that [ NG- resistor ] so its kinda the voltage drop across the resistor which is used to trip that protection relay so minor faults or overloads are not a issue .
As stated the voltage online is not effected by this resistor
But when a fault happens current is limited or held to a low point and a relay senses with some delay time the excessive voltage drop on that [ NG- resistor ] so its kinda the voltage drop across the resistor which is used to trip that protection relay so minor faults or overloads are not a issue .
As stated the voltage online is not effected by this resistor
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
To expand slightly: the neutral grounding resistor will have no effect on any of the terminal-terminal voltages; not only do L-L voltages remain unchanged (as stated above), but L-N voltages also remain unchanged.
The only thing that the resistance grounding changes is the voltage between the circuit conductors and ground (including bonded non-current carrying metal components). During a fault to bonded metal, most of the supply voltage is dropped across the grounding resistance, limiting current and voltage difference elsewhere in the system. But the L-L and L-N voltages remain unchanged, but L-G and N-G voltages may change considerably.
-Jon
The only thing that the resistance grounding changes is the voltage between the circuit conductors and ground (including bonded non-current carrying metal components). During a fault to bonded metal, most of the supply voltage is dropped across the grounding resistance, limiting current and voltage difference elsewhere in the system. But the L-L and L-N voltages remain unchanged, but L-G and N-G voltages may change considerably.
-Jon
ohmhead
Senior Member
- Location
- ORLANDO FLA
Well Winnie i agree with this but if i recall someone said once and i take back my first post at single phase a ground fault current flowing through a resistor when and if a single phase faults to ground will increase the phase to ground voltage of the other phases and that a 3 phase breaker is designed meaning its interrupt ratings at 480 /277v one pole phase to ground if voltage increases its trip setting will be effected will it work or will it not .
I read that some where in a GE page and its in the back of my mind ?
Is this why the NEC may not allow this on single phase ?
I read that some where in a GE page and its in the back of my mind ?
Is this why the NEC may not allow this on single phase ?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
One advantage to an HRG is that it keeps the L-L and L-N voltages from drastically fluctuating during a fault condition.
An ungrounded system may experience an increase in voltages.
line to line
line to line
ok, my fault after looking at the diagram when you have the first ground fault it creates a reference point and raising the the (delta or wye) point and making it the referency point, this elevates the l-g, or l-n voltage (there would be none as no line l-n loads are connected, however the l-l voltages remain the same. Sound right.. so the major advantage would be the contuinty of service, a major advantage in a mission ctitical facility.
line to line
ok, my fault after looking at the diagram when you have the first ground fault it creates a reference point and raising the the (delta or wye) point and making it the referency point, this elevates the l-g, or l-n voltage (there would be none as no line l-n loads are connected, however the l-l voltages remain the same. Sound right.. so the major advantage would be the contuinty of service, a major advantage in a mission ctitical facility.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yes, the major advantage, of an HRG is the continuity of service. It also fixes the L-G voltage at a nominal 277V.
The HRG limits any L-G fault to a maximum of about 5A (it depends on the resistor ohms).
resistor sizing
resistor sizing
What is the best way to size the resistor or is this typically done by the mfgr?? it seems like a good system in critical facilities for the 3-wire loads, and provided you have an experienced / knoweldegable staff, I thought I read somewhere where the insulation had to be increased due to the voltage and duration allowed to operate prior to tripping a breaker or the increased hazard of creating a L-L fault and instantenous tripping, on a 480V system would 600V insulation be acceptable? To me on a HRG system you would be ok on an ungrounded not so much due to high arcing ground faults and possible high transients, is this do to the distribuited capacitance in the ungrounded system (essintally grounded) thru cables, equipment etc. via a high impediance back to the source??
resistor sizing
What is the best way to size the resistor or is this typically done by the mfgr?? it seems like a good system in critical facilities for the 3-wire loads, and provided you have an experienced / knoweldegable staff, I thought I read somewhere where the insulation had to be increased due to the voltage and duration allowed to operate prior to tripping a breaker or the increased hazard of creating a L-L fault and instantenous tripping, on a 480V system would 600V insulation be acceptable? To me on a HRG system you would be ok on an ungrounded not so much due to high arcing ground faults and possible high transients, is this do to the distribuited capacitance in the ungrounded system (essintally grounded) thru cables, equipment etc. via a high impediance back to the source??
voltage
voltage
I could not find the "edit" button so I will post another reply. for th hrg system when you have a ground fault say phase "B" that becomes your reference point? therefore the voltage between the faulted phase and ground becomes zero? but the line to line voltage remains the same. however the line to ground voltage becomes the same as the l-l voltage, so the ground conductor carries the l-l voltage, now the cabinets are bonded to the ground conductor, because the faulted phase to ground becomes zero this is why the equipment does not become energized also correct?
voltage
I could not find the "edit" button so I will post another reply. for th hrg system when you have a ground fault say phase "B" that becomes your reference point? therefore the voltage between the faulted phase and ground becomes zero? but the line to line voltage remains the same. however the line to ground voltage becomes the same as the l-l voltage, so the ground conductor carries the l-l voltage, now the cabinets are bonded to the ground conductor, because the faulted phase to ground becomes zero this is why the equipment does not become energized also correct?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
There is a difference between resistance grounding in <600V and >600V installations.
You have been discussing 480V, so all of my comments have been directed to HRG <600V.
Unless there are other overwhelming reasons the most common HRG is chosen to deliver a L-G fault current of 5A on a 480Y/277 system which is about 55ohms.
You are mixing ungrounded and HRG systems.
In an HRG system the L-G voltages do not change from their nominal value of 277V even during fault conditions.
In an ungrounded system the L-G voltages will change, and in some cases (i.e. arcing and distributed capacitance) may even exceed the L-L voltage. In a solid ground fault the L-G voltage of the un-faulted phases becomes equal to the L-L voltage
You have been discussing 480V, so all of my comments have been directed to HRG <600V.
Unless there are other overwhelming reasons the most common HRG is chosen to deliver a L-G fault current of 5A on a 480Y/277 system which is about 55ohms.
You are mixing ungrounded and HRG systems.
In an HRG system the L-G voltages do not change from their nominal value of 277V even during fault conditions.
In an ungrounded system the L-G voltages will change, and in some cases (i.e. arcing and distributed capacitance) may even exceed the L-L voltage. In a solid ground fault the L-G voltage of the un-faulted phases becomes equal to the L-L voltage
??
??
Jim, thanks (480V wye 3-wire loads only) I thought on an HRG system when the first ground fault occurs the voltage becomes 480VL-G except on the faulted phase that becomes zero but that must be with ungrounded systems, so the L-G voltage always stays at 277v?
??
Jim, thanks (480V wye 3-wire loads only) I thought on an HRG system when the first ground fault occurs the voltage becomes 480VL-G except on the faulted phase that becomes zero but that must be with ungrounded systems, so the L-G voltage always stays at 277v?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Jim,
I am pretty sure that during a fault on an HRG system, the L-G voltages have to change. During normal operation the L-L voltage is 480V, the L-G voltage is 277V, and the N-G voltage is very small. During a solid fault, the faulted L-G voltage drops to zero, and the other L-L voltages climb to 480V, and the N-G voltage climbs to 277V, with that 277V driving current through the grounding resistance.
My understanding is that the HRG system acts in the same fashion as an ungrounded system for solid faults, but prevents the high voltage excursions caused by arcing or re-striking faults.
-Jon
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yes, the L-G voltage on the faulted phase will be essentially 0V and the L-X0 will be about 277V. This means that the L-G on the unfaulted phases will be 480V. An HRG behaves similar to a solidly grounded wye, in this regards.
In an ungrounded system the L-G voltage on the fault goes to zero (well actually it depends on the fault impedance). But, the L-G voltage on the unfaulted conductors can theoretically at least double, depending on the resonance of the system.
In an ungrounded system the L-G voltage on the fault goes to zero (well actually it depends on the fault impedance). But, the L-G voltage on the unfaulted conductors can theoretically at least double, depending on the resonance of the system.
- Status