We have a substation 4160 pri. to 480v sec. that had a GF indicator light on the other day.We found that someone had made a mistake hooking up a 480v 3 phase motor, one of the phases was terminated to ground at the motor and the EGC was terminated to one of the motor leads,OOPS!I don't understand how the motor ran with no problems and nothing tripping.I was told that the transformer at the substation has a floating ground.Also when you measure voltage at MCC you get 480v between all three phases but when you go to ground one of the phases I believe it was B you get 0v.My boss doesn't have the time to explain so I was hoping you guy's could point me in the right direction,Thank's!
Transformer question
- Thread starter breaux
- Start date
- Status
- Location
- Massachusetts
It's an ungrounded system, that is why it worked fine and why it has indicator lights.
Ungrounded systems are used when you do not want a ground fault opening an over current device and shutting the circuit down.
They work fine as long as someone fixes the ground fault as soon as possible after the indicator lights come on.
Ungrounded systems are used when you do not want a ground fault opening an over current device and shutting the circuit down.
They work fine as long as someone fixes the ground fault as soon as possible after the indicator lights come on.
- Location
- Massachusetts
Yeah that is a puzzler unless you drew it out.
As soon as one of the phase conductors was terminated to ground every thing grounded became that phase of the supply system.
Lets say phase A was terminated to ground, you would now have a corner grounded delta system and every thing grounded would also be at the same potential as phase A.
So now you come along and a hook up the motor with one leg to ground (which is phase A as well as also being grounded) and the other two motor legs to phases B & C.
The motor will run fine even though phase As current will be passing through the EGCs.
The motor is getting all three phases, two by way of wires one by the way of the EGC path.
As soon as one of the phase conductors was terminated to ground every thing grounded became that phase of the supply system.
Lets say phase A was terminated to ground, you would now have a corner grounded delta system and every thing grounded would also be at the same potential as phase A.
So now you come along and a hook up the motor with one leg to ground (which is phase A as well as also being grounded) and the other two motor legs to phases B & C.
The motor will run fine even though phase As current will be passing through the EGCs.
The motor is getting all three phases, two by way of wires one by the way of the EGC path.
transformer question
transformer question
OK I'm starting to understand.So one of the phases was actually the EGC and because it is a ungrounded system the motor was able to run.What about the voltage measurement at the MCC.I had 480v between all 3 phases but when I went from ground to one of the phases I had 0v and the other 2 phases I had around 240v?
transformer question
OK I'm starting to understand.So one of the phases was actually the EGC and because it is a ungrounded system the motor was able to run.What about the voltage measurement at the MCC.I had 480v between all 3 phases but when I went from ground to one of the phases I had 0v and the other 2 phases I had around 240v?
floating ground
floating ground
I heard it refered as a floating ground but I assume it's an ungrounded delta but we all know what assuming get's us.I assume again anyway that we have a corner grounded delta because I have no voltage to ground on one of the three phases.I'm going to gather more information on the transformer tommorrow.
floating ground
I heard it refered as a floating ground but I assume it's an ungrounded delta but we all know what assuming get's us.I assume again anyway that we have a corner grounded delta because I have no voltage to ground on one of the three phases.I'm going to gather more information on the transformer tommorrow.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
breaux said:
Almost.
It sounds like there were actually 3 phase conductors to the motor, but one of them was bonded to the EGC at that point.
Electrical systems do not care what conductor is bonded to ground, as long as it is only bonded in one place. Some confusion usually occurs because most people have been trained that any connection to ground (except for neutrals) is a short circuit.
- Location
- Illinois
- Occupation
- retired electrician
With one phase grounded you should see 480 phase to phase, one phase with 0 volts to ground and the other 2 phases with 480 volts to ground.
Don
quogueelectric
Senior Member
- Location
- new york
corner ground
corner ground
I have only seen one corner grounded delta and this was in a 5000 amp buss in a lipa(poco)station they told the generator grounded out on one of the phases and it was too expensive to fix so they went with the corner grounded open 5000 amp 480v buss and one leg read 0 volts. Now you know I had to test it for myself in Glenwood Landings power plant. It was creepy.
corner ground
I have only seen one corner grounded delta and this was in a 5000 amp buss in a lipa(poco)station they told the generator grounded out on one of the phases and it was too expensive to fix so they went with the corner grounded open 5000 amp 480v buss and one leg read 0 volts. Now you know I had to test it for myself in Glenwood Landings power plant. It was creepy.
transformer nameplate
transformer nameplate
I looked at the transformer nameplate and it's a delta-wye,4160vpri.-480v sec. I didn't get any information on how the system is grounded.I did read the voltage at a MCC it is feeding and I had 480v phase to phase and 240v phase to ground on all 3 phases.I'm thinking now that we either have a high resistance ground system or all 3 phases or just floating an when a ground fault occurs the fault current is contributed by the system capacitance to earth on the unfaulted phases.I asked my boss about it today and he said it's an above ground system,but he leaves it at that.I'm just trying to learn and I appreciate the help.I will try to gather more information about the system to help and determine what exactly we have.
transformer nameplate
I looked at the transformer nameplate and it's a delta-wye,4160vpri.-480v sec. I didn't get any information on how the system is grounded.I did read the voltage at a MCC it is feeding and I had 480v phase to phase and 240v phase to ground on all 3 phases.I'm thinking now that we either have a high resistance ground system or all 3 phases or just floating an when a ground fault occurs the fault current is contributed by the system capacitance to earth on the unfaulted phases.I asked my boss about it today and he said it's an above ground system,but he leaves it at that.I'm just trying to learn and I appreciate the help.I will try to gather more information about the system to help and determine what exactly we have.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Sounds like you are begining to get a good grasp on the system. I applaude your "acceptance" of the unexpected voltages caused by the coupling capacitance.
I work in an oil refinery and almost all of our substations use a high resistance ground system. This allows us to detect a ground on the system, usually a motor feeder, and plan an outage on that piece of equipment without losing it as soon as the ground appears. The important thing is to correct the problem before another ground appears on the system. No problem if another ground happens on the same phase but, if a ground comes in on any of the other two phases and we trip out the main- dead short.
Transformer vector group claculations
Transformer vector group claculations
Dear readers we at oman are installing 30 mva transformer we would like to conduct vector test at site can anybody give this details so that we can conduct this test
ours star delta system
and
main distribution transformer is delta delta 50 mva
Transformer vector group claculations
Dear readers we at oman are installing 30 mva transformer we would like to conduct vector test at site can anybody give this details so that we can conduct this test
ours star delta system
and
main distribution transformer is delta delta 50 mva
HRG system
HRG system
I found out today we have a HRG system.Just like scottk1r explained with only one of the phases grounded the system won't trip allowing you to plan downtime to correct problem.No one had ever explained this type of grounding system to me and now at the very least I have a basic understanding.Thank all of you for the information it really helped.
HRG system
I found out today we have a HRG system.Just like scottk1r explained with only one of the phases grounded the system won't trip allowing you to plan downtime to correct problem.No one had ever explained this type of grounding system to me and now at the very least I have a basic understanding.Thank all of you for the information it really helped.
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