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Discussion Starter #1 (Edited)
I know this is an unusual 3Ø 4-wire setup, but its on a ship... where shore-based codes do not apply. Ground fault monitoring is required by the regulatory body. Both panels supply mostly mixed 120V loads. The engineer and contractor have been signed off, paid, and walked away... yet the ground fault system is in continuous alarm. :blink: (Coincidentally masked by the alarm module contacts being set backwards.)



This is also the first time I'm encountering ground-fault detection using a single current transformer. It was installed with only the 3 phase conductors... which obviously registered a 'fault'. (CT was reading any unbalanced current - several amps.) I since added the neutral into each panel's CT, but I'm still left with 500-700mA faults in each panel, both de-energized.



On further inspection, the X0 point on the Tx is grounded. I ran out of time and have to dig some more tomorrow.


Main question: Can ground fault detection even work properly with a grounded star-point?


I attached a drawing of what we have. FYI, the converter turns 50Hz into 60Hz.
 

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My experience is that there are no "ground faults" on a ship, because there is no ground. At best the CT will register an imbalance and as you can guess there are any number of reasons for that.

If the panels are off, I would check to confirm that the generators are properly synchronized. When switching from shore power to generator, the synchronization module loses calibration and while you cannot see the "out of sync" in the system in general, there is enough to trip current leakage detectors or CT monitoring current in general.

Cheers
John
 

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Bilge Rat
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Ground-fault detection is done all the time using only one CT that includes all 3 phase conductors and the neutral but not the ground. It's called 'Zero Sequence'.

Proper placement is key.

If properly installed, the CT will read zero current regardless of type of load but if any phase goes to ground, it'll read the fault current.
 

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My experience is that there are no "ground faults" on a ship, because there is no ground. At best the CT will register an imbalance and as you can guess there are any number of reasons for that.

If the panels are off, I would check to confirm that the generators are properly synchronized. When switching from shore power to generator, the synchronization module loses calibration and while you cannot see the "out of sync" in the system in general, there is enough to trip current leakage detectors or CT monitoring current in general.

Cheers
John
I think you mean voltage/power factor not matched properly. If that's not what you meant, I would like some further information on what you are saying, please.
 

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Chief Flunky
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1. If there is no ground fault then obviously I1+I2+I3=0. If not there is a ground fault. This is why you can just put all three phase conductors through a single CT. Or use three CTs wired so one side on each one is tied together. This current will be the sum of the three currents or...your neutral, but it is less accurate because it sees the error of all three CTs.
2. Second normally you ground the star of a wye transformer through a resistor in a resistance grounded system. The ground fault current returns to the transformer via this conductor. At this point you can either use.a single CT again or measure the voltage across the resistor. In a lot of systems you want both. That way if the resistor ever fails you will see full ground voltage (120 V) at a ground fault so it’s a inexpensive backup protection. The resistor always goes between the star and ground.
3. A third option is to put a delta-wye transformer or a special transformer called a zig zag when the main transformer is a delta and connect the resistor, ground, and CT through the wye of this transformer. It only carries ground fault current so the kva is very low. This is really more of a retrofit option. On new systems it’s a lot cheaper to just use the proper (delta-wye) transformer in the first place.

Regardless one huge rule with resistance grounded systems is NO line-neutral loads allowed! Only balanced loads. If you need unbalanced loads put in a transformer such as 208:120! The big reason for using 208/120 over say 480/277 of course is specifically because you get 120 V “for free” but in resistance grounded systems you can’t use it.

Sounds like the engineer was an idiot that had no idea how these systems work.
 

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I think you mean voltage/power factor not matched properly. If that's not what you meant, I would like some further information on what you are saying, please.
So when the ship goes from shore power to generator, you have to sync the generator to the shore power. What happens is that when the there is switching, any combination of generators can be running, so the syncing module loses calibration.

Then what happens, when the generators are running in parallel (typically one with one on stand-by) and in certain situations two generators will be running full power (there could be many genys running actually)and if they are not synced properly, the phase angle are slightly out of sync and will show up as various faults, current fault is one of them.

This is less likely in the more modern ships with PLC programming, but not sure what is the age of the equipment that he is talking about.

Cheers
John
 

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Discussion Starter #7
Its a tricky one because its a 380/50Hz vessel with additions. The main distribution is ungrounded delta. The 120/208V(60Hz) star-point goes back to the switchboard as per the drawing (which I'll be investigating tmw.) All these 60Hz systems were installed 2 years ago, and have not had the bugs worked out fully yet.



I'll have to see if the 'leaks' change when we go to generators. The main 50Hz bus doesn't seem to have any faults/leaks. So far have been working on this while laid up, primarily on shore power.



So you don't see the grounded star-point being an inherent flaw? The leak current on the two is oddly similar in value. I'd considered the long, insulated, unshielded #2/0 'ground' conductor running the length of the ship to the switchboard to possibly be susceptible to inductance or noise.
 

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Bilge Rat
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2. Second normally you ground the star of a wye transformer through a resistor in a resistance grounded system. The ground fault current returns to the transformer via this conductor. At this point you can either use.a single CT again or measure the voltage across the resistor. In a lot of systems you want both. That way if the resistor ever fails you will see full ground voltage (120 V) at a ground fault so it’s a inexpensive backup protection. The resistor always goes between the star and ground.
While this is indeed correct, the OP stated that the loads are mixed 120V.

Of course, in this case, the neutral needs to be solidly grounded and if using a single CT, all 3 phase conductors and the neutral need to pass through it.

If the neutral is outside the CT, 120V loads will be seen as a ground-fault.
 

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Chief Flunky
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Its a tricky one because its a 380/50Hz vessel with additions. The main distribution is ungrounded delta. The 120/208V(60Hz) star-point goes back to the switchboard as per the drawing (which I'll be investigating tmw.) All these 60Hz systems were installed 2 years ago, and have not had the bugs worked out fully yet.



I'll have to see if the 'leaks' change when we go to generators. The main 50Hz bus doesn't seem to have any faults/leaks. So far have been working on this while laid up, primarily on shore power.



So you don't see the grounded star-point being an inherent flaw? The leak current on the two is oddly similar in value. I'd considered the long, insulated, unshielded #2/0 'ground' conductor running the length of the ship to the switchboard to possibly be susceptible to inductance or noise.

Actually usually it’s capacitance on the phase conductors. Every phase conductor that isn’t faulted is insulated from ground (frame). This is the definition of a capacitor (two conductors separated by an insulator) and allows a small AC current to flow to ground (capacitors block DC but pass AC). You should always see a small current on the neutral from this system capacitance and the current you are talking about which is under 1 A sounds pretty typical,
 

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Actually usually it’s capacitance on the phase conductors. Every phase conductor that isn’t faulted is insulated from ground (frame). This is the definition of a capacitor (two conductors separated by an insulator) and allows a small AC current to flow to ground (capacitors block DC but pass AC). You should always see a small current on the neutral from this system capacitance and the current you are talking about which is under 1 A sounds pretty typical,
Is there a way to calculate the actual capacitance of the phase conductors, I'd think it would be a function of voltage, length, and the characteristic capacitance of the wire. Then you would know what to expect from the capacitive coupling.

Also, you'd have that current even with the panels disconnected, right? I'd be curious what you'd see if you disconnected the panels and connected them one by one.

If there's some high impedance contact between ground and neutral anywhere, maybe even capacitance leakage in a connected load, wouldn't you see some current missing in the current transformer?
 

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Hackenschmidt
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Proper placement is key.
I am curious about this. I was taught that for a current transformer to really be accurate the conductor has to be perfectly centered in the ring. I have even seen that by making a little cardboard to center the wire in a clamp meter. To be honest I think it was working but making a small enough difference you couldn't tell from the meter.

With three conductors and a single doughnut, I'd imagine you have to bundle them and get the center of the triangle right in the middle.

With four conductors I guess you'd need the neutral in the middle of the bundle and the neutral in the center of the ring.
 
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I'm about as land-locked as you can get so I have no marine experience what so ever. But I do have some experience in generation. I find the situation described interesting but I wouldn't mind some clarification so I can understand it better. So I'm going to ask some what may appear to be dumb questions if someone would humor me.

Both ccts in the diagram shown are fed from the same switchboard?

When you turn the panels off, do you mean panel main breaker leaving the transformers/converters energized? Or the breakers at the switchboard?

Each panel has a ground fault detection CT?

Sorry if I'm missing the incredibly obvious.
 

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If the panels are de-energized, obviously the CT should see 0 amps. Just for the fun of it, still while de-energized, jump all the phases and neutral together and to ground and see what the CT reads. If it still sees current, I'm guessing it is a calibration issue rather than a real electrical issue.
 

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Is there a way to calculate the actual capacitance of the phase conductors, I'd think it would be a function of voltage, length, and the characteristic capacitance of the wire. Then you would know what to expect from the capacitive coupling.

Also, you'd have that current even with the panels disconnected, right? I'd be curious what you'd see if you disconnected the panels and connected them one by one.

If there's some high impedance contact between ground and neutral anywhere, maybe even capacitance leakage in a connected load, wouldn't you see some current missing in the current transformer?

Is there a way to calculate the actual capacitance of the phase conductors, I'd think it would be a function of voltage, length, and the characteristic capacitance of the wire. Then you would know what to expect from the capacitive coupling.

Also, you'd have that current even with the panels disconnected, right? I'd be curious what you'd see if you disconnected the panels and connected them one by one.

If there's some high impedance contact between ground and neutral anywhere, maybe even capacitance leakage in a connected load, wouldn't you see some current missing in the current transformer?
https://megaresistors.com/products/neutral-grounding-resistor/capacitance-to-ground/

Mostly shielded or conduit cables with steel conduit, transformers, and motors.

If you have a ground fault theoretically the voltage across it will be the phase to ground voltage (120 V) divided by the system resistance along the phase conductor plus bonding plus the resistor. The resistor is usually much larger so ground fault current is close to that. Usually 40-50% of this number assuming wiring resistance is zero is used as the trip setting.
 

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This is very interesting to me. I've seen a couple problems similar when i was an EM on a Coast Guard ship. Once, equipment that did not recognize two hots vs hot+neutral ended up being the problem. Still not clear on it exactly due to security clearance...
Another time, we had a fault that was the dangdest thing- it only came out at night and only when the level indicator for a water tank was on. Turned out the fault was in a light that had filled with water but you had to have the light on, water tank indicator on (it usually was left off unless taking hourly reading) and you had to have the ground fault check button depressed. Pretty specific requirements in order to find it. But we did find it. The one about land equipment on a ship still interests me because people trained only on land don't seem to see the difference in ship systems.
You've probably already done this, but you might have to go completely dark ship for a bit to determine if it's common to shore and ship power, as well as check if it's there when dark ship. Might it be an unrelated ups that was installed simultaneously?

I don't think it's a sync issue- that would prevent power source transfer but shouldn't cause a ground fault in itself, i think.
The ground system does exist, but it's different from land system as you've stated. I think we called it a perfect ground.

Is this fault present on either power source?

Very interesting.
 

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Bilge Rat
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I am curious about this. I was taught that for a current transformer to really be accurate the conductor has to be perfectly centered in the ring. I have even seen that by making a little cardboard to center the wire in a clamp meter. To be honest I think it was working but making a small enough difference you couldn't tell from the meter.

With three conductors and a single doughnut, I'd imagine you have to bundle them and get the center of the triangle right in the middle.

With four conductors I guess you'd need the neutral in the middle of the bundle and the neutral in the center of the ring.
True, if you need the best possible accuracy, the conductors need to as close to the center as possible.

With a ground-fault system, placement of the conductors within the CT isn't an issue, I've seen switchboards that had a rectangular CT installed just inside the frame of the board that was used for ground-fault protection. The conductors (busbars) were nowhere close to the center and at least a foot away from the CT.
 

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Discussion Starter #17
I am curious about this. I was taught that for a current transformer to really be accurate the conductor has to be perfectly centered in the ring.

I snugged up the cable bundling some, shifted the CT around, and watched the module readout... no detectable change


I'm about as land-locked as you can get so I have no marine experience what so ever....



Both ccts in the diagram shown are fed from the same switchboard?

When you turn the panels off, do you mean panel main breaker leaving the transformers/converters energized? Or the breakers at the switchboard?

Each panel has a ground fault detection CT?

Marine isn't a specific context IMHO. There's just so many different ways things can be engineered. Our Code is more generalized. The book isn't even 1/2" thick. You have to learn the nuances of of the electrical system on each ship you work on.



Both circuits are on the same 380V/50Hz switchboard. Then there's a distribution panelboard (split bus) after the 208/120V Tx that feeds the two 120/208V loadcenters a deck above. Both feeders in the panelboard have a CT.



De-energizing the upstream Tx's would have been a nice test to do next. Was unable to shut things down today with other work happening onboard. Will have to wait until next shift.


https://megaresistors.com/products/neutral-grounding-resistor/capacitance-to-ground/

Mostly shielded or conduit cables with steel conduit, transformers, and motors.

Based on the engineered drawings, and the vendor documentation for the modules & sensors... this is not a resistance grounded system.


We use 'shipboard' cable for just about everything. (Essentially an extra-rugged SJTW cord.) Its 600V-1kV rated (unshielded) and installed in tray, or bundled and strapped agaisnt raised runners welded to deckheads and bulkheads. Much of the main distribution runs together in tray for at least half the ship.


Is this fault present on either power source?

I found out today there's a fault on the shore-power system too (on another frequency converter) so that is likely another factor. Time to report my overall findings and pass them up the food chain. :wink:
 
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