The reason for the blackout

[shkim2000]

The PMS(Power Management System) controls and monitors power plant in ship. The powewr system is 440V, 3phase, 3 4440V generators and neutral flaoting earthing system. During earth fault from washing machine, the blackout occurs. PMS supplier said that blackout is reasonable action as sound phase-phase voltage is increased by root 3 times rated volatge while one phase is earthed. Which means that the sound phase-phase voltage exceeding preset value, i.e. overvoltage case. Then PMS start standby generator and kill running generator and during synchronzing the complete blackout occrus due to exising earth fault(overvoltage)
My question is
1. I understand neutral floating system is operable after 1st earth fault before 2nd fault occurence. This does not match with above ?
2. Should the above case reasonable ?
3. How I take actions for the above ?

 

[DanDel]

If I understand correctly, the emergency system was connected to and fed into an existing fault.
Typically, a C/B tripping on an overload or fault should be locked out so that this does not happen until the fault or the C/B lockout is cleared.

 

[waross]

It sounds as if your PMS supplier may not be familiar with floating neutral systems.
The phase to phase voltage does not change with a single ground fault.
The phase to neutral voltage (if present, eg. wye system vs. delta system) does not change with a single ground fault.
The Phase to ground voltage is normally approximately equal to the phase to neutral voltage.
During a phase to ground fault the phase to ground voltage on the other phases will rise to equal phase to phase voltage.
This is normal.
Your PMS system should indicate a ground fault.
That is all. No breakers should trip, no generators should try to synchronize.
Your PMS system should just give an indication that there is a ground fault that should be repaired.
By the way, the main reason for an ungrounded system in a shore based plant is so that the system may sustain one ground fault and stay in service.
On shipboard, the main reson for an ungrounded system is to avoid possible serious hull corrosion problems when on shore power.
You may run at sea with a ground on one phase, but it is unwise to run on shore power with a grounded phase.
respectfully

 

[coconutalley]

Agreed with Waross, if the power neutral is floating then ground fault should not trip out the generator. Suggest checking to isolate the problem:
1. Is there a line to line short, possibly a failed washer motor?
2. Is the PMS faulty or its sensors are faulty?
3. Is there any other possible explanation, e.g. lightning or other electrical equipment failure that is not obvious?

Waross,
on the reason for floating neutral, could it also be that large ships metal structure is dangerous to ground to power
systems since a ground fault could present a large current flow throught the metal frame of the ship?

 

[Hewlett]

I agree with Waross.
To me, networks protection scheme appears needing a "design review".... From you mentioning "overvoltage" it could be there is an earthfault trip device sensing phase-earth voltage. This is not sensible in a floating network, especially not on a ship, where availability is important(!)...generally, the smaller the generators, the smaller faultcurrent is needed to skew this potential, and you could have all kinds of small earthfaults releasing such a device...

 

[waross]

Re: grounding ship systems.
If there is an offset in the shore power neutral voltage for any reason and the onboard system is grounded to the hull, the hull will assume the same voltage as the neutral.
This is a low impedance connection to ground and will result in fairly large magnitude currents from the hull to the sea water with rapid consumption of the protective "Zincs" followed or accompanied by hull corrosion.
If another ship or onshore load on the same feeder is drawing a heavy neutral current there will be a voltage drop on the neutral or a voltage offset from true neutral/ground potential. This offset voltage is the source of the issues with grounded ship's systems.
respectfully

 

[Hewlett]

A story on availability...
Came to a plant with all ventilation in a production facility shut down, halting production.
Cause was found to be an LED leg (local position indicator for a damper) touching the metal of the panel it was mounted in.

History: some time earlier, a control transformer had burned on several occations. The (inexperienced) technician in charge took a sporadic "earthfault alarm" as "being the culprit" and added a relay to cut power to transformer every time the earthfault happened.
Initial design was bad, and the "fix" prevented transformer from smoking. Initially this fix made everyone happy....ventilation just had to be restarted now and then....but with the tech gone, and his modifications in no drawings, this was trouble waiting to happen...
Actual cause to initial burndowns was found to be a (rarely activated) solenoid coil that was shorted, and underprotection of its and tranformers circuits.

Valve was overhauled, fuses added to control circuits and "earthfault protection" removed.
This kind of (totally un-nescessarry) downtime is particularly unpleasant in open sea...