three phase ungrounded review 1

[bratocaster]

I need a refresher on three phase ungrounded systems on navy ships. If powering an electronic component, what is the affect of a ground on one phase?
Does it matter if it is delta or wye configured?

 

[RRaghunath]

Bratocaster,

Three phase ungrounded system will have over voltages between healthy phases & neutral in case of of grounding of one of the phases. The magnitude can be high by a factor of 1.73. I don't know whether the electronic component (in your mail) can handle the higher voltage.

Detecting an earth fault / identifying the faulty feeder is a problem with ungrounded systems.

When it comes to ungrounded 3-phase systems, delta or Wye connection doesn't make any difference.

 

[7JLAman4]

If there exists a hard single phase ground fault on an ungrounded delta system, the other two phases will continue to supply power. Effectivly, your power available is 66% of the total. In a wye ungrounded system, if a there exists a single phase ground fault, the available power is 58% of the total.
And yes troubleshooting for grounds on an ungrounded system is difficult. However each panel is equipped with ground detection test lamps.

 

[davidbeach]

A single ground fault on an ungrounded delta system does not reduce the power supplied to the load. The phase-to-phase voltages are unchanged, the current is unchanged, and the delivered power is unchanged. That is why ungrounded or high impedance grounded systems are used.

 

[Zogzog]

Try this

http://www.ibiblio.org/obp/electricCircuits/AC/AC_10.html

 

[Zogzog]

An ungrounded system has no intentional connection to ground with respect to its current-carrying conductors. Ungrounded systems are commonly distributed in a 3-phase, 3-wire delta configuration.

On low voltage systems, typical system voltages are three-phase 240, 480, or 600 volts. These systems have no intentional reference to ground but the system’s conductors are capacitively coupled to ground through the distributed capacitance of coil windings, cables, and other components.

The advantage of this system is that when a ground-fault occurs in one of the circuits, little fault current would be able to flow in the grounding path since the system has no direct ground reference. For many industrial applications such as automotive plants or paper mills with many motor loads, this provides a significant advantage. A ground-fault can occur on one circuit without causing damage or shutting down the system. Often times ground-fault detection schemes are employed that allow the faulted circuit to be identified and manually shut down for correction at the facility’s earliest convenience. Ground fault detection and alarm equipment is strongly recommended where ungrounded distribution systems are applied.

There are some disadvantages to this system. When a fault does occur in one phase, the other two phases now have an elevated voltage to ground established. For example, on an un-faulted 480 V system, the voltage level of ground might float around 200 to 300 V depending on the system capacitive coupling. Once one phase has been grounded, the other two phases are elevated to 480 V above ground. If the fault is of an arcing nature where it continually re-strikes, the voltage experienced by the other two phases can reach magnitudes several times the normal voltage. If this condition is not remedied quickly, the over voltage stressing of the insulation can break it down causing one of the other phases to fault to ground. At this point, with two phases grounded the system experiences a phase-to-ground-to-phase fault (short circuit through ground).