The difference with having an ungrounded system out on poles or underground is if there is no common conductor such as a MGN or EGC which I would just call it an Earth reference conductor since it would not be current carrying run with the ungrounded conductors, without this conductor there is no low impedance path if a fault were to happen at two different point in the system, lets say at pole one you have a conductor lets say phase A goes to ground and being an un grounded system nothing happens, as this conductor now just becomes a conductor that has a Earth reference, now at pole 2 phase B goes to ground, and there just happens to not be enough current to open the closer, but now since phase A has already reference earth you now have a 2400 volt potential around both ground points and someone who unknowingly steps in to these areas could be electrocuted, but if we have a Earth reference conductor between these two points that is also bonded to all the metal of the equipment of both areas, then the first fault to ground does nothing but make a corner grounded system, and the second fault on a different phase now has a low impedance path that will make sure the closer opens.

This is kind of the same way we do in industrial plants with a 480 volt system, even though we have a ungrounded supply, we are still required to run EGC's to each and every piece of equipment and connect it to our earth grounding electrode system so that when when we have the first fault it just turns it into a corner grounded system but when the second fault happens these EGC's will provide a low impedance between the two point of the fault so the OCPD's can open the circuit, Earth can not be depended upon to serve this function.

A ground fault protection device would not serve any purpose on a ungrounded system unless it is set to open the closer if even Earth provides enough current, but then this totally defeats the reason for having an ungrounded system in the first place, so this is why I asked if they meant a ground fault detection device since this in a must so that you would know when a phase faulted to ground so an orderly shut down can take place to repair the fault.

As far as stress on the HV cables, without a reference to Earth I do not see how an ungrounded system will place any more stress on the insulation of the cable as it would in a grounded system, if the cables insulation are rated for the phase to phase voltage or more which I think the NEC will require 5kv rated cables for the 2400 volt system, even if a phase goes to ground all that happens is you now have a grounded system.

I'm not sure what your were trying to figure in item #1

(which is 2.4kV/1.732=1.38kV)

as that is a WYE calculation for voltage difference between the X/0 and line to line voltage of a WYE you will not have this in a Delta unless you use a zig/zag grounding transformer which you wouldn't be using on a ungrounded system? remember transformers are isolating so if you use a 2400v to 480v transformer and wish to keep the 480 ungrounded then you have to install a ground detection system for the 480 volt system, it is an unusual installation to have an ungrounded system fed from an ungrounded system since any fault across the 2400/480 volt windings could result in having 2400 volt potential on the lower voltage system, the same common Earth reference conductor run with the primaries I said above, and bonded to the EGCs of the 480 volt system so that there is a low impedance fault path but the secondary loads

could be subject to the 2400v if the transformer had a primary to secondary fault, so I'm not sure if that would even be allowed? a 2400/208/120 with the X/0 bonded would protect from this as there would be a fault path since the 208/120 would be bonded to the common EGC of both systems, but not the 480 since it is also an ungrounded system, this transformer might have to be a shielded one so the 2400v would have to fault to this shield before it can fault to the secondary side

I'm just not sure and maybe some of the utility or HV guys will chime in on this as I don't normally deal with the HV side but do understand the fault current paths that must be maintained for each side of the system and even across two systems to provide a low impedance fault path.

Solution To "Overcurrent Protection"

The ground fault is occurring on a Delta system which is an ungrounded system. (The Delta system is determined by the transformer secondary connection. Since the secondary is connected in delta, the service is therefore a delta system.)

Consider the schematic of the delta system as shown below:

 

IN AN UNGROUNDED SYSTEM (DELTA OR UNGROUNDED Y), THE FIRST GROUND FAULT CAUSES NO SHORT CIRCUIT CURRENT TO FLOW.

EXPLANATION

Since the fault occurs from CØ to ground, there is no complete path for short circuit current to flow. If you trace the path of the short circuit (as shown) from CØ through the line conductor, through the ground fault, through the ground system, and back to any phase, you will notice that a return path is missing. Therefore, NO short circuit current will flow.

However, because one phase (CØ) is now grounded, the phase to ground voltage on the unfaulted phases (A & B) is now at full phase to phase voltage (i.e. 480 volts) instead of a more normal, but approximately, phase to "neutral" 277 volts.

In an ungrounded Delta system, there are stray capacitances inherently connected between each phase conductor to ground. These stray capacitances are not "solidly connected" but are innate to all systems. They are distributed pretty much evenly throughout the system, are usually equal in magnitude in all 3 phases, and cannot be removed.

(Note: Phase to phase stray capacitances also exist between all phases but these capacitances play no part in ground faults.)

It is these stray capacitances that charge up and maintain a voltage between any phase wire and ground. The ungrounded delta is not truly ungrounded. Rather it is considered to be "capacitively grounded." Notice in the schematic figure, that these stray capacitances are actually Y connected to the delta system, and their neutral point is connected to the ground.

Because of this Y connection, the "normal" phase to ground voltage in a delta is the voltage that appears across each phase to ground stray capacitance and would be measured at about 277 volts. Remember though that the ungrounded delta's Ø to ground voltage can vary since no real phase to ground connection actually exists.

Since the stray capacitances are usually equal, and are Y connected, the "normal" voltage that one would measure from each phase to ground would be around 277, which corresponds to approximately the phase to "neutral" voltage even though no neutral exists on the delta, but does exist in the Y connection.

When a ground fault occurs from CØ to ground, the stray capacitances connected from CØ to ground are shorted out. CØ is now grounded. If you measure the phase to ground voltage from AØ (or BØ) to ground, it would be the same as measuring from AØ (or BØ) to CØ. Thus, it can be seen that the phase to ground voltage on the two unfaulted phases instantly rises from 277 volts to 480 volts and maintains that voltage throughout the duration of the fault.

Because of this voltage rise, the stray capacitances on AØ and BØ to ground are now charged at 480 volts instead of 277. This causes the normal charging current of the system to increase by a factor of the square root of 3 or 1.732. The current that flows in the two unfaulted phases during the fault would be the normal load current plus the higher magnitude of charging current.

Note also, that any phase to ground insulation on the unfaulted A and B phases, in any device (transformer, motors, etc.) throughout the entire delta system, would be stressed at 480 volts instead of the normal 277, while the fault exists.

Phase to ground insulations on any ungrounded system must be specified for full line to line voltage. Lightning arresters on ungrounded systems must also be sized for full voltage.

Ground Fault Monitors make use of these stray capacitance phase to ground voltages to detect ground faults. One LED is hooked up on each phase to ground (total of three LED's are installed). The three LED's normally burn with equal brightness. When a fault occurs, the faulted phase light goes out, and the unfaulted phases' lights burn brighter.

Ground Faults on Ungrounded Systems

Floating systems are derived from a power source where there is virtually no connection to ground. 480VAC delta configured transformers are a typical supply for a floating system. Some deltas in the mining industry can be found in hoists. 480VAC deltas are also in wide spread use to supply 1000Amp - 2000Amp main feeder circuits in general industrial applications. Floating systems are often used in areas where a sudden shut down must not occur. Examples are intensive care units (ICUs) in hospitals, signal circuits, and emergency backup systems.

The magnitude of ground fault current in an ungrounded system on a first ground fault is very small. It depends on the system voltage, the resistance of the ground fault causing part and the system capacitances.

Example: If a grounded object with low resistance touches a live conductor, the resulting current flow will be negligible. The ground fault loop will be incomplete because the return path to the source is missing. Grounding may only occur through system leakage capacitance to ground. The Possible resulting current is also known as charging current.

Figure 1: The ungrounded system with single ground fault path

On a single ground fault, ungrounded systems will not produce the amount of fault current needed to trip a common GFR. The IMD is the device of choice for the protection of floating systems. BENDER IR series products are ideal for detecting ground faults on both AC and DC ungrounded systems.