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 Post subject: Solidly Grounded vs. Ungrouded as Defined by IEEE 1584 for Delta Systems
PostPosted: Wed Apr 09, 2014 1:19 pm 
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My question is whether delta systems are considered solidly grounded for calculation purposes for iEEE 1584. I think so if this statement holds true. See below.

"A 3-phase fault is the strongest-current fault, over a phase-to-phase
or phase-to-neutral fault. Remember that the latter two faults
quickly turn into a three-phase fault.

The code requires that all distribution systems be grounded.
Exceptions are glass furnaces and melters.

The phase currents from a delta secondary and a wye secondary are the
same. The three-phase fault currents are the same. The single
phase-to-ground currents are different.

Unless we are going to calculate phase-to-phase faults (which are less
than three-phase faults) we have no problem with ungrounded systems."

Please share with me your thoughts on this question. I need to know as I am doing a study on a delta system.

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PostPosted: Thu Apr 10, 2014 12:58 pm 
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First it depends on the type of grounding. Resistance grounded is treated the same as ungrounded. Second, corner grounded (with a solidly grounded phase) or mid-phase grounded would definitely be a grounded system. The traditional three wire system with only bonding and not grounding would be ungrounded.

IEEE 1584 is not the best source of understanding for the different types of grounding, especially the various impedance grounding systems. IEEE Green book is a much better reference.

By the way, I'd encourage converting ungrounded systems over to at least resistance grounded. It's not all that terribly expensive to do, is safer, greatly improves motor life, and avoids a potential burn down situation.


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PostPosted: Tue Apr 15, 2014 1:05 pm 
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IEEE 1584 does differentiate between solidly grounded and ungrounded/high resistance grounded (HRG) systems in the incident energy calculation. The actual equation has a component known as K2 that affects the exponent of the calculation. K2 = 0 if the system is ungrounded/high resistance grounded and K2 = -0.113 if it is grounded. The result is a slightly higher incident energy if the system is ungrounded/high resistance grounded.

The fact that there is a difference in calculations based on grounded vs. ungrounded/HRG is sometimes questioned since IEEE 1584 is used for calculations involving a three phase fault and three phase faults are considered balanced and use a "per phase" solution method. In reality, the three phase arc flash is erratic in nature so it is not truly balanced therefore grounding does have an effect.

The statement about "a three phase fault is the strongest fault current" is not entirely correct. Depending on the zero sequence impedance (Z0), the line-to-ground fault current could be slightly higher than the three phase current. This sometimes happens at/near the secondary of delta - wye transformers and generators. However when it comes to arc flash, the assumption is that it will escalate to three phase so 3 phases of energy will always be more than one phase, even if that one phase has a slightly larger current than three phase. Therefore for the IEEE calculations, line-to-ground fault currents are not used.

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PostPosted: Wed Apr 16, 2014 10:29 am 
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Jim Phillips (brainfiller) wrote:
IEEE 1584 does differentiate between solidly grounded and ungrounded/high resistance grounded (HRG) systems in the incident energy calculation. The actual equation has a component known as K2 that affects the exponent of the calculation. K2 = 0 if the system is ungrounded/high resistance grounded and K2 = -0.113 if it is grounded. The result is a slightly higher incident energy if the system is ungrounded/high resistance grounded.

The fact that there is a difference in calculations based on grounded vs. ungrounded/HRG is sometimes questioned since IEEE 1584 is used for calculations involving a three phase fault and three phase faults are considered balanced and use a "per phase" solution method. In reality, the three phase arc flash is erratic in nature so it is not truly balanced therefore grounding does have an effect.

The statement about "a three phase fault is the strongest fault current" is not entirely correct. Depending on the zero sequence impedance (Z0), the line-to-ground fault current could be slightly higher than the three phase current. This sometimes happens at/near the secondary of delta - wye transformers and generators. However when it comes to arc flash, the assumption is that it will escalate to three phase so 3 phases of energy will always be more than one phase, even if that one phase has a slightly larger current than three phase. Therefore for the IEEE calculations, line-to-ground fault currents are not used.



Jim, There is another type of grounded system. That is the "Virtually Grounded System". This system is generally grounded through a small value reactor rather than a resistor. As you pointed out, the line-to-ground fault current can be significantly higher near delta-wye transformer, particularly if the source impedance feeding the transformers is relatively large. We built a substation that was for 20/26/33 MVA transformer. I order to keep the LG fault current at or below the 3├ś fault current we install 3.0 ohm reactor, not resistors, in the neutral which creates a Virtually Grounded System, which can be operated as a grounded system - i.e LG transformers, lightning arrestors, etc.. The reactors have to be small or the system has to treated as an ungrounded system.


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PostPosted: Thu Apr 17, 2014 11:05 am 
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Green book treats this as any one of a number of "impedance grounded" systems. All systems are "impedance grounded" to some degree. An "ungrounded" system is actually grounded via the system capacitance. In other words we have our conductive power system insulated from the Earth ground. The two conductors separated by an insulator form a large distributed capacitor. Similarly, a "solidly grounded system" is not truly "zero" impedance, just that the impedance (usually almost a pure resistor) is intentionally made as small as practical.

In practice systems that are high impedance grounded where the allowable fault current is less than 3 times the charging current tend to take on characteristics of an ungrounded system (high transients). Above that value are "high impedance" systems. Whether this is achieved with an inductor, a resistor, or a combination of a step-down transformer and a resistor is pretty much semantics. Tripping is not an absolute necessity but is preferred. Since trip timing is not critical it becomes possible to use "pulser" systems to help locate faults as well as using trip timing as a means of coordination. These systems are recommended for "medium voltage" (1 kV-15 kV) applications, especially where faults are frequent such as in mining, though even 480 or 600 V systems are not uncommon.

Somewhere along this continuum are systems that are "resonant grounded", although these are not very popular. I have never used one and have not read good things about them.

As the allowable ground fault current reaches the point where a continuous current rating on the resistor is really no longer practical (typicall 100-400 A but one located where I currently work is 2000 A rated!), they are called "low impedance" or "low resistance" grounded systems. These have most of the attributes of a high resistance system but have physically smaller resistors/inductors. The major characteristic is that tripping becomes an absolute necessity because the resistor is usually not rated for more than 10 seconds duty cycle, and that transient performance tends to be closer to solidly grounded systems.

IEEE lists about a dozen different types of grounding systems so I've only scratched the surface. "Virtual grounding" sounds like a reactance grounded low impedance system. In low resistance systems when the resistor itself tends to be little more than a coil of high nickel/chrome alloy wound onto a ceramic support drum, I'm not sure there is really much difference between a "resistance" and "reactance" grounded system. My 22.9 kV, 2000 A resistors are only 6.6 ohms (45.8 MW), while a more typical 4160 V, 25 A resistor is 96 ohms, rated for 104 kW, and a tiny 480 V, 15 A resistor is 18 ohms and 7.2 kW. At this last size it can frequently fit inside an MCC section or in a small ventilated box on a wall, compared to the 8 foot tall, 12 feed wide enclosures for the 2000 A resistors.


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