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Grounding Question No.14 - 11kV sytem neutral grounding
Author : san - From: INDIA
 
#1
Mon, May 11, 2009 - 23:46
Usage of Solidly Grounding in low voltage systems is normal practice. But in high level short circuit current case (e.g. 97 KA which mentioned); some consideration shall be applied in grounding method and other equipment selection. Close to the grounding points, the ground-fault currents are high, in some cases exceeding the three-phase short circuit currents. In a few instances higher interrupting capacity breakers may be required over that necessary for three-phase short-circuit interruption. The higher currents also produce more conductors burning. The greater currents result in lower positive-sequence voltages with a tendency toward a lower stability limit for line-to-ground faults. The higher earth currents may in some cases interfere with communication circuits.
Most unfavorable influences from the above high-current phenomena have largely been removed, so far as system extensions are concerned, by the availability of the newer high-speed relays and circuit breakers. These comments apply particularly to such items as stability, conductor burning and communication circuit influence. The interrupting requirements of circuit breakers can be brought to equality with that for the three-phase fault condition by the addition of a moderate-sized grounding reactor where necessary. When the reduction in current is no more than this, the system will still retain the classification of “effectively grounded,” although the transformers grounded through reactance will require greater neutral insulation, but will not necessarily be fully insulated.
Older industrial plants were powered by ungrounded, 3-phase, 3-wire, delta power systems. This system choice was based on two factors. First, it made the most efficient use of conductor copper. Second, no fault current flowed when the first ground-fault occurred, which was, and still is, considered an advantage in some applications, although a shock hazard is introduced.
However, multiple motor failures in numerous industrial plants were seen and were due to severe over voltages caused by arcing or resonant ground faults on the ungrounded systems. To prevent these over voltages, many power system neutrals were grounded, usually solidly. There were many factors that contributed to the change to solidly grounded systems and these factors are still important today.
First, solid grounding very effectively limits the maximum phase-to-ground voltage. Second, it allows phase-to-neutral loads to be served without encountering dangerous neutral-to-ground voltages under ground-fault conditions. Third, simple and effective ground relaying systems can be used to isolate the defective portion of the system under ground-fault conditions.
There are, however, some limitations to solid grounding. In medium voltage systems, even with good ground-fault relaying, the damage at the point of fault can be excessive. In fact, this problem led to the common use of low-resistance grounding, which allows the passing of anywhere from several hundred to several thousand amperes of ground-fault current. This practice reduces fault damage to acceptable levels while maintaining enough ground-fault current flow to effectively relay off the defective portion of the system.
In addition, solidly grounded, low voltage systems in the 480 to 600V range have two other problems. The first problem stems from application concerns. Some users prefer to maintain service, if possible, with a ground fault present on the system, or at least to arrange for an orderly, controlled shutdown. This is especially true for such continuous process industries as electric power generation, oil refining, chemical and steel manufacturing, and the paper industry. Since many of these power systems are worked hot, electricians are exposed to a considerable flash hazard from a possible line-to-ground fault caused by a misplaced tool.
Second, since most such systems rely on the phase over current devices to protect against ground faults, it's possible to have a destructive arc of several thousand amperes in magnitude for several minutes duration without initiating an automatic trip.
To overcome the problems of unwanted shutdown, flash hazard, and burn down while still maintaining the transient over voltage protection of a grounded system, high-resistance grounding was developed.
 
Author : Hamid - From: IRAN
 
#2
Mon, February 29th, 2016 - 09:23
I am designing the ground fault protection scheme for multiple 11kv generators  connected directly to 11kv bus with dedicated breakers. The proposed scheme will consist of one common low resistance NGR and switching neutrals to avoid circulating current. The 51N relay will sense the ground fault and it will trip all generators which is not desirable.
Please guide me how can I introduce selective relying so that only the faulted source can be isolated and the remaining generators will remain online. 
Author : Muhammad Waqas - From: Pakistan
 
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