Wye-Zigzag Grounding Transformer
The common grounding assumes a wye-connected neutral to ground. As this is not always the case, and in some cases it is not a three-phase system that it is desired to be grounded. For situations of this kind a grounding transformer is used.
This may be a conventional wye-delta transformer of suitable rating or a special zigzag wye unit may be used. Once the neutral is established, any of the grounding methods may be employed, provided the rating of the grounding transformer is adequate for the amount of current permitted by the grounding method used. Figure 1 shows the setup of a zigzag wye transformer used for the grounding.
In the selection of grounding equipment and methods, many factors must be considered. It is desirable from the reduction of fault damage, repair costs, and switching equipment maintenance to limit ground fault current as much as possible.
However, the greater the limitation of current, the higher the possible transient overvoltages that will be encountered. This will determine the equipment insulation levels required and the rating of lightning arresters required to protect the equipment, and will consequently affect costs.
Therefore, these factors are in conflict with the desire for maximum fault limitation. Whether resistors or reactors are used will determine the degree of overvoltage expected on a given system for a given degree of current limitation and thus affects the selection of the use of resistors or reactors.
It is necessary that relays, fuses, or other protective devices sense and operate to clear the fault. Since the degree of current limitation employed may well have a serious effect on the ability of these devices to operate as desired, it follows that the degree of current limitation that can be employed may well be determined by the sensitivity of protective devices used, or, conversely, the type and sensitivity of the protective devices required may be determined by the degree of current limitation selected.
However, since a multiplicity of feeders at generator voltage depends upon ground overcurrent relays for their ground fault protection, ground fault current must be kept up to a value that will give adequate relay operating torque for any and all ground faults on them, with reasonable current transformer ratios and relay current ranges.
Thus, the selection of the value to which the ground fault is to be limited becomes the problem of making a selection between minimum ground fault current to limit damage, the minimum ground fault that will give adequate protective device operation, and the maximum ground fault current that the generator windings can tolerate before there is danger of the magnetic forces forcing windings out of the generator armature slots.
The extreme ground current limitation can be used only where there are no feeders at generator voltage that must have ground fault protection, and delta-wye transformers isolate the zero-sequence network for which ground fault protection at this very low current level must be provided to a very small number of equipment units. Even then, very special relaying methods must be employed.
2014 NEC – Systems and Equipment Grounding
Conclusion and 4 important points //
In conclusion, four important points with respect to impedance grounding of system neutrals are so obvious that they are often overlooked.
Point 1 //
Since grounding equipment is electrically active in a circuit only during a ground fault, considerable money can be saved by buying equipment rated for short time duty.
Under these circumstances a short time rating of the grounding equipment of 11s or less may be adequate. However, grounding equipment installed in a station having all overhead circuits will be subjected to the cumulative heating effect of perhaps many closely spaced feeder faults during severe storm conditions, each circuit outage being accompanied by several unsuccessful closing attempts.
Under these conditions, equipment having a rating on a 10 min or more basis may be inadequate.
Point 2 //
Impedance neutral grounding equipment must always be considered hot because if a ground fault occurs in the system, it will raise the neutral to full phase to ground voltage. This not only poses a safety problem but also creates the problem of how to maintain the equipment, unless the machine, bus, or station for which the impedance furnishes the ground is shut down.
Point 3 //
Where a multiplicity of grounding units is employed, care must be exercised in switching facilities for their transfer to avoid the danger that someone will get caught operating disconnects for the transfer just as a ground fault occurs.
If multiple units are used, care must be exercised to assure that the protective relaying will operate and coordinate properly through the range of conditions possible with the multiple units.
Point 4 //
Where impedance grounding is used, no other neutrals in the same zero-sequence system may be grounded except through the same impedance. To do so will shunt or short circuit the original impedance and raise the ground fault current above the desired design value.
Reference // El. power equipment maintenance and testing by P. Gill (Purchase hardcover from Amazon)