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Electrical Grounding System Explained

Introduction:

Power systems grounding is probably the most misunderstood element of any power systems design. Historically, the method of system grounding selected for various electrical system settings, e.g., industrial, commercial, etc., dates back to the early part of this century when only two methods were considered: solid grounded and ungrounded. Solid grounding with its advantage of high fault levels to drive protective devices had equally significant disadvantages such as dangers posed by arcs in hazardous areas. Also, the issue of service continuity of critical loads pointed away from this grounding method. The perception that ungrounded systems provide service continuity, at least through the first ground fault, strongly suggested ungrounded systems. In more recent times, however, well accepted, if not misapplied grounding techniques utilizing resistance or reactance, have provided the power systems engineer with other alternatives.

In order to establish a common perspective, some definitions and
short explanations of terms must be presented.

Ungrounded system:

A system, circuit, or apparatus without an intentional connection to ground, except through potential indicating or measuring devices or other very high-impedance devices. Note that although called ungrounded, this type of system is in reality coupled to ground through the distributed capacitance of its phase windings and conductors. In absence of a ground fault, the neutral of an ungrounded system under reasonably balanced load conditions will usually be held there by the balanced electrostatic capacitance between each phase conductor and ground.

Grounded system

A system of conductors in which at least one conductor or point (usually the middle wire or neutral point of a transformer or generator winding ) is intentionally grounded, either solidly or through an impedance

Solidly Groundeded System

Connected directly through an adequate ground connection in which no impedance has been intentionally inserted.


Solidly Grounded System

Solidly grounded systems are by far the most common found in industrial/commercial power systems today.

For line-to-neutral loads to be applied, the neutral point of the wye-connected source must be solidly grounded for the system to function properly and safely. If the system is not solidly grounded, the neutral point of the system would “float” with respect to ground as a function of load subjecting the line-to-neutral loads to voltag unbalances and instability.

To ensure that these systems are safe, the NEC requires that equipment grounding conductors (bare or green insulated) must extend from the source to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain a very low impedance to ground faults so that a relatively high fault current will flow, thus ensuring that circuit breakers or fuses will clear the fault quickly and therefore minimize damage. It also greatly reduces shock hazard risk to personnel.

The logic behind requiring systems with less than 150 V to ground to be solidly grounded, is that studies, laboratory experiments, and case histories have shown that it takes about 150 V across a gap in low-voltage systems, to sustain an arc. With less than 150 V, the arc is generally self healing and rarely continues. Solid grounding in this case provides equipment and personnel safety, permits the application of economical line-to-neutral loads, and in the case of a “solid” ground fault, ensures prompt actuation of phase protective devices—assuming the equipment grounding function (green insulated or bare connector in the same raceway) is intact. The historical incidence of sustained ground faults is so low in 120/208 V or 120/240 V systems that the NEC has not found it necessary to require separate system ground fault protection.

Resistance grouning

Grounded through an impedance, the principal element of which is resistance.


Resistance grounding

For large electrical systems where there is high investment in capital equipment or prolonged loss of service of equipment has a significant economic impact, resistance grounding has been selected. A resistor is connected from the system neutral point to ground and generally sized to permit only 200 A to 1200 A of ground fault current to flow. Enough current must flow such that protective devices can detect the faulted circuit and trip it off-line but not so much current as to create major damage at the fault point. Because the grounding impedance is in the form of resistance, any transient overvoltages are quickly damped out and the whole transient overvoltage phenomena is no longer applicable

Reactance grounding

Grounded through an impedance, the principal element of which is inductance.

Adding inductive reactance from the system neutral point to ground is an easy method of limiting the available ground fault from something near the maximum three-phase short-circuit capacity (thousands of amperes) to a relatively low value (200 to 800 A). However, experience and studies have indicated that this inductive reactance to ground resonates with the system shunt capacitance to the ground under arcing ground fault conditions and creates very high transient overvoltages on the system. The mechanism under which this occurs is very similar to that discussed under the ungrounded system characteristics. To control the transient overvoltages, studies have shown that the design must permit at least 60% of the three-phase to short-circuit current to flow underground fault conditions, for example, 6000 A grounding reactor for a system having 10,000 A three-phase short-circuit capacity available. Due to the high magnitude of ground fault current required to control transient overvoltages, inductance grounding is rarely used within the industry.

High resistance grounding

This is almost identical to low resistance grounding except that the ground fault current magnitude is typically limited to 10 A or less. High resistance grounding accomplishes two things. The first is that the ground fault current magnitude is sufficiently low enough such that no appreciable damage is done at the fault point. This means that the faulted circuit need not be tripped offline when the fault first occurs. It also means that once a fault does occur, you don’t know where the fault is located. In this respect, it performs just like an ungrounded system.

Grounding transformers

So far this paper has discussed system grounding where the neutral point of the source has been readily available. But what does one do when the neutral point is not available? As the ungrounded system problems became more apparent to industry, they recognized that it was to their advantage to ground their delta connected systems. Some took the approach of purposely grounding one phase. Although somewhat effective for the transient overvoltage criterion, it leaves the system with the continuous line-to-line overvoltage condition and the multiple fault (line-to-line) problems mentioned previously.

The best way to ground an ungrounded delta system (existing or new) is to derive a neutral point through grounding transformers. This may be accomplished in one of two ways as shown in Figure a, high resistance grounding is accomplished through three auxiliary transformers connected wye-broken delta. The resistor inserted in the “broken delta” leg is reflected the primary underground fault conditions and limits the current to a nominal value as dictated by its design. Under any system condition other than ground faults, the three secondary voltages add vectorially to zero. With zero voltage across the resistor, no current flows and the grounding resistor does not impact the system. However, underground fault conditions, one of the three voltages is shorted out and the voltage across the resistor now is no longer zero. Under these conditions, the resistor is now in the circuit and currently does flow with the effect of limiting the primary current to the design value. Also, sensing the voltage drop across the resistor (device 59G) can be used to signal an alarm advising that a ground fault has occurred. The three lights across each individual transformer will constitute a version of the normal ground detection scheme currently employed on ungrounded systems.

High resistance grounding can also be achieved alternatively by a zig-zag grounding transformer as shown in Figure b. The scheme in Figure a uses the flux in the transformer’s iron core to produce secondary voltages with their respective phase relationships as described previously. With the zig-zag transformer, the windings are connected in a zig-zag fashion such that the flux in the iron is vectorially summed opposed to vectorially summing the secondary voltages. Consequently, it behaves on the system just as the three auxiliary transformers do. It appears “transparent” to the system except under ground fault conditions. The resistor makes it resistance grounded. In both of these cases, either approach accomplishes the same end. Therefore, selection should be based on space, weight, size, and/ or economics as applied to the system in question. Although high resistance examples are shown, other variations are available for higher voltage systems.

In terms of grounding there are two types of grounding system available. They are Neutral grounding system and Equipment grounding system.

Neutral Grounding System:

The term Grounding or Earthing refers to the connecting of a conductor to earth. The neutral points of the generator and transformer are deliberately connected to the earth. In 3 phase a.c. systems the earthing is provided at each voltage level. If a neutral point is not available, a special Earthing Transformer is installed to obtain the neutral point for the purpose of earthing. Neutral points of star connected VTs and CTs are earthed.

The neutral earthing has several advantages such as :


— Freedom from persistent arcing grounds. The capacitance between the line and earth gets charged from supply voltage. During the flashover, the capacitance get discharged to the earth. The supply voltage charges it again. Such alternate charging and discharging produces repeated arcs called Arcing Grounds. The neutral grounding eliminates the problem of ‘arcing grounds’.

— The neutral grounding stabilizes the neutral point. The voltages of healthy phases with respect to neutral are stabilized by neutral earthing.

-The neutral earthing is useful in discharging over-voltages due to lightning to the earth.

— Simplfied design of earth fault protection.

— The grounded systems require relatively lower insulation levels as compareed with ungrounded systems.

The modern power systems are 3 phase ac systems with grounded neutrals

The Equipment Grounding System:

The Equipment Grounding System refers to the grounding of non-current carrying metal parts to earth. It is used for safety of personnel. It is a metal part is grounded, its voltage with respect to earth does not rise to a dangerously high value and the danger of a severe shock to personnel is avoided.

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