POWER SYSTEM NUTRAL GRND fundamental

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Course E-3022

Power System Neutral GroundingFundamentals

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Power System Neutral Grounding Fundamentals

2009 Louie J. Powell page 1 of 20 pages

by

Louie J. Powell, PE

Saratoga Springs, NY

One of the decisions that must be made in designing a power system is how the neutral of thesystem should be connected to ground. The designer has several options from which to choose:

No intentional connection between the neutral and ground Solid (no intentional impedance) connection between neutral and ground Insertion of resistance between neutral and ground Insertion of inductive reactance between neutral and ground

And within the resistance and inductive choices, the designer has a further decision regarding the

relative magnitude of the impedance to be inserted into the circuit. Ultimately, each of these

choices affects the way that the power system performs in response to various contingencies, so

the choice between these options is not a trivial matter.

It is possible to devote an entire career to studying the implications of these choices. Recently,

three noted authors published a nearly 600 page book1

that addresses the concerns involved ingrounding decisions in great detail. This seminal book should be included in every serious library

treating power system engineering fundamentals. IEEE has published a number of standard

references on the subject that should be available to every power system engineer.23

But it is also possible to visualize the impact of the basic choices in an intuitive fashion that does

not rely on heavy use of mathematics. This course will present the basic choices as well as the

resulting system performance characteristics.

Introduction

Figure 1 illustrates a simplified power system consisting of a three-phase voltage source

connected to a set of conductors. The three-phase source consists of three Thevenin equivalentfundamental frequency sinusoidal voltage sources that are each equal in magnitude to the phase-

to-ground system voltage, and that are displaced 120 electrical degrees. The electrical system is

represented by a set of three inductive reactances, designated as jXL, that are equal in magnitude.

There may be some unbalance on practical power systems, but the impact of unbalance between

phases is beyond the scope of the present treatment.

Practical power systems also include resistance, but in most instances the inductive reactance is

about an order of magnitude larger than the resistance. So for the sake of this development,

system resistance can be ignored.

Attention is drawn to two important aspects of Figure 1. First, two terminal points have been

identified in the figure. N is a terminal point at the neutral of the power system the neutral ofthe three-phase voltage source. G is a terminal point that is connected to ground. In this context,

ground is a reference plane that is connected to earth (in Europe, the term earthing is used to

convey the same meaning as the term grounding in North America) and that is the reference

point for all voltages throughout the system. Those two terminal points will be retained as the

figure is later transformed into an equivalent circuit for analysis so that it is possible to explore

the impact of the various choices of how N and G may be interconnected.


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Figure 1 also shows that there is a capacitance, shown here as capacitive reactance, -jXC0,

between each phase conductor and ground. This is a distributed capacitance that exists by virtue

of the fact that the power system conductor is physically in parallel with earth. The negative sign

indicates that this distributed parameter is capacitive. The suffix 0 has a meaning drawn from the

study of symmetrical components. However, it is not necessary for the reader to understand

symmetrical components, and it is sufficient to simply accept that the suffix is a convenient

handle to assign to the distributed parameter that may reappear later in some other aspect ofpower system engineering analysis.

Fig 1 Typical three-phase power system with system inductive reactance (JXL) and distributed capacitance (-JXC0), and

with terminal points that can subsequently be used to explore options for connecting the system neutral ( N) to ground(G).

System inductive reactance in ohms can be calculated from the inductance of the system using

equation 1:

LfjjXL 2= (1)

where

f is system frequency

L is the inductance in the system in henries

Likewise, the distributed capacitive reactance (also in ohms) can be calculated using equation 2.

Cfj

jXC2

10 = (2)

where

f is system frequency

C is the distributed capacitance to ground in Farads

Readers will recognize that the capacitive reactance looks a bit like a load. If fact, this capacitive

reactance is a parasitic charging capacitance through which current is always flowing. The only


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reason it is normally not considered is that jXC0 is very much larger than jXL several orders of

magnitude larger so it is normally negligible.

Because jXC0 is much larger than jXL, it really doesnt matter where the distributed capacitors are

actually connected in the equivalent circuit. In fact, we can easily move the connection point

from where it is shown in figure 1 across the series inductive reactance to a set of points between

the three voltage sources and their associated reactances. Having made that change, andrecognizing that the voltage source is a Thevenin equivalent voltage and therefore impedanceless,

we can further move those shunt capacitance elements down to the neutral of the voltage source.

However, when that is done, the distributed capacitive reactances of the three phases are in

parallel, and can be replaced with an equivalent distributed capacitance value of jXC0/3 as shown

in figure 2.

.

Fig 2 Equivalent circuit with the distributed capacitance lumped at the neutral

Note that in figure 2, terminals N and G have been retained.

Finally, we can take one further step in the evolution of this equivalent circuit by observing that

figure 2 is totally symmetrical it represents three phases that are equal in magnitude and

displaced from each other by 120 electrical degrees. Therefore, for the sake of analysis, we can

ignore two of those three phases, perform our analysis on the third phase only, and then observe

that whatever we find happening on that phase will also happen on the other two phases 120o

(approximately 5.555 msec) and 240o (about 11.11 msec) later, respectively. That then leads us

to the simple, single phase equivalent circuit shown in figure 3. And once again, we observe that

the neutral, N, and ground, G, terminals have been retained.

So now the question is: considering the simplified equivalent circuit of figure 3, what is the

consequence of the following options:

1. Open circuit between N and G2. Zero-impedance connection between N and G3. Insertion of a low magnitude of resistance between N and G4. Insertion of a high magnitude of resistance between N and G5. Insertion of a low magnitude of inductive reactance between N and G6. Insertion of a high magnitude of inductive reactance between N and G


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Fig 3 Single phase equivalent circuit for a three-phase power system

In order to investigate those six options, however, we do need to make one final modification to

the equivalent circuit. The basic problem that the options present relates to what happens whenthere is a single-phase-to-ground fault on the power system, and to represent such a fault, we need

a switch. Hence, the final equivalent circuit is shown in figure 3. Closing this switch has the

effect of applying a single-line-to-ground fault on the system. Leaving the switch open is

equivalent to having the system unfaulted.

Fig 4 Single-phase equivalent circuit for investigating system neutral grounding options

Case 1: Open circuit between N and G

This case represents the system application in which there is no intentional connection between N

and G; that is, the system is nominally ungrounded. In reality, of course, the term

ungrounded is inexact because the neutral is really connected to ground through the reactance

of the distributed charging capacitance in the system. That is, an ungrounded system is actually

capacitively grounded through jXC0/3.


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Because the impedance that actually limits ground fault current is rather high, the magnitude of

current that will flow to a ground fault is often so low that automatic tripping is not required.

That is the most-cited advantage of the ungrounded system the fact that ground fault tripping is

not required means that system availability can be attractively high. This is especially appealing

in mission critical applications such refinery and power house auxiliary systems

Earlier it was noted that jXC0 is several orders of magnitude larger than jXL. Therefore, it alsomust be true that jXC0/3 is much larger than jXL. For this reason, jXL can be ignored, and this

case then becomes a matter of capacitor switching.

When the switch is open, the voltage across the switch is equal to the single-line-to-ground

voltage on the system. But that voltage is also the same as the voltage source, V1. Therefore,

when the switch is open, the voltage across the equivalent distributed capacitance, jXC0, must be

zero (Kirchoffs voltage law requires that the voltage across the capacitor plus the voltage V1

must equal the open circuit voltage.)

Closing the switch (applying the ground fault) forces the voltage across the switch to be zero.

Therefore, the voltage across the capacitor must be V1 (again, Kirchoffs voltage law). But

principle that most of us remember from fundamental physics is that it is not possible to changethe voltage across a capacitor instantaneously. Its not possible for the voltage across the

equivalent distributed capacitance to change from zero to V1 instantaneously!

So that suggests another principle that we may remember from many years ago the switching of

an energy storage element (such as a capacitance) always involves a differential equation, and the

solution to that differential equation always includes two components:

A steady-state component A transient component

And the nature of these components depends on when the switching occurs.

Recall that V1 was described as a fundamental frequency sinusoidal voltage. If the instantaneous

value of V1 at the instant the switch is closed is zero, then the instantaneous voltage across theopen switch in the instant just prior to it being closed is also zero. Because the voltage across the

switch must remain zero after it is closed, then the voltage across the distributed capacitance will

gradually increase to a value of V1 at the same time that the driving voltage in the network

increases to V1.

The steady state component of current will be the voltage divided by the magnitude of the

distributed capacitive reactance, while the transient component will be zero. Since the distributed

capacitive reactance is very large, then the steady state current (ground fault current) will be quite

small essentially equal to the nominal capacitive charging current that would flow when the

system is otherwise healthy.

But if V1 is at its crest value at the instant the switch is closed, then a transient component mustbe generated by the switching event. That is

Prior to closing the switch, the voltage across the switch is V1 and is at its crest valueo The voltage source is also V1 and is also at its cresto Therefore the voltage across the equivalent capacitance must be zero

But in the instant after closing the switch, the voltage across the switch is zeroo The voltage source continues to be V1 and continues to be at crest.o The voltage across the equivalent capacitor must continue to be zero since it

cannot change instantaneously.


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Therefore, a transient voltage of magnitude V1 (crest) must appear in the circuit in theinstant after the switch closes.

The implications of these observations are that the application of a ground fault in a power system

with the neutral ungrounded (which really means grounded through distributed capacitance) will

result in generation of transient overvoltages. The magnitude of the transient overvoltage

depends on both system parameters (obviously on capacitance, but also on inductance andresistance) and on when during the sinusoid of voltage on the AC system the fault occurs. One

might imagine that faults are more likely to occur when the AC system voltage is nearing its crest

value (because that implies the maximum stress on insulation), and unfortunately, that is exactly

the condition that results in the most severe transient overvoltage. The phenomenon of excessive

transient overvoltages due to ground faults on nominally ungrounded systems has most often

been observed in higher voltage applications.

After the fault has been on the system for a period of time the transient overvoltage will decay

away leaving a voltage across the capacitor that is equal to V1 (because the voltage across a

closed switch must be zero). If the fault then clears, the current required to charge the distributed

capacitance will be discontinued, leaving the voltage across the capacitance at the value of V1

at the instant the fault was removed. Obviously, if this happens when V1 is at or near the crest ofthe sinusoid, then the capacitance will be left charged at this steady state magnitude. Then, if the

fault is subsequently reapplied, depending again on when on the sinusoid of the source V1 the

fault reoccurs, the transient voltage that will have to appear in the circuit can be as high as 3V1.

This repeated cycle of interruptions and reignitions would normally caused by an external means

say, vibration due to the rotation of a motor. The consequence of so-called repetitive

restriking is to cause the sustained line-to-ground voltage to ratchet up to a value that is

significantly greater than V1. Repetitive restriking has most often been observed in low-voltage

systems.

There is one very special application of the ungrounded neutral that should be mentioned

marine systems. The concern in marine systems is for cathodic corrosion of the hull of vessels,and one way to manage that risk is to completely separate earth ground from the electrical

system. This creates special problems in system design and protection that are beyond the scope

of this course.

Finally, the supposed advantage of the ungrounded system that automatic tripping for single-line-to-ground faults is not required is itself a potential problem. While the magnitude of

current for a ground fault may not necessitate tripping, the fact is that during a ground fault an

abnormally high voltage will be present on the two healthy phases, and that phenomenon may in

fact accelerate the occurrence of a second fault. And with a pre-existing ground fault on the

system, a second ground fault would create a line-to-line fault.

Conclusions: Un-grounded systems Very low ground fault current

o Essentially equal to the nominal system charging currento Generally not detectable with ordinary current-based ground fault protection

technology

o May not necessitate automatic tripping for ground faults Potential for hazardous transient overvoltages Potential for hazardous sustained overvoltages due to repetitive restriking in low voltage

systems


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Ungrounded marine systems require special consideration Potential for accelerated second fault if the first fault is not automatically cleared.

Case 2: Zero-impedance connection between N and G

Applying a zero-impedance shunt between N and G has the effect of shorting out the distributed

capacitance of the system. Effectively, this is what would be accomplished by solidlygrounding the system by connecting the neutral directly to ground with no intervening

impedance. Obviously, the transient effects associated with system capacitance are no longer a

concern (although there are other sources of transient overvoltages that would have to be

addressed).

However, there is a downside if jXC0 is removed from the circuit, the only impedance left to

control the magnitude of ground fault current is jXL. That means that that the magnitude of

ground fault current in the system would dramatically increase, and it would be the high

magnitude of fault current itself that would be troublesome. Specifically:

The single-phase-to-ground-fault current magnitude would be about the same as thethree-phase fault magnitude.

It is possible for the fault current for the current available to single-line-to-ground-faultsto greatly exceed the available three-phase magnitude. This phenomenon would most

likely be seen in association with solidly grounded generators or in close electrical

proximity to banks of wye-connected transformers.

Higher fault current implies greater stress on the circuit breakers that would have tointerrupt those fault currents.

Higher fault currents implies greater burning damage at the point of a fault. This is anespecially important consideration with rotating machines. A stator ground fault in a

motor or generator on a system where the neutral is solidly grounded will produce so

much burning damage that the machine will likely be unrepairable.

Higher fault current implies greater arc-flash hazard for employees in the workplace.This subject has gotten a lot of attention in recent years. The situation where the arc flash

energy available in the system is so high that appropriate personnel protection clothing issimply not available would likely be viewed as completely unacceptable by many

employers.

Higher ground fault current implies higher earth potential gradients. While thisphenomenon is not encountered often, it can be very disruptive for ground faults to

expose workers to distracting and potentially hazardous electric shocks even when theyare not in the immediate proximity of faulted electrical apparatus.

While these adverse consequences are a consideration, there are applications where thehigher fault ground fault current associated with a solidly grounded neutral has

advantages. Specifically, higher fault currents are usually desirable on high-voltage

transmission lines to be able to provide reliable protection over the entire length of a line.

The term solidly grounded is generically descriptive and applies to the situation where theconnection between N and G does not involve any intentional impedance. However, that term

does not provide any guidance on the resulting system performance. The term effective

grounding has been defined to apply specific technical criteria to the conditions that will exist

when the neutral is solidly grounded (equations 3 and 4)4. Systems that are effectively grounded

will have known performance characteristics with respect to transient overvoltages, and whether

or not a system is effectively grounded is a criterion in the application of surge arresters. Most

solidly grounded systems are effectively grounded. But it is possible for a solidly grounded


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system involving very long transmission or distribution circuits to not be effectively grounded,

especially at the remote ends of those circuits.

Criteria for effective grounding:

31

0 X

X (3)

11

0 X

R (4)

In these expressions,

X0 is the zero sequence inductive reactance

X1 is the positive sequence inductive reactance

R0 is the zero sequence resistance

Likewise, there are a few special instances in which effectively grounded systems are not solidly

grounded. Those will be discussed later in case 5.

There is one set of circumstances that requires special consideration. The voltage V1 in figure 4

is a sinusoidal ac voltage, that is( )tVV sin1 = (5)

For example, if the three-phase system voltage is nominally 480v, then V1 will have a range of

instantaneous values between zero and the crest value of the 277v. line-to-neutral voltage, or

392v.

There is a minimum voltage required to sustain an arc. The magnitude of this minimum voltagedepends on a variety of factors whether or not the short circuit is contained in a fashion such

that ionized gas can accumulate in the vicinity of the fault, ambient temperature and humidity,

etc. Generally, however, the minimum voltage is believed to be in the range of 200 300v.

As the system line-to-neutral driving voltage varies between zero and its crest value, there will beperiods when the voltage is unable to sustain an arc, and the arc associated with a short circuit

will be extinguished. Obviously, at a subsequent point on the sinusoid of voltage the

instantaneous magnitude may again exceed the threshold voltage required to sustain an arc, but

because the voltage is only slightly greater than the minimum, the arc may not actually restrike in

each successive half-cycle of the driving voltage.

The result is a phenomenon in which arcing ground faults are intermittent. And obviously, if the

arc associated with the short circuit is intermittent, the current that flows in the circuit will also be

intermittent. This has two consequences. First, the effective heating value of the intermittent

short circuit current (also called the rms of the short circuit current) will be lower than the

magnitude of current that one would calculate from known circuit parameters. This means that

protective devices that sense rms current are relatively less effective in detecting the currentassociated with arcing ground faults.

Second, if protective devices cannot accurately measure the current associated with arcing ground

faults, they may be unable to detect those faults, thereby allowing the faults to persist for

extended periods. And given the intermittent nature of the arc in an arcing ground fault, the result

is that extensive burning damage can result from the fault.


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The classical analysis of the arcing ground fault phenomenon would suggest that arcing ground

faults will only occur on systems where the voltage is 480v or perhaps 600v. At higher system

voltages, the actual instantaneous driving voltage is greater than the minimum voltage required to

sustain an arc for a much greater portion of the voltage half-cycle, with the result that the arc is

more likely to be re-established in every half-cycle of voltage.

Conversely, at lower voltages (for example, on three-phase 208v systems), the actual phase-to-neutral driving voltage is less than the nominal voltage required to sustain an arc. That leads to

the traditional conclusion that arcing ground faults cant occur on 208v systems. The fact is that

operators of 208v secondary network systems have experience that conflicts with this theory;

there are many documented instances of intermittent arcing ground faults in 208v network vaults.

The factor that may explain this discrepancy is that network vaults are confined spaces that allow

for accumulation of ionized gas. The effect of this accumulation is to reduce the voltage required

to sustain an arc sufficiently to allow arcing ground faults to occur.

An important consideration that strongly favors solidly or effectively grounded systems is the

ability to support single-phase-to-ground connected loads. Its not unusual for loads to be

connected single-phase-to ground. In low voltage systems, this is a very practical way to serve

individual loads; 277v lighting is very commonly applied in systems rated 480v three-phase.Also, the whole point of 208v systems is that the single-phase-to-ground voltage is 120v,

constructing three-phase systems at this voltage allows large numbers of single-phase 120v loads

to be served economically.

In medium- and high-voltage systems, it is less common to see single-phase loads. However, it is

sometimes necessary to supply small aggregations of load from these higher-voltage systems, and

it is economically more appealing to install a single-phase transformer with a single bushing that

is connected phase-to-ground than it is to install a single-phase transformer connected phase-to-

phase and must therefore have two higher-voltage bushings.

When loads are connected on a single-phase basis, the concern is that it is not always possible to

exactly balance the loads across the three phases. Whatever unbalance there may be appears as asingle-phase-to-ground load. Fig 5 is a restatement of the equivalent circuit of fig 4 for the case

of unbalanced single-phase loading.


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Fig 5 Equivalent circuit for a solidly grounded system with single-phase-to-ground connected

load

The fact that there is an essentially zero-impedance connection between neutral and ground

means that the only impedances in this circuit are the system inductance, and the impedance of

the load. More importantly, because there is no significant impedance between neutral and

ground means that there is no significant voltage drop between neutral and ground resulting fromthe flow of single-phase load current. Therefore, the neutral and ground will have essentially the

same voltage.

If the system were ungrounded, the impedance XC0/3 would appear between neutral and ground.

This would have two consequences. First, because that impedance is relatively large, it would

severely limit the ability of the system to support the single-phase load. Also, to whatever degree

the system did support single-phase loading, the fact that load current would pass through this

impedance would result in a voltage drop between neutral and ground, thereby elevating the

voltages on both the system neutral and on the phase conductor and resulting in a potentially

hazardous condition.

Conclusions: Solidly or Effectively grounded systems Very large ground fault currents

o Limited only by the impedance of the systemo In special cases, may be greater than the three-phase fault current magnitudeo High ground fault current magnitude present potential people hazards

Greater arc-flash energy requiring more aggressive personnel protection Earth potential gradient concerns

o Automatic fault detection and clearing is mandatory Transient overvoltages associated with creating or clearly ground faults controlled to

within reasonable limits allows for easier application of surge protective devices

Potential for hazardous arcing ground faults in some lower voltage applications

Readily supports single-phase-to-ground connected loading

Case 3: Low resistance connection between N and G

One of the most common options for system neutral grounding in industry involves the

application of a resistor between neutral and ground. Actually, there are two options for resistive

grounding. We will first consider the case where the resistor has a relatively low ohmic rating.In this context, low means that the resistor will have an ohmic value that is much smaller than

XC0/3 and much larger than XL. For example, on a 13.8kV system XC0/3 will have a magnitude in

the range of 1000-2000 ohms, while XL will be a fraction of one ohm. A typical situation would

then be to select a resistor with an ohmic rating of 20 ohms.

Given that

LC

XRX

>>>>3

0(6)

Then, the fault current in this system will be

R

VIfault

1= (7)


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This leads to two important observations:

1. The magnitude of ground fault current will be determined almost exclusively by theohmic rating of the resistor, and typically will be limited to a value that is not very

different from load current. For example, in a 13.8kV system, a typical 20 ohm resistor

will limit ground fault current to 400 amperes.

2. The equivalent circuit, shown in fig 6, for a ground fault is therefore a predominantlyresistive circuit. That means that there are no significant switching transients associated

with either applying or clearing ground fault currents.

Fig 6 Equivalent circuit for a system with resistance grounding

The fact that low resistance grounding limits ground fault currents to moderate levels less than

load current has a number of attractive features for industrial applications. Lower ground faultcurrent means lower arc flash hazards (although it must quickly be pointed out that the arc flash

hazards associated with phase-to-phase and three-phase faults remain a very serious concern).

Also, when less fault current is injected into the ground, the risk of earth potential gradients is

significantly reduced and in fact normally becomes a non-issue. Lower fault current also

normally implies that the burning damage at the point of a fault will be significantly reduced, andthis can be a very important consideration in industrial systems with large populations of medium

voltage motors.

The fact that the circuit is primarily resistive also has advantages. Resistance adds damping to

the equivalent circuit for the fault, so much so that there are essentially no concerns for repetitiverestriking or transient overvoltages associated with ground fault conditions.

The price that has to be paid for these attractive features is that the available ground fault current

is lower than the three-phase fault current, and in fact is not too different from load current. That

raises the concern for fault detection. Fortunately, relaying schemes have been developed that

can very easily discriminate between three-phase balanced load or fault current, and single-phase-

to-ground fault current. The most common feature of these schemes is that they employ

protective relays that measure the residual of the three phase currents rather than the individual

phase currents themselves. Note that conventional fuses tend to be less effective in low-

resistance grounded systems because they respond to currents on an actual per-phase basis.


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Also, the fact that a physical resistor is connected between neutral and ground means that under

fault conditions, the system neutral will be elevated from ground potential. As a practical matter,

the neutral-to-ground voltage can be as high as the unfaulted line-to-neutral voltage magnitude.

Resistance grounded systems are clearly NOT effectively grounded systems, and the possible

elevation of neutral voltage means that care has to be taken to insulate and isolate the actual

system neutral.

Also, the elevation of neutral voltage under ground fault conditions means that the voltage on the

unfaulted phases will be elevated to the phase-to-phase voltage magnitude. This results in the

need to apply higher-rated surge voltage protection compared with solidly or effectively grounded

system.

While limiting ground fault current can result in a reduction in fault point damage, this will be the

case only if the fault can be detected and cleared automatically and immediately. In recent years,

industry has come to recognize that faults in the stator winding of generators on low-resistance

grounded systems may actually be exposed to incrementally greater burning damage because

tripping the generator for a stator ground fault does not necessarily immediately clear the fault.5

Finally, the presence of a finite value of resistance between neutral and ground essentially

eliminates the possibility of serving single-phase-to-ground loading.

Conclusions: Low-Resistance grounded systems (typically 100-600 amperes)

Ground fault currents limited to modest valueso Limited almost entirely by the ohmic rating of the resistoro Typically on the order of load currento Automatic fault detection and clearing is mandatoryo Requires relaying schemes that measure the residual of the three phase currentso Traditional fusing may not be a practical form of protection

No significant concern for transient overvoltages associated with ground faults

Mandates special considerations in applying surge overvoltage protection Does not support single-phase-to-ground connected loading

Case 4: High resistance connection between N and G

If the resistor connected between N and G presents a high magnitude of resistance, some of the

simplifying assumptions made for the low resistance grounding case no longer apply.

As the magnitude of R approaches the scalar magnitude of XC0/3, it is no longer possible to

ignore the effect of distributed capacitance. The term high resistance grounding generally

implies the situation in which equation 8 is true.

3

COXR (8)

In this situation, the total magnitude of current that will flow to a ground fault on the system will

be

R

VIg

12 (9)


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Low voltage systems tend to be physically compact primarily because voltage drop is an

impediment to establishing longer circuits. In low voltage systems, it is not uncommon for the

capacitive charging current to be on the order of 1-2 amperes, and low resistance grounding

designed in accordance with equation (8) will result in a total current that is only slightly greater

in magnitude, perhaps 2-3 amperes. With ground faults limited to this extent, it may be possible

to forgo automatic ground fault tripping in favor of allowing the fault to remain while diagnosticprocedures determine the fault location. Since most faults originate on a single-phase-to-ground

basis, the use of high-resistance grounding therefore results in an apparent improvement in

system availability.

The process of locating a fault on a high-resistance grounded system requires that a means be

provided to cause the fault current to pulsate so that it can be distinguished from the natural

distributed charging current in the system. Traditionally, this has been done by applying a

contactor that cyclically shorts a portion of the resistance. An electrician can the trace the

pulsating current using an ordinary clamp-on ammeter.

However, it must be noted that using a clamp-on ammeter to trace a pulsating fault current may

expose the electrician to energized parts and caution must be exercised to verify that theelectrician is equipped with the appropriate personal protective clothing for the arc flash level

available from the system.

Figure 6 is appropriate for the high-resistance grounded case, but in this instance, consideration

has to be given to the fact that the distributed charging capacitance is no longer negligible. The

net impedance between N and G is significantly large. Therefore, when there is a fault on the

system, there will be a significant voltage drop between N and G the steady-state value of this

voltage will be the nominal line-to-neutral voltage, V1. That means that the system neutral will

be displaced 100% in the event of a ground fault, causing the voltages on the two healthy phases

to have a magnitude above ground equal to the line-to-line system voltage.

The fact that the voltage on the healthy phases is displaced significantly during ground faults maybe a special consideration if the application does not include automatic fault detection and

clearing. The actual insulation capability of power cables is based on the presumption that

voltages will not be significantly unbalanced for extended periods, and if the application

presumes that the fault will be traced manually rather than tripped automatically, it may be

necessary to specify cable with a higher voltage rating.

When the condition of equation (8) is satisfied exactly, the circuit between N and G becomes an

RC circuit with a time constant of one electrical radian, and that fact is very significant. Earlier,

in the discussion of the ungrounded neutral (case 1), it was observed that it was possible for

voltage to gradually build up across the equivalent system charging capacitance, XC0/3, resulting

in devastatingly high phase-to-neutral voltages. With a time constant of one electrical radian, it is

not possible for this voltage escalation effect to take place. Whatever voltage is trapped on XC0/3during a fault will decay rapidly preventing the step-wise escalation of voltage described earlier

as repetitive restriking. That consideration is another of the strong advantages of high-resistance

grounding.

Historically, high-resistance grounding was originally intended for low-voltage applications. It

has also been widely used in some higher voltage applications, but there are some serious

constraints that must be observed. The key point is the objective for using high-resistance

grounding. There are basically two scenarios here:


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If the objective for high-resistance grounding is simply to limit the available groundfault current, thereby limiting burning damage and earth potential gradients, then the

system can be applied along with equipment that automatically detects faults and

initiates automatic tripping. High resistance grounding is very commonly applied at

the neutrals of generators connected to the grid through dedicated unit step-up

transformers. In these applications, even at unit voltages of 20kV and greater, the totalnatural distributed charging current rarely exceeds 5-7 amperes, and high-resistance

grounding systems are typically designed to limit ground fault current to 10 amperes.

Another common situation for high-resistance grounding with automatic tripping is in

mining applications. There are special safety rules in the mining industry that limit the

maximum earth potential gradients, and to achieve that level of personnel protection, its

common to see high-resistance grounding used to limit ground fault currents to 25

amperes.

But if the objective for high-resistance grounding is to provide a traceable ground faultso that a diagnostic procedure can replace automatic tripping, then special

consideration has to be given to the maximum magnitude of ground fault current inhigher-voltage applications. Empirical evidence suggests that if the magnitude of

ground fault current exceeds 10 amperes, the amount of burning that will take place at

the fault point will increase and eventually lead to escalation into a multi-phase fault.

The natural charging current present at system voltages greater than 5kV is often

greater than 10 A. For that reason, when high-resistance grounding is applied at system

voltages in excess of 5kV, it is usually recommended that the system be designed to

automatically detect and trip for ground faults.

In low voltage applications, the circuit shown in fig 6 applies literally. In higher voltage

applications, however, it is common to see a small transformer connected between N and G, with

a resistor across the secondary of the transformer. In this arrangement, the effective magnitude of

resistance in the circuit is the actual ohmic rating of the resistor multiplied by the square of thetransformer turns ratio. This typically results in a lower cost, more compact installation than

would be the case if the resistor were fully rated for the required resistance. The transformer

would normally have a primary voltage rating equal to (or perhaps greater than) the line-to-

neutral voltage rating of the system, and would require a thermal (or kVA) rating sufficient to

withstand the loading associated by a ground fault for time that the fault will be allowed to persist

on the system.

Conclusions: High-Resistance grounded systems

Ground fault currents limited to very low valueso Fault current limited to slightly greater than the natural system distributed

charging current

o Fundamental design criterion:3

COXR

Traceable fault current optiono Uses a contactor to cyclically short a portion of the resistor causing the fault

current to pulsate

o Fault location is possible using a hand-held clamp-on ammeter


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o Eliminates the need for automatic ground fault tripping, resulting in an apparentincrease in system availability

o Electricians MUST be equipped with appropriate personal protection gearo May require impose special considerations in specifying power cable insulationo Should not be applied in systems rated above 5kV

In other applicationso Mining, voltages greater than 5kV, and unit-connected generatorso Objective is only to limit burning damage and potential gradientso Requires automatic tripping

No significant concern for transient overvoltages if design criterion is met Sustained overvoltage on healthy phases during ground faults mandates special

considerations in applying surge overvoltage protection

Does not support single-phase-to-ground connected loadingCase 5: Low (inductive) reactance connection between N and G

Yet another possibility is to connect an inductance between neutral (N) and ground (G) as shown

in fig 7. As in the case of resistance, this comes in two flavors low inductance and high

inductance.

Fig 7 Equivalent circuit for a low-inductance grounded system

The first observation that can be made about low reactance grounding is that while the elements

between neutral and ground form a parallel L-C circuit, the inductive reactance is many orders of

magnitude smaller than the capacitive reactance, XC0/3. Therefore, the capacitance can often be

neglected.

There is resistance in this circuit. The typical X/R ratio of an inductor falls in the range of 60-

100, while the X/R ratio of typical system inductive reactances is 5 to 15 (low voltage systems

tend to have a lower X/R than do higher voltage systems). And while the resistance is significant

and must be considered in calculating the actual magnitude of short circuit current for the purpose

of evaluating the duty on circuit breakers6, it is possible to ignore the resistive component of these

impedances while examining the fundamental concepts involved in the application.


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Because the circuit has the same fundamental R-L characteristics of the basic power system,

therefore, the practice of low reactance grounding does not introduce any new concerns with

respect to switching and transients.

While the equivalent circuit of fig 1 and its subsequent descendents is literally correct, this

discussion has intentionally overlooked the question of whether these equivalent circuits contain

all of the information needed to determine the actual magnitude of ground fault current for theapplications they represent. The fact is that there are some additional considerations that enter

into the determination of ground fault magnitudes, especially if the system neutral is solidly or

effectively grounded (Case 2, fig. 5). When those factors are considered, one can conclude that

the magnitude of ground fault current can sometimes exceed the available magnitude of current

that would flow in the same system to a balanced, three-phase fault7. Those special

circumstances include:

Applications in the immediate vicinity of the solidly-grounded wye connection ofdelta-wye transformers (or three-phase banks of single-phase transformers).

Applications at the terminals of generators whose neutrals are solidly groundedThe most common situation for applying low reactance grounding is in one of these two

situations where it is desired to fine-tune the available magnitude of ground fault current to a

slightly lower value.

Consider the multiple-transformer substation situation. While it is a relatively unusual situation,

it is possible for the single-phase-to-ground fault level to exceed the interrupting ratings of circuit

breakers even though those breakers have sufficient capability to switch the available three-phase

fault level. This situation tends to come about when there are multiple delta--grounded-wye

transformers connected together at their wye-connected terminals. In such instances, one solution

is to install neutral grounding reactors (low inductance grounding) in between the neutral

terminals of the transformers and ground. Whether the design limits the available single-line-to-ground fault level to equal the three-phase fault level, or whether an arbitrary ground fault level is

merely assumed as a design objective is a decision that the system engineer must make based on

both technical and economic considerations.

In the generator instance, any time the neutral of a generator is solidly grounded, the availableground fault level will exceed the three-phase fault level. Obviously, this is not a problem for

generators that are intended for installation in systems where the neutral is solidly grounded

(especially, low voltage generators), but it can be a problem with larger (higher voltage)

generators where NEMA standards do not require that the machines be mechanically braced for

unbalanced fault stresses that are greater than the stresses associated with balanced three-phasefaults. Again, the solution is to install low-inductance grounding at the generator neutral.

Usually, the practice is to design the low-reactance grounding installation to limit the single-line-

to-ground fault to equal the three-phase fault, but it is necessary for the system engineer go also

consider other technical and economic factors before finalizing the design.

The criteria used in designing a low-reactance grounding application depend very much on theobjectives that the system is expected to support. Design is almost always a matter of

compromising between competing objectives. For example, it is possible to design a low-

reactance grounding application that will deliver effective grounding (as defined by equations

[3] and [4]. It is also possible to design a low-reactance grounding application that supports

single-phase-to-ground loading (ie, the effective low reactance inserted between N and G is

small enough that the voltage drop between N and G due to unbalanced single-phase loading is

acceptable. On the other hand, such designs may not limit the available single-line-to-ground

fault current sufficiently to address a circuit breaker rating problem.


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And commonly, another limitation is the practicality and economics of the design. The cost of

neutral grounding reactor increases as they are designed to have lower inductance, and there is a

threshold below which it is not practical to build an inductor. As a consequence, it may not be

economically practical to design a solution that requires only a very small incremental neutral

inductance.

Low-reactance grounding is one of the least common forms of system neutral grounding. The

need to address multiple competing design objectives, and the amount of work involved in

evaluating those options, are such that it is typically reserved for one of the two special cases

cited.

Conclusions: Low Reactance grounded systems

Ground fault currents limited to meet a system design criteriono Limit ground fault current to an objective magnitudeo Limit ground fault current to no greater than three-phase fault magnitude

May or may not achieve effective grounding depending on design objectives May or may not support single-phase-to-ground connected loading depending on designobjectives A relatively uncommon practice reserved for special applications Requires extensive system engineering

Case 6: High (inductive) reactance connection between N and G

As the name implies, high reactance grounding involves installing a high magnitude of inductive

reactance between neutral (N) and ground (G). Because the inductive reactance is high, it is not

possible to ignore the fact that the presence of this inductance makes the connection between N

and G a parallel L-C circuit. In fact, it is the nature of that L-C circuit that gives this option its

most attractive features.

The design criterion for a high-inductive grounding system is given in equation 10

3

COG

XjX = [10]

When this criterion is met, the fundamental frequency impedance between N and G is infinite an open circuit. As a result, in steady state, high-reactance ground acts just like the ungrounded

case, case 1. Specifically, there can be no ground fault current. In turn, that means that there is

no fault-point damage, no mechanical distress on current-carrying conductors, no thermal

distress, no earth potential gradients, and no arc flash.

But there is still an electrical connection between N and G that presents impedances at

frequencies other than the fundamental frequency. Therefore, those undesirable consequences ofan ungrounded system that are mainly related to its performance with respect to non-fundamental

frequency phenomena such as transient response and overvoltages, also do not appear.

Historically, this form of grounding, also known as resonant neutral grounding or Peterson

Coil Grounding (in honor of its inventor), was used to some degree in high-voltage transmission

applications in North America up through the middle of the 20th century. At that point, however,

its major drawback began to become an obstacle. Unlike high-resistance grounding, where it is

necessary that the resistance only approximate the distributed charging reactance, in order for


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resonant grounding to work, the design criterion for resonant grounding must be met exactly.

That means that each installation must be tuned to the distributed capacitance that it encounters

in the system where it is installed. As transmission grids started expanding in the 1940s and

50s, it became necessary to provide a means of tuning the reactor to compensate for switching

events on the system that changed the charging capacitance. Eventually, the resonant neutral

grounding applications on the transmission system in North America were retired.

However, there were transmission-level resonant neutral grounding applications in service in

other parts of the world well into the 1970s, particularly in Asia.

Another area where resonant neutral grounding was applied was at the neutrals of generators. In

theory, these applications should have been ideal because, with the generator connected to the

system through a dedicated generator step-up transformer, the only XC0/3 that would need to be

considered was the capacitance of the generator, generator leads, and the delta-connected winding

of the transformer. Practically, that means that the XC0/3 would essentially be the surge

capacitors applied directly at the terminals of the generator. And the fact that resonant grounding

would eliminate the risk of fault-point burning was a very attractive feature.

On the other hand, experience with high-resistance grounding proved that it was just as effectivein managing fault point burning, and the overall cost of the resistive approach was lower than the

cost of a resonant solution. Even so, there were a number of power plants, mainly in the New

England area, that were built with resonant grounding of their generators8.

Today, however, resonant grounding is mainly an academic topic in North America. That is not

to say that the practice has been abandoned. In fact, it is in very widespread use in medium-

voltage utility distribution applications in Europe and the UK.

Conclusions: High Reactance grounded systems

Ground fault currents limited to essentially zeroo Design criterion

3

COG

XjX =

o Design criterion must be met exactly requires retuning if switching changesdistributed capacitance of the system

No unusual concerns for transient overvoltages Non-effective grounding requires special considerations when applying surge

protection

Does not support single-phase-to-ground connected loading No known installations in North America, but in wide-spread use in Europe and UK Also known as Peterson Coil or resonant neutral grounding

Applications Summary

It is obviously impossible to declare that there is a single set of universally-correct answers.

However, it is possible to summarize what appears to be the general application trends in system

neutral grounding.


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Grounding Practice Usual ApplicationsUngrounded neutral Generally not recommended on new systems

May exist on legacy systems although a retrofit is usuallyrecommended.

Marine systems may be a special caseSolidly grounded neutral(effective grounding)

Preferred practice for high voltage transmission systems Preferred practice on medium-voltage utility distribution

systems in North America

Commonly applied on low voltage systems serving single-phase-neutral loading

Not recommended for medium-voltage distribution in industrialworkplaces

Low resistance grounding Preferred practice for medium-voltage distribution in industrialworkplaces

High-resistance grounding Preferred practice for unit-connected generator applications(with automatic tripping)

Commonly applied in continuous process industrial applications,5kV and below in conjunction with traceable fault technology May be used at higher voltages with automatic fault detection

and tripping

Cost-effective retrofit for legacy ungrounded systemsLow reactance grounding Solution for managing high ground fault currents in substations

Solution for generator grounding to support single-phase loading Requires extensive application engineering

High reactance grounding Rarely seen in North America today Commonly applied in medium voltage utility distribution

applications in Europe and UK

Other practices including

corner of the delta and mid-

point grounding

These approaches have been used as retrofit solution for legacy

ungrounded systems. In general, these are compromise solutions and

are not currently recommended practices.

Bonding and Grounding

This course has focused on the issues and practices involved in determining how the neutral of

the electrical system should be connected to earth or ground. The primary objective of that focus

is managing the magnitude of current that will be injected into ground as a result of a fault to

ground on the electrical system.

There is a closely related set of concerns associated with the design of ground grids, and the

practices of bonding metallic structures. These concerns address management of the potential

gradients that will arise as a result of that fault current being injected into ground. The primary

criteria in establishing bonding practices is to limit the voltage to which a person can be exposed,either as a consequence of touching a metallic structure (where the voltage is the difference

between the potential where the person is standing and the potential on the structure) or as a

consequence of walking through a facility (where the voltage is the difference of potential across

the length of the persons stride).

Bonding and ground grid design is critical to personnel safety and is a specialized field in and of

itself. It is mentioned here in order to emphasize that while establishment of proper system


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neutral grounding is important, it is not the complete answer and must go hand-in-hand with

competent design of ground grids and bonding practices.

1 Dunki-Jacobs, JR; Shields, FJ and St Pierre, CR; The Industrial Power System Grounding Design

Handbook, http://groundingdesignbook.com/index.html.2

IEEE Std 80-1986, IEEE Guide for Safety in AC Substation Grounding

3 IEEE Std 142-1991, IEEE Recommended Practice for Grounding of Industrial and CommercialPower Systems (The IEEE Green Book).4

Peterson, Harold A., Transients in Power Systems, John Wiley & Sons, New York, 1951; reprinted

1966, Dover Publishing, New York.5 Powell, Louie J. The impact of system grounding practices on generator fault damage, IEEETransactions on Industry Applications, vol. IA-34, Sept./Oct. 1998, pp. 923-927.6

IEEE Std 551-2006-, IEEE Recommended Practice for Calculating Short-Circuit Currents in

Industrial and Commercial Power Systems (The IEEE Violet Book)7 Students interested in expanding their understanding of these considerations are encouraged to take the

PDHengineer course on Symmetrical Components, E-40028 The reason these installations tended to be in New England was that the main proponent of resonant

grounding of generators was the legendary Prof Eric T. B. Gross, initially at Cornell and later at Rensselaer.

Many of his students spent their careers close to home in the New England area, and served as his disciples

in advocating for this solution.

POWER SYSTEM NUTRAL GRND fundamental

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