Earthing of Power System1

8/2/2019 Earthing of Power System1

1/26


2/26

2 | P a g e

INDEX

A report on earthing of power system including-

1. Purpose of Earthing

2. Types of Earthing

3. Methods of Earthing & Types of faults

4. Transmission & Distribution line and substation Earthing.

5. Domestic House Earthing.

6. Concept of Touch and Step Potential


3/26

3 | P a g e

EARTHING OF POWER SYSTEM

Introduction:

What is earthing?

Earthing (Grounding)is a process of connecting the non-current carrying parts of

the electrical equipment (or the neutral point of the supply system) to the general

mass of earth in such a way that for all normal and adverse conditions immediate

discharge of electrical energy takes place without danger.

Purpose of Earthing: As per I.E rule- No potential points of any electrical system

must be at earth potential or zero potential. Starting from generation,

transmission and distribution the rule shall be followed for the safety of theequipment and personnels handling the equipment by making earth resistance

zero.

Objectives and conditions of Earthing (Grounding):

1. Providing a low impedance to ground for equipment protection ensuringsafety of the personnel from electric shock from non-current carrying parts

even during failure of insulation.

2. Withstand and provide path for voltage surges and surge currents due tolightning.

3. Providing corrosion allowance or corrosion to various soil chemicals toensure continuous performance and protection.

4. For providing ground connection for the system neutral.5. For providing a means of positively discharging and de-energizing feeders

before proceeding with maintenance.

Advantages of Neutral Grounding or Earthing:

1. Arcing grounds are reduced or eliminated. The arcing ground currentflowing through the neutral to ground connections is made almost equal

and opposite to the capacitive current from healthy phases to ground. So,


4/26

4 | P a g e

IR+IY+IB=0. The system is not subjected to over voltage surge due to arcing

grounds.

2. The system neutral is not shifted i.e. stable neutral point.3. The voltages of healthy phases with respect to ground remain at normal

value. They do not increase times the normal value unlike isolatedneutral system.4. The induced static charges do not cause any disturbance as they are

conducted to ground immediately.

5. Earth fault relays can be operated by utilization of the earth fault relay.6. Life of equipments, machines, installation is improved due to limitation of

voltages.

7.

Providing greater safety to personnel and equipment.

TYPES OF EARTHING:

A)Effectively Earthed System.B)Non-Effectively Earthed System.C) Isolated System or Non-Earthed System.

For the purpose of personnel, equipment and system protection earthing system

three types of earthing are:

1. Effectively Earthed System: A system is said to be effective earthed ifunder any fault condition the line to earth voltage on the healthy phase will

phase will not exceed 80% of the system line to line voltage.

The over-voltages are likely to appear on any system under fault conditions

can be calculated by the method of symmetrical components. It has been

determined that if the ratio RO/X1 is less than 1 and XO/X1 is less than 3,the

voltage from line to earth on healthy phases will not, in practice, exceed

80% of the line to line voltage. RO is the zero sequence resistance and XO is

the zero sequence reactance and X1 is the positive sequence reactance of

the system up to the point of installation of lightning arresters.


5/26

5 | P a g e

In a general system, in which all the transformers have star connected

winding with all the neutrals solidly earthed (i.e. multiple earthed system),

is regarded as effectively earthed. If only a limited number of transformers

are so earthed, the system will not necessarily be effectively earthed.

The system where the values of the short circuit current for a

system are available and the earth fault currents are found to be about 60%

(or more) of the 3-phase fault currents the system may be considered as

effectively earthed.

In a 132KV effectively earthed system for which the system highest voltage

is taken as 145KV, the voltage rating of the lightning arrester should be

145*(0.8)=116KV.

This type of earthed system is connecting the neutral point to earth without

any intentional resistance or reactance and the coefficient of earthing


6/26

6 | P a g e

Earthing system where an intentional resistance or reactance is connected

between neutral point and earth. The coefficient of earthing is >80%.

3. Isolate System or Non-Earthed System: In such systems the neutral isnot grounded and line to earth voltage of a healthy phase may exceed

100% line to line voltage. In the event of a ground fault on one phase.

However, unless there are unusual conditions (i.e. heavy charging currents

the ratio XO/X1 being negative and numerically less than 40) the voltage will

not exceed 110% of the system voltage.

For both systems (2) & (3) it is usual practice to apply

lightning arresters rated at 100% of system highest voltage and accept the

possibility of failure of lightning arrester. In 132KV non-effective earthed or

isolated neutral system with the system highest voltage 110% of the

nominal voltage, the voltage rating of the lightning arrester will be 145KV.

Here the neutral points are not earthed. The system is called Isolated

Neutral System.

Different Methods of Earthing:

The neutral grounding method of power systems can be classified as follows:a) Effective neutral grounded system:

Solidly grounded systemb) Non-effective neutral grounded system:

Ungrounded System or Isolated System. Resistance Grounding. Reactance Grounding. Resonant Grounding (Arc suppression coil or Peterson Coil).


7/26

7 | P a g e

i. Solidly earthed systemsIn a solidly earthed system a number of the transformer neutrals are directly

earthed. Figure 1 shows an earth fault in a system with a solidly earthed neutral.

Figure 1, Earth fault in a network with a solidly earthed neutral

The single-phase earth fault current in a solidly earthed system may exceed the

three phase fault current. The magnitude of the current depends on the fault

location and the fault resistance. So to limit the fault current system is used on

networks where normal impedance is quite large. One way to reduce the earth

fault current is to leave some of the transformer neutrals unearthed. The main

advantage of solidly earthed systems is low over voltages, which makes the

earthing design common at high voltage levels (HV).

Advantages of Solidly Earthed System:

i. Since fault current eliminates the effect of capacitive currents, chances ofoccurrence of arcing grounds and over-voltages are eliminated up to great

extent.

ii. Ground fault relaying is simple and satisfactory.iii. Since voltage of healthy phases does not exceed 80% of the line to line

voltage and is much less to other types of earthing. So an 84% lightning

arrester can be used instead of 105%. On system 115KV and above

additional savings are possible with transformers having less costly grade of

insulation towards the neutral.


8/26

8 | P a g e

Disadvantages:

i. The ground fault current is large. Its maximum value sometimes exceedseven the 3-phase short-circuit current.

ii. Even transient ground faults may lead to short-circuit.iii. Because of large ground fault current, the interference due toelectromagnetic induction with neighboring communication circuits may be

high.

ii. Isolated neutral systemsA system where all transformer neutrals are unearthed is called an isolated

neutral system. The only intentional connection between an unearthed neutral

and earth is via high impedance equipment for protection or measurement

purposes such as surge arresters or voltage transformers. In a power system there

are however always capacitive connections between the phases and earth. Thestrength of the capacitive connection depends on type and length of the power

system circuit. When an earth fault occurs in the system, the capacitance to earth

of the faulty phase is bypassed.

Figure 2 shows an earth fault in a system with one unearthed neutral.

Figure 2, Earth fault in a network with an unearthed neutral


9/26

9 | P a g e

Figure 3,

Figure 3 shows the Thvenin equivalent of the network with an unearthed

neutral.

In the case of a solid earth fault, the resistive connections between phase and

earth are small enough to be neglected. The earth fault current, as well as theneutral point displacement voltage, depends only on the phase to earth voltage

and capacitances.

Equation 1 gives the, therefore solely capacitive, earth fault current.

Equation 1

The maximum earth fault current of an isolated system is small providing the

systems capacitive connection to earth is weak. The presence of a fault resistancemeans a resistive part is added to the systems equivalent impedance. The

reduced fault current will therefore consist of a resistive and a capacitive part.

Equation 2 gives the earth fault current in case of a non-solid earth fault.

Equation 2


10/26

10 | P a g e

The fault current gives rise to a zero sequence voltage across the capacitances.

This voltage is called the neutral point displacement voltage. In case of a solid

earth fault this voltage equals the pre-fault phase to earth voltage of the faulty

phase. If the earth fault is non-solid, part of the phase to earth voltage will be a

across the fault resistance. Equation 3 gives the neutral point displacementvoltage.

Equation 3

Figure 4 shows the pre-fault phase voltages, the neutral point displacement

voltage and the voltage of the healthy phases during a phase-to-earth fault in an

isolated system. The voltage between the neutral point and the healthy phaseswill remain unchanged during the fault. A neutral point displacement voltage

therefore remands a change in the healthy phase to earth voltage level. The

maximum voltage of the healthy phases is 105 % of the pre-fault phase-to-phase

voltage.

Figure 4, Pre-fault voltages UA, UB, UC, neutral point displacement voltage U0

and voltage of healthy phases UB, UC during a phase-to-earth fault in an

isolated system.


11/26

11 | P a g e

In isolated neutral systems some phase-to-earth faults are cleared without

involving any relay operation. This is normally a good thing but can, in case of

intermittent faults and neutral point displacement voltage, lead to over voltages

and additional faults in the power system.

The neutral point displacement voltage and the earth fault current controls thesensitivity of the relay protection. If in an isolated system the capacitive

connection to earth is too weak, the over current relays will not be able to detect

earth faults of high enough fault resistances. The reason for the difficulties is that

the difference between the current measured during faults with high resistances

and currents due to unsymmetrical conditions at normal operation is small.

Unsymmetrical conditions at normal operation result in an unsymmetrical current

(zero sequence current) much like high fault impedance, single-phase earth fault

current. The over current relays must be programmed not to operate at this level.

Advantages of isolated systems:

Small earth fault currents, providing limited capacitive connection to earth.

Large share of the faults are self-clearing.

Disadvantages:

Strong capacitive connection to earth generates extensive earth fault currents.

Too weak capacitive connection to earth will result in difficulties detecting theearth faults.

Risk of over voltages. Because of the risk of over voltages the use of isolated

neutral is restricted to low and medium voltage.

iii. Resistance earthed systemsTo improve the earth fault detection in a power system a resistance can be

connected between a transformer neutral point and the station earthing system.

A system where at least one of the neutral points is connected to earth via a

resistor is called a resistance earthed system. The purpose of the neutral pointresistor is to increase the resistive part of the earth fault current and hence

improve the earth fault detection.

Figure 6 shows an earth fault in a system with a resistance earthed neutral.


12/26

12 | P a g e

Figure 7 shows the corresponding Thvenin equivalent.

Figure 6, Earth fault in a network with a resistance earthed neutral

Figure 7, Thvenin equivalent of a network with a resistance earthed neutral

In a system with very weak capacitive connection to earth the reactance of the

earth capacitance will be large compared to the neutral point resistance. The

neutral point resistance, instead as for the isolated systems the capacitiveconnection to earth, will therefore determine the maximum earth fault current.

Equation 4 gives the earth fault current in case of a solid earth fault.


13/26

13 | P a g e

The presence of a fault resistance reduces the earth fault current.

Equation 5 gives the earth fault current in case of a non-solid earth fault, the

phase to earth capacitance neglected.

Equation 5

As in the case of a fault in an isolated system, the fault current gives rise to a

neutral displacement voltage across the systems impedance to earth. In the case

of a resistance earthed system the impedance to earth is the neutral point

resistance in parallel to the phase to earth capacitances.

Equation 6 gives the neutral displacement voltage which in case of a solid earth

fault equals the pre-fault phase to earth voltage of the faulted phase.

Equation 6


14/26

14 | P a g e

Advantages of high resistance earthed systems:

Enables high impedance fault detection in systems with weak capacitive

connection to earth.

Some phase-to-earth faults is self-cleared.

The neutral point resistance can be chosen to limit the possible over-voltagetransients to 2.5 times the fundamental frequency maximum voltage.

Disadvantages:

Generates extensive earth fault currents when combined with strong or

moderate capacitive connection to earth.

Cost involved.

iv. Reactance Earthing systemReactance grounding means grounding through impedance which is highlyinductive.

For circuits between 3.3KV and 33KV the earth fault currents are likely to be

excessive, if solid grounding is employed. Either resistance or reactance is

connected in neutral to ground connection. There is no rule as regards which

grounding should be used- resistance or reactance.


15/26

15 | P a g e

If resistance is used fault current is limited and system reactance provides phase

opposition between capacitive ground current and fault current. The reactance

grounding provides additional reactance which provides a lagging current that

nullifies capacitive ground current.

As the value of reactance X is connected in neutral to ground connection is

increased the ground fault current decreases. If X is very small the system

behaves as an effective grounding system. If X is very large the system behaves

like an isolated system.

The transient voltage resulting from arcing ground increases as the reactance is

increased. Similarly during switching operations higher values of reactance are

expected to cause higher values of surge voltages.

Advantages:

1. The voltages across healthy phases are between 80 to 100% of the line toline voltage.

2. Arcing grounds are provided.3. Reactance grounding is very useful for grounding the neutrals of systems

where high charging currents are involved.

Disadvantages:

1. It is not applicable for low capacitive systems i.e. where groundcapacitances are weak.

Resonant earthed system

To limit the reactive part of the earth fault current in a power system a neutral

point reactor can be connected between the transformer neutral and the station

earthing system. A system in which at least one of the neutrals is connected to

earth via an inductive reactance, a Petersen coil, and the current generated by

the reactance during an earth fault approximately compensates the capacitive

component of the single phase earth fault current, is called a resonant earthedsystem. The system is hardly ever exactly tuned, i.e. the reactive current does not

exactly equal the capacitive earth fault current of the system. A system in which

the inductive current is slightly larger than the capacitive earth fault current is

over compensated. A system in which the induced earth fault current is slightly

smaller than the capacitive earth fault current is under compensated.


16/26

16 | P a g e

Figure 9 shows the earth fault current phasors of a slightly over-compensated

system.

Figure 9, Earth fault current phasors of a slightly over compensated power

system

The neutral point reactor is often combined with a neutral point resistor. In a

resonant earthed system the resulting reactive part of the earth fault current is

too small for the relay protection to measure. By using a neutral point resistance

a measurable resistive earth fault current is created as explained in the sectionabout resistance earthed systems. In addition to this, there will always be active

losses in the neutral point generator, which contributes to the active part of the

earth fault current. Typical examples of power systems with strong capacitive

connection to earth, suitable for resonant earthing, are systems consisting of an

extensive amount of cables. If the high capacitive earth fault current of such

systems is not compensated, the risk of dangerously high potential rise of

exposed parts of the power system is evident.

Figure 10 shows an earth fault in a system with a resonance earthed neutral.

Figure 10, Earth fault in a network with a resonant earthed neutral


17/26

17 | P a g e

Figure 11shows the corresponding Thvenin equivalent.

Figure 11, Thvenin equivalent of a network with a resonant earthed neutral

The earth fault current is made up of the capacitive current due to the phase toearth capacitances of the system, the inductive current generated in the neutral

point reactor, the resistive current due to losses in the reactor parallel to the

neutral point resistor. Equation 7 gives the single-phase earth fault current in case

of a solid earth fault.

Equation 7

In case of complete compensation the solid earth fault current, given by Equation

8, is solely resistive.

Equation 8

The presence of fault resistance reduces the earth fault current as given by

Equation 9.


18/26

18 | P a g e

Equation 9

In case of complete compensation the earth fault current is solely resistant as

given by Equation 10.

Equation 10

Resonance earthing makes it possible to more or less eliminate the reactive earth

fault current.

Equation 11 gives the neutral displacement voltage, the voltage across the

systems impedance to earth. The maximum, solid earth fault, neutral pointdisplacement voltage of a resonant earthed system equals the pre-fault phase to

earth voltage. In case of high fault resistance the neutral point displacement

voltage is higher than for corresponding fault resistance in an isolated system.

Equation 11

Arc Suppression Coil also known as Peterson Coil or Ground faultneutralizer:

It is an iron tapped reactor connected in neutral to ground connection. The

reactor is provided with tappings so that it can be tuned to system capacitance.


19/26

19 | P a g e

The function of the arc suppression coil is to make the arcing ground faults self

extinguishing and in case of sustained faults, to restrict the ground fault current

to a lower value so that system can be kept in operation with one line grounded.

The combination of neutral reactance L and line capacitance C acts as a parallelresonant circuit. If VP is the line to neutral voltage then,

ICR=ICY=3.VPCO ; Capacitive current is given by, IC=ICR+ICY=3.3.VPCO;

Now for balanced condition we have,

IL=IC or, (VPL) =3VPCO or, L= (132CO)


20/26

20 | P a g e

Advantages of resonant earthed systems:

Small reactive earth fault current independent of the phase to earth capacitance

of the system.

Enables high impedance fault detection.

Disadvantages: Risk of extensive active earth fault losses.

Complicated relay protection.

High costs associated.

TYPES OF FAULT:

Primarily two types of faults are prevalent in power system. They are as follows,

A. Symmetrical Faults.B. Unsymmetrical Faults.

A symmetric, symmetrical fault is a balanced fault which affects each of the

three-phases equally. In transmission line faults, roughly 5% are symmetric. This is

in contrast to an asymmetric fault, where the three phases are not affected

equally.

In practice, most faults in power systems are unbalanced. With this in mind,

symmetric faults can be viewed as somewhat of an abstraction; however, as

asymmetric faults are difficult to analyze, analysis of asymmetric faults is built up

from a thorough understanding of symmetric faults.

For symmetrical L-L-L faults it is customary to perform short circuit analysis with

the following assumptions:

i. Load currents are considered to be negligible as compared to faultcurrents.

ii. Shunt capacitances of transmission lines are neglected.iii. Shunt elements in transformers which accounts the magnetizing current

are neglected.

iv. The emfs of all the generators are assumed to be equal to 1


21/26

21 | P a g e

There are three symmetrical components in an unbalanced system:

A. Positive Sequence Network.B. Negative Sequence Network.C. Zero Sequence Network.

Generally unsymmetrical faults occurring in power system which are common andprevalent are:

1. Single Line to Ground Fault with earthed Neutral. (L-G)2. Line to Line fault. (L-L)3. Double Line to Ground Fault with Neutral earthed. (L-L-G) Single Line to Ground Fault: (L-G)

Let a 1LG fault has occurred at node kof a network. The faulted segment is then

as shown in Fig. 8.2 where it is assumed that phase-a has touched the ground

through an impedanceZf. Since the system is unloaded before the occurrence ofthe fault we have,

Also the phase-a voltage at the fault point is given by,

. -------Equation 1

This implies that the three sequence currents are in series for the 1LG fault. Let us

denote the zero, positive and negative sequence Thevenin impedance at the

faulted point as Z0, Z1, Z2 respectively. Also since the Thevenin voltage at the

faulted phase is Vfwe get three sequence circuits and accordingly equations are;


22/26

22 | P a g e

--------Equation 2, 3, 4.From the above equations we can write;

--------Equation 5

Again since, From above equation and equation 5 we get, . Where Z0, Z1, Z2 are represented asZkk1, Zkk2, Zkk3.

The Thevenin equivalent of the sequence network is shown below;

The occurrence of single line to ground fault is 70% and most prevalent

phenomenon in overhead transmission line.

Line to Line Fault: (L-L)The faulted segment for an L-L fault is shown in Fig. below, where it is assumed

that the fault has occurred at node kof the network. In this the phases b and c got

shorted through the impedance Zf . Since the system is unloaded before the

occurrence of the fault we have,


23/26

23 | P a g e

Also since phases b and c

are shorted we have,

Therefore from above two relations we have,

Hence we can summarize from above, Therefore no zero sequence current is injected into the network at bus k and

hence the zero sequence remains a dead network for an L-L fault. The positive

and negative sequence currents are negative of each other.

Now from Fig. above we get the following expression for the voltage at the

faulted point,

--Equation 1Again we have,

Equation 2


24/26

24 | P a g e

Also since we have, and and [ ] , so we can write,

, and so we get ;

--Equation 3From the above three equations we have,

Equation 4Equations 1 and 4 represents that the positive and negative sequence networks

are in parallel. The sequence network is then as shown in Fig. below.

From the network we have,

Double Line to Ground Fault: (L-L-G)The faulted segment for a 2-LG fault is shown in Fig. below where it is assumed

that the fault has occurred at node kof the network. In this, the phases b and c

got shorted through the impedanceZfto the ground. Since the system is unloaded

before the occurrence of the fault we have the same condition as for the phase-a

current in the network diagram.

Therefore, ( ) ( )


25/26

25 | P a g e

Also voltages of phases b and c are given by,

.Equation 1. Hence we have accordingly,

From the equations derived from the above matrix we have,

--Equation 2

Substituting Equations 1 & 2 in the above equation and rearranging we get,

. Also since we have ;And The Thevenin equivalent circuit is given below and we may derive the following

relations from it.

( )


26/26

Now the zero and negative sequence currents are obtained by using current

divider principle,

And,

Documents

Earthing of Power System1