NEUTRAL GROUNDING                      CHAPTER - 8

INTRODUCTION

The term earthing means connecting the non-current carrying parts of the electrical equipment or the neutral point of the supply system to the general mass of the earth in such a manner that at all times an immediate discharge of electrical energy takes place without danger.

Earthing means making a connection to the general mass of earth. The use of earthing is so widespread in the electric system that at practically every point in the system, from the generators to the consumer’s equipments, earth connections are made.

Earthing is achieved by electrically connecting the respective pats in the installations to some system of electrical conductors or electrodes placed in the intimate contact with the soil some distance below the ground level. The contacting assembly is called the Earthing.

The subject of earhing is divided into:

a)    Neutral Earthing and

b)    General Earthing

a)    Neutral Eathing is also known as “System Earhing” has the following objectives:

1.    To reduce the voltage stresses due to switching and lightening surges and

2.    To control the fault currents to satisfactory values.

b)    General Earthing is also known as “Protective Earhing” and is provided for protection against the dangers to plant and personnel associated with the use of electrical energy.

Thus, the objectives of the earthing are:

1.    For the safety of personnel from electric shock insuring that non-current carrying parts, such as equipment frames are always safely at ground potential even though insulation fails.

2.    For the safety of the equipment and personnel against lightening and voltage surges providing the discharge path for surge arrestors, gaps and other, similar devices.

3.    For providing the ground connections for the grounded neutral system.

4.    For providing a means of positively discharging and de-energizing feeders or equipment before proceeding with maintenance on them.

Power system earthing is highly important. A substantial and adequate ground that will not burn-off or permit dangerous rise in voltage under abnormal conditions is essential. Extensive damage and dangerous conditions have arisen when inadequate grounds have been provided. Multiple grounds and multiple connections to them area usually desirable to ensure ground protection, even though one connection opens due to burn off or other condition such as high resistance. The station earthing system should have low resistance.

1)            Earth Electrode: Any wire, rod, pipe, plate or an array of conductors, embedded in ground horizontally or vertically is known as the Earth electrode. In distribution system, earth electrode consists of a rod or pipe 1 metre in length and driven vertically into the ground. In generating stations and substations grounding mats are used rather than individual rods.

2)            Earth Resistance or Resistance to Earth: The resistance offered by the earth electrode to the flow of current into the ground is known as resistance to earth. The term resistance to Earth or Earth Resistance does not imply the contact resistance between the earthing electrode and soil (which is insignificant) but mainly the ‘resistance of soil’ between he electrode and the point of zero potential. Numerically, it is equal to the ratio of the potential of earth electrode w.r.t. the remote point to the current dissipated by it.

Mathematically, earth resistance is given as V/I where, V is the measured voltage between the electrode and the voltage spike and I is the injected current during the earth resistance measurement through the electrode.

As in general rule, the lower the value of earth resistance better it is but even then the following values of earth resistance [Max. permissible] values will give satisfactory results.

Large power station                : 0.5 ohms

Major power station                : 1.0 ohms

Small substations                    : 2.0 ohms

In all other cases                     : 8.0 ohms

3)            Earthing Lead: The wire which connects earth wire or any other apparatus to be earthed to the earth electrode is known as the earthing lead.

4)            Earth Current: The current dissipated by the earth electrode into the ground is called the earthing current.

Soil Resistivity: Before starting the design of earthing system, it is essential to understand the nature of the resistance of ground and the various factors which govern it.

                                                           

When an electrode is driven into ground, the region around the electrode is known as “resistance area” or the “potential gradient area” and the fault current tries to flow away from the electrode in all directions as shown in figure. It follows that the rise of grounding potential or the flow of current in the ground depends upon the “resistivity of soil” in which the earthing conductor is driven. The soil conductivity is purely electrolytic phenomenon and its resistivity is governed by nature and may vary from ”1 ohm to 10,000 ohm. Some typical values are:

1.    Sea water                    :       2.5 Ω-m

2.    Tap water                    :     20 Ω-m

3.    Clay                             :     50 Ω-m

4.    Sand-clay mixture      :    100 Ω-m

5.    Sand                            :   2000 Ω-m

6.    Wet concrete              :     100 Ω-m

7.    Dry concrete               : 10,000 Ω-m

8.    Rock                           : 10,000 Ω-m

9.    Chalk                           :      250 Ω-m

It may be a matter of interest to note that water has very poor conductivity. But its presence in the soil helps in dissolving the salts thereby improving soil conductivity. Soil conductivity increases with the increase in moisture content as shown in figure. However, when moisture exceeds 20% the change in resistivity is negligible. The amount of water which a soil can absorb depends on the soil type, grain size and compactness.

                                               

The “Temperature” of he soil also plays the vital role in varying its conductivity. The effect of the temperature on the soil resistivity as investigated by the US Bureau of Standards is given in Table:

Temperature in 0C	Resistivity in Ω-m
200C	75
100C	99
00C Water in soil	138
00C Ice formation	300
-50C	790
-10C	3300

When the temperature is more than 0⁰C its effect on soil resistivity is negligible. At 0⁰C water in the soil starts freezing and resistivity increases.

The resistivity of soil is also considerably affected by the composition and amount of soluble salts as shown in figure.

                                               

Magnitude of the current also affects the resistivity of the soil as in case the value of current being dissipated by the soil is high, it may cause significant drying of soil and increase its resistivity.

The soil resistivity at a particular location also varies with depth. Generally, the lower layers of have greater moisture content and lower resistivity. However, if the lower layer contains hard and rocky soil, resistivity may increase with depth.

Since, the soil, resistivity depends on a number of factors, it is always preferable to make proper resistivity measurements at the proposed site of grounding system.

What Can Happen if the Neutral is not Grounded

If the neutral is not grounded (i.e. kept un-grounded) then:

1)    If in any of the lines a fault develops (line to earth) the voltage to earth of the remaining two phases is increases from their “phase to neutral” value (V_P) to full line voltage (V_L). Due to this, it results in burning of insulation (break-down) Natural grounding can avoid this.

2)    The capacitive currents in the  remaining healthy phase increase to √3 times the normal value. This is avoided by neutral grounding.

3)    The capacitive current in the faulty phase is 3-times the normal value which is very harmful and can be avoided by providing neutral earthing.

4)    The capacitive fault current “I_F” flows into earth which produce “Arcing Ground” which is dangerous. Due to “I_F” high frequency oscillations are produced which are super imposed on the whole system and builds up very high voltages about 5 to 6 times the normal voltage which results in insulation break-down.

For overhead lines the value of “I_F” is found as:

           kV x km                                                                                         kV x km x 25

I_F  = --------------- Amps ,             For underground cables,              I_F  = ---------------    Amps

              260                                                                                                    260

                                                                                                                

Earth Fault

To support the above discussion, let us compare ungrounded system with neutral grounded.

 

C= capacitances between lines forming delta connection, its influence is very minor and hence neglected.

C_R = C_Y = C_B for the transpose lines

So line to neutral current

I_CB = I_CR = I_CY

     The vector diagram of voltages and currents are for the balanced condition. The currents lead by 900 with the respective voltages as shown in figure. The current V_phase/X_c for a balanced currents, the net current is zero. So no current flows to ground.

Now take the case of fault developed between a phase and ground (earth fault).

The current in the faulty line has two components. I_BR and I_B as shown through fault and capacitances. The voltage responsible to force these currents is namely V_BR and V_BY. The phase difference between the current and respective voltage is 90⁰ being passing through X_c.

                                  V_BR   √3 V_Ph

Now,    current IB_R = -----  = -------

                                   X_C        X_C

                                  V_BY   √3 V_Ph

Now,    current IB_Y = -----  = -------

                                   X_C        X_C

As seen in the vector diagram he fault current ‘I_f’ is the vector sum of I_BR and I_BY.

                                                                          √3V_ph       3V_ph

Trigonometrically,   If = √3I_BR = √3 X --------- = --------

                                                              X_c          X_c

Thus he capacitive current in the faulty phase is 3 times the normal value.

Capacitive current in the healthy phases = √3times the normal value.

Advantages of Grounding

i)              Persistent arcing is eliminated.

ii)             Faulty part can be disconnected by protective relaying.

iii)            Over voltages due to arcing can be eliminated.

iv)           Magnitude of transient voltage is reduced.

v)            System provides greater safety to the panels and equipments.

vi)           Maintenance and operational expenses can be reduced.

vii)          Improved lightening protection.

viii)         A better system fault protection system is obtained.

METHOD OF NEUTRAL GROUNDING

The drawbacks discussed above of un-grounded neutral in a situation of earth fault are eliminated by grounding the neutral by various methods discussed below.

Solid Grounding :

 In this type of grounding, a direct metallic connection is made as shown in Figure, a) from the system neutral to one or more earth electrodes consisting of rods, plat or pipes, buried in the ground.

The path for fault current ‘I_F’ is mainly inductive so it lays the faulted phase voltage (V_BN) by 90⁰ as shown in Fig.

The current in “B phase” has 3 components:

1.    “I_NR” through phase B, the fault, capacitance C_R and to _R phase conductor

2.    “I_NY” through B phase, the fault, capacitance C_Y and Y phase conductor.

3.    “I_F” through phase ‘B and fault and ground.

An analysis of fault by symmetrical components gives.

           3V_ph

If = -----------------

      Z₁ +Z₂ +Z₀

Since Z₁ + Z₂ + Z₀ is inductive, the current If lags behind by 90⁰ to phase to neutral voltage or the fault phase V_BN.

The voltages driving the current I_NR and I_NY are V_NR and V_NY  respectively and since the impedance of the circuits transverse y these current is predominantly capacitive, they lead their respective voltages by 90⁰ as shown by phases I_NR, I_NY, I_CF, the resultant of I_NR and I_NY.

Advantages:

1.    The phase to earth voltage of the faulty phase becomes zero. The other phase remains at their normal phase values.

2.    The flow of heavy fault current I_F will be completely nullify the effect of the capacitive current I_CF so no arcing ground phenomenon can occur.

3.    Low voltage lightning arrestors are sufficient.

4.    There is saving is cost of equipment.

5.    It permits the use of discriminative protective gear.

Disadvantages:

1.    If fault current is heavy, it causes disturbance in neighboring communication objects.

2.    Higher capacity C.B.S. is required as there is a possibility of burning contact material due to heavy current of fault.

3.    System has to bear large number of shocks.

Application:

 Suitably used for systems up to 33kV with total power capacity of 5000 kVA.

Resistance Grounding

A resistance ‘R’ may be of metal or may be liquid resistance (used 66 kV or above). Metal resistance is easy for maintenance but being slightly inductive the effect may create breakdown of insulation due to lightening. This drawback is absent in liquid resistance. In case of fault in the line say line B the three currents in it at the fault F are I_F, I_BR and I_BY. Fault current I_F lags V_BN by certain angle (Not 90⁰ due to resistance) I_BR lead V_BR by 90⁰ so also I_BY lead V_BY by 90⁰, I_CF is the resultant of these two. I_F may be resolved into two mutually perpendicular components (Reactive and Resistive) (as shown by I_Reac and I_Res). The value of resistance R shall be such that it will limit the fault current to the full rating of the largest generator or transformer.

                       

Merits and Demerits of Resistance Grounding:

1.    Minimizes arcing grounds so hazards are minimized.

2.    It permits the use of discriminative protective gear.

3.    Cost of equipment is more than solid type.

4.    Grounding fault current is less than first method and communication circuits are not affected much.

5.    Lightning arrestors are necessary.

6.    The equipment has to be selected for higher voltages.

7.    Extra cost due to providing resistance.

Application:

For the system of 33 kV but power capacity is more than 5000 kVA, this system is suitable.

Reactance Grounding:

This system is not used. Grounding through impedance is a reactance grounding.

                                               

In this type of grounding, over voltage or transients may occur due to arcing and higher surge voltage due to lightening and switching. But it is very good for relaying. It reduces interference with communication circuits and cost is intermediate. The circuit arrangement is similar as shown in the next type.

Arc Suppression Coil Grounding (Peterson Coil Grounding)

                       

L is a iron-cored coil inserted in the earthing. The function of this coil is to make arcing earth faults self extinguishing and in case of sustained faults it reduces the earth current to a low value. The grounding is also known as resonant grounding, B phase has an earth fault.

ICF = √3I_BR = √3 V/X_C

       = √3  V_ph/X_C – 3  V_ph/X_c

       = 3 times the line to neutral charging current.

Under the fault condition the voltage of faulty phase is impressed on the arc suppression coil and a fault current I_F lags approximately by 90⁰ and it is in phase opposition to I_CF. The coil has adjustable tapping and hence adjusting suitable. It is possible to make I_CF =I_F and hence resultant current = 0. Hence, no “arc-ground”, no power current or capacitive current can flow at fault.

Advantages:

The reactive current at the fault is neutralized to some extend in the case of cable network by the capacitance current that finds a return path to the supply point through the faulty phase byway of the fault.

Drawback: Fault current required to operate protective devices is ¼ or fault current.

Application: This method is suggested for medium voltage over-head transmission lines which suffer from lightening, borage etc.

Voltage Transformer Earthing

 

In this system, a voltage transformer V_T is connected to neutral point N and grounded. If acts as a very high reactance earthing device. It serves as a voltage measuring device to indicate a fault to earth on the system. The surge diverter is also connected as shown in figure.

The application of V_T in this circuit is confined to generator equipment which is directly connected to step up power transformer. The generator circuits are isolated from the main ‘arcing ground’. The extend of the fault can be detected by measuring voltage on the secondary winding of the transformer.

The earth neutral acts as a reflection point for the travelling waves through the machine windings. It is prevented by surge diverter.

Advantages

1.    Transient voltages due to switching and arcing grounds are reduced due to the reactance of transformer.

2.    It acts as a insulated neutral system due to transformer.