Earthed Neutral System

Earthed Neutral System

AU : May-08,10,13,15, 16,18, Dec.-15, 16, 17

In this system, the neutral is earthed either directly or through resistance or reactance depending on the requirement. Thus the system neutral can be grounded effectively or non-effectively. In effectively grounded system, the neutral is grounded directly and hence it is called solid grounding. Following methods are adopted for non-effectively grounded systems.

i) Resistance earthing

ii) Reactance earthing

iii) Arc supression coil or resonant earthing

iv) Voltage transformer earthing

v) Earthing transformer

The advantages of neutral earthing are as follows,

i) The arcing grounds are prevented from occuring by employing suitable switchgears.

ii) As the neutral point is not shifted in this system, thus the voltages of healthy phases remains nearly constant.

iii) The static charges which are induced are grounded immediately and are thus prevented from causing any disturbance.

iv) The faulty part of the system can be isolated from the remaining system with the help of earth fault relays.

v) The magnitude of transient voltage is small in this system.

vi) The discriminative type fault indicator can be installed on such systems.

vii) This system is more reliable, provides safety to personnel and equipment with reduced operational and maintenance cost than ungrounded system.

 

1. Solid Grounding

AU : May-10, 13

In this method of earthing, neutral is directly connected to earth by a metallic connection or a wire of negligible resistance and reactance. The charging currents flow through the system under normal condition similar to ungrounded system.

Because of the connection of system neutral point to earth, it always remains at earth potential at all operating conditions and under faulty conditions voltage of healthy phase will not exceed.

The solid grounding is represented in the Fig. 8.13.1.

Whenever there is earth fault on any one phase (phase B in this case), the phase to earth voltage of faulty phase is zero while voltage to earth of the remaining two healthy phases will be normal phase voltages as neutral in this case is not shifted. The phasor diagram corresponding to this condition is shown in the Fig. 8.13.2.

Let the capacitive currents flowing in the healthy phases be I_R and I_Y. the resultant capacitive current is vector sum of I_R and I_Y. The alternator in addition to capacitive current also provides the fault current I_F. This current flows from fault point through faulty phase and then return to the alternator through earth and neutral connection. The resistance of earth fault is negligible. The magnitude of fault current after the analysis is given by,

I_F = 3 V_ph / Z₁ + Z₂ + Z₀

This current is mainly dependent on zero sequence impedance of the source of power and that of phase conductor upto fault point. As the resistive component of zero sequence impedance is normally negligible, the fault current which is large can be assumed as lagging the faulty phase voltage by 90°. From the phasor diagram it can be seen that IF and Ic are exactly opposite due to which capacitive current is neutralised by high fault current which eliminates the possibility of arcing grounds and overvoltages. The discriminative types of switchgears may be used in this method.

Following are disadvantages of this method,

i) Due to high value of fault currents, the system may become unstable and there will be greater interference to neighbouring circuits. Thus this method is employed where system impedance is sufficiently large to limit fault current.

ii) With high values of fault currents, circuit breakers are difficult to handle and heavy contacts are to be provided in the circuit breakers.

The above disadvantages can be overcome by employing high rupturing capacity and high speed circuit breakers along with fast operating relays.

This method is used in high voltage systems with voltages below 33 kV with total capacity not exceeding 5000 kVA for the economic reasons.

 

2. Resistance Earthing

In the cases where it is necessary to limit the fault current then the current limiting element must be inserted in the neutral and earth. One of the ways of achieving this is the use of resistance earthing where one or more resistances are connected between neutral and earth. The resistor may be either of wire or water column resistances for voltages of 6.6 kV and above. Metallic resistors do not change with time and requires little maintenance. But owing to its inductive nature they have disadvantage with overhead lines exposed to lightning as impulses or the travelling waves are subjected to positive reflection and cause stress on insulation resulting in its breakdown. Liquid resistors are free from these advantages and have simple and robust construction.

As shown in the Fig. 8.13.3 (a) let the earth fault occurs on phase B. The corresponding phasor diagram is shown in the Fig. 8.13.3 (b).

The capacitive currents IR and Iy flow through the healthy lines. The fault current not only depends on the zero sequence impedance of the source but also on the resistance in the earth circuit. This fault current can be resolved into two components one inphase with the faulty phase voltage and other lagging the faulty phase voltage by 90°. This lagging component of current is in phase opposition to capacitive current and it changes with change in value  of earthing resistance. Thus the value of this resistance is designed in such a way that during fault on any phase, a current equal to full load current of largest alternator or transformer flows in earth resistance which will keep the overvoltages within limits. With fault current lagging component equal to capacitive current the system operation is similar to solidly earth system and no transients occur due to arcing ground.

With high value of earthing resistance and low value of reactive current than the capacitive current then system conditions approach to that of ungrounded system with chances of transient over voltages to occur. The line to earth voltage of the healthy phases at the time of fault is little more than line to earth voltage of the solidly grounded system operating under similar conditions. The duration of this voltage can be reduced by using suitable protective switchgears to avoid any harmful effect that may be caused.

The value of resistance to be inserted in earth circuit is given by,

R = VL / √3.1

where   V_L = Line to line voltage

I = Full load current of largest alternator or transformer

The advantages of this system are as follows,

1) The discriminative type of switchgears may be used for protection.

2) The hazards due to arcing grounds are minimized.

3) The influence on neighbouring communication circuits is minimized due to lower value of fault current flowing through earth as compared to that in case of solidly grounded system.

The advantages of this system are as follows,

1) The discriminative type of switchgears may be used for protection.

2) The hazards due to arcing grounds are minimized.

3) The influence on neighbouring communication circuits is minimized due to lower value of fault current flowing through earth as compared to that in case of solidly grounded system. 

The disadvantages of this method are given below,

1) As the neutral is shifted during earth faults, the equipments are to be selected for greater voltages.

2) The system is expensive than the solidly grounded system.

3) There is energy loss in neutral grounding resistor for dissipation of fault energy.

This method is normally adopted in systems with voltages from 2.2 kV and 33 kV with a power source capacity more than 5000 kVA.

 

3. Reactance Earthing

In this system, instead of resistance, a reactance is connected between neutral and earth with ratio of reactance to resistance more than 3. The system is represented in the Fig. 8.13.4. Let the earth fault occuring on phase B.

In addition to zero sequence impedance of the source and faulty phase upto point of fault, the fault current is dependent on fault current. By changing the value of reactance the fault current can be varied. In practice this method is employed to give characteristics similar to solidly earth system.

There is drawback of this system. With increase in reactance, there is increase in transient voltages resulting from arcing. Hence it is not commonly employed though it ensures satisfactory relaying, partial grading of equipment insulation, less interference with neighbouring circuits and intermediate cost.

For reactance earthing, it is necessary that the magnitude of fault current should be at least 25 % of the three phase fault current. This is higher than the requirement on resistance earthing and thus it can be seen that the resistance earthing and the reactance earthing are not similar.

 

4. Resonant Grounding

This system is also referred as arc suppression coil grounding. In the previous earthing methods that we have discussed the earth fault on any one of the phases causes total shut down of the system. So continuity of supply can not be maintained. This is not the case with ungrounded system where fault on one phase will not cause other phases to supply power. This method of grounding has this advantage of isolated neutral system along with reduced possibility of arcing grounds and numerous other advantages.

It consists of a coil called Peterson coil or Ground fault neutralizer or arc supression coil whose function is to make arcing earth faults self extinguishing and in the case of sustained faults to reduce the earth current to low value so that system can supply power with one line earthed.

This system works on the principle that when inductance and capacitance are connected in parallel, resonance takes place between them and because of the characteristics of resonance, the fault current is reduced or can be neutralized.

The system with fault on phase B is shown in the Fig. 8.13.5 (a). The corresponding phasor diagram is shown in the Fig. 8.13.5 (b).

An arc supression coil is an iron-cored reactor or similar to oil immersed transformer connected between neutral of system and earth. This coil is provided with number of tappings so that it can be tuned with the capacitance which may vary due to varying operational conditions.

As the system operation is similar to isolated neutral system, the phase to earth voltage of healthy phase is √3 times the normal phase voltage and the resultant capacitive current is 3 times the normal charging current of one phase. The resultant capacitive current will lead by 90° with faulty phase voltage while the fault current lags by 90° with faulty phase voltage.

Now we have, I_F = I_C at resonance

There is one problem with the above method. As the operating conditions vary, the capacitance of the network also vary. This can be overcome by using a tapped coil. The appropriate tapping is required to be used for each of the change in the network conditions. The current rating of the coil is given by,

I_F = 3V_ph / X_C

The time rating of coils used in systems where earth faults are located and removed is around is around ten minutes. In other systems continuous time rated coils are used.

The arc supression coil is shown in the Fig. 8.13.6.

The coil is tapped in order to select the reactance depending upon the length of transmission line and the capacitance to be neutralized. The arc supression coil is connected between neutral and ground.

The reactance of the coil can be evaluated by using the expression L = 1 / 3 ω²C

The rating of the coil is continuous and equal to the maximum earth fault current. If a double phase to ground fault or another ground fault occurs, the current flowing through the coil is more. This can be prohibited with closing of a circuit breaker after certain time lag. The earth fault current flows through the parallel circuit by passing the arc supression coil. Here the circuit breaker is normally open and closes after the closure of relay tripping circuit by passing arc supression coil.

This method of neutral grounding is used in medium voltage overhead transmission line which are connected to system generators through intermediate power transformers. This is because the higher insulation requirement on the apparatus associated with arc supression coil grounding system is easily incorporated in power transformers than in generators. Also the overhead lines are usually subjected to earth faults due to lightning. Hence protection is required.

 

Example 8.13.1 Determine the inductance of Peterson coil to be connected between the neutral and ground to neutralize the charging current of overhead line having the line to ground capacitance of 0.15 pF. If the supply frequency is 50 Hz and the operating voltage is 132 kV, Find the kVA rating of the coil.

Solution : In case of Peterson coil we have,

 

Example 8.13.2 In a 50 Hz overhead line, the capacitance of one line to earth was 1.6 pF. It was decided to use an earth fault neutralizer. Calculate the reactance to neutralize the capacitance of i) 100 % of the length of line ii) 90 % of the length of the line iii) 95 % of the length of the line.

Solution :

Inductive reactance, X_L = ω L = (314.159) (2.22) = 698.04 Ω

 

5. Voltage Transformer Earthing

In this system of earthing, the neutral point is earthed through a single phase voltage transformer. The system thus acts as an insulated neutral system. A very high reactance earthing is provided due to the voltage transformer.

The connection diagram is shown in the Fig. 8.13.7.

The voltage transformer shown in the above figure measures the voltage so that earth fault on the system is indicated. The travelling waves passing through the machine winding are reflected through voltage transformer. A surge divertor is used between neutral and earth to avoid the rise of voltage.

The voltage transformer is used normally in generator circuits which are directly connected to step up transformers. The generator circuits are physically isolated from the main distribution system. The electrostatic capacity of the circuit is negligible as the interconnecting cables between the generator and transformer windings are normally short. The risk of overvoltage conditions arising due to arcing ground is eliminated.

 

6. Earthing Transformers

When the transformers or generators are delta connected or if the neutral points are not accessible then artificially the neutral earthing point can be created with the use of star connected earthing transformer. Such transformer has no secondary. Each phase of primary has two equal parts. There are three limbs and each limb has two windings providing opposite flux during normal condition. Such a transformer is shown in the Fig. 8.13.8.

One set of windings are connected in star providing the neutral point. The other ends of this set of windings are connected to the second set of windings as shown in the Fig. 8.13.8. The directions of the currents in the two windings on each limb are opposite to each other. The small exciting current is circulated in the windings during normal operation. Under faulty condition, the transformer offers a low impedance path to the flow of zero phase sequence currents. The value of fault current is limited in some cases by the use of a resistor in series with the neutral earthing connection. This is necessary in systems with operating voltage between 2.2 kV and 3.3 kV.

These transformers are of short time ratings in the range of 10 seconds to 1 minute Hence the size of these transformers is small as compared to power transformers of same rating. If the earthing transformer is not available then a star-delta transformer is used.

Review Questions

1. What are advantages of neutral earthing?

2. What do you mean by effectively grounded and noneffectively grounded systems?

3. What are various methods adopted for non-effectively grounded systems?

4. Explain solid grounding?

5. What are disadvantages of solid grounding?

6. Explain resistance grounding?

7. State advantages of resistance grounding.

8. What are limitations of resistance grounding ?

9. Explain reactance grounding.

10. What is the drawback of reactance grounding ?

11. Write a short note on resonant grounding.

12. A 33 kV, 3 phase, 50 Hz overhead line 50 km long has a capacitance earth line equal to 0.019 µF per km.

Determine the inductance and kVA rating of the arc supression coil.

[6.75 H, 169.3 kVA]

13. In a 50 Hz, overhead line, the capacitance of one line to earth was 1.5 µF. It was decided to use an earth fault neutralizer. Calculate the reactance to neutralize the capacitance of

i) 100 % of the length of the line ii) 90 % of the length of the line iii) 95 % of the length of the line.  

[i) 2.25 H ii) 2.5 H iii) 2.37 H]

14. Determine the value of reactance to be connected in the neutral connection to neutralize the capacitance current of a overhead line to ground capacitance of each line equal to 0.015 µF. The frequency is 50 Hz. 

[22.6 H]

15. Derive the expression for the reactance of the Peterson coil. 

16. Explain the working of arc supression coil.

17. Explain voltage transformer earthing.

18. Write short note on earthing transformer.

19. Explain the various methods of power system grounding in detail.