Method of Neutral Grounding

Method of Neutral Grounding

Q. State the method of neutral grounding. Explain the solid grounding and state advantages of grounding.

Objectives (Advantages) of Grounding :

1. Persistent arcing is eliminated.
2. Faulty part can be disconnected by protective relaying.
3. Over voltage due to arcing can be eliminated.
4. Magnitude of transient voltage is reduced.
5. System provides greater safety to the panels and equipments.
6. Maintenance and operational expenses can be reduced.
7. Improved lightening protection.
8. A better system fault protection system is obtained.

Methods 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. 

1. Solid Grounding
2. Resistance Grounding
3. Reactance Grounding
4. Arc Suppression Coil Grounding
5. Voltage Transformer Earthing

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, plate 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 90° as shown in Figure B.

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 phase B, the fault, capacitance C_Y and to R phase conductor.
3. "I_F" through B phase 'B' and fault and ground.

An analysis of fault by symmetrical components gives.3 /

 I_{F =} 3.V_ph  / Z₁ + Z₂ + Z₀ 

Figure A

Figure B

Since Z₁ + Z₂ + Z₀ is inductive, the current If lags class behind by 90° phase to neutral voltage or the fault phase V_BN. The voltages driving the current I_NR and I_NY and V_NR  and v_NY  respectively and since the impedance of the circuits transversed y these current is predominantly capacitive, they lead their respective voltages by 90° as shown by phasors I_NR, I_NY, I_CF, the resultant of  I_NR and I_NY, is  in phase opposition to I_F.

1. The phase to earth voltage of the faulty phase becomes zero. The other phase remain 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 arcing ground phenomenon can occur.

Disadvantages of Solid Neutral Grounding

1. If fault current is heavy, it causes disturbance in neighbouring communication objects . 
2. Higher capacity circuit breakers are required as there is a possibility of burning contact material due to heavy current of fault. 

Application of Solid Neutral Grounding 

Suitable used for systems upto 33 kV with total power capacity of 5000 kVA .