Solid Grounding - Working, Phasor Diagram, Advantages and Disadvantages

 Working: 

In this method, the neutral is earthed using the pipe type or plate type earthing.

One or two earth electrodes are employed depending upon the need.

Fig. Shows the Solid  type earthing

We shall now discuss what happens when a single line to ground fault occurs. Let us suppose that the earth fault occurs on phase B. 

The potential of B becomes zero but there is no change in the potential of the healthy phases R and Y. There are three components of the current of phase B :

1. Current I_NR - which flows from phase B to fault to capacitance C_R to phase R.

2. Current I_NY - which flows from phase B to fault to capacitor C_Y  to phase Y.

3. Fault current I_F - which flows from phase B to fault to ground.

The value of the fault current depends on the zero sequence impedance from the phase conductor of the power source up to the fault point. 

Normally the resistive component of the zero-sequence impedance is negligible.

So the fault current I_F lags behind the faulty phase voltage V_BN by 90°. 

Vector diagram is shown in the figure. 

Current I_NR and I_NY flow due to the voltages V_NR and V_NY respectively. Vector V_NR is obtained by reversing the vector V_RN. Current vector I_NR is ahead of voltage vector V_NR.

Similarly, vector V_NY is obtained by reversing the vector V_YN. Current I_NY is ahead of voltage V_NY by 90°. 

The vector sum of currents I_NR and I_NY is current  I_CF which is 180°  out of phase by fault current I_F.

Advantages :

1. Potential of the faulty phase becomes zero but the potential of the healthy phases remains the same.

2. Fault current and the charging current becomes out of phase by 180° i.e. they get neutralized. This prevents or reduces the problem of arcing ground.

3. Discrimination type protection can be given as the heavy fault current flows.

4. There is no ill effect on the insulation of the equipment connected to the line as there is no increase in voltage of the healthy phases.

Disadvantages and limitations :

1. The system may become unstable as the value of the fault current becomes large

2. It becomes difficult to handle the circuit breaker as the heavy current has to be interrupted

3. The disturbances are produced on the nearby communication lines due to the heavy fault current.

Due to the above disadvantages, this type of earthing is used only where the impedance of the circuit is more. This system is employed up to 33 kV.