Resistance Grounding - Working, Phasor Diagram, Advantages and Disadvantages

Working: 

In this method, the neutral is earthed through resistance R'. This method is employed when the fault current has to be limited.

Resistance is of metal or liquid column. When the resistance is made of metal then it has some inductance also. 

So the insulation of the equipment is damaged when there is lightning surges on the overhead line.

This difficulty does not arise with liquid resistance. 

The figure shows the system using this type of earthing.

Let us suppose that the fault occurs on phase B. There are three components of the current of phase B. These are I_F, I_BR and I_BY. 

Resistance R' is connected between the neutral and the ground.

So their resistance also in the path of the fault current in addition to the zero sequence impedance. As a result the fault current I_F lags less than 90° behind the faulty phase voltage. (In solid earthing this angle is 90° as there is no resistance).

Vector diagram is shown in the figure.

Current I_BR is ahead of voltage V_BR by 90° and current I_BY is ahead of voltage V_BY by 90°.  The vector sum of currents I_BR and I_BY is current I_CF.

There are two components of current I_F. One component is I_FR which is in phase with the faulty voltage V_BN . Another component is I_FL which is lagging 90° behind voltage V_BN. Value of resistance R' is so adjusted that the components I_FL and I_CF get cancelled.

As a result, the problems of arcing ground and transient oscillations are avoided. But if the value of resistance R' is more, then-current I_FL cannot cancel I_CF and the condition similar to ungrounded neutral is produced which results in transient overvoltage.

Advantages :

1. Problems arising due to the arcing ground are not produced.

2. Value of the fault current is reduced.

3. Disturbances to the nearby communication line are reduced.

4. Earth fault protection can be employed.

Disadvantages :

1. The system is costly.

2. Healthy phase voltage increases to some value however it is not so high so as to cause damage to the insulation of the equipment connected to that line.

This type of earthing method is used for the system having voltages of 2.2 kV to 33 kV and where the capacity of the power source is more than 5 MVA.

Factors affecting earth resistance :

Earthing resistance depends upon the following factors.

1. Type of soil

2. Size and material of the earth electrode

3. Depth of electrode in the ground

4. Season

5. Charcoal, salt, etc. around the electrode

6. Amount of moisture in the soil