Neutral grounding also called as neutral earthing is a technique used to protect the power system from abnormal fault current and hence the stability of the power system won’t get affected. The different methods of Neutral Grounding (Neutral Earthing) techniques are discussed below.
In Solid Grounding, the neutral point is directly grounded to earth by a metallic connection or a wire which is having negligible resistance or reactance. During abnormal condition, the fault current flows through the neutral which is linked to the ground. The healthy phases won’t get affected by fault current.
The following figure shows the Solid Earthing.
Let R, Y, and B be the three phases. Consider a fault has occurred on the phase B. In Solid Grounding Technique the fault current flows through the neutral which is linked to the ground. The Healthy phases won’t get affected by the fault current.
Disadvantages of Solid Grounding:
1. The high fault current can make the system unstable.
2. The fault current can interfere with the neighboring systems.
3. The circuit breakers are difficult to high fault currents.
The highspeed circuit breakers can be used to overcome the drawbacks.
This method is only suitable for voltages below 33KV.
In Resistance Earthing, the neutral point is not directly grounded, instead, it was done by using a current limiting resistor. During abnormal condition, the fault current flows through the neutral through the current limiting resistor. The healthy phases remain healthy and don’t get affected by fault current.
The following figure shows the Resistance Earthing.
Let R, Y, and B be the three phases. Consider a fault has occurred on the phase B. In Resistance Grounding, the fault current flows the neutral via a current limiting resistor. The healthy phases won’t get affected by the fault current.
Advantages of Resistance Earthing
1. The neighboring system interference can be minimized.
2. The arcing ground hazards are minimized.
3. The system stability can be improved.
This method is suitable for the systems with voltages in the range between 2.2KV and 33KV.
In Reactance Earthing, the neutral point is grounded through a reactance. During the abnormal conditions, the fault current flows through the reactance which is connected between neutral and earth.
The Circuit representation of Reactance Earthing is shown in the figure.
Let R, Y, and B be the three phases. Consider a fault has occurred on the Phase B. The Fault current will flow through the neutral Via Reactance. The healthy phases won’t get affected by the fault current.
Drawbacks of Reactance Earthing
The increase in reactance will increase the transient. This method is only used in case of a satisfactory purpose and Less budget.
Arc Suppression Coil Grounding
This system is also referred to as Resonant Grounding. This system consists of a Peterson coil or arc suppression coil or ground fault neutralizer whose function is to self-extinguish the arcing earth fault and in the situation of sustained faults, it should reduce the earth current to a low value so that the system can supply power with one line earthed.
The system works on the principle that when inductance and capacitance are connected in a parallel configuration, resonance takes place between them and because of resonance characteristics, the fault current can be reduced or it can be neutralized.
Let R, Y, and B be the three phases. The figure shows that the fault has occurred on the phase B. The coil is tapped to select reactance according to the length of the transmission line.
The following expression is used to evaluate the reactance of the coil L = 1/ (3*ꙍ* ꙍ *C)
The circuit breaker will close and open automatically during fault current using the trip circuit.
This method of grounding technique is used in the high-end transmission line. The main aim of this grounding technique is to protect the power system from fault current which occurs due to lightning.
Voltage Transformer Earthing
In this method of earthing technique, a single-phase voltage transformer is connected to the neutral point. The system now acts as an insulated neutral system. The Voltage transformer provides a very high reactance earthing. A surge diverter is used between earth and neutral to prevent the voltage rise.
Consider a fault has occurred on the phase B. The fault current passes through the Single-Phase Transformer. A surge divider is also provided to prevent the rise of voltage.
The overvoltage which arises due to the arcing ground is eliminated.
In case if transformers or generators is delta connected or if there is no possibility for a neutral point, the neutral point can be created with the star connected earthing transformer. Those transformers don’t have secondary and each phase of primary has two equal parts. There are three limbs with each limb has two windings.
This method is suitable for the operating voltages between 2.2KV to 3.3KV. In case there is no availability of Earthing Transformer, a Star-Delta Transformer can be used.