The neutral point of a generator is typically grounded (earthed) for safety and operational reasons. Grounding the neutral helps in stabilizing the voltage levels, providing a path for fault currents, and ensuring the safety of both equipment and personnel. There are a few methods for earthing the neutral point of a generator:
Solid Grounding (Direct Grounding): In this method, the neutral point of the generator is directly connected to the earth using a low-resistance conductor. This provides a direct path for fault currents to flow into the ground. It's a simple and cost-effective method, but it may lead to high fault currents and the risk of mechanical stresses during faults.
Resistance Grounding: In resistance grounding, a resistor is connected between the neutral point and the earth. This resistor limits the fault current magnitude, reducing stress on equipment and minimizing damage during faults. This method is particularly useful in systems where a solid grounding would result in excessive fault currents. The resistor value is typically chosen to allow enough fault current to operate protective relays but limited enough to prevent high mechanical stresses.
Reactance Grounding: Similar to resistance grounding, reactance grounding involves connecting a reactor (inductor) between the neutral point and the earth. The reactor limits fault currents like a resistor but also introduces some impedance that can help reduce transient overvoltages during faults. This method is less common than solid or resistance grounding.
Low-Impedance Grounding: This method involves connecting a low-impedance grounding transformer between the neutral point and the ground. The transformer is designed to have a very low impedance, which allows a controlled amount of fault current to flow while maintaining stability and reducing mechanical stresses.
High-Resistance Grounding: This is a variation of resistance grounding where a higher value resistor is used. The goal here is not to limit fault currents to a small value but rather to identify and locate the fault while allowing the system to remain operational to some extent. It's commonly used in systems where continuity of service is crucial and complete shutdown for small faults is undesirable.
Unearthed (Floating) Neutral: In some situations, especially in systems with high reliability requirements, the generator's neutral is deliberately left ungrounded. This can prevent ground faults from causing immediate outages. However, it requires careful coordination of protection systems to detect and clear faults promptly.
The choice of method depends on factors such as the system's fault current levels, equipment requirements, safety considerations, and operational needs. Each method has its advantages and limitations, and it's important to carefully analyze the specific situation to determine the most appropriate grounding method for a generator's neutral point.