Why Do The Neutral Points of Transformers and Generators Need to Be Reliably Grounded?

Writer： admin Time：2025-09-19 13:24:34 Browse：116℃

In a three-phase AC power system, whether and how the neutral point of core equipment such as transformers and generators (the common connection point of the three-phase windings in a star connection) is grounded directly affects the insulation level of the power grid, fault-handling capability, equipment safety, and power supply reliability. The primary purpose of reliable neutral grounding is to control the neutral potential during faults and provide a path for fault current, thereby preventing insulation damage to equipment and ensuring stable operation of the power system. The following explains this in detail from both the principles and practical examples.

I. Core Principle of Neutral Grounding: Controlling Potential and Establishing Fault Path

During normal operation of a three-phase system, if the three-phase load is balanced, the neutral point potential is theoretically at “ground potential” (voltage is 0), and the three-phase voltages are balanced (each equal to the phase voltage$U_{ph}$Uph). However, when a system fault occurs—especially a single-phase-to-ground fault, which accounts for over 80% of power system faults—or if the load is unbalanced, the neutral point potential deviates from ground potential, causing a series of problems. The essence of neutral grounding is to either forcibly fix the neutral potential or guide the fault current, addressing the following key issues:

1. Suppress Neutral Potential Shift and Prevent Overvoltage on Non-faulted Phases
   When a single-phase-to-ground fault occurs in the power grid, if the neutral point is not grounded (known as an "ungrounded neutral system"), the voltage of the faulted phase drops to 0, while the voltages of the non-faulted phases rise to the line voltage ($sqrt{3} U_{ph}$). For example, in a 10 kV distribution network, the phase voltage is approximately 5.77 kV, but during a single-phase-to-ground fault, the non-faulted phase voltage rises to 10 kV—far exceeding the insulation design level of equipment (usually designed for phase voltage)—easily causing insulation breakdown and potentially triggering more severe faults such as a “two-phase short circuit.”

2. If the neutral point is reliably grounded (e.g., directly grounded, grounded through a resistor, or via a Petersen coil), the neutral potential is fixed at ground potential. When the faulted phase voltage drops to 0, the non-faulted phase voltages remain at the phase voltage U_ph avoiding overvoltage stress on equipment insulation.

2. Provide a Fault Current Path for Fast Relay Protection
   During power system faults (such as single-phase-to-ground or winding short circuits), sufficient fault current must flow for protection devices (e.g., overcurrent or zero-sequence protection relays) to quickly detect the fault and trip the circuit, isolating the faulty equipment and preventing fault escalation. If the neutral point is ungrounded, the single-phase fault current is only the network’s capacitive current (usually just a few to tens of amperes), making it difficult for protection relays to detect the fault. The fault could persist for a long time, potentially damaging equipment or causing power system oscillations.

3. With neutral grounding, a clear fault current path is established: the fault current flows from the positive terminal of the power source, through the fault point, via the grounded neutral point, and back to the power source’s negative terminal. The current magnitude is sufficient (up to several thousand amperes) for the protection system to act quickly, isolating the fault to the smallest possible area.

4. Reduce Equipment Insulation Requirements and Save Costs
   As mentioned above, in ungrounded neutral systems, equipment insulation must be designed for the line voltage ($sqrt{3} U_{ph}$) to withstand overvoltage. In grounded neutral systems, insulation only needs to withstand the phase voltage ($U_{ph}$). For example, in a 110 kV power grid, the line voltage is approximately 190.5 kV, and the phase voltage is about 63.5 kV. Designing insulation based on phase voltage can significantly reduce the amount of insulating materials (such as transformer winding insulation paper and stator insulation of generators), lowering manufacturing and operation costs.

5. Improve Power Quality under Unbalanced Three-Phase Loads
   When three-phase loads in the power grid are unbalanced (e.g., uneven distribution of single-phase appliances in residential areas), the neutral point develops a displacement voltage, leading to unequal three-phase voltages—some phases may be overvoltage, damaging connected equipment, while others may be undervoltage, causing malfunction. Reliable neutral grounding forces the displacement voltage to zero, keeping three-phase voltages balanced and ensuring stable operation of electrical equipment.