What is the function of a grounding transformer?
What is the function of a grounding transformer?
In a three-phase power grid, there is capacitance between each phase conductor and between each phase and ground. When the neutral point of the grid is not solidly grounded, the phase-to-ground voltage of the singly grounded phase is zero, while the phase-to-ground voltage of the other two phases increases to √3 times. Although the increase in phase voltage does not exceed the designed insulation strength, it leads to an increase in ground capacitance. In the case of single-phase grounding, the capacitance current is three times that of the normal operating phase-to-ground capacitance current.
When the capacitance current is large, it is prone to intermittent arcs, resulting in overvoltage that affects the oscillating circuits of grid inductance and capacitance, with values reaching 2.5 to 3 times the phase voltage. The higher the grid voltage, the greater the danger of overvoltage caused by it. Therefore, only power supply systems with neutral points below 60kV can remain ungrounded because their single-phase grounding capacitance current is relatively small. Otherwise, the neutral point should be grounded through a grounding transformer using impedance grounding.
When one side of the substation main transformer (e.g., 10kV side) has a delta or star-connected winding and there is no neutral point for grounding, a grounding transformer (reactor with a zigzag winding) is needed when the single-phase capacitance current to ground is large. This creates an artificial neutral point in the grid for grounding through a de-energized coil, as shown in Figure 114-1. The purpose of the grounding transformer is to establish an artificial neutral point in the grid.
During normal operation of the power grid, the grounding transformer withstands the symmetrical voltage of the grid, with only a small excitation current flowing in a no-load state. At this time, the potential difference between the neutral point and ground is zero (ignoring the neutral point displacement voltage of the de-energized coil), and there is no current flowing through the de-energized coil.
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