1. If the neutral point of the generator is not grounded, the connection of the PT to the neutral point may reflect the single-phase short circuit fault to the generator ground. When a single-phase ground fault occurs, the neutral point voltage of the generator changes from zero to phase voltage. At this time, an operating voltage of 100V can appear on the secondary side of the PT to drive the corresponding protection action. alarm or trigger.
2. For generators whose neutral point is directly grounded, the PT is generally not connected to the neutral point because the neutral point is connected to ground and no voltage will appear on the secondary side of the PT.
Stator grounding problem
Stator grounding is a fault during generator operation. When the insulation between the stator winding and the iron core or neutral point is damaged, the grounding is damaged. the tensionis less than the specified value, the stator is considered to be grounded. When there is only one phase to ground, zero sequence voltage occurs. Protection can be formed using this voltage.
Regarding the direction of the third harmonic phasor at the machine terminal and neutral point, generally speaking, when the generator is working normally, the direction of the third harmonic voltage phasor at the terminal of the machine and at the neutral point the point is opposite. When the single-phase stator winding of the generator is grounded, the third harmonic voltage at the machine end increases, the third harmonic voltage at the neutral point decreases, and the direction of the phasor changes.
Generator neutral point and ground capacity
The generator neutral point has a grounding transformer. The function of this transformer is to connect the neutral point to the network of terre.
As for the problem of capacitance to earth, there is indeed a capacitance between the neutral point of the generator and earth. This capacity is due to the insulating medium (such as air, insulating materials, etc.) between the neutral point and earth. Although the value of this capacitor is relatively low, it can in certain circumstances have an impact on the operation of the generator.
The neutral point grounding and grounding capacitance of the primary side of the voltage transformer
The neutral point grounding of the primary side of the voltage transformer serves to ensure its normal operation. Regarding the problem of capacitance to ground, there is also a capacitance between the neutral point of the primary side of the voltage transformer and the ground. This capacity is due to the insulating medium (such as air, insulating materials, etc.) between the neutral point and earth.
The phasor relationship between the two
For the relaytion of third harmonic phasor at the machine terminal and neutral point in the stator earth protection, when the generator operates normally, the third harmonic voltage phasors at the machine terminal and neutral point have directions opposite. This phasor relationship changes when a stator ground fault occurs. The specific phasor relationship must be judged based on specific fault conditions.