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The generator is one of the most important equipment in the power system. The generator stator winding is single-phase grounded, which is the most common electrical fault of the generator. Generator stator grounding refers to the generator stator winding circuit and the stator winding circuit connected directly to the primary system of single-phase ground short circuit.

 
Fault Analysis of Generator: Selection of Neutral Grounding Mode
After the generator stator is grounded, the ground current is connected through the fault point, the three-phase capacitance and the three-phase stator winding. When the ground current is large, can cause the arc in the fault point, resulting in stator windings and stator core burns, and even expanded to phase or inter-turn short circuit.
Therefore, in order to ensure the safety of the generator, the generator stator ground, the ground current must be limited to a certain range, so that the failure point does not produce arc or arc instantly extinguished, to avoid single-phase ground fault development into inter-phase or inter-turn short circuit, burn Bad stator core and winding insulation.

1 generator single-phase grounding hazards and the use of different neutral grounding purposes
As the generator and generator connected to the device and the device there are different sizes of ground capacitance, when the generator sets occur when the single-phase ground and other asymmetrical fault, the ground flow through the fault current that the ground capacitance current. The current is generally a few or several security. In the event of a fault, an arc overvoltage will occur at the fault, which will damage the stator of the generator, resulting in inter-turn or phase-to-phase short circuit, which will expand the accident range and will severely burn the stator core. Once the burned iron core, due to large-scale generator stator core structure is complex, difficult to repair, so downtime longer. If the stator winding insulation damage and single-phase ground fault is inevitable, which will bring disaster to the stator core should be avoided, this should try to reduce the stator winding single-phase ground current, while shortening the duration of the fault.
When the generator external components occur single-phase ground fault and other asymmetric failure, with the generator internal failure, will cause damage to the generator and other equipment.
And the neutral point of the grounding, a direct impact on the single-phase ground arc generation and limit the intensity. Generator neutral point to take a different grounding, the main purpose is to prevent the generator and other equipment from damage, specifically the following aspects:
①. Limit the fault when the stator a little ground current, to prevent the generation of arc burn iron;
②. Restrict the fault when the steady state and transient over voltage, to prevent damage to the equipment insulation;
Provide ground protection, accurate and sensitive signal or selectively disconnect the fault generator.

2 generator neutral point grounding
The grounding point of the generator is related to the size of the single-phase ground fault current, the over-voltage of the stator winding, the realization of the stator grounding protection.
With the rapid development of China's power system, the early adoption of the neutral point of the ground can not meet the needs. Therefore, there have been a variety of neutral point grounding, where we come to introduce the most widely used under several major grounding.
2.1 generator neutral point is not grounded
General units below 10MW, to meet the generator voltage circuit on the capacitor current requirements, the use of neutral point is not grounded. This method is the most simple, can reduce the neutral point equipment, stator ground current is small, can run with a fault for a period of time, it can greatly improve the reliability of power supply, but can not limit the arc over voltage. In this way, the ground fault indication signal can be given by the three-phase five-column voltage transformer opening triangular coil zero sequence voltage, can also use three single-phase voltage transformer to provide zero sequence voltage.
2.2 generator neutral point by grounding high resistance grounding
This is the current application of a more extensive way of grounding, grounding transformer primary side of the neutral point and the ground between the secondary side connected to a small resistance, the transformer by the high change ratio conversion, reflected to the high side A resistance to enlarge the resistance, which constitutes a high resistance grounding, while the cost of resistance is greatly reduced. At the same time grounding will change the neutral point of some of the voltage signals such as zero sequence voltage, the third harmonic by step-down into low-voltage signal, provided to the generator stator ground protection device, where the grounding transformer has played the role of PT. This grounding method will lead to an increase in single-phase ground current, in order to ensure the safety of the generator, it is necessary to make the stator ground protection action trip.
2.3 generator neutral point by arc suppression coil grounding
When the generator capacitor current is large, the general use of the neutral point by the arc suppression coil grounding, which mainly consider the ground current to a certain extent, the grounding point of the arc can not be automatically extinguished, and ground current if burned stator core difficult to repair. The arc suppression coil is an adjustable inductor with an iron core. It is connected to the generator (or transformer) between the neutral point and the earth, constitute the arc suppression coil grounding system. The role of the arc suppression coil is to provide an inductor current to compensate for the neutral current in the system when the single-phase ground capacitor current, so that the operating system single-phase ground fault current can be controlled in the generator to allow single-phase ground fault current , To avoid the reignition of the arc caused by the gap grounding over-voltage, and ultimately the ground point of the arc automatically extinguished to prevent the large ground current burned stator coil and core. So that the generator can be single-phase ground fault current running for some time to improve system reliability, but also to facilitate maintenance personnel troubleshooting.

3 generator neutral point grounding mode of the comprehensive choice
In China, large and medium-sized generator neutral ground grounding methods are mainly grounding and arc suppression coil grounding, because of its characteristics and characteristics of different, so the choice of neutral point grounding should be specific to the specific analysis, consider the following factors Comprehensive selection.
3.1 Ground fault current problem
Grounding high resistance to ground fault current is relatively large, is generally considered to be grounded capacitor current times, generally larger than the arc suppression coil ground fault current. The grounding device is to limit the ground fault current, so it is not as good as the arc suppression coil ground, but the fault current increases, the protection sensitivity and reliability has been improved to prevent the generator stator ground fault expansion of the role and The arc suppression coil is the same.
Arc suppression coil ground fault current is small, can often continue to run for some time. But the voltage rise to speed up the aging of the insulation, if not found in a short time and troubleshooting points, may cause the expansion of the failure.
3.2 Overvoltage problems
For over-voltage, domestic and foreign experts also have different understanding. From the results of their experimental study, the arc suppression coil grounding than the ground to become high resistance to some of the lower, in fact, China's generators arc suppression coil grounding are used under compensation, there is no resonant ground for the situation, it can not be asserted What is better?
3.3 Generator stator ground protection problem
3.3.1 Ground operation mode
Reference to China's generator single-phase ground current allowable value of the provisions of the ground current exceeds the allowable current when the trip, or cast signal. From this point of view, arc suppression coil grounding to extend the generator with the fault running time, and grounding high resistance to ground fault current is large, the instantaneous trip, the generator favorable, but the steam turbine start and stop once a greater impact on the system The China's general unit under 120MW only investment signal, 200MW unit only to vote trip.
3.3.2 Generator stator 100% protection
The 100% ground protection of the stator is usually a dual-band earth fault consisting of a fundamental zero-sequence voltage and a third harmonic voltage. For a grounded high-impedance ground, the more common view is that it will reduce the sensitivity of the third harmonic protection The arc suppression coil grounding method helps to improve the sensitivity of the fundamental zero sequence voltage ground protection.
In summary, the generator neutral grounding method is closely related to the structure of the stator grounding protection, but also with the stator single-phase ground fault current size, the stator winding over-voltage and other issues related to the choice of generator neutral grounding Must be based on the operating experience at home and abroad, taking into account the fault current, over voltage and grounding protection and other factors, choose the best way to neutral point.

in conclusion
1 due to the presence of the generator to ground capacitance, the ground current after the fault is too large. With the increase in generator capacity, the higher the voltage level, the greater the risk of failure, the generator neutral point must be considered ground.
2 grounding mode due to the generator capacity and voltage levels are different from a variety of grounding, large and medium-sized units in the country are mainly arc suppression coil and grounding high resistance to ground two ways.
3 generator neutral ground selection should refer to the experience of domestic and foreign units have been built, taking into account the fault current, over voltage and grounding protection and other factors.