The hottest neutral grounding mode and its influen

Neutral point grounding mode and its influence

Abstract: the neutral point direct grounding mode is to connect the neutral point directly to the earth. If one phase is grounded during the operation of the system, a single-phase short circuit will be formed, and its grounding current is large, which will trip the circuit breaker and remove the fault. This high current grounding system is not equipped with insulation monitoring devices

key words: neutral point grounding mode

1 neutral point direct grounding mode

neutral point direct grounding mode, that is, the neutral point is directly connected to the earth. If one phase is grounded during the operation of the system, a single-phase short circuit will be formed, and its grounding current is large, which will trip the circuit breaker and remove the fault. This high current grounding system is not equipped with insulation monitoring devices

the Internal Overvoltage generated by the neutral point direct grounding system is the lowest, and overvoltage is the basis of electrical insulation coordination. The level of electrical insulation reflects the different risk rates, and insulation coordination is an economic problem in the final analysis

the grounding current generated by the neutral point direct grounding system is large, so the interference effect on the communication system is also large. When the power line runs parallel to the communication line, the induced voltage is generated due to the coupling, which causes interference to the communication

if a single-phase grounding fault occurs in the operation of the neutral point direct grounding system, its grounding point will also produce a large step voltage and contact voltage. At this time, if the staff mistakenly board the pole or touch the live conductor, electric shock injury accidents are likely to occur. Therefore, accidents can be avoided only by strengthening safety education, correctly configuring relay protection and strict safety measures. The methods are: ① try to minimize the grounding resistance of the pole; ② Sheath the exposed part of the stay wire of the pole or the grounding down lead attached to the pole; ③ Switching operators shall strictly implement the safety work regulations of the power industry

2 neutral point ungrounded

neutral point ungrounded mode, that is, neutral point to ground insulation, simple structure, convenient operation, without any additional equipment, saving investment. It is suitable for rural 10KV overhead lines as the main radial or tree shaped power supply network under the platform of a and windows98/xp/2000 operating system. In the operation of this grounding method, if a single-phase grounding fault occurs, the current flowing through the fault point is only the capacitive current of electricity to ground, and its value is very small, which is called the small current grounding system. It is necessary to install an insulation monitoring device to detect the single-phase grounding fault in time and deal with it quickly, so as to prevent the fault from developing into a two-phase short circuit and causing a power failure

when a single-phase grounding fault occurs in a neutral ungrounded system, its grounding current is very small. If it is a transient fault, it can generally automatically extinguish the arc, and the voltage rise of the non fault phase is not large, which will not damage the symmetry of the system. Therefore, it can continuously supply power with faults for 2h, so as to obtain the troubleshooting time, and relatively improve the reliability of power supply

neutral point ungrounded mode because its neutral point is insulated, there is no release path for the energy stored in the electric capacitance to ground. When the arc light is grounded, the repeated extinction and reignition of the arc is also a process of repeated charging to the capacitor. Because the energy in the capacitance to the ground cannot be released, the voltage rises, resulting in arc grounding overvoltage or resonant overvoltage, which can reach a high multiple, posing a threat to equipment insulation

in addition, due to the existence of capacitive and inductive elements in electricity, under certain conditions, linear resonance or ferromagnetic resonance is easy to be caused due to switching operation or fault. At this time, the electricity with short feeder will excite high-frequency resonance, produce high resonance overvoltage, and lead to voltage transformer breakdown. It is easy to excite the frequency division ferromagnetic resonance for the electricity with a long feeder. During the frequency division resonance, the voltage transformer presents a small impedance, and its passing current will increase exponentially, causing fuse fusing or voltage transformer overheating and damage

3 the neutral point is grounded through the arc suppression coil

the neutral point is grounded through the arc suppression coil, that is, an inductive arc suppression coil is connected between the neutral point and the earth. When a single-phase grounding fault occurs, its grounding current is greater than 30a, and the generated arc often cannot self extinguish, resulting in an increase in the probability of arc grounding overvoltage, which is not conducive to the safe operation of electricity. Therefore, the inductance current of the arc suppression coil is used to compensate the grounding capacitance current, so that the current passing through the fault point can be reduced to the range of self arc extinguishing. By operating the no-load tap changer of arc suppression coil, it can achieve over compensation operation within a certain range, so as to reduce the grounding current. This can make the electricity run continuously for a period of time, and relatively improve the reliability of power supply

the single-phase grounding fault of this grounding mode is random due to electricity, resulting in the complex action of the single-phase grounding protection device, and it is difficult to find the fault point. The arc suppression coil adopts no-load tap changer, which is difficult to realize over compensation by manual operation and experience. The arc suppression coil itself is an inductive component, which forms a resonant circuit with the capacitance to the ground, and resonance overvoltage can occur under certain conditions. The arc suppression coil can compensate the single-phase grounding current and reduce it, which is difficult to realize relay protection

4 neutral point is grounded through resistance

neutral point is grounded through resistance, that is, the resistance with a certain resistance value is connected between the neutral point and the earth. The resistance and the system capacitance to the ground form a parallel circuit. Because the resistance is an energy consuming element, it is also a capacitive charge release element and a resonant voltage resistive element, which has certain advantages in preventing resonant overvoltage and intermittent arc grounding overvoltage. The neutral point is grounded through resistance in three ways: high resistance grounding, medium resistance grounding and low resistance grounding, which will continue to expand its business in China in the future. When the fitting clearance of these three kinds of resistance connections is 105% to 210% higher than the specified value in the product drawing, the local type has its own advantages and disadvantages, which should be selected according to the specific situation

5 conclusion

with the development of social economy and the modernization of science and technology, the degree of dependence and consumption on power is becoming higher and higher. The reliability of power supply to users is no longer guaranteed by running for 2h with single-phase grounding fault, but by electrical structure and power dispatching control

with the expansion of power scale, the single-phase grounding current also increases, which threatens the safety of equipment. Therefore, 10kV single power supply radial or tree shaped power supply must be transformed to ring dual power supply

in addition, due to the requirements of modern urban construction for city appearance, 10kV overhead lines should be transformed into cable power supply, supplemented by overhead lines, which has also become an inevitable trend. Therefore, the 10kV Electric neutral point ungrounded or grounded through arc suppression coil will change with the increase of lithium salt output of Tianqi lithium industry 26900 tons year by year and the change of electrical structure

in order to meet the needs of future power development, 10kV neutral grounding mode must be selected and determined according to power load, electrical structure, cable number, overvoltage protection, tripping mode, relay protection composition and power system stability, so as to achieve the optimization of neutral grounding mode