Neutral grounding and overvoltage
A neutral point directly grounded
Neutral grounding, that is, the neutral point of direct access to the earth. When the system is running in the event of a phase to form a single phase short circuit, the ground current to trip the circuit breaker removal failure. This current grounding system, the installation of insulation monitoring device.
Neutral grounding system generated within the overvoltage minimum, while the over-voltage grid insulation coordination, the insulation level of the grid selected, reflecting the risk-free rate, insulation coordination in the final analysis is an economic problem.
Ground current generated by the neutral grounding system, so the impact of interference on communication systems. When the power lines and communication lines parallel to the direction, the induced voltage due to coupling, cause radio interference.
Neutral grounding system in the operation of single-phase ground fault, the ground will produce a step voltage and touch voltage. At this point, staff mistakenly Teng pole or mistakenly touch the live conductor, prone to electric shock injuries. This only strengthen safety education and properly configured protection and strict safety measures, accidents can be avoided. The approach is to: (1) as far as possible, pole ground resistance to a minimum; ② The exposed part of the poles of the cable or attached to the pole grounding deflectors plus jacket; ③ switching operation should strictly enforce the electrical safe working procedures.
Two of the neutral point ungrounded
Neutral point grounding mode, that is, the neutral point-to-ground insulation, simple structure, convenient to run without any additional equipment, low investment. Applicable based on the 10kV overhead line in rural areas of radiation-shaped or tree-shaped power supply network. The ground running in the event of a single phase to ground fault, the fault point current is flowing through the grid-to-ground capacitance current only, its value is very small as a small current grounding system required the installation of insulation monitoring device in order to detect single phase to ground fault, be dealt with promptly, in order to avoid failure is the development of two-phase short circuit, caused by the blackout.
Neutral point grounding system of single phase grounding fault, the ground current is very small, if the instantaneous failure generally automatically extinction, non-fault phase voltage rise is not and will not destroy the symmetry of the system, it can be with fault continuous supply 2h, troubleshooting time and improve reliability of power supply.
Isolated neutral due to its neutral point is insulated, the grid on the capacitor stored energy release pathway. Arc grounding, arc repeatedly extinguished and rekindled, but also repeatedly to the capacitor charging process. Not release energy in the capacitance, the voltage increases, resulting in arc grounding voltage or resonance over-voltage, up to a high multiple of its value, pose a threat to the equipment insulation.
In addition, because the grid there are capacitive and inductive components, under certain conditions, due to the switching operation or failure is apt to cause linear resonance or ferroresonance, then the shorter grid feeder excited high-frequency resonance, resulting in a higher resonant voltage, resulting in the breakdown voltage transformer. But easy to arouse the grid on the feeder longer sub-frequency ferroresonance, was smaller impedance in the resonance frequency, voltage transformer, multiplied by the current, caused by a blown fuse or voltage transformer overheating and damage.
3 neutral point by the arc suppression coil grounding
The neutral point of the arc suppression coil grounding, that is, access to an inductance between the neutral point and earth arc suppression coil. When the grid single-phase ground fault occurs, the ground current greater than 30A, produced by arc often not self-extinguishing, caused by arc grounding overvoltage increase the probability is not conducive to safe operation of power grids. To this end, the inductor current of the arc suppression coil current to compensate for the capacitance to ground, so that the current through the point of failure is reduced to self-extinction range. OLTC operation of the arc suppression coil, so that within a certain range to compensate for running, so as to achieve to reduce the ground current. This allows the grid continued to run for some time, and improve supply reliability.
The grounded grid single-phase ground fault occurs, causing the ground fault protection device movement situation is complex, and to find more difficult to find the point of failure. The arc suppression coil with no-load tap more difficult to achieve compensation by artificial rule of thumb operation. Arc suppression coil is inductive components, with the capacitor form a resonant circuit, the resonance over-voltage, under certain conditions can occur. Arc suppression coil can make of single-phase current to be compensated for smaller, more difficult to achieve protection.
4 neutral point resistance grounding
Neutral point grounding mode by resistor, that is, the resistance of the neutral point between the earth and access to a certain resistance value. The resistance and system capacitance to constitute a parallel circuit, resistance is energy-consuming components, is also a capacitive charge release components and resonant resistance pressure components to prevent the resonance over-voltage and intermittent arc ground over-voltage, a certain superiority. Neutral point at the high resistance grounding resistor grounded in resistance grounded, low-resistance grounding of three ways. Resistance grounding of the three have their own advantages and disadvantages, to be selected depending on the circumstances.
Socio-economic development and technological modernization of the power dependence and consumption levels are getting higher and higher reliability of the user-powered, no longer rely on with a single-phase ground fault running 2h to ensure, but by the grid structure and power systems control assurance.
With the expansion of the grid size, single-phase ground current also increases the threat to the safety of the equipment. To this end, 10kV Single supply-shaped or tree-shaped power supply dual power supply to the ring transformation.
In addition, due to the amenity requirements of the modern urban construction, 10kV overhead lines should be converted into the main power cables, overhead lines, supplemented, which has also become an inevitable trend. 10kV network neutral point grounding or by arc suppression coil grounding will increase every year with the electricity load changes with changes in the structure of the grid.
To meet future electricity needs of the development must be based on the factors of power load, the grid structure, the cable back a few over-voltage protection trip the way, and relay the composition and power system stability, 10kV neutral grounding, select OK , so as to achieve the optimization of the neutral grounding.
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