In the power system, the neutral point operates in a variety of ways, like a dancer performing multiple dances on stage. Some choose to remain mysterious and ungrounded; Some gently touch the ground through resistance; Others are elegantly grounded with the help of arc suppression coils; Some of them are directly embraced by the earth. In our country, this dancer prefers three dance steps: ungrounded to remain independent, grounded to dance with arc suppression coils and direct grounding without reservation. And in faraway countries, the small resistance grounding system is like a local star, attracting attention. But in our country, it is only emerging and is beginning to show its talent in some areas.
1. Three-phase system with no grounding (insulation) at the neutral point
In an ideal world, when the values of the relative capacitance currents are equal and the phases differ by 120°, their vector sum is zero, no capacitance current passes through the ground, and the neutral point-to-ground potential remains zero, which is consistent with the ground potential. In this case, whether the neutral point is grounded or not has no effect on the relative ground voltage, and the system is in equilibrium. However, when the relative ground capacitance of the neutral point is not equal, the ground potential of the neutral point will deviate from the zero position even in normal operation, which is called the neutral point displacement. This is usually due to the asymmetrical arrangement of overhead lines and incomplete transposition. In a three-phase system where the neutral point is not grounded, a one-phase grounding will have a range of effects. First, the ungrounded two-phase voltage to ground will rise to the line voltage level, i.e. 3 times the phase voltage. Therefore, the relative insulation level of the system must be designed in accordance with the line voltage. Secondly, although the voltage and phase between the phases remain unchanged at this time, and the balance of the three-phase system is not disturbed, it still cannot be operated to the ground for a long time, especially in the power system where the generator is directly supplied. Because of the increase in the voltage of the ungrounded ground, a long-term one-phase ground may cause a two-phase short circuit. As a result, such systems often require the installation of insulation monitoring or grounding protection devices to signal in the event of a single-phase grounding, allowing the personnel on duty to take swift action to eliminate the fault. The maximum time allowed for continuous operation of the one-phase grounding system shall not exceed 2 hours. In addition, the current passing through the ground point is a capacitive current, which is three times the size of the original relative capacitance current. This capacitive current is not easily extinguished and may cause an arc discharge at the grounding point, resulting in periodic extinguishing and re-arcing. Continuous, intermittent arcing at arc ground is dangerous and can cause line resonances and generate overvoltages that can damage electrical equipment or develop phase-to-phase short circuits. Therefore, in this system, when the grounding current is greater than 5A, the generator, transformer and motor should be equipped with a grounding protection device that acts on tripping. 2. Three-phase system in which the neutral point is grounded through the arc suppression coil
In the case of a neutral ungrounded three-phase system, we learned that the system would remain powered despite a single-phase ground fault. However, when the single-phase ground fault current exceeds a certain threshold, such as greater than 10A in a 35kV system or greater than 30A in a 10kV system, the power supply will not be sustainable. In order to compensate for this shortcoming, people have introduced the method of grounding through arc suppression coils. Nowadays, in the 35kV power grid system, this method of grounding the neutral point through the arc suppression coil has been widely used.
The arc suppression coil, this intelligent device, is actually an adjustable inductance coil with an iron core, which is carefully placed at the neutral point of a transformer or generator. When a single-phase ground fault occurs, it is capable of generating an inductor current that is similar to the ground capacitor current but in the opposite direction. This inductor current lag voltage of 90° complements the capacitive current with a lead voltage of 90°, and finally the current flowing through the ground is negligible or even close to zero. This magical process, like a delicate dancer, with graceful steps and gestures, skillfully defuses the arc of the grounding ground and the hazards it can bring. This is also the origin of the name of the arc suppression coil.
When the capacitive current is exactly equal to the inductor current, we call it full compensation; When the capacitance current is greater than the inductor current, it is called undercompensation; When the capacitance current is less than the inductor current, it is called overcompensation. In practice, we usually prefer to overcompensate. This is because overcompensation provides a margin for the arc suppression coil to avoid resonance and overvoltage. This strategy is like putting a layer of armor on the system, making it more robust and reliable in the face of challenges.
3. The neutral point is directly grounded.
A system with a neutral point directly grounded is a representative of a high-current grounding system, and its grounding current is strong enough to cause potential damage to electrical equipment. However, when a fault strikes, the relay protection system reacts quickly and, like a heroic guardian, trips the switch to contain the spread of the fault in time. In China, the power system of 110kV and above adopts this direct grounding strategy. Different levels of power systems have different grounding methods at the neutral point, and the selection principle is like a clever dancer, gracefully adjusting the pace as the voltage level changes. The power grid of 220kV and above, like a steady dancer, firmly chooses the neutral point to be directly grounded; The 110kV grounding grid is like a flexible dancer, most of them choose to be directly grounded, and a few use arc suppression coil grounding. For the 20 60kV power grid, it is more concerned about the reliability of the power supply, so it chooses a more prudent strategy: grounded or not grounded through the arc suppression coil. However, when the single-phase grounding current exceeds 10A, it will choose to ground through the arc suppression coil for stability. For the 3 10kV power grid, the decision-making is more complicated. The reliability of the power supply and the consequences of failure, like the two ends of the scale, need to be carefully weighed. Therefore, it mostly adopts the neutral point and does not ground the way. However, when the capacitance current of the grid is greater than 30A, for safety, it will also choose to be grounded through the arc suppression coil or grounded by resistor. As for the 220 380V three-phase four-wire low-voltage power grid below 1kV, its safety performance is the primary consideration. Therefore, it adopts the neutral point direct grounding method, which effectively prevents the dangerous voltage of changing over 250V when the phase is grounded. In some special places, such as the most dangerous places or mines, in order to ensure safety, the neutral point may choose not to be grounded, and at the same time equipped with a breakdown fuse to prevent the potential danger caused by high voltage into low pressure. 4. The superiority of neutral point grounding
In the 220 380V three-phase four-wire low-voltage distribution network, most of the neutral points of the distribution transformers take working grounding measures, which is full of profound meaning and ingenuity. It is like a solid watchtower, guarding the peace of the grid at all times. First of all, in the case of normal power supply, the grounding of the neutral point maintains the constant voltage of the phase line to the ground, just like a skillful weaver girl, cleverly weaving two different voltages of 220V and 380V, to meet the power needs of single-phase and three-phase equipment, whether it is a lamp that illuminates life, or a motor that drives machinery, can get just the right amount of power. In addition, if the neutral point is not grounded, when the single phase is grounded, the voltage to the ground of the other two phases will rise sharply to several times the phase voltage, like a huge wave in a strong wind, which is difficult to deal with. After the neutral point is grounded, the ground voltage of the remaining two phases is maintained at the phase voltage level, which is gentle and pleasant like a spring breeze. This not only reduces the contact voltage of the human body, but also reduces the insulation pressure of electrical equipment, creating conditions for manufacturing and reducing costs. In addition, the grounding of the neutral point can effectively avoid the danger of high-voltage electricity channeling into the low-voltage side. Once the insulation between the high and low voltage coils is damaged, causing serious leakage or even short circuit, the grounding device can quickly build a closed loop, trigger the superior protection action, cut off the power supply, and protect the safety of the staff and equipment on the low voltage side. Therefore, the distribution neutral point of the low-voltage power grid is grounded, like a guardian saint, guarding the smooth operation of the power grid. The neutral point is divided into the neutral point of the power supply and the neutral point of the load, which will only appear when the three-phase power supply or load is connected in a Y shape. The neutral point of the power supply is the common point of the three-phase coil connected from end to end, like the foundation of a big tree, providing support for the entire power grid; The conductor that leads from the neutral point of the power supply is called the neutral wire, which is like a leafy tree branch that carries power to the grid. In the three-phase four-wire system, when the neutral point is grounded, it is called the neutral line, and when it is not grounded, it is called the neutral line. In order to ensure the safe operation of the power grid, the distribution system has also taken measures of common grounding at three points. That is, the neutral point of the transformer, the shell and the grounding lead of the arrester are connected to a grounding device, just like three warriors working together to protect against the invasion of overvoltage. When the power grid is struck by lightning, the lightning arrester acts quickly to limit the voltage on the transformer shell within the residual voltage of the lightning arrester, which effectively reduces the voltage on the grounding body and protects the safety of the power grid. To sum up, the neutral grounding measures of the distribution transformer not only ensure the stable operation of the power grid, but also bring light and power to our lives. It is like a smart engineer, carefully designing every corner of the power grid to make our lives better. Electrical Engineer