Grounding and zeroing in power systems - Solutions - Huaqiang

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In the power system, due to insulation aging, wear or overvoltage breakdown of electrical equipment, the original uncharged parts (such as metal base, metal casing, metal frame, etc.) will be charged, or the original part with low voltage will be charged. With high voltage, these accidental abnormal charging will cause electrical equipment damage and personal injury. In order to avoid such accidents, protective measures for protective grounding and protection against zero are usually adopted. Let's talk about the issue of protective grounding and protection zero.

1 Protective ground

Protective grounding refers to connecting the metal parts that are not energized under normal conditions to the grounding device to prevent the part from being suddenly charged in the event of a fault and causing harm to the human body.

1.1 The role of protective grounding and its limitations

In a system where the neutral point of the power supply is not grounded, if the metal casing of the electrical equipment is not grounded, when the insulation part of the live part of the equipment is damaged by the insulation, the outer casing is charged, and the potential is the same as the potential of the live part of the equipment. Due to the capacitance between the line and the earth, or the insulation of the line is not good, when the human body touches the casing of the charged device, the ground current will flow through the human body, which is obviously dangerous.

After the protective grounding is adopted, the grounding current will flow through both the grounding body and the human body. Because the body resistance is much larger than the protective grounding resistance, the current flowing through the human body is small, and most of the current flows from the grounding body (shunting action), thereby avoiding or reducing the electric shock.

From the voltage point of view, after the protective grounding is taken, the grounding voltage of the charged metal casing in the fault condition is equal to the product of the grounding current and the grounding resistance, and the value is much smaller than the phase voltage. The lower the ground resistance, the lower the case-to-ground voltage. When the human body touches the charged outer casing, the voltage (ie, the contact voltage) that the human body is subjected to is at most the voltage of the outer casing to the ground (the human body is 20m away from the grounding body), and is generally smaller than the voltage of the outer casing to the ground.

From the above analysis, it is known that the protective grounding is to ensure the safety of the person by limiting the voltage of the grounded casing to the ground (controlling the magnitude of the grounding resistance) or reducing the current through the human body.

In systems where the neutral point of the power supply is directly grounded, the protective ground has certain limitations. This is because in the system, when the device has a shell failure, a single-phase ground short circuit is formed, and the short-circuit current flows through the phase line and the protective ground, and the power neutral point grounding device. If the ground short-circuit current does not cause the fuse to be reliably blown or the automatic switch reliably trips, the metal casing of the leakage device will be charged for a long time, which is also dangerous.

1.2 Protection grounding application range

Protective earthing is for systems where the neutral point of the power supply is not grounded or impedance grounded. For the rural low-voltage power grid where the neutral point of the power supply is directly grounded and the low-voltage users who are powered by the urban public distribution transformer, it is not convenient to be unified and strictly managed. In order to avoid the accident caused by the protection grounding and the protection of the zero-mixing, the protective grounding method should also be adopted. . In a system with protective earthing, any part of the metal that may be charged due to insulation damage or other reasons shall be grounded unless otherwise specified. Such as transformers, motors, electrical appliances, lighting equipment shells and bases, metal frame of power distribution equipment, power equipment transmission, power wiring steel, metal sheath of AC and DC power cables.

In the dry place, the electrical equipment casing with the AC rated voltage below 127V and the DC rated voltage below 110V; and in the places with poor conductive ground such as wood and asphalt, the electrical equipment casing with the AC rated voltage below 380V and the DC rated voltage below 440V, unless otherwise In addition to the provisions, it may not be grounded.

1.3 Protective earthing resistance

The protective grounding resistance is too large, the voltage of the earth leakage device casing is high, and the risk of electric shock increases accordingly. The protective earthing resistance is too small, and the steel consumption and engineering cost are increased. Therefore, the resistance must be fully considered.

In low voltage systems where the neutral point of the power supply is not grounded or impedance grounded, the protective earthing resistance should not exceed 4 ohms. When the capacity of the distribution transformer does not exceed 100kVA, the protective grounding resistance can be relaxed to 10Ω due to the short wiring of the system. In areas with high soil resistivity (sand, rocky soil), the protective earthing resistance can be no more than 30 Ω.

In the direct neutral grounding of the power neutral point, the protective earthing resistance must be calculated and determined.

2 protection and zero

2.1 The role of protection and zero and its application range

Since the protective grounding has certain limitations, the protection is connected to zero. That is, the metal part of the electrical equipment that is not charged under normal conditions is connected with the neutral wire in the system by the metal conductor. When the insulation damage of the equipment touches the shell, a single-phase metallic short circuit is formed, and the short-circuit current flows through the phase line-zero line circuit. Without passing through the neutral point grounding device of the power supply, a sufficiently large short-circuit current is generated, so that the overcurrent protection device operates quickly, and the power supply of the leakage device is cut off to ensure personal safety. Its security effect is better than protective grounding.

The protection zero is suitable for the three-phase four-wire low-voltage system where the neutral point of the power supply is directly grounded. In this system, any metal part that may present a dangerous voltage due to insulation damage or other reasons shall be connected to zero unless otherwise specified. The equipment or parts that should be connected to zero and not connected to zero are the same as the protective ground. All factories and mines that are powered by separate distribution transformers should adopt the method of protection and zero connection.

2.2 Repeated grounding

Operating experience has shown that in a zero-connected system, it is not safe to ground the neutral wire only at the power supply. To this end, the neutral line also needs to be grounded at the terminals of the mains and branch lines of the low-voltage overhead line; when the cable or overhead line is introduced into the workshop or large building, grounding is also required (except for the grounding point not exceeding 50m); or Connect the neutral line to the grounding device of the power distribution panel and control panel in the house. This grounding is called repeated grounding.

If the short-circuit point is far away from the power supply, the phase-to-zero line loop impedance is large, and the short-circuit current is small, the overcurrent protection device cannot be operated quickly, and the power supply of the faulty segment cannot be immediately removed, which will cause the equipment casing to be charged for a long time. In addition, since the zero line section is generally smaller than the phase line section, that is, the zero line impedance is larger than the phase line impedance, the voltage drop on the zero line is larger than the voltage drop on the phase line, and is generally greater than 110V ( When the phase voltage is 220V), it is still very dangerous for the human body.

After repeated grounding, the repeated grounding and the neutral point of the power supply constitute the parallel branch of the neutral line, so that the impedance of the phase line-zero line loop is reduced, and the short-circuit current is increased, so that the overcurrent protection device operates quickly. Due to the increase of the short-circuit current, the voltage on the phase line of the low-voltage winding of the transformer increases correspondingly, so that the voltage drop on the zero line is reduced, the voltage of the equipment casing to the ground is further reduced, and the risk of electric shock is greatly reduced.

In the case of no repeated grounding, when the neutral line is broken and any electrical equipment behind the broken line is short-circuited, the voltage of all the zero-connected equipment shells after the disconnection is close to the phase voltage. It is dangerous to have a zero-to-ground voltage on the front of the line at the front of the device.

In the zero-connect system, even if there is no equipment leakage, but when the three-phase load is unbalanced, there is current on the zero line, so there is a voltage drop on the zero line, which is proportional to the zero line current and the zero line impedance. The voltage drop on the zero line is the voltage to the ground of the zero-connected device housing. When there is no repeated grounding, when the low-voltage line is too long, the neutral line impedance is large, and the three-phase load is seriously unbalanced, even if the zero line is not broken, and the equipment is not leaking, the human body often has numbness when it touches the equipment casing. a feeling of. After repeated grounding, numbness will be alleviated or eliminated.

From the above analysis, it is known that in the zero system, repeated grounding must be taken. Repeated grounding resistance should not exceed 10Ω. When the distribution transformer capacity is not more than 100kVA, and the grounding is not less than 3 places, the grounding resistance can be no more than 30Ω. Repeated grounding of the neutral line should make full use of the natural grounding body (except DC system).