Grounding network impact on the safe operation of substation - News - Global IC Trade Starts Here Free Join

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As the substation AC and DC equipment grounding and lightning protection grounding, the grounding network plays an important role in the safe operation of the system. The substation grounding includes working ground, protective grounding, and lightning protection grounding. The working ground is the grounding for the operation in the electrical system of the power system; the protective grounding is the metal casing of the electrical device, the structure of the power distribution device, and the line tower, etc., which may be charged due to insulation damage, in order to prevent it. Grounding that compromises the safety of the person and equipment; lightning protection grounding is the grounding for the lightning protection device to discharge lightning current to the earth. Therefore, the rationality of the substation grounding system is directly related to the important issues of personal and equipment safety.

1 grounding grid design

Grounding is the most important part of lightning protection technology. Whether it is direct lightning, inductive lightning or other forms of lightning, it will be introduced into the earth through the grounding device. Therefore, without a reasonable and good grounding device, lightning protection cannot be effectively prevented.

1.1 The grounding grid of the substation is connected to the grounding of the high-voltage and low-voltage electrical equipment of the whole station, the grounding of the low-voltage power system, the grounding of the cable shield, the communication, the grounding of the equipment of the computer monitoring system, and some temporary grounding during the maintenance and overhaul of the substation. If the grounding resistance is large, the ground potential may increase abnormally when a power system ground fault or other large current enters the ground. If the grid design of the grounding grid is unreasonable, the grounding system potential distribution may be uneven, local potential Exceeding the specified safety value, this will pose a threat to the safety of the operating personnel; at the same time, it may damage the low voltage or secondary equipment and cable insulation due to counterattack, causing the high voltage to enter the control protection system, substation monitoring and protection equipment, thus enabling These devices are mis-moved, refused to move, cause accidents, and even expand accidents, resulting in huge economic losses and social impact.

1.2 Substation grounding design principle: Since the grounding fault current of the voltage busbars at all levels of the substation is getting larger and larger, it is very difficult to meet R≤2000/I in the grounding design. There is a clear difference between the current standard and the original grounding procedure: the grounding resistance value is no longer specified to reach 0.5 Ω, but is allowed to be relaxed to 5 Ω. However, this does not mean that the grounding resistance can be 5Ω in any case. The grounding resistance is relaxed. That is, in order to prevent the damage caused by the transfer potential, various isolation measures should be taken; the influence of the non-periodic component of the short-circuit current should be considered. When the grounding grid potential is increased, the 3~10kV arrester should not be operated or should not be damaged after action; the pressure equalization measures should be taken, and the contact potential difference and the step potential difference should be checked to meet the requirements; Plot the potential distribution curve.

The following principles should be followed when designing a substation grounding grid:

1.2.1 As far as possible, the steel bars of the building foundation and the natural metal grounding body are connected as a grounding net;

1.2.2 As far as possible, based on natural grounding materials, supplemented by artificial grounding bodies, the shape should be closed as much as possible;

1.2.3 A unified grounding grid should be used and grounded with a single grounding.

1.3 Special requirements for lightning protection grounding during design and construction The lightning protection grounding down conductor uses existing natural conductors as much as possible. For example, the fire escape ladder, metal pillars, trusses and internal ribs of the building itself can be directly used as the down conductor.

1.3.1 When making lightning protection down conductors, there must be a reliable metal connection between various metal objects. In this case, all metal joints should be welded and a jumper at the seam.

1.3.2 If the steel bars in the concrete columns of the building are used as the down conductor, there shall be at least four columns, and the joints of at least two main ribs of each column shall be welded.

1.3.3 The lightning protection down conductor should be connected to the lightning protection grounding body with the shortest route. Minimize bending and avoid right angles and sharp corners. If it is necessary to bend, the distance at the curved opening must not be greater than 1/10 of the arc length of the curved portion to avoid increasing the resistance during lightning strikes.

1.3.4 The lightning protection down conductor should be mechanically protected within 2.5m on the ground and 200mm below the ground.

2 about grounding resistance

2.1 Definition of grounding resistance: The grounding resistance is essentially a physical value calculated by Ohm's law between a certain point on the ground flowing to a certain point on the ground. It is defined as the Ohm's law between the grounding pole and the far-reaching grounding pole with zero potential. resistance. In the lightning protection grounding resistance measurement of the substation, it is assumed that the lightning current is basically zero under the ground evacuation to 40 meters. However, the difference of the underground soil structure and the direction of the current probe and the grounding pole are different. The distance between the pin and the current probe is different, and the grounding resistance value is sometimes substantially different.

2.2 Determination of the value of the grounding resistance The determination of the value of the grounding resistance should be based on the economic benefits. The quantitative requirements should be based on certain calculation formulas. The value of the grounding resistance is closely related to the grounding current. The impedance depends on the magnitude and frequency of the grounding current. When the frequency is low, the resistance is the main component of the impedance. The grounding resistance "Technical Regulations for Grounding Design of Power Equipment" (SDJ8-79) has specific provisions for grounding resistance, generally not more than 0.5Ω. In the high soil resistivity area, when the grounding device requires the specified grounding resistance to be technically unreasonable, the grounding resistance of the large grounding short-circuit current system is allowed to reach 5Ω, but measures should be taken, such as preventing high-potential external lead. Potential isolation measures, check contact potential, step voltage, etc. According to the regulations, the grounding potential rises no more than 2000V when the ground fault occurs, and the grounding resistance is not more than 0.5Ω and 5Ω. Therefore, it is generally believed that in the 110kV and above substation, the grounding resistance value is less than 0.5Ω, which is considered to be qualified, and more than 0.5Ω is unqualified; when the grounding resistance value is qualified, no matter how much the short-circuit current is, no measures need to be taken. unreasonable. The essence of grounding is to control the ground potential of the fault point when the substation is grounded short. Because grounding is mainly to ensure the safety of people and equipment, it is the potential rather than the resistance. The grounding resistance is an important parameter to measure the qualification of the grounding grid, but it is not the only parameter. As the capacity of the power system continues to increase, the value of the single-phase short-circuit current generally increases. The short-circuit current in single-phase grounding in an effective grounding system typically exceeds 4kA. Therefore, from the perspective of safe operation, the contact potential and stride voltage of the ground net should be checked under any circumstances, and isolation measures to prevent high-potential external lead should be taken when necessary.

3 lightning protection measures

Substation is an important protection facility for lightning protection of power systems. If a lightning strike occurs, it will cause a large-scale power outage, which will seriously affect social production and people's lives. Therefore, lightning protection measures for substations must be very reliable.

The source of lightning strikes: First, the lightning strikes directly on the equipment of the substation; second, the lightning induced overvoltage of the overhead line and the lightning wave formed by the direct lightning strike overvoltage invade the substation along the line.

The lightning induced overvoltage of the overhead line and the lightning wave formed by the direct lightning strike overvoltage invade the substation along the line, which is the main cause of lightning damage in the substation. If no protective measures are taken, it will inevitably cause damage to the electrical equipment of the substation and cause an accident. The purpose of installing a lightning arrester in a substation is to limit the amplitude of the invading lightning wave so that the overvoltage of the electrical equipment does not exceed its impulse withstand voltage. The main purpose of installing the protection section in the incoming section of the substation is to limit the magnitude of the lightning current flowing through the arrester and the steepness of the invading lightning wave.

The lightning protection measures are generally summarized as two types: 1 to avoid the entry of lightning waves; 2 to use lightning protection waves to introduce lightning waves into the grounding grid.

3.1 Lightning strikes or lightning strikes Lightning strikes can only change their entry path by intercepting and guiding measures. The lightning receptor has a lightning rod and a lightning protection line. Most of the small substations use independent lightning rods. Most of the large substations use lightning rods or lightning protection lines on the substation structure, or a combination of the two. These protection methods have strict requirements on the drain line and grounding device.