Knowledge explanation of the electrical grounding

2022-08-24
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030 power system: explanation of electrical grounding system

electrical grounding system

in the power system, grounding is an important measure to protect the safety of people and power and electronic equipment. Grounding can prevent people from being shocked, equipment and lines from being damaged, fire and lightning, electrostatic damage and ensure the normal operation of the power system. Therefore, it is necessary to understand the principle and application of grounding

1. Concept explanation

electrical "ground": we call the place where the potential approaches zero as electrical "ground" or earth. In the electrical industry, usually speaking of ground is electrical ground

voltage to ground: the potential difference between the grounding part of electrical equipment, such as the grounding shell, grounding body and grounding wire, and the "Earth" with zero potential is called the voltage to ground of the grounding part

grounding: grounding is a commonly used technology in power system to meet the needs of system operation and protect equipment or personal safety. It is to make a good electrical connection between a part of electrical equipment and "ground"

2. Grounding device

grounding device refers to the device that connects the parts of electrical equipment that need to be grounded with the earth. In order to prevent the electrical equipment from being electrified due to insulation damage, it is necessary to connect the electrical equipment shell with the grounding device through metal wires. The grounding device introduces the short-circuit current, leakage current, electrostatic charge and lightning current that may be generated by electrical equipment to the earth, so as to avoid personal electric shock and other accidents

the grounding device includes grounding body and grounding wire, and its resistance is composed of grounding body resistance, grounding wire resistance and soil resistance. Since the soil resistance coefficient is relatively fixed and the resistance of the grounding wire is small, the selection of a good grounding body is the key to determine the resistance of the grounding device

3. Artificial grounding body

when the grounding resistance of the natural grounding body cannot meet the grounding resistance specified in the regulations, the special grounding body artificially buried underground is called artificial grounding body. At present (3) high thermal conductivity, common artificial grounding bodies include ionic grounding electrode, steel pipe vertically buried underground, angle steel, round steel horizontally placed, flat steel, etc. Due to the high mechanical strength of steel pipes, they are easy to be vertically driven underground. Generally, steel pipes with a diameter of 50mm and a length of 2.5m can be selected. The top of the grounding body is 500 ~ 700mm from the ground to reduce the influence of external temperature and humidity changes on its resistance value

the grounding resistance of a single grounding body is related to the type of section steel, the total length of grounding body, burial depth, size, burial method, etc. In order to achieve the expected resistance value specified in the specification, multiple grounding bodies need to be connected in parallel. At this time, a single grounding body can be connected with flat steel. The distance between vertical grounding bodies is generally twice the length of a single grounding body to reduce electromagnetic shielding

the grounding body can be centrally laid outdoors, then led indoors from the grounding trunk line, and then led to each electrical equipment from the grounding branch line, or the grounding body can be laid in a ring along the electrical equipment

4. Grounding wire

the conductor connected between the metal shell of electrical equipment and the grounding body is called grounding wire. The grounding wire is mainly the grounding trunk line and grounding branch line because the viscosity of reducer oil is too small or too large. The grounding trunk line refers to the common grounding wire laid along the surface of the building; The grounding branch line refers to the conductor LED from the grounding trunk line to the electrical equipment

5. Working grounding

in order to meet the needs of work, the neutral point of transformer or generator is often grounded directly or through special equipment. This grounding method is called working grounding. The grounding resistance shall not exceed 10 ω。

function:

① reduce the electric shock voltage of human body

② it can quickly cut off the faulty equipment. In the system with ungrounded neutral point, when one phase grounding occurs, the relay protection in the system cannot act quickly to cut off the power supply due to the small grounding current, which is very unsafe. In the neutral grounded system, when one phase grounding occurs, due to the large single-phase grounding short-circuit current, the relay protection will act quickly and the faulty equipment will be removed in the shortest time

③ it can reduce the insulation requirements of electrical equipment and lines. When the working grounding is directly led out from the neutral point of the star connection on the secondary side of the distribution transformer, it also has the function of fixing the neutral point to the ground potential and solving the power supply of single-phase electrical equipment, and when there is an inter turn short circuit between the high and low voltage windings of the transformer, it can be signaled by the monitoring device or protected by the protection device

6. Protective grounding

in order to prevent people from being exposed to the risk of electric shock due to the insulation damage of electrical equipment, make a good metal connection between the metal shell of electrical equipment and the grounding body, which is called protective grounding, also known as safety grounding. The grounding resistance shall not exceed 4 ω。

6.1 unless otherwise specified, the following metal parts of power and electrical equipment shall be grounded or connected to neutral (connected to the zero line):

metal bases and shells of motors, transformers, electrical appliances and lighting appliances

metal transmission device of electrical equipment

secondary winding of transformer

framework of switchboard and console

metal structures and reinforced concrete of indoor and outdoor power distribution devices, as well as metal fences and metal doors close to live parts

the shells of AC and DC power cable junction boxes, terminal boxes, metal sheaths of cables, steel pipes for threading, etc

metal poles and towers of power lines equipped with lightning wires

switchgear, capacitors and other electrical equipment installed on the poles of distribution lines

1 ~ 2 shielded cores of sheathed, non armored or metal sheathed cables of armored control cables

6.2 unless otherwise specified, the following metal parts of power equipment do not need to be grounded or connected to neutral:

in the dry room with poor conductor ground such as wood and asphalt, the shell of power equipment with AC rated voltage of 380V and below and DC rated voltage of 440V and below. Except when the maintenance personnel may touch the shell of power equipment and other grounding objects at the same time

in dry places, power equipment enclosures with AC rated voltage of 127V and below and DC rated voltage of 110V and below, except places with explosion hazards

enclosures of electrical measuring instruments, relays and other low-voltage appliances installed on the partition walls of distribution boards, consoles and distribution devices, as well as insulator metal bases that will not cause dangerous voltage on the supports in case of insulation damage

equipment installed on the grounded metal structure (good electrical contact shall be ensured), except for places with explosion hazards

metal support in battery room with rated voltage of 220V and below

casings of motors and electrical appliances with reliable electrical connection with the grounded machine base, except in places with explosion hazards

railway tracks from power plants, substations and industrial enterprise areas, except those transporting inflammables and explosives

in the low-voltage electricity with the neutral point directly grounded, the shell of the power equipment should adopt zero protection in this experiment, that is, connect the shell with the zero line. When there are few and scattered electrical equipment, it is really difficult to adopt neutral protection, and the soil resistivity is low, so it is easy to make grounding, grounding protection can be adopted. However, at this time, if the electric equipment leaks electricity, the equipment shell and the metal parts with electrical connection, the transformer shell and its grounding wire may be electrified, which will endanger personal safety. Necessary safety measures should be taken: such as installing relay protection devices that can automatically disconnect the grounding fault, using insulating pads, installing protective fences and other safety measures. In the low-voltage electricity in wet places and places with particularly bad conditions, the shell of the power equipment should be protected by neutral connection

portable and mobile electrical equipment shall be grounded with dedicated core wire. It is strictly forbidden to use this core wire to pass the working current at the same time, and it is strictly forbidden to use the zero line of other electrical equipment for grounding. The grounding core wire shall be multi strand soft copper wire, and its section shall not be less than 1.5mm2. The sockets of portable and mobile electrical equipment shall be equipped with special grounding contacts. The contact shall be connected to the ground wire and zero wire at the same time, and the structure of the plug used shall be able to avoid mistaking the conductive contact for the ground contact. The grounding contact of socket and plug shall be connected before the contact of conductive contact, and shall be disconnected after the disconnection of live contact. The grounding contact of the socket with metal shell and the metal shell shall have reliable electrical connection

7. Repeated grounding

repeated grounding refers to making a good metal connection with the earth again at one or more places of the zero line through the grounding body. In the low-voltage power supply system, when the neutral protection mode is adopted, in addition to the working grounding of the neutral point of the power transformer, it must also adopt the repeated grounding

the function of repeated grounding is:

(1) reduce the voltage to ground of the leakage equipment shell

(2) reduce the risk of electric shock when the zero line is disconnected

(3) reduce or eliminate the dangerous voltage to ground that may appear on the zero line when the three-phase load is seriously unbalanced

China stipulates that the grounding resistance of repeated grounding should be less than 10 ω。

in the early natural gas stations, most of the grounding bodies used galvanized angle steel and galvanized round steel, and the middle was connected with galvanized flat steel. At present, ionic grounding electrode is widely used in the grounding body of the station, and 30 is used in the middle × 5 galvanized flat steel connection, the surrounding frame adopts 50 × 5 galvanized flat steel connection. Most natural gas stations adopt joint grounding mode, that is, all grounding systems share one "ground". The combined grounding method has the advantages of low grounding resistance, saving metal materials, and occupying less land

the grounding resistance value of the grounding device in the station is required as follows:

the grounding resistance of the vent pipe and the lighthouse is less than 10 ω;

the grounding resistance of communication and automatic control equipment is less than 2 ω;

the anti-static grounding resistance of the station is less than 10 ω;

the grounding resistance of transformer in distribution room is less than 4 ω;

the grounding resistance is less than 1 when combined grounding ω。

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