High-voltage switchgear used in small substations

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1. **Functions and Purposes**
- **Control and Protection Functions**
- The high-voltage switchgear in small substations is mainly used to control and protect high-voltage electrical equipment. It can effectively control equipment such as transformers and transmission lines in the substation, and achieve normal start-up, shutdown, and quick disconnection operations in case of faults. For example, when faults such as overload and short circuit occur in the transformer, the relay protection device in the switchgear will act quickly and cut off the circuit through the circuit breaker to protect the transformer from damage.
- **Isolation Function**
- It provides electrical isolation. During equipment maintenance or repair, the isolating switches of the switchgear can reliably isolate the part to be repaired from the live part, ensuring the safety of maintenance personnel. For example, when maintaining a certain transmission line, open the corresponding isolating switch to isolate the line from other live parts of the substation, preventing the occurrence of electric shock accidents.
- **Metering Function (Some Switchgear)**
- Some high-voltage switchgear also has a metering function. It can accurately measure and record electrical energy parameters of the substation, such as voltage, current, power, and electrical energy. These metering data are very important for the operation management of the substation, electricity bill settlement, and other work.
2. **Main Components**
- **Circuit Breaker**
- The circuit breaker is one of the core components of the high-voltage switchgear. It can connect and disconnect the circuit under normal and fault conditions. When fault currents such as overload and short circuit occur in the circuit, the circuit breaker can quickly cut off the current, and its cutting speed is very fast, generally within dozens of milliseconds. For example, for a commonly used 10kV vacuum circuit breaker, its breaking time can reach about 30 - 50ms, effectively protecting electrical equipment from the damage of large currents. The rated current and short-circuit breaking current of the circuit breaker are important parameters, which need to be selected according to the actual load and short-circuit capacity of the substation.
- **Isolating Switch**
- The isolating switch is mainly used to isolate the power supply and ensure maintenance safety. Its characteristic is that it is operated under no-current or small-current conditions and cannot be used to cut off load current and short-circuit current. The isolating switch has an obvious disconnection point in the open position, which is convenient for the staff to identify whether the circuit has been cut off. For example, in the high-voltage switchgear of small substations, the operating mechanisms of isolating switches usually have two modes: manual and electric. Manual operation is simple and direct, while electric operation is more convenient for remote control.
- **Current Transformer and Voltage Transformer**
- The current transformer is used to convert the large current on the primary side into a small current on the secondary side (usually 5A or 1A) for use by measuring instruments, relay protection devices, etc. For example, for a current transformer in a 10kV substation, the rated current on the primary side may be several hundred amperes. After conversion through the transformation ratio, the secondary side is 5A, which is convenient for subsequent equipment to measure the current and judge the protection action. The voltage transformer is to convert the high voltage on the primary side into a low voltage on the secondary side (usually 100V) for voltage measurement and protection.
- **Relay Protection Device**
- The relay protection device can detect the faults and abnormal operating states of electrical equipment and send a tripping signal in a timely manner to make the circuit breaker act. It can realize various protection functions such as overcurrent protection, instantaneous trip protection, and zero-sequence protection. For example, when a short circuit occurs in the transmission line and the short-circuit current exceeds the setting value of the overcurrent protection, the relay protection device will quickly send a signal to make the circuit breaker trip, thus protecting the line and other equipment.
- **Busbars and Connection Parts**
- The busbars are conductors for collecting and distributing electrical energy. The busbars in the switchgear are usually made of copper or aluminum. The cross-sectional size of the busbars needs to be determined according to the maximum load current of the substation to ensure good electrical conductivity and current-carrying capacity. The connection parts include the connections between the busbars and equipment such as circuit breakers and isolating switches. The quality of these connection points directly affects the performance of the switchgear. It is necessary to ensure tight connections and good contact to reduce contact resistance and heating phenomena.
3. **Types and Characteristics**
- **Classification by Circuit Breaker Type**
- **Vacuum Switchgear**
- The circuit breaker of this kind of switchgear uses a vacuum arc extinguish chamber to extinguish the arc. The vacuum environment inside the vacuum arc extinguish chamber can effectively suppress the generation and reignition of the arc, and it has the advantages of good arc extinguishing performance, small volume, and simple maintenance. For example, in small 10kV substations, vacuum switchgear is widely used. Its service life is long, generally reaching 20 - 30 years, and it does not require frequent maintenance of the arc extinguish chamber during operation.
- **SF6 Switchgear**
- SF6 (sulfur hexafluoride) switchgear uses the excellent insulation and arc extinguishing performance of SF6 gas. The insulation strength of SF6 gas is several times higher than that of air, which can effectively reduce the volume of the switchgear. However, SF6 gas is a greenhouse gas, and once it leaks, it will have an impact on the environment. Therefore, this kind of switchgear needs to have good sealing performance and gas monitoring devices to ensure that SF6 gas does not leak or can be detected in time when it leaks.
- **Classification by Structural Form**
- **Fixed-Type Switchgear**
- The electrical components of the fixed-type switchgear are fixedly installed in the cabinet. Its structure is simple and the cost is relatively low. However, its flexibility is poor, and it is not very convenient for equipment maintenance or component replacement. For example, in some small substations that are more sensitive to cost, if the requirement for equipment maintenance frequency is not high, fixed-type switchgear may be selected.
- **Withdrawable-Type Switchgear**
- The main components such as the circuit breaker of the withdrawable-type switchgear can be removed from the cabinet for maintenance or replacement, which has high flexibility and maintenance convenience. This kind of switchgear is increasingly widely used in modern small substations, especially in occasions with high requirements for equipment operation reliability and maintenance efficiency. For example, when the circuit breaker fails and needs to be repaired, the withdrawable-type switchgear can remove the circuit breaker trolley from the cabinet and carry out maintenance without affecting the operation of other equipment, greatly shortening the maintenance time.
4. **Installation and Maintenance Precautions**
- **Installation Requirements**
- When installing the high-voltage switchgear, it is necessary to ensure the verticality and flatness of the cabinet. Generally, it is required that the vertical deviation of the cabinet does not exceed 1.5‰, the horizontal deviation of the tops of two adjacent cabinets does not exceed 2mm, and the horizontal deviation of the tops of a row of cabinets does not exceed 5mm. The installation location should avoid direct sunlight, humid and corrosive gas environments. For example, when installing switchgear in small substations in coastal areas, attention should be paid to taking moisture-proof and anti-corrosion measures, such as installing dehumidification equipment and using anti-corrosion coatings.
- The installation of the busbars should be carried out in strict accordance with the design requirements to ensure that the connection of the busbars is tight and the phase-to-phase distance and ground-to-ground distance meet the electrical safety standards. For example, the phase-to-phase distance of 10kV busbars is generally not less than 125mm, and the ground-to-ground distance is not less than 125mm. At the same time, the busbars should be subjected to a withstand voltage test after connection to ensure good insulation performance.
- **Maintenance Key Points**
- Regularly clean the switchgear to remove dust and debris, especially the contact points and connection parts of electrical components such as circuit breakers and isolating switches. The accumulation of dust may lead to poor contact and a decrease in insulation performance. For example, the switchgear can be cleaned once a quarter, using dry compressed air or a soft brush for cleaning.
- Check the operating status of electrical components, including the opening and closing time of the circuit breaker, the contact situation of the isolating switch, and the setting values of the relay protection device. For example, the opening and closing time of the circuit breaker can be tested once a year to ensure that it is within the specified range, otherwise it may affect the protection effect in case of a fault. At the same time, the setting values of the relay protection device should be regularly checked and adjusted to adapt to the changes in the load and operation mode of the substation.
- For SF6 switchgear, regularly check the pressure and quality of SF6 gas. If the gas pressure is lower than the specified value, the gas should be replenished in time. And check the sealing performance of the switchgear to prevent gas leakage. For example, the SF6 gas pressure can be checked once a month. Once it is found that the pressure drops too quickly or there are signs of leakage, measures should be taken immediately for processing.