Switchgear: refers to a complete set of power distribution devices that are assembled from primary equipment and secondary equipment according to a certain circuit scheme. It is used to control and protect the lines and equipment. It is divided into fixed and handcart styles. The line voltage level can be divided into high voltage switchgear (fixed and handcart) and low voltage switchgear (fixed and drawer). The structure of the switch cabinet is similar, and it is mainly divided into a bus room, a circuit breaker room, a secondary control room (instrument room), and a feeder room. Generally, steel plates are used to isolate the rooms.

　　Internal components include: bus (bus), circuit breaker, conventional relay, comprehensive relay protection device, measuring instrument, isolation knife, indicator light, grounding knife, etc.

　　Division from the application perspective

　　Line counters

　　Also known as a power receiving cabinet, it is a device used to receive electrical energy from the power grid (from the incoming line to the bus bar). It is generally installed with components such as circuit breakers, CT, PT, and isolating knives.

　　Outlet cabinet

　　Also called feeder cabinet or power distribution cabinet, it is a device used to distribute electrical energy (from the bus bar to each outgoing line). Generally, it is also equipped with circuit breakers, CT, PT, isolating knives and other components.

　　Bus contact cabinet

　　It is also called busbar breakout cabinet. It is a device used to connect two sections of busbars (from busbars to busbars). In single-segment busbar and dual-busbar systems, busbar communication is often used to meet the requirements or guarantees of different operating modes. Selective load shedding under fault conditions.

　　PT cabinet

　　The voltage transformer cabinet is usually installed directly on the bus to detect the voltage of the bus and to realize the protection function. Inside are mainly installed voltage transformers PT, isolation knives, fuses and arresters.

　　Isolation cabinet

　　Is used to isolate the busbars at both ends or to isolate the power receiving equipment from the power supply equipment. It can provide a visible endpoint for operators to facilitate maintenance and repair operations. Because the isolation cabinet does not have the ability to disconnect and connect the load current, the handcart of the isolation cabinet cannot be pushed and pulled when the circuit breaker matched with it is closed. In general applications, it is necessary to set the interlock of the auxiliary contact of the circuit breaker and the isolated handcart to prevent misoperation by the operating personnel.

　　Capacitor cabinet

　　It is also called compensation cabinet, which is used to improve the power factor of the power grid, or reactive power compensation. The main components are parallel protective capacitors, switching control loops and fuses. It is generally installed in parallel with the incoming cabinet, and one or more capacitor cabinets can be operated in parallel. After the capacitor cabinet is disconnected from the power grid, because the capacitor bank needs a period of time to complete the discharge process, you cannot directly touch the components inside the cabinet, especially the capacitor bank; within a certain time after power failure (according to the capacitor bank) The capacity depends on the capacity, such as: 1 minute). It is not allowed to re-close, so as to avoid overvoltage damage to the capacitor. When doing the automatic control function, we should also pay attention to the reasonable allocation of the switching times of each group of capacitor banks to avoid damage to one group of capacitors, while other groups rarely switch.

　　Measuring cabinet

　　It is mainly used for measuring electrical energy (kWh), and it is divided into high voltage and low voltage. Generally, it is installed with isolation switch, fuse, CT, PT, active watt-hour meter (traditional instrument or digital electric meter), reactive power Meters, relays, and some other auxiliary secondary equipment (such as load monitors, etc.).

　　GIS cabinet

　　Also known as a closed combination electrical cabinet, it is a combination of circuit breakers, disconnectors, grounding switches, CT, PT, lightning arresters, bus bars and other enclosed combinations in a metal casing, and then uses a gas with good insulation and arc extinguishing properties (generally six Sulfur fluoride SF6) is used as a phase-to-phase and ground insulation measure, and is suitable for high-voltage and high-capacity power grids for distribution and control.

　　Circuit breaker

　　Under normal operating conditions, the circuit breaker is closed (except for special applications) and the circuit is switched on. When the automatic control or protection control operation is performed, the circuit breaker can perform the circuit breaking or making operation under the control of the comprehensive protection device. The circuit breaker can not only switch on and off the normal load current, but also be able to withstand a short-circuit current (several times or even dozens of times the normal working current) for a certain time, and it can cut off the short-circuit current and cut off the faulty lines and equipment. Therefore, the main function of the circuit breaker is to open and close the circuit (including breaking and closing the normal current, breaking the short-circuit current).

　　In the process of breaking and connecting the circuit, an arc is unavoidable between the moving contact and the static contact of the circuit breaker. In order to protect the contacts, reduce the loss of contact materials and reliably break the circuit, measures must be taken to extinguish the arc as soon as possible. One of them is to use different arc extinguishing media to fill the circuit breaker between the moving and static contacts. Different circuit breakers can be divided into: oil circuit breakers (more oil, less oil), sulfur hexafluoride (SF6) circuit breakers, vacuum circuit breakers, air circuit breakers and so on. The main primary equipment in high and low voltage switch cabinets that we often come into contact with in engineering is circuit breakers.

　　Because the dynamic and static contacts of circuit breakers are usually wrapped in containers filled with arc-extinguishing medium, the opening and closing status of circuit breakers cannot be directly judged. Generally, the auxiliary devices of circuit breakers (such as the opening and closing pointers) Etc.) to judge.

　　Isolated knife gate

　　The isolating switch (or isolating switch) can be identified to be connected or disconnected due to the obvious break. It is mainly used to isolate the high-voltage power supply to ensure the safe maintenance of the line and equipment. The current that can be interrupted is very small (generally only a few ampere). Because there is no special arc extinguishing device, it cannot be used to cut off the fault current and the normal working current, and it is not allowed to carry out the breaking operation with a load.

　　Fuse

　　Fuse is a simple circuit protection electrical appliance. Its principle is that when the current flowing through the fuse reaches or exceeds a certain value for a certain period of time, the melt itself melts and cuts off the circuit. Its action principle is simple and easy to install. Generally it is not used alone. It is mainly used to cooperate with other electrical appliances.

　　Main action characteristics:

　　First, the current must reach a certain value, which has been done before the fuse leaves the factory and cannot be changed;

　　Second, it takes a certain time after the current reaches a certain value. This time is also done by the manufacturer and cannot be changed, but there are many types, including delayed action, fast action, super fast action, etc .;

　　Third, the body is damaged after the action and cannot be reused and must be replaced;

　　Whether the fuse is blown can be judged by the fuse indicator or by the appearance of the fuse; commonly used fuses and fuses belong to the scope of this type of electrical appliances.

　　Load switch

　　The load switch has a simple arc extinguishing device. The arc extinguishing medium generally uses air. It can switch on and off a certain current and overcurrent, but it cannot cut off the short-circuit current and cannot be used to cut off the short-circuit fault. Therefore, it is absolutely not allowed to simply replace the circuit breaker with a load switch; if a load switch is to be used, it must be used in conjunction with the aforementioned high-voltage fuse (in fact, fuses and load switches are often used in series for simple overload protection To reduce project costs).

　　The load switch is similar to the isolation knife in that it has a clear opening gap, and it can be easily judged whether the circuit is on or off.

　　Transformer

　　Simply put, a transformer is a device that uses alternating electromagnetic fields to achieve the conversion of different voltage levels (actually the conversion of electrical energy). The voltage before and after the conversion does not change in frequency. According to its purpose, it can be divided into many types, such as power transformers, rectifier transformers, voltage regulators, isolation transformers, and CT and PT. We often encounter power transformers at the project site.

　　Some main technical parameters related to transformers include:

　　1. Rated capacity: refers to the rated output capacity of the transformer under the rated working conditions (equal to U × I, the unit is kVA);

　　2, rated voltage: under no-load, rated tap, the value of the terminal voltage (that is, the value of the primary and secondary voltage);

　　3. No-load loss: Under no-load conditions, the loss of the transformer (also called iron loss);

　　4. No-load current: the value of the current flowing through the primary coil under no-load conditions;

　　5. Short-circuit loss: The primary side passes the rated current and the loss caused by the secondary short-circuit (mainly caused by the coil resistance);

　　6, the concept of tap (tap):

　　In order to meet the needs of power grid operation, the high voltage side of general transformers have taps. The voltage value of these taps is expressed as a percentage of the rated voltage, which is the so-called tap voltage. For example, a high-voltage 10kV transformer has a tap of ± 5%, which means that the transformer can operate at three voltage levels: 10.5kV (+ 5%), 10kV (rated), and 9.5kV (-5%). In general, the number of taps (tap points) of the on-load voltage-regulating transformer is large, such as 7 tap points (± 3 × 2.5%) and 9 tap points (± 4 × 2%). Because the synchronous switching of the tap changer cannot be completely guaranteed, the on-load voltage-regulating transformer cannot generally be operated in parallel.

　　7. Active load:

　　Loads that generate machine or thermal energy in the power system. However, the purely resistive load in the load only consumes active power, such as electric heating, electric furnace, lighting and other electrical loads are completely active loads. Asynchronous motors and synchronous motors consume active power and reactive power at the same time. The part that generates machine energy is the active load. The active load is supplied by the active power of the generator.

　　8. Reactive load:

　　The part that does not work in the electric load. Reactive power is consumed only in inductive loads. Such as: transformers, motors, air conditioners, refrigerators, etc. So while the generator outputs active power, it also needs to provide reactive power.

　　When the reactive power cannot meet the power grid, the voltage of the system will drop. In order to meet the needs of users, a reactive power compensator must be installed in the substation to maintain the reactive power balance so that the voltage level can be maintained.

　　9. Emergency reserve:

　　One of the components of reserve capacity in the power system. Due to the possibility of temporary or permanent failure of power generation equipment that affects power supply, the system must be equipped with a certain number of emergency backup power sources to ensure the safety of power facilities.

　　10. System decommissioning:

　　In order to prevent system out of step and accident expansion, a complete power system is decomposed into several independent systems that no longer operate synchronously. After decommissioning, some local systems may suffer from insufficient power, and therefore the frequency and voltage drop need to be cut off to prevent the stability of the entire system from being damaged.

　　PT (TV) / CT (AV)

　　Transformer is actually a special transformer, which is mainly used to electrically isolate the primary circuit from the control circuit, thereby expanding the use of secondary equipment (instruments, comprehensive insurance, etc.).

　　The use of PT / CT can avoid the high voltage / high current of the primary circuit from directly entering the secondary control equipment (such as instruments, comprehensive protection devices, etc.), and it can also prevent the operation of the primary circuit from being affected due to control equipment failure.

　　1. The characteristics of current transformers (CT, AV) are: the primary winding N1 is thick and small, and the secondary winding N2 is thin and large. The rated current I2 of the secondary is generally 5A (according to N1I1 = N2I2, it can be calculated approximately once Side current I1, or select a current transformer with a corresponding transformation ratio according to the primary side current I1). Because the primary and secondary windings of the CT are connected in series in the primary circuit and the secondary control circuit, respectively, according to the characteristics of the transformer U1I1 = U2I2, it can be obtained that the operating voltage of the secondary side during operation is The open circuit is very large, so CT is absolutely not allowed. According to the purpose, it can be divided into protection and measurement CT. When the CT has a short-circuit fault in the primary circuit, it is easy to saturate to limit the secondary current (secondary winding-side current I2) to be too large to achieve the purpose of protecting the comprehensive protection device. When the CT is short-circuit fault in the primary circuit, it should not A protection phenomenon occurs to ensure the reliable operation of the comprehensive protection device.

　　2. Transformation ratio: The ratio of the number of turns of the transformer's high-voltage side winding and low-voltage side winding is called the transformation ratio, which can be approximated by the ratio of the rated voltage of the high-voltage side to the low-voltage side.

　　3. The characteristics of the voltage transformer (PT, AV) are: the number of primary windings is N1 and the number of secondary windings is N2, which is equivalent to a step-down transformer (the rated voltage on the secondary side is generally 100V). Since the primary winding and the secondary winding of PT are in parallel connected to the voltage loop of the primary circuit and the secondary control circuit, respectively, and the impedance of the voltage coil is very large, the current on the secondary side of the PT is very small, and the secondary winding is approximately the same. No-load condition; but the impedance of the secondary winding itself is very small, so if the secondary winding is short-circuited, it will cause a very large secondary-side current (N1I1 = N2I2). Therefore, the secondary winding of PT must never be short-circuited.