What are the electrical components in the switchgear? What does it do?

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What is the switchgear?

Switchgear refers to a complete set of power distribution device assembled by primary equipment and secondary equipment according to a certain line scheme, which is used to control and protect lines and equipment, divided into fixed type and mobile type, and according to the input and outlet line voltage level can be divided into high voltage switchgear (fixed and mobile type) and low voltage switchgear (fixed and drawer type). The structure of the switch cabinet is roughly similar, mainly divided into bus room, circuit breaker room, secondary control room (instrument room), feeder room, there is generally steel plate isolation between the rooms.


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

From the perspective of application:

(1) Incoming cabinet:

also known as receiving cabinet, is used to receive electric energy from the power grid equipment (from incoming line to bus), generally installed with circuit breakers, CT, PT, isolation knife and other components.

(2) Outlet cabinet:

also called feed cabinet or distribution cabinet, is used to distribute electric energy equipment (from the bus bar to each outlet), generally installed with circuit breaker, CT, PT, isolation knife and other components.

(3) busbar contact cabinet:

also known as busbar breaking cabinet, it is used to connect two sections of busbar equipment (from busbar to busbar). Busbar contact is often used in single busbar segment and double busbar system, so as to meet the requirements of users to choose different modes of operation or ensure that there is a choice of cutting load in the case of failure.

(4)PT cabinet:

voltage transformer cabinet is usually installed directly on the bus to detect the voltage of the bus and realize the protection function. Inside the main installation of voltage transformer PT, isolation knife, fuse and arrester, etc.


(5) Isolation cabinet:

It is used to isolate the bus at both ends or to isolate the electrical equipment and the power supply equipment. It can provide the operator with a visible endpoint to facilitate maintenance and overhaul operations. Since the isolation cabinet does not have the ability to break and switch on the load current, it is not possible to push and pull the handcart of the isolation cabinet when the circuit breaker matched with it is closed. In the general application, it is necessary to set the auxiliary contact of the circuit breaker and the interlock of the isolation car to prevent the misoperation of the operator.

(6) Capacitor cabinet:

also known as compensation cabinet, is used to improve the power factor of the power grid, or for reactive power compensation, the main device is a group of parallel capacitor banks, switch control loop and fuse protection appliances. Generally, it is installed side-by-side with the wire inlet cabinet, and one or more capacitor cabinets can be run side-by-side. (http://www.diangon.com/ All Rights Reserved) After the capacitor cabinet is disconnected from the power grid, it takes a period of time for the capacitor bank to complete the discharge process, so you cannot directly touch the components in the cabinet, especially the capacitor bank; Within a certain period of time after power failure (depending on the capacity of the capacitor bank, such as 1 minute), it is not allowed to re-close, so as to avoid overvoltage damage to the capacitor. In the automatic control function, attention should also be paid to the rational allocation of the switching times of each capacitor bank, so as to avoid the situation that one group of capacitors is damaged while the other groups are rarely switched.

(7) Metering cabinet:

mainly used for measuring electricity (KWH), and there are high and low voltage points, generally installed with isolation switch, fuse, CT, PT, active power meter (traditional instrument or digital meter), reactive power meter, relay, and some other auxiliary secondary equipment (such as load monitoring instrument).

(8)GIS Cabinet:

Also known as closed combination electrical cabinet, it is the circuit breaker, isolation switch, grounding switch, CT, PT, arrester, bus and other closed combination in the metal shell, and then with good insulation and arc extinguishing performance of gas (generally used sulfur hexafluoride SF6) as the insulation measures between phases and ground, suitable for high voltage and high capacity grade power grid. Used for distribution and control.


(9) Circuit breaker:

under normal working conditions, the circuit breaker is in the closing state (except for special applications), and the circuit is connected. When automatic control or protective control operation is performed, the circuit breaker can be switched on and off under the control of the comprehensive protection device. Circuit breaker can not only turn off the normal load current, and can withstand a certain period of short circuit current (several times or even tens of times of the normal working current), and can break the short circuit current, cut out the fault line and equipment. Therefore, the main function of the circuit breaker is to break and connect the circuit (including the normal current and short circuit current). In the process of breaking and connecting the circuit, the arc between the moving contact and the static contact of the circuit breaker is inevitable. In order to protect 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 fill the dynamic and static contacts of the circuit breaker with different extinguishing media. According to the different circuit breakers of the arc extinguishing medium can be divided into: oil circuit breaker (more oil, less oil), sulfur hexafluoride (SF6) circuit breaker, vacuum circuit breaker, air circuit breaker, etc. The main primary equipment in the high and low voltage switchgear that we often come into contact with in the project is the circuit breaker. Because the dynamic and static contacts of the circuit breaker are generally wrapped in the container full of arc extinguishing medium, the split and close state of the circuit breaker can not be directly judged, generally through the auxiliary devices of the circuit breaker (such as split and close pointer, etc.) to judge.

(10) Isolation knife:

isolation knife (or isolation switch) because there is an obvious break can identify on or off, mainly used to isolate high-voltage power supply, to ensure the safety of the line and equipment maintenance, can break the current is very small (generally only a few amps). Since there is no special arc extinguishing device, it can not be used to break fault current and normal working current, and it is not allowed to break operation with load.

(11) Fuse:

fuse is a simple circuit protection appliance. Its principle is that when the current flowing through the fuse reaches or exceeds a certain value for a certain time, its melt melts and cuts off the circuit. Its operation principle is simple, easy to install, generally not used alone, mainly used to cooperate with other electrical appliances. Main action characteristics: first, the current to reach a certain value, the value in the fuse factory has been done, can not be changed; Second, the current reaches a certain value after a certain amount of time, the time is also done by the manufacturer, can not be changed, but there are many types, including delay action, fast action, super fast action and so on; Third, the body is damaged after the action, can not be reused, must be replaced; Whether the fuse is fused can be judged by the fuse indicator or by the appearance of the melt. Commonly used fuses and fuses belong to this kind of electrical appliance.

(12) Load switch:

the load switch has a simple arc extinguishing device, arc extinguishing medium generally adopts air, can be connected and broken off a certain current and overcurrent, but can not be broken off short circuit current, can not be used to cut off short circuit fault. Therefore, it is absolutely not allowed to use the load switch to replace the circuit breaker; If the load switch is to be used, it must be used in conjunction with the high voltage fuses mentioned above (in fact, fuses and load switches are often used in series for simple overload protection to reduce the cost of the project). The load switch is similar to the isolation knife, which has an obvious disconnect gap and can easily determine whether the circuit is on or off.

(13) Transformer:

Simply put, transformer is the use of alternating electromagnetic field to achieve the conversion of different voltage levels of equipment (in fact, the conversion of electric energy), the voltage before and after the transformation does not change in frequency. According to its use can be divided into many kinds, such as power transformer, rectifier transformer, voltage regulator, isolation transformer, as well as CT, PT and so on. What we often encounter in engineering sites is power transformers.


(14) Main technical parameters related to transformers include:

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

2, rated voltage: no load, rated, end voltage value (i.e. primary and secondary side voltage value);

3, no-load loss: no-load condition, transformer loss (also called iron consumption);

4, no-load current: under no-load condition, the current value of a side coil flow;

5, short circuit loss: primary side pass rated current, the loss caused by the second short circuit (mainly generated by coil resistance);

6, tap (tap) concept: for the operation of the power grid needs, the general transformer high voltage side has a tap, the voltage value of these taps are expressed by the percentage of rated voltage, namely the so-called tap voltage. For example, a high-voltage 10kV transformer with a +5% tap means that the transformer can operate at three voltage levels: 10.5kV(+5%), 10kV(rated), and 9.5kV(-5%). Generally speaking, the number of taps (contacts) of the load regulating transformer is more, such as 7 contacts (±3×2.5%) and 9 contacts (±4×2%). Because the synchronous switching of tap switch can not be fully guaranteed, the on-load regulating transformer generally can not run in parallel.

7. Active load: the load that generates machine energy or heat energy in the power system. But the pure resistive load in the load only consumes active power, such as electric heating, electric furnace, lighting and other power loads are completely active load. While the load of asynchronous motor and synchronous motor consumes both active power and reactive power, and the part that generates machine energy is active load. The active load is supplied by the active power of the generator.

8. Reactive load: the part of the power load that does not work. Reactive power is consumed only in inductive loads. Such as: transformer, motor, air conditioning, refrigerator and so on. So in addition to the active power output, the generator also needs to provide reactive power. When reactive power cannot meet the grid, the voltage of the system will drop. In order to meet the needs of users, reactive compensator should be installed in the substation to maintain the balance of reactive power, so as to maintain the voltage level.

9. Emergency backup: one of the components of backup capacity in the power system. The power supply may be affected by temporary or permanent faults of the power generation equipment. Therefore, the system must set a certain amount of emergency backup power to ensure the safety of the power facilities.

10. System unbundling: A measure to break up a complete power system into several independent systems that no longer operate synchronously, in order to prevent system missteps and extended accidents. Some local systems may suffer from power insufficiency and drop in frequency and voltage after unwinding. Therefore, part of the load needs to be removed to prevent the stability of the whole system from being damaged.

(15) PT (TV)/CT (AV) :

Transformer is actually a special transformer, mainly used to isolate the primary circuit from the control circuit, so as to expand the use of secondary equipment (instrumentation, comprehensive insurance, etc.). Using PT/CT can prevent the high voltage/current of the primary loop from directly entering the secondary control equipment (such as: instrument, comprehensive protection device, etc.), and prevent the failure of the control equipment from affecting the operation of the primary loop.


1. The characteristics of current transformer (CT, AV) are: The primary side winding N1 is thick and small, while the secondary side winding N2 is thin and large. The rated current I2 of the secondary side is generally 5A(the primary side current I1 can be approximately calculated according to N1I1=N2I2. Or choose the current transformer with corresponding ratio according to the primary side current I1. Since the primary winding and secondary winding are respectively connected in series in the primary loop and secondary control loop during CT operation, according to the characteristics of transformer U1I1=U2I2, the operating voltage of the secondary side during operation is very large in the open circuit, so CT is absolutely not allowed to open circuit. According to the purpose of the division, usually can be divided into protective and measurement CT. The measurement CT is easy to saturate when the primary circuit is short-circuited, so as to limit the excessive secondary current (the secondary winding side current I2) and protect the comprehensive protection device. In order to ensure the reliable operation of the comprehensive protection device, the protective CT should not be protected when there is short circuit fault in the primary circuit.

2. Variable ratio: The ratio of the number of turns between the high voltage side winding and the low voltage side winding of the transformer is called the variable ratio, which can be approximately expressed by the ratio of the rated voltage between the high voltage side and the low voltage side. 3, the characteristics of the voltage transformer (PT, AV) are: the number of primary winding turns N1, the number of secondary winding turns N2 is less, equivalent to a step-down transformer (secondary side rated voltage is generally 100V). When PT is working, the primary winding and secondary winding are in parallel with the voltage coils of the primary loop and the secondary control loop respectively. Due to the large impedance of the voltage coil, the current of the secondary side of PT is very small, and the secondary winding is similar to the state of empty load. However, the impedance of the secondary winding itself is very small, so if the secondary winding is short-circuited, it will lead to a very large secondary current (N1I1=N2I2). Therefore, the secondary winding of PT must not be short-circuited.

(16) Handcart/drawer:

The handcart and drawer are part of the high voltage switch cabinet and the low voltage switch cabinet respectively, and the high voltage circuit breaker and the low voltage circuit breaker and their relays are installed respectively. The drawer type switch cabinet (high pressure) and drawer type switch cabinet (low pressure), they and fixed switch cabinet function is basically the same, the main difference is convenient maintenance and overhaul (the hand car and drawer can be pushed through the mechanical operation mechanism to pull out). The handcart and drawer generally have three position states: work (normal operation), test (trial operation and field test) and exit (maintenance and overhaul).

(17) Grounding knife:

grounding knife (also called grounding switch) mainly: first, it is used to ensure the safety of personnel in the maintenance of lines and equipment for grounding; Second, it can be used to artificially cause the grounding short circuit of the system to achieve the purpose of control and protection. The first function is easy to understand, no introduction. The second effect is this: The grounding knife is usually connected to the high voltage side of the step down transformer, when the fault occurs at the electric end or the transformer internal fault, the grounding knife switch should be automatically closed, resulting in the grounding short circuit fault, forcing the transmission end (upper end) circuit breaker to act quickly, cut off the fault, so this is an artificial grounding short circuit fault, the purpose is to ensure that the circuit breaker at the transmission end can act quickly.

(18) Master electrical appliance:

Master electrical appliance is a mechanical control appliance, which issues control instructions to various electrical systems and is used for the conversion and transmission of various signals in the system. Commonly used transfer switches, buttons, rotary switches, position switches and signal lights all belong to the scope of master electrical appliance.

(19). Contactor:

Contactor is a kind of electrical appliance used for frequently switching on and off AC-DC main circuit and large-capacity control circuit at a distance. The main control object is motor, lighting, capacitor bank, etc., which can be divided into AC contactor and DC contactor. Compared with the circuit breaker, the difference is: the operation frequency is very high (so the electrical and mechanical life is required to be long enough); There is a higher on and on capacity, but generally used in 1kV and below the voltage level, can not be compared with the circuit breaker tens of thousands of volts, hundreds of thousands of volts.

(20). Relay:

Relay is used to control the action of other electrical appliances (usually primary electrical main equipment) in the control loop or as protection in the main circuit and for signal conversion. It is only suitable for long-distance breaking and connecting small-capacity control loop, such as: Ac/DC current relay, voltage relay, time relay, intermediate relay, thermal relay, etc.

(21). Experiment

Common tests include: 1. Type test: a test of one or more devices or equipment manufactured according to a certain design requirement to check whether the design requirements meet certain specifications.

2, conventional test: also called factory test, for each device or equipment in the manufacturing or completion of the test, to determine whether the device or equipment meets a certain standard.

3, medium test: is a general term for testing the electrical characteristics of the medium, including: insulation, static electricity, pressure resistance and so on. Sampling test: A test of a number of randomly selected samples from a batch of products, also used to determine whether the sample meets a certain standard.

4. Life test: The test to determine the possible life of the product under specified conditions, or the test to evaluate and analyze the life characteristics of the product, is a destructive test.

5, tolerance test: including a certain period of time for a certain purpose of specific operation and other specified conditions, the product test, such as repeated operation, short circuit, overvoltage, vibration, impact, etc., is a destructive test.

6. Operation test: on-site test of the product to prove that the installation is correct and the product runs normally.


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