Components and functions of high-voltage switchgear

Introduction to the components and functions of high-voltage switchgear

Switchgear is a kind of electrical equipment, which refers to a complete set of power distribution devices assembled from primary equipment and secondary equipment according to a certain circuit plan. It is used to control and protect circuits and equipment. It is divided into fixed type and handcart type. According to the input and output voltage levels, it can be divided into high voltage switch cabinet (fixed type and handcart type) and low voltage switch cabinet (fixed type and drawer type). The structure of the switchgear is roughly similar, mainly divided into busbar room, circuit breaker room, secondary control room (instrument room), and feeder room. There is generally a steel plate isolation between each room.

Internal components include: bus (busbar), circuit breakers, conventional relays, integrated relay protection devices, measuring instruments, isolating knives, indicator lights, grounding knives, etc. The following introduces the switchgear and its power distribution related applications divided by the application perspective.

(1) Incoming cabinet

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

(2) Outgoing cabinet

Also called feeder cabinet or power distribution cabinet, it is the equipment used to distribute electric energy (from the bus to each outlet). Generally, it is also installed with circuit breakers, CT, PT, isolating knives and other components.

(3) Busbar contact cabinet

It is also called a bus breaker cabinet. It is a device used to connect two sections of bus (from bus to bus). Bus connection is often used in single bus section and double bus system to meet the requirements or guarantees of users for choosing different operating modes. Selective load removal in case of failure.

(4) PT cabinet

The voltage transformer cabinet is generally installed directly on the bus bar to detect the bus bar voltage and realize the protection function. Mainly install voltage transformer PT, isolating knife, fuse and arrester etc. inside.

(5) Isolation cabinet

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

(6) Capacitor cabinet

Also called the compensation cabinet, it is used to improve the power factor of the power grid, or for reactive power compensation. The main components are groups of capacitor banks, switching control circuits, fuses and other protective electrical appliances connected in parallel. Generally installed in parallel with the incoming cabinet, 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, it is not possible to directly touch the components in the cabinet, especially the capacitor bank; within a certain period of time after the power failure (according to the capacitor bank Depending on the size of the capacitor, such as: 1 minute), re-closing is not allowed to avoid over-voltage damage to the capacitor. When performing automatic control functions, 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 on and off.

(7) Measuring cabinet

Mainly used for measuring electric energy (kwh), and divided into high voltage and low voltage, generally installed with isolating switch, fuse, CT, PT, active watt-hour meter (traditional meter or digital meter), reactive watt-hour Meters, relays, and some other auxiliary secondary equipment (such as load monitors, etc.).

(8) GIS cabinet

It is also called a closed combined electrical cabinet, which is a closed combination of circuit breakers, disconnectors, grounding switches, CT, PT, arresters, busbars, etc. in a metal shell, and then a gas with good insulation performance and arc extinguishing performance (usually six Sulfur fluoride (SF6) is used as an insulation measure between phases and the ground, and is suitable for high-voltage and high-capacity power grids for distribution and control.

(9) Circuit breaker

Under normal working conditions, the circuit breaker is in the closed state (except for special applications) and the circuit is connected. When performing automatic control or protection control operations, the circuit breaker can be operated under the control of the comprehensive protection device to break or connect the circuit. The circuit breaker can not only switch on and off the normal load current, but also can withstand a short-circuit current for a certain period of time (several or even dozens of times the normal working current), and can break the short-circuit current and remove faulty lines and equipment. Therefore, the main function of the circuit breaker is to break and make the circuit (including breaking and making the normal current, breaking the short-circuit current).

Because in the process of breaking and closing the circuit, an arc is inevitably generated 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 dynamic and static contacts of the circuit breaker. According to the different arc extinguishing media, circuit breakers can be divided into: oil circuit breakers (more oil, less oil), sulfur hexafluoride (SF6) circuit breakers, vacuum circuit breakers, air circuit breakers, 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. Since the moving and static contacts of the circuit breaker are generally wrapped in a container filled with arc extinguishing medium, the opening and closing status of the circuit breaker cannot be judged directly, usually through auxiliary devices of the circuit breaker (such as opening and closing position pointers). etc.) to judge.

(10) Isolation switch

Isolation switch (or isolating switch) is mainly used to isolate high-voltage power supply due to its obvious fracture, so as to ensure the safe inspection and repair of lines and equipment. The current that can be broken is very small (generally only a few ampere). Since there is no special arc extinguishing device, it cannot be used to break fault current and normal working current, and it is not allowed to carry out breaking operation with load.

(11) Fuse

 A 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, its own melt melts and cuts off the circuit. Its action principle is simple, easy to install, generally not used alone, mainly used to cooperate with other electrical appliances.

Main action characteristics: First, the current must reach a certain value, which has been prepared before the fuse leaves the factory and cannot be changed; second, a certain time must pass after the current reaches a certain value, which is also made by the manufacturer and cannot be changed. But there are many types, including delayed action, fast action, ultra-fast action, etc.; the third is that 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, and can also be judged by the appearance of the melt ; Commonly used fuses and insurance tubes belong to this category of electrical appliances.

(12) Load switch

The load switch has a simple arc extinguishing device. The arc extinguishing medium is generally air, which can connect and break certain currents and overcurrents, but cannot break short-circuit currents and cannot be used to cut short-circuit faults. Therefore, it is absolutely not allowed to simply use a load switch to replace a circuit breaker; if a load switch is to be used, it must be used in conjunction with the aforementioned high-voltage fuse (in fact, the fuse and the load switch are often used in series for simple overload protection, In order to reduce the project cost).

The load switch is similar to the isolating knife, and there is an obvious disconnection gap, and it is easy to distinguish whether the circuit is on or off.

(13) Transformer

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

Some of the 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, in kVA);

2.Rated voltage: the value of the terminal voltage under no-load and rated tapping (ie primary and secondary side Voltage value);

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

4.No-load current: Under no-load condition, the value of the current flowing through the primary side coil;

5.Short-circuit loss: the primary side is rated Current, the loss caused by the secondary short circuit (mainly caused by the coil resistance);

6.The concept of tapping (tap): In order to meet the needs of power grid operation, general transformers have taps on the high-voltage side, and the voltage values ​​of these taps are all It is expressed as a percentage of the rated voltage, 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%). Generally speaking, there are many taps (tap points) of the on-load tapping transformer, such as 7 taps (±3×2.5%) and 9 taps (±4×2%). Since the synchronous switching of the tap changer cannot be fully guaranteed, the on-load tapping transformers generally cannot be operated in parallel.

7. Active load: the load that generates machine energy or heat energy in the power system. But 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. The load of asynchronous motors and synchronous motors consumes both active power and reactive power. Among them, the part that generates machine energy by work is an active load. The active load shall be supplied by the active power of the generator.

8. Reactive load: the part of the electric load that does not do work. 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 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 balance of reactive power so that the voltage level can be maintained.

9. Accident reserve: one of the components of the reserve capacity in the power system. Because power generation equipment may have temporary or permanent failures that affect 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 disassembly: In order to prevent system out-of-step and accident expansion, a measure to decompose the complete power system into several independent systems that no longer operate synchronously. Some local systems may suffer from power shortage after de-loading, and the frequency and voltage drop, so part of the load needs to be removed to prevent the stability of the entire system from being damaged.

(14) PT (TV) /CT (TA)

The transformer is actually a special transformer, which is mainly used to electrically isolate the primary circuit and the control circuit, thereby expanding the scope of use of secondary equipment (instruments, comprehensive insurance, etc.). The use of PT/CT can prevent the high voltage/high current of the primary circuit from directly entering the secondary control equipment (such as meters, comprehensive protection devices, etc.), and can also prevent the operation of the primary circuit from being affected by the failure of the control equipment.

1. The characteristics of current transformers (CT, TA) are: the primary winding N1 is thick and less, and the secondary winding N2 is thin and more. The rated current I2 of the secondary side is generally 5A (according to N1I1=N2I2, it can be calculated approximately once Side current I1, or select a current transformer with corresponding transformation ratio according to the primary side current I1). Since the primary winding and the secondary winding are connected in series in the primary circuit and the secondary control circuit respectively when the CT is working, according to the characteristics of the transformer U1I1=U2I2, it can be obtained that the working voltage of the secondary side is at It is very large when open circuit, so CT is absolutely not allowed to open circuit. Divided according to the purpose, it can usually be divided into protection and measurement CT. The measuring CT is easy to saturate when a short circuit fault occurs in the primary circuit to limit the excessive secondary current (secondary winding side current I2) to achieve the purpose of protecting the comprehensive protection device; and to protect the CT when a short circuit fault occurs in the primary circuit, it should not be There is a protection phenomenon to ensure the reliable operation of the comprehensive protection device.

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

3. The characteristics of voltage transformers (PT, TV) are: the number of turns of the primary winding is more N1, and the number of turns of the secondary winding is less, which is equivalent to a step-down transformer (the rated voltage of the secondary side is generally 100V). Since the primary winding and secondary winding of the PT are connected in parallel with the voltage coil of the primary circuit and the secondary control circuit respectively, and because 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 similar In the no-load state; 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 the PT must not be short-circuited.

(15) Handcart/Drawer

The handcart and the drawer are part of the high-voltage switchgear and the low-voltage switchgear respectively, and are respectively installed with high-voltage circuit breakers, low-voltage circuit breakers and their relays and other components. This divides the handcart switchgear (high voltage) and the drawer switchgear (low voltage). Their functions are basically the same as those of the fixed switchgear. The main difference is that it facilitates maintenance and repair (both handcarts and drawers can pass through mechanical The operating mechanism shakes the handle to push in and pull out). Handcarts and drawers generally have three position states: working (during normal operation), testing (during trial operation and field testing), and exiting (during maintenance and overhaul).

(16) Grounding knifeThe grounding knife (also called earthing switch) is mainly used for grounding in order to ensure the safety of personnel during line and equipment maintenance; second, it can be used to artificially cause a system grounding short circuit to achieve the purpose of control and protection.

 The grounding knife is usually connected to the high-voltage side of the step-down transformer. When the receiving end fails or the transformer’s internal fault occurs, the grounding knife switch should automatically close, causing a grounding short-circuit fault and forcing the transmitting end ( The circuit breaker at the upper end acts quickly to cut off the fault, so this is a personal earth short-circuit fault, and the purpose is to ensure that the circuit breaker at the transmission end can act quickly.

(17) Master electrical appliances

The master electrical appliance is a mechanically operated control appliance that issues control commands to various electrical systems for the conversion and transmission of various signals in the system. Commonly used transfer switches, buttons, rotary switches, position switches, and signal lights are all Belongs to the scope of the main electrical appliances.

(18) Contactor

Contactor is an electrical appliance used to frequently switch on and off AC and DC main circuits and large-capacity control circuits at a long distance. The main control objects are motors, lighting, capacitor banks, etc., divided into AC contactors and DC contactors. Compared with the circuit breaker, the difference is: the frequency of action is very high (so the electrical life and mechanical life are required to be long enough); it has a higher breaking and making capacity, but it is generally used at voltage levels of 1kV and below It cannot be compared with the tens of kilovolts and hundreds of kilovolts of circuit breakers.

(19) Relay

Relays are used to control the action of other electrical appliances (usually primary electrical equipment) in the control loop or as electrical appliances for protection and signal conversion in the main circuit. They are only suitable for remote disconnection and connection of small-capacity control loops. , Such as: AC/DC current relay, voltage relay, time relay, intermediate relay, thermal relay, etc.

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