Electric energy has the characteristics of simultaneous production and consumption, and it is necessary for power plants and substations to make necessary adjustments and operations in a timely manner according to load changes. In order to meet the above requirements, a large number of electrical equipment are installed in power plants and substations. Its main electrical equipment can be divided into the following types:
(1) Primary equipment. Equipment for direct production and transmission and distribution is called a primary device. The electric energy is sent by the generator and sent directly to the electric appliance through a series of primary equipment, thereby completing the whole process of production, transportation and use of electric energy. Primary equipment mainly includes generators, transformers, circuit breakers , disconnectors , current limiting reactors, busbars, cables and transformers.
(2) Secondary equipment. Auxiliary equipment that monitors, measures, controls, and protects the operation of a primary device, called a secondary device. Secondary equipment mainly includes instrument poisons, signals, relays and automatic control equipment.
(3) Electrical main wiring. In power plants and substations, in order to meet the needs of various operating modes, various electrical devices are connected in a certain order to be fixedly connected circuits according to requirements. A circuit in which a device is connected is called a primary circuit or an electrical main wiring.
Knife switch
Knife gate switch is a simple low pressure switch, which only manual operation. The knife switch is mainly used in circuits that are not frequently operated. When in use, it is often connected to the circuit in series with the fuse, and the power is cut off by the fuse when an overload or short circuit occurs in the circuit.
There are many types of knife switches. According to the structural features, they can be divided into three types: single pole, double pole and three poles. According to the position characteristics of the handle, they can be divided into three types: the middle handle, the side handle and the lever operation. It is available in both an arc chute and an arc chute.
The knife switch with large rated current is often used in conjunction with components such as air automatic switch in the circuit. During operation, the device is controlled by the automatic switching of the air, such as the pull-and-close operation of the device, and the knife switch only isolates the voltage or cuts off the small current. The role.
insulator
Insulators, commonly known as insulated porcelain bottles, are widely used in power distribution units, transformers, various electrical appliances and power lines in power plants and substations. Insulators are used to support and secure the bare carrier fluid and to insulate the bare conductor from the ground or to insulate the current carrying conductors at different potentials in electrical installations and appliances. Therefore, the insulator should be required to have sufficient dielectric strength, mechanical strength, heat resistance and moisture resistance. Generally high voltage insulators should be able to operate safely at voltages exceeding 15% of their rated voltage.
The high-voltage insulator is usually an insulator made of electrical porcelain. The electrical porcelain has the advantages of compact structure, low water absorption, stable insulation performance and high mechanical strength. The ultra-high voltage insulators are made of high-density porcelain or new silicone rubber materials, which have the advantages of light weight, strong anti-fouling property, high electromechanical strength, small dispersibility, good vibration damping and fatigue resistance, and easy installation.
Insulators can be divided into indoor and outdoor types according to the installation location. Outdoor insulators should have larger sheds due to working environment conditions to increase the creeping discharge distance and block the flow of water to ensure reliable operation of the insulator in harsh climates such as rain, fog and snow. In the presence of severe dust or harmful insulating gases, anti-fouling insulators with special construction should be used. The indoor insulator has no shed structure, so it is only suitable for indoor electrical installations. In order to fix the insulator on the bracket and fix the current-carrying conductor on the insulator, the metal insulator of the insulator is firmly fixed at both ends of the insulator. The metal fittings are glued together with the ceramic insulators with cement glue. The surface of the porcelain insulator is coated with a white or dark brown hard enamel to improve its insulation and water resistance. After the surface enamel of the insulator is damaged, the insulator should be treated or replaced as soon as possible. The exposed surface of the metal attachment of the insulator and the glue of the porcelain insulator shall be coated with a moisture-proofing agent to prevent moisture from entering the adhesive. The surface of the metal fittings needs to be galvanized to prevent metal corrosion.
Busbar
In power plants and substations, conductors that connect generators, transformers, and various electrical appliances are called bus bars. The role of the busbar is to collect, distribute and transmit electrical energy.
The busbar can be divided into a dew busbar and a closed busbar according to its structure; according to its cross section, it can be divided into a rectangular busbar, a tubular busbar and a trough busbar.
Isolation switch
The isolating switch has no special arc extinguishing device and cannot be used to cut off the load current. Otherwise, an arc will be formed between the contacts, which will endanger the safety of the person and equipment and cause an accident.
The main functions of the isolating switch are as follows:
(1) Isolation voltage. Isolation switches can only be used to pull or close circuits that have no voltage and no current. In the event of power failure maintenance, the part to be repaired is reliably disconnected (insulated) from the other parts with voltage by the isolating switch to ensure that the worker can safely overhaul the electrical equipment without affecting the normal operation of the rest of the device.
(2) Switching circuit. When two sets of isolating switches are connected in parallel to the circuit, the load current is allowed to be transferred by the isolating switch under certain conditions.
(3) Pulling and closing a small current. It is allowed to use the isolating switch to pull and close small current circuits that do not generate strong arc between their contacts, such as voltage transformers and arrester circuits, arc suppression coil circuits, etc.
The contacts of the isolating switch are all exposed to the air and the break point is clearly visible. After the isolating switch is disconnected, the breakdown voltage between the moving contact and the static contact of the isolating switch must be greater than the breakdown voltage between the isolating switch phase and the grounding portion to avoid the occurrence of an overvoltage in the circuit. Breakdown to ensure the safety of the maintenance personnel.
High voltage circuit breaker
High-voltage circuit breaker is one of the most important electrical equipment in power plants and substations. It has a complete arc extinguishing device and is a special electrical appliance that connects or disconnects high-voltage circuits under normal and fault conditions.
Electrical main wiring for power plants and substations
The electrical main wiring of a power plant and a substation refers to a circuit in which a primary device in a power plant or substation is connected according to design requirements, also called a main circuit. The form of the main electrical wiring will affect the layout of the power distribution unit, power supply reliability, operational flexibility and secondary wiring, relay protection and other issues. Electrical main wiring plays an important role in the safe, reliable and economic operation of power plants and substations and power systems.
The electrical main wiring of power plants and substations must meet the following basic requirements:
(1) Guarantee the necessary power supply reliability according to the status, role and user nature of power plants and substations in the power system;
(2) The main wiring stress is simple, flexible and easy to operate;
(3) to ensure the safety and convenience of operation, maintenance and overhaul;
(4) Considering economics under the conditions of ensuring the above requirements, that is, occupying less land, minimizing initial investment and saving operating costs;
(5) Meet the requirements for expansion.
The electrical main wiring diagram is generally drawn as a one-line diagram, and the partial diagram is drawn into a three-line diagram only when it is necessary to indicate that the three-phase circuit is asymmetrically connected. In the control room of a power plant or substation, in order to indicate the actual operation of the main wiring of the plant (station), a simulation diagram of the electrical main wiring is usually provided. During operation, the operating status displayed by the various electrical devices in the simulation diagram must match the actual operating state.
Typical electrical main wiring can be roughly divided into two types: busbar and busbarless. Busbar type main wiring includes single busbar, double busbar and wiring with bypass busbar; the main busbarless busbar includes bridge, polygon and unit wiring.
Switching operation
(1) The concept of switching operation
The switching operation of an electrical device refers to a series of operations that need to be performed when the electrical device transitions from one state to another or changes the mode of operation of the system.
The main contents of the switching operation in the power system are:
(1) The stop and power transmission operation of the power line;
(2) Stop and send operation of power transformers ;
(3) Start, parallel and disengagement operations of the generator;
(4) The loop and the loop of the network;
(5) Reverse bus operation;
(6) The change of the neutral point grounding mode and the adjustment of the arc suppression coil;
(7) Changes in the state of use of relay protection and automatic devices;
(8) Installation and removal of the grounding wire.
The switching operation directly changes the operation mode and operating state of the electrical equipment, which is an important and complicated task. If the wrong operation occurs, it may cause equipment damage, endanger personal safety and cause large-scale power outage to the national economy and people. Property brings huge losses. Therefore, operators must strictly abide by the "Electric Safety Work Regulations Power Plant and Substation Electrical Part" (GB26860-2011) and operating procedures and other rules and regulations, give full play to the technical level and have a high degree of responsibility, take the necessary organization, Technical measures to ensure the safe operation of the power system.
(II) Organization and technical measures for switching operations
(1) Organizational measures means that operational personnel must establish a high sense of responsibility and strong security thinking, and conscientiously implement the operating ticket system, work ticket system, work permit system, work guardianship system, and work interruption, transfer and termination systems. When the gate is operated, the attention must be concentrated and the operating rules must be strictly observed to avoid erroneous operation.
(2) Technical measures refer to the use of anti-misoperation device to meet the requirements of five defenses, that is, to prevent accidental pulling of the circuit breaker, to prevent the load from pulling the isolation switch, to prevent the ground wire from being closed, to prevent the grounding line from being charged, and to prevent mistakes. Into the live interval.
Commonly used anti-misoperation devices are mainly:
1) Mechanical locking device. This is a locking device that achieves the intended purpose by mechanical structural constraints, that is, when one component is operated, the other component cannot be operated.
2) Electromagnetic blocking device. This is a device that uses the auxiliary contacts of the circuit breaker, the isolating switch, the equipment net door and the like to turn on or off the power supply of the isolating switch and the mesh electromagnetic lock to achieve the purpose of blocking.
3) Electrical locking device. This is a device that uses the auxiliary contacts of the circuit breaker and the isolating switch to turn the electrical operating power on or off to achieve the purpose of blocking.
4) Red and green card locking device. This is to use the control switch's split position to cooperate with the red and green cards to perform positioning and locking, so as to prevent accidental pulling and closing of the circuit breaker.
5) Microcomputer anti-misoperation device. This is a device with a computer simulation disk as its core. During operation, the operator inserts the computer key into the corresponding number lock according to the device number displayed on the computer key, and detects whether the object of the operation is correct through the probe. If it is correct, the device number of the operation is flashed and the lock circuit is opened. Or the institution can perform the switching operation. After the operation is finished, the computer key automatically displays the next operation. If the wrong position is taken, the lock cannot be unlocked, and the computer key emits a continuous alarm sound to remind the operator to achieve the purpose of forced blocking.
(3) Basic principles of switching operations
(1) The circuit breaker should be pulled and closed. In the circuit in which the circuit breaker is installed, the circuit breaker should be used for pulling and closing. It is absolutely forbidden to use the isolating switch to cut off the load current.
(2) Pulling and closing sequence of the isolating switches on both sides of the circuit breaker. When the power transmission line is out of power, first open the circuit breaker, then pull off the line side isolation switch, and finally pull off the bus side isolation switch. The order of operation at the time of power transmission is reversed.
(3) When the transformer is powered off, first pull off the load side circuit breaker, then pull the power side circuit breaker. The order of operation at the time of power transmission is reversed.
(4) Before the busbar is powered, the TV of the busbar should be put into operation; before the busbar is powered off, all the loads on the busbar should be transferred before the busbar's TV is stopped. When the inverted busbar is operated, the bus-coupled circuit breaker should be closed. After confirming that the bus-coupled circuit breaker has been assembled, remove the control fuse and then switch the busbar disconnector. Before the bus-coupled circuit breaker is disconnected, it should be checked that the load has been completely transferred. The busbar current meter indication is zero, and then the bus-coupled circuit breaker is pulled open.
(5) The parallel and disconnection operations of the ring network. When the operation is closed, the phase must be phasing first. After the ring is closed, the components can not be overloaded. The voltage of each node does not exceed the specified value. The relay protection device should adapt to the operation mode of the ring network. When the ring is opened, the components after the ring is unwrapped. Should not be overloaded, the voltage of each node does not exceed the specified value, and the relay protection device should adapt to the operation mode of the unwinding.
(6) Operation of only the fuse and the isolating switch circuit. When the power is supplied, the fuse is first applied and then the isolating switch is connected. The power outage sequence is reversed.
Causes and consequences of short-circuit current
The main cause of the short circuit is that the insulation of the current carrying portion of the electrical equipment is damaged. The causes of insulation damage are: various forms of overvoltage, such as direct lightning strikes; natural aging and fouling of insulating materials, poor maintenance of operating personnel and direct mechanical damage.
Other faults in the power system may also directly lead to short-circuits, such as transmission line disconnection and bar-breaking accidents, incorrect operation caused by operating personnel not complying with operational technical regulations and safety regulations, and bare conductors such as birds and small animals may cause Short circuit. The basic phenomenon that occurs when a short circuit occurs in a power system is that the current in the short circuit increases sharply. This current is called a short circuit current. The short-circuit current may reach several times to several tens of times, or even more, of the normal operating current, and the absolute value may reach tens of thousands or even hundreds of thousands of amps. The short-circuit current is basically an inductive current, which will generate a demagnetizing armature reaction, causing the terminal voltage of the generator to drop, and the short-circuit current increases the voltage loss when passing through equipment such as lines, and thus the system voltage increases while the current increases. Will drop significantly.
The above basic phenomena during a short circuit will cause the following serious consequences:
(1) When a short circuit occurs, an arc is often generated, which may burn out the faulty component itself.
(2) When a large short-circuit current passes through the conductor, on the one hand, the conductor may be overheated or even melted, and the insulator may be damaged; on the other hand, a large electric force is generated to act on the conductor to deform or damage the conductor and the device.
(3) When the short circuit occurs, the system voltage will drop greatly, especially the voltage near the short circuit point will drop a lot, which may affect the power supply of some or all users.
(4) When the short circuit in the power system, the sudden change of the power distribution of the system and the serious drop of the voltage may damage the stability of the parallel operation of various power plants, so that the whole system is decomposed into several asynchronous operation parts. Some generators may be overloaded at this time, so some users must be removed. The voltage drops more during a short circuit, and the longer the duration, the greater the possibility of disrupting the stable operation of the entire system.
Busbar selection
(1) Choice of form.
Commonly used busbar materials are copper and aluminum. Copper has low resistivity, corrosion resistance and high mechanical strength. It is a good conductive material, but it is expensive. Therefore, copper busbars are often used in generators with large operating currents, narrow positions, transformer outlets or places with severe corrosion to aluminum. . The price of aluminum is low and the use is common. Generally, the outdoor distribution device uses steel-cored aluminum stranded wire or hard-aluminum tubular busbar; the indoor power distribution device uses a hard bus bar, and its cross section has a rectangular shape, a trough shape and a tubular shape.
The rectangular busbar has good heat dissipation, has certain mechanical strength, is easy to install, and is widely used in power distribution devices below 4000A. In order to reduce the skin effect, the cross section of a single rectangular bus bar does not exceed 1250 mm2. The trough busbar has good mechanical strength, large current carrying capacity and small skin effect, and is generally used in power distribution devices of 4000~8000A. The tubular busbar has a small skin effect coefficient and high mechanical strength. The tube can be cooled by water or ventilation, and the surface curvature of the round tube is small and uniform, so the corona discharge voltage is high, so it is often used for large currents above 8000A and 110kV and Above the high voltage power distribution unit.
(2) Selection of busbar cross section.
For the main busbar and the shorter busbar, only the long-term heat allowable current of the busbar is selected, and the cross sections of the other busbars are generally selected according to the economic current density.
1) The current is allowed to be selected according to the long-term heating of the conductor.
The maximum continuous working current Imax in the loop where the conductor is located shall not be greater than the allowable current Ial of the long-term heating of the conductor, that is, Imax≤Ial, Ial—the long-term allowable current of the conductor at the actual ambient temperature, Ial=KθIn; In——— The allowable current at 25 ° C, that is, the rated current; Iθ - the temperature correction factor, the value can be found in the "Power Engineering Design Manual."
2) Select according to economic current density.
For circuits with a large annual average load, long busbars, and large transmission capacity (such as generators, transformer outlets, etc.), the busbar section should be selected according to the economic current density. Busbars selected by economic current density can reduce their annual calculation costs. The economic current density is related to the material of the conductor and the annual utilization hours of the maximum load.
(1) Primary equipment. Equipment for direct production and transmission and distribution is called a primary device. The electric energy is sent by the generator and sent directly to the electric appliance through a series of primary equipment, thereby completing the whole process of production, transportation and use of electric energy. Primary equipment mainly includes generators, transformers, circuit breakers , disconnectors , current limiting reactors, busbars, cables and transformers.
(2) Secondary equipment. Auxiliary equipment that monitors, measures, controls, and protects the operation of a primary device, called a secondary device. Secondary equipment mainly includes instrument poisons, signals, relays and automatic control equipment.
(3) Electrical main wiring. In power plants and substations, in order to meet the needs of various operating modes, various electrical devices are connected in a certain order to be fixedly connected circuits according to requirements. A circuit in which a device is connected is called a primary circuit or an electrical main wiring.
Knife switch
Knife gate switch is a simple low pressure switch, which only manual operation. The knife switch is mainly used in circuits that are not frequently operated. When in use, it is often connected to the circuit in series with the fuse, and the power is cut off by the fuse when an overload or short circuit occurs in the circuit.
There are many types of knife switches. According to the structural features, they can be divided into three types: single pole, double pole and three poles. According to the position characteristics of the handle, they can be divided into three types: the middle handle, the side handle and the lever operation. It is available in both an arc chute and an arc chute.
The knife switch with large rated current is often used in conjunction with components such as air automatic switch in the circuit. During operation, the device is controlled by the automatic switching of the air, such as the pull-and-close operation of the device, and the knife switch only isolates the voltage or cuts off the small current. The role.
insulator
Insulators, commonly known as insulated porcelain bottles, are widely used in power distribution units, transformers, various electrical appliances and power lines in power plants and substations. Insulators are used to support and secure the bare carrier fluid and to insulate the bare conductor from the ground or to insulate the current carrying conductors at different potentials in electrical installations and appliances. Therefore, the insulator should be required to have sufficient dielectric strength, mechanical strength, heat resistance and moisture resistance. Generally high voltage insulators should be able to operate safely at voltages exceeding 15% of their rated voltage.
The high-voltage insulator is usually an insulator made of electrical porcelain. The electrical porcelain has the advantages of compact structure, low water absorption, stable insulation performance and high mechanical strength. The ultra-high voltage insulators are made of high-density porcelain or new silicone rubber materials, which have the advantages of light weight, strong anti-fouling property, high electromechanical strength, small dispersibility, good vibration damping and fatigue resistance, and easy installation.
Insulators can be divided into indoor and outdoor types according to the installation location. Outdoor insulators should have larger sheds due to working environment conditions to increase the creeping discharge distance and block the flow of water to ensure reliable operation of the insulator in harsh climates such as rain, fog and snow. In the presence of severe dust or harmful insulating gases, anti-fouling insulators with special construction should be used. The indoor insulator has no shed structure, so it is only suitable for indoor electrical installations. In order to fix the insulator on the bracket and fix the current-carrying conductor on the insulator, the metal insulator of the insulator is firmly fixed at both ends of the insulator. The metal fittings are glued together with the ceramic insulators with cement glue. The surface of the porcelain insulator is coated with a white or dark brown hard enamel to improve its insulation and water resistance. After the surface enamel of the insulator is damaged, the insulator should be treated or replaced as soon as possible. The exposed surface of the metal attachment of the insulator and the glue of the porcelain insulator shall be coated with a moisture-proofing agent to prevent moisture from entering the adhesive. The surface of the metal fittings needs to be galvanized to prevent metal corrosion.
Busbar
In power plants and substations, conductors that connect generators, transformers, and various electrical appliances are called bus bars. The role of the busbar is to collect, distribute and transmit electrical energy.
The busbar can be divided into a dew busbar and a closed busbar according to its structure; according to its cross section, it can be divided into a rectangular busbar, a tubular busbar and a trough busbar.
Isolation switch
The isolating switch has no special arc extinguishing device and cannot be used to cut off the load current. Otherwise, an arc will be formed between the contacts, which will endanger the safety of the person and equipment and cause an accident.
The main functions of the isolating switch are as follows:
(1) Isolation voltage. Isolation switches can only be used to pull or close circuits that have no voltage and no current. In the event of power failure maintenance, the part to be repaired is reliably disconnected (insulated) from the other parts with voltage by the isolating switch to ensure that the worker can safely overhaul the electrical equipment without affecting the normal operation of the rest of the device.
(2) Switching circuit. When two sets of isolating switches are connected in parallel to the circuit, the load current is allowed to be transferred by the isolating switch under certain conditions.
(3) Pulling and closing a small current. It is allowed to use the isolating switch to pull and close small current circuits that do not generate strong arc between their contacts, such as voltage transformers and arrester circuits, arc suppression coil circuits, etc.
The contacts of the isolating switch are all exposed to the air and the break point is clearly visible. After the isolating switch is disconnected, the breakdown voltage between the moving contact and the static contact of the isolating switch must be greater than the breakdown voltage between the isolating switch phase and the grounding portion to avoid the occurrence of an overvoltage in the circuit. Breakdown to ensure the safety of the maintenance personnel.
High voltage circuit breaker
High-voltage circuit breaker is one of the most important electrical equipment in power plants and substations. It has a complete arc extinguishing device and is a special electrical appliance that connects or disconnects high-voltage circuits under normal and fault conditions.
Electrical main wiring for power plants and substations
The electrical main wiring of a power plant and a substation refers to a circuit in which a primary device in a power plant or substation is connected according to design requirements, also called a main circuit. The form of the main electrical wiring will affect the layout of the power distribution unit, power supply reliability, operational flexibility and secondary wiring, relay protection and other issues. Electrical main wiring plays an important role in the safe, reliable and economic operation of power plants and substations and power systems.
The electrical main wiring of power plants and substations must meet the following basic requirements:
(1) Guarantee the necessary power supply reliability according to the status, role and user nature of power plants and substations in the power system;
(2) The main wiring stress is simple, flexible and easy to operate;
(3) to ensure the safety and convenience of operation, maintenance and overhaul;
(4) Considering economics under the conditions of ensuring the above requirements, that is, occupying less land, minimizing initial investment and saving operating costs;
(5) Meet the requirements for expansion.
The electrical main wiring diagram is generally drawn as a one-line diagram, and the partial diagram is drawn into a three-line diagram only when it is necessary to indicate that the three-phase circuit is asymmetrically connected. In the control room of a power plant or substation, in order to indicate the actual operation of the main wiring of the plant (station), a simulation diagram of the electrical main wiring is usually provided. During operation, the operating status displayed by the various electrical devices in the simulation diagram must match the actual operating state.
Typical electrical main wiring can be roughly divided into two types: busbar and busbarless. Busbar type main wiring includes single busbar, double busbar and wiring with bypass busbar; the main busbarless busbar includes bridge, polygon and unit wiring.
Switching operation
(1) The concept of switching operation
The switching operation of an electrical device refers to a series of operations that need to be performed when the electrical device transitions from one state to another or changes the mode of operation of the system.
The main contents of the switching operation in the power system are:
(1) The stop and power transmission operation of the power line;
(2) Stop and send operation of power transformers ;
(3) Start, parallel and disengagement operations of the generator;
(4) The loop and the loop of the network;
(5) Reverse bus operation;
(6) The change of the neutral point grounding mode and the adjustment of the arc suppression coil;
(7) Changes in the state of use of relay protection and automatic devices;
(8) Installation and removal of the grounding wire.
The switching operation directly changes the operation mode and operating state of the electrical equipment, which is an important and complicated task. If the wrong operation occurs, it may cause equipment damage, endanger personal safety and cause large-scale power outage to the national economy and people. Property brings huge losses. Therefore, operators must strictly abide by the "Electric Safety Work Regulations Power Plant and Substation Electrical Part" (GB26860-2011) and operating procedures and other rules and regulations, give full play to the technical level and have a high degree of responsibility, take the necessary organization, Technical measures to ensure the safe operation of the power system.
(II) Organization and technical measures for switching operations
(1) Organizational measures means that operational personnel must establish a high sense of responsibility and strong security thinking, and conscientiously implement the operating ticket system, work ticket system, work permit system, work guardianship system, and work interruption, transfer and termination systems. When the gate is operated, the attention must be concentrated and the operating rules must be strictly observed to avoid erroneous operation.
(2) Technical measures refer to the use of anti-misoperation device to meet the requirements of five defenses, that is, to prevent accidental pulling of the circuit breaker, to prevent the load from pulling the isolation switch, to prevent the ground wire from being closed, to prevent the grounding line from being charged, and to prevent mistakes. Into the live interval.
Commonly used anti-misoperation devices are mainly:
1) Mechanical locking device. This is a locking device that achieves the intended purpose by mechanical structural constraints, that is, when one component is operated, the other component cannot be operated.
2) Electromagnetic blocking device. This is a device that uses the auxiliary contacts of the circuit breaker, the isolating switch, the equipment net door and the like to turn on or off the power supply of the isolating switch and the mesh electromagnetic lock to achieve the purpose of blocking.
3) Electrical locking device. This is a device that uses the auxiliary contacts of the circuit breaker and the isolating switch to turn the electrical operating power on or off to achieve the purpose of blocking.
4) Red and green card locking device. This is to use the control switch's split position to cooperate with the red and green cards to perform positioning and locking, so as to prevent accidental pulling and closing of the circuit breaker.
5) Microcomputer anti-misoperation device. This is a device with a computer simulation disk as its core. During operation, the operator inserts the computer key into the corresponding number lock according to the device number displayed on the computer key, and detects whether the object of the operation is correct through the probe. If it is correct, the device number of the operation is flashed and the lock circuit is opened. Or the institution can perform the switching operation. After the operation is finished, the computer key automatically displays the next operation. If the wrong position is taken, the lock cannot be unlocked, and the computer key emits a continuous alarm sound to remind the operator to achieve the purpose of forced blocking.
(3) Basic principles of switching operations
(1) The circuit breaker should be pulled and closed. In the circuit in which the circuit breaker is installed, the circuit breaker should be used for pulling and closing. It is absolutely forbidden to use the isolating switch to cut off the load current.
(2) Pulling and closing sequence of the isolating switches on both sides of the circuit breaker. When the power transmission line is out of power, first open the circuit breaker, then pull off the line side isolation switch, and finally pull off the bus side isolation switch. The order of operation at the time of power transmission is reversed.
(3) When the transformer is powered off, first pull off the load side circuit breaker, then pull the power side circuit breaker. The order of operation at the time of power transmission is reversed.
(4) Before the busbar is powered, the TV of the busbar should be put into operation; before the busbar is powered off, all the loads on the busbar should be transferred before the busbar's TV is stopped. When the inverted busbar is operated, the bus-coupled circuit breaker should be closed. After confirming that the bus-coupled circuit breaker has been assembled, remove the control fuse and then switch the busbar disconnector. Before the bus-coupled circuit breaker is disconnected, it should be checked that the load has been completely transferred. The busbar current meter indication is zero, and then the bus-coupled circuit breaker is pulled open.
(5) The parallel and disconnection operations of the ring network. When the operation is closed, the phase must be phasing first. After the ring is closed, the components can not be overloaded. The voltage of each node does not exceed the specified value. The relay protection device should adapt to the operation mode of the ring network. When the ring is opened, the components after the ring is unwrapped. Should not be overloaded, the voltage of each node does not exceed the specified value, and the relay protection device should adapt to the operation mode of the unwinding.
(6) Operation of only the fuse and the isolating switch circuit. When the power is supplied, the fuse is first applied and then the isolating switch is connected. The power outage sequence is reversed.
Causes and consequences of short-circuit current
The main cause of the short circuit is that the insulation of the current carrying portion of the electrical equipment is damaged. The causes of insulation damage are: various forms of overvoltage, such as direct lightning strikes; natural aging and fouling of insulating materials, poor maintenance of operating personnel and direct mechanical damage.
Other faults in the power system may also directly lead to short-circuits, such as transmission line disconnection and bar-breaking accidents, incorrect operation caused by operating personnel not complying with operational technical regulations and safety regulations, and bare conductors such as birds and small animals may cause Short circuit. The basic phenomenon that occurs when a short circuit occurs in a power system is that the current in the short circuit increases sharply. This current is called a short circuit current. The short-circuit current may reach several times to several tens of times, or even more, of the normal operating current, and the absolute value may reach tens of thousands or even hundreds of thousands of amps. The short-circuit current is basically an inductive current, which will generate a demagnetizing armature reaction, causing the terminal voltage of the generator to drop, and the short-circuit current increases the voltage loss when passing through equipment such as lines, and thus the system voltage increases while the current increases. Will drop significantly.
The above basic phenomena during a short circuit will cause the following serious consequences:
(1) When a short circuit occurs, an arc is often generated, which may burn out the faulty component itself.
(2) When a large short-circuit current passes through the conductor, on the one hand, the conductor may be overheated or even melted, and the insulator may be damaged; on the other hand, a large electric force is generated to act on the conductor to deform or damage the conductor and the device.
(3) When the short circuit occurs, the system voltage will drop greatly, especially the voltage near the short circuit point will drop a lot, which may affect the power supply of some or all users.
(4) When the short circuit in the power system, the sudden change of the power distribution of the system and the serious drop of the voltage may damage the stability of the parallel operation of various power plants, so that the whole system is decomposed into several asynchronous operation parts. Some generators may be overloaded at this time, so some users must be removed. The voltage drops more during a short circuit, and the longer the duration, the greater the possibility of disrupting the stable operation of the entire system.
Busbar selection
(1) Choice of form.
Commonly used busbar materials are copper and aluminum. Copper has low resistivity, corrosion resistance and high mechanical strength. It is a good conductive material, but it is expensive. Therefore, copper busbars are often used in generators with large operating currents, narrow positions, transformer outlets or places with severe corrosion to aluminum. . The price of aluminum is low and the use is common. Generally, the outdoor distribution device uses steel-cored aluminum stranded wire or hard-aluminum tubular busbar; the indoor power distribution device uses a hard bus bar, and its cross section has a rectangular shape, a trough shape and a tubular shape.
The rectangular busbar has good heat dissipation, has certain mechanical strength, is easy to install, and is widely used in power distribution devices below 4000A. In order to reduce the skin effect, the cross section of a single rectangular bus bar does not exceed 1250 mm2. The trough busbar has good mechanical strength, large current carrying capacity and small skin effect, and is generally used in power distribution devices of 4000~8000A. The tubular busbar has a small skin effect coefficient and high mechanical strength. The tube can be cooled by water or ventilation, and the surface curvature of the round tube is small and uniform, so the corona discharge voltage is high, so it is often used for large currents above 8000A and 110kV and Above the high voltage power distribution unit.
(2) Selection of busbar cross section.
For the main busbar and the shorter busbar, only the long-term heat allowable current of the busbar is selected, and the cross sections of the other busbars are generally selected according to the economic current density.
1) The current is allowed to be selected according to the long-term heating of the conductor.
The maximum continuous working current Imax in the loop where the conductor is located shall not be greater than the allowable current Ial of the long-term heating of the conductor, that is, Imax≤Ial, Ial—the long-term allowable current of the conductor at the actual ambient temperature, Ial=KθIn; In——— The allowable current at 25 ° C, that is, the rated current; Iθ - the temperature correction factor, the value can be found in the "Power Engineering Design Manual."
2) Select according to economic current density.
For circuits with a large annual average load, long busbars, and large transmission capacity (such as generators, transformer outlets, etc.), the busbar section should be selected according to the economic current density. Busbars selected by economic current density can reduce their annual calculation costs. The economic current density is related to the material of the conductor and the annual utilization hours of the maximum load.
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