The voltage levels of the power system are 220/380V(0.4 kV), 3 kV, 6 kV, 10 kV, 20 kV, 35 kV, 66 kV, 10 kV, 220 kV, 330 kV and 500 kV. With the improvement of motor manufacturing technology, 10 kV motors have been mass-produced, so 3 kV and 6 kV are rarely used, and 20 kV and 66 kV are also rarely used. The power supply system is dominated by 10 kV and 35 kV. The transmission and distribution system is mainly above 1 10 kV. There are two kinds of generators in power plants: 6 kV and 10 kV. Now 10 kV is the main generator, and all users are 220/380V(0.4 kV) low-voltage systems.
According to the urban power grid design rules, the transmission networks are 500 kV, 330 kV, 220kV, 1 10kV, the high-voltage distribution networks are 10kV, 66kV, and the medium-voltage distribution networks are 20kV, 10kV, 6kV.
The power plant generates 6 kV or 10 kV power, which can be used by the power plant itself (auxiliary power) or sent to users with 10 kV voltage near the power plant. 10 kV power supply range is 10Km, 35 kV is 20~50Km, 66 kV is 30 ~ 10km,10kv is 50~ 150Km, and 220 kV is/.
2. Type of substation
Various voltage levels in the power system are converted by the power transformer, and the voltage rises to the step-up transformer (the substation is a step-up station) and falls to the step-down transformer (the substation is a step-down station). A two-coil transformer with two coils (windings) is used to change one voltage into another, and a three-coil transformer with three coils (windings) is used to change one voltage into two voltages.
In addition to step-up and step-down, substations are also divided into hub stations, regional stations and terminal stations according to their scale. The voltage level of the hub station is generally Grade III (three-winding transformer), 550 kV/220 kV/110 kV. Generally, regional stations also have three voltage levels (three-winding transformers), 220kV/10kV/35kV or110kV/35kV/10kV. Generally, terminal stations directly access users, mostly with two voltage levels (double-coil transformers)110kv/10kv or 35 kV/10 kV. Generally, there are only two voltage levels (double-coil transformers) in users' own substations:110kv/10kv, 35kV /0.4kV, 10kV /0.4kV, of which10kv is the most.
3. Wiring diagram of primary circuit of substation
1) Main wiring type
Substation primary circuit wiring refers to the interconnection mode of all power equipment (transformers, incoming and outgoing line switches, etc.). After the transmission line enters the substation. Its wiring scheme includes: line transformer group, bridge wiring, single bus, single bus section, double bus, double bus section, ring network power supply, etc.
2) Line transformer group
There is only one incoming line and one transformer in the substation. Without further development, the line transformer group is used for wiring.
3) Bridging
There are two incoming lines and two transformers, and bridge wiring is adopted without further development. For the transformer, the tie breaker is connected to the inner bridge inside the two incoming circuit breakers, and the tie breaker is connected to the outer bridge outside the two incoming circuit breakers.
4) Single bus
When the substation has multiple incoming and outgoing lines, a single bus is used. When there are two incoming lines, usually one line supplies power and the other line is standby (when the power supply is different). Two lines can be automatically switched with the power supply of the equipment, and multiple outgoing lines are led out by a bus.
5) Single bus section
When there are more than two incoming lines and multiple outgoing lines, one bus is selected to be segmented, and the two incoming lines are connected to two buses respectively, and the two buses are connected by a bus coupler switch. Lead-out wires are respectively connected to two bus bars.
There are many kinds of single bus sectional operation modes. Generally, one is main and one is standby (not closed), and the bus is connected. When the main power supply is cut off, the standby power supply is turned off, and the main power supply and the standby power supply are interlocked with the bus. When the standby power supply capacity is small, some outgoing lines should be disconnected after the standby power supply is turned off. This is a common mode of operation.
For particularly important loads, power is mainly supplied by two incoming lines, and the bus coupler switch is turned off. When an incoming line is cut off, the bus coupler is turned on, and then the bus coupler is turned off and the online switch is turned on after the call comes in.
Single bus section is also beneficial to the internal maintenance of substation. During maintenance, a section of bus can be stopped. The single bus is not segmented, and the whole station is blacked out during maintenance. The bypass bus can only be used in power system substations.
6) Double bus
Double buses are mainly used in power plants and large substations. Each line is connected to two buses by a circuit breaker through two disconnectors, so that when the buses are overhauled, the disconnectors can be used to pour the lines onto conditional buses. There are also two kinds of double buses: segmented and non-segmented. Double-bus sectioning, with bypass circuit breaker, has complicated wiring mode, but it is very convenient to maintain, which can narrow the power outage range.
4. Secondary circuit of substation
1) type secondary circuit
The secondary loop of substation includes measurement, protection, control and signal loop. Measurement loop includes: measurement measurement and protection measurement. The control loop includes: local manual opening and closing, anti-trip interlock, test, interlock, protective tripping and opening and closing execution. The signal loop includes switch operation status signal, accident trip signal and accident alarm signal.
2) Measuring circuit
The measuring loop is divided into current loop and voltage loop. Various devices in the current loop are connected in series to the secondary side (5A) of the current transformer, which uniformly changes the primary side load current into 5A measurement current. Use their own transformers for measurement and protection (transformers for measurement require high accuracy), and the measurement is connected in series with the current terminals of ammeter, watt-hour meter, power meter and power factor meter. The protection measurement is connected in series with the current terminal of the protection relay. Microcomputer protection generally integrates measurement and protection, and has measuring current terminal and protecting current terminal respectively.
In the voltage measurement loop, the 220/380V low-voltage system is directly connected to 220V or 380V, and all the high-voltage systems above 3KV are converted into a unified 100V voltage through voltage transformers. The voltage coils of voltmeter, watt-hour meter, power meter and power factor meter are connected in parallel to the 100V voltage bus through their terminals. The metering voltage and protection voltage of microcomputer protection device are unified into one voltage terminal.
3) Control loop
(1) on/off circuit
The on-off brake is operated by an on-off switch. Conventional protection is needed to remind operators and give an alarm for accidental tripping. The switch is selected from pre-off-off and pre-on-off multistage switches. In order to use non-corresponding wiring for opening and closing alarm and accident tripping alarm, the state has standard drawing design. After using microcomputer protection, after the remote closing operation, it is necessary to carry out the contraposition operation of the transfer switch on the spot, which loses the significance of the remote closing operation. Therefore, the non-corresponding wiring should be cancelled and a three-speed transfer switch with only closing and opening in the middle should be selected.
(2) Anti-jump circuit
When the closing circuit is disconnected when there is a fault, or the short-circuit accident is not ruled out, and then it is closed (misoperation), then there will be repeated closing and opening of the circuit breaker, which will not only easily cause or expand the accident, but also cause equipment damage or personal accidents, so the control circuit of the high-voltage switch should be designed to prevent tripping. In order to prevent tripping, a double-coil relay with current start and voltage hold is usually selected. The current coil is connected in series with the open circuit as the starting coil. The voltage coil is connected to the closed circuit as a holding coil. When the brake is opened, the current coil is started by the open circuit. If there is a fault in the closing circuit, or in the manual closing position, the voltage coil starts and maintains itself through its normally open contact, and its normally closed contact immediately disconnects the closing circuit, so as to ensure that the circuit breaker cannot be closed again immediately during the opening process. The current loop of the anti-trip relay can also hold the current coil through its normally open contact, which can reduce the disconnection load of the outlet contact of the protection relay and reduce the holding time requirement of the protection relay.
Some microcomputer protection devices have their own anti-jump function, so it is not necessary to design anti-jump circuits. When the spring energy storage is selected as the operating mechanism of the circuit breaker, if the spring energy storage operating mechanism that can be switched on and off once after energy storage is selected (there are also spring energy storage operating mechanisms that can be switched on and off twice for reclosing after energy storage), because energy storage generally takes about 10 second, when the energy storage switch is always in the off position, it can be stored once and switched off once after switching on. After tripping, manual energy storage is needed to switch on. At this point, the anti-trip circuit can no longer be designed.
(3) Interlocking and control of test and mutual investment
For handcart switchgear, after the handcart is started, the circuit breaker opening and closing test should be carried out, and the opening and closing test button should be designed. When the incoming line is disconnected from the bus coupler, it should be interlocked or controlled as required.
(4) protection tripping
The protection trip outlet is connected to the trip circuit through the connector, which is used to protect some protection functions when debugging or running.
(5) Switch braking circuit
The on-off circuit generally provides power for the operating mechanism of the on-off bus, and its control circuit should be drawn separately.
4) signal loop
(1) The switch operation status signal consists of two signal lights installed on the switch cabinet, namely closing and opening instructions. After the switch operation, it is connected to the positive power supply without corresponding wiring. After using microcomputer protection, the transfer switch cancels the corresponding wiring, so the positive pole of the signal lamp can be directly connected to the positive power supply.
(2) There are two kinds of accident signals, namely, accident tripping and accident prediction. The accident trip alarm will also receive the accident trip signal on the bus after the switch does not correspond, and then lead it to the central signal system. The accident alarm signal is led to the central signal system through the contact of the signal relay. After microcomputer protection is adopted, the auxiliary contact of the circuit breaker operating mechanism and the contact of the signal relay are connected to the switch input of the microcomputer protection unit respectively. If a central signal system is needed, it can be led to the central signal system if the microcomputer protection device can provide output contacts for accident tripping and accident alarm. Otherwise, another pair of contacts of the signal relay should be used to lead to the central signal system.
(3) The central signal system is a centralized alarm system installed in the duty room, which consists of two sets of acousto-optic alarms: accident tripping and accident prediction. Optical alarms use light plates instead of signal lamps. Light plates are divided into centralized and decentralized types. After the substation integrated automation system is adopted, the central signal system can not be designed or simplified, and only the centralized alarm can be designed as the backup alarm for computer alarm.
5. Substation relay protection
1) The role of relay protection in substation.
Substation relay protection can quickly and selectively send a trip command to cut off the fault or give an alarm when there is a fault (three-phase short circuit, two-phase short circuit, single-phase grounding, etc.). ) and abnormal phenomena (overload, overvoltage, low voltage, low frequency, gas, overtemperature, disconnection of control and measurement circuits, etc.). ) in the operation of substation, so as to reduce the scope of power outage caused by fault and ensure the stable operation of power system.
2) Basic working principle of substation relay protection.
Substation relay protection is based on the phenomena of current increase, voltage increase or decrease, frequency decrease, gas appearance, temperature increase, etc., which exceed the setting value (given value) or exceeding the limit value of relay protection, and then selectively issue a trip command or alarm signal within the setting time.
According to the current value, the selective tripping is inverse time limit, and the greater the current value, the faster the tripping. Time-selective tripping is called time-limited protection. When the fault current exceeds the set value and a given time passes, a time-limited tripping command appears. Gas and temperature are non-electric protection.
The reliability coefficient is an empirical data. When calculating the relay protection action value, the calculation result should be multiplied by the reliability coefficient to ensure the accuracy and reliability of relay protection action, and its range is 1.3~ 1.5.
The ratio of the minimum value when the fault occurs to the protection action value is the sensitivity coefficient of relay protection, which is generally 1.2~2, and should be selected according to the design specification.
3) Substation relay protection is classified according to the nature of protection.
4) Substation relay protection is classified according to protection objects.
(1) generator protection
Generator protection includes stator winding phase-to-phase short circuit, stator winding grounding, stator winding turn-to-turn short circuit, generator external short circuit, symmetrical overload, stator winding overvoltage, excitation circuit grounding at one point and two points, loss of excitation fault, etc. The exit modes are shutdown, disconnection, narrowing the fault influence range and signaling.
(2) Power transformer protection
Power transformer protection includes phase-to-phase short circuit between winding and its outgoing line, single-phase short circuit at neutral point directly grounded side, winding turn-to-turn short circuit, overcurrent caused by external short circuit in neutral point directly grounded power grid, neutral point overvoltage, overload, oil level drop, transformer temperature rise, oil tank pressure rise or cooling system failure.
(3) Line protection
According to different voltage levels, the neutral grounding mode of power grid is different, and the length of transmission lines and cables or overhead lines is also different, including phase-to-phase short circuit, single-phase grounding short circuit, single-phase grounding and overload.
(4) busbar protection
Busbars in power plants and important substations should be equipped with special busbar protection.
(5) Power capacitor protection
Power capacitors have internal faults of capacitors and short circuit of their outgoing wires, short circuit of connecting wires between capacitor banks and circuit breakers, overvoltage caused by removal of faulty capacitors in capacitor banks, overvoltage of capacitor banks and voltage loss of connecting buses.
(6) High voltage motor protection
High-voltage motor has stator winding interphase short circuit, stator winding single-phase grounding, stator winding overload, stator winding low voltage, synchronous motor out of step, synchronous motor loss of excitation, synchronous motor asynchronous impulse current.
6. Microcomputer protection device
1) Advantages of microcomputer protection
(1) High reliability: A microcomputer protection unit can complete various protection and monitoring functions. It replaces a variety of protective relays and measuring instruments, simplifies the wiring between the switch cabinet and the control panel, thus reducing the fault links of related equipment and improving the reliability. The microcomputer protection unit adopts a highly integrated chip, and the software has the functions of automatic detection and automatic error correction, which also improves the reliability of protection.
(2) High precision, high speed and multifunction. The digitization of the measurement part greatly improves its accuracy. With the improvement of CPU speed, various events can be timed by m s, and the improvement of software functions can complete various protection functions through various complex algorithms.
(3) Great flexibility, the protection and control characteristics can be easily changed by software, and various interlocks can be realized by logical judgment. One type of hardware can form different types of protection by using different software.
(4) The maintenance and debugging are convenient, the hardware types are few, the lines are unified, and the external wiring is simple, which greatly reduces the maintenance workload. Using input button or upper computer for protection debugging and setting, debugging is simple and convenient.
(5) Good economy and high cost performance. Because of the universality of microcomputer protection, the comprehensive cost of measurement, control and protection of substation is reduced. High reliability and high speed can reduce power outage time, save manpower and improve economic benefits.
2) Characteristics of microcomputer protection device
In addition to the advantages of the above-mentioned microcomputer protection, the microcomputer protection device has the following characteristics compared with similar products:
(1) complete varieties: microcomputer protection devices, especially complete varieties, can meet all kinds of protection requirements of all kinds of equipment in various types of distribution stations, which provides great convenience for the design of distribution stations and computer networking.
(2) The hardware adopts the latest chips to improve the technological advancement. The CPU adopts 80C 196KB, and the measurement is 14-bit A/D conversion. There are as many as 24 analog input circuits. The collected data is processed by DSP signal processing chip, and the fundamental wave to the eighth harmonic wave is obtained by high-speed Fourier transform. Special software automatically corrects to ensure the high accuracy of measurement. Dual-port RAM and CPU are used for data conversion to form a multi-CPU system, and CAN bus is used for communication. It has the characteristics of fast communication speed (up to 100MHZ, generally working at 80 or 60MHZ) and strong anti-interference ability. Through the keyboard and LCD unit, it is convenient to observe the scene and set various protection modes and parameters.
(3) The hardware design adopts special isolation and anti-interference measures in power supply, analog input, switch input and output, communication interface, etc. , strong anti-interference ability, in addition to centralized screen installation, can be directly installed on the switchgear.
(4) The software is rich in functions. In addition to various measurement and protection functions, it can also cooperate with the upper computer to complete fault recording (1 second high-speed fault recording and 9-second fault dynamic recording), harmonic analysis and small current grounding line selection.
(5) RS232 and CAN communication modes can be selected to support various telecontrol transmission protocols and facilitate networking with various computer management systems.
(6) The large-screen LCD with 240× 128 wide temperature background is adopted, which is convenient to operate and beautiful to display.
(7) High integration, small volume and light weight, which is convenient for centralized installation of panels and decentralized installation on switchgear.
3) Application scope of microcomputer protection device
(1) Small and medium-sized power plants and their step-up substations.
(2) 1 10 kV /35 kV/10 kV regional substation.
(3) City 10 kV power grid 10 kV switching station
(4) User110 kV/10 kV or 35kV/10kV main step-down station.
(5) User 10kV Substation
4) Type of microcomputer protection device
(1) There are four types of microcomputer protection devices.
(2) Line protection device
Microcomputer line protection device microcomputer capacitance protection device microcomputer directional line protection device
Microcomputer zero sequence distance line protection device microcomputer transverse differential current direction line protection device
(3) main equipment protection device
Microcomputer differential protection device for two-winding transformer Microcomputer differential protection device for three-winding transformer
Microcomputer transformer backup protection device microcomputer generator differential protection device microcomputer generator backup protection device
Microcomputer generator backup protection device microcomputer motor differential protection device microcomputer motor protection device
Microcomputer factory (station) transformer protection device
(4) Measurement and control device
Microcomputer telemetry and remote control device microcomputer remote control signal remote control device microcomputer remote control device microcomputer automatic quasi-synchronous device
Microcomputer standby automatic switching device microcomputer PT switching device microcomputer pulse power measuring device
Microcomputer multifunctional transmission measuring device microcomputer disconnecting device
(5) Management device unit
Communication unit management unit dual-machine management unit
5) Function of microcomputer protection device
The general technical requirements and indicators (working environment, power supply, technical parameters and device structure) and main functions (protection performance indicators, main protection functions, protection principles, setting values and parameters, external terminals and secondary drawings) of microcomputer protection devices are detailed in the relevant product manuals.
Microcomputer monitoring system for 7.220/380 V low-voltage distribution system
1) Characteristics of 220/380 V low-voltage distribution system
(1) is widely used. At present, except for mines, medical treatment and dangerous goods warehouses, industrial and civil electricity consumption is 220/380V, so it is widely used.
(2) The low-voltage distribution system is generally TN-S or TN-C-S system. TN-C system consists of three-phase lines (A, B, C) and a neutral line (N), which is repeatedly grounded at the neutral point of transformer or at the entrance of building. The transmission line has four wires and the cable has four cores. There is no protective ground wire (PE) and one wire is missing. The protective grounding of the equipment shell and the metal conductive part is connected with the neutral wire (N), which is called the zero connection system. The zero connection system has poor security and great interference to electronic equipment, and it has been stipulated in the design specification that it will no longer be used.
TN-S system consists of three phase wires, a neutral wire (N) and a protective grounding wire (PE). N-line and PE-line are grounded centrally at the neutral point of transformer or repeatedly at the incoming line of building. There are five transmission lines and five cables. After the neutral wire (N) and the protective ground wire (PE) are connected together at the grounding point, there can be no connection, so the neutral wire (N) must also be insulated. If the neutral wire (N) is not insulated from the ground, or connected to the protective ground after being led out, it is also a TN-C system, so special attention should be paid. TN-S or TN-C-S system has good security and little interference to electronic equipment, so CPE can be used. After equipotential bonding, the safety is better and the interference is less. Therefore, the design specification stipulates that TN-S or TN-C-S system should be adopted except for special places.
(3) The protection of 220/380 V low-voltage distribution system still uses low-voltage circuit breakers or fuses. Therefore, 220/380V has only monitoring and no protection. The monitoring contents include current, voltage, wattage, frequency, power, power factor, temperature measurement (telemetry), switch operation state, accident trip, alarm and accident prediction (overload, overtemperature, etc.). ), alarm (remote communication), electrical switch (referred to as "three remote") unprotected remote switch operation (remote control).
(4) The primary circuit of 220/380 V low-voltage distribution system is generally single bus or single bus section, and more than two transformers are single bus sections, and several transformers are divided into several sections. This is because transformers in user substations generally do not run in parallel, in order to reduce short-circuit current and short-circuit capacity, otherwise the breaking capacity of low-voltage circuit breakers will increase.
(5) In the 220/380 V low-voltage distribution system, the incoming line, bus coupler, high-load outgoing line and low-voltage tie line usually occupy a low-voltage cabinet (1 circuit breaker) due to their large capacity. According to the magnitude of power supply load current, the low-voltage switchgear has two sockets (two circuit breakers installed), four sockets (four circuit breakers installed) and five, six, eight and ten sockets, unlike the high-voltage distribution system where one circuit breaker occupies one switchgear. Therefore, the low-voltage monitoring unit should be divided into one, two or more channels, and designed according to the number of outlet loops of each low-voltage switch and the specifications of the low-voltage monitoring unit.
(6) In addition to manual operation, low-voltage circuit breakers can also be operated electrically. Large-capacity low-voltage circuit breakers are generally operated manually and electrically. In the design, low-voltage monitoring unit with remote control and low-capacity low-voltage circuit breaker should be selected. In the design, most circuit breakers choose manual operation, which can disconnect the remote control outlet of the low-voltage monitoring unit, or choose the low-voltage monitoring unit without remote control.
2) Design of microcomputer monitoring system for 220/380 V low-voltage distribution system.
(1) The microcomputer monitoring system of 220/380 V low-voltage distribution system is designed for telemetry, remote communication and remote control according to the requirements of primary system and users.
(2) Design of measuring circuit
The secondary wiring of measuring part A is the same as that of high voltage. The current loop is connected in series with the secondary loop of the voltage transformer, and the voltage loop is connected in parallel with the voltage measuring loop. Since there is no voltage transformer in the 220/380V low-voltage distribution system, the voltage measurement can be directly connected to the 220/380V bus. Like the voltage circuit of watt-hour meter, it is generally not necessary to add fuse protection, but the wiring in the cabinet should be as short as possible, and it is best to add fuse protection when conditions permit, which is convenient for maintenance.
B pulse watt-hour meter with power supply and pulse output can be selected for electric energy measurement. For the low-voltage monitoring unit with the function of calculating power and electric quantity, the pulse watt-hour meter can not be used when it is only used for internal billing.
C choose the low-voltage monitoring unit with display function, and you can no longer design the current and voltmeter. When selecting a low-voltage monitoring unit without display function, current or voltmeter should also be designed, not both.
(3) signal loop design
In the design, a pair of normally open contacts should be added to the low-voltage circuit breaker and connected to the switch state input end of the low-voltage monitoring unit. If there is an accident trip alarm output contact, connect it to the accident prediction terminal of the low-voltage monitoring unit.
(4) Design of remote control circuit
The remote control design of low-voltage monitoring system is relatively simple. All electric low-voltage circuit breakers have a pair of on/off buttons. Just connect the on/off output terminals of the low-voltage monitoring unit to the on/off button respectively. If necessary, a local and remote control switch can be designed to prevent accidents caused by remote control operation when the switch is overhauled locally.
(5) Design of power supply and communication cable
The power supply of the low-voltage monitoring unit is 220V AC, and the power consumption is generally only a few watts. In the design, the power supply is led from the terminal to the 220V/5A two-pole low-voltage circuit breaker, then to the switch cabinet terminal, and then to the low-capacity socket of the low-voltage cabinet with KVV-3× 1.0 cable. If necessary, you can add UPS power.
Generally, KVV-3× 1.0 ordinary shielded control cable can be selected for communication cables with a distance less than 200m, and shielded twisted pair (preferably sheathed) or computer communication cable should be selected when it exceeds 200m.
8. Substation integrated automation system
1) system
High voltage is protected by microcomputer, low voltage is protected by monitoring unit, and then connected with computer through communication cable, so a modern substation management system-substation integrated automation system can be formed.
2) Design content of substation integrated automation system.
Selection of high voltage microcomputer protection device (combined screen or installed on switch cabinet) and quadratic diagram design.
B. Selection of low-voltage microcomputer monitoring unit (installed on switch cabinet) and design of quadratic diagram.
C management computer (in the duty room, can be placed in the power dispatching room when no one is on duty) selection.
D simulation screen (placed in the duty room or dispatching room) design.
E. Design of the upper computer networking scheme (networking with the factory computer or power department dispatching).
F communication cable design (including management computer and upper computer).
3) Manage the computer
The management computer can be configured according to the system requirements.
4) Analog disk
When users need an analog disk, they can design an analog disk. Small systems can be installed on the wall and large systems can be installed on the floor. The size of the simulation disk is determined according to the primary diagram of the power supply system and the area of the duty room. The simulator uses a special control unit to lead its communication cable to the management computer. The analog disk also needs an AC 220V power supply with a capacity of only tens of watts, which should be considered together with the management of computer power supply when designing.
5) Main functions of substation integrated automation system.
The management computer of substation integrated automation system exchanges information with all microcomputer protection and monitoring units installed on the site through communication cables. The management computer can issue remote control operation commands and modify related parameters, and accept telemetry, remote communication and accident information from the microcomputer protection and monitoring unit at any time. The management computer can process, save, print records and display pictures, analyze the operation of the system, find and handle accidents at any time through remote communication, reduce the power outage time, distribute the load reasonably through remote control, and realize optimal operation, which provides the necessary conditions for realizing modern management.
Management computer software should be standardized, easy to operate, good man-machine interface, convenient configuration, convenient for users to use and secondary development, and easy to master.