Power system overvoltage mainly divided into the following types: atmospheric over-voltage, over-voltage power, over-voltage operation, resonant over-voltage.
Atmospheric overvoltage: Suddenly added to the power system by direct lightning or lightning induction, so that the electrical equipment withstands voltages well beyond its rated value. Atmospheric overvoltage can be divided into direct lightning overvoltage and induced lightning overvoltage. After the power system is exposed to atmospheric overvoltage, it can cause breakdown or flashover of the insulation of the transmission and distribution lines and electrical equipment, resulting in power outages that endanger human life safety. The feature is short duration, strong impact, and has a direct relationship with the intensity of lightning activity, and has nothing to do with the voltage level of the equipment. Therefore, the insulation level of the system below 220KV is often determined by preventing atmospheric overvoltage. To prevent atmospheric over-voltage, lightning rods, lightning conductors, lightning arresters are usually installed, the insulation level of the line is reasonably improved, and automatic reclosing devices are used.
Power frequency over-voltage: The frequency that occurs in the system during an operation or ground fault is equal to the power frequency (50 Hz) or an overvoltage above the maximum operating voltage of the system near the power frequency. It is characterized by long duration and low overvoltage, which is generally not dangerous for equipment insulation, but it plays an important role in determining the insulation level at ultra-high voltage and long-distance transmission. Within hundreds of milliseconds after system operation and grounding trip, Because the magnetic flux in the generator can not be abruptly changed, the inertial effect of the automatic voltage regulator of the generator keeps the generator electromotive force constant. The over-voltage of the power frequency during this period is called the temporary power frequency over-voltage. With the increase of time, the automatic voltage regulator of the generator produces a function to make the generator electromotive force decrease and tend to be stable. At this time, the power frequency over-voltage is called the steady-state power frequency over-voltage. The main reasons for generating over-voltage are: capacitance effect of no-load long line, positive sequence, negative sequence and zero-sequence voltage component caused by asymmetrical grounding, sudden load rejection of the system, acceleration of the generator, etc. Limiting the power frequency over-voltage should adopt special measures for specific situations. Common methods include using shunt reactors to compensate the capacitance effect of no-load long-line, selecting a reasonable system neutral point operation mode, and performing rapid voltage adjustment control of the generator, etc. Wait.
Operation over-voltage: Due to the operation (such as circuit breaker closing and opening), fault or other reasons, the system parameters suddenly change, the system changes from one state to another state, the electromagnetic system itself in the transition process The overvoltage that can oscillate. The characteristics are random, but the overvoltage factor is higher in the worst case. Operating overvoltage causes and avoidance measures
1. The overvoltage of the power grid is generally caused by the following reasons:
A. Line closing and reclosing; B. No-load transformer and shunt reactor opening; C. Line asymmetric fault opening and oscillation solution; D. No-load line opening. Line closing and reclosing over-voltage have a significant impact on the insulation coordination of grid equipment. Circuit breakers with closing resistors should be used to limit this over-voltage. The lightning arrester can be used as a backup protection device for the overvoltage of the substation electrical equipment. The arrester is also the lightning overvoltage protection device of the substation. The design class A, C overvoltage should be predicted with the grid conditions.
2. Line closing and reclosing operation overvoltage
When the no-load line is closed, due to the line inductance-capacity oscillation, the closing overvoltage will be generated. When the lines are coincident, due to the high potential of the power supply and the presence of residual charges on the line, this electromagnetic oscillation process is exacerbated and the overvoltage is further increased. Therefore, circuit breakers should be installed closing resistors to effectively reduce the closing and reclosing over-voltage. The over-voltage distribution of no-load line closing, single-phase reclosing and successful and unsuccessful three-phase reclosing (if used during operation) should be obtained according to the grid prediction conditions, and the relative ground and phase between the receiving ends of the line should be obtained. Statistical operation overvoltage. The conditions for predicting such overvoltages for operation are as follows: A. No-load circuit is closed, before the circuit breaker is closed, the power bus voltage is the highest voltage in the grid; B. Before the successful three-phase reclosing, the receiver receives single-phase Ground fault; single-phase ground fault at the receiving end of the line during unsuccessful three-phase reclosing. For no-load line closing, single-phase reclosing and successful three-phase reclosing (if used during operation), the relative statistical operation overvoltage generated at the receiving end of the line shall not be greater than 22 UXG.
3, breaking the operating voltage of no-load transformers and shunt reactors
Due to the overvoltage caused by forced extinguishment when the breaker is breaking the inductive current of these devices, it should be determined according to the factors such as the circuit breaker structure, loop parameters, and the wiring and characteristics of the transformer (shunt reactor). This operating overvoltage can generally be limited by an arrester installed between the circuit breaker and the transformer (shunt reactor). For the transformer, the lightning arrester can be installed on the low-voltage side or the high-voltage side, but if the neutral point grounding mode of the high- and low-voltage power grids is different, the magnetic low-pressure side should adopt the magnetic blower type arrester. When the arrester may be frequently actuated, a circuit breaker with a high value of opening resistance should be used.
4, line asymmetric fault trip and oscillation solution operation over-voltage
Weak connections to the grid-side receivers, such as tripping due to asymmetrical line faults, or unbundling under grid conditions, will result in tripping of the line asymmetrical fault or oscillation overvoltage. Predict the overvoltage of the asymmetric fault line opening, and select the conditions for the single-phase ground fault at the receiving end of the line. When the gate is open, the power difference of the power supply and receiving ends of the line should be selected according to the actual situation. The circuit breaker with opening resistance can reduce the line asymmetrical fault opening and oscillation solution overvoltage. When this condition is not met, it should be limited by lightning arresters installed on the line.
5, for over-load line over-voltage
Use a circuit breaker that does not reignite when the power is applied to earth at a voltage of 1.3 UXG to prevent it.
6, substation should install arrester
To prevent over-voltage damage to electrical equipment. The installation position is as follows: A. On the line side of each line of the outgoing circuit breaker, the lightning arrester installed at this position is called the line arrester; B. The side of the outgoing line breaker is at the substation, and the arrester installed at this position is the substation. lightning arrester. The location and quantity of all lightning arresters should be determined in conjunction with 4.4.2. Note: When there is no shunt reactor at the entrance of the line, it is predicted (to consider the one-phase fault condition of the closing resistor when closing the circuit breaker)
Where the overvoltage does not exceed the overvoltage protection level of the arrester, it is not necessary to install the arrester there.
7. Rated voltage of arrester with series gap
Should not be lower than the installation point of the grid power frequency over-voltage level.
8, the application of metal oxide arrester to limit the operation of over-voltage
Should refer to the manufacturer's product manual, its long-term operating voltage, power frequency over-voltage, resonant over-voltage allowable duration to meet the power grid requirements.
9, arrester operation over-voltage flow capacity
The allowable energy absorption shall comply with the requirements of the power grid (conditions for the failure of one-phase closing of the closing resistor shall be taken into account for the closing of the circuit breaker). In addition, it should also check whether the voltage on the arrester exceeds its specified protection level. When it exceeds, its effect on insulation coordination should be considered.
10, to monitor the power grid over-voltage operation
Resonance over-voltage and over-voltage operation should be installed in the substation over-voltage waveform or amplitude automatic recording device, and properly collect the measured results.
The measures for limiting the overvoltage are: selecting the high voltage switch with strong arc extinguishing ability; improving the synchronous performance of the switching action; installing the parallel resistance of the switch fracture; using a lightning arrester with good performance, such as a zinc oxide arrester; directly grounding the neutral point of the power grid run.
Resonance overvoltage: Some inductance and capacitance elements in the power system can form various types of oscillation circuits when the system is operating or in the event of a fault. Under certain energy sources, series resonance phenomenon will occur, leading to severe overvoltage in some components of the system. . Resonant overvoltage is divided into the following types:
(1) Linear resonant over-voltage resonant circuits are inductive components (such as arc-extinguishing coils) and systems with iron cores (inductance of the transmission line, leakage inductance of the transformer) or excitation with near-linear excitation characteristics The capacitive element consists of.
(2) Ferro-resonant over-voltage resonant circuit consists of inductive components with iron core (such as no-load transformers, voltage transformers) and capacitive components of the system. Due to the saturation phenomenon of the core inductance component, the inductance parameter of the loop is non-linear, and the circuit containing the nonlinear inductance component will generate a ferromagnetic resonance when a certain resonance condition is satisfied.
(3) Parametric resonant overvoltage An inductive element (such as a salient-amplifier whose synchronous reactance varies between Xd and Xq) and a system capacitive element (such as an idle circuit) make a loop when an inductance parameter makes a periodic change, when the parameter matches At the same time, through the periodical change of the inductance, energy is continuously transmitted to the resonant system, resulting in a parameter resonant overvoltage.
The main measures to limit the resonant overvoltage are:
(1) Improving the synchronization of switching operations Since many resonant overvoltages are caused under non-full-phase operating conditions, the synchronization of switching operations is increased to prevent non-full-phase operation, and the occurrence of resonant overvoltage can be effectively prevented.
(2) Install a small reactance at the neutral point of the parallel high-voltage reactor. With this measure, the power frequency voltage transfer and series resonance during non-full-phase operation can be blocked. (3) Destroy the generator to generate self-excitation conditions to prevent the parameters from resonance overvoltage.
Atmospheric overvoltage: Suddenly added to the power system by direct lightning or lightning induction, so that the electrical equipment withstands voltages well beyond its rated value. Atmospheric overvoltage can be divided into direct lightning overvoltage and induced lightning overvoltage. After the power system is exposed to atmospheric overvoltage, it can cause breakdown or flashover of the insulation of the transmission and distribution lines and electrical equipment, resulting in power outages that endanger human life safety. The feature is short duration, strong impact, and has a direct relationship with the intensity of lightning activity, and has nothing to do with the voltage level of the equipment. Therefore, the insulation level of the system below 220KV is often determined by preventing atmospheric overvoltage. To prevent atmospheric over-voltage, lightning rods, lightning conductors, lightning arresters are usually installed, the insulation level of the line is reasonably improved, and automatic reclosing devices are used.
Power frequency over-voltage: The frequency that occurs in the system during an operation or ground fault is equal to the power frequency (50 Hz) or an overvoltage above the maximum operating voltage of the system near the power frequency. It is characterized by long duration and low overvoltage, which is generally not dangerous for equipment insulation, but it plays an important role in determining the insulation level at ultra-high voltage and long-distance transmission. Within hundreds of milliseconds after system operation and grounding trip, Because the magnetic flux in the generator can not be abruptly changed, the inertial effect of the automatic voltage regulator of the generator keeps the generator electromotive force constant. The over-voltage of the power frequency during this period is called the temporary power frequency over-voltage. With the increase of time, the automatic voltage regulator of the generator produces a function to make the generator electromotive force decrease and tend to be stable. At this time, the power frequency over-voltage is called the steady-state power frequency over-voltage. The main reasons for generating over-voltage are: capacitance effect of no-load long line, positive sequence, negative sequence and zero-sequence voltage component caused by asymmetrical grounding, sudden load rejection of the system, acceleration of the generator, etc. Limiting the power frequency over-voltage should adopt special measures for specific situations. Common methods include using shunt reactors to compensate the capacitance effect of no-load long-line, selecting a reasonable system neutral point operation mode, and performing rapid voltage adjustment control of the generator, etc. Wait.
Operation over-voltage: Due to the operation (such as circuit breaker closing and opening), fault or other reasons, the system parameters suddenly change, the system changes from one state to another state, the electromagnetic system itself in the transition process The overvoltage that can oscillate. The characteristics are random, but the overvoltage factor is higher in the worst case. Operating overvoltage causes and avoidance measures
1. The overvoltage of the power grid is generally caused by the following reasons:
A. Line closing and reclosing; B. No-load transformer and shunt reactor opening; C. Line asymmetric fault opening and oscillation solution; D. No-load line opening. Line closing and reclosing over-voltage have a significant impact on the insulation coordination of grid equipment. Circuit breakers with closing resistors should be used to limit this over-voltage. The lightning arrester can be used as a backup protection device for the overvoltage of the substation electrical equipment. The arrester is also the lightning overvoltage protection device of the substation. The design class A, C overvoltage should be predicted with the grid conditions.
2. Line closing and reclosing operation overvoltage
When the no-load line is closed, due to the line inductance-capacity oscillation, the closing overvoltage will be generated. When the lines are coincident, due to the high potential of the power supply and the presence of residual charges on the line, this electromagnetic oscillation process is exacerbated and the overvoltage is further increased. Therefore, circuit breakers should be installed closing resistors to effectively reduce the closing and reclosing over-voltage. The over-voltage distribution of no-load line closing, single-phase reclosing and successful and unsuccessful three-phase reclosing (if used during operation) should be obtained according to the grid prediction conditions, and the relative ground and phase between the receiving ends of the line should be obtained. Statistical operation overvoltage. The conditions for predicting such overvoltages for operation are as follows: A. No-load circuit is closed, before the circuit breaker is closed, the power bus voltage is the highest voltage in the grid; B. Before the successful three-phase reclosing, the receiver receives single-phase Ground fault; single-phase ground fault at the receiving end of the line during unsuccessful three-phase reclosing. For no-load line closing, single-phase reclosing and successful three-phase reclosing (if used during operation), the relative statistical operation overvoltage generated at the receiving end of the line shall not be greater than 22 UXG.
3, breaking the operating voltage of no-load transformers and shunt reactors
Due to the overvoltage caused by forced extinguishment when the breaker is breaking the inductive current of these devices, it should be determined according to the factors such as the circuit breaker structure, loop parameters, and the wiring and characteristics of the transformer (shunt reactor). This operating overvoltage can generally be limited by an arrester installed between the circuit breaker and the transformer (shunt reactor). For the transformer, the lightning arrester can be installed on the low-voltage side or the high-voltage side, but if the neutral point grounding mode of the high- and low-voltage power grids is different, the magnetic low-pressure side should adopt the magnetic blower type arrester. When the arrester may be frequently actuated, a circuit breaker with a high value of opening resistance should be used.
4, line asymmetric fault trip and oscillation solution operation over-voltage
Weak connections to the grid-side receivers, such as tripping due to asymmetrical line faults, or unbundling under grid conditions, will result in tripping of the line asymmetrical fault or oscillation overvoltage. Predict the overvoltage of the asymmetric fault line opening, and select the conditions for the single-phase ground fault at the receiving end of the line. When the gate is open, the power difference of the power supply and receiving ends of the line should be selected according to the actual situation. The circuit breaker with opening resistance can reduce the line asymmetrical fault opening and oscillation solution overvoltage. When this condition is not met, it should be limited by lightning arresters installed on the line.
5, for over-load line over-voltage
Use a circuit breaker that does not reignite when the power is applied to earth at a voltage of 1.3 UXG to prevent it.
6, substation should install arrester
To prevent over-voltage damage to electrical equipment. The installation position is as follows: A. On the line side of each line of the outgoing circuit breaker, the lightning arrester installed at this position is called the line arrester; B. The side of the outgoing line breaker is at the substation, and the arrester installed at this position is the substation. lightning arrester. The location and quantity of all lightning arresters should be determined in conjunction with 4.4.2. Note: When there is no shunt reactor at the entrance of the line, it is predicted (to consider the one-phase fault condition of the closing resistor when closing the circuit breaker)
Where the overvoltage does not exceed the overvoltage protection level of the arrester, it is not necessary to install the arrester there.
7. Rated voltage of arrester with series gap
Should not be lower than the installation point of the grid power frequency over-voltage level.
8, the application of metal oxide arrester to limit the operation of over-voltage
Should refer to the manufacturer's product manual, its long-term operating voltage, power frequency over-voltage, resonant over-voltage allowable duration to meet the power grid requirements.
9, arrester operation over-voltage flow capacity
The allowable energy absorption shall comply with the requirements of the power grid (conditions for the failure of one-phase closing of the closing resistor shall be taken into account for the closing of the circuit breaker). In addition, it should also check whether the voltage on the arrester exceeds its specified protection level. When it exceeds, its effect on insulation coordination should be considered.
10, to monitor the power grid over-voltage operation
Resonance over-voltage and over-voltage operation should be installed in the substation over-voltage waveform or amplitude automatic recording device, and properly collect the measured results.
The measures for limiting the overvoltage are: selecting the high voltage switch with strong arc extinguishing ability; improving the synchronous performance of the switching action; installing the parallel resistance of the switch fracture; using a lightning arrester with good performance, such as a zinc oxide arrester; directly grounding the neutral point of the power grid run.
Resonance overvoltage: Some inductance and capacitance elements in the power system can form various types of oscillation circuits when the system is operating or in the event of a fault. Under certain energy sources, series resonance phenomenon will occur, leading to severe overvoltage in some components of the system. . Resonant overvoltage is divided into the following types:
(1) Linear resonant over-voltage resonant circuits are inductive components (such as arc-extinguishing coils) and systems with iron cores (inductance of the transmission line, leakage inductance of the transformer) or excitation with near-linear excitation characteristics The capacitive element consists of.
(2) Ferro-resonant over-voltage resonant circuit consists of inductive components with iron core (such as no-load transformers, voltage transformers) and capacitive components of the system. Due to the saturation phenomenon of the core inductance component, the inductance parameter of the loop is non-linear, and the circuit containing the nonlinear inductance component will generate a ferromagnetic resonance when a certain resonance condition is satisfied.
(3) Parametric resonant overvoltage An inductive element (such as a salient-amplifier whose synchronous reactance varies between Xd and Xq) and a system capacitive element (such as an idle circuit) make a loop when an inductance parameter makes a periodic change, when the parameter matches At the same time, through the periodical change of the inductance, energy is continuously transmitted to the resonant system, resulting in a parameter resonant overvoltage.
The main measures to limit the resonant overvoltage are:
(1) Improving the synchronization of switching operations Since many resonant overvoltages are caused under non-full-phase operating conditions, the synchronization of switching operations is increased to prevent non-full-phase operation, and the occurrence of resonant overvoltage can be effectively prevented.
(2) Install a small reactance at the neutral point of the parallel high-voltage reactor. With this measure, the power frequency voltage transfer and series resonance during non-full-phase operation can be blocked. (3) Destroy the generator to generate self-excitation conditions to prevent the parameters from resonance overvoltage.
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