Vocational technical teaching and research on transformer overvoltage and its protection Liu Kai (Xuzhou Technician College, Jiangsu Xuzhou, the abnormal voltage rise of the electricity, is a kind of electromagnetic disturbance phenomenon in the power system. The insulation of electrical equipment is long-term to withstand the working voltage, At the same time, it must be able to withstand a certain range of overvoltage, so as to ensure the safe and reliable operation of the power system. Studying the causes of various overvoltages, predicting their amplitude, and taking measures to limit them is the premise for determining the insulation coordination of the power system. Electrical equipment manufacturing and power system operation are all important.
The overvoltage of the transformer means that the voltage at the time of operation of the transformer exceeds the maximum allowable operating voltage of the transformer itself. The overvoltage of the transformer is often a great hazard to the insulation of the transformer, and even breaks down the insulation.
1. The cause of atmospheric overvoltage is also called lightning overvoltage, external overvoltage, caused by thunderclouds in the atmosphere discharging to the ground. There are two types of direct lightning strike overvoltage and inductive lightning overvoltage.
The lightning strike voltage has a duration of about tens of microseconds and has a pulse characteristic, so it is often called a lightning shock wave. Direct lightning strike voltage is the overvoltage that occurs when lightning flashes directly hit the conductive part of the electrical equipment. A lightning strike hits a live conductor, such as an overhead transmission line conductor, called a direct lightning strike. A lightning strike that strikes a conductor that is normally grounded, such as a transmission line tower, causes its potential to rise and then discharges the live conductor. Direct lightning strike voltage amplitude can reach millions of volts, which will damage the insulation of electrical installations and cause short circuit ground faults.
Inductive lightning overvoltage is the overvoltage induced on the electrical equipment (including secondary equipment, communication equipment) that is not directly struck by lightning due to the sharp change of the space electromagnetic field during the lightning strike. Therefore, overhead transmission lines need to be equipped with lightning protection lines and grounding devices for protection. The lightning protection capability of the transmission line is usually indicated by the line lightning resistance level and the lightning strike rate.
Atmospheric overvoltage is caused by the drastic changes in the electromagnetic field when the transmission line is directly subjected to lightning strikes or thundercloud discharges. When the transmission line is directly subjected to lightning strikes, a large amount of charge (set as a positive charge) carried by the thundercloud falls onto the transmission line through the discharge channel, and a large amount of free charge propagates to both ends of the transmission line, causing an impact overvoltage on the transmission line. Waves, called lightning waves. The speed at which the lightning wave propagates to the ends of the transmission line is close to the speed of light. The duration is only a few tens of microseconds, and the time from zero rise to maximum is only a few microseconds. The typical waveform of a lightning wave is: the section where the curve rises from zero to the maximum is called the wave head, and the falling part is called the wave tail. If the time taken by the wave head is regarded as a quarter cycle of the periodic wave, the lightning wave can be regarded as a periodic wave with extremely high frequency. Thus, when the overvoltage wave reaches the output end of the transformer, it is equivalent to adding a very high frequency to the transformer. This transient process is very fast. At the beginning, due to the high frequency, wL is very large, 1/wC is small, and current flows only from the tantalum capacitor of the high voltage winding and the capacitance to the ground. Since the low voltage winding is close to the core, its capacitance to ground is large (ie, the capacitive reactance is small), and the low voltage winding can be approximated as grounded. When a lightning wave strikes, the voltage distribution along the winding height depends on the ratio of the inter-turn capacitance C to the capacitance. Under normal circumstances, since both capacitors exist, when an overvoltage occurs, part of the current is shunted by the capacitance to ground, so the current of each turn-to-turn capacitor current is not equal, and the current flowing through the inter-turn capacitor is as high as the bottom. The smaller the voltage becomes uneven along the height of the winding, see the figure below: UTX is the voltage across the transformer.
It can be seen from the figure that the initial voltage distribution is very uneven, and a large voltage gradient occurs between the first few turns of the A end of the power line. Therefore, in the first few turns, the insulation between the turn-to-turn insulation and the wire cake is greatly threatened, and the maximum turn-to-turn voltage may be as high as 50,200 times the rated voltage.
2. The cause of internal overvoltage causes an overvoltage caused by a change in the internal operating mode of the power system. There are transient overvoltage, operating overvoltage and resonant overvoltage.
The transient over-voltage is an over-voltage that occurs when the power system undergoes a transition process and then regains some temporary stability due to the operation of the circuit breaker or a short-circuit fault, which is also called the increase of the power frequency voltage. Common ones are: 1 no-load long-line capacitance effect (Ferrande effect). Under the action of power frequency power supply, due to the accumulation of capacitive effects of long-distance no-load lines, the voltage distribution along the line is not equal, and the terminal voltage is the highest. 2 Asymmetric short circuit grounding. When the phase 3-phase of the three-phase transmission line is short-circuited, the voltage on the b and c phases will rise. 3甩 Overload voltage, when the transmission line is forced to suddenly drop the load due to a fault, the overvoltage caused by the power supply electromotive force has not been automatically adjusted in time.
The operating overvoltage is an overvoltage with a short decay and a short duration due to the operation of the circuit breaker or a sudden short circuit. Common ones are: 1 no-load line closing and reclosing overvoltage; 2 cutting off the no-load line overvoltage ; 3 cut off the overvoltage of the no-load transformer; 4 arc grounding overvoltage.
Resonant overvoltage is an overvoltage caused by the energy storage components such as inductors and capacitors in the power system resonating with the power supply frequency under certain wiring modes. Generally divided into: 1 linear resonant overvoltage; 2 ferromagnetic resonant overvoltage; 3 parametric resonant overvoltage.
Now take the step-down transformer no-load pull as an example to illustrate the cause of the internal voltage. According to the folding algorithm of the transformer parameters, when the secondary side (low-voltage side tantalum capacitor is converted to the primary side (high-voltage side), the capacitance conversion value is twice the actual value, so the influence of the secondary side capacitance can be neglected. This is It can be said that the influence of the secondary side can be ignored at no load. As for the primary winding, since the capacitance CFe to the ground per unit length is parallel, the total capacitance to the ground is CFe=ZCFe. The inter-turn capacitor is connected in series, so its total capacitance between turns is: C: In the power transformer, usually CFeCt, so the technical teaching and research in qualitative analysis is the full self-inductance of the primary winding.
When the no-load transformer is pulled from the grid, if the instantaneous value of the no-load current is not equal to zero but a certain value ia, then the corresponding applied voltage instantaneous value is Ua. Then, at the moment of pulling the brake, the capacitor Li is stored. The magnetic field energy is the electric field energy stored in the upper part - since the circuit of the transformer is a circuit in which the inductance and the capacitance CFe are connected in parallel, an electromagnetic oscillation process will occur in the circuit at the moment of pulling. During the oscillation process, when the current is equal to zero at a certain moment, the magnetic field energy is all converted into electric field energy, which is absorbed by the capacitor, and the voltage on the capacitor rises to the maximum value U. When energy loss is not considered, according to the principle of energy conservation Third, the transformer overvoltage protection transformer in the operation, it is inevitable that overvoltage phenomenon will occur. In order to prevent the transformer winding insulation from being broken down during overvoltage, appropriate overvoltage protection measures must be taken. At present, the following measures are mainly used.
A metal oxide arrester is installed at the high voltage end of the transformer, which is characterized by sensitive action, low residual voltage and large flow capacity. When lightning waves invade from the transmission line or when an overvoltage occurs, the arrester operates, and the overvoltage wave is grounded. Bypass, the lightning wave will not invade the transformer, thus protecting the transformer. In the insulation standard of GB311,1 high-voltage transmission and distribution equipment, the insulation level of the transformer is specified as: short-time (1min) power frequency withstand voltage (effective value) of kV transformer is 25kV, lightning impulse withstand voltage (peak) ) is 60kV; 10kV transformer short-time (1min) power frequency withstand voltage (RMS) is 35kV, lightning impulse withstand voltage (peak) is 75kV. "Insulation coordination of national standard GB311.1 high-voltage transmission and distribution equipment The provisions for the insulation coordination of the operating overvoltage of equipment such as transformers are as follows.
(1) Relatively insulated, range 1 equipment, according to the statistical operation of the equipment, the overvoltage level or the operational protection level of the arrester and the insulation characteristics of the equipment, and select a certain matching factor Kc to calculate the rated operational shock resistance of the equipment. The voltage is: 4 = 2+. Therefore: Uc above shows that when the voltage of the pull-up current and the voltage on the capacitor is constant, the inductance of the winding is larger, and the smaller the capacitance to the ground, the higher the over-voltage when the brake is applied. In power systems, the pull-over overvoltage typically does not exceed 3.0.5 times the rated voltage.
Second, the transformer overvoltage hazard Transformer design insulation strength is generally considered to withstand 2.5 times overvoltage.
Therefore, over 2.5 times of overvoltage, regardless of which overvoltage, may damage the transformer insulation. The voltage distribution inside the transformer is greatly affected by the frequency of the voltage and the resistance, inductive reactance and capacitive reactance of the transformer. The capacitive reactance is very large in the case of the power frequency voltage, and the circuit composed of it is equivalent to the open circuit, so it is normal. In the case of the transformer internal voltage distribution, only the resistance and inductance can be considered, and the distribution is basically uniform. The atmospheric overvoltage or the operating overvoltage is basically a shock wave. Because the frequency of the shock wave is very high, the wave front steepness is very large, and the shock wave with a wavefront time of 1.5 ps has a frequency equivalent to 160 kHz. Therefore, under the action of the overvoltage shock wave, the transformer The capacitive reactance is very small, which has a great influence on the distribution of the internal voltage of the transformer. The hazard of the shock wave acting on the transformer winding can be divided into two stages: the initial moment and the oscillation process.
When t = 0, the capacitance of the winding plays a major role, and the effects of resistance and inductance are negligible. When the shock wave enters the high-voltage winding, due to the presence of the capacitance to the ground, the current flowing through the winding between the turns is different, and the voltage distribution at the initial moment causes a large turn-to-turn voltage between the turns of the winding end. Therefore, the insulation between the coils of the first few turns is seriously threatened, and the highest turn-to-turn voltage can reach the rated voltage. When t>0, there is an oscillation phenomenon from the initial voltage distribution to the final voltage distribution. In this process, not only capacitors but also inductors and resistors are applied, and the maximum potential (ground voltage) will appear at different points at different points of the winding. The voltage to the ground at different points of the winding can rise to twice the shock wave voltage value, and the winding may damage the ground insulation. The uniformity of the voltage distribution across the winding is related to the ratio of the winding to ground capacitance and the interturn capacitance. The smaller the ratio, the more uniform the capacitance distribution on the winding.
(2) The following factors should be considered when selecting the matching factor Kc: insulation type and its characteristics; performance index; overvoltage amplitude and distribution characteristics; atmospheric conditions; dispersion and installation quality in equipment production equipment; insulation between life expectancy Aging, test conditions and other unknown factors. For lightning shock, according to China's situation, generally select >=1.4; for operational shock, generally choose K>=1.15. Combined with the provisions of national standards, the insulation coordination factor of 6kV transformer surge arrester can be calculated: Zhao Yulin. High voltage technology. Beijing: China Electric Power Press, 2008. Lu Tiecheng. Power system overvoltage. Beijing: Water Resources and Hydropower Press, 2009.
The overvoltage of the transformer means that the voltage at the time of operation of the transformer exceeds the maximum allowable operating voltage of the transformer itself. The overvoltage of the transformer is often a great hazard to the insulation of the transformer, and even breaks down the insulation.
1. The cause of atmospheric overvoltage is also called lightning overvoltage, external overvoltage, caused by thunderclouds in the atmosphere discharging to the ground. There are two types of direct lightning strike overvoltage and inductive lightning overvoltage.
The lightning strike voltage has a duration of about tens of microseconds and has a pulse characteristic, so it is often called a lightning shock wave. Direct lightning strike voltage is the overvoltage that occurs when lightning flashes directly hit the conductive part of the electrical equipment. A lightning strike hits a live conductor, such as an overhead transmission line conductor, called a direct lightning strike. A lightning strike that strikes a conductor that is normally grounded, such as a transmission line tower, causes its potential to rise and then discharges the live conductor. Direct lightning strike voltage amplitude can reach millions of volts, which will damage the insulation of electrical installations and cause short circuit ground faults.
Inductive lightning overvoltage is the overvoltage induced on the electrical equipment (including secondary equipment, communication equipment) that is not directly struck by lightning due to the sharp change of the space electromagnetic field during the lightning strike. Therefore, overhead transmission lines need to be equipped with lightning protection lines and grounding devices for protection. The lightning protection capability of the transmission line is usually indicated by the line lightning resistance level and the lightning strike rate.
Atmospheric overvoltage is caused by the drastic changes in the electromagnetic field when the transmission line is directly subjected to lightning strikes or thundercloud discharges. When the transmission line is directly subjected to lightning strikes, a large amount of charge (set as a positive charge) carried by the thundercloud falls onto the transmission line through the discharge channel, and a large amount of free charge propagates to both ends of the transmission line, causing an impact overvoltage on the transmission line. Waves, called lightning waves. The speed at which the lightning wave propagates to the ends of the transmission line is close to the speed of light. The duration is only a few tens of microseconds, and the time from zero rise to maximum is only a few microseconds. The typical waveform of a lightning wave is: the section where the curve rises from zero to the maximum is called the wave head, and the falling part is called the wave tail. If the time taken by the wave head is regarded as a quarter cycle of the periodic wave, the lightning wave can be regarded as a periodic wave with extremely high frequency. Thus, when the overvoltage wave reaches the output end of the transformer, it is equivalent to adding a very high frequency to the transformer. This transient process is very fast. At the beginning, due to the high frequency, wL is very large, 1/wC is small, and current flows only from the tantalum capacitor of the high voltage winding and the capacitance to the ground. Since the low voltage winding is close to the core, its capacitance to ground is large (ie, the capacitive reactance is small), and the low voltage winding can be approximated as grounded. When a lightning wave strikes, the voltage distribution along the winding height depends on the ratio of the inter-turn capacitance C to the capacitance. Under normal circumstances, since both capacitors exist, when an overvoltage occurs, part of the current is shunted by the capacitance to ground, so the current of each turn-to-turn capacitor current is not equal, and the current flowing through the inter-turn capacitor is as high as the bottom. The smaller the voltage becomes uneven along the height of the winding, see the figure below: UTX is the voltage across the transformer.
It can be seen from the figure that the initial voltage distribution is very uneven, and a large voltage gradient occurs between the first few turns of the A end of the power line. Therefore, in the first few turns, the insulation between the turn-to-turn insulation and the wire cake is greatly threatened, and the maximum turn-to-turn voltage may be as high as 50,200 times the rated voltage.
2. The cause of internal overvoltage causes an overvoltage caused by a change in the internal operating mode of the power system. There are transient overvoltage, operating overvoltage and resonant overvoltage.
The transient over-voltage is an over-voltage that occurs when the power system undergoes a transition process and then regains some temporary stability due to the operation of the circuit breaker or a short-circuit fault, which is also called the increase of the power frequency voltage. Common ones are: 1 no-load long-line capacitance effect (Ferrande effect). Under the action of power frequency power supply, due to the accumulation of capacitive effects of long-distance no-load lines, the voltage distribution along the line is not equal, and the terminal voltage is the highest. 2 Asymmetric short circuit grounding. When the phase 3-phase of the three-phase transmission line is short-circuited, the voltage on the b and c phases will rise. 3甩 Overload voltage, when the transmission line is forced to suddenly drop the load due to a fault, the overvoltage caused by the power supply electromotive force has not been automatically adjusted in time.
The operating overvoltage is an overvoltage with a short decay and a short duration due to the operation of the circuit breaker or a sudden short circuit. Common ones are: 1 no-load line closing and reclosing overvoltage; 2 cutting off the no-load line overvoltage ; 3 cut off the overvoltage of the no-load transformer; 4 arc grounding overvoltage.
Resonant overvoltage is an overvoltage caused by the energy storage components such as inductors and capacitors in the power system resonating with the power supply frequency under certain wiring modes. Generally divided into: 1 linear resonant overvoltage; 2 ferromagnetic resonant overvoltage; 3 parametric resonant overvoltage.
Now take the step-down transformer no-load pull as an example to illustrate the cause of the internal voltage. According to the folding algorithm of the transformer parameters, when the secondary side (low-voltage side tantalum capacitor is converted to the primary side (high-voltage side), the capacitance conversion value is twice the actual value, so the influence of the secondary side capacitance can be neglected. This is It can be said that the influence of the secondary side can be ignored at no load. As for the primary winding, since the capacitance CFe to the ground per unit length is parallel, the total capacitance to the ground is CFe=ZCFe. The inter-turn capacitor is connected in series, so its total capacitance between turns is: C: In the power transformer, usually CFeCt, so the technical teaching and research in qualitative analysis is the full self-inductance of the primary winding.
When the no-load transformer is pulled from the grid, if the instantaneous value of the no-load current is not equal to zero but a certain value ia, then the corresponding applied voltage instantaneous value is Ua. Then, at the moment of pulling the brake, the capacitor Li is stored. The magnetic field energy is the electric field energy stored in the upper part - since the circuit of the transformer is a circuit in which the inductance and the capacitance CFe are connected in parallel, an electromagnetic oscillation process will occur in the circuit at the moment of pulling. During the oscillation process, when the current is equal to zero at a certain moment, the magnetic field energy is all converted into electric field energy, which is absorbed by the capacitor, and the voltage on the capacitor rises to the maximum value U. When energy loss is not considered, according to the principle of energy conservation Third, the transformer overvoltage protection transformer in the operation, it is inevitable that overvoltage phenomenon will occur. In order to prevent the transformer winding insulation from being broken down during overvoltage, appropriate overvoltage protection measures must be taken. At present, the following measures are mainly used.
A metal oxide arrester is installed at the high voltage end of the transformer, which is characterized by sensitive action, low residual voltage and large flow capacity. When lightning waves invade from the transmission line or when an overvoltage occurs, the arrester operates, and the overvoltage wave is grounded. Bypass, the lightning wave will not invade the transformer, thus protecting the transformer. In the insulation standard of GB311,1 high-voltage transmission and distribution equipment, the insulation level of the transformer is specified as: short-time (1min) power frequency withstand voltage (effective value) of kV transformer is 25kV, lightning impulse withstand voltage (peak) ) is 60kV; 10kV transformer short-time (1min) power frequency withstand voltage (RMS) is 35kV, lightning impulse withstand voltage (peak) is 75kV. "Insulation coordination of national standard GB311.1 high-voltage transmission and distribution equipment The provisions for the insulation coordination of the operating overvoltage of equipment such as transformers are as follows.
(1) Relatively insulated, range 1 equipment, according to the statistical operation of the equipment, the overvoltage level or the operational protection level of the arrester and the insulation characteristics of the equipment, and select a certain matching factor Kc to calculate the rated operational shock resistance of the equipment. The voltage is: 4 = 2+. Therefore: Uc above shows that when the voltage of the pull-up current and the voltage on the capacitor is constant, the inductance of the winding is larger, and the smaller the capacitance to the ground, the higher the over-voltage when the brake is applied. In power systems, the pull-over overvoltage typically does not exceed 3.0.5 times the rated voltage.
Second, the transformer overvoltage hazard Transformer design insulation strength is generally considered to withstand 2.5 times overvoltage.
Therefore, over 2.5 times of overvoltage, regardless of which overvoltage, may damage the transformer insulation. The voltage distribution inside the transformer is greatly affected by the frequency of the voltage and the resistance, inductive reactance and capacitive reactance of the transformer. The capacitive reactance is very large in the case of the power frequency voltage, and the circuit composed of it is equivalent to the open circuit, so it is normal. In the case of the transformer internal voltage distribution, only the resistance and inductance can be considered, and the distribution is basically uniform. The atmospheric overvoltage or the operating overvoltage is basically a shock wave. Because the frequency of the shock wave is very high, the wave front steepness is very large, and the shock wave with a wavefront time of 1.5 ps has a frequency equivalent to 160 kHz. Therefore, under the action of the overvoltage shock wave, the transformer The capacitive reactance is very small, which has a great influence on the distribution of the internal voltage of the transformer. The hazard of the shock wave acting on the transformer winding can be divided into two stages: the initial moment and the oscillation process.
When t = 0, the capacitance of the winding plays a major role, and the effects of resistance and inductance are negligible. When the shock wave enters the high-voltage winding, due to the presence of the capacitance to the ground, the current flowing through the winding between the turns is different, and the voltage distribution at the initial moment causes a large turn-to-turn voltage between the turns of the winding end. Therefore, the insulation between the coils of the first few turns is seriously threatened, and the highest turn-to-turn voltage can reach the rated voltage. When t>0, there is an oscillation phenomenon from the initial voltage distribution to the final voltage distribution. In this process, not only capacitors but also inductors and resistors are applied, and the maximum potential (ground voltage) will appear at different points at different points of the winding. The voltage to the ground at different points of the winding can rise to twice the shock wave voltage value, and the winding may damage the ground insulation. The uniformity of the voltage distribution across the winding is related to the ratio of the winding to ground capacitance and the interturn capacitance. The smaller the ratio, the more uniform the capacitance distribution on the winding.
(2) The following factors should be considered when selecting the matching factor Kc: insulation type and its characteristics; performance index; overvoltage amplitude and distribution characteristics; atmospheric conditions; dispersion and installation quality in equipment production equipment; insulation between life expectancy Aging, test conditions and other unknown factors. For lightning shock, according to China's situation, generally select >=1.4; for operational shock, generally choose K>=1.15. Combined with the provisions of national standards, the insulation coordination factor of 6kV transformer surge arrester can be calculated: Zhao Yulin. High voltage technology. Beijing: China Electric Power Press, 2008. Lu Tiecheng. Power system overvoltage. Beijing: Water Resources and Hydropower Press, 2009.
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