At present, the 330kV capacitive voltage transformer (hereinafter referred to as CVT), which is the main equipment for power system energy metering, relay protection and carrier communication, has been widely used in the power system of Northwest China.
Since the structure of the CVT itself contains saturated inductors and capacitor energy storage components, the transient performance of the isolation switch will affect the correct operation of the substation relay protection, which will affect the normal and safe operation of the power system. For example, in 2000, when the duty officer of the 330kV Taoqu substation in Tongchuan operated the 330kV mother CVT isolation switch, the 330kV mother C phase CVT secondary knife disconnect switch was opened, and the No. 1 main transformer microcomputer overexcitation protection action, jumped the transformer three. Side switch. The main change on the 1st exits the run. The primary and secondary equipment in the substation are operating normally before the accident. A similar accident occurred in the Tongchuan 330kV Golden Lock Substation in 2005. The author mainly studies the secondary voltage of the 330kVCVT sub-isolation switch and analyzes its influence on the over-excitation protection action of the main transformer.
1CVT secondary knife gate operation transient simulation Because there is a transformer with iron core in the CVT loop, when the system operation has a large amplitude overvoltage, the intermediate transformer core in the CVT will be saturated. After the core is saturated, the excitation branch is equivalent. The nonlinear inductance, under which the linear relationship between the normal one and the second of the CVT is destroyed. The harmonic components of different frequencies are excited in the secondary voltage and the waveform distortion occurs. The secondary voltage can not correctly reflect the voltage, and the transformer is in an unstable state (1). This situation is likely to cause system relay protection to malfunction. The electromagnetic transient calculation program (hereinafter referred to as EMTP) is an important tool for studying the transient performance of power systems. The author uses the visual version of the ATP software to simulate the transient process during the operation of the CVT isolation switch.
Since the CVT-sub-isolation switch used in the 330kV substation is a hierarchical operation, the operation of each phase will have a relatively large time difference during the operation. Only one phase CVT is operated during the simulation calculation, and the other two-phase CVTs do not operate.
1.1 CVT - secondary knife gate opening operation The instantaneous phase of the CVT - secondary isolation switch is random for the phase of the bus voltage. For the identified substation equipment and CVT, the main influencing factor of the operation of the over-voltage operation of the breaking CVT-sub-isolation switch is the voltage phase at the time of opening.
It is the secondary voltage waveform diagram of the C-phase CVT-second isolation switch when the C-phase voltage phase of the busbar C phase is 0., 45° and 90. When the A and B two-phase CVT is withdrawn, the C-phase CVT-secondary isolation switch has a secondary voltage waveform at the bus voltage phase of 90. It can be seen from the simulation calculation results that during the normal operation of the substation, the secondary voltage oscillation is reduced to zero during the operation of the CVT-second isolation switch. When breaking, the influence of the bus voltage phase on the secondary voltage is very small and can be ignored.
It is possible that the CVT-secondary isolating switch re-ignites during the breaking process. Consider the case of a re-ignition during the breaking of the isolation switch, see the secondary winding voltage waveform. It can be seen from the waveform diagram that if re-ignition occurs at the primary isolation switch during the CVT-second isolation switch, the secondary voltage will have a severe overvoltage, the highest amplitude reaches 7.4 times the normal value, and then the oscillation disappears.
1.2CVT-Second-isolation switch closing operation Considering the other parameters in the system unchanged, when the voltage phase angles are 0., 45. and 90., respectively, the secondary voltage of the CVT-sub-isolation switch is closed. The situation, see the secondary voltage waveform. It can be seen from the simulation calculation results that when the voltage phase angle is 0, the CVT secondary voltage waveform is slightly distorted, but the secondary voltage amplitude is relatively low, and the four cycles disappear. 60. The distortion has already begun and the duration is only 4 cycles. At the close of 90., the waveform distortion is the most serious, the amplitude reaches 1.5 times of the normal value, but the duration is only 5 cycles.
1.3 Factors Affecting CVT Transient Performance For a certain size of CVT products, the size of the capacitor C is certain. The factors affecting the transient performance are mainly the damper parameters and the core magnetic density of the voltage transformer.
1.3.1 Influence of Damping Parameters The speed saturation damper mainly includes a fast saturation reactor and a damping resistor. The function of the fast-saturated reactor is that when the resonance occurs, the fast-saturated reactor is saturated, and the reactance value is lowered. At this time, the resistance connected in series is turned on, and the damping resonance is achieved. The selection of the damping resistance value will directly affect the damping effect. The size of the CVT damping resistor is matched to the reactance value of the reactor under the overvoltage power frequency plus the fractional harmonics to maximize the damping power generated under the same overvoltage. At the same time, the current of the damper circuit should also be greater than the minimum current at which the reactor core reaches saturation. Otherwise, the design of the reactor should be adjusted. The selection of the damping resistor should be within a specific range. According to the calculation, the damping resistance used in the studied 330kVCVT should be less than 19.06(3)5. In the 1.2 part, the damping resistance value of 330kVCVT is 8.m. Now the damping resistance value is changed to 2D and 20D for simulation calculation. See the secondary voltage waveform after simulation calculation. It can be seen that when the damping resistance value is small, the CVT has a continuous ferromagnetic resonance, the secondary voltage waveform has a relatively large distortion, the voltage amplitude reaches 1.55 times the rated value; when the damping resistance is increased to 20D At the same time, the damping effect is significantly improved and the resonance is eliminated in 4 cycles. Within a reasonable range, taking a larger damping resistor is beneficial to the suppression of resonance.
1.3.2 Influence of core transformer magnetic core on ferromagnetic resonance 1.2 The magnetic density of CVT voltage transformer is relatively low in the simulation calculation. The magnetic density curve is shown in the middle curve. 1. Now select the magnetic core with high magnetic core to simulate the calculation. The magnetic density curve is shown in the middle curve 2. It can be seen that when the core magnetic density is large, the CVT undergoes ferromagnetic resonance when the primary knife is closed, and the secondary voltage waveform is severely distorted. The value reached 3.36 times the rated value, but due to the action of the fast saturation damper, the resonance disappeared within 6 cycles.
This will cause the main transformer to over-excitation protection action.
2CVT secondary winding induced voltage analysis When the bookmark3 substation stops a bus or CVT, the mother and mother CVT must be run side by side to prevent the running protection device from losing pressure, and then the switching operation. During the switching process, the CVT-second isolation switch operation of each phase also has a strict sequence. When withdrawing a bus CVT, it is necessary to first break the isolation switch with the auxiliary contact, so that according to the above simulation calculation results, there will be ferromagnetic resonance phenomenon during the CVT-second isolation switch closing operation, but in the The resonance phenomenon disappears quickly under the action of the fast saturation damper, and the voltage amplitude and duration do not reach the threshold value of the main over-excitation protection action. The triangular winding of the open CVT is not disconnected, and the parallel connection of the two open triangular windings is avoided. When the CVT is put into operation, it must be operated strictly in accordance with the operation sequence to avoid the parallel triangular windings being connected in parallel during the operation of one isolation switch. See the principle of parallel delta winding parallel connection.
If the operation sequence is not correct, causing the two open triangular windings in parallel during operation, an unbalanced voltage is generated on one of the open triangles, superimposed on the other open triangular winding, and the secondary winding voltage is changed by the coupling. If the mother phase CVT is withdrawn or the two phase CVT is withdrawn, the open delta winding will have a voltage of 100V. Moreover, since the six CVT residual windings of the two open triangular windings connected in parallel are connected in series, the I mother and the mother open triangular windings are each divided by 50V, and the remaining windings of each phase CVT will be divided into 50/3V voltage. The 50/3V voltage will be transmitted through the voltage transformer to generate a certain voltage on the secondary winding of the I mother CVT, and superimposed on the operating voltage of each phase of the I mother.
The secondary to tertiary transformation ratio of CVT is (100/姨)/100=0.577, and the voltage at 50/3V coupled to the secondary winding is about 9.62V.
The following two cases are analyzed.
Case 1: The I mother CVT in the substation is in normal operation, and the mother CVT is put into operation. The mother A phase CVT-second isolation switch has auxiliary contacts. Assume that the mother A phase CVT secondary knife gate is first closed, and a voltage of 100 V is generated on the open triangular winding of the mother CVT. Since the I female open delta winding is connected in series with the female open delta winding, the remaining windings of the I mother phase CVT are superimposed with a voltage of 50/3 V. At this time, the I-phase CVT secondary winding voltage vector diagram is shown. After calculation, it is known that /=9.62,=57.7V, and the rated value is 1.17 times. Also in the normal operation of the I mother CVT, when the mother CVT is withdrawn, if the Phase B and Phase C CVTs are first evacuated, leaving only the Phase A operation, the same problem will occur.
Case 2: The I mother CVT in the substation is in normal operation, and the mother CVT withdrawal operation is performed. The mother A phase CVT-second isolation switch does not have an auxiliary contact. Assume that the mother A phase CVT-secondary isolation switch is first turned off. At this time, a voltage of 100V is generated on the open triangular winding of the mother CVT. The CVT voltage vector of each phase of the I mother is seen as 0.I mother B phase and C phase CVT secondary voltage rise. It is as high as 63.1V, which is 1.09 times of the rated voltage, and the secondary voltage of the I mother A phase CVT is reduced to 48.1 V. The same happens in the process of putting the mother CVT.
According to the above analysis and the accident record provided by the substation, the main transformer over-excitation protection malfunction of 330kV Taoqu substation and 330kV Jinshen substation should belong to the above situation. When the operator withdraws the 330kV female CVT, the operation sequence is wrong. First, the C-phase CVT-second isolation switch is opened, then the A-phase CVT secondary isolation switch is opened, and the B-phase CVT-sub-isolation switch with the auxiliary contact is opened. At the end of the break, the parallel connection of the I and the female open delta windings is not broken. The secondary voltage of the I-B phase CVT rises to 72.1V, which achieves the action threshold of the main transformer over-excitation protection and protects the action. The Jinsuo substation accident occurred during the CVT process of the commissioning line. The operator's operation sequence was wrong. First, the isolation switch with the auxiliary contact was closed, causing the secondary voltage of the A-phase CVT of the Jinhan line to rise and reach the main transformer. The action threshold of the excitation protection protects the action.
3 Conclusions The simulation results show that there is a certain ferromagnetic resonance in the CVT-second isolation switch closing process, which causes the secondary side voltage waveform to be distorted and the amplitude increases. However, since the amplitude of the voltage rise is small and the duration is short, the threshold value of the main overexcitation protection action is not reached, and the main overexcitation protection does not operate. If the operator operates in the wrong order during the commissioning or evacuation of the CVT, causing the open triangular windings of the two busbar CVTs to be connected in parallel, the unbalanced voltage of an open triangular winding will be evenly distributed to six (down to page 118) ( 30 years), at the same time its cost is very expensive, which is another key factor hindering the industrialization of electronic current transformers. Therefore, the current high-energy side energy supply method generally adopts a composite power supply mode: when the primary current is large, the CT power supply mode is adopted; when the secondary current is small, the laser energy supply mode is adopted. This method can minimize the working time of high-power lasers and prolong their life. However, there are also two problems: 1 When the line is closed after the inspection, the CT energy supply needs to have a long settling time. At this time, it can only rely on the laser to supply energy. However, if the laser fails at this time, it will directly cause the transformer to fail. Normal work, so it is generally required to use two lasers: one for one, but this further increases the cost. CT and laser switching control must have a reasonable control strategy, and there should be no "vacuum" for power supply, that is, one switching, and the other has not yet started to supply energy, so it is necessary to realize the pre-judgment of switching between the two modes. Moreover, when the CT power supply mode needs to consider the short circuit of the system, the impact of the short circuit current may cause damage to the CT. The existence of these factors will directly lead to more complicated energy supply systems and lower reliability.
4 Reliability Design As mentioned above, electronic transformers have many advantages over traditional transformers, but because electronic transformers include sensing technology, electronic technology, high voltage technology, optoelectronic technology, computer network technology, etc. The knowledge system of the multidisciplinary cross-disciplinary field is technically difficult. Whether electronic transformers can ultimately replace traditional transformers depends on their long-term reliability. The reliability design of the electronic transformer includes the following main contents.
4.1 Redundant Design Redundancy design is a common method to improve equipment reliability. In the electronic current transformer, the air core coil and the A/D converter constituting the protection channel must adopt a dual redundant design.
4.2 Self-test function design For key devices, such as power modules, A/D converters (upper page 115), the remaining windings of the CVT, and coupled to the secondary winding of the CVT are superimposed with the original voltage. The voltage of the secondary winding of the one-phase CVT with the highest voltage will reach 1.17 times of the normal value. In addition, the manual grading operation time is relatively long, and the action threshold value of the main transformer over-excitation protection is reached, and the protection malfunctions.
In order to avoid such accidents, the operator should operate the CVT-secondary isolation switch in strict accordance with the operation sequence. The relevant units can also make improvements to the equipment. For example, when changing the action criterion of the main transformer over-excitation protection, when the secondary voltage of the bus CVT and the line CVT are simultaneously raised, the main transformer over-excitation is determined, and the main-variation over-excitation protection action is started. If only the line or bus CVT secondary voltage rises, it is not necessary to have a basic self-test function.
4.3 Electromagnetic compatibility design Electronic transformers using CT power supply or CT composite power supply need to take protective measures, so that the large current impact during a short circuit will not damage the power supply CT. At the same time, it is necessary to consider the impact of a short-circuit current on the subsequent sampling circuit of the low-power iron core coil and the impact of the larger di/dt on the subsequent protection circuit electronic circuit of the air-core coil.
4.4 Safety Design Because high-power lasers are used for energy supply, protective measures must be taken to prevent damage to the operation and maintenance personnel caused by high-power lasers during operation and maintenance. Recommended method: When the high-power laser is working normally, once the power module is detected to be powered down, stop the laser immediately, in case the power supply fiber loop has problems, which will endanger the safety of personnel.
5 Conclusions Electronic transformers are new types of power equipment that are currently widely concerned, and the fine research of key technologies is directly related to the fact that electronic transformers can finally meet the high safety, stability and reliability requirements of power systems. .
Since the structure of the CVT itself contains saturated inductors and capacitor energy storage components, the transient performance of the isolation switch will affect the correct operation of the substation relay protection, which will affect the normal and safe operation of the power system. For example, in 2000, when the duty officer of the 330kV Taoqu substation in Tongchuan operated the 330kV mother CVT isolation switch, the 330kV mother C phase CVT secondary knife disconnect switch was opened, and the No. 1 main transformer microcomputer overexcitation protection action, jumped the transformer three. Side switch. The main change on the 1st exits the run. The primary and secondary equipment in the substation are operating normally before the accident. A similar accident occurred in the Tongchuan 330kV Golden Lock Substation in 2005. The author mainly studies the secondary voltage of the 330kVCVT sub-isolation switch and analyzes its influence on the over-excitation protection action of the main transformer.
1CVT secondary knife gate operation transient simulation Because there is a transformer with iron core in the CVT loop, when the system operation has a large amplitude overvoltage, the intermediate transformer core in the CVT will be saturated. After the core is saturated, the excitation branch is equivalent. The nonlinear inductance, under which the linear relationship between the normal one and the second of the CVT is destroyed. The harmonic components of different frequencies are excited in the secondary voltage and the waveform distortion occurs. The secondary voltage can not correctly reflect the voltage, and the transformer is in an unstable state (1). This situation is likely to cause system relay protection to malfunction. The electromagnetic transient calculation program (hereinafter referred to as EMTP) is an important tool for studying the transient performance of power systems. The author uses the visual version of the ATP software to simulate the transient process during the operation of the CVT isolation switch.
Since the CVT-sub-isolation switch used in the 330kV substation is a hierarchical operation, the operation of each phase will have a relatively large time difference during the operation. Only one phase CVT is operated during the simulation calculation, and the other two-phase CVTs do not operate.
1.1 CVT - secondary knife gate opening operation The instantaneous phase of the CVT - secondary isolation switch is random for the phase of the bus voltage. For the identified substation equipment and CVT, the main influencing factor of the operation of the over-voltage operation of the breaking CVT-sub-isolation switch is the voltage phase at the time of opening.
It is the secondary voltage waveform diagram of the C-phase CVT-second isolation switch when the C-phase voltage phase of the busbar C phase is 0., 45° and 90. When the A and B two-phase CVT is withdrawn, the C-phase CVT-secondary isolation switch has a secondary voltage waveform at the bus voltage phase of 90. It can be seen from the simulation calculation results that during the normal operation of the substation, the secondary voltage oscillation is reduced to zero during the operation of the CVT-second isolation switch. When breaking, the influence of the bus voltage phase on the secondary voltage is very small and can be ignored.
It is possible that the CVT-secondary isolating switch re-ignites during the breaking process. Consider the case of a re-ignition during the breaking of the isolation switch, see the secondary winding voltage waveform. It can be seen from the waveform diagram that if re-ignition occurs at the primary isolation switch during the CVT-second isolation switch, the secondary voltage will have a severe overvoltage, the highest amplitude reaches 7.4 times the normal value, and then the oscillation disappears.
1.2CVT-Second-isolation switch closing operation Considering the other parameters in the system unchanged, when the voltage phase angles are 0., 45. and 90., respectively, the secondary voltage of the CVT-sub-isolation switch is closed. The situation, see the secondary voltage waveform. It can be seen from the simulation calculation results that when the voltage phase angle is 0, the CVT secondary voltage waveform is slightly distorted, but the secondary voltage amplitude is relatively low, and the four cycles disappear. 60. The distortion has already begun and the duration is only 4 cycles. At the close of 90., the waveform distortion is the most serious, the amplitude reaches 1.5 times of the normal value, but the duration is only 5 cycles.
1.3 Factors Affecting CVT Transient Performance For a certain size of CVT products, the size of the capacitor C is certain. The factors affecting the transient performance are mainly the damper parameters and the core magnetic density of the voltage transformer.
1.3.1 Influence of Damping Parameters The speed saturation damper mainly includes a fast saturation reactor and a damping resistor. The function of the fast-saturated reactor is that when the resonance occurs, the fast-saturated reactor is saturated, and the reactance value is lowered. At this time, the resistance connected in series is turned on, and the damping resonance is achieved. The selection of the damping resistance value will directly affect the damping effect. The size of the CVT damping resistor is matched to the reactance value of the reactor under the overvoltage power frequency plus the fractional harmonics to maximize the damping power generated under the same overvoltage. At the same time, the current of the damper circuit should also be greater than the minimum current at which the reactor core reaches saturation. Otherwise, the design of the reactor should be adjusted. The selection of the damping resistor should be within a specific range. According to the calculation, the damping resistance used in the studied 330kVCVT should be less than 19.06(3)5. In the 1.2 part, the damping resistance value of 330kVCVT is 8.m. Now the damping resistance value is changed to 2D and 20D for simulation calculation. See the secondary voltage waveform after simulation calculation. It can be seen that when the damping resistance value is small, the CVT has a continuous ferromagnetic resonance, the secondary voltage waveform has a relatively large distortion, the voltage amplitude reaches 1.55 times the rated value; when the damping resistance is increased to 20D At the same time, the damping effect is significantly improved and the resonance is eliminated in 4 cycles. Within a reasonable range, taking a larger damping resistor is beneficial to the suppression of resonance.
1.3.2 Influence of core transformer magnetic core on ferromagnetic resonance 1.2 The magnetic density of CVT voltage transformer is relatively low in the simulation calculation. The magnetic density curve is shown in the middle curve. 1. Now select the magnetic core with high magnetic core to simulate the calculation. The magnetic density curve is shown in the middle curve 2. It can be seen that when the core magnetic density is large, the CVT undergoes ferromagnetic resonance when the primary knife is closed, and the secondary voltage waveform is severely distorted. The value reached 3.36 times the rated value, but due to the action of the fast saturation damper, the resonance disappeared within 6 cycles.
This will cause the main transformer to over-excitation protection action.
2CVT secondary winding induced voltage analysis When the bookmark3 substation stops a bus or CVT, the mother and mother CVT must be run side by side to prevent the running protection device from losing pressure, and then the switching operation. During the switching process, the CVT-second isolation switch operation of each phase also has a strict sequence. When withdrawing a bus CVT, it is necessary to first break the isolation switch with the auxiliary contact, so that according to the above simulation calculation results, there will be ferromagnetic resonance phenomenon during the CVT-second isolation switch closing operation, but in the The resonance phenomenon disappears quickly under the action of the fast saturation damper, and the voltage amplitude and duration do not reach the threshold value of the main over-excitation protection action. The triangular winding of the open CVT is not disconnected, and the parallel connection of the two open triangular windings is avoided. When the CVT is put into operation, it must be operated strictly in accordance with the operation sequence to avoid the parallel triangular windings being connected in parallel during the operation of one isolation switch. See the principle of parallel delta winding parallel connection.
If the operation sequence is not correct, causing the two open triangular windings in parallel during operation, an unbalanced voltage is generated on one of the open triangles, superimposed on the other open triangular winding, and the secondary winding voltage is changed by the coupling. If the mother phase CVT is withdrawn or the two phase CVT is withdrawn, the open delta winding will have a voltage of 100V. Moreover, since the six CVT residual windings of the two open triangular windings connected in parallel are connected in series, the I mother and the mother open triangular windings are each divided by 50V, and the remaining windings of each phase CVT will be divided into 50/3V voltage. The 50/3V voltage will be transmitted through the voltage transformer to generate a certain voltage on the secondary winding of the I mother CVT, and superimposed on the operating voltage of each phase of the I mother.
The secondary to tertiary transformation ratio of CVT is (100/姨)/100=0.577, and the voltage at 50/3V coupled to the secondary winding is about 9.62V.
The following two cases are analyzed.
Case 1: The I mother CVT in the substation is in normal operation, and the mother CVT is put into operation. The mother A phase CVT-second isolation switch has auxiliary contacts. Assume that the mother A phase CVT secondary knife gate is first closed, and a voltage of 100 V is generated on the open triangular winding of the mother CVT. Since the I female open delta winding is connected in series with the female open delta winding, the remaining windings of the I mother phase CVT are superimposed with a voltage of 50/3 V. At this time, the I-phase CVT secondary winding voltage vector diagram is shown. After calculation, it is known that /=9.62,=57.7V, and the rated value is 1.17 times. Also in the normal operation of the I mother CVT, when the mother CVT is withdrawn, if the Phase B and Phase C CVTs are first evacuated, leaving only the Phase A operation, the same problem will occur.
Case 2: The I mother CVT in the substation is in normal operation, and the mother CVT withdrawal operation is performed. The mother A phase CVT-second isolation switch does not have an auxiliary contact. Assume that the mother A phase CVT-secondary isolation switch is first turned off. At this time, a voltage of 100V is generated on the open triangular winding of the mother CVT. The CVT voltage vector of each phase of the I mother is seen as 0.I mother B phase and C phase CVT secondary voltage rise. It is as high as 63.1V, which is 1.09 times of the rated voltage, and the secondary voltage of the I mother A phase CVT is reduced to 48.1 V. The same happens in the process of putting the mother CVT.
According to the above analysis and the accident record provided by the substation, the main transformer over-excitation protection malfunction of 330kV Taoqu substation and 330kV Jinshen substation should belong to the above situation. When the operator withdraws the 330kV female CVT, the operation sequence is wrong. First, the C-phase CVT-second isolation switch is opened, then the A-phase CVT secondary isolation switch is opened, and the B-phase CVT-sub-isolation switch with the auxiliary contact is opened. At the end of the break, the parallel connection of the I and the female open delta windings is not broken. The secondary voltage of the I-B phase CVT rises to 72.1V, which achieves the action threshold of the main transformer over-excitation protection and protects the action. The Jinsuo substation accident occurred during the CVT process of the commissioning line. The operator's operation sequence was wrong. First, the isolation switch with the auxiliary contact was closed, causing the secondary voltage of the A-phase CVT of the Jinhan line to rise and reach the main transformer. The action threshold of the excitation protection protects the action.
3 Conclusions The simulation results show that there is a certain ferromagnetic resonance in the CVT-second isolation switch closing process, which causes the secondary side voltage waveform to be distorted and the amplitude increases. However, since the amplitude of the voltage rise is small and the duration is short, the threshold value of the main overexcitation protection action is not reached, and the main overexcitation protection does not operate. If the operator operates in the wrong order during the commissioning or evacuation of the CVT, causing the open triangular windings of the two busbar CVTs to be connected in parallel, the unbalanced voltage of an open triangular winding will be evenly distributed to six (down to page 118) ( 30 years), at the same time its cost is very expensive, which is another key factor hindering the industrialization of electronic current transformers. Therefore, the current high-energy side energy supply method generally adopts a composite power supply mode: when the primary current is large, the CT power supply mode is adopted; when the secondary current is small, the laser energy supply mode is adopted. This method can minimize the working time of high-power lasers and prolong their life. However, there are also two problems: 1 When the line is closed after the inspection, the CT energy supply needs to have a long settling time. At this time, it can only rely on the laser to supply energy. However, if the laser fails at this time, it will directly cause the transformer to fail. Normal work, so it is generally required to use two lasers: one for one, but this further increases the cost. CT and laser switching control must have a reasonable control strategy, and there should be no "vacuum" for power supply, that is, one switching, and the other has not yet started to supply energy, so it is necessary to realize the pre-judgment of switching between the two modes. Moreover, when the CT power supply mode needs to consider the short circuit of the system, the impact of the short circuit current may cause damage to the CT. The existence of these factors will directly lead to more complicated energy supply systems and lower reliability.
4 Reliability Design As mentioned above, electronic transformers have many advantages over traditional transformers, but because electronic transformers include sensing technology, electronic technology, high voltage technology, optoelectronic technology, computer network technology, etc. The knowledge system of the multidisciplinary cross-disciplinary field is technically difficult. Whether electronic transformers can ultimately replace traditional transformers depends on their long-term reliability. The reliability design of the electronic transformer includes the following main contents.
4.1 Redundant Design Redundancy design is a common method to improve equipment reliability. In the electronic current transformer, the air core coil and the A/D converter constituting the protection channel must adopt a dual redundant design.
4.2 Self-test function design For key devices, such as power modules, A/D converters (upper page 115), the remaining windings of the CVT, and coupled to the secondary winding of the CVT are superimposed with the original voltage. The voltage of the secondary winding of the one-phase CVT with the highest voltage will reach 1.17 times of the normal value. In addition, the manual grading operation time is relatively long, and the action threshold value of the main transformer over-excitation protection is reached, and the protection malfunctions.
In order to avoid such accidents, the operator should operate the CVT-secondary isolation switch in strict accordance with the operation sequence. The relevant units can also make improvements to the equipment. For example, when changing the action criterion of the main transformer over-excitation protection, when the secondary voltage of the bus CVT and the line CVT are simultaneously raised, the main transformer over-excitation is determined, and the main-variation over-excitation protection action is started. If only the line or bus CVT secondary voltage rises, it is not necessary to have a basic self-test function.
4.3 Electromagnetic compatibility design Electronic transformers using CT power supply or CT composite power supply need to take protective measures, so that the large current impact during a short circuit will not damage the power supply CT. At the same time, it is necessary to consider the impact of a short-circuit current on the subsequent sampling circuit of the low-power iron core coil and the impact of the larger di/dt on the subsequent protection circuit electronic circuit of the air-core coil.
4.4 Safety Design Because high-power lasers are used for energy supply, protective measures must be taken to prevent damage to the operation and maintenance personnel caused by high-power lasers during operation and maintenance. Recommended method: When the high-power laser is working normally, once the power module is detected to be powered down, stop the laser immediately, in case the power supply fiber loop has problems, which will endanger the safety of personnel.
5 Conclusions Electronic transformers are new types of power equipment that are currently widely concerned, and the fine research of key technologies is directly related to the fact that electronic transformers can finally meet the high safety, stability and reliability requirements of power systems. .
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