1. Why do we need to install a negative sequence voltage blocking device for the demagnetization protection of the generator?
Answer: The demagnetization protection of the generator may have to act when the voltage transformer breaks one phase or breaks the two phases and the system is asymmetric. In order to prevent the mis-magnetization protection from malfunctioning under the above conditions, a negative sequence voltage blocking device is installed to make the generator's demagnetization protection when the generator is really demagnetized, which reflects the de-magnetized relay action and the negative-sequence voltage blocking relay does not act.
2. What is the reactance transformer and why does the reactance transformer have an air gap?
A: The reactance transformer is a transformer with a primary winding connected to a current source (ie, input power supply) and the secondary winding is close to an open circuit state (ie, output voltage). Its reactance value (called transfer impedance) is the excitation reactance. Because it is required that the excitation reactance Ze should be small and have good linear characteristics, there must be a gap in the magnetic circuit. The excitation impedance Ze of the reactance transformer is basically reactive, so U2 leads the primary current I1 by approximately 90°.
3. What are the main differences between current transformers and current transformers that convert current into voltage in relay protection devices? How does the former make I1 and U2: in phase and the latter makes I1 and U2 reach the required phase?
A: The main difference is in the core structure, TA no air gap, and DKB has an air gap, open circuit excitation resistance TA is large and DKB is small; in the phase of primary current and secondary voltage, TA is in phase, DKB primary current is behind secondary voltage 90°; TA secondary voltage is taken from the voltage drop across the load resistor R. To achieve the same phase and appropriate capacitance, DKB can connect the variable resistor in parallel with the secondary coil. * Change the resistance to obtain the required phase.
4. Why should the generator be equipped with negative sequence current protection?
Answer: When an asymmetric short-circuit or three-phase asymmetrical operation occurs in the power system, there is a negative sequence current in the stator winding of the generator. This current generates a reverse rotating magnetic field in the air gap of the generator, which is twice the synchronous speed with respect to the rotor. As a result, a frequency-doubling current occurs in the rotor component. This current causes local burns in certain parts of the rotor where the current density is high. In severe cases, the guard ring may be loosened by heat, causing significant damage to the generator. Another 100Hz alternating electromagnetic torque will act on the large rotor shaft and the stator frame, causing a frequency of 100Hz vibration. In order to prevent the occurrence of the above-mentioned hazardous generator problems, negative sequence current protection must be set.
5, by the A, C phase current transformer differential protection of the composition of the protection line, when the protected circuit occurs a variety of phase short-circuit fault, the three-phase short-circuit as a benchmark, the relative sensitivity of how?
Answer: Because the current flowing through the relay during the three-phase short-circuit is Ik=√3I(3), the current flowing through the relay when the two phases are short-circuited is Ik=2I(2)=2×(√3/2·I(3) ). The current flowing through the relay when the two phases of BC are short-circuited is Ik=I(2)=I(3). The current flowing through the relay when the two phases of AB are short-circuited is Ik=I(2)=I(3). Therefore, when the three-phase short-circuit sensitivity KS=1 is used as a reference, KS=1 when the AC phase is short-circuited and KS=1/2 when the AB and BC phases are short-circuited.
6. Why should bus protection devices be installed in the high-voltage power grid?
A: Although the probability of a short-circuit fault on the bus is less than the number of transmission lines, the bus is a meeting point for multiple components. If bus faults are not quickly removed, the accident will be expanded, or even the system will be destroyed, which will endanger the safe operation of the entire system. serious. In the double bus system, it is of great significance to selectively cut off the faulty busbars and ensure that the busbars continue to operate. Therefore, it is required to generally install bus protection devices in high-voltage power grids.
7. In the normally closed contact loop of the synchronous relay in the three-phase reclosing start circuit, why should the series check circuit have a voltage normally open contact?
Answer: The series KV normally open contact in the same period of reclosing the start circuit of the reclosing, the purpose is to ensure that there is a voltage on the line before the coincidence period coincides. In addition, under normal circumstances, due to some reason, in the case of no pressure overlap detection, Because the circuit breaker falls off automatically, the line voltage cannot be reclosed. At this time, if the string with KV normally open contacts in the same period of the start-up circuit is checked in parallel with the no-voltage start circuit, the start-up circuit in the same period can correct this erroneous trip. .
8. What are the conditions for quasi-synchronous periods and what are the consequences of unsatisfied conditions?
A: Quasi-synchronized parallel conditions are that the voltage of the generator to be connected and the voltage of the system are the same, the phases are the same, and the frequencies are the same.
When the above conditions are not satisfied, juxtaposition will cause an inrush current. The greater the difference in voltage, the greater the inrush current; the greater the difference in frequency, the shorter the period of the inrush current. Inrush currents are detrimental to both generators and power systems.
9. What is the maximum sensitivity angle of the directional impedance relay and why is it necessary to adjust its maximum sensitivity angle equal to the impedance angle of the protected line?
Answer: The impedance angle of the maximum operating impedance (amplitude) of the directional impedance relay is called its maximum sensitivity angle φs. When the phase-to-phase short circuit occurs in the protected circuit, the angle between the short-circuit current and the voltage at the relay installation is equal to the impedance angle φL of the circuit. When the line is short-circuited, the directional resistance relay measures the impedance of the impedance, φm, equal to the impedance angle of the line, φL. To make the relay work in the most sensitive state, the maximum sensitivity angle φS of the relay is required. Equal to the impedance angle of the circuit to be protected, φL.
10. What is the voltage transformer reverse charging and what is the effect on the protection device?
Answer: The charging of busbars that are not energized through the secondary side of the voltage transformer is called reverse charging. Such as 220kV voltage transformer, change ratio is 2200, even if the busbar is not grounded, its impedance (including bus capacitance and insulation resistance) is large, assuming 1MΩ, but the impedance seen from the secondary measurement of the voltage transformer is only 1000000/(2200)2=0.2Ω, almost short circuit, so the reverse charging current is larger (the reverse charging current is mainly determined by the cable resistance and the leakage resistance of the two voltage transformers), which will cause the secondary side of the voltage transformer during operation. If the small switch trips or the fuse blows, the protection device in operation loses voltage, which may cause the protection device to malfunction or refuse to move.
11. Why should the generator be installed with low-voltage blocking over-current protection? Why does this protection use the current transformer at the neutral point of the generator?
Answer: This is set as a differential protection of the generator or backup protection of the next component. It acts when the following two types of faults occur: (1) When an external short-circuit occurs, the protective device or circuit breaker of the faulty component refuses to act. Time. (2) When the fault is within the differential protection range of the generator and the differential protection rejects the action. In order for this protection to work after the generator is not connected to the bus after the generator has been pressurized or disconnected from the busbar (voltage not dropped), an internal short circuit will still work, so the neutral point of the generator should be used. Current transformer.
12. Why does a large-capacity generator adopt 100% stator grounding protection and explain the principle of 100% stator winding single-phase grounding protection for additional DC voltage.
A: The ground protection using the zero-sequence current and zero-sequence voltage principle can not achieve 100% protection range for the stator windings. There is a dead zone near the near-neutral point. In fact, large-capacity units are often due to mechanical Damage or water leakage in the water-cooled system may cause ground faults in the vicinity of the neutral point. If such failures cannot be detected in time, it may cause the fault to expand and cause serious damage to the generator. Therefore, 100% protection zone stator ground protection must be installed on large-capacity generators. The principle of 100% stator grounding protection of the additional DC power supply is shown in Figure F-1. When the generator is in normal operation, there is no current in the KA coil of the current relay and the protection does not act. When the stator winding of the generator is single-phase grounded, the DC voltage passes through the ground point of the stator loop and is added to the current relay KA, so that a current passes through the KA and acts and sends a signal.
13. Why large-capacity generators should use negative-sequence inverse-time overcurrent protection?
A: The asymmetry of the load or system causes a negative-sequence current to flow through the stator windings of the generator and a negative-sequence rotating magnetic field is established in the air gap of the generator, causing the rotor to induce twice the frequency of the current, causing the rotor to heat up. Because of the direct cooling type (water internal cooling and hydrogen internal cooling), a large generator requires a smaller volume increase than a larger capacity. At the same time, the thermal capacity margin of a large unit is generally lower than that of a large generator due to economic and technical reasons. Small units are small. Therefore, the negative sequence additional heating of the rotor should pay attention, the general trend is that the greater the capacity of the single machine, the smaller the value of A, the lower the ability of the rotor to withstand the negative sequence current, so we must particularly emphasize the negative sequence protection of large generators. The duration of the allowable negative sequence current of the generator is A=I2t. The larger the I2 is, the shorter the allowable time is, and the smaller the I2 is, the longer the allowable time is. Because of the inverse time characteristics of the generator to I2, negative sequence reverse time overcurrent protection should be used on large units.
14. Why should the generator excitation circuit be equipped with one-point grounding and two-point grounding protection?
Answer: The generator excitation circuit is grounded at one point. Although it will not form a fault current path, it will not cause direct damage to the generator. However, it is necessary to consider the possibility of the second point of grounding. Therefore, a signal of grounding protection is issued to strengthen the inspection. Surveillance. When the generator excitation loop occurs with two earth faults: 1 The rotor of the generator is burned due to the fault current flowing through the fault; 2 The symmetry of the air gap of the generator is damaged, causing severe vibration of the generator; 3 The rotor slowly deforms to form an eccentricity, further exacerbating the vibration. Place two points of ground protection after one point of grounding, so that when two points of grounding occurs, it will stop after a delay.
15. What is the role of the longitudinal differential protection and horizontal differential protection of the generator and the protection scope? Is it possible to replace each other?
Answer: Diffusion protection is the most effective method of protection against internal short-circuit fault protection of the generator and is the main protection against short-circuits between generator stator windings.
The horizontal differential protection is to protect the stator winding of the generator from a phase-to-turn short circuit and the inter-turn short circuit protection between the two parallel branches of the same phase. For generators with a star connection and two parallel leads per phase, the generator must be installed. Set the horizontal difference protection. When the phase windings near the stator windings or near the neutral point are short-circuited, the current in the two neutral point connections is small, and the horizontal differential protection may not operate, and the dead zone may reach 15% to 20%), so it cannot replace the longitudinal differential. protection.
16, negative sequence current relay, when the two primary winding or secondary winding of the reactance transformer and the corresponding polarity of the winding of the intermediate converter is reversed, what will happen and how to prevent it?
Answer: Negative sequence current relay reactance Transformer When the primary or secondary windings are connected to the opposite polarity of the intermediate converter winding, the negative sequence relay will become a positive sequence relay. After the relay protection is put into operation, no asymmetry fault occurs, but if the load current reaches a certain value, it will malfunction. In order to ensure the correct connection of the negative sequence current relay and prevent the above situation, the following measures must be taken:
(1) Pass the three-phase power supply to check the setting of the negative sequence current relay.
(2) When testing with a single-phase power supply, the relative polarity should be measured in accordance with the principle of the negative-sequence filter, so that it meets the requirements, and the load current must be used for inspection. After confirming that the wiring is correct, it is put into operation.
17. Why is 220kV and above systems equipped with circuit breaker failure protection and what is its role?
Answer: The transmission line above 220kV generally has large power and long transmission distance. To improve the transmission capacity of the line and the stability of the system, phase-breaking circuit breakers and rapid protection are often used. Because the circuit breaker has the possibility of operation failure, when the circuit fails and the circuit breaker refuses to move, it will pose a great threat to the power grid. Therefore, the circuit breaker failure protection device should be installed, and the circuit breaker that will selectively reject the failure will be opened. The circuit breaker of the bus where the device is located (connected) is disconnected to reduce equipment damage, reduce the scope of power outage, and improve the safety and stability of the system.
18. Why does the negative sequence starting element in distance protection change to a three-phase output type?
A: Because the three-phase negative sequence current relay has a small ripple coefficient after three-phase full-wave rectification, the required filter capacitance is small, and the reaction speed is fast. Especially when the three-phase symmetrical short circuit occurs, the DC component in the short-circuit current can be fully utilized. Ensure that the device can operate. In addition, when the system oscillates and asymmetrically operates, the DC voltage of unbalanced voltage is output, and the ripple factor is small. After the differential circuit, the voltage of the actuator is low, so that it has a higher anti-vibration capability.
19. What are the requirements for WXB-11 microcomputer protection AC current loop wiring?
Answer: Microcomputer protection is different from other types of protection devices because it is based on computer program operations to determine internal and external faults and types of faults. Therefore, if the current loops of IA, IB, and Ic are connected to the positive polarity to protect the polarity of the microcomputer, then 3I. It must also be connected to the positive polarity to protect the polarity of the zero-sequence loop of the microcomputer. No reverse polarity access is allowed. Otherwise, when an internal fault occurs, the input data and the protection program do not correspond. It will be judged as an AC data acquisition system fault. The protection device is locked, causing the protection device to refuse to act when an internal failure occurs.
20. What are the advantages of the oscillation locking device constructed by using the negative sequence plus the zero-sequence current increment principle?
Answer: The oscillation blocking device activated by the negative sequence plus zero-sequence current increment can better distinguish the oscillation and the short circuit, and can prevent the protection from malfunction due to the increase of the imbalance output of the negative sequence current filter when the system oscillates. It can also prevent unbalanced output of the filter when the line is not transposed, three-phase unbalanced, harmonic components, and non-full-phase operation are unbalanced. The locking device adopts the design principle of "non-stopping oscillation and unblocking" so that the protection will not malfunction due to the long duration of oscillation. Because the protection is only put into operation for a short time after the operation of the actuator, the system will operate without causing protection malfunction. In addition, this type of oscillating locking device also has a higher sensitivity and a faster operating speed, and thus has been widely used.
21, direction of over-current protection Why must start with the phase mode?
Answer: Directional over-current protection adopts the “phase-by-phase†wiring method in order to avoid misoperation of the device when the two-phase short circuit occurs in the opposite direction. For example, when the BC phase short-circuit occurs in the reverse direction, the relay in the phase A direction of the line will operate in the positive direction due to the load current. If the phase is not started at this time, when the C-phase current element is activated, it will cause the device to malfunction; Phase start wiring, although the A-phase direction relay operates, but the A-phase current element does not move, and the C-phase current element operates but the C-phase direction relay does not operate, so the device will not malfunction.
22, what is the connection coefficient of the current transformer? What is the effect of the wiring factor?
Answer: The ratio of the current through the relay and the secondary current of the current transformer is called the wiring coefficient of the current transformer, that is, the current in the Ik-I flow relay of the type Kc=Ik/I2; 12--the current transformer of the current transformer The secondary current connection coefficient is an important parameter in the relay protection setting calculation. The wiring factor must be taken into consideration when calculating the action values ​​of various current protection measurement components.
23, what is the high frequency blocking distance protection?
Answer: The basic principle of high-frequency blocking distance protection is to use â–³I2 â–³I. Incremental components act as starting components. They start high-frequency transceivers in the event of a fault, send high-frequency blocking signals, and use distance-II or III-direction directional impedance relays as faulty power-discriminating components. If internal faults occur, both sides of the distance protection stage or The third segment measures the action of the component, stops the high-frequency blocking signal, and instantaneously trips off the fault. If an external fault occurs, the positive-direction side contacts the impedance relay in the II- or III-direction direction to stop the transmission, but the impedance element in the opposite direction does not operate, and the relay continues to send the protection to the opposite side of the latch. This not only has the function of high-speed protection across the line, but also has the function of distance protection II for adjacent backup protection. Its main disadvantage is that the high-frequency protection and distance protection wiring are not connected to each other for easy operation and maintenance.
24. How to check the resonant frequency of the series resonance filter?
A: Connect the filter to the wiring as shown in Figure F-3. The resistance R of the circuit should be such that the output of the oscillator does not exceed its rated capacity when the impedance Z of the filter is minimum. Then use a high internal resistance voltage meter to measure the voltage on R. The voltage of the fixed oscillator is 10V, change its frequency, when the tube voltmeter indicates maximum, the corresponding frequency is the resonance frequency of the filter.
25. What is the reaction to the imbalance of active power in the power system? How to deal with?
Answer: The excess active power in the system will cause the frequency to rise, and the lack of active power will cause the frequency to drop. The solution is to adjust the output of the generator through the frequency modulation unit. When the situation is serious, some generator sets or part of the load are cut off through the automatic device or on-duty personnel operation to balance the system power.
26. For 3U. What constitutes a protection test and what are the countermeasures?
A: There are the following countermeasures for the 3Uo protection test:
(1) The 3Uo loop cannot be confirmed by checking the 3Uo loop for unbalanced voltage.
(2) It is not possible to confirm the correctness of the polarity of the directional protection formed by 3Uo by using the “hexagonal†test method alone.
(3) Tests may be conducted on the overall composition including current and voltage transformers and their secondary circuit connection and directional components to confirm that the polarity of the entire set of directional protection is correct.
(4) The most fundamental method is to find out the polarity of the voltage transformer and current transformer, and the polarity of all the connections from the transformer terminal to the relay protection screen and the polarity of the zero-sequence direction relay on the screen. judgment.
27, What is the danger of the power of the power direction relay? How to eliminate the inductive power direction relay?
A: The power direction relay may have reverse and positive creep. Reverse creep will increase the operating power. Positive creep may cause the protection to malfunction. The method to eliminate the creep of the induced power direction relay is as follows:
(1) Turn the position of the core column.
(2) Move the relative position of the voltage coil.
(3) Connect the voltage coil to the positive or reverse polarity of the compensation coil in series.
(4) Move the pole position slightly without the coil.
28. Simulate the setting of the three-phase short-circuit test negative sequence current relays. Why is the test current √3 times the value of the negative-sequence current setting? Analyze the symmetric component method.
A: Decompose a set of three-phase asymmetrical currents of A, B, and C into positive, negative, and zero-sequence symmetrical components. The negative-sequence symmetrical component is I2=1/3 (IA+a2IB+aIc). For short circuit, there is IB=-Ic=Ik(test current), IA=0 and its phasor is shown in Figure F-4. From the figure, we know a2IB+aIc=√3Ik, so I2=1/3·Ik·√3=Ik/√3. Similarly, it can be deduced that when ABCA two-phase short circuit is I2=Ik/√3, that is, the test current is a negative-sequence current. 3 times.
29. What is the blocking angle of high-frequency protection?
A: As shown in Figure F-5(a), when a through fault occurs at point k, ideally IM and IN
180 degrees difference, the protection device does not operate. In fact, when the line is externally faulty, due to various factors, the phase angle difference between IM and IN is not 180°, and the signal received by the receiver has a discontinuous angle. According to the principle of phase difference high frequency protection, the protection device will act when the line breaks and the discontinuity occurs. For this purpose, the maximum possible discontinuity angle of external faults should be found out, and this value should be used for blocking so as to ensure that the protection does not malfunction when the line fails externally. This maximum discontinuity is called the difference in the blocking angle of the high-frequency protection. As shown in Figure F-5 (b>), the protection zone φop is (180°-β)> φop> (180°+β), and the blocking angle is β.
30. What regulations should be followed in the management of the operating procedures of microcomputer relay protection devices?
Answer: The management of the operating procedures of microcomputer relay protection devices should follow the following rules:
(1) All networks (provinces) shall coordinate the procedures for the unified management of microprocessor relay protection devices within the scope of direct jurisdiction. Each network (provincial) shall set up a relay protection laboratory. After the new procedure passes the full test of the laboratory, Allow to be put into operation in the field.
(2) The same type of microcomputer high-frequency protection program version at both ends of a line should be the same.
(3) The program change of the microprocessor relay protection device shall be strictly implemented according to the procedure notice issued by the professional department of the dispatching relay protection.
Relay Protection Knowledge Q & A Compilation is provided by the Department of Biotechnology of Yangzhou Tuopu Electric Technology Co., Ltd., Hotline for consultation, website:. . Com
31. What is the criterion for how to open the impedance relay of LFP-901A protection when it fails again in the non-full-phase operation?
Answer: Open by non-full-phase oscillation locking element:
(1) When the single-phase fault reoccurs in the non-full-phase operation, the phase-selection area is not opened when the phase-opening fault occurs.
(2) When a phase-to-phase fault occurs again in a non-full-phase operation, the power-frequency variation of the difference between non-faulty two-phase currents is measured, and is opened when the current suddenly increases to a constant amplitude.
32. What problems should be noticed in the inspection of microcomputer relay protection devices?
Answer: When testing the relay protection device of the microcomputer, in order to prevent damage to the chip, the following problems should be noted:
(1) The microcomputer relay protection screen (cabinet) should have good grounding, and the grounding resistance should meet the design requirements. When measuring the circuit parameters using an AC power source electronic instrument (such as an oscilloscope, a frequency meter, etc.), the measurement terminals of the electronic instrument and the power supply side should be well insulated, and the instrument housing should be grounded at the same point as the protection panel (cabinet).
(2) The electric soldering iron should not be used in the test. If a soldering iron must be used, a special soldering iron should be used, and the soldering iron should be grounded at the same point as the protective screen (cabinet).
(3) When touching the pins of the chip by hand, measures shall be taken to prevent the damage of the IC to the IC chip.
(4) Plug and pull plugs are allowed only after the DC power is disconnected.
(5) Pull out the chip application special extractor, insert the chip should pay attention to the direction of chip insertion, after the insertion of the chip should be confirmed by the second person after the confirmation, before the electricity can be tested or used.
(6) When measuring the insulation resistance, insert the card with IC chip (except optocoupler and power plug).
33. In a small ground current system, why is single-phase ground protection used only to signal in most cases and not to trip?
Answer: In the small grounding current system, the symmetry of the system voltage is not damaged when a phase is grounded, the current through the fault point is only the capacitive current of the system, or the residual current after the arc suppression coil is compensated, and the value thereof is very small. The impact on grid operation and users' work is small. In order to prevent a short-circuit fault from occurring when grounding occurs again, the protection device is generally required to issue a warning signal in time so that the on-duty personnel can handle it as appropriate.
What is the principle of line phase-to-phase short-circuit protection configuration in 34. 5kV neutral point ungrounded networks?
Answer: The principle of phase-to-phase short-circuit protection configuration is:
(1) When using two-phase current protection, the current transformer should be mounted on the same phase of each outgoing line (for example, phase A and phase C).
(2) The protection device adopts far-back-up mode.
(3) If the line short-circuit causes the voltage of the power bus for the power plant, the bus of the main power contact point, or the voltage of the important user bus to fall below 50% to 60% of the rated voltage, the fault should be quickly removed.
35. After the new installation and overhaul of the power transformer, why should the impact closing test be conducted before the official operation?
Answer: The power transformer after the new installation and overhaul must be subjected to the impact closing test before it is officially put into operation. This is to check the insulation strength and the mechanical strength of the transformer, and to inspect the performance of the differential protection from the excitation inrush current. The newly installed equipment should be impacted 5 times and the equipment should be impacted three times after overhaul.
36, what is the automatic load reduction AFL device according to frequency? What is its role?
A: In order to improve power quality and ensure the availability of power for important users, when the frequency drop occurs due to the lack of active power in the system, depending on the degree of frequency decrease, some unimportant users are automatically disconnected, and the frequency is prevented from decreasing so that the frequency can be reduced. Quickly restored to normal, this device is called automatic frequency-reducing device, referred to as AFL device. It can not only guarantee the power supply of important users, but also avoid the collapse of the system caused by the frequency drop.
37. The two-phase current difference wiring of a relay can not be used in the case of Y, d wiring transformers.
Answer: Transformers that cannot be used in Y, D wiring. The phase winding of the transformer Y side is subjected to the phase voltage, and the phase of the delta side receives the line voltage, so the number of turns of the side winding of the triangle NΔ=√3Ny, and the ratio Iy/IΔ=Ny/NΔ=√3, such as When mounted on the Y side and a two-phase short circuit occurs on the △ side (ab phase), the A- and C-phase currents on the Y-side are both 1/√3, so the relay does not move.
38. Why does the overcurrent protection setting take into account the relay's return coefficient and the current quick-break protection does not need to be considered?
Answer: The overcurrent protection's operating current is set to escape the maximum load current and generally protects adjacent equipment. In the event of an external short-circuit, the current relay may start, but after the external fault has been removed (the current is dropped to the maximum load current), it must be *returned, otherwise false tripping will occur. The purpose of considering the return coefficient is to ensure that the protection energy * can be returned in the above case. The operating value of the current quick-break protection is set according to the maximum short-circuit current avoiding the predetermined point, and its setting value is much larger than the maximum load current, so there is no problem of not returning under the maximum load current. Furthermore, once the instant current quick-break protection starts the crap trip, there is no midway return problem, so the current quick-break protection does not consider the return coefficient.
39. Some distance protection will not immediately malfunction when the secondary circuit of the voltage transformer is out of phase. Why is there still a need to install a voltage loop off-phase locking device?
A: At present, there are some new or modified distance protections. The starting circuit is blocked by a negative sequence current element. When the secondary loop of the voltage transformer is broken, although the impedance element will malfunction, because the negative sequence current element does not start, the protection device will not immediately cause a malfunction. But when the secondary circuit of the voltage transformer is out of phase and encounters a through fault, it will still malfunction, so it is also necessary to signal when the secondary circuit of the voltage transformer is out of phase, and the time is greater than the time delay of the third stage delay. Latch protection.
40. Some distance protection will not immediately malfunction when the secondary loop of the voltage transformer fails. Why do you still need to install a voltage loop phase-locking and locking device?
A: At present, there are some new or modified distance protections. The starting circuit is blocked by a negative sequence current element. When the secondary loop of the voltage transformer is broken, although the impedance element will malfunction, because the negative sequence current element does not start, the protection device will not immediately cause a malfunction. However, when the secondary loop of the voltage transformer is out of phase and a through fault is encountered, misoperation will still occur. Therefore, the secondary transformer of the voltage transformer will also be signaled when the secondary loop is out of phase, and will start after the delay time greater than the III. Latch protection.
41. Why is it that the resonant frequency of the wave trap is about 0.2 kHz lower than the frequency of use?
A: Due to the phase-coupled high-frequency channel, other devices connected to the bus line form an impedance to ground. This impedance is connected in series with the impedance of the line trap to form a shunt of the high-frequency signal channel, resulting in shunted losses. The amount of shunt loss depends on the phasor sum of the two impedances. Experience has shown that the bus-to-ground impedance is capacitive in most cases. In order to avoid a series resonance between the impedance of the busbar and the bus-to-ground capacitor, the reactive component of the impedance of the resistor is cancelled out, so that the branch impedance is drastically reduced and the shunt attenuation is reduced. Increasing, it is required to protect the use of the trap's resonant frequency is lower than the protection of the use of frequency of about 0.2kI-Iz, to ensure that the trap in the use of frequency capacitive, so as to obtain the maximum impedance of the trap.
42. How to adjust the resonance frequency and blocking frequency band of a single frequency wave trap?
Answer: Wire as shown in Figure F-8. The voltage of the fixed oscillator is a certain value, change the frequency, when U2 appears minimum Umin, this frequency is the resonant frequency, and then change the frequency, when the U2 reading is Umin value of the upper and lower frequency difference, that is, single-phase resistance The waveband of the wave filter is â–³F.
43. How to set the current control element in circuit breaker failure protection?
Answer: When the current control element is in the minimum operating mode, the local busbar is faulty, and when the fault current at the opposite end is the minimum, there should be enough sensitivity to set the current control element. After the disconnection of the busbar breaker, the current control element should be able to operate. The setting value of the current control element should generally be greater than the load current. If the load current cannot be avoided after the sensitivity is set, the sensitivity requirement should be met.
44. Why is a non-linear resistor R connected in parallel at the entrance of the transistor transmitter channel and two diodes of opposite polarity in parallel are connected in R?
A: A non-linear resistor in parallel with the channel population is to make the device immune to the impact of channel interference signals. Because of the large static capacitance of the nonlinear resistor, a single and a non-linear resistor has a certain bypass effect on the high-frequency signal, so that the output voltage is reduced. Therefore, in the non-linear resistor R, two parallel-connected diodes of opposite polarity are connected. This reduces the total capacitance in the loop because the total capacitance between the poles of the diode is much smaller than the static capacitance of the non-linear resistor. The purpose of the diode is to prevent the positive and negative direction of the interference pulse current from being absorbed by the nonlinear resistor.
45. What is the load regulation effect? ​​If there is no load regulation effect, what will happen to the system when there is a shortage of active power?
A: When the frequency decreases, the active power drawn by the load decreases. When the frequency increases, the active power drawn by the load increases. This phenomenon of changing the active power of a load with frequency is called a load regulation effect.
Because of the existence of the load regulation effect, when the frequency changes due to the power balance failure in the power system, the load power changes with it and plays a compensating role. If the frequency drops due to the lack of active power in the system, the corresponding load power will also decrease, and some active power shortages will be compensated, which may make the system stable at a lower frequency. If there is no load regulation effect, when the frequency of the active power shortage system drops, the power shortage can not be compensated and the new active power balance will not be reached, so the frequency will decrease until the system collapses.
46. ​​What happens to the distribution of zero-sequence current when a ground fault occurs in a large ground current system?
A: The distribution of the zero-sequence currents is only related to the system's zero-sequence network and has nothing to do with the number of power supplies. When increasing or decreasing the number of neutral grounded transformers, the system zero-sequence network will change, thereby changing the zero-sequence current distribution. When increasing or decreasing the number of generators connected to the busbar and the number of ungrounded neutral transformers, and the number of neutral-grounded transformers does not change, only the magnitude of the ground current is changed, regardless of the distribution of the zero-sequence current.
47. Why is it necessary to set up a three-hop circuit breaker in high-frequency protection with remote start?
A: (1) In the event of a fault in the area, the circuit breaker on one side trips first. If the circuit breaker is not stopped immediately, the transmitter will generate continuous high-frequency signals due to no operating current, and the receiving side of the opposite side will also receive continuous signals. The high-frequency signal is latched to protect the outlet and cannot be tripped.
(2) When manually or automatically reclosed to a permanent fault, because there is no closing on the opposite side, the remote starting circuit sends a high-frequency continuous wave, so that the side to be closed first is blocked and the protection is refused. In order to ensure that the two devices can be operated in the above situation, a circuit breaker tripping trip must be set.
48. Why does a large grounding current system not use three-phase and phase current protection and zero-sequence current protection, but uses zero-sequence current protection alone?
A: The phase-to-phase current protection of the three-phase star connection can also react to the grounding short circuit, but when it is used to protect the ground from a short circuit, the maximum load current must be avoided at the fixed value, and the user must press the power supply direction in the operation time. The ladder principle is gradually increased by a time difference. The zero-sequence current protection that specifically reacts to the ground short circuit does not need to be set in accordance with this principle. Therefore, it has high sensitivity and short operation time, and because the zero sequence impedance of the line is much larger than the positive sequence impedance, the protection of the zero sequence current protection The range is long, and it is easy to fit between the lower and upper levels of protection. Therefore, phase current protection and zero-sequence current protection are generally not used.
49. Why does the three current transformers that make up the zero-sequence current filter have the same characteristics?
Answer: The output current of the zero-sequence current filter is IK=(IA+IB+IC)/nTA=3I. ×1/nTA In the equation above, the excitation current of the current transformer is ignored. Consider the output current of the excitation current filter as IK = [(IA + IB + IC) - (IAe + IBE + ICe)] / nTA where Iae + IBa + ICe)] / nTA is called unbalanced current if the three phases Current transformer saturation program is different, asymmetry of the excitation current and some differences formed in the manufacturing, in the normal operation will appear larger unbalanced current. Especially in the transient process of phase-to-phase faults, the short-circuit current contains a large non-periodic component, causing the current transformer core to be heavily saturated and a large unbalanced current appears. In order to reduce the unbalanced current, it is necessary to make the magnetization curves of the three current transformers the same, and work in the unsaturated part, while also minimizing its secondary load, so that the three-phase current transformer load is balanced as much as possible.
50. How to use the method of disconnecting one phase of working voltage and verifying the correctness of the negative sequence power direction relay wiring by means of three working currents A, B and C respectively.
Answer: (1) Measure the correct phase of the load current and the corresponding operating voltage.
(2) Disconnect the voltage of one phase of the breaker and connect the terminal of the disconnected phase to UN to simulate the negative sequence voltage. If the phase A is disconnected, U2=-1/3UA.
(3) Add the phase currents of relays A, B, and C, respectively, and disconnect the other two phases from the relay and short-circuit the power supply to observe the relay action.
(4) Use the A, B, and C phase load currents to find the corresponding negative sequence current and phase, respectively. If the A-phase current, I2 = 1/3 (IA + a2IB + aIc) = 1/3 IA, similarly, when people B-phase current I2 = IB, through the human C-phase current I2 = Ic.
(5) Using the simulated U2 as a benchmark, draw the operating characteristic diagram of the negative sequence power direction relay, and then draw the negative sequence current obtained in item (4) to the characteristic diagram of the power direction relay to analyze the relay action. Happening. If the behavior of the relay is equal to the result of the analysis, the wiring is considered correct.如以æ–å¼€A相电压求出的负åºç”µåŽ‹U2为基准绘出负åºåŠŸçŽ‡æ–¹å‘继电器的动作特性如图F-6所示(å‡è®¾çº¿è·¯åªé€æœ‰åŠŸ)。由图å¯çŸ¥ï¼Œåˆ†åˆ«é€šäººAã€C相电æµç»§ç”µå™¨åŠ¨ä½œï¼Œé€šäººB相电æµæ—¶ç»§ç”µå™¨ä¸åŠ¨ä½œã€‚继电ä¿æŠ¤çŸ¥è¯†é—®ç”汇编由扬州拓普电气科技有é™å…¬å¸ç”ŸæŠ€éƒ¨ä¾›ç¨¿ï¼Œå’¨è¯¢çƒçº¿51ã€å¦‚图F-7所示å°æŽ¥åœ°ç”µæµç³»ç»Ÿä¸ï¼Œk1(*è¿‘N侧)k2ä¸åŒåœ°ç‚¹å‘生两点接地çŸè·¯æ—¶ï¼Œä¸ºä»€ä¹ˆæ¨ªå·®æ–¹å‘ä¿æŠ¤ä¼šè¯¯åŠ¨ä½œ?
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(2)检查下列开关é‡æ˜¯å¦ä¸ºå¦‚下状æ€ï¼š
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58ã€å¾®æœºç»§ç”µä¿æŠ¤è£…置的现场检验应包括哪些内容?
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(1)测é‡ç»ç¼˜ã€‚
(2)检验逆å˜ç”µæº(拉åˆç›´æµç”µæµï¼Œç›´æµç”µåŽ‹ï¼Œç¼“慢上å‡ã€ç¼“慢下é™æ—¶é€†å˜ç”µæºå’Œå¾®æœºç»§ç”µä¿æŠ¤è£…置应能æ£å¸¸å·¥ä½œ)。
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(4)检验数æ®é‡‡é›†ç³»ç»Ÿçš„精度和平衡度(5)检验开关é‡è¾“入和输出回路。
(6)检验定值å•ã€‚
(7)整组试验。
(8)用一次电æµåŠå·¥ä½œç”µåŽ‹æ£€éªŒã€‚
59ã€åœ¨è£…设接地铜排时是å¦å¿…须将ä¿æŠ¤å±å¯¹åœ°ç»ç¼˜?
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61ã€ä¸ºä»€ä¹ˆè¦æ±‚高频阻波器的阻塞阻抗è¦å«æœ‰è¶³å¤Ÿçš„电阻分é‡?
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63ã€é«˜é¢‘阻波器的工作原ç†æ˜¯ä»€ä¹ˆ?
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Answer: The demagnetization protection of the generator may have to act when the voltage transformer breaks one phase or breaks the two phases and the system is asymmetric. In order to prevent the mis-magnetization protection from malfunctioning under the above conditions, a negative sequence voltage blocking device is installed to make the generator's demagnetization protection when the generator is really demagnetized, which reflects the de-magnetized relay action and the negative-sequence voltage blocking relay does not act.
2. What is the reactance transformer and why does the reactance transformer have an air gap?
A: The reactance transformer is a transformer with a primary winding connected to a current source (ie, input power supply) and the secondary winding is close to an open circuit state (ie, output voltage). Its reactance value (called transfer impedance) is the excitation reactance. Because it is required that the excitation reactance Ze should be small and have good linear characteristics, there must be a gap in the magnetic circuit. The excitation impedance Ze of the reactance transformer is basically reactive, so U2 leads the primary current I1 by approximately 90°.
3. What are the main differences between current transformers and current transformers that convert current into voltage in relay protection devices? How does the former make I1 and U2: in phase and the latter makes I1 and U2 reach the required phase?
A: The main difference is in the core structure, TA no air gap, and DKB has an air gap, open circuit excitation resistance TA is large and DKB is small; in the phase of primary current and secondary voltage, TA is in phase, DKB primary current is behind secondary voltage 90°; TA secondary voltage is taken from the voltage drop across the load resistor R. To achieve the same phase and appropriate capacitance, DKB can connect the variable resistor in parallel with the secondary coil. * Change the resistance to obtain the required phase.
4. Why should the generator be equipped with negative sequence current protection?
Answer: When an asymmetric short-circuit or three-phase asymmetrical operation occurs in the power system, there is a negative sequence current in the stator winding of the generator. This current generates a reverse rotating magnetic field in the air gap of the generator, which is twice the synchronous speed with respect to the rotor. As a result, a frequency-doubling current occurs in the rotor component. This current causes local burns in certain parts of the rotor where the current density is high. In severe cases, the guard ring may be loosened by heat, causing significant damage to the generator. Another 100Hz alternating electromagnetic torque will act on the large rotor shaft and the stator frame, causing a frequency of 100Hz vibration. In order to prevent the occurrence of the above-mentioned hazardous generator problems, negative sequence current protection must be set.
5, by the A, C phase current transformer differential protection of the composition of the protection line, when the protected circuit occurs a variety of phase short-circuit fault, the three-phase short-circuit as a benchmark, the relative sensitivity of how?
Answer: Because the current flowing through the relay during the three-phase short-circuit is Ik=√3I(3), the current flowing through the relay when the two phases are short-circuited is Ik=2I(2)=2×(√3/2·I(3) ). The current flowing through the relay when the two phases of BC are short-circuited is Ik=I(2)=I(3). The current flowing through the relay when the two phases of AB are short-circuited is Ik=I(2)=I(3). Therefore, when the three-phase short-circuit sensitivity KS=1 is used as a reference, KS=1 when the AC phase is short-circuited and KS=1/2 when the AB and BC phases are short-circuited.
6. Why should bus protection devices be installed in the high-voltage power grid?
A: Although the probability of a short-circuit fault on the bus is less than the number of transmission lines, the bus is a meeting point for multiple components. If bus faults are not quickly removed, the accident will be expanded, or even the system will be destroyed, which will endanger the safe operation of the entire system. serious. In the double bus system, it is of great significance to selectively cut off the faulty busbars and ensure that the busbars continue to operate. Therefore, it is required to generally install bus protection devices in high-voltage power grids.
7. In the normally closed contact loop of the synchronous relay in the three-phase reclosing start circuit, why should the series check circuit have a voltage normally open contact?
Answer: The series KV normally open contact in the same period of reclosing the start circuit of the reclosing, the purpose is to ensure that there is a voltage on the line before the coincidence period coincides. In addition, under normal circumstances, due to some reason, in the case of no pressure overlap detection, Because the circuit breaker falls off automatically, the line voltage cannot be reclosed. At this time, if the string with KV normally open contacts in the same period of the start-up circuit is checked in parallel with the no-voltage start circuit, the start-up circuit in the same period can correct this erroneous trip. .
8. What are the conditions for quasi-synchronous periods and what are the consequences of unsatisfied conditions?
A: Quasi-synchronized parallel conditions are that the voltage of the generator to be connected and the voltage of the system are the same, the phases are the same, and the frequencies are the same.
When the above conditions are not satisfied, juxtaposition will cause an inrush current. The greater the difference in voltage, the greater the inrush current; the greater the difference in frequency, the shorter the period of the inrush current. Inrush currents are detrimental to both generators and power systems.
9. What is the maximum sensitivity angle of the directional impedance relay and why is it necessary to adjust its maximum sensitivity angle equal to the impedance angle of the protected line?
Answer: The impedance angle of the maximum operating impedance (amplitude) of the directional impedance relay is called its maximum sensitivity angle φs. When the phase-to-phase short circuit occurs in the protected circuit, the angle between the short-circuit current and the voltage at the relay installation is equal to the impedance angle φL of the circuit. When the line is short-circuited, the directional resistance relay measures the impedance of the impedance, φm, equal to the impedance angle of the line, φL. To make the relay work in the most sensitive state, the maximum sensitivity angle φS of the relay is required. Equal to the impedance angle of the circuit to be protected, φL.
10. What is the voltage transformer reverse charging and what is the effect on the protection device?
Answer: The charging of busbars that are not energized through the secondary side of the voltage transformer is called reverse charging. Such as 220kV voltage transformer, change ratio is 2200, even if the busbar is not grounded, its impedance (including bus capacitance and insulation resistance) is large, assuming 1MΩ, but the impedance seen from the secondary measurement of the voltage transformer is only 1000000/(2200)2=0.2Ω, almost short circuit, so the reverse charging current is larger (the reverse charging current is mainly determined by the cable resistance and the leakage resistance of the two voltage transformers), which will cause the secondary side of the voltage transformer during operation. If the small switch trips or the fuse blows, the protection device in operation loses voltage, which may cause the protection device to malfunction or refuse to move.
11. Why should the generator be installed with low-voltage blocking over-current protection? Why does this protection use the current transformer at the neutral point of the generator?
Answer: This is set as a differential protection of the generator or backup protection of the next component. It acts when the following two types of faults occur: (1) When an external short-circuit occurs, the protective device or circuit breaker of the faulty component refuses to act. Time. (2) When the fault is within the differential protection range of the generator and the differential protection rejects the action. In order for this protection to work after the generator is not connected to the bus after the generator has been pressurized or disconnected from the busbar (voltage not dropped), an internal short circuit will still work, so the neutral point of the generator should be used. Current transformer.
12. Why does a large-capacity generator adopt 100% stator grounding protection and explain the principle of 100% stator winding single-phase grounding protection for additional DC voltage.
A: The ground protection using the zero-sequence current and zero-sequence voltage principle can not achieve 100% protection range for the stator windings. There is a dead zone near the near-neutral point. In fact, large-capacity units are often due to mechanical Damage or water leakage in the water-cooled system may cause ground faults in the vicinity of the neutral point. If such failures cannot be detected in time, it may cause the fault to expand and cause serious damage to the generator. Therefore, 100% protection zone stator ground protection must be installed on large-capacity generators. The principle of 100% stator grounding protection of the additional DC power supply is shown in Figure F-1. When the generator is in normal operation, there is no current in the KA coil of the current relay and the protection does not act. When the stator winding of the generator is single-phase grounded, the DC voltage passes through the ground point of the stator loop and is added to the current relay KA, so that a current passes through the KA and acts and sends a signal.
13. Why large-capacity generators should use negative-sequence inverse-time overcurrent protection?
A: The asymmetry of the load or system causes a negative-sequence current to flow through the stator windings of the generator and a negative-sequence rotating magnetic field is established in the air gap of the generator, causing the rotor to induce twice the frequency of the current, causing the rotor to heat up. Because of the direct cooling type (water internal cooling and hydrogen internal cooling), a large generator requires a smaller volume increase than a larger capacity. At the same time, the thermal capacity margin of a large unit is generally lower than that of a large generator due to economic and technical reasons. Small units are small. Therefore, the negative sequence additional heating of the rotor should pay attention, the general trend is that the greater the capacity of the single machine, the smaller the value of A, the lower the ability of the rotor to withstand the negative sequence current, so we must particularly emphasize the negative sequence protection of large generators. The duration of the allowable negative sequence current of the generator is A=I2t. The larger the I2 is, the shorter the allowable time is, and the smaller the I2 is, the longer the allowable time is. Because of the inverse time characteristics of the generator to I2, negative sequence reverse time overcurrent protection should be used on large units.
14. Why should the generator excitation circuit be equipped with one-point grounding and two-point grounding protection?
Answer: The generator excitation circuit is grounded at one point. Although it will not form a fault current path, it will not cause direct damage to the generator. However, it is necessary to consider the possibility of the second point of grounding. Therefore, a signal of grounding protection is issued to strengthen the inspection. Surveillance. When the generator excitation loop occurs with two earth faults: 1 The rotor of the generator is burned due to the fault current flowing through the fault; 2 The symmetry of the air gap of the generator is damaged, causing severe vibration of the generator; 3 The rotor slowly deforms to form an eccentricity, further exacerbating the vibration. Place two points of ground protection after one point of grounding, so that when two points of grounding occurs, it will stop after a delay.
15. What is the role of the longitudinal differential protection and horizontal differential protection of the generator and the protection scope? Is it possible to replace each other?
Answer: Diffusion protection is the most effective method of protection against internal short-circuit fault protection of the generator and is the main protection against short-circuits between generator stator windings.
The horizontal differential protection is to protect the stator winding of the generator from a phase-to-turn short circuit and the inter-turn short circuit protection between the two parallel branches of the same phase. For generators with a star connection and two parallel leads per phase, the generator must be installed. Set the horizontal difference protection. When the phase windings near the stator windings or near the neutral point are short-circuited, the current in the two neutral point connections is small, and the horizontal differential protection may not operate, and the dead zone may reach 15% to 20%), so it cannot replace the longitudinal differential. protection.
16, negative sequence current relay, when the two primary winding or secondary winding of the reactance transformer and the corresponding polarity of the winding of the intermediate converter is reversed, what will happen and how to prevent it?
Answer: Negative sequence current relay reactance Transformer When the primary or secondary windings are connected to the opposite polarity of the intermediate converter winding, the negative sequence relay will become a positive sequence relay. After the relay protection is put into operation, no asymmetry fault occurs, but if the load current reaches a certain value, it will malfunction. In order to ensure the correct connection of the negative sequence current relay and prevent the above situation, the following measures must be taken:
(1) Pass the three-phase power supply to check the setting of the negative sequence current relay.
(2) When testing with a single-phase power supply, the relative polarity should be measured in accordance with the principle of the negative-sequence filter, so that it meets the requirements, and the load current must be used for inspection. After confirming that the wiring is correct, it is put into operation.
17. Why is 220kV and above systems equipped with circuit breaker failure protection and what is its role?
Answer: The transmission line above 220kV generally has large power and long transmission distance. To improve the transmission capacity of the line and the stability of the system, phase-breaking circuit breakers and rapid protection are often used. Because the circuit breaker has the possibility of operation failure, when the circuit fails and the circuit breaker refuses to move, it will pose a great threat to the power grid. Therefore, the circuit breaker failure protection device should be installed, and the circuit breaker that will selectively reject the failure will be opened. The circuit breaker of the bus where the device is located (connected) is disconnected to reduce equipment damage, reduce the scope of power outage, and improve the safety and stability of the system.
18. Why does the negative sequence starting element in distance protection change to a three-phase output type?
A: Because the three-phase negative sequence current relay has a small ripple coefficient after three-phase full-wave rectification, the required filter capacitance is small, and the reaction speed is fast. Especially when the three-phase symmetrical short circuit occurs, the DC component in the short-circuit current can be fully utilized. Ensure that the device can operate. In addition, when the system oscillates and asymmetrically operates, the DC voltage of unbalanced voltage is output, and the ripple factor is small. After the differential circuit, the voltage of the actuator is low, so that it has a higher anti-vibration capability.
19. What are the requirements for WXB-11 microcomputer protection AC current loop wiring?
Answer: Microcomputer protection is different from other types of protection devices because it is based on computer program operations to determine internal and external faults and types of faults. Therefore, if the current loops of IA, IB, and Ic are connected to the positive polarity to protect the polarity of the microcomputer, then 3I. It must also be connected to the positive polarity to protect the polarity of the zero-sequence loop of the microcomputer. No reverse polarity access is allowed. Otherwise, when an internal fault occurs, the input data and the protection program do not correspond. It will be judged as an AC data acquisition system fault. The protection device is locked, causing the protection device to refuse to act when an internal failure occurs.
20. What are the advantages of the oscillation locking device constructed by using the negative sequence plus the zero-sequence current increment principle?
Answer: The oscillation blocking device activated by the negative sequence plus zero-sequence current increment can better distinguish the oscillation and the short circuit, and can prevent the protection from malfunction due to the increase of the imbalance output of the negative sequence current filter when the system oscillates. It can also prevent unbalanced output of the filter when the line is not transposed, three-phase unbalanced, harmonic components, and non-full-phase operation are unbalanced. The locking device adopts the design principle of "non-stopping oscillation and unblocking" so that the protection will not malfunction due to the long duration of oscillation. Because the protection is only put into operation for a short time after the operation of the actuator, the system will operate without causing protection malfunction. In addition, this type of oscillating locking device also has a higher sensitivity and a faster operating speed, and thus has been widely used.
21, direction of over-current protection Why must start with the phase mode?
Answer: Directional over-current protection adopts the “phase-by-phase†wiring method in order to avoid misoperation of the device when the two-phase short circuit occurs in the opposite direction. For example, when the BC phase short-circuit occurs in the reverse direction, the relay in the phase A direction of the line will operate in the positive direction due to the load current. If the phase is not started at this time, when the C-phase current element is activated, it will cause the device to malfunction; Phase start wiring, although the A-phase direction relay operates, but the A-phase current element does not move, and the C-phase current element operates but the C-phase direction relay does not operate, so the device will not malfunction.
22, what is the connection coefficient of the current transformer? What is the effect of the wiring factor?
Answer: The ratio of the current through the relay and the secondary current of the current transformer is called the wiring coefficient of the current transformer, that is, the current in the Ik-I flow relay of the type Kc=Ik/I2; 12--the current transformer of the current transformer The secondary current connection coefficient is an important parameter in the relay protection setting calculation. The wiring factor must be taken into consideration when calculating the action values ​​of various current protection measurement components.
23, what is the high frequency blocking distance protection?
Answer: The basic principle of high-frequency blocking distance protection is to use â–³I2 â–³I. Incremental components act as starting components. They start high-frequency transceivers in the event of a fault, send high-frequency blocking signals, and use distance-II or III-direction directional impedance relays as faulty power-discriminating components. If internal faults occur, both sides of the distance protection stage or The third segment measures the action of the component, stops the high-frequency blocking signal, and instantaneously trips off the fault. If an external fault occurs, the positive-direction side contacts the impedance relay in the II- or III-direction direction to stop the transmission, but the impedance element in the opposite direction does not operate, and the relay continues to send the protection to the opposite side of the latch. This not only has the function of high-speed protection across the line, but also has the function of distance protection II for adjacent backup protection. Its main disadvantage is that the high-frequency protection and distance protection wiring are not connected to each other for easy operation and maintenance.
24. How to check the resonant frequency of the series resonance filter?
A: Connect the filter to the wiring as shown in Figure F-3. The resistance R of the circuit should be such that the output of the oscillator does not exceed its rated capacity when the impedance Z of the filter is minimum. Then use a high internal resistance voltage meter to measure the voltage on R. The voltage of the fixed oscillator is 10V, change its frequency, when the tube voltmeter indicates maximum, the corresponding frequency is the resonance frequency of the filter.
25. What is the reaction to the imbalance of active power in the power system? How to deal with?
Answer: The excess active power in the system will cause the frequency to rise, and the lack of active power will cause the frequency to drop. The solution is to adjust the output of the generator through the frequency modulation unit. When the situation is serious, some generator sets or part of the load are cut off through the automatic device or on-duty personnel operation to balance the system power.
26. For 3U. What constitutes a protection test and what are the countermeasures?
A: There are the following countermeasures for the 3Uo protection test:
(1) The 3Uo loop cannot be confirmed by checking the 3Uo loop for unbalanced voltage.
(2) It is not possible to confirm the correctness of the polarity of the directional protection formed by 3Uo by using the “hexagonal†test method alone.
(3) Tests may be conducted on the overall composition including current and voltage transformers and their secondary circuit connection and directional components to confirm that the polarity of the entire set of directional protection is correct.
(4) The most fundamental method is to find out the polarity of the voltage transformer and current transformer, and the polarity of all the connections from the transformer terminal to the relay protection screen and the polarity of the zero-sequence direction relay on the screen. judgment.
27, What is the danger of the power of the power direction relay? How to eliminate the inductive power direction relay?
A: The power direction relay may have reverse and positive creep. Reverse creep will increase the operating power. Positive creep may cause the protection to malfunction. The method to eliminate the creep of the induced power direction relay is as follows:
(1) Turn the position of the core column.
(2) Move the relative position of the voltage coil.
(3) Connect the voltage coil to the positive or reverse polarity of the compensation coil in series.
(4) Move the pole position slightly without the coil.
28. Simulate the setting of the three-phase short-circuit test negative sequence current relays. Why is the test current √3 times the value of the negative-sequence current setting? Analyze the symmetric component method.
A: Decompose a set of three-phase asymmetrical currents of A, B, and C into positive, negative, and zero-sequence symmetrical components. The negative-sequence symmetrical component is I2=1/3 (IA+a2IB+aIc). For short circuit, there is IB=-Ic=Ik(test current), IA=0 and its phasor is shown in Figure F-4. From the figure, we know a2IB+aIc=√3Ik, so I2=1/3·Ik·√3=Ik/√3. Similarly, it can be deduced that when ABCA two-phase short circuit is I2=Ik/√3, that is, the test current is a negative-sequence current. 3 times.
29. What is the blocking angle of high-frequency protection?
A: As shown in Figure F-5(a), when a through fault occurs at point k, ideally IM and IN
180 degrees difference, the protection device does not operate. In fact, when the line is externally faulty, due to various factors, the phase angle difference between IM and IN is not 180°, and the signal received by the receiver has a discontinuous angle. According to the principle of phase difference high frequency protection, the protection device will act when the line breaks and the discontinuity occurs. For this purpose, the maximum possible discontinuity angle of external faults should be found out, and this value should be used for blocking so as to ensure that the protection does not malfunction when the line fails externally. This maximum discontinuity is called the difference in the blocking angle of the high-frequency protection. As shown in Figure F-5 (b>), the protection zone φop is (180°-β)> φop> (180°+β), and the blocking angle is β.
30. What regulations should be followed in the management of the operating procedures of microcomputer relay protection devices?
Answer: The management of the operating procedures of microcomputer relay protection devices should follow the following rules:
(1) All networks (provinces) shall coordinate the procedures for the unified management of microprocessor relay protection devices within the scope of direct jurisdiction. Each network (provincial) shall set up a relay protection laboratory. After the new procedure passes the full test of the laboratory, Allow to be put into operation in the field.
(2) The same type of microcomputer high-frequency protection program version at both ends of a line should be the same.
(3) The program change of the microprocessor relay protection device shall be strictly implemented according to the procedure notice issued by the professional department of the dispatching relay protection.
Relay Protection Knowledge Q & A Compilation is provided by the Department of Biotechnology of Yangzhou Tuopu Electric Technology Co., Ltd., Hotline for consultation, website:. . Com
31. What is the criterion for how to open the impedance relay of LFP-901A protection when it fails again in the non-full-phase operation?
Answer: Open by non-full-phase oscillation locking element:
(1) When the single-phase fault reoccurs in the non-full-phase operation, the phase-selection area is not opened when the phase-opening fault occurs.
(2) When a phase-to-phase fault occurs again in a non-full-phase operation, the power-frequency variation of the difference between non-faulty two-phase currents is measured, and is opened when the current suddenly increases to a constant amplitude.
32. What problems should be noticed in the inspection of microcomputer relay protection devices?
Answer: When testing the relay protection device of the microcomputer, in order to prevent damage to the chip, the following problems should be noted:
(1) The microcomputer relay protection screen (cabinet) should have good grounding, and the grounding resistance should meet the design requirements. When measuring the circuit parameters using an AC power source electronic instrument (such as an oscilloscope, a frequency meter, etc.), the measurement terminals of the electronic instrument and the power supply side should be well insulated, and the instrument housing should be grounded at the same point as the protection panel (cabinet).
(2) The electric soldering iron should not be used in the test. If a soldering iron must be used, a special soldering iron should be used, and the soldering iron should be grounded at the same point as the protective screen (cabinet).
(3) When touching the pins of the chip by hand, measures shall be taken to prevent the damage of the IC to the IC chip.
(4) Plug and pull plugs are allowed only after the DC power is disconnected.
(5) Pull out the chip application special extractor, insert the chip should pay attention to the direction of chip insertion, after the insertion of the chip should be confirmed by the second person after the confirmation, before the electricity can be tested or used.
(6) When measuring the insulation resistance, insert the card with IC chip (except optocoupler and power plug).
33. In a small ground current system, why is single-phase ground protection used only to signal in most cases and not to trip?
Answer: In the small grounding current system, the symmetry of the system voltage is not damaged when a phase is grounded, the current through the fault point is only the capacitive current of the system, or the residual current after the arc suppression coil is compensated, and the value thereof is very small. The impact on grid operation and users' work is small. In order to prevent a short-circuit fault from occurring when grounding occurs again, the protection device is generally required to issue a warning signal in time so that the on-duty personnel can handle it as appropriate.
What is the principle of line phase-to-phase short-circuit protection configuration in 34. 5kV neutral point ungrounded networks?
Answer: The principle of phase-to-phase short-circuit protection configuration is:
(1) When using two-phase current protection, the current transformer should be mounted on the same phase of each outgoing line (for example, phase A and phase C).
(2) The protection device adopts far-back-up mode.
(3) If the line short-circuit causes the voltage of the power bus for the power plant, the bus of the main power contact point, or the voltage of the important user bus to fall below 50% to 60% of the rated voltage, the fault should be quickly removed.
35. After the new installation and overhaul of the power transformer, why should the impact closing test be conducted before the official operation?
Answer: The power transformer after the new installation and overhaul must be subjected to the impact closing test before it is officially put into operation. This is to check the insulation strength and the mechanical strength of the transformer, and to inspect the performance of the differential protection from the excitation inrush current. The newly installed equipment should be impacted 5 times and the equipment should be impacted three times after overhaul.
36, what is the automatic load reduction AFL device according to frequency? What is its role?
A: In order to improve power quality and ensure the availability of power for important users, when the frequency drop occurs due to the lack of active power in the system, depending on the degree of frequency decrease, some unimportant users are automatically disconnected, and the frequency is prevented from decreasing so that the frequency can be reduced. Quickly restored to normal, this device is called automatic frequency-reducing device, referred to as AFL device. It can not only guarantee the power supply of important users, but also avoid the collapse of the system caused by the frequency drop.
37. The two-phase current difference wiring of a relay can not be used in the case of Y, d wiring transformers.
Answer: Transformers that cannot be used in Y, D wiring. The phase winding of the transformer Y side is subjected to the phase voltage, and the phase of the delta side receives the line voltage, so the number of turns of the side winding of the triangle NΔ=√3Ny, and the ratio Iy/IΔ=Ny/NΔ=√3, such as When mounted on the Y side and a two-phase short circuit occurs on the △ side (ab phase), the A- and C-phase currents on the Y-side are both 1/√3, so the relay does not move.
38. Why does the overcurrent protection setting take into account the relay's return coefficient and the current quick-break protection does not need to be considered?
Answer: The overcurrent protection's operating current is set to escape the maximum load current and generally protects adjacent equipment. In the event of an external short-circuit, the current relay may start, but after the external fault has been removed (the current is dropped to the maximum load current), it must be *returned, otherwise false tripping will occur. The purpose of considering the return coefficient is to ensure that the protection energy * can be returned in the above case. The operating value of the current quick-break protection is set according to the maximum short-circuit current avoiding the predetermined point, and its setting value is much larger than the maximum load current, so there is no problem of not returning under the maximum load current. Furthermore, once the instant current quick-break protection starts the crap trip, there is no midway return problem, so the current quick-break protection does not consider the return coefficient.
39. Some distance protection will not immediately malfunction when the secondary circuit of the voltage transformer is out of phase. Why is there still a need to install a voltage loop off-phase locking device?
A: At present, there are some new or modified distance protections. The starting circuit is blocked by a negative sequence current element. When the secondary loop of the voltage transformer is broken, although the impedance element will malfunction, because the negative sequence current element does not start, the protection device will not immediately cause a malfunction. But when the secondary circuit of the voltage transformer is out of phase and encounters a through fault, it will still malfunction, so it is also necessary to signal when the secondary circuit of the voltage transformer is out of phase, and the time is greater than the time delay of the third stage delay. Latch protection.
40. Some distance protection will not immediately malfunction when the secondary loop of the voltage transformer fails. Why do you still need to install a voltage loop phase-locking and locking device?
A: At present, there are some new or modified distance protections. The starting circuit is blocked by a negative sequence current element. When the secondary loop of the voltage transformer is broken, although the impedance element will malfunction, because the negative sequence current element does not start, the protection device will not immediately cause a malfunction. However, when the secondary loop of the voltage transformer is out of phase and a through fault is encountered, misoperation will still occur. Therefore, the secondary transformer of the voltage transformer will also be signaled when the secondary loop is out of phase, and will start after the delay time greater than the III. Latch protection.
41. Why is it that the resonant frequency of the wave trap is about 0.2 kHz lower than the frequency of use?
A: Due to the phase-coupled high-frequency channel, other devices connected to the bus line form an impedance to ground. This impedance is connected in series with the impedance of the line trap to form a shunt of the high-frequency signal channel, resulting in shunted losses. The amount of shunt loss depends on the phasor sum of the two impedances. Experience has shown that the bus-to-ground impedance is capacitive in most cases. In order to avoid a series resonance between the impedance of the busbar and the bus-to-ground capacitor, the reactive component of the impedance of the resistor is cancelled out, so that the branch impedance is drastically reduced and the shunt attenuation is reduced. Increasing, it is required to protect the use of the trap's resonant frequency is lower than the protection of the use of frequency of about 0.2kI-Iz, to ensure that the trap in the use of frequency capacitive, so as to obtain the maximum impedance of the trap.
42. How to adjust the resonance frequency and blocking frequency band of a single frequency wave trap?
Answer: Wire as shown in Figure F-8. The voltage of the fixed oscillator is a certain value, change the frequency, when U2 appears minimum Umin, this frequency is the resonant frequency, and then change the frequency, when the U2 reading is Umin value of the upper and lower frequency difference, that is, single-phase resistance The waveband of the wave filter is â–³F.
43. How to set the current control element in circuit breaker failure protection?
Answer: When the current control element is in the minimum operating mode, the local busbar is faulty, and when the fault current at the opposite end is the minimum, there should be enough sensitivity to set the current control element. After the disconnection of the busbar breaker, the current control element should be able to operate. The setting value of the current control element should generally be greater than the load current. If the load current cannot be avoided after the sensitivity is set, the sensitivity requirement should be met.
44. Why is a non-linear resistor R connected in parallel at the entrance of the transistor transmitter channel and two diodes of opposite polarity in parallel are connected in R?
A: A non-linear resistor in parallel with the channel population is to make the device immune to the impact of channel interference signals. Because of the large static capacitance of the nonlinear resistor, a single and a non-linear resistor has a certain bypass effect on the high-frequency signal, so that the output voltage is reduced. Therefore, in the non-linear resistor R, two parallel-connected diodes of opposite polarity are connected. This reduces the total capacitance in the loop because the total capacitance between the poles of the diode is much smaller than the static capacitance of the non-linear resistor. The purpose of the diode is to prevent the positive and negative direction of the interference pulse current from being absorbed by the nonlinear resistor.
45. What is the load regulation effect? ​​If there is no load regulation effect, what will happen to the system when there is a shortage of active power?
A: When the frequency decreases, the active power drawn by the load decreases. When the frequency increases, the active power drawn by the load increases. This phenomenon of changing the active power of a load with frequency is called a load regulation effect.
Because of the existence of the load regulation effect, when the frequency changes due to the power balance failure in the power system, the load power changes with it and plays a compensating role. If the frequency drops due to the lack of active power in the system, the corresponding load power will also decrease, and some active power shortages will be compensated, which may make the system stable at a lower frequency. If there is no load regulation effect, when the frequency of the active power shortage system drops, the power shortage can not be compensated and the new active power balance will not be reached, so the frequency will decrease until the system collapses.
46. ​​What happens to the distribution of zero-sequence current when a ground fault occurs in a large ground current system?
A: The distribution of the zero-sequence currents is only related to the system's zero-sequence network and has nothing to do with the number of power supplies. When increasing or decreasing the number of neutral grounded transformers, the system zero-sequence network will change, thereby changing the zero-sequence current distribution. When increasing or decreasing the number of generators connected to the busbar and the number of ungrounded neutral transformers, and the number of neutral-grounded transformers does not change, only the magnitude of the ground current is changed, regardless of the distribution of the zero-sequence current.
47. Why is it necessary to set up a three-hop circuit breaker in high-frequency protection with remote start?
A: (1) In the event of a fault in the area, the circuit breaker on one side trips first. If the circuit breaker is not stopped immediately, the transmitter will generate continuous high-frequency signals due to no operating current, and the receiving side of the opposite side will also receive continuous signals. The high-frequency signal is latched to protect the outlet and cannot be tripped.
(2) When manually or automatically reclosed to a permanent fault, because there is no closing on the opposite side, the remote starting circuit sends a high-frequency continuous wave, so that the side to be closed first is blocked and the protection is refused. In order to ensure that the two devices can be operated in the above situation, a circuit breaker tripping trip must be set.
48. Why does a large grounding current system not use three-phase and phase current protection and zero-sequence current protection, but uses zero-sequence current protection alone?
A: The phase-to-phase current protection of the three-phase star connection can also react to the grounding short circuit, but when it is used to protect the ground from a short circuit, the maximum load current must be avoided at the fixed value, and the user must press the power supply direction in the operation time. The ladder principle is gradually increased by a time difference. The zero-sequence current protection that specifically reacts to the ground short circuit does not need to be set in accordance with this principle. Therefore, it has high sensitivity and short operation time, and because the zero sequence impedance of the line is much larger than the positive sequence impedance, the protection of the zero sequence current protection The range is long, and it is easy to fit between the lower and upper levels of protection. Therefore, phase current protection and zero-sequence current protection are generally not used.
49. Why does the three current transformers that make up the zero-sequence current filter have the same characteristics?
Answer: The output current of the zero-sequence current filter is IK=(IA+IB+IC)/nTA=3I. ×1/nTA In the equation above, the excitation current of the current transformer is ignored. Consider the output current of the excitation current filter as IK = [(IA + IB + IC) - (IAe + IBE + ICe)] / nTA where Iae + IBa + ICe)] / nTA is called unbalanced current if the three phases Current transformer saturation program is different, asymmetry of the excitation current and some differences formed in the manufacturing, in the normal operation will appear larger unbalanced current. Especially in the transient process of phase-to-phase faults, the short-circuit current contains a large non-periodic component, causing the current transformer core to be heavily saturated and a large unbalanced current appears. In order to reduce the unbalanced current, it is necessary to make the magnetization curves of the three current transformers the same, and work in the unsaturated part, while also minimizing its secondary load, so that the three-phase current transformer load is balanced as much as possible.
50. How to use the method of disconnecting one phase of working voltage and verifying the correctness of the negative sequence power direction relay wiring by means of three working currents A, B and C respectively.
Answer: (1) Measure the correct phase of the load current and the corresponding operating voltage.
(2) Disconnect the voltage of one phase of the breaker and connect the terminal of the disconnected phase to UN to simulate the negative sequence voltage. If the phase A is disconnected, U2=-1/3UA.
(3) Add the phase currents of relays A, B, and C, respectively, and disconnect the other two phases from the relay and short-circuit the power supply to observe the relay action.
(4) Use the A, B, and C phase load currents to find the corresponding negative sequence current and phase, respectively. If the A-phase current, I2 = 1/3 (IA + a2IB + aIc) = 1/3 IA, similarly, when people B-phase current I2 = IB, through the human C-phase current I2 = Ic.
(5) Using the simulated U2 as a benchmark, draw the operating characteristic diagram of the negative sequence power direction relay, and then draw the negative sequence current obtained in item (4) to the characteristic diagram of the power direction relay to analyze the relay action. Happening. If the behavior of the relay is equal to the result of the analysis, the wiring is considered correct.如以æ–å¼€A相电压求出的负åºç”µåŽ‹U2为基准绘出负åºåŠŸçŽ‡æ–¹å‘继电器的动作特性如图F-6所示(å‡è®¾çº¿è·¯åªé€æœ‰åŠŸ)。由图å¯çŸ¥ï¼Œåˆ†åˆ«é€šäººAã€C相电æµç»§ç”µå™¨åŠ¨ä½œï¼Œé€šäººB相电æµæ—¶ç»§ç”µå™¨ä¸åŠ¨ä½œã€‚继电ä¿æŠ¤çŸ¥è¯†é—®ç”汇编由扬州拓普电气科技有é™å…¬å¸ç”ŸæŠ€éƒ¨ä¾›ç¨¿ï¼Œå’¨è¯¢çƒçº¿51ã€å¦‚图F-7所示å°æŽ¥åœ°ç”µæµç³»ç»Ÿä¸ï¼Œk1(*è¿‘N侧)k2ä¸åŒåœ°ç‚¹å‘生两点接地çŸè·¯æ—¶ï¼Œä¸ºä»€ä¹ˆæ¨ªå·®æ–¹å‘ä¿æŠ¤ä¼šè¯¯åŠ¨ä½œ?
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