1. How to divide the protection range of each segment of zero-sequence current protection?
Zero-sequence current I section escapes the maximum zero-sequence current setting through the short-circuit current at the end of the line; it cannot protect the entire length of the line, but should not be less than 15% to 20% of the total length of the protected line; zero-sequence section II is generally Protect the full length of the line and extend it into the I section of the adjacent line and cooperate with it. Zero-sequence III segment is a backup segment of the I and II segments and cooperates with adjacent lines.
2. What is the difference between computer protection and original relay protection?
The main difference is that the original protection input is a current and voltage signal, and comparison processing is performed directly between the analog signals, so that the analog resistance is compared with a given resistance torque in the device. Computers can only perform numerical or logical operations. Therefore, first of all, it is required to convert the instantaneous value of the input analog current and voltage into discrete digital quantities before it can be sent to the central processing unit of the computer, perform calculations according to the prescribed algorithm and program, and perform the calculation result at any time with the given number. Compare and finally make a judgment on whether to trip.
3. What is the post-acceleration of reclosing?
When the line fails, the protection acts according to the setting value, the line switch is opened, and the reclosing gate immediately moves. In the case of a transient fault, after the line switch is turned off, the fault disappears, the reclosing succeeds, and the line resumes power supply; if a permanent fault occurs, the protective time component is retired after the coincidence, causing it to become a 0 second trip, which is a reclosing action. After the fault does not disappear, the acceleration trips, and the tripping cuts off the fault point.
4. What should I do after I operate the isolation switch by mistake?
(1) When pulling the disconnect switch by mistake, the arc will occur immediately after the knife gate leaves the static contact. Immediately after closing, the arc can be eliminated and the accident can be avoided. If the knife gate is fully opened, the knife gate that will be pulled incorrectly is not allowed. Close again;
(2) When pulling the disconnect switch by mistake, even if it is wrong, even when an arc occurs at the time of closing, it is not allowed to pull open again because the load-bearing switch will cause a three-phase arc short circuit.
5, what is called R, L, C parallel resonance?
A circuit with resistors, inductors, and capacitors connected in parallel has a circuit-side voltage and a total current in phase with each other under a sinusoidal power supply of a certain frequency. The entire circuit is in a resistive special state. This state is called a parallel resonance.
6. What is the difference between negative sequence and zero sequence increment components for the distance protection starting element?
(1) High sensitivity;
(2) It can be seen that the starting element of the oscillation blocking device is made;
(3) It will not malfunction when the voltage secondary circuit breaks;
(4) The presence of symmetrical components is independent of the phase of the fault, so a single relay can be used as the starting element, so it is relatively simple.
7. What conditions do the protection devices meet to assess a class of equipment?
All protective devices of a type of equipment have good technical conditions, and their performance fully meets the requirements for safe operation of the system, and meet the following main conditions:
(1) Protection screens, relays, components, auxiliary equipment and secondary circuits are not defective.
(2) The principle, wiring and setting value of the device are correct. It complies with relevant regulations, regulations and anti-accident measures.
(3) Drawings are complete and in line with reality.
(4) Operating conditions are good.
8. What are the inspection items and contents of the control switch?
Check the control switch:
(1) The shell is clean and free from oil and dirt.
(2) The installation should be firm and inactive during operation.
(3) The sealing cap seals well.
(4) The connection of each terminal shall be firm, not loose, and not rusted.
(5) Flexible rotation, correct position and good contact.
(6) Open the seal cover and check it with a flashlight. The interior should be clean, the grease should not be dry, and the contacts should not burn. Pressure test with insulating rod pressure, the pressure should be good.
9. What is the content of the private inspection of transformer differential protection when the transformer is put into use without load?
The differential protection of the transformer must be done at the rated voltage for 5 no-load tests during the new installation. Before making no-load inputs, check the secondary wiring and make sure it is correct. The no-load test shall be conducted on the large power supply side and the low pressure side of the transformer. This is because the system impedance and the transformer leakage resistance can limit the inrush current, while the large supply side system has a low impedance, and the general transformer low-voltage winding is wound inside. Since the leakage resistance is small, the surge current is large when the large power source and the low voltage side are input. In the test, the protective device should not be acted once, otherwise the relay's operating current should be increased.
10. What measures should be taken when demolition of secondary lines?
When you remove the secondary line, you must make a record; It should be recorded on the logbook to cancel. When more changes are made to the secondary line, each thread should be tied. When removing or laying secondary cables, they should also be tumbled at the leading and trailing ends of the cable and at the turns and crossings along the line.
11. What are the requirements for anti-accident measures for gas protection?
(1) The baffle type of the lower buoy of the gas relay is changed to the vertical type to improve the reliability of the heavy gas operation.
(2) To prevent short circuiting of the gas relay due to water leakage, rainproof measures shall be taken at the ends of the gas relay and the terminals inside the cable lead terminal box.
(3) Gas line should be used for gas relay outlet.
(4) hungry ah is the relay lead and cable should be connected to the terminal in the cable lead terminal box.
12. What are the general principles of transformer protection installation?
(1) Gas protection against internal short circuit and oil level reduction of iron shell of transformer.
(2) Defense against phase-to-phase short-circuits between transformer coils and lead-out lines, grounding short-circuits on the side of large ground-current grid-side coils, and longitudinal differential protection or current-branch protection of short-circuits between coil turns.
(3) Defense against phase-to-phase short-circuits external to the transformer, and for gas protection and over-current protection for longitudinal differential protection (or overvoltage protection initiated by compound voltage, or negative-sequence current protection).
(4) Protection against the zero-sequence current protection of the external grounding short circuit in large grounding current networks.
(5) Overload protection against symmetrical overload.
13. What is the role of distance protection starting element?
(1) When the short-circuit fault occurs, quickly start the protection device;
(2) Start the oscillation locking device or double as the measuring element of the third paragraph;
(3) Switch between segments;
(4) Switch between phases;
(5) In the transistor protection, if the DC logic part fails, the entire protection is blocked.
14, 10 kV transmission lines generally installed what protection?
(1) Phase-to-phase short-circuit protection: Two-stage over-current protection is generally applied to single-supply lines, that is, current quick-break protection and definite-time over-current protection. Dual power supply lines are generally installed with or without direction of the current speed protection and over-current protection.
(2) Grounding protection: Generally installed with non-selective insulation monitoring protection, zero-sequence over-voltage protection, power direction protection.
15. What is the effect of negative feedback on the operating performance of the amplifier?
(1) Reduce the magnification
(2) Improve the stability of magnification,
(3) Improve waveform distortion
(4) Broadband passband,
(5) Change the amplifier's input and output resistances.
16. What are the causes of non-sinusoidal currents?
The generation of non-sinusoidal currents can be either a power supply or a load. There are usually the following reasons:
(1) There are several different sinusoidal electromotive forces at the same time in the circuit, or the AC and DC emf work together.
(2) The circuit has a non-sinusoidal period of electromotive force.
(3) There are nonlinear components in the circuit.
17. The lightning arrester in the 6-35kV power system is connected to the ground voltage. Why should the arrester be selected according to the rated line voltage?
The 6-35kV system is a small ground short-circuit current system. Under normal conditions, the arrester is under the influence of the relative ground voltage, but when a single-phase ground fault occurs, the voltage of the non-faulty phase rises to the line voltage, and this grounding occurs. The fault is allowed to exist for a period of time, and the arrester should not act at this time. Therefore, the rated voltage of the arrester must use the rated line voltage of the system instead of the rated phase voltage.
18, the protection device meets what conditions can be assessed as three types of equipment?
The protective devices of the three types of equipment are either incompletely equipped or have poor technical performance, thus affecting the safe operation of the system. If the main protective device is in one of the following situations, it is also rated as Class III equipment:
(1) The protection does not meet the system requirements. In the event of a failure, it can cause the system to oscillate, disrupt the accident, or severely damage the main point equipment.
(2) Failed to meet anti-accident measures requirements.
(3) Marks are provided for connecting pieces, handles, buttons, etc. operated by operating personnel.
(4) The drawings are incomplete and not in conformity with reality.
(5) The fault oscilloscope cannot be perfectly recorded or not put into operation.
19. How to master the test environment conditions when testing relays?
Test environment requirements include temperature, relative humidity, and air pressure. These conditions not only affect the basic performance of the tested relay, but also have an impact on the working status of the test equipment. The test environment requirements are as follows:
(1) Temperature: 15 to 35 degrees
(2) Humidity: 45 to 75%
(3) Air pressure: 660 to 780mmHg
20. What principles should be mastered when selecting test instruments?
(1) Select the type of instrument according to the object being measured. First, according to whether the measured relay is DC or AC, DC meter or AC meter is selected.
(2) Select the internal resistance of the instrument according to the test circuit and the impedance of the relay coil under test.
(3) Select the appropriate meter based on the measured size.
(4) Select instruments according to the location and working conditions used.
21. After the newly installed protection device is completed, what are its main acceptance items?
Acceptance items are as follows:
(1) The actual measured parameters of electrical equipment and lines are complete and correct.
(2) All the protection device as-built drawings are in line with reality.
(3) The test value is in compliance with the requirements of the setting notice.
(4) The inspection items and results are in compliance with the inspection and inspection regulations and the relevant regulations.
(5) Check the current transformer ratio and volt-ampere characteristics, and the secondary load meets the error requirements.
(6) Check the equipment in front of the screen and behind the screen is neat, in good condition, the circuit is well insulated, and the signs are complete and correct.
(7) Acceptance tests are carried out with a load current and operating voltage to determine the correctness of the transformer polarity, ratio and loop accuracy, direction of the judgement, differential, distance, and high frequency protection devices and wiring.
22. How to check the zero-sequence voltage protection circuit of large ground current system in normal operation?
In order to ensure the correct operation of the zero-sequence direction protection, the integrity of the zero-sequence voltage protection circuit should be checked. The method is to use the test small busbar pairs drawn from the secondary windings of the voltage transformer opening delta for the protection of the voltage of the zero busbar direction YMN measurement voltage is 100V, which is normal.
23. What happens to the current between a fault line and a non-fault line when a single-phase earth fault occurs in a radial power grid with a small ground current system?
The zero-sequence capacitance current measured at the faulty line sending end is equal to the sum of the zero-sequence capacitance currents of other lines and flows to the busbar. The zero-sequence current measured at the non-failed line feed end is the non-fault relative capacitance current of the line and flows out of the bus.
24. In the large grounding current system, why is the time limit of the phase-to-phase protection action longer than that of the zero-sequence protection?
The time limit for protection is generally set on the basis of a step-by-step principle. Phase-to-phase protection action time limit is formed by a time-limit step increment for each level of protection from the user to the power supply, while zero-sequence protection is mostly Y/wired due to step-down transformers. When the low-voltage side is shorted to ground, there is no zero-sequence current on the high-voltage side. The operation time limit does not need to match with the transformer low voltage user. So the zero sequence protection action time is shorter than the phase protection.
25. What is the power system oscillation? What causes the oscillation?
The phenomenon of loss of synchronization between two systems or power plants operating in parallel is called oscillation. There are many causes of oscillation, most of which are due to the dynamic and stable destruction of the system caused by a long cutting time. In a weakly connected system, it may also be due to misoperation, loss of magnetization of the generator or tripping of the generator, or failure of a circuit. Device causes oscillation.
26. What are the requirements for the modulator?
(1) When the input DC signal Ui=0, the output signal U0=0
(2) The amplitude of the output AC signal should be proportional to the size of the DC signal,
(3) When the polarity of the DC signal Ui changes, the phase of the output AC signal also changes.
27. What is the principle of line phase-to-phase short circuit protection configuration in 35kV neutral point ungrounded networks?
The principle of phase-to-phase short circuit protection configuration is:
(1) When using two-phase current protection, the current transformer should be installed on two phases with the same name (for example, A and C phases).
(2) The protection device protection device should use a far back-up mode.
(3) If the line short-circuit causes the voltage of the busbars of the power plant, the bus of the main point of the contact point of the main power source or the buses of the important users to be lower than 50%-60% of the rated voltage, the fault should be cut off quickly.
28. What is the role of high-frequency protection in high-voltage power grids?
High-frequency protection is applied to the long-distance high-voltage power transmission line. Any type of fault at any point of the protected circuit can be instantaneously cut off from both sides, which can improve the stability of power system operation and the success rate of reclosing.
29. In a large grounding current system, why is the time limit of the phase-to-phase protection action longer than the zero-sequence protection action time limit?
The time limit for protection is generally set on the basis of a step-by-step principle. Phase-to-phase protection action time limit is constituted by a time-limit difference between each level of protection by the user to the power supply, while zero-sequence protection is mostly due to the step-down transformer Y and d11. When the low-voltage side is shorted to ground, there is no zero-sequence current on the high-voltage side. The time limit of its operation does not need to cooperate with the transformer low voltage user. So the zero sequence protection action time is shorter than the phase protection.
30. What are the basic requirements for operational amplifiers?
(1) The external impedance of the input terminal and the impedance of the feedback circuit should be accurate and stable;
(2) The open-loop voltage amplification should be large enough;
(3) The open-loop input resistance ri must be large enough.
(4) The open-loop output resistance should be small;
(5) Zero drift and noise are small.
31. What is the amplifier output resistance?
At the output of the amplifier, the amplifier can be seen as a signal source with a certain internal resistance. This internal resistance is the output resistance.
32, using the principle of superposition to calculate the linear circuit should pay attention to what matters?
Applying the principle of superposition can calculate the voltage and current of each branch under the action of each voltage source and current source separately, and then add the principle of superposition. Attention should be paid when applying the superposition principle.
(1) This principle can only be used to calculate linear currents and voltages, not for nonlinear circuits
(2) When superimposing, pay attention to the direction of current and voltage, superimpose the number of substitutions and
(3) The circuit connection method and the size of each resistor in the circuit cannot be changed. When the current source is applied, the voltage source is short-circuited. When the voltage source is applied, the current source is opened.
(4) The superposition principle is only applicable to the superposition of voltage and current, and the power cannot be calculated by the superposition principle.
33. Why should the turbine generator set overvoltage protection?
Due to the slow adjustment of the hydro-turbine speed control system, after the accident rejection of load, the over-voltage is not allowed to occur. Therefore, it is required to set over-voltage protection.
34, what is the generator low excitation and generator demagnetization?
Low excitation means that the excitation current of the generator is lower than the excitation current corresponding to the static stability limit. Loss of magnetism means that the generator loses excitation current.
35. Why should the generator be installed with a load voltage overcurrent protection? Why does this protection use the current transformer at the neutral point of the generator?
This is set as a generator differential protection or back-up protection for the next component and functions when the following two faults occur:
(1) When an external short circuit occurs, the protective device or relay of the faulty component refuses to act;
(2) When the fault is within the differential protection range of the generator and the differential protection rejects the action.
36, transformer composite voltage start over-current protection of the negative sequence voltage setting according to what principle is generally set? why?
When the system is in normal operation, the three-phase voltage is basically a positive sequence component, and the negative sequence component is very small. Therefore, the negative sequence voltage component is set to the output voltage of the negative sequence voltage filter during normal operation, and generally goes to 6 to 12V. (secondary voltage value)
37. Why is the protected area of ​​distance protection basically not affected by changes in the system's operating party?
Because the distance protection uses the ratio of the voltage and current at the beginning of the line as a criterion for protection, since the short-circuit impedance only changes with the distance from the short-circuit point to the beginning of the line, the protected area is basically unaffected by changes in the operating mode of the system.
38. Why do some large-capacity transformers and systems connect transformers with negative-sequence currents and single-phase low-voltage over-current protection as back-up protection.
Because of this protection has the following advantages:
(1) The sensitivity is high when an asymmetrical short circuit occurs;
(2) When an asymmetrical short circuit occurs after the transformer, its sensitivity is independent of the connection mode of the transformer.
39. What is the role of intermediate relays in relay protection?
(1) The contact points of the measuring elements in the protective device are generally small and the number of contacts is small, and the contact capacity and quantity can be increased through the intermediate relays;
(2) When a lightning arrester is installed on the line, the delay time of the protection device can be obtained by using the intermediate relay to prevent the snap action protection caused by the discharge arrester from malfunctioning.
(3) Meet the need of protection logic circuit.
40, electromagnetic current relay and voltage relay operating conditions What is the difference?
The voltage relay is generally connected to the secondary side of the voltage transformer. Compared with the current transformer, the voltage of the relay is high, so the number of turns of the relay coil is large, the wire is thin and the impedance is large, and the reactance of the coil is increased, so that the current is reduced; On the other hand, the magnetic reactance of the magnetic circuit is reduced, and the decrease of the current and the decrease of the impedance compensate each other, so that the electromagnetic torque of the relay during the operation is not changed and the relaying characteristics are lost.
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