Analysis of electrical secondary collection problems and answers

1. What is the difference between computer protection and original relay protection?

The main difference is that the original protection input is the current and voltage signals, which are directly compared between the analog quantities, so that the analog quantity is compared with the given resistance torque in the device. The computer can only do numerical operations or logical operations.

Therefore, it is first required to convert the instantaneous value of the input analog current and voltage into a discrete digital quantity, and then send it to the central processing unit of the computer, perform operations according to a prescribed algorithm and program, and perform the operation result with a given number at any time. Compare and finally make a judgment as to whether to trip.

2. How is the protection range of each segment of zero-sequence current protection divided?

The zero-sequence current I segment avoids the maximum zero-sequence current setting of the protected grounding short circuit at the end of the line; the full length of the line cannot be protected, but should not be less than 15%-20% of the total length of the protected line; Protect the full length of the line and extend it to and fit the I segment of the adjacent line. The zero-sequence III segment is a backup segment of the I and II segments and cooperates with adjacent lines.

3. What is the post-acceleration of the reclosing?

When the line fails, the protection acts according to the setting value, the line switch is disconnected, and the reclosing is acted upon immediately. If it is a transient fault, after the line switch is disconnected, the fault disappears, the coincidence is successful, and the line resumes power supply; if it is a permanent fault, after the coincidence, the protection time component is withdrawn, causing it to trip for 0 seconds, which is the reclosing action After the fault does not disappear, it accelerates the trip and trips to clear the fault point.

4. What should I do if I operate the isolation switch incorrectly?

(1) When the isolating switch is disconnected, the arc of the knife will just leave the static contact. When it is closed immediately, it can be arc-extinguished to avoid accidents. If the knife gate is fully opened, the knife gate that is accidentally pulled is not allowed. Close again;

(2) When the disconnecting switch is wrongly pulled, even if it is wrong, even if an arc occurs during closing, it is not allowed to pull open again, because the loaded knife gate will cause a three-phase arc short circuit.

5. What is R, L, C parallel resonance?

The circuit in which the resistor, the inductor and the capacitor are connected in parallel, under the action of a sinusoidal power supply of a certain frequency, the circuit terminal voltage and the total current appear in phase, and the whole circuit is in a special state of resistance. This state is called parallel resonance.

6. What is the difference between the negative-sequence and zero-sequence incremental components of the starting component for distance protection?

(1) high sensitivity;

(2) visible starting elements for the oscillating blocking device;

(3) There is no misoperation when the voltage secondary circuit is disconnected;

(4) The occurrence of the symmetrical component is independent of the phase of the fault, so the starting component can be a single relay, so it is relatively simple.

7. What conditions can the protection device meet to evaluate one type of equipment?

All the protection devices of a class of equipment are in good technical condition, and the performance fully meets the requirements for safe operation of the system, and meets the following main conditions:

(1) Protection screens, relays, components, accessory equipment and secondary circuits are free from defects.

(2) The principle, wiring and fixed value of the device are correct, in accordance with the relevant regulations, regulations and anti-accident measures.

(3) The drawings and materials are complete and in line with the actual situation.

(4) The operating conditions are good.

8. What are the inspection items and contents of the control switch?

The contents of the control switch are:

(1) The outer casing is clean and free of grease and intact.

(2) The installation should be firm and inoperative.

(3) The sealing cover is well sealed.

(4) The connection of each terminal should be firm, not loose, and not rusted.

(5) Flexible rotation, correct position and good contact.

(6) Open the sealing cover and check it with a flashlight. The inside should be cleaned, the grease should not be dry, and the contact points are not burnt. Test the contact with an insulating rod and the pressure should be good.

9. Transformer differential protection What is privately checked when the transformer is put into no-load?

For the differential protection of the transformer, the transformer must be subjected to 5 no-load tests at rated voltage during the new installation. The secondary wiring should be inspected and ensured to be correct before making no-load inputs. The no-load input test should be carried out on the large power supply side and the low voltage side of the transformer. This is because the system impedance and the transformer's hungry resistance can limit the excitation inrush current, while the large power supply side system has small impedance and the general transformer low voltage winding is wound inside. The leakage resistance is small, so the inrush current is large when the large power source and the low voltage side are input. In the test, the protection device should not be operated at all, otherwise the operating current of the relay should be increased.

10. What measures should be taken when dismantling the secondary line?

When disassembling the secondary line, it must be recorded; when recovering. It should be written off on the record book. When the secondary line changes more, the card should be tied on each thread. When disassembling or laying the secondary cable, the card should also be placed at the first end of the cable and at the turns and crossovers along the line.

11. What are the requirements for anti-accident measures for gas protection?

(1) The baffle type of the lower float of the gas relay is changed to the vertical type to improve the reliability of the heavy gas operation. (2) In order to prevent the gas relay from being short-circuited by water leakage, rain-proof measures should be taken at the end and the terminals in the cable lead terminal box. (3) The gas relay lead wire should be oil-proof. (4) Hungry, the lead wire and cable of the relay should be connected to the terminals in the cable lead terminal box respectively.

12. What are the general principles of transformer protection installation?

(1) Protection against internal short circuit of the transformer iron shell and gas protection with reduced oil level. (2) Defying the phase-to-phase short circuit between the transformer coil and the lead-out line, the grounding short-circuit on the lead-out side of the large ground current grid side coil, and the longitudinal differential protection or current quick-break protection of the coil turn-to-turn short circuit. (3) Protection against phase-to-phase short-circuit outside the transformer and over-current protection (or over-current protection of composite voltage start or negative-sequence current protection) for gas protection and longitudinal differential protection. (4) Zero-sequence current protection against external ground short-circuit in large ground current grid. (5) Overload protection against symmetrical overload.

13. What is the role of the distance-protected starting element?

(1) When the short circuit fault occurs, the protection device is quickly started; (2) the oscillation blocking device is activated, or it also serves as the measuring component of the third segment; (3) the segment switching is performed; (4) the phase switching is performed; (5) the transistor is switched. During protection, if the DC logic part fails, the entire set of protection is blocked.

What protection is generally installed on the 14 and 10 kV transmission lines?

(1) Phase-to-phase short-circuit protection: A single-supply line is generally equipped with two-stage overcurrent protection, that is, current quick-break protection, and time-limited overcurrent protection. Dual power lines are typically equipped with current speed protection and over current trip protection with or without direction. (2) Grounding protection: general installation without selective insulation monitoring protection, zero-sequence over-voltage protection, power direction protection.

15. What is the effect of negative feedback on the performance of the amplifier?

(1) Reduce the magnification, (2) Improve the stability of the amplification factor, (3) Improve the waveform distortion, (4) Broaden the passband, and (5) Change the input and output resistance of the amplifier.

16. What is the cause of non-sinusoidal current generation?

The generation of non-sinusoidal currents can be either a power source or a load. There are usually the following reasons: (1) There are several different sinusoidal electromotive forces acting simultaneously in the circuit, or AC interacting with the DC electromotive force, and (2) having a non-sinusoidal periodic electromotive force in the circuit. (3) There are nonlinear components in the circuit.

17. The arrester in the 6~35kV power system is connected to the relative ground voltage. Why should the arrester be selected according to the rated line voltage?

The 6～35kV system is a small grounding short-circuit current system. Under normal conditions, the arrester is under the action of relative ground voltage, but when a single-phase ground fault occurs, the ground voltage of the non-fault phase rises to the line voltage, and this grounding The fault is allowed to exist for a period of time, and the arrester should not be operated. Therefore, the rated voltage of the arrester must use the rated line voltage of the system instead of the rated phase voltage.

18. What conditions can the protection device meet to be rated as three types of equipment?

The protection devices of the three types of equipment are either incomplete or have poor technical performance, thus affecting the safe operation of the system. If the primary protection device has one of the following conditions, it is also rated as three types of equipment:

(1) Protection does not meet the system requirements, can cause system oscillations in the event of failure, disintegration accidents or severe damage to the main point equipment.

(2) Failure to meet the requirements of anti-accident measures.

(3) The connecting piece, the handle, the button, etc., which are operated by the operator, are provided with signs.

(4) The drawings are incomplete and are not in line with the actual situation.

(5) The fault recorder cannot be recorded or not put into operation.

19. How do you master the test environment conditions when testing relays?

The test environmental conditions include temperature, relative humidity, and air pressure. These conditions not only affect the basic performance of the test relay, but also affect the working state of the test equipment. The test environment conditions are as follows:

(1) Temperature: 15 to 35 degrees

(2) Humidity: 45 to 75%

(3) Air pressure: 660 ~ 780mmHg

20. What are the principles to be mastered when choosing a test instrument?

(1) Select the type of meter based on the object to be measured. First, according to whether the relay under test is DC or AC, choose DC instrument or AC instrument.

(2) Select the internal resistance of the instrument according to the test circuit and the impedance of the coil of the tested relay.

(3) Select the appropriate meter according to the measured size.

(4) Select the instrument according to the place and working conditions used.

21. After the completion of the newly installed protection device, what are the main acceptance items?

The acceptance items are as follows:

(1) The measured parameters of electrical equipment and lines are complete and correct.

(2) Completed drawings of all protection devices are in line with reality.

(3) The inspection value meets the requirements of the consolidation notice.

(4) The inspection items and results are in compliance with the inspection and inspection regulations and relevant regulations.

(5) Check the current transformer ratio and volt-ampere characteristics, and the secondary load satisfies the error requirement.

(6) Check that the equipment in front of the screen and behind the screen is neat and tidy, the circuit is well insulated, and the signs are complete and correct.

(7) Carry out the acceptance test with primary load current and working voltage, judge the polarity of the transformer, the ratio of the transformer and the correctness of the circuit, and judge the correctness of the components and wiring of the protection device such as direction, differential, distance and high frequency.

22. How to check the zero-sequence voltage loop of the zero-sequence direction protection of the large ground current system in normal operation?

In order to ensure correct operation of the zero sequence direction protection, the integrity check of the zero sequence voltage loop of the zero sequence direction protection should be performed. The method is to use the test small busbar drawn from the secondary winding of the open loop connection of the voltage transformer to measure the voltage of the small busbar YMN for each set of zero-sequence direction to be 100V, which is normal.

23. What is the difference between the current of a faulty line and a non-faulty line when a single-phase earth fault occurs in a small ground current system radiating power grid?

The zero-sequence capacitor current measured at the fault line is equal to the sum of the zero-sequence capacitor currents of other lines and flows to the bus. The zero-sequence current measured by the non-faulty line is the non-fault relative ground current of the line and flows out of the bus.

24. In a large ground current system, why is the time limit for phase-to-phase protection action longer than the time limit for zero-sequence protection?

The action time limit of protection is generally set according to the principle of step. The action time limit of the phase-to-phase protection is composed of a time-order difference of each level of protection from the user to the power supply direction, while the zero-sequence protection is mostly Y/wiring due to the step-down transformer. When the low-voltage side is short-circuited to ground, there is no zero-sequence current on the high-voltage side. The action time limit does not need to match the low voltage user of the transformer. Therefore, the action time limit of zero sequence protection is shorter than that of phase protection.

25. What is the power system oscillation? What are the reasons for the oscillation?

The phenomenon that two systems or power plants operating side by side are out of synchronization is called oscillation. There are many reasons for causing oscillations. Most of them are caused by the dynamic stability of the system due to the long time of resolving the fault. In the weakly connected system, the generator may lose ground or fault trip, break a certain line or The device causes oscillation.

26. Which requirements should the modulator meet?

(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, 35kV neutral point ungrounded grid, what is the principle of line phase-to-phase short-circuit protection configuration?

The principle of phase-to-phase short-circuit protection configuration is:

(1) When two-phase current protection is used, the current transformer should be installed on each of the two phases with the same name (for example, phases A and C).

(2) The protection device protection device should adopt the remote backup mode.

(3) If the short circuit of the line causes the voltage of the busbar of the power plant, the contact point bus of the main power source or the important user busbar to be lower than 50%~60% of the rated voltage, the fault should be quickly removed.

28. What is the role of high frequency protection in high voltage power grids?

The high-frequency protection acts on the long-distance high-voltage transmission line, and all kinds of faults at any point of the protected line can be instantaneously cut off from both sides, thereby improving the stability of the power system operation and the success rate of the reclosing.

29. In the large ground current system, why is the time limit of the phase protection action longer than the zero sequence protection action time limit?

The action time limit of protection is generally set according to the principle of step. The action time limit of the phase-to-phase protection is composed of a time difference of each level of protection from the user to the power supply direction, while the zero-sequence protection is mostly due to the step-down transformer, Y, d11 wiring, when the low-voltage side is short-circuited to ground, there is no zero-sequence current on the high-voltage side. The action time limit does not need to cooperate with the transformer low voltage user. Therefore, the action time limit of zero sequence protection is shorter than that of phase protection.

30. What are the basic requirements for an operational amplifier?

(1) The external impedance of the input terminal and the impedance value of the feedback circuit should be accurate and stable;

(2) The open-loop voltage amplification factor should be large enough;

(3) The open-loop input resistance ri should be large enough;

(4) The open-loop output resistance should be small;

(5) Zero drift and noise should be small.

31. What is the amplifier output resistance?

At the output of the amplifier, the amplifier can be thought of as a signal source with a certain internal resistance. This internal resistance is the output resistance.

32. Which matter should be paid attention to when calculating the linear circuit using the superposition principle?

Applying the superposition principle, the voltage and current of each branch under the action of each voltage source and current source can be separately calculated, and then the superposition principle is added up, and attention should be paid when applying the superposition principle.

(1) This principle can only be used to calculate linear current and voltage, and is not applicable to nonlinear circuits.

(2) Pay attention to the direction of current and voltage when superimposing, and replace the number and

(3) The circuit connection method and the size of each resistor in the circuit cannot be changed. When the current source acts, the voltage source is short-circuited, and when the voltage source acts, the current source is open.

(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 hydroelectric generator be set to overvoltage protection?

Due to the slow adjustment of the turbine governing system, it is prone to over-voltage protection after the accident load-shedding, so it is required to set over-voltage protection.

34. What is the low excitation of the generator and the loss of the generator?

Low excitation is the excitation current corresponding to the generator excitation current lower than the static stability limit. Loss of magnetism means that the generator loses the excitation current.

35. Why should the generator be equipped with a load voltage and its overcurrent protection? Why does this protection use a current transformer at the neutral point of the generator?

This is intended to be used as a generator differential protection or as a backup protection for the next component, and works when the following two faults occur:

(1) When an external short circuit occurs, the protection device or relay of the faulty component refuses to operate;

(2) When the fault occurs within the differential protection range of the generator and the differential protection rejects the action.

36. The negative sequence voltage setting of the transformer composite voltage starting overcurrent protection is generally set according to what principle? why?

When the system is in normal operation, the three-phase voltage is basically the positive sequence component, and the negative sequence component is very small. Therefore, the fixed value of the negative sequence voltage component is set according to the output unbalance voltage of the negative sequence voltage filter during normal operation, generally going to 6~12V. (secondary voltage value)

37. Why do some large-capacity transformers and system contact transformers use backup current protection and single-phase low-voltage starting overcurrent protection as backup protection.

Because this protection has the following advantages:

(1) When an asymmetric short circuit occurs, its sensitivity is high;

(2) When an asymmetrical short circuit occurs after the transformer, the sensitivity is independent of the wiring mode of the transformer.

38. What is the role of the intermediate relay in relay protection?

(1) The contact points of the measuring components in the protection device are generally small and the number is small, and the contact capacity and quantity can be increased by the intermediate relay;

(2) When a tubular arrester is installed on the line, the intermediate relay can be used to obtain the delay of the action of the protection device to prevent the quick action protection caused by the arrester from being discharged;

(3) Meet the needs of the protection logic loop.

39. What is the difference between the working conditions of electromagnetic current relay and voltage relay?

The voltage relay is generally connected to the secondary side of the voltage transformer. Compared with the current transformer, due to the high voltage, the number of turns of the relay coil is large, the wire is thin, the impedance is large, and the reactance of the coil is increased, so that the current is reduced; In the aspect, the magnetic circuit magnetic field is reduced, and the current reduction and the impedance reduction are mutually compensated, so that the electromagnetic torque of the relay does not change during the operation, and the relay characteristic is lost.

40. Why is the protected area protected by distance basically unaffected by changes in system operation?

Because the distance protection uses the ratio of the voltage and current at the beginning of the line as a criterion to protect, since the short-circuit impedance only changes with the short-distance point from the beginning of the line, the protected protection area is basically unaffected by the change of the operating mode of the system.