1. What is the task of relay protection in power system?
1) When the protected power system element fails, the relay protection device of the element should quickly and accurately send a tripping command to the circuit breaker closest to the fault element, so that the fault element can be disconnected from the power system in time, so as to minimize the damage to the power system element itself and reduce the impact on the safe power supply of the power system, And meet some specific requirements of the power system (such as maintaining the transient stability of the power system, etc.).
2) Reflect the abnormal working conditions of electrical equipment, and send signals according to the different working conditions and equipment operation and maintenance conditions (such as whether there is a regular person on duty), so that the person on duty can handle it, or the device can automatically adjust it, or cut off the electrical equipment that will cause accidents if it continues to operate. The relay protection device responding to abnormal working conditions is allowed to act with a certain delay.
2. What are the basic requirements for relay protection devices? What are the requirements?
The relay protection device must meet four basic requirements: selectivity, quick action, sensitivity and reliability.
1) Selectivity means that when the system fails, the relay protection device should selectively remove the fault to ensure that the non fault part continues to operate, so as to minimize the scope of power failure.
2) The quick action refers to that the relay protection shall act at the fastest possible speed to trip the circuit breaker to disconnect the fault or stop the development of abnormal state.
3) Sensitivity refers to the ability of the relay protection device to reflect the faults within its protection range, that is, the relay protection device should be sensitive to and reflect the possible faults and abnormal operation modes of the protected equipment.
4) Reliability refers to that it can act reliably when there is a fault that it should act, that is, it does not refuse to act; In any other case that does not belong to its action, reliable inaction will not occur, that is, misoperation will not occur.
3. What is the main protection and what is the backup protection?
Main protection: meet the requirements of system stability and equipment safety, and can selectively cut off the protection of protected equipment and line faults at the fastest speed.
Backup protection: when the main protection or circuit breaker fails to operate, it is used to cut off the fault protection, including near backup and far backup.
1) Remote backup protection is the backup protection realized by the protection of adjacent power equipment or lines when the main protection or circuit breaker fails to operate.
2) When the main protection fails to operate, the backup protection is realized by another set of protection of the power equipment or line; When the circuit breaker refuses to operate, the backup protection is realized by the circuit breaker failure protection.
4. What is auxiliary protection and abnormal protection?
Auxiliary protection: a simple protection added to make up for the deficiency of main protection and backup protection, or when the main protection and backup protection are out of service. This protection is a relay protection device that plays an auxiliary role.
Abnormal operation protection: protection that reflects the abnormal operation status of the protected power equipment or line.
5. What is breaker failure protection?
It refers to a backup protection that can cut off other relevant circuit breakers in the same power plant or substation with a shorter time limit when the circuit breaker refuses to act after the protection action of the fault line sends out a tripping pulse, so as to minimize the power failure range.
6. Why should 220KV and above systems be equipped with breaker failure protection? What is its function?
220KV and above transmission lines generally transmit large power and long transmission distance. In order to improve the transmission capacity of the line and the stability of the system, split phase circuit breakers and fast protection are often used. Due to the possibility of circuit breaker operation failure, when the circuit breaker fails and refuses to operate, it will pose a great threat to the power grid. Therefore, a circuit breaker failure protection device should be installed to selectively disconnect the circuit breaker of the bus connected to the failed circuit breaker, so as to reduce equipment damage, narrow the power cut range, and improve safety and stability.
7. What is power system oscillation? What are the causes of oscillation?
The phenomenon that two systems or power plants running in parallel lose synchronization is called oscillation.
There are many reasons for oscillation, most of which are the destruction of system stability due to too long fault removal time. In the system with weak connection, oscillation may also be caused by misoperation, generator loss of excitation or fault tripping, or disconnection of a line or equipment.
8. What is the difference between power system oscillation and short circuit?
(1) During oscillation, the voltage and current values at each point of the system oscillate to and fro, while the short-circuit current and voltage are abrupt. During oscillation, the change speed of current and voltage is slow, while during short circuit, the sudden change of current and voltage is large.
(2) The phase angle between voltage and current at any point of the system changes with the change of power angle during oscillation; In case of short circuit, the phase angle between current and voltage is basically unchanged.
9. What are the effects of power system oscillation on relay protection devices? Which protective devices are not affected?
It mainly affects current relay and impedance relay. In principle, the protection not affected by oscillation is differential protection.
10. What is the maximum and minimum operation mode of the system?
In the setting calculation of relay protection, the maximum and minimum operation mode of power system should be considered generally.
1) The maximum operation mode refers to the operation mode in which the equivalent impedance of the system is the minimum and the short-circuit current through the protection device is the maximum when the end of the protected object is short circuited.
2) The minimum operation mode refers to the operation mode in which the equivalent impedance of the system is the largest and the short circuit current through the protection device is the smallest under the same short circuit condition mentioned above.
11. What are the basic requirements for distance protection?
(1) When a direct short circuit occurs on the protected line, the measured impedance of the relay shall be proportional to the distance between the bus and the short circuit point;
(2) In case of short circuit outside the positive direction area, it shall not exceed the action;
(3) There should be clear directionality;
(4) It shall still be able to operate in case of short circuit through large transition resistance in the area;
(5) No action under the minimum load impedance;
(6) It can prevent misoperation during system oscillation.
12. What is the polarization of distance relay?
Answer: In addition to the distance measurement voltage, when constituting the action quantity of the distance relay, another specific AC quantity must be connected as the reference quantity to detect the uph phase switching when the fault occurs inside and outside the protection zone. This reference AC flow is generally called the polarization of distance relay.
13. What is an impedance directional relay?
It means that it can not only measure the impedance, but also judge the fault direction.
14. Why do some distance protection sections I and II need to go through the oscillation blocking device, while section III does not go through the oscillation blocking device?
The system oscillation period is generally 0.15 to 3 seconds. The contact of a section of impedance relay affected by the system oscillation will be closed and returned again in a cycle. If the closing time is longer than the action time of the distance protection device in this section, the section will misoperate due to the system oscillation, and the action time of the distance protection sections I and II is generally short, so the oscillation period cannot be avoided, Therefore, it needs to go through the oscillation locking device, and the action time from the section III is generally more than half of the oscillation period (1.5s), so it can not go through the oscillation locking device.
15. How many starting modes of reclosing?
There are two starting modes: the control switch of the circuit breaker does not correspond to the position of the circuit breaker, and the protection starting mode.
16. What is post reclosing acceleration?
When a line fault occurs, the protection selectively acts to cut off the fault, and the reclosing carries out a reclosing to restore power supply. For example, when reclosing on a permanent fault, the protection device will open the circuit breaker without delay and non selective action, which is called post reclosing acceleration.
17. Why are overhead lines equipped with automatic reclosing devices, while cable lines are not equipped with reclosing devices?
Automatic reclosing is set to avoid power failure caused by transient faults. Many faults of overhead lines are transient faults (such as bird damage, lightning strike, pollution). In most cases, these faults can disappear immediately after tripping the circuit breaker. The effect of reclosing is very significant. Because the cable line is buried underground, the fault is mostly permanent, and the effect of reclosing is not great, so the automatic reclosing device is not installed.
17. What is the overcurrent protection of composite voltage starting?
The over-current protection of composite voltage starting refers to the protection composed of a negative sequence voltage relay and a low-voltage relay connected to the phase to phase voltage on the basis of the over-current protection. Only when current measuring element and voltage starting element act, can the protection device act on tripping.
18. What are the advantages and disadvantages of gas protection for power transformers?
Advantages: simple structure, rapid action, high sensitivity, which can reflect that when the number of turns of various inter phase short circuits and inter turn short circuits in the transformer oil tank is small, the current of the fault circuit is large, which may cause serious overheating, but the change of the phase current outside the outgoing line may be very small, and the protection of various reactive currents is difficult to act, Gas protection has special advantages for removing such faults.
Disadvantages: it can not reflect external faults of transformer oil tank, such as bushing and outgoing line faults. Therefore, the transformer cannot use it as the only main protection.
19. What kind of fault does transformer differential protection and gas protection have? Can they replace each other?
The differential protection of transformer is the protection against the phase to phase short circuit of transformer winding and outgoing line, as well as the ground fault of the winding and outgoing line at the side of large grounding current system of transformer. Gas protection is the protection against various faults inside the transformer oil tank and oil level reduction, especially for the inter turn short circuit of the transformer winding, but it cannot reflect the faults outside the oil tank, so the two cannot replace each other.
20. What are the characteristics of transformer inrush current? What are the methods to prevent the influence of magnetizing inrush current in current differential protection?
The characteristics are as follows: it contains a large aperiodic component, which tends to make the inrush deviate to the side of the time axis; It contains a large number of high-order harmonic components, and the second harmonic is the main component; The outgoing line of the magnetizing inrush current waveform is discontinuous.
Methods to prevent the influence of magnetizing inrush current: distinguish the difference between the waveform of short-circuit current and magnetizing inrush current, and the discontinuity angle is required to be 60-65 degrees; Second harmonic braking; Use the principle of waveform symmetry.
21. Why should bus differential protection be set with voltage lockout? 3/2 wiring does not need voltage locking?
Due to the action of bus differential protection under the double bus connection mode, all equipment powered by the whole bus will be cut off, affecting a wide range. In order to prevent misoperation caused by bus differential protection circuit fault, the detection of bus voltage is used to help judge the fault to ensure the reliability of protection action, but it increases the possibility of protection failure.
However, the single bus fault of 3/2 wiring mode will not affect the power transmission of equipment, so 3/2 wiring mode is mainly to prevent the bus differential protection from refusing to operate, and cancel voltage locking.
22. Why should the generator be equipped with negative sequence current protection?
When the power system has asymmetric short circuit or three-phase asymmetric operation, the negative sequence current in the generator stator winding will generate a reverse magnetic field in the generator air gap, which is twice the synchronous speed relative to the rotor. Therefore, frequency doubling current appears in the rotor parts, which makes some parts of the rotor with high current density cause local rotor burns, which may cause the retaining ring to be heated and loose, causing major damage to the generator. In addition, 100Hz alternating electromagnetic torque will act on the main shaft of rotor and stator frame, causing 100Hz vibration. In order to prevent the above problems endangering the generator, negative sequence current protection must be set.
23. Why should the generator excitation circuit be equipped with one point grounding and two point grounding protection?
Although the generator excitation circuit is grounded at one point, it will not form a fault current path, which will cause direct harm to the generator, but the possibility of the second point grounding should be considered, so the signal is sent by the one point grounding protection to strengthen inspection and monitoring.
When a two-point grounding fault occurs in the generator excitation circuit, the generator rotor body is burned due to the considerable fault current flowing through the fault point. As part of the windings are shorted, the current in the excitation winding increases, which may be burned due to overheating. As part of the windings are shorted, the air gap magnetic flux loses balance, causing unit vibration, The turbine may also magnetize the shafting and turbine. Therefore, two point grounding protection shall be put into operation after one point grounding, so that in case of two point grounding, the machine will stop after delay.
24. What is the symmetrical component method?
Because the three-phase electrical quantity system is a symmetrical rotating vector arranged at 120 degrees of electrical angle with the same frequency, when asymmetry occurs, a group of asymmetrical three-phase systems can be divided into three groups of symmetrical positive sequence, negative sequence and zero sequence three-phase systems; On the contrary, three symmetrical positive sequence, negative sequence and zero sequence three-phase systems can also be combined into one asymmetrical three-phase system. This method of analysis and calculation is called symmetrical component method.
25. What are the characteristics of power grid current and voltage when single-phase metallic grounding occurs in the neutral point ungrounded system line?
(1) The fault phase to ground voltage is zero, and the non fault phase to ground voltage rises to line voltage;
(2) When single-phase grounding occurs, zero sequence voltage will appear in the whole system.
(3) The zero sequence current at the installation of non fault line protection is the capacitive current of the line to ground, and the zero sequence current at the installation of fault line protection is the sum of capacitive current of all non fault components to ground;
(4) The actual direction of capacitive reactive power at non fault line protection installation is from bus to line, and the actual direction of capacitive reactive power at fault line protection installation is from line to bus.
26. What is the distribution of zero sequence current related to when a grounding short circuit occurs in a large grounding current system?
The distribution of zero sequence current is only related to the zero sequence network of the system, and has nothing to do with the number of power supplies. When the number of transformers with neutral grounded is increased or decreased, the zero sequence network of the system will change, thus changing the distribution of zero sequence current.
27. When is the single-phase ground fault current greater than the three-phase short circuit current?
When the zero sequence comprehensive impedance of the fault point is less than the positive sequence comprehensive impedance, the single-phase ground fault current is greater than the three-phase short circuit current.
28. When is the zero sequence current of two phase grounding fault greater than that of single phase grounding fault?
When the zero sequence comprehensive impedance of the fault point is less than the positive sequence comprehensive impedance, the zero sequence current of the two-phase grounding fault is greater than the zero sequence current of the single-phase grounding fault.
29. Why does the current transformer not allow secondary open circuit operation?
When the secondary circuit of the current transformer in operation is open, the secondary current becomes zero, and its demagnetization effect disappears. At this time, the primary current will be used for excitation, and high electromotive force will be induced in the secondary winding, whose peak value can reach several thousand volts, posing a serious threat to the safety of personnel and equipment. Moreover, the magnetizing force generated by the primary winding makes the iron core suddenly saturated, and the active power loss increases, which will cause the iron core to overheat, and may even burn out the current transformer. Therefore, open circuit is not allowed in the secondary circuit of current transformer during operation.
30. What are static stability and transient stability of power system?
Static stability refers to the ability of the power system to automatically recover to the initial operating state without non periodic out of step after a small disturbance.
Transient stability refers to the ability of synchronous motors to maintain synchronous operation and transition to a new or restore to the original stable operation mode after the power system is subject to major disturbances.
