1. What are the main differences in electrical quantity changes during system oscillation and short circuits?
– During oscillation, the electrical quantities determined by the phase angle difference between the electromotive forces of parallel running generators change in a balanced manner. In contrast, during short circuits, the electrical quantities change abruptly.
– During oscillation, the angle between voltages at any point on the grid changes with the phase angle difference between the system electromotive forces. In contrast, the angle between current and voltage remains essentially constant during a short circuit.
– The system is symmetrical during oscillation, so there is only a positive sequence component in the electrical quantities. In contrast, harmful or zero sequence components are unavoidably present in the electrical quantities during short circuits.
2. What principle is the oscillation blocking device widely used in distance protection devices based on? What are the types?
It is based on the speed of current change and the differences between sequence components during system oscillation and faults. Common ones include oscillation-blocking devices using harmful sequence components or sequence increments.
3. What is the distribution of zero-sequence current related to when a grounding short circuit occurs in a system with a directly grounded neutral point?
– The distribution of zero-sequence current is only related to the system’s zero-sequence reactance. The size of the zero-sequence reactance depends on the capacity of the grounded transformers, the number and location of neutral point grounding, and changing the number of transformer neutral point grounding will change the zero-sequence reactance network, thus changing the distribution of zero-sequence current.
4. What are the components of a high-frequency channel?
– Composed of high-frequency transceiver, high-frequency cable, high-frequency impedance, combiner filter, coupling capacitor, transmission line, and ground.
5. What is the working principle of phase difference high-frequency protection?
– It directly compares the current phase on both sides of the protected line. Suppose the positive direction of each side’s current is from the bus to the line. The phase difference between the two sides’ currents is 180 degrees during noregularperation and external short circuit faults. When an internal short circuit fault occurs, if the phase difference between the two ends’ electromotive force vectors suddenly changes, the phase difference between the two sides’ currents becomes zero. Therefore, high-frequency signals are used to transmit the phase relationship of the power frequency current to the opposite side. The protection devices installed on both sides of the line act according to the high-frequency signals representing the phase relationship of the two sides’ currents, and when the phase angle is zero, the protection device operates to trip both circuit breakers simultaneously, achieving the purpose of quickly removing the fault.
6. What is gas protection?
– When a transformer fails due to reasons such as heating or short circuit arc combustion, the volume of the transformer oil expands, producing pressure and producing or decomposing gases, causing the oil to flow towards the oil pillow, the oil level drops, and the gas relay contacts are connected, acting on the circuit breaker to trip. This protection is called gas protection.
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Features:
- Flexible Voltage and Current Output: Supports 3-phase voltage and 3-phase current output, compatible with traditional testing methods, and easily handles three-phase transformer differential tests and plant power quick transfer and backup auto-transfer tests.
- Convenient Operation: Equipped with an embedded Windows operating system, it can run independently or be connected to a laptop or desktop computer for easy and stable operation.
- High-Fidelity Linear Amplifier: Uses high-fidelity, modular linear amplifiers instead of switching amplifiers, ensuring minimal high-frequency interference and providing smooth, accurate waveforms across a wide range of currents.
- High-Performance Main Unit: DSP control technology ensures fast calculation speed, strong real-time processing capability, and high-quality output with low distortion and good linearity.
- Powerful Software Functionality: Capable of automating complex verification tasks, storing real-time test data, displaying vector graphs, and easily printing reports.
- Comprehensive Protection Features: Includes multiple hardware protection measures, software self-diagnosis, and output locking to ensure stable and safe operation during testing.
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7. What is the scope of gas protection?
- 1) Internal multi-phase short circuit fault of the transformer;
- 2) Turn-to-turn short circuit, turn-to-turn short circuit with core or external short circuit;
- 3) Core fault;
- 4) Oil level drop or oil leakage;
- 5) Poor contactor welding of tap changer switch contacts, etc.
8. What is the difference between transformer differential protection and gas protection?
– Transformer differential protection is designed according to the principle of ring current. In contrast, when an internal transformer fault occurs, gas protection is set according to oil and gas flow characteristics. Their principles are different, and the scope of protection is not the same. Differential protection is the primary protection for the transformer and its system, and the tap changer is also the scope of differential protection. Gas protection is the primary protection against internal transformer faults.
9. What is the function of reclosing?
- 1) Quickly restore the power supply when a temporary fault occurs in the line, thereby improving its reliability.
- 2) High-voltage transmission lines with dual-side power supply can improve the stability of the system’s parallel operation and increase the line’s transmission capacity.
- 3) Correct the misoperation caused by poor circuit breaker mechanism or relay misoperation.
10. What requirements should the reclosing device meet?
- 1) It should act quickly and automatically select phases.
- 2) It should not allow arbitrary multiple closings.
- 3) It should automatically reset after action.
- 4) It should not reclose when manually tripped or manually closed on a faulty line.
11. How does the comprehensive reclosing method operate?
Single-phase fault, single-phase reclosing; after reclosing a permanent fault, it jumps all three phases; phase-to-phase fault, it jumps all three phases and then recloses.
12. How does the three-phase reclosing method operate?
Any type of fault jumps all three phases, then recloses; after reclosing, if it is a permanent fault, it jumps all three phases.
13. How does the single-phase reclosing method operate?
Single-phase fault, single-phase reclosing; phase-to-phase fault, it jumps all three phases without reclosing.
14. What inspection should be done on the voltage transformer when installed or after maintenance before connecting to the system voltage?
Measure the phase-to-phase voltage, the zero-sequence voltage, and the voltage of each secondary winding, check the phase sequence, and determine the phase.
15. What circuits should the protection device withstand for a power frequency test voltage of 1500V?
110V or 220V DC circuits to ground.
16. What circuits should the protection device withstand for a power frequency test voltage of 2000V?
- 1) The primary side of the AC voltage transformer of the device to ground;
- 2) The primary side of the AC transformer of the device to ground;
- 3) The backplane wiring of the device (or panel) to the ground.
17. What circuits should the protection device withstand for a power frequency test voltage of 1000V?
The grounding circuits of each pair of contacts in the 110V or 220V DC circuit; between each pair of contacts, between the moving and static ends of the contacts.
18. What circuits should the protection device withstand for a power frequency test voltage of 500V?
- 1) DC logic circuit to ground;
- 2) DC logic circuit to high voltage circuit;
- 3) 18-24V rated voltage to ground circuit.
19. What is the construction of an electromagnetic intermediate relay?
It comprises an electromagnet, coil, armature, contacts, spring, etc.
20. What is the construction of a DX-type signal relay?
It comprises an electromagnet, coil, armature, moving and static contacts, signal plate, etc.
21. What is the essential task of a relay protection device?
When a fault occurs in the power system, some electrical automatic devices quickly cut off the faulty part from the system. When an abnormal condition occurs, a signal is sent in time to reduce the scope of the fault and the loss caused by the fault and ensure the safe operation of the system.
22. What is distance protection?
It is a protection device that reacts to the electrical distance from the protection installation to the fault point and determines the action time based on the distance.
23. What is high-frequency protection?
It is a protection device that uses one phase of the transmission line as a high-frequency channel to transmit high-frequency current and connects the two semi-sets of protection on both ends of the line, which responds to the power frequency electrical quantities (such as current phase, power direction) or other quantities, as the primary protection without reacting to external faults of the line.
24. What are the advantages and disadvantages of distance protection?
The advantages are high sensitivity, which can ensure that the faulty line is selectively removed from the system within a relatively short time and is not affected by the system’s operating mode and fault form. Its disadvantage is that when the protection suddenly loses AC voltage, it will cause a false trip. This is because impedance protection operates when the measured impedance value is equal to or less than the set impedance value. If the voltage suddenly disappears, the protection will cause a false trip, so corresponding measures must be taken.
25. What is high-frequency locking direction protection?
The basic principle of high-frequency locking direction protection is based on comparing the power direction between the two sides of the protected line. The protection operates to trip when the short-circuit power direction on both sides is from the bus
to the line. Since the high-frequency channel is typically without current, when an external fault occurs, the high-frequency locking signal is sent from the side with the negative power direction to lock both sides of the protection. This is why it is called high-frequency locking direction protection.
26. What is high-frequency locking distance protection?
High-frequency protection is a protection device that can achieve full-line rapid action but cannot act as backup protection for the bus and adjacent lines. On the other hand, distance protection can be the backup for the bus and adjacent lines, but it can only rapidly remove faults within about 80% of the line. High-frequency locking distance protection combines high-frequency and impedance protections into one. It can achieve full-line rapid action when an internal fault occurs and can also act as the backup protection for the bus and adjacent lines when faults occur.
27. During the periodic inspection of the relay protection device in our factory, which protection pressure plates should be released?
- (1) Loss of trip initiation pressure plate;
- (2) Low impedance protection of the generator-transformer set;
- (3) High voltage side zero-sequence current protection pressure plate of the main transformer.
28. When the PT (potential transformer) fails, which corresponding protection devices should be tripped?
- (1) AVR (Automatic Voltage Regulator) device;
- (2) Standby power source automatic transfer device;
- (3) Loss of excitation protection;
- (4) Stator turn-to-turn protection;
- (5) Low impedance protection;
- (6) Low voltage locking overcurrent protection;
- (7) Busbar low voltage protection;
- (8) Distance protection.
29. Which protection actions in SWTA will trip the 41MK switch?
- (1) OXP over-excitation protection stage 3 action;
- (2) 1.2 times V/HZ delay 6 seconds action;
- (3) 1.1 times V/HZ delay 55 seconds action;
- (4) ICL instantaneous current limiter stage 3 action.
30. What is the role of the inrush current blocking element in the primary transformer differential protection?
In addition to preventing the transformer from false tripping due to inrush current, it can also prevent false tripping caused by current transformer saturation when a fault occurs outside the protected area.
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