1. What are the main components of an oil-immersed transformer?

Answer: The main components of a transformer are: core, windings, tank, conservator, breather, explosion vent, radiator, insulating bushing, tap changer, gas relay, thermometer, oil purifier, etc.

2. What is a fully insulated transformer? What is a semi-insulated transformer? Answer: In a semi-insulated transformer, the main insulation of the winding near the neutral point has a lower insulation level than the winding at the ends. Conversely, a fully insulated transformer has the same insulation level at both ends of the winding.

3. What is the main function of a transformer in a power system? Answer: The main function of a transformer in a power system is to transform voltage to facilitate power transmission. After voltage is stepped up by a step-up transformer, line losses are reduced, improving the economic efficiency of power transmission and enabling long-distance power transmission. Step-down transformers reduce high voltage to various usable voltages required by users, meeting user needs.

4. What are the hazards of bushing cracks? Answer: Bushing cracks reduce insulation strength, causing further damage to the insulation until complete breakdown. Water in the cracks may also cause the bushing to crack when it freezes. Therefore, bushing cracks pose a significant threat to the safe operation of the transformer.

5. What is the function of a high-voltage circuit breaker? Answer: A high-voltage circuit breaker can not only switch on and off no-load and load currents in high-voltage circuits under normal conditions, but also, in conjunction with protective devices and automatic devices, quickly cut off faulty power sources when system faults occur, preventing accidents from escalating and ensuring safe system operation.

6. What is the function of a wave trap? Answer: A wave trap is an essential high-frequency communication component for communication and high-frequency protection. It prevents high-frequency currents from leaking to other branches, reducing high-frequency energy loss.

7. What is the use of a current transformer? Answer: A current transformer transforms a large current into a small current at a certain ratio, providing current for various instruments and relay protection, and isolating the secondary system from the high voltage. This ensures the safety of personnel and equipment, simplifies and standardizes the manufacturing of instruments and relays, and improves economic efficiency.

8. What are the different wiring methods for current transformers? Answer: The wiring methods for current transformers include two-current transformer two-phase V-connection and two-phase current differential connection; three-current transformer three-phase Y-connection, three-phase Δ-connection, and zero-sequence connection.

9. What are the types of reactive power sources in a power system? Answer: Reactive power sources in a power system include: synchronous generators; static VAR compensators; shunt compensation capacitors; series compensation capacitors; static compensators.

10. Why is a set of ZnO surge arresters installed between the power capacitor and its circuit breaker? Answer: Installing ZnO surge arresters prevents operational overvoltages that may occur during the switching operations of power capacitors, ensuring the safe operation of electrical equipment.

11. What are the differences between the readings of an energy meter and a power meter? Answer: A power meter indicates the instantaneous power generated, transmitted, and consumed by power generation, supply, and consumption equipment; while an energy meter indicates the cumulative energy generated, transmitted, and consumed over a period of time.

12. What are the requirements for connecting batteries in parallel? Answer: The electromotive forces of the batteries in a parallel battery bank must be equal; otherwise, the battery with a higher electromotive force will discharge into the battery with a lower electromotive force, creating a circulating current within the battery bank. In addition, the internal resistances of the batteries should also be the same; otherwise, the discharge current of the battery with lower internal resistance will be excessive. Batteries of different ages should not be connected in parallel.

13. What is the function of a central signaling device? Answer: Central signaling is a signaling device that monitors the operation of electrical equipment in a substation. It issues audible and light signals based on the characteristics of electrical equipment faults, informing operating personnel to quickly locate, correctly judge, and handle the faults, ensuring the safe operation of the equipment.

14. Why is there still electricity when testing with a voltage tester after a cable line is powered off? Answer: A cable line acts like a capacitor, retaining residual charge after power is cut off, maintaining a potential difference to ground. If tested immediately after power off, the voltage tester will indicate the presence of electricity. Therefore, sufficient discharge is necessary before installing ground wires after verifying the absence of electricity.

15. What is internal overvoltage?

Answer: Internal overvoltage is caused by sudden changes in the system's state due to operation, accidents, or other reasons, resulting in a transition from one stable state to another. During this process, dangerous overvoltages may occur. These overvoltages are caused by the oscillation and accumulation of electromagnetic energy within the system, hence the name internal overvoltage.

15. What is internal overvoltage? Answer: Internal overvoltage is caused by sudden changes in the system's state due to operation, accidents, or other reasons, resulting in a transition from one stable state to another. During this process, dangerous overvoltages may occur. These overvoltages are caused by the oscillation and accumulation of electromagnetic energy within the system, hence the name internal overvoltage.

16. What is the function of the upper equalizing ring on a 220KV valve-type arrester? Answer: The addition of an equalizing ring ensures uniform voltage distribution across the arrester.

17. What is protective grounding? What are its advantages? Answer: Protective grounding is a method of directly connecting the normally unenergized metal parts of equipment to the system neutral wire. Protective grounding ensures personal safety and prevents electric shock accidents.

18. What is the difference between the neutral point, zero point, and neutral wire? Answer: The common connection point of the beginning (or end) of three-phase windings is called the neutral point of the power source. When the neutral point of the power source is well connected to the grounding device, this neutral point is called the zero point; the wire from the zero point is called the neutral wire.

19. What is the function of the DC system in a substation? Answer: The DC system in a substation provides reliable DC power for control, signaling, relay protection, automatic devices, and emergency lighting. It also provides reliable operating power. The reliability of the DC system is crucial for the safe operation of the substation and is a guarantee of its safe operation.

20. To keep the battery fully charged during normal float charging, what should the terminal voltage of each battery be maintained at? Answer: To keep the battery fully charged, each battery connected to the DC busbar must maintain a voltage of 2.15V during float charging.

21. Why install a DC insulation monitoring device? Answer: In the DC system of a substation, it is not allowed for one pole to be grounded for a long time, because if another grounding occurs at another point on the same level, it may cause malfunction of signaling devices, relay protection, and control circuits. In addition, if one pole is grounded, another grounding will cause a DC short circuit.

22. What is float charging? Answer: Float charging is to install two sets of chargers, one is the main charger and the other is the float charger. Float charging is to compensate for the self-discharge loss of the battery, so that the battery pack is always in a fully charged state.

23. Why do outdoor busbar joints tend to overheat? Answer: Outdoor busbars are frequently exposed to wind, rain, snow, sun, and freezing. These can accelerate the oxidation and corrosion of busbar joints, increasing the contact resistance and temperature of the joints.

24. What are the phenomena when the system oscillates? Answer: The phenomena when the system oscillates are: (1) The pointers of ammeters, voltmeters, and wattmeters in the substation oscillate periodically. If there are connecting lines, the oscillation of the meters is most obvious. (2) The closer to the center of the system oscillation, the greater the voltage swing, and the incandescent lamps flicker obviously.

25. What is the function of the drop-out signal that does not return? Answer: The drop-out light signal is to prevent the duty personnel from missing the recording of the protection action and causing misjudgment. Attention should be paid to returning the drop-out signal in time to avoid repeated actions, so that the two actions cannot be distinguished.

26. Can low-voltage AC and DC circuits share the same cable? Why? Answer: No. Because: (1) Sharing the same cable can reduce the insulation level of the DC system; (2) If the DC insulation is damaged, the DC mixing will cause short circuits or relay protection malfunctions.

27. What size megohmmeter should be used to measure the insulation of the secondary circuit? What is the insulation standard in MΩ? Answer: It is best to use a 1000V megohmmeter to measure the insulation resistance of the secondary circuit. If there is no 1000V megohmmeter, a 500V megohmmeter can also be used. Insulation standard: no less than 1MΩ during operation, no less than 20MΩ for indoor new installations, and no less than 10MΩ for outdoor new installations.

28. What are the uses of the auxiliary contacts of the oil switch? Answer: The normally open and normally closed contacts of the oil circuit breaker itself change the opening and closing positions to connect and disconnect the circuit breaker mechanism closing and tripping circuits and sound signal circuits, achieving the purpose of opening or closing the circuit breaker circuit, and correctly issuing sound signals, starting automatic devices and protection interlock circuits, etc. When the auxiliary contacts of the circuit breaker are used in the closing and tripping circuits, they should have a delay.

29. What are the chemical properties of SF6 gas? Answer: SF6 gas is insoluble in water and transformer oil. At high temperatures, it does not react with oxygen, argon, aluminum, and many other substances. However, under the action of electric arcs and corona, SF6 gas will decompose, producing low-fluorine compounds, which can damage insulating materials, and these low-fluorine compounds are highly toxic gases. The decomposition reaction of SF6 is greatly related to moisture, so dehumidification measures are required.

30. What is the operating principle of overcurrent protection? Answer: When a phase-to-phase short circuit fault occurs in the power grid, the current will suddenly increase, and the voltage will suddenly drop. Overcurrent protection is to set the operating current of the current relay according to the selectivity requirements of the line. When the fault current in the line reaches the operating value of the current relay, the current relay operates and selectively cuts off the faulty line according to the selectivity requirements of the protection device.

31. What is the function of the oil cushion of a transformer? Answer: When the volume of the transformer oil expands or contracts with the change of oil immersion, the oil cushion plays the role of storing and replenishing oil, ensuring that the oil tank is full of oil. At the same time, due to the installation of the oil cushion, the contact area between the transformer and the air is small, which slows down the deterioration rate of the oil. An oil level gauge is also installed on the side of the oil cushion to monitor the change of the oil level.

32. Why should the core of a transformer be grounded? Answer: The core and other accessories of the transformer in operation are in the electric field around the winding. If not grounded, the core and other accessories will inevitably induce a certain voltage. Under the action of the external voltage, when the induced voltage exceeds the discharge voltage to the ground, a discharge phenomenon will occur. In order to avoid internal discharge of the transformer, the core should be grounded.

33. What is the principle of the transformer oil purifier? Answer: Due to the temperature difference between the upper and lower layers of oil in the operating transformer, the oil circulates in the oil purifier. Harmful substances in the oil, such as moisture, free carbon, and oxides, are absorbed by the silica gel in the oil purifier with the circulation of the oil, purifying the oil and maintaining good electrical and chemical properties, playing a role in regenerating the transformer oil.

34. How is the cooling effect of the forced oil-air cooling device of guided and unguided transformers? Answer: In most transformers with unguided oil-air cooling devices, most of the oil flows back through the gap between the tank wall and the windings, and a small amount of oil flows into the windings and core, and the cooling effect is not high. The cooling oil flowing into the oil tank of the guided forced oil-air cooling transformer flows through the oil flow guiding baffle, effectively flowing through the core and windings, improving the cooling effect and reducing the winding temperature.

35. What is the function of the thermometer? What are the methods of temperature measurement? Answer: The thermometer is used to measure the oil temperature in the upper layer of the oil tank, playing a role in monitoring whether the power transformer is operating normally. According to the capacity of the transformer, thermometers can be divided into three measurement methods: mercury thermometer, signal thermometer, and resistance thermometer.

36. What is the function of the closing and opening buffer of a high-voltage circuit breaker? Answer: The function of the opening buffer is to prevent the huge impact force generated by the release of spring energy from damaging the components of the circuit breaker. The function of the closing buffer is to prevent the impact force during closing from causing excessive closing and damaging the bushing.

37. What is free tripping of a circuit breaker? Answer: At any time during the closing process of the circuit breaker, if the protection action connects the tripping circuit, the circuit breaker can reliably open, which is called free tripping. Circuit breakers with free tripping can ensure that when the circuit breaker is closed to a short-circuit fault, it can quickly open to avoid expanding the accident range.

38. What are the excellent arc extinguishing properties of SF6 gas? Answer: SF6 gas has the following excellent properties: (1) High arc column conductivity, very low arc voltage, and small arc column energy. (2) When the AC current passes through zero, the dielectric strength of SF6 gas recovers quickly, about 100 times faster than air, that is, its arc extinguishing ability is 100 times higher than that of air. (3) SF6 gas has higher dielectric strength.

39. What are the characteristics of vacuum circuit breakers? Answer: Vacuum circuit breakers have the characteristics of small contact gap, short arcing time, and minimal contact burning when breaking fault current. Therefore, vacuum circuit breakers require less operating energy and are fast-acting. They also have the advantages of small size, light weight, low maintenance, fire and explosion prevention, and low operating noise.

40. What does synchronization mean for a synchronous phase shifter? Answer: When the phase shifter is running, the rotating magnetic field of the stator and the rotor rotate in the same direction and at the same speed, which is called synchronization.

41. Why is it not allowed to replace the primary fuse of a voltage transformer with an ordinary fuse after it blows? Answer: Taking a 10KV voltage transformer as an example, the rated current of the primary fuse is 0.5A. The fuse filled with quartz sand has good arc extinguishing performance and large breaking capacity, and it also has the function of limiting the short-circuit current. However, ordinary fuses cannot meet the breaking capacity requirements.

42. Why must the secondary side of voltage transformers and current transformers be grounded? Answer: Grounding the secondary side of voltage transformers and current transformers is a protective ground. Because if the insulation between the primary and secondary sides is damaged, the high voltage on the primary side will be connected to the secondary side, which will threaten the safety of personnel and equipment, so the secondary side must be grounded.

43. What is the nameplate of the electrical equipment FKL-10-2*750-6? Answer: FKL-10-2*750-6 is the nameplate of an aluminum cable split reactor with a rated reactance of 6%, a rated voltage of 10KV, and a rated current of 750A for two branches.

44. What are the functions of shunt reactors and series reactors? Answer: Line shunt reactors can compensate for the capacitive charging current of the line, limit the increase in system voltage and the generation of operating overvoltage, and ensure the reliable operation of the line. Busbar series reactors can limit the short-circuit current and maintain a higher residual voltage on the busbar. The series reactor of the capacitor bank can limit high-order harmonics and reduce reactance.

45. What are the characteristics of the single busbar sectionalized connection method? Answer: Single busbar sectionalized connection can reduce the impact range of busbar faults and improve the reliability of power supply. When a section of the busbar has a fault, the sectionalizing circuit breaker automatically trips under the coordination of the relay protection, cutting off the faulty section and keeping the non-faulty busbar in normal power supply. For important users, power can be obtained from different sections to ensure uninterrupted power supply.

46. What are the disadvantages of the double busbar connection? Answer: The double busbar connection has the following disadvantages: (1) The connection and operation are more complex, and misoperation may occur during switching operations. (2) There are more busbar isolators, and the distribution device structure is also more complex, so the economy is poor.

47. What is the function of fault recording? Answer: Fault recorders are used in power systems to automatically and accurately record the changes in various electrical quantities before and after a fault occurs. By analyzing and comparing these electrical quantities, it plays an important role in analyzing and processing accidents, judging whether the protection is working correctly, and improving the safe operation level of the power system.

48. Why should expansion joints be installed on rigid busbars? Answer: Objects have the characteristics of thermal expansion and contraction, and the busbars will change in length due to heating during operation. To avoid excessive stress and damage to the busbars and supporting insulators due to changes in thermal expansion and contraction, expansion joints should be installed on the rigid busbars.

49. What is the compensation degree of an arc suppression coil, and what is residual current? Answer: The ratio of the difference between the inductive current and the capacitive current of the arc suppression coil to the capacitive current of the power grid is called the compensation degree. After the inductive current of the arc suppression coil compensates for the capacitive current, the remaining current flowing through the grounding point is called the residual current.

50. Does the arc suppression coil carry voltage during normal operation of a system with a neutral point grounded through an arc suppression coil? Answer: During normal operation of the system, due to the imbalance of the three-phase to ground capacitance of the line, there is a certain voltage between the neutral point of the network and the ground, and the magnitude of the voltage is directly related to the degree of capacitance imbalance. Under normal circumstances, the voltage generated at the neutral point should not exceed 1.5% of the rated phase voltage.