1. The nature of electric charge
There is an interaction force between electric charges. Like charges repel each other, while opposite charges attract each other.
2. Electric field
A special substance exists around a charged body, which acts on any charge placed within it with force. This special substance is called an electric field.
3. Electrical resistance, what factors affect it?
The resistance encountered by the current flowing through a conductor is called electrical resistance, represented by R. The resistance of a conductor is directly proportional to the length of the conductor and inversely proportional to the cross-sectional area of the conductor. It also depends on the material of the conductor. The relationship between them can be represented by the following formula: R=ρL/S.
4. Characteristics of series and parallel resistances
- The current flowing through each resistance is the same.
- The voltage across a series resistance is equal to the sum of the voltage drops across each resistance.
- The total resistance of series resistances is the sum of each resistance.
Characteristics of parallel resistance
- The voltage across each parallel resistance is the same.
- The total current of parallel resistances is equal to the sum of the currents flowing through each parallel resistance.
- The reciprocal of the equivalent resistance of parallel resistances is the sum of the reciprocals of each parallel resistance.
5. Electric energy
Electric energy is used to denote the work done by electric field force over a period of time, represented by W. W=pt W: Electric energy (kw.h) p: Electric power (w) t: Time (h)
6. What is active power, and what is reactive power?
In the transmission and use of alternating electric energy, the part of energy transformed into non-electrical and magnetic forms (such as light, heat, mechanical energy, etc.) is called active power. The part of energy used for the exchange of electric and magnetic fields within the circuit is called reactive power.
7. What is power factor, and what are leading and lagging power factors?
The power factor of an alternating current system, cosФ, also known as power factor, is the ratio of active power to apparent power, i.e., cosФ=p/s. Under a certain rated voltage and rated current, the higher the power factor of a motor, the larger the proportion of active power. Synchronous generators usually generate both active and reactive power. The operation state of generating both active and reactive power is called lagging power factor or lagging. The state of delivering active power and absorbing reactive power is called leading power factor or leading.
8. What is the significance of improving the power factor of the power grid?
Most electrical equipment used in production and daily life are inductive loads, which have lower power factors. This can lead to underutilization of generating equipment and increased losses on transmission lines. Improving the power factor allows generating equipment to produce less reactive load and more active load, reducing losses on supply equipment and saving electrical energy.
9. What is electric current? How is the direction of electric current defined?
Electric current: refers to the orderly movement of free electrons or ions under the action of electric field force. The direction of motion of positive charges is defined as the direction of electric current, which is opposite to the direction of free electron movement.
10. What is the phenomenon of mutual inductance?
The phenomenon where a change in current in one circuit induces an electromotive force in an adjacent circuit is called mutual inductance.
11. Handling of voltage drop accidents
- When the voltage of various busbars drops below 95% of the rated value, electrical personnel should immediately adjust the excitation of the generator, increase reactive load to maintain the voltage within the normal range.
- When the voltage of various busbars drops below 90% of the rated value, the accident overload capability of the generator should be utilized to increase reactive to maintain voltage. (Note to check the generator temperature, record the overload multiple and duration) At the same time, the active load can be appropriately reduced, and the shift leader should be reported to contact the area dispatcher to request adjustment and load restriction.
- If after the above treatment, the voltage continues to drop below 5.4KV, the electrical personnel should ask the shift leader for permission to disconnect from the system. When the system voltage recovers to above 5.7KV, reconnect with the system as soon as possible.
12. Handling of frequency reduction accidents
- When the system frequency drops below 49.5HZ, electrical personnel should immediately report to the shift leader, contact the machine, furnace to increase the load of the unit to the maximum possible output, while contacting the area dispatcher.
- When the system frequency drops below 49HZ, in addition to increasing output, it is also required to eliminate frequency operation by the area dispatcher, so that the frequency can be restored to above 49HZ within thirty minutes
, and to above 49.5HZ within a total of one hour.
- When the system frequency drops to 48.5HZ, the low-frequency protection of the switch connected to the system in our plant should act. Otherwise, it should be executed manually. When the system frequency recovers to above 48.5HZ, reconnect with the system as soon as possible.
13. Precautions for high-voltage equipment inspection
- When inspecting high-voltage equipment, do not perform other work, do not move away from or cross barriers.
- During thunderstorms, wear insulating boots and do not approach lightning arresters and lightning rods.
- When high-voltage equipment is grounded, do not approach within 4m indoors and 8m outdoors of the fault point. Entering the area must wear insulating boots, and when touching the outer shell of the equipment, wear insulating gloves.
- When entering or leaving a high-voltage room, the door must be locked.
14. How does a synchronous generator produce three-phase alternating current?
The steam turbine drives the rotor to rotate. The rotor has an excitation winding (rotor winding) that contacts the slip ring through brushes, introducing direct current generated by the excitation system into the rotor winding to produce a stable magnetic field. Under the rotation driven by the steam turbine at a certain speed, the three-phase stator coils continuously cut through the rotor magnetic flux, generating induced electromotive force. When loaded, load current is generated, i.e., three-phase alternating current. (Alternating: Frequency, electric potential are equal but phase differs)
15. What does "synchronous" mean in synchronous generators, and what is the relationship between the frequency, speed, and number of poles of a synchronous generator?
- Synchronous means that the speed n=60ft and direction of the rotor's rotating magnetic field are the same as the speed and direction of the stator's rotating magnetic field.
- The relationship between frequency, number of poles, and rotor speed: f = pn/60 (n: rotor speed p: number of poles f: frequency)
16. What is the impact of a decrease in cosФ during generator operation?
When cosФ is below the rated value, the output of the generator should be reduced because the lower the cosФ, the greater the reactive component of the stator current. Since inductive reactive has a demagnetizing effect, the greater the effect of canceling the magnetic flux, the more the rotor current must be increased to maintain the stator voltage unchanged. If the generator output is still maintained, it will inevitably cause the rotor current to exceed the rated value, leading to an increase in temperature of the stator winding and overheating of the winding.
17. Why check the three-phase stator current meter and verify the no-load rotor voltage and current when the generator is voltage boosted without load?
Before the generator is closed and connected to the grid, the three-phase stator current should be 0. If current is found, it indicates that there is a short circuit point on the stator circuit, and the demagnetization switch should be immediately pulled open for inspection. The purpose of verifying the no-load rotor voltage and current is to check whether there is interlayer short circuit in the generator rotor winding. If the stator voltage reaches the rated value and the rotor current is greater than the value of the no-load rated voltage, it indicates that there is an interlayer short circuit in the rotor winding.
18. What are the dangers of single-phase grounding of the stator winding?
Since the neutral point of the generator is an ungrounded system, when a single-phase ground fault occurs, the current flowing through the fault point is only the smaller capacitive current of the generator system. This current does not pose much harm to the generator, so the generator can operate for a short time. However, if not dealt with in time, it may burn the stator core and even develop into a turn-to-turn or phase-to-phase short circuit.
19. Why mill grooves on the surface of the slip ring?
During operation, when the slip ring and carbon brush slide in contact, a high heat reaction is produced. The spiral grooves on the surface of the slip ring serve to increase the cooling area and improve cooling. They also improve contact with the brush and make it easier for the brush powder to be discharged along the spiral grooves.
20. What are the requirements for the excitation system of a generator?
- The excitation system should not be affected by the external power grid, otherwise, in the event of an accident, a vicious cycle may occur, causing the power grid to affect the excitation, and the excitation to affect the power grid, making the situation worse.
- The adjustment of the excitation system itself should be stable. If unstable, i.e., the change in excitation voltage is large, it will cause large fluctuations in generator voltage.
- In the event of a power system fault, when the generator terminal voltage drops, the excitation system should be able to quickly increase the excitation to the peak value.
21. What is the excitation multiplier? What should be noted after a forced excitation action?
The excitation multiplier refers to the ratio of the forced excitation voltage to the rated excitation voltage Ue of the exciter. For air-cooled excitation windings of steam turbine generators, the forced excitation voltage is twice the rated excitation voltage, and the allowed time for forced excitation is 50 seconds. After a forced excitation action, a check of the carbon brushes of the exciter should be conducted. Additionally, it is important to check whether the relay contacts of the short-circuit field resistance have opened after the voltage recovery, and whether the contact points are intact.
22. What is the function of the demagnetizing resistor in the excitation circuit?
1. To prevent overvoltage between rotor windings, ensuring it does not exceed the permissible value.
2. To convert magnetic field energy into heat energy, accelerating the demagnetization process.
23. What impact does asymmetry in generator three-phase current produce?
When the three-phase current is asymmetric, it generates a negative sequence rotating magnetic field, which sweeps over the rotor surface at twice the speed, with the main consequences being:
1. Heating of the rotor surface
2. Vibration of the rotor
24. What does a change in the temperature difference between the inlet and outlet air of a generator indicate?
Under the same load, the temperature difference between the inlet and outlet air should remain constant. If an increase in the temperature difference is found, it indicates that the internal loss of the generator has increased, or that the air volume has decreased.
25. What should be the focus when inspecting the generator through the peephole, looking at the end of the motor?
1. Inspection of the stator bar ends:
a. Whether they are loose,
b. Whether the insulation is intact,
c. Presence of corona discharge
2. Inspection of end structure components: mainly check whether the clamping rings, supports, nuts, end hoops, etc., are securely fastened.
26. Under what circumstances might the polarity of the exciter change?
1. During post-maintenance tests, such as resistance measurement or voltage adjustment tests, if the excitation circuit is not disconnected and reverse voltage is applied, the excitation can be canceled or reversed.
2. If the exciter experiences a sudden short circuit, the strong armature reaction can exceed the main magnetic field, possibly changing the polarity.
3. When the power system experiences a sudden short circuit, the sudden increase in stator current can induce a direct current component in the rotor, possibly reversing the rotor polarity.
4. If the magnetic circuit of the exciter is disconnected and then reconnected, it might cause the polarity to reverse.
27. What are the no-load characteristics of a generator?
The relationship curve between its electromotive force Eo and the excitation current Il when the rotor of the generator rotates at rated speed and the stator winding is open-circuited (unloaded).
28. Armature reaction?
In a synchronous generator under load, the rotating magnetic field (armature magnetic field) produced by the three-phase current in the stator winding affects the rotating magnetic field generated by the excitation current in the rotor winding, thus changing the air-gap magnetic field. This effect is called armature reaction.
29. How to deal with a sudden lack of indication on either the stator or rotor instrument?
1. When an indication is lost, continue to operate according to the rest of the meters without changing the operating mode of the generator, and contact relevant personnel to check the circuit and clear the fault.
2. If it affects normal operation, reduce the load according to the situation until shutdown.
30. What is the difference between a generator operating as a phase modifier and operating as a motor?
Operating as a phase modifier refers to the generator losing its prime mover (main steam valve closed), with active power dropping below 0, causing the generator to absorb active power from the system to maintain energy loss. Operating as a motor means that the generator not only loses its prime mover but also becomes demagnetized. At this point, it absorbs both active and reactive power from the system, becoming a load on the electrical grid. Both scenarios pose significant risks to the generator.
31. The regulation of top oil temperature in transformers, and when should the main plant turn on the fans?
Under the condition of an ambient air temperature of 40°C, the top oil temperature should not frequently exceed 85°C, the maximum should not exceed 95°C, and the temperature rise should not exceed 55°C. When the top oil temperature reaches 55°C, exhaust fans should be turned on to enhance ventilation and cooling.
32. Inspection during transformer operation
1. Oil color and level, the body is clean, no oil leakage
2. Bushings are clean, without cracks, damage, discharge, or other abnormalities
3. Sounds, fans, and top oil temperature are normal
4. No gas in WSJ, the silica gel in the breather has not changed color
5. The airway and protective film are intact
6. No overheating discoloration on each side's lead wires
7. Good grounding of the casing
33. Operational principles for transformer commissioning and decommissioning?
1. Transformers must be commissioned and decommissioned using a circuit breaker.
2. The main transformer should be gradually increased from zero to the rated value before paralleling with the system, and the opposite when stopping.
3. The factory transformer should first close the high-voltage side switch to charge, then close the low-voltage side switch, and the opposite when stopping.
34. Advantages and disadvantages of microcomputer protection?
Advantages: Sensitive, accurate, small equipment size, multiple functions, convenient debugging, automated operation, saves human resources.
Disadvantages: Maintenance is complex and difficult, high initial investment, high technical requirements.
35. Precautions for measuring transformer insulation?
1. Before the insulation test, clean the insulator bushing, remove all ground wires, and disconnect the neutral wire.
2. Choose the appropriate megohmmeter; record the oil temperature of the transformer at the time of the test.
3. Do not touch live conductors with hands during the test; discharge the windings after the test.
4. Testing items, measure the primary side against the secondary side and ground, and the secondary side against the primary side and ground for dual-winding transformers.
36. The function of an oil pillow?
To reduce the contact surface between oil and air, lower the possibility of oil getting damp and oxidized, and equip with gas breakers and oil thermometers.
37. The function of breathers and oil purifiers?
Breather: Prevents moisture in the air from infiltrating the oil in the oil pillow.
Oil purifier: Improves the performance of transformer oil during operation, prevents oil aging, absorbs water, sludge, acid, and oxides in the oil, keeps the oil clean, and extends the oil's service life.
38. The reason for having only one point grounded in transformers?
1. One point grounding prevents the generation of floating potentials during transformer operation and testing, causing discharge to ground.
2. Only one point is allowed to be grounded because two points grounded would form a closed loop, creating circulating currents when the main flux passes through the loop, causing local overheating.
39. Factors affecting oil temperature changes?
1. Load
2. Changes in ambient temperature
3. Internal faults in the transformer
4. Whether the heat dissipation tube is unobstructed
5. The condition of the cooling system
40. The harm of excessive voltage to transformers?
The normal voltage should not exceed 5% of Ue; excessive voltage will cause severe saturation of the magnetic flux in the core, and the core and components will generate high heat due to leakage flux. In severe cases, it will damage the transformer insulation and cause local deformation of components, shortening the transformer's lifespan.
41. Under what circumstances should a transformer be immediately shut down during operation?
1. Loud and uneven internal noise, including bursting sounds
2. Under normal load and cooling conditions, the temperature abnormally continues to rise
3. Oil spewing from the oil pillow or explosion-proof tube
4. Serious oil leakage, causing the oil level to be lower than the indicator limit of the oil gauge, oil not visible
5. Excessive change in oil color, presence of carbon in the oil
6. Serious damage to the bushing, leaking oil
42. Possible reasons for minor gas action in transformers?
1. Air ingress
2. Lower oil level
3. Secondary circuit fault
4. Minor internal fault
43. How to energize a motor after maintenance?
1. The work permit has been concluded, meeting the conditions for startup
2. No personnel working on or near the motor, the site is clean
3. Wiring is complete
4. All related safety measures have been removed
5. The insulation resistance of the motor is satisfactory
44. Reasons why a motor buzzes but does not turn on startup?
1. A phase of the stator is disconnected (a fuse of one phase is blown, cable head, switch knife blade of one phase has poor contact)
2. Rotor circuit is disconnected or terminal has poor contact
3. Mechanical jamming
4. Incorrect coil wiring
5. Stator rubbing against the rotor
45. Reasons for severe vibration in motors?
1. Misalignment between the motor and mechanical parts
2. Imbalance of the unit, mechanical damage
3. Stator rubbing against the rotor
4. Bearing damage
5. Bent shaft
6. Loose foundation bolts
46. When should the power supply to a motor be immediately cut off?
1. Personal accident
2. Mechanical damage or obvious short circuit
3. Strong vibration
4. Abnormal noise
5. Bearing temperature exceeds the specified limit
6. Motor smoking, fire, or burnt smell
7. Sudden increase in motor current, exceeding the specified limit
47. What are the disadvantages of direct starting of motors, and what are the methods for reduced voltage starting?
1. Large starting current during direct starting, significant impact on grid voltage.
2. Reduced voltage starting methods:
a. Series connection of a reactor in the stator circuit
b. Star-delta reduction
c. Auto-transformer reduction
48. How to measure transformer temperature using an alcohol thermometer?
1. Mercury is a good conductor. If placed in a strong alternating magnetic field, eddy currents will be generated, causing the temperature to rise, and thus not providing an accurate temperature reading.
2. In case the thermometer breaks during measurement, alcohol is non-toxic, easily evaporates, and is easy to clean up, whereas mercury could become a toxic gas, and is more difficult to clean.
49. What are copper loss and iron loss in transformers?
Copper loss refers to the sum of the energy consumed by the primary and secondary currents flowing through the coil resistance. Since coils are mostly made of copper wire, it is called copper loss, which is directly proportional to the square of the current. Iron loss refers to the power consumed in the core under rated voltage, including excitation loss and eddy current loss.
50. How does too large or too small an air gap affect the operation of an induction motor?
A too large air gap increases the magnetic resistance, therefore increasing the excitation current, reducing the power factor, and deteriorating motor performance. A too small air gap increases core loss, and during operation, the stator and rotor are prone to friction, causing stator rubbing against the rotor.
51. Inspection items for oil circuit breakers during operation?
1. Leads and connection parts are in good contact, without looseness, heating, or discoloration.
2. Normal oil level, the oil is clear and transparent, presenting a light yellow color.
3. Porcelain bushings should be clean and intact, without damage or discharge marks.
4. Switching indicator indicates correctly.
5. Operating mechanism is in good condition, grounding is firm, all pins are not missing, and rods have no fracture marks.
6. Secondary terminal blocks are securely installed, and the wire ends are not rotten.
7. Switch barriers are intact and locked.
52. Operational precautions for oil circuit breakers?
1. When operating the remote control switch of an oil circuit breaker, do not use excessive force to prevent damaging the control switch, nor return too quickly to prevent the switch mechanism from not closing.
2. Manual slow opening and closing of the switch during operation are prohibited.
3. After operating the switch, related signal lights and measurement instrument indications should be checked to judge the correctness of the switch action. However, this should not be used as the basis to prove the actual position of the switch. The mechanical position indicator of the switch on-site should also be checked to determine the actual opening and closing position.
53. How to deal with oil leakage and unobservable oil level in an oil circuit breaker?
1. Remove the DC control fuse.
2. Hang a "Do Not Close" sign on the operating handle of the switch.
3. Try to transfer the load and shut down the switch.
54. Inspection during the operation of a disconnecting switch?
1. Ceramic parts should be clean and intact, without damage, cracks, or discharge phenomena.
2. The moving and fixed contacts are in tight contact, without overheating or looseness.
3. The transmission mechanism should be in good condition, with no loose pins or screws.
55. What operations are performed using a disconnecting switch?
1. Cooperate with switches for reversing operations.
2. Pull and close fault-free voltage PTs and lightning arresters.
3. Connect and disconnect impedance-free parallel branches.
56. How to deal with heating of a disconnecting switch?
Depending on the importance of the location of the disconnecting switch, reduce the load current or provide ventilation and cooling to reduce its heating. If the above treatment is ineffective, report to relevant leaders for a power outage treatment.
57. Why is oil used as an arc-extinguishing medium?
When the circuit breaker cuts off the current, an arc is generated between the moving and fixed contacts. Due to the high temperature of the arc, the oil violently decomposes into gas, with hydrogen accounting for about 7% of the gas. This can rapidly reduce the temperature of the arc column and increase the insulation strength between poles, which is extremely beneficial for extinguishing the arc.
58. Why does a vacuum circuit breaker have a small volume and a long service life?
The vacuum circuit breaker has a simple structure, with a pair of contacts placed in a vacuumed glass. Due to the insulating properties of the vacuum, its arc-extinguishing performance is particularly good, allowing the moving and fixed contacts to have a very small gap (about 10mm for 10KV, compared to about 160mm for oil circuit breakers), so the volume and weight of vacuum circuit breakers are very small. Since the contacts of the vacuum circuit breaker do not oxidize and arc extinguishing is fast, the contacts are not easily damaged, so it has a long service life (about four times that of oil circuit breakers).
59. The function of high-voltage circuit breakers?
1. Under normal operation, connect or disconnect the circuit's no-load current and load current.
2. When electrical equipment or lines fail, the high-voltage circuit breaker cooperates with the protection device to quickly and automatically cut off the fault current.
60. The "Five Preventions" to avoid misoperation?
1. Prevent misoperation of closing or opening circuit breakers
2. Prevent misoperation of closing or opening load-carrying disconnecting switches
3. Prevent energizing ground disconnecting switches
4. Prevent closing with grounding wires connected
5. Prevent entering energized compartments by mistake.
61. The sequence for removing and inserting power fuses?
When removing, first remove the middle one, then the sides or top and bottom. The insertion sequence is the opposite.
62. The ampere-second characteristic of a fuse?
The melting time of the fusible element depends on the magnitude of the current flowing through it. The relationship curve between the current and the melting time is called the protection characteristic, also known as the ampere-second characteristic.
63. Selection of fuse capacity for a single directly started motor?
≥2-3 times I nominal.
64. The function of a fuse?
To protect electrical equipment from damage due to overload currents and short-circuit currents.
65. The role of a DC system?
To provide reliable direct current for control, signaling, relay protection, automatic devices, and emergency lighting, etc. It also provides reliable operating current for operations.
66. Regulations related to the DC system?
1. The line-to-line voltage of the DC system should be maintained at 230V, with an allowable variation range of 225V-235V.
2. When the battery is operated in a float charging mode, each battery should be maintained at 2.23V, with an allowable variation range of 2.18-2.28V. Discharge should be stopped when the battery discharge voltage drops to 1.85V.
3. The battery can be used normally in the temperature range of -15 to +45℃. If used in the optimal environmental temperature range of 5 to 30℃, its lifespan can be extended.
67. The role and scope of differential protection?
Serves as the main protection for internal phase-to-phase faults in generators or transformers, responding to the difference in the secondary currents of current transformers installed on both sides of the equipment.
68. What precautions should be taken when drilling holes or performing other high-vibration tasks such as using an electric drill on a distribution panel?
1. Obtain the approval of the person in charge and remove any protection that might be misoperated.
2. Prevent iron filings from falling and causing short circuits, misoperations, or ground faults.
3. Move the backside secondary wires and others away from the drilling location to prevent puncturing or damage.
4. Pay attention to the vibration on both sides of the panel where the hole is being drilled.
5. Supervision by a designated person.
69. What causes insulation aging?
During operation, the insulation of equipment is subjected to the effects of electric fields, magnetic fields, temperature, and chemicals, making it harden, become brittle, and lose elasticity, thus weakening the insulation strength and performance. This is normal aging, but unreasonable operations, such as overloading, overvoltage due to corona, etc., can accelerate aging.
70. How to delay insulation aging?
Select reasonable operating modes, enhance cooling and ventilation, reduce the temperature rise of the equipment, and isolate the insulation from air or chemicals.
71. What causes abnormal noise in a Current Transformer (CT) during operation?
Overloading of the CT, secondary side open circuit, and internal insulation damage causing discharge, among others, can cause abnormal noise. In addition, internal corona caused by uneven application of semiconductor paint and loosening of the core screw can also cause the CT to produce significant noise.
72. What is the set-trip standard for high-voltage electrical equipment?
High-voltage equipment's set-trip standards are divided into three categories:
Category I and II are for intact equipment, and Category III is for defective equipment. Category I equipment: has passed operational testing, is in good technical condition, has complete technical documentation, and can guarantee safe, economical, full output, and demand supply.
Category II equipment: is in basically good technical condition, individual components have general defects, but can still guarantee safe operation.
Category III equipment: has significant defects, cannot guarantee safe operation or has reduced output, poor efficiency, or serious leaks of oil, steam, water.
73. Why conduct operational analysis?
Operational analysis mainly involves comprehensive or thematic analysis of the operation status, safe operation, economic operation, and operation management of power generation and supply equipment. Through analysis, operational patterns can be explored, weak links identified, and targeted preventive measures formulated.
74. What is a low-voltage electrical safety device?
It is an effective protection device to prevent low-voltage electric shock. In the event of an electric shock accident or insulation damage leakage, it will immediately trigger an alarm or cut off the current to protect personnel and equipment.
75. How to maintain and store safety equipment?
1. Insulating rods should be stored vertically, placed on racks, or hung indoors, not leaning against walls.
2. Insulating gloves and boots should be positioned in cabinets, separated from other tools.
3. Rubber products of safety equipment should not come into contact with petroleum-based greases.
4. High-voltage voltage testers should be stored in boxes in a dry place after use.
5. The storage location of safety equipment should be clearly marked for easy “matching and placing” retrieval.
6. Safety equipment should not be used for other purposes.
7. Regular inspection and testing must be conducted.
76. What is protective grounding and protective earthing?
Protective grounding: In systems where the power source neutral point is not grounded, it involves reliably connecting the metal casing or frame of electrical equipment to the earth via a grounding device. Protective earthing: In systems where the power source neutral point is grounded, it involves connecting the metal casing or frame of electrical equipment to a neutral line drawn from the neutral point.
77. What is the no-load current of an induction motor?
Generally, it is 20-35% of I nominal, not exceeding 50% of I nominal.
78. What conditions ensure that a motor starts and reaches its rated speed?
When a motor operates, there are two torques: one is the electromagnetic torque that causes the motor to rotate, produced by the flow of three-phase current through the stator windings; the other is the resistance torque that hinders the motor's rotation, produced by the mechanical load on the motor and acting in the opposite direction to the rotor. To start the motor and raise it to the rated speed, the electromagnetic torque of the motor must be greater than the resistance torque over the entire range of unit speed from zero to the rated value. In a steady operating state, the electromagnetic torque equals the resistance torque.
79. How to carry out the voltage testing of equipment before the completion of maintenance work?
1. All personnel withdraw from the work area.
2. Collect all work permits, remove temporary barriers, grounding wires, and signs, and restore permanent barriers.
3. After a comprehensive check by the person in charge of the work and the duty officer without any errors, the duty officer conducts the voltage testing.
80. What safety tools should be used for operating high-voltage equipment?
When using an insulating rod to close or open a disconnecting switch (knife switch) or through a transmission mechanism to close or open a disconnecting switch (knife switch) and circuit breaker (switch), insulating gloves should be worn. When operating outdoor high-voltage equipment in rainy weather, the insulating rod should have a rain cover, and insulating boots should also be worn. If the grounding network resistance does not meet the requirements, insulating boots should be worn even in clear weather. During thunderstorms, reversing operations are prohibited. When installing or removing high-voltage fuses, protective goggles and insulating gloves should be worn, and if necessary, insulating pliers should be used, standing on an insulating mat or platform.
81. What causes excessive no-load current in an induction motor?
1. The power supply voltage is too high, causing the motor core to saturate and increasing the no-load current.
2. Improper assembly or too large an air gap.
3. Insufficient turns in the stator winding.
4. Silicon steel sheets are corroded or aged, or the insulation between sheets is damaged.
82. What causes serious imbalance in the three-phase no-load current of an induction motor?
1. Unbalanced three-phase supply voltage.
2. Stator winding open circuit.
3. Short circuit between turns in the stator winding.
4. One phase of the stator winding is connected in reverse.
83. Why are DC motors used in some areas of power plants?
1. DC motors have good regulation smoothness and a wide speed regulation range.
2. For the same output power, DC motors are lighter and more efficient than AC motors.
84. Why is the protective grounding pin of ordinary single-phase and three-phase plugs slightly longer than the other pins?
The purpose is to ensure that when the three-phase plug is inserted into the three-pole socket, the grounding pin enters the socket first, and the opposite when pulled out. This longer pin always serves as protective grounding.
85. What is the purpose of installing grounding wires during equipment maintenance?
To protect personnel at the work site from sudden electrification as a reliable safety measure, and to discharge the residual charge of the disconnected part of the equipment to the ground.
86. Steps for testing high-voltage equipment and precautions?
When testing for voltage, a voltage-appropriate and qualified voltage tester must be used, and voltage should be tested on each phase on both sides of the equipment's inlet and outlet lines. Before testing for voltage, the voltage tester should first be verified on an electrified equipment to ensure it is in good condition. When testing for voltage, insulating gloves must be worn. If an appropriate voltage tester is not available, an insulating rod can be used, and the presence of voltage can be judged based on whether there are sparks and discharge cracking sounds at the rod end.
87. The three basic measures of cardiopulmonary resuscitation?
1. Clear the airway.
2. Mouth-to-mouth (nose) artificial respiration.
3. External chest compressions (artificial circulation).
88. What to do in case of an electrical equipment fire?
Cut off the power supply to the related equipment and then proceed to extinguish the fire. Use a dry chemical fire extinguisher for electrified equipment, do not use foam extinguishers, and use foam extinguishers or dry sand for oil-filled equipment.
89. Self-induction and mutual induction phenomena?
Self-induction phenomenon: The phenomenon where an induced electromotive force is generated in a coil due to the change in its own current. Mutual induction phenomenon: The phenomenon where a change in the current of one coil induces an electromotive force in another coil.
90. What is the skin effect?
When alternating current flows through a conductor, the distribution of current density across the conductor's cross-section is uneven, with the lowest density at the center of the conductor and increasing density closer to the conductor's surface. This phenomenon, where the current tends to distribute along the conductor's surface, is called the skin effect.
91. What is the function of an automatic demagnetizing device in a generator?
The automatic demagnetizing device is used to eliminate the magnetic field of the generator and the exciter after the main switch and excitation switch of the generator trip. The purpose is to remove the generator voltage as soon as possible after the generator is disconnected, to mitigate the consequences of the fault.
92. What harm does dirt on the surface of transformer bushings cause?
Dirt on the bushing surface can easily lead to flashovers because the dielectric strength of air is less than that of the bushing. When the voltage reaches a certain value, if the bushing surface is dirty, discharge phenomena will first occur on its surface, escalating from weak to strong. The lowest voltage at which a flashover occurs is called the flashover voltage, which is even lower when the bushing surface is damp.
93. What is the purpose of the grounding brush on the large shaft of a steam turbine generator?
It is used to eliminate the static voltage of the large shaft to the ground.
94. Methods for reducing voltage during motor startup?
Star-Delta (Y-Δ), autotransformer reduction, and series reactor (in the stator winding).
95. How does a voltage transformer (VT) differ from a regular transformer?
A VT is essentially a step-down transformer, with many turns in its primary coil and fewer turns in its secondary coil. Also, because the secondary load impedance is high and the current passing through is very small, the working state of the VT is equivalent to the no-load condition of a transformer.
96. What are reactance, capacitive reactance, inductive reactance, and impedance?
When alternating current flows through an inductive component, the ability of the inductive component to limit the AC current is called inductive reactance. When alternating current flows through a capacitive component, the capacitive component's ability to limit AC is called capacitive reactance. The difference between inductive reactance and capacitive reactance is called reactance. In a circuit with resistance, inductance, and capacitance in series, the total opposition to AC is called impedance.
97. What are the inductive reactance and capacitive reactance in a DC circuit?
In a DC circuit, the frequency of the current is zero, so the inductive reactance is zero, equivalent to a short circuit, and the capacitive reactance is infinite, equivalent to an open circuit.
98. What is series resonance? Characteristics of series resonance?
In a series circuit containing resistance (R), inductance (L), and capacitance (C), the phenomenon where the circuit's terminal voltage and the total circuit current are in phase occurs. At this time, the AC source only supplies energy for the circuit's resistive losses, and no energy exchange occurs with the inductor and capacitor. This is called series resonance. Characteristics:
1. The circuit impedance is minimum, and the current is maximum;
2. Very high resonance overvoltages may occur across the inductor and capacitor.
99. Why are capacitors connected in series in transmission lines?
Transmission lines have resistance and inductance, causing not only active power losses during power transmission but also voltage drops. On long-distance, high-capacity transmission lines, part of the inductive reactance is offset by capacitive reactance, which can reduce voltage drops and improve voltage quality.
100. What reasons can cause a transformer to lack oil?
1. The transformer has been seeping or leaking oil over a long period; 2. After draining oil from the transformer, it was not refilled in time; 3. The oil pillow capacity is small and does not meet operational requirements; 4. The temperature is too low; 5. The oil pillow's oil storage is insufficient.
101. What harm does a lack of oil cause to a transformer during operation?
If the oil level in a transformer is too low, it can trigger the gas protection action, reduce the transformer's cooling capacity, and if severely lacking oil, expose the core and coils to the air, possibly causing insulation breakdown.
102. What is the function of a transformer's oil pillow?
It reduces the contact area between oil and air, thereby reducing the possibility of the oil becoming damp and oxidized. After installing an oil pillow, a gas relay can also be installed to protect against internal faults in the transformer.
103. Why is it specified that the transformer core must be grounded at only one point?
To prevent the generation of floating potentials on the core or other metal components of the transformer due to electrostatic induction during operation or testing, which could lead to discharge to ground. Only one point grounding is allowed because if there are two or more grounding points, a closed circuit could form between the two points. When the main flux passes through this closed circuit, circulating currents are generated, causing local overheating.
104. Under what circumstances are porcelain insulators likely to be damaged?
1. Improper installation or use, such as exceeding mechanical load limits, incorrect voltage levels, and not selecting according to pollution levels;
2. Damage from sudden cold, sudden heat, and hail;
3. Surface contamination causing flashovers during thunderstorms and foggy weather;
4. Equipment short-circuit causing excessive mechanical stress on the motor.
105. Why does a cable line still show voltage with a voltage tester shortly after a power outage?
A cable line acts like a capacitor; after power is cut off, the line still retains residual charge and has a potential difference to the ground. If the voltage is tested immediately after the power outage, the voltage tester will show that the line is still energized. Therefore, it must be fully discharged and verified to be de-energized before grounding wires can be installed.
106. How is the impedance voltage of a transformer defined?
The secondary winding of the transformer is short-circuited, and the primary side voltage is gradually increased until the short-circuit current of the secondary winding reaches the rated value. The percentage of the primary side voltage to the rated voltage.
107. What are the internal and external insulation of equipment?
The part of the equipment insulation that is in contact with the air is called external insulation, and the part that is not in contact with the air is called internal insulation. (In designing insulation, the strength of external insulation is made lower than that of internal insulation because external insulation has some natural recovery, whereas internal insulation is not affected by air humidity and surface contamination, relatively stable but with poor natural recovery capability. Once the insulation level declines, it will inevitably affect safe operation.)
108. How to distinguish the color coding of busbar paint?
Direct Current: Positive pole---Red Negative pole---Blue Alternating Current: A phase---Yellow B phase---Green C phase---Red Neutral wire: Non-grounded neutral wire---White Grounded neutral wire---Purple
109. What is insulation breakdown?
A certain distance of gas gap can withstand a certain voltage while maintaining its insulating properties. However, when the voltage exceeds a certain critical value, the gas medium suddenly loses its insulating ability and discharges, a phenomenon known as dielectric breakdown.
110. What is the main cause of aging transformer oil?
Mainly caused by oxidation at high temperatures.
111. What are the hazards of aging transformer oil?
The aging of transformer oil reduces its insulation strength and heat transfer performance. During operation, this affects the oil circulation, leading to poor cooling of the transformer. Therefore, the aging of the oil can cause overheating and breakdown discharge of transformers and other equipment during operation.
112. What causes aging of solid insulation?
1. Thermal effects: Overheating of solid insulation due to heat transferred from other overheated parts;
2. Chemical effects: Corrosion due to ozone, nitrogen oxides produced by partial discharge, as well as harmful gases and pollutants in the air;
3. Mechanical damage: Caused by frequent starts, sudden changes in load, vibration, and unequal thermal expansion of different parts of the insulation;
4. Electric field effects: Due to partial discharge occurring at electrode edges and inside bubbles in the insulation, the high temperature and impact of ions caused by the discharge.
113. What measures can improve the strength of solid insulation?
1. Try to remove moisture and bubbles in the insulation, treat solid insulation with varnishing or gluing. To ensure the quality of varnishing or gluing, a certain temperature or vacuum impregnation process is required; 2. Improve the electric field distribution, making the insulation components bear the voltage as reasonably as possible; 3. Improve the working conditions of insulation, such as preventing moisture intrusion, avoiding overload, enhancing heat dissipation and cooling, and preventing harmful gases and pollutants from corroding the insulation.
114. What is the function of lightning rods and needles?
Lightning rods and needles, extending above the protected object, attract lightning to themselves and safely conduct the lightning current into the ground, thus protecting electrical equipment and other installations. The function of a lightning arrester is to limit the overvoltage applied to the insulation of electrical equipment, thereby protecting the insulation of the electrical equipment.
115. Why are taps generally taken from the high-voltage side of transformers?
This is because the high-voltage winding is outside the low-voltage winding, making it more convenient to weld taps. Additionally, since the current flowing through the high-voltage side is smaller, the cross-sectional area of the lead-out wires and tap changer current-carrying parts can be smaller, and the heating issue is easier to solve.
116. What are the reasons for poor contact in transformer tap changers?
1. Insufficient contact pressure;
2. Accumulation of sludge at the switch contacts, creating a layer of sludge film between the moving and stationary contacts;
3. Small contact surface causing contact points to melt;
4. The positioning indicator does not correspond to the contact position of the switch.
117. What is the main reason for heating in transformer tap changers?
The main reason for heating in tap changers is due to poor contact, which increases the contact resistance (because the loss equals I^2R, the larger R, the greater the loss and heating. This is especially likely to occur after tap changing and when the transformer is overloaded).
118. What is parallel operation of transformers?
Parallel operation refers to the operating mode where several transformers are connected with their primary and secondary sides respectively connected to the common busbars on both sides.
119. What are the conditions for parallel operation of transformers?
The conditions are equal primary voltages, equal secondary voltages (equal turns ratios), the same phase connection groups, and equal percentage values of impedance voltages.
120. Why is a current coil connected in series with the middle relay in the protection outlet of a generator-transformer unit?
It is to prevent contact flutter, vibration, or failure of the circuit breaker to trip due to too short a closure time after the protection has acted. Therefore, the general protection outlet usually uses a contact in series with a current coil for self-holding wiring, ensuring the circuit breaker trips reliably.
121. The relay with a parallel resistance in the protection outlet of a generator-transformer unit is due to the coil of the BCJ being a voltage coil with high direct resistance. Therefore, after the protection acts, the current in the relay circuit of the signal relay is very small, which may not trigger the relay, especially when several protections act at the same time, affecting the operation of the signal relay more significantly. Often, the protection acts, the circuit breaker trips, but it does not reflect which protection acted. Therefore, a parallel resistance is necessary.
122. What are the functions of HWJ and TWJ?
HWJ: Monitors the tripping circuit; TWJ: Monitors the closing circuit.
Function: Included in the control and signal circuits of the switch, it monitors the integrity of the circuit. In the event of a fault, it uses its contact position to send a position signal or a “Control Circuit Break” sign, and also connects the various tripping and closing circuits to ensure reliable action. When the switch trips, it emits an accident sound signal.
123. Why does the 1BSJ contact need to be delayed?
Only after 2HWJ is de-energized, its contact opens, 1BSJ is de-energized, 2TWJ is energized and its contact closes, confirming that the low-voltage switch has tripped. By using the delayed opening contact of 1BSJ to connect the “BZT Outlet” circuit, 1ZJ is energized, its contact closes, and the backup transformer auto-recloses. This meets the condition that the backup transformer can auto-reclose only after the bus is de-energized and can only act once.
124. Precautions for operating an oil switch?
1. When operating the remote control switch of the switch, do not use excessive force to prevent damaging the switch, nor return too quickly to prevent the mechanism from not closing;
2. Manual slow opening and closing of the switch during operation are prohibited;
3. After operating the switch, related signal lights and measurement instruments should be checked to judge the correctness of the switch action. However, this should not be used as the basis for proving the actual position of the switch. A mechanical position indicator at the site should be checked to determine the actual opening and closing positions.
125. After how many times of breaking fault current should 6KV and 35KV oil switches be taken out of service for inspection?
6KV four times, 35KV switch three times.
126. How to deal with heating in a disconnecting switch?
Depending on the importance of the location of the disconnecting switch, reduce the load current or provide ventilation and cooling to lower its heating. If the above treatment is ineffective, report to relevant leaders for a power outage treatment.
127. What is the difference between differential and gas protection for transformers?
Differential protection is designed based on the principle of circulating current method, while gas protection is set based on the characteristic of oil gas flow generated during internal faults of the transformer. Differential protection serves as the main protection for the transformer and its system, including the lead-out wires. Gas protection serves as the main protection for internal faults of the transformer.
128. How to deal with a sudden rise in temperature at individual points on the stator core during operation?
If the temperature at individual points on the core suddenly rises during operation, analyze the trend of the rise at that point and its relationship to changes in active and reactive loads. Check if the measurement point is normal. If the core temperature, inlet and outlet air temperature, and the difference in wind temperature rise significantly, and a stator ground signal appears, reduce the load and decommission immediately to prevent the core from burning out.
129. What are the advantages of using two blades for the moving contact of a disconnecting switch?
1. To prevent the blade from dislocating due to vibration when the circuit faults; 2. To ensure tighter contact between the moving and stationary contacts, reducing resistance.
130. What is the difference between the neutral point, zero point, and neutral wire?
The common connection point where the beginning or end of a three-phase winding is connected is called the neutral point of the power source. When the neutral point of the power source is well connected to the grounding device, that neutral point is called the zero point. The conductor drawn from the zero point is called the neutral wire.
131. Why is a resistor connected in series with traffic lights and DC power supply monitoring lights?
The purpose of connecting a resistor in series with traffic lights is to prevent the switch from mistakenly tripping or closing due to a short circuit at the lamp holder. The purpose of connecting a resistor in series with a DC power supply monitoring light is to prevent a short circuit of the DC power supply due to a short circuit in the filament or lamp holder, as well as to prevent the monitoring light from burning out due to high voltage.
132. How should the live parts retained at the work site be documented?
1. To be filled out by the work permit issuer;
2. It should specify the first live part adjacent to the front, back, left, right, top, and bottom of the equipment undergoing power-off maintenance, detailing the specific live parts and the names of live equipment that pose a risk of electric shock.
133. Emergency repairs can be conducted without a work permit, but which regulations must be followed?
1. Entries must be made in the record book;
2. Before starting, safety measures must be prepared according to the technical measures specified in the safety work regulations, and a specific person must be assigned to supervise.
134. What are the advantages of using frequency change as a method of speed control for three-phase AC asynchronous motors?
According to the motor speed formula N=60(1-S)f/p (f: frequency, S: slip, p: number of pole pairs), changing any one parameter can change the motor speed. Speed control by changing p is stepped, i.e., using multi-pole motors, which is more complex; changing S for speed control is uneconomical (such as rotor series resistance speed control, etc.); speed control by changing f provides high precision, high-speed control, stepless speed regulation, and a large speed regulation range, making it very economical.
135. What is a frequency converter?
It is a device that uses the on/off action of power semiconductor devices to convert a power frequency source into another frequency of electrical energy control device.
136. What is the thermal effect of current? What factors is it related to?
The heat generated when current passes through a conductor is known as the thermal effect of current. It is proportional to the resistance of the conductor, the square of the current passing through, and the time. Q=I2RT.
137. What methods are used in switchgear to accelerate the extinguishing of an electric arc?
1. Using gas to blow the arc;
2. Using multiple breaks for arc extinguishing;
3. Making the arc come into contact with surrounding media;
4. Dividing the arc into several series short arcs in low-voltage equipment.
138. What is the main function of an automatic air switch?
It is suitable for high-capacity circuits in low-voltage AC/DC devices. It can automatically cut off the circuit when overloads, undervoltage, and short circuits, etc., occur in its control circuit.
139. What is hysteresis loss?
Hysteresis loss refers to the loss caused by the friction between magnetic domains within ferromagnetic substances as they align according to the direction of the external magnetic field. If the external magnetic field is alternating, the magnetic domains will flip back and forth, causing friction and resulting in loss.
140. What methods are used to reduce eddy current losses?
1. Adding a certain amount of silicon to the steel to increase the resistivity of the core material;
2. Using a core made of many thin steel sheets insulated from each other, which shortens the path of the eddy currents and significantly reduces the losses caused by eddy currents.
141. What are the advantages and disadvantages of quasi-synchronous paralleling?
Advantages: There are no impact currents affecting the power system;
Disadvantages: In the event of asynchronous paralleling due to some reasons, the impact current is significant, and the three-phase short-circuit current is even larger. Additionally, it's challenging to master the timing for paralleling too early.
142. What causes a generator to heat up during operation?
1. Copper loss
2. Iron loss
3. Excitation loss
4. Mechanical loss
143. What are the advantages and disadvantages of air cooling in generators?
Advantages: Inexpensive, safe, simple equipment, and easy maintenance;
Disadvantages: Poor cooling effect, and the capacity of the unit is limited.
144. What causes a generator to catch fire?
1. Short circuit
2. Dirty insulation surface
3. Local overheating at joints
4. Sparks caused by stray currents
145. Why maintain a speed of 200-300 rpm when extinguishing a fire in a generator?
To avoid bending of the shaft due to overheating on one side.
146. What is the purpose of measuring the DC resistance of stator and rotor coils during generator maintenance?
To check the quality of welding inside the coils, at the ends, and of the leads, as well as the contact conditions at the joints.
147. What are the dangers of vibration in generators?
1. Increased bearing wear
2. Carbon brushes prone to sparking, significant wear on commutators and slip rings
3. Unit components prone to damage
4. Damage to buildings
148. What is the function of transformer oil?
Insulation and cooling.
149. What is the function of a transformer oil pillow?
1. To reduce the contact area between oil and air
2. To prevent oil from becoming damp and oxidized
150. How to judge whether a transformer is normal based on its temperature?
If it is found that under the same conditions, the temperature is more than 10°C higher than usual, or the temperature keeps rising despite an unchanged load, and the cooling device is operating normally, and there is no error in the temperature gauge, it can be considered that there is an abnormality inside the transformer.
151. What is the function of an oil purifier?
The oil purifier utilizes the temperature difference of the oil to circulate it within the purifier, where water, sludge, oxides, etc., in the oil are absorbed into the silica gel. Therefore, the oil purifier not only has a thermal homogenization effect but also plays a significant role in regenerating the oil.
152. Where are transformer faults most likely to occur?
Transformer faults are prone to occur in components such as windings, cores, bushings, tap changers, and oil tanks, with phenomena like oil leakage and overheated wire joints being common.
153. What is graded insulation in transformers?
Graded insulation refers to the main insulation level near the neutral point of the transformer's winding being lower than the insulation level at the end of the winding.
154. Why install a lightning arrester at the neutral point of the #1 main transformer?
The insulation at the neutral point of the #1 main transformer is graded. When the three phases are subjected to lightning strikes, the maximum voltage that appears at the neutral point can reach about 1.8 times the discharge voltage of the lightning arrester. This voltage can damage the insulation at the neutral point, so a lightning arrester must be installed.
155. What causes electrical equipment to discharge?
1. Surface contamination
2. Harsh environment
3. Insulation damage or breakdown
4. Humid weather, dampness
156. What are the hazards of electrical equipment discharge?
Long-term discharge can cause equipment damage, insulation breakdown, ground faults, short circuits, and pose threats to personal and equipment safety.
157. How to prevent electrical equipment discharge?
Clean surface contaminants, remove moisture and bubbles from insulation, improve working conditions, and improve electric field distribution.
158. What are common electrical faults in three-phase AC asynchronous motors?
Phase loss, wiring errors, winding open circuit or short circuit, inter-turn open circuit or short circuit, insulation damage, secondary circuit fault, excessive current (broken squirrel cage bars).
159. What are common mechanical faults in three-phase AC asynchronous motors?
Stator rubbing against the rotor, bearing oil shortage or damage, high temperature rise, abnormal noise, mechanical jamming, vibration.
160. What is forced energization? In what situations can forced energization be performed?
1. For power lines with an automatic reclosure device that trips without reclosing. 2. For essential working power sources with an automatic backup power source engagement device, where the backup power source does not engage after a trip. 3. For circuit breakers that trip due to accidental touching, pulling, or without any signs of fault, and it is confirmed that there is no threat to personal or equipment safety.
161. What are the cold and hot states of a generator?
It refers to the temperature of the stator coils and the core not exceeding 50% of the rated temperature. If it exceeds 50%, it is considered to be in a hot state.
162. What is the 8-degree rule for motors?
For a motor in operation, if the coil temperature exceeds the permissible value by 8°C, it will cause the insulation to age faster, and the lifespan of the motor will be halved.
163. What is the function of the demagnetizing resistor in the excitation circuit?
One is to prevent overvoltage between the rotor windings from exceeding the allowable value, and the other is to convert magnetic field energy into thermal energy, accelerating the demagnetization process.
164. What is neutral point displacement?
When the load connected in a star configuration is unbalanced, and there is no neutral wire or the impedance of the neutral wire is relatively high, a neutral point voltage can occur. This phenomenon is called neutral point displacement.
165. Why does the voltage generally decrease after a generator is connected to the grid?
After the generator is connected to the grid and its voltage is increased, the stator winding carries a current, which is an inductive current. This current has a significant armature reaction inside the generator, which weakens the rotor magnetic field, thereby causing a decrease in terminal voltage.
166. Why should the leads used for megohm meter measurements not be twisted together?
Because the voltage of the megohm meter is relatively high, twisting two conductors together for measurement can introduce a low resistance parallel to the device being tested if the insulation of the wires is poor or below the insulation level of the device being tested, affecting the measurement result.
167. What is considered an abnormal temperature rise in a transformer?
Although the oil temperature of the transformer is within the maximum allowable value, if it rises more than 10°C under the same load and cooling conditions, it is considered an abnormal temperature rise.
168. What causes an abnormal temperature rise in a transformer?
1. Poor contact of the tap changer;
2. Inter-turn short circuit in the coil;
3. Heating of internal joints;
4. Short circuit or abnormal eddy currents between silicon steel sheets of the core;
5. Abnormal operation of the cooler.
169. What impact does the loss of excitation have on a generator itself?
1. After demagnetization, the generator absorbs reactive power, causing overcurrent in the stator winding of the demagnetized generator; 2. Slip difference currents are induced on the rotor surface, causing local overheating of the rotor; 3. During asynchronous operation, the torque undergoes periodic changes, causing severe vibration of the unit.
170. What is the protection range of zero-sequence protection for transformers?
Zero-sequence protection is installed on the directly grounded side of the transformer's neutral point. It is used to protect against ground faults inside the winding and on the lead wires of that side, and it can also serve as backup protection for corresponding bus and line ground faults.
171. After the closing and opening operations of a circuit breaker are completed, why is the corresponding operating circuit automatically disconnected?
Since the closing and opening coils of the circuit breaker are designed to pass pulse current for a short time, it is necessary to automatically disconnect the corresponding operating circuit after the closing and opening operations of the circuit breaker are completed, to avoid burning out the coils due to being energized for a long time.
172. What impact does excessively high or low primary voltage have on a transformer?
When the voltage applied to a transformer is below the rated value, it has no impact on the transformer itself but affects the quality of electrical energy. When the voltage applied to the transformer is higher, it increases the transformer's excitation inrush current, causing increased core losses and overheating, and increasing the saturation level of the core, which may produce overvoltage, causing distortion in the secondary side current waveform and damage to the equipment.
173. What is leakage current?
Leakage current is the small current that flows through the medium under the action of an electric field.
174. How does a lightning arrester protect electrical equipment from atmospheric overvoltage hazards?
A lightning arrester is a discharger that is connected in parallel with the protected equipment. Under normal working voltage, the internal gap of the lightning arrester does not break down. If there is an overvoltage that threatens the insulation of the protected equipment, the internal gap of the lightning arrester will be broken down. The breakdown voltage is lower than the breakdown voltage of the insulation of the protected equipment, thus limiting the overvoltage on the insulation.
175. Why are fuses for power applications installed on the load side of the switch and not on the power source side?
If installed on the power source side, it would be inconvenient to replace the fuse and not conducive to safety.
176. What causes shaft voltage in generators?
The reasons for generating shaft voltage include:
1. The unbalanced stator magnetic field of the generator induces an electromotive force on the shaft;
2. Poor sealing of the shaft in steam turbine generators, high-speed steam leakage along the shaft, or high-speed jetting inside the steam cylinder causes the shaft itself to carry static charges.
177. Can a transformer with a rated capacity of 100 kVA carry a load of 100 kW?
Generally not allowed. 100 kVA refers to apparent power, and it should be considered based on a power factor of 0.8, so 100*0.8=80 kW.
178. Does the speed of turning the megohmmeter affect the measured resistance value? Why?
It does not affect because the reading of the megohmmeter reflects the ratio of the generator voltage to current. When the voltage changes, the current passing through the megohmmeter current coil changes in proportion as well, so the resistance value remains unchanged. However, if the speed of the megohmmeter's generator is too slow, due to the low voltage at that time, it could cause a significant measurement error.
179. What are active and reactive power?
In the process of transmitting alternating current energy, the part of the power used to convert into non-electromagnetic forms of energy, such as light, heat, mechanical energy, etc., is called active power; the part of the power used for electromagnetic exchange within the circuit is called reactive power.
180. Why discharge to ground before measuring the insulation of a cable line?
A cable line acts as a capacitor. When the cable is operational, it is charged, and the charge accumulated on the cable core cannot be fully released in a short time after power is cut off. Touching it with hands could cause electric shock, and connecting a megohmmeter could damage the meter. Therefore, it is necessary to discharge to ground before measuring cable insulation.
181. What is dielectric breakdown?
When the voltage applied to a dielectric exceeds a certain critical value, the current through the dielectric increases sharply until the dielectric loses its inherent insulation properties.
182. What are the six key points of patrol inspection?
1. Equipment that has been repaired or newly installed and is being operated for the first time;
2. Equipment that was operated in the previous or current shift;
3. Equipment that is operated frequently on and off;
4. Equipment with defects;
5. Equipment affected by system faults;
6. Equipment that has experienced accidents or anomalies.
