1. What is a circuit?
A: The path of electric current in an electrical device. A circuit typically consists of four parts: a power source, a switch, a load, and direct wires.
2. What is electric current?
A: The directed movement of electric charges under the action of an electric field is called electric current.
3. What is an electric field?
A: The field formed around a charged body can transmit the interaction between charged bodies.
4. What is electric charge?
A: The positive or negative charge carried by an object or the particles that make up the object.
5. What is electrical potential?
A: The energy that a unit positive charge possesses at a point is called the potential at that point.
6. What is resistance?
A: When an electric current flows in a conductor, it encounters a certain resistance, which is called the resistance of the conductor.
7. What are the basic and commonly used units of resistance?
A: Its basic unit is the ohm, abbreviated as Ω. Other commonly used units include kiloohms (kΩ) and megaohms (mΩ).
8. What is voltage?
A: The potential difference between two points in a circuit is called voltage.
9. What are the basic and commonly used units of voltage?
A: Its basic unit is the volt. Abbreviated as volt, symbol v, commonly used units are kilovolt (kV) and millivolt (mV).
10. What is a capacitor?
A: The capacity of a capacitor is the ability to store electrical charge under a certain voltage.
11. What are the basic and commonly used units of capacitance?
A: Its basic unit is the farad, with the symbol F. Other commonly used symbols include microfarad (MF) and microfarad (PF). 1F = 10⁶MF = 10¹²MMf (PF).
12. What is a capacitor?
A: A circuit element that stores electric charge and electrical energy (potential energy).
13. What is an inductor?
A: The ability of a coil to generate a self-induced electromotive force when a certain amount of changing current passes through it is called the inductance of the coil. It is simply called inductance.
14. What are the basic and commonly used units of inductance?
A: Its commonly used unit is milligram (H), and another commonly used unit is millihenry (MH). 1H = 10³MH.
15. What is the function of an inductor?
A: Inductors do not play a significant role in DC circuits, but they resist sudden changes in current for sudden load changes and AC circuits.
16. What is a conductor? What are insulators and semiconductors?
A: Objects that readily conduct electric current are called conductors. Objects that can hardly conduct electric current under normal conditions are called insulators. Objects whose conductivity lies between that of conductors and insulators are called semiconductors.
17. What is capacitive reactance? What is inductive reactance? What is reactance? What is impedance? What are their basic units?
A: The impedance of a capacitor to alternating current in a circuit is called capacitive reactance. The impedance of an inductor to alternating current in a circuit is called inductive reactance.
The impedance of capacitors and inductors to alternating current in a circuit is collectively called reactance.
Resistance, capacitance, and inductance are the impedances that impede alternating current in a circuit. Their basic unit is the ohm (Ω).
18. What are the types of power plants?
A: Thermal power plants; hydroelectric power plants; thermal power plants; nuclear power plants; wind power plants; solar power plants, etc.
19. What are the components of a power system?
A: It consists of generators, transformers, power lines, and electrical equipment.
20. What are DC circuits and AC circuits?
A: A circuit containing a DC power source is called a DC circuit. A circuit containing an AC power source is called an AC circuit.
21. What are the types of substations and distribution stations?
A: There are outdoor substations, indoor substations, underground substations, mobile substations, prefabricated substations, and simple outdoor substations.
22. What are the components of an overhead power line?
A: It mainly includes poles, insulators, conductors, crossarms, hardware, grounding devices, and foundations.
23. What are the commonly used electrical equipment for overhead power lines?
A: A. Drop-out fuse; B. Disconnecting switch; C. High-voltage pole-mounted switch (load switch); D. Surge arrester; E. Pole-mounted electrical container, etc.
24. What are the types of insulators?
A: There are needle type, butterfly type, suspension type and porcelain crossbeam type, etc.
25. What are the three main categories of conductors?
A: Single-strand conductor, multi-strand stranded wire, composite multi-strand stranded wire.
26. What is a high-voltage line?
A: Overhead or cable lines that are responsible for transmitting and distributing electrical energy, with a voltage of 3~35KV and above.
27. What is a low-voltage line?
A: Overhead or cable lines that distribute electrical energy, with a voltage of 1kV or below. Typical examples are 220V/380V power lines.
28. What is a power distribution room?
A: It is a place where high and low voltage complete sets of equipment are centrally controlled, and where electrical energy is received and distributed.
29. What is a high-voltage complete set of equipment?
A: It refers to a complete set of electrical equipment, mainly switches, used in power distribution systems to receive and distribute electrical energy. It controls, measures, protects, and adjusts circuits.
30. What is the role of high-voltage electrical appliances in the power system?
A: It plays a role in control, protection and measurement during the production, transmission and distribution of electrical energy.
31. What are the main components of high-voltage primary equipment?
A: The main types include high-voltage fuses, high-voltage disconnect switches, high-voltage load switches, and high-voltage circuit breakers.
32. What is the main function of a high-voltage fuse?
A: A protective device that melts its fuse and disconnects the circuit when the current exceeds a specified value. Its main function is to provide short-circuit protection for the circuit and the equipment within it.
33. What is the main function of a high-voltage disconnector?
A: Its main function is to isolate high-voltage power supplies to ensure the safe maintenance of other electrical equipment (including wiring). Because it lacks a dedicated arc-extinguishing structure, it cannot interrupt load current and short-circuit current.
34. What are the main functions of a high-voltage load switch?
A: It has a simple arc-extinguishing device, so it can switch on and off certain loads and overload currents, but it cannot disconnect short-circuit currents. It also has the function of isolating high-voltage power supplies and ensuring safety.
35. What are the main functions of a high-voltage circuit breaker?
A: Its main function is to not only switch on and off normal load current, but also to connect and withstand short-circuit current for a certain period of time, and to automatically trip and clear short-circuit faults under the action of protection devices.
36. What are low-voltage electrical appliances?
A: It is an electrical appliance used in circuits composed of power supply systems and electrical equipment with a rated voltage of AC 1000V or DC 1500V and below, which plays a role in protection, control, regulation, conversion and switching.
37. What is a low-voltage switchgear?
A: It is a complete set of equipment consisting of low-voltage switchgear and control electrical appliances.
38. What are the two types of low-voltage switchgear?
A: There are two types: electrical control equipment and power distribution equipment (or power distribution devices).
39. What are electrical control devices? What are their uses?
A: This refers to the electric drive control equipment for various production machinery, whose direct control object is mostly an electric motor. It is mainly used in metallurgy, mining, locomotives, ships, various production machinery, and lifting and transport machinery.
40. What are power distribution equipment?
A: It refers to complete sets of equipment used for power distribution and lighting in low-voltage power distribution systems of power plants, substations and factories and mines.
41. What are the working steps for power outage operations in power distribution systems?
A: Disconnect the power supply, test for voltage, install temporary grounding wires, hang warning signs, and install barriers.
42. What are the main components of low-voltage primary equipment?
A: The main types include low-voltage fuses; low-voltage knife switches; low-voltage knife-fuse switches and load switches; and low-voltage circuit breakers.
43. What is the main function of a low-voltage fuse?
A: It mainly provides short-circuit protection for low-voltage power distribution systems, and some systems can also provide overload protection.
44. What is the main function of a low-voltage knife switch?
A: It operates under no-load conditions and is used as a disconnector.
45. What are the main functions of low-voltage knife fuse switches and load switches?
A: Low-voltage knife-fuse switches have the dual functions of a knife switch and a fuse. The main function of a load switch is to effectively switch on and off load current and provide short-circuit protection.
46. What are the main functions of a low-voltage circuit breaker?
A: It can both switch the circuit on and off under load, and automatically trip in case of short circuit, overload, and undervoltage.
47. What is a DC motor?
A: A rotating motor whose output or input is DC electrical energy is called a DC motor.
48. In which places are DC motors used?
A: It is used in large rolling mill equipment, large precision machine tools, mine winches, trams, and other similar applications.
49. What is the basic structure of a DC motor?
A: It mainly consists of two parts: the stator and the rotor.
50. What are the two types of excitation methods for DC motors?
A: There are two types: externally motivated and self-motivated.
51. What is an asynchronous motor?
A: Also known as an induction motor, it is an AC motor that generates electromagnetic torque through the interaction of the rotating magnetic field in the air gap and the induced current in the rotor winding, thereby realizing the electromechanical energy conversion.
52. According to the rotor structure, asynchronous motors are divided into two types.
A: Yes, there are squirrel-cage and wound-rotor asynchronous motors.
53. What is the basic structure of an asynchronous motor?
A: It consists of two basic parts: the stationary part is called the stator, and the rotating part is called the rotor. There is a certain air gap between the stator and the rotor.
54. What are the two ways to connect an asynchronous motor?
A: There are two connection methods: star (Y) and triangle (△).
55. What protections are typically provided for small and medium capacity asynchronous motors?
A: Short circuit protection: Fuses are generally used as short circuit protection devices; B: Undervoltage protection: The electromagnetic coil of the magnetic starter plays an undervoltage protection role in the starting motor control circuit. Automatic air switches and autotransformers are generally equipped with undervoltage tripping devices; C: Overload protection: Thermal relays are generally used as overload protection devices for motors.
56. A three-phase asynchronous motor has a rated voltage of 220/380V marked on its nameplate. How should it be connected?
A: When the power supply line voltage is 220V, use the delta (△) connection method; when the power supply line voltage is 380V, use the star (Y) connection method.
57. What is soft starting of an electric motor?
A: Soft start means gradually increasing the motor input voltage from zero according to a preset function until the start-up is complete, thus giving the motor full voltage. During soft start, the motor's starting torque and speed gradually increase.
58. What are the different types of soft starters?
Answer: (1) Ramp-up soft start. (2) Ramp-up constant current soft start. (3) Step start. (4) Pulse impact start.
59. What is a soft starter?
A: A soft starter is a new type of device used to control squirrel-cage asynchronous motors. It is a novel motor control device that integrates motor soft starting, soft stopping, light-load energy saving, and multiple protection functions.
60. What are the application ranges of soft starters?
A: In principle, squirrel-cage induction motors are suitable for all applications where speed regulation is not required. Currently, they are used in AC 380V (660V is also possible), with motor power ranging from several kilowatts to 800kW. Soft starters are particularly suitable for various pump or fan loads requiring soft starting and soft stopping.
61. What are the differences between soft start and traditional reduced pressure start?
Answer: (1) No inrush current. When starting a motor, the soft starter linearly increases the motor starting current from zero to the set value. This causes no impact on the motor, improves power supply reliability, ensures smooth starting, reduces impact torque on the load machinery, and extends the service life of the machine. (2) It has a soft stop function, that is, smooth deceleration and gradual stopping. It can overcome the drawbacks of instantaneous power failure and stop, reduce the impact on heavy-duty machinery, avoid the water hammer effect of the high-altitude water supply system, and reduce equipment damage. (3) The starting parameters are adjustable. They can be selected according to the load conditions and the characteristics of the power grid relay protection, and can be freely and steplessly adjusted to the optimal starting current.
62. What is a frequency converter?
A: A device that converts alternating current with fixed voltage and frequency into alternating current with variable voltage or frequency is called a "frequency converter".
63. What are the main differences between a soft starter and a frequency converter?
A: A frequency converter is used where speed regulation is required; its output changes both voltage and frequency. A soft starter is essentially a voltage regulator used to start a motor; its output only changes voltage, not frequency. A frequency converter has all the functions of a soft starter, but it is much more expensive and has a much more complex structure.
64. What is a transformer?
A: It is an electrical device used to change the magnitude of AC voltage. Based on the principle of electromagnetic induction, it transforms AC voltage of one level to another to meet the needs of different loads.
65. What are the different types of transformers?
A: Transformers are divided into power transformers and special transformers. Power transformers are further divided into oil-immersed and dry-type transformers. Currently, oil-immersed transformers are used as step-up transformers, step-down transformers, interconnection transformers, and distribution transformers, while dry-type transformers are only used in some distribution transformers.
66. What are the components of a transformer?
A: It generally consists of five main parts: iron core, windings, oil tank, insulating bushings, and cooling system.
67. What are the losses that occur during the operation of a transformer?
A: Yes, there are copper losses and iron losses.
68. Why do transformer taps serve an adjustment function?
A: The voltage of a power system varies with the operating mode and load size. Voltage that is too high or too low will affect the lifespan of equipment. Therefore, to ensure power quality, it is necessary to adjust the voltage according to system fluctuations. Changing the tap changer alters the number of turns in the transformer coil, thus changing the transformer's operation and consequently the voltage, thereby achieving voltage regulation.
69. What are the commonly used types of power cables?
A: Paper-insulated cables, plastic-insulated cables (polyvinyl chloride insulated cables, polyethylene insulated cables), cross-linked polyethylene insulated cables, rubber-insulated cables, etc.
What protective devices are typically installed on 70.3~10KV high-voltage power lines?
A: Common relay protection devices in 3~10KV substations include overcurrent protection, instantaneous overcurrent protection, transformer gas protection, and undervoltage protection.
71. What is the operating power supply of a substation?
A: The power supply used by the switch control, relay protection, automatic devices and signaling equipment in a substation is called the operating power supply.
72. What are the two categories of operating power supplies?
A: AC and DC operating power supplies.
73. What are the common types of relays classified according to their operating principle?
A: Electromagnetic, inductive, electric, and magnetoelectric types.
74. What types of lightning protection equipment are typically used for overvoltage protection?
A: There are lightning rods, lightning wires, and surge arresters.
75. What are electrical instruments?
A: Electrical instruments are a general term for the technical tools required to achieve electromagnetic measurement.
76. What are the classifications of commonly used electrical instruments?
A: There are indicating instruments, comparating instruments, digital instruments and roving testing devices, recording instruments and oscilloscopes, range extenders and converters.
77. What are electrical safety devices?
A: These are essential tools and equipment used to protect the safe operation of electrical work and personal safety. They can prevent accidents such as electric shock, arc burns, and falls from heights.
78. What are the electrical safety devices?
A: Insulating safety equipment (insulating boots, insulating gloves, insulating operating rods, insulating pliers), voltage detectors, sudden power surge protection equipment, temporary grounding wires, warning signs, high-altitude work safety equipment (climbing boards, foot straps) and other safety equipment.
79. What causes a short circuit?
Answer: The causes of short circuit are: (1) incorrect wiring; (2) damaged insulation; (3) incorrect operation; (4) mechanical damage.
80. What are the dangers of a short circuit?
A: Because the current does not pass through the load during a short circuit, but only flows inside the power source, the internal resistance is very small, resulting in a large current. This powerful current will produce a large thermal and mechanical effect, which may damage the power source or circuit, or cause a fire.
81. Can short circuits be used in production?
A: Welding machines use a short circuit to generate a large current to ignite an arc between the welding rod and the workpiece for welding. When the motor starts, the current is very large. The switch connected in parallel with the ammeter can be turned off to short-circuit the ammeter. The starting current of the motor does not pass through the ammeter, which protects the ammeter. After starting, the switch should be turned off.
82. What is electrical clearance?
A: The shortest distance through air between two adjacent conductors with a potential difference in an electrical appliance.
83. What is creepage distance?
A: The shortest distance along the surface of an insulator between two adjacent conductive parts with a potential difference in an electrical appliance.
84. What is the creepage distance?
A: The creepage distance is the ratio of the creepage distance to the operating voltage, expressed in m/kV.
85. What is the rated operating voltage?
A: Under specified conditions, the voltage value that ensures the normal operation of electrical appliances.
86. What is the rated insulation voltage?
A: Under specified conditions, the nominal voltage value is used to measure the insulation strength, electrical clearance, and creepage distance of different potential parts of an electrical appliance and its components.
87. What is the rated heating current?
A: Under specified conditions, the electrical appliance can withstand the maximum current when the temperature rise of each component does not exceed the limit value during testing under an 8-hour working system.
88. What are the two main categories of electrical testing?
A: There are two main categories: insulation tests and characteristic tests.
89. What are the main components of an electric drive?
A: Electric drive consists of four basic parts: electric motor, transmission device, control equipment, and production machinery.
90. What are the components of an automatic control system?
A: It generally consists of an input stage, an amplification stage, an execution stage, a feedback stage, and a comparison stage.
91. What is a three-phase symmetrical winding?
A: This refers to a three-phase winding in which all phase windings have the same structural parameters and a 120° phase difference between them in space.
92. What is the function of the arc-extinguishing chamber of a circuit breaker?
Answer: (1) Guide the arc to blow out longitudinally, thereby preventing phase-to-phase short circuits; (2) Make the arc come into contact with the insulating wall of the arc-extinguishing chamber, thereby cooling it rapidly, increasing the deionization effect, increasing the arc column voltage drop, and forcing the arc to extinguish.
93. Why are parallel compensation capacitors used in industrial and mining enterprises?
A: Parallel capacitors are used to compensate for the reactive power required by the inductive load of the power system, achieving the following: (1) improving the voltage quality of the power grid. (2) increasing the power factor. (3) reducing line losses. (4) increasing the output of transformers and lines.
94. What is electrical safety? What aspects does it include?
A: Electrical safety refers to the quality of electrical products and the prevention of accidents during installation, use, and maintenance, such as electric shock deaths, equipment damage, electrical fires, and electrical explosions. Electrical safety includes both personal safety and equipment safety. Personal safety refers to the personal safety of electricians and other personnel involved; equipment safety refers to the safety of electrical equipment and its auxiliary equipment and facilities.
95. What are some commonly used arc-extinguishing methods?
A: Commonly used arc extinguishing methods include: rapid arc extinguishing, cooling arc extinguishing, arc blowing, long arc cutting arc extinguishing, narrow groove or slit arc extinguishing, vacuum arc extinguishing, and sulfur hexafluoride arc extinguishing.
96. What is an electric power system?
A: A power system is a unified whole comprising power generation, transmission, transformation, distribution, and consumption equipment, along with corresponding auxiliary systems. A power grid is a unified whole connecting power generation and consumption, consisting of transmission, transformation, distribution equipment, and corresponding auxiliary systems. Alternatively, a power system can be described as a whole composed of power sources, the power grid, and users.
97. What is the meaning of low-voltage switchgear and controlgear assemblies (referred to as complete sets of equipment)?
A: It is an assembly consisting of one or more low-voltage switchgear and related control, measurement, signaling, protection, and regulation equipment, with all internal electrical and mechanical connections completed by the manufacturer and assembled together using structural components.
98. In a complete set of equipment, what are the meanings of main circuit and auxiliary circuit?
A: The main circuit is all the conductive circuits that transmit electrical energy; the auxiliary circuit is all the conductive circuits outside the main circuit, including those for control, measurement, signaling, and regulation.
99. What are the structural characteristics of GCS low-voltage withdrawable switchgear?
A: The frame is made of 8FM steel, and the various functional compartments are strictly separated, mainly divided into functional unit compartments, busbar compartments, and cable compartments. The cable compartment has cable outlets at both the top and bottom. Functional units are available in four sizes: half-unit, single-unit, two-unit, and three-unit. The dimensions of a single-unit are 160mm (height) × 560mm (width) × 407mm (depth). The width of the drawer in a half-unit is 280mm. The height of the two- and three-unit units varies by two and three times, respectively. The remaining dimensions are the same as the single-unit busbar clamps and insulation supports. High-strength, flame-retardant synthetic materials are used, resulting in high insulation strength, good self-extinguishing performance, and a unique structure suitable for different busbar specifications. It is highly standardized and versatile, easy to assemble, and has reliable quality assurance.
100. What standards should the design of low-voltage distribution cabinets meet?
A: IEC439-1 (Low-voltage switchgear and controlgear assemblies), GB7251 (Low-voltage switchgear assemblies), ZBK36001 (Low-voltage withdrawable switchgear assemblies)
101. What is the protection level?
A: The level of protection that the outer casing and partitions should provide to prevent the human body from approaching live parts or touching moving parts, as well as to prevent solid objects from entering the equipment.
102. What is the meaning of working grounding?
A: Functional grounding refers to the direct or indirect connection of certain equipment in a power system to the earth via arc suppression coils, reactors, resistors, etc., for operational needs.
103. What is the meaning of protective grounding?
A: Protective grounding connects the metal casing of electrical equipment to a grounding electrode.
104. What is the rated insulation voltage?
A: Under specified conditions, the standard voltage value is used to measure the insulation strength, clearance, and creepage distance of electrical appliances and their components at different potentials.
105. What are the five protections of a handcart-type device?
A: 1. Prevent switching on or off the isolation switch under load; 2. Prevent accidentally entering a energized compartment; 3. Prevent accidentally switching on or off the main switch; 4. Prevent connecting the grounding wire while the switch is energized; 5. Prevent closing the isolation switch with the grounding wire connected.
106. What are the different working states of the handcart components?
A: 1. Positioning status 2. Movement status 3. Working position 4. Experimental position 5. Storage position.
107. What are the main functions of a circuit breaker?
A: It can connect, carry, and disconnect current under normal circuit conditions, and can also connect, carry, and disconnect current for a certain period of time under specified abnormal circuit conditions.
108. What is a current transformer?
A: It is a current conversion device.
109. What is the function of a current transformer?
A: It converts high voltage and high current or low voltage and high current into low voltage and low current for use by measuring instruments and relays.
110. What is a voltage transformer?
A: It is a voltage conversion device.
111. What does PC in a power distribution cabinet refer to?
A: Power distribution center.
112. What are harmonics?
A: When power frequency voltage or current is applied to a nonlinear load, sinusoidal voltage or current of other frequencies different from the power frequency will be generated.
113. How to control harmonics?
A: Harmonic mitigation involves installing filters at the harmonic source to absorb or suppress the harmonic current generated by the harmonic source nearby.
114. The development of intelligent distribution networks is progressing rapidly. What are the two main aspects of this trend?
A: First, the intelligentization of electrical components; second, the intelligentization of power grid control technology.
115. What is a fieldbus?
A: The digital, serial, multi-point communication data bus between field devices installed in the manufacturing and process areas and automatic control devices in the control room is called a fieldbus.
116. How is the phase sequence arrangement of busbars specified?
A: When arranged from left to right, the left side is phase A, the middle side is phase B, and the right side is phase C. When arranged from top to bottom, the top side is phase A, the middle side is phase B, and the bottom side is phase C. When arranged from farthest to nearest, the farthest side is phase A, the middle side is phase B, and the nearest side is phase C.
117. What are the regulations for color coding of the phase sequence of busbars?
A: Phase A is yellow, Phase B is green, Phase C is red, PE is black, PEN is a yellow-green alternating color, positive electrode is brown, negative electrode is blue.
118. What is overcurrent protection?
A: When a short circuit occurs in a line, one of the important characteristics is a sharp increase in the current in the line. When the current flows past a certain predetermined value, the protection device that reacts to the increase in current is called overcurrent protection.
119. What is a safe distance?
A: The minimum distances that must be maintained between a person and a live conductor, and between a live conductor and the ground or other objects, to prevent electric shock and short circuit accidents.
120. What is the typical safe voltage?
A: 36V, 12V in humid environments.
121. What is the typical safe current?
A: The power frequency should be 30mA or below, 10mA in highly dangerous locations, and 5mA when working in the air or on water.
122. What is protective grounding?
A: The protective system that connects the metal casing of electrical equipment to the neutral wire is called grounding protection.
123. What is feedback?
A: Feedback is the process of sending part or all of the input electrical quantity (voltage or current) back to its input terminal and participating in the control output together with the input signal.
124. How are feedback effects classified?
A: Negative feedback and positive feedback.
125. What are the advantages of parallel operation of transformers?
Answer: (1) Improve the reliability of power supply; (2) Improve operating efficiency; (3) Reduce reserve capacity.
126. Describe the basic methods for electrical design in a factory.
A: The process involves converting the installed capacity of electrical equipment provided by the process department into the hypothetical load of the power design – the calculated load. Based on the calculated load and allowable heating conditions, the conductor cross-section of the power supply system is selected, the transformer capacity is determined, measures to improve the power factor are formulated, protection equipment is selected and set, and the voltage quality of the power supply is verified.
127. What is smart home?
A: Smart home is a living environment that uses a residence as a platform, integrating architecture, network communication, information appliances, and equipment automation, and combining systems, structure, services, and management to create an efficient, comfortable, safe, convenient, and environmentally friendly living environment.
128. What are some ways to save electricity?
A: There are three ways to save energy: management-based energy saving, structural energy saving, and technological energy saving.
129. What are TT, IN, and IT systems?
A: TT mode refers to a protection system where the metal casing of electrical equipment is directly grounded; this is called a protective grounding system, also known as a TT system. TN mode is a protection system where the metal casing of electrical equipment is connected to the neutral wire; this is called a neutral grounding protection system, denoted by TN. IT mode is a protection system where the power supply side has no working ground, or is grounded through a high-impedance circuit. The load-side electrical equipment is grounded for protection.
130. What is the standard frequency of my country's power grid?
A: It's 50Hz, also known as power frequency.
131. Classification of electrical load?
A: Level 1 load: A power outage that would cause personal injury or death; or would cause significant political or economic damage. Level 2 load: A power outage that would cause substantial political or economic damage. Level 3 load: A general electrical load.
132. What is an electric arc?
A: An electric arc is a physical phenomenon that occurs during the operation of electrical equipment. It is characterized by its intense brightness and high temperature.
133. What are some well-known international and domestic brands of high and low voltage electrical appliances?
A: Internationally, there are companies such as ABB (Sweden/Switzerland), Schneider Electric (France), ORMNO (Japan), Siemens (Germany), and GE (USA). Domestically, there are companies such as Chint Group, Delixi Group, Shanghai Renmin Huatong Group, TCL Industrial Electric, and Great Wall CNC Electric.
134. What are phase voltage and phase current? What are line voltage and line current?
A: In a three-phase four-wire circuit, the voltage between a phase line and the neutral line is called the phase voltage; the voltage between any two phase lines is called the line voltage; the line voltage is √3 times the phase voltage. The current flowing through each phase load is called the phase current; the current flowing through the phase line is called the line current.
135. What is the main function of a transformer in a power system?
A: Its main function is to transform voltage to facilitate power transmission. On the same line segment, transmitting the same power, the voltage is increased by a step-up transformer, reducing the current transmitted through the line, thus reducing line losses, improving the economics of power transmission, and achieving the goal of long-distance power transmission. Step-down, on the other hand, can meet the needs of users at various voltage levels.
136. What are the main parameters of a transformer?
Answer: (1) Rated voltage; (2) Rated current; (3) Rated capacity; (4) No-load current; (5) No-load loss; (6) Short-circuit loss; (7) Impedance voltage; (8) Winding connection diagram, phasor diagram and connection group number.
137. What is the thermal effect of short-circuit current?
A: When a short circuit occurs in a circuit, the extremely large short-circuit current will cause the temperature of the conductor to rise rapidly, which is called the thermal effect of the short-circuit current.
138. What is an eddy current? What are the harmful effects of eddy currents?
A: When alternating current flows through a conductor, an alternating magnetic field is generated around the conductor. This alternating magnetic field induces a current within the conductor itself. Because this induced current forms a closed loop within the conductor, much like a vortex in water, it is called an eddy current. Eddy currents not only waste electrical energy and reduce the efficiency of electrical equipment, but they also cause appliances (such as transformer cores) to overheat, which can severely affect the normal operation of equipment.
139. What are the definitions and classifications of secondary circuits?
A: Secondary circuits are used to monitor measuring instruments, control operating signals, and all low-voltage circuits of relays and automatic devices. Secondary circuits can be divided into the following categories according to power supply and purpose: (1) current circuit; (2) voltage circuit; (3) operating circuit; (4) signal circuit.
140. What are some common reduced-voltage starting methods for squirrel-cage induction motors?
Answer: (1) Three-phase resistor reduced voltage start (2) Reactor reduced voltage start (3) Autotransformer reduced voltage start (4) Star-delta reduced voltage start (5) Extended delta reduced voltage start.
141. What conditions must be met for transformers to operate in parallel?
A: The three conditions are equal transformation ratio, same connection group, and same short-circuit voltage.
142. Why are current transformers used to measure high voltage and high current?
Answer: (1) It expands the range of applications of the instrument. As long as the current transformers with different transformation ratios and current ratios are used in conjunction with a voltmeter with a full-scale voltage of 100V and an ammeter with a full-scale current of 5A, various voltages and currents can be measured.
(2) To make the specifications of instruments and relays uniform, easy to standardize, and mass-produce.
(3) Using a current transformer as an intermediate element between the primary and secondary circuits isolates the danger of high voltage and prevents secondary circuit faults from affecting the primary circuit.
143. What is a relay protection device?
A: A device that can detect faults or abnormal operating conditions of electrical equipment in a power system and trigger a switch to trip or send a signal.
144. What are the commonly used types of relays?
A: Current relays, voltage relays, signal relays, time relays, intermediate relays, differential relays, and gas relays, etc.
145. How many types of protection devices are there for a transformer?
A: Overcurrent protection, instantaneous overcurrent protection, differential protection, and gas protection.
146. What are the requirements for relay protection?
A: In order to correctly and quickly isolate faults, relay protection is required to have sufficient selectivity, speed, sensitivity and reliability.
147. What are the relay protection features of small and medium-sized substations?
A: A. Overcurrent protection; B. Instantaneous overcurrent protection; C. Time-limited instantaneous overcurrent protection; D. Undervoltage protection.
148. What role does the DC system play in a substation?
A: In substations, DC systems provide reliable DC power for control signals, relay protection, automatic devices, and emergency lighting, as well as reliable operating power. The reliability of the DC system plays a crucial role in the safe operation of the substation and is its guarantee.
149. What are the commonly used DC power supplies for substations?
A: Storage batteries, silicon current, and capacitors.
150. What is phase sequence?
A: It refers to the phase sequence, the order in which the instantaneous value of the alternating current changes from negative to positive, passing through zero.
151. When a grounding fault occurs in high-voltage equipment, what is the safe distance for a person to be grounded?
A: Indoors, it should be greater than 4m, and outdoors, it should be greater than 8m.
152. Which electrical devices must be grounded or connected to neutral for protection?
A: ① Bases and housings of generators, transformers, motors, high and low voltage electrical appliances and lighting fixtures; ② Secondary coils of instrument transformers; ③ Frames of distribution panels and control panels; ④ Transmission devices of electrical equipment; ⑤ Metal structures, concrete frames and metal fences of indoor and outdoor power distribution equipment; ⑥ Cable heads and cable box housings, cable sheaths and conduits; ⑦ Pole towers of power lines and switching equipment and capacitors installed on power line poles.
153. What should you do if you discover someone has been electrocuted?
A: First, quickly remove the person from the power source. For low-voltage electric shocks, methods such as pulling, cutting, prying, dragging, or padding can be used to separate or cut off the power. During these operations, care should be taken to avoid electrocuting the rescuer. Use dry, insulated sharp objects to disconnect the power or isolate the person from the source. For high-voltage electric shocks, notify the power supply department to cut off the power to the circuit, or use an insulated lever of the appropriate voltage rating to pull open a drop-out fuse to cut off the circuit. Alternatively, short-circuit the line to trip the circuit breaker. Also, ensure the safety of the rescuer to prevent step voltage shock. If the person is electrocuted at a height, take precautions to prevent falls. After removing the person from the power source, transport them to a hospital or emergency room as quickly as possible, depending on the severity of their injuries.
