A selection of nearly 100 key electrical technology issues
1. What is protective grounding?
A: It involves connecting the metal casing of electrical equipment that is not normally powered to the ground via a grounding electrode.
2. What is repeated grounding?
A: In a power distribution system where the neutral point of the low-voltage side of the transformer is grounded, or in a power distribution system where one or more points on the neutral wire are grounded, the neutral wire is tightly connected to the earth through a grounding device.
3. What is a ground fault?
A: When live parts of electrical equipment and power lines in operation come into contact with grounded metal components or become directly connected to the ground due to insulation damage, it is called a grounding short circuit.
4. Which electrical devices must be grounded or connected to neutral for protection?
A: 1) Generators, transformers, high and low voltage electrical appliances; 2) Power equipment transmission devices; 3) Instrument transformer secondary coils; 4) Frames of distribution panels and control panels.
5. What is ground fault current?
A: When a ground fault occurs, the short-circuit current flows into the ground through the connection point.
6. What is voltage to ground?
A: When an electrical device experiences a phase ground fault, the potential difference between the grounded portion and the point where the high potential is zero is...
7. What is the purpose of a reversing switch?
A: It is mainly used to control small-capacity electric motors to start in both forward and reverse directions under full voltage.
8. What is the purpose of repeated grounding?
A: 1) Reduce the voltage to ground on the casing of the leaking equipment; 2) Reduce the danger of a broken neutral wire.
9. What is overload protection?
A: It is a type of protection that can delay cutting off the power supply when the load on a line or device exceeds the allowable range.
10. What are the different situations in which electrical equipment overheats?
A: The main cause is the heat generated by the electric current, including the following:
(1) Short circuit; (2) Overload; (3) Poor contact; (4) Core overheating; (5) Poor heat dissipation.
11. What are the differences between high-voltage and low-voltage electrical equipment?
A: High voltage: Equipment with a voltage to ground of 250V or higher. Low voltage: Equipment with a voltage to ground of 250V or lower.
12. What is the load factor? How can the load factor be improved?
A: The load factor is the percentage of the average active load to the peak active load over a certain period of time. It is used to measure the difference between the average load and the peak load. To improve the load factor, the main approach is to reduce peak load and increase average load.
13. What are the causes of abnormal noises during the operation of an electric motor?
A: Electrical aspects: (1) Winding grounding or phase-to-phase short circuit; (2) Winding turn-to-turn short circuit; (3) Incorrect polarity connection of winding or part of the coil; (4) Phase loss operation. Mechanical aspects: (1) Fan blades touching the end cover; (2) Severe bearing wear or ball bearing damage; (3) Weak contact between the inner ring of the bearing and the shaft; (4) Stator rubbing.
14. What should I do if a three-phase asynchronous motor fails to start after the power is turned on?
Answer: (1) Use a test pen to check if the power supply is working; (2) Check if the power switch is working properly; (3) Check the motor fuse; (4) Check the connectors on the motor terminal block; (5) Check if the motor is open-circuited; (6) Check if there is a short circuit in the stator and rotor windings.
15. What should I do if the electric motor cannot be started using the direct starting method?
Answer: (1) Star-delta starter; (2) Autotransformer starter; (3) Resistor and reactance reduced voltage starter.
16. What is the utilization rate of a transformer?
A: The ratio of the actual output power of a transformer to its rated output power.
17. How to determine the four basic states of electrical equipment?
A: 1) In "Operating" state, the circuit breaker and disconnector are closed, and the power supply is connected to the equipment. 2) In "Hot Standby" state, the circuit breaker is open, the power supply is interrupted, and the equipment is shut down, but the disconnectors on both sides of the circuit breaker remain closed. 3) In "Cold Standby" state, the equipment circuit breaker and disconnector are open. 4) In "Under Maintenance" state, the equipment circuit breaker and disconnector are open, and a sign is displayed; there are no barriers, and the grounding wire is connected.
18. How to improve the power factor?
Answer: Natural adjustment is adopted: (1) Minimize the floating capacity of transformers and motors; (2) Improve equipment utilization and reduce no-load operation; (3) Change delta-connected motors to star-connected motors, manually adjust them, and install capacitors, which is the most economical and effective method.
19. How to choose the wiring method for a capacitor bank?
A: The selection should be based on the capacitor's voltage, protection method, and capacity. When the capacitor's rated voltage matches the network's rated voltage, a delta connection is used; when the capacitor's rated voltage is lower than the network's rated voltage, a star connection is used.
20. What is overcurrent protection?
A: When a short circuit occurs on a line, its key characteristic is a sharp increase in the current in the line. When the current exceeds a certain predetermined value, the protection device will activate the current switch.
21. What are the components of a secondary circuit?
A: 1) According to the nature of the power supply: (1) AC current circuit, all circuits powered by the secondary side of the current transformer (2) AC voltage circuit, all circuits powered by the secondary side of the voltage transformer and the three-phase five-limb voltage, and the open delta of the transformer (3) DC circuit, all circuits of the DC system; 2) According to the purpose of the circuit: (1) Measurement circuit (2) Relay protection circuit
22. What is a capacitor?
A: A capacitor, also known as a capacitive device, is a static electrical device that can store electrical charge. When the charge is released, an electric current is generated, making it an energy storage device.
23. What are electric charge, current, voltage, resistance, circuit, and electric power?
A: Electric charge – The amount of electric charge carried by an object is called electric charge, and its unit is coulomb. Current – The directed movement of electric charge in a conductor forms an electric current, and its unit is ampere. The amount of electricity passing through the cross-sectional area of a conductor in one second is 1 coulomb, and its current intensity is 1 ampere. Voltage – The potential difference between two points in a circuit is called voltage, and its unit is volt. Resistance – The resistance that current encounters in a conductor is called resistance. Its unit is ohm. Circuit – The path that current travels is called a circuit. A circuit consists of four basic parts: voltage, electrical appliances, switches, and connecting wires. A power source is a device that generates electrical energy; electrical appliances include light bulbs and electric fans; connecting wires connect the power source, electrical appliances, and switches, and must form a closed circuit. Electric power – Indicates how much work is done by electricity per unit time, and its unit is watt. One kilowatt of electrical energy consumed in one hour is called "1 kilowatt-hour" (kWh).
24. What is an electric motor?
A: An electric motor is an electrical device that converts electrical energy into mechanical energy and outputs that mechanical energy. The most common type is the three-phase AC asynchronous motor. Various types of cotton spinning machinery widely use electric motors for driving.
25. What is a contactor? What is the function of a contactor?
A: A contactor is an automatic electromagnetic switch that uses electromagnetic force to close and open contacts, thus connecting and disconnecting current. Furthermore, contactors also have a low-voltage release function, making them suitable for frequent operation and distance control.
26. What is a fuse?
A: A fuse, also called a circuit breaker or fusible link, is an effective electrical protection device that prevents damage from short circuits.
27. What is a thermal relay?
A: Thermal relays utilize the effect of electric current to create electrical appliances for overload protection.
28. What is a live wire?
A: The three phase coils of a generator or transformer are connected in a star configuration. Three wires are drawn from the beginning of the coils and are called phase wires, also known as live wires.
29. What is the neutral wire?
A: The common point of the three-phase coils is called the neutral point. The conductors drawn from the neutral point are at the same potential as the ground. Since the ground has zero potential, the neutral point is called the zero point. The neutral wire is called the neutral line.
30. What is a grounding wire?
A: To prevent electric shock due to damage to electrical insulation, electrical wires are usually connected to the ground. This wire is called a protective ground wire.
31. What are the measures for safe electricity use?
A: The live wire must be connected to the switch so that the switch is in the open state and the appliance is not energized.
32. What is a low-voltage electric shock protector?
A: It is an effective protective device to prevent low-voltage electric shock. If an electric shock accident or insulation failure and leakage occurs in a low-voltage network, it will immediately issue an alarm signal or command to cut off the power supply, thus protecting people or equipment.
33. What is air gap?
A: To prevent people from getting too close to live conductors or to prevent vehicles or other objects from colliding with them and getting too close, and to prevent electrical short circuits and fires caused by them, a certain safe distance must be maintained between live conductors and the ground, between live conductors, and between equipment.
34. What is safe voltage?
A: A safe voltage is a voltage that will not cause any damage to any tissues of the human body when it comes into contact with electricity.
35. What are the dangers of unbalanced three-phase power supply?
A: Due to the three-phase voltage imbalance, a reverse-sequence rotating magnetic field will be generated on the stator of the induction motor. At this time, the induction motor will run under the action of both positive and negative magnetic fields. Since the positive-sequence rotating magnetic field is much larger than the negative-sequence magnetic field, the motor rotates in the positive-sequence direction. However, the rotor's reverse-sequence resistance is very small, so the reverse-sequence current is large. Due to the presence of the reverse-sequence current and the magnetic field, a large braking torque is generated, which will reduce the power.
36. What are the three electrical components (electricity, power, and communication)?
A: The three electricity uses refer to planned electricity use, economical electricity use, and safe electricity use.
37. Star-delta (Y-△) reduced voltage starting?
A: For motors that are delta connected when running at a rated voltage of 380V, the stator windings can be first connected in a star configuration, and then connected in a delta configuration after startup. This method is called star-delta reduced voltage starting.
38. What is the earth?
A: When an electrical equipment experiences a grounding fault, the grounding current flows into the ground through the grounding wire and grounding electrode, dissipating in a hemispherical pattern. Near the grounding electrode, the hemisphere is smaller, the resistance is higher, and the voltage drop through this area is also greater, resulting in a higher potential. Further away from the grounding electrode, the hemisphere is larger, the resistance is lower, the voltage drop is smaller, and the potential is lower. With the grounding electrode as the center, the radius of this sphere is approximately 20 meters, and its potential practically approaches zero; this is generally referred to as the earth or ground.
39. What is a grounding electrode?
A: A metal conductor or group of metal conductors that is in direct contact with the earth's soil is called a grounding electrode or grounding group.
40. What are grounding wires and grounding devices?
A: The metal conductor connecting the grounding part of electrical equipment to the grounding electrode is called a grounding wire. The grounding electrode and the grounding wire together are called a grounding system.
41. What is protective grounding?
A: Connect the non-energized metal parts of the electrical equipment to the neutral wire of the electrical grid.
42. What is the function of protective grounding?
A: The neutral point of a transformer or generator grounding system is called the zero point, and the conductor connected to the zero point is called the neutral wire. Protective grounding involves connecting the normally non-energized metal casing of electrical equipment to the neutral wire. However, in power supply systems with a directly grounded neutral point below 1000V, protective grounding is generally not used; instead, protective neutral grounding is employed.
43. What are the key points for connecting to zero?
A: When connecting to neutral, the following requirements should be met: (1) In the same system, it is strictly forbidden to ground one part of the electrical equipment and connect another part of the electrical equipment to neutral, which is extremely unsafe. (2) Switches and fuses are not allowed to be installed on the neutral wire of a three-phase four-wire system. (3) Pay attention to the quality of the neutral wire installation. In order to prevent the neutral wire from breaking, sufficient repeated grounding devices should be installed. The grounding resistance of repeated grounding should not be greater than 10 ohms. (4) All electrical equipment connected to neutral wire should be connected in parallel on the neutral wire or branch line. (5) The cross-sectional area of the neutral wire should not be less than 1/2 of the cross-sectional area of the phase wire.
44. What is a residual current device (RCD)?
A: When a single-phase ground fault occurs in an electrical line or equipment, a residual current is generated. The protective device that uses this residual current to cut off the power supply to the faulty line or equipment is called a residual current device, also known as a leakage current protection device.
45. What is the commonly used three-level protection method for leakage current protection?
A: Overcurrent, grounding, short circuit.
46. What are the causes of electrical leakage? What are the consequences of electrical leakage?
A: Causes: Insulation damage (including aging, environmental factors, mechanical factors, and toxic gases). Consequences: Induces stray currents, causing losses and increasing line losses.
47. What are the causes of leakage in lighting circuits, and what are the methods and steps for checking for leakage?
A: There are two ways for lighting circuits to leak electricity: one is that the insulation resistance between the live wire and the neutral wire is damaged, resulting in leakage between the live wire and the neutral wire. The other is that the insulation between the live wire and the ground is damaged, resulting in leakage between the live wire and the ground. The inspection steps can be carried out as follows: (1) First, determine whether there is leakage. Use a megohmmeter to measure the value of the insulation resistance, or connect an ammeter to the main switch of the building being inspected, remove all light bulbs, turn on all light switches, and if the ammeter pointer swings, it indicates that there is leakage. (2) Determine whether there is leakage between the live wire and the neutral wire, or between the live wire and the ground, or both. Taking the ammeter check as an example: cut off the neutral wire and observe the change in current. If the ammeter reading does not change, it is leakage between the live wire and the ground. If the ammeter reading is zero, it is leakage between the live wire and the neutral wire. If the ammeter reading decreases but is not zero, it indicates that the live wire is the neutral wire and there is leakage between the live wire and the ground. (3) Determine the range of leakage. If the ammeter reading remains unchanged after removing the branch circuit breaker or opening the knife switch, it indicates that the main circuit is leaking current. If the ammeter reading decreases but is not zero, it indicates that both the main circuit and the branch circuit are leaking current. (4) Locate the leakage point, and check each branch circuit one by one by opening the switch to find and eliminate the leakage point.
48. What are the methods of leakage current protection?
A: Total network protection refers to installing a protector at the low-voltage power source; for mobile electrical equipment, temporary electrical equipment, and household electricity use, final-level protection should be installed; for larger low-voltage power grids, multi-level protection is required. In all three protection methods, the power supply is automatically cut off after the residual current device (RCD) trips.
49. What are electrical safety devices? What are their specific contents?
A: Electrical safety equipment is categorized by function into operating equipment and protective equipment. Operating equipment includes: voltage detectors, insulating rods, and insulating pliers. Protective equipment includes: insulating gloves, insulating boots, insulating mats, insulating platforms, safety barriers, and safety equipment for working at heights.
50. What are the dangers of static electricity?
A: Explosions and fires, electric shocks, and disruptions to production.
51. What are some measures to eliminate the hazards of static electricity?
A: Grounding method, leakage method, neutralization method, and process control method.
52. What is a short circuit? What are the consequences of a short circuit?
A: A short circuit occurs when two power lines come into direct contact or touch without passing through a load. A short circuit can cause electrical equipment to overheat, potentially burning it out and causing a fire. The short circuit current also generates a large electrodynamic force, which can damage electrical equipment. Severe short circuits can disrupt system stability and waste electrical energy.
53. How are safe voltage values defined?
A: In my country, the safe voltage value is specified according to different environmental conditions as follows:
65 volts in buildings where there is no high risk of electric shock;
In buildings with a high risk of electric shock, the voltage is 36 volts.
In buildings with a particularly high risk of electric shock, the voltage is 12 volts.
54. What are the safety requirements for grounding and neutral connection?
Answer: (1) Continuity of electrical conductivity; (2) Reliable connection; (3) Sufficient mechanical strength; (4) Sufficient electrical conductivity and thermal stability; (5) The neutral wire must be repeatedly grounded; (6) Prevent corrosion; (7) Prevent mechanical damage; (8) The underground installation distance must meet the relevant requirements; (9) The grounding and neutral wires cannot be connected in series.
55. What are the components of ensuring the normal operation of a transformer?
A: This includes observing whether the current, voltage, power factor, oil level, oil color, temperature indication, and contact status are normal; observing whether the equipment and wiring are damaged or severely dirty; observing whether auxiliary facilities such as doors, windows, and fences are intact; listening to whether the sounds are normal, paying attention to whether there are any discharge sounds; and smelling whether there is any burning smell or other abnormal odors.
56. What is screen protection?
A: Shielding is the use of shielding devices to control unsafe factors, that is, using barriers, covers, shields, gates, etc. to isolate live parts from the outside world.
57. What is the main function of transformer oil?
A: The main functions of transformer oil are cooling and insulation.
58. What is the function of an automatic air circuit breaker?
A: Automatic air circuit breakers, also known as automatic switches, are important devices for switching circuits in low-voltage distribution networks. They provide overcurrent time protection, short-circuit instantaneous protection, and undervoltage protection.
59. What are the main types of high-voltage electrical equipment?
A: High-voltage electrical equipment mainly includes high-voltage fuses, high-voltage disconnect switches, high-voltage load switches, high-voltage circuit breakers, high-voltage switchgear, and power transformers.
60. What are the components of a circuit?
A: A circuit is the path through which current flows. It consists of a power source, a load, control and protection devices, and wires.
61. What is Ohm's Law?
A: The current through a resistive element is directly proportional to the voltage across the resistor and inversely proportional to the resistance, i.e., I = U/R.
62. What is alternating current (AC)?
A: Alternating current refers to electromotive force, the magnitude and direction of which change periodically with time.
63. What is the operating principle of an asynchronous motor?
A: An asynchronous motor works using a magnetic field. When three-phase alternating current is applied to the stator windings of the motor, the magnetic field generated by the constant changes in the three-phase current in the stator windings rotates continuously in space. As the rotor conductors cut the magnetic lines of force during the rotation of the magnetic field, (http://www.diangon.com/electrical learning network electrical learning WeChat public account copyright) and since the two ends of all the rotor conductors are connected together by two copper rings, they form a closed loop. Therefore, a rotor current is generated. This current is subjected to electromagnetic force in the rotating magnetic field, which creates an electromagnetic torque on the shaft, causing the rotor to rotate in the direction of the rotating magnetic field.
64. What are some commonly used wireless insulation materials?
A: Mica, porcelain, asbestos, marble, glass, etc.
65. What are some commonly used organic insulating materials?
A: Rubber, resin, quartz sand, silk, etc.
66. What is the importance of the live wire going to the switch?
A: When installing lighting circuits, the light switch must control the live wire. This ensures that the lamp holder is not energized when the switch is off, preventing electric shock when replacing bulbs or repairing lamp holders.
67. What are the reasons for severe overheating of the electric motor?
Answer: (1) The power grid voltage is too low or too high, the driven machinery is stuck or the lubrication is poor; (2) The motor is poorly ventilated and the fan cover is blocked by clothes and debris; (3) The rotor bar is broken; (4) Frequent forward and reverse rotation or too many starts; (5) Stator and rotor rub against each other; (6) There is a small short circuit in the stator winding or a local grounding in the stator winding.
68. What are some commonly used insulating tools?
A: We have insulating rods, insulating pliers, rubber insulating gloves and insulating boots, rubber blankets and insulating platforms.
69. What are the characteristics of a diode?
A: The characteristic of a diode is unidirectional conduction.
70. What are some common lightning protection devices?
A: Lightning rod, lightning wire, lightning protection network, lightning protection strip, lightning arrester.
71. What are the different insulation classes of electric motors? What are their respective temperature ratings?
A: Insulation classes are divided into: Class A, Class E, Class B, Class F, and Class H.
The permissible temperature limit for Class A is 105℃, with a temperature rise of 60℃.
Class E allows a maximum permissible temperature of 120°C and a temperature rise of 75°C.
Class B allows a maximum temperature of 130℃ and a temperature rise of 80℃.
Class F allows a maximum permissible temperature of 155°C and a temperature rise of 100°C.
Class H allows a maximum permissible temperature of 180℃ and a temperature rise of 125℃.
72. What are the components of a handheld power tool?
A: It consists of a drive section, a transmission section, and insulation and mechanical protection sections.
73. What are the different types of lighting methods?
Answer: (1) General lighting; (2) Local lighting; (3) Mixed lighting.
74. What are the installation requirements for lighting switches?
Answer: (1) The height of the lever switch from the ground is 1.2-1.4m and the distance from the cabinet is 150-200mm; (2) The height of the pull switch is 2.2-2.8m and the distance from the cabinet is 150-200mm; (3) The concealed switch is securely installed in the switch box, and the switch box should have a complete cover.
75. What is the minimum cross-sectional area of the suspension wire for incandescent and fluorescent lamps?
A: It should not be less than 0.75 mm2.
76. Braking of a three-phase asynchronous motor?
Answer: (1) Mechanical braking; (2) Reverse braking; (3) Energy-consuming braking.
77. What are the lubrication requirements for electric motor bearings?
A: For motors with a speed of 3000 rpm, it is advisable to fill the motor with oil to a level slightly less than 2/3 full; for motors with a speed of 1500 rpm, it is advisable to fill the motor with oil to a level slightly less than 2/3 full; for motors with a speed of 1500 rpm or higher, it is generally advisable to add calcium-sodium grease high-speed grease; for motors with a speed of less than 1000 rpm low-speed grease, it is usually advisable to add calcium-sodium grease grease.
78. What is the difference between direct current (DC) and alternating current (AC)?
A: Direct current (DC) refers to electric current whose direction does not change with time. In a circuit with constant resistance, applying a power source with constant voltage produces a current whose magnitude and direction remain constant. Alternating current (AC) refers to electric current whose magnitude and direction change periodically with time.
79. What are some common circuit protection devices?
A: Undervoltage protection, short circuit protection, limit protection, overload protection, and interlock protection.
80. What are the different types of guy wires?
Answer: (1) Ordinary guy wire (2) Four-sided guy wire (3) Two-sided guy wire (4) Passageway guy wire (5) Common guy wire (6) V-shaped guy wire (7) Bow-shaped guy wire
81. In what situations is phase determination necessary?
Answer: (1) Newly installed equipment; (2) Two or more incoming cable lines; (3) Two or more overhead lines; (4) After one of the two lines is overhauled; (5) After a transformer is overhauled, it is running in parallel with another transformer.
82. How to distinguish between operating lines and decommissioned lines?
Answer: (1) Switch status; (2) Whether there is a voltage indicator; (3) Signal light display status; (4) Direct observation with an electroscope.
83. What are the common types of equipment and line faults?
(1) Short circuit; (2) Open circuit; (3) Incorrect wiring; (4) Incorrect operation.
84. What are the basic requirements for selecting conductors?
Answer: (1) Select according to the allowable current carrying capacity; (2) Select according to the allowable voltage drop; (3) Select according to the mechanical strength.
85. What are the different types of grounding? What is their function?
A: Methods include protective grounding, functional grounding, shockproof grounding, neutral grounding, and repeated grounding. Purpose: To protect the safety of people and equipment; therefore, electrical equipment needs to be grounded.
86. What are the different types of electric shock?
A: It is divided into single-phase electric shock, two-phase electric shock, and step voltage electric shock.
87. What is phase-to-phase electric shock?
A: When the human body is insulated from the ground, it simultaneously comes into contact with two different phase wires, or the human body simultaneously comes into contact with two live parts of different phases of an electrical device. In this case, the current flows from one phase wire through the human body to the other phase wire.
88. What are safety colors?
A: Safety colors are colors used to convey safety information, such as prohibition, warning, instruction, and reminder. The nationally designated safety colors are red, blue, yellow, and green. Red indicates prohibition or stop; blue indicates instruction or mandatory regulations; yellow indicates warning or caution; and green indicates indication, safe status, or passage.
89. What is the spacing between the lines?
A: Power lines should be placed above communication lines, and high-voltage lines above low-voltage lines. The distance between communication lines and low-voltage lines must not be less than 1.5 meters; between low-voltage lines, not less than 0.6 meters; between low-voltage lines and 10kV high-voltage lines, not less than 1.2 meters; and between 10kV high-voltage lines, not less than 0.8 meters. The distance between 10kV service lines and the ground must not be less than 4.0 meters; the distance between low-voltage service lines and the ground must not be less than 2.5 meters; when low-voltage service lines cross streets with traffic, the distance to the ground must not be less than 6 meters; when crossing streets with difficult traffic or sidewalks, the distance must not be less than 3.5 meters. The burial depth of directly buried cables must not be less than 0.7 meters.
Do you really understand power quality?
As a commodity, the quality of electricity is the measure of its value, and different people have different perceptions of the quality of electricity.
In this episode, we'll delve into what power quality is.
What is power quality?
In a general sense, power quality refers to high-quality power supply, including voltage quality, current quality, power supply quality, and power consumption quality.
It can be defined as: deviations in voltage, current or frequency that cause electrical equipment to malfunction or not work properly. Its contents include frequency deviation, voltage deviation, voltage fluctuation and flicker, three-phase imbalance, temporary or transient overvoltage, waveform distortion (harmonics), voltage sag, interruption, swell and power supply continuity, etc.
In modern power systems, voltage sags and interruptions have become the most critical power quality issues. Among them:
1. Voltage quality. This refers to the deviation between the actual voltage and the ideal voltage, reflecting whether the electrical energy supplied by the power company to the user is up to standard. This definition can cover most power quality problems, but it cannot cover power quality problems caused by frequency, nor can it cover the impact and pollution of power grid power quality by electrical equipment.
2. Current quality. This reflects the current variation that is closely related to voltage quality. In addition to requiring a constant frequency and sinusoidal waveform for AC power, electricity users also require the current waveform to be in phase with the supply voltage to ensure high power factor operation. This definition helps improve power grid quality and reduce line losses, but it cannot encompass most power quality problems caused by voltage issues.
3. Power supply quality. Technically, this refers to voltage quality and power supply reliability; non-technically, it refers to service quality. This includes the speed at which the power company responds to user complaints, as well as the reasonableness and transparency of electricity pricing.
4. Power quality. This includes current quality and reflects the rights, responsibilities, and obligations of the power user in the interaction and influence between the power supplier and the user. It also includes whether the power user pays the electricity bill on time and in full.
Key indicators for measuring power quality
The metrics for measuring power quality include: voltage deviation, frequency deviation, three-phase voltage imbalance, harmonics and interharmonics, voltage fluctuations, and voltage flicker.
1. Voltage deviation: Voltage deviation refers to the instability of the power supply voltage, with voltage rises or falls;
2. Frequency deviation: All power grids have the same frequency requirements, which will not change due to different power users. Each country has its own regulations regarding frequency deviation.
3. Three-phase voltage imbalance: The value of the three-phase voltage exceeds the specified standard;
4. Harmonics and Interharmonics: Currents or voltages whose frequencies are integer multiples of the fundamental frequency and exhibit sinusoidal behavior are called harmonics. Those that are not integer multiples of the fundamental frequency are collectively called interharmonics.
5. Voltage fluctuations and flicker: Regular variations in voltage within the power grid are called voltage fluctuations, or random variations with a change range between 0.9 and 1.1. Flicker refers to the visual impact of voltage instability on light bulb illumination.
Small industries and main products involved in power quality
The main sub-sectors involved in power quality include:
1. Reactive power compensation: Classified by voltage level: mainly divided into high-voltage reactive power compensation and low-voltage reactive power compensation. Classified by different products and functions, the main reactive power compensation equipment includes: TSC, LC type passive power filter, SVC, STATCOM (SVG);
2. Harmonic mitigation: Active power filters (APF), passive filters;
3. Power quality testing;
4. Other power quality issues.
What are the national standards for power quality?
1. Power supply voltage deviation (GB/T12325—2008)
2. Voltage fluctuations and flicker (GB/T12326—2008)
3. Harmonics in public power grids (GB/T14549—1993)
4. Three-phase voltage imbalance (GB/T15543—2008)
Characteristics of power quality
Power generation companies cannot fully control power quality. Some changes in power quality are caused by power users (such as harmonics, voltage fluctuations, and flicker), or by natural disasters and uncontrollable factors.
The electrical energy indicators supplied and used at different times are usually different, meaning that the quality of electrical energy is constantly changing in space and time.
Causes of power quality
1. Nonlinearity Issues in Power System Components: Nonlinearity issues in power supply systems include: harmonics generated during normal generator operation; harmonics generated by various transformers in the power grid; harmonics generated by DC transmission; and the amplification effect of high-voltage transmission lines on harmonics. Additionally, factors such as the installation of parallel capacitors in substations also contribute to harmonic generation. Among these factors, DC transmission is currently the primary contributor to harmonic generation in power systems.
2. Nonlinear load
Nonlinear loads constitute a large proportion of industrial and residential electricity consumption, and are a major source of harmonics in power systems. The primary nonlinear load is the electric arc furnace, whose arc ignition delay and severe nonlinearity generate harmonics. In residential and industrial loads, the nonlinear volt-ampere characteristics of fluorescent lamps also contribute to significant harmonic current generation, with the third harmonic being the most prevalent. Furthermore, the use of high-power rectifiers and frequency converters also generates substantial harmonic currents, seriously impacting power grid safety.
3. Power system failure
Power quality is also affected by internal and external faults during the operation of the power system. For example, various natural disasters, abnormal human operations, various line short circuits, and changes in the working state of generators and excitation systems when the power grid fails can all have a significant impact on power quality.
The dangers of harmonics
The main hazards of power harmonics include:
1. It can cause series and parallel resonance, amplify harmonics, and create dangerous overvoltages or overcurrents;
2. It generates harmonic losses, reducing the efficiency of power generation, transformation, and power consumption equipment;
3. Accelerates the aging of electrical equipment insulation, making it more prone to breakdown and thus shortening its service life;
4. Causes equipment (such as motors, relay protection, automatic devices, measuring instruments, power electronic devices, computer systems, precision instruments, etc.) to malfunction or fail to operate correctly;
5. Interferes with communication systems, reduces signal transmission quality, disrupts the correct transmission of signals, and may even damage communication equipment.
Methods for controlling power harmonics
There are three main types of harmonic mitigation measures: first, active mitigation, which starts from the harmonic source itself and improves the electrical equipment to prevent or reduce the generation of harmonics; second, end-of-line mitigation, which starts from the equipment or system affected by harmonics and improves their ability to resist harmonic interference; and third, passive mitigation, which involves installing power filters to prevent harmonics generated by the harmonic source from being injected into the power grid or to prevent harmonics from the power system from flowing into the load end.
Due to the widespread and complex nature of harmonic sources, active mitigation methods are limited by factors such as equipment structure, efficiency, cost, and reliability, and can only solve some problems. Receiving-end mitigation and passive mitigation methods remain the main approaches to managing power harmonics. Examples include using series detuned reactors to suppress harmonic resonance amplification caused by reactive power compensation capacitors, and installing passive and active power filters in the system for filtering.
Measures to improve power quality
First, adjust the load and reduce its sensitivity. If the power company cannot meet the requirements of power users with particularly high power quality in the short term by relying solely on the measures taken by the power company, the power company must work with the power users to take necessary measures to reduce the load sensitivity and the degree of poor power quality.
Second, improve the power grid and have power companies install necessary equipment to suppress or eliminate power disturbances.
Commonly seen power quality conditioning devices have relatively simple functions, such as active power filters (APF) and dynamic voltage restorers (DVR). The device that comprehensively improves the power quality for electricity users is the power quality conditioner, which is mainly composed of a capacitor that couples a parallel inverter and a series inverter together.
Parallel inverters use PWM current control technology to compensate for nonlinear load harmonic currents and reactive power, thereby regulating the DC voltage of the capacitors. Series inverters, on the other hand, use PWM voltage control technology, which primarily controls the output voltage to suppress harmonics and reduce load sensitivity.
Because a power quality regulator consists of a series and a parallel inverter, it possesses the structural characteristics of both. It can simultaneously regulate the waveforms of current and voltage in the network, and the application of power quality regulators has greatly solved the problem of power quality in the power grid.
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